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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)

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.

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

2006-01-01

2

Frozen magma lenses below the oceanic crust.  

Science.gov (United States)

The Earth's oceanic crust crystallizes from magmatic systems generated at mid-ocean ridges. Whereas a single magma body residing within the mid-crust is thought to be responsible for the generation of the upper oceanic crust, it remains unclear if the lower crust is formed from the same magma body, or if it mainly crystallizes from magma lenses located at the base of the crust. Thermal modelling, tomography, compliance and wide-angle seismic studies, supported by geological evidence, suggest the presence of gabbroic-melt accumulations within the Moho transition zone in the vicinity of fast- to intermediate-spreading centres. Until now, however, no reflection images have been obtained of such a structure within the Moho transition zone. Here we show images of groups of Moho transition zone reflection events that resulted from the analysis of approximately 1,500 km of multichannel seismic data collected across the intermediate-spreading-rate Juan de Fuca ridge. From our observations we suggest that gabbro lenses and melt accumulations embedded within dunite or residual mantle peridotite are the most probable cause for the observed reflectivity, thus providing support for the hypothesis that the crust is generated from multiple magma bodies. PMID:16121179

Nedimovi?, Mladen R; Carbotte, Suzanne M; Harding, Alistair J; Detrick, Robert S; Canales, J Pablo; Diebold, John B; Kent, Graham M; Tischer, Michael; Babcock, Jeffrey M

2005-08-25

3

Thin oceanic crust and flood basalts: India-Seychelles breakup  

Science.gov (United States)

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.

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

2011-05-01

4

Magnetization of the oceanic crust: TRM or CRM?  

Science.gov (United States)

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.

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

1987-01-01

5

Microbial Life of North Pacific Oceanic Crust  

Science.gov (United States)

Information on the microbiology of the deep subsurface is necessary in order to understand the factors controlling the rate and extent of the microbially catalyzed reactions that influence the geophysical properties of these environments. Drilling into 45-Ma oceanic basaltic crust in a deepwater environment during ODP Leg 200 provided a promising opportunity to explore the abundance, diversity and activity of micro-organisms. The combined use of culture-independent molecular phylogenetic analyses and enrichment culture techniques is an advantageous approach in investigating subsurface microbial ecosystems. Enrichment culture methods allow the evaluation of potential activities and functions. Microbiological investigations revealed few aerobic cultivable, in part hitherto unknown, micro-organisms in deep submarine sediments and basaltic lava flows. 16S rDNA sequencing of isolates from sediment revealed the next relatives to be members of the genera Halomonas, Pseudomonas, and Lactobacillus. Within the Pseudomonadaceae the closest relative is Acinetobacter sp., which was isolated from a deep subsurface environment. The next phylogenetical relatives within the Halomonadaceae are bacteria typically isolated from Soda lakes, which are considered as model of early life conditions. Interestingly, not only sediment bacteria could be obtained in pure culture. Aerobic strains could also be successfully isolated from the massive tholeiitic basalt layer at a depth of 76.16 mbsf (46 m below the sediment/basement contact). These particular isolates are gram-positive with low G+C content of DNA, phylogenetically affiliated to the phylum Firmicutes. The closest neighbors are e.g. a marine Bacillus isolated from the Gulf of Mexico and a low G+C gram-positive bacterium, which belongs to the microbial flora in the deepest sea mud of the Mariana Trench, isolated from a depth of 10,897 m. Based on the similarity values, the isolates represent hitherto undescribed species of the deep biosphere. Molecular microbial diversity is currently determined by cloning und comparative 16S rRNA gene analyses. The first results will also be presented. In summary, the low number of isolates, cultivated under aerobic conditions, is in good agreement with the common opinion that most of the bacteria within the deep biosphere are anaerobic. Thus, studies of microbial community structure in solid geological materials are feasible and constitute further evidence that continuing microbiological activity in the challenging exploration of the deep sub-seafloor biosphere environment is absolutely promising.

Schumann, G.; Koos, R.; Manz, W.; Reitner, J.

2003-12-01

6

The African Plate: A history of oceanic crust accretion and subduction since the Jurassic  

Science.gov (United States)

Initially part of Gondwana and Pangea, and now surrounded almost entirely by spreading centres, the African plate moved relatively slowly for the last 200 million years. Yet both Africa's cratons and passive margins were affected by tectonic stresses developed at distant plate boundaries. Moreover, the African plate was partly underlain by hot mantle (at least for the last 300 Ma) - either a series of hotspots or a superswell, or both - that contributed to episodic volcanism, basin-swell topography, and consequent sediment deposition, erosion, and structural deformation. A systematic study of the African plate boundaries since the opening of surrounding oceanic basins is presently lacking. This is mainly because geophysical data are sparse and there are still controversies regarding the ages of oceanic crust. The publication of individual geophysical datasets and more recently, global Digital Map of Magnetic Anomalies (WDMAM, EMAG2) prompted us to systematically reconstruct the ages and extent of oceanic crust around Africa for the last 200 Ma. Location of Continent Ocean Boundary/Continent Ocean Transition and older oceanic crust (Jurassic and Cretaceous) are updates in the light of gravity, magnetic and seismic data and models of passive margin formation. Reconstructed NeoTethys oceanic crust is based on a new model of microcontinent and intr-oceanic subduction zone evolution in this area.The new set of oceanic palaeo-age grid models constitutes the basis for estimating the dynamics of oceanic crust through time and will be used as input for quantifying the paleo-ridge push and slab pull that contributed to the African plate palaeo-stresses and had the potential to influence the formation of sedimentary basins.

Gaina, C.; Torsvik, T. H.; Labails, C.; van Hinsbergen, D.; Werner, S.; Medvedev, S.

2012-04-01

7

Age variations of oceanic crust Poisson's ratio: Inversion and a porosity evolution model  

Science.gov (United States)

Porosity in the oceanic crust is one of the most important factors influencing measured seismic velocities. Porosity is particularly important in the uppermost young crust, where rapid variations in velocities with depth and crustal age are observed. Knowledge of the concentration and aspect ratios of inferred crack populations can be improved considerably if estimates of Poisson's ratio are available from observations of compressional and shear velocities nu(sub p) and nu(sub s). In this paper I present a joint seismic waveform inversion for nu(sub p) and nu(sub s); velocities are found while maximizing or minimizing Poisson's ratio using a hypothesis-testing mechanism. I apply this method to ocean bottom hydrophone data in 140 Ma Atlantic crust; the resulting solution corridor agrees with laboratory measurements without the low Poisson's ratio anomalies at depths of 0.8-1.5 km found by Spudich and Orcutt (1980) and Au and Clowes (1984) on younger (less than 15 Ma) Pacific crust. Compiling other published nu(sub p) and nu(sub s) solutions, an age-dependent pattern emerges: none of the solutions for crust older than 60 Ma display the Poisson's ratio anomaly. I propose a simple crustal evolution model, using thin and thick cracks, to explain these observations: thin cracks preferentially close at shallow depths in the crust, producing the localized Poisson's ratio anomaly. Sealing of all cracks by hydrothermal deposits as the crust ages restores the seismic velocities to consistency with laboratory measurements. This model is consistent with similar models of crack populations and their evolution from shallow measurements.

Shaw, Peter R.

1994-02-01

8

Primitive layered gabbros from fast-spreading lower oceanic crust.  

Science.gov (United States)

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

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

2014-01-01

9

Changes in erosion and ocean circulation recorded in the Hf isotopic compositions of North Atlantic and Indian Ocean ferromanganese crusts  

Science.gov (United States)

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.

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

2000-01-01

10

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

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Isotopic variations within upper oceanic crust at IODP Site 1256: Implications for crustal recycling and the formation of ocean island basalts  

Science.gov (United States)

The origin of ocean island basalts (OIBs) is a fundamental question facing Earth scientists. It is commonly agreed that lithospheric material recycled in the mantle is involved in the magma source of OIBs. The relative importance of 1) subducted altered oceanic basaltic crust (AOC), 2) subducted marine sediments and/or 3) delaminated metasomatised subcontinental lithosphere and continental lower crust remains to be resolved. We examine the geochemical composition of a complete in situ section of oceanic crust drilled at Site 1256 during IODP Expeditions 309 and 312. It includes the extrusive layer, sheeted dikes and gabbros of ca. 15 Ma old oceanic crust of the Cocos Plate formed during a period of superfast spreading at the East Pacific Rise. Modeling in the Sr-Nd-Pb-isotope space and comparison with present day radiogenic isotope ratios of OIBs provides constraints on the significance of recycled oceanic crust in the OIB mantle source(s). Our study shows that the generation of sulphides during low- and high-temperature alteration of oceanic crust has a strong influence on U/Pb and Th/Pb ratios and whether an AOC domain evolves relatively low or high Pb-isotope ratios over geological timescales. The model suggests that AOC as the sole precursor material, modified during the subduction process, and after relatively low to moderate recycling ages of ca. 300-800 Ma, is sufficient to explain the Sr-Nd-Pb-isotopic composition of OIBs with Pb-isotopic compositions along or below the Northern Hemisphere Reference Line (NHRL) and relatively high Nd-isotope ratios (e.g. Canaries, Galapagos, Iceland, Madeira). This indicates that additional EM-components, potentially associated with recycled lithospheric material such as subducted sediments, lower continental crust or subcontinental lithosphere, are not required for an array of OIBs, but are only necessary to explain OIBs with Pb-isotope ratios above the NHRL and relatively low Nd- isotope ratios (e.g. Pitcairn, Tristan, Samoa).

Duggen, S.; Hoernle, K.; Geldmacher, J.; Hauff, F.

2007-12-01

12

Layered basic complex in oceanic crust, romanche fracture, equatorial atlantic ocean.  

Science.gov (United States)

A layered, basic igneous intrusion, analogous in mineralogy and texture to certain large, continental layered complexes, is exposed in the Romanche Fracture, equatorial Atlantic Ocean. Crustal intrusion of large masses of basic magmas with their subsequent gravity differentiation is probably one of a number of major processes involved in the formation of new oceanic crust during sea-floor spreading. PMID:17768910

Melson, W G; Thompson, G

1970-05-15

13

Primary carbonatite melt from deeply subducted oceanic crust.  

Science.gov (United States)

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

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

2008-07-31

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Noble gases in the Oceanic Crust: Preliminary results from ODP Hole 1256D  

Science.gov (United States)

Noble gas isotopes and abundance ratios have been extensively used as tracers of oceanic mantle sources and fluxes. Most of the existing data are from seafloor basalt glasses and hydrothermal fluids, and there are very few studies of noble gases in the oceanic crust, which is an important component in global subduction flux estimates. In an effort to determine the relative contributions of mantle, radiogenic, and atmospheric/hydrothermal noble gas components in the ocean crust, we have performed helium, neon and argon measurements on a suite of gabbros and granoblastic dikes collected during IODP Expeditions 312 and 335 to Hole 1256D, a deep crustal borehole drilled into 15 Ma ocean crust formed at the East Pacific Rise during an episode of superfast spreading (>200 mm/yr). All measurements were carried out by coupled vacuum crushing and melting of whole rock samples, in order to determine the distribution of noble gases within the ocean crust. Total helium abundances in the gabbros range from 0.46 to 1.22 micro cc STP/gram, which is 2 to 5 times higher than literature data, all of which are from the slow spreading Southwest Indian Ridge (Kumagai et al., 2003; Moreira et al., 2003). These strikingly higher helium concentrations place constraints on the thermal crustal history (due to rapid helium diffusivity) and are assumed to reflect fundamentally different emplacement/degassing processes within crust formed at a super fast spreading rate. Crushing releases 12-25 % of the total helium in the gabbros demonstrating that most of the helium resides in the solid mineral phases. Contact metamorphosed granoblastic dikes have total helium contents lower than the gabbros (typically ~ 0.15 micro cc STP/gram), but significantly higher than the assumed degassed basaltic protolith, thus suggesting that metamorphism actually adds helium to the crust, an important hypothesis that requires further testing. The helium isotopes obtained by crushing of both the gabbros and granoblastic dikes are dominated by mantle helium, with average 3He/4He = 6.5 ± .2 times atmosphere (Ra). This value is at the low end of the range for normal Pacific MORB helium data and is interpreted to represent the mantle source. 3He/4He values obtained by melting are slightly lower and are consistent with a small radiogenic component, and suggest that helium will be useful for geochronology of the ocean crust. In contrast with the helium isotopic data, neon and argon are dominated by atmospheric isotopic compositions, which is consistent with mineralogical and petrological evidence for extensive alteration of the crust. Crushing in vacuum releases a larger fraction of total neon and argon (28 to 64 %), suggesting that atmospheric/hydrothermal/alteration neon and argon are loosely bound, most likely in secondary alteration minerals. Small mantle argon isotopic components are only found in a few samples, and only during the heating experiments. These data suggest that the atmospheric noble gas components are most likely to be expelled during subduction of the ocean crust.

Kurz, M. D.; Curtice, J. M.; IODP Expedition 335 Science Party

2011-12-01

15

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.)

16

Contrasting subduction structures within the Philippine Sea plate: Hydrous oceanic crust and anhydrous volcanic arc crust  

Science.gov (United States)

show contrasting subduction structures within the Philippine Sea plate inferred from active-source wide-angle reflection data. Previous studies showed that large-amplitude reflections from the slab are observed in southwest Japan and indicated that a thin low-velocity layer with a high fluid content is formed along the top of the subducting oceanic crust. On the contrary, we found that the slab reflections have smaller amplitudes in the Izu collision zone, central Japan, where the Izu-Bonin volcanic arc has been colliding/subducting, suggesting that such a low-velocity layer does not exist beneath the collision zone. This structural difference is also supported by P-wave and S-wave velocity anomalies by passive-source tomography and electrical conductivity, and correlates with the regional distribution of deep tremors and intraslab earthquakes, both of which are induced by dehydration processes within the downgoing slab. Based on these comparisons, we suggest that the original structure of the incoming plate controls the contrasting subducting systems: typical oceanic plate absorbs water by hydrothermal circulation at spreading centers and/or seawater infiltration at outer rises, whereas volcanic arc crust consumes a large amount of hydrous minerals for melt production and metamorphoses to more stable, anhydrous forms before subduction.

Arai, Ryuta; Iwasaki, Takaya; Sato, Hiroshi; Abe, Susumu; Hirata, Naoshi

2014-05-01

17

Chemical complexity of hotspots caused by cycling oceanic crust through mantle reservoirs  

Science.gov (United States)

Lavas erupted at ocean island hotspots such as Hawaii have diverse geochemical signatures. These ocean island basalts are thought to be derived from many sources with different chemical compositions within Earth's mantle and contain components of more primitive, less degassed material, as well as several recycled oceanic crustal components. Furthermore, the recycled oceanic crustal components display vastly different ages. The various components may be derived from different mantle reservoirs that are entrained and carried to the surface by mantle plumes, but it is unclear how individual plumes could successively sample each of these reservoirs or why the recycled oceanic crust would have variable ages. Here we use high-resolution numerical simulations to investigate the interaction between mantle plumes, subducted oceanic crust and a more primitive lower mantle reservoir. In our simulations, some subducted oceanic crust is entrained directly into mantle plumes, but a significant fraction of the crust--up to 10%--enters the more primitive reservoirs. As a result, mantle plumes entrain a variable combination of relatively young oceanic crust directly from the subducting slab, older oceanic crust that has been stirred with ancient more primitive material and background, depleted mantle. Cycling of oceanic crust through mantle reservoirs can therefore reconcile observations of different recycled oceanic crustal ages and explain the chemical complexity of hotspot lavas.

Li, Mingming; McNamara, Allen K.; Garnero, Edward J.

2014-05-01

18

Recycling of geochemically heterogenous oceanic crust: Significance for the origin of ocean island basalts  

Science.gov (United States)

Explaining the isotopic signature and origin of ocean island basalts (OIBs) is a challenge in Earth sciences. There is general agreement that lithospheric material, recycled into the Earth's mantle, is involved in the mantle sources of OIBs. The relative roles, however, of 1) subducted marine sediments, 2) altered oceanic basaltic crust (AOC), 3) oceanic lithospheric mantle and/or 4) delaminated metasomatised subcontinental lithosphere and continental lower crust, however, are much debated. We present results from geochemical modeling in the Sr-Nd-Pb-isotope space following a new approach that takes into account the trace element and isotope heterogeneity of subducted oceanic crust (sediments + AOC). By means of backward and forward modeling, we examine how a geochemically heterogeneous package of oceanic crust may evolve in terms of Sr-Nd-Pb-isotopic composition through time and compare the results with present day radiogenic isotope ratios of OIBs. Our study suggests that recycling of AOC, modified during the subduction process, and stored in the Earth's mantle for several hundreds of millions of years can explain the Sr-Nd-Pb-isotopic composition of OIBs with relatively high Nd-isotope ratios that form elongated fields along or below the Northern Hemisphere Reference Line (NHRL) in the Pb-isotopic diagrams (e.g. Canaries, Galapagos, Iceland, Madeira). Explaining the origin of OIBs with relatively low Nd-isotope ratios and Pb-isotopic composition above the NHRL, and thus geochemical affinity to enriched mantle (EM) components (e.g. Pitcairn, Tristan, Samoa), however, seems to also require recycling of other lithospheric material such as subducted sediments, lower continental crust and/or subcontinental lithosphere.

Duggen, S.; Hoernle, K.; Hauff, F.; Park, S.-H.; Geldmacher, J.

2009-04-01

19

Early Carboniferous (˜357 Ma) crust beneath northern Arabia: Tales from Tell Thannoun (southern Syria)  

Science.gov (United States)

Continental crust beneath northern Arabia is deeply buried and poorly known. To advance our knowledge of this crust, we studied 8 xenoliths brought to the surface by Neogene eruptions of Tell Thannoun, S. Syria. The xenolith suite consists of two peridotites, one pyroxenite, four mafic granulites, and one charnockite. The four mafic granulites and charnockite are probably samples of the lower crust, and two mafic granulites gave 2-pyroxene equilibration temperatures of 780-800 °C, which we take to reflect temperatures at the time of formation. Peridotite and pyroxenite gave significantly higher temperatures of ?900 °C, consistent with derivation from the underlying lithospheric mantle. Fe-rich peridotite yielded T?800 °C, perhaps representing a cumulate layer in the crust. Three samples spanning the lithologic range of the suite (pyroxenite, mafic granulite, and charnockite) yielded indistinguishable concordant U-Pb zircon ages of ?357 Ma, interpreted to approximate when these magmas crystallized. These igneous rocks are mostly juvenile additions from the mantle, as indicated by low initial 87Sr/86Sr (0.70312 to 0.70510) and strongly positive initial ?Nd(357 Ma) (+4 to +9.5). Nd model ages range from 0.55 to 0.71 Ga. We were unable to unequivocally infer a tectonic setting where these melts formed: convergent margin, rift, or hotspot. These xenoliths differ from those of Jordan and Saudi Arabia to the south in four principal ways: 1) age, being least 200 Ma younger than the presumed Neoproterozoic (533-1000 Ma) crust beneath Jordan and Saudi Arabia; 2) the presence of charnockite; 3) abundance of Fe-rich mafic and ultramafic lithologies; and 4) the presence of sapphirine. Our studies indicate that northern Arabian plate lithosphere contains a significant proportion of juvenile Late Paleozoic crust, the extent of which remains to be elucidated. This discovery helps explain fission track resetting documented for rocks from Israel and provides insights into the nature of Late Paleozoic (Hercynian) deformation that affected Arabia near the Persian Gulf.

Stern, Robert J.; Ren, Minghua; Ali, Kamal; Förster, Hans-Jürgen; Al Safarjalani, Abdulrahman; Nasir, Sobhi; Whitehouse, Martin J.; Leybourne, Matthew I.; Romer, Rolf L.

2014-05-01

20

Partial separation of halogens during the subduction of oceanic crust  

Science.gov (United States)

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.

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

2014-05-01

21

The difficulty for subducted oceanic crust to accumulate at the Earth's core-mantle boundary  

Science.gov (United States)

tomography has revealed two large low shear velocity provinces (LLSVPs) in the lowermost mantle beneath the central Pacific and Africa. The LLSVPs are further shown to be compositionally different from their surroundings. Among several hypotheses put forth in recent years to explain the cause of the LLSVPs, one postulates that they are thermochemical piles caused by accumulation of subducted oceanic crust at the core-mantle boundary (CMB). Mineral physics experiments indicate that oceanic crust becomes denser than the surrounding mantle at lower mantle pressures. In addition, seismic observations provide evidence of subducted slabs arriving at the CMB. However, a major question pertains to whether subducted oceanic crust can survive viscous stirring associated with mantle plumes and accumulate into piles with the same spatial scale as LLSVPs. We perform a set of high-resolution convection calculations to examine this hypothesis by investigating the interaction of thin oceanic crust (6 km) with mantle plumes. Our results show that as subducted oceanic crust is swept toward upwelling plume regions, the majority of it is viscously stirred into the surrounding mantle. Only a small amount of oceanic crust may accumulate at the base of plumes, but it is consistently entrained away into the plume at a rate equal to or greater than it is accumulated. We find that it is difficult for subducted oceanic crust to accumulate into large thermochemical piles at the CMB.

Li, Mingming; McNamara, Allen K.

2013-04-01

22

Widespread Occurrence of Zircon in Slow- and Ultraslow Spreading Ocean Crust: A Tool for Studying Ocean Lithospheric Processes  

Science.gov (United States)

The presence of igneous zircon in oceanic gabbro and peridotite provides a new opportunity to constrain absolute ages, and the processes and rates of crustal accretion in oceanic environments. Our recent investigations show zircon to be common in slow and ultraslow spreading oceanic crust including several locations along the Mid-Atlantic Ridge (MAR) and Southwest Indian Ridge (SWIR), and in rock types ranging from trondjhemite dikes to peridotite. Zircon is typically found in felsic intrusions and oxide gabbro, and in many cases may be due to late stage saturation in small pockets of residual melt. We report the morphologic and chemical characteristics of zircon grains collected from >100 rock samples recovered both from the seafloor by manned submersible and ROV, and with depth by ODP/IODP drilling. Grains range from euhedral and faceted to anhedral and fractured, with internal zonation that may be homogeneous, concentric, or patchy, and rarely contain relict cores. Sizes range from 1 mm. Measurements of major, minor, and trace element concentrations and high-resolution Pb/U ages were collected with the SHRIMP-RG. Chondrite-normalized rare earth element (REE) patterns for more than 50 zircon grains are uniform in shape and closely resemble patterns for known terrestrial igneous zircon. This is in contrast to mantle affinity zircon (e.g. kimberlite), which typically show depleted and relatively unfractionated patterns. Observed total REE concentrations range from 330-3765 ppm. Patterns are convex upward and rise sharply towards the HREE, with normalized Sm/La ratios = 16-320 and Lu/Gd ratios = 20-51. Positive Ce and negative Eu anomalies are ubiquitous. Hf abundances range from 5988 to 14,266 ppm. Other elements occurring at minor abundance levels include Y (463-6949 ppm), P (253-2288 ppm), U (7-2827 ppm), and Th (3-7403 ppm). Preliminary Ti concentrations range from 13 to 270 ppm, indicating crystallization temperatures of 765 to 1147°C based on Ti in zircon thermometry. Enrichment of U and Th (>100 ppm) is frequently observed, and allows for the application of isotopic dating techniques on rocks collected near ridge axes. SHRIMP-RG Pb/U age determinations reveal resolution on the order of 2% corresponding to 40,000 years in crust as young as 2 Ma, and errors of <2% for samples up to 13 Ma. Attempts to employ the (U-Th)/He dating method on oceanic zircon have also been successful. With a closure temperature (Tc) of 180±20°C, this technique can be used in conjunction with U/Pb dating (Tc = 900±50°C) to bracket magnetic ages (Tc = 450-580°C). By applying multi-temperature chronometers to zircon collected from boreholes we can measure cooling rates related to emplacement, denudation, and/or tectonic rotation. Finally, the relatively common presence of zircon in oceanic crust precludes the simple assumption that detrital zircons in Archean meta-sedimentary rocks demonstrate the existence of continental crust.

Grimes, C. B.; John, B. E.; Cheadle, M. J.; Schwartz, J. J.

2005-12-01

23

Chlorine Stable Isotope Composition of Altered Oceanic Crust: Empirical and Experimental Results  

Science.gov (United States)

Chlorine is an excellent geochemical tracer of fluid-rock interactions because it strongly partitions into the aqueous fluid phase. Chlorine can be used to study the migration of fluids in the crustal environment, volatiles in subduction zones, and the interaction between oceanic lithosphere and seawater-derived hydrothermal fluids. Cl is only a useful tracer, however, if 1) the ?37Cl values of potential chlorine reservoirs and 2) the relevant equilibrium chlorine isotope fractionation factors are both well constrained. Poor constraints on both 1 and 2 for altered oceanic crust (AOC) severely limit our understanding of the global Cl cycle. Here we present ?37Cl values of AOC sampled by the Ocean Drilling Program (ODP). Samples from the Southwest Indian Ridge (ODP Hole 735B) have ?37Cl values ranging from -0.2 to +0.2‰ (error Chloride, defined as 0‰). Samples from the Western Pacific (ODP Hole 801C) have ?37Cl values ranging from -0.4 to +0.8‰. ODP Site 735 samples a 11 Ma lower section of slow spreading (0.6-1.0 cm/yr) oceanic crust. In contrast, ODP Site 801 is located in ~170 Ma fast-spreading crust (16 cm/yr). Despite those differences in age and tectonic setting, the ?37Cl values of AOC are remarkably similar, implying similar sources and mechanisms of hydration. The only previously reported AOC ?37Cl values are from the Costa Rica Rift (ODP Hole 504B). Site 504 was drilled into 5.9 Ma crust from an intermediate spreading center (~3 cm/yr). ?37Cl values range from -1.6 to -0.9‰ (Bonifacie et al., 2007). Our study expands the range of ?37Cl values reported for AOC, and can be used to reevaluate mass balance calculations improving our understanding of subduction recycling. Experimental and theoretical constraints on chlorine isotope fractionation in inorganic systems are limited to only a handful of studies. Theoretical calculations estimate that at 25°C substances in which Cl bonds with 2+ cations will be ~ +2-3‰ heavier than those in which Cl bonds with 1+ cations (Schauble et al., 2003). These calculations have led to the hypothesis that silicates should have higher 37Cl/35Cl ratios than co-existing brines at room temperature (Schauble et al., 2003). Preliminary Cl isotope fractionation experiments between pargasitic amphibole and either seawater or brine (25 wt% NaCl solution) were run in cold-seal pressure vessels at 600°C and 500 bars. Preliminary results suggest that ?37Clpargasite-Cl(aq) is within analytical error of the theoretically predicted fractionation factor, as well as measured ?37Cl values of AOC samples. Further work is planned to investigate the role of fluid-rock ratio and amphibole composition. Additional experimental data may allow us to infer the fluid-rock ratio and temperature during hydration of natural samples and to unravel the fluid histories recorded in the alteration minerals.

Barnes, J.; Gardner, J. E.

2010-12-01

24

Deformation and rupture of the oceanic crust may control growth of Hawaiian volcanoes  

Science.gov (United States)

Hawaiian volcanoes are formed by the eruption of large quantities of basaltic magma related to hot-spot activity below the Pacific Plate. Despite the apparent simplicity of the parent process - emission of magma onto the oceanic crust - the resulting edifices display some topographic complexity. Certain features, such as rift zones and large flank slides, are common to all Hawaiian volcanoes, indicating similarities in their genesis; however, the underlying mechanism controlling this process remains unknown. Here we use seismological investigations and finite-element mechanical modelling to show that the load exerted by large Hawaiian volcanoes can be sufficient to rupture the oceanic crust. This intense deformation, combined with the accelerated subsidence of the oceanic crust and the weakness of the volcanic edifice/oceanic crust interface, may control the surface morphology of Hawaiian volcanoes, especially the existence of their giant flank instabilities. Further studies are needed to determine whether such processes occur in other active intraplate volcanoes. ??2008 Nature Publishing Group.

Got, J.-L.; Monteiller, V.; Monteux, J.; Hassani, R.; Okubo, P.

2008-01-01

25

IODP Expedition 345: Structural characteristics of fast spread lower ocean crust, implications for growth and cooling of ocean crust  

Science.gov (United States)

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 (hydraulic fracturing.

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

2013-12-01

26

Asymmetric generation of oceanic crust at the ultra-slow spreading Southwest Indian Ridge, 64°E  

Science.gov (United States)

We describe topographic, gravity, magnetic, and sonar data from a Southwest Indian Ridge spreading segment near 64°E, 28°S. We interpret these to reveal crustal structure, spreading history, and volcanic and tectonic processes over the last 12 Myr. We confirm that the crust is some 2 km thicker north of the ridge axis, though it varies along and across axis on scales of ˜10 km and 4 Myr. The plate separation rate remained approximately constant at 13 ± 1 km Myr-1, but half-spreading rates were up to 40% asymmetric, varying between faster-to-the-north and faster-to-the-south on a 4 Myr timescale. Topography shows a dominant E-W lineation normal to the N-S spreading direction. This is superficially similar to faulted abyssal hill terrain of the Mid-Atlantic Ridge (MAR), but inferred fault scarps are 3-4 times more widely spaced and have greater offsets. Conjugate pairs of massifs on either plate are interpreted as volcanic constructions similar to the large volcano currently filling the median valley at the segment center. They have temporal spacings of ˜4 Myr and are thought to reflect episodic melt focusing along an otherwise melt-poor ridge. Additionally, there are places, mainly on the southern plate, where lineated topography is replaced by a much blockier topography and embryonic ocean core complexes similar to those recently reported on the MAR near 13°N. There is generally more extrusive volcanism on the northern plate and more tectonism on the southern one. Extrusive volcanism has propagated westward from the segment center since 2 Ma. The FUJI Dome core complex and adjacent seafloor to its east and west appear to be part of a single coherent block, capped by extrusive rock near the segment center, exposing gabbro via a detachment fault over the Dome and probably exposing deeper crust or upper mantle farther west near the segment end. Magnetic anomalies are continuous along this block. We suggest that at its eastern boundary the detachment is simply welded onto magmatically emplaced crust to the east in a similar way to young crust being welded to the old plate at ridge-transform intersections.

Searle, R. C.; Bralee, A. V.

2007-05-01

27

Contraction or expansion of the Moon's crust during magma ocean freezing?  

Science.gov (United States)

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

Elkins-Tanton, Linda T; Bercovici, David

2014-09-13

28

Diversity of microbial communities in ocean crust below ancient hotspot seamounts along the Louisville Seamount Chain  

Science.gov (United States)

The goal of Integrated Ocean Drilling Expedition 330, Louisville Seamount Trail, was to understand the motion of the Louisville hotspot during 50-80 Ma. As such, >1 km of volcanic basement was collected from five sites on four seamounts, providing an excellent chance to study how microbial populations are effected by different lithologies, different seamounts and age of basement rock along the Louisville Seamount Chain (LSC). Analysis of bacteria growing in enrichment incubations that targeted oligotrophs (with 1% or 10% Marine Broth 2216 diluted with 3% NaCl) and sulfur oxidizers reveals the presence of a diverse array of bacteria, including ?-proteobacteria closely related to Sulfurimonas autotrophica, ?-proteobacterial methylotrophs, ?-proteobacteria and Bacteroidetes most closely related to organisms cultured from sediments. Many of these sequences are Halomonas sulfidaeris str. Esulfude1, a bacterium originally isolated from a hydrothermal sulfide chimney. A second isolate may be a new species of Bacillus. Initial molecular analysis of bacterial communities by pyrosequencing of the 16S rRNA gene as part of the Census of Deep Life (CoDL) supports the data from the culturing work; in one sample collected 174 meters below seafloor, the most abundant bacteria detected include species from the genera Pseudomonas, Sulfurimonas, Methyloversatilis and Desulfocapsa. More CoDL samples will be analyzed in the near future. We will describe results to date on subsurface microbial diversity along the Louisville Seamount Chain from the culturing work and CoDL project and draw comparisons to data derived from younger crustal sites to try to understand how the LSC ecosystem fits into our global picture of life in ocean crust.

Sylvan, J. B.; Edwards, K. J.

2012-12-01

29

Transition from oceanic to continental lithosphere subduction in southern Tibet: Evidence from the Late Cretaceous-Early Oligocene (~ 91-30 Ma) intrusive rocks in the Chanang-Zedong area, southern Gangdese  

Science.gov (United States)

Little is known about the detailed processes associated with the transition from oceanic to continental lithosphere subduction in the Gangdese Belt of southern Tibet (GBST). Here, we report zircon U-Pb age, major and trace element and Sr-Nd-Hf isotopic data for Late Cretaceous-Early Oligocene (~ 91-30 Ma) intermediate-acid intrusive rocks in the Chanang-Zedong area immediately north of the Yarlung-Tsangpo suture zone. These rocks represent five magmatic episodes at ~ 91, ~ 77, ~ 62, ~ 48, and ~ 30 Ma, respectively. The 91-48 Ma rocks have slightly lower initial 87Sr/86Sr (0.7037 to 0.7047), and higher ?Nd(t) (+ 1.8 to + 4.3) and ?Hf(t) (+ 3.5 to + 14.7) values in comparison with those (0.7057 to 0.7062, - 3.3 to - 2.5 and + 2.2 to + 6.6) of the ~ 30 Ma intrusive rocks. The ~ 91, ~ 62 and ~ 30 Ma rocks are geochemically similar to slab-derived adakites. The ~ 91 Ma Somka adakitic granodiorites were likely derived by partial melting of the subducting Neo-Tethyan oceanic crust with minor oceanic sediments, and the ~ 91 Ma Somka dioritic rocks with a geochemical affinity of adakitic magnesian andesites likely resulted from interactions between adakitic magmas and overlying mantle wedge peridotite. The ~ 77 Ma Luomu diorites were probably generated by partial melting of juvenile basaltic lower crust. The ~ 62 Ma Naika and Zedong adakitic diorites and granodiorites were likely generated mainly by partial melting of thickened juvenile mafic lower crust but the source region of the Zedong adakitic rocks also contained enriched components corresponding to Indian continental crust. The ~ 48 Ma Lamda granites were possibly generated by melting of a juvenile basaltic crust. The younger (~ 30 Ma) Chongmuda adakitic quartz monzonites and minor granodiorites were most probably derived by partial melting of Early Oligocene northward-subducted Indian lower crust beneath the southern Lhasa Block. Taking into account the regional tectonic and magmatic data, we suggest that the Gangdese Belt of southern Tibet (GBST) underwent a tectonodynamic transition from oceanic subduction to continental subduction between 100 and 30 Ma. It evolved through four stages: 100-65 Ma roll-back of subducted Neo-Tethyan oceanic lithosphere; 65-60 Ma initial collision between Indian and Asian continents; 60-40 Ma breakoff of subducted Neo-Tethyan oceanic lithosphere; and ~ 30 Ma northward subduction of the Indian continent.

Jiang, Zi-Qi; Wang, Qiang; Wyman, Derek A.; Li, Zheng-Xiang; Yang, Jin-Hui; Shi, Xiao-Bing; Ma, Lin; Tang, Gong-Jian; Gou, Guo-Ning; Jia, Xiao-Hui; Guo, Hai-Feng

2014-05-01

30

Hydrothermal fluid fluxes calculated from the isotopic mass balance of thallium in the ocean crust  

OpenAIRE

Hydrothermal fluids expelled from the seafloor at high and low temperatures play pivotal roles in controlling seawater chemistry. However, the magnitude of the high temperature water flux of mid-ocean ridge axes remains widely disputed and the volume of low temperature vent fluids at ridge flanks is virtually unconstrained. Here, we determine both high and low temperature hydrothermal fluid fluxes using the chemical and isotopic mass balance of the element thallium (Tl) in the ocean crust. Th...

Nielsen, Sg; Rehkamper, M.; Teagle, Dah; Butterfield, Da; Alt, Jc; Halliday, An

2006-01-01

31

Europa's Crust and Ocean: Origin, Composition, and the Prospects for Life  

Science.gov (United States)

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.

Kargel, Jeffrey S.; Kaye, Jonathan Z.; Head, James W.; Marion, Giles M.; Sassen, Roger; Crowley, James K.; Ballesteros, Olga Prieto; Grant, Steven A.; Hogenboom, David L.

2000-11-01

32

Distribution of hydrous minerals in the Cocos oceanic crust inferred from receiver function analysis  

Science.gov (United States)

Receiver functions (RFs) are used to estimate the mineralogy as a function of depth along the Cocos slab in central and southern Mexico. The RF image includes a thin low-velocity layer (lower than normal oceanic crustal velocities) on the top of the subducting oceanic lithosphere beneath Mexico. By inverting the amplitudes of the converted phases for the shear-wave velocity (Vs) and density, we produced detailed maps of the seismic properties (Vs, density, Vp/Vs ratio, and Poisson’s ratios) of the upper and lower oceanic crust. In central Mexico, Vs decreases with increasing depth from 40 to 120 km along with elevated Vp/Vs and Poisson’s ratios, which suggests that the downgoing oceanic crust contains a hydrous mineral assemblage. The release of H2O via dehydration could be in the form of a fluid phase or could lower the melting temperature of surrounding phases, where either scenario correlates well with the arc volcanism directly above the slab. Using Vp/Vs as a function of Vs in a range of likely temperatures and depths for candidate low-pressure hydrated phases, we determined that the major hydrous minerals in upper the oceanic crust are likely to be talc and lawsonite. By using similar analyses, we found that the region under southern Mexico is less hydrous than under Central Mexico and not fully eclogitized. This difference may provide a clue as why the slab in central Mexico is flat and in southern Mexico it is not.

Kim, Y.; Clayton, R. W.; Jackson, J. M.

2010-12-01

33

Deep mantle cycling of oceanic crust: evidence from diamonds and their mineral inclusions.  

Science.gov (United States)

A primary consequence of plate tectonics is that basaltic oceanic crust subducts with lithospheric slabs into the mantle. Seismological studies extend this process to the lower mantle, and geochemical observations indicate return of oceanic crust to the upper mantle in plumes. There has been no direct petrologic evidence, however, of the return of subducted oceanic crustal components from the lower mantle. We analyzed superdeep diamonds from Juina-5 kimberlite, Brazil, which host inclusions with compositions comprising the entire phase assemblage expected to crystallize from basalt under lower-mantle conditions. The inclusion mineralogies require exhumation from the lower to upper mantle. Because the diamond hosts have carbon isotope signatures consistent with surface-derived carbon, we conclude that the deep carbon cycle extends into the lower mantle. PMID:21921159

Walter, M J; Kohn, S C; Araujo, D; Bulanova, G P; Smith, C B; Gaillou, E; Wang, J; Steele, A; Shirey, S B

2011-10-01

34

Microbial community on oceanic ferro-manganese crusts from Takuyo-Daigo Seamount and Ryusei Seamount  

Science.gov (United States)

Background and Purpose Iron and manganese oxide deposits are often found on deep seafloor. Rocks covered with these oxides are called ferro-manganese crusts (Mn crusts), and are ubiquitously distributed on deep seafloor (Rona 2003). Because Mn crusts contain rare metals such as Co, Pt and rare earth element, it can be resources in the future. Mn crusts and microbes on Mn crusts may contribute to material, especially carbon and nitrogen circulation between hydrosphere and lithosphere. Mechanism of Mn crust formation is not completely understood. Wang et al. propose a model that microorganisms associate with initial Mn mineral deposition (Wang et al., 2011). There is a possibility that microbes may contribute to formation of Mn crust relying on their ability to oxidize Fe and Mn. However, there is limited information about diversity, spatial distribution and abundance of microbes on Mn crust surface. Our purpose is to clarify microbial community composition, spatial distribution, diversity and abundance of microbes on Mn crusts collected from Takuyo-Daigo seamount and Ryusei seamount. Method We collected Mn crusts, sediments and ambient seawater from Takuyo-Daigo seamount at the depth of 1200 m, 1419 m, 2209 m and 2991 m during NT09-02 cruise in Feb 2009 and Ryusei seamount at the depth of 1194 m, 2079 m during KY11-02 in Feb 2011 with remotely operated vehicle Hyper-Dolphin (JAMSTEC). Genomic DNA was extracted from each sample using Fast DNA kit for soil (Qbiogene). Partial 16S rRNA gene and amoA gene were amplified by PCR with prokaryote-universal primer set (Uni516F-Uni1407R) and bacterial and archaeal amoA specific primer sets. PCR products were cloned. The nucleotide sequences of randomly selected clones were determined. We performed phylogenetic and statistical analysis to determine microbial community compositions, and estimated diversity indices. We also estimated the copy numbers of 16S rRNA and amoA genes of Bacteria and Archaea by quantitative PCR. Results and Discussion We estimated the numbers of bacterial and archaeal cell on Mn crusts from Takuyo-Daigo seamount by QPCR. Bacterial cell number on Mn crust was estimated to be approximately 10^7 cells/g. Those of archaea were estimated to be between 10^6 and 10^7 cells/g. Archaea dominated in three of four Mn crust samples (50~83 % of total cell numbers). Microbial community of Mn crusts was different from those of sediment and seawater. This suggests that unique microbial community present on Mn crusts. Many phylotypes related to uncultured group were detected. Phylotypes closely related to Marine Group I (MGI) were detected from six Mn crust samples, collected from Takuyo-Daigo and Ryusei seamounts. MGI includes Ammonia-Oxidizing Archaea (AOA) and is ubiquitously distributed in ocean (Karner et al., 2001). Phylotypes closely related to Nitrosospira, ammonia-oxidizing bacteria (AOB), were detected from four Mn crusts collected from Takuyo-Daigo seamount. Presence of these ammonia oxidizers was supported by detection of bacterial and archaeal amoA genes. The copy numbers of bacterial and archaeal amoA genes were estimated to be approximately 10^5 -10^6 copy/g by QPCR. These facts suggest that ammonia oxidizers are present abundantly on Mn crusts. MGI and Nitrosospira include autotrophic ammonia oxidizers. These groups may play a role as primary producers in Mn crust ecosystems.

Nitahara, S.; Kato, S.; Yamagishi, A.

2012-12-01

35

Glacial cycles drive variations in the production of oceanic crust.  

Science.gov (United States)

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. PMID:25766231

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

2015-03-13

36

Thin and layered subcontinental crust of the great Basin western north America inherited from Paleozoic marginal ocean basins?  

Science.gov (United States)

The seismic profile of the crust of the northern part of the Basin and Range province by its thinness and layering is intermediate between typical continental and oceanic crust and resembles that of marginal ocean basins, especially those with thick sedimentary fill. The geologic history of the Great Basin indicates that it was the site of a succession of marginal ocean basins opening and closing behind volcanic arcs during much of Paleozoic time. A long process of sedimentation and deformation followed throughout the Mesozoic modifying, but possibly not completely transforming the originally oceanic crust to continental crust. In the Cenozoic, after at least 40 m.y. of quiescence and stable conditions, substantial crustal and upper-mantle changes are recorded by elevation of the entire region in isostatic equilibrium, crustal extension resulting in Basin and Range faulting, extensive volcanism, high heat flow and a low-velocity mantle. These phenomena, apparently the result of plate tectonics, are superimposed on the inherited subcontinental crust that developed from an oceanic origin in Paleozoic time and possibly retained some of its thin and layered characteristics. The present anomalous crust in the Great Basin represents an accretion of oceanic geosynclinal material to a Precambrian continental nucleus apparently as an intermediate step in the process of conversion of oceanic crust into a stable continental landmass or craton. ?? 1974.

Churkin, M., Jr.; McKee, E.H.

1974-01-01

37

Structure of modern oceanic crust and ophiolites and implications for faulting and magmatism at oceanic spreading centers  

Science.gov (United States)

A review of the internal structure of the upper and lower crust in modern oceanic lithosphere and in well-preserved ophiolites leads to some conclusions on the nature of interactions between magmatism and faulting during the construction of oceanic lithosphere at spreading centers. Sheeted dike complexes are made of subparallel vertical intrusions of magma parallel to the axial plane of an oceanic spreading center, and they display structures whose nature are strongly controlled by the mode of and interplay between magmatism and faulting at different seafloor spreading rates. Drilled core samples from the slow-spreading MARK area and Site 735B (Southwest Indian Ridge) record a complex history of solid-state deformation and attendant alteration of the lower crust at temperatures in excess of 700°C and continuing down to 180°C. Ductile shear zones, brittle faults, and detachment surfaces observed in the core samples and on the seafloor consistently indicate normal sense of shearing associated with tectonic extension and crustal stretching. Gabbroic rocks and serpentinized peridotites are exposed on the seafloor in the MARK area, suggesting that upper crustal units have been stripped away due to amagmatic extension. The sheeted dike complex of the intermediate-spreading oceanic crust along the Costa Rica Rift displays intense microfracturing at discrete depth intervals, as observed in ODP cores, that are possibly associated with faults or localized deformation zones. The seismic layer 2/3 boundary occurs within the sheeted dike complex and corresponds to changes in physical properties over a depth interval, rather than to the presence of a lithological change from dikes to gabbros. The dike-gabbro boundary is probably tectonic, corresponding to the fault zone drilled into in the borehole and coincides with one of the half-graben bounding and gently dipping normal faults depicted on single channel seismic reflection profiles. The fast-spreading oceanic crust drilled in the Hess Deep area in the eastern equatorial Pacific Ocean is relatively undeformed with well-preserved igneous contacts between the sheeted dike complex and the gabbros and a transition zone between the lower crustal and mantle sequences. Gabbroic core samples mainly display magmatic flow structures (subvertical foliation and associated lineation defined by anisotropic plagioclase crystals), rather than solid-state ductile deformation features. Widespread fracturing of the crust was associated with subsolidus cooling and thermal contraction after axial magma emplacement within the crustal accretion zone. The sheeted dike complexes in the Troodos (Cyprus) and Kizildag (Turkey) ophiolites, both inferred to be of slow-spreading origin, show numerous planar to listric normal faults and structural grabens suggestive of tectonic extension, and complex intrusive relations in their plutonic units indicative of recurring and intermittent magmatic activities. Detachment surfaces within the lower crustal sequence (Troodos) or at the boundary between the crustal and mantle sequences (Kizildag) define a brittle-plastic transition zone along which the upper crustal units above was accommodated by mylonitization and denudation in lower crustal units and mantle rocks below. Sheeted dike complexes in the Semail (Oman) and Solund-Stavfjord (Norway) ophiolites, interpreted to be of fast- to intermediate-spreading origin (respectively), have steeply dipping dike intrusions and show minor dike-parallel and dike-perpendicular faults. Sheeted dikes in these ophiolites root into the underlying plutonic sequences and locally display mutually intrusive relations with differentiated plutonic rocks. These relations together with the relatively undeformed nature of the remnant oceanic crust imply robust magmatism that kept pace with seafloor spreading and associated extension during the evolution of these ophiolites. The internal structure of the sheeted dike complexes and the nature of dike-gabbro boundary in oceanic crust are affected by the vertical temperature gradient in the ocean

Dilek, Yildirim; Moores, Eldridge M.; Furnes, Harald

38

Global distribution of beryllium isotopes in deep ocean water as derived from Fe-Mn crusts  

Science.gov (United States)

The direct measurement of the ratio of cosmogenic 10Be (T1/2 = 1.5 Ma) to stable terrigenously sourced 9Be in deep seawater or marine deposits can be used to trace water mass movements and to quantify the incorporation of trace metals into the deep sea. In this study a SIMS-based technique has been used to determine the 10Be/9Be ratios of the outermost millimetre of hydrogenetic ferromanganese crusts from the worlds oceans. 10Be/9Be ratios, time-corrected for radioactive decay of cosmogenic 10Be using 234U/ 238U, are in good agreement with AMS measurements of modern deep seawater. Ratios are relatively low in the North and equatorial Atlantic samples (0.4-0.5 ?? 10-7). In the Southwest Atlantic ratios increase up to 1 ?? 10-7, they vary between 0.7 and 1.0 ?? 10-7 in Indian Ocean samples, and have a near constant value of 1.1 ?? 0.2 ?? 10-7 for all Pacific samples. If the residence time of 10Be (??10Be) in deep water is constant globally, then the observed variations in 10Be/9Be ratios could be caused by accumulation of 10Be in deep water as it flows and ages along the conveyor, following a transient depletion upon its formation in the Northern Atlantic. In this view both 10Be and 9Be reach local steady-state concentration in Pacific deep water and the global ??10Be ??? 600 a. An alternative possibility is that the Be isotope abundances are controlled by local scavenging. For this scenario ??10Be would vary according to local particle concentration and would ??? 600 a in the central Pacific, but ??10Be ??? 230 a in the Atlantic. Mass balance considerations indicate that hydrothermal additions of 9Be to the oceans are negligible and that the dissolved riverine source is also small. Furthermore, aeolian dust input of 9Be appears insufficient to provide the dissolved Be inventory. The dissolution of only a small proportion (2%) of river-derived particulates could in principle supply the observed seawater Be content. If true, ocean margins would be the sites for 9Be addition. Due to the particle-reactive nature of Be, these would also be the primary sites of Be removal. A possible net result of horizontal water masses passing through these marginal areas might be a decrease in seawater 10Be/9Be, and establishment of a relatively constant 9Be concentration. As ??10Be ( ??? 600 a) is less than the apparent age of deep water in the Pacific ( ??? 1500 a), the Pacific record of 10Be/ 9Be is not expected to show secular variations due to changes in deep-water flow, despite the large variations in 10Be/ 9Be between different water masses. Because of this insensitivity to deep-water flow, however, it is suggested that the 10Be/ 9Be ratio, determined in the authigenic phase of marine sediments or hydrogenetic precipitates, should be a suitable tool for monitoring changes in continental input or cosmic ray intensity on longer time scales.

Von Blanckenburg, F.; O'Nions, R. K.; Belshaw, N.S.; Gibb, A.; Hein, J.R.

1996-01-01

39

Structure of Mesozoic oceanic crust in the vicinity of the Cape Verde Islands from seismic reflection profiles  

OpenAIRE

Multichannel seismic reflection profile data have been used to determine the internal structure of Mesozoic oceanic crust in the vicinity of the Cape Verde islands. The data show the oceanic crust to be characterized by both dipping and sub-horizontal reflectors. Several lines of evidence argue against the reflectors being scattering artifacts arising, for example, from rough basement topography. Instead, the reflectors are attributed to tectonic and magmatic processes associated with the acc...

Ali, My; Watts, Ab; Hill, I.

2003-01-01

40

The Fate of Eclogitized Oceanic Crust During Subduction: Implications for Subduction Zone Dynamics  

Science.gov (United States)

The Monviso ophiolite is composed of two main tectonic slices: the Monviso Unit (MU) to the west, which overlies the Lago Superiore Unit (LSU). Our PT estimates show that the MU has been subducted down to 480°C-23kbar (~70km) during Alpine subduction while the LSU reached slightly deeper conditions (540°C-26kbar, i.e. ~80km). This ophiolite, which comprises large (10-20km long) ophiolite fragments therefore does not correspond to a serpentinite mélange, and may be the southern extension of the Zermatt-Saas ophiolite (Angiboust et al., 2009; 2011). The well-preserved LSU constitutes an almost continuous upper fragment of oceanic lithosphere subducted between 50 and 40 Ma and later exhumed along the subduction interface. It therefore provides a unique opportunity to study strain partitioning and deep mechanical behaviour of the subducting lithosphere. The LSU comprises (i) several hundred meters of eclogitized basaltic crust (+ minor calcschist lenses) overlying a 100-400m thick metagabbroic body and (ii) a serpentinite sole (c. 1km thick). We herein focus on eclogite-facies shear zones, which are found at the boundary between basalts and gabbros, and between gabbros and serpentinites, i.e. between material with marked rheological contrasts. Eclogite facies blocks within the shear zones display intense fracturation, fragment rotation and dispersion in the serpentinite schists which line up the shear zones. We also report the first finding of eclogite-facies breccias, constituted of rotated eclogite mylonitic clasts cemented within an eclogite-facies matrix. Local fracturation of garnet within these breccias is attested by the presence of numerous fracture networks within garnet, generally healed by a Mg-enriched composition. The shear zones also preserve clear evidence of pervasive and channelized fluid flow (of variable duration) leading to alteration of bulk rock composition, weakening of the rock and widespread crystallization of lawsonite. Our results provide new constraints for deep mechanical coupling processes and meter-scale fluid-rock interaction occurring at depth in present-day subduction zones. In particular, we emphasize that (i) rheological contrasts in the field qualitatively support those inferred from experimental flow laws, (ii) fluid flow is channelized along deep eclogite facies shear zones (iii) brittle fracturing recorded by eclogite breccias and garnet fractures could be associated with deep interplate seismicity.

Angiboust, S.; Agard, P.; Langdon, R.; Waters, D.; Raimbourg, H.; Yamato, P.; Chopin, C.

2011-12-01

41

Osmium isotope variations in the oceans recorded by Fe-Mn crusts  

Science.gov (United States)

This study presents osmium (Os) isotope data for recent growth surfaces of hydrogenetic ferromanganese (Fe-Mn) crusts from the Pacific, Atlantic and Indian Oceans. In general, these data indicate a relatively uniform Os isotopic composition for modern seawater, but suggest that North Atlantic seawater is slightly more radiogenic than that of the Pacific and Indian Oceans. The systematic difference in the Os isotopic composition between the major oceans probably reflects a greater input of old continental material with a high Re/Os ratio in the North Atlantic Ocean, consistent with the distribution of Nd and Pb isotopes. This spatial variation in the Os isotope composition in seawater is consistent with a residence time for Os of between 2 and 60 kyr. Indian Ocean samples show no evidence of a local source of radiogenic Os, which suggests that the present-day riverine input from the Himalaya-Tibet region is not a major source for Os. Recently formed Fe-Mn crusts from the TAG hydrothermal field in the North Atlantic yield an Os isotopic composition close to that of modern seawater, which indicates that, in this area, the input of unradiogenic Os from the hydrothermal alteration of oceanic crust is small. However, some samples from the deep Pacific (???4 km) possess a remarkably unradiogenic Os isotope composition (187Os/186Os ratios as low as 4.3). The compositional control of Os incorporation into the crusts and mixing relationships suggest that this unradiogenic composition is most likely due to the direct incorporation of micrometeoritic or abyssal peridotite particles, rather than indicating the presence of an unradiogenic deep-water mass. Moreover, this unradiogenic signal appears to be temporary, and local, and has had little apparent effect on the overall evolution of seawater. These results confirm that input of continental material through erosion is the dominant source of Os in seawater, but it is not clear whether global Os variations are due to the input of mantle or meteoritic material, or simply indicate that the continental source itself is not uniform.

Burton, K.W.; Bourdon, B.; Birck, J.-L.; Allegre, C.J.; Hein, J.R.

1999-01-01

42

Seismic structure of the crust and uppermost mantle of South America and surrounding oceanic basins  

Science.gov (United States)

We present a new set of contour maps of the seismic structure of South America and the surrounding ocean basins. These maps include new data, helping to constrain crustal thickness, whole-crustal average P-wave and S-wave velocity, and the seismic velocity of the uppermost mantle (Pn and Sn). We find that: (1) The weighted average thickness of the crust under South America is 38.17 km (standard deviation, s.d. ±8.7 km), which is ˜1 km thinner than the global average of 39.2 km (s.d. ±8.5 km) for continental crust. (2) Histograms of whole-crustal P-wave velocities for the South American crust are bi-modal, with the lower peak occurring for crust that appears to be missing a high-velocity (6.9-7.3 km/s) lower crustal layer. (3) The average P-wave velocity of the crystalline crust (Pcc) is 6.47 km/s (s.d. ±0.25 km/s). This is essentially identical to the global average of 6.45 km/s. (4) The average Pn velocity beneath South America is 8.00 km/s (s.d. ±0.23 km/s), slightly lower than the global average of 8.07 km/s. (5) A region across northern Chile and northeast Argentina has anomalously low P- and S-wave velocities in the crust. Geographically, this corresponds to the shallowly-subducted portion of the Nazca plate (the Pampean flat slab first described by Isacks et al., 1968), which is also a region of crustal extension. (6) The thick crust of the Brazilian craton appears to extend into Venezuela and Colombia. (7) The crust in the Amazon basin and along the western edge of the Brazilian craton may be thinned by extension. (8) The average crustal P-wave velocity under the eastern Pacific seafloor is higher than under the western Atlantic seafloor, most likely due to the thicker sediment layer on the older Atlantic seafloor.

Chulick, Gary S.; Detweiler, Shane; Mooney, Walter D.

2013-03-01

43

Hydrothermal and tectonic processes recorded in fault rocks from the upper oceanic crust  

Science.gov (United States)

Faulting and fracturing along mid-ocean ridges play a crucial role in hydrothermal systems and the mechanical behavior of the oceanic crust. Fault and fracture systems resulting from explosive hydrothermal events may differ in permeability and mechanical strength from those that accommodate axial extension. To explore the potential differences, images and samples have been investigated from a range of spreading environments, including the Hess and Pito Deep rifts in East Pacific Rise-spread crust, the SMARK area (22°N) on the Mid Atlantic Ridge, the Troodos Ophiolite, and the Icelandic rift system. These exposures of lavas and dikes contain fault-zone units with contrasting lithologic and mechanical properties and geochemical compositions. To further understand the deformation mechanisms of ocean crustal faulting, image analysis of the fault-zone units from micro- to meso-scales provides a quantitative assessment of grain size, orientation, and fracture density. A key measure is the Particle Size Distribution (PSD), found in continental fault rocks to be a power-law probability function distribution reflecting incremental grain fracture. However, the PSDs of ocean crustal fault rocks are not power law, suggesting a departure from continental fault-zone deformation patterns. Controls on PSD in ocean crustal fault rocks include the initial fracture development in otherwise massive basalts, importance of fluid-rock interaction, and distinctive tectonic strain and stress conditions for seafloor spreading. Here it is further suggested that structures resulting from intense hydrothermal activity may result from different brittle deformation mechanisms (and thereby have different PSDs and SPOs) than those resulting from dominantly tectonic strain. Identifying these different mechanisms of ocean crustal fault-zone deformation establishes length scales, grain-scale deformation mechanisms, the geologic record of fluid-rock interaction, and could lead to in situ constraints on permeability and mechanical properties. These are important steps in understanding the subsurface controls on hydrothermal events and tectonic extension otherwise only observed at the seafloor.

Browne, C. M.; Hayman, N. W.

2011-12-01

44

Nickel isotopic compositions of ferromanganese crusts and the constancy of deep ocean inputs and continental weathering effects over the Cenozoic  

Science.gov (United States)

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.

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

2013-08-01

45

Thickened Ocean Crust or Basal Island Arc Origin for the Pitka Mafic-Ultramafic Complex, Northern Alaska  

Science.gov (United States)

Scattered occurrences of Mesozoic mafic-ultramafic rocks that extend over 1000 km across northern Alaska occupy a tectonic position similar to many suprasubduction zone ophiolites. The mafic-ultramafic (mum) complexes occur as klippe; the structural layers range downward from ocean protoliths, immediately below the mum complexes, to distal continentally-derived metasediments, to more proximal metasediments. The mum complexes lack the classic full or even partial ophiolite section and consist only of ultramafic and mafic plutonic rocks which are not genetically related to structurally underlying cherts, metagabbros and metabasites. Most of the mum complexes do have metamorphic soles and/or retrograde shear zones, which have strikingly similar ages ( ~166 Ma) throughout northern Alaska. The protoliths and metamorphic histories of the mum complexes and the rocks immediately beneath them vary, which may provide the key to understanding the origin and emplacement of these "ophiolites". The Pitka complex, one of the easternmost of the northern Alaska mum complexes, has a similar protolith to other complexes, containing predominantly layered gabbros, harzburgite, and lherzolite. It is anomalous compared to those in northern Alaska in that the mafic-ultramafic rocks are high-P granulite facies. Metamorphosed mafic rocks contain plagioclase (pl)-clinopyroxene (cpx)-garnet (grt)-orthopyroxene (opx)-hornblende (hb). Scapolite is present locally but quartz (qtz) is absent. Metamorphosed ultramafic rocks contain olivine-cpx-opx-spinel. Grt-cpx Fe-Mg exchange temperatures (T) range from 770 to 860 C at 10 kbar. Opx-cpx geothermometry yields T's from 715 to 810 C at 10 kbar. Maximum pressure based upon grt-pl-cpx equilibria without qtz is near 10 kbar at 800 C. Ar/Ar dates from hornblende range from 169.5+/- 0.3 Ma in the granulite facies rocks to 166 +/- 0.6 and 164.8 +/- 1.1 Ma in epidote-amphibolite shear zones. Many rocks show little evidence of retrogression (symplectites are rare) and apparently underwent rapid, relatively dry cooling. The Pitka complex appears to correlate with the Kanuti complex to the west, which also preserves evidence for high-P and high-T metamorphism. We prefer an interpretation that the metamorphism reflects conditions present when the Kanuti and Pitka complexes crystallized, rather than subsequent tectonic burial. The pressures require a model for origin of either basal island arc or anomalously thick ocean crust. Final assembly of the mum klippe over oceanic and passive margin sequences occurred during Late Jurassic-Early Cretaceous collision of the Arctic margin with an island arc.

Roeske, S. M.; Ghent, E. D.; Stout, M. Z.; Bradshaw, J. Y.

2001-12-01

46

Imaging the Moho and Subducted Oceanic Crust at the Isthmus of Tehuantepec, Mexico, from Receiver Functions  

Science.gov (United States)

Using teleseismic data recorded along a transect, which we call VEOX (for Veracruz-Oaxaca seismic line), of 46 broadband stations installed across the Isthmus of Tehuantepec in southern Mexico, we obtained receiver functions and stacked them to study the Moho topography and back projected them to visualize the subducted slab geometry beneath the isthmus. We observed a back-azimuth dependent Moho thickness across the transect, particularly beneath the Los Tuxtlas Volcanic Field. Also, we observed the Cocos plate which subducts with an angle of 26° between 140 and 310 km from the trench. Comparison with regional seismicity indicates that it occurs below the oceanic crust.

Melgar, Diego; Pérez-Campos, Xyoli

2011-08-01

47

The Jamestown Ophiolite Complex, Barberton mountain belt - A section through 3.5 Ga oceanic crust  

Science.gov (United States)

The Jamestown Ophiolite Complex of the Barberton greenstone belt, South Africa, is investigated, and the intrusive nature of mafic-ultramafic units from the Komati and Kromberg formations into overlying pillow lavas and sediments is documented. Evidence is presented for multiple intrusive events within the igneous sections, including crosscutting intrusives, multiple injection of magma in the Komati section, and sheeted intrusions in the Kromberg section. The thinness of the Jamestown complex suggests that, locally at least, the ca 3.5 Ga oceanic crust was also thin, consistent with the regionally extensive metasomatic alteration.

de Wit, Maarten J.; Hart, Roger A.; Hart, Rodger J.

48

Osmium isotopic evidence for recycled oceanic crust in East African Rift Volcanism  

Science.gov (United States)

The composition of volcanic products in the East African Rift System suggest there are two separate mantle plumes supplying heat and material to the region. The Afar plume, currently located beneath north-central Ethiopia, consistently produces high 3He/4He (R/Ra 9-19) lavas with strong contributions from the C-mantle component. The Kenya plume, currently located beneath the Tanzanian craton, produces low 3He/4He (R/Ra 5-7) lavas with a conspicuous HIMU component. The origin of the HIMU component has been a subject of debate; interpretations focus on three models (1) deeply recycled oceanic crust, (2) lithospheric contamination and (3) metasomatic overprinting of mantle material. Because Os isotopes have proven to be an effective way to detect the presence of recycled oceanic crust, we measured the 187Os/188Os of uncontaminated picrites best representing the composition of the Afar and Kenya plumes to determine the possible influence of recycled material. For comparison we also analyzed lavas from other parts of the East African Rift System that have been affected by lithospheric / crustal contamination, metasomatism, or mixing / hybridization between the two plumes. Our results demonstrate that the Kenya plume contains radiogenic Os (187Os/188Os = 0.1450-0.1483) consistent with contributions from recycled oceanic crust. In contrast, Afar plume lavas display a mildly radiogenic Os signature (187Os/188Os = 0.1239-0.1311) inconsistent with a significant recycled component but typical of “normal” mantle plumes. Mixing models demonstrate crustal and lithospheric contamination cannot cause the Os isotopic signature of one plume to resemble the other. Therefore, the Afar and Kenya plumes are compositionally distinct. The origin of the compositional differences is not clear. Geochemical analyses cannot distinguish between oceanic crust recycled near the core-mantle boundary, recycled near the 660 km discontinuity, or fused to the base of the lithosphere. While geophysical data cannot definitively link the Afar and Kenya mantle plumes to the African superplume, global seismic studies demonstrate the superplume contains density heterogeneities related to compositionally distinct material (i.e. eclogite and peridotite). Therefore, the chemical differences between the Afar and Kenya plume could ultimately be derived from heterogeneities within the African superplume.

Nelson, W. R.; Furman, T.; Shirey, S. B.

2009-12-01

49

Deep-Ocean Crusts as Telescopes: Using Live Radioisotopes to Probe Supernova Nucleosynthesis  

OpenAIRE

Live 60Fe has recently been detected in a deep-ocean ferromanganese crust, isolated in layers dating from about 3 Myr ago. Since 60Fe has a mean life of 2.2 Myr, a near-Earth supernova is the only likely source for such a signal, and we explore here the consequences of a supernova origin. We combine the 60Fe data with several supernova nucleosynthesis models to calculate the supernova distance as a function of progenitor mass, finding an allowed range of 15-120 pc. We also p...

Fields, Brian D.; Hochmuth, Kathrin A.; Ellis, John

2004-01-01

50

Physical properties and logging of the lower oceanic crust: Hole735B  

Science.gov (United States)

Results from downhole logging instrumentation and physical properties measurements on samples recovered from a 500-m-thick section of gabbros at Site 735 on the SW Indian Ridge are compared. Here we emphasize particularly the seismic, electrical, and nuclear logging measurements to deduce the physical state and evolution of this crustal section over the 11-12 m.y. since its formation. Various seismic methods give compressional velocities ranging from 6.5 to >7 km/s, typical of lower oceanic crustal velocities determined from marine refraction measurements. Except for the unusually low intrinsic electrical resistivity (neutron log. The decrease with depth of thin, relatively high porosity (20-25%) zones, low temperature (sea water) rock alteration, and fluid permeability suggests that overburden stress is an important factor maintaining closed fractures in young ocean crust.

Von Herzen, R. P.; Goldberg, D.; Manghnani, M.

51

Microbial phylogeny of igneous minerals and glasses in deep ocean crust  

Science.gov (United States)

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.

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.

2011-12-01

52

Positive geothermal anomalies in oceanic crust of Cretaceous age offshore Kamchatka  

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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.

G. Delisle

2011-09-01

53

Positive geothermal anomalies in oceanic crust of Cretaceous age offshore Kamchatka  

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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 years 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 descend 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.

G. Delisle

2011-05-01

54

Araxa Group in the type-area: A fragment of Neoproterozoic oceanic crust in the Brasilia Fold Belt  

International Nuclear Information System (INIS)

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 ?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 ?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)basement complex. (author)

55

New observations of the shallow seismic structure of young oceanic crust  

Science.gov (United States)

The results are presented of three new experiments carried out on the Mid-Atlantic Ridge (MAR) near latitude 23°N using a unique method of studying the seismic structure of the uppermost few hundred meters of the oceanic crust. The data were collected using a fixed ocean floor hydrophone receiver and a controllable explosive source that was towed within a few tens of meters of the rugged bottom topography. These 1- to 2-km-long refraction lines produced for the first time direct observations of the compressional wave velocity structure of the uppermost 200-300 m of the young igneous crust. One experiment was carried out over the site of hole 648B of the Ocean Drilling Program on a small volcano within the median valley of the MAR. The seafloor velocity was observed to be 2.1 km s-1 underlain by an approximately linear velocity gradient of 4 s-1. Given that we know the crust at this location consists of fresh basalt lavas with laboratory-measured velocities in excess of 5.8 km s-1, porosities at the seafloor of as high as 30-50% are inferred. The two other experiments were located over 7-m.y.-old crust near Deep Sea Drilling Project site 395 west of the MAR, separated by only 14 km laterally but by over 1400 m in water depth. The experiment positioned in the ˜100-m-thick sediment pond and that located 14 km to the south atop a prominent topographic high produced results that within the data resolution were indistinguishable. The velocity of the uppermost basaltic basement was 4.1 km s-1, and the velocity gradient was less than 0.5 s-1. This ˜2 km s-1 difference in velocity between the two sites is assumed to be a consequence of age-related modifications to the physical properties of the crust. If the primary change is in only the total porosity, then a 15-20% reduction is required to explain these observations. This seems unlikely because any such pervasive process would have been previously observed in geophysical and sampling data (e.g., off-axis volcanism, secondary mineral deposition, tectonic compression). A combination of many processes acting in unison would be necessary to produce such a decrease in porosity. If the geometry of the cracks and voids were modified in just the right way, then no change in total porosity is required to explain the velocity difference. These on-bottom refraction profiles are proven to be practical experiments that can provide precise information on a scale appropriate to many of the ocean floor's geologic features. They produce results that may be correlated in a meaningful way with results of ocean drilling and downhole measurements.

Purdy, G. M.

1987-08-01

56

Age estimates of the seaward-dipping volcanic wedge, earliest oceanic crust, and earliest drift-stage sediments along the North American Atlantic continental margin  

Science.gov (United States)

Owing to their depths of burial along and adjacent to the North American continental margin, there is no direct evidence obtained from boreholes for the ages of the seaward-dipping volcanic wedge, earliest drift-stage sediments overlying the wedge, and the earliest Atlantic oceanic crust between the East Coast (ECMA) and Blake Spur (BSMA) magnetic anomalies. Maximum ages of late Sinemurian for drift-stage sediments have been determined from exploration wells in the Scotian Basin. A similar age is postulated for those sediments in the Georges Bank Basin, but palynomorphs from exploration wells may indicate that earliest drift-stage sediments, in places associated with volcanic rocks, are of Bajocian age and occur higher in the section above the postrift unconformity as recognized on seismic lines. In the Southeast Georgia Embayment of the Blake Plateau Basin, the oldest drift-stage sediments overlying the postrift unconformity that were drilled are of Kimmeridgian-Tithonian age. In the Baltimore Canyon Trough, the volcanic wedge overlies the postrift unconformity which truncates buried synrift rocks that may be as young as Sinemurian. In the Carolina Trough and Blake Plateau Basin, a possible offshore flood basalt marking the postrift unconformity and traced as a reflector to the volcanic wedge may correspond to a subsurface flood basalt onshore that may be part of CAMP (Hettangian). Alternatively, its magmatic source may have been that of the possibly younger volcanic wedge. Sea-floor-spreading-rate lines based on the latest Jurassic time scales and extended to the BSMA and ECMA indicate ages of 166 and 171 Ma for the BSMA and 172 and 179 Ma for the ECMA. An alternative model suggests a middle Pliensbachian/early Toarcian age (188-190 Ma) for the igneous activity that produced the volcanic wedge and earliest oceanic crust.

Benson, Richard N.

57

Static and fault-related alteration in the lower ocean crust, IODP Expedition 345, Hess Deep  

Science.gov (United States)

IODP Expedition 345 drilled the first holes in the lower plutonic crust at a fast-spreading ridge, recovering primitive layered gabbros (Gillis et al 2014). Alteration can be subdivided into two series: 1) a largely static pseudomorphic alteration affecting predominantly olivine. This began in the amphibolite facies with minor secondary cinopyroxene and hornblendic amphibole replacing primary pyroxene, and sporadically developed corona textures with tremolite and chlorite replacing olivine and plagioclase respectively, but was predominantly in the greenschist and sub-greenschist facies with talc, serpentine, clay minerals,oxides andsulphides replacing olivine, and prehnite and locally other calcsilicates replacing plagioclase, commonly in micro-vein networks. Albitic plagioclase is sporadically developed, and locally zeolite and carbonate. 2) An overprinting metasomatic alteration under sub-greenschist or perhaps lowermost greenschist conditions(occurs as patches replacing plagioclase along grain boundaries, locally associated with carbonate and amphibole needles. Metamorphosed dykes show chilled margins within the cataclasites, and are affected by cataclastic deformation. Faults, dykes and overprinting alteration are all inferred to be related to the westward propagation of Cocos-Nazca spreading that formed Hess Deep. Samples of different alteration and cataclastic domains were cut out of this section chips for isotopic analysis. 87Sr/86Sr ratios 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. ?18O values range from +1 to + 6 per mil, indicating alteration at temperatures generally >200 °C. Preliminary modelling using Comsol Multiphysics suggests that the temperatures of the overprinting alteration could be achieved in a permeable fault slot cutting through crust 0.5 to 1 m.y. old. Our study reveals a low temperature alteration assemblage dominated by prehnite and chlorite that is not normally associated with the lower oceanic crust. Yet it is likely to be common in any location where faults intersect the Moho off-axis, including transform faults, near axis normal faults at slow spreading ridges, and bending faults at subduction zones, and would be accompanied by serpentinites in upper mantle rocks, as seen at ODP site 895 in Hess Deep. This prehnite + chlorite assemblage may therefore be significant in the release of volatiles in subduction zones. Gillis, K.M., Snow J. E. and Shipboard Science Party (2014) Primitive layered gabbros from fast-spreading lower oceanic crust. Nature, 505,204-207, doi: 10.1038/nature12778

McCaig, Andrew; Faak, Kathrin; Marks, Naomi; Nozaka, Toshio; Python, Marie; Wintsch, Robert; Harigane, Yumiko; Titarenko, Sofya

2014-05-01

58

Geochemical relationship between PREMA, FOZO and HIMU: link to chemical heterogeneity of MORB and layered structure of oceanic crust  

Science.gov (United States)

One of important concepts of the geochemistry is a mantle reservoir model, in which isotopic composition of the ocean island basalts (OIBs) are explained by mixing of distinct and isolated reservoirs, those are, depleted MORB mantle (DMM), HIMU (high-?) and EMs (Enriched Mantles). In addition to these reservoirs, importance of reservoirs whose isotopic compositions are intermediate has been pointed out, these are, FOZO (Focal Zone), C (Common component) and PREMA (Prevalent Mantle). Although the existences of these 'intermediate reservoirs' are still in debated, the isotopic compositions of these reservoirs have been used to describe the isotopic distribution of OIBs. Therefore, clarifying the origin of these reservoirs should be significant for the better understanding of cause of mantle heterogeneity. For the evaluation of origin and genetic linkage between these reservoirs, geochemical modeling has been conducted from the perspective of chemical fractionation at mid-ocean ridges and subduction zones. For the modeling, MORB compositions from Mid-Atlantic ridge are compiled for seven trace elements (Rb, Sr, Nd, Sm, Pb, Th and U) and used as representative oceanic crust compositions. Effect of chemical fractionation at a mid-ocean ridge is estimated based on magnesium number and frequency distribution. The results suggest that the chemical fractionation can produce small isotopic variation with moderately depleted isotopic signature that is suitable for PREMA if the age of recycled MORBs is 1.5 Ga. A mixing of dehydrated and dry MORBs can produce isotopic variation from PREMA to FOZO during 1.5 Ga recycling. For the production of typical HIMU (206Pb/204Pb > 21), Pb depletion due to removal of sulfur during subduction and/or U and Th enrichment owing to crystal fractionation at mid-ocean ridge is required. A mixing of dehydrated and dry MORBs can be explained by vertical difference in water content of oceanic crust that might correspond to surface hydrated and deep dry layers, respectively. If this is the case, origin of FOZO could be explained by recycling of commonly exist dehydrated surface layer of oceanic crust. In addition, PREMA may exemplify deep dry layer of oceanic crust. Origin of HIMU can be a totally hydrated evolved oceanic crust that experienced least degassing process to preserve sulfur. Magma evolution at mid-ocean ridges and different degree of dehydration in an oceanic crust beneath subduction zones play an essential role in producing the isotopic variations between PREMA, FOZO and HIMU.

Shimoda, G.; Kogiso, T.

2013-12-01

59

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

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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.

KatrinaEdwards

2012-02-01

60

Controls on thallium uptake during hydrothermal alteration of the upper ocean crust  

Science.gov (United States)

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.

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

2014-11-01

61

Geological storage of CO2 within the oceanic crust by gravitational trapping  

Science.gov (United States)

rise of atmospheric carbon dioxide (CO2) principally due to the burning of fossil fuels is a key driver of anthropogenic climate change. Mitigation strategies include improved efficiency, using renewable energy, and capture and long-term sequestration of CO2. Most sequestration research considers CO2 injection into deep saline aquifers or depleted hydrocarbon reservoirs. Unconventional suggestions include CO2 storage in the porous volcanic lavas of uppermost oceanic crust. Here we test the feasibility of injecting CO2 into deep-sea basalts and identify sites where CO2 should be both physically and gravitationally trapped. We use global databases to estimate pressure and temperature, hence density of CO2 and seawater at the sediment-basement interface. At previously suggested sites on the Juan de Fuca Plate and in the eastern equatorial Pacific Ocean, CO2 is gravitationally unstable. However, we identify five sediment-covered regions where CO2 is denser than seawater, each sufficient for several centuries of anthropogenic CO2 emissions.

Marieni, Chiara; Henstock, Timothy J.; Teagle, Damon A. H.

2013-12-01

62

Petrophysical properties of the root zone of sheeted dikes in the ocean crust: A case study from Hole ODP/IODP 1256D, Eastern Equatorial Pacific  

Science.gov (United States)

ODP (Ocean Drilling Program)/IODP (Integrated Ocean Drilling Program) Site 1256 is located on the Cocos Plate in the Eastern Equatorial Pacific Ocean, in a 15 Ma old oceanic lithosphere formed at the EPR during a period of superfast spreading (> 200 mm/yr). ODP/IODP Hole 1256D reached for the first time the contact between sheeted dikes and underlying gabbros. It consequently offers a unique opportunity to study in situ, in present-day oceanic crust, the root zone of the sheeted dike complex. This root zone is a thin, 100 m thick boundary layer between the magmatic system (i.e., the axial melt lens, ~ 1100 °C), and the overlying high temperature hydrothermal system (? 450 °C). The understanding of interactions within this boundary layer is critical to that of crustal processes along mid-ocean ridges. This work focuses on the petrophysical characterization of the root zone of the sheeted dike complex in order to further constrain the hydrothermal circulation system in the vicinity of the axial melt lens, as recorded in non-granoblastic dikes, granoblastic dikes, and varitextured gabbros. The petrophysical properties were determined from sample measurements in the laboratory and were compared to in situ downhole geophysical probing. The porosity structure is bipolar, depending on lithology, resulting in a layered system. Non-granoblastic dikes are generally altered in the greenschist facies (~>250 °C) with relatively high and interconnected (cementation index m ~ 1.72, electrical tortuosity ? ~ 28.3) porosity (1.5%). In contrast, gabbros are retrogressively metamorphosed in the amphibolite (~>450 °C) and greenschist facies, with lower porosity (1.3%) that involves numerous fissures and cracks, resulting in a more connected medium ( m ~ 1.58, ? ~ 11.8) than non-granoblastic dikes. These cracks are more abundant but also tend to close with increasing depth as indicated in downhole geophysical data. Porosity and alteration, as viewed from surface electrical conductivity, appear to be directly correlated.

Violay, Marie; Pezard, Philippe A.; Ildefonse, Benoît; Belghoul, Akram; Laverne, Christine

2010-10-01

63

Alteration of basaltic glass: Mechanisms and significance for the oceanic crust-seawater budget  

Science.gov (United States)

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.

Staudigel, Hubert; Hart, Stanley R.

1983-03-01

64

Generation of mantle heterogeneity by oceanic crust recycling: how well can we match geochemical and geophysical observations? (Invited)  

Science.gov (United States)

The heterogeneity of the Earth's mantle is expressed in complementary geochemical and geophysical signatures, where the geochemistry provides a time-integrated signal and the geophysics tends to see a recent snapshot of the Earth's interior. While the geophysical evidence tends to support a form of whole mantle convection that is moderated by rheological and phase changes below the transition zone, the geochemical observations have been generally used to support the presence of long-lived and isolated reservoirs. Recent dynamical modeling (Brandenburg et al., EPSL, 2008) employed high resolution finite modeling of mantle convection using an energetically consistent simulation of tectonic plates. A suite of models was developed with a dynamic vigor similar to that of the present day earth. The recycling of oceanic crust combined with a two-stage formation of the continental crust leads to a satisfactory match to the observed spread between HIMU-DMM-EM1 in multiple isotope systems without invoking recycling of continental crust. Due to the rheological contrast between upper and lower mantle there is a natural occurrence of a well-mixed upper mantle overlaying a chemically more heterogeneous lower mantle. The pooling of dense oceanic crust provides the formation of dense piles at the base of the mantle. Together with the occurrence of slabs that thicken and/or stagnate at the 670 discontinuity we find reasonable correspondance with the present day tomographic signatures. At present the models fail to explain noble gas systematics, even when taking the suggested high compatibility of helium into account.

van Keken, P. E.; Brandenburg, J. P.; Hauri, E. H.; Ballentine, C. J.

2009-12-01

65

Salinity changes in the North West Pacific Ocean during the late Pliocene/early Quaternary from 2.73 Ma to 2.52 Ma  

OpenAIRE

Recent research has increasingly advocated a role for the North Pacific Ocean in modulating global climatic changes over both the last glacial cycle and further back into the geological record. Here a diatom ?18O record is presented from Ocean Drilling Program Site 882 over the Pliocene/Quaternary boundary from 2.73 Ma to 2.52 Ma (MIS G6-MIS 99). Large changes in ?18Odiatom of c. 4‰ from 2.73 Ma onwards are documented to occur on a timeframe broadly coinciding with glacial-int...

Swann, George E. A.

2010-01-01

66

Predictions of hydrothermal alteration within near-ridge oceanic crust from coordinated geochemical and fluid flow models  

Science.gov (United States)

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.

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

2001-01-01

67

Structure, porosity and stress regime of the upper oceanic crust: Sonic and ultrasonic logging of DSDP Hole 504B  

Science.gov (United States)

The layered structure of the oceanic crust is characterized by changes in geophysical gradients rather than by abrupt layer boundaries. Correlation of geophysical logs and cores recovered from DSDP Hole 504B provides some insight into the physical properties which control these gradient changes. Borehole televiewer logging in Hole 504B provides a continuous image of wellbore reflectivity into the oceanic crust, revealing detailed structures not apparent otherwise, due to the low percentage of core recovery. Physical characteristics of the crustal layers 2A, 2B and 2C such as the detailed sonic velocity and lithostratigraphic structure are obtained through analysis of the sonic, borehole televiewer and electrical resistivity logs. A prediction of bulk hydrated mineral content, consistent with comparison to the recovered material, suggests a change in the nature of the alteration with depth. Data from the sonic, borehole televiewer, electrical resistivity and other porosity-sensitive logs are used to calculate the variation of porosity in the crustal layers 2A, 2B and 2C. Several of the well logs which are sensitive to the presence of fractures and open porosity in the formation indicate many zones of intense fracturing. Interpretation of these observations suggests that there may be a fundamental pattern of cooling-induced structure in the oceanic crust. ?? 1985.

Newmark, R.L.; Anderson, R.N.; Moos, D.; Zoback, M.D.

1985-01-01

68

Magnetic mapping of (carbonated) oceanic crust-mantle boundary: New insights from Linnajavri, northern Norway  

Science.gov (United States)

The contribution of lower oceanic crust and upper mantle to marine magnetic anomalies has long been recognized, but the detailed magnetic character of this non-volcanic source layer remains to be fully defined. Here, we report preliminary results of a magnetic survey and source characterization of a "carbonated" oceanic Moho (petrological "Mohorovicic discontinuity") sequence observed at the Linnajavri Serpentinite Complex (LSC), northern Norway. The LSC is located at 67° 36'N and 16° 24'E within the upper Allochthon of the Norwegian Caledonides and represents a dismembered ophiolite. Particularly in the southern ("Ridoalggicohkka") area of the LSC, gabbro, serpentinite and its talc-carbonate (soapstone) and quartz-carbonate (listvenite) altered equivalents are extraordinarily well-exposed [1]. An intact oceanic Moho is exposed here, despite its complex tectonic setting. The small degree of arctic rock weathering (? 2 mm weathering surface) allowed for detailed regional-scale surface magnetic mapping across alteration fronts (serpentinite-soapstone; soapstone-listvenite) and lithological contacts (soapstone-gabbro). Magnetic mapping was conducted using a handheld 3-axis magnetometer, surface-towed resistivity meter and Teka surface magnetic susceptometer with sample spacing of 1 m. Geophysical field mapping was combined with petrological observations and scanning SQUID microscopy (SM) mapping conducted on thin sections from rock samples that were drilled along the survey lines. Regional scale magnetic mapping indicates that the total magnetic field across both the "carbonated" Moho and the soapstone-serpentinite interfaces show higher frequency changes in their magnetic anomaly character and amplitudes than the surface-towed resistivity data. SQUID microscopy mapping of both natural remanence magnetization (NRM) and anhysteretic remanence magnetization (ARM) on gabbro, serpentinite, soapstone, and listvenite samples, with a sensor-sample separation of ?190 ?m, show that the distribution of microscopically measurable ferromagnetic and possibly sulfide minerals produces a different bulk intensity for each of the rock types. SM vector magnetic field maps of these samples also reveal that the magnetization associated with these grains (observed as dipole-like fields in SM maps) is variable in direction from grain to grain, which may result from different alteration histories for each grain. These complex magnetization patterns acquired through thermal and chemical alteration history may explain the short wavelength magnetic anomalies observed along our traverse lines. [1] Beinlich, A., Plümper, O., Hövelmann, J., Austrheim, H. and Jamtveit, B. (2012), Terra Nova, in press.

Tominaga, M.; Beinlich, A.; Tivey, M.; Andrade Lima, E.; Weiss, B. P.

2012-12-01

69

The Generation of Oceanic Lithosphere in an Embryonic Oceanic Crust : the Example of the Chenaillet Ophiolite in the Western Alps  

Science.gov (United States)

The Chenaillet Ophiolite exposed in the Franco-Italian Alps represents a well-preserved ocean-floor sequence that was only weakly affected by later Alpine convergence. Based on the similarity between rock types and structures reported from ultraslow spreading ridges and those observed in the Chenaillet Ophiolite, it may represent a field analogue for slow to ultraslow spreading ridges such as the Gakkel Ridge or the Southwest Indian Ridge. Mapping of the Chenaillet Ophiolite enabled to identify an oceanic detachment fault that extends over a surface of about 16 km2 capping exhumed mantle and gabbros onto which clastic sediments have been deposited. The footwall of the detachment is formed by mafic and ultramafic rocks. The mantle rocks are strongly serpentinized lherzolites and subordinate harzburgites and dunites. Microstructures reminiscent of impregnation, and cpx major and trace element chemistry indicate that spinel peridotite is (locally) replaced by plagioclase-bearing assemblages. Pyroxene thermometry on primary minerals indicates high temperatures of equilibration ( max 1200°C) for the mantle rocks. Gabbros range from troctolite and olivine-gabbros to Fe-Ti gabbros and show clear evidence of syn-magmatic deformation, partially obliterated by retrograde amphibolite and low-grade metamorphic conditions. In sections perpendicular to the detachment within the footwall, syn-tectonic gabbros and serpentinized peridotites grade over some tens of meters into cataclasites that are capped by fault gouges. Petro-structural investigations of the fault rocks reveal a syn-tectonic retrograde metamorphic evolution. Clasts of dolerite within the fault zone suggest that detachment faulting was accompanied by magmatic activity. Hydrothermal alteration is indicated by strong mineralogical and chemical modifications. Gabbro and serpentinized peridotite, together with serpentinite cataclasites occur as clasts in tectono-sedimentary breccias overlying directly the detachment fault. Across the whole Chenaillet Ophiolite, volcanic rocks directly overlie either the detachment fault or the sediments. In several places, N-S trending high-angle normal faults have been mapped. These faults truncate and displace the detachment fault leading to small domino-like structures. The basins, limited by these high-angle faults, are some hundreds to a few kilometres wide and few tens to some hundreds of meters deep. Because these high- angle faults are sealed locally by basalts and obliterated by volcanic structures, we interpret them as oceanic structures being active during the emplacement of the basalts. The alignment of porphyritic basaltic dykes parallel to, and their increasing abundance towards the high-angle faults suggest that they may have served as feeder channels for the overlying volcanic rocks. The complex poly-phase tectonic and magmatic processes observed in the Chenaillet Ophiolite are reminiscent of those reported from slow to ultraslow spreading ridges. The key result from our study is that mantle exhumation along detachment faults is followed by syn-magmatic normal faulting resulting in the emplacement of laterally variable, up to 300 meters thick massive lavas and pillow basalts covering the exhumed detachment fault. This implies that off-axis processes are more important as previously assumed and that large-scale detachment faults may be buried under massive volcanic sequences suggesting that detachment faulting is presumably more common than suggested by dredging or morpho-structural investigations of ultra- to slow- spreading oceanic crust.

Masini, E.; Manatschal, G.; Muntener, O.

2007-12-01

70

Uranium isotope systematics of ferromanganese crusts in the Pacific Ocean: Implications for the marine 238U/235U isotope system  

Science.gov (United States)

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.

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

2014-12-01

71

Increase of seismic velocities in upper oceanic crust: The “superfast” Spreading East Pacific Rise at 14°14'S  

Science.gov (United States)

One of the most important observations of oceanic crustal evolution is that seismic compressional wave velocities in the upper igneous crust, layer 2A, increase as the crust ages. The most viable explanation for this phenomenon is decreasing porosity due to filling of open void spaces with hydrothermally generated minerals. But the idea that seismic velocity could be a function of plate age, however, depends primarily on airgun/sonobuoy profiles collected more than 20 years ago [Houtz and Ewing, 1976]. Therefore, in late 1995 new seismic refraction measurements were obtained on the eastern flank of the “superfast” spreading East Pacific Rise south of the Garrett transform. We found that velocities rapidly increase by about 45-50% within only 0.5-1 m.y., double within 5 m.y. or less and remain constant thereafter.

Grevemeyer, Ingo; Weigel, Wilfried

72

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

73

10Be-dating of manganese crust from Central North Pacific and implications for ocean palaeocirculation  

International Nuclear Information System (INIS)

Mass spectrometric measurement of a 10Be profile across the manganese crust VA 13-2 from the Central Pacific, yields growth rates of 2.7 and 4.8 mm Myr-1 for the layers of the crust accumulated between Recent and 6 Myr BP and between 6 and 11 Myr, respectively. Using these measurements as well as 230Th dating, the authors have been able to distinguish boundaries between zones in the crust with different structures and chemical compositions and to assign ages to them. These ages are 0.12, 2.9-3.4, 5.7-6.7, 7-9, 10-12 and 13-16 Myr BP. All of these boundaries apparently coincide with the times reported for Quaternary and late Tertiary palaeoclimatic events, suggesting that the crust growth has been strongly influenced by palaeoclimate. (author)

74

Active-Source Wide-Angle Seismic Observations of a Shallow Mantle Discontinuity Within the 8-9-Ma Juan de Fuca Oceanic Plate Off Cascadia Subduction Zone  

Science.gov (United States)

In June-July 2012 we conducted an active-source seismic study of the Juan de Fuca (JdF) oceanic plate. This two-ship experiment (cruises RV Langseth MGL1211 and RV Oceanus OC1206A) acquired coincident two-dimensional multichannel seismic reflection data and long-offset wide-angle reflection/refraction data using ocean bottom seismometers (OBSs) to study the evolution and state of hydration of this young oceanic plate prior to subduction beneath the Cascadia margin. Data were collected along two spreading-parallel profiles extending from zero-age crust at the JdF Ridge to the Cascadia deformation front, and along a margin-parallel profile a few km seaward off the deformation front over 8-9 Ma crust. OBS record sections from the margin-parallel profile show the expected crustal (Pg) and shallow mantle (PmP and Pn) P-wave arrivals, with the Pg-PmP-Pn triplication typically observed at 40-50 km source-receiver offsets. For larger offsets, data from OBSs deployed along the southern portion of the profile show clear sub-Moho reflections characterized by a high-amplitude triplication at 140-160 km source-receiver offsets. We will use a combination of travel-time tomography and waveform forward modeling to constrain some of the properties of the upper mantle seismic discontinuity that produces the observed wide-angle reflections: its depth, thickness, and P-wave velocity structure (i.e., velocity inversion or positive impedance contrast). We expect that our observations and models will provide new constraints on the nature of upper mantle discontinuities, contributing to our understanding of the development of the lithosphere-asthenosphere boundary and/or other intra-lithospheric structures beneath young oceanic plates.

Canales, J.; Carton, H. D.; Carbotte, S. M.; Nedimovic, M. R.

2012-12-01

75

Nd isotopic structure of the Pacific Ocean 70-30 Ma and numerical evidence for vigorous ocean circulation and ocean heat transport in a greenhouse world  

Science.gov (United States)

The oceanic meridional overturning circulation (MOC) is a crucial component of the climate system, impacting heat and nutrient transport, and global carbon cycling. Past greenhouse climate intervals present a paradox because their weak equator-to-pole temperature gradients imply a weaker MOC, yet increased poleward oceanic heat transport appears to be required to maintain these weak gradients. To investigate the mode of MOC that operated during the early Cenozoic, we compare new Nd isotope data with Nd tracer-enabled numerical ocean circulation and coupled climate model simulations. Assimilation of new Nd isotope data from South Pacific Deep Sea Drilling Project and Ocean Drilling Program Sites 323, 463, 596, 865, and 869 with previously published data confirm the hypothesized MOC characterized by vigorous sinking in the South and North Pacific ~70 to 30 Ma. Compilation of all Pacific Nd isotope data indicates vigorous, distinct, and separate overturning circulations in each basin until ~40 Ma. Simulations consistently reproduce South Pacific and North Pacific deep convection over a broad range of conditions, but cases using strong deep ocean vertical mixing produced the best data-model match. Strong mixing, potentially resulting from enhanced abyssal tidal dissipation, greater interaction of wind-driven internal wave activity with submarine plateaus, or higher than modern values of the geothermal heat flux enable models to achieve enhanced MOC circulation rates with resulting Nd isotope distributions consistent with the proxy data. The consequent poleward heat transport may resolve the paradox of warmer worlds with reduced temperature gradients.

Thomas, Deborah J.; Korty, Robert; Huber, Matthew; Schubert, Jessica A.; Haines, Brian

2014-05-01

76

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

DEFF Research Database (Denmark)

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. ?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 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.

Dahl, Tais Wittchen; Canfield, Donald Eugene

2011-01-01

77

Microbial communities in recent and 10 - 28 Ma ocean floor basalt (ODP Leg 187)  

Science.gov (United States)

Previous studies have shown that microbial communities are harboring ocean crust basalt (e.g., Thorseth et al. 1995). The non-hydrothermal regions of ocean ridges are largely unstudied with respect to microbial diversity and physiology. In the present study, the microbial communities resident in samples of recent (Jaccard's algorithm, and species richness was estimated using Shannon's index. Furthermore, individual DNA bands were excised from the gel and sequenced to evaluate the phylogenetic affiliation of the endolithic microbes. Shannon indices show that the species richness of microbial communities in basalt is higher for seafloor samples (Arctic Ridges) than for subsurface samples (Southeast Indian Ridge). The microbial population in the Arctic Ridge basalt samples affiliates with ten major lineages of the domain Bacteria and 1 major lineage of Archaea. Bacteria in the ODP Leg 187 basalt samples affiliate with six major lineages of the domain Bacteria, whereas no archaeal sequences were retrieved from these samples. Many sequences from both areas appear to be unaffiliated with any previously isolated microbes. The uncultured green nonsulfur bacterium Chloroflexales Arctic 96BD-6, and the three gamma proteobacteria Acinetobacter junii, Pseudoalteromonas sp., and Shewanella frigidimarina affiliate with sequences from both the Arctic Ridges and the Southeast Indian Ridge. The physiological groups of iron and manganese reducers appear to be common in basalt samples, but the major part of the microbial populations can not be assigned to a specific metabolic pathway. There is probably a significant overlap in microbial communities between basalt, sediment, and seawater due to the close contact of the environments. However, part of the microbial population appear to be unique to the basalt environment. Reference: Thorseth, I.H., Torsvik, T., Furnes, H., and Muehlenbachs, K. 1995. Microbes play an important role in the alteration of oceanic crust. Chem. Geol. 126: 137-146.

Lysnes, K.; Steinsbu, B. O.; Einen, J.; Thorseth, I. H.; Pedersen, R. B.; Torsvik, T.

2003-04-01

78

Growth of continental crust and its episodic reworking over >800 Ma: evidence from Hf-Nd isotope data on the Pietersburg block (South Africa)  

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The formation and evolution of the continental crust during the Precambrian, and in particular during the Archaean eon (4.0-2.5 Ga), is still a matter of debate. In particular, it is not yet clear in which tectonic environment the genesis of crust took place and how the large volume of granitoid rocks that form ~70% of the Archaean crust were extracted from the mantle. Many studies highlighted that radiogenic isotope systems, especially Lu-Hf and Sm-Nd, are powerful tools to unravel the respective extent of crustal growth and recycling in Archaean terranes. This work presents coupled Hf and Nd isotope data (analyzed both in situ in accessory minerals and in whole rock samples) of Meso- to Neoarchaean granitoids, applied to unravel the processes of crust formation and evolution of the Pietersburg crustal block in South Africa. This crustal segment, the northermost one of the Archaean Kaapvaal Craton, is separated from older crust (3.65-3.10 Ga) by a large-scale suture zone, and the processes related to amalgamation of both blocks and their subsequent evolution are still unclear. The Pietersburg block is made up of a wide range of Archaean granitoid rocks, including tonalite-trondhjemite-granodiorite (TTG) series, high-K monzogranites as well as (grano)diorites belonging to the so-called "sanukitoid" group [1], all intruded by late Paleoproterozoic alkaline complexes. Age determinations highlighted two stages of granitoid formation: (1) TTG magmatism took place episodically over >400 Ma between 3.34 and 2.89 Ga, with a major pulse at 2.97-2.90 Ga; while (2) all the other (high-K) granitoid types emplaced subsequently between 2.84 and 2.69 Ga before a long magmatic shutdown until the intrusion of alkaline complexes at ~2.00 Ga [2-3]. Isotope systematics reveal that these two stages are related to juvenile crust formation and crust reworking, respectively. Indeed, all Hf-Nd isotope data from TTG gneisses are suprachondritic, pointing to a juvenile origin and precluding any incorporation of older crust from the core of the Kaapvaal craton. In contrast, all data from the younger granitoids, including ~2 Ga-old alkaline complexes, plot along a single, well-defined trend of decreasing ?Hf-Nd towards youngest ages. This trend points to model ages around 2.95-3.05 Ga, which is that of the youngest TTGs, indicating that all ?2.84 Ga-old granitoids formed by episodic reworking, over >800 Ma, of a single crustal reservoir represented by these TTGs. Interestingly enough, this reworking took place "in situ" by both (1) classical intracrustal differentiation (formation of high-K monzogranites) and (2) erosion, sedimentation and recycling of the resulting detrital material in the mantle, producing a hybrid mantle source from which derived the "sanukitoid" magmas and, much later, the alkaline complexes. All these observations are consistent if the Pietersburg block is regarded as an accretionary, Cordilleran-type orogen, formed along the northern margin of the Kaapvaal block by successive amalgamation of small TTG terranes between 3.34 and 2.90 Ga, as a result of episodic subduction dynamics. This gave rise to (1) a juvenile, accretionary crustal terrane and (2) a (isotopically similar) lithospheric mantle source, hybridized with crustal components derived from it. All the younger granitoids thus reult from significant reworking of these two reservoirs, first during ongoing convergence and continental collision between 2.84 and 2.69 Ga; and second, in response to lithosphere heating (intrusion of the Bushveld Complex?) in a within-plate environment at 2.00 Ga. [1] Laurent O. et al., submitted to Lithos [2] Zeh A. et al., 2009. Journal of Petrology 50(5), 933-966 [3] Laurent O. et al., 2013. Precambrian Research 230, 209-226

Laurent, Oscar; Zeh, Armin; Moyen, Jean-François; Doucelance, Régis; Martin, Hervé

2014-05-01

79

Continental growth through time by underplating of subducted oceanic crust: Evidence from kimberlites in South Africa and SW Pacific  

Science.gov (United States)

In the dynamic model of plate tectonics, it is evident that crustal components are returned to the mantle by subduction. Chemical signatures of these subducted components were identified in ocean island volcanics and in island arc volcanics. Indeed, an origin involving a subducted protolith was postulated for certain types of xenoliths in kimberlite, including diamonds. Recent studies of eclogite xenoliths in kimberlite from southern Africa and megacrysts form the Malaitan alnoite, Solomon islands, indicate that lithospheric underplating by subducted oceanic crust has occurred in these two contrasting areas. The results of new eclogite studies from the Bellsbank kimberlite, South Africa, and isotopic data from the Malaitan alnoite megacryst suite. This forms the basis for discerning the role of lithospheric underplating in the growth of cratons and in the evolution of mantle-derived magma.

Taylor, Lawrence A.; Neal, Clive R.

1988-01-01

80

Continental growth through time by underplating of subducted oceanic crust: evidence from kimberlites in South Africa and SW Pacific  

International Nuclear Information System (INIS)

In the dynamic model of plate tectonics, it is evident that crustal components are returned to the mantle by subduction. Chemical signatures of these subducted components were identified in ocean island volcanics and in island arc volcanics. Indeed, an origin involving a subducted protolith was postulated for certain types of xenoliths in kimberlite, including diamonds. Recent studies of eclogite xenoliths in kimberlite from southern Africa and megacrysts form the Malaitan alnoite, Solomon islands, indicate that lithospheric underplating by subducted oceanic crust has occurred in these two contrasting areas. The results of new eclogite studies from the Bellsbank kimberlite, South Africa, and isotopic data from the Malaitan alnoite megacryst suite. This forms the basis for discerning the role of lithospheric underplating in the growth of cratons and in the evolution of mantle-derived magma

81

Atypical oceanic crust and uppermost mantle around the Oki-Daito Escarpment in the West Philippine Basin  

Science.gov (United States)

The Oki-Daito Escarpment (ODE), formerly identified as Lapu-Lapu Ridge, is a 300 km-long WNW-ESE trending submarine fault with a maximum offset over 1,000 m and located at about 250 km south from the Oki- Daito Ridge in the West Philippine Basin. The depths of the oceanic basin around ODE range 5,500-6,100 m and the distinctive seafloor fabric in the basin is apparently associated with seafloor spreading. However, the trend of the fabric is not simple: south of the escarpment, the abyssal hills trend NW-SE, whereas to the north of it, they are N-S direction. The minimum water depth of the ODE is less than 5,000 m and low Bouguer gravity anomaly beneath the ODE indicates the existence of a thicker crust. In the previous seismic experiment in 2004, we obtained a P-wave velocity model to the north of the ODE. Two remarkable differences from normal oceanic crust/mantle were found: (1) a low velocity layer in the lower crust and (2) a very high P-wave velocity layer of 8.6 km/s at about 2 km below the Moho. In 2005, we conducted five wide-angle seismic refraction and single/multi-channel reflection experiments in order to investigate the extent of such peculiar crust and uppermost mantle in this region. The results showed the low velocity layer in the lower crust was not detected beneath the profile perpendicular to the 2004 experiment and other four profiles. The uppermost mantle structure varies in places and depths. Some profiles show 8.5-8.6 km/s layer exists just beneath the Moho, and in the other profiles, over 8.5 km/s layer appears within 5 km deep from the Moho. These out of the ordinary characteristics of the seismic velocity structures can provide important clues on the evolution model of the West Philippine Basin.

Nishizawa, A.; Oikawa, M.; Kaneda, K.; Kasahara, J.

2006-12-01

82

Elastic anisotropy in subducted oceanic crust and sediments: estimates from a paleo-subduction channel, Eastern Alps  

Science.gov (United States)

In many subduction zones on Earth oceanic plates are subducted below continents. The downgoing oceanic plates consist of a variably thick sediment layer, pillow basalts, sheeted dykes and gabbros of the oceanic crust, and serpentinite and peridotite of the lithospheric mantle. Globally, most sediment is subducted towards great depth along with the oceanic crust in the so-called subduction channel, and both rock types suffer high-strain deformation and high-pressure metamorphism. Thickness, internal structure and physical properties of subduction channels remain enigmatic in seismic imaging, yet a better understanding is vital to constrain models of deformation behaviour, stress build-up and release, and earthquake generation. Here we report the results of a comprehensive analysis of textures (determined by neutron diffraction) and elastic anisotropy (P and S waves) of eclogites, retrogressed eclogites and metasediments from the south side of the Tauern Window, Eastern Alps (Austria), which constituted part of a subduction channel in the Tertiary, and was rapidly exhumed during later continental collision, preserving high-pressure deformation fabrics. All studied rock types show evidence for substantial plastic deformation, and exhibit corresponding crystallographic- (CPO) and shape- (SPO) preferred orientations. CPO (quartz, calcite, micas) is strongest in the metasediments, and elastic anisotropy is 10-12 % (P-waves), and up to 28% (S-waves) in experiments conducted under up to 600 MPa confining pressure. Fresh eclogite has anisotropies of about 11% (P-waves) and 5% (S-waves). Retrogression to amphibolite reduces these values to 4% (P-waves) and 3% (S-waves). In the metasediments about half of the anisotropy may be attributed to the CPO. In the eclogites this relation is less clear, and most of the anisotropy is probably related to the SPO fabric. In general we conclude that subduction channel fills may be highly anisotropic indeed, and wave propagation models may need to take this possibility into account. _

Behrmann, J. H.; Ullemeyer, K.; Stipp, M.

2011-12-01

83

Intermediate crust (IC); its construction at continent edges, distinctive epeirogenic behaviour and identification as sedimentary basins within continents: new light on pre-oceanic plate motions  

Science.gov (United States)

Introduction. The plate tectonics paradigm currently posits that the Earth has only two kinds of crust - continental and oceanic - and that the former may be stretched to form sedimentary basins or the latter may be modified by arc or collision until it looks continental. But global analysis of the dynamics of actual plate motions for the past 150 Ma indicates [1 - 3] that continental tectospheres must be immensely thicker and rheologically stiffer than previously thought; almost certainly too thick to be stretched with the forces available. In the extreme case of cratons, these tectospheric keels evidently extend to 600 km or more [2, 3]. This thick-plate behaviour is attributable, not to cooling but to a petrological 'stiffening' effect, associated with a loss of water-weakening of the mineral crystals, which also applies to the hitherto supposedly mobile LVZ below MORs [4, 5]. The corresponding thick-plate version of the mid-ocean ridge (MOR) process [6 - 8], replacing the divergent mantle flow model, has a deep, narrow wall-accreting axial crack which not only provides the seismic anisotropy beneath the flanks but also brings two outstanding additional benefits:- (i) why, at medium to fast spreading rates, MOR axes become straight and orthogonally segmented [6], (ii) not being driven by body forces, it can achieve the sudden jumps of axis, spreading-rate and direction widely present in the ocean-floor record. Furthermore, as we will illustrate, the crack walls push themselves apart at depth by a thermodynamic mechanism, so the plates are not being pulled apart. So the presence of this process at a continental edge would not imply the application of extensional force to the margin. Intermediate Crust (IC). In seeking to resolve the paradox that superficially extensional structures are often seen at margins we will first consider how this MOR process would be affected by the heavy concurrent sedimentation to be expected when splitting a mature continent. I reason that, by blocking the hydrothermal cooling widely seen along MOR axes this must inhibit the freezing-in of diagnostic spreading-type magnetic anomalies and would prolong magmagenesis to give a thicker-than-oceanic mafic crust. I have called this Intermediate Crust (IC) [9, 10], to distinguish it from Mature Continental Crust (MCC). Plate separation will continue to generate IC along the margins for as long/far as the sedimentation input is sufficient to have this effect. Transition to the MOR process will then follow. But if, contrary to the general plate tectonics assumption, based on body forces, plate separation ceases after a limited separation (or perhaps several in differing directions), without proceeding to the oceanic condition, the resulting IC areas will be incorporated within the continent [11]. Where does this lead us? With examples drawn from 40 years' study, I will contend that this is indeed the way the Earth has worked and that it offers potential plate kinematic explanation of the origin of the block-and-sedimentary basin layouts abundantly present in the non-craton areas of continents. I will show that in some cases the intricacy of block outlines and the precision with which they can be fitted together in a kinematically consistent manner rules out that this was purely by chance. The evidently meaningful character of those outlines means that they have been drawn by a narrow-crack separative mechanism which reflects that of our new MOR model. To provide a basis for such Plate Kinematic Analysis (PKA) we now link and compare some features of IC-formation at continental edges and of the crust of sedimentary basins. Characteristics of IC and of sedimentary basin crust (SBC). 1. IC basement, with expected seismic Vp around 6km/s, must look deceptively like that assigned to supposedly stretched MCC. 2. For thermodynamic reasons, the hydrous metamorphic content of deep MCC and of deeply subducted UHP slices of it gives them a big thermal epeirogenic sensitivity which IC lacks. Calculation [8, 9] shows that this type of process yields some 12-30 t

Osmaston, Miles F.

2014-05-01

84

The magmatic-hydrothermal transition in the lower oceanic crust: Clues from the Ligurian ophiolites, Italy  

Science.gov (United States)

The gabbroic bodies from the Jurassic Ligurian ophiolites are structurally and compositionally similar to the gabbroic sequences from the oceanic core complexes of the Mid Atlantic Ridge. Initial cooling of the Ligurian gabbros is associated with local development of hornblende-bearing felsic dykes and hornblende vein networks. The hornblende veining is correlated with the widespread development of hornblende as coronas/pseudomorphs after the clinopyroxene in the host gabbros. In addition, the studied gabbroic body includes a mantle sliver locally containing hornblende gabbros and hornblendite veins. The hornblendes from the felsic dykes and the hornblende-rich rocks within the mantle sliver show a similar geochemical signature, characterized by low Mg#, CaO and Al2O3, negligible Cl, and high TiO2, K2O, REE, Y, Zr and Nb concentrations. The whole-rock Sm-Nd isotopic compositions of the felsic dykes and the hornblende-rich rocks define a Sm-Nd isochron corresponding to an age of 154 ± 20 Ma and an initial ?Nd of 9.2 ± 0.5. The ?18O of the hornblendes and coexisting zircons from these rocks (about +4.5‰ and +5.8‰, respectively) do not indicate the presence of a seawater component in these melts. The formation of the felsic dykes and of the hornblende-rich rocks within the mantle sliver involved SiO2-rich silicate melts with negligible seawater component, which presumably were derived from high degree fractional crystallization of MOR-type basalts. The vein and the coronitic/pseudomorphic hornblendes show high Mg# and CaO, significant Cl (0.02-0.17 wt%) and low TiO2 and K2O concentrations. The coronitic/pseudomorphic hornblendes have trace element compositions similar to those of the clinopyroxenes from the gabbros and ?18O values (+1.0‰ to 0.7‰) close to seawater, suggesting an origin by reaction between migrating seawater-derived fluids and the host gabbros. The vein hornblendes commonly show slight LREE enrichment, relatively high concentrations of Nb (up to 2.5 ppm) and ?18O ranging from +3.7‰ to +0.8‰. The crystallization of these hornblendes most likely involved both seawater and magmatic components.

Tribuzio, Riccardo; Renna, Maria Rosaria; Dallai, Luigi; Zanetti, Alberto

2014-04-01

85

Petrology and Geochemistry of Eclogite Xenoliths from the Colorado Plateau: Implications for the Evolution of Subducted Oceanic Crust  

Science.gov (United States)

Eclogite xenoliths from the Colorado Plateau, interpreted as representing fragments of the subducted Farallon plate, are used to infer the evolution of trace element and isotopic compositions of oceanic crust subducted into the subarc mantle. The xenoliths consist of almandine-rich garnet, Na-clinopyroxene, lawsonite and zoisite with minor amounts of phengite, rutile, pyrite and zircon. They have essentially basaltic bulk-rock major element compositions, except for significantly higher Na2O, but similar K2O, with increasing SiO2 contents, compared to altered MORB. These major element characteristics are explained by spilitization/albitization during hydrothermal alteration at a mid-ocean ridge and by subduction zone metasomatism in the forearc region. Whole-rock trace elements and Sr, Nd, and Pb isotopic compositions of the xenoliths are variable and exhibit significantly enriched characteristics compared to altered MORB, except for similar Zr/Hf ratios of 36.9 to 37.6. Mass balance for the Colorado Plateau eclogite xenoliths can be achieved for 26 trace elements, Rb, Cs, Sr, Ba, Y, REE, HFSE (Zr, Hf, Nb, and Ta), Pb, Th and U. The mass balance calculations and mineralogical observations corroborate that whole-rock chemistry of the xenoliths were contaminated by near-surface processes after eruption and limited interaction with the serpentinized ultramafic microbreccia host magma. Thus, Sr, Nd and Pb isotopic compositions of separated minerals from the xenoliths were measured to avoid these secondary effects; these separates yield distinctively enriched isotopic compositions in the range of 0.70502 to 0.70590 for 87Sr/86Sr, -1.5 to -3.1 for ? Nd and 18.928 to 19.052 for 206Pb/204Pb. This suggests that the xenoliths were metasomatized by a fluid equilibrated with the sedimentary layer probably covering the Farallon plate in the forearc region. This metasomatism resulted in the xenoliths acquiring distinctively enriched isotopic compositions compared with those of altered MORB. Some of the distinct isotopic signatures observed in OIBs compared to those from MORBs have been interpreted as a result of oceanic sediment subducted deep into the mantle. Our results, on the contrary, suggest an alternative possibility that these anomalous isotopic reservoirs in the mantle are formed by the subduction of oceanic crust modified by the metamorphic fluid from the covering sedimentary rocks.

Usui, T.; Nakamura, E.; Helmstaedt, H.

2004-12-01

86

The magma ocean from the Fra Mauro shoreline - An overview of the Apollo 14 crust  

Science.gov (United States)

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.

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

1983-01-01

87

The magma ocean from the Fra Mauro shoreline - an overview of the Apollo 14 crust  

Science.gov (United States)

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.

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

1983-11-01

88

Eoarchean crust is not that rare: Widespread pre-3750 Ma supracrustal rocks from the Nuvvuagittuq supracrustal belt, northern Québec.  

Science.gov (United States)

Geochemistry and U-Pb ion microprobe zircon geochronology guided by high-resolution mapping (1:50 scale) supports a minimum age of ca. 3750 Ma for supracrustal rocks from the Nuvvuagittuq supracrustal belt (NSB) in the Inukjuak domain of the northern Superior Province (Minto Block), Québec (Canada). Field relations, mineralogy and geochemistry at the Porpoise Cove locality describe a supracrustal succession of mafic amphibolites and ultramafic rocks, finely banded quartz-magnetite units with intermixed coarse-grained ferruginous quartz-pyroxene rocks, and quartz-biotite schists that superficially resemble polymict meta- conglomerates with large (up to 10 cm) deformed polycrystalline quartz and mafic clasts. All units preserve sharp lithological contacts. Narrow (dominantly trondhjemitic) orthogneiss sheets locally preserve intrusive contact relationships to the supracrustal enclave. The enclave is strongly deformed and the full deformation history appears to be shared by all units with later modifications from leucogranitoid intrusions. The quartz- biotite schists record complex zircon growth at 3500 and 2800 Ma, interpreted to reflect metamorphic events. Zircons separated from orthogneisses in the enclave including one sheet that transects a banded quartz- pyroxene(magnetite) unit yield magmatic 207Pb/206Pb ages of ca. 3750 Ma. These ages are slightly younger than earlier provisional reports for an NSB orthogneiss from Porpoise Cove. The Nuvvuagittuq supracrustals are the oldest rocks thus far reported for the Minto Block, they are locally abundant in the Inukjuak domain, they overlap in age with the ca. 3.7-3.83 Ga Isua supracrustal belt and Akilia association rocks in West Greenland, and represent an important new data set for exploration of the early Earth. Every year more and more pre-3.6 billion year old rocks of sedimentary origin are discovered. This means that our understanding of this pivotal time period will surely continue to grow.

Cates, N. L.; Mojzsis, S. J.

2006-12-01

89

Lithium isotope as a proxy for water/rock interaction between hydrothermal fluids and oceanic crust at Milos, Greece  

Science.gov (United States)

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.

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

2014-05-01

90

Can Fractional Crystallization of a Lunar Magma Ocean Produce the Lunar Crust?  

Science.gov (United States)

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

Rapp, Jennifer F.; Draper, David S.

2013-01-01

91

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

Science.gov (United States)

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. The ages determined by this technique (site 261 - 121.4 ±1.6m.y.; site 462A - 105.1 ±2.8m.y.; site 516F - 69.9 ±2.4m.y.) are 34, 54 and 18 m.y. younger, respectively, than the age of crust formation at the site; in the case of site 462A, the young age is clearly related to off-ridge emplacement of a massive sill/flow complex. At the other sites, either the hydrothermal circulation systems persisted longer than for normal crust (10-15 m.y.), or were reactivated by off-ridge igneous activity. Celadonites show U and Pb contents and Pb isotopic compositions little changed from their basalt precursors, while Th 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 seawaterdominated systems with effective water/rock ratios of 20-200.

Hart, S. R.; Staudigel, H.

1986-12-01

92

Subduction-modified oceanic crust in the sources of continental picrite dikes from the Karoo LIP?  

Science.gov (United States)

The Ahlmannryggen mountain range in East Antarctica hosts unusual LILE-depleted, but Fe- and Ti-enriched ultramafic dikes (Group 3) that belong to the Jurassic (~180 Ma) Karoo continental flood basalt (CFB) province. Their high initial ?Nd (+5 to +9) indicates their origin within the sublithospheric mantle beneath the Gondwana supercontinent. Using the new Pb and Os isotopic data and previously published geochemical and mineral chemical data, we try to constrain their mantle sources. The dikes that lack evidence of crustal contamination exhibit very radiogenic ?Nd (+8.6 to +9.0), relatively radiogenic 206Pb/204Pb (18.2-18.4) and 87Sr/86Sr (0.7035-0.7037), and unradiogenic 187Os/188Os (0.124-0.125) at 180 Ma. These isotopic compositions are unlike those typical of MORBs, excluding depleted mantle as the sole source contributor. The Pb isotopic composition of the dikes plots close to the 4.43 Ga geochron and hence is compatible with derivation from an early-depleted reservoir (EDR), recently suggested to be a major source component in CFBs. However, the high ?Nd of the dikes exceeds the ?Nd estimated for EDR (+4.9 to +8.5 at 180 Ma) and the relative Nb, Fe, and Ti enrichment (pyroxenite fingerprint) of the dikes is not readily ascribed to EDR source. Based on our isotopic and trace element modeling, we regard that the mantle source of the picrite dikes contained seawater-altered and subduction-modified MORB with a recycling age of 0.8 Ga. Such a source component would explain the unusual combination of elevated initial 87Sr/86Sr, ?Nd, and 206Pb/204Pb, relative depletion in fluid-mobile LILE, U, Th, Pb, and LREE, and relative enrichment in Nb, Fe, Ti, and other HFSE. Behavior of Re and Os in subduction environments is not well constrained, but loss of Re from recycled MORB, as observed in some subduction-associated eclogites and blueschists, and predominant contribution of Os from depleted peridotite matrix could have produced the observed low 187Os/188Os. Pyroxenite sources also are consistent with mineral chemical data (e.g., high-Ni olivine) for the picrite dikes. Such peculiar sources were likely not a predominant component in Karoo magmatism in general. Nevertheless, less subduction-modified or more enriched (e.g., additional sediment component) recycled crustal signatures would be difficult to distinguish from the 'lithospheric signatures' of many common CFBs. In addition to depleted mantle or EDR components that have been identified in the high-Mg dikes of the adjacent Vestfjella mountain range, a variety of recycled source components could thus be hiding in the geochemical jungle of the Karoo (and other) CFBs.

Heinonen, J. S.; Carlson, R. W.; Riley, T. R.; Luttinen, A. V.; Horan, M. F.

2013-12-01

93

Multichannel seismic images of the oceanic crust's internal structure: Evidence for a magma chamber beneath the Mesozoic Mid-Atlantic Ridge  

Science.gov (United States)

Near the base of the Mesozoic oceanic crust of the western North Atlantic we recognize a discontinuous, diffractive layer, Horizon R. Moho is a distinct, uniform, and more continuous horizon beneath. Variations in “seismic thickness” of the crust occur primarily through changes in the R-to-Moho layer, the maximum thickness occurring midway between fracture zones. We propose that Horizon R may mark the top of a layered cumulate sequence that formed on the floor of a relatively persistent crustal magma chamber. The crust having minimum thickness is spatially restricted to a small strip near fracture zones and comprises only the super-R unit. We further propose that this implies the sustained accretion of normal oceanic crust at the Mesozoic Mid-Atlantic Ridge required the presence of a magma chamber. *P. Buhl, J. B. Diebold, J. W. Ladd (Lamont-Doherty Geological Observatory, Palisades, New York 10964); P. L. Stoffa, J. D. Phillips (University of Texas, Institute for Geophysics, Austin, Texas 77550); K. Hinz, H. Meyer (Bundesanstalt für Geo-wissenschaften und Rohstoffe, Hannover, Federal Republic of Germany); R. S. Detrick (University of Rhode Island, Graduate School of Oceanography, Kingston, Rhode Island 02881)

Mutter, John C.; North Atlantic Transect (Nat) Study Group

1985-09-01

94

Oceanic crust production in the Dinarides during the Senonian: combined U-Pb in situ laser ablation ICP(MC)-MS zircon and mineral separates Ar-Ar dating  

International Nuclear Information System (INIS)

Full text: Serbian ophiolitic belts are part of ophiolite occurrences traced from the Alps to the Himalayas. Even though they represent one of the largest ophiolitic areas in the world, they are only poorly studied. Our ongoing project presents a comprehensive study of remnants of magmatic members of oceanic crust from Bosnia, Serbia and Macedonia. Here, we present new geochemical data for the area with focus on new U-Pb and Ar-Ar ages for mafic rocks from the Dinarides. It is thought that closure of oceanic domains present in the area of the Dinarides finished latest in early Cretaceous time, based on K-Ar ages of amphibolites from metamorphic sole and over-step sequence conglomerates. The severely altered rocks (LOI up to 9 %, high mobile element content) are characterized by low Ni and Cr contents and Mg number (66-44), implying that fractionation of mafic minerals was one of the processes modifying these magmas. Based on trace element geochemistry, especially REE patterns, rocks display MORB (depleted LREE compared to HREE,) and marginal basin basalt (somewhat elevated LREE over HREE,) signatures. The isotopic data, measured on leached samples, show more radiogenic 87Sr/86Sr (0.703854 - 0.704317), unradiogenic 143Nd/144Nd (0.512860 - 0.512733), and radiogenic 206Pb/204Pb (18.478 - 20.284), 208Pb/204Pb (38.485 - 39.957), and 207Pb/204Pb (15.611 - 15.709) vap>Pb/204Pb (15.611 - 15.709) values when compared with recent MORB. Although the Sr and Pb composition could reflect see water contamination, the Nd isotopic composition suggests a subduction-related setting for the generation of these magmas. We used a novel approach in ophiolite research by combining U-Pb and Lu-Hf zircon analyses by in situ laser ablation ICP-(MC)-MS and Ar-Ar dating. Combined Hf (in zircon) and Nd (on whole rock samples) data give the most accurate information about the composition of the source. First data show that zircon from a gabbro have 176Hf/177Hf slightly higher than chondritic values (0.282730 ± 0.000028; 2SD; ?Hf of 0.8 ± 1.0) which is compatible with a subduction-related setting.Our combined Ar- Ar and U-Pb dating show that - at least locally in the Dinarides - oceanic crust formed in the Senonian. The U-Pb zircon ages show that gabbros crystallized at 103.2 ± 1.5 Ma and confirm the amphibole Ar-Ar ages (109 ± 17 Ma). Pillow lavas from another Dinaric locality gave even younger plagioclase Ar-Ar ages (61 ± 14 Ma). Commonly it was thought that oceanic domains in the Dinarides had closed no later than the early Cretaceous. Our data, however clearly, suggest that oceanic crust was produced for a long time and lasted at least till Campanian/Maastrichtian times. It is currently too early to make new geotectonic interpretation but these results give completely new insights in our understanding of geodynamic development of this part of Balkans. (author)

95

Rock uplift and exhumation of continental margins by the collision, accretion, and subduction of buoyant and topographically prominent oceanic crust  

Science.gov (United States)

Understanding the causes of rock and surface uplift is important because they control the location of mountain building, depocenters, and drainage characteristics and can influence climate. Here we combine previous thermochronological data with field observations to determine the amount of exhumation, rock, and surface uplift that occurs in the upper plate of Central and South American subduction zones during the collision, accretion, and subduction of oceanic plateaus and aseismic ridges. The collision of buoyant and topographically prominent oceanic plateaus and ridges can drive at least 5 km of rock uplift within 2 Ma. Uplift appears to be an immediate response to collision and is generally independent of the slab dip. The amount of rock uplift is controlled mainly by excess topography associated with the ridge (ultimately linked to buoyancy) and erosion, while it is also influenced by the strength of the subduction interface related to the presence of volcanic asperities and overpressured sediments in the subduction channel. The quantity of exhumation is strongly dependant on climate-induced erosion and the lifespan over which the topography is uplifted and supported. Sediment draining into the trench may leave the elevated ridge axis sediment starved, increasing the shear stresses at the ridge subduction interface, leading to positive feedback between ridge subduction, rock uplift, and exhumation. Trench-parallel variations in exhumation have a direct impact on exploration paradigms for porphyry-related metalliferous deposits, and it is likely that porphyry systems are completely eroded by the impingement of plateaus and aseismic ridges within temperate and tropical climates.

Spikings, Richard; Simpson, Guy

2014-05-01

96

Osmium isotope stratigraphy of a marine ferromanganese crust  

Science.gov (United States)

Ferromanganese crusts provide records of long term change in ocean circulation and continental weathering. However, calibrating their age prior to 10 Ma has been entirely based on empirical growth rate models using Co concentrations, which have inherently large uncertainties and fail to detect hiatuses and erosional events. We present a new method for dating these crusts by measuring their osmium (Os) isotope record and matching it to the well-known marine Os isotope evolution of the past 80 Ma. The well-characterised crust CD29-2 from the central Pacific, was believed to define a record of paleooceanographic change from 50 Ma. Previous growth rate estimates based on the Co method are consistent with the new Os isotope stratigraphy but the dating was grossly inaccurate due to long hiatuses that are now detectable. The new chronology shows that it in fact started growing prior to 70 Ma in the late Cretaceous and stopped growing or was eroded between 13.5 and 47 Ma. With this new technique it is now possible to exploit the full potential of the oceanographic and climatic records stored in Fe-Mn crusts. ?? 2005 Elsevier B.V. All rights reserved.

Klemm, V.; Levasseur, S.; Frank, M.; Hein, J.R.; Halliday, A.N.

2005-01-01

97

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)

98

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

Scientific Electronic Library Online (English)

Full Text Available SciELO Brazil | Language: English Abstract in portuguese 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.

Emilson Pereira, Leite; Naomi, Ussami.

2006-09-01

99

Characterization of the microbial community in a legacy borehole in the igneous ocean crust  

Science.gov (United States)

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.

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

2012-12-01

100

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  

Science.gov (United States)

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.

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

2014-05-01

101

Relics of Triassic-Jurassic Meliata Ocean crust in the Western Carpathians: some constraints on the back-arc basin closure in the western Tethysn  

Science.gov (United States)

Hypothetical Meliata (or Meliata-Hallstatt) Ocean was deduced from isolated occurrences of small tectonic slices, mélanges and recycled sedimentary material (clasts) located along the northwest and southeast borders of the lithospheric block referred to as ALCAPA, which included the area of the Eastern Alps, Western Carpathians and also Panonnian area northwest of the Middle Hungarian Zone. Fragmentary character of preserved relics of the Meliata Ocean crust in the form of an incomplete dismembered ophiolite sequence is typical feature. Deep-sea sediments, basaltic pillow lavas and lava flows are most widespread rock types. Most outcrops of these rocks are concentrated in the inner Western Carpathians (Meliatic Unit). Trace element distribution in basalts reveal suprasubduction zone environment of the Meliata Ocean formation. This ocean was probably founded and opened inboard of the Permian volcanic arc, the vestiges of which have been identified in the inner and southern central Western Carpathians. Analogically to recent back arc basins arc-like (IAT) or back-arc basin basalts (BABB) were generated during initial stage of the basin opening and were erupted in the environment of carbonaceous and pelitic clastic sediments. In the evolved stage of opening dated as Ladinian basalts close to typical N-MORB occur in association with red radiolarites. The Meliata Ocean closure in the middle Jurassic was related with accretionary prism formation and HP/LT metamorphism of the subducted slab. Basinal margin rocks produced originally in the initial stage of opening represent the majority of preserved blueschists, whereas LP/LT metamorphosed basalts of the accretionary wedge came originally from the central part of the basin. In the final stage of the Meliata Ocean closure also continental crust complexes represented by arc-related volcano-sedimentary complexes of the Permian and Early Paleozoic(?) age were involved in the subduction zone and metamorphosed in HP/LT conditions. The Meliata Ocean ophiolite suture in its original position was destroyed during the following tectonic evolution and nappe formation in the Cretaceous time. The present-day geographic position of the Meliata Ocean relics is a result of their dextral displacement by ALCAPA microplate motion to the northeast in the Miocene. Direct relation of the Meliata Ocean relics to Hellenide-Dinaride ophiolites as their northwestern continuation seems to be probable.

Ivan, P.

2003-04-01

102

Global occurrence of tellurium-rich ferromanganese crusts and a model for the enrichment of tellurium  

Science.gov (United States)

Hydrogenetic ferromanganese oxyhydroxide crusts (Fe-Mn crusts) precipitate out of cold ambient ocean water onto hard-rock surfaces (seamounts, plateaus, ridges) at water depths of about 400 to 4000 m throughout the ocean basins. The slow-growing (mm/Ma) Fe-Mn crusts concentrate most elements above their mean concentration in the Earth's crust. Tellurium is enriched more than any other element (up to about 50,000 times) relative to its Earth's crustal mean of about 1 ppb, compared with 250 times for the next most enriched element. We analyzed the Te contents for a suite of 105 bulk hydrogenetic crusts and 140 individual crust layers from the global ocean. For comparison, we analyzed 10 hydrothermal stratabound Mn-oxide samples collected from a variety of tectonic environments in the Pacific. In the Fe-Mn crust samples, Te varies from 3 to 205 ppm, with mean contents for Pacific and Atlantic samples of about 50 ppm and a mean of 39 ppm for Indian crust samples. Hydrothermal Mn samples have Te contents that range from 0.06 to 1 ppm. Continental margin Fe-Mn crusts have lower Te contents than open-ocean crusts, which is the result of dilution by detrital phases and differences in growth rates of the hydrogenetic phases. Correlation coefficient matrices show that for hydrothermal deposits, Te has positive correlations with elements characteristic of detrital minerals. In contrast, Te in open-ocean Fe-Mn crusts usually correlates with elements characteristic of the MnO2, carbonate fluorapatite, and residual biogenic phases. In continental margin crusts, Te also correlates with FeOOH associated elements. In addition, Te is negatively correlated with water depth of occurrence and positively correlated with crust thickness. Q-mode factor analyses support these relationships. However, sequential leaching results show that most of the Te is associated with FeOOH in Fe-Mn crusts and ???10% is leached with the MnO2. Thermodynamic calculations indicate that Te occurs predominantly as H5TeO6- in ocean water. The speciation of Te in ocean water and charge balance considerations indicate that Te should be scavenged by FeOOH, which is in agreement with our leaching results. The thermodynamically more stable Te(IV) is less abundant by factors of 2 to 3.5 than Te(VI) in ocean water. This can be explained by preferential (not exclusive) scavenging of Te(IV) by FeOOH at the Fe-Mn crust surface and by Fe-Mn colloids in the water column. We propose a model in which the extreme enrichment of Te in Fe-Mn crusts is likely the result of an oxidation reaction on the surface of FeOOH. A similar oxidation process has been confirmed for Co, Ce, and Tl at the surface of MnO2 in crusts, but has not been suggested previously to occur in association with FeOOH in Fe-Mn crusts. Mass-balance considerations indicate that ocean floor Fe-Mn deposits are the major sink for Te in the oceans. The concentration and redox chemistry of Te in the global ocean are likely controlled by scavenging on Fe-Mn colloids in the water column and Fe-Mn deposits on the ocean floor, as is also the case for Ce. ?? 2003 Elsevier Science Ltd.

Hein, J.R.; Koschinsky, A.; Halliday, A.N.

2003-01-01

103

75 FR 34929 - Safety Zones: Neptune Deep Water Port, Atlantic Ocean, Boston, MA  

Science.gov (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...

2010-06-21

104

Characterizing borehole fluid flow and formation permeability in the ocean crust using linked analytic models and Markov chain Monte Carlo analysis  

Science.gov (United States)

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.

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

2013-09-01

105

Petrogenesis of the Kaikomagatake granitoid pluton in the Izu Collision Zone, central Japan: implications for transformation of juvenile oceanic arc into mature continental crust  

Science.gov (United States)

The Miocene Kaikomagatake pluton is one of the Neogene granitoid plutons exposed in the Izu Collision Zone, which is where the juvenile Izu-Bonin oceanic arc is colliding against the mature Honshu arc. The pluton intrudes into the Cretaceous to Paleogene Shimanto accretionary complex of the Honshu arc along the Itoigawa-Shizuoka Tectonic Line, which is the collisional boundary between the two arcs. The pluton consists of hornblende-biotite granodiorite and biotite monzogranite, and has SiO2 contents of 68-75 wt%. It has high-K series compositions, and its incompatible element abundances are comparable to the average upper continental crust. Major and trace element compositions of the pluton show well-defined chemical trends. The trends can be interpreted with a crystal fractionation model involving the removal of plagioclase, biotite, hornblende, quartz, apatite, and zircon from a potential parent magma with a composition of ~68 wt% SiO2. The Sr isotopic compositions, together with the partial melting modeling results, suggest that the parent magma is derived by ~53% melting of a hybrid lower crustal source comprising ~30% Shimanto metasedimentary rocks of the Honshu arc and ~70% K-enriched basaltic rocks of the Izu-Bonin rear-arc region. Together with previous studies on the Izu Collision Zone granitoid plutons, the results of this study suggest that the chemical diversity within the parental magmas of the granitoid plutons reflects the chemical variation of basaltic sources (i.e., across-arc chemical variation in the Izu-Bonin arc), as well as a variable contribution of the metasedimentary component in the lower crustal source regions. In addition, the petrogenetic models of the Izu Collision Zone granitoid plutons collectively suggest that the contribution of the metasedimentary component is required to produce granitoid magma with compositions comparable to the average upper continental crust. The Izu Collision Zone plutons provide an exceptional example of the transformation of a juvenile oceanic arc into mature continental crust.

Saito, Satoshi; Arima, Makoto; Nakajima, Takashi; Tani, Kenichiro; Miyazaki, Takashi; Senda, Ryoko; Chang, Qing; Takahashi, Toshiro; Hirahara, Yuka; Kimura, Jun-Ichi

2012-04-01

106

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)

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)

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

2001-09-01

107

Low-velocity Oceanic Crusts at the top of Subducting Plates Beneath Japan Islands Imaged by Tomographic Method Using the NIED Hi-net Data  

Science.gov (United States)

In the original grid-type tomographic method (Zhao et al., 1992), the resolution of the images is equal to the grid spacing. It is more realistic to use as many grid nodes as possible for representing heterogeneous velocity distribution. However, the increasing of grid nodes introduces the instability in tomographic image having an artificially rough structure. We introduce correlation among velocities at surrounding grid nodes (Matsubara et al., 2001) to stable the solution. Thus the obtained structure might be more close to the real. We apply this modified method to 1,125,272 P- and 854,717 S-wave arrival times from 21,656 earthquakes recorded by 664 stations of the high-sensitivity seismograph network of Japan, NIED Hi-net, during 2000 to 2002. In the inversion process, the configuration of the Moho discontinuity is fixed according to Yoshii et al. (2002) without considering the upper boundary of the Pacific plate. A high-velocity (high-V) zone is imaged as the Pacific plate in the northeastern (NE) Japan arc. A low-velocity (low-V) layer above the high-V zone with about -(4-10)% velocity anomalies, considered as oceanic crust, coincides with the upper part of the double-seismic zone in the upper part of the slab. A high-V zone at depths of 20-60 km beneath the southwestern (SW) Japan is imaged as the Philippine Sea plate. Low-V zones, as oceanic crust, are found beneath the central and SW Japan except around the collision zone of Izu Peninsula at the northern edge of the Izu-Bonin arc. In the Kanto region, the central part of Japan, the Pacific and Philippine Sea plates subduct beneath the North-American plate and the velocity structure is complicate. We also find that the low-V zone of the Pacific plate is harmonious with the existence of the S-wave reflector found by Obara and Sato (1988). The low-V oceanic crust of the Philippine Sea plate is consistent with the results obtained from the block-type tomographic method (Ohmi and Hurukawa, 1996). The VP/V_S ratio above the Philippine Sea plate is about 1.90 corresponding to the serpentined wedge mantle by Kamiya and Kobayashi (2000). Beneath the Tohoku region, NE Japan, low-V zones of the Pacific plate coincide with the oceanic crust with velocity 6% lower than that in the mantle wedge by the analysis of P-to-S converted waves (Matsuzawa et al, 1986). Low-V oceanic crusts of the Philippine Sea plate beneath Chubu, southern Kinki, and western Shikoku region, the central and SW Japan, are consistent with the result of the analysis by channel waves (Hori, 1990). The low-V zone beneath the Shukoku region is harmonious with the model derived from receiver function analysis (Shiomi, 2002). Low-V zones are also continuously subducted beneath the volcanic front at a depth of 40 km in the shallower part of the mantle wedge with -(3-7)% anomalies in the NE Japan.

Matsubara, M.; Sekine, S.; Obara, K.; Kasahara, K.

2003-12-01

108

BATOLITO DE SABANALARGA, PLUTONISMO DE ARCO EN LA ZONA DE SUTURA ENTRE LAS CORTEZAS OCEÁNICA Y CONTINENTAL DE LOS ANDES DEL NORTE / THE SABANALARGA BATHOLITH, ARC PLUTONISM IN THE SUTURE ZONE BETWEEN CONTINENTAL AND OCEANIC CRUST IN THE NORTHERN ANDES  

Scientific Electronic Library Online (English)

Full Text Available SciELO Colombia | Language: Spanish Abstract in spanish 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.

GABRIEL, RODRÍGUEZ G.; MARÍA ISABEL, ARANGO; JOSÉ GILBERTO, BERMÚDEZ.

2012-12-01

109

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  

Science.gov (United States)

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.

Coogan, Laurence A.; Dosso, Stan E.

2015-04-01

110

From the lavas to the gabbros: 1.25 km of geochemical characterization of upper oceanic crust at ODP/IODP Site 1256, eastern equatorial Pacific  

Science.gov (United States)

Here we present trace element and Sr-Nd-Hf-Pb (double spike) isotopic data covering the entire igneous section of oceanic crust drilled at Ocean Drilling Program (ODP)/Integrated Ocean Drilling Program (IODP) Site 1256 on the Cocos Plate. The penetrated interval extends from the upper lavas through the sheeted dike complex to the gabbroic plutonic rocks, formed during superfast spreading at the mid-Miocene equatorial East Pacific Rise. The data are used to characterize the effects of chemical alteration, resulting from convection of seawater and hydrothermal fluids, on the trace element and isotopic composition of oceanic crust. Compared to normal mid-ocean-ridge basalt, the igneous basement of Site 1256 (Holes 1256C/D) is isotopically slightly enriched but shows only narrow downhole variations in Nd-Hf-Pb isotope ratios: 143Nd/144Nd = 0.513089 ± 0.000028 (2?), 176Hf/177Hf = 0.283194 ± 0.000033 (2?), 206Pb/204Pb = 18.61 ± 0.11 (2?), 207Pb/204Pb = 15.521 ± 0.014 (2?), 208Pb/204Pb = 38.24 ± 0.15 (2?). We believe that this minor variability is mainly of primary (magmatic) origin. The Sr isotopic composition shows considerably larger variation and, as expected, serves as sensitive tracer of seawater influence, which is particularly pronounced in the lava-dike transition zone and the sheeted dikes. The seawater influence is most prominent in a highly metal sulfide-enriched breccia layer encountered in the transition zone with 87Sr/86Sr of ~ 0.706, indicating a maximum water-rock mixing ratio of ~ 12. However, compared to the igneous section drilled at Site 504 (Hole 504B), which formed at intermediate, i.e., slower spreading rates at the Galápagos Spreading Center and hosting a much thicker sulfide-rich stockwork zone, the average intensity of water-rock interaction is lower. This is expressed by lesser mobility of base metals, narrower variability of alteration-sensitive incompatible elements, and less radiogenic Sr isotopic compositions on average at Site 1256. The amount of metal sulfide precipitation seems to be positively correlated with the degree of hydrothermal overprint. The less intense alteration of the Site 1256 transition zone, compared to Site 504, most likely reflects the higher rate of spreading, eventually resulting in a shorter period of time of continuous exposure to hydrothermal convection at the ridge crest. The observed seafloor alteration, leading to modified radiogenic parent/daughter ratios in the Site 1256 rocks, is ultimately not sufficient to develop time-integrative high 206Pb/204Pb and moderate 87Sr/86Sr ratios, as being characteristic of the HIMU (high ? = high 238U/204Pb) mantle signature proposed to originate from hydrothermally altered, subducted oceanic crust. Therefore, additional modification during the subduction process must be taken into account.

Höfig, Tobias W.; Geldmacher, Jörg; Hoernle, Kaj; Hauff, Folkmar; Duggen, Svend; Garbe-Schönberg, Dieter

2014-12-01

111

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

DEFF Research Database (Denmark)

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.

Artemieva, Irina M.; Meissner, Rolf

2012-01-01

112

Provenance of Proterozoic garnet-biotite gneiss recovered from Elan Bank, Kerguelen Plateau, southern Indian Ocean  

OpenAIRE

At Elan Bank of the Kerguelen Plateau in the southeast Indian Ocean, Leg 183 of the Ocean Drilling Program recovered clasts of garnet-biotite gneiss in a fluvial conglomerate intercalated with basalt flows. U-Pb and Pb-Pb dates of zircons and monazites in these clasts and an overlying sandstone range from 534 to 2547 Ma, which is much older than the surrounding Indian Ocean seafloor. These dates show that old continental crust resides in the shallow crust of the oceanic Kerguelen Plateau and ...

Nicolaysen, K.; Bowring, S.; Frey, F.; Weis, D.; Ingle, S.; Pringle, Ms; Coffin, Mf; Antretter, M.; Arndt, N.; Barling, J.; Boehm, F.; Borre, M.; Coxall, H.; Damasceno, D.; Damuth, J.

2001-01-01

113

The growth of continental crust  

Science.gov (United States)

The petrological and geochemical composition of the mantle-derived igneous products that will eventually form the continental crust (protolith), the episodic nature, and the geodynamic sites of crustal growth are discussed. Models in which crustal growth takes place at converging boundaries from orogenic magmas contrast with those in which basaltic plume material is involved (underplating, loose-plate loading, oceanic plateaus). Because some chemical components of the crust are either preferentially returned to the mantle at subduction zones (Mg, Ca) or sequestered in the crust (Si, Al, Na, K), the composition of the crust and that of its protolith are probably very different. Continental crust may therefore form from basaltic magmas and not necessarily from intermediate (e.g., andesitic) magmas. Because subduction is a continuous process, the episodic pattern of crust formation ages is a strong argument against crustal growth at converging boundaries. The preferred model is based on major mantle instabilities (superplumes) and their surface expression, the oceanic plateaus where thick piles of plume basalts rapidly erupted on the ocean floor reach the buoyancy threshold that defines the status of continental crust. The plateaus are accreted against the continents, and the felsic magmas that stand out as the most conspicuous feature of continental crust chemistry, are produced subsequently upon subduction erosion and possibly by gravitational instability of thin hot young lithospheric plates.

Albarède, Francis

1998-10-01

114

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.

115

Channelling of hydrothermal fluids during the accretion and evolution of the upper oceanic crust: Sr isotope evidence from ODP Hole 1256D  

Science.gov (United States)

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.

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

2015-04-01

116

Late Quaternary sediment deposition of core MA01 in the Mendeleev Ridge, the western Arctic Ocean: Preliminary results  

Science.gov (United States)

Late Quaternary deep marine sediments in the Arctic Ocean are characterized by brown layers intercalated with yellowish to olive gray layers (Poore et al., 1999; Polyak et al., 2004). Previous studies reported 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 (~450 ka) of core MA01 was tentatively estimated by correlation of brown layers with an adjacent core HLY0503-8JPC (Adler et al., 2009). A total of 22 brown layers characterized by low L* and b*, high Mn concentration, and abundant foraminifera were identified. Corresponding 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 occurred with the coarse-grained (>0.063 mm) fractions (i.e., IRD) both in brown and gray layers. IRDs are transported presumably by sea ice for the deposition of brown layers and by iceberg for the deposition of gray layers (Polyak et al., 2004). A strong 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 which mainly originated from the Canadian Arctic Archipelago. In addition, high kaolinite/chlorite (K/C) ratios mostly correspond to CaCO3 peaks, which suggests that the fine-grained particles in the Mendeleev Ridge are transported from the north coast Alaska and Canada where Mesozoic and Cenozoic strata are widely distributed. Thus, the Beaufort Gyre, the predominant surface current in the western Arctic Ocean, played an important role in the sediment delivery to the Mendeleev Ridge. It is worthy of note that the 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. References Adler, R.E., Polyak, L., Ortiz, J.D., Kaufman, D.S., Channell, J.E.T., Xuan, C., Grottoli, A.G., Sellén, E., and Crawford, K.A., 2009. Global and Planetary Change 68(1-2), 18-29. Polyak, L., Curry, W.B., Darby, D.A., Bischof, J., and Cronin, T.M., 2004. Palaeogeography, Palaeoclimatology, Palaeoecology 203, 73-93. Poore, R., Osterman, L., Curry, W., and Phillips, R., 1999. Geology 27, 759-762.

Park, Kwang-Kyu; Kim, Sunghan; Khim, Boo-Keun; Xiao, Wenshen; Wang, Rujian

2014-05-01

117

Rock uplift and exhumation of continental margins by the collision, accretion, and subduction of buoyant and topographically prominent oceanic crust  

OpenAIRE

Understanding the causes of rock and surface uplift is important because they control the location of mountain building, depocenters, and drainage characteristics and can influence climate. Here we combine previous thermochronological data with field observations to determine the amount of exhumation, rock, and surface uplift that occurs in the upper plate of Central and South American subduction zones during the collision, accretion, and subduction of oceanic plateaus and aseismic ridges. Th...

Spikings, Richard Alan; Simpson, Guy

2014-01-01

118

A 4D Framework for Ocean Basin Paleodepths and Eustatic Sea Level Change  

Science.gov (United States)

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.

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

2006-12-01

119

The hydrothermal power of oceanic lithosphere  

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Full Text Available 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 (<1 Ma is about 50% of the total, significantly higher than previous estimates. This low hydrothermal power estimate originates from the thermally insulating properties of oceanic crust in relation to the mantle. Since the crust is relatively insulating, the effective properties of the lithosphere are "crust dominated" near ridge axes (yielding lower heat flow, and gradually approach mantle values over time. Thus, cooling models with crustal insulation predict low heat flow over young seafloor, implying that the difference of modeled and measured heat flow is due to the heat transport properties of the lithosphere, in addition to ventilated hydrothermal circulation as generally accepted. These estimates may bear on 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.

C. J. Grose

2015-03-01

120

The trajectory of India towards Eurasia recorded by subducted slabs: evidence for southward subduction of the Tethys Ocean under India after 130 Ma  

Science.gov (United States)

At the ~130 Ma breakup of India from Australia-Antarctica, published paleomagnetic data estimate that India was located south of 30°S. As India began its northward journey that continues today, the 5000 km of Tethys Ocean that stood in the expanse between India and Eurasia gradually disappeared, leaving only traces of its existence in ophiolite sutures and accreted terranes at the Himalayas, the largest orogeny on Earth today. Here we present newly mapped, sub-horizontal slabs under the Indian Ocean at depths of 1600 to 2100 km and at latitudes between 35°S to 15°N. These slabs are further south than any published paleolatitudinal estimates of initial India-Asia collision and thus, the existence of these slabs cannot be explained by the popular idea of northward Tethys subduction under Eurasia. Instead, our slab constraints show for the first time that the majority of the Tethys Ocean was subducted southward after 130 Ma, overrun by a northward-moving India. When restored to the surface of a spherical model Earth, the slabs closely correspond to the well-known track of India from Eastern Gondwanaland to Eurasia that began at ~130 Ma, viewed in a mantle reference. We present a plate reconstruction that includes other restored slabs from the India-Eurasia collision zone, which are at shallower depths and ubiquitously located north of the equator. The reconstruction implies that the Tethys Ocean was subducted under Greater India, India and the post-breakup ocean at the eastern margin of the Indian plate - now identified as the enigmatic 'Burma slab'. Slab geometries were mapped from global P- and S-wave models and restored to the surface of a spherical Earth model. Gplates software was used to reconstruct the mapped slabs.

Suppe, John; Wu, Jonny; Lin, Chris D. J.; Kanda, Ravi V. S.

2014-05-01

121

Microbial borehole observatories deployed within the oceanic crust: Design considerations and initial results from long-term colonization experiments (Invited)  

Science.gov (United States)

Borehole observatories developed for long-term sampling and monitoring in the subseafloor of the deep ocean must satisfy design and operation requirements that are similar to systems deployed on land. Many of these systems are used to achieve simultaneous hydrologic, geochemical and microbiological goals, requiring innovative design, installation, and operation. There are major logistical challenges for subseafloor observatories, the foremost being having to remotely access sites kilometers underwater using multiple oceanographic platforms (drill ship, surface ship, submersible, remotely-operated vehicle) and reliance on autonomous devices that are serviced only after several years. Contamination of the analytical environment is probable during installation operations, requiring vigilance during analysis for interpretation. Subseafloor observatories also require self-contained and robust instrumentation that can withstand long-term exposure to seawater at high pressures, elevated temperatures, a variety of redox conditions, and little to no access to external power. Although subseafloor borehole observatories have been in development for hydrologic monitoring for two decades, the inclusion of experimentation to examine the deep biosphere in the marine subsurface has only recently been developed. Results from some of the first microbial colonization experiments in young basaltic rocks on the eastern flank of the Juan de Fuca Ridge demonstrate in situ microbial-mineral interactions that can be identified using complementary geochemical and microbiological techniques. Mineral surfaces were first colonized by iron oxidizing bacteria, and as fluid composition changed, the microbial community became dominated by Firmicutes bacteria, some of which are phylogenetically similar to microbial communities observed in the terrestrial deep biosphere.

Orcutt, B. N.; Bach, W.; Becker, K.; Fisher, A. T.; Hulme, S.; Toner, B. M.; Wheat, C. G.; Edwards, K. J.; Iodp Expedition 327 Shipboard Party

2010-12-01

122

Lunar and terrestrial crust formation  

International Nuclear Information System (INIS)

Planetary crusts may be accreted, produced in primordial differentiation, or built up piecemeal by serial magmatism. The existence of old, polygenetic, laterally heterogeneous, partial melt rocks in the lunar highlands suggests that the moon produced its early crust by serial magmatism. This view can be reconciled with lunar Eu anomalies, previously thought to support the magma ocean model of crust formation, if complications in the fractionation of mare basalts are reconized. Phase equilibrium and magmatic density information for mare basalts suggest a model in which plagioclase fractionation can occur even though plagioclase is not a near-liquidus phase. The crytic fractionation of clinopryoxene in MORB provides a precedent for this model. The necessity for a lunar magma ocean is questioned, but a role for a terrestrial magma ocean of sorts at depth is suggested

123

Subduction-modified oceanic crust mixed with a depleted mantle reservoir in the sources of the Karoo continental flood basalt province  

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The great majority of continental flood basalts (CFBs) have a marked lithospheric geochemical signature, suggesting derivation from the continental lithosphere, or contamination by it. Here we present new Pb and Os isotopic data and review previously published major element, trace element, mineral chemical, and Sr and Nd isotopic data for geochemically unusual mafic and ultramafic dikes located in the Antarctic segment (Ahlmannryggen, western Dronning Maud Land) of the Karoo CFB province. Some of the dikes show evidence of minor contamination with continental crust, but the least contaminated dikes exhibit depleted mantle - like initial ?Nd (+9) and 187Os/188Os (0.1244-0.1251) at 180 Ma. In contrast, their initial Sr and Pb isotopic compositions (87Sr/86Sr = 0.7035-0.7062, 206Pb/204Pb = 18.2-18.4, 207Pb/204Pb = 15.49-15.52, 208Pb/204Pb = 37.7-37.9 at 180 Ma) are more enriched than expected for depleted mantle, and the major element and mineral chemical evidence indicate contribution from (recycled) pyroxenite sources. Our Sr, Nd, Pb, and Os isotopic and trace element modeling indicate mixed peridotite-pyroxenite sources that contain ˜10-30% of seawater-altered and subduction-modified MORB with a recycling age of less than 1.0 Ga entrained in a depleted Os-rich peridotite matrix. Such a source would explain the unusual combination of elevated initial 87Sr/86Sr and Pb isotopic ratios and relative depletion in LILE, U, Th, Pb and LREE, high initial ?Nd, and low initial 187Os/188Os. Although the sources of the dikes probably did not play a major part in the generation of the Karoo CFBs in general, different kind of recycled source components (e.g., sediment-influenced) would be more difficult to distinguish from lithospheric CFB geochemical signatures. In addition to underlying continental lithosphere, the involvement of recycled sources in causing the apparent lithospheric geochemical affinity of CFBs should thus be carefully assessed in every case.

Heinonen, Jussi S.; Carlson, Richard W.; Riley, Teal R.; Luttinen, Arto V.; Horan, Mary F.

2014-05-01

124

Microstructures and petro-fabrics of lawsonite blueschist in the North Qilian suture zone, NW China: Implications for seismic anisotropy of subducting oceanic crust  

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We conducted a detailed study on the microstructures and petro-fabrics of massive and foliated lawsonite blueschist (LBS) in North Qilian suture zone, NW China. The lattice preferred orientation (LPO) of glaucophane and lawsonite in foliated lawsonite blueschist (LBS) is considered to be dominantly formed by the deformation mechanism of dislocation creep and rigid-body rotation, respectively. The LPO of glaucophane is mainly characterized by the [001] axis aligning parallel to lineation and the [100] axis and (110) pole plunging perpendicular to foliation. In contrast, the LPO of lawsonite features the maximum [010] axis concentrated close to lineation and the [001] axis strongly clustered normal to foliation. The preferred orientation of [010] axis of lawsonite parallel to lineation is supported by a two-dimensional numerical modeling using the finite-volume method (FVM). The mineral LPOs are much stronger in foliated LBS than in massive LBS. In addition, a kinematic vorticity analysis suggests that both pure shear dominant (Wm = 0.18-0.26) and simple shear dominant (Wm = 0.86-0.93) deformation regimes are present in foliated LBS. The [001] axis and (010) pole of glaucophane, and the [100] and [010] axes of lawsonite, tend to distribute in a foliation-parallel girdle in the pure shear dominant samples, but simple shear dominant samples display more lineation-parallel concentrations of a [001] axis of glaucophane and a [010] axis of lawsonite. Because the whole-rock seismic anisotropies in foliated LBS are significantly higher than those in massive LBS and a counteracting effect on seismic anisotropies occurs between glaucophane and lawsonite, the delay time of fast S-wave polarization anisotropy induced by an actual subducting oceanic crust with a high subducting angle (> 45-60°) is expected to range from 0.03 to 0.09 s (lower bound for massive LBS) and from 0.1 to 0.3 s (upper bound for foliated epidote blueschist).

Cao, Yi; Jung, Haemyeong; Song, Shuguang

2014-07-01

125

Evolution of biogeochemical cycling of phosphorus during 45~50 Ma revealed by sequential extraction analysis of IODP Expedition 302 cores from the Arctic Ocean  

Science.gov (United States)

The modern Arctic Ocean plays crucial roles in controlling global climate system with the driving force of global thermohaline circulation through the formation of dense deep water and high albedo due to the presence of perennial sea-ice. However, the Arctic sea-ice has not always existed in the past. Integrated Ocean Drilling Program (IODP) Expedition 302 Arctic Coring Expedition (ACEX) has clarified that global warming (water temperature: ca. 14~16?C) during 48~49 Ma Azolla Event induced the loss of sea-ice and desalination of surface ocean, and that sea-ice formed again some million years later (45 Ma). In the Arctic Ocean, warming and cooling events repeated over and over (e.g., Brinkhuis et al., 2006; Moran et al., 2006; März et al., 2010). Large variations in the extent of thermohaline circulation through time often caused stagnation of seawater and appearance of anaerobic environment where hydrogen sulfide was produced by bacterial sulfate reduction. Ogawa et al. (2009) confirmed occurrence of framboidal pyrite in the ACEX sediments, and suggested that the Arctic Ocean at the time was anoxic, analogous to the modern Black Sea, mainly based on sulfur isotope analysis. To further clarify the variations in the nutrient status of the Arctic Ocean, we focus on the geochemical cycle of phosphorus. We performed sequential extraction analysis of sedimentary phosphorus in the ACEX sediments, using the method that we improvped based on the original SEDEX method by Ruttenberg (1992) and Schenau et al. (2000). In our method, phosphorus fractions are divided into five forms; (1) absorbed P, (2) Feoxide-P, (4) carbonate fluorapatite (CFAP) + CaCO3-P + hydroxylapatite (HAP), (4) detrital P, and (5) organic P. Schenau et al. (2000) divided the (3) fraction into non-biological CFAP and biological HAP and CaCO3-P. When the Arctic Ocean was closed and in its warming period, the water mass was most likely stratified and an anaerobic condition would have prevailed where bacterial sulfate reduction was active. In this case, most of the phosphorus in sediment was stored as organic P, which was originally derived as sinking particles of detrital plankton from the surface ocean. Increased rainfalls during such a warming period would have enhanced continental weathering and delivery of phosphorus to the surface ocean, and biological activity using increased amounts of phosphorus supply would also have increased. Feoxide-P is considered to be less important as a sink for phosphorus because of the likely formation of pyrite through the reductive dissolution of Fe oxide. CFAP could be a sink for phosphorus, because the formation of CFAP tends to increase with increasing age and depth.

Hashimoto, S.; Yamaguchi, K. E.; Takahashi, K.

2012-12-01

126

Synchronization of Glacial-Interglacial SST changes in the Tropical Oceans: Evidence for Amplification of Climate Sensitivity by Carbon Dioxide Over the Past 2.7 Ma  

Science.gov (United States)

Understanding the links between tropical sea surface temperatures (SST) and changes in high latitude climate across ice age cycles has challenged paleoclimatologists for some time. On the one hand, the tropical oceans should be shielded from processes that produce large temperature sensitivity in the high latitudes (ice-albedo feedback, sea-ice feedback, steering of wind fields by ice sheets, etc.). However, the high latitudes exert an influence on tropical ocean temperatures through the thermocline, and, perhaps via the atmosphere by modulating the CO2 greenhouse effect. The tropics, in turn could provide important feedbacks to glacial cycles via their influence on water vapor and clouds. Work with recently developed SST proxies (Mg/Ca, alkenone Uk'37) has proven that substantial sensitivity exists in tropical SST on a glacial-interglacial timescale over the course of the last ~ 1Ma. Over the extent of the EPICA ice core record, this sensitivity is coherent with glacial-interglacial variations in CO2. In this study, we have extended the tropical SST record to the mid-Pliocene (3.5 Ma) in tropical locations in the Atlantic, Indian and Pacific oceans. We use the alkenone unsaturation index, recorded at ~ 3kyr resolution at each of three sites, in conjunction with benthic foraminiferal ?18O measured at the same sites, which synchronizes the records and allows for a direct comparison of tropical SST to ice volume changes. We find that all three tropical sites show long-term cooling over the last three million years, although the rate of cooling is the steepest in the eastern equatorial Pacific. Glacial-interglacial SST variation is strikingly similar in the three tropical basins after 2.7 Ma, with a succession of 41 kyr (obliquity) cycles persisting up to the transition to 100 kyr cycles at about 800 ka. Prior to 2.7 Ma, temperatures at the three sites behave idiosyncratically on the orbital timescale. A further intriguing observation is the existence of coherent 300-500 kyr cycles in SST in all three basins (also observed in Mg/Ca data) that may not be related to orbital forcing. We evaluate and reject the case that the tropical SST cycles originate dominantly from upwelling/thermocline- driven processes, and argue instead that the SST data suggest that a coherent, substantial CO2-glaciation feedback began at about 2.7 Ma, synchronizing to first order the tropical SST variations on orbital timescales in late Pliocene and all of Pleistocene time. The key event at 2.7 Ma, therefore was the phase locking of CO2 and ice/sea ice feedbacks to orbital forcing. Furthermore, the large tropical SST memory observed (the 300-500 kyr wavelength) argues for the importance of a long response time element in the climate system, most likely ocean biogeochemical cycles and CO2, in modulating tropical SST on supra-orbital timescales. Prior to 2.7 Ma, the combination of glacial and CO2 feedbacks may have been much weaker, diminishing the similarity of tropical SST patterns between the different ocean basins, and lessening their sensitivity to high latitude processes.

Herbert, T. D.; Cleaveland, L.; Liu, Z.; Lawrence, K. T.

2007-12-01

127

Earth science: when crust is bred.  

OpenAIRE

An analysis of the distribution of helium-isotope ratios in oceanic extrusions from Earth's mantle seems to establish a connection with the spread of ages in continental crust. What mechanism might underlie this? ©2007 Nature Publishing Group.

Porcelli, D.

2007-01-01

128

The Gop Basin - A Possible Imprint of Early Oceanic Spreading Between Greater Seychelles and India  

Science.gov (United States)

The Arabian and its conjugate Eastern Somali basins were formed by the seafloor spreading at the Carlsberg Ridge since Early Tertiary (anomaly 28n; ~62.5 Ma). The reconstruction model at anomaly 28n suggested existence of a wide swath of deep offshore region (Gop and Laxmi basins) between the Laxmi Ridge and the India-Pakistan continental shelf. In the present study we focus on the Gop Basin, where the important constraints about the early geodynamic evolution of the Arabian Sea appear to exist. The nature of the crust underlying this basin remains a matter of debate, with views varying from volcanics-intruded thinned continental crust to oceanic crust formed by a now extinct spreading centre. Our interpretation of an updated compilation of marine geophysical data supports the oceanic nature of the crust underlying the Gop Basin, where the Palitana Ridge represents the extinct spreading centre related to an episode of early oceanic spreading between Greater Seychelles (Seychelles-Laxmi Ridge block) and India. Our magnetic modelling shows that the well correlatable, prominent but short sequence of magnetic anomalies in the Gop Basin does not allow a unique identification; it can be reasonably explained either as A31r - A25r (~69 - 56 Ma) or as A29r - A25r (~65 - 56 Ma) sequence. Both the models suggest that spreading in the Gop Basin was significantly affected by the nearby onset of the Reunion hotspot at ~65 Ma, which formed the Deccan Traps on the adjacent western Indian mainland.

Bhattacharya, G. C.; Yatheesh, V.; Dyment, J.

2009-04-01

129

Gondwanaland from 650-500 Ma assembly through 320 Ma merger in Pangea to 185-100 Ma breakup: supercontinental tectonics via stratigraphy and radiometric dating  

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Gondwanaland lasted from the 650-500 Ma (late Neoproterozoic-Cambrian) amalgamation of African and South American terranes to Antarctica-Australia-India through 320 Ma (mid-Carboniferous) merging with Laurussia in Pangea to breakup from 185 to 100 Ma (Jurassic and Early Cretaceous). Gondwanaland straddled the equator at 540 Ma, lay wholly in the Southern Hemisphere by 350 Ma, and then rotated clockwise so that at 250 Ma Australia reached the S pole and Africa the equator. By initial breakup of Pangea at 185 Ma, Gondwanaland had moved northward such that North Africa reached 35°N. The first clear picture of Gondwanaland, in the Cambrian, shows the assembly of continents with later Laurentian, European and Asian terranes along the "northern" margin, and with a trench along the "western" and "southern" margins, reflected by a 10,000-km-long chain of 530-500 Ma granites. The interior was crossed by the Prydz-Leeuwin and Mozambique Orogenic Belts. The shoreline lapped the flanks of uplifts generated during this complex terminal Pan-Gondwanaland (650-500 Ma) deformation, which endowed Gondwanaland with a thick, buoyant crust and lithosphere and a nonmarine siliciclastic facies. During the Ordovician, terranes drifted from Africa as the first of many transfers of material to the "northern" continents. Central Australia was crossed by the sea, and the eastern margin and ocean floor were flooded by grains of quartz (and 600-500 Ma zircon) from Antarctica. Ice centres in North Africa and southern South America/Africa waxed and waned in the latest Ordovician, Early Silurian, latest Devonian, and Early Carboniferous. In the mid-Carboniferous, Laurussia and Gondwanaland merged in the composite called Pangea by definitive right-lateral contact along the Variscan suture, with collisional stress and subsequent uplift felt as far afield as Australia. Ice sheets developed on the tectonic uplands of Gondwanaland south of 30°S. In the Early Permian, the self-induced heat beneath Pangea drove the first stage of differential subsidence of the Gondwanaland platform to intercept sediment from the melting ice, then to accumulate coal measures with Glossopteris, and subsequently Early Triassic redbeds. An orogenic zone along the Panthalassan margin propagated from South America to Australia and was terminally deformed in the mid-Triassic. Coal deposition resumed during Late Triassic relaxation in the second stage of Pangean extension. In the Early Jurassic, the vast ˜200 Ma Central Atlantic magmatic province of tholeiite anticipated the 185 Ma breakup in the Central Atlantic. Another magmatic province was erupted at this time between southern Africa and southeastern Australia. The northeastern Indian Ocean opened from 156 Ma, and the western Indian Ocean from 150 Ma. By the 100 Ma mid-Cretaceous, the Gondwanaland province of Pangea had split into its five constituents, and the Earth had entered the thalassocratic state of dispersed continents. The 650-500 Ma "Pan-Gondwanaland" events (? by mafic underplating) rendered Gondwanaland permanently geocratic. Pangean (320-185 Ma) tectonics, driven by pulses of self-induced heat, promoted widespread subsidence at 300 Ma Early Permian and 230 Ma Late Triassic. Pangea initially broke up at 185 Ma and the five continental pieces of Gondwanaland had broken apart by the 100 Ma mid-Cretaceous. Another long-lasting feature of Gondwanaland was subduction beneath the "southern" margin and export of terranes from the "northern" and "northwestern" margins. Export of terranes was promoted by Gondwanaland-induced heat, and internal breakup by Pangea-induced heat.

Veevers, J. J.

2004-12-01

130

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.)

131

Late Cretaceous (~ 81 Ma) high-temperature metamorphism in the southeastern Lhasa terrane: Implication for the Neo-Tethys ocean ridge subduction  

Science.gov (United States)

An integrated study of Usbnd Pb zircon dating, geochemical and Srsbnd Ndsbnd Hf isotopic compositions of garnet-bearing granulite and marble from the granulite-facies domain of the Nyingchi Complex (eastern Himalayan syntaxis) has provided insights into the tectonic evolution of the southern Lhasa terrane. The peak metamorphism of the garnet-bearing granulite is marked by a mineral assemblage of garnet + orthopyroxene + high-Ti amphibole + plagioclase + quartz + rutile. Abundant exsolved rutile needles are observed within amphibole, garnet and quartz. The peak metamorphic temperatures are estimated at 803-924 °C. Geochemical data from the garnet-bearing granulites provide evidence for a basaltic protolith that formed in a continental-margin arc setting. Srsbnd Ndsbnd Hf isotopic compositions indicate that this protolith was sourced from partial melting of a depleted mantle. LA-ICP MS Usbnd Pb zircon dating shows that the protolith age and metamorphic age of the garnet-bearing granulites are 89.3 ± 0.6 Ma and 81.1 ± 0.8 Ma, respectively. The detrital magmatic zircons from the marble yield ages from 86.3 to 167 Ma. The age distribution and Hf isotopic composition (?Hf(t) = + 5.9 to + 17.5) of the detrital magmatic zircon in the marble are consistent with the isotopic data of zircons from the Jurassic-Cretaceous Gangdese batholiths, suggesting that the clastic sediments were partially derived from these intrusives or associated volcanic rocks, and deposited in the fore-arc basin of the Gangdese arc. The metamorphic zircons in the marble yield a metamorphic age of 81.4 ± 0.5 Ma. These results show that both the arc magmatic rocks and forearc sedimentary rocks underwent high-temperature (HT) granulite-facies metamorphism at ~ 81 Ma, indicating anomalously high heat input in the forearc region. A range of tectonic observations, including a coeval hiatus in arc magmatism and a period of regional uplift, indicate that HT metamorphism resulted from the subduction of the Neo-Tethys ocean ridge beneath the southern Lhasa terrane during the Late Cretaceous.

Guo, Liang; Zhang, Hong-Fei; Harris, Nigel; Pan, Fa-Bin; Xu, Wang-Chun

2013-11-01

132

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  

Science.gov (United States)

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.

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

2013-10-01

133

Growth of the continental crust: a planetary-mantle perspective  

International Nuclear Information System (INIS)

The lack of earth rocks older than about 3.8 Ga is frequently interpreted as evidence that the earth formed little or no subduction-resistant continental crust during the first 700 My of its history. Such models obviously imply that the pre-3.8 Ga earth was covered entirely or almost entirely by smoothly subducting oceanic crust. On the other hand, the thermal regime of the early earth probably tended to cause the oceanic crust at this time to be comparatively thin and comparatively mafic. The present earth is covered by about 50 percent oceanic crust, averaging about 7 km in thickness, and 41 percent continental crust, averaging roughly 40 km in thickness. Thus continentless-early-earth models would seem to imply a total mass of crust less than 1/3 that of the present day earth. Possible explanations are examined

134

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

CERN Document Server

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...

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

2003-01-01

135

Channelized lava flows at the East Pacific Rise crest 9??-10??N: The importance of off-axis lava transport in developing the architecture of young oceanic crust  

Science.gov (United States)

Submarine lava flows are the building blocks of young oceanic crust. Lava erupted at the ridge axis is transported across the ridge crest in a manner dictated by the rheology of the lava, the characteristics of the eruption, and the topography it encounters. The resulting lava flows can vary dramatically in form and consequently in their impact on the physical characteristics of the seafloor and the architecture of the upper 50-500 m of the oceanic crust. We have mapped and measured numerous submarine channelized lava flows at the East Pacific Rise (EPR) crest 9?? - 10??N that reflect the high-effusion-rate and high-flow-velocity end-member of lava eruption and transport at mid-ocean ridges. Channel systems composed of identifiable segments 50 - 1000 m in length extend up to 3 km from the axial summit trough (AST) and have widths of 10 - 50 m and depths of 2 - 3 m. Samples collected within the channels are N-MORB with Mg# indicating eruption from the AST. We produce detailed maps of lava surface morphology across the channel surface from mosaics of digital images that show lineated or flat sheets at the channel center bounded by brecciated lava at the channel margins. Modeled velocity profiles across the channel surface allow us to determine flux through the channels from 0.4 to 4.7 ?? 103 m3/s, and modeled shear rates help explain the surface morphology variation. We suggest that channelized lava flows are a primary mechanism by which lava accumulates in the off-axis region (1 - 3 km) and produces the layer 2A thickening that is observed at fast and superfast spreading ridges. In addition, the rapid, high-volume-flux eruptions necessary to produce channelized flows may act as an indicator of the local magma budget along the EPR. We find that high concentrations of channelized lava flows correlate with local, across-axis ridge morphology indicative of an elevated magma budget. Additionally, in locations where channelized flows are located dominantly to the east or west of the AST, the ridge crest is asymmetric, and layer 2A appears to thicken over a greater distance from the AST toward the side of the ridge crest where the channels are located. Copyright 2005 by the American Geophysical Union.

Soule, S.A.; Fornari, D.J.; Perfit, M.R.; Tivey, M.A.; Ridley, W.I.; Schouten, Hans

2005-01-01

136

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)

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.

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

2013-07-01

137

Nitrogen content and isotopic composition of oceanic crust at a superfast spreading ridge: A profile in altered basalts from ODP Site 1256, Leg 206  

Science.gov (United States)

The present paper provides the first measurements of both nitrogen content and isotopic composition of altered oceanic basalts. Samples were collected from Ocean Drilling Program Site 1256 located at the eastern flank of the East Pacific Rise. Twenty-five samples affected by low temperature alteration were analyzed. They include moderately altered basalts together with veins and related alteration halos and host rocks, as well as unique local intensely altered basalts showing green (celadonite-rich) and red (iron oxyhydroxide-rich) facies. Nitrogen contents of moderately altered basalts range from 1.4 to 4.3 ppm and are higher than in fresh MORB. Their ?15N values vary in a large range from +1.6 to +5.8‰. Veins, halos, and host rocks are all enriched in N relative to moderately altered basalts. Notably, veins show particularly high N contents (354 and 491 ppm) associated with slightly low ?15N values (+0.4 and -2.1‰). The intensely altered red and green facies samples display high N contents of 8.6 and 9.7 ppm, respectively, associated with negative ?15N values of -3.8 and -2.7‰. Detailed petrological examination coupled with N content suggests that N of altered basalts occurs as ammonium ion (NH4+) fixed in various secondary minerals (celadonite, K- and Na-feldspars, smectite). A body of evidence indicates that N is enriched during alteration of oceanic basalts from ODP Site 1256, contrasting with previous results obtained on basalts from DSDP/ODP Hole 504B (Erzinger and Bach, 1996). Nitrogen isotope data support the interpretation that N in metasomatizing fluid occurred as N2, derived from deep seawater and likely mixed with magmatic N2 contained in basalt vesicles.

Busigny, Vincent; Laverne, Christine; Bonifacie, Magali

2005-12-01

138

Ocean Drilling Program  

Science.gov (United States)

This site describes the Ocean Drilling Program (ODP). The ODP conducts basic research into the history of the ocean basins and the overall nature of the crust beneath the ocean floor using the scientific drill ship JOIDES Resolution. There are also links to photographs, core data, and educational material on the site.

Ocean Drilling Program

139

Physical and chemical processing of eclogite: protolith and pre-subduction inheritance versus subduction-related deformation and alteration of oceanic crust  

Science.gov (United States)

Subduction-related eclogites are products of their igneous, metamorphic, fluid-rock, and tectonic histories. Changes of bulk rock chemistry, mineral assemblages and compositions, as well as microstructures operate in concert both before and during subduction to transmogrify ocean floor basalt into subduction zone eclogite. We present textural and chemical characterizations of suites of blueschists and eclogites from three subduction complexes to evaluate the extent of interactions of these processes and determine the degree to which consequences of chemical and physical features along a complex P-T-t-D may be preserved. Samples of coronitic and mylonitic eclogite and blueschist from subduction complexes in the Franciscan (CA, U.S.A.), Monviso (Western Alps) and Gruppo di Voltri (Ligurian Alps) were analyzed for major and trace elements of both rocks and minerals. Comparisons of these areas can illustrate different depths of subduction, and degrees of metamorphism. In addition, all of these suites show some degree of retrogression to blueschist or greenchist assemblages. Textural observations suggest that some primary magmatic mineral domains in gabbroic rocks are preserved through subduction, metamorphism and exhumation (e.g., Omp replacement of domains that were originally Pl). In effect some blocks in subduction complexes show evidence for multiple metamorphic episodes that transform seafloor metamorphic rocks to HP ones. In finer-grained rocks, primary domains have been totally obscured by full recrystallization and/or deformation. Trace element distributions within minerals testify to which minerals equilibrated with each other, and/or with exotic fluids. Minerals may acquire trace element signatures from precursor minerals, which themselves equilibrated with an earlier fluid, or with the bulk rock following mass transfer on a large scale. For example, the trace element chemistry of talc inclusions in omphacite within gabbroic FeTi-rich eclogites from Monviso indicates interaction of mantle-derived fluid with ocean floor Fe-Ti gabbro, preserved through subduction, eclogite facies metamorphism and exhumation. The mineral and trace element chemistry of phengite indicates that the trace element signature of this mineral is entirely controlled by exotic components in fluids that interacted with their host rocks, and not by deformation or bulk chemistry. The REE in eclogite from the Alpine Monviso and Voltri Massifs, the Franciscan Complex, and the Catalina Schist garnet amphibolites reside primarily in apatite, epidote-group minerals, garnet and titanite. The relative stabilities of the accessory minerals apatite and titanite, along with epidote, and titanite, which in some instances appear to be controlled by relative solubilities of Ca-rich silicate or phosphate minerals in aqueous solutions and brines, provide critical evidence for understanding how L- and MREE are retained, lost, or introduced during eclogite-fluid interaction upon retrogression.

Brouwer, F. M.; Sorensen, S. S.; Philippot, P.

2011-12-01

140

Early formation of evolved asteroidal crust.  

Science.gov (United States)

Mechanisms for the formation of crust on planetary bodies remain poorly understood. It is generally accepted that Earth's andesitic continental crust is the product of plate tectonics, whereas the Moon acquired its feldspar-rich crust by way of plagioclase flotation in a magma ocean. Basaltic meteorites provide evidence that, like the terrestrial planets, some asteroids generated crust and underwent large-scale differentiation processes. Until now, however, no evolved felsic asteroidal crust has been sampled or observed. Here we report age and compositional data for the newly discovered, paired and differentiated meteorites Graves Nunatak (GRA) 06128 and GRA 06129. These meteorites are feldspar-rich, with andesite bulk compositions. Their age of 4.52 +/- 0.06 Gyr demonstrates formation early in Solar System history. The isotopic and elemental compositions, degree of metamorphic re-equilibration and sulphide-rich nature of the meteorites are most consistent with an origin as partial melts from a volatile-rich, oxidized asteroid. GRA 06128 and 06129 are the result of a newly recognized style of evolved crust formation, bearing witness to incomplete differentiation of their parent asteroid and to previously unrecognized diversity of early-formed materials in the Solar System. PMID:19129845

Day, James M D; Ash, Richard D; Liu, Yang; Bellucci, Jeremy J; Rumble, Douglas; McDonough, William F; Walker, Richard J; Taylor, Lawrence A

2009-01-01

141

Uranium, Ce and Fe(III) enrichments in Archean igneous rocks indicating the early development of the oxygenated atmosphere-ocean-crust system  

Science.gov (United States)

We report new high-precision trace element data obtained for well-preserved boninites from: the 3.12 G Whundo volcanic sequence and the 2.97-2.95 Ga Mallina sequence, Pilbara Craton; the 3.0 Ga Olondo greenstone belt, Aldan Shield; the 2.8 Ga Karelian greenstone belt, Baltic Shield; the 2.7 Ga Gadwall greenstone belt, Eastern Dhawar Craton; the 2.8-2.7 Ga Superior Province. These Archean-aged boninites record systematically low primary magmatic Th/U ratios adsorption on ferric-iron (hydr)oxides; (3) positive Ce anomalies associated with ferric-iron enrichment; and (4) anomalous enrichment of uranogenic Pb isotopes. The positive Ce-anomalies in hydrothermally altered Archean submarine volcanic rocks in open oceans contrast the negative Ce-anomalies in coeval shale-BIF units that accumulated in chemically-stratified (semi)closed basins. These data suggest that the geochemical cycles of Fe, U, and Ce (and many other redox-sensitive elements) during the 3.5-2.7 Ga period were fundamentally the same as today. Collectively, these results require revisiting conventional accounts for the mantle Pb-paradox's to accommodate recycling of U into the asthenosphere from ~ 3.5 Ga, as well as revision of the ~2.4 Ga Great Oxygenation Event (GOE) hypothesis.

Kerrich, R.; Ohmoto, H.

2012-12-01

142

Does subduction zone magmatism produce average continental crust  

International Nuclear Information System (INIS)

The question of whether present day subduction zone magmatism produces material of average continental crust composition, which perhaps most would agree is andesitic, is addressed. It was argued that modern andesitic to dacitic rocks in Andean-type settings are produced by plagioclase fractionation of mantle derived basalts, leaving a complementary residue with low Rb/Sr and a positive Eu anomaly. This residue must be removed, for example by delamination, if the average crust produced in these settings is andesitic. The author argued against this, pointing out the absence of evidence for such a signature in the mantle. Either the average crust is not andesitic, a conclusion the author was not entirely comfortable with, or other crust forming processes must be sought. One possibility is that during the Archean, direct slab melting of basaltic or eclogitic oceanic crust produced felsic melts, which together with about 65 percent mafic material, yielded an average crust of andesitic composition

143

On the dynamics and the geochemical mechanism of the evolution of the continental crust. 1  

International Nuclear Information System (INIS)

An investigation of the isotopic composition of oxygen in the continental crust, in the oceans, in the oceanic crust and in the upper mantle shows the dynamics of plate tectonics and continental growthto be more or less constant during the last three or four aeons independent on the geochemical mechanism of continental growth. (author)

144

Refined Estimates of the Depths of Magma Chambers Beneath the Reykjanes and Kolbeinsey Ridges, and Implications for the Structure of Oceanic Crust  

Science.gov (United States)

The mid-Atlantic ridge is the divergent plate boundary between North and South America to the west and Europe and Africa to the east. Plate spreading is accompanied by intrusion of dikes and eruption of lava along the ridge axis. The dikes are fed by magma chamber (s) located beneath the ridge. It has been suggested that the depth of magma chambers is related to the rate of spreading. In order to examine this hypothesis we determined the depths of magma chambers beneath the slow spreading Reykjanes Ridge that extends form the Charlie Gibbs fracture zone at 53° north to the southern tip of Iceland at 64° north and the Kolbeinsey Ridge that extends from north of Iceland at about 66° north to the west Jan Mayon ridge at about 71° degrees north in the North Atlantic . Pressures of partial crystallization were calculated from the compositions of natural liquids (glasses) with those of liquids in equilibrium with olivine, plagioclase, and clinopyroxene different pressures and temperature. Chemical analyses of mid-ocean ridge basalts (MORB) glasses collected along the Reykjanes and Kolbeinsey Ridge were used as liquid compositions. The glasses form by rapid cooling of magma when quenched by contact with seawater, and provide unambiguous samples of natural basalt liquids The calculated pressures were used to estimate the depths of partial crystallization of liquids in sub-crustal chambers or reservoirs. The results indicate that the depth of magma chambers of the Reykjanes Ridge decreases from 4 to 8 km (±0.8 km) near the Charlie Gibbs fracture zone to 1.2±0.5 km at 55.67° N. As the Ridge approaches Iceland the depth of chambers increases to 9.7±3 km. The limited data available for the Kolbeinsey Ridge provides only an approximate estimate of the depth of magma chambers (average, 8.2km) but the depths also seem to increase towards Iceland.. The shallow depths obtained for chambers beneath the southern part of the Reykjanes ridge and the average depth of chambers beneath the Kolbeinsey ridge is in contrast with results obtained for slow-spreading ridges elsewhere. This may reflect increased magma flux associated with the Iceland plume, and this is consistent with crustal thickening towards Iceland as suggested by the northerly increase in the maximum depths of chambers along the Reykjanus ridge. The influence of the Iceland plume is apparent from the chemical analyses of the glasses. The abundances of Ti, Na, K, P, and Fe increase whereas the abundances of Si, Mg, Al, and Ca decrease as Iceland is approached. These chemical data can also be interpreted in terms of increased magma flux reflecting the thermal effects of the Iceland plume.

Scott, J. L.; Panero, W. R.; Barton, M.

2009-05-01

145

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  

Science.gov (United States)

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.

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

2015-03-01

146

Zircon xenocrysts from the Kambalda volcanics: Age constraints and direct evidence for older continental crust below the Kambalda-Norseman greenstones  

International Nuclear Information System (INIS)

The Hangingwall Basal at Kambalda, Western Australia, contains zircons that have been shown by ion microprobe analyses to have very high U and Th contents and a wide variety of crystallization ages. Nearly all of these zircons certainly are xenocrysts; a few might relate to intrusive veinlets. The age of the youngest, 2693+-50 Ma (2sigma), shows that the eruptive age of the basalt cannot exceed 2743 Ma. This confirms that the apparent Sm-Nd isochron giving 3200 Ma for Kambalda mafic and ultramafic rocks is a mixing-line between unrelated components enriched and depleted in light rare earth elements. Mixing probably occurred at depth by erosion of 3200-3500 Ma old felsic crust from the walls of HWB conduits. The zircon xenocryst ages are the first direct evidence for the presence of very old felsic crust in the eastern Yilgarn Block. The latter implies that the Kalgoorlie-Norsemann greenstone sequences were formed in a continental rather than an oceanic environment. (orig.)

147

The Continental Crust: A Geophysical Approach  

Science.gov (United States)

Nearly 80 years ago, Yugoslavian seismologist Andrija Mohorovicic recognized, while studying a Balkan earthquake, that velocities of seismic waves increase abruptly at a few tens of kilometers depth , giving rise to the seismological definition of the crust. Since that discovery, many studies concerned with the nature of both the continental and oceanic crusts have appeared in the geophysical literature.Recently, interest in the continental crust has cascaded. This is largely because of an infusion of new data obtained from major reflection programs such as the Consortium for Continental Reflection Profiling (COCORP) and British Institutions Reflection Profiling Syndicate (BIRPS) and increased resolution of refraction studies. In addition, deep continental drilling programs are n ow in fashion. The Continental Crust: A Geophysical Approach is a summary of present knowledge of the continental crust. Meissner has succeeded in writing a book suited to many different readers, from the interested undergraduate to the professional. The book is well documented , with pertinent figures and a complete and up-to-date reference list.

Christensen, Nikolas I.

148

Alvin, Jason II, DSL-120 Investigation of Super-Fast EPR Crust Exposed at the Pito Deep Rift, Easter Microplate, SE Pacific  

Science.gov (United States)

Rift propagation along the NE corner of the Easter Microplate over the past 1 Ma has created a tectonic window into 3-Ma-old oceanic crust generated at the EPR at a full-rate of 144 mm/yr. Rift-related faults cut EPR abyssal hill lineaments and magnetic anomaly boundaries at a high angle revealing the internal structure and composition of the upper oceanic crust. Nested-scale investigations with the submersible Alvin, ROV Jason II, DSL-120 side-scan sonar and near-bottom magnetic surveys were carried out in 2 study areas each several kilometers long and spanning a depth range of about 1500 m. Direct observations, imaging, and sampling with these systems augment results from earlier Nautile dives in both areas. As anticipated, the major rock units mapped in the study areas include (from top down): 1) poorly consolidated pelagic ooze and chalk (900 m exposed above talus at the base of the scarp). Transitional units both above and below the sheeted dikes are highly variable but generally in the range of 100-250 m vertically. The internal structure of the lavas and sheeted dike units is complex with crustal sections tilted slightly toward or away from the EPR spreading axis. Faulting is highly localized and marked by intense brecciation and hydrothermal alteration, locally with abundant syn- to post-tectonic quartz vein networks. Variations in local crustal structure are interpreted in terms of temporal fluctuations in magma supply, faulting, and hydrothermal processes. Compared to crustal sections in other tectonic windows and deep drill holes in the Pacific region, super-fast EPR crust at Pito Deep has a relatively thin basaltic lava unit, a relatively thick sheeted dike unit, and less intense fracturing. These results provide a different perspective on the internal structure, composition, and variability of crust formed at very high spreading rates and processes at very fast-spreading ridges like the southern EPR.

Varga, R. J.; Karson, J. A.; Francheteau, J.; Gee, J. S.; Gillis, K.; Hekinian, R.; Hey, R.; Klein, E.; Naar, D.

2005-12-01

149

Variscan to eo-Alpine events recorded in European lower-crust zircons sampled from the French Massif Central and Corsica, France  

Science.gov (United States)

Ion-microprobe U-Pb zircon dating of lower-crust metasedimentary granulite are reported on samples from two localities in Europe in order to determine (a) how this environment recorded the Variscan and eo-Alpine events, and (b) whether the transition between the two orogenic cycles was continuous or separated by a gap. The samples come from enclaves hosted by Miocene volcanoes at Bournac in the French Massif Central, and from the granulitic metasedimentary basement of the Alpine Santa Lucia nappe in Corsica, on the South European paleomargin of the Ligurian branch of the Tethys Sea. The zircon ages from Bournac range between 630 and 430 Ma and between 380 and 150 Ma with a major frequency peak at 285 Ma; the zircons older than 430 Ma are interpreted as detrital, whereas those younger than 380 Ma are considered to have formed by metamorphic processes after burial in the lower crust. Zircon ages from Santa Lucia range from to 356 to 157 Ma, with exception of one inherited Archean grain, and are interpreted like the younger Bournac zircons as having been formed by metamorphic processes. In a granulite metamorphic environment, as opposed to an anatectic environment, new zircon growth can occur in the solid state. Once Zr has been incorporated into zircon, however, it is difficult to remobilize without dissolution; thus Zr available for new zircon growth must result from the breakdown of Zr-bearing minerals during prograde and/or retrograde events. In this light, the U-Pb zircon-age probability curves are interpreted as markers for major tectonometamorphic events, as suggested by the close correspondence between peaks in the curve and geological events recorded in the upper-crust, such as magma emplacement and basin subsidence. Evidence of a tectonometamorphic gap between the Variscan and Alpine orogeneses is provided by the Santa Lucia zircon-age probability curve, which reveals a probable interlude during the Variscan-Alpine transition between 240 and 210 Ma. Here, the peak at 240 Ma is interpreted as the very beginning of crustal extension and the low at 210 Ma as a period of quiescence prior to the formation of an active margin and oceanization.

Rossi, Philippe; Cocherie, Alain; Fanning, C. Mark; Deloule, Étienne

2006-04-01

150

Continental collision zones are primary sites for net continental crust growth — A testable hypothesis  

Science.gov (United States)

The significance of the continental crust (CC) on which we live is self-evident. However, our knowledge remains limited on its origin, its way and rate of growth, and how it has acquired the "andesitic" composition from mantle derived magmas. Compared to rocks formed from mantle derived magmas in all geological environments, volcanic arc rocks associated with seafloor subduction share some common features with the CC; both are relatively depleted in "fluid-insoluble" elements (e.g., Nb, Ta and Ti), but enriched in "fluid-soluble" elements (e.g., U, K and Pb). These chemical characteristics are referred to as the "arc-like signature", and point to a possible link between subduction-zone magmatism and CC formation, thus leading to the "island arc" model widely accepted for the origin of the CC over the past 45 years. However, this "island-arc" model has many difficulties: e.g., (1) the bulk arc crust (AC) is basaltic whereas the bulk CC is andesitic; (2) the AC has variably large Sr excess whereas the CC is weakly Sr deficient; and (3) AC production is mass-balanced by subduction erosion and sediment recycling, thus contributing no net mass to the CC growth, at least in the Phanerozoic. Our recent and ongoing studies on granitoid rocks (both volcanic and intrusive) formed in response to the India-Asia continental collision (~ 55 ± 10 Ma) show remarkable compositional similarity to the bulk CC with the typical "arc-like signature". Also, these syncollisional granitoid rocks exhibit strong mantle isotopic signatures, meaning that they were recently derived from a mantle source. The petrology and geochemistry of these syncollisional granitoid rocks are most consistent with an origin via partial melting of the upper ocean crust (i.e., last fragments of underthrusting ocean crust upon collision) under amphibolite facies conditions, adding net mantle-derived materials to form juvenile CC mass. This leads to the logical and testable hypothesis that continental collision produces and preserves the juvenile crust, and hence maintains net CC growth.

Niu, Yaoling; Zhao, Zhidan; Zhu, Di-Cheng; Mo, Xuanxue

2013-12-01

151

Central Tibetan Meso-Tethyan oceanic plateau  

Science.gov (United States)

We report the occurrences of the remnants of a Meso-Tethyan oceanic plateau, encompassing an area of ~ 2 × 105 km2 in central Tibet. The plateau remnants include large volumes of pillow basalt formed largely by emergent to subaerial eruption, minor ultramafic intrusives and cumulates, exotic blocks of limestone, radiolarian chert, graywacke, and shale. Isotopic and paleontological dating suggest two major plateau eruptive events at 193-173 Ma and at 128-104 Ma, respectively. The basalts are characterized by enrichment of incompatible elements and a wide range of Sr-Nd isotope composition (initial ?Nd from -3.71 to + 7.9, initial 87Sr/86Sr from 0.703927 to 0.707618). The trace element and Sr-Nd isotopic data suggest that these basalts are of affinity with those from the Kerguelen and Tethyan plumes, indicative of a plume mantle upwelling origin with involvement of continental material. The wholesale obduction of the Meso-Tethyan oceanic plateau, along with the dismembered normal oceanic crustal fragments, over the Tibetan continental crust could have given rise to perhaps 2 km elevation of central Tibet during the Late Cretaceous.

Zhang, Kai-Jun; Xia, Bin; Zhang, Yu-Xiu; Liu, Wei-Liang; Zeng, Lu; Li, Jian-Feng; Xu, Li-Feng

2014-12-01

152

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)

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.

R. Banerjee

2010-07-01

153

Birth of an ocean in the Red Sea: Initial pangs  

Science.gov (United States)

We obtained areal variations of crustal thickness, magnetic intensity, and degree of melting of the sub-axial upwelling mantle at Thetis and Nereus Deeps, the two northernmost axial segments of initial oceanic crustal accretion in the Red Sea, where Arabia is separating from Africa. The initial emplacement of oceanic crust occurred at South Thetis and Central Nereus roughly ˜2.2 and ˜2 Ma, respectively, and is taking place today in the northern Thetis and southern Nereus tips. Basaltic glasses major and trace element composition suggests a rift-to-drift transition marked by magmatic activity with typical MORB signature, with no contamination by continental lithosphere, but with slight differences in mantle source composition and/or potential temperature between Thetis and Nereus. Eruption rate, spreading rate, magnetic intensity, crustal thickness and degree of mantle melting were highest at both Thetis and Nereus in the very initial phases of oceanic crust accretion, immediately after continental breakup, probably due to fast mantle upwelling enhanced by an initially strong horizontal thermal gradient. This is consistent with a rift model where the lower continental lithosphere has been replaced by upwelling asthenosphere before continental rupturing, implying depth-dependent extension due to decoupling between the upper and lower lithosphere with mantle-lithosphere-necking breakup before crustal-necking breakup. Independent along-axis centers of upwelling form at the rifting stage just before oceanic crust accretion, with buoyancy-driven convection within a hot, low viscosity asthenosphere. Each initial axial cell taps a different asthenospheric source and serves as nucleus for axial propagation of oceanic accretion, resulting in linear segments of spreading.

Ligi, Marco; Bonatti, Enrico; Bortoluzzi, Giovanni; Cipriani, Anna; Cocchi, Luca; Caratori Tontini, Fabio; Carminati, Eugenio; Ottolini, Luisa; Schettino, Antonio

2012-08-01

154

Temperature distribution in the crust and mantle  

Science.gov (United States)

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.

Jeanloz, R.; Morris, S.

1986-01-01

155

Hf Isotope Constraints on the Longevity of the Indian Ocean Mantle Signature: New Results from Tethyan Ophiolites  

Science.gov (United States)

Studies over the last decade of drilled seafloor basalts have shown that the distinct Sr-Nd-Pb isotope compositions exhibited by Indian Ocean MORB extend well into the geologic past. Investigations of Tethyan seafloor exposed in ophiolites extending from eastern Europe to China indicate that the 'Indian Ocean' isotopic signature existed well before the opening of the Indian Ocean basin. The Lu-Hf system is particularly useful for testing the longevity and magnitude of the Indian Ocean isotopic signature because of its resistance to the effects of alteration and because the Indian Ocean signature is strongly expressed as having radiogenic Hf for a given Nd isotopic composition relative to oceanic basalts worldwide. Hf isotope data will be presented for MORB-like basalts from 20 DSDP sites throughout the Indian Ocean and for Neotethyan ophiolites (up to 200 Ma) from Iran and the Indus-Yarlung Suture Zone in Tibet. These data show that Indian Ocean type Nd-Hf isotopic compositions (outside the range of Pacific & Atlantic MORB) do indeed extend back to at least 200 Ma, but the maximum decoupling observed between Hf and Nd isotopes in Tethyan seafloor basalts (e.g., from ophiolites and Wharton Basin) is less than that measured in Indian Ocean seafloor basalts. Thus, although similar processes (e.g., assimilation of subduction modified mantle wedge and/or lower continental crust) may have been responsible for the distinct Sr-Nd-Pb-Hf isotopic composition of both Indian Ocean and Tethyan seafloor basalts, the closing of the Tethyan ocean and opening of the Indian Ocean basin appears to have significantly increased the heterogeneity of the upper mantle now located beneath the Indian Ocean.

Janney, P. E.

2008-12-01

156

Perspectives of Electromagnetic Sounding in the Arctic Ocean (Invited)  

Science.gov (United States)

The Arctic Ocean differs from other oceans by the fact that crust of continental type can be found in some parts of its area on typical oceanic depths. 3D modeling shows that MTS from ice cover reliably distinguishes the type of crust, but mapping of the ocean structures is possible only from the ocean floor. From the analysis of experimental MVP data it follows that more complex model of the source than the plane wave is needed.

Velikhov, E.; Korotaev, S.; Kruglyakov, M.; Orekhova, D.; Popova, I.; Schors, Y.; Shneyer, V.; Trofimov, I.; Zhdanov, M. S.

2010-12-01

157

Neutron star crusts  

Science.gov (United States)

We calculate properties of neutron star matter at subnuclear densities using an improved nuclear Hamiltonian. Nuclei disappear and the matter becomes uniform at a density of about 0.6n(s), where n(s) of about 0.16/cu fm is the saturation density of nuclear matter. As a consequence, the mass of matter in the crusts of neutron stars is only about half as large as previously estimated. In about half of that crustal mass, nuclear matter occurs in shapes very different from the roughly spherical nuclei familiar at lower densities. The thinner crust and the unusual nuclear shape have important consequences for theories of the rotational and thermal evolution of neutron stars, especialy theories of glitches.

Lorenz, C. P.; Ravenhall, D. G.; Pethick, C. J.

1993-01-01

158

The effect of hydrothermal circulation on subsidence on ocean basins : evidence from the South East Indian Ocean  

Science.gov (United States)

While it is generally accepted that off-axis hydrothermal circulation is responsible for the majority of the observed oceanic heat flow anomalies (e. g. for the "missing" conductive heatflow through oceanic lithosphere), the effect of water circulation on the subsidence of ocean basins at the scale of tens of millions years has not been thoroughly recognized. We present a very simple model (based on the half-space model) showing that, that in some particular circumstances, hydrothermal circulation may be held responsible for unexplained subsidence rate anomalies at the scale of ocean basins. An example is given for the poorly sedimented South-East Indian Ridge flanks of age Ma1/2 ) can be more readily explained by the observed sedimentation pattern and the subsequent pattern in hydrothermal circulation than by ad hoc variations in the mantle thermal parameters. Our model assumes that in absence of sedimentation, hydrothermal circulation convectively maintains low temperatures within the upper crust at ages as old as 20 to 30 Ma. Agreement with re-assesed subsidence estimates supports the model hypothesis.

Géli, L.; Francheteau, J.

2003-04-01

159

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

OpenAIRE

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...

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

1997-01-01

160

Arc-continent collision and the formation of continental crust: A new geochemical and isotopic record from the Ordovician Tyrone Igneous Complex, Ireland  

Science.gov (United States)

Collisions between oceanic island-arc terranes and passive continental margins are thought to have been important in the formation of continental crust throughout much of Earth's history. Magmatic evolution during this stage of the plate-tectonic cycle is evident in several areas of the Ordovician Grampian-Taconic orogen, as we demonstrate in the first detailed geochemical study of the Tyrone Igneous Complex, Ireland. New U-Pb zircon dating yields ages of 493 2 Ma from a primitive mafic intrusion, indicating intra-oceanic subduction in Tremadoc time, and 475 10 Ma from a light rare earth element (LREE)-enriched tonalite intrusion that incorporated Laurentian continental material by early Arenig time (Early Ordovician, Stage 2) during arc-continent collision. Notably, LREE enrichment in volcanism and silicic intrusions of the Tyrone Igneous Complex exceeds that of average Dalradian (Laurentian) continental material that would have been thrust under the colliding forearc and potentially recycled into arc magmatism. This implies that crystal fractionation, in addition to magmatic mixing and assimilation, was important to the formation of new crust in the Grampian-Taconic orogeny. Because similar super-enrichment of orogenic melts occurred elsewhere in the Caledonides in the British Isles and Newfoundland, the addition of new, highly enriched melt to this accreted arc terrane was apparently widespread spatially and temporally. Such super-enrichment of magmatism, especially if accompanied by loss of corresponding lower crustal residues, supports the theory that arc-continent collision plays an important role in altering bulk crustal composition toward typical values for ancient continental crust. ?? 2009 Geological Society of London.

Draut, Amy E.; Clift, Peter D.; Amato, Jeffrey M.; Blusztajn, Jerzy; Schouten, Hans

2009-01-01

161

A model of oceanic development by ridge jumping: Opening of the Scotia Sea  

Science.gov (United States)

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.

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

2014-12-01

162

Late Mesozoic magmatic records in NE Russia and coeval events in Arctic ocean  

Science.gov (United States)

Magmatic records onshore of Arctic Siberia and Alaska may provide constrains on formation of Arctic ocean including Amerasia and Eurasia basins, helps to interpret geophysical data and better understand meaning of spars dredged samples. We used new geochemistry and geochronology (U-Pb and Ar-Ar) data collected in the last decade across North East Russia and partly Alaska to better understand timing and tectonic setting of main magmatic events. It is widely accepted that opening of the Canada basin could have occurred as early as the late Jurassic-earliest Cretaceous (Grantz et al., 1998). Some of the models suggest initial rifting in the early Jurassic and seafloor spreading initiated at 145-142 Ma (Seton et al., 2012). Much older Devonian oceanic crust fragments found in the continental part of Arctic Alaska (Angaucham) as well as in Chukotka (Billings-Provideniya suture zone). Recently we mapped spars outcrops of metaultramafic rocks located within Velitkenay granite-migmatite complex and within Koolen metamorphic dome and Senyavin uplift. Metamorphic growth zircon from ultramafic rocks have U-Pb ages between 360 to 400 Ma. The most important Late Jurassic - Cretaceous collision and subduction-related magmatic provinces in NE Russia related mostly to paleo-Pacific events but some coincide with tectonic episodes in Arctic: (1) 155-145 Ma granitoids of Main Kolyma batholith belt and coeval Uyandino-Yasachnaya volcanic arc (partly coeval with closure of the Anyui Ocean around c. 160-145 Ma); (2) 150-140 Ma volcanics of subduction related Uda-Murgal and Nutesyn margin continental arcs; (3) 130-135 Ma Northern belt granites and oldest granitic complexes in Eastern Chukotka (coeval with beginning of HALIP volcanism); (4) 118-124 Ma Tytylveem continental volcano-plutonic belt in Chukotka coeval with suggested cessation of spreading in the Canada basin; (5) 100-109 Ma extension-related granite-metamorphic core complexes along Arctic cost of Chukotka (coeval with within-plate alkaline basalts of De-Longy archipelago) (6) 106-78 Ma subduction-related Okhotsk-Chukotka volcanic belt (final stage of volcanism in Chukotka at 88-90 Ma coeval with Iceland plume and opening of the Labrador Sea and Baffin Bay between 90 and 55 Ma, which might have affected the Central Arctic region). Small volume alkali basalts eruption happened during 54 to to 37 Ma in the Chersky seismic belt triggered by extension and thinning of the lithosphere combined with adiabatic upwelling of the underlying mantle (coeval with opening of the Eurasia basin at 55-33 Ma).

Akinin, V. V.

2013-12-01

163

Lithospheric structure of the Costa Rican Isthmus: Effects of subduction zone magmatism on an oceanic plateau  

Science.gov (United States)

We present the results of a multidisciplinary geophysical study, conducted to investigate the lithospheric structure of the Costa Rican Isthmus. The physical properties of the lithosphere are resolved by three-dimensional (3-D) simultaneous inversion of velocity anomalies and hypocenter parameters using local earthquakes and 2-D forward modeling of onshore and offshore seismic refraction and gravity data. According to our results, the northern half of the Costa Rican Isthmus is constituted by a ˜40-km-thick crust, with a 6- to 7-km-thick oceanic crust subducting under it. The uppermost level of the basement and most of the marginal wedge show intermediate velocities and high densities, in good agreement with those described for flood basalts. The midlevel shows velocities and densities representative of oceanic crust. The bottommost level (20-40 km) shows high velocities and densities, typical of mafic rocks, and the upper mantle displays anomalously low densities and velocities. Intracrustal heterogeneities at intermediate wavelengths are indicated by prominent velocity anomalies. These results are consistent with a basement beneath the Costa Rican Isthmus being part of the Caribbean plateau, originated at 85-90 Ma with the onset of the Galápagos hotspot. The upper level corresponds to the flood basalts extruded during this phase, and it includes most of the marginal wedge. The second level represents the preexisting oceanic crust. The mafic lower crust, intracrustal heterogeneities, and anomalous upper mantle are interpreted to be built up by underplating, intrusion, and crystallization of basaltic melts, formed under the influence of subducting lithosphere dehydration.

SallarèS, Valentí; DañObeitia, Juan José; Flueh, Ernst R.

2001-01-01

164

Dating Detrital Zircons from Paleozoic Strata on the Northern Margin of the Tarim Craton: Constraints on the Timing of Closure of Paleo-Asian Ocean  

Science.gov (United States)

The Central Asian Orogenic Belt (CAOB) is underwent a long-lived and complicated accretion/collision during Neoproterozoic to Paleozoic times. As the western part of the suture zone of the Paleo-Asian Ocean (PAO) whose closure to have formed the CAOB, the Chinese South Tianshan (STS) belt is considered to have resulted from the closure of the western sector of the PAO, called the STS ocean, between the Tarim craton and the Yili-Central Tianshan block. However, there is a hot debate on the timing of the final closure of the PAO and its subduction polarity. Sedimentary basins at the northwestern margin of the Tarim craton preserve successive Paleozoic deposits, and enable us to determine the timing of the final closure of the STS ocean by tracing the provenance of clastic sediments and especially U-Pb ages of detrital zircons from the Paleozoic strata. Detrital zircons from Late Silurian sandstone gave the youngest age of 417×7 Ma, defining its maximum depositional age. The dating results are dominated by four major populations: 417-470 Ma, 750-1007 Ma, 1740-2100 Ma and 2400-2700 Ma. The later three populations are correlated well with major tectono-magmatic events in the Tarim craton, suggesting that they resulted from the reworking of the old crust of the craton. The age spectrum is consistent with previous dating results of sedimentary rocks in the Central Tianshan (CTS) block, implying the block was part of the Tarim craton prior to ~417 Ma. Five sandstones collected from the Late Devonian, the Early Carboniferous and the middle Carboniferous yielded the youngest zircon U-Pb ages ranging from 413×6 Ma to 442×7 Ma, which are interpreted as their maximum depositional ages. The age spectra of these samples are remarkably similar and show four major age populations at 415-485Ma, 750-1020Ma, 1750-2100Ma and 2400-2680Ma, coinciding with those of the Late Silurian sample. This suggests a long-term recycling of pre-Devonian sediments without addition of juvenile materials, implying a passive continental margin setting. Intense magmatism at 380-340 Ma in the CTS and Yili blocks was not recorded in the Devonian-middle Carboniferous sedimentary rocks at the northwestern margin of the Tarim craton, suggesting that the CTS block was separated from the Tarim craton by the STS Ocean during Devonian to middle Carboniferous times. Zircon grains from two Middle Permian sandstones yielded the youngest age of 264×5 Ma. The two samples preserve detrital zircons with major age populations at 270-330Ma, 410-490 Ma, 720-1100 Ma, 1730-2120 Ma and 2400-2590 Ma. The last four populations are consistent with those of the Silurian-Carboniferous samples, resulting from the recycling of old crust. The population of 270-330 Ma could be related to the Permian mantle plume-related large igneous province in the Tarim craton and the collision-related magmatism and ultra-high pressure metamorphism in the CTS and Yili blocks. Therefore, our data suggest that the final closure of the STS Ocean and the collision between the Tarim craton and CTS-Yili block may have happened in the Late Paleozoic. Acknowledgements: NSFC (41190070, 41190075)

Han, Y.; Zhao, G.

2013-12-01

165

Earth’s earliest evolved crust generated in an Iceland-like setting  

Science.gov (United States)

It is unclear how the earliest continental crust formed on an Earth that was probably originally surfaced with oceanic crust. Continental crust may have first formed in an ocean island-like setting, where upwelling mantle generates magmas that crystallize to form new crust. Of the oceanic plateaux, Iceland is closest in character to continental crust, because its crust is anomalously thick and contains a relatively high proportion of silica-rich (sialic) rocks. Iceland has therefore been considered a suitable analogue for the generation of Earth’s earliest continental crust. However, the geochemical signature of sialic rocks from Iceland is distinct from the typical 3.9- to 2.5-billion-year-old Archaean rocks discovered so far. Here we report the discovery of an exceptionally well-preserved, 4.02-billion-year-old tonalitic gneiss rock unit within the Acasta Gneiss Complex in Canada. We use geochemical analyses to show that this rock unit is characterized by iron enrichment, negative Europium anomalies, unfractionated rare-earth-element patterns, and magmatic zircons with low oxygen isotope ratios. These geochemical characteristics are unlike typical Archaean igneous rocks, but are strikingly similar to those of the sialic rocks from Iceland and imply that this ancient rock unit was formed by shallow-level magmatic processes that include assimilation of rocks previously altered by surface waters. Our data provide direct evidence that Earth’s earliest continental crust formed in a tectonic setting comparable to modern Iceland.

Reimink, Jesse R.; Chacko, Thomas; Stern, Richard A.; Heaman, Larry M.

2014-07-01

166

Paleo-elevation and subsidence of ˜145Ma Shatsky Rise inferred from CO2 and H2O in fresh volcanic glass  

Science.gov (United States)

Shatsky Rise, a large Mesozoic oceanic plateau in the northwest Pacific, consists of three massifs (Tamu, Ori, and Shirshov) that formed near a mid-ocean-ridge triple junction. Published depth estimates imply that Shatsky Rise has not subsided normally, like typical oceanic lithosphere. We estimated paleo-eruption depths of Shatsky Rise massifs on the basis of dissolved CO2 and H2O in volcanic glass and descriptions of cores recovered from five sites of Integrated Ocean Drilling Program Expedition 324. Initial maximum elevations of Shatsky Rise are estimated to be 2500-3500 m above the surrounding seafloor and the ensuing subsidence of Shatsky Rise is estimated to be 2600-3400 m. We did not observe the anomalously low subsidence that has been reported for both Shatsky Rise and the Ontong Java Plateau. Although we could not resolve whether Shatsky Rise originated from a hot mantle plume or non-plume fusible mantle, uplift and subsidence histories of Shatsky Rise for the both cases are constrained based on the subsidence trend from the center of Tamu Massif (˜2600 m) toward the flank of Ori Massif (˜3400 m). In the case of a hot mantle plume origin, Shatsky Rise may have formed on young (˜5 Ma) pre-existing oceanic crust with a total crustal thickness of ˜20 km. For this scenario, the center of Shatsky Rise is subsequently uplifted by later (prolonged) crustal growth, forming the observed ˜30 km thickness crust. For a non-plume origin, Shatsky Rise may have formed at the spreading ridge center as initially thick crust (˜30 km thickness), with later reduced subsidence caused by the emplacement of a buoyant mass-perhaps a refractory mantle residuum-beneath the center of Shatsky Rise.

Shimizu, Kenji; Shimizu, Nobumichi; Sano, Takashi; Matsubara, Noritaka; Sager, William

2013-12-01

167

The crust of neutron stars  

OpenAIRE

The structure of the crust of a neutron star is completely determined by the experimentally measured nuclear masses up to a density of the order of 10^11 g.cm^{-3}. At higher densities, the composition of the crust still remains uncertain, mainly due to the presence of ``free'' superfluid neutrons which affect the properties of the nuclear ``clusters''. After briefly reviewing calculations of the equilibrium structure of the crust, we point out that the current approach base...

Chamel, Nicolas

2006-01-01

168

Heat flux through an old (˜175 Ma) passive margin: Offshore southeastern United States  

Science.gov (United States)

New heat flow data on the southeastern United States passive margin show that measured, uncorrected flux averages 49±11.8 mW m-2 through old (˜175 Ma) oceanic crust. Nonuniform thermal gradients were measured at about half of the 114 penetrations that comprise the data set and over the full range of water depths (1900 m to 4250 m) at which data were collected. With the simplifying assumption that the nonuniform gradients were caused by a step function change in bottom water temperatures at some time before the heat flow cruises, concave down (decreasing dT/dz with depth) and concave up (increasing dT/dz) sediment thermal gradients can be explained by respective average temperature decreases and increases of 0.1-0.2 K amplitude occurring 28-35 days before the measurements. Thermal gradients throughout the entire region are strongly influenced by oceanographic phenomena and locally by the presence of subsurface diapiric structures, while sediment thickness variations and lateral differences in sedimentation rate and sediment lithology appear to exercise relatively little control over thermal regimes. Mean reduced heat flow in the study area is estimated at ˜49 mW m-2 by decompacting and back stripping the 5-8 km of sediment deposited on the margin since the formation of the underlying oceanic crust. This value agrees with previous measurements made on younger crust in the Blake Ridge area but is significantly higher than the reduced heat flow value in a similar passive margin setting at the Baltimore Canyon Trough. We use the calculated average thermal gradient value and an assumption of constant conductivity to estimate temperatures of 19.5°C to 24.5°C at the bottom-simulating reflector (BSR) on the Blake Ridge slope.

Ruppel, C.; von Herzen, R. P.; Bonneville, A.

1995-10-01

169

Update on CRUST1.0 - A 1-degree Global Model of Earth's Crust  

Science.gov (United States)

Our new 1-by-1 degree global crustal model, CRUST1.0, was introduced last year and serves as starting model in a comprehensive effort to compile a global model of Earth's crust and lithosphere, LITHO1.0 (Pasyanos et al., 2012). The Moho depth in CRUST1.0 is based on 1-degree averages of a recently updated database of crustal thickness data from active source seismic studies as well as from receiver function studies. In areas where such constraints are still missing, for example in Antarctica, crustal thicknesses are estimated using gravity constraints. The compilation of the new crustal model initially followed the philosophy of the widely used crustal model CRUST2.0 (Bassin et al., 2000; http://igppweb.ucsd.edu/~gabi/crust2.html) to assign elastic properties in the crystalline crust according to basement age or tectonic setting (loosely following an updated map by Artemieva and Mooney (2001; http://www.lithosphere.info). For cells with no local seismic or gravity constraints, statistical averages of crustal properties, including crustal thickness, were extrapolated. However, in places with constraints the depth to basement and mantle are given explicitly and no longer assigned by crustal type. This allows for much smaller errors in both. In each 1-degree cell, boundary depth, compressional and shear velocity as well as density is given for 8 layers: water, ice, 3 sediment layers and upper, middle and lower crystalline crust. Topography, bathymetry and ice cover are taken from ETOPO1. The sediment cover is based on our sediment model (Laske and Masters, 1997; http://igppweb.ucsd.edu/~sediment.html), with some near-coastal updates. In an initial step toward LITHO1.0, the model is then validated against new global surface wave disperison maps and adjusted in areas of extreme misfit. This poster presents the next validation step: compare the new Moho depths with in-situ active source and receiver function results. We also present comparisons with CRUST2.0. CRUST1.0 is available for download. References: Pasyanos, M.E., Masters, G., Laske, G. and Ma, Z., LITHO1.0 - An Updated Crust and Lithospheric Model of the Earth Developed Using Multiple Data Constraints, Abstract T11D-09 presented at 2012 Fall Meeting, AGU, San Francisco, Calif., 3-7 Dec, 2012. Artemieva, I.M. and Mooney, W.D., Thermal thickness and evolution of Precambrian lithosphere: A global study, J. Geophys. Res., 106, 16,387-16,414, 2001. Bassin, C., Laske, G. and Masters, G., The Current Limits of Resolution for Surface Wave Tomography in North America, EOS Trans AGU, 81, F897, 2000. Laske, G. and Masters, G., A Global Digital Map of Sediment Thickness, EOS Trans. AGU, 78, F483, 1997. URL: http://igppweb.ucsd.edu/~gabi/crust1.html

Laske, Gabi; Masters, Guy; Ma, Zhitu; Pasyanos, Mike

2013-04-01

170

Diverse Manifestations of Convective Upwelling Beneath the North Atlantic Ocean  

Science.gov (United States)

The Icelandic Plume dominates the North Atlantic Ocean. Residual depth anomalies of oceanic lithosphere, long wavelength gravity anomalies, and seismic tomographic models show that this large upwelling reaches from Baffin Bay to Western Norway, and from offshore Newfoundland to Spitzbergen. At continental margins, there is excellent evidence for present-day dynamic support of crust beneath Scotland and Western Norway. It is generally agreed that the Icelandic Plume started at 62 Ma. In recent years, a quantitative understanding of the temporal evolution of this upwelling has begun to emerge. The best evidence occurs in the oceanic basins north and south of Iceland. Since the mid-oceanic ridge straddles the plume, it acts as a linear sampler of transient activity over the last 40-50 Ma. A pair of seismic reflection flowlines acquired in 2010 have enabled us to determine the detailed history of transient activity. The implications of this history are profound. Waxing and waning of convective upwelling beneath this important oceanic gateway appears to have modulated the overflow of the ancient precursor to North Atlantic Deep Water (NADW). The growth of contourite drifts which plaster deep-water margins can also be directly linked to changing vertical motions at this gateway. Finally, there is increasing evidence that the otherwise uniform thermal subsidence of sedimentary basins, which fringe both sides of the North Atlantic Ocean, has been periodically interrupted by transient uplift events which generated ephemeral landscapes. These geologic manifestations of convective activity should lead to improved insights into the fluid dynamics of the mantle.

White, Nicky; Parnell-Turner, Ross

2013-04-01

171

Physics of Neutron Star Crusts  

OpenAIRE

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.

Chamel Nicolas; Haensel Pawel

2008-01-01

172

Physics of Neutron Star Crusts  

Directory of Open Access Journals (Sweden)

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.

Chamel Nicolas

2008-12-01

173

Triassic "adakitic" rocks in an extensional setting (North China): Melts from the cratonic lower crust  

Science.gov (United States)

Adakite was originally defined as a specific type of magmatic rock derived from melting of subducted oceanic plates (Defant, M.J., Drummond, M.S., 1990. Derivation of some modern arc magmas by melting of young subducted lithosphere. Nature 347 (6294), 662-665), producing unique chemical signatures with high Sr/Y and La/Yb. However, widespread occurrences of igneous rocks that are geochemically similar to the adakites, but from diverse tectonic settings, suggest that "adakitic" rocks may have a variety of origins. Late Triassic high Sr/Y lavas, mainly trachytes with minor pyroxene andesite and rhyolite, are found at Shuiquangou, in the Yanshanian fold-and-thrust belt on the northern margin of the North China Craton. Data on mineral chemistry, major and trace elements and Sr-Nd isotopes of whole rocks, and in situ U-Pb age and Hf-isotope analyses of zircons are reported here. The Shuiquangou volcanic rocks with high Sr/Y (> 72) and (La/Yb)N (> 24) also show enrichment in light rare-earth elements and large-ion lithophile elements (e.g., Rb, Ba and Pb), and depletion in high-field-strength elements (e.g., Nb, Ta and Ti). They have low Ce/Pb (< 4.3) and Nb/U (< 4.8) and moderate (Gd/Yb)N (2.8-3.9). U-Pb dating of zircons yields concordant and lower-intercept ages of ~ 220 Ma, indicating that they erupted during the late Triassic. Concordant grains and an upper intercept age of ~ 2.50 Ga suggest that Neoarchean materials may have been involved in their petrogenesis. The relatively low initial 87Sr/86Sr (0.70529 to 0.70540) and negative ?Nd(t) (- 3.9 to - 9.9) of the these rocks, and the negative ?Hf(t) (- 8.6 to - 1.1) of their zircons, suggest that the magmas were derived by partial melting of the cratonic lower crust, induced by continuous magmatic underplating under an extensional regime following the southward subduction of the Paleo-Asian Ocean. Their high Sr/Y is inherited from their source, and does not necessarily imply melting at great depths (e.g., garnet-bearing lower crust). We suggest that partial melting of the ancient lower crust may be important for the petrogenesis of "adakitic" magmas in a continental extensional setting.

Ma, Qiang; Zheng, Jianping; Griffin, W. L.; Zhang, Ming; Tang, Huayun; Su, Yuping; Ping, Xianquan

2012-09-01

174

The Evolution of the Lunar Highlands Crust: A Complicated History  

Science.gov (United States)

More than 30 years after the first Lunar Highlands Crust conference, lunar petrology and remote sensing experts from around the world gathered to discuss and debate the formation and evolution of the lunar highlands crust. Huge strides in orbital remote sensing have enabled researchers to put the samples gathered during the Apollo missions into a larger, global context, yet many of the original, key questions remain. What was the extent and fate of the lunar magma ocean (LMO)? What is the nature of the lower lunar crust? Do lunar sample ages still suggest that the inner solar system was subject to an increase in impact flux around 3.9 billion years ago? At the heart of these questions is the desire to understand not only the formation and evolution of the Moon but of Earth, the terrestrial planets, and the inner solar system as a whole.

Klima, Rachel

2013-01-01

175

Tectonic relations between shallow and deep crust in the southeastern Brazilian continental margin: low temperature thermochronology, gravimetry and seismic reflection  

Science.gov (United States)

Low-temperature thermochronology studies, gravimetric and seismic reflection modeling, developed on the southeastern Brazil has been approached independently and without apparent connection. This paper correlates data from shallow and deep crust in the region that includes the Serra do Mar and Mantiqueira. This region is formed by Precambrian rocks with steep topography resulted of intense reworking during the Mesozoic and Cenozoic. Fission tracks data on zircon, apatite and U-Th/He methodology record a polycyclic history with tectonic peaks at temperatures below 240oC in 90, 60 and 45 Ma. Uplift and exhumation alternated heterogeneously along the margin, related to a E-W extensional process with strong vertical movements. Associated with the history of the Eocene, Precambrian rocks, forms structure of the Southeastern Brazilian Continental Rift of totaling approximately 2 000 km along the continental and submerged margin of the southeastern South America Gravimetric modeling shows an alignment of denser rocks at the base of the crust along the Rift. Interpretation of reflexion seismic section in the Campos Basin, shows syn-rift, post-rift stratigraphic sequences and Precambrian basement, postulating an tectonic evolution with an crustal stretching (Cainelli, C., Mohriak, W.U.,1998; Macedo, J.M., 1989). This process would be associated with the drift phase (Post-Albian) responsible for the large amount of clastic sediments to the marginal basins and can be observed in the interpretations of seismic profiles and wells. The correlation of the thermochronological, seismic and gravimetric tools allows us to consider an E-W stretching with thinning of the continental, until the oceanic crust, in SE Brazil, with uplift of the lithospheric mantle and consequent formation of the Serra do Mar and Mantiqueira, erosion and deposition of sediments of the Southeastern Brazilian Continental Rift, all occurring, after the drift phase of the South Atlantic Rifting. Concepts of plume and delamination can be attributed to these modeling.

Hackspacher, P. C.; Souza, I. A.; Almeida, S. H.; Glasmacher, U. A.

2012-04-01

176

Lithospheric cooling as a basin forming mechanism within accretionary crust.  

Science.gov (United States)

Widely accepted basin forming mechanisms are limited to flexure of the lithosphere, lithospheric stretching, lithospheric cooling following rifting and, possibly, dynamic topography. In this work forward models have been used to investigate lithospheric growth due to cooling beneath accretionary crust, as a new basin forming mechanism. Accretionary crust is formed from collision of island arcs, accretionary complexes and fragments of reworked older crust at subduction zones, and therefore has thin lithosphere due to melting and increased convection. This is modeled using a 1D infinite half space cooling model similar to lithospheric cooling models for the oceans. The crustal composition and structure used in the models has been varied around average values of accretionary crust to represent the heterogeneity of accretionary crust. The initial mantle lithosphere thickness used in the model was 20 km. The model then allows the lithosphere to thicken as it cools and calculates the subsidence isostatically. The model produces sediment loaded basins of 2-7 km for the various crustal structures over 250 Myrs. Water-loaded tectonic subsidence curves from the forward models were compared to tectonic subsidence curves produced from backstripping wells from the Kufrah and Ghadames basins, located on the accretionary crust of North Africa. A good match between the subsidence curves for the forward model and backstripping is produced when the best estimates for the crustal structure, composition and the present day thickness of the lithosphere for North Africa are used as inputs for the forward model. This shows that lithospheric cooling provides a good method for producing large basins with prolonged subsidence in accretionary crust without the need for initial extension.

Holt, P. J.; Allen, M.; van Hunen, J.; Björnseth, H. M.

2009-04-01

177

Evolution of the earth's crust: Evidence from comparative planetology  

Science.gov (United States)

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.

Lowman, P. D., Jr.

1973-01-01

178

Spreading behaviour of the Pacific-Farallon ridge system since 83 Ma  

Science.gov (United States)

We present improved rotations, complete with uncertainties, for the Pacific-Farallon Ridge (PFR) between geomagnetic chrons 34y (83 Ma) and 10y (28.28 Ma). Despite substantial shortening since ˜55 Ma, this ridge system and its remnants (e.g. the East Pacific Rise) have produced as much as 45 per cent of all oceanic lithosphere created since the Late Cretaceous, but reconstructions face the twin challenges of extensive subduction of Farallon crust-which precludes reconstruction by fitting conjugate magnetic anomaly and fracture zone (FZ) traces-and asymmetric spreading behaviour for at least the past 51 Myr. We have calculated best-fit `half'-angle stage rotations between nine geomagnetic chron boundaries (34y, 33y, 29o, 24.3o, 20o, 18.2o, 17.1y, 13y and 10y) using combined anomaly and FZ data from both the northern and southern Pacific Plate. For rotations younger than chron 24.3o, estimates for spreading asymmetry, derived using anomaly picks from yet-to-be subducted Farallon/Nazca crust in the south Pacific, allow full stage rotations to be calculated. Between 50 and 83 Ma, where no direct constraints on spreading asymmetry are possible, a `best-fit' full stage rotation was calculated based on the net Nazca:Pacific spreading asymmetry (Pacific spreading fraction fPAC = 0.44) over the past 50 Myr, with conservative lower and upper bounds, based on variability in the degree of spreading asymmetry over periods of relatively constant trend of the major Pacific FZs. This stability spans at least one episode of Farallon Plate fragmentation caused by subduction of PFR segments beneath the Americas, at 55-48 Ma, which appears to have greatly accelerated divergence on the surviving ridge without significantly affecting the location of the instantaneous rotation pole. Together with quasi-periodic 15-20 Myr variations in the degree of spreading asymmetry that also appear to correlate with changes in spreading rate, this indicates that forces other than slab pull may be a factor in determining Pacific-Farallon Plate motions.

Rowan, Christopher J.; Rowley, David B.

2014-06-01

179

Crustal thickening prior to 220 Ma in the East Kunlun Orogenic Belt: Insights from the Late Triassic granitoids in the Xiao-Nuomuhong pluton  

Science.gov (United States)

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.

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

2014-10-01

180

Global maps of the CRUST 2.0 crustal components stripped gravity disturbances:  

OpenAIRE

We use the CRUST 2.0 crustal model and the EGM08 geopotential model to compile global maps of the gravity disturbances corrected for the gravitational effects (attractions) of the topography and of the density contrasts of the oceans, sediments, ice, and the remaining crust down to the Moho discontinuity. Techniques for a spherical harmonic analysis of the gravity field are used to compute both the gravity disturbances and the topographic and bathymetric corrections with a spectral resolution...

Tenzer, R.; Hamayun, K.; Vajda, P.

2009-01-01

181

Processes of Magma-crust Interaction : Insights from Geochemistry and Experimental Petrology  

OpenAIRE

This work focuses on crustal interaction in magmatic systems, drawing on experimental petrology and elemental and isotope geochemistry. Various magma-chamber processes such as magma-mixing, fractional crystallisation and magma-crust interaction are explored throughout the papers comprising the thesis. Emphasis is placed on gaining insights into the extent of crustal contamination in ocean island magmas from the Canary Islands and the processes of magma-crust interaction observed both in natur...

Deegan, Frances M.

2010-01-01

182

Nature of crust in the central Red Sea  

Science.gov (United States)

A transition between continental crust in the northern Red Sea and oceanic crust in the southern Red Sea coincides broadly with a southward increase in plate tectonic separation rate and with a decrease in upper mantle seismic velocity. We re-evaluate here the nature of crust in the intervening central Red Sea with the results of legacy seismic refraction experiments and recently released marine gravity anomalies derived from satellite altimeter measurements. In the refraction data, collected east of Thetis Deep, velocities of 6.6-6.9 km s- 1 of a deep refracting layer, which are similar to measured velocities of unaltered gabbro samples, extend outside the deep to 65 km from the axis. The new version of the marine gravity field reveals trends crossing the central Red Sea. Whereas some of them connect with major lineaments in the surrounding African-Arabian shield, those around Thetis Deep die out towards the coastlines. They can be paired across the ridge and lie slightly oblique to plate motions, as is typical of oceanic fracture zones or non-transform discontinuities migrating away from hotspots. Taken together these observations support the view that an oceanic rather than extended continental crust underlies this part of the central Red Sea. The crestal mountains around the median valleys of slow-spreading ridges are typically 500-1000 m lower at spreading discontinuities. Around Thetis Deep, the similar pattern in the gravity field to those of slow-spreading ridges suggests that the crestal mountains may variably block or impede flowage of evaporites towards the spreading centre, whereas the discontinuities may mark areas where flowage is unobstructed. Limited multibeam data collected in transits outside Thetis Deep show oblique fabrics as expected from these predicted movements.

Mitchell, Neil C.; Park, Yongcheol

2014-07-01

183

Durability of neutron star crust  

Energy Technology Data Exchange (ETDEWEB)

How long do neutron star mountains last? The durability of elastically deformed crust is important for neutron star physics including pulsar glitches, emission of gravitational waves from static mountains, and flares from star quakes. The durability is defined by the strength properties of the Yukawa crystals of ions, which make up the crust. In this paper we extend our previous results [Mon. Not. R. Astron. Soc. 407, L54 (2010)] and accurately describe the dependence of the durability on crust composition (which can be reduced to the dependence on the screening length {lambda} of the Yukawa potential). We perform several molecular dynamics simulations of crust breaking and describe their results with a phenomenological model based on the kinetic theory of strength. We provide an analytical expression for the durability of neutron star crust matter for different densities, temperatures, stresses, and compositions. This expression can also be applied to estimate durability of Yukawa crystals in other systems, such as dusty plasmas in the laboratory (copyright 2011 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

Chugunov, A.I. [Ioffe Institute, Saint-Petersburg (Russian Federation); Horowitz, C.J. [Nuclear Theory Center and Dept. of Physics, Indiana University, Bloomington (United States)

2012-02-15

184

Gaps below strange star crusts  

International Nuclear Information System (INIS)

The gap caused by a strong electric field between the quark surface and nuclear crust of a strange star is studied in an improved model including gravity and pressure as well as electrostatic forces. The transition from gap to crust is followed in detail. The properties of the gap are investigated for a wide range of parameters assuming both color-flavor locked and noncolor-flavor locked strange star cores. The maximally allowed crust density is generally lower than that of neutron drip. Finite temperature is shown to increase the gap width, but the effect is significant only at extreme temperatures. Analytical approximations are derived and shown to provide useful fits to the numerical results

185

The crust of neutron stars  

International Nuclear Information System (INIS)

The structure of the crust of a neutron star is completely determined by the experimentally measured nuclear masses up to a density of the order of 1011 g.cm-3. At higher densities, the composition of the crust still remains uncertain, mainly due to the presence of 'free' superfluid neutrons which affect the properties of the nuclear 'clusters'. After briefly reviewing calculations of the equilibrium structure of the crust, we point out that the current approach based on the Wigner-Seitz approximation does not properly describe the unbound neutrons. We have recently abandoned this approximation by applying the band theory of solids. We have shown that the dynamical properties of the free neutrons are strongly affected by the clusters by performing 3D calculations with Bloch boundary conditions

186

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

OpenAIRE

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...

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

2009-01-01

187

Palaeomagnetism and the continental crust  

Energy Technology Data Exchange (ETDEWEB)

This book is an introduction to palaeomagnetism offering treatment of theory and practice. It analyzes the palaeomagnetic record over the whole of geological time, from the Archaean to the Cenozoic, and goes on to examine the impact of past geometries and movements of the continental crust at each geological stage. Topics covered include theory of rock and mineral magnetism, field and laboratory methods, growth and consolidation of the continental crust in Archaean and Proterozoic times, Palaeozoic palaeomagnetism and the formation of Pangaea, the geomagnetic fields, continental movements, configurations and mantle convection.

Piper, J.D.A.

1987-01-01

188

An Introduction to Biological Soil Crusts  

Science.gov (United States)

Biological soil crusts are the feature of this USGS Canyonlands Research Station website. The site provides an introduction to biological soil crusts with linked images, divided into the following topics: nomenclature of the crusts- crytogamic, microbiotic, crytobiotic, and microphytic crusts, structure and formation, species composition, ecological functions, response to disturbance, future, and a glossary. In addition, the site provides links to other Canyonlands Research Station webages including an advanced page with a downloadable 90-page report on soil crusts, a gallery of biological soil crust images and figures, references, the Canyon Country Ecosystems Research Site (CCERS), and other related links.

USGS Canyonlands Research Station

189

Cyanobacteria and the Cryptobiotic Crust  

Science.gov (United States)

Cryptobiotic crusts are featured in this web page, with discussion on cryptobiotic communities, cyanobacteria, and other organisms that comprise this environment. The site provides numerous images of cryptobiotic soil and its inhabiting microorganisms. In addition, this site provides links to a wealth of information on deserts including regions of deserts, organisms found in deserts, and selected deserts in the world.

Jim Deacon

190

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

DEFF Research Database (Denmark)

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.

Kellett, Dawn; Cottle, John

2014-01-01

191

CHIC - Coupling Habitability, Interior and Crust  

Science.gov (United States)

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.

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

2014-05-01

192

Continental Crust Formation in Costa Rica and Nicaragua.  

Science.gov (United States)

Much work has been done in Costa Rica and Nicaragua on relating the variation in modern arc volcanism to variations in crustal type and thickness, sediment input and nature of the mantle wedge. However, little work has been done on the origin of the voluminous silicic ignimbrites in these sections of the arc. This is important because of the unusual abundance of silicic volcanism in a mature island arc environment where no continental crust occurs. The southern part of the Central American arc is the Chorotega block, which has developed on thickened oceanic crust. The boundary between the Chorotega block and the northern continental Chortis block is still a matter of debate. What is clear is that the Paleocene-Eocene island arc of the Chorotega block was built on a primitive basaltic crust (perhaps oceanic plateau?), which has evolved to the modern arc-an evolutionary process that has led to the development of abundant silicic magmas. In most island arc settings, abundant, high-silica volcanic deposits are not common, and this is generally attributed to the absence of continental crust. However, in Costa Rica and parts of Nicaragua, there are extensive silicic ignimbrite deposits. Because there is no recycling of continental crust in these sections of the arc, these silicic deposits represent the formation of new continental crust. This is in contrast to the northern part of the Central American Volcanic Arc where silicic magmatism is associated with older continental crust. In Costa Rica the extensive ignimbrite sheets erupted from the Miocene to Pleistocene and occurred in three main areas: the Guanacaste province in the northwestern Costa Rica, Cordillera de Tillaran and Valle Central in central Costa Rica. In Nicaragua widespread ignimbrites developed from Eocene to as young as 6,000 yr bp (Ehrenborg, 1996). An important observation is that trace element ratios of these silicic volcanic deposits mimic the trace element ratios of basaltic to andesitic deposits from the modern arc. For example in southeastern Nicaragua and northwestern Costa Rica, samples from both the modern arc (basalt to andesite) and the ignimbrites have low Ce/Pb and high Ba/Nb ratios, indicating a large input from slab fluids. In Central Costa Rica samples from the modern arc (basalt to andesite) and the silicic ignimbrites have high Ce/Pb and low Ba/Nb ratios indicating a lower input from slab fluids. We interpret these data to indicate that the silicic melts were produced from partial melts of rocks chemically similar to the modern arc lavas, thus preserving the incompatible element ratio signatures (e.g. Ce/Pb and Ba/Nb ratios). There are no systematic variations of these trace element ratios in the different units with respect to age. There are differences in the chemical and mineralogical composition among the ignimbrites and these differences most likely resulted from differences among source rocks and to crystallization processes. For example, although all of the samples are enriched in light rare earth elements, some units display positive Eu anomalies, whereas others do not. In addition, there are differences in the behavior of heavy REEs. These data most likely reflect the variability of hornblende and plagioclase in the sources for the melts, indicating to us that melting took place at the base of a thickened arc crust near the stability limit of plagioclase. Processes that formed the silicic volcanic rocks in this overthickened island arc or ocean plateau may be similar to processes that formed continental crust in the Archean.

Vogel, T. A.; Patino, L. C.; Alvarado, G. E.

2001-12-01

193

The case for nearly continuous extension of the West Antarctic Rift System, 105-25 Ma (Invited)  

Science.gov (United States)

It is a common perception that extension in the West Antarctic Rift System (WARS) was a two-phase process, with a Cretaceous phase ending when the Campbell Plateau rifted from West Antarctica (~80 Ma), and a mid-Cenozoic phase synchronous with sea floor spreading in the Adare trough (~45-25 Ma). Several lines of evidence indicate that significant extension probably occurred in the intervening 80-45 Ma interval. The strongest evidence comes from subsidence rates on the Central High and Coulman High structures in the central-western Ross Sea, where DSDP Site 270 and other areas with shallow basement have subsided 1 km or more since Oligocene time. With sediment load, these subsidence rates are reasonable for thermal subsidence resulting from extension with a stretching factor of about 2.0-2.5 at about 50-70 Ma, but are hard to reconcile with an extension age around 90 Ma. The seismic velocity structure of the WARS inferred from global surface-wave dispersion is similar to that of oceanic lithosphere of age 40-60 Ma [Ritzwoller et al., 2001 JGR]. Geometric relations of sea floor between Adare Trough and Iselen Bank, northwest Ross Sea, suggest sea floor spreading of about 130 km during early Cenozoic, before the Adare Trough spreading episode started. Numerous cooling ages in the Transantarctic Mountains in the range of 55-45 Ma [Fitzgerald, 1992 Tectonics; Miller et al., 2010 Tectonics] support the interpretation of significant extension prior to 45 Ma. Present crustal thickness of about 22 km near DSDP Site 270 [Trey et al., 1999 Tectonophysics] suggests a pre-extension crustal thickness exceeding 50 km. A simple overall interpretation follows that the WARS has a tectonic history similar to the Basin and Range of western North America: a thick-crust orogenic highland extended for many tens of million years. The main difference between the WARS and the Basin and Range is the post-tectonic cooling and subsidence in the WARS.

Wilson, D. S.; Luyendyk, B. P.

2010-12-01

194

Seasonal Methane Oxidation Potential in Manure Crusts  

OpenAIRE

Organic crusts on liquid manure storage tanks harbor ammonia- and nitrite-resistant methane oxidizers and may significantly reduce methane emissions. Methane oxidation potential (0.6 mol CH4 m?2 day?1) peaked during fall and winter, after 4 months of crust development. Consequences for methane mitigation potential of crusts are discussed.

Nielsen, Daniel A.; Schramm, Andreas; Nielsen, Lars P.; Revsbech, Niels P.

2013-01-01

195

Mid-Ocean Ridge Observations  

Science.gov (United States)

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.

Jamie Levine

196

The extent of continental crust beneath the Seychelles  

Science.gov (United States)

The granitic islands of the Seychelles Plateau have long been recognised to overlie continental crust, isolated from Madagascar and India during the formation of the Indian Ocean. However, to date the extent of continental crust beneath the Seychelles region remains unknown. This is particularly true beneath the Mascarene Basin between the Seychelles Plateau and Madagascar and beneath the Amirante Arc. Constraining the size and shape of the Seychelles continental fragment is needed for accurate plate reconstructions of the breakup of Gondwana and has implications for the processes of continental breakup in general. Here we present new estimates of crustal thickness and VP/VS from H-? stacking of receiver functions from a year long deployment of seismic stations across the Seychelles covering the topographic plateau, the Amirante Ridge and the northern Mascarene Basin. These results, combined with gravity modelling of historical ship track data, confirm that continental crust is present beneath the Seychelles Plateau. This is ˜30-33 km thick, but with a relatively high velocity lower crustal layer. This layer thins southwards from ˜10 km to ˜1 km over a distance of ˜50 km, which is consistent with the Seychelles being at the edge of the Deccan plume prior to its separation from India. In contrast, the majority of the Seychelles Islands away from the topographic plateau show no direct evidence for continental crust. The exception to this is the island of Desroche on the northern Amirante Ridge, where thicker low density crust, consistent with a block of continental material is present. We suggest that the northern Amirantes are likely continental in nature and that small fragments of continental material are a common feature of plume affected continental breakup.

Hammond, J. O. S.; Kendall, J.-M.; Collier, J. S.; Rümpker, G.

2013-11-01

197

Seismic evidence of hyper-stretched crust and mantle exhumation offshore Vietnam  

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We study the evolution of the Eocene-Recent Phu Khanh Basin opened during the rifting of the South China Sea (SCS). This sub-basin formed when continental crust ruptured along the East-Vietnam Boundary Fault (EVBF) at the western edge of the SCS. Using high quality long-streamer seismic lines we interpret structures that highlight the different phases of the SCS rifting and processes related to crustal boudinage. Extreme crustal thinning and mantle uplift that sometimes places sediments in contact with the Moho discontinuity mark the central part of the basin. The mantle is shallowest there and marks the final rupture of the continental crust during an intense phase of mantle upwelling. There, a low-angle detachment fault separates several crustal blocks from the Moho. The cylindrical axis of the Moho rise is roughly parallel to the trend of the South China Sea propagator. Above the mantle, the upper and lower crusts form large crustal boudins. The network of normal faults is dense in the upper crust and occasionally propagates into the lower crust. However, the lower crust is missing at some places. The seismic facies above the Moho rise is poorly stratified and might have been affected by a certain degree of metamorphism. At the apex of mantle uplift, there are frequent indications of fluid circulations, including volcanic edifices and gas escapes features. Three stages of extension are clearly identifiable, with ages of the two youngest constrained by well calibration: the first and oldest rift sequence is situated between the tilted pre-rift basement and the Oligocene horizons (32 Ma); the second is delimited by the Oligocene to the Mid Miocene (15.5 Ma) horizons, and the third is bound by the Mid Miocene and the Upper Miocene (before 10.5 Ma) horizons. These three rift episodes formed in at least two extension directions, the first N-S and the second NW-SE. The distinct Mid Miocene (15.5 Ma) horizon is tilted and the above layers show a diverging reflection. These are in turn sealed by an erosional unconformity before 10.5 Ma. Although tectonic activity appears diachronous from north to south, we suggest that cessation of rifting did not occur before 12-10.5 Ma. This differs from models derived from magnetic anomalies observed in the South China Sea (15.5 to 20 Ma).

Savva, D.; Meresse, F.; Pubellier, M.; Chamot-Rooke, N.; Lavier, L.; Po, K. Wong; Franke, D.; Steuer, S.; Sapin, F.; Auxietre, J. L.; Lamy, G.

2013-11-01

198

Opening of the Gulf of Mexico and the Nature of the Crust in the Deep Gulf: New Evidence from Seafloor Spreading Magnetic Anomalies  

Science.gov (United States)

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.

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

2013-12-01

199

Evolution Of The Alpha Ride, The Arctic Ocean, On The Basis Of The Geohistorical Analysis Of The Magnetic Anomalies  

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A new magnetic anomaly map of the Amerasian Basin has been created owing to a joint reprocessing of the Russian and American aeromagnetic data [Glebovsky, Kovacs at all., 2000]. This model produced the base for the magnetic data interpretation on the more qualitative level. As a result three series of seafloor spreading-type magnetic anomalies have been identified within the area of the Alpha Ridge and the adjacent part of the Canada Basin [Gurevich et all, 2003]. Their sources were formed from three spreading centers (SC). Two spreading centers: the western and the eastern, are situated at the axial part of the Alpha Ridge, the third one - the southern, is located on the southern slope of the Alpha Ridge and on the adjacent part of the Canada Basin. The triple junction of these SC had been located in the central part of the recent Alpha Ridge. The geohistorical analysis of these magnetic anomalies is fulfilled using an original computer programs. In consequence of this analysis: the geochronological characteristics are specified; the kinematic characteristics of the oceanic floor movement are determined and the main stages of the area evolution are found. The magnetic anomalies M16r (140 Ma), which signify the position of all three SC, and pair anomalies M20r (146.5 Ma) and M23r (151.5 Ma) are identified enough sure for all three SC and pair anomalies M30r (157.5 Ma) - fore the eastern and the southern SC. Finite and differential Euler poles of the lithospheric plates rotation were calculated for all three SC from best-fit pair magnetic anomalies. All the poles are concentrated around the Nares strait and at the northeastern part of the Ellesmere island. Angle and linear spreading rates were calculated using Euler poles. The calculation has showed that all three SC had low spreading rates. Three stages of the area evolution are found on the basis of the plate tectonic reconstruction for the periods 146.5, 151.5 and 157.5 Ma ago. The first stage, slightly earlier 157.5 Ma ago: the initiation of the oceanic crust formation to the north-west from the recent shelf of the Prince Patrick island. The second stage, from about 157.5 Ma ago: SC had advanced to the north-east; the oceanic crust was forming from one SC. The third stage, from slightly earlier 151.5 Ma ago to 140 Ma ago: the oceanic floor spreading from three SC took place, 140 Ma ago spreading ceased in this area. During the third stage the triple junction of the spreading centers was not stable and changed from type "ridge - ridge - ridge" to type " ridge - ridge - transform". The intraplate volcano-tectonic activity of the oceanic floor that created the Alpha Ridge was t8he fourth stage of the area evolution The kinematic characteristics of the spreading imply of crustal compression in the north of the Greenland and in the north-east of the Ellesmere island and of crustal stretching in the area of the Queen Elizabeth Islands, that agrees with their geological structure. The main stages of the Amerasian Basin evolution correspond by age to unconformities that A. Embry determined in the Mesozoic strata of the Sverdrup Basin [Embry, 1991]. The work has been supported by the Russian Foundation for basic Research (Grant 01-05-65481).

Gurevich, N. I.; Merkouriev, S. A.

2004-12-01

200

Seawater osmium isotope evidence for a middle Miocene flood basalt event in ferromanganese crust records  

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Three ferromanganese crusts from the northeast, northwest and central Atlantic were re-dated using osmium (Os) isotope stratigraphy and yield ages from middle Miocene to the present. The three Os isotope records do not show evidence for growth hiatuses. The reconstructed Os isotope-based growth rates for the sections older than 10??Ma are higher than those determined previously by the combined beryllium isotope (10Be/9Be) and cobalt (Co) constant-flux methods, which results in a decrease in the maximum age of each crust. This re-dating does not lead to significant changes to the interpretation of previously determined radiogenic isotope neodymium, lead (Nd, Pb) time series because the variability of these isotopes was very small in the records of the three crusts prior to 10??Ma. The Os isotope record of the central Atlantic crust shows a pronounced minimum during the middle Miocene between 15 and 12??Ma, similar to a minimum previously observed in two ferromanganese crusts from the central Pacific. For the other two Atlantic crusts, the Os isotope records and their calibration to the global seawater curve for the middle Miocene are either more uncertain or too short and thus do not allow for a reliable identification of an isotopic minimum. Similar to pronounced minima reported previously for the Cretaceous/Tertiary and Eocene/Oligocene boundaries, possible interpretations for the newly identified middle Miocene Os isotope minimum include changes in weathering intensity and/or a meteorite impact coinciding with the formation of the No??rdlinger Ries Crater. It is suggested that the eruption and weathering of the Columbia River flood basalts provided a significant amount of the unradiogenic Os required to produce the middle Miocene minimum. ?? 2008 Elsevier B.V.

Klemm, V.; Frank, M.; Levasseur, S.; Halliday, A.N.; Hein, J.R.

2008-01-01

201

Melt anomalies of the northern Atlantic Ocean basin  

Science.gov (United States)

We investigated the melt anomalies and lithosphere dynamics of the northern Atlantic Ocean between 76°N and 8°S through combined analysis of seafloor bathymetry, shipboard and satellite-derived gravity, and sediment thickness. Residual mantle Bouguer anomaly (RMBA) was calculated by removing from free-air gravity anomaly the predicted attractions of water-sediment, sediment-crust, and crust-mantle interfaces as well as the effect of lithospheric plate cooling. Residual bathymetry anomaly (RBA) was obtained by subtracting from observed seafloor topography the predicted effects of plate cooling and the observed sediment load. Our analysis indicates that more than 50% of the seafloor has been affected by melt anomalies. The most prominent features that we observe include: (1) A pronounced negative RMBA associated with the Iceland hotspot, the Reykjanes Ridge, and the Mid-Atlantic Ridge (MAR) north of Iceland. The region of enhanced magma supply extends southward to the Charlie Gibbs F.Z., northward to the Jan Mayen F.Z., and to both the eastern and western basin margins. The strong negative RMBA associated with the submarine part of the Iceland hotspot reaches -450 mGal, corresponding to modeled crustal thickness of more than 22 km. (2) A widespread effect of the Azores hotspot on crustal accretion at the MAR since 40-50 Ma, as reflected in negative RMBA and positive RBA that extend southward to at least 26.5°N and northward to 44°N. The strongest RMBA anomaly associated with the Azores melt anomaly reaches about -230 mGal, corresponding to crustal thickening about half of that in Iceland. (3) A ~ 500 km wide corridor of negative RMBA is found along the west African margin between 40°N and 6°S, indicating that this region was influenced extensively by melt anomalies associated with the Horseshoe Seamounts, Madeira Islands, Canary Islands, and Cape Verde Islands. Negative RMBA of -100 to -180 mGal is also associated with the Bermuda Rise in the western Atlantic. Large, but areally restricted negative RMBA anomalies are found along a west-east extending corridor at 28°-36°N, correlating with the New England and Corner seamounts in the western Atlantic and the Cruiser - Great Meteor seamount chain in the eastern Atlantic. (4) The only major, contiguous region of the Atlantic basin that has been relatively uninfluenced by melt anomalies lies between 10°N and 26°N at crustal ages to about 110 Ma. Much smaller regions, particularly in Upper Cretaceous crust but also locally in older and younger crust, are also relatively unaffected in the Atlantic basin between 8°S and the Charlie Gibbs F.Z. at 52°N.

Wang, T.; Lin, J.; Tucholke, B. E.

2009-12-01

202

Proterozoic zircon growth in Archean lower crustal xenoliths, southern Superior craton - a consequence of Matachewan ocean opening  

Science.gov (United States)

Granulite-grade, anorthositic and mafic xenoliths recovered from a Jurassic kimberlite pipe near Kirkland Lake, Ontario are fragments of the lower crust that underlies the ca. 2.7 Ga Abitibi greenstone belt of the Superior craton. Cathodoluminescence imaging and/or backscatter electron microscopy of zircon from four individual xenoliths reveals a complex crystallization history, characterized by two main stages of zircon growth. The age of the two stages has been constrained by combining imaging results with isotope dilution U-Pb dating of grain fragments and single grains. Minimum ages for the first crystallization stage in individual xeno liths are 2584+/-7 Ma, 2629+/-8 Ma, 2633+/-3 Ma, whereas an approximate crystallization age for a fourth sample is 2788+/-57 Ma. The second main stage of growth consists of chemically and isotopically distinct metamorphic zircon overgrowths. Times of solid-state zircon growth are most broadly constrained in three samples to the interval between 2.52 Ga to 2.40 Ga, and most precisely dated in a meta-anorthosite at 2416+/-30 Ma. These complex zircons are intergrown with garnet and clinopyroxene of the host granulite-facies assemblage, and thus the Paleoproterozoic ages of the metamorphic overgrowths are interpreted to reflect an interval of isobaric, granulite-grade metamorphism of the lower crust beneath the greenstone belt approximately 150 million years after craton formation. This interval of metamorphism is broadly coeval with the intrusion of the Matachewan dyke swarm across the southern Superior craton, and with mafic magmatism and deposition of Huronian rift-margin sediments 200 km to the south during the opening of the Matachewan ocean. It is proposed that a significant volume of magma intruded the crust-mantle interface during rifting, promoting isobaric metamorphism and zircon growth in the deep levels of the Superior craton. Subsequent major rifting events along this margin apparently failed to produce a similar lower crustal response. The results have important implications for the structure of lithosphere beneath Archean continental crust.

Moser, D. E.; Heaman, L. M.

203

Metamorphism in the Martian crust  

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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.

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

2014-07-01

204

Constraints on hydrothermal heat flux through the oceanic lithosphere from global heat flow  

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A significant discrepancy exists between the heat flow measured at the seafloor and the higher values predicted by thermal models of the cooling lithosphere. This discrepancy is generally interpreted as indicating that the upper oceanic crust is cooled significantly by hydrothermal circulation. The magnitude of this heat flow discrepancy is the primary datum used to estimate the volume of hydrothermal flow, and the variation in the discrepancy with lithospheric age is the primary constraint on how the hydrothermal flux is divided between near-ridge and off-ridge environments. The resulting estimates are important for investigation of both the thermal structure of the lithosphere and the chemistry of the oceans. We reevaluate the magnitude and age variation of the discrepancy using a global heat flow data set substantially larger than in earlier studies, and the GDHI (Global Depth and Heat Flow) model that better predicts the heat flow. We estimate that of the predicted global oceanic heat flux of 32 x 10(exp 12) W, 34% (11 x 10(exp 12) W) occurs by hydrothermal flow. Approximately 30% of the hydrothermal heat flux occurs in crust younger than 1 Ma, so the majority of this flux is off-ridge. These hydrothermal heat flux estimates are upper bounds, because heat flow measurements require sediment at the site and so are made preferentially at topographic lows, where heat flow may be depressed. Because the water temperature for the near-ridge flow exceeds that for the off-ridge flow, the near-ridge water flow will be even a smaller fraction of the total water flow. As a result, in estimating fluxes from geochemical data, use of the high water temperatures appropriate for the ridge axis may significantly overestimate the heat flux for an assumed water flux or underestimate the water flux for an assumed heat flux. Our data also permit improved estimates of the 'sealing' age, defined as the age where the observed heat flow approximately equals that predicted, suggesting that hydrothermal heat transfer has largely ceased. Although earlier studies suggested major differences in sealing ages for different ocean basins, we find that the sealing ages for the Atlantic, Pacific, and Indian oceans are similar and consistent with the sealing age for the entire data set, 65 +/- 10 Ma. The previous inference of a young (approximately 20 Ma) sealing age for the Pacific appears to have biased downward several previous estimates of the global hydrothermal flux. The heat flow data also provide indirect evidence for the mechanism by which the hydrothermal heat flux becomes small, which has often been ascribed to isolation of the igneous crust from seawater due to the hydraulic conductivity of the intervening sediment. We find, however, that even the least sedimented sites show the systematic increase of the ratio of observed to predicted heat flow with age, although the more sedimented sites have a younger sealing age. Moreover, the heat flow discrepancy persists at heavily sedimented sites until approximately 50 Ma. It thus appears that approximately 100-200 m of sediment is neither necessary nor sufficient to stop hydrothermal heat transfer. We therefore conclude that the age of the crust is the primary control on the fraction of heat transported by hydrothermal flow and that sediment thickness has a lesser effect. This inference is consistent with models in which hydrothermal flow decreases with age due to reduced crustal porosity and hence permeability.

Stein, Carol A.; Stein, Seth

1994-01-01

205

Foundering of lower island-arc crust as an explanation for the origin of the continental Moho.  

Science.gov (United States)

A long-standing theory for the genesis of continental crust is that it is formed in subduction zones. However, the observed seismic properties of lower crust and upper mantle in oceanic island arcs differ significantly from those in the continental crust. Accordingly, significant modifications of lower arc crust must occur, if continental crust is indeed formed from island arcs. Here we investigate how the seismic characteristics of arc crust are transformed into those of the continental crust by calculating the density and seismic structure of two exposed sections of island arc (Kohistan and Talkeetna). The Kohistan crustal section is negatively buoyant with respect to the underlying depleted upper mantle at depths exceeding 40 kilometres and is characterized by a steady increase in seismic velocity similar to that observed in active arcs. In contrast, the lower Talkeetna crust is density sorted, preserving only relicts (about ten to a hundred metres thick) of rock with density exceeding that of the underlying mantle. Specifically, the foundering of the lower Talkeetna crust resulted in the replacement of dense mafic and ultramafic cumulates by residual upper mantle, producing a sharp seismic discontinuity at depths of around 38 to 42?kilometres, characteristic of the continental Mohorovi?i? discontinuity (the Moho). Dynamic calculations indicate that foundering is an episodic process that occurs in most arcs with a periodicity of half a million to five million years. Moreover, because foundering will continue after arc magmatism ceases, this process ultimately results in the formation of the continental Moho. PMID:24305163

Jagoutz, Oliver; Behn, Mark D

2013-12-01

206

Origin and evolution of a submarine large igneous province: the Kerguelen Plateau and Broken Ridge, southern Indian Ocean  

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Oceanic plateaus form by mantle processes distinct from those forming oceanic crust at divergent plate boundaries. Eleven drillsites into igneous basement of Kerguelen Plateau and Broken Ridge, including seven from the recent Ocean Drilling Program Leg 183 (1998-99) and four from Legs 119 and 120 (1987-88), show that the dominant rocks are basalts with geochemical characteristics distinct from those of mid-ocean ridge basalts. Moreover, the physical characteristics of the lava flows and the presence of wood fragments, charcoal, pollen, spores and seeds in the shallow water sediments overlying the igneous basement show that the growth rate of the plateau was sufficient to form subaerial landmasses. Most of the southern Kerguelen Plateau formed at ˜110 Ma, but the uppermost submarine lavas in the northern Kerguelen Plateau erupted during Cenozoic time. These results are consistent with derivation of the plateau by partial melting of the Kerguelen plume. Leg 183 provided two new major observations about the final growth stages of the Kerguelen Plateau. 1: At several locations, volcanism ended with explosive eruptions of volatile-rich, felsic magmas; although the total volume of felsic volcanic rocks is poorly constrained, the explosive nature of the eruptions may have resulted in globally significant effects on climate and atmospheric chemistry during the late-stage, subaerial growth of the Kerguelen Plateau. 2: At one drillsite, clasts of garnet-biotite gneiss, a continental rock, occur in a fluvial conglomerate intercalated within basaltic flows. Previously, geochemical and geophysical evidence has been used to infer continental lithospheric components within this large igneous province. A continental geochemical signature in an oceanic setting may represent deeply recycled crust incorporated into the Kerguelen plume or continental fragments dispersed during initial formation of the Indian Ocean during breakup of Gondwana. The clasts of garnet-biotite gneiss are the first unequivocal evidence of continental crust in this oceanic plateau. We propose that during initial breakup between India and Antarctica, the spreading center jumped northwards transferring slivers of the continental Indian plate to oceanic portions of the Antarctic plate.

Frey, F. A.; Coffin, M. F.; Wallace, P. J.; Weis, D.; Zhao, X.; Wise, S. W.; Wähnert, V.; Teagle, D. A. H.; Saccocia, P. J.; Reusch, D. N.; Pringle, M. S.; Nicolaysen, K. E.; Neal, C. R.; Müller, R. D.; Moore, C. L.; Mahoney, J. J.; Keszthelyi, L.; Inokuchi, H.; Duncan, R. A.; Delius, H.; Damuth, J. E.; Damasceno, D.; Coxall, H. K.; Borre, M. K.; Boehm, F.; Barling, J.; Arndt, N. T.; Antretter, M.

2000-02-01

207

Complex evolution of the lower crust beneath the southeastern North China Craton: the Junan xenoliths and xenocrysts  

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Knowledge of the lower crust beneath the southeastern parts of North China Craton (NCC) is still sparse. The Junan basalts (67 Ma) in the southeastern NCC contain abundant xenoliths of lower crustal granulites, pyroxenites and mantle peridotites. We present integrated in-situ U-Pb ages and Hf isotopes of zircons from the Junan basalts and granulite xenoliths, to investigate accretion and modification processes in the lower crust. The granulite xenoliths define three distinct U-Pb age populations of ca 2.3 Ga, ca 2.0 Ga and 114-126 Ma. The ca 2.3 Ga zircons have widely variable ?Hf(t) and Paleo-Neoarchean model ages (Tcrust = 2.6-4.0 Ga), whereas the ca 2.0 Ga structureless grains give negative ?Hf(t) and Tcrust of 2.7-3.3 Ga. In addition to a few discordant Early Paleoproterozoic xenocrysts, zircons from the basalts are dominantly Early Cretaceous (115-125 Ma), with some Neoproterozoic (550-800 Ma) and Early Paleozoic (437-493 Ma), as well as minor Late Triassic and Late Jurassic grains. These results, combined with previous petrological and geochemical studies and P-T estimates, suggest that the upper part of the Junan lower crust consists of mafic-intermediate granulites, mainly formed at ca 2.3 Ga by crystallization of depleted-mantle-derived magmas that assimilated ancient crust and then fractionated. More significantly, zircon ages and Hf isotopes imply that this lower crust had a complex history of accretion and modification, including initial growth at 3.0-4.0 Ga and 2.5-2.7 Ga, conversion to the dominant granulitic assemblages in the Early Paleoproterozoic (ca 2.3 Ga), modification or metamorphism in the Late Paleoproterozoic (1.8-2.0 Ga) and possibly slight heating in Neoproterozoic time. Episodic thermal events during the Early Paleozoic, Late Triassic and Late Jurassic may also have reworked this Precambrian lower crust. Most of the Early Cretaceous zircons and xenocrysts have uniform ?Hf(t) values similar to those of the nearby coeval magmatic rocks that derived from the enriched lithospheric mantle. This suggests that the Early Cretaceous basaltic underplating, which was contemporary with extensive partial melting of the enriched parts of the NCC lithospheric mantle at the peak of lithospheric thinning, might have substantially modified the Paleoproterozoic granulitic lower crust, and finally gave the diverse cumulate pyroxenites that now make up the deeper lower crust. The discovery of ca 2.1-2.3 Ga lower crust in the southeastern NCC also highlights the heterogeneous nature of the Precambrian lower crust across the eastern NCC.

Tang, Huayun; Zheng, Jianping; Griffin, William L.; O?Reilly, Suzanne Y.; Yu, Chunmei; Pearson, Norman J.; Ping, Xianquan; Xia, Bing; Yang, Huaben

2014-10-01

208

Revisiting Seafloor-Spreading in the Red Sea: Basement Nature, Transforms and Ocean-Continent Boundary  

Science.gov (United States)

A new marine geophysical survey on the Saudi Arabian side of the Red Sea confirms early inferences that ~ 2/3 of the eastern Red Sea is floored by oceanic crust. Most seismic profiles south of 24°N show a strongly reflective, landward-deepening volcanic basement up to ~ 100 km east of the axial ridge, beneath thick evaporitic deposits. This position of the Ocean-Continent Boundary (OCB) is consistent with gravity measurements. The low amplitudes and long wavelengths of magnetic anomalies older than Chrons 1-3 can be accounted for by low-pass filtering due to thick sediments. Seafloor-spreading throughout the Red Sea started around 15 Ma, as in the western Gulf of Aden. Its onset was coeval with the activation of the Aqaba/Levant transform and short-cutting of the Gulf of Suez. The main difference between the southern and northern Red Sea lies not in the nature of the crust but in the direction and modulus of the plate motion rate. The ~ 30° counterclockwise strike change and halving of the spreading rate (~ 16 to ~ 8 mm/yr) between the Hermil (17°N) and Suez triple junctions results in a shift from slow (? North Atlantic) to highly oblique, ultra-slow (? Southwest Indian) ridge type. The obliquity of spreading in the central and northern basins is taken up by transform discontinuities that stop ~ 40 km short of the coastline, at the OCB. Three large transform fault systems (Jeddah, Zabargad, El Akhawein) nucleated as continental transfer faults reactivating NNE-trending Proterozoic shear zones. The former two systems divide the Red Sea into three main basins. Between ~15 and ~5 Ma, for about 10 million years, thick evaporites were deposited directly on top of oceanic crust in deep water, as the depositional environment, modulated by climate, became restricted by the Suez and Afar/Bab-el-Mandeb volcano-tectonic 'flood-gates.' The presence of these thick deposits (up to ~ 8 km) suffices to account for the difference between the Red Sea and the Gulf of Aden. Widespread salt tectonics was triggered by the flow of large evaporite sheets and salt glaciers toward the ridge axis. Such flow was more pervasive in the north, where slower spreading resulted in a deeper trough, and was guided by the rugged topography of the oceanic seafloor. The Red Sea may represent the best model for comparably deep evaporitic basins along the Earth's passive margins, particularly in the South Atlantic.

Tapponnier, P.; Dyment, J.; Zinger, M. A.; Franken, D.; Afifi, A. M.; Wyllie, A.; Ali, H. G.; Hanbal, I.

2013-12-01

209

The Dupal isotopic anomaly in the southern Paleo-Asian Ocean: Nd-Pb isotope evidence from ophiolites in Northwest China  

Science.gov (United States)

It has been suggested that the Dupal isotopic anomaly in the mantle can be traced in the Paleozoic ophiolites from the Neo- and Paleo-Tethyan Ocean (275-350 Ma). The Karamaili ophiolite (KO) and Dalabute ophiolite (DO) in the eastern and western corners, respectively, of the Junggar basin in NW China represent remnants of the relatively older (> 350 Ma) Paleo-Asian Ocean (PAO) crust. Thus, these ophiolites can provide additional constraints on the long-term composition and evolution of the Paleozoic suboceanic mantle. We present new major-trace element and Sr, Nd and high-precision Pb isotope data for the basalts, gabbros and a plagioclase separate from the KO and DO. Our results indicate that the PAO crust indeed has a Dupal-like isotopic signature. In detail, all samples have relatively low ?Nd(t) and high 208Pb/204Pb(t) for given 206Pb/204Pb(t) ratios (i.e., positive ?8/4 values), similar to the Dupal isotopic characteristics of Indian Ocean mid-ocean ridge basalts (MORB). The trace element signature of DO mafic rocks is similar to that of normal- and enriched-MORB whereas that of the KO is transitional between MORB and arc basalt. Therefore, the DO mantle domain reflects the PAO asthenosphere and the KO domain additionally shows the influence of the subduction process. Geochemical modeling using Th/Nd as well as Nd and Pb isotopic ratios indicates that up to 2% subduction component had been added to a depleted Indian MORB-type mantle to produce the bulk of KO rocks. The subduction component in the KO rocks consisted of variable proportions of ? 1% partial melt of unradiogenic sediment similar to modern Izu-Bonin trench sediment and hydrous fluid dehydrated from the subducted altered oceanic crust. The Devonian asthenospheric mantle beneath the southern PAO is isotopically heterogeneous, but lends support to the idea that the Dupal isotopic anomaly existed prior to the opening of the Indian Ocean. Finally, plate tectonic reconstruction indicates that the anomaly was present in the Neo- and Paleo-Tethyan oceans in the southern hemisphere and in the southern part of PAO in the northern hemisphere during the late Paleozoic.

Liu, Xijun; Xu, Jifeng; Castillo, Paterno R.; Xiao, Wenjiao; Shi, Yu; Feng, Zuohai; Guo, Lin

2014-02-01

210

The formation of deep basins in High Arctic from metamorphism in continental crust  

Science.gov (United States)

In the East Barents and North Chukchi basins, 16-20 km deep, the crystalline crust is attenuated to 12-18 km (reference profiles 2-AR, 4-AR and 5-AR). P-wave velocities and densities in this layer are characteristic of the oceanic crust. However, the subsidence history in the basins is quite different from that typical of the oceanic crust. In both basins the subsidence continued for several hundred million years and one half of the deposits or more was formed long after the start of the subsidence when cooling of the oceanic plate would be already over. Moreover, the basins are 4-5 km deeper than it could be expected according to the thickness of the crystalline crust above the Moho boundary. In the absence of large free-air gravity anomalies, joint analysis of the gravity and seismic data indicates the existence under the Moho of thick layers of high-density and high-velocity eclogites. As can be seen in high resolution seismic profiles, the intensity of crustal stretching did not exceed 10% in the basins, and their formation can be predominantly attributed to a high-grade metamorphism in the mafic lower part of continental crust. At some episodes, strong increase in the rate of subsidence occurred in the basins. This indicates acceleration of metamorphism catalyzed by infiltration of mantle fluids. A set of the above features, abnormally large depth, long subsidence history with its acceleration at the late stages, and episodes of pronounced acceleration of the subsidence represent characteristic features of some other large hydrocarbon basins, e.g., of the North and South Caspian basins. These features can be used for prospecting new prolific provinces on the Arctic shelf. The Lomonosov ridge, Mendeleev high and the Makarov basin pertain to the same structural type. In the Oligocene they underwent erosion near to sea level with the formation of pronounced unconformity. Then at the end of Oligocene deep-water basins were formed in these regions. Rapid crustal subsidence after a long period of relative stability is atypical of oceanic crust. It can be produced either by intense stretching of continental crust or by a density increase due to metamorphism in this layer. Recent seismic reflection profiles demonstrate only minor stretching of the crystalline basement in the regions. Then metamorphism should be the main cause of formation of deep basins in these regions. This can explain attenuation of crystalline crust and an increase in P-wave velocities in this layer that are typical for many deep basins formed due to intense metamorphism in continental crust.

Artyushkov, Eugene; Belyaev, Igor; Chekhovich, Peter; Petrov, Eugene; Poselov, Viktor

2014-05-01

211

Composition of the Primary Crust of Mars: Observations of Deeply Excavated Crater Central Peaks  

Science.gov (United States)

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.

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

2011-12-01

212

Pushing the margins : research in the Ocean Margins LINK programme  

OpenAIRE

Continental margins are areas of intense exploration interest at the present day, both in NW Europe and across the world. Investigation of the processes involved in shaping margin geology, from crustal geometry to sub-seismic scale reservoir configuration, have contributed to the greater understanding of geological structure in areas of transition from continental crust to oceanic crust. The Ocean Margin LINK Programme is a research programme co-funded by NERC and industry focusing on the ...

Leslie, Alick

2005-01-01

213

Nature of the Levantine (eastern Mediterranean) crust from multiple-source Werner deconvolution of Bouguer gravity anomalies  

Science.gov (United States)

The nature of the Levantine (eastern Mediterranean) crust has been the subject of controversy for many years, revolving around two hypotheses: the continental crust hypothesis and the oceanic crust hypothesis. The proponents of the first hypothesis suggest that the Levantine (eastern Mediterranean) basin is characterized by a thick sedimentary succession overlying thinned crust of continental origin, through which a number of aborted Mesozoic rifts were etched. However, multiple-source Werner deconvolution (MSWD) estimates and other geophysical data, integrated with earlier geological and geophysical results, provide further support to the second hypothesis (oceanic crust) and lead to the following conclusions: (1) The depth to Moho ranges from about 20 km to about 28 km below sea level, with an average crustal thickness of about 22 km. (2) The large thickness (about 10 km) of Phanerozoic section leaves only about 12 km of thickness for the igneous/metamorphic (basement) complex. (3) The northern boundary of the Levantine (easternmost Mediterranean) lithosphere is delineated by an arcuate belt of seismic activities along the southern margin of Cyprus. (4) The formation of the Phoenician and Latakia basins and the Iskenderun Bay is probably controlled by the counterclockwise rotation of the lithospheric slices southeast of Cyprus. (5) The apparent absence of magnetic anomaly lineations (reversals) is due probably to the thick Phanerozoic cover, and/or the formation of the oceanic crust during a long magnetic chron.

Khair, Kamal; Tsokas, Gregory N.

1999-11-01

214

Strange crusts on strange stars  

International Nuclear Information System (INIS)

We re-examine the surface composition of strange stars. It is widely accepted that they are characterized by an enormous density gradient (1026 g cm-4) and large electric fields at surface. By investigating the possibility of realizing a heterogeneous crust, comprised of nuggets of strange quark matter embedded in a uniform electron background, we find that the strange star surface has a much reduced density gradient and negligible electric field. We discuss the role of Debye screening in estimating the critical surface which will disfavour the nugget phase. We comment on how our findings will impact various proposed observable signatures for strange stars

215

Microbial communities at the borehole observatory on the Costa Rica Rift flank (Ocean Drilling Program Hole 896A)  

OpenAIRE

The microbiology of subsurface, hydrothermally-influenced basaltic crust flanking mid-ocean ridges has remained understudied, due to the difficulty in accessing the subsurface environment. The instrumented boreholes resulting from scientific ocean drilling offer access to samples of the formation fluids circulating through oceanic crust. We analyzed the phylogenetic diversity of bacterial communities of fluid and microbial mat samples collected in situ from the observatory at Ocean Drilling P...

AndreasTeske; BethOrcutt; KeirBecker

2012-01-01

216

Petrogenesis of oceanic kimberlites and included mantle megacrysts: the Malaitan alnoite  

International Nuclear Information System (INIS)

The study of unambiguous suboceanic mantle was facilitated by the occurrence of anomalous kimberlite-type intrusives on Malaita in the Solomon Islands. The pseudo-kimberlites were termed alnoites, and are basically mica lamprophyres with melilite in the ground mass. Alnoitic magmas were explosively intruded into the Ontong Java Plateau (OJP) 34 Ma ago. The OJP is a vastly overthickened portion of the Pacific plate which now abuts the Indo-Australian plate. Malaita is considered to be the obducted leading edge of the OJP. Initial diapiric upwelling beneath the OJP produced the proto-alnoite magma. After impingement on the rigid lithosphere, megacrysts fractionation occurred, with augites precipitating first, representing the parental magma. Sea water-altered oceanic crust, which underplated the OJP, was assimilated by the proto-alnoite magma during megacrysts fractionation

217

The petrogenesis of oceanic kimberlites and included mantle megacrysts: The Malaitan alnoite  

Science.gov (United States)

The study of unambiguous suboceanic mantle was facilitated by the occurrence of anomalous kimberlite-type intrusives on Malaita in the Solomon Islands. The pseudo-kimberlites were termed alnoites, and are basically mica lamprophyres with melilite in the ground mass. Alnoitic magmas were explosively intruded into the Ontong Java Plateau (OJP) 34 Ma ago. The OJP is a vastly overthickened portion of the Pacific plate which now abuts the Indo-Australian plate. Malaita is considered to be the obducted leading edge of the OJP. Initial diapiric upwelling beneath the OJP produced the proto-alnoite magma. After impingement on the rigid lithosphere, megacrysts fractionation occurred, with augites precipitating first, representing the parental magma. Sea water-altered oceanic crust, which underplated the OJP, was assimilated by the proto-alnoite magma during megacrysts fractionation (AFC).

Neal, Clive R.

1988-01-01

218

Temperature distribution in magnetized neutron star crusts  

OpenAIRE

We investigate the influence of different magnetic field configurations on the temperature distribution in neutron star crusts. We consider axisymmetric dipolar fields which are either restricted to the stellar crust, ``crustal fields'', or allowed to penetrate the core, ``core fields''. By integrating the two-dimensional heat transport equation in the crust, taking into account the classical (Larmor) anisotropy of the heat conductivity, we obtain the crustal temperature dis...

Geppert, U.; Kueker, M.; Page, D.

2004-01-01

219

Biogenic crust dynamics on sand dunes  

OpenAIRE

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 ...

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

2012-01-01

220

Generation of Continental Crust in Central America: New Field and Geochemical Observations on Silicic Magmatism in Costa Rica  

Science.gov (United States)

Explaining the occurrence of high-silica arc magmatism in the absence of continental crust remains a fundamental problem in igneous petrology. Recent work in the southern portion of the Central American volcanic arc has expanded the database for the abundant high-silica ash-flow tuffs erupted on top of thick oceanic basement in Costa Rica and southern Nicaragua. Regional differences in geochemistry are observed in data from central and northern Costa Rica. In addition, local heterogeneities among units are demonstrated in plots of both major and trace elements. High-silica ash-flow tuffs in central Costa Rica include the Tiribi Tuff (~0.33 Ma) and Alto Palomo formation (~0.56 Ma). In northern Costa Rica, numerous large silicic ash-flow sheets are found in the Guanacaste province, ranging from late Miocene (Guanacaste region. In both chemistry and geographical position, the Alto Palomo formation appears to represent a transition between tuffs in the Valle Central and those in Guanacaste. Incompatible trace element ratios for these units are nearly identical to regional trends observed in basaltic to andesitic lavas of the modern Costa Rican arc (e.g. Ba/Nb). The Papagayo sequence is an example of chemical variation within one vertical section. The sequence is a ~21 m section of well-exposed tuff that represents an essentially continuous sampling of an evolving magma body. Major-element analyses from a systematic vertical sampling of the section support a model of crystal fractionation, eruption, and mafic replenishment of the magma chamber. Samples range from 60.1 to 70.2 wt.% SiO2, with the most mafic sample occurring at the top of sequence as a visibly mafic-silicic mingled pumice. The Rio Liberia (~1.47 Ma) and Salitral (~1.3 Ma) formations in the Guanacaste region form a series of tuffs, related by the same inferred vent. Despite overlapping silica content, the units have distinct mineral compositions. The Salitral formation includes plagioclase- and amphibole-rich units that appear very similar in the field, while the Rio Liberia contains biotite. Chemically, the units are distinct, forming several separate trends in trace element plots. These heterogeneities most likely reflect differences in both source and/or processes of magma evolution.

Szymanski, D. W.; Patino, L. C.; Vogel, T. A.; Alvarado, G. E.

2002-12-01

221

Pulsar glitches: The crust is not enough  

CERN Document Server

Pulsar glitches are traditionally viewed as a manifestation of vortex dynamics associated with a neutron superfluid reservoir confined to the inner crust of the star. In this Letter we show that the non-dissipative entrainment coupling between the neutron superfluid and the nuclear lattice leads to a less mobile crust superfluid, effectively reducing the moment of inertia associated with the angular momentum reservoir. Combining the latest observational data for prolific glitching pulsars with theoretical results for the crust entrainment we find that the required superfluid reservoir exceeds that available in the crust. This challenges our understanding of the glitch phenomenon, and we discuss possible resolutions to the problem.

Andersson, N; Ho, W C G; Espinoza, C M

2012-01-01

222

The hydrothermal power of oceanic lithosphere  

Science.gov (United States)

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 (power estimate originates from the thermally insulating properties of oceanic crust in relation to the mantle. Since the crust is relatively insulating, the effective properties of the lithosphere are "crust dominated" near ridge axes (yielding lower heat flow), and gradually approach mantle values over time. Thus, cooling models with crustal insulation predict low heat flow over young seafloor, implying that the difference of modeled and measured heat flow is due to the heat transport properties of the lithosphere, in addition to ventilated hydrothermal circulation as generally accepted. These estimates may bear on 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.

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

2015-03-01

223

Geodynamic Models of Melt Generation and Extraction at Mid-Ocean Ridges  

OpenAIRE

It is widely accepted that plate divergence at mid-ocean ridges drives mantle flow, mantle melting, and the formation of new oceanic crust. However, many of the details of this process remain obscure because of the inaccessibility of the mantle to direct observation. Thus, geodynamic models are needed to provide insight into the processes that control the formation of new crust and hydrothermal circulation at mid-ocean ridges. These models allow us to test governing parameters and investigate...

Gregg, Patricia M.; Hebert, Laura B.; Monte?si, Laurent G. J.; Katz, Richard F.

2012-01-01

224

Metagabbros of the Gangdese arc root, south Tibet: Implications for the growth of continental crust  

Science.gov (United States)

The Gangdese batholith of the southern Tibet is regarded as a principal component of the pre-Himalayan collision, Andean-type continental margin. However, the Gangdese arc root remains poorly understood because most of previous studies concentrated on granitoids from the middle and upper crust of the arc. In this paper, we report new petrochemical and geochronological characteristics of metagabbros occurred as a part of the Late Cretaceous Lilong batholith in the eastern Gangdese arc. The gabbros with protolith ages of ?82-95 Ma, underwent slightly late (68-77 Ma) granulite-facies metamorphism and nearly coeval anatexis, typical of lower arc crusts. The metagabbros and diorites of the Lilong batholith show geochemical features of juvenile crust in continental magmatic arcs, and an original magmatic differentiation trend, with gabbros formed as the cumulates, whereas diorites as the crystalline products of evolved magmas. The Lilong batholith forms an arc crustal section with a possible thickness of 20-30 km, and may have an overall andesitic composition, providing a support for the “andesite model” of continental crust growth.

Zhang, Zeming; Dong, Xin; Xiang, Hua; He, Zhenyu; Liou, J. G.

2014-10-01

225

Post-collisional ore-bearing adakitic porphyries from Gangdese porphyry copper belt, southern Tibet: Melting of thickened juvenile arc lower crust  

Science.gov (United States)

Porphyry Cu-Mo deposits, related to the Miocene adakitic porphyries from the Gangdese porphyry copper belt in the southern Tibet, formed in a post-collisional setting. Here, we present the new zircon U-Pb ages, whole-rock chemical, and Sr-Nd and zircon Hf isotopic data for the ore-bearing adakitic porphyries from Gangdese porphyry copper belt. LA-ICP-MS zircon dating for six samples yielded ages ranging from 19 Ma to 14 Ma, indicating they formed in the Miocene. The ore-bearing adakitic porphyries show SiO 2 of 61.47-71.67%, K 2O of 3.29-4.74%, and high Sr content (394-1106 ppm), high Sr/Y ratios (63-158), and low Y (6.12-10.3 ppm) and heavy rare earth element contents (e.g. Yb = 0.52-0.91 ppm). They show steep fractionated REE and flat HREE patterns, and strong enrichment in large ion lithophile elements (Cs, Rb, Ba, Th, and U) and depletion of high field strength elements (Nb), with positive Sr and negative Ti anomalies. There are no linear variations of Ba, La, Sr/Y, Dy/Yb, ( 87Sr/ 86Sr) i, and ?Nd(t) with increasing SiO 2 content. Combined with the zircon positive ?Hf(t) values, and wide range of ( 87Sr/ 86Sr) i (0.70559 to 0.70908) and of ?Nd(t) (- 6.8 to 0) values for all the adakitic samples, they were likely derived from the melting of garnet-bearing amphibolite in the juvenile arc mafic lower crust. Additionally, the adakitic porphyries with low Th/Nb ratios have lower initial 87Sr/ 86Sr, and higher ?Nd(t) and ?Hf(t) values, and those with high Th/Nb show higher initial 87Sr/ 86Sr, and lower ?Nd(t) and ?Hf(t) values. This indicates that the juvenile arc lower crust is derived from the melting of the mantle wedge that was metasomatized by slab fluid or sediment melt during the Neotethyan ocean subduction. Significantly, the juvenile arc lower crust possibly inherited the arc magma characteristics of abundant in F, Cl, and Cu and high oxidation state, which contributed to the porphyry Cu-Mo deposits in the Gangdese porphyry copper belt.

Li, Jin-Xiang; Qin, Ke-Zhang; Li, Guang-Ming; Xiao, Bo; Chen, Lei; Zhao, Jun-Xing

2011-10-01

226

Apulian crust: Top to bottom  

Science.gov (United States)

We investigate the crustal seismic structure of the Adria plate using teleseismic receiver functions (RF) recorded at 12 broadband seismic stations in the Apulia region. Detailed models of the Apulian crust, e.g. the structure of the Apulian Multi-layer Platform (AMP), are crucial for assessing the presence of potential décollements at different depth levels that may play a role in the evolution of the Apenninic orogen. We reconstruct S-wave velocity profiles applying a trans-dimensional Monte Carlo method for the inversion of RF data. Using this method, the resolution at the different depth level is completely dictated by the data and we avoid introducing artifacts in the crustal structure. We focus our study on three different key-elements: the Moho depth, the lower crust S-velocity, and the fine-structure of the AMP. We find a well defined and relatively flat Moho discontinuity below the region at 28-32 km depth, possibly indicating that the original Moho is still preserved in the area. The lower crust appears as a generally low velocity layer (average Vs = 3.7 km/s in the 15-26 km depth interval), likely suggestive of a felsic composition, with no significant velocity discontinuities except for its upper and lower boundaries where we find layering. Finally, for the shallow structure, the comparison of RF results with deep well stratigraphic and sonic log data allowed us to constrain the structure of the AMP and the presence of underlying Permo-Triassic (P-T) sediments. We find that the AMP structure displays small-scale heterogeneities in the region, with a thickness of the carbonates layers varying between 4 and 12 km, and is underlain by a thin, discontinuous layer of P-T terrigenous sediments, that are lacking in some areas. This fact may be due to the roughness in the original topography of the continental margins or to heterogeneities in its shallow structure due to the rifting process.

Amato, Alessandro; Bianchi, Irene; Agostinetti, Nicola Piana

2014-12-01

227

The magma ocean as an impediment to lunar plate tectonics  

Science.gov (United States)

The primary impediment to plate tectonics on the moon was probably the great thickness of its crust and particularly its high crust/lithosphere thickness ratio. This in turn can be attributed to the preponderance of low-density feldspar over all other Al-compatible phases in the lunar interior. During the magma ocean epoch, the moon's crust/lithosphere thickness ratio was at the maximum theoretical value, approximately 1, and it remained high for a long time afterwards. A few large regions of thin crust were produced by basin-scale cratering approximately contemporaneous with the demise of the magma ocean. However, these regions probably also tend to have uncommonly thin lithosphere, since they were directly heated and indirectly enriched in K, Th, and U by the same cratering process. Thus, plate tectonics on the moon in the form of systematic lithosphere subduction was impeded by the magma ocean.

Warren, Paul H.

1993-01-01

228

Ocean Planet: Ocean Market  

Science.gov (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.

229

Thermalization time of hot neutron star crust  

Energy Technology Data Exchange (ETDEWEB)

We discuss the thermalization 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 temperature. In this framework we analyze the dependence of the crust thermalization on pairing properties and on cluster structure of inner crust matter. It is shown that the pairing correlations reduce the crust thermalization time by a large fraction. The calculations show also that the nuclear clusters have a non-negligible influence on the time evolution of the surface temperature of the neutron star.

Margueron, J [Institut de Physique Nucleaire, IN2P3-CNRS and Universie Paris-Sud, F-91406 Orsay CEDEX (France); Fortin, M [Ecole Normale Superieure, Departement de Physique, 24 rue Lhomond, 75005 Paris (France); Grill, F [Dipartimento di Fisica, Universita degli Studi di Milano, Via Celoria 16, 20133 Milan (Italy); Page, D [Departamento de Astrofisica Teorica, Instituto de AstronomIa, Universidad Nacional Autonoma de Mexico, 04360 Mexico D.F. (Mexico); Sandulescu, N, E-mail: jerome.margueron@ipno.in2p3.fr [National Institute of Physics and Nuclear Engineering, 76900, Bucharest (Romania)

2011-09-16

230

Neutron star inner crust: Nuclear physics input  

International Nuclear Information System (INIS)

A fully self-consistent model of the neutron star inner crust based upon models of the nucleonic equation of state at zero temperature is constructed. The results nearly match those of previous calculations of the inner crust given the same input equation of state. The extent to which the uncertainties in the symmetry energy, the compressibility, and the equation of state of low-density neutron matter affect the composition of the crust are examined. The composition and pressure of the crust is sensitive to the description of low-density neutron matter and the nuclear symmetry energy, and the latter dependence is nonmonotonic, giving larger nuclei for moderate symmetry energies and smaller nuclei for more extreme symmetry energies. Future nuclear experiments may help constrain the crust and future astrophysical observations may constrain the nuclear physics input

231

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  

Science.gov (United States)

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.

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

2014-06-01

232

Exhumation structures in the Tauern Window and their relation to present-day crust-mantle structure in the Eastern Alps  

Science.gov (United States)

The Alps east of the Guidicarie and Brenner Lines comprise a Late Cretaceous, NE-vergent nappe stack that overlies a Cenozoic N-vergent duplex in the Tauern Window (Scharf et al., this session). The Tauern Window exposes exhumed oceanic (Alpine Tethys) and basement units of the distal European margin. The MOHO attains a maximum depth of 50-52 km at the W end of the Tauern Window, where it appears to be sinistrally offset by the Guidicarie Line. To the E, this thickness gradually decreases to about 44 km at the E end of the Tauern Window, and to as little as 38-40 km towards the Pannonian Basin. The Tauern Window also coincides broadly with a positive Vp mantle anomaly, which in tomographic studies is variously interpreted as a lithospheric slab dipping steeply to the N (Lippitsch et al. 2003) or to the S (Mitterbauer et al. 2011). Our goal is to relate the kinematics of crustal thickening and exhumation in the Tauern Window to these first-order geophysical anomalies. Accretion of units exposed in the Tauern Window began with Late Cretaceous thrusting of the Adriatic margin onto oceanic units of Alpine Tethys. This continued throughout the early Cenozoic with the subduction (D1) and early exhumation (D2) of imbricated oceanic and distal European units, as recorded by Eocene high-pressure metamorphic assemblages (e.g., Kurz et al. 2008 and refs.). The onset of collision at ca. 35 Ma is marked by nappes and duplex formation (D3, Venediger nappe complex) in the European basement units forming the core of the Tauern Window. Rapid exhumation began no later than 20-23 Ma and lasted until about 17 Ma in the E Tauern Window, and 11 Ma in the W Tauern Window. This exhumation was accommodated by a combination of km-scale upright-folding, doming and low-angle normal faulting (D4) at both ends of the Tauern Window. Continued N-S shortening was accommodated increasingly by eastward lateral escape along a conjugate system of strike-slip faults that affected progressively more E parts of the Eastern Alps (Wölfler et al. 2011). We attribute the formation of the MOHO root beneath the Tauern Window to a combination of Oligocene nappe stacking and Miocene late-orogenic doming. Doming coincided broadly with two kinematically related events: (1) sinistral Guidicarie faulting which reaches down to the base of the thickened crust and deforms the MOHO; (2) E-W stretching of the orogenic crust in the footwall of the normal faults bounding the Tauern Window. This supports the notion that pronounced exhumation and lateral escape were triggered by indentation of the eastern part of the rigid Adriatric microplate. This indentation may be related to the high Vp mantle anomaly beneath the Eastern Alps.

Favaro, S.; Handy, M. R.; Scharf, A.; Schuster, R.; Pestal, G.

2012-04-01

233

Thallium isotopes in Iceland and Azores lavas — Implications for the role of altered crust and mantle geochemistry  

Science.gov (United States)

Mantle plumes are commonly perceived to have both a chemical and dynamic link with the subduction of ocean crust into the mantle. In principle, this should lead to the observation of chemical and isotopic signatures that are characteristic of ocean crust and marine sediments in ocean island basalts. This study investigates the thallium (Tl) isotope systematics of lavas from Iceland and the Azores archipelago, in order to determine if their compositions were affected by admixing of ferromanganese sediments or upper ocean crust altered at low temperature. Such materials are known to display strongly fractionated Tl isotope signatures relative to the ambient upper mantle. Two samples from the island of Terceira in the Azores archipelago have Tl isotope compositions significantly different from normal mantle, and this suggests the presence of Fe-Mn sediments. Combined Pb and Tl isotope modelling indicates that the Tl anomalies are not a feature of the Azores plume but produced by assimilation of modern Fe-Mn sediments during magma ascent through the ocean crust. Excluding these two anomalous lavas from Terceira, the Iceland and Azores samples have identical Tl isotope compositions, with an overall mean of ?205Tl = - 1.5 ± 1.4 (2SD, n = 30) that is indistinguishable from the previously estimated upper mantle average ( ?205Tl = - 2.0 ± 0.5). The near-constant Tl isotope compositions of the Iceland and Azores lavas may indicate that the respective mantle plume sources contain virtually no Fe-Mn sediments or altered upper ocean crust. Alternatively, it is possible that the lack of Tl isotope variation reflects quantitative removal of fractionated Tl from the slab during subduction and dehydration. A less straightforward explanation is that past marine environments produced sediments and altered marine basalts with nearly unfractionated Tl isotope compositions. All three scenarios have important implications and future Tl isotope studies will be able to identify the most feasible interpretation.

Nielsen, Sune G.; Rehkämper, Mark; Brandon, Alan D.; Norman, Marc D.; Turner, Simon; O'Reilly, Suzanne Y.

2007-12-01

234

Macquarie island and the cause of oceanic linear magnetic anomalies.  

Science.gov (United States)

Macquarie Islands is formed of probably Pliocene oceanic crust. Intruded into pillow lavas is a belt of harzburgite and layered gabbro mnasses cut by dike swarms. Similar belt-like structures may cause the linear magnetic anomalies of the ocean. PMID:17731490

Varne, R; Gee, R D; Quilty, P G

1969-10-10

235

Microphytic crusts: 'topsoil' of the desert  

Science.gov (United States)

Deserts throughout the world are the home of microphytic, or cryptogamic, crusts. These crusts are dominated by cyanobacteria, previously called blue-green algae, and also include lichens, mosses, green algae, microfungi and bacteria. They are critical components of desert ecosystems, significantly modifying the surfaces on which they occur. In the cold deserts of the Colorado Plateau (including parts of Utah, Arizona, Colorado, and New Mexico), these crusts are extraordinarily well-developed, and may represent 70-80% of the living ground cover.

Belnap, Jayne

1990-01-01

236

Superfluidity and entrainment in neutron star crusts  

OpenAIRE

Despite the absence of viscous drag, the neutron superfluid permeating the inner crust of a neutron star can still be strongly coupled to nuclei due to non-dissipative entrainment effects. Neutron superfluidity and entrainment have been systematically studied in all regions of the inner crust of a cold non-accreting neutron star in the framework of the band theory of solids. It is shown that in the intermediate layers of the inner crust a large fraction of “free” neutrons are actually ent...

Chamel, Nicolas; Pearson, Michael J.; Goriely, Ste?phane

2013-01-01

237

Biogenic crust dynamics on sand dunes  

CERN Document Server

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.

Kinast, Shai; Yizhaq, Hezi; Ashkenazy, Yosef

2012-01-01

238

Origin of the oceanic basalt basement of the Solomon Islands arc and its relationship to the Ontong Java Plateau-insights from Cenozoic plate motion models  

Science.gov (United States)

Cenozoic global plate motion models based on a hotspot reference frame may provide a useful framework for analyzing the tectonic evolution of the Solomon Islands convergent margin. A postulated late Miocene collision of the Ontong Java Plateau (OJP) with a NE-facing arc is consistent with the predicted path of the OJP across the Pacific Basin and its Miocene arrival at the trench. Late-stage igneous activity (65-30 Ma) predicted for the OJP as it rode over the Samoan hotspot occurred in correlative stratigraphic sections on Malaita, the supposed accreted flake of OJP in the Solomon Islands arc. Convergence similar to the present velocities between Australia and the Pacific plates was characteristic of the last 43 million years. Prior to 43 Ma Pacific-Australia plate motions were divergent, seemingly at odds with geologic evidence for early Tertiary convergence, particularly in Papua New Guinea. A postulated South Pacific plate may have existed between Australia and the Pacific plate and would have allowed implied northward subduction along the northeastern Australia plate boundary that lasted into the early Eocene. Subsequent reorganization of plate motions in the middle Eocene correlates with middle Eocene marginal basin formation along ridges oblique to the main plate boundary. Cessation of spreading on the Pacific-South Pacific Ridge and its subsequent subduction beneath Asia followed the change in Pacific plate motion at 43 Ma. A trapped remnant of the extinct, NW-trending ridge may still lie beneath the western Philippine Sea. The terminal deformation, metamorphism and ophiolite obduction in the Eocene orogen of the southwest Pacific also correlates with the major change in Pacific plate motion at 43 Ma and the subsequent compression of the dying Eocene arc against outlying continental and oceanic crustal blocks of the Australian plate. The Solomon Islands oceanic basement may represent juxtaposition of oceanic plateaus of the Australian plate beneath overthrust, dismembered ophiolite derived from adjacent marginal basin crust. ?? 1989.

Wells, R.E.

1989-01-01

239

UV - BOSTON MA  

Science.gov (United States)

Brewer 103 is located in Boston MA, measuring ultraviolet solar radiation. Irradiance and column ozone are derived from this data. Ultraviolet solar radiation is measured with a Brewer Mark IV, single-monochrometer, spectrophotometer manufactured by SCI-TEC Instruments, Inc. of S...

240

Seismicity, metamorphism and rheology of the lower continental crust  

Science.gov (United States)

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.

Austrheim, Håkon

2014-05-01

241

Investigation of thallium fluxes from subaerial volcanism-Implications for the present and past mass balance of thallium in the oceans  

Science.gov (United States)

A suite of 34 volcanic gas condensates and particulates from Kilauea (Hawaii), Mt. Etna and Vulcano (Italy), Mt. Merapi (Indonesia), White Island and Mt. Nguaruhoe (New Zealand) were analysed for both Tl isotope compositions and Tl/Pb ratios. When considered together with published Tl-Pb abundance data, the measurements provide globally representative best estimates of Tl/Pb = 0.46 ?? 0.25 and ??205Tl = -1.7 ?? 2.0 for the emissions of subaerial volcanism to the atmosphere and oceans (??205Tl is the deviation of the 205Tl/203Tl isotope ratio from NIST SRM 997 isotope standard in parts per 10,000). Compared to igneous rocks of the crust and mantle, volcanic gases were found to have (i) Tl/Pb ratios that are typically about an order of magnitude higher, and (ii) significantly more variable Tl isotope compositions but a mean ??205Tl value that is indistinguishable from estimates for the Earth's mantle and continental crust. The first observation can be explained by the more volatile nature of Tl compared to Pb during the production of volcanic gases, whilst the second reflects the contrasting and approximately balanced isotope fractionation effects that are generated by partial evaporation of Tl during magma degassing and partial Tl condensation as a result of the cooling and differentiation of volcanic gases. Mass balance calculations, based on results from this and other recent Tl isotope studies, were carried out to investigate whether temporal changes in the volcanic Tl fluxes could be responsible for the dramatic shift in the ??205Tl value of the oceans at ???55 Ma, which has been inferred from Tl isotope time series data for ferromanganese crusts. The calculations demonstrate that even large changes in the marine Tl input fluxes from volcanism and other sources are unable to significantly alter the Tl isotope composition of the oceans. Based on modelling, it is shown that the large inferred change in the ??205Tl value of seawater is best explained if the oceans of the early Cenozoic featured significantly larger Tl output fluxes to oxic pelagic sediments, whilst the sink fluxes to altered ocean crust remained approximately constant. ?? 2009 Elsevier Ltd.

Baker, R.G.A.; Rehkamper, M.; Hinkley, T.K.; Nielsen, S.G.; Toutain, J.P.

2009-01-01

242

Growth of the Afanasy Nikitin Seamount, Central Indian Ocean - the product of short-lived hotspots  

Science.gov (United States)

The Afanasy Nikitin seamount (ANS) is a major structural feature in the Central Indian Basin. An understanding of its evolution using multibeam bathymetry, magnetic and seismic reflection data provides new insights on growth of the seamount through time, emplacement of the 85°E Ridge in the Bay of Bengal and deformation of the lithosphere in the equatorial Indian Ocean. The seafloor morphology and internal structure of the ANS show that the seamount consists of extensive plateaus extending from 2°15' to 5°30'S in water depths of 3000-4500 m, numerous elevated features (seamount highs) pierce through the northern part of the seamount plateau reaching up to 1600 m water depth and faulted blocks up to 1.0 s TWT throw in the southern part of the seamount plateau. Model studies of magnetic profiles suggest that the main plateau of the seamount was emplaced during the normal magnetisation period between the formation of seafloor spreading anomalies 33-32n.2 (79-73 Ma) and that the seamount high was formed in a reverse magnetisation period later than the main seamount plateau formation. Integrated geophysical results clearly demonstrate that the ANS was constructed in two phases, initially coeval with the formation of the oceanic crust during 79-73 Ma, and later at about 55 Ma in an intraplate setting. Based on present geophysical results and published plate reconstruction results of the Indian Ocean from Late Cretaceous to Early Cenozoic, we believe that the Conrad hotspot has emplaced the main plateau of the ANS and Conrad Rise during the period 79-73 Ma in an on-ridge setting, after which the hotspot has continued its activity in Antarctica plate leaving the main plateau of the ANS as an isolated feature on the Indian plate. Subsequently another hotspot that formed the 85°E Ridge and buried hills in the Bay of Bengal has rebuilt the existing main plateau of the ANS at around 55 Ma and eventually the hotspot became defunct in the vicinity of the ANS.

Krishna, K. S.; Scrutton, R. A.; Bull, J. M.; Shankar, S. Jai; Banakar, V. K.

2009-04-01

243

The nature of the crust beneath the Afar triple junction: Evidence from receiver functions  

Science.gov (United States)

The Afar depression is an ideal locale to study the role of extension and magmatism as rifting progresses to seafloor spreading. Here we present receiver function results from new and legacy experiments. Crustal thickness ranges from ˜45 km beneath the highlands to ˜16 km beneath an incipient oceanic spreading center in northern Afar. The crust beneath Afar has a thickness of 20-26 km outside the currently active rift segments and thins northward. It is bounded by thick crust beneath the highlands of the western plateau (˜40 km) and southeastern plateau (˜35 km). The western plateau shows VP/VS ranging between 1.7-1.9, suggesting a mafic altered crust, likely associated with Cenozoic flood basalts, or current magmatism. The southeastern plateau shows VP/VS more typical of silicic continental crust (˜1.78). For crustal thicknesses 2.0) can only be explained by significant amounts of magmatic intrusions in the lower crust. This suggests that melt emplacement plays an important role in late stage rifting, and melt in the lower crust likely feeds magmatic activity. The crust between the location of the Miocene Red Sea rift axis and the current rift axis is thinner (2.0) than beneath the eastern part of Afar (>26 km, VP/VS region contains less partial melt, has undergone less stretching/extension and has preserved a more continental crustal signature than west of the current rift axis. The Red Sea rift axis appears to have migrated eastward through time to accommodate the migration of the Afar triple junction.

Hammond, J. O. S.; Kendall, J.-M.; Stuart, G. W.; Keir, D.; Ebinger, C.; Ayele, A.; Belachew, M.

2011-12-01

244

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

DEFF Research Database (Denmark)

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 (

Frassetto, Andrew; Thybo, Hans

2013-01-01

245

Spectral expressions for modelling the gravitational field of the Earth’s crust density structure:  

OpenAIRE

We derive expressions for computing the gravitational field (potential and its radial derivative) generated by an arbitrary homogeneous or laterally varying density contrast layer with a variable depth and thickness based on methods for a spherical harmonic analysis and synthesis of gravity field. The newly derived expressions are utilised in the gravimetric forward modelling of major known density structures within the Earth’s crust (excluding the ocean density contrast) beneath the geoid ...

Tenzer, R.; Novak, P.; Hamayun; Vajda, P.

2011-01-01

246

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

Science.gov (United States)

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.

Weis, D.

2013-12-01

247

The breaking strain of neutron star crust  

Energy Technology Data Exchange (ETDEWEB)

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.

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

2009-01-01

248

Early Cretaceous intra-oceanic rifting in the Proto-Indian Ocean recorded in the Masirah Ophiolite, Sultanate of Oman  

OpenAIRE

The Masirah Ophiolite (Sultanate of Oman) was part of an oceanic basin (Proto-Indian Ocean) formed by the break-up of Gondwana in Late Jurassic times similar to the Somali basin. It was obducted onto the Arabian continental margin in the Early Paleocene, 100 Ma after its formation. Hence, it is possible to investigate the different tectonic and magmatic processes that have affected the oceanic lithosphere during these 100 Ma. Tithonian ridge magmatism, tectonism and hydrothermal alteration ar...

Marquer, Didier; Mercolli, Ivan; Peters, Tjerk

2007-01-01

249

Shear modulus of neutron star crust  

OpenAIRE

Shear modulus of solid neutron star crust is calculated by thermodynamic perturbation theory taking into account ion motion. At given density the crust is modelled as a body-centered cubic Coulomb crystal of fully ionized atomic nuclei of one type with the uniform charge-compensating electron background. Classic and quantum regimes of ion motion are considered. The calculations in the classic temperature range agree well with previous Monte Carlo simulations. At these temper...

Baiko, D. A.

2011-01-01

250

Neutron Star Crust and Molecular Dynamics Simulation  

CERN Document Server

In this book chapter we review plasma crystals in the laboratory, in the interior of white dwarf stars, and in the crust of neutron stars. We describe a molecular dynamics formalism and show results for many neutron star crust properties including phase separation upon freezing, diffusion, breaking strain, shear viscosity and dynamics response of nuclear pasta. We end with a summary and discuss open questions and challenges for the future.

Horowitz, C J; Schneider, A; Berry, D K

2011-01-01

251

Crystals stirred up: 2. Numerical insights into the formation of the earliest crust on the Moon  

Science.gov (United States)

This is the second paper in a two-part series examining the fluid dynamics of crystal settling and flotation in the lunar magma ocean. In the first paper, we develop a direct numerical method for resolving the hydrodynamic interactions between crystals and their feedback on the flow field in magmatic liquid. In this paper, we use this computational technique to test the leading model for the formation of the earliest crust on the Moon. The anorthositic lithology of the lunar crust is thought to have been formed by the flotation of buoyant plagioclase crystals at a time when the lunar mantle was still wholly or largely molten. This model is appealing from an observational point of view, but its fluid dynamical validity is not obvious, because (1) plagioclase probably started crystallizing very late (i.e., when the magma ocean was already 80% solidified) and (2) a significant portion of the shallow lunar crust consists of almost pure plagioclase (>90 vol. %), requiring very efficient plagioclase segregation. The goal of this study is to better understand the fluid dynamical conditions that hinder or facilitate crystal settling or flotation. Our approach complements earlier studies by explicitly linking the petrological and fluid dynamical evolution and by focusing on the effect of increasing crystal fraction. We find that crystal settling was probably possible throughout the entire solidification history of the lunar magma ocean as long as crystal sizes were sufficiently large (r > 1 mm) and crystal fraction sufficiently low (? 13%).

Suckale, Jenny; Elkins-Tanton, Linda T.; Sethian, James A.

2012-08-01

252

RHUM-RUM investigates La Réunion mantle plume from crust to core  

Science.gov (United States)

RHUM-RUM (Réunion Hotspot and Upper Mantle - Réunions Unterer Mantel) is a French-German passive seismic experiment designed to image an oceanic mantle plume - or lack of plume - from crust to core beneath La Réunion Island, and to understand these results in terms of material, heat flow and plume dynamics. La Réunion hotspot is one of the most active volcanoes in the world, and its hotspot track leads unambiguously to the Deccan Traps of India, one of the largest flood basalt provinces on Earth, which erupted 65 Ma ago. The genesis and the origin at depth of the mantle upwelling and of the hotspot are still very controversial. In the RHUM-RUM project, 57 German and French ocean-bottom seismometers (OBS) are deployed over an area of 2000 km x 2000 km2 centered on La Réunion Island, using the "Marion Dufresne" and "Meteor" vessels. The one-year OBS deployment (Oct. 2012 - Oct. 2013) will be augmented by terrestrial deployments in the Iles Eparses in the Mozambique Channel, in Madagascar, Seychelles, Mauritius, Rodrigues and La Réunion islands. A significant number of OBS will be also distributed along the Central and South West Indian Ridges to image the lower-mantle beneath the hotspot, but also to provide independent opportunity for the study of these slow to ultra-slow ridges and of possible plume-ridge interactions. RHUM-RUM aims to characterize the vertically ascending flow in the plume conduit, as well as any lateral flow spreading into the asthenosphere beneath the western Indian Ocean. We want to establish the origin of the heat source that has been fueling this powerful hotspot, by answering the following questions: Is there a direct, isolated conduit into the deepest mantle, which sources its heat and material from the core-mantle boundary? Is there a plume connection to the African superswell at mid-mantle depths? Might the volcanism reflect merely an upper mantle instability? RHUM-RUM also aims at studying the hotspot's interaction with the neighboring ridges of the Indian Ocean. There is in particular a long-standing hypothesis, not yet examined seismically, that channelized plume flow beneath the aseismic Rodrigues Ridge could feed the Central Indian Ridge at 1000 km distance. The RHUM-RUM group (www.rhum-rum.net): * IPG Paris & Géosciences Réunion: G. Barruol, J.P. Montagner, E. Stutzmann, F.R. Fontaine, C. Deplus, M. Cannat, G. Roult, J. Dyment, S. Singh, W. Crawford, C. Farnetani, N. Villeneuve, L. Michon. V. Ferrazzini, Y. Capdeville. * Univ. Munich (LMU): K. Sigloch, H. Igel. AWI Bremerhaven: V. Schlindwein. Univ. Frankfurt: G. Rümpker. Univ. Münster: C. Thomas. Univ. Bonn: S. Miller. * Géosciences Montpellier: C. Tiberi, A. Tommasi, D. Arcay, C. Thoraval. * Mauritius Oceanography Institute: D. Bissessur. Univ. Antananarivo: G. Rambolamanana. SEYPEC Seychelles Petroleum: P. Samson, P. Joseph. * Other institutes: A. Davaille, M. Jegen, M. Maia, G. Nolet, D. Sauter, B. Steinberger.

Sigloch, Karin; Barruol, Guilhem

2013-04-01

253

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)

254

Thermal models of dyke intrusion during development of continent-ocean transition  

Science.gov (United States)

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.

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

2014-01-01

255

Implications of Continental Crust Extension for Plate Reconstruction: AN Example from the Gulf of Mexico  

Science.gov (United States)

The prerift reconstruction of continental plates bounded by rifted margins requires the closure of relative motion accomplished both by seafloor spreading and by the extension of continental crust during the rift phase of continental breakup. Continental extension is not expected to be pervasive throughout the plate, but to be confined to a zone of up to several hundred kilometers in width. The direction of particle motion in this zone is expected to be parallel to flow lines followed by the rigid portions of the plates. The "best fit" between rifted continental margins is then described by the rotation angle about an Euler pole which best closes both the oceanic crust and the extension within the continental crust, along small circles about the Euler pole. As an example application of these concepts, the pattern of Late Mesozoic crust extension within the Gulf of Mexico basin is used to constrain the location of the Euler pole and angle of rotation of the Yucatan block with respect to North America. Both crust type and the degree of extension within the transitional crust surrounding the basin are estimated on a point-by-point basis from bathymetry and basement depth. The root-mean-square (rms) misfit in the apparent total closure of both oceanic crust and extension within the continental crust is computed for a range of possible poles of opening. A contour map of misfit versus pole location reveals a global minimum rms misfit of ±47 km for a 45° counterclockwise opening of the Gulf of Mexico basin about an Euler pole at 25°N, 79°W. An elongate trend of poles, for which the misfit is less than 100 km, extends from the Bahama Islands eastward across the Atlantic and through North Africa. The reconstruction of the Gulf of Mexico for the best fit pole places the palinspastically restored Yucatan block in contact with North America along the Texas and Louisiana shelf. The resulting restored southern margin of North America fits against the the northern margin of South America in its prerift position to within the uncertainty in the North America-Africa-South America plate circuit. We conclude that the prerift placement of the Yucatan block between North and South America along the Texas-Louisiana shelf is consistent with both the pattern of extension within the Gulf of Mexico basin and seafloor spreading history of the central Atlantic basin.

Dunbar, John A.; Sawyer, Dale S.

1987-12-01

256

Galicia Bank ocean-continent transition zone: New seismic reflection constraints  

Science.gov (United States)

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.

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

2015-03-01

257

Recognition of Intermediate Crust (IC), its construction and its distinctive epeirogenic behaviour: an exciting new tool for plate kinematic analysis (PKA) of the Arctic margins and western Siberia  

Science.gov (United States)

Identification of a microcontinental block within or near a continental margin raises two questions, addressed in this talk - How did it get there? What is the nature of the intervening crust? I will then illustrate briefly how, in the Arctic, the answers, although by no means restricted to that region, do seem to help us a lot to begin unravelling the ancient plate kinematics of its wide margins. The plate tectonics paradigm currently posits that the Earth has only two kinds of crust - continental and oceanic - and that the former may be stretched to form sedimentary basins or the latter may be modified by arc or collision until it looks continental. But global analysis of the dynamics of actual plate motions for the past 150 Ma indicates [1, 2, 3] that continental tectospheres must be immensely thicker than previously thought and almost certainly too thick to be stretched with the forces available. In the extreme case of cratons, these tectospheric keels may commonly extend to 600 km or more [3]. This thick-plate behaviour is attributable, not to cooling but to a petrological 'stiffening' effect, associated with a loss of water-weakening, which also applies to the LVZ below MORs [4, 5, 6]. The corresponding thick-plate version of the MOR process [1, 6] has a deep, narrow wall-accreting axial crack which inherently brings two outstanding additional benefits:- (i) why, at medium to fast spreading rates, MOR axes become straight and orthogonally segmented [7], (ii) not being driven by body forces, it can achieve the sudden jumps of axis, spreading-rate and direction widely recorded in mid-ocean and are necessary after generating the limited separations of microplates near margins. So in seeking the 'continent-ocean boundary' (COB) along passive margins, a site where stretching has often been invoked, we need instead to consider how this MOR process would be affected by the heavy concurrent sedimentation to be expected when splitting a continent. I reason that, by blocking the hydrothermal cooling, this must inhibit magnetic anomaly formation and prolong magmagenesis to give a thicker-than-oceanic mafic crust, which I have called Intermediate Crust (IC) [8, 9], to distinguish it from Mature Continental Crust (MCC). Seismologically, IC basement must look deceptively like that assigned to stretched MCC. For thermodynamic reasons [8, 9] the hydrous content of deep MCC and of deeply subducted UHP crustal slices gives them a big thermal epeirogenic sensitivity which IC lacks. The NE Atlantic offers an example of this distinction. Structurally, the MCC of Greenland and Norway must have been intimately juxtaposed by the Scandian collision, so it was concluded [9] that the crust of the Greenland-Norway continental shelves must mostly be IC of post-Scandian (early Devonian?) age, a character confirmed by their lack of epeirogenic response to laterally conducted heat from the opening N Atlantic, although drainage systems in Norway proper clearly show it. Geometrically, this separation appears to have changed direction sharply, the second and bigger stage also involving separation of Svalbard from near Tromsø, where it had provided northward continuation of a complete Caledonian transect, so it has an IC implication for much of the Barents Sea area (bar the Bjørnøya block). Moving quickly round to the NE side of Baltica, we can begin to trace the separative motions of the Novaya Zemlya - Pay Khoy (NZPK) strip of less-mature MCC, transverse to the Timanian belt, and the associated evolution of the Pechora basin system. In places, faulted IC/MCC epeirogenic contrasts seem to define the size and direction of the IC-generating separation with remarkable precision. A crucial opening-up of this analysis is provided by realizing that the Polar Ural stretch is not MCC, but is merely the huge 585 Ma Voykar-Synya ophiolite, with its metamorphics, resting on a now-crumpled boundary between IC of very different ages. For further understanding we need briefly to extend the analysis, first to the formation of the West Siberian Basin, the IC nat

Osmaston, M. F.

2012-04-01

258

The atypical Caribbean-Colombia oceanic plateau and its role in the deformation of the Northern Andes  

Science.gov (United States)

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.

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

2013-05-01

259

Uplift at lithospheric swells-I: seismic and gravity constraints on the crust and uppermost mantle structure of the Cape Verde mid-plate swell  

OpenAIRE

Wide-angle seismic data have been used to determine the velocity and density structure of the crust and uppermost mantle beneath the Cape Verdes mid-plate swell. Seismic modelling reveals a 'standard' oceanic crust, ?8 km in thickness, with no direct evidence for low-density bodies at the base of the crust. Gravity anomaly modelling within the constraints and resolution provided by the seismic model, does not preclude, however, a layer of crustal underplate up to 3 km thick beneath the swel...

Wilson, Dj; Peirce, C.; Watts, Ab; Grevemeyer, I.; Krabbenhoeft, A.

2010-01-01

260

Spatial and Temporal Relationships Between Anatexis and Deformation in the Himalayan Mid-Crust (Invited)  

Science.gov (United States)

Partial melting of mid- to lower-crustal rocks within collisional orogens, such as the Himalaya, has the potential to dramatically alter the rheology (effective bulk viscosity) and therefore strength of the crust. Understanding the processes that lead to generation and mobilization of anatectic melts is important in defining the spatial and temporal scales over which flow or extrusion of crustal material can occur as well as defining the linkages and potential feedbacks between melting and other fundamental orogenic processes such as deformation and exhumation. To gain a more complete picture of the magmatic history of the Himalayan mid-crust we compare and contrast the thermobarometric, geochronologic and geochemical record of melting and its relationship to deformation in the three spatially distinct, yet ultimately related, geologic components of the Himalaya: the Greater Himalayan Series (GHS); North Himalayan domes; and younger domes exhumed during orogen-parallel extension. Within the GHS, as exemplified by the Mt. Everest transect in central Nepal / Southern Tibet and the Leo Pargil Dome in NW India, the magmatic history is protracted, with multiple phases of melt-generation via muscovite dehydration of metapelitic source rocks over the period 26 - 12 Ma. The northern most exposures of GHS equivalent rocks - the North Himalayan domes - also record a complex melting history with leucogranite crystallization ages ranging from Oligocene to as young as ~8 Ma. Our ongoing petrologic work indicates that in addition to muscovite dehydration, early melt production via biotite dehydration appears to be volumetrically important in generating leucogranites. The age and petrogenesis of leucogranites within the domes imply that they potentially record an earlier part of the melting history than is preserved in the majority of the GHS to the south. In contrast to the GHS rocks, orogen-parallel domes such as the Ama Drime Massif (ADM) record melting over a relatively restricted time interval of 2 - 3 Ma. Within the ADM, syn-kinematic melting of granitic orthogneiss at granulite-facies Pressure-Temperature (P-T) conditions of 750°C and 0.8GPA occurred between 13 and 12 Ma. This was immediately followed by emplacement of post-kinematic dykes along steep semi-brittle structures at 12-11 Ma. Observations from these three tectonic domains suggest that the timing, duration and sources of melt generation varied considerably throughout the history of the orogen. Significantly, during the late Miocene, as a result of a fundamental shift in the kinematic configuration, the locus of melting migrated from widespread anatexis and associated south-directed extrusion of the mid-crust (GHS) to structurally controlled isolated zones of melting within the lower-crust (ADM) during east-west extension.

Cottle, J. M.; Jessup, M. J.

2010-12-01

261

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  

Science.gov (United States)

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.

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

2010-10-01

262

Basin Excavation, Lower Crust, Composition, and Bulk Moon Mass balance in Light of a Thin Crust  

Science.gov (United States)

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.

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

2013-01-01

263

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  

Science.gov (United States)

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.

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

2015-01-01

264

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  

Science.gov (United States)

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.

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

2014-08-01

265

The Subduction of Continental Crust, the Variscan Evolution of the Bohemian Massif, and the Origin of PO Granitoids  

Science.gov (United States)

Slices of continental crust subducted into the mantle during collisional orogeny may either undergo metamorphism and exhumation towards the surface as coherent slab-like or domal high pressure/ultrahigh pressure (HP/UHP) terranes or, if stalled or delayed in the mantle, melt and return towards the surface as magmas, or undergo a combination of exhumation and melting. Some exhumed HP/UHP terranes contain synorogenic granitoid bodies demonstrating melting does occur during exhumation. Therefore, crust that remains trapped in the mantle will also melt when temperatures reach the appropriate solidi through adiabatic decompression and/or conductive heating and/or radioactive decay. Subducted terranes with hydrous phases will undergo hydrate-breakdown melting and could melt during subduction, when stalled in the mantle or during exhumation. Terranes lacking hydrous phases probably require melting by adiabatic decompression as heated crust becomes ductile and rises as diapirs through the mantle wedge. The generated magmas will intrude through the overlying mantle wedge and into the overlying continental crust to form late orogenic and post orogenic (PO) granitoids depending on the time required to reach solidus temperatures. Geochemical characteristics will depend on P-T conditions, the age/chemistry/mineralogy of the subducted terrane (especially the presence or lack of hydrous phases), and the degree of melt interaction (i.e. the traverse length) with the mantle wedge. Melts that significantly traverse the wedge will acquire the hybrid mantle/crust nature of many PO granitoids. Melts generated by adiabatic decompression close to or within the continental crust will retain ancient crustal signatures. The Variscan evolution of the Bohemian Massif involved two episodes of subduction of continental crust: (1), the southward (present coordinates) subduction of Saxo-Thuringia beneath Bohemia (aka Tapla-Barrandia) along an east-west suture at 400-370 Ma followed by, (2), the east-northeast subduction of Moldanubia beneath Bohemia at 340-320 Ma1. The first subduction was followed by the intrusion of granitoids, including the Central Bohemian Batholith, between 370-340 Ma, all occurring south of the Saxo-Thuringian - Bohemia suture suggesting elements of the subducted Saxo-Thuringian continental crust melted and intruded the overlying Bohemian Craton. The second subduction event was accompanied and followed by multiple intrusions of granites (340 Ma synorogenic granitoids, 340-310 Ma post-orogenic S and high-K granitoids, and 310-290 I-type granitoids2) all occurring west of the of the Moldanubia-Bohemia suture suggesting elements of the subducted Moldanubian crust were melted and intruded the overlying Bohemian/Saxo-Thuringian Craton. Many of the HP/UHP terranes in the Bohemian Massif occur in domal structures suggesting exhumation and melting occurred in part by diapiric upwelling3. It is proposed that both the Saxo-Thuringian and Moldanubian terranes were exhumed by a combination of slab-return and diapiric upwelling and that elements of both terranes were stranded long enough in the mantle to heat up and melt to form most or all of the PO granitoids of the Bohemian Massif. This model involves successive re-distillation of the continental crust and may play a significant role in the evolution of both the continental crust and upper mantle. 1Medaris et al., 2005, Lithos 82. 2Finger et al., 1997, Min & Pet 61. 3Stípská et a., 04, J. Met. Geol. 22.

Brueckner, H. K.

2012-04-01

266

Nuclei in Strongly Magnetised Neutron Star Crusts  

CERN Document Server

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...

Nandi, Rana

2012-01-01

267

Unified Structural Representation of the southern California crust and upper mantle  

Science.gov (United States)

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 (

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

2015-04-01

268

Factors controlling the variations in the mid-ocean ridge segmentation  

Science.gov (United States)

Mid-ocean ridge systems show striking difference according to their spreading rates and associated thermal structures. The geometry of ridge-transform fault intersections and the spacing of transform fault zones (TFZ) are among the defining characteristics of a mid-ocean ridge system. We first address the mechanics responsible for the variations in geometry made by mid-ocean ridges and transform faults using numerical models. Secondly, the role of internal loading in the mantle is studied as a possible cause to the variation in the TFZ spacing as observed in Antarctic-Australian Discordance (AAD). In models of ridge-transform fault geometry, the driving forces are thermal stresses arising from the cooling of young oceanic crust and extensional kinematic boundary conditions. The model domain is a fully 3-D block of oceanic crust with elasto-visco-plastic rheology. Thermal stress can exert ridge-parallel tension comparable to spreading-induced stress when selectively released by ridges and ridge-parallel structure. Two modes of ridge segment growth have been identified in plan view: An overlapping mode where ridge segments overlap and bend toward each other and a connecting mode where two ridge segments are connected by a transform-like fault. As the ratio of thermal stress to spreading-induced stress (?) increases, the patterns of localized plastic strain on the top surface change from the overlapping to connecting mode. The orthogonal pattern marks the transition from one mode to the other. Besides the amount of stress from each driving force, the rate of stress accumulation is crucial in determining the emergent pattern. This rate-dependence is characterized by the spreading rate normalized by a reference-cooling rate (Pe^'). When Pe^' is paired with the ratio of thermal stress to a reference spreading-induced stress (?^'), they define stability fields of the two modes. Models for the TFZ spacing have a cold temperature anomaly placed at the bottom of the domain. The anomaly represents a subducted slab crossing perpendicular to a 1000-km long straight ridge segment. Extension at the full spreading rate of 7 cm/yr and a sea floor age variation up to 10 Ma are imposed as boundary and initial conditions. We show the dynamic response of the lithosphere to the internal loading. Surface topography and plastic strain localization from the models will be compared with the residual bathymetry and transform faults in AAD, respectively.

Choi, E.; Gurnis, M.; Lavier, L.

2006-12-01

269

Earth's Crust: Thinner Than You Think  

Science.gov (United States)

This demonstration illustrates the thickness of the crust of the earth in relation to the radius of the earth. A postage stamp stuck to a soccer ball represents the thickness of the crust. Assuming the model is somewhat accurate, students use the fact that the radius of the Earth averages about 6400 km and the crust 15 km to work out the thickness of a postage stamp. Alternatively, they may use a micrometer to measure the thickness of the postage stamp and devise a method for measuring the radius of a soccer ball to check whether this analogy is valid. Pupils could discuss the use of models such as this in science education as an aid to learning and understanding.

270

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)

271

The Inner Crust and its Structure  

CERN Document Server

In this chapter we discuss some possible physical pictures that describe the constitution of the inner crust of compact objects. Different relativistic models both with constant couplings and density dependent ones are used. We calculate the liquid-gas phase transition in asymmetric nuclear matter from the thermodynamic and dynamic instabilities. The equations of state used to describe the crust are related to the crust-core transition properties. Cold and warm pasta phases with and without alpha particles are constructed. The influence of the pasta phase and its internal structure on the diffusion coefficients associated with Boltzman transport equations used to simulate the evolution of protoneutron stars are shown. Finally, the possible existence of bare quark stars and the effects of strong magnetic fields on quark matter are considered. Open questions are pointed out.

Menezes, Débora P; Providência, Constança; Alloy, Marcelo D

2011-01-01

272

Millennial-scale ocean acidification and late Quaternary  

Energy Technology Data Exchange (ETDEWEB)

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.

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

2014-01-01

273

Fractionation of the geochemical twins Zr-Hf and Nb-Ta during scavenging from seawater by hydrogenetic ferromanganese crusts  

Science.gov (United States)

In contrast to igneous systems, the geochemical twins Zr and Hf are decoupled from each other in seawater, and specific Zr/Hf ratios appear to be characteristic of individual marine water masses. Hydrogenetic marine ferromanganese (Fe-Mn) crusts which accumulate trace metals from seawater may be an archive of Zr/Hf ratios that reveal changes in oceanic paleocirculation over millions of years. To verify whether Fe-Mn crusts truly reflect the Zr-Hf distribution in seawater, we studied these particle-reactive elements together with Nb and Ta (another geochemical twin pair) in bulk Fe-Mn crusts and their surface layers from different locations in the Atlantic and Pacific oceans. Zirconium (400-1000 mg kg-1), Hf (5-18 mg kg-1), Nb (42-83 mg kg-1) and Ta (0.5-1.5 mg kg-1) are significantly enriched in Fe-Mn crusts relative to the average continental crust, and their Zr/Hf and Nb/Ta ratios are super-chondritic (57-87 and 35-96, respectively), whereas the continental crust shows ratios close to those of chondrites. We emphasize that neither bulk Fe-Mn crusts nor their surface layers match the Zr/Hf or Nb/Ta ratios of modern deep seawater, but are lower and higher, respectively. The presence of aluminosilicate detritus cannot explain the different Zr/Hf ratios of crusts and ambient seawater, as potential detritus has much lower Zr and Hf concentrations. Consequently, these geochemical twins must be fractionated during their removal from seawater and their incorporation into Fe-Mn (oxyhydr)oxides. Hafnium is preferentially scavenged as shown by Zr/Hf ratios of crust surface layers (75-100) that are always below those of modern deep seawater (150-300). The decoupled behavior of geochemical twins during sorption, which is also observed for Nb-Ta, can be related to differences in the electron structures of these elements. Iron-normalized concentrations of Zr, Hf, Nb, and Ta increase with increasing size of the positive Ce anomaly (known to increase with decreasing growth rate), which is accompanied by decreasing Zr/Hf and Nb/Ta ratios. Concentrations and ratios of Zr-Hf and Nb-Ta in the crusts are controlled by the Fe/Mn ratio, the growth rate of a crust, and by the composition of ambient seawater. On a basin-wide scale, the variation of Zr/Hf ratios in Fe-Mn crusts, although different from those of ambient seawater, generally reflects the variation of the regional seawater. Compared to Fe-Mn crusts from the Central Pacific, crusts from the NE Atlantic display lower Zr/Hf and Nb/Ta ratios. This likely reflects the lower Zr/Hf ratios in Atlantic deep water compared to Pacific deep water. Although the Zr/Hf ratio of Fe-Mn crusts broadly reflects major changes in seawater circulation, the Zr/Hf signatures of individual Fe-Mn crust layers cannot be used as a paleoceanographic proxy because they are continuously modified by ongoing sorption. Sorption of Zr and Hf onto Fe and/or Mn (oxyhydr)oxides that occur either as discrete particles or as particle coatings, appear to be an efficient way to fractionate Zr and Hf and may significantly contribute to the continuous fractionation of dissolved Zr and Hf observed in seawater.

Schmidt, Katja; Bau, Michael; Hein, James R.; Koschinsky, Andrea

2014-09-01

274

The nature of the acoustic basement on Mendeleev and northwestern Alpha ridges, Arctic Ocean  

Science.gov (United States)

The Alpha-Mendeleev ridge complex, over 1500 km long and 250-400 km wide, is the largest submarine structure in the Arctic Ocean basin. Its origin is unknown, but often inferred to represent a large igneous province where domains of continental crust may also be a possibility. We investigate the basement geology of part of this large scale feature using 1100 km of multichannel seismic reflection data, sonobuoy recordings and marine gravity data acquired in 2005 from USCG icebreaker Healy. The sonobuoy results show top and intra-acoustic basement velocities in the range of 2.3-4.0 km/s and the seismic reflection attributes define three main acoustic facies: 1) continuous high amplitude reflections often with abrupt breaks, 3) weak wedge geometry and 3) segmented, disrupted to chaotic reflections. The acoustic characteristics and seismic velocities compare more closely with basement on Ontong Java Plateau than normal ocean crust or wedges of seaward dipping reflections at volcanic margins. The acoustic facies are interpreted to represent basalt flows and sills capping voluminous tuff deposits and possible sediments. At least two volcanic centres are identified. The upper volcanic carapace on the surveyed part of Mendeleev and northwestern Alpha ridges was emplaced during a brief igneous episode no later than Campanian (80 Ma) and most likely part of wider Late Cretaceous circum Arctic volcanism. The horst and graben morphology on Mendeleev Ridge is largely a result of post-emplacement faulting where a number of the major extensional faults remained active until a late Miocene intrusive event.

Bruvoll, Vibeke; Kristoffersen, Yngve; Coakley, Bernard J.; Hopper, John R.; Planke, Sverre; Kandilarov, Aleksandre

2012-01-01

275

Nagssugtoqidian mobile belt of West Greenland: A cryptic 1850 Ma suture between two Archaean continents - chemical and isotopic evidence  

International Nuclear Information System (INIS)

New chemical and isotopic data permit the recognition of a cryptic suture zone between two Archean continental masses within the Nagssugtoqidian mobile belt of West Greenland. This discovery has important implications for Precambrian crustal evolution: suture zones may not always be identifiable from geologic field observations, with the consequence that mobile belts in which undetected sutures exist may be mis-identified as ensialic, and thought to require special non-plate tectonic models to account for their development. The Nagssugtoqidian belt consists mainly of Archaean gneisses reworked during the Proterozoic, with metamorphic grade and degree of isotopic disturbance increasing towards the center of the belt. At the centre of the belt the Nagssugtoqidian includes metasediments and calc-alkaline volcanic and plutonic rocks of Proterozoic age, almost always strongly deformed and metamorphosed. From isotopic evidence (Sri ca. 0.703; model ?1 values ca. 8.0; initial ?Nd ca. 0) it is clear that the Proterozoic igneous rocks do not include any significant contributions derived from the Archaean crust, and the chemistry of rocks, together with the isotope data, suggests that they were formed at a destructive plate margin. The Proterozoic rocks are found in a narrow zone (up to 30 km wide) between the Archaean gneisses to the north and south of Nordre Stroemfjord, and are interpreted as reflecting the existence of a suture between twong the existence of a suture between two Archaean continental blocks. Zircon U-Pb data and other isotope evidence show that subduction started before ca. 1920 Ma ago, and lasted until ca. 1850 Ma when collision occurred, with consequent crustal thickening, high-grade metamorphism and local anatexis. Given the time-span for the operation of subduction, the existence of a wide Nagssugtoqidian ocean can be inferred, even for slow rates of plate motion. (orig./SHOE)

276

The Continental Distillery: Building Thick Continental Crust in the Central Andes (Invited)  

Science.gov (United States)

The formation of stable continental crust and the associated development and destruction of mantle lithospheric roots is central to our understanding of plate tectonics, both at its inception and as an ongoing process today. Subduction zones play an important role in the creation and refinement of continental crust, and also serve as a possible mechanism for the removal of residual mantle material. The central Andes provide an intriguing laboratory for the study of these processes. Up to 400 km wide, 1500 km long, and with an average elevation of 4 km, the Altiplano Plateau is the largest orogen on earth associated with an ocean-continent subduction zone. This is much larger than adjacent 'normal' sections of the Andes, raising the question of why this portion of South American crust became so much more substantial than surrounding areas. Over the past several years, new seismic data have made it possible for us to develop a more complete picture of the lithospheric and asthenospheric processes involved in the development of the Altiplano Plateau and the adjacent narrower orogen further to the north. The 'Central Andean Uplift and the Geodynamics of High Topography' (CAUGHT) comprises in part a broadband deployment of 50 stations across the northern flank of the Altiplano Plateau in southern Peru and northern Bolivia. The adjacent 'PerU Lithosphere and Slab Experiment' (PULSE) includes 40 broadband stations that cover the region directly north of the CAUGHT deployment, encompassing the northern edge of the Altiplano, the transition to 'normal' width orogen, and the transition in slab geometry from normal to flat from south to north across the study area. Uplift of the Altiplano Plateau is likely due to some combination shortening, isostasy due to lithospheric destruction or changes in crustal density, magmatic addition to the crust, and/or flow within the thickened crust. Our studies indicate pervasive low velocities across the Altiplano consistent with a dominantly felsic composition, together with even lower velocities in the mid crust beneath the Western Cordillera and Altiplano that are best explained as the intrusion of a Neogene - to - recent batholith. The lithospheric mantle appears to be highly variable across the study area, with some regions appearing to have lost most of their original roots and others indicating the persistence of a high velocity mantle lithosphere. These studies and others in progress will help us form a better idea of the processes involved in generating thick continental crust from otherwise 'normal' ocean-continent subduction zones, and those responsible for the development and destruction of continental lithospheric roots.

Wagner, L. S.; Beck, S. L.; Zandt, G.; Long, M. D.; Tavera, H.; Minaya, E.; Biryol, C. B.; Bishop, B.; Eakin, C. M.; Franca, G.; Knezevic Antonijevic, S.; Kumar, A.; Ryan, J. C.; Scire, A. C.; Ward, K. M.; Young, B. E.

2013-12-01

277

Resonant shattering of neutron star crusts  

OpenAIRE

The resonant excitation of neutron star (NS) modes by tides is investigated as a source of short gamma-ray burst (sGRB) precursors. We find that the driving of a crust-core interface mode can lead to shattering of the NS crust, liberating ~10^46-10^47 erg of energy seconds before the merger of a NS-NS or NS-black hole binary. Such properties are consistent with Swift/BAT detections of sGRB precursors, and we use the timing of the observed precursors to place weak constraints...

Tsang, D.; Read, J. S.; Hinderer, T.; Piro, A. L.; Bondarescu, R.

2011-01-01

278

Evolution of the lunar highland crust  

Science.gov (United States)

The evolution of three distinct element associations in the lunar highland crust is discussed in terms of the Taylor-Jakes model which involves melting of most of the moon during accretion. Sources for (1) high Ca, Al, Sr, Eu, (2) high Mg and Cr, and (3) high K, REE, Zr, Hf, Nb are suggested. Bombardment by large projectiles during the differentiation process causes melting and mixing, which produces a wide range of compositions in the crust. The formation of dunite, troctolite, high-, medium-, and low-K Fra Mauro basalts, and rocks close to the olivine-spinel-plagioclase peritectic point is considered.

Taylor, S. R.; Bence, A. E.

1975-01-01

279

What governs the enrichment of Pb in the continental crust? An answer from the Mexican Volcanic Belt  

Science.gov (United States)

One of Al Hofmann’s many important contributions to our understanding of geochemical cycling in the Earth is the observation that Pb behaves like the light rare earth elements Ce and Nd during melting to form oceanic basalts, but is enriched in the continental crust compared to the LREE by nearly an order of magnitude (Hofmann et al. 1986). This is unusual behavior, and has been called one of the Pb paradoxes, since in most cases, the ratios of elements are effectively the same in the continental crust and oceanic basalts if they show similar mantle melting behavior. One of several mechanisms suggested to mediate this special enrichment is hydrothermal circulation at ocean ridges, which preferentially transports Pb compared to the REE from the interior of the ocean crust to the surface. We confirm the importance of hydrothermal processes at the East Pacific to mediate Pb enrichment at the Trans-Mexican Volcanic Belt (TMVB, through comparison of Pb isotope and Ce/Pb ratios of TMVB lavas with sediments from DSDP Site 487 near the Middle America trench. The lavas of the Trans-Mexican Volcanic Belt include “high Nb” alkali basalts (HNAB), whose trace element patterns lack subduction signatures. The HNAB basalts and hydrothermally affected sediments from DSDP 487, form end-members that bound calcalkaline lavas from volcanoes Colima, Toluca, Popocatépetl, and Malinche in Ce/Pb versus Pb isotope space. The HNAB represent the high Ce/Pb and high Pb-isotope end-member. The hydrothermal sediments have Pb isotopes like Pacific MORB but Ce/Pb ratios typical of the arcs and the continental crust, and an order of magnitude lower than MORB. No analyzed calcalkaline lavas are have compositions outside of the bounds formed by the HNAB and the hydrothermal sediments. The Ce/Pb and Pb isotope ratios show that the calcalkaline lava compositions are inconsistent with contributions from HNAB and EPR MORB, rather the contributions are from HNAB upper mantle and subducted hydrothermal sediments. The Trans-Mexican Volcanic Belt data confirm the two-step process of Pb enrichment in the arc lavas (and more generally in the continental crust). In the first step, hydrothermal processes at the East Pacific Rise preferentially transport Pb from the basaltic oceanic crust to surface sediments. In the second step, during subduction, these sediments are the main source of asthenospheric mantle-derived Pb to the lavas. Our data also confirm the importance of subduction contributions to the Quaternary Mexican arc, despite the >40 km thick continental crust. Ref: Hofmann et al. (1986) EPSL 79 p. 33-45.

Goldstein, S. L.; Lagatta, A.; Langmuir, C. H.; Straub, S. M.; Martin-Del-Pozzo, A.

2009-12-01

280

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)

281

Folding and the Displacements and Emplacements of Partial Melts in Late Archean Crust  

Science.gov (United States)

The discussion on the links between between rock deformation and the displacements of partial melts in the crust is strongly focused on shear zones. In the Late Archean crust of SE Superior Province, Canadian Shield, field observations show that displacements of partial melts should be viewed in a larger context of folding, within which shear zones formed mainly on the limbs of folds. In the study area, the Late Archean crust underwent periodic, or sustained, partial melting and magmatic intrusion, during a period of about 30 Ma, ca. 2723 Ma through ca. 2690 Ma. As a result, several generations of tonalite and granodiorite are recognized. I document field evidence that displacements and accumulations of tonalite and granodiorite partial melts, in the lower and middle crust, were controlled by stresses asscociated with folding. Veins of frozen partial melt are aligned parallel to axial surfaces of folds and parallel to folded surfaces of lithological contacts. Dikes of the same materials were emplaced on fold limbs, where strain and fabric development were intense. I insist on the critical point that the shear zones are sheared fold limbs; their formation and spatial distribution are controlled by the scales of folding. The field evidence leads to the conclusion that displacements of partial melts were strongly controlled by the stresses associated with heterogeneous deformation of the crust, namely folding. The heterogeneous nature of folding deformation may, in fact, be a requirement for efficient partial-melt transfer. It is the heterogeneous nature of the stress field, and of the resulting strain, that drives the melts from smaller veins, generally distributed in the folding rock mass, and oriented parallel to axial surfaces and to lithological surfaces, into the sheared fold limbs. The sheared limbs are upright, allowing for efficient vertical partial-melt migration to sites of magma emplacement. The observations are made at the outcrop-scale. I also present structural profiles through a region of the Late Archean Abitibi Subprovince that are consistent with larger-scale partial-melt displacements, and pluton emplacements, controlled by regional folding.

Benn, K.

2006-12-01

282

Fusion of neutron rich oxygen isotopes in the crust of accreting neutron stars  

CERN Document Server

Fusion reactions in the crust of an accreting neutron star are an important source of heat, and the depth at which these reactions occur is important for determining the temperature profile of the star. Fusion reactions depend strongly on the nuclear charge $Z$. Nuclei with $Z\\le 6$ can fuse at low densities in a liquid ocean. However, nuclei with $Z=8$ or 10 may not burn until higher densities where the crust is solid and electron capture has made the nuclei neutron rich. We calculate the $S$ factor for fusion reactions of neutron rich nuclei including $^{24}$O + $^{24}$O and $^{28}$Ne + $^{28}$Ne. We use a simple barrier penetration model. The $S$ factor could be further enhanced by dynamical effects involving the neutron rich skin. This possible enhancement in $S$ should be studied in the laboratory with neutron rich radioactive beams. We model the structure of the crust with molecular dynamics simulations. We find that the crust of accreting neutron stars may contain micro-crystals or regions of phase sep...

Horowitz, C J; Berry, D K

2007-01-01

283

Fusion of neutron-rich oxygen isotopes in the crust of accreting neutron stars  

International Nuclear Information System (INIS)

Fusion reactions in the crust of an accreting neutron star are an important source of heat, and the depth at which these reactions occur is important for determining the temperature profile of the star. Fusion reactions depend strongly on the nuclear charge Z. Nuclei with Z?6 can fuse at low densities in a liquid ocean. However, nuclei with Z=8 or 10 may not burn until higher densities where the crust is solid and electron capture has made the nuclei neutron rich. We calculate the S factor for fusion reactions of neutron rich nuclei including 24O+24O and 28Ne+28Ne. We use a simple barrier penetration model. The S factor could be further enhanced by dynamical effects involving the neutron rich skin. This possible enhancement in S should be studied in the laboratory with neutron rich radioactive beams. We model the structure of the crust with molecular dynamics simulations. We find that the crust of accreting neutron stars may contain micro-crystals or regions of phase separation. Nevertheless, the screening factors that we determine for the enhancement of the rate of thermonuclear reactions are insensitive to these features. Finally, we calculate the rate of thermonuclear 24O+24O fusion and find that 24O should burn at densities near 1011 g/cm3. The energy released from this and similar reactions may be important for the temperature profile of the starre profile of the star

284

Crust and Upper Mantle of North Africa Using Libyan Seismic Data  

Science.gov (United States)

We investigate the crust and upper mantle structure of North Africa using Libyan seismic data. Libya sits at the transition between the relatively aseismic continental crust of the African plate and the seismically active oceanic crust under the Mediterranean Sea which is subducting under the Eurasian Plate along the Calabrian, Hellenic, and Cyprean Arcs. The country also encompasses the Sirte Basin to the north and the smaller Murzuk and Kufra basins in the south. Broadband data from several seismic stations in Libya provide an opportunity for studying the velocity structure of the region. We have made some preliminary dispersion measurements from these stations and have found notable improvements in the group velocity tomography model by incorporating the additional measurements. We will be adding to this analysis by making dispersion measurements from regional events and receiver functions for teleseismic events. Recently, we have been employing methods to jointly invert both surface wave dispersion data and teleseismic receiver functions. The technique holds great promise in accurately estimating seismic structure, including important tectonic parameters such as basin thickness, crustal thickness, upper mantle velocity, as well as more detail about the upper mantle (lithospheric thickness and presence of anisotropy). We propose to apply this method to data from several Libyan stations where we can and, in the absence of receiver functions, invert the dispersion data only. The technique holds the promise of improving our understanding of the crust and upper mantle in Libya and how it fits into the larger tectonic picture of North Africa.

Pasyanos, M. E.; Eshwehdi, A.

2005-12-01

285

Reconciling evidence for Tethyan intra-oceanic subduction and a two-stage collision between India and Eurasia  

Science.gov (United States)

We present a plate tectonic model for the India-Eurasia collision that includes a time-dependent network of evolving plate boundaries with synthetic plates constructed for now-subducted Tethyan ocean floor, including back-arc basins that formed on the southern Eurasian margin. Southern Eurasia and Southeast Asia are riddled with dismembered oceanic arcs indicating long-lived intra-oceanic subduction. This intra-oceanic subduction may have extended further west into the India-Eurasia convergence zone in the NeoTethys, which was consumed during Greater India's northward trajectory towards Eurasia from the Early Cretaceous. Fragments of obducted oceanic crust within the Himalayan Yarlung-Tsangpo Suture Zone, between India and Eurasia, cluster around two age groups, the Late Jurassic and mid Cretaceous (Barremian-Aptian). The adakitic, boninitic and MORB-affinities of the various ophiolites along strike suggest that there was at least one generation of intra-oceanic subduction, whose plate boundary configuration remains uncertain, though it is best preserved in the Kohistan-Ladakh Arc. Paleomagnetic and magmatic characterisation studies from the ophiolites suggest that the intra-oceanic arc was as far south as the equator during the Early Cretaceous before subduction resumed further north beneath the southern Eurasian margin (Lhasa terrane) to consume the back-arc basin. During ~80-65 Ma, a hiatus in subduction-related magmatism along the southern Lhasa terrane may indicate the approach of the back-arc spreading centre towards the active Andean-style margin. We incorporate these observations into a regional, self-consistent plate tectonic model for the dispersal of East Gondwana, simultaneously considering geophysical data and seafloor spreading histories from abyssal plains offshore West Australia and East Antarctica, including Jurassic seafloor age data from offshore NW Australia that limits northern Greater India to a maximum of ~1000 km. This Greater India collided with the Tethyan intra-oceanic arc, including the Kohistan and Ladakh arcs, from the Mid Paleocene. Greater India's leading edge, bearing the intra-oceanic arc, finally closed the Tethyan seaway with progressive suturing to Eurasia from the Mid-Late Eocene, which coincides with the age of the youngest marine deposits found between India and Eurasia. Our model of mid-ocean ridge and subduction zone geometries, locations and divergence/convergence vectors through time can be represented as a time-dependent plate velocity mesh and is testable by combining coupled plate-mantle simulations with mantle seismic tomography. The model also provides a basis for future modifications in order to assimilate new data and test alternative tectonic scenarios.

Gibbons, Ana D.; Zahirovic, Sabin; Dietmar Müller, R.; Whittaker, Joanne M.; Yatheesh, Vadakkeyakath

2014-05-01

286

Central Andean Giant Ore Deposits: Links to Forearc Subduction Erosion, Shallowing Subduction and Thickening Crust  

Science.gov (United States)

An outstanding question on the Central Andean margin is the relationship between tectonic processes like ebbing arc volcanism, shallowing of the subducting slab and crustal thickening, and the origin of giant porphyry and epithermal Cu, Au and Ag deposits. Another potentially important factor in forming these major mineral deposits is forearc subduction erosion, which is postulated to have removed up to ~250 km of Central Andean forearc crust since the Jurassic. Geochemical and geophysical studies provide insights into possible links. Evidence for partial melts of removed and subducted forearc crust reaching the arc magma source and thus the magmas that host the ore deposits comes from the chemistry of late Neogene volcanic rocks on both the northern and southern margin of the Chilean-Pampean flat-slab (28°-33°S), where the frontal arc was displaced ~50 km into the foreland between ~10 and 3 Ma. This chemical evidence consists of transient ultra-steep REE patterns, elevated Mg, Cr and Ni contents and steps in isotopic ratios that are particularly notable in the glassy adakitic 8-3 Ma (Pircas Negras) andesites on the northern flat-slab margin at 27°-28°S. Well constrained reconstructions of the margin near 26-28°S that assume a sustained 300 km wide arc-trench gap and ~50 km of forearc removal suggest an accelerated average forearc subduction erosion rate over 150 km3/my/km between 8 and 3 Ma. Noting that the late Miocene arc is now at least ~ 260 km from the trench from 26°S to 34°S and that the active arc extrapolates through the amagmatic flat-slab region (28°-33°S) at 300 km from the trench, accelerated forearc removal could be inferred from ~34°S to 26°S at ~10 to 3 Ma. Geophysical evidence for forearc crust entering the mantle wedge as the flatslab shallowed could come from low Vp/Vs seismic ratios in the mantle wedge under the flatslab, which Wagner et al. (2010) attribute to orthopyroxene. Formation of this orthopyroxene could be explained by forearc crust reacting with the mantle wedge. Thus, the slab shallowing, crustal thickening and forearc subduction erosion in the flatslab region, which began at ca 20-18 Ma and accelerated after 11-10 Ma could have set the stage for the formation of the Los Pelambres, Rio Blanco and El Teniente giant Cu porphyries between ~ 11-4 Ma. The backarc 8-6 Ma Bajo de la Alumbrera Cu-Au district near 27°S, also formed east of the migrating volcanic arc on the northern flatslab margin at this time. This deposit is notable for now being above a high Qp mantle seismic anomaly overlying the slab, which is at a depth of ~150 km. Elsewhere, Ag-Zn mineralization in the ~14-12 Ma Potosi district near 19.5°S in the Altiplano backarc, which has been suggested to have occurred in the early stages of steepening of a shallow slab, would potentially predate flushing of eroded forearc material from an expanding mantle wedge. In the same vein, a lack of known big Cu-Au-Ag deposits associated with the late Neogene giant plateau ignimbrite complexes, considered to be fomed over steepening subduction zones characterized by low Vp and Vs and high Qp tomographic seismic anomalies, could also partially reflect loss of forearc subducted components from an expanding wedge.

Kay, S. M.; Mpodozis, C.

2013-05-01

287

Anomalous Subsidence at the Ocean Continent Transition of the Gulf of Aden Rifted Continental Margin  

Science.gov (United States)

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.

Cowie, Leanne; Kusznir, Nick; Leroy, Sylvie

2013-04-01

288

Exotic crust formation on Mercury: Consequences of a shallow, FeO-poor mantle  

Science.gov (United States)

The range in density and compressibility of Mercurian melt compositions was determined to better understand the products of a possible Mercurian magma ocean and subsequent volcanism. Our experiments indicate that the only mineral to remain buoyant with respect to melts of the Mercurian mantle is graphite; consequently, it is the only candidate mineral to have composed a primary floatation crust during a global magma ocean. This exotic result is further supported by Mercury's volatile-rich nature and inexplicably darkened surface. Additionally, our experiments illustrate that partial melts of the Mercurian mantle that compose the secondary crust were buoyant over the entire mantle depth and could have come from as deep as the core-mantle boundary. Furthermore, Mercury could have erupted higher percentages of its partial melts compared to other terrestrial planets because magmas would not have stalled during ascent due to gravitational forces. These findings stem from the FeO-poor composition and shallow depth of Mercury's mantle, which has resulted in both low-melt density and a very limited range in melt density responsible for Mercury's primary and secondary crusts. The enigmatically darkened, yet low-FeO surface, which is observed today, can be explained by secondary volcanism and impact processes that have since mixed the primary and secondary crustal materials.

Vander Kaaden, Kathleen E.; McCubbin, Francis M.

2015-02-01

289

A relatively reduced Hadean continental crust  

Science.gov (United States)

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.

Yang, Xiaozhi; Gaillard, Fabrice; Scaillet, Bruno

2014-05-01

290

“Imaging” the cross section of oceanic lithosphere: The development and future of electrical microresistivity logging through scientific ocean drilling  

Science.gov (United States)

A detailed understanding of the architecture of volcanic and magmatic lithologies present within the oceanic lithosphere is essential to advance our knowledge of the geodynamics of spreading ridges and subduction zones. Undertaking sub-meter scale observations of oceanic lithosphere is challenging, primarily because of the difficulty in direct continuous sampling (e.g., by scientific ocean drilling) and the limited resolution of the majority of geophysical remote sensing methods. Downhole logging data from drillholes through basement formations, when integrated with recovered core and geophysical remote sensing data, can provide new insights into crustal accretion processes, lithosphere hydrogeology and associated alteration processes, and variations in the physical properties of the oceanic lithosphere over time. Here, we introduce an alternative approach to determine the formation architecture and lithofacies of the oceanic sub-basement by using logging data, particularly utilizing downhole microresistivity imagery (e.g. Formation MicroScanner (FMS) imagery), which has the potential to become a key tool in deciphering the high-resolution internal architecture of the intact upper ocean crust. A novel ocean crust lithostratigraphy model based on meticulously deciphered lava morphology determined by in situ FMS electrofacies analysis of holes drilled during Ocean Drilling Program legs (1) advances our understanding of ocean crust formation and accretionary processes over both time and space; and (2) allows the linking of local igneous histories deciphered from the drillholes to the regional magmatic and tectonic histories. Furthermore, microresistivity imagery can potentially allow the investigation of (i) magmatic lithology and architecture in the lower ocean crust and upper mantle; and, (ii) void space abundances in crustal material and the determination of complex lithology-dependent void geometries.

Tominaga, Masako

2013-11-01

291

From a collage of microplates to stable continental crust - an example from Precambrian Europe  

Science.gov (United States)

Svecofennian orogen (2.0-1.7 Ga) comprises the oldest undispersed orogenic belt on Baltica and Eurasian plate. Svecofennian orogenic belt evolved from a series of short-lived terrane accretions around Baltica's Archean nucleus during the formation of the Precambrian Nuna supercontinent. Geological and geophysical datasets indicate W-SW growth of Baltica with NE-ward dipping subduction zones. The data suggest a long-lived retreating subduction system in the southwestern parts whereas in the northern and central parts the northeasterly transport of continental fragments or microplates towards the continental nucleus is also documented. The geotectonic environment resembles that of the early stages of the Alpine-Himalayan or Indonesian orogenic system, in which dispersed continental fragments, arcs and microplates have been attached to the Eurasian plate margin. Thus the Svecofennian orogeny can be viewed as proxy for the initial stages of an internal orogenic system. Svecofennian orogeny is a Paleoproterozoic analogue of an evolved orogenic system where terrane accretion is followed by lateral spreading or collapse induced by change in the plate architecture. The exposed parts are composed of granitoid intrusions as well as highly deformed supracrustal units. Supracrustal rocks have been metamorphosed in LP-HT conditions in either paleo-lower-upper crust or paleo-upper-middle crust. Large scale seismic reflection profiles (BABEL and FIRE) across Baltica image the crust as a collage of terranes suggesting that the bedrock has been formed and thickened in sequential accretions. The profiles also image three fold layering of the thickened crust (>55 km) to transect old terrane boundaries, suggesting that the over-thickened bedrock structures have been rearranged in post-collisional spreading and/or collapse processes. The middle crust displays typical large scale flow structures: herringbone and anticlinal ramps, rooted onto large scale listric surfaces also suggestive of spreading. Close to the original ocean-continent plate boundary, in the core of the Svecofennian orogen, the thickened accretionary crust carries pervasive stretching lineations at surface and seismic vp-velocity anisotropy in the crust. The direction of spreading and crustal flow seems to be diverted by shapes of the pre-existing boundaries. It is concluded that lateral spreading and midcrustal flow not only rearrange the bedrock architecture but also stabilize the young accreted continental crust in emerging internal orogenic systems. Pre-existing microplate/terrane boundaries will affect the final architecture of the orogenic belt.

Korja, Annakaisa

2013-04-01

292

Resonant shattering of neutron star crusts.  

Science.gov (United States)

The resonant excitation of neutron star (NS) modes by tides is investigated as a source of short gamma-ray burst (SGRB) precursors. We find that the driving of a crust-core interface mode can lead to shattering of the NS crust, liberating ?10{46}-10{47}??erg of energy seconds before the merger of a NS-NS or NS-black-hole binary. Such properties are consistent with Swift/BAT detections of SGRB precursors, and we use the timing of the observed precursors to place weak constraints on the crust equation of state. We describe how a larger sample of precursor detections could be used alongside coincident gravitational wave detections of the inspiral by Advanced LIGO class detectors to probe the NS structure. These two types of observations nicely complement one another, since the former constrains the equation of state and structure near the crust-core boundary, while the latter is more sensitive to the core equation of state. PMID:22304251

Tsang, David; Read, Jocelyn S; Hinderer, Tanja; Piro, Anthony L; Bondarescu, Ruxandra

2012-01-01

293

Collective excitations in neutron-star crusts  

OpenAIRE

We explore the spectrum of low-energy collective excitations in the crust of a neutron star, especially in the inner region where neutron-proton clusters are immersed in a sea of superfluid neutrons. The speeds of the different modes are calculated systematically from the nuclear energy density functional theory using a Skyrme functional fitted to essentially all experimental atomic mass data.

Chamel, N.; Page, D.; Reddy, S.

2013-01-01

294

Superfluid dynamics in neutron star crusts  

OpenAIRE

A simple description of superfluid hydrodynamics in the inner crust of a neutron star is given. Particular attention is paid to the effect of the lattice of nuclei on the properties of the superfluid neutrons, and the effects of entrainment, the fact that some fraction of the neutrons are locked to the motion of the protons in nuclei.

Pethick, C. J.; Chamel, Nicolas; Reddy, S.

2010-01-01

295

A new crystalline phase in magnetar crusts  

Science.gov (United States)

We show that ions at the low densities and high magnetic fields relevant to the outer crust of magnetars form a novel crystalline phase made of strongly coupled filaments of ions along the magnetic field. This is due to a long-range oscillatory behavior in the ion-ion potential along the magnetic field that is caused by anisotropic electronic screening of ions in this direction (found by Sharma & Reddy 2011). We show that this long-range oscillatory term in the ion-ion potential (Friedel oscillations), that has been neglected in previous studies of neutron star structure at high magnetic fields, has a strong effect on the structure and elastic properties of the crystal lattice in the outer crust. In fact, we find that due to this long-range force the elastic constants (the bulk and shear moduli) that are dominated by the longitudinal structure of the lattice are significantly larger than that of a usual bcc Coulomb crystal at comparable densities. This has potentially interesting implications for the Quasi-periodic Oscillations (QPOs) seen in the X-ray flux of magnetars during their giant flares. These QPOs have been linked to global torsional vibrations within the star's crust, the frequencies of which are a function of the shear modulus of the crust.

Mahmoodifar, Simin; Bedaque, P. F.; Sen, S.

2014-01-01

296

Neutron Star Crust in Strong Magnetic Fields  

International Nuclear Information System (INIS)

We discuss the effects of strong magnetic fields through Landau quantization of electrons on the structure and stability of nuclei in neutron star crust. In strong magnetic fields, this leads to the enhancement of the electron number density with respect to the zero field case. We obtain the sequence of equilibrium nuclei of the outer crust in the presence of strong magnetic fields adopting most recent versions of the experimental and theoretical nuclear mass tables. For B ? 1016G, it is found that some new nuclei appear in the sequence and some nuclei disappear from the sequence compared with the zero field case. Further we investigate the stability of nuclei in the inner crust in the presence of strong magnetic fields using the Thomas-Fermi model. The coexistence of two phases of nuclear matter - liquid and gas, is considered in this case. The proton number density is significantly enhanced in strong magnetic fields B ? 1017G through the charge neutrality. We find nuclei with larger mass number in the presence of strong magnetic fields than those of the zero field. These results might have important implications for the transport properties of the crust in magnetars.

297

Crust and subduction zone structure of Southwestern Mexico  

Science.gov (United States)

Southwestern Mexico is a region of complex active tectonics with subduction of the young Rivera and Cocos plates to the south and widespread magmatism and rifting in the continental interior. Here we use receiver function analysis on data recorded by a 50 station temporary deployment of seismometers known as the MARS (MApping the Rivera Subduction zone) array to investigate crustal structure as well as the nature of the subduction interface near the coast. The array was deployed in the Mexican states of Jalisco, Colima, and Michoacan. Crustal thickness varies from 20 km near the coast to 42 km in the continental interior. The Rivera plate has steeper dip than the Cocos plate and is also deeper along the coast than previous estimates have shown. Inland, there is not a correlation between the thickness of the crust and topography indicating that the high topography in northern Jalisco and Michoacan is likely supported by buoyant mantle. High crustal Vp/Vs ratios (greater than 1.82) are found beneath the trenchward edge of magmatism including below the Central Jalisco Volcanic Lineament and the Michoacan-Guanajuato Volcanic Field implying a new arc is forming closer to the trench than the Trans Mexican Volcanic Belt. Elsewhere in the region, crustal Vp/Vs ratios are normal. The subducting Rivera and Cocos plates are marked by a dipping shear wave low-velocity layer. We estimate the thickness of the low-velocity layer to be 3 to 4 km with an unusually high Vp/Vs ratio of 2.0 to 2.1 and a drop in S velocity of 25%. We postulate that the low-velocity zone is the upper oceanic crust with high pore pressures. The low-velocity zone ends from 45 to 50 km depth and likely marks the basalt to eclogite transition.

Suhardja, Sandy Kurniawan; Grand, Stephen P.; Wilson, David; Guzman-Speziale, Marco; Gomez-Gonzalez, Juan Martin; Dominguez-Reyes, Tonatiuh; Ni, James

2015-02-01

298

Pulsar glitches: The crust may be enough  

Science.gov (United States)

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.

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

2014-07-01

299

Crust-mantle contribution to Andean magmatism  

International Nuclear Information System (INIS)

There has long been great interest in quantifying the contributions of the continental crust to continental arc magmas, such as those of the Andes using osmium isotopes (Alves et al., 1999; Borg et al., 2000; Brandon et al., 1996; McInnes et al., 1999). In general, Andean volcanic rocks of all compositions show relatively low Sr-isotope ratios and positive to mildly negative epsilon Nd values. Nonetheless, in the Southern Volcanic Zone of central Chile, basalt-andesite-dacite volcanoes along the Quaternary volcanic front were shown (by Hildreth and Moorbath, 1988) to have latitudinally systematic chemical variations, as well as a monotonic increase in 87Sr/Sr86 from ca. 0.7035 to 0.7055 and a decrease in epsilon Nd values from ca. +3 to -1. The isotopic variations correlate with basement elevation of the volcanic edifices and with Bouguer gravity anomalies, both of which are thought to reflect along-arc variations in thickness and average age of the underlying crust. Volcanoes with the most evolved isotopic signatures were fed through the thickest crust. Correlation of chemical and isotopic variations with crustal thickness was interpreted to be caused by Melting (of deep-crustal host rocks), Assimilation, Storage, and Homogenization (MASH) of mantle-derived magmas in long-lived lower-crustal reservoirs beneath each center prior to eruption. We have now determined Os-isotope ratios for a sample suite from these volcanoes (33-36 S lat.), representing a range of crustal6 S lat.), representing a range of crustal thickness from ca. 60-35 km. The samples range in MgO from ca. 8-4% and in SiO2 from 51-57%. The most evolved eruptive products occur above the thickest crust and have 87Sr/86Sr ratios of 0.7054 and epsilon Nd values of -1.5. The 187Os/188Os ratios correlate with the other isotopic systems and with crustal thickness. Volcanoes on the thinnest crust have 187Os/188Os ratios of 0.18-0.21. Those on the thickest crust have 187Os/188Os ratios as high as 0.64. All the Os values are much too radiogenic to represent purely mantle-derived melts (nominally ca. 0.13). The Os-isotopic system mimics the variations recorded by the other isotopic systems but is significantly magnified, demonstrating its power for evaluating crustal contributions to arc-magma genesis. Mixing calculations suggest that the Os isotopic values of the Chilean samples represent mixing of mantle-derived magmas with 20% or more of material derived from mafic lower crust (au)

300

Stability of clathrate hydrates in Martian crust  

Science.gov (United States)

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.

Gloesener, Elodie; Karatekin, Özgür; Dehant, Véronique

2014-05-01

301

Discovery and utilization of sorghum genes (Ma5/Ma6)  

Science.gov (United States)

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.

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

2012-11-13

302

Fault reactivation in the central Indian Ocean and the rheology of oceanic lithosphere  

Science.gov (United States)

THE intraplate deformation in the central Indian Ocean basin is a well-known example of a deviation from an axiom of plate tectonics: that of rigid plates with deformation concentrated at plate boundaries. Here we present multichannel seismic reflection profiles which show that high-angle reverse faults in the sediments of the central Indian Ocean extend through the crust and possibly into the uppermost mantle. The dip of these faults, which we believe result from the reactivation of pre-existing faults formed at the spreading centre, is ~40° in the basement, which is consistent with the distribution and focal mechanisms of earthquakes on faults now forming at spreading centres. This style of deformation, coupled with the observation of large earthquakes in the mantle lithosphere, indicates that brittle failure of the oceanic lithosphere may nucleate in the vicinity of the brittle/ductile transition and propagate through the crust.

Bull, Jonathan M.; Scrutton, Roger A.

1990-04-01

303

CRUST1.0: An Updated Global Model of Earth's Crust  

Science.gov (United States)

We present an updated global model of Earth's crustal structure. The new model, CRUST1.0, serves as starting model in a more comprehensive effort to compile a global model of Earth's crust and lithosphere, LITHO1.0. CRUST1.0 is defined on a 1-degree grid and is based on a new database of crustal thickness data from active source seismic studies as well as from receiver function studies. In areas where such constraints are still missing, for example in Antarctica, crustal thicknesses are estimated using gravity constraints. The compilation of the new crustal model initially follows the philosophy of the widely used crustal model CRUST2.0 (Bassin et al., 2000; http://igppweb.ucsd.edu/~gabi/crust2.html). Crustal types representing properties in the crystalline crust are assigned according to basement age or tectonic setting. The classification of the latter loosely follows that of an updated map by Artemieva and Mooney (2001) (http://www.lithosphere.info). Statistical averages of crustal properties in each of these crustal types are extrapolated to areas with no local seismic or gravity constraint. In each 1-degree cell, boundary depth, compressional and shear velocity as well as density is given for 8 layers: water, ice, 3-layer sediment cover and upper, middle and lower crystalline crust. Topography, bathymetry and ice cover are taken from ETOPO1. The sediment cover is essentially that of our sediment model (Laske and Masters, 1997; http://igppweb.ucsd.edu/~sediment.html), with several near-coastal updates. In the sediment cover and the crystalline crust, updated scaling relationships are used to assign compressional and shear velocity as well as density. In an initial step toward LITHO1.0, the model is then validated against our new global group velocity maps for Rayleigh and Love waves, particularly at frequencies between 30 and 40 mHz. CRUST1.0 is then adjusted in areas of extreme misfit where we suspect deficiencies in the crustal model. These currently include some near-coastal areas with thick sediment cover and several larger orogenic belts. Some remaining discrepancies, such as in backarc basins, may result from variations in the deeper uppermost mantle and remain unchanged in CRUST1.0 but will likely be modified in LITHO1.0. CRUST1.0 is available for download.

Laske, G.; Masters, G.; Ma, Z.; Pasyanos, M. E.

2012-04-01

304

Thin crust as evidence for depleted mantle supporting the Marion Rise.  

Science.gov (United States)

The global ridge system is dominated by oceanic rises reflecting large variations in axial depth associated with mantle hotspots. The little-studied Marion Rise is as large as the Icelandic Rise, considering both length and depth, but has an axial rift (rather than a high) nearly its entire length. Uniquely along the Southwest Indian Ridge systematic sampling allows direct examination of crustal architecture over its full length. Here we show that, unlike the Icelandic Rise, peridotites are extensively exposed high on the rise, revealing that the crust is generally thin, and often missing, over a rifted rise. Therefore the Marion Rise must be largely an isostatic response to ancient melting events that created low-density depleted mantle beneath the Southwest Indian Ridge rather than thickened crust or a large thermal anomaly. The origin of this depleted mantle is probably the mantle emplaced into the African asthenosphere during the Karoo and Madagascar flood basalt events. PMID:23389441

Zhou, Huaiyang; Dick, Henry J B

2013-02-14

305

Compositional Freeze-Out of Neutron Star Crusts  

OpenAIRE

We have investigated the crustal properties of neutron stars without fallback accretion. We have calculated the chemical evolution of the neutron star crust in three different cases (a modified Urca process without the thermal influence of a crust, a thick crust, and a direct Urca process with a thin crust) in order to determine the detailed composition of the envelope and atmosphere as the nuclear reactions freeze out. Using a nuclear reaction network up to technetium, we c...

Hoffman, Kelsey; Heyl, Jeremy

2009-01-01

306

Ocean Bioturbation  

Science.gov (United States)

Ocean mixing is thought to control the climatically important oceanic overturning circulation. Here we argue the marine biosphere, by a mechanism like the bioturbation occurring in marine sediments, mixes the oceans as effectively as the winds and tides. Over-fishing is thus suggested as a mechanism of global climate change.

Dewar, W. K.; Bingham, R. J.; Iverson, R. L.; Nowacek, D. P.; St. Laurent, L. C.; Wiebe, P. H.

2004-12-01

307

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)

308

MoMA: SOUNDINGS  

Science.gov (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.

2013-01-01

309

Dynamics of the Precambrian Continental Crust  

Science.gov (United States)

The Precambrian continental crust is mainly composed of (1) granite greenstone belts (GGB) and (2) granulite facies complexes (GFC). The GFC are often separated from GGB by inward dipping crustal scale shear zones with characteristic sense of movements reflecting thrusting of GFC onto cratonic rocks. The isotope age of the shear zones is identical to GFC, while the latter are always younger than the granite greenstone belts. The dynamics relationships between these two geological units strongly determine tectonic evolution of the Precambrian continental crust. Numerous thermobarometric studies of magmatic and metamorphic rocks show that the Archaean to Early Protorozoic crust as well as the Mantle were hot and therefore relatively soft. Such geothermal regimes may limit separation and movement of micro continents, limiting collisional mechanisms in evolution of the Precambrian crust. The goal of this paper is to show evidence for an alterative model that is based on the mechanism of gravitational redistribution of rocks within the Precambrian continental crust, which might be initiated by a fluid/heat flow related to mantle plumes. The model is tested on the basis of geological, geochemical, geophysical and petrologic data for many paired GFT GGB complexes around the word. Studied granulite complexes are located in between Archaean GGB from which they are separated by inward dipping crustal scale shear zones with reverse sense of movements. The most important evidence for this mechanism is: (i) the near isobaric cooling (IC) and (ii) decompression cooling (DC) shapes of the retrograde P T paths recorded in GFC, while rocks from the juxtaposed GGB in footwalls of the bounding shear zones record P T loops. The Pmax of the loops corresponds to the Pmin, recorded in GFC. Thus the GGB P T loop reflects the burial and ascending of the juxtaposed GGB while the GFC P T path records the exhumation only. The identical isotopic age of GFC and contacting rocks from the shear zone strongly indicates simultaneous dynamics of both contacting complexes: while GFC move up to the surface, relatively cool the GGB metabasalts and metakomatiites therefore move down, cooling the granulites next to the bounding shear zone. The existence of both DC and IC paths in the same GGB can thus be explained by differences in the movement of different crustal 3blocks* during their ascent. The results of our numerical modelling suggest that GFC may develop as crustal scale intrusive shaped bodies in a period of about 8.5 Myr. Seismic tomography shows that the mantle underneath the Limpopo GFC exhibits a cratonic signature, indicating intracratonic origin. Thus, the studied GFC were initially formed from GGB and then exhumed within relatively thin (30 35 km thick) continental crust.

Perchuk, L. L.; Gerya, T. V.; van Reenen, D. D.; Smit, C. A.

2003-04-01

310

Kinematic model for decoupling orogenic crust from upper mantle during a switch in subduction polarity at the junction of the Alps, Carpathians and Dinarides  

Science.gov (United States)

We depict the Alpine belt in a series of tectonic maps for critical time slices (10, 20, 35, 67, 84 Ma) to show how its evolution can be related to the geometry of subducting lithosphere as imaged by seismic tomography. These maps are constructed by deriving shortening and extensional values from cross sections across key parts of the chain in a stepwise fashion (external to internal parts), then applying these values as successive retrotranslations of points on stable parts of the Adriatic microplate. All motions thus obtained for Adria are with respect to a European reference frame. This approach works best in the Alps due to an abundance of geologic data, but less so in parts of the Carpathians and Dinarides where age and structural information are still limited. To test the kinematic plausibility of a N- to NE-dipping slab fragment beneath the Eastern Alps (Lippitsch et al. 2003), we first horizontalized the presently inclined positive Vp-anomaly, then applied the above sequence of retrotranslations to this image (taken to be the leading edge of the subducted Adriatic margin) to obtain the putative trace of the Adriatic margin at the time slices above. The following features characterize our model: (1) From 67-35 Ma, a N-S trending Alps-Dinarides Transfer (ADT) fault delimited the eastern end of the Alps, linking 465 km of SSE-directed "European" subduction of Alpine Tethys with a still unconstrained amount of oblique dextral convergence and N- to NE-directed "Dinaric" subduction of the Adriatic margin of Neotethys. The trace of the ADT in map view coincides with the reconstructed eastern edge of the future Adriatic slab fragment, suggesting that the ADT may have nucleated along an inherited structure (e.g., transform fault) of the Adriatic margin; (2) Beginning at c. 35 Ma, collision in the Alps coincided with an increased in the obliquity of dextral convergence in the Dinarides and with breakoff of the European slab beneath the Alps. Separation of the Adriatic slab fragment from the rest of the Adriatic slab beneath the Dinarides may have begun already in late Paleogene time; (3) Counter-clockwise rotation (20°) and oblique NE-ward subduction of the Adriatic slab fragment beneath the Alpine orogenic wedge began at c. 20 Ma as constrained by the onset of indentation, rapid exhumation and lateral escape in the Tauern Window and Eastern Alps, and shortening in the Southern Alps. We speculate that vertical tearing of this subducting lithospheric fragment led to asthenospheric upwelling and thermal erosion of the Adriatic slab beneath the northern Dinarides. This favoured wholesale decoupling of the crust and mantle in Miocene time, thereby facilitating lateral escape of orogenic crust in the Eastern Alps, rollback subduction of the Carpathian oceanic embayment and associated extension of the Pannonian Basin.

Handy, M. R.; Ustaszewski, K.; Kissling, E.; Spakman, W.; Rosenberg, C. L.

2012-04-01

311

Critical Metals In Western Arctic Ocean Ferromanganese Mineral Deposits  

Science.gov (United States)

Little exploration for minerals has occurred in the Arctic Ocean due to ice cover and the remote location. Small deposits of seafloor massive sulfides that are rich in copper and zinc occur on Gakkel Ridge, which extends from Greenland to the Laptev Sea, and on Kolbeinsey and Mohns ridges, both located between Greenland and mainland Europe. However, rocks were recently collected by dredge along the western margin of the Canada Basin as part of the U.S. Extended Continental Shelf (ECS) program north of Alaska. Sample sites include steep escarpments on the Chukchi Borderland, a newly discovered seamount informally named Healy seamount, the southern part of Alpha-Mendeleev Ridge, and several basement outcrops in Nautilus Basin. These dredge hauls yielded three types of metal-rich mineralized deposits: ferromanganese crusts, ferromanganese nodules, and hydrothermal iron and manganese deposits. Chemical analyses of 43 crust and nodule samples show high contents of many critical metals needed for high-technology, green-technology, and energy and military applications, including cobalt (to 0.3 wt.%), vanadium (to 0.12 wt.%), zirconium (to 459 grams/tonne=ppm), molybdenum (to 453 g/t), the rare-earth elements (including scandium and yttrium; yttrium to 229 g/t), lithium (to 205 g/t), tungsten (to 64 g/t), and gallium (to 26 g/t). The metal contents of these Arctic Ocean crusts and nodules are comparable to those found throughout the global ocean, however, these Arctic Ocean samples are the first that have been found to be enriched in rare metal scandium. The metal contents of these samples indicate a diagenetic component. Crusts typically form by precipitation of metal oxides solely from seawater (hydrogenetic) onto rock surfaces producing a pavement, whereas nodules form by accretion of metal oxides, from both seawater and pore waters (diagenetic), around a nucleus on the surface of soft sediment. The best evidence for this diagenetic input to the crusts is that crusts typically have low lithium contents, 1-10 g/t while diagenetic nodules can have contents up to 600 g/t; the Arctic Ocean crusts have relatively high lithium contents of up to 205 g/t, indicating that these crusts may be only the second yet discovered to acquire some elements from sediment pore waters. A potential avenue for acquisition of diagenetic metals would be via release from pore waters into the bottom waters that bathe the crusts, or alternatively by partial burial of the crusts in mud. However, the overall composition of the crusts indicates predominantly a hydrogenetic origin. Hydrothermal iron hydroxide samples from the Arctic Ocean were dated using argon isotopes, which produced a Paleozoic age. This indicates that the Chukchi Platform in the SW Arctic Ocean is a piece of continental crust. This age also indicates that hydrothermal iron and manganese deposits are not temporally related to the Neogene ferromanganese crusts and nodules. Our preliminary results suggest that additional exploration in the Arctic Ocean for mineral deposits is warranted.

Hein, J. R.; Spinardi, F.; Conrad, T. A.; Conrad, J. E.; Genetti, J.

2013-12-01

312

Sensitivity of climate and atmospheric CO2 to deep-ocean and shallow-ocean carbonate burial  

Science.gov (United States)

A model of the carbonate-silicate geochemical cycle is presented that distinguishes carbonate masses produced by shallow-ocean and deep-ocean carbonate burial and shows that reasonable increases in deep-ocean burial could produce substantial warmings over a few hundred million years. The model includes exchanges between crust and mantle; transients from burial shifts are found to be sensitive to the fraction of nondegassed carbonates subducted into the mantle. Without the habitation of the open ocean by plankton such as foraminifera and coccolithophores, today's climate would be substantially colder.

Volk, Tyler

1989-01-01

313

Oceanic Plateau Overview and Look Ahead  

Science.gov (United States)

Oceanic plateaus result from fundamental processes in the Earth's interior, and have been implicated as instigators of major worldwide environmental changes. Although the plate tectonics paradigm successfully explains volcanic activity on the Earth's surface associated with seafloor spreading and plate subduction, it does not elucidate the massive flood volcanism that produces oceanic plateaus. Temporal correlations between flood basalts and environmental phenomena such as mass extinctions and oceanic anoxic events (OAEs) are well documented, yet the underlying mechanisms causing these global catastrophes are only beginning to be grasped. Focused investigations of oceanic plateaus have targeted the two largest features globally, the ~120 Ma Ontong Java Plateau (Pacific Ocean) and ~120-95 Ma Kerguelen Plateau/Broken Ridge (Indian Ocean), and the ~145-130 Ma Shatsky Rise (Pacific Ocean). These three features constitute the only oceanic plateaus where igneous basement has been drilled at more than one site. Multiple models - plume, bolide impact, and upwelling eclogite - have been proposed for Ontong Java's origin. The feature correlates temporally with OAE-1a, and interpretation of Sr, Os, and Pb isotopic systems during the time of OAE-1a points to a close linkage between the two, with CO2, Fe, and trace metal emissions from the massive magmatism potentially triggering the event. The Kerguelen Plateau/Broken Ridge is a composite feature that includes flood basalts, depleted mid-ocean ridge basalt (MORB)-related asthenosphere, and continental lithosphere. Models for the Shatsky Rise include mantle plume and fast seafloor spreading. Future studies of oceanic plateaus have the potential to transform our understanding of the Earth system through investigating: 1) magma (and hence mantle source) variability through times; 2) the nature of melting anomalies, i.e., compositional vs. thermal, that produce oceanic plateaus; 3) the precise durations of oceanic plateau events; 4) modes of eruption, i.e., constant effusion over one to several million years, or several discrete pulses over the same time interval; and 5) relationships among oceanic plateaus, OAEs, extinction events, and other major environmental changes (e.g., ocean acidification and fertilization). Increased knowledge of all will contribute to understanding and forecasting regional and global environmental changes during the Anthropocene. Advancing knowledge of oceanic plateaus and the Earth system requires integrated multi-disciplinary and cross-disciplinary approaches involving mantle geodynamics, plume modeling, petrology, geochemistry, environmental impacts, paleoceanography, micropaleontology, physical volcanology, geophysics, and tectonics. Oceanic plateaus must be studied in concert with oceanic (volcanic divergent margin) and continental counterparts to better understand emplacement mechanisms and environmental effects of their formation.

Coffin, M. F.

2011-12-01

314

Towards a metallurgy of neutron star crusts.  

Science.gov (United States)

In the standard picture of the crust of a neutron star, matter there is simple: a body-centered-cubic lattice of nuclei immersed in an essentially uniform electron gas. We show that, at densities above that for neutron drip (? 4 × 1 0(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 BaTiO3. As a consequence, the properties of matter in the inner crust are expected to be much richer than previously appreciated, and we mention possible consequences for observable neutron star properties. PMID:24702357

Kobyakov, D; Pethick, C J

2014-03-21

315

Towards a metallurgy of neutron star crusts  

CERN Document Server

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.

Kobyakov, D

2013-01-01

316

Nuclear heating in neutron star crusts  

International Nuclear Information System (INIS)

A fluid element deposited onto the surface of an accreting neutron star is compressed by continual accretion. Here we describe calculations of the nucleosynthetic evolution of an accreted element from deposition into the atmosphere down to neutron drip density. These calculations are coupled with a solution of the thermal structure in the neutron star crust. In particular, we include temperature-dependent continuum electron capture rates and realistic sources of heat loss by thermal neutrino emission from the crust and core. In contrast to previous calculations, electron captures to excited states and subsequent ?-emission significantly reduces the local heat loss due to weak-interaction neutrinos. These reactions release a large amount of heat at densities < 1011g cm-3, which can raise the local temperature and reduce the critical mass needed for unstable ignition of 12C

317

Coulomb forces in neutron star crusts  

OpenAIRE

We study the role the proton-electron gas interaction has on the formation of nuclear structures in neutron star crusts. Using a classical molecular dynamics model we study isospin symmetric and asymmetric matter at subsaturation densities and low temperatures varying the Coulomb interaction strength. The effect of such variation is quantified on the fragment size multiplicity, the inter-particle distance, the isospin content of the clusters, the nucleon mobility and cluster...

Dorso, C. O.; Molinelli, P. A. Gime?nez; Lo?pez, J. A.; Rami?rez-homs, E.

2012-01-01

318

Topological characterization of neutron star crusts  

OpenAIRE

Neutron star crusts are studied using a classical molecular dynamics model developed for heavy ion reactions. After the model is shown to produce a plethora of the so-called "pasta" shapes, a series of techniques borrowed from nuclear physics, condensed matter physics and topology are used to craft a method that can be used to characterize the shape of the pasta structures in an unequivocal way.

Dorso, C. O.; Molinelli, P. A. Gime?nez; Lo?pez, J. A.

2012-01-01

319

Excited nuclei in neutron star crusts  

OpenAIRE

The paper considers the chains of successive electron capture reactions by nuclei of the iron group which take place in the crystal structures of neutron star envelopes. It is shown that as a result of such reactions the daughter nuclei in excited states accumulate within certain layers of neutron star crusts. The phonon model of interactions is proposed between the excited nuclei in the crystalline structure, as well as formation of highly excited nuclear states which emit ...

Takibayev, N.; Kato, K.; Nasirova, D.

2012-01-01

320

Mesoscopic pinning forces in neutron star crusts  

OpenAIRE

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...

Seveso, Stefano; Pizzochero, Pierre M.; Grill, Fabrizio; Haskell, Brynmor

2014-01-01

321

Seismic Structure of Eastern Anatolia Crust  

International Nuclear Information System (INIS)

Regional crustal structure, which is mainly, affected by the collision of the Eurasian and the Arabian Plates beneath Eastern Anatolia plateau has been investigated using seismological data. P-wave first arrivals and P-S waveforms of the earthquakes recorded by ETSE (1999-2001) and KOERI (Kandilli) stations were simulated. The crust has an average depth of 38 - 42 km and low velocity zones due to the partially melting were modeled

322

Excited nuclei in neutron star crusts  

Directory of Open Access Journals (Sweden)

Full Text Available The paper considers the chains of successive electron capture reactionsby nuclei of the iron group which take place in the crystal structuresof neutron star envelopes. It is shown that as a result of suchreactions the daughter nuclei in excited states accumulate within certainlayers of neutron star crusts. The phonon model of interactions isproposed between the excited nuclei in the crystalline structure, as wellas formation of highly excited nuclear states which emit neutrons andhigher energy photons.

N. Takibayev

2013-01-01

323

Breaking stress of neutron star crust  

OpenAIRE

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...

Chugunov, A. I.; Horowitz, C. J.

2010-01-01

324

r-process in neutron star crust  

International Nuclear Information System (INIS)

Understanding the formation of heavy and superheavy nuclei in the universe is a challenging problem. It is predicted that the rapid neutron-capture process (r-process) could be responsible for the synthesis of heavy and superheavy elements in supernova explosions and neutron star crust under extreme physical conditions. However, there is much debate about the astrophysical sites, in which ideal r-process conditions are met. Supernova, could not reproduce the observed solar abundances of r-process elements and has been abandoned by recent studies. Neutron-rich ejecta of compact binary mergers are now believed to be a perfect candidate for an astrophysical r-process. In the events of merging of binary neutron star or a neutron star with a black hole, the crust matter can be dynamically stripped and ejected. Once the density decreases below neutron drip, ?-decay channels begin opening in full, and a conventional r-process begins leading to the formation of very heavy nuclei when the seed nuclei rapidly capture the free neutrons. Recently it has been shown that the decompression of the neutron star matter from the outer crust provides suitable condition for nucleosynthesis of r-nuclei with A ? 140. The decompression is triggered by a phase transition to strange quark matter at the core of a neutron star that ejects neutron-rich matter at the surface

325

Pyrolysis of waste plastic crusts of televisions.  

Science.gov (United States)

The disposal of waste plastic crusts of televisions is an issue that is gaining increasing interest around the world. In this investigation, the pyrolysis and catalytic cracking of the waste television crusts mainly composed of acrylonitrile--butadiene-styrene copolymer was studied. Thermogravimetric analysis was used for initial characterization of the pyrolysis of the waste plastic, but most of the investigations were carried out using a 600 mL tubing reactor. Effects of temperature, reaction time and catalyst on the pyrolysis of the waste television crusts were investigated. The results showed that the oil yield increased with increasing temperature or with prolongation of reaction time. With increasing temperature, the generating percentage of gasoline and diesel oil increased, but the heavy oil yield decreased. Zinc oxide, iron oxide and fluid catalytic cracking catalyst (FCC catalyst) were employed to perform a series of experiments. It was demonstrated that the liquid product was markedly improved and the reaction temperature decreased 100 degrees C when FCC was used. The composition ofpyrolysis oils was analysed using gas chromatography-mass spectrometry, and they contained 36.49% styrene, 19.72% benzenebutanenitrile, 12.1% alpha-methylstyrene and 9.69% dimethylbenzene. PMID:23240191

Liu, Xinmin; Wang, Zhen; Xu, Dongyan; Guo, Qingjie

2012-09-01

326

Geological cycles and a two-stage history of the Continental Crust (Robert Wilhelm Bunsen Medal Lecture)  

Science.gov (United States)

The continental crust is the archive of Earth history, and the apparently cyclical nature of geological evolution is a feature of the geological record. The advent of radiometric ages has highlighted that the spatial and temporal distribution of the Earth's record of rock units and events is heterogeneous with distinctive peaks and troughs in the distribution of ages of igneous crystallization, metamorphism, continental margins and mineralization. It is argued that the temporal distribution largely reflects the different preservation potential of rocks generated in different tectonic settings, rather than fundamental pulses of activity, and the peaks of ages are linked to the timing of supercontinent assembly. In contrast there are other signals, such as the Sr isotope ratios of seawater, mantle temperatures, and redox conditions on the Earth, where the records are regarded as primary because they are not sensitive to the numbers of samples of different ages that have been analysed. Models based on the U-Pb, Hf and O isotope ratios of detrital zircons suggest that at least ~60-70% of the present volume of the continental crust had been generated by 3 Ga. The sedimentary record is biased by preferential sampling of relatively young material in their source terrains. The implication is that there were greater volumes of continental crust in the Archaean than might be inferred from the compositions of detrital zircons and sediments. The growth of continental crust was a continuous rather than an episodic process, but the rates of continental growth were significantly higher before 3 Ga than subsequently. The time-integrated Rb/Sr ratios, and the average SiO2 contents, indicate that new continental crust was largely mafic over the first 1.5 Ga of Earth's evolution, and that significant volumes of pre-3 Ga crust may have been associated with intraplate magmatism. Since ~3 Ga there has been an increase in Rb/Sr, SiO2, and the inferred thickness of new crust, consistent with an increase of continental input into the oceans and the onset of plate tectonics. The 60-70% of the present volume of the continental crust estimated to have been present at 3 Ga, contrasts markedly with the <10% of crust of that age apparently still preserved and it requires ongoing destruction (recycling) of early formed crust and subcontinental mantle lithosphere back into the mantle through processes such as subduction and delamination.

Hawkesworth, Chris J.; Cawood, Peter A.; Dhuime, Bruno

2014-05-01

327

Molecular Dynamics Simulations of Non-accreting Neutron Star Crusts  

Science.gov (United States)

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.

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

2010-02-01

328

The Dynamics of Slab detachment: Process Initiated by Melting of Subducted Crust  

Science.gov (United States)

It is well recognized that slab detachment or breakoff is a realistic geological process, as shown by recent tomographic imaging [1]. Using 2-D upper-mantle model with an area of 660 km deep and 2000 km wide we have investigated with a finite-difference and marker numerical technique the multi-resolutional character of thermomechanical phenomena related to this complex geological process. We have used up to 50 million markers on a shared-memory computer for delineating the complex multiscale structures in the composition, viscosity, accumulated strain, shear heating, and other field variables. Our experiments show that this process can be initiated by slab weakening due to the thermal relaxation of the slab and related melting of the subducted oceanic crust. The melting propagates within the subducted oceanic crust at the top of the slab occurring at the restricted depth interval of 100 to 200 km defined by the non-linear dependence of wet solidus temperature of the oceanic crust with pressure. The detachment process is self-accelerating due to the strain and thermal erosion focussing and strong thermal feedback from the shear heating. Slab detachment develops around 10% faster with viscous dissipation included, thus showing the importance of shear heating in this process. Detached slab rapidly fall down having a tendency of coherent rotation. This may produce near horizontal relatively cold slab fragments laying on denser mantle at 660 km discontinuity. Influence of a temperature- and pressure-dependent thermal conductivity for the process of thermal relaxation of the slab is significant. Overall 20% increase in thermal conductivity of mantle produce 20% decrease in timescale of detachment. This support the idea that breakoff process is mainly driven by focussed thermal erosion with timescale linearly dependent on heat conductivity. Rapid changes in topography and significant volcanic activity due to the massive melting of subducted oceanic crust during the slab detachment process are plausible consequences of this vigorous geodynamic scenario. [1] Levin, V., Shapiro, N., Park, J. and M. Ritzwoller, Seismic evidence for catastrophic slab loss beneath Kamchatka, Nature, 418, 763-767, 2002.

Gerya, T.; Yuen, D.

2003-12-01

329

Experimental Fractional Crystallization of the Lunar Magma Ocean  

Science.gov (United States)

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].

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

2012-01-01

330

Tectonomagmatic evolution of the Earth: from the primordial crust to Phanerozoic type of activit  

Science.gov (United States)

There are two dominating hypotheses about composition of the primordial Earth's crust now: basic or sialic. Both models require a global melting of primary chondritic material, and final result would depend on degree of melt differentiation during hardening of global magma ocean some hundreds km deep. Such solidification, due to difference in adiabatic and melting point gradients proceeded in bottom-top direction and resulted in accumulation of low-temperature derivates in outer shell of the planet. Geological data, namely granite-dominated Archean crust, composed mainly by tonalite-trondhjemite-granodiorite (TTG) rocks, and Hadean detrital zircons from Australia with U-Pb age 4.4-4.2 Ga supports the primordial-sialic crust hypothesis. Formation of the sialic crust was responsible for the depletion of the upper mantle matter. Tectonomagmatic activity in the Early Precambrian was rather different from the Phanerozoic. Granite-greenstone terranes (GGTs) and their separating granulite belts were major Archean tectonic structures. The GGTs consisting of irregular network of greenstone belts with high-Mg komatiite-basaltic and boninite-like magmatism, "submerged" in TTG granite-gneiss matrix, probably, strongly reworked primordial sialic crust. They were areas of extension, uplifting and denudation, whereas the granulite belts were dominated by compression, sinking and sedimentation. By the Proterozoic the crust became rigid resulting in formation of rift structures, huge dike swarms and large mafic-ultramafic layered intrusions. In early Paleoroterozoic character of the tectonomagmatic activity remained almost the same: cratons, separated by granulite belts, appeared on the place of GGTs. Magmatism was dominated by siliceous high-Mg (boninite-like) series (SHMS), which formed large igneous provinces. SHMS are close in composition to the Phanerozoic subduction-related magmas; however, instead of them, SHMS had intracontinental tectonic settings. Negative ?Nd in these rocks suggests an important assimilation of the Archean lower-crustal rocks. We assume that origin of the SHMS magmas was linked with floating up of magma chambers of high-temperature mantle-derived ultramafic melts through the crust according to zone refinement principle, i.e. by melting of roof accompanied by crystallization at bottom. It suggests that the Early Precambrian tectonomagmatic activity was linked with ascending of the first generation mantle superplumes, composed by depleted ultramafic material Cardinal change of tectonomagmatic processes occurred in the period of 2.35 to 2.0 Ga, which was characterized by voluminous eruption of Fe-Ti picrites and basalts similar to the Phanerozoic within-plate magmas, derived from geochemical-enriched mantle source. Simultaneously, important compositional changes occurred in the atmosphere, hydrosphere and biosphere (Melezhik et al., 2005). The first Phanerozoic-type orogens (Svecofennian of the Baltic Shield, Trans-Hudson and others of the Canadian Shield, etc.) appeared ca. 2 Ga. Since then, subduction of the ancient sialic continental crust (together with newly-formed oceanic crust) is a permanent process and the crustal material has stored in the "slab graveyard", revealed in the mantle by seismic tomography. We believe that the ascending of the second generation mantle plumes (thermochemical), enriched in Fe, Ti, P, LREE, etc., was responsible for those changes. Those plumes were generated at the core-mantle boundary and this process is active so far. The thermochemical plume matter possessed less density and could reach shallower depths; triggering plate tectonics processes. So, previously absent geochemical-enriched material started to involve from ~2.3 Ga in the Earth's tectonomagmatic processes. Where such material was "conserved" and how it was activated? The established succession of events could be provided by a combination of two independent factors: (1) the Earth originally was heterogeneous, and (2) the downward heating of the Earth (from the surface to the core) was f

Sharkov, Evgenii

2010-05-01

331

Diversity in early crustal evolution: 4100 Ma zircons in the Cathaysia Block of southern China.  

Science.gov (United States)

Zircons are crucial to understanding the first 500?Myr of crustal evolution of Earth. Very few zircons of this age (>4050?Ma) have been found other than from a ~300?km diameter domain of the Yilgarn Craton, Western Australia. Here we report SIMS U-Pb and O isotope ratios and trace element analyses for two ~4100?Ma detrital zircons from a Paleozoic quartzite at the Longquan area of the Cathaysia Block. One zircon ((207)Pb/(206)Pb age of 4127 ± 4?Ma) shows normal oscillatory zonation and constant oxygen isotope ratios (?(18)O = 5.8 to 6.0‰). The other zircon grain has a ~4100?Ma magmatic core surrounded by a ~4070?Ma metamorphic mantle. The magmatic core has elevated ?(18)O (7.2 ± 0.2‰), high titanium concentration (53 ± 3.4?ppm) and a positive cerium anomaly, yielding anomalously high calculated oxygen fugacity (FMQ + 5) and a high crystallization temperature (910°C). These results are unique among Hadean zircons and suggest a granitoid source generated from dry remelting of partly oxidizing supracrustal sediments altered by surface waters. The ~4100?Ma dry melting and subsequent ~4070?Ma metamorphism provide new evidence for the diversity of the Earth's earliest crust. PMID:24888297

Xing, Guang-Fu; Wang, Xiao-Lei; Wan, Yusheng; Chen, Zhi-Hong; Jiang, Yang; Kitajima, Kouki; Ushikubo, Takayuki; Gopon, Phillip

2014-01-01

332

Active Long-Lived Faults Emerging Along Slow-Spreading Mid-Ocean Ridges  

OpenAIRE

In the classic mid-ocean ridge model, new seafloor is generated through a combination of magmatic diking feeding lava flows at the spreading axis, and the formation of short-offset, high-angle normal faults that dip toward the axis. These processes lead to the formation of a layered magmatic crust and linear, ridge-parallel abyssal hills on both ridge flanks. This model of ocean crust generation applies well to fast-spreading mid-ocean ridges (i.e., > 80 mm yr–1), but it is not always valid...

Smith, Deborah K.; Javier Escartín; Hans Schouten; Cann, Johnson R.

2012-01-01

333

Rejuvenation vs. recycling of Archean crust in the Gawler Craton, South Australia: Evidence from U-Pb and Hf isotopes in detrital zircon  

Science.gov (United States)

Zircon grains from modern and ancient drainages of the Gawler Craton, South Australia have been analysed to define the relative contributions of juvenile sources and recycled crust to magmatic activity as a function of time, and to constrain the role of mantle inputs during the Proterozoic rejuvenation of the Archean Gawler Craton. The integration of Hf-isotope data with U-Pb age spectra shows that the crustal evolution was dominated by long periods of crustal reworking, and that crust generated in the Paleoarchean (3.2-3.5 Ga) was largely reworked during Proterozoic time. Four periods of juvenile mantle input can be recognised at ca 2540 Ma, 1853 Ma, 1595 Ma and 1169 Ma. The 1169 Ma event is not previously recognised in the Gawler Craton, but the absence of other important age peaks argues against the transport of these zircons from the Musgrave Block. Only minor proportions of the magmatic zircons with Paleoproterozoic to Mesoproterozoic ages have strongly juvenile signatures ( ?Hf ? 4), indicating that crustal reworking was the dominant process, with relatively minor mantle input. Comparison of the crustal evolution with that of nearby Australian terranes suggests that although the Gawler Craton shows some similarities with the Mount Isa Block and the Georgetown Inlier in Neoarchean-Paleoproterozoic time, it evolved independently between ca 1800 and 1550 Ma.

Belousova, E. A.; Reid, A. J.; Griffin, W. L.; O'Reilly, Suzanne Y.

2009-12-01

334

Asymmetric ocean basins  

Science.gov (United States)

While the superficial expression of oceanic ridges manifests a general symmetry, the deep nature may rather present a relevant asymmetry. Based on a recent surface-wave tomographic 3D model of the upper Earth's 300 km (Shapiro and Ritzwoller, 2002), we constructed a cross-section parallel to the so-called tectonic equator. The shear wave velocity indicates a systematic difference between the western and eastern flanks of the three major oceanic rift basins (Pacific, Atlantic and Indian ridges). They have a faster velocity and thicker lithosphere in the western limb with respect to the eastern or northeastern one, whereas the upper asthenosphere is faster i