Sample records for ma oceanic crust

  1. Recycled oceanic crust in the source of 90-40 Ma basalts in North and Northeast China: Evidence, provenance and significance (United States)

    Xu, Yi-Gang


    Major, trace element and Sr-Nd-Pb isotopic data of basalts emplaced during 90-40 Ma in the North and Northeast China are compiled in this review, with aims of constraining their petrogenesis, and by inference the evolution of the North China Craton during the late Cretaceous and early Cenozoic. Three major components are identified in magma source, including depleted component I and II, and an enriched component. The depleted component I, which is characterized by relatively low 87Sr/86Sr (1.1) and HIMU-like trace element characteristics, is most likely derived from gabbroic cumulate of the oceanic crust. The depleted component II, which distinguishes itself by its high ?Nd (?8) and moderate 87Sr/86Sr (?0.7038), is probably derived from a sub-lithospheric ambient mantle. The enriched component has low ?Nd (2-3), high 87Sr/86Sr (>0.7065), low 206Pb/204Pb (17), excess Sr, Rb, Ba and a deficiency of Zr and Hf relative to the REE. This component is likely from the basaltic portion of the oceanic crust, which is variably altered by seawater and contains minor sediments. Comparison with experimental melts and trace element modeling suggest that these recycled oceanic components may be in form of garnet pyroxenite/eclogite. These components are young (Li, 2010). The western end of this stagnant slab does not go beyond the NNE-trending NSGL (Huang and Zh

  2. Field and geochemical characterisitics of the Mesoarchean (~3075 ma) Ivisaartoq greenstone belt, southern West Greenland: Evidence for seafloor hydrothermal alteration in a supra-subduction oceanic crust.

    DEFF Research Database (Denmark)

    Polat, A.; Appel, P.W.U.


    The Mesoarchean (ca. 3075 Ma) Ivisaartoq greenstone belt in southern West Greenland includes variably deformed and metamorphosed pillow basalts, ultramafic flows (picrites), serpentinized ultramafic rocks, gabbros, sulphide-rich siliceous layers, and minor siliciclastic sedimentary rocks. Primary magmatic features such as concentric cooling-cracks and drainage cavities in pillows, volcanic breccia, ocelli interpreted as liquid immiscibility textures in pillows and gabbros, magmatic layering in gabbros, and clinopyroxene cumulates in ultramafic flows are well preserved in low-strain domains. The belt underwent at least two stages of calc-silicate metasomatic alteration and polyphase deformation between 2963 and 3075 Ma. The stage I metasomatic assemblage is composed predominantly of epidote (now mostly diopside) + quartz + plagioclase ± hornblende ± scapolite, and occurs mainly in pillow cores, pillow interstitials, and along pillow basalt-gabbro contacts. The origin of this metasomatic assemblage is attributed to seafloor hydrothermal alteration. On the basis of the common presence of epidote inclusions in diopside and the local occurrence of epidote-rich aggregates, the stage I metasomatic assemblage is interpreted as relict epidosite. The stage II metasomatic assemblage occurs as concordant discontinuous layered calc-silicate bodies to discordant calc-silicate veins commonly associated with shear zones. The stage II metasomatic assemblage consists mainly of diopside + garnet + amphibole + plagioclase + quartz ± vesuvianite ± scapolite ± epidote ± titanite ± calcite ± scheelite. Given that the second stage of metasomatism is closely associated with shear zones and replaced rocks with an early metamorphic fabric, its origin is attributed to regional dynamothermal metamorphism. The least altered pillow basalts, picrites, gabbros, and diorites are characterized by LREE-enriched, near-flat HREE, and HFSE (especially Nb)-depleted trace element patterns, indicating a subduction zone geochemical signature. Ultramafic pillows and cumulates display large positive initial eNd values of + 1.3 to + 5.0, consistent with a strongly depleted mantle source. Given the geological similarities between the Ivisaartoq greenstone belt and Phanerozoic forearc ophiolites, we suggest that the Ivisaartoq greenstone belt represents Mesoarchean supra-subduction zone oceanic crust.

  3. Drilling to gabbro in intact ocean crust. (United States)

    Wilson, Douglas S; Teagle, Damon A H; Alt, Jeffrey C; Banerjee, Neil R; Umino, Susumu; Miyashita, Sumio; Acton, Gary D; Anma, Ryo; Barr, Samantha R; Belghoul, Akram; Carlut, Julie; Christie, David M; Coggon, Rosalind M; Cooper, Kari M; Cordier, Carole; Crispini, Laura; Durand, Sedelia Rodriguez; Einaudi, Florence; Galli, Laura; Gao, Yongjun; Geldmacher, Jörg; Gilbert, Lisa A; Hayman, Nicholas W; Herrero-Bervera, Emilio; Hirano, Nobuo; Holter, Sara; Ingle, Stephanie; Jiang, Shijun; Kalberkamp, Ulrich; Kerneklian, Marcie; Koepke, Jürgen; Laverne, Christine; Vasquez, Haroldo L Lledo; Maclennan, John; Morgan, Sally; Neo, Natsuki; Nichols, Holly J; Park, Sung-Hyun; Reichow, Marc K; Sakuyama, Tetsuya; Sano, Takashi; Sandwell, Rachel; Scheibner, Birgit; Smith-Duque, Chris E; Swift, Stephen A; Tartarotti, Paola; Tikku, Anahita A; Tominaga, Masako; Veloso, Eugenio A; Yamasaki, Toru; Yamazaki, Shusaku; Ziegler, Christa


    Sampling an intact sequence of oceanic crust through lavas, dikes, and gabbros is necessary to advance the understanding of the formation and evolution of crust formed at mid-ocean ridges, but it has been an elusive goal of scientific ocean drilling for decades. Recent drilling in the eastern Pacific Ocean in Hole 1256D reached gabbro within seismic layer 2, 1157 meters into crust formed at a superfast spreading rate. The gabbros are the crystallized melt lenses that formed beneath a mid-ocean ridge. The depth at which gabbro was reached confirms predictions extrapolated from seismic experiments at modern mid-ocean ridges: Melt lenses occur at shallower depths at faster spreading rates. The gabbros intrude metamorphosed sheeted dikes and have compositions similar to the overlying lavas, precluding formation of the cumulate lower oceanic crust from melt lenses so far penetrated by Hole 1256D. PMID:16627698

  4. Thin oceanic crust and flood basalts: India-Seychelles breakup (United States)

    Armitage, J. J.; Collier, J. S.; Minshull, T. A.; Henstock, T. J.


    Recent seismic experiments showed that separation of India from the Seychelles occurred in two phases of rifting. The first brief phase of rifting between India and the Laxmi Ridge formed the Gop Rift, which is characterized by thick oceanic crust and underplating of the adjacent continental margins. The age of the Gop Rift is uncertain, initiation of seafloor spreading being some time between 71 and 66 Ma. This was then followed by rifting and seafloor spreading between the Laxmi Ridge and the Seychelles, the onset of which is well dated by magnetic anomalies at 63.4 Ma and characterized by thin oceanic crust. Both of these rift events occurred within 1000 km of the center of the Deccan flood basalts, which formed at 65 ± 1 Ma. To constrain the age of the Gop Rift and to explore the reasons for the change in crustal structure between the Gop Rift and Seychelles-Laxmi Ridge margins, we employ a geodynamic model of rift evolution in which melt volumes, seismic velocity, and rare earth element (REE) chemistry of the melt are estimated. We explore the consequences of different thermal structures, hydration, and depletion on the melt production during the India-Seychelles breakup to understand the reasons behind the thin oceanic crust observed. Magmatism at the Gop Rift is consistent with a model in which the seafloor spreading began at 71 Ma, ca. 6 Myr prior to the Deccan. The opening occurred above a hot mantle layer (temperature of 200°C, thickness of 50 km) that we interpret as incubated Deccan material, which had spread laterally beneath the lithosphere. This scenario is consistent with observed lower crustal seismic velocities of 7.4 km s-1 and 12 km igneous crustal thickness. The model indicates that when the seafloor spreading migrated to the Seychelles-Laxmi Ridge at 63 Ma, the thermal anomaly was reduced significantly but not sufficient to explain the observed reduction in breakup magmatism. From observations here of 5.2 km oceanic crust, lower crustal seismic velocities of 6.9 km s-1 and a flat REE profile, we infer that breakup occurred in a region of mantle that became depleted by prior extension related to the Gop Rift.

  5. Magnetization of the oceanic crust: TRM or CRM? (United States)

    Raymond, C. A.; Labrecque, J. L.


    A model was proposed in which chemical remanent magnetization (CRM) acquired within the first 20 Ma of crustal evolution may account for 80% of the bulk natural remanent magnetization (NRM) of older basalts. The CRM of the crust is acquired as the original thermoremanent magnetization (TRM) is lost through low temperature alteration. The CRM intensity and direction are controlled by the post-emplacement polarity history. This model explains several independent observations concerning the magnetization of the oceanic crust. The model accounts for amplitude and skewness discrepancies observed in both the intermediate wavelength satellite field and the short wavelength sea surface magnetic anomaly pattern. It also explains the decay of magnetization away from the spreading axis, and the enhanced magnetization of the Cretaceous Quiet Zones while predicting other systematic variations with age in the bulk magnetization of the oceanic crust. The model also explains discrepancies in the anomaly skewness parameter observed for anomalies of Cretaceous age. Further studies indicate varying rates of TRM decay in very young crust which depicts the advance of low temperature alteration through the magnetized layer.

  6. Helium isotopes in ferromanganese crusts from the central Pacific Ocean (United States)

    Basu, S.; Stuart, F.M.; Klemm, V.; Korschinek, G.; Knie, K.; Hein, J.R.


    Helium isotopes have been measured in samples of two ferromanganese crusts (VA13/2 and CD29-2) from the central Pacific Ocean. With the exception of the deepest part of crust CD29-2 the data can be explained by a mixture of implanted solar- and galactic cosmic ray-produced (GCR) He, in extraterrestrial grains, and radiogenic He in wind-borne continental dust grains. 4He concentrations are invariant and require retention of less than 12% of the in situ He produced since crust formation. Loss has occurred by recoil and diffusion. High 4He in CD29-2 samples older than 42 Ma are correlated with phosphatization and can be explained by retention of up to 12% of the in situ-produced 4He. 3He/4He of VA13/2 samples varies from 18.5 to 1852 Ra due almost entirely to variation in the extraterrestrial He contribution. The highest 3He/4He is comparable to the highest values measured in interplanetary dust particles (IDPs) and micrometeorites (MMs). Helium concentrations are orders of magnitude lower than in oceanic sediments reflecting the low trapping efficiency for in-falling terrestrial and extraterrestrial grains of Fe-Mn crusts. The extraterrestrial 3He concentration of the crusts rules out whole, undegassed 4–40 ?m diameter IDPs as the host. Instead it requires that the extraterrestrial He inventory is carried by numerous particles with significantly lower He concentrations, and occasional high concentration GCR-He-bearing particles.

  7. Post-glacial ocean acidification and the decline of reefal microbial crusts (United States)

    Riding, R.; Liang, L.; Braga, J.


    Data from Pacific, Indian Ocean and Caribbean coral reefs indicate marked Late Pleistocene to Holocene decline in the maximum thickness of microbial carbonate crusts in reef cavities. Using estimated values of pH, temperature, CO2, and ionic composition, we calculated calcite saturation ratio (?calcite) of tropical surface seawater for the past 16 Ka. This shows a declining trend of ?calcite, paralleling that of reefal microbial crust thickness. We suggest that thinning of reefal microbial crusts could reflect decrease in seawater carbonate saturation due to ocean acidification in response to deglacial CO2 increase. Previously, decline in reefal microbial crusts, for example at Tahiti in the Pacific Ocean, has mainly been attributed to changes in nutrient supply associated with ocean upwelling and/or terrestrial run-off. Ocean acidification does not preclude such effects on microbial crust development produced by localized changes, but two features in particular are consistent with a global link with carbonate saturation state. Firstly, post-glacial decline in reefal microbial crust thickness affected tropical coral reefs in several oceans. Secondly, seawater carbonate saturation is a major long-term control on microbial carbonate abundance; microbially-induced biocalcification requires elevated seawater saturation for CaCO3 minerals and can be expected to fluctuate with carbonate saturation. In addition to compiling published crust thickness data, we measured thicknesses of microbial carbonate crusts in cavities in Tahiti reefs sampled by Integrated Ocean Drilling Program coring in 2005. This indicates halving of maximum crust thickness, during the same period as steep decline in mean-ocean calcite saturation, near the Pleistocene-Holocene transition. Reefal microbial crusts have been common since skeletal reefs became widespread during the Ordovician Period, 475 Ma ago. The habitat for cryptic crusts expanded as scleractinian corals developed cavernous frameworks. These typically form late-stage stromatolitic veneers on coral and other reef skeletons in framework cavities, and can make a substantial contribution to reef structure. In some tropical reefs of the past 10 Ma, microbial crusts constitute up to 80% of the CaCO3 framework. The thinning and/or elimination of crusts affects the wave-resistance and mechanical stability of skeletal frames, and can be expected to influence overall patterns of coral reef growth and architecture. If the deglacial decline in reefal microbial crusts recorded by our data does reflect reduction in carbonate saturation ratio due to CO2 increase, then this natural ocean acidification can be expected to have occurred with similar effects during earlier interglacial periods.

  8. Stable Chlorine Isotopes in Ocean Crust Processes (United States)

    Bach, W.; Layne, G.; Kent, A.


    The study of natural variations of Cl isotopic composition in ocean crustal rocks has large potential to further our understanding of geochemical cycling of volatiles and elements soluble in saline aqueous solutions. Studies of oceanic basalt suites to date confirm that Cl abundances are highly sensitive to the addition of saline components - either from addition of subduction-related volatile fluxes in back-arc basins and volcanic arcs or via interaction between magmas and Cl-rich seawater-derived components during melting, magma storage and transport. Recent data suggest that ? 37Cl is much more variable in the marine environment than originally thought, with strongly negative ? 37Cl values (down to -7.5 ‰ ) in marine pore waters and positive values (up to +7 ‰ ) in hydrothermal fluids from oceanic spreading centers. Moreover, mantle-derived magmatic rocks reveal large variations in ? 37Cl (-3 to +11 ‰ ), reflecting mantle heterogeneity as well as assimilation of exogenic Cl by crystallizing magmas. The large isotopic variation in low-Cl basalts has been explained by isotopic heterogeneities of the mantle, with very light ? 37Cl values in rocks from the southwest Chile Ridge that have island arc geochemical affinities and heavy ? 37Cl values in Reykjanes Ridge samples (Stewart, 2000, PhD Thesis, Duke University). The inference is that a slab-flux carries a negative ? 37Cl signature while recycled ocean crust in mantle plumes carries a strongly positive ? 37Cl signature, although this is not well constrained at present. Preferential release of isotopically light Cl from the dewatering sediments is suggested by pore water data from the Barbados and Nankai accretionary prisms with ? 37Cl values down to -7.5 ‰ (Ransom et al. 1995, Geology, 23, 715). Volcanic fumaroles also appear to have negative ? 37Cl values. If this is the case then residual Cl in the subducting slab should become isotopically heavier as 35Cl is preferentially released in the shallow subduction zone. The depleted MORB mantle is believed to have a ? 37Cl between 4 and 7 ‰ , similar to C1-chondrite (Magenheim et al., 1995, EPSL, 131, 427). MORB with high Cl and Cl/K tend to have ? 37Cl close to 0 ‰ , which has been explained by contamination of basaltic magmas with seawater-derived Cl. However, the most evolved ferrobasalts and andesites from oceanic spreading ridges have negative ? 37Cl values, down to -1.7 ‰ (Magenheim, unpublished data). Together with data for oceanic gabbros, the ? 37Cl-[Cl] data for these highly evolved rocks form a trend that could be explained by an AFC-like process, although the fact that the trend extends to negative ? 37Cl values cannot be reconciled with batch mixing of magma and salt or brine. Rather, it indicates that 35Cl is preferentially incorporated into the magma and may be related to diffusive exchange between Cl in brine pools above the melt lens of an axial magma system. A more comprehensive global dataset as well as spot analyses of Cl isotope ratios by IMP-SIMS (e.g., of melt inclusions) and the combination of ? 37Cl with other stable isotope systems (B, Li, O, H) are required before these tentative models for global chlorine cycling and crustal assimilation at spreading ridges can be rigorously evaluated.

  9. Extremely thin oceanic crust in the Proto-Indian Ocean: Evidence from the Masirah Ophiolite, Sultanate of Oman (United States)

    Peters, Tjerk; Mercolli, Ivan


    The Masirah Ophiolite is a good example of thin oceanic crust. Below pillow lavas and a sheeted dike complex with a relatively normal thickness of 1-1.5 km, the gabbroic lower crust barely exceeds 500 m in thickness. In spite of this reduced thickness, the oceanic crust preserves all members of a model ophiolite in a coherent lithostratigraphic sequence. The crust was formed during the uppermost Jurassic (circa 150 Ma) when the Indian-Madagascar plate separated from the African-Arabian plate and is therefore related to the opening of the coeval Somali basin. Geological relationships indicate that this portion of oceanic crust was formed at a ridge-transform intersect. The peculiarly reduced thickness of the gabbro layer is interpreted as the result of a weak magma supply at the edge of a ridge segment, rather than the consequence of a tectonic thinning. The cooling effect due to the vicinity of two large continental lithospheric blocks (Indian-Madagascar and African-Arabian plates) during this initial stage of the oceanization might have been an additional factor contributing to the reduction of the crustal thickness.

  10. Estimation of seismic velocities of upper oceanic crust from ocean bottom reflection loss data. (United States)

    Dong, Hefeng; Chapman, N Ross; Hannay, David E; Dosso, Stan E


    This paper describes a Bayesian inversion of acoustic reflection loss versus angle measurements to estimate the compressional and shear wave velocities in young uppermost oceanic crust, Layer 2A. The data were obtained in an experiment on the thinly sedimented western flank of the Endeavor segment of the Juan de Fuca Ridge, using a towed horizontal hydrophone array and small explosive charges as sound sources. Measurements were made at three sites at increasing distance from the ridge spreading center to determine the effect of age of the crust on seismic velocities. The inversion used reflection loss data in a 1/3-octave band centered at 16 Hz. The compressional and shear wave velocities of the basalt were highly sensitive parameters in the inversion. The compressional wave velocity increased from 2547 + or - 30 to 2710 + or - 18 m/s over an age span of 1.4 million years (Ma) from the spreading center, an increase of 4.5 + or - 1.0%/Ma. The basalt shear wave velocity increased by nearly a factor of 2, from approximately 725 to 1320 m/s over the same age span. These results show a decreasing trend of Poisson's ratio with age, from a value of 0.46 at the youngest site closest to the ridge axis. PMID:20369999

  11. Experimental Constraints on Recycling of Potassium from Subducted Oceanic Crust (United States)



    Petrological experiments on oceanic crust samples characterize the recycling of potassium from mid-ocean ridge basalts and sediments. Metasomatism could develop directly and continuously from subducted potassium-bearing crust from shallow levels to a maximum depth of 300 kilometers. Phengite (a potassium-rich mica) is the principal potassium host at subsolidus conditions. It transports potassium and water to depths of up to 300 kilometers and could yield over the entire depth range potassium-rich fluids or melts (depending on the specific geotherm), which are likely to constitute one of the primary metasomatic agents for generation of calc-alkaline magmas. PMID:8662494

  12. Chemical Composition of Ferromanganese Crusts in the World Ocean: A Review and Comprehensive Database. U.S. Geological Survey.

    National Oceanic and Atmospheric Administration, Department of Commerce — The USGS Ferromanganese Crust data set was compiled by F.T. Manheim and C.M. Lane-Bostwick of the U.S. Geological Survey, Woods Hole, MA. The data set consists of...

  13. Continental Growth and Recycling in Convergent Orogens with Large Turbidite Fans on Oceanic Crust


    Ben D. Goscombe; David A. Foster


    Convergent plate margins where large turbidite fans with slivers of oceanic basement are accreted to continents represent important sites of continental crustal growth and recycling. Crust accreted in these settings is dominated by an upper layer of recycled crustal and arc detritus (turbidites) underlain by a layer of tectonically imbricated upper oceanic crust and/or thinned continental crust. When oceanic crust is converted to lower continental crust it represents a juvenile addition to th...

  14. Primitive layered gabbros from fast-spreading lower oceanic crust. (United States)

    Gillis, Kathryn M; Snow, Jonathan E; Klaus, Adam; Abe, Natsue; Adrião, Alden B; Akizawa, Norikatsu; Ceuleneer, Georges; Cheadle, Michael J; Faak, Kathrin; Falloon, Trevor J; Friedman, Sarah A; Godard, Marguerite; Guerin, Gilles; Harigane, Yumiko; Horst, Andrew J; Hoshide, Takashi; Ildefonse, Benoit; Jean, Marlon M; John, Barbara E; Koepke, Juergen; Machi, Sumiaki; Maeda, Jinichiro; Marks, Naomi E; McCaig, Andrew M; Meyer, Romain; Morris, Antony; Nozaka, Toshio; Python, Marie; Saha, Abhishek; Wintsch, Robert P


    Three-quarters of the oceanic crust formed at fast-spreading ridges is composed of plutonic rocks whose mineral assemblages, textures and compositions record the history of melt transport and crystallization between the mantle and the sea floor. Despite the importance of these rocks, sampling them in situ is extremely challenging owing to the overlying dykes and lavas. This means that models for understanding the formation of the lower crust are based largely on geophysical studies and ancient analogues (ophiolites) that did not form at typical mid-ocean ridges. Here we describe cored intervals of primitive, modally layered gabbroic rocks from the lower plutonic crust formed at a fast-spreading ridge, sampled by the Integrated Ocean Drilling Program at the Hess Deep rift. Centimetre-scale, modally layered rocks, some of which have a strong layering-parallel foliation, confirm a long-held belief that such rocks are a key constituent of the lower oceanic crust formed at fast-spreading ridges. Geochemical analysis of these primitive lower plutonic rocks--in combination with previous geochemical data for shallow-level plutonic rocks, sheeted dykes and lavas--provides the most completely constrained estimate of the bulk composition of fast-spreading oceanic crust so far. Simple crystallization models using this bulk crustal composition as the parental melt accurately predict the bulk composition of both the lavas and the plutonic rocks. However, the recovered plutonic rocks show early crystallization of orthopyroxene, which is not predicted by current models of melt extraction from the mantle and mid-ocean-ridge basalt differentiation. The simplest explanation of this observation is that compositionally diverse melts are extracted from the mantle and partly crystallize before mixing to produce the more homogeneous magmas that erupt. PMID:24291793

  15. Changes in erosion and ocean circulation recorded in the Hf isotopic compositions of North Atlantic and Indian Ocean ferromanganese crusts (United States)

    Piotrowski, Alexander M.; Lee, Der-Chuen; Christensen, John N.; Burton, Kevin W.; Halliday, Alex N.; Hein, James R.; Günther, Detlef


    High-resolution Hf isotopic records are presented for hydrogenetic Fe–Mn crusts from the North Atlantic and Indian Oceans. BM1969 from the western North Atlantic has previously been shown to record systematically decreasing Nd isotopic compositions from about 60 to ?4 Ma, at which time both show a rapid decrease to unradiogenic Nd composition, thought to be related to the increasing influence of NADW or glaciation in the northern hemisphere. During the Oligocene, North Atlantic Hf became progressively less radiogenic until in the mid-Miocene (?15 Ma) it reached +1. It then shifted gradually back to an ?Hf value of +3 at 4 Ma, since when it has decreased rapidly to about ?1 at the present day. The observed shifts in the Hf isotopic composition were probably caused by variation in intensity of erosion as glaciation progressed in the northern hemisphere. Ferromanganese crusts SS663 and 109D are from about 5500 m depth in the Indian Ocean and are now separated by ?2300 km across the Mid-Indian Ridge. They display similar trends in Hf isotopic composition from 20 to 5 Ma, with the more northern crust having a composition that is consistently more radiogenic (by ?2 ?Hf units). Paradoxically, during the last 20 Ma the Hf isotopic compositions of the two crusts have converged despite increased separation and subsidence relative to the ridge. A correlatable negative excursion at ?5 Ma in the two records may reflect a short-term increase in erosion caused by the activation of the Himalayan main central thrust. Changes to unradiogenic Hf in the central Indian Ocean after 5 Ma may alternatively have been caused by the expanding influence of NADW into the Mid-Indian Basin via circum-Antarctic deep water or a reduction of Pacific flow through the Indonesian gateway. In either case, these results illustrate the utility of the Hf isotope system as a tracer of paleoceanographic changes, capable of responding to subtle changes in erosional regime not readily resolved using other isotope systems.

  16. Variation of young oceanic crust and upper mantle structure

    International Nuclear Information System (INIS)

    Seismic refraction and single-channel reflection data taken along 0.5-, 2.5-, and 4.5-m.y.-age isochrons near the East Pacific Rise during Project ROSE are used to determine if a systematic change in the P velocity-depth function with age can be resolved. Inversion of these data suggests that any change in crustal P velocity structure related to age is smaller than variability in the seismic velocity-depth function along an isochron. The emergence of a 'normal' crust-mantle transition by 4.5-m.y.-age is seen in these data. Crust and crust-mantle transition zone heterogeneity along these isochrons may be related to the along strike variability in processes at the ridge crest. The velocity-depth functions for the threee split profile refractions lines are compared with velocity-depth functions for the Samail ophiolite, which is thought to represent oceanic crust of similar age. The velocity-depth functions for the ROSE data are bounded by different velocity-depth models for the Samail ophiolite; this suggests that those models are not in disagreement but represent the lateral heterogeneity that can be expected in young oceanic crust

  17. Magnetization of the oceanic crust - Thermoremanent magnetization of chemical remanent magnetization? (United States)

    Raymond, C. A.; Labrecque, J. L.


    A model was proposed in which chemical remanent magnetization (CRM) acquired within the first 20 Ma of crustal evolution may account for 80 percent of the bulk natural remanent magnetization (NRM) of older basalts. The CRM of the crust is acquired as the original thermoremanent magnetization (TRM) is lost through low temperature alteration. The CRM intensity and direction are controlled by the post-emplacement polarity history. This model explains several independent observations concerning the magnetization of the oceanic crust. The model accounts for amplitude and skewness dicrepancies observed in both the intermediate wavelength satellite field and the short wavelength sea surface magnetic anomaly pattern. It also explains the decay of magnetization away from the spreading axis, and the enhanced magnetization of the Cretaceous Quiet Zones while predicting other systematic variations with age in the bulk magnetization of the oceanic crust. The model also explains discrepancies in the anomaly skewness parameter observed for anomalies of Cretaceous age. Further studies indicate varying rates of TRM decay in very young crust which depicts the advance of low temperature alteration through the magnetized layer.

  18. Do Two Deep Drill Holes Into the Upper Ocean Crust Quantify the Hydrothermal Contribution to Global Geochemical Cycles? (United States)

    Teagle, D. A. H.; Alt, J.; Coggon, R. M.; Harris, M.; Smith-Duque, C. E.; Rehkamper, M.


    Vigorous circulation of seawater at the ocean ridges is required to cool and crystallize magma to form new ocean crust. Axial and ridge flank hydrothermal fluid circulation is accompanied by seawater-basalt exchanges over a spectrum of temperatures that buffer the chemistry of seawater, provide unique microbial niches, alter the chemistry and mineralogy of the ocean crust, and through subduction return surface-derived geochemical tracers to the interior of our planet. In many models of axial and ridge flank hydrothermal circulation, most fluid-rock interaction occurs in the upper oceanic crust. Hence inventories of seawater exchange should be captured by relatively shallow (drilling we have just two deep drill holes that sample the lava and dike layers of intact upper oceanic crust. DSDP Hole 504B on 6.9 Ma ocean crust produced at the intermediate spreading rate Costa Rica Rift penetrates 1836 m into basement through a complete sequence of lavas to near the base of the sheeted dike complex. In isolation, Hole 504B became the 'reference section' for upper oceanic crust from which hydrothermal contributions to global geochemical cycles were determined. The recent drilling of Hole 1256D in 15 Ma superfast spreading rate Pacific crust penetrated through the complete volcanic and sheeted dike layers into the underlying gabbroic rocks in intact ocean crust for the first time. These boreholes are complemented by observations from seafloor tectonic windows, fracture zones, and ophiolites, but these are imperfect analogs. Although Holes 504B and 1256D formed at different spreading rates, crust from both sites is expected to conform to textbook Penrose-type layering, albeit with different thicknesses of lavas and dikes. However, what was not anticipated was the contrasting distribution and nature of elemental and isotopic hydrothermal exchanges. Differences reflect the influence of local crustal structure, such as lava morphology and flow thicknesses, and thermal gradients on hydrothermal processes. These contrasts highlight the importance of further deep drilling to at least the upper gabbros in a range of spreading rates and ages to robustly extrapolate the results from what will always be a limited number of bore holes to quantify global hydrothermal exchanges.

  19. Thickened juvenile lower crust-derived ~ 90 Ma adakitic rocks in the central Lhasa terrane, Tibet (United States)

    Sun, Gao-Yuan; Hu, Xiu-Mian; Zhu, Di-Cheng; Hong, Wen-Tao; Wang, Jian-Gang; Wang, Qing


    The questions of why the Late Cretaceous magmatism generated and how the nature of the lower crust evolves in central Tibet remain poorly constrained. In this paper, we report the presence of early Late Cretaceous adakitic rocks from the Azhang area, northern edge of the central Lhasa subterrane, central Tibet. These rocks are rhyodacites/dacites in composition and have geochemical characteristics of adakitic rocks, e.g., high Sr (554-836 ppm), Sr/Y (66-100), and (La/Yb)N (20-21), low Y (7.96-8.96 ppm) and heavy rare earth elements (HREE). In situ zircon U-Pb dating for two samples yields an early Late Cretaceous age (90 ± 1 Ma and 87 ± 1 Ma). The low MgO (1.4-1.9 wt.%) contents and compatible element abundances (e.g., Cr = 22-30 ppm; Ni = 19-25 ppm) indicate that these rocks were most likely derived from the partial melting of a garnet-bearing amphibolite under a thickened lower crust condition. The positive whole-rock ?Nd(t) (+ 2.5 to + 5.6) and zircon ?Hf(t) (+ 8.9 to + 16.0) values suggest that this thickened lower crust was juvenile. The crust beneath the central Lhasa subterrane may have been significantly thickened due to tectonic shortening in response to the Lhasa-Qiangtang collision and magma underplating before the emplacement of Azhang adakitic rocks (~ 90 Ma). We argue that regional lithospheric delamination at ~ 90 Ma triggered the partial melting of the lowermost garnet-bearing crust that is still attached to the middle crust to generate the Azhang adakitic rocks. The presence of the ~ 90 Ma Azhang adakitic rocks provides valuable constraints on the origin of the early Late Cretaceous magmatism in the central Lhasa subterrane and on the crustal evolution beneath the Lhasa-Qiangtang collisional zone prior to the Cenozoic India-Asia collision.

  20. Primary carbonatite melt from deeply subducted oceanic crust. (United States)

    Walter, M J; Bulanova, G P; Armstrong, L S; Keshav, S; Blundy, J D; Gudfinnsson, G; Lord, O T; Lennie, A R; Clark, S M; Smith, C B; Gobbo, L


    Partial melting in the Earth's mantle plays an important part in generating the geochemical and isotopic diversity observed in volcanic rocks at the surface. Identifying the composition of these primary melts in the mantle is crucial for establishing links between mantle geochemical 'reservoirs' and fundamental geodynamic processes. Mineral inclusions in natural diamonds have provided a unique window into such deep mantle processes. Here we provide experimental and geochemical evidence that silicate mineral inclusions in diamonds from Juina, Brazil, crystallized from primary and evolved carbonatite melts in the mantle transition zone and deep upper mantle. The incompatible trace element abundances calculated for a melt coexisting with a calcium-titanium-silicate perovskite inclusion indicate deep melting of carbonated oceanic crust, probably at transition-zone depths. Further to perovskite, calcic-majorite garnet inclusions record crystallization in the deep upper mantle from an evolved melt that closely resembles estimates of primitive carbonatite on the basis of volcanic rocks. Small-degree melts of subducted crust can be viewed as agents of chemical mass-transfer in the upper mantle and transition zone, leaving a chemical imprint of ocean crust that can possibly endure for billions of years. PMID:18668105

  1. Primary carbonatite melt from deeply subducted oceanic crust

    Energy Technology Data Exchange (ETDEWEB)

    Walter, M.J.; Bulanova, G.P.; Armstrong, L.S.; Keshav, S.; Blundy, J.D.; Gudfinnesson, G.; Lord, O.T.; Lennie, A.R.; Clark, S.M.; Smith, C.B.; Gobbo, L.


    Partial melting in the Earth's mantle plays an important part in generating the geochemical and isotopic diversity observed in volcanic rocks at the surface. Identifying the composition of these primary melts in the mantle is crucial for establishing links between mantle geochemical 'reservoirs' and fundamental geodynamic processes. Mineral inclusions in natural diamonds have provided a unique window into such deep mantle processes. Here they provide exper8imental and geochemical evidence that silicate mineral inclusions in diamonds from Juina, Brazil, crystallized from primary and evolved carbonatite melts in the mantle transition zone and deep upper mantle. The incompatible trace element abundances calculated for a melt coexisting with a calcium-titanium-silicate perovskite inclusion indicate deep melting of carbonated oceanic crust, probably at transition-zone depths. Further to perovskite, calcic-majorite garnet inclusions record crystallization in the deep upper mantle from an evolved melt that closely resembles estimates of primitive carbonatite on the basis of volcanic rocks. Small-degree melts of subducted crust can be viewed as agents of chemical mass-transfer in the upper mantle and transition zone, leaving a chemical imprint of ocean crust that can possibly endure for billions of years.

  2. Ultramafic rock/seawater interaction in the oceanic crust

    International Nuclear Information System (INIS)

    A deposit consisting almost exclusively of the Mg-silicate sepiolite has been sampled from the Owen transform zone in the western Indian Ocean. This deposit is associated with basaltic, gabbroic and ultramafic rocks. On the basis of major, trace and rare earth elemental composition, strontium and oxygen isotopic ratios, as well as of theoretical considerations, the sepiolite deposit appears to have formed neither from seawater nor from solutions resulting from the interaction of seawater with the basaltic crust. Interaction of ultramafic rocks with seawater circulating in the crust, particularly in transform zones, may produce solutions which at low temperature (0C) become enriched in Mg and/or Si, and can give rise to precipitation of sepiolite on or below the sea floor. The ratio of Mg to Si in the solutions is probably a factor in determining whether sepiolite or another Mg-silicate (i.e., one of the serpentine polymorphs) is deposited. (orig.)

  3. Periodic deformation of oceanic crust in the central Indian Ocean

    Digital Repository Service at National Institute of Oceanography (India)

    Krishna, K.S.; Ramana, M.V.; Rao, D.G.; Murthy, K.S.R.; Rao, M.M.M.; Subrahmanyam, V.; Sarma, K.V.L.N.S.


    New seismic reflection profiles of ~~ 5370 km, running through the Ocean Drilling Program Leg 116 sites and Deep Sea Drilling Project Sites 215 and 218, were obtained to investigate the spatial extent, timing, and nature of the Tertiary deformation...

  4. Cretaceous evolution of the Indian Plate and consequences for the formation, deformation and obduction of adjacent oceanic crust (United States)

    Gaina, C.; Van Hinsbergen, D. J.; Spakman, W.


    As part of the gradual Gondwana dispersion that started in the Jurassic, the Indian tectonic block was rifted away from the Antarctica-Australian margins, probably in the Early-Mid Cretaceous and started its long journey to the north until it collided with Eurasia in the Tertiary. In this contribution first we will revise geophysical and geological evidences for the formation of oceanic crust between India and Antarctica, India and Madagascar, and India and Somali/Arabian margins. This information and possible oceanic basin age interpretation are placed into regional kinematic models. Three important compressional events NW and W of the Indian plate are the result of the opening of the Enderby Basin from 132 to 124 Ma, the first phase of seafloor spreading in the Mascarene basin approximately from 84 to 80 Ma, and the incipient opening of the Arabian Sea and the Seychelles microplate formation around 65 to 60 Ma. Based on retrodeformation of the Afghan-Pakistan part of the India-Asia collision zone and the eastern Oman margin, the ages of regional ophiolite emplacement and crystallization of its oceanic crust, as well as the plate tectonic setting of these ophiolites inferred from its geochemistry, we evaluate possible scenarios for the formation of intra-oceanic subduction zones and their evolution until ophiolite emplacement time. Our kinematic scenarios are constructed for several regional models and are discussed in the light of global tomographic models that may image some of the subducted Cretaceous oceanic lithosphere.

  5. Energetics of hydrothermal convection in heterogeneous ocean crust (United States)

    Ruepke, Lars; Hasenclever, Joerg; Andersen, Christine


    Recent advances in hydrothermal flow modeling have revealed the key thermodynamic and fluid-dynamic controls on hydrothermal convection and vent temperatures at oceanic spreading centers. The observed upper limit to black smoker vent temperatures of approx. 400°C can be explained by the thermodynamic properties of water (Jupp and Schultz, 2000). Likewise, 3D models of hydrothermal flow at fast-spreading ridges show cylindrical upwellings with closely interwoven recharge flow (Coumou et al., 2008, Hasenclever et al., 2014). While these studies provide a robust theoretical basis for hydrothermal flow observations at fast-spreading ridges, the situation at slow-spreading ridges is different. The slow-spreading Mid-Atlantic Ridge produces highly heterogeneous crust along its tectonic and magmatic segments with significant permeability contrasts across structural and lithological interfaces. The sub-seafloor permeability structure has a strong control on vent field location such that off-axis hydrothermal systems are apparently consistently located at outcropping fault zones. We have recently shown that preferential flow along high-permeability conduits inevitably leads to the entrainment of cold ambient seawater (Andersen et al., 2014), which causes a temperature drop that is difficult to reconcile with fault-related high-temperature venting. A fundamental question is therefore how hydrothermal fluids can maintain their high temperature while flowing kilometers from a driving heat source through highly heterogeneous crust to a vent site at the seafloor? We address this question by exploring the energetics of hydrothermal convection in heterogeneous ocean crust using 2D and 3D flow simulations. In our analysis we focus on the energy balance of rising hydrothermal plumes and on mixing processes at permeability boundaries, with the aim to establish a more robust theoretical framework for hydrothermal flow through highly heterogeneous seafloor.

  6. Native Cu from the oceanic crust: Isotopic insights into native metal origin


    Dekov, Vesselin; Rouxel, Olivier; Asael, Dan; Halenius, Ulf; Munnik, Frans


    Ocean drilling has revealed that, although a minor mineral phase, native Cu ubiquitously occurs in the oceanic crust. Cu isotope systematics for native Cu from a set of occurrences from volcanic basement and sediment cover of the oceanic crust drilled at several sites in the Pacific, Atlantic and Indian oceans constrains the sources of Cu and processes that produced Cu0. We propose that both hydrothermally-released Cu and seawater were the sources of Cu at these sites. Phase stability diagram...

  7. Partial separation of halogens during the subduction of oceanic crust (United States)

    Joachim, Bastian; Pawley, Alison; Lyon, Ian; Henkel, Torsten; Clay, Patricia L.; Ruzié, Lorraine; Burgess, Ray; Ballentine, Christopher J.


    Incompatible elements, such as halogens, have the potential to act as key tracers for volatile transport processes in Earth and planetary systems. The determination of halogen abundances and ratios in different mantle reservoirs gives us the ability to better understand volatile input mechanisms into the Earth's mantle through subduction of oceanic crust. Halogen partition coefficients were experimentally determined between forsterite, orthopyroxene and silicate melt at pressures ranging from 1.0 to 2.3 GPa and temperatures ranging from 1500-1600°C, thus representing partial melting conditions of the Earth's mantle. Combining our data with results of recent studies (Beyer et al. 2012; Dalou et al. 2012) shows that halogen partitioning between forsterite and melt increases by factors of about 1000 (fluorine) and 100 (chlorine) between 1300°C and 1600°C and does not show any pressure dependence. Chlorine partitioning between orthopyroxene and melt increases by a factor of about 1500 for a temperature increase of 100°C (anywhere between 1300°C and 1600°C), but decreases by a factor of about 1500 for a pressure increase of 1.0 GPa (anywhere between 1.0 GPa and 2.5 GPa). At similar P-T conditions, a comparable effect is observed for the fluorine partitioning behaviour, which increases by 500-fold for a temperature increase of 100°C and decreases with increasing pressure. Halogen abundances in mid-ocean ridge basalts (MORB; F=3-15, Cl=0.5-14ppm) and ocean island basalts (OIB; F=35-65, Cl=21-55 ppm) source regions were estimated by combining our experimentally determined partition coefficients with natural halogen concentrations in oceanic basalts (e.g. Ruzié et al. 2012). The estimated chlorine OIB source mantle concentration is in almost perfect agreement with primitive mantle estimates (Palme and O'Neill 2003). If we expect an OIB source mantle slightly depleted in incompatible elements, this suggests that at least small amounts of chlorine are recycled deep into the mantle through subduction of oceanic crust, possibly via marine pore fluids (Sumino et al. 2010). The OIB source region is, however, significantly enriched in fluorine relative to the primitive mantle by a factor of 1.4-3.6, which indicates that significantly larger amounts of fluorine are transported deep into the Earth's mantle through subduction. An explanation for the partial separation of chlorine and fluorine during subduction is that the heavy halogens are more likely to escape from the subducting slab in hydrous fluids at an early subduction stage whereas significant amounts of fluorine are likely to remain in the slab, possibly incorporated in the lattice of hydrous amphibole or mica, or in anhydrous high-pressure phases of eclogite. The MORB source mantle is degassed in fluorine (17-88%) and chlorine (22-99%) relative to primitive mantle estimates. Preliminary data suggest that the bromine partitioning behaviour between forsterite and melt is roughly comparable to the behaviour of fluorine and chlorine. If true, this would imply that the Earth's upper mantle is presumably degassed of all halogens despite the more likely escape of heavy halogens from the slab at an early subduction stage, implying that these halogens are at least partly accumulating in the crust after leaving the slab. Beyer C, Klemme S, Wiedenbeck M, Stracke A, Vollmer C (2012) Earth Planet Sci. Lett. 337-338, pp. 1-9. Dalou C, Koga KT, Shimizu N, Boulon J, Devidal JL (2012) Contrib. Mineral. Petrol. 163, pp. 591-609. Palme H, O'Neill HSTC (2003) Treatise Geochem. 2, pp. 1-38. Ruzié L, Burgess R, Hilton DR, Ballentine CJ (2012) AGU Fall Meeting 2012. V31A-2762 (abstr.). Sumino H, Burgess R, Mizukami T, Wallis SR, Holland G, Ballentine CJ (2010) Earth Planet. Sci. Lett. 294, pp. 163-172.

  8. Geochemistry of hydrothermally altered oceanic crust: DSDP/ODP Hole 504B - Implications for seawater-crust exchange budgets and Sr- and Pb-isotopic evolution of the mantle (United States)

    Bach, Wolfgang; Peucker-Ehrenbrink, Bernhard; Hart, Stanley R.; Blusztajn, Jerzy S.


    This paper presents petrographic, chemical, and isotopic (Sr, S) analyses of whole rock samples from a 1.8 km section of upper ocean crust (DSDP/ODP Hole 504B). The samples were selected to cover all lithologies (pillows, flows, breccias, dikes) and alteration/mineralization styles. The chemical and petrographic data were used to calculate weighted averages for upper crustal composition, based on which seawater-ocean crust exchange fluxes were calculated. These results confirm earlier estimates that identify the upper crust as a significant sink for K and Mg and a source of Ca and Si to the oceans. Changes in trace element geochemistry implies that the upper ocean crust in 504B is a sink for CO2, Rb, Cs, and U, although the flux rates are an order of magnitude smaller than suggests by previous estimates for DSDP Sites 417 and 418 in 118 Ma Atlantic crust. Fluxes of these components are similar, within a factor of four, to flux rates estimated for the Juan de Fuca Ridge flank, which may relate to similarities in the thermal and hydrogeological evolution at both sites that is controlled by rapid termination of fluid circulation and conductive reheating of the upper crust. The contrast between the fluxes of trace elements derived for those settings and the open-ocean sites 417/418 likely reflects prolonged fluid-rock interaction at the latter location. If the Mg uptake and Sr exchange reconstructed from 504B core is representative, ridge flank hydrothermal alteration cannot account for the imbalance in the Mg and Sr budgets of the oceans. Up to 10% of the crustal Pb resides in the mineralized parts of the transition zone between the volcanic section and the sheeted dike complex. Combined, the Pb mobilized in the deepest parts of the hydrothermal systems (probably not penetrated in 504B) and hosted in metalliferous sediments and mineralized stockwork may account for the Pb surplus of the continental crust and the evolution of Ce/Pb of the mantle. Hydrothermal alteration results in net increases of Rb/Sr and U/Pb, in particular in the uppermost 600 m of crust, but the increases are not large enough to make altered upper ocean crust a plausible precursor for the HIMU mantle component. Moreover, the fractionation between Th and Pb, if any, is insufficient to account for the development of highly radiogenic 208Pb/204Pb in a HIMU mantle source. Potential HIMU precursors can be derived from altered ocean crust after 1-2 Ga, if on the order of 80-90% Pb, 40-55% Rb, 40% Sr, and 35-40%U are removed during partial dehydration in subduction zones.

  9. Metasomatic modification of oceanic crust during early stages of subduction recorded in Mariana blueschist (United States)

    Zack, Thomas; Savov, Ivan P.; Pabst, Sonja; Schmitt, Axel K.


    Serpentine mud volcanoes from the Mariana forearc bear unique witness of metasomatic processes in an active subduction zone in the form of centimeter-size blueschist-facies xenoliths. Charcateristic metamorphic assemblages point to conditions of ca 400°C and a formation depth of 27 km. Bulk rock compositions of amphibole-talc schists and chlorite-rich schists lie on a mixing line, extending from typical MORB towards SiO2-enriched mantle. Such mixing trends are remarkably similar to findings from the amphibolite-facies assemblages of the Catalina schist, although they equilibrated at much lower temperatures (Pabst et al. 2012). These observations demonstrate that the material experienced severe metasomatic changes at the slab-mantle interface in the shallow forearc. Further supporting evidence derives from ?11B measurements: phengite, amphibole and chlorite within the clasts have boron isotope values of -6±4‰, significantly lighter than oceanic crust, requiring isotopic fractionation by fluids carrying an isotopically heavy B component (Pabst et al. 2012). Although most current models assume that the Mariana blueschists record conditions of the ongoing subduction process, our recent findings indicate otherwise. Large (>100 µm) rutiles with high U (ca 20 ppm) found in one blueschist clast were dated by HR-SIMS at UCLA employing recently established U/Pb dating techniques (Schmitt & Zack 2012). Rutile concordia ages were tightly constrained at 48.1±2.9 Ma and are reproduced by concordia ages of low Th/U zircons at 47.5±1.5 Ma in the same sample. As those ages are interpreted to be formation ages of metasomatically modified blueschists and are only a few million years older than subduction initiation (at ca 50-52 Ma), we draw the following conclusions: (1) fast cooling of the downgoing oceanic crust must occur right after subduction initiation; (2) effective metasomatic and mechanical mixing processes (subduction channels?) must be established early in subduction zones and (3) the forearc mantle (source region of serpentine mud volcanoes) must contain stable areas where 48 Ma old low-grade samples are not being reset. Pabst S et al. 2012: Lithos 132-133, pp. 162-179; Schmitt AK & Zack T 2012: Chem Geol 332-333, pp. 65-73.

  10. Rock magnetic characterization through an intact sequence of oceanic crust, IODP hole 1256 and comparison with DSDP 504 B

    International Nuclear Information System (INIS)

    Complete text of publication follows. One goal of drilling a complete oceanic crust section is to determine the source of marine magnetic anomalies. For crust generated by fast seafloor spreading, is the signal dominated by the upper extrusive layer, do the sheeted dikes play a role, what role do the gabbros play relative to slow spreading centers, and what is the timing of acquisition of the magnetization? To address these questions, we are conducting a comprehensive set of rock magnetic, paleomagnetic measurements and microscopic studies that extend through the intervals drilled on Leg 206 and Expeditions 309 and 312. Recent drilling in the Eastern Pacific Ocean in Hole 1256D reached gabbro within seismic layer 2, 1157 meters into crust formed at a superfast spreading rate (i.e. up to 200mm/year full rate) on the Cocos-Pacific plate boundary between 19 and 12 million years ago. Sampling an intact sequence of oceanic crust through lavas, dikes, and gabbros is necessary to advance the understanding of the formation and evolution of crust formed at mid-ocean ridges, but it has been an elusive goal of scientific ocean drilling for decades. Continuous downhole variations in magnetic grain size, coercivity, mass-normalized susceptibility, Curie temperatures, and composition have been mapped. Compositionally, we have found that the iron oxides vary from being titanium-rich (TM60) to titanium-poor magnetite as determined semi-quantitatively from Curie temperature analyses. tatively from Curie temperature analyses. Magnetic grain sizes vary from few Single Domain (SD), to the majority of them being Pseudo Single Domain (PSD) and some on the Multi Domain (MD) area of the Day diagram. The low-Ti magnetite or stoichiometric magnetite is present mainly in the lowest part of the section and is associated with higher Curie temperatures (550 A deg C to near 580 A deg C) and higher coercivities than in the extrusive basalts. Skeletal titanomagnetites with varying degrees of alteration is the most common magnetic mineral throughout the section and is often bordered by large iron sulfide grains. Last but not least, absolute paleointensity experiments have been determined on several samples, although the success rate is low as has been found in other studies of oceanic basalts. We also compare our rock magnetic results with the results obtained from DSDP hole 504B to have a better characterization of the rock magnetic properties of the two sampling sites within the Cocos Plate (?15 Ma) and Nazca Plates (?5.9 Ma) respectively.

  11. Evidence for unusually thin oceanic crust and strong mantle beneath the Amazon Fan


    Rodger, M; Watts, Ab; Greenroyd, CJ; Peirce, C.; Hobbs, RW


    We used seismic and gravity data to determine the structure of the crust and mantle beneath the Amazon Fan. Seismic data suggest that the crust is of oceanic-type and is unusually thin (< ?4 km) compared to elsewhere in the Atlantic. We attribute the thin crust to ultraslow seaffoor spreading following the breakup of South America and Africa during the Early Cretaceous. Gravity data suggest that the fan was emplaced on lithosphere that increased its elastic thickness, Te, and hence strength, ...

  12. Inconsistent correlation of seismic layer 2a and lava layer thickness in oceanic crust. (United States)

    Christeson, Gail L; McIntosh, Kirk D; Karson, Jeffrey A


    At mid-ocean ridges with fast to intermediate spreading rates, the upper section of oceanic crust is composed of lavas overlying a sheeted dyke complex. These units are formed by dykes intruding into rocks overlying a magma chamber, with lavas erupting at the ocean floor. Seismic reflection data acquired over young oceanic crust commonly image a reflector known as 'layer 2A', which is typically interpreted as defining the geologic boundary between lavas and dykes. An alternative hypothesis is that the reflector is associated with an alteration boundary within the lava unit. Many studies have used mapped variability in layer 2A thickness to make inferences regarding the geology of the oceanic crust, including volcanic construction, dyke intrusion and faulting. However, there has been no link between the geologic and seismological structure of oceanic crust except at a few deep drill holes. Here we show that, although the layer 2A reflector is imaged near the top of the sheeted dyke complex at fast-spreading crust located adjacent to the Hess Deep rift, it is imaged significantly above the sheeted dykes section at intermediate-spreading crust located near the Blanco transform fault. Although the lavas and underlying transition zone thicknesses differ by about a factor of two, the shallow seismic structure is remarkably similar at the two locations. This implies that seismic layer 2A cannot be used reliably to map the boundary between lavas and dykes in young oceanic crust. Instead we argue that the seismic layer 2A reflector corresponds to an alteration boundary that can be located either within the lava section or near the top of the sheeted dyke complex of oceanic crust. PMID:17251977

  13. Continental Growth and Recycling in Convergent Orogens with Large Turbidite Fans on Oceanic Crust

    Directory of Open Access Journals (Sweden)

    Ben D. Goscombe


    Full Text Available Convergent plate margins where large turbidite fans with slivers of oceanic basement are accreted to continents represent important sites of continental crustal growth and recycling. Crust accreted in these settings is dominated by an upper layer of recycled crustal and arc detritus (turbidites underlain by a layer of tectonically imbricated upper oceanic crust and/or thinned continental crust. When oceanic crust is converted to lower continental crust it represents a juvenile addition to the continental growth budget. This two-tiered accreted crust is often the same thickness as average continental crustal and is isostatically balanced near sea level. The Paleozoic Lachlan Orogen of eastern Australia is the archetypical example of a tubidite-dominated accretionary orogeny. The Neoproterozoic-Cambrian Damaran Orogen of SW Africa is similar to the Lachlan Orogen except that it was incorporated into Gondwana via a continent-continent collision. The Mesozoic Rangitatan Orogen of New Zealand illustrates the transition of convergent margin from a Lachlan-type to more typical accretionary wedge type orogen. The spatial and temporal variations in deformation, metamorphism, and magmatism across these orogens illustrate how large volumes of turbidite and their relict oceanic basement eventually become stable continental crust. The timing of deformation and metamorphism recorded in these rocks reflects the crustal thickening phase, whereas post-tectonic magmatism constrains the timing of chemical maturation and cratonization. Cratonization of continental crust is fostered because turbidites represent fertile sources for felsic magmatism. Recognition of similar orogens in the Proterozoic and Archean is important for the evaluation of crustal growth models, particularly for those based on detrital zircon age patterns, because crustal growth by accretion of upper oceanic crust or mafic underplating does not readily result in the addition of voluminous zircon-bearing magmas at the time of accretion. This crust only produces significant zircon when and if it partially melts, which may occur long after accretion.

  14. Renormalisation of Global Mantle Dynamic Topography Predictions using Residual Topography Measurements for "Normal" Ocean Crust (United States)

    Cowie, Fergus; Kusznir, Nick; Cowie, Leanne


    The best constraint on model predictions of present day mantle dynamic topography are measurements of residual topography. Residual topography is calculated by removing the isostatic effects of bathymetry, sediments, ice, crustal thickness variation and lithosphere thermal anomalies from the observed topography. Comparison of global model predictions of mantle dynamic topography with global compilations of residual topography, while showing a broadly similar pattern at long wavelengths, differ substantially in amplitude. A strong contribution to the present day surface topographic signal arises from crustal thickness variation. As a consequence it is difficult to accurately determine residual topography for continental crust and for oceanic regions with substantially thicker than average oceanic crust (e.g. oceanic crust adjacent to volcanic rifted margins, oceanic plume tracks, volcanic plateaux, micro-continents). Residual topography is best measured on ocean crust of "normal" oceanic thickness. We use global mapping of crustal thickness using gravity inversion to identify crust with thicknesses greater than that of "normal" oceanic crust in order than we can eliminate the less accurate measurements of residual topography for these thicker crustal regions. Comparison of model predicted mantle dynamic topography with residual topography measurements for the remaining regions of thinner "normal" oceanic crust shows an improved correlation but with a dynamic topography showing a positive bias with respect to residual topography and a greater amplitude. We use residual topography measurements for "normal" oceanic crust to downward shift (by approximately 600 m) and rescale (by 0.6) predicted global mantle dynamic topography. We present maps of the renormalised model predictions of global mantle topography from Steinberger (2007) and Flament et al. (2013). One consequence of renormalization is to reduce the amplitude of predicted mantle dynamic topographic uplift in the Pacific. The gravity inversion methodology includes a correction for the elevated geothermal gradient of oceanic and rifted continental margin lithosphere and sediment thickness. Caveats on this methodology are (i) that the gravity inversion methodology used to determine crustal thickness for screening out thick crust is itself dependent on mantle dynamic topography (but fortunately only weakly so) and (ii) that the renormalization procedure is biased towards oceanic regions.

  15. IODP Expedition 345: Structural characteristics of fast spread lower ocean crust, implications for growth and cooling of ocean crust (United States)

    John, B. E.; Ceuleneer, G.; Cheadle, M. J.; Harigane, Y.


    IODP Expedition 345 to the Hess Deep Rift sampled ~1 Ma, fast-spread East Pacific Rise gabbroic crust exposed as a dismembered, lower crustal section. Sixteen holes were drilled at Site U1415, centered on a sub-horizontal, 200-m wide E-W-trending bench between 4675 and 4850 mbsl. The bench was formed as a rotational slide within a 1km high slump along the southern wall of the intra-rift ridge. Primitive olivine gabbro and troctolite (Mg# 76-89) were sampled in four discrete, 30 to ? 65 m sized blocks formed by the mass wasting that dominates the southwestern slope of the ridge. Igneous fabric orientations (both layering and foliation) in the blocks vary from sub-vertical to gently dipping, suggesting some of the blocks have rotated at least 90°. Magmatic fabrics including spectacular modal and/or grain size layering are prevalent in >50% of the recovered core. Magmatic foliation in all blocks is defined by plagioclase crystal shape, but may also be defined by olivine and, to a lesser extent, orthopyroxene and clinopyroxene when the crystals have suitable habits. In all cases, this foliation is controlled by both the preferred orientation and shape anisotropy of the crystals. Fabric intensity varies from moderate to strong in the block with simple modal layering, weak to absent in the two blocks of troctolite, and largely absent in the block with heterogeneous textures and/or diffuse banding. Intrinsic to the layering and banding is the common development of dendritic and/or skeletal olivine textures (grain size up to 3 cm). The preservation of these delicate olivine grains showing only limited subgrain formation, and no kinking precludes significant low melt fraction (sub-solidus crystal plastic deformation and/or shear zones are virtually absent from the recovered core. Significant lower temperature (<400°C), localized, cataclastic deformation, in part synchronous with basaltic dike intrusion, is clearly displayed in one troctolitic block. Brittle fabrics include dense-anastomosing fracture networks, well-developed breccias and cataclasite, all commonly cut by prehnite and minor chlorite veins. Locally, early prehnite and chlorite veins are cut by a second period of cataclastic deformation and crosscut by later prehnite and chlorite veins, indicating a complex succession of vein formation and brittle deformation. This cataclastic deformation is likely related to the formation of the Cocos-Nazca Rift. Alteration veins though ubiquitous, are volumetrically insignificant (cutting <5% of the recovered core) of the rocks, but reflect late stages of cracking, fluid circulation, and fluid-rock interaction. Vein density is low except in the zones of extensive cataclasis. The alteration veins form networks with no preferred orientation consistent with hydraulic fracturing.

  16. First Investigation of the Microbiology of the Deepest Layer of Ocean Crust


    Mason, Olivia U.; Nakagawa, Tatsunori; Rosner, Martin; Van Nostrand, Joy D; Zhou, Jizhong; Maruyama, Akihiko; Fisk, Martin R.; Giovannoni, Stephen J.


    The gabbroic layer comprises the majority of ocean crust. Opportunities to sample this expansive crustal environment are rare because of the technological demands of deep ocean drilling; thus, gabbroic microbial communities have not yet been studied. During the Integrated Ocean Drilling Program Expeditions 304 and 305, igneous rock samples were collected from 0.45-1391.01 meters below seafloor at Hole 1309D, located on the Atlantis Massif (30 °N, 42 °W). Microbial diversity in the rocks was a...

  17. Brittle versus ductile deformation as the main control of the deep fluid circulation in oceanic crust (United States)

    Violay, M.; Gibert, B.; Mainprice, D.; Burg, J.-P.


    The brittle to ductile transition may strongly influence hydraulic properties of rocks at the depth and temperature ranges that hydrothermal fluids circulate. To examine this transition in the context of the oceanic crust, we conducted a series of deformation experiments on a natural basalt sample at in situ oceanic crust conditions. Dilatancy was measured during deformation. The method consisted in monitoring the volume of pore fluid that flows into or out of the sample at constant pore pressure. Mechanical and microstructural observations at experimental constant strain rate of 10-5 s-1 indicated that the basalt was brittle and dilatant up to 800°C. At higher temperature, the deformation mode became macroscopically ductile and samples compacted. These observations have important implications on heat transfer and fluid migration in oceanic crust.

  18. Contraction or expansion of the Moon's crust during magma ocean freezing? (United States)

    Elkins-Tanton, Linda T; Bercovici, David


    The lack of contraction features on the Moon has been used to argue that the Moon underwent limited secular cooling, and thus had a relatively cool initial state. A cool early state in turn limits the depth of the lunar magma ocean. Recent GRAIL gravity measurements, however, suggest that dikes were emplaced in the lower crust, requiring global lunar expansion. Starting from the magma ocean state, we show that solidification of the lunar magma ocean would most likely result in expansion of the young lunar crust, and that viscous relaxation of the crust would prevent early tectonic features of contraction or expansion from being recorded permanently. The most likely process for creating the expansion recorded by the dikes is melting during cumulate overturn of the newly solidified lunar mantle. PMID:25114310

  19. Tectonic slicing of subducted oceanic crust along plate interfaces: numerical modeling (United States)

    Ruh, Jonas B.; Burov, Evgenii; Gerya, Taras; Agard, Philippe; Le Pourhiet, Laetitia


    Remains of high-pressure low-temperature metamorphic oceanic crust are observed within orogenic belts and along recent subduction zones all around the globe. Such blueshist and eclogite "slivers" can strongly vary in extend and experienced P-T-t evolution. To reveal the surficial occurrence of oceanic rocks that occasionally have been at depths down to ~ 80 km within subduction zones, two main processes have to be investigated individually: (i) Detachment of oceanic slivers from the down going plate preventing rocks to be consumed by the mantle, and (ii) exhumation of detached oceanic material to the surface. It has often been stated that the process of slicing and exhumation of oceanic blueshists and eclogites is closely related to continental subduction. Nevertheless, several examples worldwide show exhumation (and therefore latest possible detachment from slab) occurring early or intermediate of the subduction process. We present new insight into the mechanical processes of detachment of oceanic slivers and their exhumation independent of continental collision by using numerical modelling tools. Large-scale thermo-mechanical models (600 * 200 km) based on finite difference, marker-in-cell technique, are applied to test how serpentinised upper slab mantle (mantle serpentinisation at oceanic ridges or/and along to outer-rise normal faults) influences shallow and deep crustal detachment. Preliminary results show that a through-going serpentinite layer below the oceanic crust, if serpentinisation exceeds 50%, triggers slicing at wedge depths. Even if mechanical coupling mainly occurs at intermediate depths along the subduction interface, stresses within the oceanic crust are dispersed upward due to the weak, decoupling serpentinite layer. Unconnected patches of serpentinised upper slab mantle eventually lead to deep slicing, depending on upper plate serpentinisation due to slab dewatering. Furthermore, we investigated the effect of pressure-dependent fluid migration and according weakening (increasing fluid pressure) of subducted crust and its implications on slicing.

  20. Velocity and attenuation in young oceanic crust: New downhole log results from DSDP/ODP/IODP Holes 504B and 1256D (United States)

    Guerin, Gilles; Goldberg, David S.; Iturrino, Gerardo J.


    To characterize the acoustic properties of oceanic crust and investigate the nature of the oceanic layer 2/3 seismic boundary, we analyze sonic logging waveforms recorded in the two deepest holes ever drilled in intact oceanic crust: Ocean Drilling Program (ODP)/Integrated Ocean Drilling Program (IODP) Hole 1256D and Deep Sea Drilling Program (DSDP)/ODP Hole 504B. Hole 1256D penetrated 15 Ma old crust on a superfast spreading segment of the Cocos plate, while Hole 504B was drilled in a ˜6 Ma old section of an intermediate spreading segment of the Nazca plate. Careful reprocessing of different sets of waveforms allows us to draw depth profiles of the primary parameters controlling wave propagation in the crust: compressional and shear velocities (Vp and Vs), and attenuation. The results underline the importance of proper filtering and processing parameters for the acquisition of acoustic logs in hard rocks. While the reprocessed Vp logs improve previous data, particularly at the bottom of Hole 1256D, the Vs logs represent significant enhancements in quality and in extent of previous profiles in the two holes. Attenuation results provide the first measure of intrinsic attenuation in these holes. The velocity and attenuation logs confirm that Hole 504B had reached oceanic layer 3 without recovering gabbros. Conversely, while gabbros were recovered in Hole 1256D, no layer 3 velocity values were recorded in the interval logged. However, the reprocessed data suggest that the transition to layer 3 might occur within the deepest 87 m of the hole that were not logged or immediately below. Accordingly, the layer 2/3 seismic boundary does not appear to be related to the top of the plutonic crust. Despite different ages, spreading rates, and mineralogy, however, the two sites display remarkably similar velocity and attenuation profiles through the entire crustal sections logged, suggesting that they can be reliably considered representative of the acoustic properties of the Nazca/Cocos spreading system and more generally of undisturbed oceanic crust.

  1. Yttrium and rare earth element contents in seamount cobalt crusts in the Indian Ocean

    Digital Repository Service at National Institute of Oceanography (India)

    Balaram, V.; Banakar, V.K.; Subramanyam, K.S.V.; Roy, P.; Satyanarayan, M.; RamMohan, M.; Sawant, S.S.


    reports high concentrations of rare earth elements (REE) and yttrium ranging from 1,727 to 2,511 mu g/g in the crust samples collected from the Afanasy Niktin Seamount (ANS) in the Eastern Equatorial Indian Ocean. The concentrations of REE in the ANS Fe-Mn...

  2. Role of hydrology in the formation of Co-rich Mn crusts from the equatorial N Pacific, equatorial S Indian Ocean and the NE Atlantic Ocean

    Digital Repository Service at National Institute of Oceanography (India)

    Glasby, G.P.; Mountain, B.; Vineesh, T.C.; Banakar, V.K.; Rajani, R.; Ren, X.


    Co-rich Mn crusts from four different locations of the world ocean have been studied to understand the role of dissolved oxygen of the ambient seawater in the formation of Co-rich Mn crusts. WOCE (World Ocean Circulation Experiment) oxygen profiles...

  3. Europa's Crust and Ocean: Origin, Composition, and the Prospects for Life (United States)

    Kargel, J.S.; Kaye, J.Z.; Head, J. W., III; Marion, G.M.; Sassen, R.; Crowley, J.K.; Ballesteros, O.P.; Grant, S.A.; Hogenboom, D.L.


    We have considered a wide array of scenarios for Europa's chemical evolution in an attempt to explain the presence of ice and hydrated materials on its surface and to understand the physical and chemical nature of any ocean that may lie below. We postulate that, following formation of the jovian system, the europan evolutionary sequence has as its major links: (a) initial carbonaceous chondrite rock, (b) global primordial aqueous differentiation and formation of an impure primordial hydrous crust, (c) brine evolution and intracrustal differentiation, (d) degassing of Europa's mantle and gas venting, (e) hydrothermal processes, and (f) chemical surface alteration. Our models were developed in the context of constraints provided by Galileo imaging, near infrared reflectance spectroscopy, and gravity and magnetometer data. Low-temperature aqueous differentiation from a carbonaceous CI or CM chondrite precursor, without further chemical processing, would result in a crust/ocean enriched in magnesium sulfate and sodium sulfate, consistent with Galileo spectroscopy. Within the bounds of this simple model, a wide range of possible layered structures may result; the final state depends on the details of intracrustal differentiation. Devolatilization of the rocky mantle and hydrothermal brine reactions could have produced very different ocean/crust compositions, e.g., an ocean/crust of sodium carbonate or sulfuric acid, or a crust containing abundant clathrate hydrates. Realistic chemical-physical evolution scenarios differ greatly in detailed predictions, but they generally call for a highly impure and chemically layered crust. Some of these models could lead also to lateral chemical heterogeneities by diapiric upwellings and/or cryovolcanism. We describe some plausible geological consequences of the physical-chemical structures predicted from these scenarios. These predicted consequences and observed aspects of Europa's geology may serve as a basis for further analys is and discrimination among several alternative scenarios. Most chemical pathways could support viable ecosystems based on analogy with the metabolic and physiological versatility of terrestrial microorganisms. ?? 2000 Academic Press.

  4. Mission Moho: Rationale for drilling deep through the ocean crust into the upper mantle (United States)

    Ildefonse, B.; Abe, N.; Kelemen, P. B.; Kumagai, H.; Teagle, D. A. H.; Wilson, D. S.; Moho Proponents, Mission


    Sampling a complete section of the ocean crust to the Moho was the original inspiration for scientific ocean drilling, and remains the main goal of the 21st Century Mohole Initiative in the IODP Science Plan. Fundamental questions about the composition, structure, and geophysical characteristics of the ocean lithosphere, and about the magnitude of chemical exchanges between the mantle, crust and oceans remain unresolved due to the absence of in-situ samples and measurements. The geological nature of the Mohorovi?i? discontinuity itself remains poorly constrained. "Mission Moho" is a proposal that was submitted to IODP in April 2007, with the ambition to drill completely through intact oceanic crust formed at a fast spreading rate, across the Moho and into the uppermost mantle. Although, eventually, no long-term mission was approved by IODP, the scientific objectives related to deep drilling in the ocean crust remain essential to our understanding of the Earth. These objectives are to : - Determine the geological meaning of the Moho in different oceanic settings, determine the in situ composition, structure and physical properties of the uppermost mantle, and understand mantle melt migration, - Determine the bulk composition of the oceanic crust to establish the chemical links between erupted lavas and primary mantle melts, understand the extent and intensity of seawater hydrothermal exchange with the lithosphere, and estimate the chemical fluxes returned to the mantle by subduction, - Test competing hypotheses of the ocean crust accretion at fast spreading mid-ocean ridges, and quantify the linkages and feedbacks between magma intrusion, hydrothermal circulation and tectonic activity, - Calibrate regional seismic measurements against recovered cores and borehole measurements, and understand the origin of marine magnetic anomalies, - Establish the limits of life in the ocean lithosphere. The "MoHole" was planned as the final stage of Mission Moho, which requires non-riser and riser drilling, geophysical site surveys and the development of new technology including the construction of a +4000 m riser. Initial expeditions would use existing drilling capabilities to sample shallow and deep targets in increasingly hostile conditions in ocean crust formed at both fast and slow spreading rates, allowing us to deliver major short-term science returns while we develop the equipment, technology and experience to tackle a full crustal penetration. The first, short-term milestone is to return in IODP Hole 1256D, into intact crust formed during a period of superfast spreading (> 200 mm/yr) on the East Pacific rise 15 million years ago, and drill as deeply as possible with non-riser technology. The first gabbroic rocks below the sheeted dikes were encountered at the end of IODP expedition 312 at 1407 meters below seafloor. They mark the interface between the axial melt lens and the base of the hydrothermal system. Future deepening to a minimum of a few hundred meters should recover cumulate gabbros that will further constraint accretion mechanisms of the lower, igneous crust.

  5. Glacial cycles drive variations in the production of oceanic crust

    CERN Document Server

    Crowley, John W; Huybers, Peter; Langmuir, Charles H; Park, Sung-Hyun


    Glacial cycles redistribute water between the oceans and continents causing pressure changes in the upper mantle, with potential consequences for melting of Earth's interior. A numerical model of mid-ocean ridge dynamics that explicitly includes melt transport is used to calculate the melting effects that would be caused by Plio-Pleistocene sea-level variations. Model results interpreted in the context of an analytical approximation predict sea-level induced variations in crustal thickness on the order of hundreds of meters. The specifics of the response depend on rates of sea-level change, mid-ocean ridge spreading rates, and mantle permeability. Spectral analysis of the bathymetry of the Australian-Antarctic ridge shows significant spectral energy near 23, 41, and 100 ky periods, consistent with model results and with the spectral content of Pleistocene sea-level variability. These results support the hypothesis that sea-floor topography records the magmatic response to changes in sea level, reinforcing the...

  6. Glacial cycles drive variations in the production of oceanic crust (United States)

    Crowley, John W.; Katz, Richard F.; Huybers, Peter; Langmuir, Charles H.; Park, Sung-Hyun


    Glacial cycles redistribute water between oceans and continents, causing pressure changes in the upper mantle, with consequences for the melting of Earth’s interior. Using Plio-Pleistocene sea-level variations as a forcing function, theoretical models of mid-ocean ridge dynamics that include melt transport predict temporal variations in crustal thickness of hundreds of meters. New bathymetry from the Australian-Antarctic ridge shows statistically significant spectral energy near the Milankovitch periods of 23, 41, and 100 thousand years, which is consistent with model predictions. These results suggest that abyssal hills, one of the most common bathymetric features on Earth, record the magmatic response to changes in sea level. The models and data support a link between glacial cycles at the surface and mantle melting at depth, recorded in the bathymetric fabric of the sea floor.

  7. Hydrous magmatism triggered by assimilation of hydrothermally altered rocks in fossil oceanic crust (northern Oman ophiolite) (United States)

    France, Lydéric; Ildefonse, Benoit; Koepke, Juergen


    Mid-ocean ridges magmatism is, by and large, considered to be mostly dry. Nevertheless, numerous works in the last decade have shown that a hydrous component is likely to be involved in ocean ridges magmas genesis and/or evolution. The petrology and geochemistry of peculiar coarse grained gabbros sampled in the upper part of the gabbroic sequence from the northern Oman ophiolite (Wadi Rajmi) provide information on the origin and fate of hydrous melts in fast-spreading oceanic settings. Uncommon crystallization sequences for oceanic settings (clinopyroxene crystallizing before plagioclase), extreme mineral compositions (plagioclase An% up to 99, and clinopyroxene Mg # up to 96), and the presence of magmatic amphibole, imply the presence of a high water activity during crystallization. Various petrological and geochemical constraints point to hydration, resulting from the recycling of hydrothermal fluids. This recycling event may have occurred at the top of the axial magma chamber where assimilation of anatectic hydrous melts is recurrent along mid-ocean ridges or close to segments ends where fresh magma intrudes previously hydrothermally altered crust. In ophiolitic settings, hydration and remelting of hydrothermally altered rocks producing hydrous melts may also occur during the obduction process. Although dry magmatism dominates oceanic magmatism, the dynamic behavior of fast-spreading ocean ridge magma chambers has the potential to produce the observed hydrous melts (either in ophiolites or at spreading centers), which are thus part of the general mid-ocean ridges lineage.

  8. Glacial cycles drive variations in the production of oceanic crust (United States)

    Crowley, J. W.; Katz, R. F.; Langmuir, C. H.; Huybers, P. J.


    Changes in sea level accompanying glacial cycles affect the static pressure within the asthenosphere; these variations could modulate melting rates beneath the mid-ocean ridge system as well as crustal thickness. These effects can be investigated and quantified using models of ridges based on conservation of mass, momentum, energy, and composition for two phases (magma & mantle) and two thermodynamic components (enriched & depleted). The models predict that the sensitivity of crustal thickness to oscillations in sea-level depends on the period of oscillation, the spreading rate of the ridge, and the assumed permeability scale of the melting regime. In contrast to previous studies (Huybers & Langmuir, 2009 and Lund & Asimow, 2011), the new results indicate that effects are larger for ridges with faster spreading rates. They also show that the dominant period of variations in crustal thickness changes with spreading rate and permeability. Sea-level variations with periods in the range of 10 ky - 100 ky can result in significant changes in crustal thickness that are orders of magnitude larger than the sea-level variations that drive them. Accurately modelling this process requires the inclusion of two previously unaccounted for processes: (1) determining the volume of the melting regime that is consistent with the ridge spreading rate and (2) properly treating the transport of melt. These enable us to capture the non-linear dependencies on spreading rate and other model parameters. Spectral analysis of bathymetry at two ridge segments that have a symmetric bathymetric signal and hence are undisturbed by off-axis volcanism or ridge jumps reveals the presence of variability at frequencies associated with precession, obliquity, and the 100 ky glacial/inter-glacial variability. Furthermore, the faster spreading ridge has larger amplitude responses to changes in sea level and shows a proportionately greater response at higher frequencies. These observations reinforce the possible links among climate cycles at the surface, mantle melting at depth and the crustal fabric of the sea floor.

  9. IODP Expedition 345: Primitive Layered Gabbros From Fast-Spreading Lower Oceanic Crust (United States)

    Ildefonse, Benoit; Gillis, Kathryn M.; Snow, Jonathan E.; Klaus, Adam


    Three-quarters of the ocean crust formed at fast-spreading ridges is composed of plutonic rocks whose mineral assemblages, textures and compositions record the history of melt transport and crystallization between the mantle and the seafloor. However, owing to the nearly continuous overlying extrusive upper crust, sampling in situ the lower crust is challenging. Hence, models for understanding the formation of the lower crust are based essentially on geophysical studies and ophiolites. Integrated Ocean Drilling Program (IODP) Expedition 345 recovered the first significant sections of primitive, modally layered gabbroic rocks from the lowermost plutonic crust formed at a fast-spreading ridge, and exposed at the Hess Deep Rift (Gillis et al., Nature, 2014, doi:10.1038/nature12778). Drilling Site U1415 is located along the southern slope of the intrarift ridge. The primary science results were obtained from coring of two ~110 m deep reentry holes and one 35-m-deep single-bit hole, all co-located within an ~100-m-wide area. Olivine gabbro and troctolite are the dominant plutonic rock types recovered, with minor gabbro, clinopyroxene oikocryst-bearing gabbroic rocks, and gabbronorite. All rock types are primitive to moderately evolved, with Mg# 89-76, and exhibit cumulate textures similar to ones found in layered mafic intrusions and some ophiolites. Spectacular modal and grain size layering, prevalent in >50% of the recovered core, confirm a long held paradigm that such rocks are a key constituent of the lowermost ocean crust formed at fast-spreading ridges. Magmatic foliation is largely defined by the shape-preferred orientation of plagioclase. It is moderate to strong in intervals with simple modal layering but weak to absent in troctolitic intervals and typically absent in intervals with heterogeneous textures and/or diffuse banding. Geochemical analysis of these primitive lower plutonics, in combination with previous geochemical data for shallow-level plutonics, sheeted dikes and lavas, provides the best constrained estimate to date of the bulk composition of crust formed at a fast-spreading ridge. Simple crystallization models using this bulk crustal composition as the parental melt accurately predict the composition of both the lavas and plutonics. However, the recovered plutonic rocks show unanticipated early crystallization of orthopyroxene, challenging current models of melt extraction from the mantle and mid-ocean ridge basalt differentiation. The core recovered at Site U1415 originated at a stratigraphic level at least 2 km beneath the sheeted dike-plutonic transition, representing intervals of the lower half to one third of the EPR plutonic crust. A more precise depth cannot be assigned as the results of Expedition 345 (e.g., magnetic inclinations) and site survey indicate that the sampled units are tilted, mass-wasted blocks. However, sampling four large blocks of relatively fresh rocks proved facilitated observations of the wide variety and complexity of rock types and textures present in fast spread primitive lowermost crust.

  10. Nickel isotopic compositions of ferromanganese crusts and the constancy of deep ocean inputs and continental weathering effects over the Cenozoic (United States)

    Gall, L.; Williams, H. M.; Siebert, C.; Halliday, A. N.; Herrington, R. J.; Hein, J. R.


    The global variability in nickel (Ni) isotope compositions in ferromanganese crusts is investigated by analysing surface samples of 24 crusts from various ocean basins by MC-ICPMS, using a double-spike for mass bias correction. Ferromanganese crusts have ?Ni60 isotopic compositions that are significantly heavier than any other samples thus far reported (-0.1‰ to 0.3‰), with surface scrapings ranging between 0.9‰ and 2.5‰ (relative to NIST SRM986). There is no well resolved difference between ocean basins, although the data indicate somewhat lighter values in the Atlantic than in the Pacific, nor is there any evidence that the variations are related to biological fractionation, presence of different water masses, or bottom water redox conditions. Preliminary data for laterite samples demonstrate that weathering is accompanied by isotopic fractionation of Ni, which should lead to rivers and seawater being isotopically heavy. This is consistent with the slightly heavier than average isotopic compositions recorded in crusts that are sampled close to continental regions. Furthermore, the isotopic compositions of crusts growing close to a hydrothermal source are clustered around ?1.5‰, suggesting that hydrothermal fluids entering the ocean may have a Ni isotopic composition similar to this value. Based on these data, the heavy Ni isotopic compositions of ferromanganese crusts are likely due to input of isotopically heavy Ni to the ocean from continental weathering and possibly also from hydrothermal fluids. A depth profile through one crust, CD29-2, from the north central Pacific Ocean displays large variations in Ni isotope composition (1.1-2.3‰) through the last 76 Myr. Although there may have been some redistribution of Ni associated with phosphatisation, there is no systematic difference in Ni isotopic composition between deeper, older parts and shallower, younger parts of the crust, which may suggest that oceanic sources and sinks of Ni have largely remained in steady state over the Cenozoic. Additionally, the isotope profile is in agreement with a profile of Mn concentration through the same crust. This implies a link between the Ni isotopic composition recorded in ferromanganese crusts and the release of Ni into the ocean through hydrothermal activity. This supports the conclusions drawn from surface data, that Ni isotope ratios in ferromanganese crusts are largely controlled by the isotopic compositions of the Ni oceanic input sources.

  11. IODP Exp 345: Primitive Layered Gabbros From Fast-Spreading Lower Oceanic Crust (United States)

    Gillis, K. M.; Snow, J. E.; Klaus, A.


    Plutonic rocks from the lower ocean crust formed at fast-spreading ridges provide a record of the history of melt transport and crystallization between the mantle and the seafloor. Despite the significance of these rocks, sampling them in situ has proven extremely challenging. This means our models for understanding the formation of the lower crust are based largely on geophysical studies and ophiolites that did not form at typical mid-ocean ridges. Integrated Ocean Drilling Program (IODP) Expedition 345 recovered the first significant recovery of primitive modally layered gabbroic rocks from the lowermost plutonic crust from a fast-spreading ridge exposed at the Hess Deep Rift. Drilling was focused at Site U1415, located along the southern slope of the intrarift ridge. The primary science results were obtained from coring of two ~110 m deep reentry holes and one 35-m-deep single-bit hole, all co-located within an ~100-m-wide area. Olivine gabbro and troctolite are the dominant plutonic rock types recovered, with minor gabbro, clinopyroxene oikocryst-bearing gabbroic lithologies, and gabbronorite. All rock types are primitive (Mg# 76-89) and exhibit cumulate textures similar to ones found in layered mafic intrusions and some ophiolite complexes. Spectacular modal and grain size layering, prevalent in >50% of the recovered core, confirm a long held paradigm that such rocks are a key constituent of the lowermost ocean crust formed at fast-spreading ridges. Magmatic foliation is largely defined by the shape-preferred orientation of plagioclase. It is moderate to strong in intervals with simple modal layering but weak to absent in troctolitic intervals and typically absent in intervals with heterogeneous textures and/or diffuse banding. Geochemical analysis of these primitive lower plutonics, in combination with previous geochemical data for shallow-level plutonics, sheeted dikes and lavas, provides the first robust estimate of the bulk composition of crust formed at a fast-spreading ridge. Simple crystallization models using this bulk crustal composition as the parental melt accurately predict the composition of both the lavas and plutonics. However, the recovered plutonic rocks show unanticipated early crystallization of orthopyroxene, challenging current models of melt extraction from the mantle and mid-ocean ridge basalt differentiation. The core recovered at Site U1415 originated at a stratigraphic level at least 2 km beneath the sheeted dike-plutonic transition, representing intervals of the lower half to one third of the EPR plutonic crust. A more precise depth cannot be assigned as the results of Expedition 345 (e.g., magnetic inclinations) and site survey data (Ferrini et al., 2013) indicate that the southern slope of the intrarift ridge formed by mass wasting. However, sampling four large blocks of relatively fresh rocks proved advantageous, as it facilitated observations of the wide variety and complexity of rock types and textures present in fast spread primitive lowermost crust. [Ferrini et al., Mar. Geol., 339, 13-21, 2013

  12. Seismic wave velocity of rocks in the Oman ophiolite: constraints for petrological structure of oceanic crust (United States)

    Saito, S.; Ishikawa, M.; Shibata, S.; Akizuki, R.; Arima, M.; Tatsumi, Y.; Arai, S.


    Evaluation of rock velocities and comparison with velocity profiles defined by seismic refraction experiments are a crucial approach for understanding the petrological structure of the crust. In this study, we calculated the seismic wave velocities of various types of rocks from the Oman ophiolite in order to constrain a petrological structure of the oceanic crust. Christensen & Smewing (1981, JGR) have reported experimental elastic velocities of rocks from the Oman ophiolite under oceanic crust-mantle conditions (6-430 MPa). However, in their relatively low-pressure experiments, internal pore-spaces might affect the velocity and resulted in lower values than the intrinsic velocity of sample. In this study we calculated the velocities of samples based on their modal proportions and chemical compositions of mineral constituents. Our calculated velocities represent the ‘pore-space-free’ intrinsic velocities of the sample. We calculated seismic velocities of rocks from the Oman ophiolite including pillow lavas, dolerites, plagiogranites, gabbros and peridotites at high-pressure-temperature conditions with an Excel macro (Hacker & Avers 2004, G-cubed). The minerals used for calculations for pillow lavas, dolerites and plagiogranites were Qtz, Pl, Prh, Pmp, Chl, Ep, Act, Hbl, Cpx and Mag. Pl, Hbl, Cpx, Opx and Ol were used for the calculations for gabbros and peridotites. Assuming thermal gradient of 20° C/km and pressure gradient of 25 MPa/km, the velocities were calculated in the ranges from the atmospheric pressure (0° C) to 200 MPa (160° C). The calculation yielded P-wave velocities (Vp) of 6.5-6.7 km/s for the pillow lavas, 6.6-6.8 km/s for the dolerites, 6.1-6.3 km/s for the plagiogranites, 6.9-7.5 km/s for the gabbros and 8.1-8.2 km/s for the peridotites. On the other hand, experimental results reported by Christensen & Smewing (1981, JGR) were 4.5-5.9 km/s for the pillow lavas, 5.5-6.3 km/s for the dolerites, 6.1-6.3 km/s for the plagiogranites, 6.5-7.7 km/s for the gabbros and 6.3-7.9 km/s for the peridotites. Although the two results are broadly comparable to each other for plagiogranites and gabbros, the calculated velocities are considerably higher than the experimental ones for pillow lavas, dolerites and peridotites. The discrepancy for the pillow lavas and dolerites can be attributed to the presence of pore-spaces in the experimental samples. On the other hand, serpentinization of peridotite samples likely resulted in lower velocities in experiments than in calculation. We compared our results with Vp structure of the oceanic crust and mantle (White et al. 1992, JGR). The calculated Vp of peridotites and gabbros are comparable to those of mantle and layer-3, respectively. The calculated Vp of dolerites is comparable to layer-3 and considerably higher than layer-2 velocities. However, recent deep drilling results (Holes 504B and 1256D) indicate the seismic layer-2 of oceanic crust mainly composed of dolerites, which is consistent with the experimental P-wave velocities of dolerites (Christensen & Smewing, 1981, JGR). These results imply that the velocity structure of seismic layer-2 reflects the distribution of pore-spaces in the upper oceanic crust.

  13. Insights into magmatic processes and hydrothermal alteration of in situ superfast spreading ocean crust at ODP/IODP site 1256 from a cluster analysis of rock magnetic properties (United States)

    Dekkers, Mark J.; Heslop, David; Herrero-Bervera, Emilio; Acton, Gary; Krasa, David


    analyze magnetic properties from Ocean Drilling Program (ODP)/Integrated ODP (IODP) Hole 1256D (6°44.1' N, 91°56.1' W) on the Cocos Plate in ˜15.2 Ma oceanic crust generated by superfast seafloor spreading, the only drill hole that has sampled all three oceanic crust layers in a tectonically undisturbed setting. Fuzzy c-means cluster analysis and nonlinear mapping are utilized to study down-hole trends in the ratio of the saturation remanent magnetization and the saturation magnetization, the coercive force, the ratio of the remanent coercive force and coercive force, the low-field magnetic susceptibility, and the Curie temperature, to evaluate the effects of magmatic and hydrothermal processes on magnetic properties. A statistically robust five cluster solution separates the data predominantly into three clusters that express increasing hydrothermal alteration of the lavas, which differ from two distinct clusters mainly representing the dikes and gabbros. Extensive alteration can obliterate magnetic property differences between lavas, dikes, and gabbros. The imprint of thermochemical alteration on the iron-titanium oxides is only partially related to the porosity of the rocks. Thus, the analysis complements interpretation based on electrofacies analysis. All clusters display rock magnetic characteristics compatible with an ability to retain a stable natural remanent magnetization suggesting that the entire sampled sequence of ocean crust can contribute to marine magnetic anomalies. Paleointensity determination is difficult because of the propensity of oxyexsolution during laboratory heating and/or the presence of intergrowths. The upper part of the extrusive sequence, the granoblastic dikes, and moderately altered gabbros may contain a comparatively uncontaminated thermoremanent magnetization.

  14. High-Albedo Salt Crusts on the Tropical Ocean of Snowball Earth: Measurements and Modeling (United States)

    Carns, R.; Light, B.; Warren, S. G.


    During a Snowball Earth event, almost all of the ocean surface first freezes as sea ice. As in modern sea ice, trapped inclusions of liquid brine permeate the ice cover. As the ice grows and cools, salt crystals precipitate within the inclusions. At -23C, the most abundant salt in seawater, sodium chloride, begins to precipitate as the dihydrate mineral hydrohalite (NaCl·2H2O). Crystals of hydrohalite within the sea ice scatter light. Measurements of cold, natural sea ice show a broadband albedo increase of 10-20% when salt precipitates. Such snow-free natural sea ice with a surface temperature below -23C is rare on modern Earth, but would have been common in tropical regions of a Snowball Earth where evaporation exceeded precipitation. The persistent cold and lack of summer melt on the Snowball ocean surface, combined with net evaporation, is hypothesized to yield lag deposits of hydrohalite crystals on the ice surface. To investigate this process, we prepared laboratory-grown sea ice in a 1000 liter tank in a walk-in freezer laboratory. The ice was cooled below -23 C and the surface sprayed with a 23% NaCl solution to create a layer of hydrohalite-enriched ice, a proxy for lag deposits that would have formed over long periods of surface sublimation. We have developed a novel technique for measuring the spectral albedo of ice surfaces in the laboratory; this technique was used to monitor the evolution of the surface albedo of our salt crust as the ice matrix sublimated away leaving a layer of fine-grained hydrohalite crystals. Measurements of this hydrohalite surface crust show a very high albedo, comparable to fresh snow at visible wavelengths and significantly larger than fresh snow at near infrared wavelengths. Broadband albedos are 0.55 for bare artificial sea ice at -30C, 0.75 for ice containing 25% hydrohalite by volume, 0.84 after five days of desiccation and 0.93 after 47 days of desiccation. Using our laboratory measurements, along with estimates of grain size and crust optical depth, as inputs to Mie scattering and radiative transfer models allowed us to infer the imaginary refractive index of hydrohalite. The model can calculate albedo for pure hydrohalite crusts of varying thickness and for mixtures of ice and hydrohalite. A parameterization is presented for albedo as a function of the thickness of the hydrohalite crust.

  15. The influence of plate movement on the evolution of hydrothermal convection cells in the oceanic crust (United States)

    Fehn, Udo; Cathles, Lawrence M.


    Two types of hydrothermal circulation can be distinguished in the oceanic crust, a very intense convection at spreading centers caused by the intrusion processes at mid-ocean ridges, and a much less vigorous, but more common convection in older, sediment covered crust driven by heat coming from the underlying, cooling plate. We investigated the relation between these two types of convection under normal spreading conditions, i.e. oceanic plate moving away from a stationary spreading center. In particular, we studied the stability of convection cells in a moving plate, the transition between on- and off-axis convection and the temperature distribution in these convection cells. The study is based on numerical calculations using the theory of flow through porous media. Our results show that convection cells associated with the intrusion processes in the accretion zone of a spreading center are stationary with respect to the ridge axis. Convective heat transport in the stationary, on-axis cells is sufficient to remove all the heat released at the ridge axis. The other convection cells, which are not immediately associated with these intrusion processes, are not stationary. Most of them, especially once they are more than 30 km away from the ridge axis, move with the moving plate. This movement occurs regardless of the permeability distribution in the crust. As a consequence, individual segments of the crust are exposed either to continuous upwelling or to continuous downwelling flow once they have left the vicinity of the ridge axis; high water-to-rock ratios can be reached in these long-lived cells in spite of the relatively slow flow velocities. Temperatures in the off-axis cells, even in close proximity to the ridge axis, are low, in general below 100°C. The low-temperature alteration found commonly in the oceanic crust is evidence for the widespread occurrence of these off-axis cells. More specifically, the distinct differences in degree of alteration observed in some closely spaced DSDP cores are in good agreement with the concept of convection cells attached to the moving plate. Because of the movement of the off-axis cells away from the stationary axial cells, a transition zone where new convection cells are formed exists at distances between 5 and 25 km from the spreading center. The formation of a new convection cell is accompanied by a maximum in low-temperature upwelling flow. When the heat transport by the plate and that by fluid convection is of similar magnitude, off-axis cells shift position with respect to the ridge axis and the moving plate. During this stage two cells can merge, which event causes a short (ca. 5000 years) episode of intense upwelling. These episodes, which are not directly related to any intrusive activity, can occur several times in or near a particular segment of the crust. Only during these short upwelling episodes are temperatures up to 200°C reached in the off-axis cells. Chains of hydrothermal mounds near active spreading centers as well as low-temperature hydrothermal deposits such as the Mn lenses commonly associated with ophiolite sequences may be related to the formation of these off-axis convection cells.

  16. Positive geothermal anomalies in oceanic crust of Cretaceous age offshore Kamchatka

    Directory of Open Access Journals (Sweden)

    G. Delisle


    Full Text Available Heat flow measurements were carried out in 2009 offshore Kamchatka during the German-Russian joint-expedition KALMAR. An area with elevated heat flow in oceanic crust of Cretaceous age – detected ~30 yr ago in the course of several Russian heat flow surveys – was revisited. One previous interpretation postulated anomalous lithospheric conditions or a connection between a postulated mantle plume at great depth (>200 km as the source for the observed high heat flow. However, the positive heat flow anomaly – as our bathymetric data show – is closely associated with the fragmentation of the western flank of the Meiji Seamount into a horst and graben structure initiated during descent of the oceanic crust into the subduction zone offshore Kamchatka. This paper offers an alternative interpretation, which connects high heat flow primarily with natural convection of fluids in the fragmented rock mass and, as a potential additional factor, high rates of erosion, for which evidence is available from our collected bathymetric image. Given high erosion rates, warm rock material at depth rises to nearer the sea floor, where it cools and causes temporary elevated heat flow.

  17. Microbial phylogeny of igneous minerals and glasses in deep ocean crust (United States)

    Smith, A. R.; Popa, R.; Fisk, M. R.; Nielsen, M. E.; Wheat, C. G.; Jannasch, H. W.; Fisher, A. T.; Becker, K.; Sievert, S. M.; Flores, G. E.


    Ocean crust basalts exposed to oxidant-laden seawater are ideal habitats for iron-oxidizing microbes. Evidence of iron oxidizers in the form of iron mats, twisted stalks, or DNA is commonly found on basalts at or very near the seafloor, and several strains have been cultured. However, microbial abundances and diversities associated with the various igneous minerals and glasses that make up basalt (a mineralogically heterogeneous rock) have not previously been investigated. Therefore, we initiated a four year subseafloor incubation of twelve different igneous minerals and glasses in IODP borehole site 1301A on the eastern flank of the Juan de Fuca Ridge. Mineral sands were placed inside flow cell chambers that were attached to an osmotic pump and suspended in the borehole at 278 meters below the seafloor. We found that Fe(II)-rich minerals, such as olivine, host a greater abundance of total microbial cells and organotrophs than other igneous minerals and glasses in the subsurface ocean crust. Isolates also grew on olivine in culture. Here we highlight the community diversity and abundance of microbes associated with the incubated minerals and glasses to reveal the intimate link between a mineral's composition and its microbial community.

  18. Investigation of the Oceanic Crust and Mantle in the Eastern Mid Atlantic Next to a Major Transform Fault (Gloria Fault) By Receiver Function Analysis (United States)

    Hannemann, K.; Krueger, F.; Dahm, T.


    Within the project Deep OCean Test ARray (DOCTAR), we want to test how much we can enhance the signal to noise ratio (SNR) of teleseismic and regional events recorded at the ocean bottom by using broad band array methods. Furthermore, we want to learn more about the structure of the oceanic crust and mantle 100 km North of the Gloria Fault (major transform fault at the plate boundary between Eurasian and African plate in the Atlantic ocean). For the latter, we employ receiver functions and apparent P-wave incidence angles. We deployed 12 ocean bottom stations (OBS) as a mid aperture array (75 km) in the deep Eastern Mid Atlantic (4-6 km) in 2011. Each free fall station consists of a broad band seismometer and a hydrophone. After 10 month of recording, the stations were recovered. We use P phase and Rayleigh phase polarization to estimate the orientation of the stations. Different data quality and site effects at the stations need a careful review of the processing parameters (filter, deconvolution length) used for the calculation of the receiver functions. We defined different criteria as relative spike position within the deconvolution time window, and energy ratios of several time windows of the deconvolved traces to assess an evaluation of the receiver function quality in dependence on the used processing parameters. Additionally, we had a look at the relationship between the apparent incidence angle and the S-velocity and find that it differs for the ocean bottom in comparison to the free surface. Surprisingly, the densities of the oceanic crust and the water column, as well as the P-velocity of the water column have also an influence on the apparent incidence angle. We measured incidence angles for several events and find that the angles show a dependence on the dominant frequency of the event. By comparison with synthetic receiver functions, we find that water multiples have a small or no influence at all on the real data receiver functions. We identify the MOHO conversion which indicates a crustal thickness of about 6 to 9 km which could be expected for 70 Ma old oceanic crust. Furthermore, there exist several features which are related to mantle structures. Finally, we use the Zhu and Kanamori (2000) algorithm, the apparent incidence angle, and the comparison with synthetic receiver functions to come up with a preliminary 1D velocity model.

  19. Hydrothermal circulation in fast spread ocean crust - where and how much? Insight from ODP Hole 1256D (United States)

    Harris, M.; Coggon, R. M.; Smith-Duque, C. E.; Teagle, D. A. H.


    Understanding and quantifying hydrothermal circulation is critical to testing models of the accretion of lower ocean crust and quantifying global geochemical cycles. However, our understanding is principally limited by a lack of direct observations from intact ocean crust. Key questions remain about the magnitude of hydrothermal fluid fluxes, the nature and distribution of fluid pathways and their global variability. ODP Hole 1256D in the eastern equatorial Pacific samples a complete section of 15 Myr old upper ocean crust down to the dike/gabbro transition zone. A high spatial resolution Sr isotope profile is integrated with wireline studies, volcanostratigraphy, petrography and mineral geochemistry to document fluid pathways and develop a model for the evolving hydrothermal system during volcanic construction of the crust. Major off-axis fluid conduits in the volcanic sequence are restricted to the flow margins of two anomalously thick (>25 m) massive flows, indicating that massive flows act as a permeability barrier for fluid flow. Dike margins are pathways for both recharge and discharge hydrothermal fluids. Sub-horizontal channeling of high temperature fluids at the dike/gabbro boundary is a common attribute of most cartoons of mid ocean ridge hydrothermal systems. Hole 1256D provides the first in situ observations of the dike/gabbro transition zone and records lateral fluid transport along intrusive boundaries. The time-integrated fluid flux in the sheeted dikes of Hole 1256D calculated using Sr isotope mass balance is ~1.8 x 106 kg/m2. This is similar to fluid fluxes from other studies (Hole 504B, Pito Deep, Hess Deep) despite large variations in the thickness and Sr isotope profiles of the sheeted dike complexes, suggesting that hydrothermal fluid fluxes are remarkably uniform and independent of the local structure of the crust. This fluid flux is not large enough to completely remove the heat flux from crystallizing and cooling the lower crust and requires deeper hydrothermal circulation in the lower crust.

  20. A Cross-Hole, Multi-Year Tracer Injection Experiment in the Volcanic Ocean Crust (United States)

    Fisher, A. T.; Neira, N. M.; Wheat, C. G.; Clark, J. F.; Becker, K.; Hsieh, C. C.; Rappe, M. S.


    We present preliminary results from the first cross-hole tracer injection experiment in the volcanic ocean crust. The test site is on 3.5 to 3.6 M.y. old seafloor on the eastern flank of the Juan de Fuca Ridge. Six borehole subseafloor observatories (CORKs) were installed during three scientific ocean drilling expeditions, five arrayed along a 1 km profile aligned with the strike of underlying abyssal hills (Holes 1026B, 1301A/B, and 1362A/B), and one offset 2.4 km to the east (1027C). Before installing the sixth CORK in Hole 1362B, in 2010, we injected a mixture of tracers (dissolved gas, metal salts, particles) during 24 hours into the upper ocean crust. Seafloor samplers connected CORKs, sampling from different locations in the crust, were recovered during servicing expeditions in 2011 and 2013; downhole samplers that contain records from the full four years following tracer injection will be recovered in Summer 2014. Analyses of dissolved gas tracers collected with wellhead samplers through 2013 suggest that the dominant flow direction in upper basement is south to north, as inferred from regional thermal data and the chemistry of geochemical (pore fluid and borehole) samples. The apparent tracer flow rate in upper basement is on the order of meters/day, but calculations are complicated by an incomplete CORK seal in Hole 1301A, which resulted in discharge from this system that also "pulled" water and tracer to the south. Samples were collected from the tracer injection borehole, Hole 1362B, and a sampling site 200 m to the north, Hole 1362A, beginning one year after tracer injection, after opening a large-diameter ball valve on the wellhead of Hole 1362B to initiate a long-term free flow experiment. Analyses of these samples suggest that much of the tracer injected in 2010 remained close to Hole 1362B rather than being advected and dispersed into the formation. It also appears that much of the tracer transport to Hole 1362A occurred within one or more relatively thin/isolated zones, because tracer concentrations remain relatively high and there is a long tail of gradually decreasing values during the last two years of sampling. This interpretation is consistent with the highly layered and laterally continuous volcanic stratigraphy observed in basement boreholes drilled at Sites 1301 and 1362.

  1. The Ocean and Crust of a Rapidly Accreting Neutron Star Implications for Magnetic Field Evolution and Thermonuclear Flashes

    CERN Document Server

    Brown, E F; Brown, Edward F.; Bildsten, Lars


    We investigate the atmosphere, ocean, and crust of neutron stars accreting at rates sufficiently high (typically in excess of the local Eddington limit) to stabilize the burning of accreted hydrogen and helium. For hydrogen-rich accretion at global rates in excess of 10^-8 solar masses per year (typical of a few neutron stars), we discuss the thermal state of the deep ocean and crust and their coupling to the neutron star core, which is heated by conduction (from the crust) and cooled by neutrino emission. We estimate the Ohmic diffusion time in the hot, deep crust and find that it is noticeably shortened (to less than 10^8 yr) from the values characteristic of the colder crusts in slowly accreting neutron stars. We speculate on the implications of these calculations for magnetic field evolution in the bright accreting X-ray sources. We also explore the consequences of rapid compression at local accretion rates exceeding ten times the Eddington rate. This rapid accretion heats the atmosphere/ocean to temperat...

  2. The deep subsurface biosphere in igneous ocean crust: frontier habitats for microbiological exploration

    Directory of Open Access Journals (Sweden)



    Full Text Available We discuss ridge flank environments in the ocean crust as habitats for subseafloor microbial life. Oceanic ridge flanks, areas far from the magmatic and tectonic influence of seafloor spreading, comprise one of the largest and least explored microbial habitats on the planet. We describe the nature of the ridge flank crustal environments, and present a framework for delineating a continuum of conditions and processes that are likely to be important for defining subseafloor microbial "provinces." The basis for this framework is three governing sets of conditions that help to determine the nature of subseafloor biomes: crustal age, extent of fluid flow, and thermal state. We present a brief overview of subseafloor conditions, within the context of these three characteristics, for five field sites where microbial studies have been done, are underway, or have been proposed. Technical challenges remain and likely will limit progress in studies of microbial ridge-flank ecosystems, which is why it is vital to select and design future studies so as to leverage as much general understanding as possible. A characterization framework such as presented in this paper, perhaps including alternative or additional physical or chemical characteristics, is essential for achieving the greatest benefit from multidisciplinary microbial investigations of the oceanic ridge flanks.

  3. Metamorphic controls on seismic velocity of subducted oceanic crust at 100-250 km depth (United States)

    Connolly, J. A. D.; Kerrick, D. M.


    Most circum-Pacific subduction zones at 100-250 km depth contain layers in which seismic velocities are ca. 5% slower than in the adjacent mantle. We compute seismic velocities from thermodynamic data for equilibrium metabasalt mineralogies, determined by free energy minimization, at subduction zone conditions. Lawsonite stability has a profound effect on seismic velocities of subducted oceanic metabasalts. Velocity reductions of 3-7% are estimated for lawsonite-eclogites derived by metamorphism of hydrothermally altered oceanic basalt subducted along relatively cool geotherms, whereas a 2-4% velocity increase is characteristic of anhydrous eclogites within the coesite stability field. The restricted depth extent of low-velocity layers is explicable through the influence of the coesite-stishovite transition, which reduces lawsonite stability at high pressure. This transition also increases the positive velocity anomaly in anhydrous eclogites to 4-6%, an effect that may account for deep high-velocity layers. The quality of the match between the properties of lawsonite-eclogite and low-velocity layers supports the contention that significant quantities of volatiles are retained within the oceanic crust beyond sub-arc depths. Because the velocity anomalies are explicable in terms of equilibrium phase relations, we find no reason to invoke metastability of metamorphic reactions to explain the low-velocity layers.

  4. Formation of Oceanic Crust Geostructurs and Relation Between Submarine landslides and Tsunamis (United States)

    Harutyunyan, Albert V.


    Numerous geological and geophysical data proved the presence of oceanic crust relicts of Tethys in the territory of Lesser Caucasus. To discover the deep structure, composition and evolution of the modern Earth crust, the elastic and density properties of basites, ultrabasites and serpentinites of lesser Caucasus at high pressures and temperatures were investigated. On the basis of this data, and numerous geological-geophysical factual data concerning Mid ridges, Transform faults, Subdaction zones, Island arcs and Marginal seas are presented as a possible mechanism of their formations and relation between submarine landslides and tsunamis. The numerous volcanic and seismic centers, serpentinized protrusions and also hydrothermal sources are dated in the rang of Mid ridges and Transform faults. The formation of serpentinized ultrabasites 3-rd layer affects an infiltration of oceanic waters on ultrabasites of the upper mantle. At the same time, on an axial part of the ridge, the horizontally cramping forces, on the 5-6?? depth, are established, which step-by-step pass on expanding in the top of the ridge. Analyzing the data about composition and properties of oceanic crust, we suspect that during the formation of Mid ridges, the main role belongs to serpentinized rocks of the 3-rd oceanic layer. Owing to high plasticity and low density, the serpentinized masses, by tectonic faults, in the central zone of Mid ridges, from both parties, float up and by means of protrusions implanted in the oceanic crust, then the serpentinized masses are grasp by basalts lavas. Accumulation in the axial zone of Mid ridges large masses of basalts and serpentinits, under influence of gravitation forces make slides to downwards on the serpentinized layer to the foot of ridge and low-powered sedimentary layers between these masses are saved. In the proposed model we attempted to interpret the above mentioned phenomena in the following sense. 1. Because of serpentinization of ultrabasits, the horizontal stress growth is more than 40- 50%. Spreading of oceanic floor can be partially conditioned by these forces. At impossibility to distensible in the lateral direction, the serpentinized masses increase vertically, both in the axial part of the ridge and in transform faults. Serpentinized blocks of the 3-rd layer from the axial part of ridge, are involved in basalt lava and under gravitation forces slide downwards. The data of drilling has shown the availability of serpentinized blocks in the second volcanogenic layer. 2 .On formation transform faults a definite role is played the processes of basalt masse slides, which naturally takes place permanently, owing to which one between blocks the faults are reshaped. In the transform faults the serpentinized protrusions and hydrotherms are also dated. 3. The chemical composition of basalts of Island arcs and Mid ridges basically coincide, which give us the basis to suppose that they are source commune. We suppose that in definite depth of subdaction zone there exist connection with astenosphere. In the upper mantle molten substrate is enriched by rare elements and erupted in the rift zones of Mid ridges. 4. Marginal seas extension speed is 5-7cm/year. Esteeming the proposed model, it is possible to mark that spreading of Marginal seas takes place owing to eruption of magmatic masses as directly from in the region of Island arcs, as well as from astenosphere in the Mid oceanic ridges. 5. The studies of catastrophic tsunami origin in 2004 has resulted in judgment, that the formation of 9-m surges took place because the sliding process in the bottom of ocean and even catastrophic earthquake at the bottom can't provoke a tsunami with such force. According a to the proposed model, in Mid ridges, and in other parts of ocean, as well sliding process took place, which could provoke a high-power tsunami. The volume of sliding structures, between transform faults can reach several million of km3. The sliding masses at the bottom of the ocean should invoke a movement of oceanic water and provoke the sur

  5. Reactive overprint of the Central Indian Ridge mantle and formation of hybrid troctolites: reassessing the significance of bulk oceanic crust (United States)

    Sanfilippo, A.; Morishita, T.; Kumagai, H.; Nakamura, K.; Okino, K.; Tamura, A.; Arai, S.


    The idea that hybridized mantle rocks can contribute to the oceanic crust composition has recently emerged thanks to studies on primitive (olivine-rich) troctolites [e.g. 1]. These rocks are considered to be formed by melt-rock interaction, but the exact reaction process by which they originate is still debated and their role on the bulk oceanic crust composition has been never defined. Olivine-rich troctolites have been mostly found at slow spreading ridges [2] or at their fossil analogues [3]. Similar rocks have been recently collected in the 25ºS area of the intermediate spreading Central Indian Ridge (CIR), and rarely characterize the crust mantle boundary at fast spreading ridges [4]. We show that textural and chemical inheritances of the pre-existing mantle are preserved in the CIR troctolites. In particular, the local occurrence of granular, mantle-derived orthopyroxenes and the composition of the associated clinopyroxene indicate that these crustal rocks formed through a direct (one-stage) conversion of a mantle peridotite. We use chemical evidence to infer the same origin of the olivine-rich troctolites worldwide, concluding that the reactive overprint of the oceanic mantle is a process diffused over the entire spreading rate spectrum. Bulk oceanic crust estimates of the Hess Deep (Pacific) and Atlantis Massif (Atlantic) crustal sections are used to quantify and compare the effect of these rocks on the bulk crust composition at fast and slow spreading ridges. Our inferences suggest that the significance of the bulk oceanic crust should be reassessed. When hybrid troctolites are included at crustal levels, the oceanic crust cannot be considered equal to the composition of the melt extracted from the mantle, but it results more primitive and importantly thicker. References: [1] Suhr G., Hellebrand E., Johnson K., Brunelli D., 2008, Geochem. Geophys. Geosyst. 9, doi:10.1029/2008GC002012; [2] Drouin M., Godard M., Ildefonse B., Bruguier O., Garrido C.J. , 2009, Chem. Geol. 264, 71-88; [3] Renna M. R., Tribuzio R., 2011, J. Petrol. 52, 1763-1790; [4] Dick H.J.B. and Natland J.H., 1996, Proceedings Ocean Drill. Prog., 147, 103-134.

  6. Controls on thallium uptake during hydrothermal alteration of the upper ocean crust (United States)

    Coggon, Rosalind M.; Rehkämper, Mark; Atteck, Charlotte; Teagle, Damon A. H.; Alt, Jeffrey C.; Cooper, Matthew J.


    Hydrothermal circulation is a fundamental component of global biogeochemical cycles. However, the magnitude of the high temperature axial hydrothermal fluid flux remains disputed, and the lower temperature ridge flank fluid flux is difficult to quantify. Thallium (Tl) isotopes behave differently in axial compared to ridge flank systems, with Tl near-quantitatively stripped from the intrusive crust by high temperature hydrothermal reactions, but added to the lavas during low temperature reaction with seawater. This contrasting behavior provides a unique approach to determine the fluid fluxes associated with axial and ridge flank environments. Unfortunately, our understanding of the Tl isotopic mass balance is hindered by poor knowledge of the mineralogical, physical and chemical controls on Tl-uptake by the ocean crust. Here we use analyses of basaltic volcanic upper crust from Integrated Ocean Drilling Program Hole U1301B on the Juan de Fuca Ridge flank, combined with published analyses of dredged seafloor basalts and upper crustal basalts from Holes 504B and 896A, to investigate the controls on Tl-uptake by mid-ocean ridge basalts and evaluate when in the evolution of the ridge flank hydrothermal system Tl-uptake occurs. Seafloor basalts indicate an association between basaltic uptake of Tl from cold seawater and uptake of Cs and Rb, which are known to partition into K-rich phases. Although there is no clear relationship between Tl and K contents of seafloor basalts, the data do not rule out the incorporation of at least some Tl into the same minerals as the alkali elements. In contrast, we find no relationship between the Tl content and either the abundance of secondary phyllosilicate minerals, or the K, Cs or Rb contents in upper crustal basalts. We conclude that the uptake of Tl and alkali elements during hydrothermal alteration of the upper crust involves different processes and/or mineral phases compared to those that govern seafloor weathering. Furthermore, a correlation between the Tl and S concentrations of upper crustal basalts from Holes U1301B, 504B and 896A indicates that Tl is primarily incorporated into secondary sulfides. Given that some of these secondary sulfides formed as a result of microbial sulfate reduction, microbial action is at least indirectly responsible for Tl-uptake. Thallium-enrichment of ridge flank basalts requires a Tl-bearing fluid and physical, chemical and microbial conditions that favor secondary sulfide formation. Uptake of Tl occurs in reducing environments in the background rocks away from fluid flow pathways during early 'open' circulation of oxidizing seawater but more pervasively throughout the system during later 'restricted' circulation of reducing fluids. The Tl-isotope system is therefore a useful tracer of the fluid flux through both the 'open' and 'restricted' ridge flank hydrothermal regimes.

  7. Observations of anomalous oceanic crust in the Canada Basin, Arctic Ocean (United States)

    Jackson, H. R.; Grantz, A.; Reid, I.; May, S. D.; Hart, P. E.


    We present a seismic reflection frofile and coincident refraction data in deep water north of the Northwind Ridge, in the Canada Basin. The reflection profile shows a package of bright events, which are locally nonparallel and discontinuous, about 2.5 s below the seafloor. These bright reflectors are of interest because previous reflection profiles in the Canada Basin have not penetrated the coherent flat-lying events, which are typical of bedded sedimentary rocks. These prominent reflections are recorded on the refraction profile as wide-angle reflections, and the locally nonparallel and discontinuous event as a refraction. On the refraction profile the nonparallel and discontinuous event exhibits lower frequencies and has an associated refraction with a velocity of 4.5 km/s and a high gradient. This event is interpreted as representing the top of oceanic layer 2, which is observed for the first time in the Canada Basin. Three kilometres beneath layer 2 a velocity of 7.2-7.5 km/s is measured. This layer could represent either a high-velocity layer 3 or serpentinized mantle. Based on comparison with the eastern Grand Banks and the west Iberian and Labrador Sea margins the interpretation of serpentinized mantle is preferred.

  8. Compositional variation and genesis of ferromanganese crusts of the Afanasiy-Nikitin Seamount, Equatorial Indian Ocean

    Digital Repository Service at National Institute of Oceanography (India)

    Rajani, R.P.; Banakar, V.K.; Parthiban, G.; Mudholkar, A.V.; Chodankar, A.R.


    Eight ferromanganese crusts (Fe-Mn crusts) with igneous and sedimentary substrates collected at different water depths from the Afanasiy-Nikitin Seamount are studied for their bulk major, minor and rare earth element composition. The Mn/Fe ratios...

  9. Transient Hydrothermal Alteration In Fault Zones Cutting The Lower Oceanic Crust, Hess Deep Rift (United States)

    McCaig, A. M.; Titarenko, S.; Cliff, R. A.; Savov, I. P.; Boyce, A.; Dutt, R.


    IODP Expedition 345 drilled the first holes in the lower plutonic crust at a fast-spreading ridge, recovering primitive layered gabbros [1]. Alteration occurred as: 1) a largely static pseudomorphic alteration, predominantly in the greenschist and sub-greenschist facies with mainly talc and serpentine replacing olivine, and prehnite replacing plagioclase. Talc sometimes overprints serpentine mesh texture. 2) an overprinting metasomatic alteration, spatially related to cataclastic fault zones and macroscopic veins, dominated by prehnite and chlorite. Secondary clinopyroxene and epidote locally overprint the prehnite-chlorite assemblage, but the last events are veins of prehnite and zeolite. Metamorphosed dykes show chilled margins within the cataclasites, and are themselves affected by cataclastic deformation. Faults, dykes and overprinting alteration are inferred to be related to the westward propagation of Cocos-Nazca spreading forming Hess Deep. 87Sr/86Sr ratios of small whole rock samples of cataclasites and dyke rocks are in the range 0.7037 - 0.7048, indicating alteration by seawater at moderate integrated fluxes. The highest values were in cataclasites overprinted by prehnite. Sampling of individual minerals has been undertaken using a microscope mounted drill, and shows that alteration is mainly affecting secondary minerals, with late prehnite veins ranging up to 0.7054. ?18O values range from +1 to + 6 per mil. Combined with metamorphic data this indicates alteration at temperatures between 200 and 400 °C. Secondary clinopyroxene and talc replacing serpentine are interpreted to indicate transient prograde hydrothermal events. Preliminary modelling using Comsol Multiphysics suggests that the temperatures of the overprinting alteration, as well as transient prograde events, could be achieved in a permeable fault slot cutting through crust 0.5 to 1 m.y. old. The prehnite-chlorite assemblage is predicted to be important in off-axis alteration, common in any location where faults intersect the Moho, including transform faults, near axis normal faults at slow spreading ridges, and perhaps bending faults at subduction zones. [1] Gillis, K.M., Snow J. E. and Shipboard Science Party (2014) Primitive layered gabbros from fast-spreading lower oceanic crust. Nature, 505, 204-207.

  10. Alteration of basaltic glass: Mechanisms and significance for the oceanic crust-seawater budget (United States)

    Staudigel, Hubert; Hart, Stanley R.


    Alteration of basaltic glass to palagonite is characterized by a nearly isomolar exchange of SiO 2, Al 2O 3, MnO, MgO, CaO, Na 2O, P 2O 5, Zn, Cu, Ni, Cr, Hf, Sc, Co and REE for H 2O and K 2O, whilst TiO 2 and FeO are passively accumulated during removal of the remaining cations. The network forming cations Al and Si are removed from the glass in proportion to the gain in Ti and Fe, whilst the other cations do not show a significant relationship to the amount of Ti and Fe accumulation. Sr isotopic data show that during palagonite formation approximately 85% of the basaltic Sr is lost to the hydrous solutions and 40% of seawater Sr is added to the glass, yielding an average loss of the same order of magnitude as of the network forming cations. Losses and gains of oxides yield an average increase of +105% TiO 2. K, Rb, and Cs show high increases, but K/Rb and K/Cs ratios indicate two different alteration processes: (1) formation of palagonite involves a drastic decrease in these ratios, indicating structural similarities between palagonite and smectite; (2) surface alteration of glass is characterized by an increase in K/Rb and K/Cs ratios, probably best interpreted as sorption of alkalies in ratios approximating those of seawater. The total fluxes involved in alteration of glass in the upper portion of the oceanic crust are estimated from the modal abundance of palagonite in the oceanic crust and the abundance of the vein materials smectite and carbonate. Smectite and carbonates act as a sink for a significant portion of the elements liberated up during alteration of basaltic glass except for Na and Al, which are probably taken up by zeolites and/or albite, possibly hidden in the macroscopic estimate of carbonate. Formation of the observed quantity of secondary phases requires additional sources for Si, Fe. Ca and K. K is provided in excess from the inflowing seawater at reasonable water/rock ratios. The remaining excess Ca, Si and Fe required may be derived by alteration of interstitial glass and breakdown of anorthite rich plagioclase and titano-magnetite, and/or by supply of deeper seated metamorphic reactions.

  11. Assimilation of sediments embedded in the oceanic arc crust: myth or reality? (United States)

    Bezard, Rachel; Davidson, Jon P.; Turner, Simon; Macpherson, Colin G.; Lindsay, Jan M.; Boyce, Adrian J.


    Arc magmas are commonly assumed to form by melting of sub-arc mantle that has been variably enriched by a component from the subducted slab. Although most magmas that reach the surface are not primitive, the impact of assimilation of the arc crust is often ignored with the consequence that trace element and isotopic compositions are commonly attributed only to varying contributions from different components present in the mantle. This jeopardises the integrity of mass balance recycling calculations. Here we use Sr and O isotope data in minerals from a suite of volcanic rocks from St Lucia, Lesser Antilles arc, to show that assimilation of oceanic arc basement can be significant. Analysis of 87Sr/86Sr in single plagioclase phenocrysts from four Soufrière Volcanic Complex (SVC; St Lucia) hand samples with similar composition (87Sr/86Sr = 0.7089-0.7091) reveals crystal isotopic heterogeneity among hand samples ranging from 0.7083 to 0.7094 with up to 0.0008 difference within a single hand sample. ?O18 measurements in the SVC crystals show extreme variation beyond the mantle range with +7.5 to +11.1‰ for plagioclase (n=19), +10.6 to +11.8‰ for quartz (n=10), +9.4 to +9.8‰ for amphibole (n=2) and +9 to +9.5‰ for pyroxene (n=3) while older lavas (Pre-Soufriere Volcanic Complex), with less radiogenic whole rock Sr composition (87Sr/86Sr = 0.7041-0.7062) display values closer to mantle range: +6.4 to +7.9‰ for plagioclase (n=4) and +6 to +6.8‰ for pyroxene (n=5). We argue that the 87Sr/86Sr isotope disequilibrium and extreme ?O18 values provide compelling evidence for assimilation of material located within the arc crust. Positive correlations between mineral ?O18 and whole rock 87Sr/86Sr, 143Nd/144Nd and 206,207,208Pb/204Pb shows that assimilation seems to be responsible not only for the isotopic heterogeneity observed in St Lucia but also in the whole Lesser Antilles since St Lucia encompasses almost the whole-arc range of isotopic compositions. This highlights the need for detailed mineral-scale investigation of oceanic arc suites to quantify assimilation that could otherwise lead to misinterpretation of source composition and subduction processes.

  12. High Temperature Logging and Monitoring Instruments to Explore and Drill Deep into Hot Oceanic Crust. (United States)

    Denchik, N.; Pezard, P. A.; Ragnar, A.; Jean-Luc, D.; Jan, H.


    Drilling an entire section of the oceanic crust and through the Moho has been a goal of the scientific community for more than half of a century. On the basis of ODP and IODP experience and data, this will require instruments and strategies working at temperature far above 200°C (reached, for example, at the bottom of DSDP/ODP Hole 504B), and possibly beyond 300°C. Concerning logging and monitoring instruments, progress were made over the past ten years in the context of the HiTI ("High Temperature Instruments") project funded by the european community for deep drilling in hot Icelandic geothermal holes where supercritical conditions and a highly corrosive environment are expected at depth (with temperatures above 374 °C and pressures exceeding 22 MPa). For example, a slickline tool (memory tool) tolerating up to 400°C and wireline tools up to 300°C were developed and tested in Icelandic high-temperature geothermal fields. The temperature limitation of logging tools was defined to comply with the present limitation in wireline cables (320°C). As part of this new set of downhole tools, temperature, pressure, fluid flow and casing collar location might be measured up to 400°C from a single multisensor tool. Natural gamma radiation spectrum, borehole wall ultrasonic images signal, and fiber optic cables (using distributed temperature sensing methods) were also developed for wireline deployment up to 300°C and tested in the field. A wireline, dual laterolog electrical resistivity tool was also developed but could not be field tested as part of HiTI. This new set of tools constitutes a basis for the deep exploration of the oceanic crust in the future. In addition, new strategies including the real-time integration of drilling parameters with modeling of the thermo-mechanical status of the borehole could be developed, using time-lapse logging of temperature (for heat flow determination) and borehole wall images (for hole stability and in-situ stress determination) as boundary conditions for the models. In all, and with limited integration of existing tools, to deployment of high-temperature downhole tools could contribute largely to the success of the long awaited Mohole project.

  13. Using pulse shapes of intermediate depth seismicity to locate sources relative to subducting oceanic crust showing low seismic velocity (United States)

    Martin, S.; Rietbrock, A.


    We are proposing a method to locate intermediate depth earthquakes in respect to the position of a relatively low velocity subducted crust by classifying pulse shapes recorded at specific receiver locations. P wave onsets of intermediate depth earthquakes at subduction zones indicate that low velocity layering of less than 10 km thickness is present in the vicinity of nearly all circum pacific slab surfaces. Whether low velocity is caused by hydrous metabasalt phases persisting to depth, released fluids, dehydration within the subducting slab, a serpentinized layer or even partial melts on top of subducted crust is a subject of ongoing discussion. Defining source locations relative to such a low velocity structure will help to rule out part of these interpretations of intermediate depth seismicity and shed more light on the source processes involved. We use 2D finite difference modelling to calculate full synthetic wave propagation in subduction zones. Our simulations show that a low velocity structure of km scale brings forth characteristic p onsets at near coastal receiver locations for sufficiently long propagation distances along the slab surface. Pulse shapes and frequency of these synthetic onsets are analysed regarding source location relative to the slab surface. We test Double Couple (DC) source position and orientation in an up dip slab geometry derived from local seismicity in Northern Chile. The layered slab consists of eclogized upper crust (5 km) and a layer of slow basaltic lower crust (2 km) on top of fast oceanic mantle. The tests account for velocity undulations in subducted crust, roughness of the low velocity structure and slab geometry as well as scale of layering. We find that sources of intermediate depth events at 21° S in Northern Chile are situated within or directly below the lower crust at distances of less than 2 km from the oceanic Moho.

  14. Dike intrusion controls on permeability and hydrothermal circulation of oceanic crust at IODP Hole 1256D (United States)

    Gilbert, L. A.; Tartarotti, P.; Fontana, E.; Bona, M. L.; Gross, D.; LaPier, G.; Dempsey, C.


    We examine the hydrothermal structure of the lava-dike transition zone in oceanic crust of Integrated Ocean Drilling Program (IODP) Hole 1256D using detailed sample measurements of permeability, porosity, metamorphic minerals, and structures. The transition zone consists of basaltic sheet and massive flows, a cataclastic unit, and hyaloclastitic breccias. Structural investigations show that this transition occurs through a larger depth interval than that previously defined, extending 254 m upward from the top of the Sheeted Dike Complex (811.4 to 1065.7 meters below seafloor (mbsf)). Through the transition zone, models predict a general decrease in permeability, based on a corresponding decrease in porosity with depth. Laboratory measurements of physical properties show porosity decreases drastically, as does permeability. Thin sections reveal no open pore space, and all structures are filled: veins (mm- to cm-thick) or sets of parallel veins or vein networks, Riedel-deformation bands, cataclasites (sub-vertical structures may represent original cooling fractures and/or might be related to the regional tectonics (i.e., extensional tectonics of the rift zone) or to the local tensional stress field created at the top of the dikes as a direct consequence of dike intrusion. Since this transition zone is located near the boundary between the Low Temperature Alteration Zone (above) and the Hydrothermal Alteration Zone (below), we postulate that fracturing and hydrothermal alteration in the investigated crustal interval likely occurred during dike intrusion. Consequently, the lower part of the lavas (below 811.4 mbsf or 561 meters sub-basement) seems to mark a boundary layer between different stress fields, permeability regimes, and metamorphic imprints.

  15. Oceanic crust within the paleozoic Granjeno Schist, northeastern Mexico. Remnants of the Rheic and paleo-Pacific Ocean. (United States)

    Torres Sanchez, Sonia Alejandra; Augustsson, Carita; Rafael Barboza Gudiño, Jose; Jenchen, Uwe; Torres Sanchez, Dario; Aleman Gallardo, Eduardo; Abratis, Michael


    Late Paleozoic metamorphic rocks in Mexico are related to the Laurentia-Gondwana collision in Carboniferous time, during Pangaea amalgamation. Vestiges of the Mexican Paleozoic continental configuration are present in the Granjeno Schist, the metamorphic basement of the Sierra Madre Oriental. Field work and petrographic analysis reveal that the Granjeno Schist comprises metamorphic rocks with both sedimentary (psammite, pelite, turbidite, conglomerate, black shale) and igneous (tuff, lava flows, pillow lava and ultramafic bodies) protoliths. The chlorite geothermometer and the presence of phengite in the metasedimentary units as well as 40Ar/39Ar ages on metavolcanic and metaultramafic rocks indicate that the Granjeno Schist was metamorphosed under sub-greenschist to greenschist facies with temperatures ranging from 250-345°C with 2.5 kbar during Carboniferous time (330±30 Ma). The presence of metabasalt, metacumulate, serpentinite and talc bodies suggests an oceanic tectonic setting for the evolution of the Granjeno Schist. Serpetinite rocks have mesh, granular and ribbon textures which indicate recrystallization and metasomatic events. The serpentinite rocks are enriched in the very large incompatible elements Cs, U, and Zr and depleted in Ba, Sr, Pb, Zr and Ce. Normalized REE patterns (LaN/YbN = 0.51 - 19.95 and LaN/SmN = 0.72 - 9.08) of the serpentinite and talc/soapstone are characteristic of peridotite from both suprasubduction and mid-ocean ridge zones. Serpentinite from the Granjeno Schist have spinel content which can reveal different stages of evolution in host serpentinite. The composition of chromite indicates that they belong to podiform chromite that may have crystallized from mid-ocean ridge magma. Al-chromite in the serpentinite is characterized by #Cr 0.48 to 0.55, which indicates a depleted mantle source affected by 17 to 18% of partial melting. The ferritchromite has #Cr values of 0.93 to 1.00 which indicates a metamorphic origin. Our study suggests at least two serpentinization stages. The first serpentinization stage is related to an ocean-floor environment. At this stage, mesh-textured serpentinite formed under static conditions under subgreenschist to greenschist conditions. The second serpentinization stage occurred under greenschist to low amphibole conditions. During this stage Cr-spinel progressively was replaced by ferritchromite with magnetite rims due to regional metamorphism. Tectonic contact of the serpentinite with metavolcanic and metasedimentary rocks indicates lithospheric mantle slivers juxtaposed during the metamorphism of the Granjeno Schist during Pennsylvanian time. This metamorphic event occurred in an active continental margin. It represents the last events of the southern closure of the Rheic Ocean and Permo-Carboniferous convergence of Pacific plates on the western margin of Pangea.

  16. Evolution of the Late Cretaceous crust in the equatorial region of the Northern Indian Ocean and its implication in understanding the plate kinematics

    Digital Repository Service at National Institute of Oceanography (India)

    Desa, M.; Ramana, M.V.; Ramprasad, T.


    anomalies suggests the presence of fossil spreading ridge segments and extra oceanic crust on the Indian plate that has been transferred from the Antarctica plate by discrete southward ridge jumps. These ridge jumps are caused by thermal instability...

  17. Predictions of hydrothermal alteration within near-ridge oceanic crust from coordinated geochemical and fluid flow models (United States)

    Wetzel, L.R.; Raffensperger, J.P.; Shock, E.L.


    Coordinated geochemical and hydrological calculations guide our understanding of the composition, fluid flow patterns, and thermal structure of near-ridge oceanic crust. The case study presented here illustrates geochemical and thermal changes taking place as oceanic crust ages from 0.2 to 1.0 Myr. Using a finite element code, we model fluid flow and heat transport through the upper few hundred meters of an abyssal hill created at an intermediate spreading rate. We use a reaction path model with a customized database to calculate equilibrium fluid compositions and mineral assemblages of basalt and seawater at 500 bars and temperatures ranging from 150 to 400??C. In one scenario, reaction path calculations suggest that volume increases on the order of 10% may occur within portions of the basaltic basement. If this change in volume occurred, it would be sufficient to fill all primary porosity in some locations, effectively sealing off portions of the oceanic crust. Thermal profiles resulting from fluid flow simulations indicate that volume changes along this possible reaction path occur primarily within the first 0.4 Myr of crustal aging. ?? 2001 Elsevier Science B.V. All rights reserved.

  18. Detachment Fault Initiation and Control by Partially Molten Zones in the Lower Ocean Crust (United States)

    Dick, H. J.; Natland, J. H.; MacLeod, C. J.; Robinson, P. T.


    The close association of oxide gabbro and deformation in interleaved ferrogabbro and olivine gabbro at Atlantis Bank on the SW Indian Ridge explains the formation of this enormous single-domed gabbroic oceanic core complex. ODP Holes 735B and 1105A show that the stratigraphy is defined by 100's of zones of intense deformation and strain localization in the upper 500-m where various melts percolated including late-stage iron-titanium rich melts. The latter created highly deformed oxide-rich gabbro zones at scales from millimeters to over 100 meters. Mapping by ROV, over-the-side rock drilling, dredging, and submersible shows that this stratigraphy exists uniformly over the bank. Deep drilling and sampling up the headwalls of major landslips cutting into the core complex show that the fault zone was imbricate, likely reflecting relocation of the active slip plane due to cyclic intrusion in the lower crust. The detachment originated as a high-angle fault on the rift valley wall that propagated into a zone of partially molten gabbro beneath the sheeted dikes. This zone then pinned the footwall block, creating a plutonic growth fault along which gabbro intruded beneath the ridge axis was continuously uplifted and exposed on the Antarctic plate for ~3.9 myr. The overlying basaltic carapace spread more slowly to the north on the African Plate. Textural evidence, particularly that provided by iron-titanium oxides, shows that melts migrated along complex shear zones in which several creep mechanisms operated, ranging from crystal plastic dislocation creep, diffusion creep, grain boundary sliding, and brittle deformation. More than one of these mechanisms may have occurred concurrently. Subsequently, these zones localized later solid-state creep, often producing texturally complex rocks where separation of the timing and duration of different creep mechanisms is difficult to unravel. As uplift of the plutonic section progressed, the footwall passed through the zone of diking beneath the rift valley, and as a consequence numerous inliers of the dike gabbro transition are found exposed across the gabbro massif. Olivine gabbros that underwent localized deformation and shearing were strongly affected both by crystallization of abundant oxides and new more sodic plagioclase and iron-rich pyroxene, and by local re-equilibration of existing calcic plagioclase and pyroxene with the invading iron-titanium rich melt. This was likely accompanied by dissolution of existing olivine and crystallization of secondary igneous clinopyroxene. Brown amphibole after clinopyroxene and plagioclase also formed, with some amphibole rimming relict olivine. The latter processes makes it hard to differentiate between ferrogabbros that crystallized directly from a melt, and those produced by hybridization of pre-existing olivine gabbro by migrating melts in the shear zone. Dredge samples collected at numerous localities at oceanic transforms along the SW Indian, American-Antarctic, and Mid-Atlantic Ridge with similar fabrics show that the relationship between oxide gabbros and deformation is widely associated with the exposure of plutonic rocks at oceanic core complexes. Atlantis Bank therefore provides the general case for the formation of the numerous large gabbro massifs exposed on the ocean floor at all slow and ultraslow spreading ridges.

  19. Uranium isotope systematics of ferromanganese crusts in the Pacific Ocean: Implications for the marine 238U/235U isotope system (United States)

    Goto, Kosuke T.; Anbar, Ariel D.; Gordon, Gwyneth W.; Romaniello, Stephen J.; Shimoda, Gen; Takaya, Yutaro; Tokumaru, Ayaka; Nozaki, Tatsuo; Suzuki, Katsuhiko; Machida, Shiki; Hanyu, Takeshi; Usui, Akira


    Variations of 238U/235U ratio (?238U) in sedimentary rocks have been proposed as a possible proxy for the paleo-oceanic redox conditions, although the marine ?238U system is not fully understood. Here we investigate the spatial variation of ?238U in modern ferromanganese (Fe-Mn) crusts by analyzing U isotopes in the surface (0-3 mm depth) layer of 19 Fe-Mn crusts collected from 6 seamounts in the Pacific Ocean. ?238U values in the surface layers show little variation and range from -0.59‰ to -0.69‰. The uniformity of ?238U values is consistent with the long residence time of U in modern seawater, although the ?238U values are lighter than that of present-day seawater by ?0.24‰. The light ?238U values are consistent with the isotope offset observed in previously reported adsorption experiment of U to Mn oxide. These results indicate that removal of U from seawater to Mn oxide is responsible for the second largest U isotope fractionation in the modern marine system, and could contribute to isotopically heavy U to seawater. Depth profiles of U isotopes (?234U and ?238U) in two Fe-Mn crusts (MR12-03_D06-R01 and MC10_CB07_B), dated by Os isotope stratigraphy, were investigated to reconstruct the evolution of the oceanic redox state during the Cenozoic. The ?238U depth profiles show very limited ranges (-0.57‰ to -0.67‰ for MR12-03_D06-R01 and -0.56‰ to -0.69‰ for MC10_CB07_B), and have values that are similar to those of the surface layers of Fe-Mn crusts. The absence of any resolvable variation in the ?238U depth profiles may suggest that the relative amounts of oxic and reducing U sinks have not varied significantly over the past 45 Myr. However, the ?234U depth profiles of the same samples show evidence for the possible redistribution of 234U after deposition. Therefore, the depth profile of ?238U in Fe-Mn crusts may have been also overprinted by later chemical exchange with pore-water or seawater, and may not reflect the paleo-oceanic environmental changes. To assess the potential role of U removal by Mn oxides on seawater ?238U, we calculated seawater ?238U with different U sink fluxes into Mn oxides using a simple mass balance model. The results of these calculations suggest that seawater ?238U could have varied significantly throughout Earth's history due to changes in the accumulation rate of Mn oxides.

  20. Folding instabilities and cracking of thin coatings on a soft polymer substrate as a model of the oceanic crust

    Directory of Open Access Journals (Sweden)

    S. L. Bazhenov


    Full Text Available Nucleation and development of microrelief and fragmentation of coating under tensile extension of polymer films coated with a thin rigid layer is studied, and the mechanisms responsible for the development of both types of structures are discussed. The development of regular folding is controlled by compression-induced buckling instabilities in a rigid coating on a compliant support. Parallel cracks are due to features of mechanical stress transfer from a soft substratum to a rigid coating via an interface. Micro-relief is similar to relief of the oceanic floor in the vicinity of mid-oceanic ridges. We suggest that the young oceanic crust and the upper mantle may behave as a solid coating on a soft basement system.

  1. Radiocarbon dating of basalts from middle oceanic ridges using lithified carbonate crust samples

    International Nuclear Information System (INIS)

    Results of radiocarbon dating of basalts using lithified carbonate crust samples are presented. Core samples were samples in Red sea deep-water cavities during joint Picar expedition of two scientific research ships ''Academic Kurchatov'' and ''Professor Shtockman''. Crust samples were dated according to benzene variant. Age values obtained are varied in the interval from 2980 to 20700 yeras. Dating of basalts using lithified carbonate crusts is efficient in the range of radiocarbon dating (up to 40000-45000 years). This range is inaccessible for other methods of nuclear geochronology that makes the above method more valuable

  2. Ocean crust formation age in the Southern Urals: example of the Mindyak ophiolite complex (United States)

    Tesalina, S.; Gannoun, A.; Bourdon, B.; Capmas, F.; Garuti, G.; Pushkarev, E.; Orgeval, J.-J.; Birck, J.-L.; Allegre, C.


    The Mindyak lherzolite complex belongs to the allochthonous ophiolite belt, which marks the main Urals suture zone. This structure separates the East European continental margin and Uralian Palaeozoic volcano-sedimentary formations. The Mindyak complex consists of lherzolites, which gradually change into more depleted harzburgites and dunites toward a transitional clinopyroxenite-wehrlite zone. Gabbro dykes crosscut the mantle unit. Rhenium-osmium isotope composition has been measured in the peridotites and mafic dykes. Osmium contents decrease sharply from the peridotites to the transition sequence and mafic dykes. The 187Os/188Os isotope ratio of the peridotites (187Os/188Os = 0.118-0.125) is typical of the depleted mantle, slightly increasing toward the clinopyroxenite-wehrlite transition zone sequence (187Os/188Os = 0,17-0,64) and reaches a maximum values in the mafic dykes (187Os/188Os up to 29). These radiogenic samples define an isochron with a best-fit age of about 485±14 Ma. This age corresponds to the Ordovician age determined on the basis of the conodonts occurrences in the sediments intercalated with the lavas. The initial 187Os/188Os isotope ratio (about 0.5) is too high for mantle material and could be indicative of a crustal contribution, which could be explained by the formation of these complexes during intra-continental rifting. Thus the Early Ordovician Re-Os model age provides insights into the early history of Urals related to the processes of the lherzolite ophiolite sequences formation. According to paleoreconstruction, at this time the rifting affected the passive East European continental margin with subsequent opening of the early Palaeozoic oceanic basin in the Southern Urals. The previous U-Pb, Pb-Pb and Sm-Nd radiometric data on garnet metagabbros in serpentinite melange from the same ophiolite complex define an age range of 410-415 and 467 Ma, which corresponds to the peak of metamorphic event in this area. This research work is supported by Inco-Copernicus MinUrals project.

  3. Age, spreading rates, and spreading asymmetry of the world's ocean crust

    National Oceanic and Atmospheric Administration, Department of Commerce — The authors present four companion digital models of the age, age uncertainty, spreading rates and spreading asymmetries of the world's ocean basins as geographic...

  4. Molybdenum evidence for expansive sulfidic water masses in ~ 750 Ma oceans (United States)

    Dahl, Tais W.; Canfield, Donald E.; Rosing, Minik T.; Frei, Robert E.; Gordon, Gwyneth W.; Knoll, Andrew H.; Anbar, Ariel D.


    The Ediacaran appearance of large animals, including motile bilaterians, is commonly hypothesized to reflect a physiologically enabling increase in atmospheric and oceanic oxygen abundances (pO 2). To date, direct evidence for low oxygen in pre-Ediacaran oceans has focused on chemical signatures in the rock record that reflect conditions in local basins, but this approach is both biased to constrain only shallower basins and statistically limited when we seek to follow the evolution of mean ocean chemical state through time. Because the abundance and isotopic composition of molybdenum (Mo) in organic-rich euxinic sediments can vary in response to changes in global redox conditions, Mo geochemistry provides independent constraints on the global evolution of well-oxygenated environments. Here, we establish a theoretical framework to access global marine Mo cycle in the past from the abundance and isotope composition of ancient seawater. Further, we investigate the ~ 750 Ma Walcott Member of the Chuar Group, Grand Canyon, which accumulated in a rift basin with open connection to the ocean. Iron speciation data from upper Walcott shales indicate that local bottom waters were anoxic and sulfidic, consistent with their high organic content (up to 20 wt.%). Similar facies in Phanerozoic successions contain high concentrations of redox-sensitive metals, but in the Walcott Member, abundances of Mo and U, as well as Mo/TOC (~ 0.5 ppm/wt.%) are low. ? 98Mo values also fall well below modern equivalents (0.99 ± 0.13‰ versus ~ 2.35‰ today). These signatures are consistent with model predictions where sulfidic waters cover ~ 1-4% of the global seafloor, corresponding to a ~ 20-80 fold increase compared to the modern ocean. Therefore, our results suggest globally expansive sulfidic water masses in mid-Neoproterozoic oceans, bridging a nearly 700 million-year gap in previous Mo data. We propose that anoxic and sulfidic (euxinic) conditions governed Mo cycling in the oceans even as ferruginous subsurface waters re-appeared 800-750 Ma, and we interpret this anoxic ocean state to reflect a markedly lower atmospheric and oceanic O 2 level, consistent with the hypothesis that pO 2 acted as an evolutionary barrier to the emergence of large motile bilaterian animals prior to the Ediacaran Period.

  5. Molybdenum evidence for expansive sulfidic water masses in ~ 750 Ma oceans

    DEFF Research Database (Denmark)

    Dahl, Tais Wittchen; Canfield, Donald Eugene


    The Ediacaran appearance of large animals, including motile bilaterians, is commonly hypothesized to reflect a physiologically enabling increase in atmospheric and oceanic oxygen abundances (pO2). To date, direct evidence for low oxygen in pre-Ediacaran oceans has focused on chemical signatures in the rock record that reflect conditions in local basins, but this approach is both biased to constrain only shallower basins and statistically limited when we seek to follow the evolution of mean ocean chemical state through time. Because the abundance and isotopic composition of molybdenum (Mo) in organic-rich euxinic sediments can vary in response to changes in global redox conditions, Mo geochemistry provides independent constraints on the global evolution of well-oxygenated environments. Here, we establish a theoretical framework to access global marine Mo cycle in the past from the abundance and isotope composition of ancient seawater. Further, we investigate the ~ 750 Ma Walcott Member of the Chuar Group, Grand Canyon, which accumulated in a rift basin with open connection to the ocean. Iron speciation data from upper Walcott shales indicate that local bottom waters were anoxic and sulfidic, consistent with their high organic content (up to 20 wt.%). Similar facies in Phanerozoic successions contain high concentrations of redox-sensitive metals, but in the Walcott Member, abundances of Mo and U, as well as Mo/TOC (~ 0.5 ppm/wt.%) are low. d98Mo values also fall well below modern equivalents (0.99 ± 0.13‰ versus ~ 2.35‰ today). These signatures are consistent with model predictions where sulfidic waters cover ~ 1–4% of the global continental shelf area, corresponding to a ~ 400–800 fold increase compared to the modern ocean. Therefore, our results suggest globally expansive sulfidic water masses in mid-Neoproterozoic oceans, bridging a nearly 700 million-year gap in previous Mo data. We propose that anoxic and sulfidic (euxinic) conditions governed Mo cycling in the oceans even as ferruginous subsurface waters re-appeared 800–750 Ma, and we interpret this anoxic ocean state to reflect a markedly lower atmospheric and oceanic O2 level, consistent with the hypothesis that pO2 acted as an evolutionary barrier to the emergence of large motile bilaterian animals prior to the Ediacaran Period.

  6. Seismic structure of subducted oceanic crust near the slow-earthquake source region in the southern Ryukyu arc (United States)

    Nakamura, Mamoru


    Seismic tomography and receiver function analysis were carried out to investigate the relation between the slab structure in the southern Ryukyu arc region and the occurrence of slow-slip events that repeat biannually. For calculation of the receiver function, 212 teleseismic earthquakes with magnitudes larger than 6.0 were selected, and teleseismic waveforms were observed using two short-period seismometers and one broadband seismometer. Assuming that each later phase in a receiver function was a wave converted from P to S at depth, the time-domain receiver function was transformed into depth domain along each ray path using a reference velocity model. P- and S-wave arrival times, selected manually by the Japan Meteorological Agency (JMA), were used for the analysis of seismic tomography. In all, 6,750 earthquakes from January 2002 to March 2014 were used. The results showed that a cluster of slab earthquakes is distributed about 10 km below the plate interface. This suggests that the slab earthquakes occur in the lower part of the oceanic crust near the oceanic Moho within the slab. Moreover, the fault of the slow-slip events corresponds with the plate interface. The results also showed that a low Vp and high Vp/Vs area is distributed within the subducted oceanic crust underlying the fault associated with the slow-slip events. These structures are similar to those in the Tokai district, which suggests that the thermal condition of the southern Ryukyu arc is similar to the case of a hot-slab area where slow-slip events occur.

  7. Baltican versus Laurentian Crust in the Norwegian Caledonides between Latitudes 67° and 69° N: Implications for Mountains across oceans (United States)

    Steltenpohl, Mark G.; Yaw Nana Yaw, Nana; Andresen, Arild; Verellen, Devon


    Field and geochronological data (U-Pb ID-TIMS, SHRIMP, and LA ICPMS) on granitoids and their metasedimentary hosts are reported for rocks of the Bodø and Ofoten regions of north-central Norway documenting the distribution of Baltican versus Laurentian crust and allowing for tectonostratigraphic correlations across the EW-trending Tysfjord basement culmination. In the Bodø region, large areas previously interpreted as domes cored by Baltic basement (ca. 1.8 Ga; e.g., Heggmovatn and Landegode domes) are in fact Caledonian thrust sheets belonging to the exotic (Laurentian) Uppermost Allochthon. The Bratten orthogneiss, the Landegode augen gneiss, and the batholithic Tårnvika augen gneiss each has a ca. 950 Ma age of crystallization, and are together called the Rørstad complex. Orthogneisses that intrude metasedimentary units of the Heggmo allochthon (formerly the Heggmovatn dome) are dated to ca. 930 Ma, and these are intruded by 430 Ma leucogranites; U-Pb analysis of detrital zircons from metasiliciclastic rocks constrain the age of deposition to between 1100-930 Ma. We lithologically correlate the metasedimentary rocks between the Heggmo and Rørstad complexes. The Rørstad complex was migmatized at ca. 450 Ma and then was intruded by 430 Ma granitoids. Ordovician migmatites have not been documented in the Heggmo unit but such relics might have been masked by intense Scandian magmatic and metamorphic activity. The Rørstad and Heggmo units have straightforward age correlations to Mesoproterozoic to Neoproterozoic rock complexes in southern East Greenland and in other parts of the North Atlantic realm (i.e., Krummedal sequence and Eleonore Bay Supergroup). Laurentian Grenville-continental crust preserved in the Uppermost Allochthon of the Bodø region, therefore, records tectonic events that took place on the northeastern Laurentian continental margin prior to its Scandian continent-continent collision with Baltica. In Ofoten, ~150 km north of Bodø, the basal units of the Uppermost Allochthon comprise a thick sequence of platformal marbles (Evenes Group) that overlie a fragmented ophiolite complex dated at ca. 474 Ma. Multiple suites of felsic intrusions occur within the overlying Bogen and Niingen nappes and in the underlying Narvik nappe (Upper Allochthon) but none are found in the Evenes Group. A geochemically distinct (A-type) suite of ca. 470 Ma granites (Snaufjell granite) intrudes the Bogen Group and implies correlation to parts of the Uppermost Allochthon in the Helgeland nappe far to the south of Bodø; lithologically correlative units also occur directly south of Tysfjord in the Engeløy synform, the southern counterpart to the Ofoten synform. Ordovician magmatism and metamorphism and Grenville detrital zircon age populations within rocks of Lofoten-Vesterålen imply slivers of the Uppermost Allochthon exist far to the west of exposed Baltic basement. Although we have not identified Tonian-aged plutonic rocks north of Tysfjord, U-Pb detrital zircon age populations in quartzites from Lofoten-Vesterålen (Leknes and Gullesfjord), and the Evenes, Bogen, and Niingen groups indicate that they too likely have Laurentian origins. We speculate on potential Laurentian source areas and possible interactions between the two conjugate continental sides of the orogen that could have resulted in the Scandian amalgamation of these orphaned terranes.

  8. A Record of Oceanic Lithium Isotope Composition for the Last 7Ma (United States)

    Marriott, C. S.; Henderson, G. M.


    Continental weathering plays an important role in global climate change but has proved difficult to reconstruct for the past. New geological tools with which to assess the past rate and style of weathering are therefore urgently required. One such tool is Li isotope fractionation. Recent studies [1,2] have shown preferential release of 7Li into the aqueous phase and retention/adsorption of 6Li during weathering processes such as partial dissolution and secondary mineral formation. Lithium behaves conservatively in the oceans, with a residence time of ˜1Ma, so that a history of ocean Li isotope composition provides information about the average rate and style of global continental weathering on long timescales. The incorporation of lithium as a trace element in marine carbonates enables the construction of a record of oceanic Li-isotopic variation and is the focus of this work. Carbonate Li-isotope compositions are lighter than seawater by ˜8 per mil, but this fractionation is not temperature dependent. This has been demonstrated by measurement of Li isotopes in inorganically precipitated calcites (5-30° C) [3], in coralline aragonite (25-30° C) [3] and in benthic foraminifera Uvigerina (7-23° C). This lack of T-dependent fractionation suggests that the variation in the isotope composition of planktonic foraminifera will solely reflect changes in oceanic Li isotope composition, which in turn is strongly influence by changes in continental weathering. ODP site 758, located on the Ninetyeast Ridge in the Indian Ocean (5° N, 90° E; 2925m), was sampled at 2m intervals, over a depth corresponding to the last 7Ma, to produce 55 samples with a temporal resolution of approximately 130Ka. Site 758 is previously well studied with an existing chronology and high resolution Sr, O and Nd isotope data. Individual foram species in the core top were first investigated to assess inter-species fractionation effects. Down core lithium isotope variation was examined by measurement of hand picked and fully-reductively cleaned G. menardii. G. menardii was chosen because of its abundance throughout the core enabling the required 30-40mg of carbonate to be readily picked. Lithium isotope composition will be measured using a well established MC-ICP-MS technique [2,3] with a precision of better than +/-1 per mil to provide a record of the Li isotope composition of seawater for the last 7Ma and to identify changes in continental weathering during this period. 1. Huh Y. et al. (2001) EPSL 194, 189-199. 2. Pistiner J. S. & Henderson G. M. (In press) EPSL. 3. Marriott C. S. et al. (2002) GCA S1 66, A485.

  9. Incorporation of transition and platinum group elements (PGE) in Co-rich Mn crusts at Afanasiy-Nikitin Seamount (AFS) in the equatorial S Indian Ocean

    Digital Repository Service at National Institute of Oceanography (India)

    Glasby, G.P.


    Of the 12 elements enriched in Co-rich Mn crusts from the Afanasiy-Nikitin Seamount in the Equatorial S Indian Ocean, Mn, Fe and Co are enriched by a factor of approx. 10 sup(9) compared to their concentrations in seawater whereas Ni and Cu...

  10. New ichthyoliths from ferromanganese crusts and nodules from the Central Indian Ocean Basin

    Digital Repository Service at National Institute of Oceanography (India)

    Gupta, S.M.


    Ferromanganese encrusted hardgrounds, their intraclasts and the nuclei of manganese nodules collected from the Central Indian Ocean basin have yielded plentiful numbers of ichthyoliths. Forty well-knon ichthyoliths, one new type and 35 new subtypes...

  11. OS isotopic variation in basalts from Haleakala Volcano, Maui, Hawaii: A record of magmatic processes in oceanic mantle and crust (United States)

    Martin, Candace E.; Carlson, Richard W.; Shirey, Steven B.; Frey, Fred A.; Chen, Chu-Yung


    Os isotopic data are presented for a suite of tholeiitic and alkalic basaltic lavas from the Honomanu Gulch section of Haleakala Volcano, Maui spanning the shield-building, post-shield and post-erosional stages of the typical Hawaiian volcanic cycle. Os is shown to behave compatibly during fractional crystallization of the baslts, with a bulk crystal-melt partition coefficient of 10 +/- 5. Os isotopic compositions are inversely related to Os concentrations and exhibit no simple temporal trend, in contrast to Sr, Nd, Pb and He isotopic dta from the suite. The Os, Sr and He isotopic systematics in the Haleakala basalts are explained by the presence of three components. The dominant component has Os-187/Os-188 = 0.132 +/- 0.002 (Os-187/Os-186 = 1.10 +/- 0.02), Sr-87/Sr-86 = 0.704 and elevated He-3/He4 and is interpreted to reflect melts derived from the Hawaiian mantle plume. A second component with low Os-187/Os-186, Sr-87/Sr-86 and He-3/He-4 is considered representative of depleted upper mantle (Mid-ocean ridge basalts (MORB) source). Temporal and petrologic trends in the isotopic data support a model for progressive mixing between plume-derived melts and up to 30% of small-degree partial melts of the MORB source during the later stages of volcanic activity. Direct exchange of plume-derived melts with MORB-source peridotite is precluded. A third component with Os-187/Os-188 greater than 0.135 (Os-187/Os-186 greater than 1.12) is diagnostic of old crustal materials with high time-integrated Re/Os. It is present only in some of the most differentiated basalts and is probably derived through contamination of melts with aged oceanic crust beneath the volcano. This study demonstrated that Os isotopes in ocean island basaltic magmas are sensitive to interactions with oceanic crust and mantle. As a consequence, the Os isotopic compositions of such basalts can record a variety of magmatic processes both within and outside their mantle plume source regions.

  12. The magma ocean from the Fra Mauro shoreline - An overview of the Apollo 14 crust (United States)

    Hunter, R. H.; Taylor, L. A.


    Petrographic and mineralogical studies of coarse grained, polymineralic, texturally monominct igneous clasts from the Apollo 14 breccias show troctolite and troctolitic anorthosite to be the most abundant rock types. The second most abundant group consists of plagioclase cumulates with more evolved mineral compositions than the Mg-rich trend anorthosite cumulates. Coarse grained ilmenite gabbros and mineralogically evolved monzonoritic and granitic clasts are widespread in occurrence, but not abundant. The present data provide further support for widespread regional heterogeneities within the early lunar crust.

  13. Pliocene (3.2-2.4 Ma) ostracode faunal cycles and deep ocean circulation, North Atlantic Ocean (United States)

    Cronin, T. M.; Raymo, M.E.; Kyle, K.P.


    Ostracode assemblages from Deep Sea Drilling Project Sites 607 (western Mid-Atlantic Ridge) and 610 (southeast Rockall Plateau) show rapid, systematic shifts during late Pliocene glacial-interglacial cycles that reflect deep-sea environmental change. Progressive decreases in North Atlantic deep-water taxa and increases in Southern Ocean taxa occur from 3.4 to 2.4 Ma, and high-amplitude faunal cycles begin near 2.8 Ma. Four ostracode assemblages, each with a characteristic phase relative to 41 k.y. obliquity glacial-interglacial ??18O cycles, characterize the benthic faunal record at Site 607. Cross-spectral analysis shows that the Site 607 glacial assemblage has a 41 k.y. periodicity significant at the 95% level; other assemblages show a less significant, but still obvious, concentration of variance at 41 k.y. Faunal patterns suggest climatically controlled reorganization of deep-sea benthic communities during glacial-interglacial cycles due to oscillating deep-sea environments.

  14. Hess Deep Interactive Lab: Exploring the Structure and Formation of the Oceanic Crust through Hands-On Models and Online Tools (United States)

    Kurtz, N.; Marks, N.; Cooper, S. K.


    Scientific ocean drilling through the International Ocean Discovery Program (IODP) has contributed extensively to our knowledge of Earth systems science. However, many of its methods and discoveries can seem abstract and complicated for students. Collaborations between scientists and educators/artists to create accurate yet engaging demonstrations and activities have been crucial to increasing understanding and stimulating interest in fascinating geological topics. One such collaboration, which came out of Expedition 345 to the Hess Deep Rift, resulted in an interactive lab to explore sampling rocks from the usually inacessible lower oceanic crust, offering an insight into the geological processes that form the structure of the Earth's crust. This Hess Deep Interactive Lab aims to explain several significant discoveries made by oceanic drilling utilizing images of actual thin sections and core samples recovered from IODP expeditions. . Participants can interact with a physical model to learn about the coring and drilling processes, and gain an understanding of seafloor structures. The collaboration of this lab developed as a need to explain fundamental notions of the ocean crust formed at fast-spreading ridges. A complementary interactive online lab can be accessed at for students to engage further with these concepts. This project explores the relationship between physical and on-line models to further understanding, including what we can learn from the pros and cons of each.

  15. Lithium isotope as a proxy for water/rock interaction between hydrothermal fluids and oceanic crust at Milos, Greece (United States)

    Lou, U.-Lat; You, Chen-Feng; Wu, Shein-Fu; Chung, Chuan-Hsiung


    Hydrothermal activity at Milos in the Aegean island (Greece) is mainly located at rather shallow depth (about 5 m). It is interesting to compare these chemical compositions and the evolution processes of the hydrothermal fluids at deep sea hydrothermal vents in Mid-ocean Ridge (MOR). Lithium (Li) is a highly mobile element and its isotopic composition varies at different geological settings. Therefore, Li and its isotope could be used as an indicator for many geochemical processes. Since 6Li preferential retained in the mineral phase where 7Li is leached into fluid phase during basalt alteration, the Li isotopic fractionation between the rocks and the fluids reflect sensitively the degree of water-rock interaction. In this study, Bio-Rad AG-50W X8 cation exchange resin was used for purifying the hydrothermal fluids to separate Li from other matrix elements. The Li isotopic composition (?7Li) was determined by Multi-collector Inductively Coupled Plasma Mass Spectrometry (MC-ICP-MS) with precision better than 0.2‰ (2?, n=20). The Li concentration in the hydrothermal fluids falls between 0.02 to 10.31 mM. The ?7Li values vary from +1.9 to +29.7‰, indicating significant seawater contamination have occurred. These hydrothermal fluids fit well with seawater and brine two end-member binary mixing model. During phase separation, lithium, boron, chlorine, iodine, bromine, sodium and potassium were enriched in the brine phase. On the other hand, aluminum, sulphur and iron were enriched in the vapor phase. There is no significant isotope fractionation between the two phases. The water/rock ratio (W/R) calculated is low (about 1.5 to 1.8) for the Milos fluids, restricted seawater recharge into the oceanic crust. Moreover, the oceanic crust in the region becomes less altered since the W/R is low. The ?7Li value of the hydrothermal fluids can be used as a sensitive tool for studying water-rock interaction.

  16. Generation of felsic melts within fast-spreading oceanic crust: Experimental partial melting of hydrothermally altered sheeted dike (United States)

    Fischer, L. A.; Erdmann, M.; France, L.; Deloule, E.; Koepke, J.


    In recent oceanic crust and in ophiolites, felsic lithologies are observed. Different processes, like fractional crystallization of MORB and partial melting of mafic rocks are discussed to form these lithologies. Partial melting is expected as a major process in forming felsic lithologies at the base of the sheeted dike complex of fast-spreading ridges, where the axial melt lens is assumed to be located directly beneath the sheeted dikes.It is widely accepted that this melt lens has the potential to trigger partial melting of mafic lithologies at the gabbro/dike transition zone. In this experimental study, the influence of partial melting on the generation of felsic lithologies is examined. Therefore, partial melting experiments at a pressure of 100 MPa were performed. As starting material, a natural basalt from the IODP (Integrated Ocean Drilling Program) drilling at Site 1256 (equatorial East Pacific Rise) was chosen, which is representative for the lower sheeted dike complex. It is characterized as a moderately altered dolerite containing plagioclase (An50-57), clinopyroxene (Mg# 0.55-0.60) and quartz, with chlorite as secondary phase; sulfides and Fe-Ti-oxides are present as accessory minerals. The partial melting experiments were conducted in an H2-controlled IHPV at the Institute of Mineralogy in Hanover, Germany. To investigate the evolution of the partial melts, different experiments were performed at temperatures between 1030°C and 910°C and a constant pressure of 100 MPa. All experiments were water saturated leading to a fO2 corresponding to QFM +1 (QFM = quartz-fayalite-magnetite oxygen buffer). This is slightly more oxidized than MORB crystallization due to the influence of a hydrous fluid which generally increases the oxygen activity. The experimental products were analyzed using electron microprobe for major elements, and a SIMS (CRPG Nancy, France) for trace elements. We present here our first results on phase relations and mineral compositions. The analyzed melts vary in their SiO2 contents between ~50wt% (1030°C) and ~71wt% (910°C) and can be classified as basaltic respectively dacitic. The residual mineral assemblage in the experiments performed at 1030°C contains olivine + plagioclase + clinopyroxene. At lower temperatures, orthopyroxene crystallizes at the expense of olivine, and Fe-Ti-oxides are stabilized. A comparison with corresponding naturally formed lithologies shows similarities both in terms of melt composition and residual phase compositions in those experimental runs performed at lower temperature. First trace element analyses of the experimental melts reveal similar trace element patterns known from naturally formed felsic lithologies recorded in the gabbro/dike transition zone of fast-spread oceanic crust.

  17. South Australian record of a Rodinian epicontinental basin and its mid-neoproterozoic breakup (˜700 Ma) to form the Palaeo-Pacific Ocean (United States)

    Powell, C. McA.; Preiss, W. V.; Gatehouse, C. G.; Krapez, B.; Li, Z. X.


    The Neoproterozoic Adelaidean System and overlying Cambrian rocks are a thick succession of terrestrial and shallow-marine sediments that were deposited on older continental crust near the eastern edge of the exposed Precambrian rocks in Australia. The Adelaidean System records four major tectonic regimes: (1) early Neoproterozoix formation of an epicontinental, partly rifted basin; (2) mid-Neoproterozoic (˜700 Ma) continental breakup and accumulation of a passive margin wedge; (3) latest Neoproterozoic/Early Cambrian renewed rifting, and (4) Middle to Late Cambrian conversion of the passive margin to a convergence zone during the development of the Delamerian Orogen. Deposition of the Adelaidean System commenced by ˜830 Ma, when Australia was contiguous with Laurentia in the Rodinia supercontinent. Sandstone and evaporitic carbonate of an initial epicontinental basin are preserved as the lowest succession of the Adelaide Geosyncline and the Amadeus, georgina, Ngalia, Officer and Savory-Yeneena basins. Mafic volcanics were extruded under NE-SW continental extension during the Willouran, which was overprinted by a more widespread E-W Torrensian extension. Continental breakup occurred around 700 Ma, during accumulation of the Sturtian glaciogenic deposits in rift valleys. The succeeding siltstone, sandstone and carbonate were deposited on a passive continental margin that faced the newly formed Palaeo-Pacific Ocean as Laurentia rotated clockwise away from Australia. A wide Palaeo-Pacific Ocean existed at the time of the younger Ice Brook (Laurentia) and Marinoan (Australian) glaciations. Dextral shear between northern and southern Australia along the Paterson-Petermann Ranges Orogen in the 600-550-Ma interval, broke up the continuuity of the central Australian basins through uplift of the Musgrave Block, and led to renewed rifting, reflecting the latest Neoproterozoic continental breakup that formed the eastern margin of Laurentia. The South Australian passive margin persisted until the Middle Cambrian when convergence along the Pacific Ocean margin led to development of the Delamerian Orogebn. Deposition of the marine siliciclastic Kanmantoo Group commenced in the late Early Cambrian in a rapidlly subsiding trough, for which both extensional (rift-basin) and compressional (foredeep) origins gave been proposed. Reversal of the eastward-dipping Early Cambrian palaeoslope in the Middle Cambrian shed siliciclastic sediments back onto the continent from an initial uplift in advance of Delamerian deformation.

  18. Detachment and steep normal faulting in Atlantic oceanic crust west of Africa (United States)

    Reston, T.J.; Ruoff, O.; McBride, J.H.; Ranero, C.R.; White, Robert S.


    Improved images of the internal structure of Early Cretaceous North Atlantic crust reveal both probable detachment faults and more steeply dipping normal faults. The detachment faults occur as subhorizontal structures passing ???1.5 km beneath fault blocks without offset; several steeper block-bounding faults appear to detach onto these structures. However, the detachments are bounded to the west (ridgeward) by presumably younger, more steeply west-dipping normal faults. In one possible interpretation, the detachment and the steep faults belong to the same "rolling-hinge" extension system. An intriguing alternative is that a phase of detachment faulting, perhaps related to increased magmatic activity, was succeeded by localized amagmatic extension along steeper and more deeply penetrating faults.

  19. Does seismic activity control carbon exchanges between transform-faults in old ocean crust and the deep sea? A hypothesis examined by the EU COST network FLOWS (United States)

    Lever, M. A.


    The European Cooperation in Science and Technology (COST)-Action FLOWS ( was initiated on the 25th of October 2013. It is a consortium formed by members of currently 14 COST countries and external partners striving to better understand the interplay between earthquakes and fluid flow at transform-faults in old oceanic crust. The recent occurrence of large earthquakes and discovery of deep fluid seepage calls for a revision of the postulated hydrogeological inactivity and low seismic activity of old oceanic transform-type plate boundaries, and indicates that earthquakes and fluid flow are intrinsically associated. This Action merges the expertise of a large number of research groups and supports the development of multidisciplinary knowledge on how seep fluid (bio)chemistry relates to seismicity. It aims to identify (bio)geochemical proxies for the detection of precursory seismic signals and to develop innovative physico-chemical sensors for deep-ocean seismogenic faults. National efforts are coordinated through Working Groups (WGs) focused on 1) geophysical and (bio)geochemical data acquisition; 2) modelling of structure and seismicity of faults; 3) engineering of deep-ocean physico-chemical seismic sensors; and 4) integration and dissemination. This poster will illustrate the overarching goals of the FLOWS Group, with special focus to research goals concerning the role of seismic activity in controlling the release of carbon from the old ocean crust into the deep ocean.

  20. Untangling Magmatic Processes and Hydrothermal Alteration of in situ Superfast Spreading Ocean Crust at ODP/IODP Site 1256 with Fuzzy c-means Cluster Analysis of Rock Magnetic Properties (United States)

    Dekkers, M. J.; Heslop, D.; Herrero-Bervera, E.; Acton, G.; Krasa, D.


    Ocean Drilling Program (ODP)/Integrated ODP (IODP) Hole 1256D (6.44.1' N, 91.56.1' W) on the Cocos Plate occurs in 15.2 Ma oceanic crust generated by superfast seafloor spreading. Presently, it is the only drill hole that has sampled all three oceanic crust layers in a tectonically undisturbed setting. Here we interpret down-hole trends in several rock-magnetic parameters with fuzzy c-means cluster analysis, a multivariate statistical technique. The parameters include the magnetization ratio, the coercivity ratio, the coercive force, the low-field susceptibility, and the Curie temperature. By their combined, multivariate, analysis the effects of magmatic and hydrothermal processes can be evaluated. The optimal number of clusters - a key point in the analysis because there is no a priori information on this - was determined through a combination of approaches: by calculation of several cluster validity indices, by testing for coherent cluster distributions on non-linear-map plots, and importantly by testing for stability of the cluster solution from all possible starting points. Here, we consider a solution robust if the cluster allocation is independent of the starting configuration. The five-cluster solution appeared to be robust. Three clusters are distinguished in the extrusive segment of the Hole that express increasing hydrothermal alteration of the lavas. The sheeted dike and gabbro portions are characterized by two clusters, both with higher coercivities than in lava samples. Extensive alteration, however, can obliterate magnetic property differences between lavas, dikes, and gabbros. The imprint of thermochemical alteration on the iron-titanium oxides is only partially related to the porosity of the rocks. All clusters display rock magnetic characteristics in line with a stable NRM. This implies that the entire sampled sequence of ocean crust can contribute to marine magnetic anomalies. Determination of the absolute paleointensity with thermal techniques is not straightforward because of the propensity of oxyexsolution during laboratory heating and/or the presence of intergrowths. The upper part of the extrusive sequence, the granoblastic portion of the dikes, and moderately altered gabbros may contain a comparatively uncontaminated thermoremanent magnetization.

  1. Himalayan sedimentary pulses recorded by silicate detritus within a ferromanganese crust from the Central Indian Ocean

    Digital Repository Service at National Institute of Oceanography (India)

    Banakar, V.K.; Galy, A.; Sukumaran, N.P.; Parthiban, G.; Volvaiker, A.Y.


    (area with slanting lines) Bengal Fan sediment [7,20,21,30,32,33], the Andaman Fan sediment (area without ¢ll) [30,31], Indian Ocean sediment south of Bengal Fan (shaded area) [21], modern Ganges and Brahmaputra sedi- ment (open squares) [17...], and Eastern Indian slope sediment (¢lled square) [28]. The mixing curve A corresponds to the mixture of the average of the Himalayan feldspar recovered from the distal Bengal Fan (open circles) [20] and the arc- type feldspar [32,33]. The mixing curve B...

  2. Can Fractional Crystallization of a Lunar Magma Ocean Produce the Lunar Crust? (United States)

    Rapp, Jennifer F.; Draper, David S.


    New techniques enable the study of Apollo samples and lunar meteorites in unprecedented detail, and recent orbital spectral data reveal more about the lunar farside than ever before, raising new questions about the supposed simplicity of lunar geology. Nevertheless, crystallization of a global-scale magma ocean remains the best model to account for known lunar lithologies. Crystallization of a lunar magma ocean (LMO) is modeled to proceed by two end-member processes - fractional crystallization from (mostly) the bottom up, or initial equilibrium crystallization as the magma is vigorously convecting and crystals remain entrained, followed by crystal settling and a final period of fractional crystallization [1]. Physical models of magma viscosity and convection at this scale suggest that both processes are possible. We have been carrying out high-fidelity experimental simulations of LMO crystallization using two bulk compositions that can be regarded as end-members in the likely relevant range: Taylor Whole Moon (TWM) [2] and Lunar Primitive Upper Mantle (LPUM) [3]. TWM is enriched in refractory elements by 1.5 times relative to Earth, whereas LPUM is similar to the terrestrial primitive upper mantle, with adjustments made for the depletion of volatile alkalis observed on the Moon. Here we extend our earlier equilibrium-crystallization experiments [4] with runs simulating full fractional crystallization

  3. Ocean crust vein mineral deposition: Rb/Sr ages, U-Th-Pb geochemistry, and duration of circulation at DSDP sites 261, 462 and 516

    International Nuclear Information System (INIS)

    Cation exchange experiments (ammonium acetate and cation resin) on celadonite-smectite vein minerals from three DSDP holes demonstrate selective removal of common Sr relative to Rb and radiogenic Sr. This technique increases the Rb/Sr ratio by factors of 2.3 to 22 without significantly altering the age of the minerals, allowing easier and more precise dating of such vein minerals. Celadonites show U and Pb contents and Pb isotopic compositions little changed from their basalt precursors, while Tb contents are significantly lower. Celadonites thus have unusually high alkali/U,Th ratios and low Th/U ratios. If this celadonite alteration signature is significantly imprinted on oceanic crust as a whole, it will lead to very distinctive Pb isotope signatures for any hot spot magmas which contain a component of aged subducted recycled oceanic crust. Initial Sr isotope ratios of ocean crust vein minerals (smectite, celadonite, zeolite, calcite) are intermediate between primary basalt values and contemporary sea water values, and indicate formation under seawater-dominated systems with effective water-rock ratios of 20-200. (author)

  4. Linear inversion of a negative gravity anomaly in se Rio Grande cone: a graben on oceanic crust?

    Scientific Electronic Library Online (English)

    Emilson Pereira, Leite; Naomi, Ussami.


    Full Text Available Uma anomalia ar-livre com amplitude negativa de 23 mGal em uma região no oceano Atlântico Sul, centrada em 48ºW e 35ºS, foi observada pela primeira vez devido à integração de dados de gravimetria marinha convencionais e dados de gravidade derivados de altimetria por satélite, adquiridos pela missão [...] GEOSAT/ERM. O limite norte desta anomalia coincide com o Lineamento Chuí e o limite sul indica outro lineamento, que é uma extensão da Zona de Fratura Meteoro. A anomalia tem direção NE-SW, sua largura é de 400 km e seu comprimento é de 600 km. Foi utilizada uma metodologia de inversão linear bidimensional, com vínculos relativos e absolutos, para calcular a distribuição de densidades ao longo de três perfis paralelos ao eixo principal da anomalia. O resultado sugere que a espessura de sedimentos na parte mais profunda da bacia é de no mínimo 3,0 km onde a batimetria oceânica é de 4.800 m. Esta feição tectônica, um semi-gráben assimétrico formado entre dois lineamentos, provavelmente situa-se sobre uma crosta oceânica. O volume de sedimentos estimado para esta bacia é de cerca de 50% do volume de sedimentos pós-Mioceno depositados no Rio Grande Cone, onde hidratos de gás foram encontrados. Abstract in english We detect, for the first time, a negative free-air gravity anomaly of 23 mGal amplitude over a region in the South Atlantic Ocean centered at 48ºW and 35ºS. To this end, we used the integration of conventional shipborne gravity data and gravity data derived from GEOSAT/ERM satellite altimetry. The n [...] orth bound of this anomaly coincides with the Chuí Lineament and the south bound indicates another lineament, which is the extension of the Meteor Fracture Zone. The anomaly trend is NE-SW, its width is 400 km and its length is 600 km. Two-dimensional linear inversion with relative and absolute equality constraints was used to calculate the density distribution along three profiles perpendicular to the main axis of the anomaly. The result suggests that the sediment thickness in the deepest part of the basin is at least 3.0 km where the ocean bathymetry is 4,800 m. This tectonic feature, an asymmetric half-graben formed between two lineaments, probably lies over an oceanic crust. The estimated volume of sediments in this basin is approximately 50% of the post-Miocene sediments volume deposited in the Rio Grande Cone where gas-hydrates were found.

  5. Characterization of the microbial community in a legacy borehole in the igneous ocean crust (United States)

    Salas, E. C.; Bhartia, R.; Hug, W. F.; Reid, R.; Edwards, K. J.


    The deep subsurface continues to hold promise as a significant reservoir of the Earth's microbiota. However, the extent and nature of microbial communities in the subsurface is still uncertain. Current efforts at elucidating the scope of deep subsurface communities include development of methods for enumeration of cells and characterization of metabolic niches. These methods typically rely on bulk analysis of extracted core material or in situ enrichment studies. Legacy boreholes, such as 395A, which have been isolated from the overlying ocean and sediment, have been proposed as good model systems to study the subsurface in its native state. However, current methods for exploring these environments do not allow for real-time analysis and, in the case of molecular work which rely on dyes to produce fluorescence signals, can be challenging due to issues such as mineral auto-fluorescence and non-specific binding. The Deep Exploration Biosphere Investigative tool (DEBI-t) was developed to explore legacy boreholes and provide near real-time characterization of borehole environments. DEBI-t utilizes deep ultraviolet (224nm) excitation to induce fluorescence (280nm - 400nm) enabling detection and classification of microbes and organics in their native environment, without the need for tagging or sample processing. This capability will be discussed using results from IODP Expedition 336.

  6. Fluid flow pathways through the oceanic crust: reaction permeability and isotopic tracing (United States)

    McCaig, Andrew; Castelain, Teddy; Klein, Frieder


    It is generally assumed that the dominant means of creating permeability in ocean floor hydrothermal systems is fracturing, induced either by cooling or by tectonic stress. Here we show textural evidence that metamorphic reactions can create a hierarchy of permeable pathways through gabbroic rocks similar to a fracture hierarchy. Isotopic microsampling shows that just as with fractures, most flow occurs through the larger channelways, and that even at the microscale, flow can be extremely heterogeneous with alteration affecting only certain minerals in the framework, leaving others untouched. Reaction permeability is created in three ways; dissolution creating open porosity, microcracking due to volume increase reactions involving olivine, and expansion of water due to rapid heating in dyke margins, particularly when intruded into brecciated rocks. Our data comes from IODP Hole U1309D, which was drilled to 1400 mbsf in the footwall of the Atlantis Massif detachment fault at the Mid-Atlantic Ridge 30°N. The core is composed of gabbroic rocks interlayered with olivine rich troctolites, with several basalt/diabase sills in the top 130 m. The dominant alteration occurred in the greenschist facies, at depths at least 1 km below seafloor, and decreases in intensity downhole. Whole rock oxygen isotope values range from +5.5 permil to +1.5 permil, indicating variable degrees of interaction with seawater at temperatures generally > 250 °C. Gabbroic rocks and diabases exhibit a range of Sr isotope ratios from MORB values (0.70261) to intermediate ratios (0.70429). Microsampling shows that amphiboles are often more radiogenic than coexisting plagioclase and can sometimes be isotopically altered in the same rock as completely unaltered primary minerals. Large (10 cm) amphibole-filled vugs show values ranging up to 0.708, close to seawater. In some cases however the secondary minerals are virtually unaltered indicating low fluid fluxes in pervasive alteration. SEM textures in broken surfaces reveal extensive evidence for dissolution reactions creating porosity, particularly in diabase where pyroxene is selectively dissolved and the porosity partially filled by actinolite needles. If far-from-equilibrium fluid (such as black smoker fluid) interacts with pyroxene at 300-400 °C, dissolution rates of several microns/day are possible. Fluid volume increase in dyke margins due to heating provides space nearby for dissolved components to precipitate without immediately closing the dissolution porosity, which may be an important part of the process. Amphibole-filled vugs in gabbro are interpreted as the final result of the positive feedback between dissolution and permeability - creating fluid flow tubes analogous to karst in limestone. But in contrast, permeability created by volume increase cracking is self-limiting once the primary phase responsible (olivine) is gone, and hence leads to pervasive olivine replacement but little fluid flux.

  7. Reconstruction of seawater chemistry from deeply subducted oceanic crust; hydrogen and oxygen isotope of lawsonite eclogites preserving pillow structure (United States)

    Hamabata, D., VI; Masuyama, Y.; Tomiyasu, F.; Ueno, Y.; Yui, T. F.; Okamoto, K.


    In order to understand evolution of life, change of seawater chemistry from Hadean, Archean to present is significant. Pillow structure is well-preserved in the Archean greenstone belt (e.g. Komiya et al., 1999). Oxygen and hydrogen isotope of rims in the pillow is useful conventional tool to decipher chemistry of Paleao-seawater from Archean to Present. However, Archean greenstone belt suffered regional metamorphism from greenschist to Amphibolite facies conditions. Therefore, it is necessary to testify the validity of pillow chemistry from recent (Phanerozoic) metamorphosed greenstone. We have systematically collected pillowed greenstone from blueschist and eclogites. Two eclogite exhibiting pillow structures were chosen for oxygen and hydrogen isotope analysis. One is from Corsica (lawsonite eclogite collected with Dr. Alberto Vidale Barbarone) and another is from Cazadero, Franciscan belt (collected by Dr. Tatsuki Tsujimori). The both are ascribed as MORB from major and trace bulk chemistry and Ca is rich in the core and Na is poor in the rims. The former exhibits garnet, omphacite, lawsonite, and glacophane. Phengite is in core of the pillow and chlorite is in the rims. In the latter, besides garnet, omphacite, epdiote and glaucophane, chlorite is recognized with phengite in the core. Glaucophane is richer in the rims from the both samples, therefore istope analysis of glaucophane was done. Mineral separation was carefully done using micro-mill, heavy liquid and isodynamic separator. 20 mg specimens were used for oxygen isotope analysis and 2mg were for hydrogen analysis. ?18O of the all analysis (7.7 to 8.3) is within the range of unaltered igneous oceanic crust and high temperature hydrothermal alteration although rims (8.3 for Franciscan and 8.0 for Corsica) are higher than cores (7.7 for Franciscan and Corsica). ?D data is also consistent with hydrothermal alteration. It is relative higher in core from the Corsica and Franciscan (-45 and -56) than of the rims (-49 and -57, respectively), suggesting dehydration in deep subduction zone.

  8. Mafic granulite xenoliths in the Chilka Lake suite, Eastern Ghats Belt, India: evidence of deep-subduction of residual oceanic crust

    Directory of Open Access Journals (Sweden)

    S. Bhattacharya


    Full Text Available Granulite xenoliths preserve key geochemical and isotopic signatures of their mantle source regions. Mafic granulite and pyroxinite xenoliths within massif-type charnockitic rocks from the Eastern Ghats Belt have recently been reported by us. The mafic granulite xenoliths from the Chilka Lake granulite suite with abundant prograde biotite are geochemically akin to Oceanic Island Basalt (OIB. They can be distinguished from the hornblende-mafic granulite xenoliths with signatures of Arc-derived basalt occurring in the other suites of the Eastern Ghats Belt. These two groups of xenoliths in the Paleoproterozoic Eastern Ghats Province have quite distinct Nd-model ages- 1.9 Ga and 2.5 Ga respectively, which may be interpreted as their crustal residence ages. Strong positive Nb anomalies, indicating subducted oceanic crust in the source, LREE enrichment and strongly fractionated REE pattern are key geochemical signatures attesting to their origin as OIB-type magma. Also low Yb and Sc contents and high (La / YbN ratios can be attributed to melting in the presence of residual garnet and hence at great depths (> 80 km. The variable enrichment in radiogenic 87Sr, between 0.70052 and 0.71092 at 1.9 Ga and less radiogenic 143Nd between ?-1.54 and 7.46 are similar to those of the OIBs compared to MORBs. As OIBs commonly contain some recycled oceanic crust in their sources, we suggest that the residue of the oceanic crust from a previous melting event (~ 2.5 Ga that produced the Arc-derived basalts (protoliths of hornblende-mafic granulite xenoliths could have subducted to great depths and mechanically mixed with the mantle peridotite. A subsequent re-melting event of this mixed source might have occurred at ca. 1.9 Ga as testified by the crustal residence ages of the biotite-mafic granulite xenoliths of the Chilka Lake granulite suite.

  9. Mafic granulite xenoliths in the Chilka Lake suite, Eastern Ghats Belt, India: evidence of deep-subduction of residual oceanic crust (United States)

    Bhattacharya, S.; Chaudhary, A. K.; Saw, A. K.; Das, P.; Chatterjee, D.


    Granulite xenoliths preserve key geochemical and isotopic signatures of their mantle source regions. Mafic granulite and pyroxinite xenoliths within massif-type charnockitic rocks from the Eastern Ghats Belt have recently been reported by us. The mafic granulite xenoliths from the Chilka Lake granulite suite with abundant prograde biotite are geochemically akin to Oceanic Island Basalt (OIB). They can be distinguished from the hornblende-mafic granulite xenoliths with signatures of Arc-derived basalt occurring in the other suites of the Eastern Ghats Belt. These two groups of xenoliths in the Paleoproterozoic Eastern Ghats Province have quite distinct Nd-model ages- 1.9 Ga and 2.5 Ga respectively, which may be interpreted as their crustal residence ages. Strong positive Nb anomalies, indicating subducted oceanic crust in the source, LREE enrichment and strongly fractionated REE pattern are key geochemical signatures attesting to their origin as OIB-type magma. Also low Yb and Sc contents and high (La / Yb)N ratios can be attributed to melting in the presence of residual garnet and hence at great depths (> 80 km). The variable enrichment in radiogenic 87Sr, between 0.70052 and 0.71092 at 1.9 Ga and less radiogenic 143Nd between ?-1.54 and 7.46 are similar to those of the OIBs compared to MORBs. As OIBs commonly contain some recycled oceanic crust in their sources, we suggest that the residue of the oceanic crust from a previous melting event (~ 2.5 Ga) that produced the Arc-derived basalts (protoliths of hornblende-mafic granulite xenoliths) could have subducted to great depths and mechanically mixed with the mantle peridotite. A subsequent re-melting event of this mixed source might have occurred at ca. 1.9 Ga as testified by the crustal residence ages of the biotite-mafic granulite xenoliths of the Chilka Lake granulite suite.

  10. Updated maps of Moho topography and the earth crust thickness in the Deep Arctic Ocean based on results of potential field zoning and 3-D gravity modeling (United States)

    Glebovsky, Yury; Astafurova, Ekaterina; Chernykh, Andrey; Egorova, Alena; Kaminsky, Valeriy; Korneva, Mariya; Redko, Anton


    Both initial (Glebovsky et al., 2013) and updated maps and digital models (DM) of Moho topography and earth crust thickness in the deep Arctic Ocean were compiled using the same procedure. It included several steps: analysis of potential fields information compiled under CAMPGM and ArcGP projects and updating by new Russian data; separation of the study area into individual geostructures; calculation of gravitational effects from two main boundaries lying above Moho, presented by IBCAO grid, and by grid of basement relief (Kaminsky et al., 2012); subtraction of these effects from observed gravity anomalies, and converting of residual anomalies to depths to Moho using Parker's (1974) algorithm. Averaged depth to Moho required by Parker's algorithm to estimate its relative variations was determined from available deep refraction seismic data. It varies for different regional geological structures (basins, ridges and rises) which boundaries were contoured based on results of potential fields zoning. Modeling process for each structure was iterative and calibrated by seismic data. Results that best fit with seismic sections were merged to compile the grid of depths to Moho. This grid was specified by estimation of gravitational effects related both with increasing of density of sediments with depth and with uplift of asthenosphere beneath the Gakkel Ridge (GR). Grids of total and consolidated crust thickness were computed by sequential subtracting the IBCAO and sediment thickness grids from the final grid of depths to Moho. Updated versions of maps and DM of Moho topography and earth crust thickness are specified by recent Russian multi-channel and DSS seismic data collected in 2011-2012. It is confirmed the significant differences in crustal structure between the Eurasian (EB) and Amerasian Basins (AB). The thickness of the consolidated crust in the EB shows a fairly clear bilateral symmetry with respect to the GR. In the Nansen and Amundsen basins it varies from 3 to 8 km. The GR is underlain by the thinnest crust (2 km or less km). The crustal thickness of the Lomonosov Ridge varies along its strike within 16-26 km which may indicate its block structure. Within the AB there are a number of large highs and deep basins. The Mendeleev and Alpha ridges are underlain by crust that varies from 24 to 30 km and reaches maximum values of 30-32 km, respectively. The thickest crust that reaches 32-34 km is observed at the Northwind Ridge and Chukchi Plateau. Thickness of crust in deep basins of the AB varies widely. In the Canada and Makarov basins, it ranges from 10 to 16 km, and in the Podvodnikov Basin, from 16 to 20 km.

  11. Sulfur geochemistry and microbial sulfate reduction during low-temperature alteration of uplifted lower oceanic crust: Insights from ODP Hole 735B (United States)

    Alford, Susan E.; Alt, Jeffrey C.; Shanks, Wayne C., III


    Sulfide petrography plus whole rock contents and isotope ratios of sulfur were measured in a 1.5 km section of oceanic gabbros in order to understand the geochemistry of sulfur cycling during low-temperature seawater alteration of the lower oceanic crust, and to test whether microbial effects may be present. Most samples have low SO4/?S values (? 0.15), have retained igneous globules of pyrrhotite ± chalcopyrite ± pentlandite, and host secondary aggregates of pyrrhotite and pyrite laths in smectite ± iron-oxyhydroxide ± magnetite ± calcite pseudomorphs of olivine and clinopyroxene. Compared to fresh gabbro containing 100–1800 ppm sulfur our data indicate an overall addition of sulfide to the lower crust. Selection of samples altered only at temperatures ? 110 °C constrains microbial sulfate reduction as the only viable mechanism for the observed sulfide addition, which may have been enabled by the production of H2 from oxidation of associated olivine and pyroxene. The wide range in ?34Ssulfide values (? 1.5 to + 16.3‰) and variable additions of sulfide are explained by variable ?sulfate-sulfide under open system pathways, with a possible progression into closed system pathways. Some samples underwent oxidation related to seawater penetration along permeable fault horizons and have lost sulfur, have high SO4/?S (? 0.46) and variable ?34Ssulfide (0.7 to 16.9‰). Negative ?34Ssulfate–?34Ssulfide values for the majority of samples indicate kinetic isotope fractionation during oxidation of sulfide minerals. Depth trends in sulfide–sulfur contents and sulfide mineral assemblages indicate a late-stage downward penetration of seawater into the lower 1 km of Hole 735B. Our results show that under appropriate temperature conditions, a subsurface biosphere can persist in the lower oceanic crust and alter its geochemistry.

  12. Early (pre–8 Ma) fault activity and temporal strain accumulation in the central Indian Ocean

    Digital Repository Service at National Institute of Oceanography (India)

    Krishna, K.S.; Bull, J.M.; Scrutton, R.A.


    The diffuse deformation zone in the central Indian Ocean is the classical example of distributed deformation of the oceanic lithosphere, where shortening between the Indian and Capricorn plates is manifest as reverse faulting (5-10-km-spaced faults...

  13. Distribution of various components in a hydrogeneous ferromanganese nodule and an Afanasiy Nikitin Seamount crust from Indian Ocean - A geochemical study using micro-PIXE

    International Nuclear Information System (INIS)

    The present study emphasizes the geochemical features pertaining to the distribution of the major and minor elements in a hydrogenous ferromanganese nodule and a seamount crust originating from the Indian Ocean. The micro-PIXE elemental maps indicate the successive layer formation of Fe and Mn in these deposits. A Ni association with Mn has been further confirmed by observing a Ni-Mn spatial correlation together with their compositional correlation. In addition, the core or the nucleus of the ferromanganese nodule was found to be rich in Fe and not Mn, which strengthens the assumptions made earlier that the nodule formation started with Fe deposition which catalysed the growth of a ferromanganese nodule or crust by successive deposition of Mn and Fe. Irregular patterns of Mn and Fe layers were observed and discussed. Instead of the more often studied Co-Fe association in nodules and crusts, the intra nodule microanalysis revealed a Co-Mn correlation that will be discussed in this paper

  14. Characterizing borehole fluid flow and formation permeability in the ocean crust using linked analytic models and Markov chain Monte Carlo analysis (United States)

    Winslow, D. M.; Fisher, A. T.; Becker, K.


    Thermal records from boreholes in young oceanic crust, in which water is flowing up or down, are used to assess formation and borehole flow properties using three analytic equations that describe the transient thermal and barometric influence of downhole or uphole flow. We link these calculations with an iterative model and apply Markov chain Monte Carlo (MCMC) analysis to quantify ranges of possible values. The model is applied to two data sets interpreted in previous studies, from Deep Sea Drilling Project Hole 504B on the southern flank of the Costa Rica Rift and Ocean Drilling Program Hole 1026B on the eastern flank of the Juan de Fuca Ridge, and to two new records collected in Integrated Ocean Drilling Program Holes U1301A and U1301B, also on the eastern flank of the Juan de Fuca Ridge. Our calculations indicate that fluid flow rates when thermal logs were collected were ˜2 L/s in Holes 504B, 1026B, and U1301A, and >20 L/s in Hole U1301B. The median bulk permeabilities determined with MCMC analyses are 4 to 7 × 10-12 m2 around the uppermost parts of Holes 504B, 1026B, and U1301A, and 1.5 × 10-11 m2 around a deeper section of Hole U1301B, with a standard deviation of 0.2 to 0.3 log cycles at each borehole. The consistency of permeability values inferred from these four holes is surprising, given the range of values determined globally and the tendency for permeability to be highly variable in fractured crystalline rock formations such as the upper oceanic crust.

  15. Magmatic processes in developing oceanic crust revealed in a cumulate xenolith collected at the East Pacific Rise, 9°50'N (United States)

    Ridley, W. Ian; Perfit, Michael R.; Smith, Matthew C.; Fornari, Daniel J.


    The petrology and geochemistry of a xenolith, a fragment of a melt-bearing cumulate, within a recently erupted mid-ocean ridge (MOR) lava flow provide information on petrogenetic processes occurring within the newly forming oceanic crust beneath the northern East Pacific Rise (NEPR). The xenolith reveals important petrologic information about MOR magmatic systems concerning (1) melt distribution in a crystal-dominated mush; (2) melt-crystal reactions within the mush; (3) the chemistry of melts that have contributed to the cumulate lithology; and (4) the chemistry of axial melts that enter the axial magma system. The xenolith was enclosed within a moderately primitive, normal mid-ocean ridge basalt (NMORB) erupted in 1991 within the neovolcanic zone of the NEPR, at approximately 9°50'N. The sample is a matrix-dominated, cumulate olivine anorthosite, composed of anorthite (An94-90) and bytownite (An89-70), intergranular olivine (Fo86±0.3), minor sulfide and spinel, and intergranular glass. Marginal corrosion of plagioclase, and possibly olivine, and internal remelting of plagioclase indicate syntexis. It is surmised that the pore volume was eviscerated several times with moderately primitive basaltic melts and reduced by intergranular crystallization of forsteritic olivine. The presence of anorthite as a cumulate phase in the xenolith and the observation of anorthite xenocrysts in NMORB lavas, and as a cumulate phase in ophiolite gabbros, indicate that Ca-rich melts that are not a part of the NMORB lineage play an important role in the construction of the oceanic crust.

  16. Araxa Group in the type-area: A fragment of Neoproterozoic oceanic crust in the Brasilia Fold Belt; Grupo Araxa em sua area tipo: um fragmento de crosta oceanica Neoproterozoica na faixa de dobramentos Brasilia

    Energy Technology Data Exchange (ETDEWEB)

    Seer, Hildor Jose [Centro Federal de Educacao Tecnologica de Araxa, (CEFET), MG (Brazil); Brod, Jose Affonso; Fuck, Reinhardt Adolfo; Pimentel, Marcio Martins; Boaventura, Geraldo Resende; Dardenne, Marcel Auguste [Brasilia Univ., DF (Brazil). Inst. de Geociencias


    This study reviews the geological characteristics and puts forward a new evolution model for the Araxa Group in its type-area, the southern segment of the Neo proterozoic Brasilia Belt, Minas Gerais, Brazil. The Araxa Group is confined within a thrust sheet belonging to a syn formal regional fold, the Araxa Syn form, overlying two other thrust sheets made of the Ibia and Canastra Groups. The Araxa Group is described as a tectono stratigraphic terrane in the sense of Howell (1993). It comprises an igneous mafic sequence, with fine and coarse grained amphibolites, associated with pelitic meta sedimentary rocks, and subordinate psanmites. All rocks were metamorphosed to amphibolite facies at ca. 630 Ma ago and were intruded by collisional granites. The amphibolites represent original basaltic and gabbroic rocks, with minor ultramafic (serpentinite/ amphibole-talc schist). The basalts are similar to high Fe O tholeiites, with REE signatures that resemble E-MORB and {epsilon}{sub Nd(T)} =+ 1.1. The meta sedimentary rocks are interpreted as the result of a marine deep-water sedimentation. They have Sm-Nd model ages of 1,9 Ga, and {epsilon}{sub Nd(T)} = -10.21. The amphibolites and metasediments could represent a fragment of back-arc oceanic crust. The data presented here differ significantly from the original definition of Barbosa et al. (1970) who describe the Araxa Group as a pelitic/psanmitic sequence and the collisional granites as a basement complex. (author)

  17. Chemistry and possible resource potential of cobalt rich ferromanganese crust from Afanasiy-Nikitin seamount in the Indian Ocean

    Digital Repository Service at National Institute of Oceanography (India)

    Parthiban, G.; Banakar, V.K.


    , Ni, Co, Zn and V and found to be rich in Co (upto 0.88%) mineralogically, the studied Fe-Mn crust samples contain poorly crystalline d-MnO sub(2) and minor amount of Carbonate Fluorapatite (CFA) and feldspars. The low Mn/Fe ratios (less than 2), d-MnO...

  18. Platinum group elements and gold in ferromanganese crusts from Afanasiy-Nikitin seamount, equatorial Indian Ocean: Sources and fractionation

    Digital Repository Service at National Institute of Oceanography (India)

    Banakar, V.K.; Hein, J.R.; Rajani, R.P.; Chodankar, A.R.


    in ANS Fe–Mn crusts are derived from seawater and are mainly of terrestrial origin, with a minor cosmogenic component. The Ru/Rh (0.5–2) and Pt/Ru ratios (7-28) are closely comparable to ratios in continental basalts, whereas Pd/Ir ratios exhibit values...


    Scientific Electronic Library Online (English)



    Full Text Available El Batolito de Sabanalarga es a un cuerpo alargado de 410 Km2 que se extiende entre las Cordilleras Central y Occidental de Colombia, intruye en el borde occidental la Formación Barroso y las Diabasas de San José de Urama y en el borde oriental las rocas metamórficas del Complejo Cajamarca. El Batol [...] ito de Sabanalarga está formado por al menos dos pulsos magmáticos: uno inicial representado por gabros y dioritas de afinidad subalcalina toleítica y un segundo pulso constituido por cuarzodioritas y tonalitas de afinidad subalcalina de la serie calcoalcalina baja en K. El magmatismo se generó en un ambiente localizado por encima de la zona de subducción, en un arco volcánico plutónico localizado en el borde de sutura entre la corteza continental y la corteza oceánica, afectando ambas cortezas. Hace parte del arco las rocas volcánicas de la Formación Barroso. La edad del Batolito y del arco en general, en concordancia con los datos radiométricos, las relaciones intrusivas y el registro fósil, ocurrió dentro del rango comprendido entre el Cenomaniano-Aptiano superior, localizándose el plutonismo en el rango de edad entre 83 M.a y 102 M.a. Magmatismo como el de la Diorita de Altavista, el Gabro de San Diego y el Batolito Antioqueño, es contemporáneo con el arco que generó el Batolito de Sabanalarga, pudiendo ser parte del mismo evento magmático. Abstract in english The Sabanalarga Batholith is a long shape body reaching 410 Km2, located between the Central and Western cordilleras of Colombia. It intrudes rocks of the Barroso Formation and San Jose de Urama diabases along its western margin and rocks belonging to the Cajamarca complex towards the eastern side o [...] f the pluton. The Sabanalarga batholith is formed by at least two magmatic pulses. The first pulse is represented by gabbros and diorites with tholeiitic sub-alkaline affinity. The second pulse corresponds to cuarzodiorites and tonalites with sub-alkaline to calco-alkalyne low-K affinity. Magmatism is interpreted as being of supra-subduction environment, where the plutonic-volcanic arc is located right into the suture zone bonding and affecting both, continental and oceanic crust. The batholith is part of the volcanic arc of the Barroso Formation. The age of the batholith and related arc, according to available radiometric data, intrusive character and fossil record, occurred between the Cenomanian-Upper Aptian range, constraining the plutonism in the age range between 83 Ma and 102 Ma. The magmatic arc represented by the Sabanalarga batholith is contemporary with the Altavista diorite, San Diego Gabbro and Antioquian batholith and eventually belong to the same magmatic event.

  20. Tectonic model for the evolution of oceanic crust in the northeastern Indian Ocean from the Late Cretaceous to the Early Tertiary

    Digital Repository Service at National Institute of Oceanography (India)

    Krishna, K.S.; Rao, D.G.; Ramana, M.V.; Subrahmanyam, V.; Sarma, K.V.L.N.S.; Pilipenko, A.I.; Shcherbakov, V.S.; Murthy, I.V.R.


    Bathymetry and magnetic studies (part of the Trans Indian Ocean Geotraverse investigations) in the northeastern Indian Ocean revealed seafloor topographic features, magnetic Lineations (19 through 32B) and abandonaed spreading centers. The seafloor...

  1. 75 FR 34929 - Safety Zones: Neptune Deep Water Port, Atlantic Ocean, Boston, MA (United States)


    ...1625-AA00 Safety Zones: Neptune Deep Water Port, Atlantic Ocean...for review and comment at the Web site http://www.regulations...rulemaking docket titled: Neptune Deep Water Port, Atlantic Ocean...infrastructure of the Neptune Deep Water Port. Extensive...

  2. Late Quaternary Sedimentary Records of Core MA01 in the Mendeleev Ridge, the Western Arctic Ocean: Preliminary Results (United States)

    Park, K.; Kim, S.; Khim, B. K.; Wang, R.; Mei, J.; Xiao, W.; Polyak, L. V.


    Late Quaternary deep sea sediments in the Arctic Ocean are characterized by brown layers intercalated with yellowish to olive gray layers. It has been known that the brown and gray layers were deposited during interglacial (or interstadial) and glacial (or stadial) periods, respectively. A 5.5-m long gravity core MA01 was obtained from the Mendeleev Ridge in the western Arctic Ocean by R/V Xue Long during scientific cruise CHINARE-V. Age (~1.0 Ma) of core MA01 was tentatively decided by correlation of sediment color cycles, XRF Mn and Ca cycles, and geomagnetic inclinations with core HLY0503-8JPC (Adler et al., 2009) and core HLY0503-06JPC(Cronin et al., 2013) that were also collected from the Mendeleev Ridge area. A total of 23 brown layers are characterized by low L* and b*, high Mn concentration, and abundant foraminifera. In contrast, gray layers are characterized by high L* and b*, low Mn concentration, and few foraminiferal tests. Foraminifera abundance peaks are not well correlated to CaCO3 peaks which are accompanied with the coarse-grained (>63 ?m) fractions (i.e., IRD) both in brown and gray layers. A strong positive correlation coefficient (r2=0.89) between TOC content and C/N ratio indicates that the major source of organic matter is terrestrial. The good correlations of CaCO3 content to TOC (r2=0.56) and C/N ratio (r2=0.69) imply that IRDs contain detrital CaCO3 fraction which mainly originated from the Canadian Arctic Archipelago. In addition, high kaolinite/chlorite (K/C) ratios mostly correspond to CaCO3 peaks, also suggesting that the fine-grained particles in the Mendeleev Ridge were transported from the northern coasts of the Alaska and Canada. Thus, the Beaufort Gyre, the predominant surface current in the western Arctic Ocean, has played an important role in the sediment delivery to the Mendeleev Ridge. It is worthy of note that TOC and CaCO3 peaks are obviously distinct in the upper part of core MA01, whereas these peaks are reduced in the lower part of the core. More study on these contrasting features is in progress.

  3. Alteration of ocean crust provides a strong temperature dependent feedback on the geological carbon cycle and is a primary driver of the Sr-isotopic composition of seawater (United States)

    Coogan, Laurence A.; Dosso, Stan E.


    On geological timescales there is a temperature dependent feedback that means that increased degassing of CO2 into the atmosphere leads to increased CO2 drawdown into rocks stabilizing Earth's climate. It is widely considered that this thermostat largely comes from continental chemical weathering. An alternative, or additional, feedback comes from dissolution of seafloor basalt in low-temperature (tens of °C), off-axis, hydrothermal systems. Carbonate minerals precipitated in these systems provide strong evidence that increased bottom water temperature (traced by their O-isotopic compositions) leads to increased basalt dissolution (traced by their Sr-isotopic compositions). Inversion of a simple probabilistic model of fluid-rock interaction allows us to determine the apparent activation energy of rock dissolution in these systems. The high value we find (92 ± 7 kJmol-1) indicates a strong temperature dependence of rock dissolution. Because deep-ocean temperature is sensitive to global climate, and the fluid temperature in the upper oceanic crust is strongly influenced by bottom water temperature, increased global temperature must lead to increased basalt dissolution. In turn, through the generation of alkalinity by rock dissolution, this leads to a negative feedback on planetary warming; i.e. off-axis, hydrothermal systems play an important role in the planetary thermostat. Changes in the extent of rock dissolution, due to changes in bottom water temperature, also lead to changes in the flux of unradiogenic Sr into the ocean. The decreased flux of unradiogenic Sr into the ocean due to the cooling of ocean bottom water over the last 35 Myr is sufficient to explain most of the increase in seawater 87Sr/86Sr over this time.

  4. Crustal thickness controlled by plate tectonics : a review of crust-mantle interaction processes illustrated by European examples

    DEFF Research Database (Denmark)

    Artemieva, Irina M.; Meissner, Rolf


    The continental crust on Earth cannot be extracted directly from the mantle, and the primary crust extracted directly from an early magma ocean is not preserved on Earth. We review geophysical and geochemical aspects of global crust–mantle material exchange processes and examine the processes which, on one side, form and transform the continental crust and, on the other side, chemically modify the mantle residue from which the continental crust has been extracted. Major mechanisms that provide crust–mantle material exchange are oceanic and continental subduction, lithosphere delamination, and mafic magmatism. While both subduction and delamination recycle crustal material into the mantle, mafic magmatism transports mantle material upward and participates in growth of new oceanic and continental crusts and significant structural and chemical modification of the latter. We discuss the role of basalt/gabbro–eclogite phase transition in crustal evolution and the links between lithosphere recycling, mafic magmatism, and crustal underplating. We advocate that plate tectonics processes, together with basalt/gabbro–eclogite transition, limit crustal thickness worldwide by providing effective mechanisms of crustal (lithosphere) recycling. The processes of crust–mantle interaction have created very dissimilar crustal styles in Europe, as seen by its seismic structure, crustal thickness, and average seismic velocities in the basement. Our special focus is on processes responsible for the formation of the thin crust of central and western Europe, which was largely formed during the Variscan (430–280 Ma) orogeny but has the present structure of an “extended” crust, similar to that of the Basin and Range province in western USA. Major geophysical characteristics of the Variscan lithosphere are discussed within the frame of possible sequences of crust–mantle material exchange mechanisms during and after main orogenic events in the European Variscides.

  5. Icelandic-type crust (United States)

    Foulger, G. R.; Du, Z.; Julian, B. R.


    Numerous seismic studies, in particular using receiver functions and explosion seismology, have provided a detailed picture of the structure and thickness of the crust beneath the Iceland transverse ridge. We review the results and propose a structural model that is consistent with all the observations. The upper crust is typically 7 +/- 1 km thick, heterogeneous and has high velocity gradients. The lower crust is typically 15-30 +/- 5 km thick and begins where the velocity gradient decreases radically. This generally occurs at the Vp~ 6.5 km s-1 level. A low-velocity zone ~10 000 km2 in area and up to ~15 km thick occupies the lower crust beneath central Iceland, and may represent a submerged, trapped oceanic microplate. The crust-mantle boundary is a transition zone ~5 +/- 3 km thick throughout which Vp increases progressively from ~7.2 to ~8.0 km s-1. It may be gradational or a zone of alternating high- and low-velocity layers. There is no seismic evidence for melt or exceptionally high temperatures in or near this zone. Isostasy indicates that the density contrast between the lower crust and the mantle is only ~90 kg m-3 compared with ~300 kg m-3 for normal oceanic crust, indicating compositional anomalies that are as yet not understood. The seismological crust is ~30 km thick beneath the Greenland-Iceland and Iceland-Faeroe ridges, and eastern Iceland, ~20 km beneath western Iceland, and ~40 km thick beneath central Iceland. This pattern is not what is predicted for an eastward-migrating plume. Low attenuation and normal Vp/Vs ratios in the lower crust beneath central and southwestern Iceland, and normal uppermost mantle velocities in general, suggest that the crust and uppermost mantle are subsolidus and cooler than at equivalent depths beneath the East Pacific Rise. Seismic data from Iceland have historically been interpreted both in terms of thin-hot and thick-cold crust models, both of which have been cited as supporting the plume hypothesis. This suggests that the plume model for Iceland is an a priori assumption rather than a hypothesis subject to testing. The long-extinct Ontong-Java Plateau, northwest India and Paraná, Brazil large igneous provinces, beneath which mantle plumes are not expected, are all underlain by mantle low-velocity bodies similar to that beneath Iceland. A plume interpretation for the mantle anomaly beneath Iceland is thus not required.

  6. Center for Ocean Sciences Education Excellence Mid-Atlantic (COSEE-MA) (United States)

    COSEE-MA's goal is to integrate research and education programs to encourage lifelong learning experiences for everyone. They especially want to reach out to K-12 educators, students (K-16), coastal managers, families, and underserved audiences. The site features information for all of the above audiences, as well as the latest news and upcoming events.

  7. Canterbury Drifts, SW Pacific Ocean: Record of Antarctic Intermediate Water Flow Since 24 Ma (United States)

    Carter, R. M.


    The Canterbury Drifts were deposited in water depths between 400 and 1500 m by northward flowing, cold, intermediate depth water masses - Subantarctic Mode Water, Antarctic Intermediate Water, and their predecessor current flows. Drift accumulation started at 24 Ma, fed by terrigenous sediment derived from the newly rising Alpine Fault plate boundary in the west, which has built a progradational shelf-slope sediment prism up to 130 km wide at rates of eastward advance of up to 5.4 km/My. These regionally extensive, intermediate-depth sediment drifts can be examined in outcrop, in marine drillcore (ODP Site 1119) and at the modern seabed. The drifts comprise planar-bedded units up to several metres thick. Sand intervals have either reverse graded or sharp, erosive bases and normally graded tops. Bioturbation is moderate and the sands occur within a pervasive background of cm-scale, planar or wispy alternating muddy and sandy silts, consistent with deposition from rhythmically fluctuating bottom currents. In the Plio-Pleistocene, the sand:silt lithological rhythmicity occurs in synchroneity with Milankovitch-scale climate cycling; periods of inferred faster current flow (sand beds) mostly correspond to warm climatic intervals. The drifts vary in thickness from 300 m near the early Miocene shoreline, where they were accumulating in limited shallow water accommodation, to 2000+ m under the modern shelf edge. Mounded drifts first occur at 15 Ma (Middle Miocene), their appearance perhaps reflecting more vigorous intermediate water flow consequent upon the worldwide climatic deterioration between 15 and 13 Ma. A further change from large (more than 10 km wide) to smaller (1-3 km wide) mounded slope drifts occurs at 3.1 Ma, marking further cooling, the inception of discrete SAMW flows, and initiation of the Subantarctic Front. The natural gamma ray record from Site 1119 contains a history since 3.9 Ma of the waxing and waning of the New Zealand mountain ice cap. Back to 0.37 Ma, this record closely mirrors the climate history of Antarctica, as manifested in the Vostok ice core, at 0.1-0.6 ky resolution. Beyond, and back to 3.9 Ma, the gamma record reflects southern mid-latitude ice-volumes and perhaps Antarctic polar plateau temperature at a resolution of 1.3 ky.

  8. Seismic evidence for the presence of Jurassic oceanic crust in the central Gulf of Cadiz (SW Iberian margin)


    Sallarès, Valentí; Gailler, Audrey; Gutscher, Marc-André; Graindorge, David; Bartolomé, Rafael; Gràcia, Eulàlia; Díaz, Jordi; Dañobeitia, Juan José; Zitellini, N.


    We investigate the crustal structure of the SW Iberian margin along a 340 km-long refraction and wide-angle reflection seismic profile crossing from the central Gulf of Cadiz to the Variscan continental margin in the Algarve, Southern Portugal. The seismic velocity and crustal geometry model obtained by joint refraction and reflection travel-time inversion reveal three distinct crustal domains: the 28-30 km-thick Variscan crust in the north, a 60 km-wide transition zone offshore, where the cr...

  9. Crusted scabies. (United States)

    Karthikeyan, Kaliaperumal


    Crusted scabies is a rare manifestation of scabies characterized by uncontrolled proliferation of mites in the skin. In immunocompromised patients, this infestation is characterized by crusted lesions. The occurrence of the disease in human immunodeficiency virus-infected patients and the widespread use of immunosuppressive agents has led to a renewed interest in the disease. Early recognition and treatment is necessary to avoid an outbreak of scabies. This review highlights the pathogenesis, predisposing factors, clinical features and treatment of crusted scabies. PMID:19584457

  10. Channelling of hydrothermal fluids during the accretion and evolution of the upper oceanic crust: Sr isotope evidence from ODP Hole 1256D (United States)

    Harris, Michelle; Coggon, Rosalind M.; Smith-Duque, Christopher E.; Cooper, Matthew J.; Milton, James A.; Teagle, Damon A. H.


    ODP Hole 1256D in the eastern equatorial Pacific is the first penetration of a complete section of fast spread ocean crust down to the dike-gabbro transition, and only the second borehole to sample in situ sheeted dikes after DSDP Hole 504B. Here a high spatial resolution record of whole rock and mineral strontium isotopic compositions from Site 1256 is combined with core observations and downhole wireline geophysical measurements to determine the extent of basalt-hydrothermal fluid reaction and to identify fluid pathways at different levels in the upper ocean crust. The volcanic sequence at Site 1256 is dominated by sheet and massive lava flows but the Sr isotope profile shows only limited exchange with seawater. However, the upper margins of two anomalously thick (>25 m) massive flow sequences are strongly hydrothermally altered with elevated Sr isotope ratios and appear to be conduits of lateral low-temperature off-axis fluid flow. Elsewhere in the lavas, high 87Sr/86Sr are restricted to breccia horizons. Mineralised hyaloclastic breccias in the Lava-Dike Transition are strongly altered to Mg-saponite, silica and pyrite, indicating alteration by mixed seawater and cooled hydrothermal fluids. In the Sheeted Dike Complex 87Sr/86Sr ratios are pervasively shifted towards hydrothermal fluid values (?0.705). Dike chilled margins display secondary mineral assemblages formed during both axial recharge and discharge and have higher 87Sr/86Sr than dike cores, indicating preferential fluid flow along dike margins. Localised increases in 87Sr/86Sr in the Dike-Gabbro Transition indicates the channelling of fluids along the sub-horizontal intrusive boundaries of the 25 to 50 m-thick gabbroic intrusions, with only minor increases in 87Sr/86Sr within the cores of the gabbro bodies. When compared to the pillow lava-dominated section from Hole 504B, the Sr isotope measurements from Site 1256 suggest that the extent of hydrothermal circulation in the upper ocean crust may be strongly dependent on the eruption style. Sheet and massive flow dominated lava sequences typical of fast spreading ridges may experience relatively restricted circulation, but there may be much more widespread circulation through pillow lava-dominated sections. In addition, the Hole 1256D sheeted dikes display a much greater extent of Sr-isotopic exchange compared to dikes from Hole 504B. Because seawater-derived hydrothermal fluids must transit the dikes during their evolution to black smoker-type fluids, the different Sr-isotope profiles for Holes 504B and 1256D suggest there are significant variations in mid-ocean ridge hydrothermal systems at fast and intermediate spreading ridges, which may impact geochemical cycles of elements mobilised by fluid-rock exchange at different temperatures.

  11. Investigating the link between an iron-60 anomaly in the deep ocean's crust and the origin of the Local Bubble

    International Nuclear Information System (INIS)

    Supernova explosions responsible for the creation of the Local Bubble (LB) and its associated HI cavity should have caused geological isotope anomalies via deposition of debris on Earth. The discovery of a highly significant increase of 60Fe (a radionuclide that is exclusively produced in explosive nucleosynthesis) in layers of a deep sea ferromanganese crust corresponding to a time of 2.2 Myr before present, appears very promising in this context. We report on our progress in relating these measurements to the formation of the LB by means of 3D hydrodynamical adaptive mesh refinement simulations of the turbulent interstellar medium in the solar neighborhood. Our calculations are based on a sophisticated selection procedure for the LB's progenitor stars and take advantage of passive scalars for following the chemical mixing process.

  12. A documentation on burrows in hard substrates of ferromanganese crusts and associated soft sediments from the Central Indian Ocean

    Digital Repository Service at National Institute of Oceanography (India)

    Banerjee, R.


    megafauna on the floor of the CIB, much away from the actively spreading ridges. Future deep- ocean mining for the extraction of sea-floor ferroman- ganese deposits should take enough care to protect and preserve such extensive deep-sea benthic life. 1...

  13. Crusted scabies

    Directory of Open Access Journals (Sweden)

    Karthikeyan Kaliaperumal


    Full Text Available Crusted scabies is a rare manifestation of scabies characterized by uncontrolled proliferation of mites in the skin. In immunocompromised patients, this infestation is characterized by crusted lesions. The occurrence of the disease in human immunodeficiency virus-infected patients and the widespread use of immunosuppressive agents has led to a renewed interest in the disease. Early recognition and treatment is necessary to avoid an outbreak of scabies. This review highlights the pathogenesis, predisposing factors, clinical features and treatment of crusted scabies.

  14. Preservation and Recycling of Crust during Accretionary and Collisional Phases of Proterozoic Orogens: A Bumpy Road from Nuna to Rodinia

    Directory of Open Access Journals (Sweden)

    Kent C. Condie


    Full Text Available Zircon age peaks at 2100–1650 and 1200–1000 Ma correlate with craton collisions in the growth of supercontinents Nuna and Rodinia, respectively, with a time interval between collisions mostly <50 Myr (range 0–250 Myr. Collisional orogens are two types: those with subduction durations <500 Myr and those ?500 Myr. The latter group comprises orogens with long-lived accretionary stages between Nuna and Rodinia assemblies. Neither orogen age nor duration of either subduction or collision correlates with the volume of orogen preserved. Most rocks preserved date to the pre-collisional, subduction (ocean-basin closing stage and not to the collisional stage. The most widely preserved tectonic setting in Proterozoic orogens is the continental arc (10%–90%, mean 60%, with oceanic tectonic settings (oceanic crust, arcs, islands and plateaus, serpentinites, pelagic sediments comprising <20% and mostly <10%. Reworked components comprise 20%–80% (mean 32% and microcratons comprise a minor but poorly known fraction. Nd and Hf isotopic data indicate that Proterozoic orogens contain from 10% to 60% of juvenile crust (mean 36% and 40%–75% reworked crust (mean 64%. Neither the fraction nor the rate of preservation of juvenile crust is related to the collision age nor to the duration of subduction. Regardless of the duration of subduction, the amount of juvenile crust preserved reaches a maximum of about 60%, and 37% of the volume of juvenile continental crust preserved between 2000 and 1000 Ma was produced in the Great Proterozoic Accretionary Orogen (GPAO. Pronounced minima occur in frequency of zircon ages of rocks preserved in the GPAO; with minima at 1600–1500 Ma in Laurentia; 1700–1600 Ma in Amazonia; and 1750–1700 Ma in Baltica. If these minima are due to subduction erosion and delamination as in the Andes in the last 250 Myr; approximately one third of the volume of the Laurentian part of the GPAO could have been recycled into the mantle between 1500 and 1250 Ma. This may have enriched the mantle wedge in incompatible elements and water leading to the production of felsic magmas responsible for the widespread granite-rhyolite province of this age. A rapid decrease in global Nd and in detrital zircon Hf model ages between about 1600 and 1250 Ma could reflect an increase in recycling rate of juvenile crust into the mantle; possibly in response to partial fragmentation of Nuna.

  15. From continent to intra-oceanic arc: zircon xenocrysts record the crustal evolution of the Solomon island arc


    Tapster, Simon; Roberts, N.M.W.; Petterson, M.G.; Saunders, A.D.; Naden, J.


    The first U-Pb ages from a ca. 26–24 Ma pluton on Guadalcanal, in the intra-oceanic Solomon island arc (southwest Pacific Ocean), reveal Eocene- to Archean-aged zircon xenocrysts. Xenocryst populations at ca. 39–33 Ma and ca. 71–63 Ma correlate with previously obtained ages of supra-subduction magmatism within the arc. A ca. 96 Ma zircon population may be derived from Cretaceous ophiolite basement crust or region-wide continental rift-related magmatism. Xenocryst age populations alterna...

  16. Origin of gabbro sills in the Moho transition zone of the Oman ophiolite: Implications for magma transport in the oceanic lower crust (United States)

    Korenaga, Jun; Kelemen, Peter B.


    The Moho transition zone (MTZ) of the Oman ophiolite commonly includes a number of gabbro sills surrounded by dunites. The petrology and geochemistry of these sills are investigated to provide constraints on how magma migrates from the subridge mantle to the oceanic crust. The gabbro sills have millimeter-scale to tens of centimeter-scale modal layering that closely resembles layering in lower crustal gabbros of the ophiolite. Variations in mineral compositions correlate with the modal layering, but there are no overall trends within the sills. The gabbroic sills and the layered gabbros have clear covariations among mineral compositions, which can be interpreted as a fractional crystallization path from a common parental magma. Together with constraints from mid-ocean ridge thermal evolution and crustal accretion dynamics, the petrological and geochemical observations on the gabbro sills indicate that they formed from small, open-system, melt-filled lenses within the MTZ. The thermal evolution of the MTZ melt lenses, buffered by the ambient mantle, is characterized by a slow cooling rate (gravity currents and oscillatory nucleation, are unlikely in such a thermal environment, and we propose that open-system evolution of the melt lenses is essential to produce the observed layering. The formation of the MTZ melt lenses may be a consequence of porous flow with low Peclet number entering a conductively cooling regime, where porosity becomes "clogged" by crystallized plagioclase. Preservation of fine-scale vertical variation in mineral composition, together with correlated compositions of different minerals, rules out diffuse porous flow as the primary mechanism of melt transport above these melt lenses. Instead, melt extraction must have been focused into porous channels or melt-filled fractures. Melt lenses drained by fractures would experience repetitious expulsion with continuous melt replenishment. Modal layering could develop through the expulsion cycles, probably via in situ crystallization at the margins of melt lenses.

  17. Subduction-zone crust-mantle interaction is a common mechanism for the origin of oceanic arc and island basalts (United States)

    Zheng, Y. F.; Zhao, Z. F.


    We present a generalized model for the origin of oceanic arc basalts (OAB) and oceanic island basalts (OIB). This is realized by an integrated study of their major-trace element and stable-radiogenic isotope compositions. Many continental basalts are geochemically indistinguishable from common OIB, a fact that requires part of the upper mantle to have been a common reservoir beneath both oceans and continents. In addition, this reservoir must have been isolated from the convective asthenosphere for preservation of geochemical anomalies. Common OAB and OIB show consistent enrichment of LILE and LREE relative to normal MORB. On the primitive mantle-normalized spidergram, however, OAB are characterized by negative Nb and Ta anomalies but a positive Pb anomaly, whereas OIB show positive or no Nb and Ta anomalies but a negative Pb anomaly. Such differences are attributed to the difference in the property of metasomatic agents (aqueous solutions, hydrous melts and supercritical fluids) derived from subducting crustal rocks. The metasomatic agents are highly enriched in fluid/melt-mobile incompatible trace elements such as LILE and LREE, transferring enriched components from the crustal rocks to the mantle sources of OAB and OIB. The stability of rutile in the subducting crustal rocks dictates the abundance of Nb and Ta in the metasomatic agents. Lead is preferentially partitioned into the metasomatic agents when released at subarc depths, whereas dehydrated Pb-poor restites were subducted to greater depths. This explains the positive Pb anomaly in OAB but the negative Pb anomaly in OIB. We accept the assumption that normal MORB are derived from partial melting of the normal asthenospheric mantle, a common reservoir of isotopic depletion. We extend the chemical reaction at the slab-mantle interface in subduction channel from subarc depths to those above the mantle transition zone, generating metasomatic ultramafic rocks (metasomes) in the upper mantle. The reaction at subarc depths produces serpentinized to chloritized peridotites, serving as the source of OAB; the reaction at greater depths produces pyroxene-rich peridotites, pyroxenite and hornblendite, serving as the source of OIB. These metasomes have low solidii and thus are more susceptible to partial melting than the peridotite at the same P-T conditions.

  18. The Sabzevar blueschists of the North-Central Iranian micro-continent as remnants of the Neotethys-related oceanic crust subduction (United States)

    Omrani, Hadi; Moazzen, Mohssen; Oberhänsli, Roland; Altenberger, Uwe; Lange, Manuela


    The Sabzevar ophiolites mark the Neotethys suture in east-north-central Iran. The Sabzevar metamorphic rocks, as part of the Cretaceous Sabzevar ophiolitic complex, consist of blueschist, amphibolite and greenschist. The Sabzevar blueschists contain sodic amphibole, epidote, phengite, calcite ± omphacite ± quartz. The epidote amphibolite is composed of sodic-calcic amphibole, epidote, albite, phengite, quartz ± omphacite, ilmenite and titanite. The greenschist contains chlorite, plagioclase and pyrite, as main minerals. Thermobarometry of a blueschist yields a pressure of 13-15.5 kbar at temperatures of 420-500 °C. Peak metamorphic temperature/depth ratios were low (~12 °C/km), consistent with metamorphism in a subduction zone. The presence of epidote in the blueschist shows that the rocks were metamorphosed entirely within the epidote stability field. Amphibole schist samples experienced pressures of 5-7 kbar and temperatures between 450 and 550 °C. The presence of chlorite, actinolite, biotite and titanite indicate greenschist facies metamorphism. Chlorite, albite and biotite replacing garnet or glaucophane suggests temperatures of >300 °C for greenschist facies. The formation of high-pressure metamorphic rocks is related to north-east-dipping subduction of the Neotethys oceanic crust and subsequent closure during lower Eocene between the Central Iranian Micro-continent and Eurasia (North Iran).

  19. Carbonate stability and fluid composition in subducted oceanic crust: an experimental study on H 2O-CO 2-bearing basalts (United States)

    Molina, Josè F.; Poli, Stefano


    Carbonates and hydrates are common products of the alteration of the upper basaltic crust in modern oceans. However, phase relationships and devolatilization reactions in altered CO 2-bearing metabasalts during the subduction process are still poorly known. A series of fO 2-buffered piston cylinder experiments were performed on three basaltic model compositions in the presence of a H 2O-CO 2 mixed fluid, at pressures from 1.0 to 2.0 GPa and temperatures from 665 to 730°C. Experimental results on a tholeiite composition demonstrate that amphibole coexists with calcite at P?1.4 GPa, with dolomite at 1.4? P?1.8 GPa, and with dolomite+magnesite at pressures higher than 1.8 GPa. The stability of calcite increases with pressure with increasing Fe/(Fe+Mg) of the bulk composition. Omphacite was found in tholeiite only at 2.0 GPa, 730°C. Garnet, plagioclase, paragonite, epidote and kyanite further complicate phase relationships in the pressure range investigated. Estimates of the coexisting fluid compositions, on the basis of mass-balance and thermodynamic calculations, demonstrate the continuous H 2O enrichment with increasing pressure and decreasing temperature. An almost purely aqueous fluid ( X CO 2imbalance at convergent margins.

  20. A 4D Framework for Ocean Basin Paleodepths and Eustatic Sea Level Change (United States)

    Muller, R.; Sdrolias, M.; Gaina, C.


    A digital framework for paleobathymetry of the ocean basins requires the complete reconstruction of ocean floor through time, including the main ocean basins, back-arc basins, and now subducted ocean crust. We reconstruct paleo-oceans by creating "synthetic plates", the locations and geometry of which is established on the basis of preserved ocean crust (magnetic lineations and fracture zones), geological data, and the rules of plate tectonics. We reconstruct the spreading histories of the Pacific, Phoenix, Izanagi, Farallon and Kula plates, the plates involved in the Indian, Atlantic, Caribbean, Arctic, Tethys and Arctic oceanic domains and all plates involved in preserved backarc basins. Based mainly on the GML-standards compliant GPlates software and the Generic Mapping Tools, we have created a set of global oceanic paleo-isochrons and paleoceanic age and depth grids. We show that the late-Cretaceous sea level highstand and the subsequent long-term drop in sea level was primarily caused by the changing age-area distribution of Pacific ocean floor through time. The emplacement of oceanic plateaus has resulted in a 40 m sealevel rise between 125 and 110 Ma, and a further 60 m rise after 110 Ma, whereas the oceanic age and latitude dependence of marine sediments has resulted in a 40m sealevel rise since about 120Ma, offsetting the gradual post-80Ma drop in sealevel due to the ageing and deepening mainly of the Pacific ocean basin, with the net effect being an about 200m drop after 80 Ma. Between 140 Ma and the present, oceanic crustal production dropped by over 40% in the Pacific, but stayed roughly constant in the remaining ocean basins. Our results suggest that the overall magnitude of 1st order sealevel change implied by Haq's sea level curve is correct.

  1. Ocean crust deformation at the North America-South America plate boundary: Results of the 2007 ANTIPLAC marine survey (United States)

    Patriat, M.; Benard, F.; Deville, E.; Le Drezen, E.; Loubrieu, B.; Maltese, L.; Roest, W.; Thereau, E.; Umber, M.; Vially, R.


    East of the Lesser Antilles active margin, the area of the Barracuda and Tiburon ridges is resulting from of a multidirectional and polyphase tectonic history at the diffuse plate boundary between the North and South American plates. These WNW-ESE trending ridges control the sediment distribution and they are bounded by sedimentary trenches, both ridges and trenches trending parallel to the Mid-Atlantic oceanic fracture zones. A marine survey (called ANTIPLAC) conducted in the beginning of the year (January 2007) has provide new evidences (multibeam and seismic acquisition) of the deformation processes which occurred at this plate boundary. On the seismic lines, a major angular unconformity can be recognized in the whole area of the survey. Interpreting the acquired seismic grid, the lower part of the stratigraphic series can be easily tied to the DSDP/ODP holes of legs 78A, 110, 156, 171A, especially with wells 543 and 672. Thus a Maastrichtian-Pliocene age can be attributed to the geological formations located below the regional unconformity. The very recent geological formations located above the unconformity (attributed to the Late Pliocene-Pleistocene) tend to fill the main depressions of the area and show very heterogeneous thickness. These recent deposits can be more than 3 s(TWT) thick in the Barracuda trough (north of Barracuda ridge). Globally they show clear onlapping characters above the older levels, but in some places these levels show spectacularly evidences of syntectonic deposition. This is notably the case of a narrow WNW-ESE trending fold and fault system trending along the axis of the Barracuda trough. South of Barracuda ridge the recent deposits show also locally spectacular fan geometries characterizing deposition during significant tilting. Also, between Barracuda and Tiburon ridges several fracture zones show evidences of very recent (and probably active) reactivation. This recent deformation is also characterized by recent basin inversion structures. Finally and more generally, the data acquired during the ANTIPLAC survey demonstrate that high deformation occurred at the boundary between the North and South American plates during much more recent times than previously thought, and that notably spectacular compressional structures resulting from the convergence between the two american plates developed recently during Late Pliocene-Quaternary times. The subduction of this structural pattern and its partial incorporation within the Barbados tectonic wedge has widely influenced the deformation processes within the accretionary prism and has also induced segmentation within the overriding Caribbean plate.

  2. Chapter 50: Geology and tectonic development of the Amerasia and Canada Basins, Arctic Ocean (United States)

    Grantz, A.; Hart, P.E.; Childers, V.A.


    Amerasia Basin is the product of two phases of counterclockwise rotational opening about a pole in the lower Mackenzie Valley of NW Canada. Phase 1 opening brought ocean-continent transition crust (serpentinized peridotite?) to near the seafloor of the proto-Amerasia Basin, created detachment on the Eskimo Lakes Fault Zone of the Canadian Arctic margin and thinned the continental crust between the fault zone and the proto-Amerasia Basin to the west, beginning about 195 Ma and ending prior to perhaps about 160 Ma. The symmetry of the proto-Amerasia Basin was disrupted by clockwise rotation of the Chukchi Microcontinent into the basin from an original position along the Eurasia margin about a pole near 72??N, 165 Wabout 145.5-140 Ma. Phase 2 opening enlarged the proto-Amerasia Basin by intrusion of mid-ocean ridge basalt along its axis between about 131 and 127.5 Ma. Following intrusion of the Phase 2 crust an oceanic volcanic plateau, the Alpha-Mendeleev Ridge LIP (large igneous province), was extruded over the northern Amerasia Basin from about 127 to 89-75 Ma. Emplacement of the LIP halved the area of the Amerasia Basin, and the area lying south of the LIP became the Canada Basin. ?? 2011 The Geological Society of London.

  3. Precambrian U-Pb zircon ages in eclogites and garnet pyroxenites from South Brittany (France): An old oceanic crust in the West European Hercynian belt

    International Nuclear Information System (INIS)

    U-Pb zircon ages have been determined for tow eclogites from the Vendee and for two garnet pyroxenites from the Baie d'Audierne. In an episodic Pb loss model, the two discordia could give upper intercept ages around 1300-1250 Ma and lower intercepts ages of 436-384 Ma. (orig.)

  4. Underplating generated A- and I-type granitoids of the East Junggar from the lower and the upper oceanic crust with mixing of mafic magma: Insights from integrated zircon U-Pb ages, petrography, geochemistry and Nd-Sr-Hf isotopes (United States)

    Liu, Wei; Liu, Xiu-Jin; Liu, Li-Juan


    Whole rock major and trace element, Nd-Sr and zircon Hf isotopic compositions and secondary-ion mass spectrometry zircon U-Pb ages of eleven granitoid intrusions and dioritic rocks from the East Junggar (NW China) were analyzed in this study. The East Junggar granitoids were emplaced during terminal Early to Late Carboniferous (325-301 Ma) following volcanic eruption of the Batamayi Formation. Zircons from the East Junggar granitoids yielded 210 concordant 206Pb/238U ages which are all younger than 334 Ma and exhibit ?Hf(t) values distinctly higher than Devonian arc volcanic-rocks. Seismic P-wave velocities of deep crust of the East Junggar proper resemble those of oceanic crust (OC). These characteristics suggest absence of volcanic rock and volcano-sedimentary rock of Devonian and Early Carboniferous from the source region. The East Junggar granitoids show ?Nd(t) and initial 87Sr/86Sr values substantially overlapping those of the Armantai ophiolite in the area. The Early Paleozoic OC with seamount-like composition as the Zhaheba-Armantai ophiolites remained in the lower crust and formed main source rock of the East Junggar granitoids. Based on petrography and geochemistry, the East Junggar granitoids are classified into peralkaline A-type in the northern subarea, I-type (I1 and I2 subgroups) mainly in the north and A-type in the south of the southern subarea. The perthitic or argillated core and oligoclasic rim with an argillated boundary of feldspar phenocrysts and inclusion of perthites or its overgrowth by matrix plagioclase, in the monzogranites (northern subarea), suggest mixing of peralkaline granitic magma with mafic magma. In the north of the southern subarea, the presence of magmatic microdioritic enclaves (MMEs) in the I1 subgroup granitoids, transfer of plagioclase phenocrysts and hornblendes between host granodiorite and the MME across the boundary and a prominent resorption surface in the plagioclase phenocrysts indicate mixing of crustal magma (I2 subgroup granitoids) with mafic magma. Magma mixing shifted (87Sr/86Sr)i of the I1 subgroup granitoids towards the mantle array. Two generations of hornblende with zonal distribution and similar mineral and geochemical compositions of quartz monzodiorite and hosted MME with unfractionated rare earth elements (REE) suggest extended magma mixing with onset probably at or near source region. These observations imply concurrency of mantle input and the crustal melting and, hence, a causal relationship between underplating/intraplating and the lower OC/upper OC melting. The I-type granitoids experienced plagioclase and hornblende fractionations, whereas fractionated phases of the two groups of A-type granites were alkali feldspar and albite-oligoclase with significant involvement of F--rich fluid. Granodioritic parent magmas of the I2 subgroup granitoids stemmed from the hydrous upper OC. Parent magmas of the two A-type groups possess syenogranitic or quartz syenitic compositions. The peralkaline A-type granites stemmed from the lower OC, whereas the A-type granites from dehydrated upper OC left behind after extensive partial melting and extraction of I-type granitoids. Based on comparison in the ternary system Mg2SiO4-CaAl2SiO6-SiO2, most of the Batamayi volcanic rocks with affinity to ocean-island basalts were derived from asthenospheric upwelling. The gabbro-dioritic rocks with higher light to heavy REE ratios stemmed from metasomatized lithospheric mantle. Both of the above mafic rocks contain subducted slab component.

  5. Southern Ocean Surface and Intermediate Water Temperature from Alkenones and Mg/Ca of Infaunal Foraminifera for the last 1.5 Ma (United States)

    Elmore, Aurora; McClymont, Erin; Elderfield, Harry; Kender, Sev


    The reconstruction of past surface (SST), intermediate, and deep-water temperatures is critical to our understanding of feedbacks within the ocean-climate system. Intermediate water temperature (IWT) reconstruction is particularly important since intermediate waters, including Antarctic Intermediate Water (AAIW), are proposed to be an important driver in high-low latitude teleconnections, despite limited intermediate-depth records through the Pliocene and Pleistocene. Paleotemperature proxies have caveats, including the 'Carbonate Ion Effect' on the Magnesium to Calcium ratio (Mg/Ca) of benthic foraminifera. However, recent studies demonstrated that the infaunal species, Uvigerina peregrina, co-precipitates Mg independent of secondary effects, affording the use of U.peregrina Mg/Ca as a paleotemperature proxy (Elderfield et al., 2010). We present the first 1.5 Ma record of IWT from Mg/CaU.peregrina coupled with an alkenone- derived UK37' SST record from a sediment core in the Southwest Pacific (DSDP site 593; 1068m water depth), in the core of modern AAIW. Our new data reconstruct interglacial IWTs at ~7°C before and after the Mid-Pleistocene Transition (MPT), whereas values of ~5°C occur in the later Pleistocene. Glacial IWT remained fairly constant (~2°C) throughout the last 1 Ma. These results are in apparent disagreement with the typical idea that glacial-interglacial temperature fluctuations were smaller in the '41-kyr world' before the MPT, than during the '100-kyr world', after the MPT. At proximal ODP site 1123 (3290m water depth; Elderfield et al., 2012), interglacial deepwater temperatures increase by ~1°C after the MPT, with relatively constant glacial deepwater temperatures (~-2°C) over the last 1 Ma. New results from DSDP 593 therefore imply that the mechanisms that drive intermediate and deep water temperatures varied, suggesting that at least one of these watermasses has properties driven by something other than Northern Hemisphere glaciation patterns.

  6. Global variations in gravity-derived oceanic crustal thickness: Implications on oceanic crustal accretion and hotspot-lithosphere interactions (United States)

    Lin, J.; Zhu, J.


    We present a new global model of oceanic crustal thickness based on inversion of global oceanic gravity anomaly with constrains from seismic crustal thickness profiles. We first removed from the observed marine free-air gravity anomaly all gravitational effects that can be estimated and removed using independent constraints, including the effects of seafloor topography, marine sediment thickness, and the age-dependent thermal structure of the oceanic lithosphere. We then calculated models of gravity-derived crustal thickness through inversion of the residual mantle Bouguer anomaly using best-fitting gravity-modeling parameters obtained from comparison with seismically determined crustal thickness profiles. Modeling results show that about 5% of the global crustal volume (or 9% of the global oceanic surface area) is associated with model crustal thickness 8.6 km and is interpreted to have been affected by excess magmatism. The percentage of oceanic crustal volume that is associated with thick crustal thickness (>8.6 km) varies greatly among tectonic plates: Pacific (33%), Africa (50%), Antarctic (33%), Australia (30%), South America (34%), Nazca (23%), North America (47%), India (74%), Eurasia (68%), Cocos (20%), Philippine (26%), Scotia (41%), Caribbean (89%), Arabian (82%), and Juan de Fuca (21%). We also found that distribution of thickened oceanic crust (>8.6 km) seems to depend on spreading rate and lithospheric age: (1) On ocean basins younger than 5 Ma, regions of thickened crust are predominantly associated with slow and ultraslow spreading ridges. The relatively strong lithospheric plate at slow and ultraslow ridges might facilitate the loading of large magmatic emplacements on the plate. (2) In contrast, crustal thickness near fast and intermediately fast spreading ridges typically does not exceed 7-8 km. The relatively weak lithosphere at fast and intermediately fast ridges might make it harder for excess magmatism to accrete. We further speculate that the relatively wide partial melting zones in the upper mantle beneath the fast and intermediately fast ridges might act as "buffer" zones, thus diluting the melt anomalies from the underlying hotspots or regions of mantle heterogeneities. (3) As the crustal age increases and the lithospheric plate thickens, regions of thickened crust start to develop on ocean basins that were originally created at fast and intermediately fast ridges. The integrated crustal volume for fast and intermediately fast ocean crust appears to reach peak values for certain geological periods, such as 40-50 Ma and 70-80 Ma. The newly constructed global models of gravity-derived crustal thickness, combining with geochemical and other constraints, can be used to investigate the processes of oceanic crustal accretion and hotspot-lithosphere interactions.

  7. A case for Australia-Antarctica separation in the Neocomian (ca. 125 Ma) (United States)

    Stagg, H. M. J.; Willcox, J. B.


    The age of separation of Australia and Antarctica (that is, the onset of seafloor spreading) has usually been determined by the identification of seafloor-spreading magnetic anomalies adjacent to the margin, or by extrapolation of the spreading rate/time span between the oldest identified anomaly and the continent-ocean boundary (COB) as interpreted from magnetic and single-channel seismic data. The most recent estimate of the age of breakup using these methods is 95 ± 5 Ma, in the Cenomanian. However, identification of the oldest magnetic anomalies, formed during the early phase of slow drift between Australia and Antarctica, is tenuous, particularly between southwest Australia and the central Great Australian Bight (GAB). An alternative approach for establishing the age of breakup is to determine the relationship of the continental margin sequences and the oceanic crust, using seismic data. Application of this technique to the western Great Australian Bight demonstrates that it is likely that the oldest interpreted oceanic crust is overlain by several hundred metres of sediment that is no younger than Valanginian (ca. 125 Ma); that is, separation of Australia and Antarctica must have occurred prior to this time, perhaps contemporaneously with the separation of Australia and "Greater India" off Western Australia. The similarity of structural and magnetic anomaly trends between the southern and western margins of Australia further suggests that breakup and spreading on those two margins are closely linked. However, there may be further complicating factors in the Australian-Antarctic extension and breakup history. High-quality migrated seismic data from the central Great Australian Bight, across what has been previously interpreted as oceanic crust, show the presence of listric faulting and apparent syn-rift sedimentation within "basement". The gross structure is similar in appearance to published models of a metamorphic core complex. Other possibilities are that the observed structuring is the product of the re-rifting of oceanic crust of undetermined (pre-Valanginian) age, or that the zone oceanwards of the "COB" consists of highly thinned lower plate, probably injected by ribbons of oceanic basalt towards its southern edge. If such structures can also be identified in the western GAB, then it is clear that the age of the emplacement of the first oceanic crust between Australia and Antarctica is still undetermined.

  8. Phase separation in the crust of accreting neutron stars


    Horowitz, C. J.; Berry, D. K.; Brown, E. F.


    Nucleosynthesis, on the surface of accreting neutron stars, produces a range of chemical elements. We perform molecular dynamics simulations of crystallization to see how this complex composition forms new neutron star crust. We find chemical separation, with the liquid ocean phase greatly enriched in low atomic number elements compared to the solid crust. This phase separation should change many crust properties such as the thermal conductivity and shear modulus. The concen...

  9. Investigating the link between an iron-60 anomaly in the deep ocean's crust and the origin of the Local Bubble

    Energy Technology Data Exchange (ETDEWEB)

    Schulreich, Michael; Breitschwerdt, Dieter [Zentrum fuer Astronomie und Astrophysik, TU Berlin, Berlin (Germany)


    Supernova explosions responsible for the creation of the Local Bubble (LB) and its associated HI cavity should have caused geological isotope anomalies via deposition of debris on Earth. The discovery of a highly significant increase of {sup 60}Fe (a radionuclide that is exclusively produced in explosive nucleosynthesis) in layers of a deep sea ferromanganese crust corresponding to a time of 2.2 Myr before present, appears very promising in this context. We report on our progress in relating these measurements to the formation of the LB by means of 3D hydrodynamical adaptive mesh refinement simulations of the turbulent interstellar medium in the solar neighborhood. Our calculations are based on a sophisticated selection procedure for the LB's progenitor stars and take advantage of passive scalars for following the chemical mixing process.

  10. Absolute palaeointensity of Oligocene (28-30 Ma) lava flows from the Kerguelen Archipelago (southern Indian Ocean)

    CERN Document Server

    Plenier, G; Coe, R S; Perrin, M; Plenier, Guillaume; Camps, Pierre; Coe, Robert S.; Proxy, Mireille Perrin


    We report palaeointensity estimates obtained from three Oligocene volcanic sections from the Kerguelen Archipelago (Mont des Ruches, Mont des Tempetes, and Mont Rabouillere). Of 402 available samples, 102 were suitable for a palaeofield strength determination after a preliminary selection, among which 49 provide a reliable estimate. Application of strict a posteriori criteria make us confident about the quality of the 12 new mean-flow determinations, which are the first reliable data available for the Kerguelen Archipelago. The Virtual Dipole Moments (VDM) calculated for these flows vary from 2.78 to 9.47 10e22 Am2 with an arithmetic mean value of 6.15+-2.1 10e22 Am2. Compilation of these results with a selection of the 2002 updated IAGA palaeointensity database lead to a higher (5.4+-2.3 10e22 Am2) Oligocene mean VDM than previously reported, identical to the 5.5+-2.4 10e22 Am2 mean VDM obtained for the 0.3-5 Ma time window. However, these Kerguelen palaeointensity estimates represent half of the reliable Ol...

  11. Mafic granulite xenoliths in the Chilka Lake suite, Eastern Ghats Belt, India: evidence of deep-subduction of residual oceanic crust


    Bhattacharya, S.; Chaudhary, A. K.; Saw, A. K.; Das, P.; Chatterjee, D.


    Granulite xenoliths preserve key geochemical and isotopic signatures of their mantle source regions. Mafic granulite and pyroxinite xenoliths within massif-type charnockitic rocks from the Eastern Ghats Belt have recently been reported by us. The mafic granulite xenoliths from the Chilka Lake granulite suite with abundant prograde biotite are geochemically akin to Oceanic Island Basalt (OIB). They can be distinguished from the hornblende-mafic granulite xenoliths with signatures of Arc-derive...

  12. Fine Structure of Icelandic Crust: A Critical Re-evaluation of the Plume Hypothesis (United States)

    Foulger, G. R.; Du, Z. J.; Julian, B. R.


    Numerous seismic experiments have studied the crust beneath Iceland and the Greenland-Iceland-Faroe ridge. The upper crust is typically 7 +/- 1 km thick, with velocity (Vp) gradients typically 0.5 s-1. The lower crust is ~ 15 - 30 +/- 5 km thick, with a typical velocity gradient an order of magnitude lower than that of the upper crust. Its top is at the Vp ~ 6.5 km/s level. It commonly contains low-velocity zones (LVZ). A LVZ ~ 10,000 km2 in area, up to ~ 15 km thick occupies the lower crust beneath central Iceland, and may indicate a local high temperature gradient or small degree of partial melt. The crust-mantle boundary is a transition zone up to ~ 5 +/- 3 km thick throughout which average velocity increases from Vp ~ 7.2 to 8.0 km/s. It may be gradational or a stack of thin layers with alternating high and low velocities. It may represent a gradual transition from crustal to mantle rocks, with a progressive downward increase in the frequency of high-velocity layers e.g. dunite and harzburgite compared with lower-velocity crustal rocks. Such high-velocity basal crustal transition zones are probably a general characteristic of oceanic large igneous provinces (LIPs). The top of the upper mantle beneath Iceland may have a negative velocity gradient, indicating a high temperature gradient. Because the crust-mantle boundary is gradational, and temperature gradients apparently vary laterally, the seismological base of the crust is difficult to define meaningfully. Also the depth to any laterally-extensive seismological horizon is difficult to determine accurately because the crust contains LVZs and multiple reflectors. Best estimates suggest that the seismological crust is ~ 30 km thick beneath the Greenland-Iceland and Iceland-Faroe ridges, ~ 20 km beneath W Iceland, and ~ 40 km beneath central Iceland. Low attenuation and normal Vp/Vs ratios suggest that the crust is subsolidus and that the mantle is cooler than at equivalent depths beneath the East Pacific Rise. The constancy of Vp at the base of the crust implies that at a given depth the temperature is lowest where the crust is thickest, suggesting that central Iceland is underlain by a cold spot, not a hot spot. The pattern of crustal thickness over Iceland is not as expected for an eastwards-migrating plume, and provides no evidence for melt channeling or pulses of plume productivity, as are suggested to explain the elevation of the Reykjanes ridge and the existence of its flanking V-shaped ridges. Spreading in Iceland has probably proceeded along a double zone since ~ 26 Ma, with the westernmost zone persistently subsidiary. Seismic data from Iceland have been interpreted in terms of thin-hot and thick-cold crustal models, both of which have been cited in support of the plume hypothesis. This suggests that no conceivable crustal structure would be considered inconsistent with this hypothesis. Lateral density variations in the mantle are not required by gravity data when crustal low-velocity zones and variations in the thickness of the high-velocity crust-mantle transition zone are taken into account. The long-extinct Ontong-Java Plateau, northwest India and Parana, Brazil LIPs, where mantle plumes are not expected, are all underlain by upper-mantle low-velocity bodies similar to that beneath Iceland. A mantle plume interpretation for the upper-mantle anomaly beneath Iceland is thus probably non-unique.

  13. High-resolution geology, petrology and age of a tectonically accreted section of Paleoarchean oceanic crust, Barberton greenstone belt, South Africa (United States)

    Grosch, Eugene; Vidal, Olivier; McLoughlin, Nicola; Whitehouse, Martin


    The ca. 3.53 to 3.29 Ga Onverwacht Group of the Barberton greenstone belt (BGB), South Africa records a rare sequence of exceptionally well-preserved volcanic, intrusive and volcani-clastic Paleaoarchean rocks. Numerous conflicting models exist for the geologic evolution and stratigraphy of this early Archean greenstone belt, ranging from plume-type dynamics to modern-style plate tectonics. Although much work has focussed on the komatiites of the ca. 3.48 Ga Komati Formation since their discovery in 1969, far less petrological attention has been given to the younger oceanic rock sequences of the Kromberg type-section in the mid-Onverwacht Group. In this study, we present new field observations from a detailed re-mapping of the Kromberg type-section, and combine this with high-resolution lithological observations from continuous drill core of the Barberton Scientific Drilling Project [1]. The new mapping and field observations are compared to a recent preliminary study of the Kromberg type-section [2]. A U-Pb detrital provenance study was conducted on a reworked, volcani-clastic unit in the upper Kromberg type-section for the first time. This included U-Pb age determination of 110 detrital zircons by secondary ion microprobe analyses (SIMS), providing constraints on maximum depositional age, provenance of the ocean-floor detritus, and timing for the onset of Kromberg ocean basin formation. These new zircon age data are compared to a previous U-Pb detrital zircon study conducted on the structurally underlying sediments of the ca. 3.43 Ga Noisy formation [3]. A multi-pronged petrological approach has been applied to various rock units across the Kromberg, including thermodynamic modelling techniques applied to metabasalts and metapyroxenites for PT-estimates, bulk- and in-situ isotope geochemistry providing constraints on protolith geochemistry and metamorphic history. Consequently, it is shown that this previously poorly studied Kromberg oceanic rock sequence of the mid-Onverwacht, is a key area in resolving stratigraphy models and understanding the geologic evolution of the BGB. The combined field and petrological data provide new insight into mid-Paleoarchean ocean basin formation and subsequent tectonic destruction. [1]. Grosch et al., (2009b). EOS 90, 350-351. [2] Furnes et al., (2011) Precam. Research 186, 28-50. [3] Grosch et al. (2011) Precam. Research 191, 85-99.

  14. Core and early crust formation on Mars (United States)

    Golabek, G. J.; Keller, T.; Gerya, T.; Tackley, P. J.; Connolly, J.; Zhu, G.


    One of the most striking surface features on Mars is the crustal dichotomy. It is the oldest geological feature on Mars and was formed more than 4.1 Ga ago by either exogenic or endogenic processes [1,2]. In order to find an internal origin of the crustal dichotomy, located within a maximum of 400 Ma of planetary differentiation, the thermal state of the planet resulting from core formation needs to be considered. Additionally, it was suggested that a primordial crust with up to 45 km thickness can be formed already during the Martian core formation [3]. We suggest that the sinking of iron diapirs delivered by predifferentiated impactors induced impact- and shear heating-related temperature anomalies in the mantle that fostered the formation of early Martian crust. Thus, the crustal thickness distribution would largely be a result of planetary core formation, late impact history and the onset of mantle convection. To test this hypothesis we use numerical models to simulate the formation of the Martian iron core and the resulting mantle convection pattern, while peridotite melting is enabled to track melting caused by shear and radioactive heating. We perform 2D simulations using the spherical-Cartesian code I2ELVIS for planetary accretion and the spherical code STAGYY for the consequent onset of mantle convection. We apply a temperature-, stress- and melt-fraction dependent viscoplastic rheology. Radioactive and shear heating as well as consumption of latent heat by silicate melting are taken into account. The depth of neutral buoyancy of silicate melt with respect to solid silicates is determined by the difference in compressibility of the liquid and solid phase. To self-consistently simulate the silicate phase changes expected inside a Mars-sized body, we use the thermodynamical database Perple_X. As initial condition for core formation, we apply randomly distributed iron diapirs with 75 km radius inside the planet, representing the cores of stochastically distributed impactors. Additionally, we explore the effect of one giant impactor core on the planetary evolution. Results indicate that the presence of a large impactor core induces hemispherically asymmetrical core formation. The amplitude of shear heating anomalies often exceeds the solidus of primitive mantle material and thus, the formation of a considerable amount of silicate melt is observed. The resulting temperature field after core formation is then read into the mantle convection code STAYY. The hemispherical magma ocean induced by one late giant impactor favours a dichotomous crust formation during and shortly after core formation. Afterwards, the extraction of excess heat produced by the sinking of the giant impactor through the mantle leads to a localized region of massive magmatism, comparable to Tharsis, which is sustained during later evolution by a single plume forming beneath the province. The rest of the mantle is dominated by a sluggish convection pattern with limited crust formation that preserves the early formed dichotomous crustal structure until recent time. References [1] Nimmo, F. et al., Nature, 453, 1220-1223, 2008. [2] Keller, T. & Tackley, P.J., Icarus, 202, 429-443, 2009. [3] Norman, M.D., Meteorit. Planet. Sci., 34, 439-449, 1999.

  15. 60 Myr records of major elements and Pb-Nd isotopes from hydrogenous ferromanganese crusts: Reconstruction of seawater paleochemistry

    Digital Repository Service at National Institute of Oceanography (India)

    Frank, M.; O; Hein, J.R.; Banakar, V.K.


    The time series of major element geochemical and Pb- and Ni-isotopic composition obtained for seven hydrogenous ferromanganese crusts from the Atlantic, Indian, and Pacific Oceans which cover the last 60 Myr are compared. Average crust growth rates...

  16. Variability of low temperature hydrothermal alteration in upper ocean crust: Juan de Fuca Ridge and North Pond, Mid-Atlantic Ridge (United States)

    Rutter, J.; Harris, M.; Coggon, R. M.; Alt, J.; Teagle, D. A. H.


    Over 2/3 of the global hydrothermal heat flux occurs at low temperatures (< 150°) on the ridge flanks carried by fluid volumes comparable to riverine discharge. Understanding ridge flank hydrothermal exchange is important to quantify global geochemical cycles. Hydrothermal chemical pathways are complex and the effects of water-rock reactions remain poorly constrained. Factors controlling fluid flow include volcanic structure, sediment thickness, and basement topography. This study compares the effects of low temperature alteration in two locations with contrasting hydrogeological regimes. The intermediate spreading Juan de Fuca ridge flank (JdF) in the northeast Pacific sports a thick sediment blanket. Rare basement outcrops are sites of fluid recharge and discharge. The average alteration extent (~10% secondary minerals), oxidation ratio (Fe3+/FeTOT=34%), and alteration character (orange, green, grey halos) of basement is constant with crustal age and depth along a 0.97-3.6 m.yr transect of ODP basement holes. However, vesicle fills record an increasingly complex history of successive alteration with age. In contrast, North Pond, a ~8 m.yr-old sediment-filled basin at 22N on the slow spreading Mid Atlantic Ridge, hosts rapid, relatively cool SE to NW basinal fluid flow. Average alteration extent (~10%) and oxidation ratio (33%) of Hole 395A basalts are similar to JdF. However, 395A cores are dominated by orange alteration halos, lack celadonite, but have abundant zeolite. Vesicle fill combinations are highly variable, but the most common fill progression is from oxidising to less oxidising secondary assemblages. The comparable extent of alteration between these two sites and the absence of an age relationship on the JdF suggests that the alteration extent of the upper crust is uniform and mostly established by 1 Myr. However, the variable alteration character reflects the influence of regional hydrology on hydrothermal alteration.

  17. Crustal magnetization and accretion at the Southwest Indian Ridge near the Atlantis II fracture zone, 0-25 Ma (United States)

    Hosford, A.; Tivey, M.; Matsumoto, T.; Dick, H.; Schouten, Hans; Kinoshita, H.


    We analyze geophysical data that extend from 0 to 25-Myr-old seafloor on both flanks of the Southwest Indian Ridge (SWIR). Lineated marine magnetic anomalies are consistent and identifiable within the study area, even over seafloor lacking a basaltic upper crust. The full spreading rate of 14 km/Myr has remained nearly constant since at least 20 Ma, but crustal accretion has been highly asymmetric, with half rates of 8.5 and 5.5 km/Myr on the Antarctic and African flanks, respectively. This asymmetry may be unique to a ???400 km wide corridor between large-offset fracture zones of the SWIR. In contrast to the Mid-Atlantic Ridge, crustal magnetization amplitudes correlate directly with seafloor topography along the present-day rift valleys. This pattern appears to be primarily a function of along-axis variations in crustal thickness, rather than magnetic mineralogy. Off-axis, magnetization amplitudes at paleo-segment ends are more positive than at paleo-segment midpoints, suggesting the presence of an induced component of magnetization within the lower crust or serpentinized upper mantle. Alteration of the magnetic source layer at paleo-segment midpoints reduces magnetization amplitudes by 70-80% within 20 Myr of accretion. Magnetic and Ocean Drilling Program (ODP) Hole 735B data suggest that the lower crust cooled quickly enough to lock in a primary thermoremanent magnetization that is in phase with that of the overlying upper crust. Thus magnetic polarity boundaries within the intrusive lower crust may be steeper than envisioned in prior models of ocean crustal magnetization. As the crust ages, the lower crust becomes increasingly important in preserving marine magnetic stripes.

  18. Coupling, decoupling and metasomatism: Evolution of crust-mantle relationships beneath NW Spitsbergen (United States)

    Griffin, W. L.; Nikolic, N.; O'Reilly, Suzanne Y.; Pearson, N. J.


    The Bockfjord area of NW Spitsbergen (Norwegian Arctic) exposes a long history of crustal evolution, culminating in the Caledonian (400-500 Ma) orogeny; prior to the opening of the N. Atlantic Ocean, this area was part of the Laurentian (Greenland) side of the orogen. The N-striking Breibogen-Bockfjorden (BB) fault marks the western margin of a large graben filled with Devonian redbeds. West of the fault the basement consists of gneisses, schists and granites of the Hekla Hoek formation, inferred to represent a Caldeonian thrust sheet. U-Pb and Hf-isotope data for detrital zircons from this area show that the Hekla Hoek protoliths formed at ca 1.8 Ga, but were heavily reworked ca 800-1000 Ma ago, and again during the Caledonian orogeny. Quaternary alkali-basalt volcanism has provided abundant xenoliths of mantle and crustal rocks from both sides of the BB fault. Lower-crustal xenoliths are mainly mafic to intermediate granulites. Whole-rock Sr-, Nd- and Hf-isotope data for the granulites from both sides of the BB fault show significant disequilibrium, implying the removal of melts late in the evolution of the lower crust. Most zircons from eight xenoliths have Neoarchean/Paleoproterozoic and Paleozoic U-Pb ages; several also contain zircons with ages and/or Hf model ages > 3.2 Ga. The peridotite xenoliths are dominantly spinel lherzolites, metasomatized to varying extents. Xenoliths from basalts east of the BB fault commonly contain metasomatic amphibole, phlogopite and apatite; peridotites from west of the fault rarely display these phases. West of the fault, there is no clear correlation between cpx REE patterns and whole-rock Al contents; east of the fault there is a clear negative correlation between LREE enrichment and whole-rock Al2O3. In-situ Re-Os isotope analysis of sulfides in the peridotites shows another dichotomy. Xenoliths from west of the fault contain sulfides with Re depletion (TRD) model ages extending back to 3.3 Ga, with major populations at 2.4-2.6 Ga, 1.6-1.8 Ga and 1.2-1.3 Ga; the Caledonian orogeny is weakly represented. East of the BB fault the peaks in the TRD spectrum are at 900-1100 Ma and 400-500 Ma; only a few grains have TMA > 2.5 Ga. The data demonstrate a major disjunct, on both sides of the BB fault, between the Archean lower crust and a Proterozoic-Paleozoic upper crust; this suggests that the original Archean upper (and middle?) crust was detached and replaced by thrust sheets of younger material during the Caledonian orogeny. The striking differences in the sub-continental lithospheric mantle (SCLM) on either side of the BB fault suggest that major transcurrent movement has juxtaposed lithospheric sections that evolved independently. West of the fault, the presence of Archean lower crust overlying Archean SCLM suggests coupling of the crust and mantle for ? 3 Ga. East of the BB, similar Archean lower crust overlies an apparently younger, more fertile SCLM. We suggest that pervasive melt-related metasomatism refertilized the SCLM east of the BB fault, largely obscuring its Archean origin. Typical Archean SCLM is 150-250 km thick, whereas the Bockfjord volcanoes carry no samples from deeper than 80 km. The detached lower part of the Archean SCLM may now lie beneath the Gakkel Ridge to the NW, as proposed by Goldstein et al. (2008) on the basis of basalt geochemistry. This proposition is supported by the mean TRD of the Sverrefjell sulfides (1740 ± 718 Ma), which is similar to the oldest whole-rock TRD values for peridotites from the Gakkel Ridge (1.8-2.2 Ga; Liu et al., 2010).

  19. A unified history of the ocean around southern Africa (United States)

    Reeves, Colin; Master, Sharad


    The movement with respect to Africa of the hotspot marked by present-day Bouvet island is extrapolated backward in time to a position in the Lower Limpopo Valley at the time of the Karoo-Ferrar basalt event (183 Ma). In a tight reconstruction of the Precambrian fragments of Gondwana at this time, the triangular gap that remains between South Africa's Precambrian, that of Dronning Maud Land, Antarctica, and the eastward-extrapolated front of the Cape Fold Belt we fill with the Precambrian fragments of South Patagonia and the Falkland Islands. We postulate that the 183 Ma mantle upwelling produced a triple junction-type fracture marked by the alignments of the Lebombo, the SE margin of the Zimbabwe craton and the giant Botswana dyke swarm (178 Ma) that was rather quickly followed by the expulsion of the South Patagonia terranes from the Gondwana assembly along the alignment of the Falklands-Agulhas Fault Zone (FAFZ) as a transform margin. The space created was filled with igneous material akin to the present day Afar triangle. The magma supply generated not only oceanic crust but also overlying igneous deposits, much probably erupted subaerially. These developed progressively into the Falklands Plateau, the Mozambique Plains, the Mozambique Rise and the Explora Wedge of Antarctica. Not until the early Cretaceous did the growth of normal ocean crust start to exceed the ability of the declining mantle plume to cover the new ocean crust in a confined space with subaerial deposits that substantially thickened otherwise ‘oceanic' crust. When Antarctica and Africa began to separate before about 167 Ma, the future Mozambique Rise moved with Antarctica until, at about 125 Ma, a modest ridge reorganization east of Africa left Madagascar and the Mozambique Rise as part of the Africa Plate. An increasing westerly component to the movement of Antarctica against Africa preceded the initial opening of the South Atlantic and the fusing of the South Patagonia terranes with the bulk of South America. The triple junction jumped from off Cape St Lucia to immediately south of the Mozambique Rise as part of this 125 Ma reorganization and normal ocean crust grew from each of the three ridges emanating from it. The Agulhas Bank represents a reactivation of the mantle plume at about 100 Ma and the Maurice Ewing Bank and other submarine features east of the Falklands Plateau later and smaller ones. The model conforms with limited magnetic anomaly evidence in the oceans and the direction of preserved transforms before Anomaly 34 time (84 Ma). After Anomaly 34, events around the triple junction are well-defined by both magnetic anomalies and preserved transforms. The model may be demonstrated by a geometrically correct animation and offers simple solutions to a number of geological enigmas concerning (a) the Falkland Islands, (b) supposedly ‘continental' plateaus off South Africa, (c) the sudden ending of the Karoo igneous episode well before substantive Gondwana disruption and (d) the exotic nature of the Precambrian rocks of South Patagonia in the context of South American geology. The central role of the Bouvet mantle plume suggests that it has produced a volume of magma comparable with - and a longevity 50 myrs in excess of - that demonstrated by the Kerguelen plume.

  20. Late Triassic Batang Group arc volcanic rocks in the northeastern margin of Qiangtang terrane, northern Tibet: partial melting of juvenile crust and implications for Paleo-Tethys ocean subduction (United States)

    Zhao, Shao-Qing; Tan, Jun; Wei, Jun-Hao; Tian, Ning; Zhang, Dao-Han; Liang, Sheng-Nan; Chen, Jia-Jie


    The Batang Group (BTG) volcanic rocks in the Zhiduo area, with NW-trending outcrops along the northeastern margin of the Qiangtang terrane (northern Tibet), are mainly composed of volcaniclastic rocks, dacite and rhyolite. Major and trace element, Sr and Nd isotope, zircon U-Pb and Hf isotope data are presented for the BTG dacites. Laser ablation inductively coupled plasma mass spectrometry zircon U-Pb dating constrains the timing of volcanic eruption as Late Triassic (221 ± 1 Ma). Major and trace element geochemistry shows that the BTG volcanic rocks are classified as calc-alkaline series. All samples are enriched in large-ion lithophile elements and light rare earth elements with negative-slightly positive Eu anomalies (Eu/Eu* = 0.47-1.15), and depleted in high field strength elements and heavy rare earth elements. In addition, these rocks possess less radiogenic Sr [(87Sr/86Sr) i = 0.7047-0.7078], much radiogenic Nd (?Nd( t) = -4.2 to -1.3) and Hf (?Hf( t) = 4.0-6.6) isotopes, suggesting that they probably originated from partial melting of a crustal source containing a mantle-derived juvenile component. The inferred magma was assimilated by crustal materials during ascending and experienced significant fractional crystallization. By combining previously published and the new data, we propose that the BTG volcanic rocks were genetically related to southwestward subduction of the Ganzi-Litang ocean (a branch of Paleo-Tethys) in the northeastern margin of the Qiangtang terrane. Given the coeval arc-affinity magmatic rocks in the region, we envisage that the Ganzi-Litang ocean may extend from the Zhongdian arc through the Yidun terrane to the Zhiduo area, probably even further northwest to the Tuotuohe area.

  1. An alternative early opening scenario for the Central Atlantic Ocean (United States)

    Labails, C.; Roest, W. R.; Olivet, J.; Aslanian, D.


    The overall kinematic history of the Central Atlantic Ocean is reasonably well documented. Although published plate kinematic models are able to reproduce most of the broad scale features of the formation of the continental margins and the tectonic history, the initial breakup of Pangaea and the early evolution of the Central Atlantic Ocean are still debated. Here, we propose an alternative scenario for the early opening of the Central Atlantic Ocean. The new model is based on 1) recently published results which demonstrate that the opening of the Central Atlantic Ocean started as early as the Late Sinemurian (190 Ma) - i.e. about 15 myr earlier than most previous models and only 10 myr after of the Central Atlantic Magmatic Province (CAMP) event, 2) the identification of the African conjugate of the Blake Spur Magnetic Anomaly (BSMA) based on all available magnetic data and on the similarity in shape of its counterpart as well as on seismic data, 3) an analysis of the most recent geophysical data (including seismic lines, an interpretation of the newly compiled magnetic data and satellite derived gravimetry), and 4) an analysis of important tectonic events observed in the geology of Morocco, related to the opening of the Central Atlantic Ocean. The early opening history for the Central Atlantic Ocean can be described by three distinct phases spanning the time interval from the Late Sinemurian (190 Ma) to Chron M0 (125 Ma). During the initial breakup and the first 25 myr, from 190 Ma to the base Callovian (BSMA, 165 Ma), oceanic crust was formed during a significant oblique motion which differs drastically from previous studies and with a spreading rate which was significantly slower. In contrast to previous studies, which consider an eastern ridge jump at Blake Spur time (165 Ma), the newly identified African conjugate of BSMA suggests a plate reorganization phase both in the relative plate motion direction (from NNW-SSE to NW-SE) and in spreading rate. This hypothesis is strengthening by the observation of a significant change in basement topography at Blake Spur time. From Chron M22 (150 Ma) onwards, the spreading rate slowed down and remained fairly constant until Chron M0 (125 Ma). Finally, kinematic reconstructions show a significant and widespread spreading asymmetry, from the initial breakup to at least Chron M0. This spreading asymmetry is consistent along the entire ridge which seems to favor an interpretation related to an asymmetric thermal structure of the mantle underlying the separating plates rather than a succession of ridge jumps.

  2. Evidence of Himalayan erosional event at approx. 0.5 Ma from a sediment core from the equatorial Indian Ocean in the vicinityof ODP Leg 116 sites

    Digital Repository Service at National Institute of Oceanography (India)

    Nath, B.N.; Gupta, S.M.; Mislankar, P.G.; Rao, B.R.; Parthiban, G.; Roelandts, I.; Patil, S.K.


    in weathering environment and increased sediment ?uxes (Derryand France- Lanord, 1996). An additional erosional event of high energy (butlowerintensitythan0.8Ma)seemstohave occurred at 0.5Ma. To our knowledge, the 0.5Ma event is not reported in the literature... byeolian deposition in the deep sea: the geologic historyof wind. Reviews of Geophysics 32, 159–195. ARTICLE IN PRESS B.N. Nath et al. / Deep-Sea Research II 52 (2005) 2061–2077 2077 Nath, B.N., Rao, V.P., Becker, K.P., 1989. Geochemical evidence...

  3. Corium crust strength measurements

    International Nuclear Information System (INIS)

    Corium strength is of interest in the context of a severe reactor accident in which molten core material melts through the reactor vessel and collects on the containment basemat. Some accident management strategies involve pouring water over the melt to solidify it and halt corium/concrete interactions. The effectiveness of this method could be influenced by the strength of the corium crust at the interface between the melt and coolant. A strong, coherent crust anchored to the containment walls could allow the yet-molten corium to fall away from the crust as it erodes the basemat, thereby thermally decoupling the melt from the coolant and sharply reducing the cooling rate. This paper presents a diverse collection of measurements of the mechanical strength of corium. The data is based on load tests of corium samples in three different contexts: (1) small blocks cut from the debris of the large-scale MACE experiments, (2) 30 cm-diameter, 75 kg ingots produced by SSWICS quench tests, and (3) high temperature crusts loaded during large-scale corium/concrete interaction (CCI) tests. In every case the corium consisted of varying proportions of UO2, ZrO2, and the constituents of concrete to represent a LWR melt at different stages of a molten core/concrete interaction. The collection of data was used to assess the strength and stability of an anchored, plant-scale crust. The results indicate that such a crust is likely to be too weak to support itself a likely to be too weak to support itself above the melt. It is therefore improbable that an anchored crust configuration could persist and the melt become thermally decoupled from the water layer to restrict cooling and prolong an attack of the reactor cavity concrete.

  4. Is ash layer of ~ 8 Ma at ODP-758 from Bay of Bengal and Fe-Mn oxide coated pumice from Central Indian Ocean Basin are from same eruption ? (United States)

    Pattan, J. N.; Jumaila, C.; Ahmad, S. M.; Khedekar, V.; Parthiban, G.; Padmakumari, V.


    An ash layer of ~8 Ma old is recovered from the Bay of Bengal ODP 758 site. Glass shards from this ash layer have been studied for their morphology and chemical composition to trace its source. Earlier, in the Central Indian Ocean Basin occurrence of uncoated and Fe-Mn oxide coated pumice which are of younger (few thousand years) and older (few Ma) in age were reported. Glass grains were separated from the Fe-Mn oxide coated pumice and a studied for morphology and chemical composition. In the present study, an attempt has been made to compare both ash layer and glass from the pumice to trace their source and relation between them if any. Glass shards from the ash layer exhibit bubble wall type morphology and presence of pumice shards while, glass from pumice have pumice shards, suggesting magmatic type of eruption. Electron Probe Micro Analysis of glass shards and glass shows has high content of SiO2 (79%) and total alkalies (6%) indicating rhyolite nature and rich in K- cal-alkaline series. Potassium content in pumice is relatively low as compared to glass shards probably suggest leaching of K by the sea water interaction. Other major oxide contents (Al2O3, TiO2, FeO, MnO and MgO) are nearly similar in both shard and pumice glass. Total rare earth element (?REE) of glass shard (147 ppm) and glass (158 ppm) are nearly same, enriched in light REE, with a pronounced negative Eu-anomaly and exhibit flat heavy REE distribution. Tectnomagmatic and triangular plots suggest that both glass shards and glass are of arc type origin. The nearest Indonesian Arc is most probably acting as the source for both of them. Pumice in the present study has 3-4 mm thick Fe-Mn oxide coatings. In Central Indian Ocean Basin earlier report suggested that Fe-Mn oxide accumulates at ~ 2 mm/Ma. This indicates that pumice are of ~6-8 Ma old which is nearly similar in age (~8 Ma) of ash layer encountered at ODP site 758. Therefore, the present study suggest that ash layer of ~ 8 Ma old and Fe-Mn oxide coated pumice may probably correspond to a single eruption from Indonesian Arc Volcanism. However, precise dating of these pumice needs to be carried out for further confirmation.

  5. A ~400 ka supra-Milankovitch cycle in the Na, Mg, Pb, Ni, and Co records of a ferromanganese crust from the Vityaz fracture zone, central Indian ridge

    Directory of Open Access Journals (Sweden)

    R. Banerjee


    Full Text Available A ~400 ka (kilo years supra-Milankovitch cycle, recorded in the sodium, magnesium, lead, nickel and cobalt contents of a 32 mm thick ferromanganese crust from Vityaz fracture zone, central Indian ridge is reported here. To arrive at the geological ages, we used both 230Thexeccs and Co-chronometric datings. The correlation coefficient between the 230Thexeccs based dates and Co-chronometric dates for the top 0–8 mm is very high (r=0.9734, at 99.9% significance. The cobalt chronometric age for the bottom most oxide layer of this crust is computed as 3.5 Ma. Red-fit and multi-taper spectral analyses of time series data revealed the existence of the significant ~400 ka cycle, representing the changes in the hydrogeochemical conditions in the ocean due to the Earth's orbital eccentricity related summer insolation at the equator. This is the first report of such cycle from a hydrogenous ferromanganese crust from equatorial Indian ocean.

  6. Melting of continental crust during subduction initiation: A case study from the Chaidanuo peraluminous granite in the North Qilian suture zone (United States)

    Chen, Yuxiao; Song, Shuguang; Niu, Yaoling; Wei, Chunjing


    The Chaidanuo granite batholith is a ˜500 km2 homogeneous, high-level intrusion in the North Qilian oceanic suture zone. Three types of enclaves have been recognized, including (1) supracrustal xenoliths, (2) biotite gneiss that represents restite of the upper crustal melting, and (3) coeval mantle-derived mafic magmatic enclave (MME). The batholith consists dominantly of peraluminous biotite monzogranite with SiO2 69-73 wt.% and A/CNK 1.05-1.28 and shows geochemical affinity with the upper-continental crust, e.g., enrichment of large ion lithophile elements (LILEs; K, Rb, U, Th, Pb), depletion of high field strength elements (HFSEs; Nb, Ta, Ti), P, Eu, and Sr, and enriched Sr ([87Sr/86Sr]i, 0.731 to 0.744)-Nd (?Nd (t), -6.0 to -7.1) isotopes. Zircon U-Pb dating indicates that this batholith formed at 516-505 Ma, coeval with the MMEs (510 Ma), which represent the early stage of seafloor subduction in the North Qilian suture zone. The Hf isotopic composition of the MME (?Hf (t) = -6.0 to +2.1) is more depleted than that of the host granite (?Hf (t) = -14.8 to -4.7), suggesting mixing of mantle- and crust-derived melts. The inherited zircon cores in the host granite yield an age peak at ˜750 Ma with a few detritals of 591-1683 Ma. Zircons from a biotite gneiss enclave yield a protolith age of 744 Ma and a metamorphic/melting event at 503 Ma. Sr-Nd isotopic modeling reveals that the batholith was generated by melting of Neoproterozoic granitic rocks with the addition of 10-18% mantle-derived magma. Therefore, peraluminous granite can be produced by melting of crustal materials heated by mantle-derived magmas during the early stage of subduction initiation at the site of a prior passive continental margin.

  7. H11346_INTERP.SHP: Interpretation of Bottom Features from National Oceanic and Atmospheric Administration (NOAA) Survey H11346 of Edgartown Harbor, MA (Geographic, WGS84)

    U.S. Geological Survey, Department of the Interior — The U.S. Geological Survey (USGS), in cooperation with the National Oceanic and Atmospheric Administration (NOAA) and the Massachusetts Office of Coastal Zone...

  8. H11076_INTERP.SHP: Interpretations of Bottom Features from National Oceanic and Atmospheric Administration (NOAA) Survey H11076 of Quicks Hole, MA (Geographic)

    U.S. Geological Survey, Department of the Interior — The U.S. Geological Survey (USGS), in cooperation with the National Oceanic and Atmospheric Administration (NOAA) and the Massachusetts Office of Coastal Zone...

  9. H11077_INTERP.SHP: Interpretation of Bottom Features from National Oceanic and Atmospheric Administration (NOAA) Survey H11077 of Woods Hole, MA (Geographic)

    U.S. Geological Survey, Department of the Interior — The U.S. Geological Survey (USGS), in cooperation with the National Oceanic and Atmospheric Administration (NOAA) and the Massachusetts Office of Coastal Zone...

  10. Temperature distribution in the crust and mantle (United States)

    Jeanloz, R.; Morris, S.


    In an attempt to understand the temperature distribution in the earth, experimental constraints on the geotherm in the crust and mantle are considered. The basic form of the geotherm is interpreted on the basis of two dominant mechanisms by which heat is transported in the earth: (1) conduction through the rock, and (2) advection by thermal flow. Data reveal that: (1) the temperature distributions through continental lithosphere and through oceanic lithosphere more than 60 million years old are practically indistinguishable, (2) crustal uplift is instrumental in modifying continental geotherms, and (3) the average temperature through the Archean crust and mantle was similar to that at present. It is noted that current limitations in understanding the constitution of the lower mantle can lead to significant uncertainties in the thermal response time of the planetary interior.

  11. Deep Ocean Circulation and Nutrient Contents from Atlantic-Pacific Gradients of Neodymium and Carbon Isotopes During the Last 1 Ma (United States)

    Piotrowski, A. M.; Elderfield, H.; Howe, J. N. W.


    The last few million years saw changing boundary conditions to the Earth system which set the stage for bi-polar glaciation and Milankovich-forced glacial-interglacial cycles which dominate Quaternary climate variability. Recent studies have highlighted the relative importance of temperature, ice volume and ocean circulation changes during the Mid-Pleistocene Transition at ~900 ka (Elderfield et al., 2012, Pena and Goldstein, 2014). Reconstructing the history of global deep water mass propagation and its carbon content is important for fully understanding the ocean's role in amplifying Milankovich changes to cause glacial-interglacial transitions. A new foraminiferal-coating Nd isotope record from ODP Site 1123 on the deep Chatham Rise is interpreted as showing glacial-interglacial changes in the bottom water propagation of Atlantic-sourced waters into the Pacific via the Southern Ocean during the last 1 million years. This is compared to globally-distributed bottom water Nd isotope records; including a new deep western equatorial Atlantic Ocean record from ODP Site 929, as well as published records from ODP 1088 and Site 1090 in the South Atlantic (Pena and Goldstein, 2014), and ODP 758 in the deep Indian Ocean (Gourlan et al., 2010). Atlantic-to-Pacific gradients in deep ocean neodymium isotopes are constructed for key time intervals to elucidate changes in deep water sourcing and circulation pathways through the global ocean. Benthic carbon isotopes are used to estimate deep water nutrient contents of deep water masses and constrain locations and modes of deep water formation. References: Elderfield et al. Science 337, 704 (2012) Pena and Goldstein, Science 345, 318 (2014) Gourlan et al., Quaternary Science Reviews 29, 2484-2498 (2010)

  12. Oceanic plateau and island arcs of southwestern Ecuador: their place in the geodynamic evolution of northwestern South America (United States)

    Reynaud, Cédric; Jaillard, Étienne; Lapierre, Henriette; Mamberti, Marc; Mascle, Georges H.


    Coastal Ecuador is made up of an oceanic igneous basement overlain by Upper Cretaceous to Lower Paleocene (?98-60 Ma) volcaniclastic and volcanic rocks of island-arc affinities. The igneous basement, known as the Piñón Formation, locally dated at 123 Ma, consists of olivine-free basalts and dolerites. Relative to N-MORB, both types of rocks exhibit high concentrations in Nb (0.3-10.75 ppm), Ta (0.03-0.67 ppm), Th (0.11-1.44 ppm), light and medium rare earth elements, and low Zr (22-105 ppm) and Hf (0.59-2.8 ppm) contents, thus showing oceanic plateau basalts affinities. Most of these oceanic plateau basalts tholeiites display rather homogeneous ? Nd ( T = 123 Ma) ratios (˜+7), with the exception of two rocks with higher (+10) and lower (+4.5) ? Nd ( T = 123 Ma) , respectively. All these basalts plot, with one exception, within the ocean island basalts field. Their ( 87Sr/ 86Sr) i ratios are highly variable (0.7032-0.7048), probably due to hydrothermal oceanic alteration or assimilation of altered oceanic crust. The rocks of the Piñón Formation are geochemically similar to the oceanic plateau tholeiites from Nauru and Ontong Java Plateaus and to the Upper Cretaceous (92-88 Ma) Caribbean Oceanic Plateau lavas. The basalts and dolerites of the Upper Cretaceous-Lower Paleocene island arcs show calc-alkaline affinities. The ? Nd ratios (+6.1 to +7.1) of these arc-rocks are very homogenous and fall within the range of intra-oceanic island-arc lavas. The Upper Cretaceous-Lower Paleocene calc-alkaline and tholeiitic rocks from coastal Ecuador share similar high ? Nd ratios to Cretaceous intra-oceanic arc rocks from north, central and South America and from the Greater Antilles. Since the Piñón oceanic plateau tholeiites are locally overlain by early-Late Cretaceous sediments (˜98-83 Ma) and yielded locally an Early Cretaceous age, they do not belong to the Late Cretaceous Caribbean Oceanic Plateau. The basement of coastal Ecuador is interpreted as an accreted fragment of an overthickened and buoyant oceanic plateau. The different tectonic units of coastal Ecuador cannot be easily correlated with those of western Colombia, excepted the Late Cretaceous San Lorenzo and Ricaurte island arcs. It is suggested that northwestern South America consists of longitudinally discontinuous terranes, built by repeated accretionary events and significant longitudinal displacement of these terranes.

  13. Psoriasis or crusted scabies. (United States)

    Goyal, N N; Wong, G A


    We describe a case of a 67-year-old woman with a 1-year history of nail thickening and a non-itchy erythematous scaly eruption on the fingertips. She was diagnosed with psoriasis and started on methotrexate after having had no response to topical calcipotriol. The diagnosis was reviewed after it was revealed by another consultant that the patient's husband had been attending dermatology clinics for several years with chronic pruritus, which had been repeatedly thought to be due to scabies. Our patient was found to have crusted scabies after a positive skin scraping showed numerous mites. She was treated with topical permethrin, keratolytics and oral ivermectin. We also review the literature on crusted scabies and its management, with recommendations. PMID:18257840

  14. Buoyancy-driven, rapid exhumation of ultrahigh-pressure metamorphosed continental?crust


    Ernst, W. G.; Maruyama, S.; Wallis, S.


    Preservation of ultrahigh-pressure (UHP) minerals formed at depths of 90–125 km require unusual conditions. Our subduction model involves underflow of a salient (250 ± 150 km wide, 90–125 km long) of continental crust embedded in cold, largely oceanic crust-capped lithosphere; loss of leading portions of the high-density oceanic lithosphere by slab break-off, as increasing volumes of microcontinental material enter the subduction zone; buoyancy-driven return toward midcrustal levels of a...

  15. Primary estimation of forming date for carbonate weathering crust in Guizhou province

    International Nuclear Information System (INIS)

    The problem of directed dating of carbonate weathering crust in Guizhou Province hasn't been resolved. On the base of our previous study, we tested in detail the ages of antigenic quartz grains by fission track dating method and give a limitation of the forming date to carbonate weathering crust. The results show that the age of Xinpu profile is younger than 8.5 Ma, and the age of Guanba profile is younger than 7.3 Ma, and the age of Daxing profile is younger than 4.6 Ma. (authors)

  16. Hydroacoustic Monitoring of Oceanic Spreading Centers: Past, Present, and Future


    Dziak, Robert P; Bohnenstiehl, DelWayne R.; Deborah K. Smith


    Mid-ocean ridge volcanism and extensional faulting are the fundamental processes that lead to the creation and rifting of oceanic crust, yet these events go largely undetected in the deep ocean. Currently, the only means available to observe seafloor-spreading events in real time is via the remote detection of the seismicity generated during faulting or intrusion of magma into brittle oceanic crust. Hydrophones moored in the ocean provide an effective means for detecting these small-magnitude...

  17. A model of oceanic development by ridge jumping: Opening of the Scotia Sea (United States)

    Maldonado, Andrés; Bohoyo, Fernando; Galindo-Zaldívar, Jesús; Hernández-Molina, Fº. Javier; Lobo, Francisco J.; Lodolo, Emanuele; Martos, Yasmina M.; Pérez, Lara F.; Schreider, Anatoly A.; Somoza, Luis


    Ona Basin is a small intra-oceanic basin located in the southwestern corner of the Scotia Sea. This region is crucial for an understanding of the early phases of opening of Drake Passage, since it may contain the oldest oceanic crust of the entire western Scotia Sea, where conflicting age differences from Eocene to Oligocene have been proposed to date. The precise timing of the gateway opening between the Pacific and Atlantic oceans, moreover, has significant paleoceanographic and global implications. Two sub-basins are identified in this region, the eastern and western Ona basins, separated by the submarine relief of the Ona High. A dense geophysical data set collected during the last two decades is analyzed here. The data include multichannel seismic reflection profiles, and magnetic and gravimetric data. The oceanic basement is highly deformed by normal, reverse and transcurrent faults, as well as affected by deep intrusions from the mantle. The initial extension and continental thinning, with subsequent oceanic spreading, were followed by compression and thrusting. Several elongated troughs, bounded by faults, depict a thick sequence of depositional units in the basin. Eight seismic units are identified in a deep trough of the eastern Ona Basin. The deposits reach a thickness of 5 km, a consistent value not previously reported from the Scotia Sea. A body of chaotic seismic facies is also observed above the thinned continental crust of the Ona High. Magnetic seafloor anomalies older than C10 (~ 28.5 Ma) may be present in the region. The anomalies could include up to chron C12r (~ 32 Ma), although their identification is difficult, since the amplitude is subdued and the original oceanic crust was highly deformed by later faulting and thrusting. The magnetic anomaly distribution is not congruent with seafloor spreading from a single ridge. The basin plain is tilted and subducted southwestward below the South Shetland Islands Block, particularly in the western part, where an accretionary prism is identified. Such tectonics, locally affecting up to the most recent deposits, imply that a portion of the primitive oceanic crust is absent. Based on the stratigraphy of the deposits and the magnetic anomalies, an age of 44 Ma is postulated for the initiation of oceanic spreading in the eastern Ona basin, while spreading in the western Ona Basin would have occurred during the early Oligocene. The tectonics, depositional units and the age of the oceanic crust provide additional evidence regarding the Eocene opening of Drake Passage. The initial tectonic fragmentation of the South America-Antarctic Bridge, followed by oceanic spreading, was characterized by jumping of the spreading centers. An Eocene spreading center in the eastern Ona Basin was the precursor of the Scotia Sea. A model comprising four tectonic evolutionary phases is proposed: Phase I, Pacific subduction - Paleocene to middle Eocene; Phase II, eastern Ona back-arc spreading - middle to late Eocene; Phase III, ridge jumping and western Ona back-arc spreading - early Oligocene; and Phase IV, ridge jumping and West Scotia Ridge spreading - early Oligocene to late Miocene. The development of shallow gateways allowed for an initial connection between the Pacific and Atlantic oceans and, hence, initiated the thermal isolation of Antarctica during the middle and late Eocene. Deep gateways that enhanced the full isolation of Antarctica developed in Drake Passage from the Eocene/Oligocene transition onward. A significant correlation is observed between the tectonics, stratigraphic units and major climate events, thereby indicating the influence of the local tectonic and paleoceanographic events of the Southern Ocean on global evolution.

  18. Origin of the 'Gabbro' Signature in Ocean Island Basalts: Constraints from Osmium Isotopic Ratios of Galapagos Basalts (United States)

    Gibson, S. A.; Dale, C. W.; Geist, D.; Harpp, K. S.


    The Re-Os isotope system has become increasingly used as a tracer of lithological heterogeneity in the convecting mantle, with radiogenic 187Os/188Os in high-Os oceanic basalts and picrites widely interpreted as evidence of a melt contribution from ancient recycled oceanic crust. When combined with 206Pb/204Pb and O isotopes, 187Os/188Os ratios have been used to identify distinct lithological units (i.e. sediments, gabbros and basalts). We report new 187Os/188Os for basalts with high Os (>40 ppt) and MgO from Galápagos, which range from near primitive mantle values (0.130) to highly radiogenic (0.155). While co-variations in 187Os/188Os and 206Pb/204Pb for some Galápagos basalts (Floreana-type) are HIMU like, and consistent with melting of ancient recycled oceanic crust, others have variable 187Os/188Os ratios and primitive to depleted mantle like 206Pb/204Pb. Similar variations in Os and Pb isotopic space have been interpreted in other OIB suites as melts from recycled ancient oceanic gabbros, entrained by upwelling mantle plumes. Nevertheless, a marked east-west spatial variation in 187Os/188Os of Galápagos basalts does not correlate with postulated lithological variations in the Galápagos plume (Vidito et al., 2013). We show that basalts in eastern Galápagos with elevated 187Os/188Os and positive Sr anomalies occur in the vicinity of over-thickened 10 Ma gabbroic crust, that formed when the Galápagos plume was on-axis. We propose the elevated 187Os/188Os of Galápagos basalts are due to in-situ assimilation of young gabbroic lower crust, with high Re/Os, rather than melting of ancient recycled material in the Galápagos plume. In western Galápagos recent plume accreted crust is thick but more mafic, the melt flux higher and assimilation more sporadic. The contamination thresholds of Os and MgO in Galápagos basalts occur at higher contents than for many global OIBs (Azores, Iceland, Hawaii) and may reflect both a relatively low melt flux into the crust from the weak Galápagos plume (Tp=1400 oC) and excess thickness of ridge-formed gabbro in the east of the archipelago. Similar in-situ assimilation of lower oceanic crust by high-Os and MgO-rich OIBs suites may have been overlooked in the quest for establishing melting of ancient recycled oceanic gabbro in hotspots and heterogeneity in the convecting mantle.

  19. Continued Evidence for Input of Chlorine into the Martian Crust from Degassing of Chlorine-Rich Martian Magmas with Implications for Potential Habitability (United States)

    Filiberto, J.; Gross, J.


    The chlorine-concentration (or salinity) of a fluid affects the potential for that fluid to be a habitable environment, with most known terrestrial organisms preferring low salinity fluids [1, 2]. The Martian crust (as analyzed by the Gamma Ray Spectrometer) is chlorine-rich with up to 0.8 wt% Cl; while the MER rovers Spirit and Opportunity as well as MSL Curiosity have analyzed rocks with even higher chlorine concentrations [e.g., 3]. This suggests that any potential fluid flowing through the crust would have high chlorine concentrations and therefore high salinity. Here we investigate the bulk and mineral chemistry of the SNC meteorites to constrain the pre-eruptive chlorine concentrations of Martian magmas as the potential source of chlorine in the Martian crust. Bulk SNC meteorites have Cl concentrations similar to terrestrial Mid Ocean Ridge Basalts which would suggest a Cl content of the Martian interior similar to that of the Earth [4]. However, based on Cl/La ratios, the Martian interior actually has 2-3 times more Cl than the Earth [5]. This is also reflected in the composition of Cl-rich minerals within the SNC meteorites [5, 6] and suggests that the pre-eruptive parental magmas to the SNC meteorites were Cl-rich. Eruption and degassing of such Cl-rich magmas would have delivered Cl to the Martian crust, thereby increasing the salinity of any fluids within the crust. [1] Rothschild L.J. and R.L. Mancinelli (2001) Nature. 409: 1092-1101. [2] Sharp Z.D. and D.S. Draper (2013) EPSL. 369-370: 71-77. [3] Taylor G.J. et al. (2010) GRL. 37: L12204. [4]. Burgess R. et al (2013) GCA 77: 793. [5] Filiberto J. and A.H. Treiman (2009) Geology. 37: 1087-1090. [6] McCubbin F.M. et al. (2013) MaPS. 48: 819-853.

  20. Age and isotope evidence for the evolution of continental crust

    International Nuclear Information System (INIS)

    Irreversible chemical differentiation of the mantle's essentially infinite reservoir for at least the past 3800 Ma has produced new continental, sialic crust during several relatively short (ca. 100-300 Ma) episodes which were widely separated in time and may have been of global extent. During each episode (termed 'accretion-differentiation superevent'), juvenile sial underwent profound igneous, metamorphic and geochemical differentiation, resulting in thick (ca. 25-40 km), stable, compositionally gradational, largely indestructible, continental crust exhibiting close grouping of isotopic ages of rock formation, as well as mantle-type initial Sr and Pb isotopic ratios for all major constituents. Isotopic evidence suggests that within most accretion-differentiation superevents - and especially during the earlier ones - continental growth predominated over reworking of older sialic crust. Reworking of older sialic crust can occur in several types of geological environment and appears to have become more prevalent with the passage of geological time. It is usually clearly distinguishable from continental growth, by application of appropriate age and isotope data. (author)

  1. ConcepTest: Oceans by South America #6 (United States)

    Examine the image of part of the South American continent and neighboring oceans. (Image courtesy the National Geophysical Data Center.) Which sites are most likely to be located on oceanic crust? a. A, B, C, & ...

  2. Physics of Neutron Star Crusts

    Directory of Open Access Journals (Sweden)

    Chamel Nicolas


    Full Text Available The physics of neutron star crusts is vast, involving many different research fields, from nuclear and condensed matter physics to general relativity. This review summarizes the progress, which has been achieved over the last few years, in modeling neutron star crusts, both at the microscopic and macroscopic levels. The confrontation of these theoretical models with observations is also briefly discussed.

  3. High-precision TIMS U-Pb dating and SHRIMP trace element analyses of zircons from plutonic crust from ODP Hole 735B, Atlantis Bank, Southwest Indian Ridge (United States)

    Rioux, M. E.; Cheadle, M. J.; John, B. E.; Bowring, S. A.; Wooden, J. L.; Baines, G.


    Ocean Drilling Program Hole 735B at Atlantis Bank on the Southwest Indian Ridge is the deepest drill hole (1508m) into plutonic oceanic crust. The recovered core provides the opportunity to study both the processes and timescales of lower crustal accretion at a slow-spreading mid-ocean ridge. Major element chemistry suggests that the crust is made up of three 200-1000m thick igneous series (Natland and Dick, 2002). Previous SHRIMP U/Pb dating of zircons from oxide gabbro and felsic/dioritic dikes/veins from the length of the core found no resolvable age differences, suggesting rapid crustal growth; SHRIMP Th-corrected weighted mean 206Pb/238U dates ranged from 11.86 ± 0.20 to 12.13 ± 0.21 Ma (Baines et al., 2009). Here we report combined SHRIMP chemical analyses and high precision TIMS U-Pb geochronology on zircons from a suite of fifteen samples from depths of 26-1430 mbsf. The samples are from each of the three main intrusive series and range from oxide gabbro to diorite and granodiorite dikes/veins. Single grain TIMS 206Pb/238U date uncertainties for most analyses range from ~0.01-0.2 Ma and weighted mean 206Pb/238U date uncertainties range from ~0.004-0.07 Ma, providing precise constraints on the timing and duration of magmatism. Zircon chemistry is variable between samples (John et al., this meeting). Ti and Hf from spot analyses within individual samples range from tight clusters of data to linear trends of decreasing Ti with increasing Hf. Apparent Ti-in-zircon temperature variations within samples range from ~60-230°C, and variations within single grains are as large as 160°C. For zircons with significant chemical zoning, the cores are typically higher in Ti and lower in Hf than the rims. Th-corrected single grain 206Pb/238U dates from individual samples typically overlap within uncertainty, consistent with crystallization of a single batch of magma with no evidence for assimilation of older crust or protracted crystallization, as has been seen in high precision dates from the Mid-Atlantic Ridge and East Pacific Rise (Lissenberg et al., 2009; Rioux et al., 2012). However, two diorite dikes each contain populations of younger zircons with dates of ~11.9 Ma and a single older zircon with a date of ~12.4 Ma, suggesting that these magmas entrained zircons from older but so far unrecognized wall rocks. Resolvable age differences between the most precisely dated rocks suggest that the upper-two magmatic series (0-540 mbsf) under went final crystallization before the lowest series (540-1508 mbsf). Two precisely dated samples from the top two magmatic series have weighted mean 206Pb/238U dates of 12.00 ± 0.02 Ma and 11.96 ± 0.02 Ma. Six precisely dated samples from a range of depths in the deepest magmatic series all have younger weighted mean 206Pb/238U dates of 11.94 ± 0.02 to 11.91 ± 0.01 Ma. The current data do not show resolvable correlations between Th-corrected 206Pb/238U dates and zircon chemistry within individual samples.

  4. Evaluating Complex Magma Mixing via Polytopic Vector Analysis (PVA in the Papagayo Tuff, Northern Costa Rica: Processes that Form Continental Crust

    Directory of Open Access Journals (Sweden)

    Guillermo E. Alvarado


    Full Text Available Over the last forty years, research has revealed the importance of magma mixing as a trigger for volcanic eruptions, as well as its role in creating the diversity of magma compositions in arcs. Sensitive isotopic and microchemical techniques can reveal subtle evidence of magma mixing in igneous rocks, but more robust statistical techniques for bulk chemical data can help evaluate complex mixing relationships. Polytopic vector analysis (PVA is a multivariate technique that can be used to evaluate suites of samples that are produced by mixing of two or more magma batches. The Papagayo Tuff of the Miocene-Pleistocene Bagaces Formation in northern Costa Rica is associated with a segment of the Central American Volcanic Arc. While this segment of the arc is located on oceanic plateau, recent (<8 Ma ignimbrites bear the chemical signatures of upper continental crust, marking the transition from oceanic to continental crust. The Papagayo Tuff contains banded pumice fragments consistent with one or more episodes of mixing/mingling to produce a single volcanic deposit. The PVA solution for the sample set is consistent with observations from bulk chemistry, microchemistry and petrographic data from the rocks. However, without PVA, the unequivocal identification of the three end-member solution would not have been possible.

  5. Protracted construction of fast-spread gabbroic crust: Constraints from SHRIMP Pb/U dates and chemical analysis of zircon from Hess Deep (United States)

    Pettiette, R.; John, B. E.


    Core samples from ODP Hole 894G and IODP Hole 1415N provide the opportunity to study both the processes and timescales of crustal accretion of fast spread ocean crust initially formed at the East Pacific Rise. Here we report combined SHRIMP chemical analyses and U-Pb geochronology on zircon from a suite of ten samples from depths of 26-148 mbsf in ODP Hole 894G, and one sample from IODP Hole U1415N. U-Pb zircon dating of gabbro, olivine gabbro, gabbronorite and olivine gabbronorite (1-10% oxide) collected from Hole 894G yield 230Th-corrected 206U/238Pb ages from 1.090 to 1.311 Ma with errors as low as 2.5%. In the uppermost 40 m of Hole 894G the one zircon-bearing sample yielded a date of 1.211±0.041Ma (MSWD =0.07). The next five sample dates from 50-100mbsf are older, with a weighted mean age of 1.281±0.019 Ma (MSWD=1.19). In the deepest section between 100-150 mbsf, two zircon-bearing samples have a weighted mean age of 1.234±0.050Ma (MSWD=3.5). These changes in zircon age (at ~40 and 100 mbsf) coincide with marked petrologic, textural and geochemical variations. Together with petrologic observations, these Pb/U dates define 3 macro-units, each tens of meters thick, showing no systematic pattern of age downhole. The range of zircon solidus dates implies this ~120m (true thickness) of fast spread crust was constructed over a protracted period of time (? 70ka), as thin pulses of magma emplaced randomly with depth, via a 'piecemeal' mode of crustal accretion. The one sample of lower crust hosting zircon sampled from IODP Hole U1415N (olivine- and orthopyroxene-bearing gabbro w/ 0.5% oxide) yields a weighted mean solidus age of 1.321± 0.097Ma (MSWD 1.06). Zircon chemistry is variable between samples downhole. Ti and Hf concentrations from spot analyses within individual samples range from tight clusters to linear trends of decreasing Ti with increasing Hf. Measured Ti concentrations in all zircon from 894G vary from 5 to 50 ppm (mean 23 ppm); calculated Ti-in-zircon temperatures vary by sample (Tmean= 783°; Tmax= 864°; Tmin =680°), with intrasample temperature ranges from 40-165°C (DTmean= 120°C). Zircons from U1415N have measured Ti concentrations of 19-50 ppm (calculated Tmean= 829°; Tmax= 966°; Tmin=777°; DT=189°C), consistent with the more primitive chemistry of this lower crustal sample.

  6. Isotopically-diverse rhyolites coeval with the Columbia River Basalts Large Igneous Province: evidence for widespread mantle-plume driven hydrothermal alteration and remelting of the crust (United States)

    Colon, D.; Bindeman, I. N.; Stern, R. A.; Fisher, C. M.


    The formation of the most recent flood basalt province on Earth, the Columbia River Flood Basalts (CRBs) of the northwestern USA, was accompanied by eruptions of several thousand km3 of rhyolite in a short time window from 16.7 to 15 Ma. These rhyolites span from low (+1‰) to high (+11‰) in ?18O values as recorded by major phenocrysts, and alteration-resistant zircons within each rhyolite commonly display diversity of up to 6‰ ?18O, indicative of batch assembly prior to eruption. Significant variation in ?Hf also exists in zircons, ranging from -39 to 0 in rhyolites erupted through the North American cratonic crust, and from -1 to +9 in rhyolites erupted through accreted oceanic terranes to the east of the Sr87/86Sr = 0.706 line. This isotopic diversity cannot be accounted for by fractionation of a CRB-like parent magma, demonstrating that the syn-CRB rhyolites must have been derived from melting of the crust. Abundant low-?18Omelt values among syn-CRB rhyolites further constrains this crustal melting to shallow depths of 5-10 km, due to the shallow depths of the necessary hydrothermal alteration of the protolith. By contrast, high-?18O rhyolites must have been formed by remelting of sedimentary or metasedimentary rocks. Low-?18O rhyolites are also most common in the vicinity of the crustal suture between the thick lithosphere of the Archean craton and the thin lithosphere of the accreted terranes. Thermomechanical modeling suggests that this contrast concentrates crustal heating and deformation, creating pathways for meteoric water to penetrate the crust and cause extensive hydrothermal alteration less than 1 Ma before those same rocks remelt to form low-?18O rhyolites. Finally, we suggest that this extensive crustal hydrothermal alteration and melting may be typical of continental flood basalt provinces world wide, and particularly when there is syn-volcanic extension.

  7. Evolution of the earth's crust: Evidence from comparative planetology (United States)

    Lowman, P. D., Jr.


    Geochemical data and orbital photography from Apollo, Mariner, and Venera missions were combined with terrestrial geologic evidence to study the problem of why the earth has two contrasting types of crust (oceanic and continental). The following outline of terrestrial crustal evolution is proposed. A global crust of intermediate to acidic composition, high in aluminum, was formed by igneous processes early in the earth's history; portions survive in some shield areas as granitic and anorthositic gneisses. This crust was fractured by major impacts and tectonic processes, followed by basaltic eruptions analogous to the lunar maria and the smooth plains of the north hemisphere of Mars. Seafloor spreading and subduction ensued, during which portions of the early continental crust and sediments derived therefrom were thrust under the remaining continental crust. The process is exemplified today in regions such as the Andes/Peru-Chile trench system. Underplating may have been roughly concentric, and the higher radioactive element content of the underplated sialic material could thus eventually cause concentric zones of regional metamorphism and magmatism.

  8. Cretaceous volcanic rocks in south Qiangtang Terrane: Products of northward subduction of the Bangong-Nujiang Ocean? (United States)

    Li, Yalin; He, Juan; Wang, Chengshan; Han, Zhongpeng; Ma, Pengfei; Xu, Ming; Du, Kaiyuan


    The subduction of the Bangong-Nujiang Ocean is a long-standing problem in the study of the geological evolution of the Tibetan Plateau. Based on recent data acquired from the Abushan volcanic rocks in the Qiangtang Terrane, we will discuss the relationship between the Cretaceous volcanism and the evolution of the Bangong-Nujiang Ocean. The Abushan volcanic rocks are mainly composed of trachy-andesites, dacites, and rhyolites. Zircon U-Pb dating constrains the time of emplacement as the Middle Cretaceous (102.6 ± 1.6-96.1 ± 2.4 Ma). Major element geochemistry shows that the volcanic rocks belong to the high-K calc-alkaline and calc-alkaline series. All of the volcanic rocks are enriched in light rare earth elements (LREE) and some large ion lithophile elements (LILE, Rb, Th, and U) and are depleted in heavy rare earth elements (HREE) and some high field strength elements (HFSE, Nb, Ta, and Ti). The geochemical characteristics of the Abushan volcanic rocks are correlated with those of the volcanic arc rocks in the subduction zone. The petrogenesis of the Abushan volcanic rocks suggests that they were derived from the partial melting of the relict subducted oceanic crust combined with the input of oceanic sediments. Oceanic crust will melt upon reaching its hydrous solidus in the amphibolite facies after plate collision; this induces the formation of the Abushan volcanic rocks. Our data, compared with the previous studies on volcanism in the Lhasa Terrane, leads us to propose that the Bangong-Nujiang Ocean crust was subducted beneath the Qiangtang Terrane and underwent bidirectional subduction during the Cretaceous.

  9. Color characterization of Arctic Biological Soil Crusts (United States)

    Mele, Giacono; Gargiulo, Laura; Ventura, Stefano


    Global climate change makes large areas lacking the vegetation coverage continuously available to primary colonization by biological soil crusts (BSCs). This happens in many different environments, included high mountains and Polar Regions where new areas can become available due to glaciers retreat. Presence of BSCs leads to the stabilization of the substrate and to a possible development of protosoil, with an increase of fertility and resilience against erosion. Polar BSCs can exhibit many different proportions of cyanobacteria, algae, microfungi, lichens, and bryophytes which induce a large variability of the crust morphology and specific ecosystem functions. An effective and easy way for identifying the BSCs in the field would be very useful to rapidly recognize their development stage and help in understanding the overall impact of climate change in the delicate polar environments. Color analysis has long been applied as an easily measurable physical attribute of soil closely correlated with pedogenic processes and some soil functions. In this preliminary work we used RGB and CIE-L*a*b* color models in order to physically characterize fourteen different BSCs identified in Spitsbergen island of Svalbard archipelago in Arctic Ocean at 79° north latitude. We found that the "redness parameter "a*" of CIE-L*a*b* model was well correlated to the succession process of some BSCs at given geomorphology condition. Most of color parameters showed, moreover, a great potential to be correlated to photosynthetic activity and other ecosystem functions of BSCs.

  10. Crust Formation in Aluminum Cells (United States)

    Oedegard, R.; Roenning, S.; Rolseth, S.; Thonstad, J.


    This paper examines the catalytic effects offlourides on the ???-Al2O3 phase transformation by heat treating commercial alumina samples with 2wt% additions of different flouride compounds. The various additives were ranked according to their effect on transformation temperature. Experiments were conducted to explain the high temperature coherence of crusts. The findings indicate that an alumina network is formed during ??? phase transformation, which reinforces the crust on top of the cryolite bath.

  11. Seismic reflection images of a near-axis melt sill within the lower crust at the Juan de Fuca ridge. (United States)

    Canales, J Pablo; Nedimovi?, Mladen R; Kent, Graham M; Carbotte, Suzanne M; Detrick, Robert S


    The oceanic crust extends over two-thirds of the Earth's solid surface, and is generated along mid-ocean ridges from melts derived from the upwelling mantle. The upper and middle crust are constructed by dyking and sea-floor eruptions originating from magma accumulated in mid-crustal lenses at the spreading axis, but the style of accretion of the lower oceanic crust is actively debated. Models based on geological and petrological data from ophiolites propose that the lower oceanic crust is accreted from melt sills intruded at multiple levels between the Moho transition zone (MTZ) and the mid-crustal lens, consistent with geophysical studies that suggest the presence of melt within the lower crust. However, seismic images of molten sills within the lower crust have been elusive. Until now, only seismic reflections from mid-crustal melt lenses and sills within the MTZ have been described, suggesting that melt is efficiently transported through the lower crust. Here we report deep crustal seismic reflections off the southern Juan de Fuca ridge that we interpret as originating from a molten sill at present accreting the lower oceanic crust. The sill sits 5-6 km beneath the sea floor and 850-900 m above the MTZ, and is located 1.4-3.2 km off the spreading axis. Our results provide evidence for the existence of low-permeability barriers to melt migration within the lower section of modern oceanic crust forming at intermediate-to-fast spreading rates, as inferred from ophiolite studies. PMID:19571883

  12. Crustal thickening prior to 220 Ma in the East Kunlun Orogenic Belt: Insights from the Late Triassic granitoids in the Xiao-Nuomuhong pluton (United States)

    Xia, Rui; Wang, Changming; Deng, Jun; Carranza, Emmanuel John M.; Li, Wenliang; Qing, Min


    The East Kunlun Orogenic Belt (EKOB) played an important role in plate tectonics, magma generation, and crustal evolution. Late Triassic granodiorites and their mafic micro-granular enclaves (MMEs) from Xiao-Nuomuhong in the EKOB were studied for geochemistry and geochronology to constrain their petrogenesis. Zircon LA-ICP-MS dating indicates that the Xiao-Nuomuhong granodiorites are coeval with their MMEs (?222 Ma). The granodiorites are high-K calc-alkaline rocks that are enriched in Rb, Th, U and LREE, and depleted in Cr, Ni and HFSE, with high Sr/Y ratios (82.2-85.3) and geochemically resemble the lower crust-derived adakites. The MMEs are also high-K calc-alkaline rocks, with high Al2O3 (16.8-18.8 wt.%), low Mg# (30-40), Nb, Zr and Hf, with weak negative Eu anomalies (Eu/Eu# = 0.8-0.9). We suggest the MMEs are mafic magmatic globules that were injected into the felsic host magma. The adakitic rocks from the Xiao-Nuomuhong pluton were generated by partial melting of thickened crust, while the primitive compositions of the MMEs were most likely from the lithospheric mantle beneath the EKOB. The Late Triassic Xiao-Nuomuhong pluton is important evidence that crustal thickening in the EKOB occurred prior to 220 Ma. The pluton is interpreted as the result of mixing between thickened lower crust-derived melts and lithospheric mantle-derived mafic melts and the protracted magmatic response to the break-off of the Paleo-Tethys oceanic slab at ?232 Ma.

  13. Tectonic History of the Amerasia Basin, Arctic Ocean (United States)

    Grantz, A.; Hart, P. E.; Childers, V. A.


    Seismic reflection, refraction and potential field data from Amerasia Basin in conjunction with piston cores from Northwind Ridge suggest that the basin was formed by four rotational extensions. The first event stretched and thinned Pangea on a westerly-dipping crustal scale detachment fault system of Sinemurian to no later than Early Hauterivian age that day-lighted on the east along the continental margin of Northwestern Canada. This event rotated Eastern Siberia about 50° anticlockwise from Northwest Canada about a pole in the lower Mackenzie Valley and created transitional crust, which lacks seafloor spreading magnetic anomalies, beneath the marginal areas of the present Amerasia Basin. The second event, 9° or 10° of anticlockwise rotational seafloor spreading, split the earlier-formed transitional crust and emplaced MORB (mid-ocean-ridge basalt) along a northerly trending symmetry axis in the center of the Amerasia Basin. The resultant fan of magnetic anomalies, estimated to be of Late Hauterivian to Late Barremian age (136-125 Ma), is geometrically symmetrical with the first spreading event and likewise converges toward a pole of rotation in the lower Mackenzie Valley. Approximately 45° of clockwise rotation of Chukchi Microplate out of the East Siberian shelf about a pole near 72.5° N, 170° W constitutes the third rotational event, which probably occurred during or shortly following the Late Barremian. This event thrust the northeastern corner of the Chukchi Microplate across the boundary between event 1 and event 2 crusts in the western Canada Basin and created North Chukchi Basin in its wake. North Chukchi Basin is partially filled with post-Barremian to Early Campanian oceanic basalts of the Alpha-Mendeleev Large Igneous Province (125-80 Ma). In the absence of well-defined aeromagnetic anomalies or crustal-scale reflection data we can only speculate, on the basis of morphology, that North Chukchi Basin is a product of localized rotational seafloor spreading The fourth event, mildly rotational Paleocene extension, created basin and range structural morphology and the northerly-trending Northwind Basin in the axial region of the Chukchi Microplate. This extension thinned the continental crust of the microplate beneath the Northwind Basin by about 35 percent and created accommodation space for >2,000 m of water and >4,500 m of clastic sediment within the basin. The pole of this rotation was apparently located on the central Chukchi shelf. Following the four extensional events the southeastern margin of the Amerasia Basin was subject to far-field convergence of Middle Eocene to Quaternary age that appears to have originated at the Pacific Rim and created large thrust-related detachment folds that may be significant for hydrocarbon exploration.

  14. Eocene deep crust at Ama Drime, Tibet : Early evolution of the Himalayan orogen

    DEFF Research Database (Denmark)

    Kellett, Dawn; Cottle, John


    Granulitized eclogite-facies rocks exposed in the Ama Drime Massif, south Tibet, were dated by Lu-Hf garnet geochronology. Garnet from the three samples analyzed yielded Lu-Hf ages of 37.5 ± 0.8 Ma, 36.0 ± 1.9 Ma, and 33.9 ± 0.8 Ma. Eclogitic garnet growth is estimated at ca. 38 Ma, the oldest age for burial of the lower Indian crust beneath Tibet reported from the central-eastern Himalaya. Granulite-facies overprinting followed at ca. 15–13 Ma, as indicated by U-Pb zircon ages. Unlike ultrahigh-pressure eclogites of the northwest Himalaya, the Ama Drime eclogites are not characteristic of rapid burial and exhumation of a cold subducted slab. The rocks instead resulted from crustal thickening during the early stages of continental collision, and resided in the lower-middle crust for >20 m.y. before they were exhumed and reheated. These new data provide solid evidence for the Indian crust having already reached at least ?60 km thickness by the late Eocene.

  15. Accreted oceanic terranes in Ecuador: Southern edge of the Caribbean plate ?


    Jaillard, Etienne; Lapierre, Henriette; Ordonez, Martha; Toro Alava, Jorge; Amortegui, Andrea; Vanmelle, Jeremie


    The western part of Ecuador is made of several oceanic terranes, which comprise two oceanic plateaus, of Early (? 120 Ma), and Late Cretaceous age (? 90 Ma), respectively. The older oceanic plateau was accreted to the andean margin in the Late Campanian (? 75 Ma). Fragments of the Turonian-Coniacian plateau were accreted to the ecuadorian margin in the Late Maastrichtian (? 68 Ma, Guaranda terrane) and Late Paleocene (? 58 Ma, Piñón-Naranjal terrane). The Guaranda terrane received...

  16. Galenicals in the treatment of crusted scabies

    Directory of Open Access Journals (Sweden)

    Sugathan P


    Full Text Available Crusted scabies is rare. It is a therapeutic challenge, as the common drugs used against scabies are unsatisfactory. The successful use of galenicals in a 10-year-old girl with crusted scabies is reported.

  17. Galenicals in the treatment of crusted scabies. (United States)

    Sugathan, P; Martin, Abhay Mani


    Crusted scabies is rare. It is a therapeutic challenge, as the common drugs used against scabies are unsatisfactory. The successful use of galenicals in a 10-year-old girl with crusted scabies is reported. PMID:20606896

  18. CHIC - Coupling Habitability, Interior and Crust (United States)

    Noack, Lena; Labbe, Francois; Boiveau, Thomas; Rivoldini, Attilio; Van Hoolst, Tim


    We present a new code developed for simulating convection in terrestrial planets and icy moons. The code CHIC is written in Fortran and employs the finite volume method and finite difference method for solving energy, mass and momentum equations in either silicate or icy mantles. The code uses either Cartesian (2D and 3D box) or spherical coordinates (2D cylinder or annulus). It furthermore contains a 1D parametrised model to obtain temperature profiles in specific regions, for example in the iron core or in the silicate mantle (solving only the energy equation). The 2D/3D convection model uses the same input parameters as the 1D model, which allows for comparison of the different models and adaptation of the 1D model, if needed. The code has already been benchmarked for the following aspects: - viscosity-dependent rheology (Blankenbach et al., 1989) - pseudo-plastic deformation (Tosi et al., in preparation phase) - subduction mechanism and plastic deformation (Quinquis et al., in preparation phase) New features that are currently developed and benchmarked include: - compressibility (following King et al., 2009 and Leng and Zhong, 2008) - different melt modules (Plesa et al., in preparation phase) - freezing of an inner core (comparison with GAIA code, Huettig and Stemmer, 2008) - build-up of oceanic and continental crust (Noack et al., in preparation phase) The code represents a useful tool to couple the interior with the surface of a planet (e.g. via build-up and erosion of crust) and it's atmosphere (via outgassing on the one hand and subduction of hydrated crust and carbonates back into the mantle). It will be applied to investigate several factors that might influence the habitability of a terrestrial planet, and will also be used to simulate icy bodies with high-pressure ice phases. References: Blankenbach et al. (1989). A benchmark comparison for mantle convection codes. GJI 98, 23-38. Huettig and Stemmer (2008). Finite volume discretization for dynamic viscosities on Voronoi grids. PEPI 171(1-4), 137-146. King et al. (2009). A Community Benchmark for 2D Cartesian Compressible Convection in the Earth's Mantle. GJI 179, 1-11. Leng and Zhong (2008). Viscous heating, adiabatic heating and energetic consistency in compressible mantle convection. GJI 173, 693-702.

  19. Relamination and the Differentiation of Continental Crust (United States)

    Hacker, B. R.; Kelemen, P. B.; Behn, M. D.


    Most immature crust must be refined to attain the composition of mature continental crust. This refining may take the form of weathering, delamination, or relamination. Although delamination and relamination both call upon gravity-driven separation of felsic rock into the crust and mafic rock into the mantle, delamination involves foundering of rock from the base of active magmatic arcs, whereas relamination involves the underplating/diapirism of subducted sediment, arc crust, and continent crust to the base of the crust in any convergence zone. Relamination may be more efficient than lower crustal foundering at generating large volumes of material with the major- and trace-element composition of continental crust, and may have operated rapidly enough to have refined the composition of the entire continental crust over the lifetime of Earth. If so, felsic rocks could form much of the lower crust, and the bulk continental crust may be more silica rich than generally considered. Seismic wavespeeds require that only ~10-20% of the lowermost 5-15 km of continental crust must be mafic; combined heat-flow and wavespeed constraints permit continental lower crust to have 50 to 65 wt% SiO2.

  20. Identification of hyper-extended crust east of Davie Ridge in the Mozambique Channel (United States)

    Klimke, Jennifer; Franke, Dieter


    Davie Ridge is a ~1200 km wide, N-S trending bathymetrical high in the Mozambique Channel. Today, it is widely accepted that Davie Ridge is located along a fossil transform fault that was active during the Middle Jurassic and Early Cretaceous (~165-120 Ma). This transform fault results from the breakup of Gondwana, when Madagascar (together with India and Antarctica) drifted from its northerly position in the Gondwana Supercontinent (adjacent to the coasts of Tanzania, Somalia and Kenya) to its present position (e.g. Coffin and Rabinowitz, 1987; Rabinowitz et al., 1983; Segoufin and Patriat, 1980). The southward motion of Madagascar relative to Africa is constrained by the interpretation of magnetic anomalies in the Western Somali Basin, located north of Madagascar (e.g. Rabinowitz et al., 1983). According to Bird (2001), sheared margins share typical characteristics and a common evolution: 1. The transition from continental to oceanic crust is relatively abrupt (~ 50-80 km). 2. Along the continental side of the margin, complex rift basins form that display a wide range of faults. 3. Prominent marginal ridges form along the sheared margin that probably originate from the propagation of the oceanic spreading center along the plate boundary (Bird, 2001). In February and March 2014, a dense geophysical dataset (multichannel seismic, magnetics, gravimetry and bathymetry) with a total of 4300 profile km along the sheared margin was acquired with the R/V Sonne by the Federal Institute for Geosciences and Natural Resources (BGR). A special objective of the project, amongst others, is the characterization and interpretation of the continent-ocean transition seaward of Davie Ridge in the Mozambique Channel. Seismic profiles located east of Davie Ridge in the Western Somali Basin reveal a wide sequence of half-grabens bounded by listric normal faults. We tentatively suggest that this crust is of continental origin and results from rifting between Africa and Madagascar during the breakup of Gondwana. This implies that the continent-ocean transition is located at least ~ 150 km east of Davie Ridge. References Bird, D., 2001. Shear margins: Continent-ocean transform and fracture zone boundaries. The Leading Edge, 150-159. Coffin, M. F., und Rabinowitz, P. D., 1987. Reconstruction of Madagascar and Africa: Evidence from the Davie Fracture Zone and Western Somali Basin. Journal of Geophysical Research: Solid Earth, vol. 92, no. B9, 9385-9406. Rabinowitz, P.D., Coffin, M.F. and Falvey, D.A., 1983. The separation of Madagascar and Africa. Science 220, 67-69. Segoufin, J., und Patriat, P., 1980. Existence d'anomalies mesozoiques dans le bassin de Somalie. Implications pour les relations Afrique-Antarctique-Madagascar: C.R. Acad. Sci. Paris, v. 291, p. 85-88.

  1. Continental crust subducted deeply into lithospheric mantle: the driving force of Early Carboniferous magmatism in the Variscan collisional orogen (Bohemian Massif) (United States)

    Janoušek, Vojt?ch; Schulmann, Karel; Lexa, Ondrej; Holub, František; Fran?k, Jan; Vrána, Stanislav


    The vigorous Late Devonian-Early Carboniferous plutonic activity in the core of the Bohemian Massif was marked by a transition from normal-K calc-alkaline, arc-related (~375-355 Ma), through high-K calc-alkaline (~346 Ma) to (ultra-)potassic (343-335 Ma) suites, the latter associated with mainly felsic HP granulites enclosing Grt/Spl mantle peridotite bodies. The changing chemistry, especially an increase in K2O/Na2O and 87Sr/86Sri with decrease in 143Nd/144Ndi in the basic end-members, cannot be reconciled by contamination during ascent. Instead it has to reflect the character of the mantle sources, changing over time. The tectonic model invokes an oceanic subduction passing to subduction of the attenuated Saxothuringian crust under the rifted Gondwana margin (Teplá-Barrandian and Moldanubian domains). The deep burial of this mostly refractory felsic metaigneous material is evidenced by the presence of coesite/diamond (Massonne 2001; Kotková et al. 2011) in the detached UHP slices exhumed through the subduction channel and thrusted over the Saxothuringian basement, and by the abundance of felsic HP granulites (> 2.3 GPa), some bearing evidence for small-scale HP melt separation, in the orogen's core (Vrána et al. 2013). The subduction channel was most likely formed by 'dirty' serpentinites contaminated by the melts/fluids derived from the underlying continental-crust slab (Zheng 2012). Upon the passage through the orogenic mantle, the continental crust-slab derived material not only contaminated the adjacent mantle forming small bodies/veins of pyroxenites (Becker 1996), glimmerites (Becker et al. 1999) or even phlogopite- and apatite-bearing peridotites (Naemura et al. 2009) but the felsic HP-HT granulites also sampled the individual peridotite types at various levels. Eventually the subducted felsic material would form an (U)HP continental wedge under the forearc/arc region, to be later redistributed under the Moldanubian crust by channel flow and crustal relamination mechanisms. The presence of refractory light material rich in radioactive elements under the denser upper plate would eventually result in gravity-driven overturns in the thickened crust. The contaminated lithospheric mantle domains yielded, soon thereafter, ultrapotassic magmas whose major- and compatible-trace element signatures point to equilibration with the mantle peridotite, while their LILE contents and radiogenic isotope signatures are reminiscent of the subducted continental crust. This research was financially supported by the GA?R Project P210-11-2358 (to VJ) and Ministry of Education of the Czech Republic program LK11202 (to KS). Becker, H. 1996. Journal of Petrology 37, 785-810. Kotková, J. et al. 2011. Geology 39, 667-670. Massonne, H.-J. 2001. European Journal of Mineralogy 13, 565-570. Naemura, K. et al. 2009. Journal of Petrolology 50, 1795-1827. Schulmann, K., et al., 2014. Geology, in print. Vrána, S. 2013. Journal of Geosciences 58, 347-378. Zheng, Y. F. 2012. Chemical Geology 328, 5-48.

  2. Opening of the Gulf of Mexico and the Nature of the Crust in the Deep Gulf: New Evidence from Seafloor Spreading Magnetic Anomalies (United States)

    Harry, D. L.; Eskamani, P. K.


    The seafloor spreading history in the Gulf of Mexico is poorly constrained due to a lack of recognized seafloor spreading magnetic anomalies, a paucity of deep penetrating seismic data, and absence of drilling to constrain crystalline ocean floor composition and ages. We have identified lineated magnetic anomalies in the eastern Gulf on profiles collected during the Woods Hole R/V Farnella FRNL85-2 cruise that correlate with magnetic chrons M21R to M10. Forward modeling shows that these anomalies formed during creation of weakly magnetized new seafloor in the eastern Gulf between 149-134 Ma at an average half-spreading rate of 3.2 cm/yr. The oldest anomalies are located against stretched continental crust beneath the western Florida shelf on the east and the Yucatan shelf on the west. The youngest anomalies form a juxtaposed conjugate pair that mark the location of an extinct spreading ridge between Yucatan and Florida. Seismic velocities of the crust in the eastern Gulf and the amplitude of the magnetic anomalies are similar to the Iberian and Newfoundland rifted margins, where the early stages of continental breakup were accommodated by exhumation of subcontinental lithosphere rather than creation of new basaltic oceanic crust. We infer that the eastern Gulf of Mexico is underlain by exhumed sub-continental peridotitic mantle intruded by lesser volumes of basaltic igneous rocks generated by decompression melting of the asthenosphere during the late stages of opening of the Gulf. The long wavelength characteristics of the magnetic and gravity fields in the eastern Gulf, as well as the seismic velocity structure of the crust, differ from those in the central and western Gulf, which are more similar to typical magmatic rifted margins. This suggests that the character of the Gulf changes along strike, from a magmatic western portion to an amagmatic eastern portion. Paleogeographic restoration of the lineated magnetic anomaly pattern suggests a 4-phase model for opening of the Gulf. During phase 1 (Early Permian-Late Triassic), Yucatan and associated tectonic blocks that now comprise eastern Mexico were translated eastward from the Pacific realm into positions near the modern western Gulf. During phase 2 (Late Triassic-ca. 160 Ma) Yucatan and the South Florida block were translated southeastward relative to North America, rotating 6.7? counterclockwise about a pole located at 34?N, 74?W. This resulted in ca. 430 km of southeastward extension on the North American coastal plain, 120 km of southward extension on the northern Yucatan shelf, and displacement of the South Florida Block from a pre-rift position on the northwest Florida shelf to its modern position. During phase 3 (ca. 160-149 Ma), Yucatan rotated counterclockwise 46? relative to North America about a pole located at 27.6?N, 84.0?W. Phase 3 may have coincided with seafloor spreading in the central and western Gulf, but predated seafloor spreading in the eastern Gulf. During phase 4 (149-134 Ma), Yucatan moved southwestward relative to North America, rotating counterclockwise 2.2? about a pole located at 17.6?N, 74.2?W and completing opening of the Gulf.

  3. Mid-Ocean Ridge Observations (United States)

    Jamie Levine

    Students work in partners during class to make observations about the East Pacific Rise and Mid-Atlantic Ridge from Google Earth images. They also examine samples of typical oceanic crust and upper mantle and use relative density to predict the appropriate rock sequence.

  4. Metamorphism in the Martian crust (United States)

    McSween, Harry Y.; Labotka, Theodore C.; Viviano-Beck, Christina E.


    Compositions of basaltic and ultramafic rocks analyzed by Mars rovers and occurring as Martian meteorites allow predictions of metamorphic mineral assemblages that would form under various thermophysical conditions. Key minerals identified by remote sensing roughly constrain temperatures and pressures in the Martian crust. We use a traditional metamorphic approach (phase diagrams) to assess low-grade/hydrothermal equilibrium assemblages. Basaltic rocks should produce chlorite + actinolite + albite + silica, accompanied by laumontite, pumpellyite, prehnite, or serpentine/talc. Only prehnite-bearing assemblages have been spectrally identified on Mars, although laumontite and pumpellyite have spectra similar to other uncharacterized zeolites and phyllosilicates. Ultramafic rocks are predicted to produce serpentine, talc, and magnesite, all of which have been detected spectrally on Mars. Mineral assemblages in both basaltic and ultramafic rocks constrain fluid compositions to be H2O-rich and CO2-poor. We confirm the hypothesis that low-grade/hydrothermal metamorphism affected the Noachian crust on Mars, which has been excavated in large craters. We estimate the geothermal gradient (>20 °C km-1) required to produce the observed assemblages. This gradient is higher than that estimated from radiogenic heat-producing elements in the crust, suggesting extra heating by regional hydrothermal activity.

  5. The evolution of climatically driven weathering inputs into the western Arctic Ocean since the late Miocene: Radiogenic isotope evidence (United States)

    Dausmann, Veit; Frank, Martin; Siebert, Christopher; Christl, Marcus; Hein, James R.


    We present the first continuous records of dissolved radiogenic neodymium, hafnium, and lead isotope compositions of deep waters in the western Arctic Ocean, spanning the time from the late Miocene to the present. The data were obtained from three hydrogenetic ferromanganese (Fe-Mn) crusts recovered from seamounts along the northernmost edge of the Northwind Ridge in the Canada Basin from water depths of 2200, 2400, and 3600 m. Dating the crusts using cosmogenic 10Be documents undisturbed present-day growth surfaces and yields growth rates between 27 and 2.2 mm/Myr. The Nd (Hf) isotope time series of the three crusts show similar evolutions from ?Nd (?Hf) of -8.5 (+4) in the oldest parts to -11.5 (-4) at the surfaces and a pronounced trend to less radiogenic values starting at ?4 Ma. This coincided with a trend of the Pb isotope evolution towards more radiogenic 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb. It is inferred that climatically controlled changes in weathering regime and sediment transport along the North American continent were responsible for the major change of the radiogenic isotope composition of the Arctic Deep Water (ADW) in the Canada Basin. Based on these records we conclude that weathering inputs from the North American continent linked to enhanced glacial conditions started to increase and to influence the radiogenic isotope composition of ADW ?4 million years ago and were further intensified at ?1 Ma. These new time series differ markedly from the radiogenic isotope evolution of Arctic Intermediate Water recorded on the Lomonosov Ridge and suggest that much larger isotopic differences between the water masses of the Arctic Ocean than today prevailed in the past.

  6. Petrological, geochemical and geochronological evidence for a Neoproterozoic ocean basin recorded in the Marlborough terrane of the northern New England Fold Belt

    International Nuclear Information System (INIS)

    Petrological, geochemical and radiogenic isotopic data on ophiolitic-type rocks from the Marlborough terrane, the largest (?700 km2) ultramafic-mafic rock association in eastern Australia, argue strongly for a sea-floor spreading centre origin. Chromium spinel from partially serpentinised mantle harzburgite record average Cr/(Cr + Al) = 0.4 with associated mafic rocks displaying depleted MORB-like trace-element characteristics. A Sm/Nd isochron defined by whole-rock mafic samples yields a crystallisation age of 562 ± 22 Ma (2?). These rocks are thus amongst the oldest rocks so far identified in the New England Fold Belt and suggest the presence of a late Neoproterozoic ocean basin to the east of the Tasman Line. The next oldest ultramafic rock association dated from the New England Fold Belt is ca 530 Ma and is interpreted as backarc in origin. These data suggest that the New England Fold Belt may have developed on oceanic crust, following an oceanward migration of the subduction zone at ca 540 Ma as recorded by deformation and metamorphism in the Anakie Inlier. Fragments of late Neoproterozoic oceanic lithosphere were accreted during progressive cratonisation of the east Australian margin. Copyright (1999) Geological Society of Australia

  7. Protracted construction of gabbroic crust at a slow spreading ridge: Constraints from 206Pb/238U zircon ages from Atlantis Massif and IODP Hole U1309D (30°N, MAR) (United States)

    Grimes, Craig B.; John, Barbara E.; Cheadle, Michael J.; Wooden, Joseph L.


    Sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon ages of 24 samples from oceanic crust recovered in Integrated Ocean Drilling Program (IODP) Hole U1309D and from the surface of Atlantis Massif, Mid-Atlantic Ridge (MAR) (30°N) document a protracted history of accretion in the footwall to an oceanic detachment fault. Ages for 18 samples of evolved Fe-Ti oxide gabbro and felsic dikes collected 40–1415 m below seafloor in U1309D yield a weighted mean of 1.20 ± 0.03 Ma (mean square of weighted deviates = 7.1). However, the ages range from 1.08 ± 0.07 Ma and 1.28 ± 0.05 Ma indicating crustal construction occurred over a minimum of 100–200 ka. The zircon ages, along with petrologic observations, indicate at least 2 major periods of intrusive activity with age peaks separated by 70 ka. The oldest ages are observed below 600 mbsf, an observation inconsistent with models requiring constant depth melt intrusion beneath a detachment fault. The data are most consistent with a “multiple sill” model whereby sills intrude at random depths below the ridge axis over a length scale greater than 1.4 km. Zircon ages from broadly spaced samples collected along the southern ridge of Atlantis Massif yield a detachment fault slip rate of 28.7 ± 6.7 mm/a and imply significant asymmetric plate spreading (up to 100% on the North American plate) for at least 200 ka during core complex formation.

  8. Composition of the Primary Crust of Mars: Observations of Deeply Excavated Crater Central Peaks (United States)

    Skok, J. R.; Mustard, J. F.; Tornabene, L. L.; Murchie, S. L.


    It is predicted that the primary crust of Mars crystallized from a magma ocean and would be well preserved at depth on a single plate planet but poorly exposed as impacts, volcanism and alteration has reworked the upper crust. In a few select locations, extensive excavation by impact or erosion has exposed unaltered mafic minerals of the Martian crust. The majority of these exposures occur within the uplifted central peaks and peak rings of Southern Highland craters. We examine the mafic compositions of these deeply excavated crustal rocks in an attempt to constrain the composition of the Martian crust and test models of planetary formation. The search for deeply excavated bedrock from HiRISE images is ongoing and has so far resulted in nearly 200 potential locations. Over half of these currently have CRISM spectroscopic observations with ~50 locations having good exposures of crustal rocks showing little to no alteration. It is this combination of deeply excavated minerals that has potential to tap the preserved primary crust of Mars. We focus our analysis on olivine and pyroxene as crustal formation models predict that these two minerals would dominate the modal mineralogy of the crystallizing crust with a garnet layer potentially stable at depth. The high-resolution visible and near-infrared spectroscopic data provided by the CRISM instrument is ideally suited for examining these compositional characteristics. Initial in-depth analysis of the central peak of Alga Crater shows excellent exposures of lithologies characterized by both olivine and pyroxene. The olivine-bearing unit here has a fayalitic composition and a dunite lithology. This ancient Fe-rich olivine is in stark contrast to the Mg-enriched olivine of the primitive mantle of Earth. The primary pyroxene-bearing unit was determined to be a low-calcium, high-Fe enstatite orthopyroxenite, consistent with the mineralogy of the ancient Mars meteorite ALH84001. These observations suggest that the crust crystallized into compositionally homogeneous units in close proximity to allow single impact to sample multiple lithologies. The units are consistent with the late-stage crystallization of a hot magma ocean enriched in Fe. Here we expand the results to all suitable Southern Highland exposures to check regional compositional consistency and examine global trends. Initial results support similar compositions in excavated crust throughout the southern highlands, though the presence of both olivine and pyroxene lithologies are rarely well exposed in the same central peak. Additional analysis will continue to test the emerging hypothesis that the upper primary crust is the direct result of the late stage crystallization of a magma ocean, with no density driven overturn, that results in compositionally segregated fayalite and enstatite rich crust.

  9. Petrogenesis of Cretaceous adakite-like intrusions of the Gangdese Plutonic Belt, southern Tibet: Implications for mid-ocean ridge subduction and crustal growth (United States)

    Zheng, Yuan-chuan; Hou, Zeng-qian; Gong, Ying-li; Liang, Wei; Sun, Qing-Zhong; Zhang, Song; Fu, Qiang; Huang, Ke-Xian; Li, Qiu-Yun; Li, Wei


    We have conducted a whole-rock geochemical, U-Pb zircon geochronological, and in situ zircon Hf-O isotopic compositional study of rocks in southern Tibet from the Langxian igneous suite (including a lamprophyre dyke, mafic enclaves, a granodiorite, and a two-mica granite) and the Nuri igneous suite (a quartz-diorite). U-Pb zircon dating indicates that the timing of crystallization of the mafic enclaves and host granodiorite of the Langxian suite are ca. 105 Ma and 102 Ma, respectively, that the Langxian lamprophyre dyke and the two-mica granite were emplaced at ca. 96 Ma and 80-76 Ma, respectively, and that the Nuri quartz-diorite was emplaced at ca. 95 Ma. With the exception of the lamprophyre dyke and mafic enclaves in the Langxian area, felsic rocks from the Langxian and Nuri igneous suites all show signs of a geochemical affinity with adakite-like rocks. The high Mg-numbers, high abundance of compatible elements, high ?Nd(t) (2.7 and 2.8) and ?18O (8.9 and 9.2‰) values, elevated zircon ?Hf(t) (11.0-17.0) values, and low 87Sr/86Sr(i) ratios (0.7040), collectively indicate that the Nuri adakite-like quartz-diorite was derived from partial melting of the low temperature altered Neo-Tethyan oceanic crust, and that these dioritic magmas subsequently interacted with peridotite as they rose upwards through the overlying mantle wedge. The observation of identical differentiation trends, similar whole-rock Sr-Nd and zircon Hf isotopic compositions, and consistently low (Dy/Yb)N ratios among the Langxian igneous suite rocks, indicates that the adakite-like granodiorite was produced by low-pressure fractional crystallization of precursor magmas now represented by the (relict) mafic enclaves. However, relatively high Al2O3 contents, low MgO, Cr and Ni contents, and low (La/Yb)N and (Dy/Yb)N values indicate that the two-mica granite was derived from partial melting of the southern Tibetan mafic lower crust in the absence of garnet, while isotopic data suggest that at least 70% of the magma source region was juvenile materials. Combined with the presence of HT (high temperature) charnockitic magmatism, HT granulite facies metamorphism, and large volumes of Late Cretaceous batholiths, the oceanic-slab-derived Nuri adakitic rocks indicate a substantial high heat flux in the Gangdese batholith belt during the Late Cretaceous, which may have been related to subduction of a Neo-Tethyan mid-ocean ridge system. According to this model, hot asthenosphere would rise up through the corresponding slab window, and come into direct contact with both the oceanic slab and the base of the overlying plate. This would cause melting of both the oceanic slab and the overlying plate by the addition of heat that was ultimately linked with peak magmatism and the significant growth and chemical differentiation of juvenile crust in southern Tibet during the Late Cretaceous (105-76 Ma). In addition, the petrogenesis of the Langxian adakite-like two-mica granite indicates that the southern Tibetan crust was still of normal thickness prior to the emplacement of these intrusions at ca. 76 Ma. This probably means that large parts of southern Tibet were not very highly elevated prior to the Indian-Asian collision.

  10. Segmentation of mid-ocean ridges (United States)

    Schouten, Hans; Klitgord, Kim D.; Whitehead, J.A.


    Studies of mid-ocean ridges in the Pacific and Atlantic oceans show that the volcanism that forms the oceanic crust along the spreading-plate boundaries is concentrated at regular intervals related to spreading rate. This observation and a new calculation for a Rayleigh-Taylor type of gravitational instability of a partially molten mantle region growing under spreading centres yield reasonable estimates of upper mantle viscosities. ?? 1985 Nature Publishing Group.

  11. Modelling the volatile and organic content of Enceladus' ocean (United States)

    Sotin, Christophe; Lunine, Jonathan


    A variety of Cassini data on Enceladus suggest that the jets of material spraying out of the south polar region of this Saturnian moon are connected to a regional or global ocean below an icy crust -- an ocean in contact with a rocky core [1, 2, 3]. In this study we construct models of the mineralogical and elemental composition of Enceladus' rocky core so as to predict the abundance of species that may provide more detail on the extent of hydrothermal evolution of the ocean and its interaction with the rocky core. Using equations of state of the relevant material, the models are made consistent with the values of the gravity coefficients [1]. We investigate the amount of organic molecules (amino acids and fatty acids) as well as the amount of 40Ar that could be present in Enceladus' deep ocean and, therefore, in the icy grains expelled into space. Some models show that the conditions in Enceladus rocky core can be very similar to those existing in the Earth's oceanic crust, suggesting that all of Enceladus rocky core would have been leached over the age of the solar system. The amount of 40Ar dissolved in the ocean provides constraints on the amount of K in the building blocks of Enceladus, the amount of leaching of the silicate fraction, and the extent of the ocean. Based on chondritic abundances for K, we have calculated that the total potential of 40Ar is about 5.6x1012 kg. We also investigate the amount of organic material that would have been concentrated in the ocean. The Murchison meteorite contains about 60 ppm of amino acids, mainly glycine [4]. Assuming that all the rocky core has been leached by water, the modelled concentration of amino acids exceeds 150 ppm. Carboxylic acids were detected in the Asuka carbonaceous chondrites in Antarctica with values, for example, of 90 nmol/g of benzoic acid. Assuming this value, about 35 ppm of benzoic acid would be present in Enceladus' ocean. The concentrations are larger if the ocean is not global, but rather limited to the South Pole [1], and assuming that all the leached material is concentrated in the regional ocean (and not partially trapped elsewhere in ice). A future mission to Enceladus could eventually measure these quantities in order to assess the extent of evolution of Enceladus, and the potential presence of the building blocks of life in its ocean. This work has been performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract to NASA. [1] Iess L. et al. (2014) Science, 344, 78. [2] Porco C. et al. (2014) The Astronomical Journal, 148:45. [3] Hedman M. M. et al. (2013) Nature, 500, 182-184. [4] Sephton M.A. (2002) Nat. Prod. Rep., 19, 292-311.

  12. Biogenic crust dynamics on sand dunes


    Kinast, Shai; Meron, Ehud; Yizhaq, Hezi; Ashkenazy, Yosef


    Sand dunes are often covered by vegetation and biogenic crusts. Despite their significant role in dune stabilization, biogenic crusts have rarely been considered in studies of dune dynamics. Using a simple model, we study the existence and stability ranges of different dune-cover states along gradients of rainfall and wind power. Two ranges of alternative stable states are identified: fixed crusted dunes and fixed vegetated dunes at low wind power, and fixed vegetated dunes ...

  13. H11076_GEO_1MSSS.TIF: Composite Grayscale Image of the Sidescan Sonar Data From National Oceanic and Atmospheric Administration (NOAA) Survey H11076 of the Sea Floor in Quicks Hole, MA (Geographic)

    U.S. Geological Survey, Department of the Interior — The U.S. Geological Survey (USGS), in cooperation with the National Oceanic and Atmospheric Administration (NOAA) and the Massachusetts Office of Coastal Zone...

  14. H11076_GEO_1MMBES.TIF: Color Shaded-Relief GeoTIFF Image Showing the 1-m Bathymetry Generated from National Oceanic and Atmospheric Administration (NOAA) Survey H11076 in Quicks Hole, Elizabeth Islands, MA (Geographic)

    U.S. Geological Survey, Department of the Interior — The U.S. Geological Survey (USGS), in cooperation with the National Oceanic and Atmospheric Administration (NOAA) and the Massachusetts Office of Coastal Zone...

  15. Dry and strong quartz during deformation of the lower crust in the presence of melt


    Menegon, Luca; Nasipuri, Pritam; Stunitz, Holger; Behrens, Harald; Ravna, Erling J. Krogh


    Granulite facies migmatitic gneisses from the Seiland Igneous Province (northern Norway) were deformed during deep crustal shearing in the presence of melt, which formed by dehydration melting of biotite. Partial melting and deformation occurred during the intrusion of large gabbroic plutons at the base of the lower crust at 570 to 520 Ma in an intracontinental rift setting. The migmatitic gneisses consist of high-aspect-ratio leucosome-rich domains and a leucosome-poor, restitic domain of qu...

  16. Symmetry energy effects on location of the inner edge of neutron star crusts


    Moustakidis, Ch C.


    The symmetry energy effects on the location of the inner edge of neutron star crusts are studied. Three phenomenological models are employed in order to check the accuracy of the well known parabolic approximation of the equation of state for asymmetric nuclear matter in the determination of the transition density $n_t$ and transition pressure $P_t$. The results corroborate the statement that the error due to the assumption that a priori the equation of state is parabolic ma...

  17. Presence of ca. 43 Ma highly fractionated and normal calc-alkaline granites first identified in the Gangdese Batholith, southern Tibet (United States)

    Wang, Qing; Zhu, Di-Cheng; Cawood, Peter A.; Zhao, Zhi-Dan; Liu, Sheng-Ao; Chung, Sun-Lin; Liu, Dong; Dai, Jin-Gen; Mo, Xuan-Xue


    The Gangdese Batholith in southern Tibet has long been linked with the subduction of Neo-Tethyan oceanic lithosphere and subsequent India-Asia collision. However, the specific processes of magmatic evolution and petrogenesis of the batholith remain poorly constrained because most existing studies have focused on its geochronological framework. New integrated whole-rock major and trace element, zircon U-Pb age, and zircon Hf isotopic data of granites from Dajia in the western Gangdese Batholith, southern Tibet establish the presence of highly fractionated syenogranite (HFS) and normal calc-alkaline monzogranite (NCM). One syenogranite sample has been dated by LA-ICP-MS zircon U-Pb method to be 43.9 ± 0.3 Ma and three monzogranite samples yielded ages of 42.6 ± 0.3 Ma, 42.7 ± 0.4 Ma, and 43.6 ± 0.3 Ma, representing the late-phase magmatism of the Gangdese Batholith. Six NCM samples display SiO2 of 69-72 wt.%, K2O of 4.9-5.5 wt.%, and Na2O of 3.2-3.8 wt.%, with differentiation index (DI) in the range of 84-93. These rocks are enriched in Rb, Th, U, and LREE and depleted in Ba, Nb, Sr, P, and Ti, with (La/Yb)N = 18.1-26.9 and Eu/Eu* = 0.6-0.83. Eight HFS samples are characterized by high SiO2 (75-78 wt.%) and DI (95-97), and significant negative Eu anomalies (Eu/Eu* = 0.27-0.70), although K2O (4.8-5.3 wt.%) and Na2O (3.2-3.6 wt.%) contents and (La/Yb)N (12.5-27.2) ratios are comparable to those of the NCM samples. These HFS samples display marked concave-upward middle rare earth element (MREE; Gd-Ho) patterns that are not observed in the NCM samples. The NCM and HFS samples have similar zircon Hf isotopic compositions with zircon ?Hf(t) of -5.6 to +6.3 and -1.6 to +4.6, respectively. Such isotopic compositions, together with low heavy REE and Y abundances, indicate that both the NCM and HFS samples were most likely derived from partial melting of the thickened juvenile crust beneath the southern Lhasa subterrane with varying contributions from ancient continental crust material. The absence of fractionation trends between the NCM and HFS samples (e.g., SiO2 vs. Sc and SiO2 vs. Dy/Yb) suggests that the HFS samples with concave-upward MREE patterns can be interpreted as resulting from partial melting of basaltic lower crust leaving an amphibole-rich residuum followed by significant fractional crystallization of feldspar, plagioclase and apatite. The presence of HFS with low heavy REE and Y (6.2-15.3 ppm) abundances identified for the first time in the Gangdese Batholith corroborates that the southern Lhasa crust had already been thickened by ca. 43 Ma. High whole-rock zircon saturation temperatures (815° C-869° C) for the NCM samples suggest high heat supply likely associated with rising asthenospheric flow in response to post-collision breakoff of the Neo-Tethyan oceanic lithosphere.

  18. Rb-Sr and Sm-Nd isotope systematics and geochemical studies on metavolcanic rocks from Peddavura greenstone belt: Evidence for presence of Mesoarchean continental crust in easternmost part of Dharwar Craton, India (United States)

    Rajamanickam, M.; Balakrishnan, S.; Bhutani, R.


    Linear, north-south trending Peddavura greenstone belt occurs in easternmost part of the Dharwar Craton. It consists of pillowed basalts, basaltic andesites, andesites (BBA) and rhyolites interlayered with ferruginous chert that were formed under submarine condition. Rhyolites were divided into type-I and II based on their REE abundances and HREE fractionation. Rb-Sr and Sm-Nd isotope studies were carried out on the rock types to understand the evolution of the Dharwar Craton. Due to source heterogeneity Sm-Nd isotope system has not yielded any precise age. Rb-Sr whole-rock isochron age of 2551 ± 19 (MSWD = 1.16) Ma for BBA group could represent time of seafloor metamorphism after the formation of basaltic rocks. Magmas representing BBA group of samples do not show evidence for crustal contamination while magmas representing type-II rhyolites had undergone variable extents of assimilation of Mesoarchean continental crust (>3.3 Ga) as evident from their initial ? Nd isotope values. Trace element and Nd isotope characteristics of type I rhyolites are consistent with model of generation of their magmas by partial melting of mixed sources consisting of basalt and oceanic sediments with continental crustal components. Thus this study shows evidence for presence of Mesoarchean continental crust in Peddavura area in eastern part of Dharwar Craton.

  19. Ocean Ridges and Oxygen (United States)

    Langmuir, C. H.


    The history of oxygen and the fluxes and feedbacks that lead to its evolution through time remain poorly constrained. It is not clear whether oxygen has had discrete steady state levels at different times in Earth's history, or whether oxygen evolution is more progressive, with trigger points that lead to discrete changes in markers such as mass independent sulfur isotopes. Whatever this history may have been, ocean ridges play an important and poorly recognized part in the overall mass balance of oxidants and reductants that contribute to electron mass balance and the oxygen budget. One example is the current steady state O2 in the atmosphere. The carbon isotope data suggest that the fraction of carbon has increased in the Phanerozoic, and CO2 outgassing followed by organic matter burial should continually supply more O2 to the surface reservoirs. Why is O2 not then increasing? A traditional answer to this question would relate to variations in the fraction of burial of organic matter, but this fraction appears to have been relatively high throughout the Phanerozoic. Furthermore, subduction of carbon in the 1/5 organic/carbonate proportions would contribute further to an increasingly oxidized surface. What is needed is a flux of oxidized material out of the system. One solution would be a modern oxidized flux to the mantle. The current outgassing flux of CO2 is ~3.4*1012 moles per year. If 20% of that becomes stored organic carbon, that is a flux of .68*1012 moles per year of reduced carbon. The current flux of oxidized iron in subducting ocean crust is ~2*1012 moles per year of O2 equivalents, based on the Fe3+/Fe2+ ratios in old ocean crust compared to fresh basalts at the ridge axis. This flux more than accounts for the incremental oxidizing power produced by modern life. It also suggests a possible feedback through oxygenation of the ocean. A reduced deep ocean would inhibit oxidation of ocean crust, in which case there would be no subduction flux of oxidized material, and oxygen could rise. Once the ocean becomes fully oxidized, a substantial subduction flux operates as a negative feedback. Plate tectonic geochemical cycles may have played a very significant role in the oxygen balance in both the ancient and modern Earth.

  20. The lunar magma ocean - A transient lunar phenomenon (United States)

    Minear, J. W.


    The time required for the solidification of a lunar magma ocean is considered. In the absence of a thermal boundary crust, heat loss is by radiation and the ocean will solidify in a few decades. However, only a few millimeters of crust would effectively limit radiation. The current investigation has the objective to present estimates of conduction boundary (crust) thickness based on several models of the mechanism of crustal formation. Three different models for estimating conduction boundary layer thickness are discussed. They are based on the formation of a chill margin, a plagioclase flotation layer, and a layer which is continually destroyed by meteorite impacts and regrows by conductive heat loss. Based on these models, the maximum solidification time for a 200 km thick ocean is about 100 million years. A more probable solidification time based on a plagioclase flotation crust model is about 60 million years.

  1. Hydroacoustic Monitoring of Oceanic Spreading Centers: Past, Present, and Future

    Directory of Open Access Journals (Sweden)

    Robert P. Dziak


    Full Text Available Mid-ocean ridge volcanism and extensional faulting are the fundamental processes that lead to the creation and rifting of oceanic crust, yet these events go largely undetected in the deep ocean. Currently, the only means available to observe seafloor-spreading events in real time is via the remote detection of the seismicity generated during faulting or intrusion of magma into brittle oceanic crust. Hydrophones moored in the ocean provide an effective means for detecting these small-magnitude earthquakes, and the use of this technology during the last two decades has facilitated the real-time detection of mid-ocean ridge seafloor eruptions and confirmation of subseafloor microbial ecosystems. As technology evolves and mid-ocean ridge studies move into a new era, we anticipate an expanding network of seismo-acoustic sensors integrated into seafloor fiber-optic cabled observatories, satellite-telemetered surface buoys, and autonomous vehicle platforms.

  2. Biogenic crust dynamics on sand dunes

    CERN Document Server

    Kinast, Shai; Yizhaq, Hezi; Ashkenazy, Yosef


    Sand dunes are often covered by vegetation and biogenic crusts. Despite their significant role in dune stabilization, biogenic crusts have rarely been considered in studies of dune dynamics. Using a simple model, we study the existence and stability ranges of different dune-cover states along gradients of rainfall and wind power. Two ranges of alternative stable states are identified: fixed crusted dunes and fixed vegetated dunes at low wind power, and fixed vegetated dunes and active dunes at high wind power. These results suggest a cross-over between two different forms of desertification.

  3. Receiver function analysis of the crust and upper mantle in Fennoscandia - isostatic implications

    DEFF Research Database (Denmark)

    Frassetto, Andrew; Thybo, Hans


    The mountains across southern Norway and other margins of the North Atlantic Ocean appear conspicuously high in the absence of recent convergent tectonics. We investigate this phenomenon with receiver functions calculated for seismometers deployed across southern Fennoscandia. These are used to constrain the structure and seismic properties of the lithosphere and primarily to measure the thickness and infer the bulk composition of the crust. Such parameters are key to understanding crustal isostasy and assessing its role, or lack thereof, in supporting the observed elevations. Our study focuses on the southern Scandes mountain range that has an average elevation >1.0 km above mean sea level. The crust-mantle boundary (Moho) is ubiquitously imaged, and we occasionally observe structures that may represent the base of the continental lithosphere or other thermal, chemical, or viscous boundaries in the upper mantle. The Moho resides at similar to 25-30 km depth below mean sea level in southeastern coastal Norwayand parts of Denmark, similar to 35-45 km across the southern Scandes, and similar to 50-60 km near the Norwegian-Swedish border. That section of thickest crust coincides with much of the Transscandinavian Igneous Belt and often exhibits a diffuse conversion at the Moho, which probably results from the presence of a high wave speed, mafic lower crust across inner Fennoscandia. A zone of thinned crust (

  4. Seismicity, metamorphism and rheology of the lower continental crust (United States)

    Austrheim, Håkon


    Seismological data document that both normal earthquakes and tremors occur in the lower continental crust. Pseudotachylytes (frictional melts and ultracommunited rocks) have been described from several high grade metamorphic terrains and may be the geological manifestation of this seismicity. The Grenville (c. 930Ma) granulite facies complex (T: 800 °C; P: ?10kbar) of the Lindås Nappe in the Bergen Arcs, W-Norway underwent a fluid induced partial eclogite (T: 600-650 °C; P: 15-20 kbar) and amphibolite facies metamorphism during the Caledonian (c.400-430 Ma) continent collision. Pseudotachylyte fault and injection veins formed in the dry granulites at or close to the reaction fronts both in the eclogitized (western parts) and the amphibolitized (eastern parts) of the Nappe. They are locally recrystalized with the development of amphibolite and eclogite facies assemblages demonstrating that they formed pre or syn the Caledonian metamorphism. The pseudotachylytes transect lithologies ranging from peridotite to anorthosite and consequently the influence of the seismic energy release on a range of granulite facies minerals including garnet, pyroxenes, olivine, plagioclase, hornblende and scapolite can be observed. The seismic energy released promotes the Caledonian metamorphism and change the petrophysical properties of the lower crust in the following ways: The melting and the ultracommunition of the granulite facies minerals increased the reactive surface area and produce local pathways for fluid. S-rich scapolite, a common mineral in granulities play a key role in this process by releasing S and C to form sulfides and carbonates. Small sulfide grains impregnate the pseudotachylyte veins which may lead to an increased electrical conductivity of the deep crust. The pseudotachylyte veins impose inhomogeneities in the massive rocks through grain size reduction and lead to strain localization with development of amphibolite and eclogite facies shear zones. Formation of eclogite facies breccias where meter size blocks of rotated granulites are enclosed in eclogite may have initiated by the seismic events as indicated by fractures in the relict granulite facies garnet. The seismic events may have been important in large scale transport of fluid required to bring about the metamorphism of the dry granulite facies complex.

  5. Ocean Planet: Ocean Market (United States)

    Unit from Smithsonian multidisciplinary ocean curriculum. Lesson plan focuses on foods, materials and medicines that comes form marine life, how these resources are harvested and processed and the impacts of fisheries. Students identify and classify consumer goods from the ocean and calculate their cost. Unit includes: background essay; teacher instructions; forms for student activity; discussion questions; all online in PDF format. Resources include online version of Smithsonian Ocean Planet exhibition.

  6. On the possible role of chemical boundary layers in regulating the thermal thickness of continents and oceans (United States)

    Lee, C.; Lenardic, A.; Cooper, C.; Niu, F.; Alan, L.


    One of the most important observations made during the early developments of plate tectonic theory was that the depth of the seafloor initially (for the first ~70 Ma) increases with the square root of the crust's age and that the accompanying heat flow decreases as the inverse of the square root of age. It was subsequently shown that the sqrt(t) relationships could be explained by approximating the growth of the upper thermal boundary layer (TBL: the boundary layer over which the mode of heat transfer changes from advective to conductive) of the Earth's convecting interior by an infinite half-space conductive cooling model. This model, which we term the boundary layer model, predicts that the thickness of the TBL increases monotonically with the square root of seafloor age according to the relationship, L = sqrt(4kt), where L is the thermal thickness, t is time, and k is thermal diffusivity. For a thermal diffusivity of ~30 km2/Ma, this relationship takes the form, 11 sqrt(t), where L is in km and t is in Ma. By accounting for thermal contraction and the decrease in thermal gradient across the growing TBL, the evolution of seafloor depth and heat flow with time follow accordingly. The success of the boundary layer model in linking the kinematics of seafloor spreading to the depth and heatflow of <70 Ma old lithosphere represents the strongest evidence so far that plate tectonics and convection are linked. However, the boundary layer model breaks down at 70 Ma after which the heat flow and seafloor depth saturate at constant values. At the same time, it appears from seismic studies that the TBL thickness also saturates at a maximum value of 90-100 km. Many models have been proposed to explain the discrepancy between the predictions of the boundary layer model and the geologic features of post-70 Ma lithosphere. Most of these models, however, are difficult to test. Here, we present a testable model that explains the evolution of oceanic TBLs by invoking a pre-existing chemical boundary layer (CBL). We base this hypothesis on a growing understanding of the deep thermal and compositional structure of continents. Continents are underlain by a thick melt-depleted and dehydrated mantle layer, the former resulting in buoyancy and the latter resulting in increased viscosity. Radiogenic isotopic studies indicate that these CBLs do not significantly deform over billion year timescales, implying that on the timescales of mantle convection, such CBLs act as rigid lids resting on top of and separated from the convecting mantle. For this reason, the upper TBL of the convecting mantle therefore consists of a purely conductive layer (represented by the rigid CBL) and a convective sub-layer (CS-L), which lies just beneath the CBL and represents the actively convecting part of the TBL. We show using petrologic and geodynamic arguments that the thickness of the CBL beneath continents may limit the thickness of the convective sublayer and accordingly, the thickness of continental TBLs. Petrologic observations require that the seafloor also be underlain by a melt-depleted and dehydrated mantle layer, albeit thinner than that beneath continents. The base of this layer roughly coincides with the thermal thickness at which the boundary layer model breaks down. By analogy with our continental studies, we suggest that the presence of a CBL beneath oceanic crust may also be responsible for maintaining a constant thickness of oceanic TBLs beyond ~70 Ma. A future test of this hypothesis would be to seismically map whether there exists a crossover between the CBL and TBL beneath oceans and, if so, whether the crossover occurs at 70 Ma.

  7. The breaking strain of neutron star crust

    Energy Technology Data Exchange (ETDEWEB)

    Kadau, Kai [Los Alamos National Laboratory; Horowitz, C J [INDIANA UNIV


    Mountains on rapidly rotating neutron stars efficiently radiate gravitational waves. The maximum possible size of these mountains depends on the breaking strain of neutron star crust. With multimillion ion molecular dynamics simulations of Coulomb solids representing the crust, we show that the breaking strain of pure single crystals is very large and that impurities, defects, and grain boundaries only modestly reduce the breaking strain to around 0.1. Due to the collective behavior of the ions during failure found in our simulations, the neutron star crust is likely very strong and can support mountains large enough so that their gTavitational wave radiation could limit the spin periods of some stars and might be detectable in large scale interferometers. Furthermore, our microscopic modeling of neutron star crust material can help analyze mechanisms relevant in Magnetar Giant and Micro Flares.

  8. Crustal Structure and Lithospheric Rupture Process of the Continent-Ocean Boundary of the South China Sea (United States)

    Song, T. R.; Li, C. F.; Shi, H.; Ding, W.; Li, J.


    Seismic reflection profiles acquired in the continent-ocean transition zone (COT) of South China Sea provide a detailed view of Moho and deep crustal reflectors and continental lithosphere extension and breakup styles. At the north margin, rift basins are often bounded by listric normal faults, most of which are terminated at the base of the upper crust. The upper-lower crust interface corresponds to the brittle-ductile transition zone, where listric faults tend to converge into a low angle detachment fault. According to calculated stretch factors in different depth scales (upper crust, lower crust) along several profiles, hyper-extended continental crust is widespread, with the upper crust being often preferentially more thinned than the lower crust. The ductile lower crust is more resistant to faulting and therefore stretching. A high-velocity lower crustal layer represents either magmatic underplating or pervasive lower crustal intrusions in the northern margin. The possible lower crust ductile flow and the high-velocity lower crustal layer may have contributed to extension discrepancy, leading to direct exposure of lower crust material landward of the continent-ocean boundary. Hyper-extended continental crust and thick syn-rift sequence developed during a long period of rifting prior to the inception of seafloor spreading are also observed in the southern continental margin, further suggesting depth-dependent continental extension in the South China Sea. Basement highs and discontinuity in Moho reflector are common features around the continent-ocean boundary of South China Sea. The basement ridges are located at the landward edge of the continent-ocean boundary and possibly composed by lower crust material. The COT is ~50 km wide, where the gravity anomaly is approximately zero and the Moho reflector is discontinuous. The COT here is narrower than those found in other magma-poor margins (e.g., Iberia-Newfoundland type), indicating that normal oceanic crust started to establish soon after crustal breakup.

  9. Some seismic signatures in the Romanian crust (United States)

    R?ileanu, V.; Diaconescu, C. C.


    Some new seismic reflection data in the W of the Moesian platform, Foc?ani depression and Transylvania depression show seismic patterns which reflect the local structure and evolution of the each area. These local areas were affected by subsidence which over-printed the changes on the old pattern. The old and more or less reflective lower crust was fractured and some rigid blocks have preserved the reflectivity up to the present. The B?ile?ti section illustrates a transparent or diffractive upper crust and a well-marked lower crust by an alternation of reflective and non-reflective zones. The crystalline crust of the Râmnicu S?rat section has two patterns: one relatively reflective, near the bottom of the depression (the W side), whilst the other, on the E flank of depression, is transparent. The E flank seems to be more fractured due to some tensile fractures originating in the bending stress during subsidence. The Târgu Mure? section has a specific crustal pattern. A near-transparent crystalline crust overlies a very large crust-mantle transition zone which is fragmented in alternated reflective and transparent blocks.

  10. Platinum stable isotopes in ferromanganese crust and nodules (United States)

    Corcoran, Loretta; Seward, Terry; Handler, Monica R.


    Hydrogenetic ferromanganese (Fe-Mn) crust and nodules are slow-growing chemical sediments that form by direct precipitation from seawater, resulting in a record of changing seawater chemistry. These sediments are the primary sink for platinum in the modern oxic marine environment, hosting well-documented enrichments over other platinum-group elements (PGEs): the Pt anomaly [1]. Platinum is a non-bio-essential, highly siderophile, transition metal with six stable isotopes (190Pt, 192Pt, 194Pt, 195Pt, 196Pt, and 198Pt) with several oxidation states (Pt0, Pt2+ and Pt4+). Platinum is generally considered to exist in the hydrosphere as Pt2+ although its behaviour in the marine environment is poorly constrained, and Pt4+may also be present. Variations in ocean redox state, together with changes in source fluxes to the oceans, may therefore lead to small variations (stable isotopic composition of marine platinum, raising the potential of adding platinum to the growing arsenal of paleoceanographic tracers. A method has been developed to measure the platinum isotopic composition using double spike MC-ICPMS analysis [2]and applied to a global suite of modern Fe-Mn crust and nodules. Combining synchrotron XAFS analyses of platinum adsorbed onto Fe-Mn oxide and oxyhydroxide surfaces to determine oxidation state and bonding environment, with platinum stable isotopic measurements allowing us to evaluate both platinum incorporation onto these sediments and the associated degree of platinum isotopic fractionation. Leaching experiments conducted on platinum rich terrestrial materials underwent platinum stable isotopic measurement as an analogue for the Pt isotopic fractionation associated with continental weathering. [1] Hodge, V.F. et al. (1985) Earth and Planetary Science Letters, 72, 158-162. [2] Creech, J. et al. (2013) Journal of Analytical Atomic Spectrometry, 28. 853-865.

  11. Fission track dating of authigenic quartz in red weathering crusts of carbonate rocks in Guizhou province

    International Nuclear Information System (INIS)

    The Cenozoic evolution history of Guizhou Province, which is located on the southeastern flank of the Qinghai-Tibet Plateau, is unclear because of the lack of sedimentation records. The red weathering crusts widespread on the Yunnan-Guizhou Plateau may bear critical information about their evolution history. This work firstly determined the ages of four red weathering crusts in eastern, central and northern Guizhou. The material used in fission track dating is well-crystallized quartz occurring in many in-situ weathering crusts of carbonate rocks. The results showed that the fission track ages of quartz vary over a wide range from 1 Ma to 25 Ma in the four profiles, significantly younger than the ages of Triassic and Cambrian parent rocks. In combination with the regionally geological evolution history during the period from 25 Ma to 1 Ma, the ages of quartz can exclude the possibility that the origin of quartz has nothing to do with primary clastic minerals in parent rocks, authigenesis during diagenesis and hydrothermal precipitation or replacement by volcanic activities. It is deduced that the well-crystallized quartz was precipitated from Si-rich weathering fluids during weathering processes of carbonate rocks. The recorded ages of quartz from the four profiles are consistent with the episodes of planation surfaces on the Qinghai-Tibet Plateau, the stages of red soil in the tropics of South China, the tectonically stable periods in Guizhou, and the ages of weathereriods in Guizhou, and the ages of weathering in other parts of the world during the Cenozoic era. That is to say, the ages of authigenic quartz dated by the fission track method are well feasible and credible. (authors)

  12. Rapid syn-convergent Exhumation of Miocene-aged Lower Orogenic Crust in the Eastern Himalaya (United States)

    Grujic, D.; Warren, C. J.; Wooden, J. L.


    Rare granulitized eclogites exposed in the eastern Himalaya provide an insight into conditions and processes deep within the orogen. SHRIMP U-Pb, Ti and REE data from zircons in mafic granulitized eclogites located in the upper structural levels of the GHS in Bhutan show that zircon crystallized under eclogite facies metamorphic conditions, between 15.3 ± 0.3 and 14.4 ± 0.3 Ma, within a couple of million years of the later granulite-facies overprint. In conjunction with pressure estimates of the eclogite and granulite facies stages of metamorphism, the age data suggest that initial exhumation occurred at plate tectonic rates (cm yr-1). These extremely rapid syn-convergence exhumation rates during the later stages of the India-Asia collision require a revision of theories about how crustal materials are transported and exhumed during continental collisions. In contrast to western Himalayan examples, the eastern Himalayan eclogites cannot be tectonically related to steep subduction of India beneath Asia. Instead, they more likely represent fragments from the base of the over-thickened Tibetan crust. Based on the zircon age and trace element data we hypothesize that the protolith of the mafic granulites are Middle Miocene mafic intrusions into the lower crust of southern Tibet, linked to the Miocene volcanism in the Lhasa block. We suggest that a transient tectonic event, possibly the indenting of a strong Indian crustal ramp into crust under southern Tibet weakened by partial melting, may have promoted exhumation of the eclogitized lower crust under Tibet. Mafic magmatism and volcanism themselves may be related to the convective thinning of the lithospheric mantle triggered by the reduction in India-Eurasia convergence rate during the Middle Miocene, which in turn could have helped the rapid extrusion of the lower crust over the earlier exhumed middle crust.

  13. Ferrobasalts from the Central Indian Ocean Basin

    Digital Repository Service at National Institute of Oceanography (India)

    Iyer, S.D.; Mukhopadhyay, R.; Popko, D.C.


    The occurrence of ferrobasalts recovered from the Central Indian Ocean Basin crust generated at the Southeast Indian Ridge during a phase of moderate to fast spreading accretion (approx 110-190 mm/yr, full rate) is reported. FeO (13-19%), and TiO...

  14. Role of the Deep Mantle in Generating EM-I in Ocean Island Basalts: Insight from the Kerguelen Archipelago (Indian Ocean) (Invited) (United States)

    Weis, D.


    The Kerguelen Plume is responsible for one of the longest (both in length and time) hotspot tracks on Earth, starting at ~120 Ma with the formation of the Rajmahal Traps in India. The tectonic setting evolved from continental break-up at ~120 Ma, to a position above the Southeast Indian Ridge (SEIR) at ~40 Ma, to a purely oceanic environment today. The Cretaceous record of volcanism on the Central and Southern Kerguelen Plateau shows interaction of mantle plume-derived magmas with continental-related material (5000 km-long Ninetyeast Ridge (82-38 Ma) lacks any evidence of such shallow contamination. Upper mantle components (depleted, SEIR-type) participated in the formation of the submarine Northern Kerguelen Plateau (~34 Ma). The Kerguelen Archipelago is covered (>80%) by flood basalts erupted between 30 and 24 Ma. With decreasing age and increasing distance from the SEIR, the compositions evolved from tholeiitic in the northwest, to transitional in the central part of the archipelago, and to alkaline in the Southeast Province. The transition from tholeiitic to mildly alkalic compositions primarily reflects changes in melting conditions (lower extents of partial melting at higher pressures), associated with crust and lithosphere thickening as the distance from the SEIR increased. High-precision Pb-Sr-Nd-Hf isotopic data reveal that the archipelago flood basalts were derived from melting of an enriched component (EM-I) in the plume source, without any trace of continental contamination during eruption. The enriched component dominates the chemistry of the alkalic basalts (25-24 Ma), whereas the older (28-26 Ma) tholeiitic-transitional basalts contain a higher proportion of a depleted-SEIR component. In binary isotope plots, Kerguelen compositions form subparallel trends that are distinctly more enriched than those from Hawaii. Seismic data shows the presence of two large-low-shear-velocity-provinces (LLSVP) in the deep mantle, one centred in the Pacific, the other below Africa. When projected down to the core-mantle boundary (CMB), Kerguelen and Tristan (Atlantic Ocean) are located on the eastern and western edges of the tall steep-sided LLSVP African anomaly, respectively. The other two islands with EM-I signatures, Hawaii and Pitcairn, overlie the edges of the Pacific LLSVP. We infer that these deep zones with velocity anomalies at the CMB are the repositories for enriched components in the mantle that are brought to the surface by strong mantle plumes. Kerguelen and Tristan carry the strongest enriched signature, also referred to as the ';DUPAL anomaly', whereas Pitcairn and Hawaii have a distinct, slightly less pronounced enriched signature. The differences in EM-I compositions indicate that some of the material constituting the LLSVP at the base of the mantle is different in the African and Pacific anomalies. In Hawaii, the EM-I signature can be traced back to at least 5 Ma, whereas in Kerguelen it can be traced back until 34 Ma on the archipelago and Northern Kerguelen Plateau, and until 82 Ma along the Ninetyeast Ridge. This implies that the LLSVPs are long-lived features of the deep mantle; in the case of the African anomaly, on the order of 100 million years.

  15. Laser ablation MC-ICP-MS U/Pb geochronology of ocean basement calcium carbonate veins (United States)

    Harris, M.; Coggon, R. M.; Teagle, D. A. H.; Roberts, N. M. W.; Parrish, R. R.


    Given the vast areas of mid ocean ridge flanks, even small chemical changes dues to fluid-rock interaction on the flanks may significantly influence global geochemical cycles. A conductive heat flow anomaly associated with hydrothermal circulation in ocean crust exists until on average 65Ma, but it is not known whether the thermal signature is accompanied by continued fluid-rock chemical exchange. Constraining the duration of fluid-rock chemical exchange is critical for calculating robust chemical fluxes associated with ridge flank hydrothermal circulation. Calcium carbonate veins form during relatively late-stage hydrothermal alteration and can be used to estimate the duration of ridge flank hydrothermal circulation. LA-MC-ICP-MS U/Pb geochronology provides a novel and independent approach to date calcium carbonate veins, and is advantageous over using the seawater Sr isotope curve that is in part non-unique and requires assumptions about the contribution of MORB Sr from fluid-rock exchange. LA-MC-ICP-MS U/Pb analyses have been undertaken on a suite of calcium carbonate veins from a range of basement ages (1.6 - 170 Ma), spreading rates and sediment thickness. Preliminary results indicate that the temperature of formation of calcium carbonate veins place a strong control on achieving a successful U/Pb isochron. This is likely related to the temperature dependent geochemical evolution of basement fluids due to fluid-rock reaction, and the partitioning of U and Pb into calcite/aragonite. Successful U/Pb isochrons have been achieved for a range of crustal ages and spreading rates, and indicate that calcium carbonate precipitation occurs within 25Myrs of crustal formation. This is substantially shorter than 65Ma, the average extent of the conductive heat flow anomaly, and will allow for more robust estimates of the contribution of hydrothermal chemical fluxes to global geochemical cycles.

  16. Thermal models of dyke intrusion during development of continent-ocean transition (United States)

    Daniels, K. A.; Bastow, I. D.; Keir, D.; Sparks, R. S. J.; Menand, T.


    A consensus has emerged in recent years from a variety of geoscientific disciplines that extension during continental rifting is achieved only partly by plate stretching: dyke intrusion also plays an important role. Magma intrusion can accommodate extension at lower yield stresses than are required to extend thick, strong, unmodified continental lithosphere mechanically, thereby aiding the breakup process. Dyke intrusion is also expected to heat and thereby weaken the plate, but the spatial extent of heating and the effect of different rates of magmatic extension on the timescales over which heating occurs are poorly understood. To address this issue, a numerical solution to the heat-flow equation is developed here to quantify the thermal effects of dyke intrusion on the continental crust during rifting. The thermal models are benchmarked against a priori constraints on crustal structure and dyke intrusion episodes in Ethiopia. Finite difference models demonstrate that magmatic extension rate exerts a first-order control on the crustal thermal structure. Once dyke intrusion supersedes faulting and stretching as the principal extensional mechanism the crust will heat and weaken rapidly (less than 1 Ma). In the Main Ethiopian Rift (MER), the majority of present-day extension is focused on ?20 km-wide Quaternary-Recent axial magmatic segments that are mostly seismogenic to mid-crustal depths and show P-wave seismic velocities characteristic of heavily intruded continental crust. When reviewed in light of our models, these observations require that no more than half of the MER's extension since ?2 Ma has been achieved by dyke intrusion. Magmatic heating and weakening of the crust would have rendered it aseismic if dyke intrusion accounted for the entire 6 mm/yr extension rate. In the older, faster extending (16 mm/yr) Red Sea rift (RSR) in Afar, dyke intrusion is expected to have had a more dramatic impact on crustal rheology. Accordingly, effective elastic plate thickness and Moho depth in the Danakil region of northernmost Afar are markedly reduced and seismicity is shallower than in the MER. Thermally driven variations in crustal rheology over time in response to dyke intrusion thus play an important role in the development of continent-ocean transition.

  17. Galicia Bank ocean-continent transition zone: New seismic reflection constraints (United States)

    Dean, S. L.; Sawyer, D. S.; Morgan, J. K.


    The West Iberia continental margin is a type locale for magma-poor rifting, and studies there have been instrumental in changing the classical view of the ocean-continent transition (OCT) from a discrete boundary juxtaposing continental and oceanic crust, into a more complicated zone of varying width that can include exhumed mantle. This study examines two new seismic lines in the Galicia Bank area extending west of the Peridotite Ridge, showing high resolution images of five new ridges. These ridges could be hyperextended continental crust, exhumed continental mantle, or rough ultra-slow spreading oceanic crust. There are no tilted fault blocks with pre-syn rift stratigraphy that would indicate continental crust. There are also no faults indicating mid-ocean spreading with seismic layer stratigraphy indicating normal oceanic crust. The ridges have no coherent internal seismic structure, and some resemble the topographic profile of the Peridotite Ridge. Therefore, it is likely the western ridges are also mainly composed of serpentinized mantle. These western ridges are also similar to small oceanic core complexes observed along the active part of the Mid-Atlantic Ridge, which also contain exhumed serpentinized mantle. This implies that there is a gradual transition within our study area from continental extension to seafloor spreading. Exhumation of continental mantle results in the formation of peridotite ridges, then transitions to episodic volcanism, which produces local thin basaltic crust, and exhumation of oceanic core complexes. Asymmetric processes during initial rifting and spreading result in contrasting structures on the two resulting margins.

  18. The Moho beneath western Tibet: Shear zones and eclogitization in the lower crust (United States)

    Zhang, Zhongjie; Wang, Yanghua; Houseman, Gregory A.; Xu, Tao; Wu, Zhenbo; Yuan, Xiaohui; Chen, Yun; Tian, Xiaobo; Bai, Zhiming; Teng, Jiwen


    The Tibetan Plateau is formed by continuing convergence between Indian and Asian plates since ?50 Ma, involving more than 1400 km of crustal shortening. New seismic data from western Tibet (the TW-80 experiment at 80°E) reveal segmentation of lower crustal structure by the major sutures, contradicting the idea of a mobile lower crust that flows laterally in response to stress variations. Significant changes in crustal structure and Moho depth occur at the mapped major tectonic boundaries, suggesting that zones of localized shear on sub-vertical planes extend through the crust and into the upper mantle. Converted waves originating at the Moho and at a shallower discontinuity are interpreted to define a partially eclogitized layer that extends 200 km north of the Indus-Yarlung Suture Zone, beneath the entire Lhasa block at depths of between 50 and 70 km. This layer is thinner and shallower to the north of the Shiquanhe Fault which separates the northern Lhasa block from the southern part, and the degree of eclogitization is interpreted to increase northward. The segmentation of the Tibetan crust is compatible with a shortening deformation rather than shear on horizontal planes. Unless the Indian-plate mantle lithosphere plunges steeply into the mantle beneath the Indus-Yarlung suture, leaving Indian-plate crust accreted to the southern margin of Tibet, then it too must have experienced a similar shortening deformation.

  19. Basin Excavation, Lower Crust, Composition, and Bulk Moon Mass balance in Light of a Thin Crust (United States)

    Jolliff, B. L.; Korotev, R. L.; Ziegler, R. A.


    New lunar gravity results from GRAIL have been interpreted to reflect an overall thin and low-density lunar crust. Accordingly, crustal thickness has been modeled as ranging from 0 to 60 km, with thinnest crust at the locations of Crisium and Moscoviense basins and thickest crust in the central farside highlands. The thin crust has cosmochemical significance, namely in terms of implications for the Moon s bulk composition, especially refractory lithophile elements that are strongly concentrated in the crust. Wieczorek et al. concluded that the bulk Moon need not be enriched compared to Earth in refractory lithophile elements such as Al. Less Al in the crust means less Al has been extracted from the mantle, permitting relatively low bulk lunar mantle Al contents and low pre- and post-crust-extraction values for the mantle (or the upper mantle if only the upper mantle underwent LMO melting). Simple mass-balance calculations using the method of [4] suggests that the same conclusion might hold for Th and the entire suite of refractory lithophile elements that are incompatible in olivine and pyroxene, including the KREEP elements, that are likewise concentrated in the crust.

  20. The atypical Caribbean-Colombia oceanic plateau and its role in the deformation of the Northern Andes (United States)

    Ferrari, L.; Lopez-Martinez, M.; Petrone, C. M.; Serrano, L.


    The Late Cretaceous to Early Tertiary tectono-magmatic evolution of the Northern Andes has been strongly influenced by the dextral oblique interaction of the Caribbean-Colombian oceanic plateau (CCOP) with northwestern South America. This complex interaction has resulted in several pulses of transpressional deformation and crustal accretion to the South America plate but also in a widespread deformation in the plateau itself. In this peculiar type of orogeny one of the factors controlling the deformation is the crustal structure and thus the rheological profiles of the two lithospheric sections that interact. The genesis of the CCOP has been traditionally associated to the melting of the Galapagos plume head when it impacted the Farallon plate, which is supposed to have built an unsubductable and thick crustal section. This interpretation was based on the apparent clustering of ages at ~91-89 Ma for several obducted fragments of the CCOP in northwestern South America and in the Caribbean islands. However, seismic profiles show that magmatism added a very variable amount but no more than 10 km of igneous material to the original crust of the Farallon plate, making the CCOP much more irregular than other oceanic plateaus. Recent studies of key areas of the obducted part of the CCOP contradict the notion that the plateau formed by melting of a plume head at ~ 90 Ma. Particularly, new geochronologic data and petrologic modeling from the small Gorgona Island document a magmatic activity spanning the whole Late Cretaceous (98.7±7.7 to 64.4±5 Ma) and a progressive increase in the degree of melting and melt extraction with time. Multiple magmatic pulses over several tens of Ma in small areas like Gorgona, are also recognized in other areas of the CCOP, documenting a long period of igneous activity with peaks at 74-76, 80-82, and 88-90 Ma in decreasing order of importance. Even older, Early Cretaceous ages, have been reported for fragments in Costa Rica and Curaçao. A prolonged period of igneous activity over several tens of Ma is not consistent with a short, voluminous outburst of magmatism from a plume head at ~91-89 Ma and the geographic distribution of ages does not point to a definite pattern of migration as it would be expected if magmatism would be the result of the passage of the Farallon plate over a stationary, or slowly moving, hotspot. However, the age span of this magmatism is broadly concurrent with the existence of the Caribbean slab window, formed by the intersection of the proto- Caribbean spreading ridge with the Great Caribbean Arc. During this time span the Farallon oceanic lithosphere advanced eastward ~1500 km, overriding the astenosphere feeding the proto-Caribbean spreading ridge. This hotter mantle flowed westward into, and mixed with, the opening mantle wedge, promoting increasing melting with time. This mechanism may explain the irregularly thickened oceanic crust of the CCOP and its internal deformation but also the evidence of partial subduction of some of its parts.

  1. Dredged bedrock samples from the Amerasia Basin, Arctic Ocean (United States)

    Brumley, K. J.; Mukasa, S. B.; O'Brien, T. M.; Mayer, L. A.; Chayes, D. N.


    Between 2008-2012, as part of the U.S. Extended Continental Shelf project in the Amerasia Basin, Arctic Ocean, 17 dredges were successfully collected sampling the first rock outcrops in the Chukchi Borderland and surrounding regions for the purpose of describing the geologic nature of the bathymetric features in this area. Multiple lines of evidence indicate that the specimens were collected from submarine rock exposures and were not samples of ice rafted debris, common in the ice covered waters of the Arctic Ocean. Using the USCGC Healy, each dredge was collected along very steep slopes (>35 degrees) measured with high resolution multibeam swath bathymety data. Each haul yielded samples of similar lithologies and identical metamorphic grade with manganese crusts on the surfaces exposed to seawater and fresh surfaces where the rocks were broken from outcrop. High tension pulls on the dredge line also indicated sampling of bedrock exposures. Dredged samples from a normal fault scarp in the central Chukchi Borderland consisted of Silurian (c. 430 Ma) orthogneisses that intruded older (c. 487-500 Ma) gabbros and luecogranties that were all metamorphosed to amphibolite grade (Brumley et al., 2011). Samples from the northern Northwind Ridge consisted of metasediments (greenschist facies) interpreted to have been deposited in a proximal arc setting with detrital zircon U-Pb age peaks at 434, 980 Ma with lesser peaks between 500-600, 1100-2000 Ma, and rare 2800 Ma grains (Brumley et al, 2010). Other dredges in the region of the Northwind Ridge yielded deformed and metamorphosed calcareous sandstones and low-grade phyllites (O'Brien et al., 2013). Taken together these rocks indicate a relationship to the Pearya Terrane of northern Ellesmere Island and S.W. Svalbard that were thought to represent a Cambro-Ordovician volcanic arc terrane that was involved in Caledonian orogenesis (Brumley et al., 2011). These findings constrain plate tectonic reconstruction models and bring into question long held ideas that the Chukchi Borderland was made up of an undeformed platformal sequence that was part of the Laurentian passive margin from Proterozoic through Jurassic time (e.g. Grantz et al., 1998). Dredges collected along fault scarps that border the edges of the Nautilus Basin yielded undeformed but highly altered volcaniclastic rocks that were deposited in a shallow water setting and contain primary potassium feldspar phenocrysts that are not associated with mafic magmas. Also in this region, several dredges contained undeformed Late Cretaceous (112, 88-80 Ma) basalts (Andronikov et al., 2008; Mukasa et al., 2009) interpreted to have been derived from a continental lithospheric source similar to continental flood basalts from other regions (Mukasa et al., 2009). These dredged rock samples not only have implications for the Extended Continental Shelf projects of Arctic nations, but add greatly to the body of geologic information about the history of the Arctic Ocean and provide the first ground truth as to the nature of the bathymetric features within the Amerasia Basin.

  2. Formation of the giant Chalukou porphyry Mo deposit in northern Great Xing'an Range, NE China: Partial melting of the juvenile lower crust in intra-plate extensional environment (United States)

    Li, Zhen-Zhen; Qin, Ke-Zhang; Li, Guang-Ming; Ishihara, Shunso; Jin, Lu-Ying; Song, Guo-Xue; Meng, Zhao-Jun


    The Chalukou porphyry Mo deposit (2.46 Mt @ 0.087% Mo), located in the northern Great Xing'an Range, NE China, is the largest Mo deposit discovered in China so far. The host rocks consist of aplite porphyry, granite porphyry and quartz porphyry, and are intruded into Lower Ordovician intermediate-felsic volcanic-sedimentary rocks and pre-ore monzogranite and are cut by post-ore feldspar porphyry, diorite porphyry and quartz monzonite porphyry. Here, we present the zircon U-Pb ages, whole-rock geochemistry, Sr-Nd isotopic and zircon Hf isotopic data for the pre-ore, syn-ore and post-ore intrusive rocks. The Chalukou ore-forming porphyries intruded during 147-148 Ma and have high-silica, alkali-rich, metaluminous to slightly peraluminous compositions and are oxidized. They are enriched in large ion lithophile elements (e.g. K, Rb, U and Th), light REE and depleted in high-field strength elements (e.g. Nb, P and Ti). Depletions in Eu, Ba, Sr, Nb, Ta, P and Ti suggest that they have experienced strong fractional crystallization of plagioclase, biotite, hornblende and accessory minerals. The pre-ore monzogranite (~ 172 Ma) also belongs to the high-K calc-alkaline series. Highly fractionated REE patterns ((La/Yb) N = 19.6-21.7), high values of Sr/Y (54-69) and La/Yb (29-32), are adakite-like geochemical features. The post-ore rocks (~ 141-128 Ma) have similar geochemical characteristics with ore-forming porphyries except that quartz monzonite porphyry shows no Ba-Sr negative anomaly. All intrusive rocks have relative low initial 87Sr/86Sr (0.705413-0.707889) and ?Nd (t) values (- 1.28 to + 0.92), positive ?Hf (t) values (+ 2.4 to + 10.1) and young two-stage Nd and Hf model ages (TDM2 (Nd) = 863-977 Ma, TDM2 (Hf) = 552-976 Ma). These geochemical and isotopic data are interpreted to demonstrate that the ore-forming porphyries formed by partial melting of the juvenile lower crust caused by underplating of mafic magmas in an intra-plate extensional setting. The pre-ore monzogranite formed by partial melting of thickened lower crust in a collisional setting caused by closure of Mongol-Okhotsk Ocean. The post-ore feldspar porphyry shares a similar magma source with ore-forming porphyry, but the quartz monzonite porphyry has a relatively deeper magma source region and has not experienced as much fractional crystallization. The transformation from middle Jurassic compression to late Jurassic extension created favorable conditions for the generation and emplacement of the ore-forming magma. The juvenile lower crust provided the main source of molybdenum for Chalukou deposit. Enrichment of Mo by fractional crystallization played an important role in concentrating Mo during formation of the Chalukou Mo deposit. The age (~ 147 Ma), high fluorine, and associated Pb-Zn deposits are all different from other major porphyry Mo deposits in NE China; Chalukou is a new mineral deposit type in the Great Xing'an Range.

  3. Unified Structural Representation of the southern California crust and upper mantle (United States)

    Shaw, John H.; Plesch, Andreas; Tape, Carl; Suess, M. Peter; Jordan, Thomas H.; Ely, Geoffrey; Hauksson, Egill; Tromp, Jeroen; Tanimoto, Toshiro; Graves, Robert; Olsen, Kim; Nicholson, Craig; Maechling, Philip J.; Rivero, Carlos; Lovely, Peter; Brankman, Charles M.; Munster, Jason


    We present a new, 3D description of crust and upper mantle velocity structure in southern California implemented as a Unified Structural Representation (USR). The USR is comprised of detailed basin velocity descriptions that are based on tens of thousands of direct velocity (Vp, Vs) measurements and incorporates the locations and displacement of major fault zones that influence basin structure. These basin descriptions were used to developed tomographic models of crust and upper mantle velocity and density structure, which were subsequently iterated and improved using 3D waveform adjoint tomography. A geotechnical layer (GTL) based on Vs30 measurements and consistent with the underlying velocity descriptions was also developed as an optional model component. The resulting model provides a detailed description of the structure of the southern California crust and upper mantle that reflects the complex tectonic history of the region. The crust thickens eastward as Moho depth varies from 10 to 40 km reflecting the transition from oceanic to continental crust. Deep sedimentary basins and underlying areas of thin crust reflect Neogene extensional tectonics overprinted by transpressional deformation and rapid sediment deposition since the late Pliocene. To illustrate the impact of this complex structure on strong ground motion forecasting, we simulate rupture of a proposed M 7.9 earthquake source in the Western Transverse Ranges. The results show distinct basin amplification and focusing of energy that reflects crustal structure described by the USR that is not captured by simpler velocity descriptions. We anticipate that the USR will be useful for a broad range of simulation and modeling efforts, including strong ground motion forecasting, dynamic rupture simulations, and fault system modeling. The USR is available through the Southern California Earthquake Center (SCEC) website (

  4. Nuclei in Strongly Magnetised Neutron Star Crusts

    CERN Document Server

    Nandi, Rana


    We discuss the ground state properties of matter in outer and inner crusts of neutron stars under the influence of strong magnetic fields. In particular, we demonstrate the effects of Landau quantization of electrons on compositions of neutron star crusts. First we revisit the sequence of nuclei and the equation of state of the outer crust adopting the Baym, Pethick and Sutherland (BPS) model in the presence of strong magnetic fields and most recent versions of the theoretical and experimental nuclear mass tables. Next we deal with nuclei in the inner crust. Nuclei which are arranged in a lattice, are immersed in a nucleonic gas as well as a uniform background of electrons in the inner crust. The Wigner-Seitz approximation is adopted in this calculation and each lattice volume is replaced by a spherical cell. The coexistence of two phases of nuclear matter - liquid and gas, is considered in this case. We obtain the equilibrium nucleus corresponding to each baryon density by minimizing the free energy of the c...

  5. Tectonomagmatic setting and provenance of the Santa Marta Schists, northern Colombia: Insights on the growth and approach of Cretaceous Caribbean oceanic terranes to the South American continent (United States)

    Cardona, A.; Valencia, V.; Bustamante, C.; García-Casco, A.; Ojeda, G.; Ruiz, J.; Saldarriaga, M.; Weber, M.


    Metamorphosed volcano-sedimentary rocks accreted to the northern South American continental margin are major vestiges of the Caribbean oceanic plate evolution and its interactions with the continent. Selected whole rock geochemistry, Nd-Sr isotopes and detrital zircon geochronology were obtained in metabasic and metasedimentary rocks from the Santa Marta and San Lorenzo Schists in northernmost Colombia. Trace element patterns are characterized by primitive island arc and MORB signatures. Similarly initial 87Sr/ 86Sr-? Nd isotopic relations correlate with oceanic arcs and MORB reservoirs, suggesting that the protoliths were formed within a back-arc setting or at the transition between the inta-oceanic arc and the Caribbean oceanic crust. Trace element trends from associated metasedimentary rocks show that the provenance was controlled by a volcanic arc and a sialic continental domain, whereas detrital U/Pb zircons from the Santa Marta Schists and adjacent southeastern metamorphic units show Late Cretaceous and older Mesozoic, Late Paleozoic and Mesoproterozoic sources. Comparison with continental inland basins suggests that this arc-basin is allocthonous to its current position, and was still active by ca. 82 Ma. The geological features are comparable to other arc remnants found in northeastern Colombia and the Netherland Antilles. The geochemical and U/Pb detrital signatures from the metasedimentary rocks suggest that this tectonic domain was already in proximity to the continental margin, in a configuration similar to the modern Antilles or the Kermadec arc in the Pacific. The older continental detritus were derived from the ongoing Andean uplift feeding the intra-oceanic tectonic environment. Cross-cutting relations with granitoids and metamorphic ages suggest that metamorphism was completed by ca. 65 Ma.

  6. Early Cretaceous adakitic magmatism in the Dachagou area, northern Lhasa terrane, Tibet: Implications for slab roll-back and subsequent slab break-off of the lithosphere of the Bangong-Nujiang Ocean (United States)

    Wu, Hao; Li, Cai; Xu, Mengjing; Li, Xingkui


    The late Mesozoic geodynamic evolution of the Lhasa terrane (Tibet) remains controversial due to a lack of systematic geochemical and chronological data. Here we present the results of geochemical and zircon U-Pb geochronological studies of two granodiorite plutons in the Dachagou area of the northern Lhasa terrane. The zircon U-Pb dating yielded magmatic crystallization ages of 104 and 117 Ma, indicating pluton emplacement in the Early Cretaceous. On the basis of their K2O contents, the granodiorites can be divided into low-K calc-alkaline granodiorites (LKG, 104 Ma) and high-K calc-alkaline granodiorites (HKG, 117 Ma), and both types are characterized by the adakitic signatures of high Na2O and Sr contents, low Yb and Y abundances, and high Sr/Y ratios. The fact that the LKG and HKG formed at different times and have different geochemical characteristics suggests different petrogenetic mechanisms. Specifically, the LKG were generated by the interaction of an oceanic-slab-derived melt with enriched mantle, while the HKG were derived via the partial melting of the lower crust followed by hybridization with a significant amount of sediment and a minor amount of mantle. Based on these data and the regional tectonic setting, we propose that the LKG and HKG resulted from different geodynamic mechanisms: the HKG were the product of slab roll-back, while the LKG resulted from the subsequent slab break-off during the southwards subduction of the Bangong-Nujiang Ocean seafloor.

  7. Models of strange stars having a crust (United States)

    Vartanyan, Yu. L.; Grigoryan, A. K.


    Models of strange quark stars with a crust consisting of atomic nuclei and degenerate electrons, maintained by an electrostatic barrier at the surface of the strange quark matter, are investigated for a realistic range of parameters of the MIT bag model. The density at which neutrons escape from nuclei, ? = ?drip, is taken as the maximum possible boundary density of the crust. Series of strange stars are calculated as a function of central density. Configurations with masses of 1.44 and 1.77 M{ie 330-1} and a gravitational redshift Zs = 0.23, corresponding to the best-known observational data, are investigated. The presence of a crust results in the existence of a minimum mass for strange stars, and also helps to explain the glitch phenomenon of pulsars within the framework of the existence of strange quark matter.

  8. Magnetic susceptibility of a neutron star crust

    International Nuclear Information System (INIS)

    The magnetic susceptibility of the degenerate free electrons in the crust of a neutron star is computed for a range of densities, temperatures, and field strengths. It is shown that when the temperature is low enough (typically less than 107 K for densities of about 107 g cm-3 and 1012 G fields), the susceptibility undergoes de Haas-van Alphen oscillations. The crust is then unstable to the formation of layers of domains of alternating magnetisation. Associated with these domains are magnetic field fluctuations of a few per cent amplitude and anisotropic magnetostrictive stresses which may be large enough to crumble the crust. It is argued that these domains are unlikely to directly influence the surface properties of the neutron star but may possibly be coupled indirectly to observable effects. (author)

  9. Elemental composition of the Martian crust. (United States)

    McSween, Harry Y; Taylor, G Jeffrey; Wyatt, Michael B


    The composition of Mars' crust records the planet's integrated geologic history and provides clues to its differentiation. Spacecraft and meteorite data now provide a global view of the chemistry of the igneous crust that can be used to assess this history. Surface rocks on Mars are dominantly tholeiitic basalts formed by extensive partial melting and are not highly weathered. Siliceous or calc-alkaline rocks produced by melting and/or fractional crystallization of hydrated, recycled mantle sources, and silica-poor rocks produced by limited melting of alkali-rich mantle sources, are uncommon or absent. Spacecraft data suggest that martian meteorites are not representative of older, more voluminous crust and prompt questions about their use in defining diagnostic geochemical characteristics and in constraining mantle compositional models for Mars. PMID:19423810

  10. The hydrothermal power of oceanic lithosphere (United States)

    Grose, C. J.; Afonso, J. C.


    We have estimated the power of ventilated hydrothermal heat transport, and its spatial distribution, using a set of recently developed plate models which highlight the effects of hydrothermal circulation and thermal insulation by oceanic crust. Testing lithospheric cooling models with these two effects, we estimate that global advective heat transport is about 6.6 TW, significantly lower than previous estimates, and that the fraction of that extracted by vigorous circulation on the ridge axes (important problems in the physics and chemistry of the Earth because the magnitude of hydrothermal power affects chemical exchanges between the oceans and the lithosphere, thereby affecting both thermal and chemical budgets in the oceanic crust and lithosphere, the subduction factory, and convective mantle.

  11. Ocean Mammals (United States)

    Miss Teschner


    What are the different types of mammals that live in the ocean? First, you will need to use the Ocean Mammals Table 1. This website is here for you to learn about ocean mammals. Mammals 2. This website will help you learn about the different mammals that live in the ocean. Ocean Mammals 3. Here is some information about how oil spills effect animal skin in the ocean. Oil Spills 4. This link ...

  12. Crusted scabies and multiple dosages of ivermectin. (United States)

    Ortega-Loayza, Alex G; McCall, Calvin O; Nunley, Julia R


    We present the case of a bone marrow transplant patient who was diagnosed with crusted scabies but did not respond to the usual approach with topical permethrin and ivermectin. The Centers for Disease Control and Prevention were contacted and suggested a 7-dose regimen of ivermectin. The patient started to improve remarkably after the third dose, and the skin eruption was resolved after 7 doses. This case supports the use of a more prolonged course of oral ivermectin for crusted scabies in those who fail the conventional approach. PMID:23652958

  13. Tungsten Stable Isotope Compositions of Ferromanganese Crusts (United States)

    Abraham, K.; Barling, J.; Hein, J. R.; Schauble, E. A.; Halliday, A. N.


    We report the first accurate and precise data for mass-dependent fractionation of tungsten (W) stable isotopes, using a double spike technique and MC-ICPMS. Results are expressed relative to the NIST 3136 W isotope standard as per mil deviations in 186W/184W (?186W). Although heavy element mass-dependent fractionations are expected to be small, Tl and U both display significant low temperature isotopic fractionations. Theoretical calculations indicate that W nuclear volume isotopic effects should be smaller than mass-dependent fractionations at low temperatures. Hydrogenetic ferromanganese (Fe-Mn) crusts precipitate directly from seawater and have been used as paleoceanographic recorders of temporal changes in seawater chemistry. Crusts are strongly enriched in W and other metals, and are a promising medium for exploring W isotopic variability. Tungsten has a relatively long residence time in seawater of ~61,000 years, mainly as the tungstate ion (WO42-). Water depth profiles show conservative behaviour. During adsorption on Fe-Mn crusts, W species form inner-sphere complexes in the hexavalent (W6+) state. The major host phase is thought to be Mn oxides and the lighter W isotope is expected to be absorbed preferentially. Surface scrapings of 13 globally distributed hydrogenetic Fe-Mn crusts display ?186W from -0.08 to -0.22‰ (±0.03‰, 2sd). A trend toward lighter W isotope composition exists with increasing water depth (~1500 to ~5200m) and W concentration. One hydrothermal Mn-oxide sample is anomalously light and Mn nodules are both heavy and light relative to Fe-Mn crusts. Tungsten speciation depends on concentration, pH, and time in solution and is not well understood because of the extremely slow kinetics of the reactions. In addition, speciation of aqueous and/or adsorbed species might be sensitive to pressure, showing similar thermodynamic stability but different effective volumes. Thus, W stable isotopes might be used as a water-depth barometer in marine environments; time-series in Fe-Mn crusts may show a heavier isotope composition in older crust layers due to the shallower water environments in the early history of the seamounts on which the crusts grow.

  14. Millennial-scale ocean acidification and late Quaternary

    Energy Technology Data Exchange (ETDEWEB)

    Riding, Dr Robert E [University of Tennessee (UT); Liang, Liyuan [ORNL; Braga, Dr Juan Carlos [Universidad de Granada, Departamento de Estratigraf?a y Paleontolog?a, Granada, Spain


    Ocean acidification by atmospheric carbon dioxide has increased almost continuously since the last glacial maximum (LGM), 21 000 years ago. It is expected to impair tropical reef development, but effects on reefs at the present day and in the recent past have proved difficult to evaluate. We present evidence that acidification has already significantly reduced the formation of calcified bacterial crusts in tropical reefs. Unlike major reef builders such as coralline algae and corals that more closely control their calcification, bacterial calcification is very sensitive to ambient changes in carbonate chemistry. Bacterial crusts in reef cavities have declined in thickness over the past 14 000 years with largest reduction occurring 12 000 10 000 years ago. We interpret this as an early effect of deglacial ocean acidification on reef calcification and infer that similar crusts were likely to have been thicker when seawater carbonate saturation was increased during earlier glacial intervals, and thinner during interglacials. These changes in crust thickness could have substantially affected reef development over glacial cycles, as rigid crusts significantly strengthen framework and their reduction would have increased the susceptibility of reefs to biological and physical erosion. Bacterial crust decline reveals previously unrecognized millennial-scale acidification effects on tropical reefs. This directs attention to the role of crusts in reef formation and the ability of bioinduced calcification to reflect changes in seawater chemistry. It also provides a long-term context for assessing anticipated anthropogenic effects.

  15. The crust role at Paramillos Altos intrusive belt: Sr and Pb isotope evidence

    International Nuclear Information System (INIS)

    Paramillos Altos Intrusive Belt (PAIB) (Ostera, 1996) is located in the thick skinned folded-thrust belt of Malargue, southwestern Mendoza, Argentina. Geochemical, geochronologic and isotopic studies were carried out in it (Ostera 1996, 1997, Ostera et al. 1999; Ostera et al. 2000) and these previous papers suggested a minor involvement of the crust in the genesis of the PAIB. According with Ostera et al. (2000) it is composed by stocks, laccoliths, dykes and sills which range in composition from diorites to granodiorites, and from andesites to rhyolites, and divided in five Members, which range in age from Middle Miocene to Early Miocene: a- Calle del Yeso Dyke Complex (CYDC), with sills and dykes of andesitic composition (age: 20±2 Ma). b- Puchenque-Atravesadas Intrusive Complex (PAIC), composed by dykes and stocks ranging from diorites to granodiorites (age: 12.5±1 Ma). c- Arroyo Serrucho Stock (SAS), an epizonal and zoned stock, with four facies, with K/Ar and Ar/Ar dates of 10±1 and 9.5±0.5 Ma. d- Portezuelo de los Cerros Bayos (PCB), that includes porphyritic rocks of rhyolitic composition, of 7.5±0.5 Ma. e- Cerro Bayo Vitrophyres (CBV), with andesitic sills and dykes (age: 4.8±0.2 Ma). We present in this paper new Sr and Pb isotopes data that constrain the evolution of the PAIB (au)

  16. Crust and Mantle Anisotropy Variations from the Coast to Inland In Central and Southern Mexico (United States)

    Castillo, J. A.; Perez-Campos, X.; Husker, A. L.; Valenzuela Wong, R.


    We examine radial and tangential receiver functions (RFs) obtained along the Meso America Subduction Experiment (MASE), a profile from the Pacific coast in central-southern Mexico to the Gulf of Mexico. Tangential RFs show waveform variations in timing and polarity, both in the Moho and the slab Ps phases in function of its backazimuth; also, radial RFs show arrivals timing variation; all of which suggests a non-homogeneous horizontal layer medium. Using a particle motion analysis and a cross-correlation procedure, we are able to quantify the shear wave splitting in the continental crust, the subducted oceanic crust and the mantle below each station of the array in terms of a time delay, and a fast azimuth direction. From these variations, we distinguish between three major regions: 1) dipping subducted slab, 2) horizontal subducted slab, 3) absence of subducted slab. Results for region 1 are consistent with the geometry of the Cocos plate previously determined by other studies, showing a minimum energy content in the tangential RFs for the NE-SW geographic quadrants. In region 2, we identify a strong azimuthal dependence with a variable periodicity of 180° and 360° as well as the existence of "split" Ps phases in our data, possibly related to the presence of fluids and the ultra low velocity layer (ULVL) localized between the continental and oceanic crust. We compare these results with previous silent earthquakes (SSE) and non-volcanic tremors (NVT) studies in the area.

  17. A relatively reduced Hadean continental crust (United States)

    Yang, Xiaozhi; Gaillard, Fabrice; Scaillet, Bruno


    Among the physical and chemical parameters used to characterize the Earth, oxidation state, as reflected by its prevailing oxygen fugacity (fO2), is a particularly important one. It controls many physicochemical properties and geological processes of the Earth's different reservoirs, and affects the partitioning of elements between coexisting phases and the speciation of degassed volatiles in melts. In the past decades, numerous studies have been conducted to document the evolution of mantle and atmospheric oxidation state with time and in particular the possible transition from an early reduced state to the present oxidized conditions. So far, it has been established that the oxidation state of the uppermost mantle is within ±2 log units of the quartz-fayalite-magnetite (QFM) buffer, probably back to ~4.4 billion years ago (Ga) based on trace-elements studies of mantle-derived komatiites, kimberlites, basalts, volcanics and zircons, and that the O2 levels of atmosphere were initially low and rose markedly ~2.3 Ga known as the Great Oxidation Event (GOE), progressively reaching its present oxidation state of ~10 log units above QFM. In contrast, the secular evolution of oxidation state of the continental crust, an important boundary separating the underlying upper mantle from the surrounding atmosphere and buffering the exchanges and interactions between the Earth's interior and exterior, has rarely been addressed, although the presence of evolved crustal materials on the Earth can be traced back to ~4.4 Ga, e.g. by detrital zircons. Zircon is a common accessory mineral in nature, occurring in a wide variety of igneous, sedimentary and metamorphic rocks, and is almost ubiquitous in crustal rocks. The physical and chemical durability of zircons makes them widely used in geochemical studies in terms of trace-elements, isotopes, ages and melt/mineral inclusions; in particular, zircons are persistent under most crustal conditions and can survive many secondary processes such as metamorphism, weathering and erosion. Thus, zircons in granites of shallow crust may record the chemical/isotopic composition of the deep crust that is otherwise inaccessible, and offer robust records of the magmatic and crust-forming events preserved in the continental crust. In fact, due to the absence of suitable rock records (in particular for periods older than ~4.0 Ga), studies in recent years concerning the nature, composition, growth and evolution of the continental crust, and especially the Hadean crust, have heavily relied on inherited/detrital zircons. Natural igneous zircons incorporate rare-earth elements (REE) and other trace elements in their structure at concentrations controlled by the temperature, pressure, fO2 and composition of their crystallization environment. Petrological observations and recent experiments have shown that the concentration of Ce relative to other REE in igneous zircons can be used to constrain the fO2 during their growth. By combining available trace-elements data of igneous zircons of crustal origin, we show that the Hadean continental crust was significantly more reduced than its modern counterpart and experienced progressive oxidation till ~3.6 billions years ago. We suggest that the increase in the oxidation state of the Hadean continental crust is related to the progressive decline in the intensity of meteorite impacts during the late veneer. Impacts of carbon- and hydrogen-rich materials during the formation of Hadean granitic crust must have favoured strongly reduced magmatism. The conjunction of cold, wet and reduced granitic magmatism during the Hadean implies the degassing of methane and water. When impacts ended, magma produced by normal decompression melting of the mantle imparted more oxidizing conditions to erupted lavas and the related crust.

  18. Compositional diversity of ca. 110 Ma magmatism in the northern Lhasa Terrane, Tibet: Implications for the magmatic origin and crustal growth in a continent-continent collision zone (United States)

    Sui, Qing-Lin; Wang, Qing; Zhu, Di-Cheng; Zhao, Zhi-Dan; Chen, Yue; Santosh, M.; Hu, Zhao-Chu; Yuan, Hong-Lin; Mo, Xuan-Xue


    The Yanhu area in the northern Lhasa subterrane exposes diverse rock types including basalts, rhyolites, quartz dioritic porphyries, and associated dioritic enclaves. The basalts and rhyolites occur as a bimodal volcanic suite, and the quartz dioritic porphyries intrude into the older basalts as a small apophysis. In this paper, we report for the first time the zircon LA-ICP-MS U-Pb age and Hf isotopic composition data, whole-rock major and trace element composition data, and Sr-Nd isotopic data from the diverse Early Cretaceous magmatic rocks from Yanhu. The three basalt samples that we dated yielded zircon U-Pb ages of 110 ± 0.7 Ma, 108.9 ± 1.1 Ma, and 111.8 ± 3.2 Ma. The zircons from one quartz diorite porphyry yielded an age of 109.7 ± 0.8 Ma, which is coeval with the dioritic enclave (110.4 ± 1.4 Ma). The basalts show a high-K calc-alkaline signature, enriched in Rb, Th, U, and light rare earth elements (REEs) and depleted in Nb, Ta, Ti, Zr, and Hf. These rocks possess varying whole-rock ?Nd(t) (- 0.6 to + 2.1) and zircon ?Hf(t) (+ 0.6 to + 8.9) values. The rhyolite samples are high-K calc-alkaline and are metaluminous to slightly peraluminous. These rocks yielded whole-rock ?Nd(t) of + 0.1 to + 0.9 and zircon ?Hf(t) of + 5.1 to + 12.4. The quartz dioritic porphyries are characterized by high Al2O3 content (15.9-16.1 wt.%), high Sr (466-556 ppm), low Yb (1.36-1.41 ppm), and low Y (13.8 ppm) abundances. Similar geochemical signatures are also present in the dioritic enclaves, revealing that both the quartz dioritic porphyries and the dioritic enclaves have an affinity with adakitic rocks. Moreover, the host rocks and the enclaves display homogeneous ?Nd(t) (+ 3.5 to + 3.6 and + 3.4 to + 4.0, respectively) and zircon ?Hf(t) values (+ 12.4 to + 16.9 and + 11.5 to + 15.7, respectively). Our geochemical data indicate that the magmatic rocks from Yanhu were derived from the partial melting of distinct source regions, i.e., a heterogeneous metasomatized mantle wedge source for basalts, a juvenile crust source for rhyolites, and a thickened mafic lower crust source that mixed with basaltic magmas for the adakitic rocks (including quartz dioritic porphyries and dioritic enclaves). Compared to typical arc basalts, the basalts from Yanhu are relatively enriched with high field strength elements (HFSEs) (e.g., Zr, Nb), resembling those of within-plate basalts elsewhere. In combination with the presence of a coeval bimodal volcanic rock suite, the ca. 110 Ma magmatism in Yanhu is inferred to have occurred in an extensional setting. Our new data, together with recently published data, enable us to correlate the generation of the compositional diversity of the ca. 110 Ma Yanhu magmatic rocks that formed in an extensional setting to the slab break-off during the southward subduction of the Bangong-Nujiang Ocean lithosphere. The presence of basaltic magmatism and coeval silicic magmatic rocks with positive zircon ?Hf(t) indicate that the extensive magmatism at ca. 110 Ma have contributed significantly to the crustal growth of the northern Lhasa subterrane.

  19. MaXi Avisen

    DEFF Research Database (Denmark)

    Kanstrup, Anne Marie; SØrensen, Marianne

    maXi-projektets vision er at sprænge rammerne for sundhedsstøtte med it ved at sætte diabetikere og deres familier i centrum og ved at flytte fokus fra sygdom og hospitaler til samfund, hverdagsliv og services. maXi-projektet har til formål at afprøve og gennemføre brugerdreven innovation som metode til konceptudvikling af it-services til kronikere med diabetikere som fokusgruppe. Målet er at kunne fremlægge en række koncepter for it-services til støtte af kronikere i deres hverdagsliv. I år 2008 deltager 8 familier i innovationsprocessen gen-nem interviews, dagbøger, workshops og eksperimenter i et 'living lab' - som etableres i Skagen. I 2009 udvælges nye brugere til deltagelse i projektet. maXi-projektet opbygges som et modelprojekt i samar-bejde mellem Aalborg Universitet, Fonden Skagen Helse, Teknologisk Institut og Edvantage Group. Se Projektet er støttet af Erhvervs- og Byggestyrelsen - pro-grammet for brugerdreven Udgivelsesdato: juni

  20. The morphostructure of the atlantic ocean floor its development in the meso-cenozoic

    CERN Document Server

    Litvin, V M


    The study of the topography and structure of the ocean floor is one of the most important stages in ascertaining the geological structure and history of development of the Earth's oceanic crust. This, in its turn, provides a means for purposeful, scientifically-substantiated prospecting, exploration and development of the mineral resources of the ocean. The Atlantic Ocean has been geologically and geophysically studied to a great extent and many years of investigating its floor have revealed the laws governing the structure of the major forms of its submarine relief (e. g. , the continental shelf, the continental slope, the transition zones, the ocean bed, and the Mid-Oceanic Ridge). The basic features of the Earth's oceanic crust structure, anomalous geophysical fields, and the thickness and structure of its sedimentary cover have also been studied. Based on the investigations of the Atlantic Ocean floor and its surrounding continents, the presently prevalent concept of new global tectonics has appeared. A g...

  1. Crust-mantle interaction beneath the Luxi Block, eastern North China Craton: Evidence from coexisting mantle- and crust-derived enclaves in a quartz monzonite pluton (United States)

    Lan, Ting-Guang; Fan, Hong-Rui; Santosh, M.; Hu, Fang-Fang; Yang, Kui-Feng; Yang, Yue-Heng; Liu, Yongsheng


    The Laiwu quartz monzonite in the Luxi Block of eastern North China Craton (NCC) is characterized by the presence of abundant plagioclase amphibolite and gabbro-diorite enclaves. Here we present LA-ICPMS zircon U-Pb ages which show that the host quartz monzonite was emplaced at 129.8 ± 1.0 Ma, whereas the protolith of the plagioclase amphibolite enclaves formed during early Paleoproterozoic. The gabbro-diorite enclaves were produced simultaneously with or slightly earlier than the formation of the host quartz monzonite. Combined with the Archean and Paleoproterozoic zircons as well as the low ?Nd(0) values (- 18.4 to - 18.0) in the plagioclase amphibolite enclaves, the equilibrium temperature and pressure conditions (645-670 °C and 4.8-6.5 Kb) suggest that the plagioclase amphibolite enclaves are fragments of the middle crust. The gabbro-diorite enclaves mainly originated from an enriched lithospheric mantle metasomatized by melts/fluids derived from the continental crust, as indicated by their low SiO2 (54.4-54.7 wt.%) and high MgO (10.9-11.1 wt.%) contents as well as the negative ?Nd(t) values (- 13.5 to - 10.7) and enrichment of LILEs (e.g., Ba and Sr) and depletion of HFSEs (e.g., Nb, Ta, P and Ti). Compared with the ancient crustal rocks and the mafic plutons considered to have been derived from lithospheric mantle in the Luxi Block, the moderate ?Nd(t) (- 15.7 to - 15.1) and ?Hf(t) (- 20.7 to - 13.0) values of the quartz monzonite in our study suggest that both mantle- and crust-derived melts were involved in the magma generation. Thus we propose a model involving magma mixing between mantle- and crust-derived melts for the formation of the quartz monzonite. Since significant crust-mantle interaction is recorded not only in the quartz monzonite and its enclaves in the Luxi Block but also in the other granitoids widespread in the NCC, it is considered that large-scale crust-mantle interaction and magmatic underplating were associated with the Mesozoic lithospheric mantle thinning and crustal reactivation in the eastern NCC.

  2. Velocity structure of the oceanic lithosphere in the western Pacific obtained from Rayleigh waves (United States)

    Sotirov, T. A.; Weeraratne, D. S.; Forsyth, D. W.; Ramirez, C.; Takeo, A.


    We present results from a seafloor seismic tomography study that is designed to test existing models for the evolution of the oceanic lithosphere with time. When the newly created seafloor spreads away from the mid ocean ridge it cools and subsides. This process is described by a model of conductive cooling of an infinite half space, which predicts that seafloor will subside in proportion to the square root of age. This is consistent with bathymetry data for regions up to 80 Ma. However for seafloor that is older than 80 Ma, subsidence departs significantly from this relationship and is shallower than predicted by the conductive cooling model. This flattening has been explained by several models including the cooling plate model that incorporates a constant temperature boundary condition at base of the plate, and small scale convection. The PLATE project (Pacific Lithosphere Anisotropy and Thickness Experiment) deployed two arrays of ocean bottom seismometers on each arm of a magnetic bight in the western Pacific (150 -160 Ma) to exploit the contrast in lithospheric fabric expected on either side of the ancient triple junction. We will measure seismic velocities and anisotropy in each array and expect to observe a lateral contrast in velocities and crystal alignment within the lithosphere in each region as well as a vertical contrast between the lithosphere and the asthenosphere due to mantle flow induced by Pacific plate motion. We use approximately 100 earthquake events with excellent azimuthal distribution that is only sparse from the NW azimuth. Seismic records are filtered with a series of narrow bandpass filters at periods from 18 to 78 s, and then windowed around the Rayleigh wave group arrival at each period. This period range samples shear velocity structure from the crust to the upper asthenosphere. Tomographic imaging is achieved by a method that uses two plane waves to approximate complexity in the incoming wave field from each source event and finite frequency kernels to represent the local response. We will compare our velocities with results from previous global tomography studies that employed only long paths from earthquakes to land stations as well as previous regional ocean-bottom seismometer studies. Predictions by previous models for old seafloor including the infinite half-space cooling, the plate model, and small scale convection will be tested with results from this study located on the oldest seafloor in the western Pacific that has identifiable magnetic anomalies and has not been altered by volcanic activity since its initial creation.

  3. Geodetic And Geological Analysis Of The Tandilia Crust (United States)

    Del Cogliano, D.; Dallasalda, L.


    Keywords: Tandilia-Geoid-Anomaly-Collision-Transamazonic The oldest Precambrian rocks of the south-western Gondwana in South America are cropping out in the Río de la Plata craton, it encompasses the western region of Uruguay, the Martín Garcia island and the Tandilia Ranges in the Buenos Aires Province, Argentina. The Tandil Ranges are the oldest region in Argentina (1.8- 2.2Ga); however, some features of the crust still remain unknown. These rocks evolved during two main events:Transamazonian and Brasilian tectonic cycles. The local and regional gravitational effects were analyzed on gravity and height anomalies. The studied are extended on 400 km x 400 km area which includes three geological units: the Tandilia ranges, and the Claromecó and Salado basins. Due the dependence of gravity and height anomalies with the distance, the seconds are more suitable to analyze the crust interior. For this reason a very precise cuasi geoid model was calculated using the point masses method, from gravity and GPS/leveling data. Taking into account the topography (less than 500 m high hills) and the Bouguer anomalies values (| AB | < 35 mGal), the differences between the cuasi geoid and the geoid will are less than 1 cm. In consequence the geoid undulations (N) were used. The EIGEN-CG01C geopotential model allowed to remove the wavelengths of more than 1000 km of the local geoid (Nobs=N-Neigen). This made it possible to analyze the signals attributed to the structure of Tandilia, and the Claromecó and Salado basins influence on it. In Tandilia geophysical, geodetic and geologic analysis allowed to postulate an isostatic compensated Airy model (Nisost) with a 42 km thick crust (2.84 gr/cm3) resting on a 3.24 gr/cm3 density mantel. Residuals geoid distribution (Nobs-Nisost) shows a tendency to the eastern edge of the ranges. This anomaly is interpreted as the presence of an upper crust excess of mass, which, from Olavarría and Azul hills (north of Tandilia) it extends to the southwest running mainly along the eastern edge of the ranges until the continental platform. This anomaly is attributed to a basic-ultrabasic tectonic slab, a relict of a suture (oceanic bottom and astenosphere) from a continent-continent collision (Transamazonian orogeny). This collisional model was previously based on the presence of wide areas of gneisses, migmatites and granitoids (leucogranites), of sub volcanic and lava flows, of a polifase metamorphic-deformational style, of swarms of pre-metamorphic belts, and of a strong piling up of the crust, associated to thrusting and transcurrence; as well as minor lenses of ultrabasic rocks, that seem to also be part of the suture as a result of a "mantel pinching" during collision.

  4. Norwegian crusted scabies: an unusual case presentation. (United States)

    Maghrabi, Michael M; Lum, Shireen; Joba, Ameha T; Meier, Molly J; Holmbeck, Ryan J; Kennedy, Kate


    Scabies is a contagious condition that is transmitted through direct contact with an infected person and has been frequently associated with institutional and healthcare-facility outbreaks. The subtype Norwegian crusted scabies can masquerade as other dermatologic diseases owing to the heavy plaque formation. Successful treatment has been documented in published reports, including oral ivermectin and topical permethrin. Few case studies documenting the treatment of Norwegian crusted scabies have reported the use of surgical debridement as an aid to topical and/or oral treatment when severe plaque formation has been noted. A nursing home patient was admitted to the hospital for severe plaque formation of both feet. A superficial biopsy was negative for both fungus and scabies because of the severity of the plaque formation on both feet. The patient underwent a surgical, diagnostic biopsy of both feet, leading to the diagnosis of Norwegian crusted scabies. A second surgical debridement was then performed to remove the extensive plaque formation and aid the oral ivermectin and topical permethrin treatment. The patient subsequently made a full recovery and was discharged back to the nursing home. At 2 and 6 months after treatment, the patient remained free of scabies infestation, and the surgical wound had healed uneventfully. The present case presentation has demonstrated that surgical debridement can be complementary to the standard topical and oral medications in the treatment of those with Norwegian crusted scabies infestation. PMID:24370484

  5. Crusted Scabies In An HIV Seropositive Woman

    Directory of Open Access Journals (Sweden)

    Thappa Devinder Mohan


    Full Text Available A 27 year old female presented with itchy lesions over the trunk and extremities of six month duration. She had typical lesions of scabies in hands, besides crusted hyperkeratotic lesions over the lumber and gluteal areas, and was found to have underlying HIV infection. The case is reported for unusual presentation and its rarity.

  6. Pulsar Glitches: The Crust may be Enough

    CERN Document Server

    Piekarewicz, J; Horowitz, C J


    Pulsar glitches-the sudden spin-up in the rotational frequency of a neutron star-suggest the existence of an angular-momentum reservoir confined to the inner crust of the neutron star. Large and regular glitches observed in the Vela pulsar have originally constrained the fraction of the stellar moment of inertia that must reside in the solid crust to about 1.4%. However, crustal entrainment-which until very recently has been ignored-suggests that in order to account for the Vela glitches, the fraction of the moment of inertia residing in the crust must increase to about 7%. This indicates that the required angular momentum reservoir may exceed that which is available in the crust. We explore the possibility that uncertainties in the equation of state provide enough flexibility for the construction of models that predict a large crustal thickness and consequently a large crustal moment of inertia. Given that analytic results suggest that the crustal moment of inertia is sensitive to the transition pressure at ...

  7. COOL: Crust of the Oman Ophiolite and its Lithosphere - a passive seismic experiment (United States)

    Weidle, Christian; Agard, Philippe; Ducassou, Céline; El-Hussain, Issa; Prigent, Cécile; Meier, Thomas


    Plate tectonics has established a framework for geoscientists to understand most geologic/tectonic processes that shaped our present-day Earth. 'Obduction', the emplacement of young, dense oceanic lithosphere (ophiolites) on top of older lighter continental lithosphere remains, however, a rather odd phenomenon. Some ophiolites are fundamentally similar to young oceanic crust and it is hence assumed that they were obducted as thrust sheets at the onset of continental subduction in a previously intra-oceanic subduction setting. The Peri-Arabic obduction corresponded to a spectacular, almost synchronous thrust movement along thousands of km from Turkey to Oman. At the eastern margin of the Arabian plate, the world's largest and best preserved ophiolite was emplaced in only a few My during Upper Cretaceous and is exposed today atop the Oman Mountain range. Although being the best studied ophiolite in the world, rather little is still known about the internal structure of the ophiolite and the Oman Mountains. The dimension of the ophiolite is large enough (~700 km) to be studied with seismological methods, providing thus a rare setting to investigate oceanic crust on land without ocean bottom installations. We have deployed a network of 40 broadband seismometers across the Oman Mountains in Oct/Nov 2013 for passive seismic registration for a duration of ca. 15 months. The network is complemented by 10 permanent stations in the area operated by the Earthquake Monitoring Center in Oman. Aims of the project include: - Seismological imaging of the geometry and internal properties of obducted oceanic, and its underlying continental lithosphere. - Regional tomographic velocity models will provide constraints on geodynamic processes that led to large scale obduction. - Investigating the "quiet" Makran subduction zone for local seismicity will improve understanding of seismic hazard on the eastern Arabian plate.

  8. Pulsar glitches: The crust may be enough (United States)

    Piekarewicz, J.; Fattoyev, F. J.; Horowitz, C. J.


    Background: Pulsar glitches—the sudden spin-up in the rotational frequency of a neutron star—suggest the existence of an angular-momentum reservoir confined to the inner crust of the neutron star. Large and regular glitches observed in the Vela pulsar have originally constrained the fraction of the stellar moment of inertia that must reside in the solid crust to about 1.4%. However, crustal entrainment—which until very recently has been ignored—suggests that in order to account for the Vela glitches, the fraction of the moment of inertia residing in the crust must increase significantly; to about 7-9 %. This indicates that the required angular momentum reservoir may exceed that which is available in the crust. Purpose: We explore the possibility that uncertainties in the equation of state provide enough flexibility for the construction of models that predict a large crustal thickness and consequently a large crustal moment of inertia. Methods: Moments of inertia—both total and crustal—are computed in the slow-rotation approximation using a relativistic mean-field formalism to generate the equation of state of neutron-star matter. Results: We compute the fractional moment of inertia of neutron stars of various masses using a representative set of relativistic mean-field models. Given that analytic results suggest that the crustal moment of inertia is sensitive to the transition pressure at the crust-core interface, we tune the parameters of the model to maximize the transition pressure, while still providing an excellent description of nuclear observables. In this manner we are able to obtain fractional moments of inertia as large as 7% for neutron stars with masses below 1.6 solar masses. Conclusions: We find that uncertainties in the equation of state of neutron-rich matter are large enough to accommodate theoretical models that predict large crustal moments of inertia. In particular, we find that if the neutron-skin thickness of Pb208 falls within the (0.20-0.26) fm range, large enough transition pressures can be generated to explain the large Vela glitches—without invoking an additional angular-momentum reservoir beyond that confined to the solid crust. Our results suggest that the crust may be enough.

  9. Global Moho from the combination of the CRUST2.0 model and GOCE data (United States)

    Reguzzoni, Mirko; Sampietro, Daniele; Sansò, Fernando


    The discontinuity surface between Earth crust and mantle, the so-called Moho, is commonly estimated by means of seismic or gravimetric methods. Usually these methods do not yield the same result since they are based on different geological and geophysical hypotheses, as well as different data types, also in terms of quality and spatial distribution. In particular, global crust models based only on seismic data (e.g. the CRUST2.0 model) can be locally very accurate since seismic profiles give an almost direct observation of the actual crust structure, but can be quite uninformative in large regions where no data are available or they are too inhomogeneous. On the contrary, when using satellite gravity observations like those provided by the ESA mission GOCE, information on the Moho can be inferred from a uniform and global data set. However, Moho models estimated by gravity data are in general characterized by simplified hypotheses to guarantee the uniqueness of the solution of the inverse gravitational problem. The aim of this work is to attenuate these drawbacks by combining the seismic global model CRUST2.0 with gravity observations from the GOCE satellite mission. More specifically, the used GOCE data are grid values at mean satellite altitude estimated by the so-called space-wise approach. After reducing the data to a two-layer model by removing the effect of topography, bathymetry and sediments, a combined inversion driven by a priori information on the CRUST2.0 accuracy and by the error covariance structure of the GOCE grids is performed. In addition, the observation errors as well as the error due to the data reduction are tentatively taken into account to estimate the accuracy of the final Moho model. The result is an update of the CRUST2.0 Moho model with a 0.5° × 0.5° resolution, which at the same time contains seismic and geological information and it is consistent, at 20 mE level, to the actually observed gravity field. A first comparison with the CRUST2.0 Moho shows that in the continental crust the mean difference between the two models is of the order of 1.5 km with standard deviations depending on the considered region. As expected, the main variations (standard deviation of the order of 7 km) are located in South America, Africa and Antarctica where very few data in the CRUST2.0 were originally used. In the rest of the world, differences have a standard deviation of about 4 km. As for the oceanic crust, it can be noted that the corrections to the CRUST2.0 model are of the order of 3 km (mean value) with a standard deviation of 6 km. Finally, the solution computed in this paper has been compared with a set of Moho models at different scales from global to local ones showing that it is reasonably consistent (differences of about 5 km standard deviation) also with seismic observations.

  10. Anomalous Subsidence at the Ocean Continent Transition of the Gulf of Aden Rifted Continental Margin (United States)

    Cowie, Leanne; Kusznir, Nick; Leroy, Sylvie


    It has been proposed that some rifted continental margins have anomalous subsidence and that at break-up they were elevated at shallower bathymetries than the isostatic response predicted by classical rift models (McKenzie, 1978). The existence of anomalous syn- or early-post break-up subsidence of this form would have important implications for our understanding of the geodynamics of continental break-up and sea-floor spreading initiation. We have investigated subsidence of the young rifted continental margin of the eastern Gulf of Aden, focussing on the western Oman margin (break-up age 17.6 Ma). Lucazeau et al. (2008) have found that the observed bathymetry here is approximately 1 km shallower than the predicted bathymetry. In order to examine the proposition of an anomalous early post break-up subsidence history of the Omani Gulf of Aden rifted continental margin, we have determined the subsidence of the oldest oceanic crust adjacent to the continent-ocean boundary (COB) using residual depth anomaly (RDA) analysis corrected for sediment loading and oceanic crustal thickness variation. RDAs corrected for sediment loading using flexural backstripping and decompaction have been calculated by comparing observed and age predicted oceanic bathymetries in order to identify anomalous subsidence of the Gulf of Aden rifted continental margin. Age predicted bathymetric anomalies have been calculated using the thermal plate model predictions of Crosby and McKenzie (2009). Non-zero RDAs at the Omani Gulf of Aden rifted continental margin can be the result of non standard oceanic crustal thickness or the effect of mantle dynamic topography or a non-classical rift and break-up model. Oceanic crustal basement thicknesses from gravity inversion together with Airy isostasy have been used to predict a "synthetic" gravity RDA, in order to determine the RDA contribution from non-standard oceanic crustal thickness. Gravity inversion, used to determine crustal basement thickness, incorporates a lithosphere thermal gravity anomaly correction and uses sediment thicknesses from 2D seismic data. Reference Moho depths used in the gravity inversion have been calibrated against seismic refraction Moho depths. The difference between the sediment corrected RDA and the "synthetic" gravity derived RDA gives the component of the RDA which is not due to variations in oceanic crustal thickness. This RDA corrected for sediment loading and crustal thickness variation has a magnitude between +600m and +1000m (corresponding to anomalous uplift) and is comparable to that reported (+1km) by Lucazeau et al. (2008). We are unable to distinguish whether this anomalous uplift is due to mantle dynamic topography or anomalous subsidence with respect to classical rift model predictions.

  11. High Tech High interns develop a mid-ocean ridge database for research and education (United States)

    Staudigel, D.; Delaney, R.; Staudigel, H.; Koppers, A.; Miller, S.


    Mid-ocean ridges (MOR) represent one of the most important geographic features on planet Earth. MORs are the locations where plates spread apart, they are the locations of most of the earths' volcanoes that harbor some of the most extreme life forms. These concepts attract much research, but mid-ocean ridges are still effectively not represented in the earth science class rooms. We began an internship at Scripps to develop a database for mid-ocean ridges as a resource for science and for education. Major research goals of this project include the development of an archival structure for data, images or any other arbitrary digital objects relating to MORs, and to compile a global data set for some of the most defining characteristics of every ridge segment. One of the challenges included the need of making MOR data useful to the scientist as well as the teacher in the class room. While this data base remains a long term project, we completed a series of first order steps that establish an archival structure and lay out the defining information for each ridge segment. To create this database we used existing maps of the age of the ocean floor (University of Sidney) and the MOR locations from the University of Texas database. We divided the global MOR system into segments through their end-point coordinates, using the mid-point lat/lon as a generic name and we digitized the area of the 11 Ma isochron. Each ridge segment was also characterized with the ocean it is in, and the names of the plates. This allowed us to create a database structure for MOR segments, similar to the seamount catalogue ( The data compiled allow us to determine the length of the ridges, spreading rates, the ocean crust production rates, and plate motion vectors for every ridge segment, ocean or the global ocean ridge system. The process of creating this data base introduced us to the excitement of MOR research, allowing us to create a resource that is equally useful for researchers as well as the members of the educational community.

  12. Stability of clathrate hydrates in Martian crust (United States)

    Gloesener, Elodie; Karatekin, Özgür; Dehant, Véronique


    Clathrate hydrates are crystalline compounds constituted by cages formed by hydrogen-bonded water molecules inside of which guest gas molecules are trapped. These materials are typically stable at high pressure and low temperature and are present on Earth mainly in marine sediments and in permafrost. Moreover, clathrate hydrates are expected to exist on celestial bodies like the icy moons Titan, Europa or Enceladus. Current conditions in the Martian crust are favourable to the presence of clathrate hydrates. In this study, we focused on the stability of methane and carbon dioxide clathrates in the Martian crust. We coupled the stability conditions of clathrates with a 1D thermal model in order to obtain the variations of the clathrate stability zone in the crust of Mars with time and for different crust compositions. Indeed, the type of soil directly controls the geothermal conditions and therefore the depth of clathrates formation. Unconsolidated soil acts as a thermal insulator and prevents the clathrates formation in the crust except on a small part of a few tens of meters thick. In contrast, sandstone or ice-cemented soil allows the clathrates formation with a stability zone of several kilometers. This is explained by the fact that they evacuate heat more efficiently and thus maintain lower temperatures. We also studied the stability zone of clathrates formed from a mixture of methane and hydrogen sulphide as well as from a mixture of methane and nitrogen. Contrary to the addition of N2, the addition of H2S to CH4 clathrates extends the stability zone and thus brings it closer to the surface. Therefore, mixed clathrates CH4-H2S will be more easily destabilized by changes in surface temperature than CH4 clathrates.

  13. Crust and upper mantle structure in the Caribbean region by group velocity tomography and regionalization

    International Nuclear Information System (INIS)

    An overview of the crust and upper mantle structure of the Central America and Caribbean region is presented as a result of the processing of more than 200 seismograms recorded by digital broadband stations from SSSN and GSN seismic networks. By FTAN analysis of the fundamental mode of the Rayleigh waves, group velocity dispersion curves are obtained in the period range from 10 s to 40 s; the error of these measurements varies from 0.06 and 0.10 km/s. From the dispersion curves, seven tomographic maps at different periods and with average spatial resolution of 500 km are obtained. Using the logical combinatorial classification techniques, eight main groups of dispersion curves are determined from the tomographic maps and eleven main regions, each one characterized by one kind of dispersion curves, are identified. The average dispersion curves obtained for each region are extended to 150 s by adding data from the tomographic study of and inverted using a non-linear procedure. As a result of the inversion process, a set of models of the S-wave velocity vs. depth in the crust and upper mantle are found. In six regions, we identify a typically oceanic crust and upper mantle structure, while in the other two the models are consistent with the presence of a continental structure. Two regions, located over the major geological zones of the accretionary crust of the Caribbean region, are characterized by a peculiar crust and upper mantle structure, indicating the presence of lithospheric roots reaching, at least, about 200 km of depth. (author)

  14. Widespread, Miocene, Large-Magnitude Exhumation of the Pamir Deep Crust Driven by the India-Asia Collision (United States)

    Hacker, B. R.; Ratschbacher, L.; Stearns, M.; McGraw, J.; Stübner, K.; Kylander-Clark, A. R.; Pfander, J.; Weise, C.; Minaev, V.; Gadoev, M.; Oimahmadov, I.


    Thermobarometry, thermochronology, and structural geology provide insight into the formation and exhumation histories of the high-grade crustal domes across the Pamir. P-T histories were reconstructed from thermobarometry based on major elements and pseudosections. Intrusion, recrystallization, and cooling histories were determined by SIMS and LA-MC-ICP-MS U-Th-Pb zircon, monazite, titanite, and rutile; Rb-Sr mica; 40Ar/39Ar hornblende and mica; apatite fission-track, and (U-Th)/He apatite dating. For the northern Pamir Kurgovat dome we find peak conditions of 600-650 °C and 6.5-8.2 kbar. Hornblende and biotite 40Ar/39Ar ages indicate that this metamorphism is Jurassic and Early Cretaceous (ca. 200 and ca. 130 Ma), overprinting Devonian arc intrusions (350 Ma U-Pb zircon). The western central Pamir Yazgulom dome yields P-T conditions of 575 °C and 9.4 kbar; U-Pb zircon ages of 21-18 Ma from igneous rocks; a U-Pb titanite age of 19 Ma and 40Ar/39Ar biotite ages of 17-16 Ma tightly constrain an early Miocene exhumation. The eastern central Pamir Muskol-Sares dome yields hotter P-T conditions of 700-800 °C and 9.1-11.7 kbar; U-Pb zircon ages as young as 23-17 Ma, U-Pb titanite ages of ca. 17 Ma, 40Ar/39Ar hornblende ages of 22-15 Ma, and 40Ar/39Ar mica ages of 19-13 Ma indicate an essentially identical exhumation history. U-Pb zircon and titanite ages indicate a Triassic magmatic protolith, intruding Paleozoic meta-sedimentary strata. The enormous Shakhdara dome in the southwestern Pamir gives higher peak metamorphic conditions at 6.5-14.6 kbar and 700-800 °C. U-Pb zircon ages indicate ca. 1.8 and 2.5 Ga basement, strongly remobilized by 134-73 Ma Cretaceous magmatism. U-Pb metamorphic zircon ages of 22-12 Ma, U-Pb titanite ages of 18-10 Ma, Th-Pb metamorphic monazite ages of 30-18 Ma, U-Pb titanite ages of 18-10 Ma, 40Ar/39Ar biotite ages of 18-10 Ma, and apatite fission-track ages of 8-5 Ma imply early to late Miocene exhumation. These data, combined with those of earlier studies (e.g., Hubbard, 1989; Schwab et al., 2004; Robinson et al., 2007), indicate that the bulk of the Pamir high-grade crystalline rocks were exhumed from ~35 km depth, beginning in the central Pamir in the early Miocene and progressing north and south in the middle Miocene. If the crust of the Pamir prior to the India-Asia collision was <30 km thick and convergence within the Pamir was <600 km, shortening could have been accommodated by homogeneous plane-strain vertical thickening; each of the Pamir domes then represents a zone of unusually great exhumation compensated by a corresponding zone of less exhumation. Alternatively, if the pre-collisional crust was thicker or the amount of convergence was larger, vast amounts of crust must have been removed from the Pamir orogenic system by i) subhorizontal extrusion along the strike of the orogenic belt, ii) erosion, or-most likely-iii) recycling into the mantle.

  15. Forward modelling of oceanic lithospheric magnetization (United States)

    Masterton, S. M.; Gubbins, D.; Müller, R. D.; Singh, K. H.


    We construct a model of remanence for the oceans, combine it with a model of induced magnetization for the whole Earth from a previous study, compute the predicted lithospheric geomagnetic field and compare the result with a model, MF7, that is based on satellite data. Remanence is computed by assigning magnetizations to the oceanic lithosphere acquired at the location and time of formation. The magnetizing field is assumed to be an axial dipole that switches polarity with the reversal time scale. The magnetization evolves with time by decay of thermal remanence and acquisition of chemical remanence. The direction of remanence is calculated by Euler rotation of the original geomagnetic field direction with respect to an absolute reference frame, significantly improving previous results which did not include realistic oceanic magnetization computed this way. Remanence only accounts for 24 per cent of the energy of the oceanic magnetization, the induced magnetization being dominant, increasing slightly to 30 per cent of the part of the magnetization responsible for generating geomagnetic anomalies and 39 per cent of the Lowes energy of the geomagnetic anomalies. This is because our model of oceanic crust and lithosphere is fairly uniform, and a uniform layer magnetized by a magnetic field of internal origin produces no external field. The largest anomalies are produced by oceanic lithosphere magnetized during the Cretaceous Normal Superchron. Away from ridges and magnetic quiet zones the prediction fails to match the MF7 values; these are also generally, but not always, somewhat smaller than the observations. This may indicate that the magnetization estimates are too small, in which case the most likely error is in the poorly-known magnetization deep in the crust or upper mantle, or it may indicate some other source such as locally underplated continental lithosphere or anomalous oceanic crust, or even small-scale core fields.

  16. [Crusted scabies (Norwegian scabies) a case report]. (United States)

    Fernández-Tamayo, Nora; Flores-Villa, Rebeca; Blanco-Aguilar, Jaime; Dueñas-Arau, Maria de los Angeles; Peña-Flores, María del Pilar Cristal; Rubio-Calva, Carolina; Santos-Marcial, Edgar


    Different types of scabies have been described based on their clinical outcome, one of which is the Crusted (Norwegian) type. This is an extreme manifestation of scabies that can be observed mainly among immunosupressed patients. A case ofa 42 year-old homosexual man is described. The patient was diagnosed with HIV, presenting pruritic lesions with a 4 month evolution in trunk and extremities. Lesions included xerosis, decapitated papules, badges with erythema, residual hyperchromic stains, multiple abrasions and ungueal pigmentation in both feet. At the beginning it was treated as apsorasiform dermatitis with steroids and antipruritics without success. Through a biopsy the suspected diagnosis of Crusted (Norwegian) scabies was confirmed. The patient was treated with a dose of oral ivermectin and topical benzyl benzoate and showed remission after two days. PMID:17201114

  17. Towards a metallurgy of neutron star crusts

    CERN Document Server

    Kobyakov, D


    In the standard picture of the crust of a neutron star, matter there is simple: a body-centered-cubic (bcc) lattice of nuclei immersed in an essentially uniform electron gas. We show that at densities above that for neutron drip ($\\sim4\\times10^11$) g cm$^{-3}$ or roughly one thousandth of nuclear matter density, the interstitial neutrons give rise to an attractive interaction between nuclei that renders the lattice unstable. We argue that the likely equilibrium structure is similar to that in displacive ferroelectric materials such as BaTiO$_3$. As a consequence, properties of matter in the inner crust are expected to be much richer than previously appreciated and we mention consequences for observable neutron star properties.

  18. Nature of the crust in the Laxmi Basin (14°-20°N), western continental margin of India

    Digital Repository Service at National Institute of Oceanography (India)

    Krishna, K.S.; Rao, D.G.; Sar, D.


    observed west of the Laxmi Ridge (Figures 2 and 6) are not associated with basement structures. The velocity-depth results obtained from seismic refraction experiments of Laxmi Ridge, Laxmi Basin, Western Basin, Seychelles Bank and Indian continental... combined (Figure 10) with a view to compare the Laxmi Basin velocity structure with that of known oceanic (Western Basin) and continental (Seychelles Bank, Laxmi Ridge and Indian subcontinent) crust. No refraction signals have been received from Moho...

  19. Molecular Dynamics Simulations of Non-accreting Neutron Star Crusts (United States)

    Hoffman, Kelsey L.; Heyl, J. S.


    Neutron stars which do not accrete material can still have impurities in their crust, due to nuclear reactions in the crust. These impurities in the crust could affect the mechanical properties of the neutron star crust. In order to investigate the properties of the crust of a non-accreting neutron star we are performing molecular dynamic simulations of the crust. We are using the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS), where simulations are run at fixed energy and volume, with the isotopes interacting via a repulsive Yukawa potential. Here we are presenting the preliminary results of the equilibrium structure of the solid neutron star crust using abundances of a non-accreting source.

  20. Millennium Ecosystem Assessment: MA Scenarios

    National Aeronautics and Space Administration — The Millennium Ecosystem Assessment: MA Scenarios provide useful insight into the complex factors that drive ecosystem change, estimating the magnitude of regional...

  1. Syn-volcanic cannibalisation of juvenile felsic crust: Superimposed giant 18O-depleted rhyolite systems in the hot and thinned crust of Mesoproterozoic central Australia (United States)

    Smithies, R. H.; Kirkland, C. L.; Cliff, J. B.; Howard, H. M.; Quentin de Gromard, R.


    Eruptions of voluminous 18O-depleted rhyolite provide the best evidence that the extreme conditions required to produce and accumulate huge volumes of felsic magma can occur in the upper 10 km of the crust. Mesoproterozoic bimodal volcanic sequences from the Talbot Sub-basin in central Australia contain possibly the world's most voluminous accumulation of 18O-depleted rhyolite. This volcanic system differs from the better known, but geochemically similar, Miocene Snake River Plain - Yellowstone Plateau of North America. Both systems witnessed 'super' sized eruptions from shallow crustal chambers, and produced 18O-depleted rhyolite. The Talbot system, however, accumulated over a much longer period (>30 Ma), at a single depositional centre, and from a magma with mantle-like isotopic compositions that contrast strongly with the isotopically evolved basement and country-rock compositions. Nevertheless, although the Talbot rhyolites are exclusively 18O-depleted, the unavoidable inference of an 18O-undepleted precursor requires high-temperature rejuvenation of crust in an upper-crustal chamber, and in this respect the evolution of the Talbot rhyolites and 18O-depleted rhyolites of the Snake River Plain - Yellowstone Plateau is very similar. However, instead of older crustal material, the primary upper-crustal source recycled into Talbot rhyolites was comagmatic (or nearly so) felsic rock itself derived from a contemporaneous juvenile basement hot-zone. Whereas giant low ?18O volcanic systems show that voluminous melting of upper crust can occur, our studies indicate that felsic magmas generated at lower crustal depths can also contribute significantly to the thermal and material budget of these systems. The requirement that very high-temperatures be achieved and sustained in the upper crust means that voluminous low ?18O magmatism is rare, primarily restricted to bimodal tholeiitic, high-K rhyolite (A-type) magmatic associations in highly attenuated lithosphere. In the case of the Talbot system, at least, our data suggest that an unusually hot pre-history might also be required to thermally prime the crust.

  2. Discovery and utilization of sorghum genes (Ma5/Ma6) (United States)

    Mullet, John E; Rooney, William L; Klein, Patricia E; Morishige, Daryl; Murphy, Rebecca; Brady, Jeff A


    Methods and composition for the production of non-flowering or late flowering sorghum hybrid. For example, in certain aspects methods for use of molecular markers that constitute the Ma5/Ma6 pathway to modulate photoperiod sensitivity are described. The invention allows the production of plants having improved productivity and biomass generation.

  3. Ocean Animals (United States)


    There are many types of Ocean Animals, today we wil be going to identify several Ocean Anumals through specific body parts that makeOcean Animals different from one another. To begin examine the links below to see what different types of ocean animals there are and what makes those animals different from one another Beluga Whales- National Geographic Kids Dolphins- Who lives in the sea? Puffer fish- National Geographic Stingrays- National Geographic Kids ...

  4. Evolution of the long-wavelength, subduction-driven topography of South America since 150 Ma (United States)

    Flament, N. E.; Gurnis, M.; Williams, S.; Bower, D. J.; Seton, M.; Müller, D.


    Subduction to the west of South America spans 6000 km along strike and has been active for over 250 Myr. The influence of the history of subduction on the geodynamics of South America has been profound, driving mountain building and arc volcanism in the Andean Cordillera. Here, we investigate the long-wavelength changes in the topography of South America associated with subduction and plate motion and their interplay with the lithospheric deformation associated with the opening of the South Atlantic. We pay particular attention to the topographic expression of flat-lying subduction zones. We develop time-dependent geodynamic models of mantle flow and lithosphere deformation to investigate the evolution of South American dynamic and total topography since the late Jurassic (150 Ma). Our models are semi-empirical because the computational cost of fully dynamic, evolutionary models is still prohibitive. We impose the kinematics of global plate reconstructions with deforming continents in forward global mantle convection models with compositionally distinct crust and continental lithosphere embedded within the thermal lithosphere. The shallow thermal structure of subducting slabs is imposed, allowing us to investigate the evolution of dynamic topography around flat slab segments in time-dependent models. Multiple cases are used to investigate how the evolution of South American dynamic topography is influenced by mantle viscosity, the kinematics of the opening of the South Atlantic and alternative scenarios for recent and past flat-slab subduction. We predict that the migration of South America over sinking oceanic lithosphere resulted in continental tilt to the west until ~ 45 Ma, inverting to an eastward tilt thereafter. This first-order result is consistent with the reversal of the drainage of the Amazon River system. We investigate which scenarios of flat-slab subduction since the Eocene are compatible with geological constraints on the evolution of the Solimoes Basin, the Chaco Basin, the Sierras Pampeanas and the Central Patagonian Basin. To broadly constrain mantle viscosity, we compare models to the total subsidence inferred from well data offshore Argentina and Brazil, and to mantle tomography, since the initial and boundary conditions are based on independent plate reconstructions.

  5. Lunar Magma Ocean Crystallization: Constraints from Fractional Crystallization Experiments (United States)

    Rapp, J. F.; Draper, D. S.


    The currently accepted paradigm of lunar formation is that of accretion from the ejecta of a giant impact, followed by crystallization of a global scale magma ocean. This model accounts for the formation of the anorthosite highlands crust, which is globally distributed and old, and the formation of the younger mare basalts which are derived from a source region that has experienced plagioclase extraction. Several attempts at modelling the crystallization of such a lunar magma ocean (LMO) have been made, but our ever-increasing knowledge of the lunar samples and surface have raised as many questions as these models have answered. Geodynamic models of lunar accretion suggest that shortly following accretion the bulk of the lunar mass was hot, likely at least above the solidus]. Models of LMO crystallization that assume a deep magma ocean are therefore geodynamically favorable, but they have been difficult to reconcile with a thick plagioclase-rich crust. A refractory element enriched bulk composition, a shallow magma ocean, or a combination of the two have been suggested as a way to produce enough plagioclase to account for the assumed thickness of the crust. Recently however, geophysical data from the GRAIL mission have indicated that the lunar anorthositic crust is not as thick as was initially estimated, which allows for both a deeper magma ocean and a bulk composition more similar to the terrestrial upper mantle. We report on experimental simulations of the fractional crystallization of a deep (approximately 100km) LMO with a terrestrial upper mantle-like (LPUM) bulk composition. Our experimental results will help to define the composition of the lunar crust and mantle cumulates, and allow us to consider important questions such as source regions of the mare basalts and Mg-suite, the role of mantle overturn after magma ocean crystallization and the nature of KREEP

  6. Experimental Fractional Crystallization of the Lunar Magma Ocean (United States)

    Rapp, J. F.; Draper, D. S.


    The current paradigm for lunar evolution is of crystallization of a global scale magma ocean, giving rise to the anorthositic crust and mafic cumulate interior. It is thought that all other lunar rocks have arisen from this differentiated interior. However, until recently this paradigm has remained untested experimentally. Presented here are the first experimental results of fractional crystallization of a Lunar Magma Ocean (LMO) using the Taylor Whole Moon (TWM) bulk lunar composition [1].

  7. The Lunar Highland Crust: The Origin of the MG Suite (United States)

    Taylor, S. R.; Norman, M. D.; Esat, T.


    Norites, troctolites, dunites, spinel troctolites and gabbronorites form the Mg suite, which constitutes perhaps 20% of the highland crust. Their ages range from about 4.43 b.y. down to about 4.17 b.y. The Mg suite does not appear to be related to crystallization from the magma ocean. These rock types commonly have Mg# > 90 and so are "primitive," but also contain high concentrations of incompatible elements, typical of highly "evolved" igneous rocks. An origin by mixing of these two distinct components, one "primitive" to account for the major elements (particularly the high Mg#), and the other "evolved" to account for the high trace-element abundances, is suggested by these contradictory petrochemical characteristics. The source of the highly evolved trace-element component is clearly KREEP. The source of the "primitive" Mg-rich component is less clear. Many theories propose that the Mg-suite rocks are derived from different plutons that intruded the crust as separate igneous intrusions. However, all Mg-suite rocks have REE patterns parallel to those of KREEP and the ferroan anorthosites. This characteristic is compatible with mixing, but should not be expected to be duplicated in many separate igneous intrusions. The Mg suite also contains Mg-rich orthopyroxene, a mineral lacking in most mare basalts, so that the source regions of the mare basalts were distinct from those of the Mg suite. During crystallization of the lunar magma ocean, Mg-rich minerals accumulate on the bottom of the magma chamber at depths probably exceeding 400 km. It has been suggested that massive overturning of the crystal pile has occurred to bring these Mg-rich minerals close to the surface. The source regions of the mare basalts were solid by 4400 m.y. with only the minor KREEP component remaining liquid until about 4360 m.y. Thus the lunar interior was effectively solid, although still hot, at the time of the formation of the Mg suite. It thus seems difficult to envisage a massive overturning of the upper 400 km of the solid lunar interior after 4400 m.y. Near-surface remelting of such early refractory Mg-rich cumulates would be difficult in the absence of an obvious internal source of energy. Subsequent melting in the lunar interior that produced the mare basalts took place much later within differentiated cumulates. These lavas do not have the primitive and evolved characteristics nor the mineralogy of the Mg-suite. The total amount of mare basalt melt was very small, about 0.1% of lunar volume. This was produced over 1000 m.y. in more than 20 separate locations and was essentially trivial on a Moon-wide scale. The Mg suite with a volume about 20 times that of the mare basalts, was produced in 100-200 m.y. The formation of the Mg suite thus requires a major source of energy and a large volume of a primitive component. Large-scale overturning of the lunar mantle seems unlikely to produce the compositionally distinct and voluminous Mg-suite magmas. If the primitive compositions cannot be derived from the interior, then they may have come from above. The giant impact hypothesis for lunar origin spins out the silicate mantle of the impactor into Earth orbit. It thus provides a ring of debris of primitive lunar composition from which the Moon accreted. If accretion of this material into the Moon is not 100% efficient, sweep-up of some left-over bodies at relatively low velocities could occur following the formation of the lunar crust. The impact of such bodies could result in mixing of whole Moon, hence "primitive" compositions with some remelted ferroan anorthosite and with the residual KREEP liquid. The magmas so formed could then pond beneath the ferroan anorthositic crust and subsequently intrude the crust. Such a model can account for the mineralogy of the Mg suite, for the old ages, and provide both the primitive "whole Moon" component and an adequate energy source.

  8. MoMA: SOUNDINGS (United States)


    Soundings is MoMA's first major exhibition of sound art, presenting work by 16 contemporary artists, all born in the 1960s, 70s, and 80s, who work with sound. The artists include: Luke Fowler, Toshiya Tsunoda, Marco Fusinato, Richard Garet, Florian Hecker, Christine Sun Kim, Jacob Kirkegaard, Haroon Mirza, Carsten Nicolai, Camille Norment, Tristan Perich, Susan Philipsz, Sergei Tcherepnin, Hong-Kai Wang, Jana Winderen, and Stephen Vitiello. Probably the best way to view the online exhibition is to skip right to the Artists section (URL listed above), where there is a list of linked artists names. Following the links will lead visitors to images of each artist's works, biographical information, and of course, sound. For example, look at and listen to Susan Philipsz's Study of Strings, 2012, an installation at Kassel Hauptbahnhof. This piece is a reinterpretation of a 1943 composition by Pavel Haas (Czech, 1899â??1944), who composed the score while imprisoned at the Theresienstadt concentration camp. A performance was filmed in 1944 for a Nazi propaganda film. Philipsz's piece omits the instruments - and musicians - who were executed at the camp.

  9. IODP Expeditions 304 & 305 Characterize the Lithology, Structure, and Alteration of an Oceanic Core Complex

    Directory of Open Access Journals (Sweden)

    Christopher J. MacLeod


    Full Text Available More than forty years after the Mohole Project (Bascom, 1961, the goal of drilling a complete section through in situ oceanic crust remains unachieved. Deep Sea Drilling Project – Ocean Drilling Program (DSDP-ODP Hole 504B within the eastern Pacifi c (Alt et al., 1993 is the deepest hole ever drilled into ocean crust (2111 mbsf, but it failed to reach lower crustal plutonic rocks below the pillow basalts and sheeted dikes. IODP Expeditions 309 and 312 eventuallyrecovered the long-sought transition from sheeted dikes into underlying gabbros by drilling into very fast-spreading Pacifi c crust (Wilson et al., 2006. The lithology and structure of oceanic crust produced at slow-spreading ridges are heterogeneous (e.g., Cannat et al., 1997 and offer unique drilling access to lower crust and upper mantle rocks. After ODP Hole 735B penetrated 1500 m of gabbro at the Southwest Indian Ridge (Dick et al., 2000, IODP Expeditions 304 and 305 recently recovered just over 1400 m of little-deformed, gabbroic lower crust from a tectonic window along the slowspreading Mid-Atlantic Ridge.

  10. Paleomagnetism continents and oceans

    CERN Document Server

    McElhinny, Michael W; Dmowska, Renata; Holton, James R; Rossby, H Thomas


    Paleomagnetism is the study of the fossil magnetism in rocks. It has been paramount in determining that the continents have drifted over the surface of the Earth throughout geological time. The fossil magnetism preserved in the ocean floor has demonstrated how continental drift takes place through the process of sea-floor spreading. The methods and techniques used in paleomagnetic studies of continental rocks and of the ocean floor are described and then applied to determining horizontal movements of the Earth''s crust over geological time. An up-to-date review of global paleomagnetic data enables 1000 millionyears of Earth history to be summarized in terms of the drift of the major crustal blocks over the surface of the Earth. The first edition of McElhinny''s book was heralded as a "classic and definitive text." It thoroughly discussed the theory of geomagnetism, the geologicreversals of the Earth''s magnetic field, and the shifting of magnetic poles. In the 25 years since the highly successful first editio...

  11. Evolution of high Arctic ocean basins and continental margins

    Energy Technology Data Exchange (ETDEWEB)

    Engen, Oeyvind


    Taking advantage of the much increased detail offered by new data, the dissertation attempts to answer some of the remaining questions about the ocean basins and continental margins flanking the Eurasia-North America plate boundary. Its four constituent papers result from integrated geophysical analysis of gravity and magnetic anomalies, bathymetry, seismic reflection and refraction profiles, earthquake locations and focal mechanisms, and onshore and offshore geological data. The overall objectives are to: 1) Elucidate aspects of the structure, composition and evolution of the Eurasia Basin and Norwegian-Greenland Sea and their passive continental margins. 2) Relate the findings to fundamental Earth processes, specifically associated with lithospheric break-up and seafloor spreading. Summary of Papers: The present-day global seismograph network is capable of detecting earthquakes with nearly uniform magnitude threshold throughout the Eurasia Basin region. Given that the location of each earthquake is constrained by at least 12 recording stations, global earthquake catalogues confidently show that 1) earthquakes along the oceanic part of the plate boundary occur in swarms; 2) plate boundary stress decreases eastwards, in accordance with decreasing spreading rates; and 3) deformation takes place in a narrow zone in the oceanic domain but is abruptly defocused at the transition to the Laptev Sea continental rift system. When integrated with bathymetry and potential field data, the earthquake distribution indicates four distinct plate boundary provinces. The Spitsbergen Transform System is a series of oblique ridges and transform faults where the seismicity becomes increasingly diffuse to the north. The western Gakkel Ridge (west of 60{sup E}) has clustered and focused seismicity, accentuated topography and highamplitude magnetic anomalies, whereas the eastern Gakkel Ridge has smoother topographic relief, lower magnetic amplitudes, and slightly more focused seismicity. At the Laptev Sea continental slope, the change from ultra-slow seafloor spreading to active continental rifting takes place over a less than 60-km-wide continent-ocean transition featuring a 150-200-km-long sheared margin segment. The western Gakkel Ridge province is magmatically segmented. The central, sparsely magmatic segment is characterised by discrete magmatic centres that have been stationary with respect to the spreading axis since at least Chron 6 times (apprx. 19.6 Ma) and possibly since before Chron 18 times (apprx. 39.9 Ma). The westernmost, volcanic segment may have been amagmatic during Chron 13-5 times (apprx. 33.3-9.8 Ma). Sedimentary rocks in the Nansen Basin comprise four turbidite units with typical seismic velocities of 2.3, 2.2, 1.9 and 1.8 km s-1. The upper unit is associated with glaciomarine deposition in the Franz-Victoria Fan system and dates accordingly to approx. 2.3 Ma. The deeper, regional velocity contrast from 2.2 to 1.9 km s-1 probably represents a late Miocene (apprx. 10 Ma) response to major paleoceanographic changes during the opening of the Fram Strait gateway. A location of the continent-ocean transition (COT) on conjugate margins of the western Eurasia Basin and the northern Norwegian-Greenland Sea is proposed from the relation between seismically observed crustal thinning and seaward increasing mantle Bouguer anomalies. A refined location of the COT around the Hovgaard and Greenland ridges is also provided. The new COT location indicates that the distinct segmentation of the western Barents Sea margin is mirrored on the conjugate northeast Greenland margin. The Hinlopen margin north of Svalbard is characterised by a steep boundary fault on the COT and may be a sheared margin segment. The present geological and geophysical data base favours a continental origin of the Yermak Plateau and the Morris Jesup Rise, but a firm conclusion on their crustal structure cannot yet be drawn. A continuous oceanic corridor formed through shear-rifted continental crust in the Fram Strait between Chron 5B (14.8 Ma) and Chron 5 times (9.8

  12. A Continuous History of Plume-Influenced Rifting in the North Atlantic Ocean (United States)

    Parnell-Turner, Ross; White, Nicky; Henstock, Tim; Murton, Bramley; Maclennan, John; Jones, Stephen


    Evolution of the North Atlantic Ocean has been dominated the Iceland mantle plume. Here we present an unbroken record of variable mantle plume activity stretching back 55 Ma, through analysis of regional seismic reflection images. Residual depth anomalies of oceanic lithosphere, long wavelength gravity anomalies and seismic tomographic models show that this convective upwelling reaches from Baffin Bay to Western Norway, and from offshore Newfoundland to Spitzbergen. At fringing passive margins, there is strong evidence for present-day dynamic support of the crust (e.g. Scotland, Western Norway). The Iceland plume is bisected by a mid-oceanic ridge, which provides a record of the temporal evolution of the plume. Transient behavior of the plume is indirectly recorded within the fabric of oceanic floor south of Iceland. We exploit regional seismic reflection profiles that traverse the oceanic basin between northwest Europe and Greenland. A diachronous pattern of V-shaped ridges is imaged beneath a thickening blanket of sediment, revealing a complete record of transient periodicity that can be traced continuously. This periodicity increases from ~3 to ~8 Myr with clear evidence for minor, but systematic, asymmetric crustal accretion. V-shaped ridges grow with time and reflect small (e.g. 5-30°C) changes in mantle temperature, consistent with quasi-periodic generation of hot solitary waves triggered by growth of thermal boundary layer instabilities within the mantle. Our continuous record of convective activity suggests that the otherwise uniform thermal subsidence of sedimentary basins, which fringe the North Atlantic Ocean, has been periodically interrupted by transient uplift events. These elevation changes can explain a suite of diverse observations from the geologic record. Regional Paleogene erosion surfaces in the Faroe-Shetland Basin, the punctuated deposition of contourite drifts, and the history of denudation on the UK continental shelf can all be explained by transient mantle plume behaviour. These manifestations of convective activity should lead to improved insights into the fluid dynamics of the mantle, with implications for the subsidence history of sedimentary elsewhere.

  13. Scaly scalp associated with crusted scabies: case series. (United States)

    Anbar, T S; El-Domyati, M B; Mansour, H A; Ahmad, H M


    The diagnosis of crusted scabies is becoming more relevant due to the increase in number of immunocompromised patients. To date, more than 200 cases have been reported in the literature. However, crusted scabies seems to be under-diagnosed because of its unusual presentations. In this case series we present history, clinical manifestations, KOH smear, and histopathological findings of a series of four patients with crusted scabies. Scaly scalp was a prominent feature of the disease in all cases. Examination of and treatment of the scalp of patients with suspected crusted scabies should not be neglected. A KOH smear from the scalp offers a simple and reliable technique for diagnosis. PMID:18328212

  14. Thinning and flow of Tibetan crust constrained by seismic anisotropy. (United States)

    Shapiro, Nikolai M; Ritzwoller, Michael H; Molnar, Peter; Levin, Vadim


    Intermediate-period Rayleigh and Love waves propagating across Tibet indicate marked radial anisotropy within the middle-to-lower crust, consistent with a thinning of the middle crust by about 30%. The anisotropy is largest in the western part of the plateau, where moment tensors of earthquakes indicate active crustal thinning. The preferred orientation of mica crystals resulting from the crustal thinning can account for the observed anisotropy. The middle-to-lower crust of Tibet appears to have thinned more than the upper crust, consistent with deformation of a mechanically weak layer that flows as if confined to a channel. PMID:15247475

  15. Upper and lower crust recycling in the source of CAMP basaltic dykes from southeastern North America (United States)

    Callegaro, Sara; Marzoli, Andrea; Bertrand, Hervé; Chiaradia, Massimo; Reisberg, Laurie; Meyzen, Christine; Bellieni, Giuliano; Weems, Robert E.; Merle, Renaud


    The densest dykes swarm of the Central Atlantic magmatic province (CAMP) occur in southeastern North America (SENA) and were intruded between 202 and 195 Ma during Pangea break-up. New combined geochemical data (major and trace elements, Sr-Nd-Pb-Os isotopes) constrain the mantle source of these magmatic bodies and their evolution path. While Sr-Nd isotopic compositions for SENA rocks (87Sr/86Sr200Ma 0.70438-0.70880 and 143Nd/144Nd200Ma 0.51251-0.51204) fall within the low-Ti CAMP field, Pb-Pb isotopes (206Pb/204Pb200Ma 17.46-18.85, 207Pb/204Pb200Ma 15.54-15.65, 208Pb/204Pb200Ma 37.47-38.76) are peculiar to this area of the CAMP and cover a considerable span of compositions, especially in 206Pb/204Pb200Ma. Given the generally unradiogenic Os isotopic compositions (187Os/188Os200Ma 0.127-0.144) observed and the lack of correlation between these and other geochemical markers, crustal contamination during the evolution of SENA dykes must have been limited (less than 10%). Thus the isotopic variation is interpreted to reside primarily within the mantle source. These observations, coupled with typical continental signatures in trace elements (positive anomaly in Pb and negative anomalies in Ti and Nb), require another means of conveying a continental flavor to these magmas, which is here hypothesized to be the shallow recycling within the upper mantle of subducted lower and upper crustal materials. Pseudo-ternary mixing models show that a maximum of 10% recycled crust is enough to explain their trace element patterns as well as their isotopic heterogeneity. Looking at the larger picture of the origin of the CAMP, the thermal contribution of a mantle plume cannot be ruled out due to the relatively high mantle potential temperatures (1430-1480 °C) calculated for high-Fo SENA olivines. Nevertheless, our results suggest that the chemical involvement of a mantle plume is negligible (less than 5%) if either a C- or an EM-flavored plume is considered. Rather, the possibility of a PREMA-flavored mantle plume, enriched by 5-20% recycled crustal material, remains a possible, though less plausible, source for these tholeiites.

  16. Crusted Scabies in the Burned Patient

    DEFF Research Database (Denmark)

    Berg, Jais Oliver; AlsbjØrn, Bjarne


    The objectives of this study were 1) to describe a case of crusted scabies (CS) in a burned patient, which was primarily undiagnosed and led to a nosocomial outbreak in the burn unit; 2) to analyze and discuss the difficulties in diagnosing and treating this subset of patients with burn injury; and 3) to design a treatment strategy for future patients. Case analysis and literature review were performed. The index patient had undiagnosed crusted scabies (sive Scabies norvegica) with the ensuing mite hyperinfestation when admitted to the department with minor acute dermal burns. Conservative healing and autograft healing were impaired because of the condition. Successful treatment of the burns was only accomplished secondarily to scabicide treatment. An outbreak of scabies among staff members indirectly led to diagnosis. CS is ubiquitous, and diagnosis may be difficult. This is the first report of a burned patient with CS in the English language literature. CS is also highly contagious and may lead to a nosocomial outbreak. Furthermore, CS seems to have a detrimental impact on the burned patient's course of treatment. A scabicide treatment is necessary to guarantee successful treatment of the burns.

  17. Crusted scabies in the burned patient. (United States)

    Berg, Jais Oliver; Alsbjørn, Bjarne


    The objectives of this study were 1) to describe a case of crusted scabies (CS) in a burned patient, which was primarily undiagnosed and led to a nosocomial outbreak in the burn unit; 2) to analyze and discuss the difficulties in diagnosing and treating this subset of patients with burn injury; and 3) to design a treatment strategy for future patients. Case analysis and literature review were performed. The index patient had undiagnosed crusted scabies (sive Scabies norvegica) with the ensuing mite hyperinfestation when admitted to the department with minor acute dermal burns. Conservative healing and autograft healing were impaired because of the condition. Successful treatment of the burns was only accomplished secondarily to scabicide treatment. An outbreak of scabies among staff members indirectly led to diagnosis. CS is ubiquitous, and diagnosis may be difficult. This is the first report of a burned patient with CS in the English language literature. CS is also highly contagious and may lead to a nosocomial outbreak. Furthermore, CS seems to have a detrimental impact on the burned patient's course of treatment. A scabicide treatment is necessary to guarantee successful treatment of the burns. PMID:21427595

  18. Mesoscopic pinning forces in neutron star crusts

    CERN Document Server

    Seveso, Stefano; Grill, Fabrizio; Haskell, Brynmor


    The crust of a neutron star is thought to be comprised of a lattice of nuclei immersed in a sea of free electrons and neutrons. As the neutrons are superfluid their angular momentum is carried by an array of quantized vortices. These vortices can pin to the nuclear lattice and prevent the neutron superfluid from spinning down, allowing it to store angular momentum which can then be released catastrophically, giving rise to a pulsar glitch. A crucial ingredient for this model is the maximum pinning force that the lattice can exert on the vortices, as this allows us to estimate the angular momentum that can be exchanged during a glitch. In this paper we perform, for the first time, a detailed and quantitative calculation of the pinning force \\emph{per unit length} acting on a vortex immersed in the crust and resulting from the mesoscopic vortex-lattice interaction. We consider realistic vortex tensions, allow for displacement of the nuclei and average over all possible orientation of the crystal with respect to...

  19. New Tectonic Map of the Arctic (TeMAr) and the Question of Distinguishing the Paleo-Asian Ocean (United States)

    Petrov, Oleg; Shokalsky, Sergey; Morozov, Andrey; Kashubin, Sergey; Sobolev, Nikolay; Petrov, Evgeny


    Over the last decade in the framework of the international project "Atlas of Geological Maps of the Circumpolar Arctic at 1:5 M scale" under the auspices of UNESCO/CGMW, a new Tectonic Map of the Arctic (TeMAr) has been compiled; its first version (draft) was displayed at the 34th IGC in Brisbane. To date, the international working groups of the geological surveys of Arctic states involving France and Germany have already compiled the geological map and geophysical maps of magnetic anomaly and gravity fields of the Arctic, set of geophysical maps and sections reflecting the deep structure of the Arctic region up to 60°N. The set includes: zoning map of the Circumpolar Region by nature of potential fields, thickness maps of the sedimentary cover, consolidated crust, and the Earth's crust in general, schematic map of the Earth's crust types in the Circumpolar region showing the distribution of areas with oceanic, continental, and transitional crust, seismic velocity models of tectonic structures of the Arctic. It has been revealed during TeMAr compilation that the basement in the central Arctic region is one of the largest on the planet accretion polychronous collages clamped by three cratons - Siberian, North American, and East European. It combines orogenic belts of different ages from 1 Ga (Timan, Yenisei, Central Taimyr, Chukchi-Seward orogens) to 205-135 Ma (Pai-Khoi-Novaya Zemlya, Novosibirsk orogens). These fold belts enclose and cement the Early Precambrian cratonic blocks (North-Kara, Alpha-Mendeleev ridges etc.). Arctic accretionary collage (mobile belt) is built up to the south by the Ural-Mongolian (Central Asian) also polychronous mobile belt, that formed on the place of the Paleo-Asian Ocean in the age range from Neoproterozoic to Permian. Thus one can observe the largest Arctic-Paleo-Asian mobile belt, which corresponds to the paleo-ocean comparable in size to the modern Atlantic and Indian oceans. This mobile belt is characterized by a complex combination of accretionary and rifting tectonic-magmatic processes, with their gradual rejuvenation to the north and east until the junction with the Pacific mobile belt structures. At its early stages, accretionary tectonics with a wide development of volcanic belts dominated; at the late ones (in the Late Paleozoic, Mesozoic, and Cenozoic) stretching, rifting and postrift subsidence were widely shown with the formation of oil and gas sedimentary basins with a thick sedimentary cover (East Barents, West Siberian, South Kara, Yenisei-Khatanga, North Chukchi, Beaufort), large igneous provinces (East and West Siberian, Central Arctic) and rift systems (Canada Basin, Laptev Sea, etc.). During the Cenozoic, penetration of the North Atlantic rift system, accompanied by intraplate volcanism of NALIP, into the Central Arctic is observed. As a result, suprarift Eurasian oceanic basin crossing across the strike structures of the Arctic-Paleo-Asian mobile belt was formed. Spatial and age boundaries, the history of origination and development of this belt as a global-level structure should be the subject of further investigations under the international Asian and Arctic projects.

  20. Transition metal isotope fractionation in marine hydrothermal deposits of the Mohns Ridge, North Atlantic Ocean


    Mo?ller, Kirsten


    Seafloor hydrothermal vent systems form along mid-ocean ridges in all of the Earth’s oceans. They have a major impact on the chemical exchange between the lithosphere and the hydrosphere, as vast volumes of seawater cycle through these systems, thereby interacting with young, oceanic crust. Furthermore, seafloor hydrothermal vent systems provide an excellent environment for organisms to thrive, resulting in diverse and unique vent faunas. Due to their favourable ecological co...

  1. Late Cretaceous (ca. 90 Ma) adakitic intrusive rocks in the Kelu area, Gangdese Belt (southern Tibet): Slab melting and implications for Cu-Au mineralization (United States)

    Jiang, Zi-Qi; Wang, Qiang; Li, Zheng-Xiang; Wyman, Derek A.; Tang, Gong-Jian; Jia, Xiao-Hui; Yang, Yue-Heng


    The Gangdese Belt in southern Tibet (GBST) is a major Cu-Au-Mo mineralization zone that mostly formed after the India-Asia collision in association with the small-volume, though widespread, Miocene (18-10 Ma) adakitic porphyries. Cu-Au mineralization has scarcely been found in the regional Jurassic-Early Tertiary batholiths related to subduction of the Neo-Tethyan oceanic plate. Here, we report petrological, zircon geochronological and geochemical data for Late Cretaceous (˜90 Ma) intrusive rocks that contain Cu-Au mineralization from the Kelu area in the GBST. These rocks consist of quartz monzonites and diorites. The quartz monzonites, with SiO2 of 58-59 wt.% and Na2O/K2O of 1.1-1.2, are geochemically similar to slab-derived adakites characterized by apparent depletions in heavy rare earth elements (e.g., Yb = 1.4-1.5 ppm) and Y (16-18 ppm) contents, positive Sr but negative Nb and Ti anomalies on multi-element variation diagrams. They have relatively low (87Sr/86Sr)i (0.7038-0.7039) ratios and high ?Nd(t) (+3.4 to +3.9) and in situ zircon ?Hf(t) (+9.3 to +15.8) values. The diorites exhibit high Mg-numbers (0.57-0.61) similar to those of magnesian andesites, and have (87Sr/86Sr)i (0.7040-0.7041) and ?Nd(t) (+3.0 to +4.4) values similar to those of the quartz monzonites. We suggest that the quartz monzonitic magmas were most likely generated by partial melting of the subducted Neo-Tethyan basaltic oceanic crust and minor associated oceanic sediments, with subsequent melt-mantle interaction, and the dioritic magmas were mainly derived by the interaction between slab melts and mantle wedge peridotites, with fractionation of apatite and hornblende. These slab-derived adakitic magmas have high oxygen fugacity that may have facilitated Cu-Au mineralization. The close association of the Late Cretaceous adakitic intrusive rocks and Cu-Au mineralization in the Kelu area suggests that the arc magmatic rocks in the GBST may have higher potential than previously thought for Cu-Au mineralization.

  2. Structures and evolution of the northernmost South China Sea continental margin and ocean basin (United States)

    Chang, I.-Ting; Liu, Char-Shine


    The South China Sea (SCS), a marginal sea offshore Southeast Asia, covers a surface area of 2.32×106km2. It is bounded by the passive China continental margin to the north, Taiwan and the Luzon arc to the east, north Palawan to the south, and Indo-China peninsula to the west. Tectonically, the SCS is at the junction of Eurasia, India-Australia, and Philippine Sea plates. Based on marine magnetic studies, seafloor spreading of the South China Sea was previously to take place between 32-15 Ma, associated with magnetic anomalies C11 to C5c. In general, the South China Sea can be divided into three portions, the northern continental margin, the central oceanic basin and the southern continental margin. The northernmost SCS abuts the Southeastern Asia continental margin and is contiguous to the Taiwan accretionary wedge in the northeast. Morphology of the northern SCS continental margin shows distinctive variation, from wide and gentle dipping continental slope to the west of 118° E to steeply dipping and narrow continental slop near Taiwan. It has been suggested that the oldest oceanic crust of the SCS lies in the northeastern most corner of SCS, and there might exist a NW-SE trending fossil transform fault at 118° E, but the evidences are inconclusive. The tectonic evolution of the northernmost part of the South China Sea thus has been both interesting and perplexing. Multichannel seismic (MCS) reflection profile data are used to characterize the crustal structures in the northernmost South China Sea from about 115° E to 120° E. We have mapped structures of the northern SCS continental margins where normal faults, igneous bodies, slope basins are prevailing west of 118° E, on the other hand, few slope basins and igneous bodies are identified east of 118° E. From the large-offset MCS data collected by the R/V Marcus G. Langseth during the 2009 TAIGER survey, we have identified some strong deep reflectors at some profiles underneath the northern SCS continental margin and beneath a thickened oceanic crust in the northeastern corner of the SCS that maybe represents Moho reflections. We suggest that volcanic activities played a key role in shaping the northern SCS continental margin to the west of 118° E, and igneous activities may have large influence on the whole Cenozoic evolution of this region.

  3. Ocean Terracing


    Cathcart, Richard; Bolonkin, Alexander


    Artworks can improve humanity ability to apply macro-engineering principles which skirt or correct oceanographic problems impairing the economic usefulness of coastal land, the overhead airshed, and seawater temperature and salinity stability. A new form of Art, Ocean Art, is here proposed which centers on deliberate terracing of appropriate regions of our world ocean; a proposed example of macro-engineered useful Ocean Art is the technically possible 21-st Century terracing...

  4. The feasibility of MA transmutation in CEFR

    International Nuclear Information System (INIS)

    The feasibility of MA transmutation in CEFR (China Experimental Fast Reactor) is described. The nuclear characteristics of reference core and those of MA-loaded core are compared, the MA-transmutation amount is presented. Although the amount of MA transmutation in CEFR is limited, CEFR still has a significant role in MA fuel irradiation tests and MA transmutation technique studies. (author). 6 refs, 1 fig., 3 tabs

  5. Ocean Color

    National Aeronautics and Space Administration — Satellite-derived Ocean Color Data sets from historical and currently operational NASA and International Satellite missions including the NASA Coastal Zone Color...

  6. Ocean FEST (United States)

    Barbara Bruno

    Ocean FEST family science nights feature hands-on, standards-based, ocean-themed science activities for students in grades 3-6 and their families. Our goals are to: (1) educate participants about ocean and earth science issues that are relevant to their communities; and (2) inspire students - especially those from underrepresented groups - to pursue careers in the ocean and earth sciences. The teacher guide provides all information (including supply lists) necessary to perform these activities in a classroom. Some supply funding is available--see Teacher Resources section for more information.

  7. Millennium Ecosystem Assessment: MA Population

    National Aeronautics and Space Administration — The Millennium Ecosystem Assessment: MA Population data sets provide baseline population information as one of the drivers of ecosystem change. The data helped in...

  8. Crustal structure, restoration and evolution of the Greater Himalaya in Nepal-South Tibet: implications for channel flow and ductile extrusion of the middle crust


    Searle, MP; Law, RD; Jessup, MJ


    Recent suggestions that the Greater Himalayan Sequence (GHS) represents a mid-crustal channel of low viscosity, partially molten Indian plate crust extruding southward between two major ductile shear zones, the Main Central thrust (MCT) below, and the South Tibetan detachment (STD) normal fault above, are examined, with particular reference to the Everest transect across Nepal -south Tibet. The catalyst for the early kyanite ± sillimanite metamorphism (650-680°C, 7-8 kbar, 32-30 Ma) was cru...

  9. Reference crust-mantle density contrast beneath Antarctica based on the Vening Meinesz-Moritz isostatic inverse problem and CRUST2.0 seismic model

    Scientific Electronic Library Online (English)

    Robert, Tenzer; Mohammad, Bagherbandi.


    Full Text Available El contraste de densidad de la discontinuidad de Mohorovicic (Moho) debajo de la Antártida fue estimado con base en la solución del problema isostático Vening Meinesz-Moritz y a partir de datos obtenidos con el modelo sísmico de la corteza global (CRUST2.0). La solución se encontró a través de un aj [...] uste al método de mínimos cuadrados por el método de elementos. El modelo geopotencial global (GOCO02S), el modelo topográfico/batimétrico (DTM2006.0), los datos de espesor del hielo para la Antártida (reunidos por el proyecto BEDMAP) y el modelo sísmico de corteza global (CRUST2.0) fueron utilizados para calcular las anomalías gravitatorias isostáticas. Ya que los datos de CRUST2.0 para las estructuras de la corteza en la Antártida no son exactos (debido a la falta de información sísmica para esta parte del planeta), el contraste de densidad de la Discontinuidad de Mohorovicic fue determinado a partir de un modelo de corteza homogéneo que tiene una densidad constante de 2,670 kg/m³. Los valores estimados del contraste de densidad de la Moho se encontraron entre 160 y 682kg/m³. La distribución espacial del contraste de densidad de la Moho exhibe mayores rasgos en la configuración de la plancha tectónica de la Antártida continental y su alrededor oceánico. El valor máximo encontrado excede los 500 kg/m³ y se ubica en la parte Este continental, con extensión en las Montañas Transantárticas. El contraste de densidad de la Moho (zona de transición entre la corteza y el manto terrestre) en el Oeste de la Antártida osciló entre 400-500 kg/m³, excepto para la máxima local de ? 550 kg/m³, en el centro de la Península Antártida. Abstract in english The crust-mantle (Moho) density contrast beneath Antarctica was estimated based on solving the Vening Meinesz-Moritz isostatic problem and using constraining information from a seismic global crustal model (CRUST2.0). The solution was found by applying a least-squares adjustment by elements method. [...] Global geopotential model (GOCO02S), global topographic/bathymetric model (DTM2006.0), ice-thickness data for Antarctica (assembled by the BEDMAP project) and global crustal model (CRUST2.0) were used for computing isostatic gravity anomalies. Since CRUST2.0 data for crustal structures under Antarctica are not accurate (due to a lack of seismic data in this part of the world), Moho density contrast was determined relative to a reference homogenous crustal model having 2,670 kg/m³ constant density. Estimated values of Moho density contrast were between 160 and 682 kg/m³. The spatial distribution of Moho density contrast resembled major features of the Antarctic’s continental and surrounding oceanic tectonic plate configuration; maxima exceeding 500 kg/m³ were found throughout the central part of East Antarctica, with an extension beneath the Transantarctic mountain range. Moho density contrast in West Antarctica decreased to 400-500 kg/m³, except for local maxima up to ? 550 kg/m³ in the central Antarctic Peninsula.

  10. Highly Faulted Upper Crust Caused by Continental Rifting: Evidence from Seafloor Compliance Inversion in the Okinawa Trough (United States)

    Kuo, B. Y.


    Seafloor compliance measures the elastic response of the ocean bottom to water pressure fluctuation, and is a function of the elasticity of the sediment/crust. We have measured seafloor compliance from OBS data at two sites on the Okinawa trough (OT), which results from continental rifting, and one site on the Ryukyu arc (RA), a crustal block separated by the OT from the Eurasian plate, in the vicinity of Taiwan. Because the water depth is known and the sediment thickness can be estimated from seismic studies, this study offers a comparison in crustal properties between rifting and non-rifting environment. We performed both grid search and minimum-structure inversion for Vs. The models that best explain the compliance data indicate that the shear velocity of the upper crust of the RA is higher than that in the OT. This implies that the extension of the OT may cause significant faulting/fracturing of the brittle part of the crust and lower its shear modulus. Between the 2 OT sites, the Vs is lower for the OBS more distant from Taiwan. This observation is consistent with the scenario in which the crust is faulted and weakened more as rifting process intensifies from the edge of the Okinawa trough.

  11. Structure, tectonic and petrology of mid-oceanic ridges and the Indian scenario

    Digital Repository Service at National Institute of Oceanography (India)

    Iyer, S.D.; Ray, Dwijesh


    elements to long - time (or permanent) storage in the continental crust and litho - sphere and (ii) a portion of the subducted oceanic crust is stored at the base of the convecting system and thus sequesters incompatible elements temporarily... process producing MORB of essentially constant composition through time, detailed studies conducted at the segment length scale have linked patterns of compositional varia bility at the EPR and at the southern Juan de Fuca Ridge 59 . An orderly spatial...

  12. Structure of the Crust Beneath Cameroon, West Africa, from the Joint Inversion of Rayleigh Wave Group Velocities and Receiver Functions

    International Nuclear Information System (INIS)

    The joint inversion of Rayleigh wave group velocities and receiver functions was carried out to investigate the crustal and uppermost mantle structures beneath Cameroon. This was achieved using data from 32 broadband seismic stations installed for 2 years across Cameroon. The Moho depth estimates reveal that the Precambrian crust is variable across the country and shows some significant differences compared to other similar geologic units in East and South Africa. These differences suggest that the setting of the Cameroon Volcanic Line (CVL) and the eastward extension of the Benue Trough have modified the crust of the Panafrican mobile belt in Cameroon by thinning beneath the Rift area and CVL. The velocity models obtained from the joint inversion show at most stations, a layer with shear wave velocities ? 4.0 km/s, indicating the presence of a mafic component in the lower crust, predominant beneath the Congo Craton. The lack of this layer at stations within the Panafrican mobile belt may partly explain the crustal thinning observed beneath the CVL and rift area. The significant presence of this layer beneath the Craton, results from the 2100 Ma magmatic events at the origin of the emplacement of swarms of mafic dykes in the region. The CVL stations are underlain by a crust of 35 km on average except near Mt-Cameroon where it is about 25 km. The crustal thinning observed beneath Mt. Cameroon supported by the observed positive gravity anomalies here, suggests the presence of dense astenospheric material within the lithosphere. Shear wave velocities are found to be slower in the crust and uppermost mantle beneath the CVL than the nearby tectonic terrains, suggesting that the origin of the line may be an entirely mantle process through the edge-flow convection process. (author)

  13. Moored systems designed to sense deep ocean earthquakes

    International Nuclear Information System (INIS)

    The ability to predict earthquakes and tsunamis is becoming increasingly important as world population continues to grow in high-density coastal metropolitan areas. Earthquakes which occur in and near undersea subduction zones where the earth's crust slides under continental masses generate highly destructive tsunamis. Deep ocean buoy systems and sensor implantation techniques are being developed to obtain seismic data from the earth's crust in water depths of 6000 m. For the first time, deep-sea drilling, high-resolution seismic sensors, and long-term, deep-ocean mooring technology are being combined to provide systems which continuously monitor earthquake activity in the deep ocean. Such systems provide vital seismic research information to the scientific community

  14. Growth of the continental crust: constraints from radiogenic isotope geochemistry

    International Nuclear Information System (INIS)

    Most models for evolution of continental crust are expressed in the form of a diagram illustrating the cumulative crustal mass (normalized relative to the present crustal mass) as a function of time. Thus, geochronological data inevitably play a major role in either constructing or testing crustal growth models. For all models, determining the start-time for effective crustal accretion is of vital importance. To this end, the continuing search for, and reliable characterization of, the most ancient crustal rock-units remains a worthy enterprise. Another important role for geochronology and radiogenic isotope geochemistry is to assess the status of major geological events as period either of new crust generation or of reworking of earlier formed continental crust. For age characterization of major geological provinces, using the critieria outined, the mass (or volume) of crust surviving to the present day should be determinable as a function of crust formation age. More recent developments, however, appear to set severe limitations on recycling of crust, at least by the process of sediment subduction. In modeling crustal growth without recycling, valuable constaints on growth rate variations through time can be provided if variations in the average age of the continental crust can be monitored through geological history. The question of the average age of the exposed continental crust was addressed by determining Sm-Nd crustal residence model ages (T-CR) for fine-grainresidence model ages (T-CR) for fine-grained sediment loads of many of the world's major rivers

  15. Ocean Update (United States)

    SeaWeb's monthly newsletter summarizing recent news, views and events concerning marine and coastal environments and wildlife. Site also features The Ocean Report, a series of ninety-second radio slots highlighting a wide range of news and issues relating to the ocean, and Give Swordfish a Break, a successful campaign that helped restore depleted North Atlantic swordfish populations.

  16. Ocean Temperatures (United States)

    NBC Learn


    Bermuda may be known as a luxurious vacation destination, but it also houses one of the world's leading institutes for ocean studies, called BIOS. Dr. Tony Knap explains how climate change is causing ocean temperatures to rise, and what impacts it may bring around the world. "Changing Planet" is produced in partnership with the National Science Foundation.

  17. Ocean technology

    Digital Repository Service at National Institute of Oceanography (India)

    Peshwe, V.B.


    stream_size 2 stream_content_type text/plain stream_name Voices_Oceans_1996_113.pdf.txt stream_source_info Voices_Oceans_1996_113.pdf.txt Content-Encoding ISO-8859-1 Content-Type text/plain; charset=ISO-8859-1 ...

  18. Crusting susceptibility in some allic Colombian soils

    International Nuclear Information System (INIS)

    Many lab methods were used: dry and water soil aggregates stability, instability index and erosion index and their results were related with soil characteristics like texture, Fe and Al oxides and organic matter. Soil samples collected within 0-2.5 and 2.5-5 cm of the soil surface came from terrains with many kinds of both forest and savanna intervened systems. Those results were analyzed like a completely randomized designed. It was found that significative changes in oxides content could increase soil-crusting susceptibility unless soil humus was up to was up to 4%. In this sense, pastures or its rotation with rice and leguminous offer a best alternative for intervening these natural systems. Intensive land husbandry or monocultures with low stubble soil incorporation caused an increase in physical instability at the top of soil. Dry soil stability test and instability index were most adequate for these soils

  19. Breaking stress of neutron star crust

    CERN Document Server

    Chugunov, A I


    The breaking stress (the maximum of the stress-strain curve) of neutron star crust is important for neutron star physics including pulsar glitches, emission of gravitational waves from static mountains, and flares from star quakes. We perform many molecular dynamic simulations of the breaking stress at different coupling parameters (inverse temperatures) and strain rates. We describe our results with the Zhurkov model of strength. We apply this model to estimate the breaking stress for timescales ~1 s - 1 year, which are most important for applications, but much longer than can be directly simulated. At these timescales the breaking stress depends strongly on the temperature. For coupling parameter <200, matter breaks at very small stress, if it is applied for a few years. This viscoelastic creep can limit the lifetime of mountains on neutron stars. We also suggest an alternative model of timescale-independent breaking stress, which can be used to estimate an upper limit on the breaking stress.

  20. Element recycling from subducting slabs to arc crust: A review (United States)

    Spandler, Carl; Pirard, Cassian


    Subduction zones not only return oceanic lithosphere into the mantle, but are also sites where chemical components are transferred from the downgoing plate back to the surface in arc crust and, to a lesser extent, fore-arc and back-arc basins. Understanding of subduction-zone processes has evolved significantly over a relatively brief 40-year research history, thanks to combined insights from experimental petrology, geophysics, numerical and thermodynamic modelling, arc magma geochemistry and studies of high- and ultrahigh-pressure metamorphic rocks. Early models considered aqueous fluids produced by metamorphic devolatilisation of the slab to be responsible for directly transferring chemical components of the slab into the overlying mantle wedge, as well as fluxing melting of the mantle wedge to produce arc basalt. Subducting crustal rocks were considered too cold to melt under most circumstances. However, the latest generation of thermal modelling combined with improved understanding of the chemistry and phase petrology of subduction-zone fluids and melts indicates that conditions for deep slab melting are likely met in subduction zones, provided that free fluid is available at sub-arc depths. We outline a model to explain element transfer out of subducting slabs that involves serpentinite subduction and slab partial melting. Serpentinite is likely to comprise part of the subducting slab, either as downgoing oceanic lithosphere that was hydrated at, or near, the seafloor, or as down-dragged fore-arc mantle wedge that was initially hydrated at shallow levels by aqueous fluids emanating from underthrust crustal rocks. Slab coupling with convecting asthenospheric mantle at sub-arc depths leads to slab heating and devolatilisation of deep slab serpentinite and/or hydrated mélange atop of the slab. Interaction between these fluids and coesite-phengite eclogite at the top of the slab produces hydrous slab melts, which then migrate out of the slab to ultimately contribute to arc magma generation. In this scenario hydrous slab melts dominate element transfer from the slab to arc magmas, although serpentinites (and/or related hybrid mélange rocks) are the initial source of H2O and some trace elements (e.g., B, Cl, As, and Sb). This model conforms to petrological and geophysical constraints on deep subduction conditions, and in general is consistent with the geology of blueschist-and eclogite-facies terranes and key geochemical and isotopic features of arc lavas.

  1. Pb Isotopes from Paleogene Fe-Mn Oxide Coatings from the Southern Ocean as a Proxy for Deep Water Circulation and Weathering Intensity. (United States)

    Basak, C.; Martin, E. E.


    Pb isotopes extracted from Fe-Mn oxide coatings on bulk sediment are investigated as proxies for seawater Pb isotopes on Cenozoic time scales. Previous studies investigating long-term variations in weathering inputs and circulation patterns used Pb isotopes derived from Fe-Mn crust, but these crusts grow very slowly, are difficult to date, and have limited spatial resolution. Due to the slow growth rates they record a signal that is averaged over ~ 104-105 years, limiting the application to studies of long term changes. Initial results suggest that Pb isotopes extracted from Fe-Mn oxide coatings may be a proxy for more rapid variations in weathering and circulation. Comparison between the Pb isotopic values derived from these coatings and from fossil fish teeth for ~25 Ma old samples from ODP Site 1090 on the Aghulus Ridge (Southern Ocean) shows similar values, although there is a small systematic offset when plotted in 207Pb/204Pb vs. 206Pb/204Pb and 208Pb/204Pb vs.206Pb/204Pb space. This may be due to initial Pb concentrations in the hydroxyapatite of fossil fish teeth. Initial data from oxide coatings indicate a well-defined inverse correlation between values for ?Nd compared to 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb for samples ranging from 15-45 Ma at ODP Site 1090. Previous studies argue that the Nd isotopic variations at this site are largely driven by circulation, specifically the opening of Drake Passage followed by the influx of North Atlantic waters in response to development of the Antarctic Circumpolar Current (ACC) (Scher and Martin 2004, 2006 and in review). This correlation between the two isotopic systems suggests the Pb isotopic record is also dominated by a circulation signal. A 207Pb/204Pb vs. 206Pb/204Pb plot illustrates that Pb isotopic values varied over this interval as a function of the relative influx of Atlantic and Pacific waters, with the introduction of less radiogenic Pacific values indicating opening of Drake Passage, followed by the introduction of more radiogenic Atlantic waters. It is intriguing that Atlantic Pb signals are carried as far as the Southern Oceans given the short residence time of Pb (50-200yrs). Increased variability of Pb isotopic data and a decreased correlation to ? Nd values from 20 Ma and younger may record variations in more local weathering inputs.

  2. Persistent crust-core spin lag in neutron stars

    CERN Document Server

    Glampedakis, Kostas


    It is commonly believed that the magnetic field threading a neutron star provides the ultimate mechanism (on top of fluid viscosity) for enforcing long-term corotation between the slowly spun down solid crust and the liquid core. We show that this argument fails for axisymmetric magnetic fields with closed field lines in the core, the commonly used `twisted torus' field being the most prominent example. The failure of such magnetic fields to enforce global crust-core corotation leads to the development of a persistent spin lag between the core region occupied by the closed field lines and the rest of the crust and core. We discuss the repercussions of this spin lag for the evolution of the magnetic field, suggesting that, in order for a neutron star to settle to a stable state of crust-core corotation, the bulk of the toroidal field component should be deposited into the crust soon after the neutron star's birth.

  3. Persistent crust-core spin lag in neutron stars (United States)

    Glampedakis, Kostas; Lasky, Paul D.


    It is commonly believed that the magnetic field threading a neutron star provides the ultimate mechanism (on top of fluid viscosity) for enforcing long-term corotation between the slowly spun-down solid crust and the liquid core. We show that this argument fails for axisymmetric magnetic fields with closed field lines in the core, the commonly used `twisted torus' field being the most prominent example. The failure of such magnetic fields to enforce global crust-core corotation leads to the development of a persistent spin lag between the core region occupied by the closed field lines and the rest of the crust and core. We discuss the repercussions of this spin lag for the evolution of the magnetic field, suggesting that, in order for a neutron star to settle to a stable state of crust-core corotation, the bulk of the toroidal field component should be deposited into the crust soon after the neutron star's birth.

  4. Shear wave velocity structure of the Anatolian Plate: anomalously slow crust in southwestern Turkey (United States)

    Delph, Jonathan R.; Biryol, C. Berk; Beck, Susan L.; Zandt, George; Ward, Kevin M.


    The Anatolian Plate is composed of different lithospheric blocks and ribbon continents amalgamated during the closure of the Paleotethys Ocean and Neotethys Ocean along a subduction margin. Using ambient noise tomography, we investigate the crustal and uppermost mantle shear wave velocity structure of the Anatolian Plate. A total of 215 broad-band seismic stations were used spanning 7 yr of recording to compute 13 778 cross-correlations and obtain Rayleigh wave dispersion measurements for periods between 8 and 40 s. We then perform a shear wave inversion to calculate the seismic velocity structure of the crust and uppermost mantle. Our results show that the overall crustal shear wave velocities of the Anatolian crust are low (˜3.4 km s-1), indicative of a felsic overall composition. We find that prominent lateral seismic velocity gradients correlate with Tethyan suture zones, supporting the idea that the neotectonic structures of Turkey are exploiting the lithospheric weaknesses associated with the amalgamation of Anatolia. Anomalously slow shear wave velocities (˜3.15 km s-1 at 25 km) are located in the western limb of the Isparta Angle in southwestern Turkey. In the upper crust, we find that these low shear wave velocities correlate well with the projected location of a carbonate platform unit (Bey Da?lari) beneath the Lycian Nappe complex. In the lower crust and upper mantle of this region, we propose that the anomalously slow velocities are due to the introduction of aqueous fluids related to the underplating of accretionary material from the underthrusting of a buoyant, attenuated continental fragment similar to the Eratosthenes seamount. We suggest that this fragment controlled the location of the formation of the Subduction-Transform Edge Propagator fault in the eastern Aegean Sea during rapid slab rollback of the Aegean Arc in early Miocene times. Lastly, we observe that the uppermost mantle beneath continental Anatolia is generally slow (˜4.2 km s-1), indicating higher than usual temperatures consistent with the influx of asthenosphere to shallow depths as a result of the segmentation and break-up of the subducting African lithosphere.

  5. El Hierro's floating stones as messengers of crust-magma interaction at depth (United States)

    Burchardt, S.; Troll, V. R.; Schmeling, H.; Koyi, H.; Blythe, L. S.; Longpré, M. A.; Deegan, F. M.


    During the early stages of the submarine eruption that started on October 10 2011 south of El Hierro, Canary Islands, Spain, peculiar eruption products were found floating on the sea surface. These centimetre- to decimetre-sized "bombs" have been termed "restingolites" after the nearby village La Restinga and consist of a basaltic rind and a white to light grey core that resembles pumice in texture. According to Troll et al. (2011; see also Troll et al. EGU 2012 Abstracts), this material consists of a glassy matrix hosting extensive vesicle networks, which results in extremely low densities allowing these rocks to float on sea water. Mineralogical and geochemical analyses reveal that the "restingolites" originate from the sedimentary rocks (sand-, silt-, and mudstones) that form layer 1 of the oceanic crust beneath El Hierro. During the onset and early stages of the eruption, magma ponded at the base of this sedimentary sequence, breaking its way through the sedimentary rocks to the ocean floor. The textures of the "restingolites" reveal that crust-magma interaction during fragmentation and transport of the xenoliths involved rapid partial melting and volatile exsolution. Xenoliths strikingly similar to those from El Hierro are known from eruptions on other Canary Islands (e.g. La Palma, Gran Canaria, and Lanzarote). In fact, they resemble in texture xenoliths of various protoliths from volcanic areas worldwide (e.g. Krakatao, Indonesia, Cerro Quemado, Guatemala, Laacher See, Germany). This indicates that the process of partial melting and volatile exsolution, which the "restingolites" bear witness of, is probably occurring frequently during shallow crustal magma emplacement. Thermomechanical numerical models of the effect of the density decrease associated with the formation of vesicle networks in partially molten xenoliths show that xenoliths of crustal rocks initially sink in a magma chamber, but may start to float to the chamber roof once they start to heat up and vesiculate. The "floating stones" from El Hierro thus represent the products of crust-magma interaction beneath the Canary Islands, but is probably relevant in most volcanic areas and tectonic settings. In addition, xenolith devolatilisation has important general implications for the mechanics of crustal recycling, magma emplacement into the upper crust and volatile release from active volcanic systems.

  6. Effect of water activity on fracture and acoustic characteristics of a crust model


    Primo Martin, C.; Sözer, N.; Hamer, R.J.; van Vliet, T


    A crust model is described that is suited to study crispness of bread crusts as a function of steady state water activity. The study of crispness of this type of products as a function of water activity is complicated since the way a bread crust fractures does not depend on the crust only but also on the properties of the crumb, the curvature of the crust and on the gradient of water activity/content inside the crust. To circumvent these problems a baked crust model was developed. The crust m...

  7. Multipolarity remanences in lower oceanic crustal gabbros recovered by drilling at Hess Deep (Integrated Ocean Drilling Program Expedition 345) (United States)

    Morris, Antony; Horst, Andrew; Friedman, Sarah; Nozaka, Toshio


    A long-term goal of the scientific ocean drilling community is to understand the processes by which the ocean crust is constructed through magmatism, deformation, metamorphism and hydrothermal cooling. Insights into the magnetic properties of the lower crust have come from drilling at oceanic core complexes and in tectonic windows. At the Hess Deep Rift, propagation of the Cocos-Nazca Ridge into young, fast-spreading East Pacific Rise crust exposes a dismembered, but nearly complete lower crustal section. Here, IODP Expedition 345 (Site U1415) recovered primitive plutonic lithologies including gabbro, troctolitic gabbro and olivine gabbronorite. These rocks exhibit cumulate textures similar to those found in layered basic intrusions and some ophiolite complexes. Metamorphism is dominated by background greenschist facies alteration associated with cataclastic deformation that likely results from Cocos-Nazca rifting. Some intervals display complex, multiple remanence components within individual samples. A high temperature component unblocks above 500°-520°C and an intermediate temperature component of nearly antipodal direction unblocks between 425°-450°C and 500°-520°C. In addition, a few samples display a third component that unblocks between 100-350°C that is nearly parallel to the highest temperature component. These multiple, nearly antipodal components suggest that remanence was acquired in different geomagnetic chrons, and represent the first multipolarity remanences seen in Pacific lower oceanic crust. Similar remanence structures, however, have been reported in lower crustal gabbros recovered from slow-spreading rate crust along the Mid-Atlantic Ridge, and have been interpreted to reflect protracted accretion or protracted cooling. In contrast, at Hess Deep unblocking temperatures appear consistent with temperatures inferred for successive phases of alteration, suggesting an alteration history spanning at least two polarity chrons.

  8. Widespread and synchronous change in deep-ocean circulation in the North and South Atlantic during the Late Cretaceous


    Robinson, Sa; Vance, D.


    Modern thermohaline circulation plays a role in latitudinal heat transport and in deep-ocean ventilation, yet ocean circulation may have functioned differently during past periods of extreme warmth, such as the Cretaceous. The Late Cretaceous (100-65Ma) was an important period in the evolution of the North Atlantic Ocean, characterized by opening ocean gateways, long-term climatic cooling and the cessation of intermittent periods of anoxia (oceanic anoxic events, OAEs). However, how these phe...

  9. Open Oceans.

    Czech Academy of Sciences Publication Activity Database

    Mikuláš, Radek

    Vol. 1. EOLSS Publishers, 2002, s. - Institutional research plan: CEZ:AV0Z3013912 Keywords : environment * ocean * paleogeography Subject RIV: DB - Geology ; Mineralogy

  10. The giant Dexing porphyry Cu-Mo-Au deposit in east China: product of melting of juvenile lower crust in an intracontinental setting (United States)

    Hou, Zengqian; Pan, Xiaofei; Li, Qiuyun; Yang, Zhiming; Song, Yucai


    The Dexing porphyry Cu-Mo-Au deposit in east China (1,168 Mt at 0.45 % Cu) is located in the interior of the South China Craton (SCC), made up of two lithospheric blocks, the Yangtze and Cathaysia blocks. The Cu-Mo-Au mineralization is associated with mid-Jurassic granodioritic porphyries with three high-level intrusive centers, controlled by a series of lineaments at the southeastern edge of the Yangtze block. Available age data define a short duration (172-170 Ma) of the felsic magmatism and the mineralization (171 ± 1 Ma). The deposit shows broad similarities with deposits in volcanoplutonic arcs, although it was formed in an intracontinental setting. Porphyries associated with mineralization are mainly granodiorites, which contain abundant phenocrysts (40-60 %) and carry contemporaneous microgranular mafic enclaves (MMEs). They are mainly high-K calc-alkaline and show geochemical affinities with adakite, characterized by relatively high MgO, Cr, Ni, Th, and Th/Ce ratios. The least-altered porphyries yielded relatively uniform ? Nd( t) values from -0.9 to +0.6, and wide (87Sr/86Sr)i range between 0.7046 and 0.7058 partially overlapping with the Sr-Nd isotopic compositions of the MMEs and mid-Jurassic mafic rocks in the SCC. Zircons from the porphyries have positive ? Hf( t) values (3.4 to 6.9), and low ?18O values (4.7 to 6.3 ‰), generally close to those of depleted mantle. All data suggest an origin by partial melting of a thickened juvenile lower crust involving mantle components (e.g., Neoproterozoic mafic arc magmas), triggered by invasion of contemporaneous mafic melts at Dexing. The MMEs show textural, mineralogical, and chemical evidence for an origin as xenoliths formed by injection of mafic melts into the felsic magmas. These MMEs usually contain magmatic chalcopyrite, and have original, variable contents of Cu (up to 500 ppm). Their geochemical characteristics suggest that they were derived from an enriched mantle source, metasomatized by Proterozoic slab-derived fluids, and supplied a part of Cu, Au, and S for the Dexing porphyry system during their injection into the felsic magmas. The 171 ± 1 Ma magmatic-hydrothermal event at Dexing is contemporaneous with the mid-Jurassic extension in the SCC, followed by 160-90 Ma arc-like magmatism in southeastern China. With respect to the tectono-magmatic evolution of the SCC, the emplacement of Cu-bearing porphyries and the associated Cu mineralization occurred in response to the transformation from a tensional regime, related to mid-Jurassic extension, to a transpressional regime, related to the subduction of the Paleo-Pacific oceanic lithosphere.

  11. Models Of Strange Stars With A Crust And Strange Dwarfs


    Vartanyan, Yu. L.; Grigoryan, A. K.; Sargsyan, T. R.


    Strange quark stars with a crust and strange dwarfs consisting of a compact strange quark core and an extended crust are investigated in terms of a bag model. The crust, which consists of atomic nuclei and degenerate electrons, has a limiting density of ro_ cr=ro_drip=4.3*10^11g/cm^3. A series of configurations are calculated for two sets of bag model parameters and three different values of ro_cr (10^9g/cm^3

  12. Treatment of crusted scabies with albendazole: A case report. (United States)

    Douri, Thaer; Shawaf, A Z


    Crusted scabies is a severe variant of scabies caused by the ectoparasite Sarcoptes scabiei. It is characterized by high mite burden, extensive hyperkeratotic scaling, crusted lesions, variable pruritus, generalized lymphadenopathy, erythroderma, and eosinophilia, in some cases. There is an increased incidence of crusted scabies, particularly among patients with HIV infection. We describe a 22-year-old Syrian immunocompetent female who had hyperkeratotic psoriasiform plaques and hyperkeratosis without itching. She was treated with oral albendazole and topical crotamiton with salicylic acid 5 percent. PMID:19951635

  13. Collective Modes in the Superfluid Inner Crust of Neutron Stars

    CERN Document Server

    Urban, Michael


    The neutron-star inner crust is assumed to be superfluid at relevant temperatures. The contribution of neutron quasiparticles to thermodynamic and transport properties of the crust is therefore strongly suppressed by the pairing gap. Nevertheless, the neutron gas still has low-energy excitations, namely long-wavelength collective modes. We summarize different approaches to describe the collective modes in the crystalline phases of the inner crust and present an improved model for the description of the collective modes in the pasta phases within superfluid hydrodynamics.

  14. LOCV approach and core-crust transition in neutron stars

    Energy Technology Data Exchange (ETDEWEB)

    Bigdeli, M.; Elyasi, S. [University of Zanjan, Department of Physics, Zanjan (Iran, Islamic Republic of)


    In this paper, we have calculated the core-crust transition parameters and the location of inner edge for crust in the neutron stars. We have also investigated the structural properties of neutron stars, such as mass and radius for the core and crust, the moment of inertia, and its crustal fraction. Here we have employed the lowest-order constrained variational approach and used the UV{sub 14} + TNI and AV{sub 18} potentials to compute the equation of state of nuclear matter. Finally, we have compared our results with those of other techniques. (orig.)

  15. Comparing Oceans (United States)

    A variety of classroom activities and lessons that compare the world's oceans. Activities included: The Gulf of Maine, Satellite Comparisons, Design a Fish, What Migrations, Incredible Feasting of Whales, Paddle to the Sea, and Ocean Soundings. Discover why weather at identical latitudes is not always the same, learn the different ways whales eat, and find out the temperature difference between the Gulf Stream and surrounding water. Links to other Aquarium modules.

  16. Anomalous Subsidence at Rifted Continental Margins: Distinguishing Mantle Dynamic Topography from Anomalous Oceanic Crustal Thickness (United States)

    Cowie, L.; Kusznir, N. J.


    It has been proposed that some continental rifted margins have anomalous subsidence histories and that at breakup they were elevated at shallower bathymetries than the isostatic response of classical rift models (McKenzie 1978) would predict. The existence of anomalous syn or post breakup subsidence of this form would have important implications for our understanding of the geodynamics of continental breakup and rifted continental margin formation, margin subsidence history and the evolution of syn and post breakup depositional systems. We have investigated three rifted continental margins; the Gulf of Aden, Galicia Bank and the Gulf of Lions, to determine whether the oceanic crust in the ocean-continent transition of these margins has present day anomalous subsidence and if so, whether it is caused by mantle dynamic topography or anomalous oceanic crustal thickness. Residual depth anomalies (RDA) corrected for sediment loading, using flexural backstripping and decompaction, have been calculated by comparing observed and age predicted oceanic bathymetries in order to identify anomalous oceanic bathymetry and subsidence at these margins. Age predicted bathymetric anomalies have been calculated using the thermal plate model predictions from Crosby & McKenzie (2009). Non-zero sediment corrected RDAs may result from anomalous oceanic crustal thickness with respect to the global average, or from mantle dynamic uplift. Positive RDAs may result from thicker than average oceanic crust or mantle dynamic uplift; negative RDAs may result from thinner than average oceanic crust or mantle dynamic subsidence. Gravity inversion incorporating a lithosphere thermal gravity anomaly correction and sediment thickness from 2D seismic data has been used to determine Moho depth and oceanic crustal basement thickness. The reference Moho depths used in the gravity inversion have been calibrated against seismic refraction Moho depths. The gravity inversion crustal basement thicknesses together with Airy isostasy have been used to predict a "synthetic" gravity derived RDA. Sediment corrected RDA for oceanic crust in the Gulf of Aden are positive (+750m) indicating anomalous uplift with respect to normal subsidence. Gravity inversion predicts normal thickness oceanic crust and a zero "synthetic" gravity derived RDA in the oceanic domain. The difference between the positive sediment corrected RDA and the zero "synthetic" gravity derived RDA, implies that the anomalous subsidence reported in the Gulf of Aden is the result of mantle dynamic uplift. For the oceanic crust outboard of Galicia Bank both the sediment corrected RDA and the "synthetic" gravity derived RDA are negative (-800m) and of similar magnitude, indicating anomalous subsidence, which is the result of anomalously thin oceanic crust, not mantle dynamic topography. We conclude that there is negligible mantle dynamic topography influencing the Galicia Bank region. In the Gulf of Lions, gravity inversion predicts thinner than average oceanic crust. Both sediment corrected RDA (-1km) and "synthetic" gravity derived RDA (-500m) are negative. The more negative sediment corrected RDA compared with the "synthetic" gravity derived RDA implies that the anomalous subsidence in the Gulf of Lions is the result of mantle dynamic subsidence as well as thinner than average oceanic crust.

  17. Late Triassic adakitic plutons within the Archean terrane of the North China Craton: Melting of the ancient lower crust at the onset of the lithospheric destruction (United States)

    Wang, Chao; Song, Shuguang; Niu, Yaoling; Su, Li


    We present the results of a geochemical and geochronological study for Late Triassic (230?220 Ma) adakitic plutons within the Archean terrane of the eastern part of the North China Craton (NCC). These plutons show adakitic signatures with high Sr, Sr/Y, (La/Yb)N, and low Cr and Ni. The enriched Nd-Hf isotopic compositions (?Nd(t) = - 13.3 to - 12.9; ?Hf(t) = - 17.4 to - 14.6) and old Nd (TDM2 = 2078-2037 Ma) and Hf (TDM2 = 2366-2192 Ma) isotope model ages suggest that the adakitic pluton may be derived from the underplated mafic lower crust of Paleoproterozoic age. The relatively low Cr and Ni contents and lower ?Nd(t) and ?Hf(t) values of the Taili adakitic plutons imply negligible input of mantle materials. Calculations of equilibrium mineral assemblages and modeling of trace element partition between melts and residual phases at different pressures confirm the interpretation that the petrogenesis of the Taili adakitic plutons is consistent with partial melting of the Paleoproterozoic mafic lower crust at 10-12 kbar (36-43 km) with a garnet granulite residue. Melting of the ancient mafic lower crust may be triggered by excess heating of the upwelling mantle in an extensional setting evoked by the contemporary subduction toward beneath the NCC from both north and south, which could serve as one possible mechanism for the destruction or lithospheric thinning of the NCC. Complex mantle-crust interaction through various mechanisms may have been responsible for the long-lived process of destruction or lithospheric thinning, which might have begun as early as in the late Triassic.

  18. Microbial Turnover of Fixed Nitrogen Compounds in Oceanic Crustal Fluids (United States)

    Kraft, B.; Wankel, S. D.; Glazer, B. T.; Huber, J. A.; Girguis, P. R.


    Oceanic crust is the largest aquifer on Earth, with a massive volume of seawater advecting through the basaltic crust. The microbiome of this deep marine subsurface biosphere has been estimated to be substantial, and consequently their metabolic activity may have major influences on global biogeochemical cycles. While earlier and recent studies provide insight into the microbial community composition of oceanic crustal fluids, information on the microbial ecophysiology is broadly missing. Therefore, to investigate the microbial transformation of fixed nitrogen compounds in crustal aquifer fluids, fluids were sampled from different horizons of two neighbouring CORK (Circulation Obviation Retrofit Kit) observatories at the North Pond sediment pond. This site is located on the western flank of the Mid Atlantic Ridge and is characterized by relatively young oceanic crust and cold fluids. The crustal fluids contain oxygen and nitrate, which potentially both may serve as electron acceptor for respiration. In a multidisciplinary approach we combined stable isotope incubations, determination of the natural isotopic compositions and plan to analyse relevant functional genes from a metagenomic dataset to investigate the nitrogen cycling at North Pond. The turnover of fixed nitrogen in oceanic crustal fluids may have important implications for the understanding of the global nitrogen cycle.

  19. Temporal Evolution of the Upper Continental Crust: Implications for the Mode of Crustal Growth and the Evolution of the Hydrosphere (United States)

    Rudnick, R. L.; Gaschnig, R. M.; Li, S.; Tang, M.; Qiu, L.; Valley, J. W.; Zurkowski, C.; McDonough, W. F.


    The upper continental crust (UCC), the interface between the atmosphere and solid Earth, is the site of weathering that produces sedimentary rocks, influences ocean chemistry through runoff of soluble elements, and affects climate through CO2 draw-down. The UCC also contains more than 50% of the crust's highly incompatible element budget (including K, Th, and U). Therefore, understanding its composition and evolution provides insight into how continents have formed, evolved, and interacted with the hydrosphere. New major and trace element compositions of >100 glacial diamictites and >100 Archean shales, plus ?7Li and ?18O for a subset of these samples, combined with data from the literature, show that the average composition of the UCC has changed through time, reflecting both the rise of atmospheric oxygen and its attendant effects on weathering, as well as the mode of crust formation and differentiation. Some changes that occur as a step function near the Archean/Proterozoic boundary (increased Th/U, decreased Mo/Pr, V/Lu) reflect the rise of oxygen at the great oxidation event (GOE) and its influence on chemical weathering signatures in the UCC. Other changes are more gradual with time (e.g., higher Th/Sc and ?18O, lower Ni/Co, La/Nb, Eu/Eu* and transition metal abundances) and reflect an UCC that has transitioned from a more mafic to a more felsic bulk composition, and which experienced increased interaction with the hydrosphere with time. The gradual nature of these compositional changes likely reflects the waning heat production of the Earth, rather than an abrupt change in tectonics or style of crust formation. These more gradual changes in crust composition, which contrast with the abrupt changes associated with the GOE, suggest that a fundamental change in the nature of crust differentiation is unlikely to be responsible for the rise of atmospheric oxygen (cf. Keller and Schoene, 2012). Indeed, it appears that the opposite may be true: that the rise of oxygen has influenced crust composition (and possibly differentiation).

  20. Estimation of seismic velocity in the subducting crust of the Pacific slab beneath Hokkaido, northern Japan by using guided waves (United States)

    Shiina, T.; Nakajima, J.; Toyokuni, G.; Kita, S.; Matsuzawa, T.


    A subducting crust contains a large amount of water as a form of hydrous minerals (e.g., Hacker et al., 2003), and the crust plays important roles for water transportation and seismogenesis in subduction zones at intermediate depths (e.g., Kirby et al., 1996; Iwamori, 2007). Therefore, the investigation of seismic structure in the crust is important to understand ongoing physical processes with subduction of oceanic lithosphere. A guided wave which propagates in the subducting crust is recorded in seismograms at Hokkaido, northern Japan (Shiina et al., 2014). Here, we estimated P- and S-wave velocity in the crust with guided waves, and obtained P-wave velocity of 6.6-7.3 km/s and S-wave velocity of 3.6-4.2 km/s at depths of 50-90 km. Moreover, Vp/Vs ratio in the crust is calculated to be 1.80-1.85 in that depth range. The obtained P-wave velocity about 6.6km/s at depths of 50-70 km is consistent with those estimated in Tohoku, northeast Japan (Shiina et al., 2013), and this the P-wave velocity is lower than those expected from models of subducting crustal compositions, such as metamorphosed MORB model (Hacker et al., 2003). In contrast, at greater depths (>80 km), the P-wave velocity marks higher velocity than the case of NE Japan and the velocity is roughly comparable to those of the MORB model. The obtained S-wave velocity distribution also shows characteristics similar to P waves. This regional variation may be caused by a small variation in thermal regime of the Pacific slab beneath the two regions as a result of the normal subduction in Tohoku and oblique subduction in Hokkaido. In addition, the effect of seismic anisotropy in the subducting crust would not be ruled out because rays used in the analysis in Hokkaido propagate mostly in the trench-parallel direction, while those in Tohoku are sufficiently criss-crossed.

  1. Emplacement of ultramafic rocks into the continental crust monitored by light and other trace elements: An example from the Geisspfad body (Swiss-Italian Alps)


    Pelletier, Laure; Müntener, Othmar; Kalt, Angelika; Vennemann, Torsten W.; Belgya, Tamás


    In order to evaluate the influence of continental crustal rocks on trace element budgets of serpentinized peridotites incorporated into the continental crust, we have analyzed the chemical composition of whole rock samples and minerals of the Geisspfad ultramafic complex (Swiss-Italian Alps). This complex represents a relict oceanic succession composed of serpentinites, ophicarbonates and metabasic rocks, emplaced into crustal gneisses during Alpine collision. Following peak metamorphic amphi...

  2. A Record of Magmatic and Hydrothermal Fluids in the Irish Crust: Evidence from Caledonian Granites (United States)

    Conliffe, J.; Feely, M.


    Fluid inclusions studies of granite quartz can provide evidence for major syn and post emplacement fluid influxes in and around granite bodies. Late Caledonian granites form a major component of the Irish crust, and facilitate the study of major hydrothermal fluid trapping events within the crust. We report the results of a regional scale fluid inclusion study of late Caledonian granite quartz. A variety of fluid signatures have been recorded, including evidence of magmatic, meteoric and low-temperature fluid influxes during evolution of the Irish crust over the past 400 Ma. Three main fluid inclusion types have been recognised in granite quartz. The earliest fluid type identified is a H2O-CO2-NaCl ± CH4 fluid, with high homogenisation temperatures (~300-450°C) and moderate salinities (3-16 eq. wt% NaCl). These inclusions display characteristic of primary fluid inclusions and their origin remains enigmatic. However the presence of these inclusions is restricted to granite bodies originating from subduction related melts, or lower crustal melts. Evidence for a later influx of H2O-NaCl-KCl fluids is preserved in all granites. Stable isotope analysis has identified this fluid as being meteoric in origin, and linked this fluid to the widespread retrograde alteration observed in some of these granite bodies. Finally a H2O-NaCl-CaCl2 fluid has been recorded in some Caledonian granites. This fluid displays variable salinity (8-28 eq. wt% NaCl) and homogenisation temperatures (125-205°C, and reflects the late penetration of basinal brines into the granites after their final crystallisation. These fluids are similar to CaCl2-bearing waters associated with major Pb-Zn deposits hosted within the Carboniferous limestones of the Irish midlands. Genetic models for the formation of these deposits involve the circulation of these fluids within the Caledonian basement rocks, and it is believed the presence of these fluids is recorded within the Caledonian granites. The presence or absence of these fluids within various Caledonian granites may be related to the spatial extent of this major fluid influx in the Irish crust.

  3. Natural constraints on the rheology of the lower continental crust (Musgrave Ranges, Central Australia) (United States)

    Hawemann, Friedrich; Mancktelow, Neil; Wex, Sebastian; Camacho, Alfredo; Pennacchioni, Giorgio


    Current models and extrapolated laboratory data generally predict viscous flow in the lower continental crust and any localized brittle deformation at these depths has been proposed to reflect downward propagation of the frictional-viscous transition zone during short-term seismic events and related high strain rates. Better natural constraints on this proposed rheological behaviour can be obtained directly from currently exposed lower crust that has not been strongly overprinted during its exhumation. One of the largest and best preserved lower crustal sections is located in the Musgrave Ranges, Central Australia. The Petermann Orogeny (550 Ma) in this area is characterized by the development of localized shear zones on a wide range of scales, overprinting water-deficient granulites of Musgravian age (1.2 Ga) as well as younger granites and gabbros. Shearing is rarely localized on lithological inhomogeneities, but rather on precursor fractures and on commonly associated pseudotachylytes. The only exception is that older dolerite dykes are often exploited, possibly because they are planar layers of markedly smaller grain size. Sheared pseudotachylyte often appears caramel-coloured in the field and has a fine grained assemblage of Grt+Cpx+Fsp. Multiple generations of pseudotachylyte formed broadly coeval with shearing are indicated by clasts of sheared pseudotachylyte within pseudotachylyte veins that then themselves subsequently sheared. The ductile shear zones formed under sub-eclogitic conditions of ca. 650°C and 1.2 GPa, generally typical of the lower continental crust. However, the P-T conditions during pseudotachylyte formation cannot be readily determined using classical geothermobarometry, because of the fine grain sizes and possible disequilibrium. The software "Xmaptools" (by Pierre Lanari) allows the quantification of X-ray maps produced by EDS or WDS. It provides both very precise definition of local mineral compositions for exchange geothermobarometry on a statistical basis, and an estimate of the bulk pseudotachylyte composition for small areas, avoiding clasts and heterogeneous composition of the former melt. The combination with thermodynamic modelling using PerpleX is used to test the results from geothermobarometry. The estimated conditions are similar to the ductile shear zones and support evidence for synchronous action of brittle faulting and viscous shearing in the lower crust.

  4. a Reappraisal of the Density and Thickness of the Martian Crust (United States)

    Baratoux, D.; Monnereau, M.; Wieczorek, M. A.; Michaut, C.; Garcia, R.; samuel, H.; Toplis, M. J.


    Terrestrial planets and some of the largest asteroids are known to be chemically differentiated into a dense metallic core, a silicate mantle and a crust. The term crust generally applies to the light outermost envelope extracted from the silicate mantle, a geochemical reservoir that may be formed in a number of different ways. For example, a primary crust may start to form as the result of the crystallization of a magma ocean early in the planet's history, whereas protracted crustal growth is possible if internal energy is sufficient to maintain internal convection and the generation of zones of partial melting at depth. In this respect, Mars has a long and rich volcanic history but has also preserved old crustal material, exposed essentially in the southern hemisphere (Grott et al., 2013). Insights into crustal composition of Mars have been provided by the martian meteorites and remote sensing and in-situ observations. However, it is not known from the analysis of these data if Mars has preserved a significant proportion of its primary crust or if subsequent crustal growth generated by partial melting of the mantle accounts for most of the crustal material (Baratoux et al., 2013). In addition to chemical and mineralogical data, inversion of geophysical data (moment of inertia, average density, or gravity field) provides direct constraints on mass distributions and therefore on the crustal thickness and density. However, the number of unknowns in the approach is large. Geophysical parameters were estimated and analyzed before strong petrologic constraints were available. As a consequence, conservative assumptions concerning the crustal density were generally made. Despite the fact that Martian meteorites were known to be generally denser than 3200 kg/m3 a range of density limited to 2700 - 3100 kg/m3 was preferred (Zuber, 2001, McGovern et al., 2002, Neumann et al., 2004, Wieczorek and Zuber, 2004, Sohl et al., 2005). The Martian meteorites were considered to be a non-representative set of samples of the Martian crust biased toward young ages. Ten years later, these assumptions need to be revised in the light of new available constraints. For example, the young age of the basaltic shergottites has been contested (Bouvier et al., 2009), while the number of samples and the quality of chemical analyses obtained on Martian meteorites has significantly increased in comparison with the last survey made by Lodders (1998). Furthermore, numerous chemical and mineralogical observations have been made directly on the Martian crust (Grott et al., 2013). In light of all this new data, we propose a reappraisal of the crustal density of Mars and argue for values near 3200 kg/m3. This implies a three-fold reduction of the density contrast with the mantle in comparison with previous studies. We then discuss the implications of our results for the thickness, nature, evolution and recycling of the Martian crust. This work has also been motivated by the preparation of the Insight mission that will provide seismological observations that will yield unprecedented constraints on the crustal structure of Mars.

  5. Micrometer-scale porosity as a biosignature in carbonate crusts (United States)

    Bosak, Tanja; Souza-Egipsy, Virginia; Corsetti, Frank A.; Newman, Dianne K.


    We formed calcite crusts in the presence and absence of the heterotrophic bacterium Desulfovibrio desulfuricans strain G20 to investigate microbial morphological signatures in fast-accreting carbonate precipitates. Submicrometer- to micrometer-sized pores (micropores) were present and ubiquitous in the G20 crusts but absent in abiotically precipitated crusts. Bacterial micropores resemble inclusions under transmitted light, but have distinct size, biological shapes and patterns (swirling or dendritic) and are distributed differently from common fluid inclusions. We observed similar porosity in both modern and ancient carbonate crusts of putative biotic origin. Our experiments support the microbial origin of micropores and help define specific criteria whereby to recognize these features as biosignatures in the rock record.

  6. Biological soil crusts in post-mining areas.

    Czech Academy of Sciences Publication Activity Database

    Lukešová, Alena; Zahradníková, M.; Frouz, J.

    Boca Raton : Taylor & Francis CRC Press, 2013, s. 53-65. ISBN 978-1-4665-9931-4 Institutional support: RVO:60077344 Keywords : biological soil crusts * post-mining areas Subject RIV: DF - Soil Science

  7. Yellow sorediate crusts called Caloplaca citrina in England.

    Czech Academy of Sciences Publication Activity Database

    Powell, M.; Vondrák, Jan

    Ro?. 2012, ?. 110 (2012), s. 20-24. ISSN 0300-4562 Institutional research plan: CEZ:AV0Z60050516 Institutional support: RVO:67985939 Keywords : biodiversity * crytic species * sorediate crusts Subject RIV: EF - Botanics

  8. Nuclear superfluidity and cooling time of neutron-star crust

    International Nuclear Information System (INIS)

    We analyse the effect of neutron superfluidity on the cooling time of inner crust matter in neutron stars, in the case of a rapid cooling of the core. The specific heat of the inner crust, which determines the thermal response of the crust, is calculated in the framework of HFB approach at finite temperature. The calculations are performed with two paring forces chosen to simulate the pairing properties of uniform neutron matter corresponding respectively to Gogny-BCS approximation and to many-body techniques including polarisation effects. Using a simple model for the heat transport across the inner crust, it is shown that the two pairing forces give very different values for the cooling time. (authors)

  9. Shallow Moho with aseismic upper crust and deep Moho with seismic lower crust beneath the Japanese Islands obtained by seismic tomography using data from dense seismic network (United States)

    Matsubara, Makoto; Obara, Kazushige


    P-wave seismic velocity is well known to be up to 7.0 km/s and over 7.5 km/s in the lower crust and in the mantle, respectively. A large velocity gradient is the definition of the Moho discontinuity between the crust and mantle. In this paper, we investigates the configuration of Moho discontinuity defined as an isovelocity plane with large velocity gradient derived from our fine-scale three-dimensional seismic velocity structure beneath Japanese Islands using data obtained by dense seismic network with the tomographic method (Matsubara and Obara, 2011). Japanese Islands are mainly on the Eurasian and North American plates. The Philippine Sea and Pacific plates are subducting beneath these continental plates. We focus on the Moho discontinuity at the continental side. We calculate the P-wave velocity gradients between the vertical grid nodes since the grid inversion as our tomographic method does not produce velocity discontinuity. The largest velocity gradient is 0.078 (km/s)/km at velocities of 7.2 and 7.3 km/s. We define the iso-velocity plane of 7.2 km/s as the Moho discontinuity. We discuss the Moho discontinuity above the upper boundary of the subducting oceanic plates with consideration of configuration of plate boundaries of prior studies (Shiomi et al., 2008; Kita et al., 2010; Hirata et al, 2012) since the Moho depth derived from the iso-velocity plane denotes the oceanic Moho at the contact zones of the overriding continental plates and the subducting oceanic plates. The Moho discontinuity shallower than 30 km depth is distributed within the tension region like northern Kyushu and coastal line of the Pacific Ocean in the northeastern Japan and the tension region at the Cretaceous as the northeastern Kanto district. These regions have low seismicity within the upper crust. Positive Bouguer anomaly beneath the northeastern Kanto district indicates the ductile material with large density in lower crust at the shallower portion and the aseismic upper crust. The Moho discontinuity deepens over 35 km in the collision zone like as Kanto Mountains, the volcanic underplating zone as the Tohoku backbone range, and non-tension region like as Chugoku Mountains. These regions associated with deep Moho are characterized by the crustal seismicity within the depth range from 20 to 30 km. The iso-depth contour of 35 km beneath the southwestern Japan is consistent with that derived from the receiver function method (Shiomi et al. 2006). There are nonvolcanic tremors and short-time slow slip events (SSE) beneath the southwestern Japan (eg. Obara, 2002). Matsubara et al. (2009) consider that the tremors and SSEs occur along the contact zone of Moho discontinuity beneath the Eurasian plate and the subducting Philippine Sea plate beneath southwestern Japan. Our Moho model is consistent with this since they exist along the southern edge of the Moho discontinuity of the continental Eurasian plate. Reference: Hirata, N., Sakai, S., Nakagawa, S., Ishikawa, M., Sato, H., Kasahara, K., Kimura, H. and Honda, R. (2012) A new tomographic image on the Philippine Sea Slab beneath Tokyo - Implication to seismic hazard in the Tokyo metropolitan region, EOS, Transactions, AGU, T11C-06. Kita, S., T. Okada, A. Hasegawa, J. Nakajima, and T. Matsuzawa (2010) Anomalous deepening of a seismic belt in the upper-plane of the double seismic zone in the Pacific slab beneath the Hokkaido corner: Possible evidence for thermal shielding caused by subducted forearc crust materials, Earth Planet. Science Lett., 290, 415-426. Matsubara, M. and K. Obara (2011) The 2011 Off the Pacific Coast of Tohoku earthquake related to a strong velocity gradient with the Pacific plate, Earth Planets Space, 63, 663-667. Matsubara, M., K. Obara, and K. Kasahara (2009) High-Vp/Vs zone accompanying non-volcanic tremors and slow-slip events beneath southwestern Japan, Tectonophysics, 472, 6-17, doi:10.1016/j.tecto.2008.06.013. Obara, K. (2002) Nonvolcanic deep tremor associated with subduction in southwest Japan. Science 296, 1679-1681. Shiomi, K., K. Obara, and H. Sato (2006) Moh

  10. Thermalisation time and specific heat of neutron stars crust


    Fortin, M.; Grill, F.; Margueron, J.; Sandulescu, N.


    We discuss the thermalisation process of the neutron stars crust described by solving the heat transport equation with a microscopic input for the specific heat of baryonic matter. The heat equation is solved with initial conditions specific to a rapid cooling of the core. To calculate the specific heat of inner crust baryonic matter, i.e., nuclear clusters and unbound neutrons, we use the quasiparticle spectrum provided by the Hartree-Fock-Bogoliubov approach at finite temp...

  11. Magnetar activity mediated by plastic deformations of neutron star crust


    Lyutikov, Maxim


    We advance a "Solar flare" model of magnetar activity, whereas a slow evolution of the magnetic field in the upper crust, driven by electron MHD (EMHD) flows, twists the external magnetic flux tubes, producing persistent emission, bursts and flares. At the same time the neutron star crust plastically relieves the imposed magnetic field stress, limiting the strain $ \\epsilon_t $ to values well below the critical strain $ \\epsilon_{crit}$ of a brittle fracture, $ \\epsilon_t \\s...

  12. Melt anomalies in the Indian Ocean basin: Constraints on dynamic interaction between the Marion/Crozet hotspots and the Southwest Indian Ridge (United States)

    Zhang, T.; Lin, J.; Gao, J.


    We investigate melt anomalies created by hotspots in the Indian Ocean and their implications on the dynamic interaction between hotspots and the Southwest Indian Ridge (SWIR). Residual mantle Bouguer anomalies (RMBA) were calculated by subtracting from satellite-derived free-air gravity (Sandwell and Smith, version 18.1) the predicted attractions of water-sediment, sediment-crust, and crust-mantle interfaces as well as the effects of cooling lithosphere. Residual bathymetry anomaly (RBA) was calculated by subtracting from the shipboard and predicted bathymetry (Sandwell and Smith, version 11.1) the effects of sediment unloading and seafloor subsidence due to lithospheric cooling. The results of analysis reveal profound negative RMBA in several regions: (1) The Conrad Rise, which is south of the SWIR on crustal age of 90 - 84 Ma, is associated with negative RMBA with amplitude of up to -360 mGal, corresponding to maximum crustal thickening of 19 km for an end-member model in which the RMBA is caused only by crustal thickness variations. The Madagascar Ridge, which is on the conjugate crust to the north of the SWIR, is associated with negative RMBA of similar amplitude. (2) A region of negative RMBA or thickened crust is found in the vicinity of the Marion Island, Del Cano Rise, and the Crozet plateau, with the calculated RMBA of -300, -320, and -400 mGal, respectively. (3) Along the present-day SWIR axis, the mean RMBA is most negative (about -110 mGal) between the Andrew Bain and Gallieni FZ, moderate (about -50 mGal) between the Gallieni and Atlantis II FZ, and most positive (about zero mGal) east of the Atlantis II FZ approaching the Rodrigues triple junction. (4) Plate reconstruction based on matching crustal isochrones reveals that the eastern part of the Del Cano Rise was created by the interaction of the Marion hotspot with the paleo-Rodrigues triple junction at 74 - 69 Ma; the central part of the Del Cano Rise was formed when the Marion hotspot was centered on the SWIR at 69 - 50 Ma; while the western end of the Del Cano Rise might most be created by intra-plate volcanism of Marion hotspot at 36 - 21 Ma. The above interpretation is consistent with the observations that the eastern and central portions of the Del Cano Rise are well explained by Airy isostatic compensation models, indicating their formation on relatively weak lithosphere in proximity of a seafloor spreading center. In contrast, the western Del Cano Rise is best explained by plate flexural models of intra-plate loading on relatively thick lithosphere. Together these results attest the importance of hotspot melt anomalies and ridge-hotspot interaction in the lithosphere accretion of the Southwest Indian Ridge.

  13. Neutron Star Asteroseismology. Axial Crust Oscillations in the Cowling Approximation

    CERN Document Server

    Samuelsson, L; Samuelsson, Lars; Andersson, Nils


    Recent observations of quasi-periodic oscillations in the aftermath of giant flares in soft gamma-ray repeaters suggest a close coupling between the seismic motion of the crust after a major quake and the modes of oscillations in a magnetar. In this paper we consider the purely elastic modes of oscillation in the crust of a neutron star in the relativistic Cowling approximation (disregarding any magnetic field). We determine the axial crust modes for a large set of stellar models, using a state-of-the-art crust equation of state and a wide range of core masses and radii. We also devise useful approximate formulae for the mode-frequencies. We show that the relative crust thickness is well described by a function of the compactness of the star and a parameter describing the compressibility of the crust only. Considering the observational data for SGR 1900+14 and SGR 1806-20, we demonstrate how our results can be used to constrain the mass and radius of an oscillating neutron star.

  14. Recent Advances in Multichannel Seismic Imaging for Academic Research in Deep Oceanic Environments

    Directory of Open Access Journals (Sweden)

    Juan Pablo Canales


    Full Text Available Academic research using marine multichannel seismic (MCS methods to investigate processes related to Earth's oceanic crust has made substantial advances in the last decade. These advances were made possible by access to state-of-the-art MCS acquisition systems, and by development of data processing and modeling techniques that specifically deal with the particularities of oceanic crustal structure and the challenges of subseafloor imaging in the deep ocean. Among these methods, we highlight multistreamer three-dimensional (3D imaging, streamer refraction tomography, synthetic ocean bottom experiments (SOBE, and time-lapse (4D studies.

  15. Ocean Motion (United States)


    This Web site offers a review of the surface circulation of Earth's ocean and classroom investigations appropriate for various disciplines at the high school level. Articles and video interviews about ocean current research, interactive data visualizes, news articles, simplified models, teacher and student guides are included. This site highlights use on-line satellite data of Earth for understanding patterns of ocean surface currents and how they relate to human exploration, commerce, science, weather/climate, and pollution. The interdisciplinary investigations included are classroom-ready, aiming to help high school students practice science, mathematics and writing skills in accordance to national standards. The topics covered also align with the traditional high school curriculum. Each investigation is keyed to the stages of the 5 E's teacher/learning model.

  16. Ocean energy

    International Nuclear Information System (INIS)

    There are 5 different ways of harnessing ocean energy: tides, swells, currents, osmotic pressure and deep water thermal gradients. The tidal power sector is the most mature. A single French site - The Rance tidal power station (240 MW) which was commissioned in 1966 produces 90% of the world's ocean energy. Smaller scale power stations operate around the world, 10 are operating in the European Union and 5 are being tested. Underwater generators and wave energy converters are expanding. In France a 1 km2 sea test platform is planned for 2010. (A.C.)

  17. Ocean Acidification (United States)

    Vicki Osis

    The purpose of the lessons is to teach about ocean acidification, its causes and impacts on marine life especially zooplankton, an essential part of marine food webs. Included in the materials is background information on ocean acidification. There are four different activities included in this document. To do all four you should plan on at least two 45 minute periods. The activities define and explain the process of acidification as well as its impacts on shelled organism. The materials can be adapted and used for grades 5-6 and adding more indepth information makes it suitable for middle and high school students.

  18. Dispatch from the Deep: Shaping the Ocean Floor at the Mid-Ocean Ridges (United States)

    This text reports on the experiences of a science teacher who joined a research expedition to study deep sea vents in the Pacific Ocean. It offers an engaging and informative overview of topics including the peaks, valleys, and plains that form the floor of the ocean and the internal heat that cracks the crust and causes it to move along plate boundaries. It also explains why submarine volcanoes form at mid-ocean ridges and how the intense pressure and cold water of the deep seas change the way lava flows to produce pillow lava. The author makes these observations from the deck of the research vessel Atlantis while the submersible research vessel Alvin dives below.

  19. H11077_MB0.5M_GEO.TIF: Color Shaded-Relief GeoTIFF Image Showing the 0.5-m Bathymetry Generated from National Oceanic and Atmospheric Administration (NOAA) Survey H11077 in Woods Hole, MA (Geographic)

    U.S. Geological Survey, Department of the Interior — The U.S. Geological Survey (USGS), in cooperation with the National Oceanic and Atmospheric Administration (NOAA) and the Massachusetts Office of Coastal Zone...

  20. H11077_MB1.5M_GEO.TIF: Color Shaded-Relief GeoTIFF Image Showing the 1.5-m Bathymetry Generated from National Oceanic and Atmospheric Administration (NOAA) Survey H11077 in Woods Hole, MA (Geographic)

    U.S. Geological Survey, Department of the Interior — The U.S. Geological Survey (USGS), in cooperation with the National Oceanic and Atmospheric Administration (NOAA) and the Massachusetts Office of Coastal Zone...

  1. H11346_MB25M_GEO.TIF: Color Shaded-Relief GeoTIFF Image Showing the 25-m Bathymetry Generated from National Oceanic and Atmospheric Administration (NOAA) Survey H11346 in the vicinity of Edgartown Harbor, MA (Geographic, WGS84)

    U.S. Geological Survey, Department of the Interior — The U.S. Geological Survey (USGS), in cooperation with the National Oceanic and Atmospheric Administration (NOAA) and the Massachusetts Office of Coastal Zone...

  2. H11346_MB25M_UTM19.TIF: Color Shaded-Relief GeoTIFF Image Showing the 25-m Bathymetry Generated from National Oceanic and Atmospheric Administration (NOAA) Survey H11346 in the vicinity of Edgartown Harbor, MA (UTM Zone 19, WGS84)

    U.S. Geological Survey, Department of the Interior — The U.S. Geological Survey (USGS), in cooperation with the National Oceanic and Atmospheric Administration (NOAA) and the Massachusetts Office of Coastal Zone...

  3. H11077_MB1.5M_UTM19.TIF: Color Shaded-Relief GeoTIFF Image Showing the 1.5-m Bathymetry Generated from National Oceanic and Atmospheric Administration (NOAA) Survey H11077 in Woods Hole, MA (UTM Zone 19)

    U.S. Geological Survey, Department of the Interior — The U.S. Geological Survey (USGS), in cooperation with the National Oceanic and Atmospheric Administration (NOAA) and the Massachusetts Office of Coastal Zone...

  4. H11077_MB0.5M_UTM19.TIF: Color Shaded-Relief GeoTIFF Image Showing the 0.5-m Bathymetry Generated from National Oceanic and Atmospheric Administration (NOAA) Survey H11077 in Woods Hole, MA (UTM Zone 19)

    U.S. Geological Survey, Department of the Interior — The U.S. Geological Survey (USGS), in cooperation with the National Oceanic and Atmospheric Administration (NOAA) and the Massachusetts Office of Coastal Zone...

  5. Sea surface temperature and salinity in the south Atlantic subtropical gyre over the last 4 Ma (United States)

    Wojcieszek, D. E.; Dekens, P. S.


    The early Pliocene (3-5 Ma) is the most recent time in the Earth's history when climate was significantly warmer than today. Even though atmospheric pCO2 (360-410 ppm) was similar to today, the average global temperature was 3-4°C warmer, ice sheets were smaller, and sea level was approximately 35 m higher than today. Recent studies show that reduced zonal and meridional sea surface temperature (SST) gradients played a role in sustaining Pliocene warmth and increasing SST gradients coincides with Pliocene-Pleistocene climate cooling. Ocean circulation is another important factor in climate transitions. Changes in shallow overturning circulation (ventilated thermocline) and ocean heat transport are hypothesized to be responsible for increasing Earth's sensitivity to orbital forcing and the onset of Pleistocene glaciations. However, most early Pliocene SST records are focused in the northern hemisphere and tropical upwelling regions, while data from the subtropical southern hemisphere is scarce. We present G.sacculifer Mg/Ca and ?18O records from ODP site 1264 (28.53°S; 2.85°E, 2505 m water depth), located on the Walvis Ridge in the south Atlantic subtropical gyre and within the influence of the Agulhas leakage. The Mg/Ca SST record displays no long-term trend over the past 4 Ma. The ?18O record indicates that sea surface salinity (SSS) gradually decreased from the early Pliocene to today. In the modern ocean the Agulhas Current is a western boundary current in the Indian Ocean that sheds rings of salty Indian Ocean waters into the SE Atlantic. The current is driven by a high air pressure system (the downward portion of the Hadley cell) over the south Indian Ocean and therefore responds to climatic shifts. Previous work indicates that weaker Hadley circulation in the early Pliocene lead to expanded subtropical gyres. This would have enhanced inter-ocean flow from the Indian to the Atlantic Ocean, resulting in higher SSS. Contraction of the south Indian Ocean subtropical gyre due to strengthening Hadley circulation from the early Pliocene to today would have resulted in a decreasing influence of the Agulhas Current, and therefore reduced SSS in the SE Atlantic. Increasing foraminifera abundance and decreasing nannofossil content from 4 Ma to today at ODP site 1264 implies a change from turbulent conditions in the Pliocene to calm gyre-like conditions in the modern ocean, consistent with a reduced Agulhas current influence. We conclude that the SST and SST records at ODP site 1264 reflect a decreasing influence of the Agulhas Current on the south-eastern Atlantic over the last 4 Ma.

  6. Galactic-cosmic-ray-produced 3He in a ferromanganese crust: any supernova 60Fe excess on earth? (United States)

    Basu, S; Stuart, F M; Schnabel, C; Klemm, V


    An excess of 60Fe in 2.4-3.2 x 10(6) year old ferromanganese crust (237 KD) from the deep Pacific Ocean has been considered as evidence for the delivery of debris from a nearby supernova explosion to Earth. Extremely high ;{3}He/;{4}He (up to 6.12 x 10(-3)) and 3He concentrations (up to 8 x 10(9) atoms/g) measured in 237 KD cannot be supernova-derived. The helium is produced by galactic cosmic rays (GCR) and delivered in micrometeorites that have survived atmospheric entry to be trapped by the crust. 60Fe is produced by GCR reactions on Ni in extraterrestrial material. The maximum (3)He/(60)Fe of 237 KD (80-850) is comparable to the GCR (3)He/(60)Fe production ratio (400-500) predicted for Ni-bearing minerals in iron meteorites. The excess 60Fe can be plausibly explained by the presence of micrometeorites trapped by the crust, rather than injection from a supernova source. PMID:17501264

  7. Stress analysis and scaling studies of corium crusts

    International Nuclear Information System (INIS)

    In a severe accident in a light water reactor, a molten mixture of fuel and metals could be deposited onto the concrete floor below the vessel. The termination of the subsequent molten core-concrete interaction (MCCI) by the addition of water is of major concern. This core coolability question is the primary focus of the tests being conducted by Argonne National Labs, i.e., those designated as MACE. The experiments to date have resulted in relatively thick solidified crusts that act as thermal barriers, limiting the heat transferred upward into the water pool. A number of technical issues need to be addressed to fully investigate the coolability issue when there is the possibility of crust formation, e.g., (a) under what conditions or circumstances will a crust develop (and be stable), (b) if such a crust does form, to what extent will it reduce the upward heat loss and retard core materials quench and (c) what are the scaling implications of such a crust as one extrapolates from test conditions to full scale accident conditions. This paper focuses on a methodology to answer the structurally related questions of (a) and (c) above. A companion paper discusses additional details on (a) and provides supporting calculations for (b) and (c). If a crust does form between the molten pool and the water layer being added, the two primary loadings (considering the crust as a structural component) consist of a uniform pressure over the entire surface and a temperature gradient tntire surface and a temperature gradient through the thickness. The absolute pressure acting on the crust can be found by taking into account the pressure from the overlying water and from the gases released from the molten fuel. For the analysis which follows, the worst case is often assumed, where the pressure loading is shown on the top surface and is equated with a water depth of 0.5 m. The bending stresses associated with this particular loading can be superimposed with the thermal stresses produced by the temperature loading, in effect decoupling the problem. A number of structural models of the crust have been developed to evaluate the stability of the crust under these external loads. In addition, the models were used to determine the effect of geometric features on the quasi-static state of stress present in the crust. Consequently, these analyses provide a predictive tool to study the effects of scaling. Bending stresses and membrane stresses due to pressure loadings and the temperature differential are considered in the analyses to investigate the stability of the crust as a function of the time, thickness and span. The results from parametric studies show the conditions under which a crust could develop, and how such structural models could be used to determine scaling effects and provide correlations to prototypic accident situations. In order to provide more quantitative data (rather than qualitative) the material properties of the crust as a function of temperature and porosity must be identified

  8. Evidence from gabbro of the Troodos ophiolite for lateral magma transport along a slow-spreading mid-ocean ridge. (United States)

    Abelson, M; Baer, G; Agnon, A


    The lateral flow of magma and ductile deformation of the lower crust along oceanic spreading axes has been thought to play a significant role in suppressing both mid-ocean ridge segmentation and variations in crustal thickness. Direct investigation of such flow patterns is hampered by the kilometres of water that cover the oceanic crust, but such studies can be made on ophiolites (fragments of oceanic crust accreted to a continent). In the Oman ophiolite, small-scale radial patterns of flow have been mapped along what is thought to be the relict of a fast-spreading mid-ocean ridge. Here we present evidence for broad-scale along-axis flow that has been frozen into the gabbro of the Troodos ophiolite in Cyprus (thought to be representative of a slow-spreading ridge axis). The gabbro suite of Troodos spans nearly 20 km of a segment of a fossil spreading axis, near a ridge-transform intersection. We mapped the pattern of magma flow by analysing the rocks' magnetic fabric at 20 sites widely distributed in the gabbro suite, and by examining the petrographic fabric at 9 sites. We infer an along-axis magma flow for much of the gabbro suite, which indicates that redistribution of melt occurred towards the segment edge in a large depth range of the oceanic crust. Our results support the magma plumbing structure that has been inferred indirectly from a seismic tomography experiment on the slow-spreading Mid-Atlantic Ridge. PMID:11343114

  9. Sr and Nd isotope geochemistry of oceanic basalts and mantle evolution

    International Nuclear Information System (INIS)

    Sr and Nd isotope ratios are reported for 17 mid-ocean ridge basalts and for 11 oceanic islands and island groups. Data from the Azores, Samoa and the Society Islands diverge significantly from the mantle array. These results are not explained by binary mixing of depleted and undepleted mantle reservoirs or by variable magmatic depletion of a planetary reservoir, but support mantle evolution models involving re-injection of crust material into the mantle. (author)

  10. An alternative early opening scenario for the Central Atlantic Ocean


    Labails, Cinthia; Olivet, Jean-louis; Aslanian, Daniel; Roest, Walter


    The opening of the Central Atlantic Ocean basin that separated North America from northwest Africa is well documented and assumed to have started during the Late Jurassic. However, the early evolution and the initial breakup history of Pangaea are still debated: most of the existing models are based on one or multiple ridge jumps at the Middle Jurassic leaving the oldest crust on the American side, between the East Coast Magnetic Anomaly (ECMA) and the Blake Spur Magnetic Anomaly (BSMA). Acco...

  11. Origin of deep ocean microseisms by using teleseismic body waves.


    Campillo, Michel; Paul, Anne; Shapiro, Nikolai M.; Lande?s, Matthieu; Hubans, Fabien


    Recent studies of oceanic microseisms have concentrate on fundamental-mode surface waves. Extraction of fundamental-mode Rayleigh and Love wave Green functions from station-station correlations of ambient seismic noise has recently been demonstrated to be a very powerful tool for imaging of the Earth's crust and uppermost mantle. In this study we concentrate on energetic arrivals in two frequency bands around the primary (14s) and the secondary (7s) microseismic peaks that appear at near-zero...

  12. Ocean energies

    International Nuclear Information System (INIS)

    This timely volume provides a comprehensive review of current technology for all ocean energies. It opens with an analysis of ocean thermal energy conversion (OTEC), with and without the use of an intermediate fluid. The historical and economic background is reviewed, and the geographical areas in which this energy could be utilized are pinpointed. The production of hydrogen as a side product, and environmental consequences of OTEC plants are considered. The competitiveness of OTEC with conventional sources of energy is analysed. Optimisation, current research and development potential are also examined. Separate chapters provide a detailed examination of other ocean energy sources. The possible harnessing of solar ponds, ocean currents, and power derived from salinity differences is considered. There is a fascinating study of marine winds, and the question of using the ocean tides as a source of energy is examined, focussing on a number of tidal power plant projects, including data gathered from China, Australia, Great Britain, Korea and the USSR. Wave energy extraction has excited recent interest and activity, with a number of experimental pilot plants being built in northern Europe. This topic is discussed at length in view of its greater chance of implementation. Finally, geothermal and biomass energy are considered, and an assessment of their future is given. The authors also distinguished between energy schemes which might be valuable in less-industrialized regiht be valuable in less-industrialized regions of the world, but uneconomical in the developed countries. A large number of illustrations support the text. This book will be of particular interest to energy economists, engineers, geologists and oceanographers, and to environmentalists and environmental engineers

  13. Abandoned Paleocene spreading center in the northeastern Indian Ocean: evidence from magnetic and seismic reflection data

    Digital Repository Service at National Institute of Oceanography (India)

    Krishna, K.S.; Rao, D.G.


    of the Wharton Ridge and India-Antarctica Ridge, after the first major plate reorganization of the Indian Ocean (about 95 plus or minus 5 Ma), but ceased shortly after formation of anomaly 30 (about 65 Ma). Then it jumped southward between anomalies 32n.2 and 33...

  14. Crust-mantle density distribution in the eastern Qinghai-Tibet Plateau revealed by satellite-derived gravity gradients (United States)

    LI, Honglei; Fang, Jian; Braitenberg, Carla; Wang, Xinsheng


    As the highest, largest and most active plateau on Earth, the Qinghai-Tibet Plateau has a complex crust-mantle structure, especially in its eastern part. In response to the subduction of the lithospheric mantle of the Indian plate, large-scale crustal motion occurs in this area. Despite the many previous studies, geodynamic processes at depth remain unclear. Knowledge of crust and upper mantle density distribution allows a better definition of the deeper geological structure and thus provides critically needed information for understanding of the underlying geodynamic processes. With an unprecedented precision of 1-2 mGal and a spatial resolution better than 100 km, GOCE (Gravity field and steady-state Ocean Circulation Explorer) mission products can be used to constrain the crust-mantle density distribution. Here we used GOCE gravitational gradients at an altitude of 10km after reducing the effects of terrain, sediment thickness variations, and Moho undulations to image the density structures of eastern Tibet up to 200 km depths. We inverted the residual satellite gravitational gradients using a least square approach. The initial density model for the inversion is based on seismic velocities from the tomography. The model is composed of rectangular blocks, having a uniform density, with widths of about 100 km and variable thickness and depths. The thickness of the rectangular cells changes from10 to 60km in accordance with the seismic model. Our results reveal some large-scale, structurally controlled density variations at depths. The lithospheric root defined by higher-density contrast features from southwest to northeast, with shallowing in the central part: base of lithosphere reaches a depth of180 km, less than 100km, and 200 km underneath the Lhasa, Songpan-Ganzi, and Ordos crustal blocks, respectively. However, these depth values only represent a first-order parameterization because they depend on model discretization inherited from the original seismic tomography model. For example, the thickness of the uniform density blocks centered at140 km depth is as large as 60 km. Low-density crustal anomalies beneath the southern Lhasa and Songpan-Ganzi blocks in our model support the idea of weak lower crust and possible crustal flow, as a result of the thermal anomalies caused by the upwelling of hot deep materials. The weak lower crust may cause the decoupling of the upper crust and the mantle. These results are consistent with many other geophysical studies, confirming the effectiveness of the GOCE gravitational gradient data. Using these data in combination with other geodynamic constraints (e.g., gravity and seismic structure and preliminary reference Earth model), an improved dynamic model can be derived.

  15. Th, Pb and Sr isotope variations in young island arc volcanics and oceanic sediments

    International Nuclear Information System (INIS)

    Destructive plate margin rocks have (230Th/232Th) values in the range 0.5-2.5, which is similar to those in MORB and OIB. Approximately 60% of the available data on subduction-related rocks plot within 10% of the (230Th/232Th)-(238U/232Th) equiline, and most of the remaining analyses are displaced to high (238U/230Th). Moreover, despite the large range of Th isotope ratios in sediments and altered MORB in the subducted slab, the majority (?70%) of destructive margin rocks plot within the Th-Sr and Th-Pb mantle arrays defined by MORB-OIB. Th/U ranges from 1.0 to 6.6 in the rocks analysed, and both the measured and the source Th/U inferred from initial (230Th/232Th) correlate with 208Pb*/206Pb*. Thus, the major (six-fold) difference in Th/U between different arc suites was established several 100 Ma prior to subduction, and it does not appear to reflect element fractionation processes associated with Recent subduction. Oceanic sediments exhibit a wide range in Th/U but relatively restricted 208Pb*/206Pb* suggesting that the average residence age of pelagic sediments is less than 350 Ma. Th isotopes are sensitive tracers of subducted carbonate, hydrothermally altered oceanic crust and detrital sediments. However, only those from Nicaragua and Tonga exhibit unusually high (230Th/232Th) consistent 30Th/232Th) consistent with a significant contribution of Th from altered MORB and/or carbonate. Most of the water in the subducted slab is in sediments and altered MORB, and so they should dominate the slab-derived contribution. Their bulk (230Th/232Th) values are likely to be >4, which suggests that subducted materials constitute <15% of the Th in arc magmas. Finally, Th/Nb and Ba/Nb ratios are high in subduction-related rocks, but whereas the latter primarily reflects a high Ba/Nb slab flux, the high Th/Nb ratios also reflect partial melting processes in the mantle wedge. (orig./WL)

  16. Models Of Strange Stars With A Crust And Strange Dwarfs

    CERN Document Server

    Vartanyan, Y L; Sargsyan, T R; Vartanyan, Yu. L.


    Strange quark stars with a crust and strange dwarfs consisting of a compact strange quark core and an extended crust are investigated in terms of a bag model. The crust, which consists of atomic nuclei and degenerate electrons, has a limiting density of ro_ cr=ro_drip=4.3*10^11g/cm^3. A series of configurations are calculated for two sets of bag model parameters and three different values of ro_cr (10^9g/cm^3<=ro_cr<=ro_drip) to find the dependence of a star's mass M and radius R on the central density. Sequences of stars ranging from compact strange stars to extended strange dwarfs are constructed out of strange quark matter with a crust. The effect of the bag model parameters and limiting crust density ro_cr on the parameters of the strange stars and strange dwarfs is examined. The strange dwarfs are compared with ordinary white dwarfs and observational differences between the two are pointed out.

  17. Crusted scabies-associated immune reconstitution inflammatory syndrome

    Directory of Open Access Journals (Sweden)

    Fernández-Sánchez Mónica


    Full Text Available Abstract Background Despite the widely accepted association between crusted scabies and human immunodeficiency virus (HIV-infection, crusted scabies has not been included in the spectrum of infections associated with immune reconstitution inflammatory syndrome in HIV-infected patients initiating antiretroviral therapy. Case presentation We report a case of a 28-year-old Mexican individual with late HIV-infection, who had no apparent skin lesions but soon after initiation of antiretroviral therapy, he developed an aggressive form of crusted scabies with rapid progression of lesions. Severe infestation by Sarcoptes scabiei was confirmed by microscopic examination of the scale and skin biopsy. Due to the atypical presentation of scabies in a patient responding to antiretroviral therapy, preceded by no apparent skin lesions at initiation of antiretroviral therapy, the episode was interpreted for the first time as “unmasking crusted scabies-associated immune reconstitution inflammatory syndrome”. Conclusion This case illustrates that when crusted scabies is observed in HIV-infected patients responding to antiretroviral therapy, it might as well be considered as a possible manifestation of immune reconstitution inflammatory syndrome. Patient context should be considered for adequate diagnosis and treatment of conditions exacerbated by antiretroviral therapy-induced immune reconstitution.

  18. Anisotropic Rayleigh-wave tomography of Ireland's crust: Implications for crustal accretion and evolution within the Caledonian Orogen (United States)

    Polat, G.; Lebedev, S.; Readman, P. W.; O'Reilly, B. M.; Hauser, F.


    The Irish landmass, now at the western extremity of the Eurasian Plate, was formed in the Caledonian Orogeny during the Palaeozoic assembly of Pangea. The associated closure of the Iapetus Ocean is recorded in the NE-SW structural trends that dominate the tectonic set-up of Ireland today. The deep-crustal dynamics of the orogeny and the effect on the crust of the subsequent extension and magmatism in the North Atlantic are debated. Fabrics within deep crustal rocks preserve a record of deformation during and after the continental collisions. Here, we measured Rayleigh-wave phase velocities using seismograms recorded by permanent and temporary intermediate-band stations in Ireland and inverted the data for phase-velocity maps, including azimuthal anisotropy. The observed isotropic phase-velocity heterogeneity reflects moderate crustal thickness and seismic velocity variations across Ireland. Anisotropy of Rayleigh waves at 10-20 s periods shows a NE-SW fast-propagation direction and is largest (up to 2%) at a 15 s period, at which Rayleigh waves sample primarily the middle and lower crust. The NE-SW trend of the deep-crustal anisotropic fabric is parallel to tectonic trends, in particular the Iapetus Suture Zone, which indicates that suture-parallel flow in the middle and lower crust accommodated the continental collision. The apparent preservation of the Caledonian-age fabric also shows that the deep crust of the Eurasian margin in Ireland was neither stretched by the NW-SE extension associated with the opening of the North Atlantic, nor modified significantly by the Cenozoic magmatism in the region.

  19. Mid-ocean ridges, InRidge and the future

    Digital Repository Service at National Institute of Oceanography (India)

    Iyer, S.D.; Mukhopadhyay, R.; Drolia, R.K.; Ray, Dwijesh


    and Africa. Large scale plate reorganization occurred ~ 65 Ma ago, during which time the North Atlantic and Indian Oceans were shaped, t he South Atlantic widened and Australia was still attached to Antarctica. Gradually a number of events came about... such as the separation of Australia from Antarctica and of Laurasia into North America and Eurasia and the initiation of collision between Ind ia and Eurasia (> 65 Ma to ~ 33 Ma (ref. 4)). The driving mecha - nism to carry the floundered continents was unknown...

  20. Propagation of coupled Rayleigh-gravity waves on the ocean floor

    Directory of Open Access Journals (Sweden)

    Vladimir V. Grimalsky


    Full Text Available It is shown that the Ocean-Earth crust interface can propagate gravity-sound Rayleigh waves. Dispersion properties of waves and flux of energy are derived. It is shown that the waves split into low and fast velocity branches. The fast branch has a multimode structure and has a cutoff in frequency and wave number. Numerical solutions are discussed.

  1. Ocean bottom seismometers deployed in Tyrrhenian Sea (United States)

    Dahm, Torsten; Thorwart, M.; Flueh, E. R.; Braun, Th.; Herber, R.; Favali, P.; Beranzoli, L.; D'Anna, G.; Frugoni, F.; Smriglio, G.

    The Institute for Geophysics (IfG) at Hamburg University and the Research Center for Marine Geoscience (GEOMAR) of Kiel University have developed new, wideband ocean bottom seismic stations (OBS) for long-term, deep-sea deployments of up to 1 year. A first long-term pilot experiment of these stations was conducted in the Tyrrhenian Sea north of Sicily, in-cooperation with the first long-term, deep-sea test of the European Ocean Bottom Observatory GEOSTAR [Beranzoli et al., 2000] by Istituto Nazionale di Geofisica e Vulcanologia (INGV). The seismic data retrieved prove that the new OBSs are useful for seismological studies. A large number of tele-seismic earthquakes have been recorded in good quality; waves originating from such events pass the mantle and crust below the network, and thus provide important constraints on their structure.

  2. Harnessing Ocean Energy by Tidal Current Technologies


    Nasir Mehmood; Zhang Liang; Jawad Khan


    The world is heavily dependent on fossil fuels since most of its energy requirements are fulfilled by conventional methods of burning these fuels. The energy demand is increasing by day with growing population. The energy production by fossil fuels is devastating the environment and survival of life on globe is endangered. The renewal energy technologies are vital to ensure future energy sustenance and environmental issues. Ocean is a vast resource of renewable energy. The technology today ma...

  3. Using Google Earth to Visualize the Core, Mantle, and Crust in Four Dimensions (Invited) (United States)

    de Paor, D. G.; Dordevic, M.; Wild, S. C.; Scientific Team Of Digitalplanet. Org


    Google Earth is generally recognized as a powerful tool for visualizing the Earth’s atmosphere, hydrosphere, and solid surface. However, it’s opaque terrain imagery would seem to preclude most solid earth applications. We have previously described how to visualize the subsurface using emergent COLLADA models. The KML "timespan" element was intended for time series but it can be redeployed to lift models out of the sub surface. We have used this technique mainly to display relatively shallow crustal and lithospheric structures. For studies of the Earth’s interior in four dimensions, we need to use the time slider as originally intended. Therefore, we make the surface transparent in the Primary Layers sidebar and represent deep structures with in situ COLLADA models. This approach enables us to model major stages in Earth’s internal evolution, including differentiation of the liquid core and magma ocean, the formation of the Moon, and the growth of a solid inner core and magnetic field. We can also follow subducted oceanic crust into the lower mantle and model the rise of deep mantle plumes from the core-mantle boundary. There are alternative virtual globes and 3D or 4D modeling tools that may be more powerful than Google Earth but none competes in terms of usability and universal accessibility. The extra effort required at the content creation stage is therefore well worthwhile.

  4. Pb isotopic evidence for early Archaean crust in South Greenland

    International Nuclear Information System (INIS)

    The results of an isotopic remote sensing study focussed on delineating the extent of Early Archean crust north and south of the Nuuk area and in south Greenland is presented. Contamination of the Late Archean Nuk gneisses and equivalents by unradiogenic Pb uniquely characteristic of Amitsoq gneiss was detected as far south as Sermilik about 70 km south of Nuuk and only as far north as the mouth of Godthabsfjord. This study was extended to the southern part of the Archean craton and the adjoining Early Proterozoic Ketilidian orogenic belt where the Pb isotopes suggest several episodes of reworking of older uranium depleted continental crust. The technique of using the Pb isotope character of younger felsic rocks, in this case Late Archean and Early Proterozoic gneisses and granites to sense the age and isotopic character of older components, is a particularly powerful tool for reconstructing the evolutionary growth and development of continental crust

  5. Melting of the Continental Crust: Products and Processes (United States)

    Brown, M.; Johnson, T.


    Earth's continental crust is unique compared to the 'basaltic' crusts of other inner planets; this is a consequence of free water on Earth and recycling of Earth's primary basaltic crust. Although there is no agreement about the average chemical composition of bulk continental crust or about secular change in composition, there is agreement that upper crust is more felsic than lower crust. Further, it is widely accepted that intracrustal differentiation by melting is responsible for the difference in composition, even though controversy remains about whether part of the inventory of lower crust has been lost by delamination. Magmatic additions from the mantle, recycling through erosion, sedimentation and burial via subduction and/or orogenesis, and regeneration through multiple orogenic events have developed a diversity of crustal rocks that neither melt nor behave uniformly. Water is an important constituent of crustal rocks, but the lower crust may be essentially 'dry', so that melting is controlled by successive dehydration of muscovite, biotite and hornblende in bulk compositions that vary from 'granodioritic' or 'pelitic' to 'dioritic' or 'basaltic'. Thus, the products of melting of continental crust are varied, both in terms of melt compositions, and in terms of the residue. Further, since H2O is partitioned in a hydrous silicate melt, cooling of melt bearing crust in a closed system potentially will lead to retrogression by back reaction between melt and residue, unless segregation has separated them sufficiently to avoid reaction during cooling. This raises the issue of equilibration domains and local equilibrium. The common occurrence of pristine to weakly retrogressed residual granulites suggests melt loss from the lower crust, consistent with the presence of leucogranites in the upper crust. We illustrate the 'reaction principle' (Bowen, N.L., 1922, The Reaction Principle in Petrogenesis. Journal of Geology, 30, 177-198) with reference to microstructures in natural systems, and by the use of pseudosections [including T-X and P-X pseudo-binaries (or projections) of fixed bulk compositions] modeled in the MnNCKFMASH system under suprasolidus conditions using an internally consistent thermodynamic database and the software THERMOCALC. Such diagrams are potentially powerful interpretative tools, and it is instructive to consider the consequences of a range of possible paths through them. For example, univariant reactions are rarely intersected ('seen') by single rocks (fixed bulk compositions) during burial and exhumation, even in relatively simple sub-systems (e.g., KFMASH). Modeling melt loss from metapelitic compositions in the MnNCKFMASH system shows that both bulk composition and proportion of melt lost give rise to a wide variation in retrograde behavior consistent with observation of lower crustal rocks. This point is important in understanding evolution of the melts produced, since retrograde (back) reactions occur at peritectics in the system. Thus, modeling of fractionation processes must take into account the possibility of such reactions and the consequent change in phase asssemblages. In the absence of a robust thermodynamic model for sapphirine, and for Ti in model granite melt compositions, we are not yet able to model metapelite compositions under ultra-high temperature metamorphic conditions. Further, in the absence of a robust thermodynamic model for the amphiboles, we are not yet able to model crustal melting of metabasaltic compositions. Experimental data are available that partly remedy this deficiency and provide some information about melt compositions and solid residues, but only for a limited range of fixed bulk compositions or for compositionally restricted sub-systems.

  6. The onshore-offshore ENCENS project: Imaging the stretching of the continental lithosphere and inception of oceanic spreading in the eastern Gulf of Aden (United States)

    Leroy, S.; Ebinger, C.; D'Acremont, E.; Stuart, G.; Al-Lazki, A.; Tiberi, C.; Autin, J.; Watremez, L.; Beslier, M.; Bellahsen, N.; Lucazeau, F.; Perrot, J.; Mouthereau, F.; Courrèges, E.; Huchon, P.; Rouzo, S.; Balahaf, S.; Sholan, J.; Unternehr, P.; Hello, Y.; Anglade, A.; Desprez, O.; Beguery, L.; Aouji, O.; Daniel, R.; Al Toubi, K.


    The eastern Gulf of Aden exemplifies several extensional processes that began 35 Ma ago from continental rifting to seafloor spreading at 2 cm/yr. Thin post-breakup sediment cover reveals the syn- and pre-rift basement fabric and the variable styles of conjugate margins along and across strike. A comprehensive multi- disciplinary study of the eastern part of the northern margin of the Gulf of Aden has been undertaken recently in the framework of the French margins program (GDR Marges) and the NERC with the long term objective to integrate in a consistent model of evolution, field observations where the margins crop out, results of marine geophysical survey where the margins are submerged and seismological observations of deep interior structure. A deep onshore-offshore seismic survey operated by IFREMER-GENAVIR was conducted between February, 3rd and March 14th 2006 off the Dhofar coast (southern Oman). We collect 67 multichannel seismic reflection profiles and 15 seismic refraction profiles acquired thanks to 60 stations. The shots have been recorded by 35 Ocean Bottom Seismometers (15 from IRD-Villefranche sur Mer and 20 from INSU Paris) and 25 seismological stations deployed onland in Southern Oman (18 stations from Encens-UK project funded by NERC and managed by RHUL ; 2 temporary and 4 permanent stations from Sultan Qaboos University ; 1 from ENS Paris). The network has been extended to the southern conjugate margin with 2 broadband stations deployed in Socotra island. The preliminary results show the evolution of the thickness of the crust from 35 km in the upper part of the margin to about 5 km in the ocean-continent transition. From our observations (low angle faults dipping toward the ocean or the continent imaged) we will propose rifting models responsible for the lithospheric stretching. The segmentation of the margins should be also precisely defined thanks to the multibeam bathymetry, the magnetism and the gravity acquired during the cruise.

  7. Modeling of Perturbations in Mid-Ocean Hydrothermal Systems (United States)

    Singh, S.; Lowell, R. P.


    Mid-ocean ridge hydrothermal systems are complex fluid circulation systems straddling the locations of formation of oceanic crust. Due to the dynamic nature of the crust building process, these systems are episodically subject to magmatic and seismic perturbations. Magma may be emplaced deep or shallow in the oceanic crust thereby changing the thermal structure and permeability of the system. Such events would enhance hydrothermal venting resulting in an increase in vent temperature and heat output along with a decrease in vent salinity in a phase separating system. Event plumes, which may be associated with dike intrusions into the shallow crust, are an important class of such perturbations. In this case, the formation of low salinity vapor may add to the thermal buoyancy flux and allow the plume to rise rapidly to a considerable height above the seafloor. Additionally, seismic or tectonic disturbances may occur both deep and shallow in the crust, changing the fluid-flow structure in the system. Upon knowledge of a major magmatic or seismotectonic event, temporary surveillance at the respective mid ocean ridge site is often increased as a result of rapid response cruises. One of the most common observations made after such events is the temperature of vent fluids, which is then correlated to time of observed activity and used to estimate the residence time of fluids in the system. However, our numerical results indicate that for deep-seated perturbations, surface salinity may show quicker response than temperature. This result serves as our motivation to seek better understanding of propagation mechanism of perturbations through hydrothermal systems. We construct analytical models for fluid flow, heat and salt transfer in both single cracks and through porous media to investigate how perturbations affect both heat and salt transfer to the surface. Our preliminary results for simplified fluid circulation systems tend to support the results from numerical modeling. We plan to develop further our analytical models to understand the variability in the response time of hydrothermal systems to perturbations and its relation to the site of the event within the crust. By incorporating the effects of phase separation near the seafloor on fluid buoyancy, this work will also provide us insight into formation of event plumes. We also plan to investigate whether surface salinity is a better indicator than temperature of hydrothermal disturbances beneath the seafloor.

  8. Evidence for Andean Uplift and Changes in Ocean Circulation from Ceara Rise during the middle Miocene to Pliocene (United States)

    Newkirk, D. R.; Martin, E. E.


    The Ceara Rise is ideally situated to monitor the interplay between northern and southern component waters (NCW and SCW) as a result of changing rates of deep water production in both hemispheres. Initial seawater Nd isotopic results from Ocean Drilling Program (ODP) Sites 925 (3,040 m), 926 (3,600 m) and 929 (4,360 m) illustrated that the two shallowest sites record values of ~-15 to -16 from 18 to 8 Ma, while the deepest site records values of ~-13. These values are less radiogenic than NCW or SCW throughout the Cenozoic. At 8 Ma, Nd values at all three sites increase rapidly to ~-11.5, a value similar to contemporaneous NCW. To better understand the cause of these low seawater Nd values, we generated Pb and Nd isotopic records for detrital fractions and Pb isotopes for seawater (using extractions of bulk sediments) from all three sites. Detrital fraction Pb isotopes all shift to less radiogenic values across an interval from 11 Ma to 7 Ma. A similar, but smaller shift is observed in seawater Pb isotopes, although seawater values are consistently less radiogenic than detrital values. Detrital Nd isotopes also show a dramatic shift at 8 Ma with values prior to the shift ~-18 and values after the shift ~-13. The dramatic increase in Nd and decrease in Pb isotopes of the detrital fractions at ~8 Ma coincides with documented uplift events of the Andes Mountains and associated changes in clay mineralogy at Ceara Rise (Harris and Mix, 2002, Geology), believed to reflect a shift in terrigenous sources from the Amazon lowlands (South American shield material) to the Andean highlands (volcanic arc material). Unlike most deep sea locations, Ceara Rise receives large fluxes of terrigenous material from the Amazon River. As a result, this region is likely to be more susceptible to boundary exchange and reversible scavenging than typical deep sea environments. Less radiogenic seawater Nd isotopes at the shallower sites compared to the deeper site is consistent with the pattern predicted for reversible scavenging associated with continental inputs. Seawater Nd isotopes for all three sites merge at values typical of NCW at ~8 Ma, which argues for stronger NCW production and suggests more effective reversible scavenging during the early part of the record when circulation was sluggish and a more homogeneous deep water column following enhanced NCW production. Thus, the seawater isotopic record at Ceara Rise appears to be a combination of Amazon drainage and circulation, with enhanced NCW production documented ~8 Ma at Ceara Rise, Walvis Ridge and the Romanche fracture zone (ROM). It is intriguing that the timing of Andean uplift and enhanced NCW production seem to be so closely linked. Nd and Pb isotopic values for deep waters at Ceara Rise also represent a potential previously unidentified endmember for values recorded in Fe-Mn crusts in ROM that were difficult to explain in terms of SCW and NCW endmembers (Frank et al., 2003, Paleoceanography).

  9. Introduction to Ocean Models (United States)


    Oceans cover over 70% of the surface of the earth, yet many details of their workings are not fully understood. To better understand and forecast the state of the ocean, we rely on numerical ocean models. Ocean models combine observations and physics to predict the ocean temperature, salinity, and currents at any time and any place across the ocean basins. This module will discuss what goes into numerical ocean models, including model physics, coordinate systems, parameterization, initialization, and boundary conditions.

  10. Density constraints of lunar upper crust from gravity and topography (United States)

    Huang, Q.; Wieczorek, M. A.; Ping, J.


    Remote sensing and in situ measurements have shown that there are large lateral and vertical variations in the composition of the lunar crust. These crustal density diversities can be investigated using newly obtained global gravity and topography data. We applied a localized spectral admittance technique to various crustal regions by windowing the free-air gravity and surface topography with the band-limited localization windows of Wieczorek and Simons (2005, 2007). These admittances were interpreted using a geophysical model that includes both surface and subsurface loads that are supported by an elastic lithosphere. By varying the crustal density, elastic thickness and loading ratio in certain ranges, the best fitting bulk densities for a number of homogeneous regions were constrained to vary laterally from 2590 kg m-3 to 3010 kg m-3, with a mean value of 2700 kg m-3. Assuming the composition of the upper crust is uniform, the porosity of the upper crust could be determined if the pore-free surface density is known. Based on the known compositions of lunar rocks and estimated mineralogical norms, we found that the pore-free densities of lunar rocks were highly correlated with FeO and TiO2 abundance. The rock density can vary from 2884 to 3038 kg m-3 in estimated regions by using the iron and titanium abundances from Lunar Prospector gamma-ray spectrometer. We calculated the porosity of each region and found a mean value of ~7.4±3.4%, with permissible values from 0 to 14%. Furthermore, we took into account the vertical variation of crust density, and developed a novel technique that the density profile of the crust could be inverted using higher resolution gravity models. Since all these analyses are challenging using the recent Kaguya gravity models, higher resolution gravity data expected from NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission would place tight constrains on both the lateral and vertical density variations of the lunar crust.

  11. MoMA: Cindy Sherman (United States)

    Sherman, Cindy


    Everyone's abuzz about Cindy Sherman - her current retrospective at MoMA was reviewed in the Arts sections of the "New York Times" on February 23rd, she's in the February 27th "New Yorker," and was even mentioned in the "Wall Street Journal" on March 5, in an article by Pia Catton, who admits to being skeptical of Sherman's elevated status in the art world. So it's a good thing that we can all use the online version of MoMA's exhibition to do a reality check. All of the characters in Sherman's photographs are the artist herself, in different guises. Sherman has gone large for the first time in this exhibition - creating larger-than-life, floor-to-ceiling murals for the lofty museum space. Another new thing is her use of Photoshop to digitally alter her features in addition to the tricks of hair and makeup she has always employed. At the website, visitors can view works chronologically or by Gallery. There's also a set of videos, My Favorite Cindy Sherman, that consists of art critics, curators, and other artists commenting on Sherman's work.

  12. Magma mixing and crust-mantle interaction in the Triassic monzogranites of Bikou Terrane, central China: Constraints from petrology, geochemistry, and zircon U-Pb-Hf isotopic systematics (United States)

    Yang, Li-Qiang; Deng, Jun; Qiu, Kun-Feng; Ji, Xing-Zhong; Santosh, M.; Song, Kai-Rui; Song, Yao-Hui; Geng, Jian-Zhen; Zhang, Chuang; Hua, Bei


    Mafic microgranular enclaves (MMEs) are common in the monzogranites from the Yangba pluton in the Bikou Terrane of central China. Zircon LA-ICP-MS U-Pb dating yields ages of 208.7 ± 0.7 Ma and 209.3 ± 0.9 Ma for the host monzogranites, and 211.9 ± 0.8 Ma for the MMEs, indicating formation from coeval magmas. The field occurrence and textures including spheroidal shapes, transitional contacts, igneous mineral assemblages, acicular apatites, and oscillatory zoning with repeated resorption surfaces in plagioclase, indicate that the MMEs crystallized from mafic magma that was injected into and mingled with the host felsic magma. The host monzogranites are intermediate-felsic, metaluminous, and lack typical peraluminous minerals or alkaline mafic minerals, suggesting their I-type affinity. The monzogranites display depletion in high field strength elements, with Nb and Ta anomalies, and enrichment in large ion lithophile elements and light rare earth elements (LREE), with slightly negative Eu anomalies (Eu/Eu? = 0.72-0.93). The MMEs from the Yangba pluton are intermediate, metaluminous, and have higher K2O, Al2O3, Fe2O3T, MgO, Ni, and Cr contents than the pluton. The MMEs also display much higher total rare earth element (REE) concentrations and REE patterns that are sub-parallel to those of the host rocks, enrichments in Rb, Th, U, K, and LREE, and depletions in Ba, Nb, Ta, and Ti. These features are similar to those of the host rocks, indicating crust-mantle interaction during their petrogenesis. Zircons from the MMEs have highly variable ?Hf(t) values (-5.5 to +8.7) with corresponding two-stage Hf model ages (TDM2) of 1.14 to 1.42 Ga, indicating they were derived from depleted mantle with crustal contamination. In contrast, zircons from host monzogranite show ?Hf(t) values ranging from -1.7 to +2.7, with TDM2 of 1.13 to 1.21 Ga, corresponding to that of the basement rocks in the Bikou Terrane, which indicates that these granitic magmas were probably derived from the Neoproterozoic juvenile lower crust with minor contribution of Mesoproterozoic ancient lower crust. The integrated petrology and elemental and isotopic geochemistry suggest that the MMEs and host monzogranites were generated by the interaction of a granitic magma and a mafic magma in the lower crust. We envisage that Neoproterozoic subcontinental lithosphere mantle-derived mafic magma intruded the lower continental crust during Late Triassic, following the Late Permian to Early Triassic collision between the South China and North China Blocks.

  13. Big Impacts and Transient Oceans on Titan (United States)

    Zahnle, K. J.; Korycansky, D. G.; Nixon, C. A.


    We have studied the thermal consequences of very big impacts on Titan [1]. Titan's thick atmosphere and volatile-rich surface cause it to respond to big impacts in a somewhat Earth-like manner. Here we construct a simple globally-averaged model that tracks the flow of energy through the environment in the weeks, years, and millenia after a big comet strikes Titan. The model Titan is endowed with 1.4 bars of N2 and 0.07 bars of CH4, methane lakes, a water ice crust, and enough methane underground to saturate the regolith to the surface. We assume that half of the impact energy is immediately available to the atmosphere and surface while the other half is buried at the site of the crater and is unavailable on time scales of interest. The atmosphere and surface are treated as isothermal. We make the simplifying assumptions that the crust is everywhere as methane saturated as it was at the Huygens landing site, that the concentration of methane in the regolith is the same as it is at the surface, and that the crust is made of water ice. Heat flow into and out of the crust is approximated by step-functions. If the impact is great enough, ice melts. The meltwater oceans cool to the atmosphere conductively through an ice lid while at the base melting their way into the interior, driven down in part through Rayleigh-Taylor instabilities between the dense water and the warm ice. Topography, CO2, and hydrocarbons other than methane are ignored. Methane and ethane clathrate hydrates are discussed quantitatively but not fully incorporated into the model.

  14. OESbathy version 1.0: a method for reconstructing ocean bathymetry with realistic continental shelf-slope-rise structures (United States)

    Goswami, A.; Olson, P. L.; Hinnov, L. A.; Gnanadesikan, A.


    We present a method for reconstructing global ocean bathymetry that uses a plate cooling model for the oceanic lithosphere, the age distribution of the oceanic crust, global oceanic sediment thicknesses, plus shelf-slope-rise structures calibrated at modern active and passive continental margins. Our motivation is to reconstruct realistic ocean bathymetry based on parameterized relationships of present-day variables that can be applied to global oceans in the geologic past, and to isolate locations where anomalous processes such as mantle convection may affect bathymetry. Parameters of the plate cooling model are combined with ocean crustal age to calculate depth-to-basement. To the depth-to-basement we add an isostatically adjusted, multicomponent sediment layer, constrained by sediment thickness in the modern oceans and marginal seas. A continental shelf-slope-rise structure completes the bathymetry reconstruction, extending from the ocean crust to the coastlines. Shelf-slope-rise structures at active and passive margins are parameterized using modern ocean bathymetry at locations where a complete history of seafloor spreading is preserved. This includes the coastal regions of the North, South, and Central Atlantic Ocean, the Southern Ocean between Australia and Antarctica, and the Pacific Ocean off the west coast of South America. The final products are global maps at 0.1° × 0.1° resolution of depth-to-basement, ocean bathymetry with an isostatically adjusted, multicomponent sediment layer, and ocean bathymetry with reconstructed continental shelf-slope-rise structures. Our reconstructed bathymetry agrees with the measured ETOPO1 bathymetry at most passive margins, including the east coast of North America, north coast of the Arabian Sea, and northeast and southeast coasts of South America. There is disagreement at margins with anomalous continental shelf-slope-rise structures, such as around the Arctic Ocean, the Falkland Islands, and Indonesia.

  15. El Margen Atlántico Ibérico al W de Galicia. Evolución en régimen extensional y sedimentación. (Resultados preliminares del Leg. 103, Ocean Drilling Program.

    Directory of Open Access Journals (Sweden)

    Boillot, G.


    Full Text Available Leg 101 of the Ocean Drilling Program (ODP was devoted discovering the tectonic and sedimentary evolution of the Atlantic Margin of the lberian Peninsula. A transect of five sites, with a total of 14 drill-holes was undertaken to the South of the Galicia Bank on the seaward edge of the margin. The data obtained revealed a complex history of subsidence and rifting preceding the initiation of sea floor spreading between Newfoundland and Iberia. The main findings include: 1 The Upper Jurassic-Lowermost Cretaceous shallow-water carbonate platform are the first Messozoic deposits at the margin. The «basement seismic reflector» is made-up of these carbonates. 2 The platform drowning, tilting of fault blocks and rapid subsidence preceded the spreading by as much as 25 million years. 3 A ridge of serpentiniced peridotites is located near the boundary between the oceanic and continental crusts. 4 The seismic reflector «S» does not, as widely believed represent a ductile-brinle boundary within the continental crust but is instead a reflector at the base of the synrift sediments.

    La campaña oceanográfica 103 del Ocean Drilling Program (ODP ha estado dedicada a dilucidar la evolución tectónica y sedimentación del Margen Atlántico-Ibérico. Se realizaron un total de 14 sondeos., en cinco puntos de posicionamiento, sobre el extremo más profundo del margen; al S del Banco de Galicia. Los resultados obtenidos revelan que previamente al inicio de la expansión oceánica entre Terranova e Iberia ocurrió una historia compleja de distensión cortical, fracturación y subsidencia asociadas. Los resultados fundamentales son los siguientes: 1 Carbonatos de plataforma marina somera, de edad Jurásico superior-Cretáceo basal constituyen los primeros depósitos mesozóicos en ese ámbito del margen y dan lugar a un reflector sísmico considerado interiormente como basamento, 2 El hundimiento de la plataforma, fallamiento y basculamiento de los bloques ocurre desde 25 m.a. antes de iniciarse la acreción oceánica. 3 En el límite entre corteza oceánica-corteza continental se ubica una cresta constituida por peridotitas serpetinizadas. 4 El reflector sísmico «S», generalmente considerado como el límite dúctil-frágil en la corteza continental, corresponde realmente a la base de los depósitos sinrift.

  16. Crustal-Scale Images of the Continent-Ocean Transition Across the Eastern Canadian Margins (United States)

    Louden, K.; Gerlings, J.


    The acquisition and analysis of ~10, 400-500-km-long, deep MCS reflection and wide-angle reflection/refraction (WAR/R) profiles across the eastern Canadian continental margins from Nova Scotia to Baffin Is. have been accomplished over the past 20 years during a number of joint Canadian and international programs. The combination of both reflectivity and velocity images from separate MCS and WAR/R profiles have detailed the large-scale patterns of crustal extension, mantle serpentinization and exhumation, and ocean crustal formation both within and between rifted segments from full thickness continental crust to oceanic crust produced by sea-floor spreading. A number of striking features are documented by these crustal-scale sections. In particular, a wide transition region with very thin seismic crust is delineated by a well-defined upper mantle zone with reduced velocities interpreted as partially serpentinized peridotite. The geological nature of the transitional crust is quite complex and may consist of various regions dominated by highly stretched continental crust, highly serpentinized continental mantle or thin ultra-slow spread ocean crust. It is difficult to define the nature of this region from its velocity structure alone, however, since it is only poorly resolved by standard travel-time methods. One robust characteristic that is generally observed is an abrupt change to typical ocean crust at the seaward edge of the transition zone. This boundary shows characteristic and coincident variations in both velocity structure and basement morphology. New results from the eastern margin of Flemish Cap demonstrate such a pattern particularly well. This observation suggests that once melt begins to form it causes an abrupt shift from a diffuse pattern of lithospheric extension to a focused zone of melt formation. Based on our profiles, we suggest that such transitions have occurred at a number of discrete pulses, which progress in age from south to north and may be linked to previously documented pulses of continental extension in adjacent regions. Future improvements in characterizing the nature of the transition zone and its boundaries will require new detailed seismic profiles in which reflection and refraction data are analyzed simultaneously, including use of full waveform techniques. This will necessitate use of longer towed MCS arrays (10-15 km) and a greater number of ocean bottom receivers (50) spaced at smaller intervals (2-5 km). These requirements are well within our present capacity, but will require new international initiatives with significant new funding. Since the nature of the transitional crust has important implications for both resources and political boundaries, it is hoped that such potential can be realized within the next 5-10 years.

  17. Oxygen Distribution and Potential Ammonia Oxidation in Floating, Liquid Manure Crusts

    DEFF Research Database (Denmark)

    Nielsen, Daniel Aagren; Nielsen, Lars Peter


    Floating, organic crusts on liquid manure, stored as a result of animal production, reduce emission of ammonia (NH3) and other volatile compounds during storage. The occurrence of NO2- and NO3- in the crusts indicate the presence of actively metabolizing NH3 oxidizing bacteria (AOB) which may be partly responsible for this mitigation effect. Six manure tanks with organic covers (straw and natural) were surveyed to investigate the prevalence and potential activity of AOB and its dependence on the O2 availability in the crust matrix as studied by electrochemical profiling. Oxygen penetration varied from <1 mm in young, poorly developed natural crusts and old straw crusts, to several centimeters in the old, natural crusts. AOB were ubiquitously present in all crusts investigated but nitrifying activity could only be detected in old natural crusts and young straw crust with high O2 availability. In old natural crusts total potential NH3 oxidation rates were similar to reported fluxes of NH3 from slurry without surface crust. These results indicate that old, natural surface crusts may develop into a porous matrix with high O2 availability that harbors an active population of aerobic microorganisms, including AOB. The microbial activity may thus contribute to a considerable reduction of ammonia emissions from slurry tanks with well-developed crusts.

  18. Dynamic Topography in the Oceanic Realm of West Africa, India, and the Gulf of Mexico (United States)

    Hoggard, M. J.; Roberts, G.; White, N. J.


    It is generally agreed that convection in the mantle can generate vertical motions at the Earth's surface. Consequently, the recorded history of subsidence and uplift holds important clues about mantle convection. We use the well-established relationship between seafloor subsidence and age to map present-day residual depth anomalies in the oceanic realm. This map yields estimates of the spatial variation of dynamic topography, providing care is taken to rule out other potential causes of subsidence or uplift such as flexure adjacent to seamounts and subduction zones. Global analysis indicates that anomalies typically vary between ±1 km, over wavelengths of ~1000 km. This analysis of residual topography is concentrated on the oldest oceanic crust that abuts continental margins in order to provide a leg up onto the continents, where measuring absolute values of dynamic topography is considerably more complicated. Here we begin by looking at three areas in more detail - the west coast of Africa, India and the Gulf of Mexico. Residual depths along the west coast of Africa capture two full wavelengths of dynamic topography, which correlate well with the long-wavelength free air gravity anomaly. To constrain the temporal evolution of dynamic topography, we focus on regions such as the Gulf of Mexico which is currently drawn-down by ~2 km at its centre. Backstripping stratigraphic data from wells implies the majority of the anomalous subsidence has occurred in the last ~15 Ma. The east coast of India shows a drawdown of 2 km beneath the Bengal fan and the development of this anomaly is clearly recorded in the transition from progradational to aggradational behaviour within the margin's clinoform architecture. Analysis of adjacent river profiles indicates recent onshore uplift has provided high quantities of clastic detritus that have been deposited on the margin. Oceanic residual depths along the west African margin overlain by the long wavelength filtered free air gravity anomaly in contours of 10 mGal. The filigree corresponds to estimates calculated from published global grid datasets of water and sediment thickness. The points are based on primary data such as reflection seismic profiles. Circles are our most accurate residual depth measurements where crustal thickness is known. Downwards pointing triangles are maximum estimates and upwards pointing triangles are minimum.

  19. Partially melted, mica-bearing crust in Central Tibet (United States)

    Hacker, B. R.; Ritzwoller, M. H.; Xie, J.


    Surface wave tomography shows that the central Tibetan Plateau (the Qiangtang block) is characterized by S wave speeds as slow as 3.3 km/s at depths from 20-25 km to 45-50 km and S wave radial anisotropy of at least 4% (VSH > VSV) that is stronger in the west than the east. The depth of the Curie temperature for magnetite inferred from satellite magnetic measurements, the depth of the ?-? quartz transition inferred from VP/VS ratios, and the equilibration pressures and temperatures of xenoliths erupted from the middle to deep crust indicate that the Qiangtang crust is hot, reaching 1000°C at the Moho. This inferred thermal gradient crosses the dehydration melting solidi for crustal rocks at 20-30 km depth, implying the presence or former presence of melt in the Tibetan middle to deep crust. These temperatures do not require the wholesale breakdown of mica at these depths, because F and Ti can stabilize mica to at least 1300°C. Petrology suggests, then, that the Qiangtang middle to deep crust consists of a mica-bearing residue from which melt has been extracted or is being extracted. Wave speeds calculated for mica-bearing rocks with a subhorizontal to gently dipping foliation and 2% silicate melt are a good match to the wave speeds and anisotropy observed by seismology.

  20. Structure of the crust and the lithospheric mantle in Siberia

    DEFF Research Database (Denmark)

    Cherepanova, Yulia


    The study addresses the structure of the lithosphere of Siberia, including the Siberian Craton and the West Siberian Basin, with a particular focus on the crustal structure of the entire region and the density structure of its lithospheric mantle. The first part of the study included the development of a new regional crustal model, SibCrust, that is a digital crustal model for both the Siberian Craton and the West Siberian Basin. The SibCrust model, constrained by digitizing of all available seismic profiles and crustal velocity models across the Siberia, also includes a critical quality assessment of regional seismic data and crustal regionalization based on seismic structure of the crust. The second part of the study included the development of the SibDensity model that is the density model of the lithospheric mantle calculated by the mass balance method. Mantle density modeling involved calculation of the crustal correction based on the SibCrust model. The SibDensity model is made of two independent parts calculated for two major tectonic provinces with contrasting geodynamic evolution: the West Siberian Basin and the Siberian Craton. The modeling results, that are lateral variations in the lithospheric mantle density, are interpreted in terms of regional tectonic evolution, namely the mechanism by which the Paleozoic intracontinental basin has been formed and the tectono-magmatic processes by which the Archean-Proterozoic craton has been modified as reflected in the composition of its mantle.

  1. Structure of the Crust and the Lithosperic Mantle in Siberia

    DEFF Research Database (Denmark)

    Cherepanova, Yulia


    The study addresses the structure of the lithosphere of Siberia, including the Siberian Craton and the West Siberian Basin, with a particular focus on the crustal structure of the entire region and the density structure of its lithospheric mantle. The first part of the study included the development of a new regional crustal model, SibCrust, that is a digital crustal model for both the Siberian Craton and the West Siberian Basin. The SibCrust model, constrained by digitizing of all available seismic profiles and crustal velocity models across the Siberia, also includes a critical quality assessment of regional seismic data and crustal regionalization based on seismic structure of the crust. The second part of the study included the development of the SibDensity model that is the density model of the lithospheric mantle calculated by the mass balance method. Mantle density modeling involved calculation of the crustal correction based on the SibCrust model. The SibDensity model is made of two independent parts calculated for two major tectonic provinces with contrasting geodynamic evolution: the West Siberian Basin and the Siberian Craton. The modeling results, that are lateral variations in the lithospheric mantle density, are interpreted in terms of regional tectonic evolution, namely the mechanism by which the Paleozoic intracontinental basin has been formed and the tectono-magmatic processes by which the Archean-Proterozoic craton has been modified as reflected in the composition of its mantle.

  2. Acoustic techniques for studying soil-surface seals and crusts (United States)

    The impact of raindrops on a soil surface during a rainstorm may cause soil-surface sealing and upon drying, soil crusting. Soil-surface sealing is a result of the clogging of interaggregate pores by smaller suspended particles in the water and by structural deformation of the soil fabric, which red...

  3. Geoelectrical and geological structure of the crust in Western Slovakia.

    Czech Academy of Sciences Publication Activity Database

    Bezák, V.; Pek, Josef; Vozár, J.; Bielik, M.; Vozár, J.


    Ro?. 58, ?. 3 (2014), s. 473-488. ISSN 0039-3169 Institutional support: RVO:67985530 Keywords : magnetotellurics * MT15 profile * Western Carpathians * applied geophysics * Earth’s crust Subject RIV: DE - Earth Magnetism, Geodesy, Geography Impact factor: 0.752, year: 2013

  4. A Comparison of Microbial Communities from Deep Igneous Crust (United States)

    Smith, A. R.; Flores, G. E.; Fisk, M. R.; Colwell, F. S.; Thurber, A. R.; Mason, O. U.; Popa, R.


    Recent investigations of life in Earth's crust have revealed common themes in organism function, taxonomy, and diversity. Capacities for hydrogen oxidation, carbon fixation, methanogenesis and methanotrophy, iron and sulfur metabolisms, and hydrocarbon degradation often predominate in deep life communities, and crustal mineralogy has been hypothesized as a driving force for determining deep life community assemblages. Recently, we found that minerals characteristic of the igneous crust harbored unique communities when incubated in the Juan de Fuca Ridge flank borehole IODP 1301A. Here we present attached mineral biofilm morphologies and a comparison of our mineral communities to those from a variety of locations, contamination states, and igneous crustal or mineralogical types. We found that differences in borehole mineral communities were reflected in biofilm morphologies. Olivine biofilms were thick, carbon-rich films with embedded cells of uniform size and shape and often contained secondary minerals. Encrusted cells, spherical and rod-shaped cells, and tubes were indicative of glass surfaces. We also found that the attached communities from incubated borehole minerals were taxonomically more similar to native, attached communities from marine and continental crust than to communities from the aquifer water that seeded it. Our findings further support the hypothesis that mineralogy selects for microbial communities that have distinct phylogenetic, morphological, and potentially functional, signatures. This has important implications for resolving ecosystem function and microbial distributions in igneous crust, the largest deep habitat on Earth.

  5. Growth response of a deep-water ferromanganese crust to evolution of the Neogene Indian Ocean

    Digital Repository Service at National Institute of Oceanography (India)

    Banakar, V.K.; Hein, J.R.


    began (older zone) with the formation of isolated cusps of Fe-Mn oxide during a time of high detritus influx, probably due to the early-Miocene intense erosion associated with maximum exhumation of the Himalayas (op. cit.)...

  6. Mission Moho Workshop: Drilling Through the Oceanic Crust to the Mantle

    Directory of Open Access Journals (Sweden)

    David M. Christie


    Full Text Available The Mission Moho workshop was held in Portland Oregon on 7–9 September 2006. It was funded by the IODP, the Joint Oceanographic Institutions (JOI, the Ridge 2000 program, and the InterRidge initiative. This report builds on many fruitful and passionate discussions during the workshop, and we express our deepest thanks to all workshop participants. Several of them contributed to the writing of the full workshop report.

  7. Variability in microbial communities attached to minerals and glasses incubated in young ocean crust (United States)

    Smith, A.; Colwell, F. S.; Popa, R.; Fisk, M. R.


    To investigate whether mineral chemistry influences the distribution and composition of attached subseafloor microbial communities, we initiated a four-year incubation of twelve igneous minerals and glasses in IODP Hole 1301A on the eastern flank of the Juan de Fuca Ridge. Minerals were suspended at 278 meters below seafloor, within the warm basalt basement, with an osmotic pump to ensure fluid flow. Post-recovery, we performed high-throughput sequencing of the V4-V6 region of the 16S rRNA gene from genomic DNA extracted from all minerals and glasses using bacterial and archaeal primers. Archaeoglobaceae was the dominant family on all samples; however almost no archaeal sequences were recovered from the high-Fe mineral fayalite. Bacteria were diverse, and included Clostridia, Ralstonia, Thermosipho, and representatives of the deep-branching candidate division OP1. Low-Fe olivine had the most diverse community, and the least diverse community was found on diopside. Several mineral communities were similar to each other, yet many were dissimilar to all other communities. Our results suggest that the composition of microbial communities colonizing subseafloor basalts is influenced by the chemistry of the minerals present, and that communities are heterogeneously distributed in basalts. These findings have important implications for understanding the role of attached subseafloor communities in geochemical cycling of elements and the weathering of basalts.

  8. Mossbauer studies and oxidised manganese ratio in ferromanganese nodules and crusts from the Central Indian Ocean

    Digital Repository Service at National Institute of Oceanography (India)

    Pattan, J.N.; Mudholkar, A.V.


    stream_size 5 stream_content_type text/plain stream_name Geo-Mar_Lett_11_51.pdf.txt stream_source_info Geo-Mar_Lett_11_51.pdf.txt Content-Encoding ISO-8859-1 Content-Type text/plain; charset=ISO-8859-1 ...

  9. Global distribution of azimuthal anisotropy within the upper mantle and the crust (United States)

    Schaeffer, Andrew; Lebedev, Sergei


    We present our new global, azimuthally anisotropic model of the upper mantle and the crust. We compare two versions of this new model, the rough SL2013svAr and smooth SL2013svA, which are constrained by a larger, updated waveform fit dataset (>900, 000 vertical component seismogram fits) than that used in the construction of the isotropic model SL2013sv (Schaeffer and Lebedev, 2013). These two anisotropy models are computed using a more precise regularization of anisotropy, which is tuned to honour the both the amplitude and orientation of the anisotropic terms uniformly, including near the poles. Automated, multimode waveform inversion was used to extract structural information from surface and S wave forms, yielding resolving power from the crust down to the transition zone. Our unprecedentedly large waveform dataset, with complementary high-resolution regional arrays in additional to global networks, produces improved resolution of global azimuthal anisotropy patterns. The model also reveals smaller scale patterns of 3D anisotropy variations related to regional lithospheric deformation and mantle flow, in particular in densely sampled regions. In oceanic regions, we examine the strength of azimuthal anisotropy, as a function of depth, spatial position with respect to the spreading ridge, and deviation in fast axis orientation from the current and fossil spreading directions. In continental regions, azimuthal anisotropy is more complex. Reconciling complementary observations given by shear wave splitting, surface-wave array analysis, and large-scale, global 3D models offers new insights into the mechanisms of continental deformation and the architecture and evolution of the lithosphere. Finally, quantitative comparisons with other recently published models demonstrate which features are consistently resolved across the different models, and therefore provide a means to estimate the robustness of anisotropic patterns and amplitudes. Reference: Schaeffer, A. J., and S. Lebedev. Global shear-speed structure of the upper mantle and transition zone. Geophys. J. Int., 194, 417-449, 2013.

  10. Hydrocode simulation of Ganymede and Europa cratering trends - How thick is Europa’s crust? (United States)

    Bray, Veronica J.; Collins, Gareth S.; Morgan, Joanna V.; Melosh, H. Jay; Schenk, Paul M.


    One of the continuing debates of outer Solar System research centers on the thickness of Europa’s ice crust, as it affects both the habitability and accessibility of its sub-surface ocean. Here we use hydrocode modeling of the impact process in layered ice and water targets and comparison to Europan cratering trends and Galileo-derived topographic profiles to investigate the crustal thickness. Full or partial penetration of the ice crust by an impactor occurred in simulations in which the ice thickness was less than 14 times the projectile radius. Craters produced in these thin-shell simulations were consistently smaller than for larger ice thicknesses, which will complicate inference of large impactor population sizes. Simulations in which the resultant crater was 3 times the ice layer thickness resulted in summit-pit morphology. This work supports that summit pit craters noted on both rocky and icy bodies, can be created by the presence of a weaker layer at depth. We suggest that floor pits, seen only on ice-rich bodies, require a different formation mechanism to summit pits. Pristine craters formed in a target with high heat flow were shallower than for the same impact into a target of lesser heat flow, suggesting that the ‘starting’ crater morphology for viscous relaxation, isostatic readjustments and erosion rate studies is different for craters formed in times of different heat flow. We find that the crater depth-diameter trend of Europa can only be recreated when simulating impact into an upper brittle ice layer of 7 km depth, with a corresponding geothermal gradient of 0.025 K/m. As this ice thickness estimate is below ?10 km, results from this work suggest that convective overturn of the surface ice may occur, or have occurred, on Europa making the development of indigenous life a possibility.

  11. Enhanced anatexis as a consequence of mantle-derived magma intrusion in the middle crust: a case study from the Eastern French Massif Central (United States)

    Couzinié, Simon; Moyen, Jean-François; Villaros, Arnaud; Paquette, Jean-Louis; Scarrow, Jane H.; Marignac, Christian


    The post-collisional stage of orogens corresponds to a dramatic change in mountain belts dynamics. During this period, large volumes of granitic melts are generated in the crust thus impacting its rheology and overall behavior. Evolving from compression/transpression to extension/transtension enhances exhumation of high-grade metamorphic rocks and subsequent decompression crustal melting. However, other processes can trigger anatexis such as heat or fluid fluxes from the mantle and the crust. The Early Carboniferous nappe stack of the Eastern French Massif Central (EFMC) underwent two successive low-pressure melting events at the end of its evolution, during the Late Carboniferous. They are particularily evident in the southern edge of the Velay Complex, a 100 km-diameter migmatite-granite dome. The M3 'pre-Velay' event corresponds to water-saturated melting in the amphibolite facies at T cordierite-free, stromatic migmatites and was quite long-lasting since available U-Th-Pb monazite ages span from 335 to 310 Ma. At that time, crustal melts mainly remained trapped in the source and few granite plutons were associated with this event. Contrarily, the M4 'Velay' anatexis occurred under granulite-facies conditions at 760 cordierite-bearing migmatites are nebulitic to diatexitic as a consequence of biotite breakdown which led to disruption of the solid framework of melanosomes and enhanced melt extraction. This widespread melting event is synchronous with emplacement of the cordierite-bearing restite-rich S-type Velay granite at ca. 305 Ma. Then, the EFMC records an evolution in melting conditions with a clear heat input at the M3-M4 transition. The EFMC anatectic crust is intruded by widespread, Mg-K-rich biotite-rich diorites locally called 'vaugnerites'. These mantle-derived melts emplaced in a partially molten setting, as evidenced by mingling features between vaugnerites and anatectic melts, as well as the presence of hybrid granitoids including a 'vaugnerite' component. In situ (LA-ICP-MS) U-Pb zircon and monazite dating of vaugnerites or coeval granites in the Southern Velay area yielded ages mostly indistinguishable within analytical uncertainties, spanning from 307.4 ± 1.8 to 303.7 ± 3.1 Ma. Thus, mantle-derived magmas emplaced at ca. 305 Ma which is the very transition from M3 to M4. This striking synchronism between enhanced crustal melting and mantle-derived magmatism suggests that vaugnerites could be the cause of the M3-M4 transition. Depending on the volume involved, the emplacement of hot (ca. 1000 ° C) melts in mid crustal levels would have supplied significant amounts of heat. Vaugnerites could also be the manifestation of a (yet unconstrained) process enhancing the conductive mantle heat flux to the crust. For instance, delamination of a lithospheric mantle root or slab break-off would result in generation of mantle-derived melts as well as increase the heat conduction into the crust. Therefore, the relevant system that must be considered to study late-orogenic periods is not only the crust but the whole lithosphere, taking into account mass/heat transfer from the mantle to the overlying crust.


    Rainfall in deserts occurs in discrete events or pulses. The frequency and magnitude of precipitation pulses may differentially drive ecosystem components, such as soil microbes, plants, and microbial crusts. As significant components of desert ecosystems, microbial crusts are photosynthetic commu...

  13. Ocean Surface Currents Glossary (United States)

    This glossary provides short definitions of the oceanographic jargon used to describe ocean surface currents. It is designed to accompany the website "Ocean Surface Currents", a reference that provides information on surface currents in the world's oceans.

  14. Ocean Maps Coordinate Planes

    CERN Document Server

    Wall, Julia


    Learn about ocean maps and the concepts surrounding coordinate planes as Ocean Maps explores geography under the sea, early and current navigation practices, and the variety of ways the ocean can be mapped such as sonar, submersibles, and satellites.

  15. Low velocity and low electricalresistivity layers in the middle crust

    Directory of Open Access Journals (Sweden)

    N. I. Pavlenkova


    Full Text Available Some Deep Seismic Sounding (DSS revealed low velocity layers in the upper and middle crust of old platforms. The layers are often characterised by a lower electrical resistivity. It is not clear, however, how reliable the layers recognized from DSS data are, if they are regular or occasional events and how they correlate with other geophysical parameters. To answer these questions the experimental DSS data obtained in the Baltic and Ukranian shields by different institutions were reinterpreted by the author with unified methods. The shield areas are well studied using both the DSS and high-frequency magnetotelluric sounding (MTS methods. As a result a marked velocity inversion (waveguide was observed in a 10 to 20 km depth range in the majority of the DSS profiles. An increase in the electrical conductivity is typical for the waveguide. A comparison of the results with the data from other platform regions allow the conclusion that this low velocity and high electrical conductivity layer has a global significance. In the continental crust, the layer is characterised by changes in the reflectivity pattern, earthquakes number and changes in velocity pattern where the block structure is transformed into a subhorizontal layering. These structural features suggest that the layers separate brittle and weak parts of the crust. Usually they play the role of detachment zones at crustal block moving. A possible factor responsible for this phenomenon is an increase in porosity and in the salinity of the waveguide pore water compared with the upper crust. This suggestion is confirmed by the Kola superdeep borehole data. Porosity increasing in the middle crust is explained by the change in rock mechanical properties with depth, by fracturing porosity and by dilatancy effect, at a depth of 10-20 km.

  16. Geochemistry and Geochronology of the Lower Crust Beneath Central Mongolia (United States)

    Ancuta, L. D.; Carlson, R. W.; Ionov, D. A.; Zeitler, P. K.


    Two-pyroxene granulite xenoliths recovered from the Shavaryn-Tsaram Quaternary basaltic breccia pipe near Tariat Mongolia provide a snapshot of the modern lower crust beneath the elevated Hangay Dome in central Mongolia. Two-pyroxene thermometry indicates the xenoliths equilibrated at temperatures of 840 ± 30ºC. Previous studies using the Grt-Opx-Pl barometer indicated an upper limit on pressures between 12.5 and 15.5 kbar for samples collected from the same locality (Stosch et al., 1995). Whole-rock trace-element data for the xenoliths show similar trends to arc-derived magmatic rocks, with enrichments in the large-ion lithophile elements and rare earth elements compared to depletion in the high field-strength elements. The arc-like geochemical signature of the lower crust suggests it did not form through underplating during the most recent phase of Cenozoic volcanism, which has geochemical characteristics typical of intraplate volcanism. Instead, the lower crust beneath the Hangay dome may have formed during the accretion of the Central Asian Orogenic Belt (CAOB) in the Late Paleozoic to Early Mesozoic. Alternatively, it could include remnants of older Precambrian basement accreted during the formation of the CAOB. Abundant zircon observed in the samples will be dated by U-Pb laser ablation ICP-MS and ID-TIMS to better determine the timing of lower-crust formation and metamorphism. To the extent that a crustal root supports the high elevation of Hangay region, constraining the age of the root will help place constraints on the timing of crustal thickening and the attainment of high elevation. The post-orogenic, long-term thermal evolution of the lower crust can be assessed using U-Pb analyses of a number of trace phases, including monazite and apatite. Results from these measurements will address the timing and evolution of the high topography in central Mongolia.

  17. "Ma veel ei tea, kes olen ma..." : [luuletused] / Mihhail Fedotov ; tlk. Arvo Valton

    Index Scriptorium Estoniae

    Fedotov, Mihhail


    Sisu: "Ma veel ei tea, kes olen ma..." ; Multani lokulaud ; "Need külateed siit linna lähevad..." ; "Linna kohal laiub mürgipilv..." ; Lahkumine ; "Te sõbrad, ärge langetage pead..." ; Jamps ; "Tuul öine ulus kurja hundina...". Luuletused paralleelselt udmurdi ja eesti keeles

  18. Microprobe monazite constraints for and early (ca. 790 Ma) Braziliano orogeny: The Embu Terrane, southeastern Brazil

    International Nuclear Information System (INIS)

    The evolution of the Mantiqueira Orogenetic System, Southeastern Brazil, comprises discrete episodes of tectonic collage and docking of remnants of Rodinia break-up in the borders of the Sao Francisco Craton. This system is related to the closure of the Adamastor ocean and assemblage of the western Gondwana super-continent during Neoproterozoic times (ca. 610-530 Ma, Brito Neves et al., 1999; Campos Neto, 2000). This report presents monazite microprobe dating results for metassediments from the Embu Complex, an important lithological unit from the Ribeira Belt, currently included in the Juiz de Fora terrane, a unit added to the Sao Francisco Craton at ca. 600-580 Ma. (Campos Neto, 2000). The age results unravel a main metamorphic episode and related orogeny at ca. 790 Ma and bring new insights concerning the agglutination of Gondwana in this region during the Neoproterozoic (au)

  19. IODP Expeditions 309 and 312 Drill an Intact Section of Upper Oceanic Basement into Gabbros

    Directory of Open Access Journals (Sweden)

    Douglas S. Wilson


    Full Text Available The Integrated Ocean Drilling Program’s (IODP Expeditions 309 and 312 successfully completed the first sampling of an intact section of upper oceanic crust, through lavas and the sheeted dikes into the uppermost gabbros. Hole 1256D, which was initiated on the Ocean Drilling Program’s (ODP Leg 206, now penetrates to >1500 mbsf and >1250 m sub-basement. The first gabbroic rocks were encountered at 1407 mbsf. Below this, the hole penetrates ~100 m into a complex zone of fractionated gabbros intruded into contact metamorphosed dikes.

  20. Fuzzy comprehensive evaluation of earth crust stability of Beishan and nerghbouring areas, Gansu province

    International Nuclear Information System (INIS)

    The evaluation of earth crust stability is a research project priot to siting of the repository of high-radioactive waste disposal. Based on fuzzy mathematics authors have developed a fuzzy comprehensive evaluation model of earth crust stability, then divided the studied region into eight a reas with different stability of earth crust on the basis of analysing the structure of earth crust, seismic characteristics, active faults and recent tectonic stress field. (author)

  1. The Alpha-Mendeleev Magmatic Province, Arctic Ocean: A New Synthesis (United States)

    Vogt, P. R.; Jung, W.; Jakobsson, M.; Mayer, L.; Williamson, M.


    Since the 1970s, the Alpha-Mendeleev Ridge (AMR) has generally been considered oceanic, with a thickened (ca. 35km) crustal or/and abnormally low-density mantle root, a "hotspot"-type, Cretaceous-aged aseismic ridge perhaps generated by the controversial Iceland hotspot. The high-amplitude AMR aeromagnetic anomalies (locally > 1000nT) are sublinear, largely correlated with ±20mgal free-air gravity anomalies and bathymetric/basement topography. Such correlation is consistent with most magma emplaced during the long Cretaceous normal polarity interval (120-83Ma). (Present basement topography may post-date the magmatism). Based on multibeam bathymetry from the 2003 Healy expedition and published sources, we synthesize the mid-Cretaceous-age Alpha-Mendeleev Ridge (AMR) complex, a ca.300-700km X 1500km Arctic Basin rise (>700,000 km sq; mininum basement depths 10 million km3 excess mafic materials under the AMR, a volume exceeded only by the Ontong Java Plateau. Multibeam bathymetry collected on USCGC Healy in 2003 discovered probable volcanic seamounts in the Northwind Basin and mapped a seamount off the tip of the Chukchi Rise. We suggest these edifices, and linear aeromagnetic anomalies - suggesting major dikes crossing the continental Chukchi Rise - are part of the same extensive igneous episode that created the AMR. Other evidence for a more extensive "AMR Magmatic Province" (AMRMP) includes: 1) Basement peaks/seamounts in the Sever and Peary spurs and Nautilus and Stefansson basins, which together would bring the total AMRMP area to ca. 10 million km2; 2) Aeromagnetics, showing AMR-type magnetic and gravity anomaly patterns well beyond the AMR; and 3) Mafic rocks (125-89Ma) of the Sverdrup Basin Magmatic Province (SBMP), as shown by M-C. Williamson and her colleagues. The northeastern SBMP adjoins the polar margin, not far from the poorly dated AMR. The two final SBMP igneous episodes are coeval with AMR magmatism, and comprise ferrogabbroic sills and thin successions of ferrobasaltic lavas, a clue that high AMR magnetic anomalies may have a similar origin, as first suggested by Williamson and Van Wagoner in 1985. However, simple amplitude comparison would be incorrect - the higher geomagnetic field intensity near the poles, shallow AMR basement, high AMR basement relief, and possibly stronger middle Cretaceous dipole would all contribute to increasing anomaly amplitudes relative to typical Cenozoic oceanic crust created at lower latitudes. We hypothesize that the AMRMP constitutes a vast mass of anomalously fractionated, highly magnetized FeTi basalts and ferrograbbros. However,we cannot exclude the possibility of admixed continental crust, especially at the Siberian end of the AMR, and only deep drilling into AMR basement will provide firm answers.

  2. Eclogites with oceanic crustal and mantle signatures from the Bellsbank kimberlite, South Africa. Pt. 2

    International Nuclear Information System (INIS)

    The Sr, Nd, and O isotopic compositions of garnet and clinopyroxene mineral separates from nine eclogite xenoliths from the Bellsbank kimberlite (erupted at 120 Ma) define three groups. Group A eclogites, considered to be mantle cumulates, are characterized by ?18O and 87Sr/86Sr values typical of mantle-derived materials (+5.1 to +5.6per mille and 0.7042-0.7046, respectively), and very low Sm/Nd ratios, (apparent) Rb/Sr ratios and ?Nd[120] values (0.057-0.078, 0.00005-0.00136 and -14 to -16 respectively). The REE and isotopic data for these eclogites can be modelled in terms of crystallization from a Group II kimberlite magma at ? 1-1.5 Ga. Group B and C eclogites, believed to be the metamorphosed products of ancient subducted oceanic crust, are characterized by low ?18O (+2.9 to +4.7), extremely high ?Nd[120] (?+40 to +219), and radiogenic 87Sr/86Sr ratios (0.708-0.710). The Sm/Nd ratios of the Group B eclogites are very high (up to 1.6). The data for Group B and C eclogites define a linear correlation on Sm/Nd and 1/Nd vs. ?Nd[120] diagrams. These relationships are consistent with mixing of the Bellsbank kimberlite (?Nd[120] = -10; Sm/Nd = 0.10) with a depleted eclogite end-member (?Nd[120] + 219; Sm/Nd = 1.6) during a cryptic metasomatic event. The Sr isotopic variations in Group B and C eclogites cannot be generated by simple two-compbe generated by simple two-component mixing. The Sr, Nd, and O isotope data for Group B and C eclogittes