WorldWideScience
1

Warm springs discovered on 3.5 Ma oceanic crust, eastern flank of the Juan de Fuca Ridge  

Science.gov (United States)

We have located warm springs on an isolated basement outcrop on 3.5 Ma crust on the eastern flank of the Juan de Fuca Ridge in the northeast Pacific Ocean. These are the first ridge-flank hydrothermal springs discovered on crust older than 1 Ma. The springs are venting altered seawater at 25.0 °C along a fault near the summit of Baby Bare outcrop, a high point along a ridge-axis-parallel basement ridge that is otherwise buried by turbidite sediment. Baby Bare is a small volcano that probably erupted off-axis ca. 1.7 Ma; it is thermally extinct, but acts as a high-permeability conduit for venting of basement fluids. The springs have been sampled from the manned submersible Alvin. Compared with the ambient ocean bottom water, they are heavily depleted in Mg, alkalinity, CO2, sulfate, K, Li, U, O2, nitrate, and phosphate, and enriched in Ca, chlorinity, ammonia, Fe, Mn, H2S, H2, CH4, 222Rn, and 226Ra. The springs appear to support a community of thysirid clams. Although we saw no obvious bacterial mats, the surficial sediments contain the highest biomass concentrations ever measured in the deep sea, based on their phospholipid phosphate content. Areal integration of Alvin heat-flow and pore-water velocity data yields flux estimates of 4 13 L/s and 2 3 MW for the total (diffuse and focused) hydrothermal output from Baby Bare, comparable to that from a black smoker vent on the ridge axis. Warm springs such as those on Baby Bare may be important for global geochemical fluxes.

Mottl, M. J.; Wheat, G.; Baker, E.; Becker, N.; Davis, E.; Feely, R.; Grehan, A.; Kadko, D.; Lilley, M.; Massoth, G.; Moyer, C.; Sansone, F.

1998-01-01

2

Recycled oceanic crust in the source of 90-40 Ma basalts in North and Northeast China: Evidence, provenance and significance  

Science.gov (United States)

Major, trace element and Sr-Nd-Pb isotopic data of basalts emplaced during 90-40 Ma in the North and Northeast China are compiled in this review, with aims of constraining their petrogenesis, and by inference the evolution of the North China Craton during the late Cretaceous and early Cenozoic. Three major components are identified in magma source, including depleted component I and II, and an enriched component. The depleted component I, which is characterized by relatively low 87Sr/86Sr (1.1) and HIMU-like trace element characteristics, is most likely derived from gabbroic cumulate of the oceanic crust. The depleted component II, which distinguishes itself by its high ?Nd (?8) and moderate 87Sr/86Sr (?0.7038), is probably derived from a sub-lithospheric ambient mantle. The enriched component has low ?Nd (2-3), high 87Sr/86Sr (>0.7065), low 206Pb/204Pb (17), excess Sr, Rb, Ba and a deficiency of Zr and Hf relative to the REE. This component is likely from the basaltic portion of the oceanic crust, which is variably altered by seawater and contains minor sediments. Comparison with experimental melts and trace element modeling suggest that these recycled oceanic components may be in form of garnet pyroxenite/eclogite. These components are young (slab. Eu/Eu? and 87Sr/86Sr of the 90-40 Ma magmas increases and decreases, respectively, with decreasing emplacement age, mirroring a change in magma source from upper to lower parts of subducted oceanic crust. Such secular trends are created by dynamic melting of a heterogeneous mantle containing recycled oceanic crust. Due to different melting temperature of the upper and lower ocean crust and progressive thinning of the lithosphere, the more fertile basaltic crustal component is preferentially sampled during the early stage of volcanism, whereas the more depleted gabbroic lower crust and lithospheric mantle components are preferentially sampled during a late stage. This model is consistent with a protracted destruction process of the lithosphere beneath eastern China. The presence of significant recycled oceanic crust components in the 90-40 Ma basalts highlights the influence of Pacific subduction on the deep processes in the North China Craton, which can be traced back at least to the late Cretaceous. This, along with the conjugation of crustal deformation pattern in this region with the movement of the Pacific plate, makes the Pacific subduction as a potential trigger of the destruction of the North China Craton. Geophysical investigations and morphological analyses indicate that decratonization is largely confined to east of the NSGL, whereas to west of NSGL, in particular the Ordos basin, characteristics typical of a craton are observed (Menzies et al., 2007; Zhu et al., 2011). This spatial pattern of craton destruction, together with NE-NNE-oriented extensional basins, main structural alignments and metamorphic core complexes (Zheng et al., 1978; Ye et al., 1987; Ren et al., 2002; Liu et al., 2006; Zhu G et al., 2012), is consistent with the subduction direction of the Pacific plate. Two main episodes of late Mesozoic magmatism have been identified in the Jurassic and the early Cretaceous. These correspond to the subduction of the Pacific plate underneath the Eurasian content and to subsequent extensions, respectively (Wu et al., 2005, 2006). Global tomography studies indicate that the subducted Pacific oceanic slab has become stagnant within the mantle transition zone and extended subhorizontally westward beneath the East Asian continent (Fukao et al., 1992; Huang and Zhao, 2006; Chen and Ai, 2009; Van der Hilst and Li, 2010). The western end of this stagnant slab does not go beyond the NNE-trending NSGL (Huang and Zh

Xu, Yi-Gang

2014-10-01

3

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

4

Evolution from fore-arc oceanic crust to island arc crust: A seismic study along the Izu-Bonin fore arc  

Science.gov (United States)

Petrological studies have suggested that oceanic crust is formed during the initial stage of subduction. However, there is little geophysical evidence for the formation of oceanic crust in the fore-arc region. We conducted an active-source seismic survey in the fore-arc region of the Izu-Bonin intraoceanic arc to examine processes of crustal formation associated with the initiation of subduction. We used seismic refraction tomography to obtain a detailed seismic velocity model and diffraction-stack migration of picked reflection travel times to derive reflectivity images. These data show a remarkably thin crust (<10 km thick) along the northern half of the Bonin ridge and abrupt crustal thickening (to ˜20 km) toward the south of the ridge. Comparison of velocity-depth profiles of the thin fore-arc crust of the Bonin ridge with those of typical oceanic crusts showed them to be seismologically identical. Boninitic magmatism is evident in the area of thin crust and tholeiitic-calcalkaline andesitic volcanism in the area of thick crust. High-precision dating of the volcanic rocks showed that the thin fore-arc oceanic crust was created soon after initiation of subduction (48-45 Ma) and that the nonoceanic thick crust was created with tholeiitic-calcalkaline andesitic magmatism after the boninitic magmatism was ceased. Our seismological image strongly supports the view that fore-arc oceanic crust was formed by fore-arc spreading during the initial stage of subduction along the Izu-Bonin intraoceanic arc.

Kodaira, Shuichi; Noguchi, Naoto; Takahashi, Narumi; Ishizuka, Osamu; Kaneda, Yoshiyuki

2010-09-01

5

How Variable Slow-Spread Ocean Crust?  

Science.gov (United States)

Field studies at slow and ultra-slow spreading ocean ridges reveal a diversity of crustal architecture. At the extreme, ultra-slow spreading ridges consist of linked magmatic and amagmatic accretionary segments. The latter replace transform faults and en-echelon magmatic segments to accommodate ridge obliquity. They are sparsely volcanic and expose large areas of peridotite on the sea floor. Gabbros are largely absent in dredges and dikes uncommon. At magmatic accretionary segments, sheeted dikes and pillow lavas, and gabbros are exposed in abundance, including primitive layered gabbros, and the crustal section appears to fit the Penrose ophiolite model. At slow spreading ridges, by contrast, en-echelon magmatic segments linked to non-transform discontinuities and transforms accommodate ridge obliquity. Diabase, gabbro and peridotite are dredged largely from transform walls and locally at non-transform offsets. Lithologic relationships inferred from dives and dredges suggest a non-Penrose stratigraphy. However, mapping at the Atlantis Bank oceanic core complex suggests that this may be due to sampling fault faces that localize late serpentine diapirism and intrusions of mostly highly differentiated gabbro. There, detachment faulting rooted at the dike-gabbro transition has dismembered Penrose type oceanic crust exposing a 400 km2 gabbro massif less than ~5 km thick, that tapers out at variable distances towards the fracture zone. Large mantle outcrops, however, are exposed along rift valley walls away from transforms in the 15° 20'N region of the MAR. ODP Leg 209 drilling at 8 widely spaced sites found more abundant gabbro than expected from the dredging. Here, a non-Penrose stratigraphy consists of numerous gabbro plugs intruded at varying depth in a mantle section beneath the ridge axis. These, in turn, are locally cross-intruded by dikes and covered by a veneer of pillow basalts. Most gabbros are highly evolved, suggesting more primitive gabbros are abundant at depth. The abundance of dredged peridotite likely reflects strain localization in the roof zone where peridotites are intruded by gabbros, commonly exposing deformed intrusion breccias and net vein complexes in massive metamorphosed peridotites, rather than the main gabbro body. With this exception, oceanic crust at slow spreading ridge magmatic accretionary segments likely largely conforms to a Penrose model, while that at amagmatic accretionary segments, spreading discontinuities, and transforms have an attenuated stratigraphy, often quite different than the Penrose model of massive pillow basalt, sheeted dikes, isotropic and layered gabbro overlying a massive peridotite tectonite.

Dick, H. J.; Kelemen, P.; Kikawa, E.; Cheadle, M. J.; Michael, P. J.; Snow, J.; Schouten, H.; Lin, J.; Hirth, G.; Leg 209 Scientific Party, .

2003-12-01

6

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

OpenAIRE

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 prog...

Piotrowski, Am; Lee, Dc; Christensen, Jn; Burton, Kw; Halliday, An; Hein, Jr; Gunther, D.

2000-01-01

7

Growth response of a deep-water ferromanganese crust to evolution of the Neogene Indian Ocean  

Science.gov (United States)

A deep-water ferromanganese crust from a Central Indian Ocean seamount dated previously by 10Be and 230Th(excess) was studied for compositional and textural variations that occurred throughout its growth history. The 10Be/9Be dated interval (upper 32 mm) yields an uniform growth rate of 2.8 ?? 0.1 mm/Ma [Frank, M., O'Nions, R.K., 1998. Sources of Pb for Indian Ocean ferromanganese crusts: a record of Himalayan erosion. Earth Planet. Sci. Lett., 158, pp. 121-130.] which gives an extrapolated age of ~ 26 Ma for the base of the crust at 72 mm and is comparable to the maximum age derived from the Co-model based growth rate estimates. This study shows that Fe-Mn oxyhydroxide precipitation did not occur from the time of emplacement of the seamount during the Eocene (~ 53 Ma) until the late Oligocene (~ 26 Ma). This paucity probably was the result of a nearly overlapping palaeo-CCD and palaeo-depth of crust formation, increased early Eocene productivity, instability and reworking of the surface rocks on the flanks of the seamount, and lack of oxic deep-water in the nascent Indian Ocean. Crust accretion began (older zone) with the formation of isolated cusps of Fe-Mn oxide during a time of high detritus influx, probably due to the early-Miocene intense erosion associated with maximum exhumation of the Himalayas (op. cit.). This cuspate textured zone extends from 72 mm to 42 mm representing the early-Miocene period. Intense polar cooling and increased mixing of deep and intermediate waters at the close of the Oligocene might have led to the increased oxygenation of the bottom-water in the basin. A considerable expansion in the vertical distance between the seafloor depth and the CCD during the early Miocene in addition to the influx of oxygenated bottom-water likely initiated Fe-Mn crust formation. Pillar structure characterises the younger zone, which extends from 40 mm to the surface of the crust, i.e., ~ 15 Ma to Present. This zone is characterised by > 25% higher content of oxide-bound elements than in the older zone, possibly corresponding to further increased oxygenation of bottom-waters, increased stability of the seamount slope, and gradually reduced input of continental detritus from the erosion of the Himalayas. Middle Miocene Antarctic glaciation, which peaked ~ 12-13 Ma ago, increased the oxic bottom-water influx to the basin resulting in accretion of the crust with low detritus. Therefore, the younger crust started to accrete in response to a shift in bottom-water circulation towards the contemporary pattern, which produced a uniform growth rate and pillar structure up to the present. (C) 2000 Published by Elsevier Science B.V.

Banakar, V.K.; Hein, J.R.

2000-01-01

8

Deep-sea mud volcanoes - a window to alteration processes in old oceanic crust?  

Science.gov (United States)

A number of deep sea mud volcanoes (>4700 m water depth) were discovered during a recent expedition with the German research vessel Meteor along a prominent WSW-ENE trending strike-slip fault (SWIM 1; Zitellini et al., 2009) in the western extension of the Gulf of Cadiz (NE Atlantic). Mud volcanism was unambiguously related to tectonic activity along the fault and fluids expelled at these sites show a very distinct geochemical composition that has not been reported from any other mud volcano to date. In previous studies on deep-water mud volcanoes in the Western Gulf of Cadiz accretionary wedge it was hypothesized that the discharge fluids were affected by alteration processes occurring in the old (>140 Ma) and deeply buried (>4 km) oceanic crust (Scholz et al., 2009; Sallarès et al, 2011). This hypothesis is supported by recent findings at the mud volcanoes located to the west of the realm of tectonic deformation driven by the accretionary wedge of the Gulf of Cadiz. Pore water geochemical analyses revealed fluid sources from oceanic crust and oldest sedimentary strata. Regardless of the ultimate source, these findings suggest that large strike-slip faults may play a significant, yet unrecognized role in terms of fluid circulation and element redistribution. To date, hot vents and cold seeps occurring at active spreading centers and forearcs of subduction zones have been pinpointed as hotspots of fluid activity. However, bearing in mind that transform-type plate boundaries are equal in length compared to other types of plate boundaries, fluid exchange at this type of plate boundary may provide a similarly important pathway for water and element exchange between the lithosphere and ocean. Sallarès V., Gailler A., Gutscher M.-A., Graindorge D., Bartolomé R., Gràcia E., Díaz J., Dañobeitia J.J. and Zitellini N. (2011) Seismic evidence for the presence of Jurassic oceanic crust in the central Gulf of Cadiz (SW Iberian margin), Earth and Planetary Science Letters 311(1-2), 112-123. Scholz F., Hensen C., Reitz A., Romer R.L., Liebetrau V., Meixner A., Weise S.M., and Haeckel M. (2009) Isotopic evidence (87Sr/86Sr, ?7Li) for alteration of the oceanic crust at deep-rooted mud volcanoes in the Gulf of Cadiz, NE Atlantic Ocean. Geochimica et Cosmochimica Acta 73, 5444-5459. Zitellini N., Gràcia E., Matias L., Terrinha P., Abreu M.A., Dealteriis G., Henriet J.P., Dañobeitia J.J., Masson D.G., Mulder T., Ramella R., Somoza L., and Diez S. (2009) The quest for the Africa-Eurasia plate boundary west of the Strait of Gibraltar. Earth and Planetary Science Letters 280, 13-50.

Hensen, Christian; Scholz, Florian; Nuzzo, Marianne; Valadares, Vasco; Terrinha, Pedro; Liebetrau, Volker; Kaul, Norbert; Manzoni, Sonia; Schmidt, Mark; Gràcia, Eulàlia

2013-04-01

9

Geochemical evidence for the nature of the crust beneath the eastern North Penninic basin of the Mesozoic Tethys ocean  

Science.gov (United States)

The North Penninic basin was a subbasin in the northern part of the Mesozoic Tethys ocean. Its significance within the framework of this ocean is controversial because it is not clear whether it was underlain by thinned continental or oceanic crust. Remnants of the eastern North Penninic basin are preserved in the Alps of eastern Switzerland (Grisons) as low metamorphic "Bündnerschiefer" sediments and associated basaltic rocks which formed approximately 140-170 Ma ago (Misox Bündnerschiefer zone, Middle Jurassic to Early Cretaceous). Nb/U, Zr/Nb, and Y/Nb ratios, as well as Nd-Sr isotopic and REE data of most of the metabasalts point to a depleted MORB-type mantle origin. They have been contaminated by magmatic assimilation of Bündnerschiefer sediments and by exchange with seawater, but do not prove the existence of a subcontinental lithospheric mantle or continental crust beneath the North Penninic basin. This suggests that the studied part of the North Penninic realm was underlain by oceanic crust. Only the metabasalts from two melange zones (Vals and Grava melanges) show a more important contamination by crustal material. Since this type of contamination cannot be observed in the other tectonic units, we suggest that its occurrence is related to melange formation during the subduction of the North Penninic basin in the Tertiary. The North Penninic basin was probably, despite the occurrence of oceanic crust, smaller than the South Penninic ocean where the presence of oceanic crust is well established. Modern analogues for the North Penninic basin could be the transitional zone of the Red Sea or the pull-apart basins of the southernmost Gulf of California where local patches of oceanic crust with effusive volcanism have been described.

Steinmann, M.; Stille, P.

10

Chlorine cycling during subduction of altered oceanic crust  

Science.gov (United States)

Eclogitic rocks that have experienced devolatilization, with little or no interaction with external fluid sources, can be viewed as analogues of crustal material which may be transferred back into the mantle during subduction. Thus they can be used to evaluate the extent of the recycling of volatile elements, such as chlorine. We report new oxygen isotope ratios of omphacite, and fluid inclusion data determined from eclogitic metagabbros, of the Rocciavré massif (Italian Alps). The data are compared with those obtained for the Monviso, Cyclades and the Franciscan Complex high-pressure rocks. In all localities, relics of early dehydration fluids are preserved as primary fluid inclusions in the cores of omphacite megacrysts (Rocciavré, Monviso and Franciscan Complex) or garnet (Cyclades). Salinity estimates of the inclusion fluids range from 32 to 45 wt% NaCl in Rocciavré, 17 to 21 wt% NaCl in Monviso, and are similar to seawater in other areas. Omphacite and bulk-rock ? 18O values of Rocciavré (5.1-6.8‰) and Monviso (3.0-5.3‰) metagabbros are markedly lower than those of Cyclades (6.8-14.3‰) and the Franciscan (6.7-13.1‰) metabasites. The fluid salinity-? 18O systematics of eclogitic rocks is similar to that documented along a typical section of the altered oceanic crust and unmetamorphosed ophiolites. This suggests that high-pressure metamorphism, and associated processes, did not modify significantly the variability in chlorine concentrations and oxygen isotope ratios, inherited from a stage of sea-floor hydrothermal alteration under low- (basaltic layer) and high-temperature (gabbroic layer) conditions, respectively. Extrapolating the estimated H 2O and Cl contents of eclogitic rocks to a representative section of the subducted oceanic crust indicates that a minimum of 100 to 200 ppm Cl could be recycled into the mantle during subduction. This yields a Cl/H 2O ratio of 3.6 to 7.5×10 -2 for the subducted oceanic crust, which is similar to E-MORB. On the basis of available Cl isotopic data, we infer that a large proportion (70%) of the Cl stored in the altered crust should be recycled to the mantle to generate an isotopic composition of the subducted crust equivalent to the source of unaltered mid-ocean ridge basalt (? 37Cl = 4.7‰).

Philippot, Pascal; Agrinier, Pierre; Scambelluri, Marco

1998-09-01

11

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

12

Samples from the Jurassic ocean crust beneath Gran Canaria, La Palma and Lanzarote (Canary Islands)  

Science.gov (United States)

Gabbro and minor metabasalt fragments of MORB composition were found on three of the seven Canary Islands. On Gran Canaria, they occur as metamorphosed (greenschist facies) metabasalt and metagabbro clasts in Miocene fanglomerates and sandstones overlying the shield basalts. On Lanzarote and La Palma, MORB gabbros occur as xenoliths in Pleistocene and historic basanite scoria cones and lava flows. The MORB xenoliths are interpreted as fragments of layers 2 and 3 of the underlying Mesozoic oceanic crust, based on mineral compositions (An-rich plagioclase, Ti- and Al-poor clinopyroxene, ± orthopyroxene ± olivine), depleted major and trace element signatures, and Jurassic ages (ca. 180 Ma) determined on single primary plagioclase and secondary amphibole crystals using the 40Ar/ 39Ar laser technique. The Lanzarote gabbros are very mafic (mg# 87 to 89 in clinopyroxene), moderately deformed, and highly depleted. Gran Canaria gabbros are more evolved (mg# 69 to 83 in clinopyroxene) and texturally mostly isotropic. La Palma MORB gabbros have a range of compositions (mg# 68 to 83 in clinopyroxene), some rocks being strongly metasomatized by interaction with basanite magma. The occurrence of MORB fragments on Lanzarote provides definite evidence that oceanic crust beneath the Canary Island archipelago continues at least as far east as the eastern Canary Islands. We postulate that MORB gabbros on Lanzarote which are commonly associated with peridotite xenoliths, represent the base of oceanic layer 3 where gabbros and peridotites were possibly tectonically interleaved. Such tectonic mixing would explain the enigmatic seismic velocities in this area. Gabbro xenoliths from La Palma were derived from within layer 3, probably from wall rock close to magma reservoirs emplaced during the Pleistocene/Holocene growth of La Palma. The Gran Canaria xenoliths are interpreted to represent the metamorphosed layer 2 and upper layer 3. The abundance of lower crustal xenoliths emphasizes the importance of the lower crust and crust-mantle boundary zone as a major level of magma accumulation.

Schmincke, Hans-Ulrich; Klügel, Andreas; Hansteen, Thor H.; Hoernle, Kaj; van den Bogaard, Paul

1998-11-01

13

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

14

Was the Valaisan basin floored by oceanic crust? Evidence of Permian magmatism in the Versoyen unit (Valaisan domain, NW Alps)  

OpenAIRE

The Versoyen Unit (Western Alps) and its mafic rocks have been long considered the remnants of the oceanic crust that supposedly floored the Valaisan basin during the Cretaceous. Here we present U-Pb dating of zircons from a metaleucogabbro and a metagranite from the Versoyen Unit challenging this view. Magmatic zircon cores yield Permian ages of 267±1 and 272±2 Ma, respectively, which are interpreted as dating the crystallization of the magmas. Older inherited crystals and rare Cretaceous ...

Compagnoni, Roberto; Beltrando, Marco

2007-01-01

15

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

OpenAIRE

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

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

2013-01-01

16

Boron isotope exchange between seawater and the oceanic crust  

International Nuclear Information System (INIS)

Dissolved boron in seawater from the Atlantic and Pacific is isotopically homogeneous at 39.5 per mil(11B/10B ratios are expressed as per mil deviations from NBS SRM 951). Unaltered mid-ocean ridge basalts (MORB) from the crest of the East Pacific Rise (EPR) at 210 and 130N have B contents of 0.39 +- 0.03 and 0.46 +-0.03 ppm (about one order of magnitude lower than previous estimates) and delta11B of -3.6 +- 0.4 and -2.2 +- 0.6 per mil respectively. Large scale B exchange between seawater and the oceanic crust has been demonstrated at both high and low temperature. Hydrothermal solutions from nine separate vent fields at 210 and 130N (EPR) have variable B enrichments, relative to seawater (416 ?moles/kg), of between 8 and 32% and have 11B values between 30.0 +- 0.4 and 36.8 +- 0.4 per mil. Boron has been extracted from the basalts with no resolvable isotopic fractionation. High temperature water/rock ratios, based on the B concentrations, are between 0.28 and 3.0. The hydrothermal flux of B into the oceans is between 0.4 and 0.8 x 109 moles/yr, assuming that only pristine basalts are present in the reaction zone. Basalts altered at low temperature and serpentinites are variably enriched in B. The B content of altered whole rocks correlates strongly with 18O, and increases with degree of alteration. Altered basalts (n = 7) containing between 8.9 and 69 ppm B h 7) containing between 8.9 and 69 ppm B have 11B between 0.1 and 9.2 +- 0.4 per mil. Model calculations give water/rock ratios greater than 100 for rocks recovered from DSDP Hole 418A. Serpentinized peridotites (n = 4) with between 50 and 81 ppm B have 11B between 8.3 +- 0.4 and 12.6 +- 0.4 per mil. The flux of B into the crust during low temperature alteration is about 13 x 109 moles/yr. The maximum diffusive flux of B into the crust from sediment pore waters is 0.8 x 109 moles/yr with a 11B less than 43 per mil. (author)

17

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

18

Partial separation of halogens during the subduction of oceanic crust  

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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

19

Metasomatic modification of oceanic crust during early stages of subduction recorded in Mariana blueschist  

Science.gov (United States)

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

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

2013-04-01

20

Drilling a complete in situ section of upper oceanic crust formed at a superfast spreading rate: Hole 1256D  

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The Superfast Spreading Rate Crust mission is a multi-cruise program to drill a complete section of the upper oceanic crust into the underlying gabbros. Hole 1256D was initiated during Ocean Drilling Program Leg 206 in the eastern equatorial Pacific in 15 Ma crust that formed at the East Pacific Rise during a period of superfast spreading (~220 mm per year). This site was chosen to exploit the inverse relationship between spreading rate and the depth to axial low-velocity zones, thought to be magma chambers now frozen as gabbros, observed from seismic experiments. During Integrated Ocean Drilling Program (IODP) Expedition 309 in Jul- Aug 2005, Hole 1256D was deepened to a total depth of 1255 meters below seafloor (mbsf; 1005 m subbasement). Expedition 312 returned to Hole 1256D in Nov-Dec 2005 and deepened it to 1507 mbsf. The hole now extends through 810 m of extrusive normal mid-ocean-ridge basalt, 345 m of sheeted dikes, and 101 m into plutonic rocks, completing the first penetration of an intact section of the upper oceanic crust. Gabbros were encountered at 1407 mbsf, precisely within the depth range predicted from the extrapolation of multichannel seismic results at modern mid-ocean ridges to this superfast spreading rate. The uppermost crust at Site 1256 comprises a >74 m thick ponded lava overlying massive, sheet, and minor pillow flows. Dike contacts, and mineralized breccias indicate a lithologic transition from 1004 to 1061 mbsf. Below the transition zone, massive basalts with doleritic textures, commonly with brecciated and mineralized chilled margins, dominate the sheeted dikes. The transition zone marks a change from predominantly low temperature alteration minerals to greenschist hydrothermal assemblages. Actinolite, hornblende, and secondary plagioclase occur within 100-200 meters of the dike transition indicating a very steep thermal gradient in the dikes. The lowermost ~70 m of dikes are strongly recrystallized to granoblastic minerals resulting from intrusion of underlying gabbros. The plutonic complex comprises a ~60 m thick upper gabbroic body that intrudes the sheeted dikes, separated from a lower gabbroic body by a screen of granoblastic dikes. Gabbroic rocks are highly altered, fine to coarse grained and range from gabbro to oxide gabbro and gabbronorite with some differentiated rocks (quartz-rich oxide diorite and trondhjemite). The gabbro compositions are evolved compared to primary magmas in equilibrium with mantle olivine but similar to the overlying dikes and lavas, precluding the formation of the lower oceanic crust from the geophysically imaged melts lens.

Teagle, D. A.; Wilson, D. S.; Alt, J. C.; Banerjee, N. R.; Umino, S.; Miyashita, S.; Acton, G. D.; Expedition 312 Scientific Parties, L.

2006-12-01

21

Methane deep in ocean crust could feed chemical-hungry microorganisms  

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This University of Washington news article details the work of oceanographer Deborah Kelley regarding microorganisms found in "layer 3" of the ocean crust. These microbes live deep beneath the sea floor and are thought to survive on methane gas trapped in fractures in the crust.

Sandra Hines

22

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

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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

23

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

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

Ben D. Goscombe

2013-07-01

24

Peculiarities in the Fabric of Oceanic Crust Generated at the Gakkel Ridge, Arctic Ocean.  

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The Gakkel Ridge, northern boundary of the American and Eurasian plates, presents the lowest spreading rate (Basin. Reflection seismics and gravity records aquired during the ARCTIC'91 expedition across the Gakkel Ridge and the adjacent Nansen and Amundsen Basin are used. The data are combined with results of former refraction seismic experiments to constrain starting-points for gravity modeling. The topography of the basements surface, buried under more than 3000m thick sediments in the central parts of the basins, appears to be very rough. It varies from several hundred meters up to 1000 m. The RMS-roughness ranges from 450 m in the central Amundsen Basin to 584 m in the southern Eurasian Basin. These values agree reasonably well with RMS-roughness values derived by an empirical model from spreading rates. The gravity models reveal a 5-6 km thick oceanic crust (density of 2900 kg/cm) in the central part of the Amundsen Basin, increasing to 9 km towards the Gakkel Ridge. At the southwestern end of the Eurasian Basin, oceanic crust is only 2-5 km thick and thickens towards the Gakkel Ridge. In our model the ridge is composed of a 2 km thick upper layer with a density of 2600 kg/cm, underlain by an 8 km thick zone with a density of 2900 kg/cm. This is a surprising result, contradicting most theoretical models from which crustal thickness is supposed to decrease with decreasing spreading rate.

Weigelt, E.; Jokat, W.

2001-12-01

25

Evidence of oceanic crust in the southern Baffin Bay from a seismic refraction experiment  

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The Baffin Bay is located north of the Labrador Sea and Davis Strait, separating Canada and Greenland. The nature of the Baffin Bay crust is still enigmatic due to the lack of clearly identified oceanic crust and its spreading centers. The position of an extinct spreading axis has been proposed from gravity data, but magnetic spreading anomalies were not yet identified. Alternative models have suggested thinned continental crust. New geophysical data were acquired from RV Maria S. Merian in 2008 in the southern Baffin Bay and Davis Strait. Multichannel seismic and wide-angle reflection and refraction profiling as well as magnetic and gravity surveying reveal characteristics of the southern Baffin Bay crust which points to an oceanic and transitional stage. One of three seismic refraction lines with 24 deployed ocean-bottom seismometers (OBS) will be presented here. The line is 470 km long and runs from Baffin Island in the SSW to the West Greenland margin in the NNE across the assumed position of an extinct north-south trending spreading center (based on gravity data). Forward modelling shows thick sedimentary sequences of 5-6 km thickness in the basin, underlain by basement of oceanic character. The crystalline crust is 7 km thick on average and shows a two layered architecture with a velocity structure typical for oceanic crust. A spreading center can be identified close to the formerly proposed spreading axis, derived from gravity data. An irregular Moho topography and velocity variations in the lower crust are interpreted to relate to major fracture zones and a variable magma production.

Gohl, K.; Suckro, S.; Funck, T.; Ehrhardt, A.; Heyde, I.; Schreckenberger, B.; Damm, V.

2010-12-01

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Millennial-scale ocean acidification and late Quaternary decline of cryptic bacterial crusts in tropical reefs.  

Science.gov (United States)

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. PMID:25040070

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

2014-09-01

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Peculiarities of roughness and thickness of oceanic crust in the Eurasian Basin, Arctic Ocean  

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The Gakkel Ridge, northern boundary of the American and Eurasian plates, has the slowest spreading rate of the global ridge system. Therefore, it provides an excellent opportunity to study any dependence of crustal fabric on spreading rate. Two parameters, crustal surface roughness and crustal thickness, at the super-slow-spreading Gakkel Ridge (Nansen and Amundsen basins during the ARCTIC'91 expedition are used. The surface of the basement, imaged along the seismic multichannel profiles, is very rough and varies in its topography from several hundreds of metres up to 1000m. Its RMS-roughness ranges from 450m in the central Amundsen Basin to 584m in the southern Eurasian Basin. These values agree reasonably well with RMS-roughness values derived by an empirical model from spreading rates. The gravity models reveal a 5-6km thick oceanic crust (density of 2900kgm-3) in the central part of the Amundsen Basin, increasing to 9km towards the Gakkel Ridge. At the southwestern end of the Eurasian Basin, the oceanic crust is only 2-5km thick and thickens towards the Gakkel Ridge. In our model the ridge is composed of a 2km thick upper layer with a density of 2600kgm-3, underlain by an 8km thick zone with a density of 2900kgm-3. The increase of crustal thickness does not confirm theoretical models for the relation between spreading rate and crustal thickness. The results indicate that the super-slow spreading rate of the Gakkel Ridge may have caused lateral variations in the crustal thickness of the Eurasian Basin.

Weigelt, Estella; Jokat, Wilfried

2001-05-01

28

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  

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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

29

A glimpse of Earth's primordial crust: The Nuvvuagittuq greenstone belt as a vestige of mafic Hadean oceanic crust  

Science.gov (United States)

Investigation of Earth’s primitive crust is biased towards felsic rocks because they contain zircons that provide robust geochronological constraints. Felsic rocks, however, cannot be derived directly from the mantle thus the first crust had to be mafic in composition. Obtaining precise ages on old mafic rocks is however difficult due to their lack of zircon and the metamorphic overprinting they have suffered. 143Nd and 142Nd analyses on the Nuvvuagittuq greenstone belt suggests that its mafic components formed more than 4 billion years ago and that the dominant lithology of the belt, known as the “faux-amphibolite”, represents the only relict of Hadean crust formed at ~4.3 Ga. Its protolith is interpreted to be mafic volcanic rocks and volcanic pyroclastic deposits. The faux-amphibolite can be divided into three distinct geochemical group stratigraphically superimposed. The faux-amphibolite at the base of the sequence is tholeiitic with a composition indicating derivation of primary melts from an undepleted mantle and fractionation under dry conditions. Towards the top of the volcanic sequence, the faux-amphibolites are characterized by higher Al/Ti ratios. They appear to have fractionated under elevated water pressure and are geochemically similar to modern boninite and calc-alkaline volcanic rocks. A new series of faux-amphibolite was analysed for 142Nd isotopic composition. 21 samples have deficits in 142Nd ranging from -7 to -18 ppm compared to the terrestrial standard. These deficits have now been confirmed by measurements of the same samples at ETH (Roth et al., GCA, A886, 2010). A 146Sm-142Nd isochron constructed for all faux-amphibolite yields an age of 4368 +72-142 Ma (n=30). A line fit only to the faux-amphibolite compositional group that shows the widest range in LREE enrichment, including corresponding co-genetic ultramafic sills gives a 146Sm-142Nd age of 4381 +67-123 Ma (n=21). The Hadean age for the faux-amphibolite is supported by a 4079 ± 110 Ma (n=15) 147Sm-143Nd age for intruding gabbro sills. The supracrustal sequence has been highly metamorphosed with isotopic data suggesting metamorphic events at circa 3.8 Ga and 3.1 Ga with the last significant heating event recorded at ~2.65 Ga in Sm-Nd data for garnets from the faux-amphibolite. This supports a mafic composition for Earth’s primitive crust with a compositional transition from tholeitic to boninitic and calc-alkaline volcanic rocks. The presence of pillow basalts supports a subaqueous eruption for at least a portion of the province. The Nuvvuagittuq faux-amphibolite shares some chemical characteristics with the 3.8 Ga Garbenschiefer unit of the Isua greenstone belt in SW Greenland. Both the Garbenschiefer and the faux-amphibolite are chemically similar to modern-day suprasubduction volcanic arc lavas suggesting that modern plate tectonic regime already may have been established on Earth by the Hadean/Eoarchean. Regardless of the tectonic regime, the similarities between the 3.8 Ga Garbenschiefer and the Nuvvuagittuq 4.3 Ga faux-amphibolite suggest that their chemical features are perhaps characteristic of Earth’s early mafic crust.

O'Neil, J.; Carlson, R. W.

2010-12-01

30

Coupling of oceanic and continental crust during Eocene eclogite-facies metamorphism: evidence from the Monte Rosa nappe, western Alps  

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High precision U Pb geochronology of rutile from quartz carbonate white mica rutile veins that are hosted within eclogite and schist of the Monte Rosa nappe, western Alps, Italy, indicate that the Monte Rosa nappe was at eclogite-facies metamorphic conditions at 42.6 ± 0.6 Ma. The sample area [Indren glacier, Furgg zone; Dal Piaz (2001) Geology of the Monte Rosa massif: historical review and personal comments. SMPM] consists of eclogite boudins that are exposed inside a south-plunging overturned synform within micaceous schist. Associated with the eclogite and schist are quartz carbonate white mica rutile veins that formed in tension cracks in the eclogite and along the contact between eclogite and surrounding schist. Intrusion of the veins at about 42.6 Ma occurred at eclogite-facies metamorphic conditions (480 570°C, >1.3 1.4 GPa) based on textural relations, oxygen isotope thermometry, and geothermobarometry. The timing of eclogite-facies metamorphism in the Monte Rosa nappe determined in this study is identical to that of the Gran Paradiso nappe [Meffan-Main et al. (2004) J Metamorphic Geol 22:261 281], confirming that these two units have shared the same Alpine metamorphic history. Furthermore, the Gran Paradiso and Monte Rosa nappes underwent eclogite-facies metamorphism within the same time interval as the structurally overlying Zermatt-Saas ophiolite [˜50 40 Ma; e.g., Amato et al. (1999) Earth Planet Sci Lett 171:425 438; Mayer et al. (1999) Eur Union Geosci 10:809 (abstract); Lapen et al. (2003) Earth Planet Sci Lett 215:57 72]. The nearly identical P T t histories of the Gran Paradiso, Monte Rosa, and Zermatt-Saas units suggest that these units shared a common Alpine tectonic and metamorphic history. The close spatial and temporal associations between high pressure (HP) ophiolite and continental crust during Alpine orogeny indicates that the HP internal basement nappes in the western Alps may have played a key role in exhumation and preservation of the ophiolitic rocks through buoyancy-driven uplift. Coupling of oceanic and continental crust may therefore be critical in preventing permanent loss of oceanic crust to the mantle.

Lapen, Thomas J.; Johnson, Clark M.; Baumgartner, Lukas P.; Piaz, Giorgio V. Dal; Skora, Susanne; Beard, Brian L.

2007-02-01

31

Evidence for Archean ocean crust with low high field strength element signature from diamondiferous eclogite xenoliths  

Science.gov (United States)

Late Archean (2.57 Ga) diamond-bearing eclogite xenoliths from Udachnaya, Siberia, exhibit geochemical characteristics including variation in oxygen isotope values, and correlations of ? 18O with major elements and radiogenic isotopes which can be explained by an origin as subducted oceanic crust. Trace element analyses of constituent garnet and clinopyroxene by Laser-ICPMS are used to reconstruct whole-rock trace element compositions, which indicate that the eclogites have very low high field strength element (HFSE) concentrations and Zr/Hf and Nb/Ta ratios most similar to modern island arcs or ultradepleted mantle. Although hydrothermal alteration on the Archean sea floor had enough geochemical effect to allow the recognition of its effects in the eclogites and thus diagnose them as former oceanic crust, it was not severe enough to erase many other geochemical features of the original igneous rocks, particularly the relatively immobile HFSEs. Correlations of the trace element patterns with oxygen isotopes show that some, generally Mg-richer, eclogites originated as lavas, whereas others have lower ? 18O and higher Sr and Eu contents indicating an origin as plagioclase-bearing intrusive rocks formed in magma chambers within the ocean crust. Major and trace element correlations demonstrate that the eclogites are residues after partial melting during the subduction process, and that their present compositions were enriched in MgO by this process. The original lava compositions were picritic, but not komatiitic, whereas the intrusives had lower, basaltic MgO contents. The HFSE signature of the eclogites may indicate that ocean floor basalts of the time were relatively close to island arcs and recycled material, which would be consistent with a larger number of smaller oceanic plates. Their composition appears to indicate that komatiitic ocean crust compositions were restricted to the early Archean which is not known to be represented among the eclogite xenolith population.

Jacob, Dorrit E.; Foley, Stephen F.

1999-09-01

32

Bacterial Diversity of Young Seafloor Basalts: A Potential Role for Microorganisms in Ocean Crust Weathering  

Science.gov (United States)

A growing number of studies indicate that microbial communities exist within the oceanic crust on mid-ocean ridge flanks. Young ocean crust that is exposed at the seafloor or in the shallow subseafloor interacts directly with low-temperature, oxygenated fluids and undergoes alteration. The free energy associated with oxidation of reduced species in the basalt could potentially be used by microbial communities for growth. Basaltic rock habitats at and below the seafloor, however, remain poorly studied with respect to the physiological and phylogenetic diversity of microbial communities that may be supported by oxidative weathering reactions. In this study, we have investigated the diversity of microorganisms living on or within basaltic crust at the seafloor, and the changes in these microbial communities with increasing oxidative rock alteration. Seafloor lavas representing various flow morphologies, alteration states, and ages (up to 20 kyrs) were collected from the East Pacific Rise between 9°28'N and 9°50'N. Total community DNA was extracted and bacterial 16S rRNA was amplified by PCR. Clone libraries were constructed and sequenced for phylogenetic analyses. To assess the overall extent of basalt alteration and quantify cell abundance in relation to surfacial weathering products, a combination of confocal laser scanning microscopy and scanning electron microscopy was used on natural, unprocessed samples. Phylogenetic and microscopic analyses indicate that diverse, yet distinct populations of bacteria inhabit different lavas, and these microbial communities shift with changes in basalt alteration state. A general trend from metal and sulfur-oxidizing autotrophic communities towards metal- and sulfur-reducing populations correlates with apparent increasing accumulation of weathering products (oxides, clays, etc.). These results provide insight into phylogenetic population trends among bacterial communities harbored in basalt during ocean crust weathering.

Santelli, C. M.; Edgcomb, V.; Bach, W.; Edwards, K.

2005-12-01

33

Activity and phylogenetic diversity of sulfate-reducing microorganisms in low-temperature subsurface fluids within the upper oceanic crust  

Science.gov (United States)

The basaltic ocean crust is the largest aquifer system on Earth, yet the rates of biological activity in this environment are unknown. Low-temperature (subsurface marine crustal fluids on marine and global biogeochemical carbon cycling.

Robador, Alberto; Jungbluth, Sean P.; LaRowe, Douglas E.; Bowers, Robert M.; Rappé, Michael S.; Amend, Jan P.; Cowen, James P.

2015-01-01

34

Melt flow and hypersolidus deformation in the lower ocean crust: Preliminary observations from IODP Leg 345  

Science.gov (United States)

Models for the construction of the fast-spreading lower ocean crust include the gabbro glacier model (GGM), in which most crystallization occurs within a shallow melt lens and the resulting crystal mush subsides downwards and outwards by crystal sliding. Second, the Sheeted Sill Model (SSM) predicts magmatic injection at many levels in the crust, and requires rapid cooling of the lithosphere. A second set of models seeks to reconcile the relatively unevolved nature of most MORB with the existence of an extensive lower crust with both layering (in the lower crust) and highly evolved gabbros (in the upper plutonic sequence). The mechanisms involved here are melt aggregation during vertical porous flow in the lower crust as opposed to lateral sill injection and in-situ crystallization. Here we report new observations from IODP Expedition 345 to the Hess Deep Rift, where propagation of the Cocos Nazca Ridge (CNR) into young, fast-spreading East Pacific Rise (EPR) crust exposes a dismembered lower crustal section. Drilling in ~4850 m water depth produced 3 holes of 35 to 100 mbsf with ~30% recovery of primitive (Mg# 79-87) plutonic lithologies including troctolite, olivine gabbro, and olivine gabbronorite, showing cumulate textures found in layered mafic intrusions and some ophiolite complexes including: 1. Spectacular modal layering 2. Orthopyroxene very early on the liquidus compared to canonical MORB. 3. Delicate large (2-5 cm) skeletal and hopper structures in olivine. 4. Oikocrystic clinopyroxene enclosing chadacrysts different from the host assemblage. These complex relationships are only hinted at in the existing observations from the ocean floor, and will require significant lab study, however some preliminary inferences can be drawn from the petrographic observations. First, the textures observed in olivine throughout the cores are consistent with rapid crystallization, possibly due to steep thermal gradients in the lower crust. They occur early in the crystallization sequence and survive until crystallization is completed, suggesting that limited hypersolidus deformation occurred during their crystallization. This inference is essentially incompatible with the prediction of substantial hypersolidus flow implicit in the GGM Second, while some authors (Korenaga and Kelemen, 1998) suggest that widespread layering is incompatible with pervasive melt flow models, some of the banding observed is exactly consistent with predictions of such models (Sanfilippo and Dick, 2013). Third, orthopyroxene early on the liquidus cannot be achieved simply by lower crustal processes. High level melt-rock reaction in the mantle combined with dry crystallization in the lower crust may do so, however this requires that some melts crossing the MOHO are not aggregated.

Snow, J. E.; Koepke, J.; Falloon, T.; Abe, N.; Hoshide, T.; Akizawa, N.; Maeda, J.; Jean, M. M.; Cheadle, M. J.

2013-12-01

35

33 CFR 165.T01-0542 - Safety Zones: Neptune Deepwater Port, Atlantic Ocean, Boston, MA.  

Science.gov (United States)

...Safety Zones: Neptune Deepwater Port, Atlantic Ocean, Boston, MA. 165.T01-0542 Section 165.T01-0542 Navigation and...Safety Zones: Neptune Deepwater Port, Atlantic Ocean, Boston, MA. (a) Location. The following areas are safety zones: All...

2010-07-01

36

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

Science.gov (United States)

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

France, Lydéric; Ildefonse, Benoit; Koepke, Juergen

2013-08-01

37

Small, monogenetic volcanoes: building blocks of the upper oceanic crust  

Science.gov (United States)

The study of slow-spreading mid-ocean ridge volcanism provides important insights into the mechanisms of oceanic crustal accretion. This study uses a combination of sidescan sonar and recently developed methods of high resolution bathymetry and video data collection to describe the volcanic features on the Mid-Atlantic Ridge axis at 45°N in more detail than has previously been possible. Within most axial valleys lie axial volcanic ridges (AVRs), linear volcanic features thought to be the focus of volcanism at slow spreading ridges. AVR volcanic morphologies have been described independently in a number of studies, through combinations of remote sensing (predominantly through the use of sidescan sonar) and deep towed cameras or submersibles. These different methods have led to classification of volcanic features on two very different scales. While the resolution of the sidescan sonar studies allows only for the identification and classification of features tens to hundreds of metres in size, the photographic and submersible studies describe features from centimetre to metre scale. Until now it has been difficult to reliably link these observations together as no intermediate sensing method has been available. This study uses 1m resolution ROV multibeam bathymetry to address this problem and link features identified at different scales together. We identify a prominent 22km long axial volcanic ridge within a 1km deep axial valley that ranges from 6 to 14km across. We find that 'hummocks' described in previous sidescan sonar studies (of which the AVR is composed) are individual, monogenetic volcanic cones. These cones range from 2 to 200m in height and 40 to 400m in diameter and we identify over 8000 of them on the surface of the AVR. We calculate the average volume of a cone to be 220,000m3 and estimate the AVR is built of approximately 73,000 such cones. We estimate these edifices form on time scales ranging from less than one hour to several months so are likely the products of single eruptions. Cones of all heights, but particularly those over 70m, are prone to collapse soon after forming. A variety of mechanisms are examined and collapse triggers may include: a) flank over-steepening, b) building on unstable material, and c) cutting by fissuring. Collapse scarps show two strong alignments, one ridge parallel and one at 30° to the ridge trend; however as cones always collapse downslope, these alignments may be due to the slope angles produced as a result of cone emplacement rather than first order controls on collapses themselves. We estimate the minimum magmatic flux to the surface for this segment to be at least 64,000m3 yr -1, which is equivalent to producing one average volume cone every 3.5 years.

Yeo, Isobel A.; Achenbach, Kay L.; Searle, Roger C.; Le Bas, Tim P.

2010-05-01

38

Glacial cycles drive variations in the production of oceanic crust  

Science.gov (United States)

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

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

2013-12-01

39

IODP Expedition 345: Primitive Layered Gabbros From Fast-Spreading Lower Oceanic Crust  

Science.gov (United States)

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

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

2014-05-01

40

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

41

IODP Exp 345: Primitive Layered Gabbros From Fast-Spreading Lower Oceanic Crust  

Science.gov (United States)

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

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

2013-12-01

42

Downdip velocity changes in subducted oceanic crust beneath Northern Japan—insights from guided waves  

Science.gov (United States)

Dispersed P-wave arrivals observed in the subduction zone forearc of Northern Japan suggest that low velocity subducted oceanic crustal waveguide persists to depths of at least 220 km. First arrivals from events at 150-220 km depth show that the velocity contrast of the waveguide reduces with depth. High frequency energy (>2 Hz) is retained and delayed by the low velocity crustal waveguide while the lower frequency energy (tests using 2-D elastic waveform simulations show that the dispersion can be accounted for by a 6-8 km thick low velocity oceanic crust, with a velocity contrast that varies with depth. The velocities inferred for this variable low velocity oceanic crust can be explained by lawsonite bearing assemblages, and suggest that low velocity minerals may persist to greater depth than previously thought. 2-D simulations are benchmarked to 3-D full waveform simulations and show that the structures inferred by the 2-D approximation produce similar dispersion in 3-D. 2-D viscoelastic simulations show that including elevated attenuation in the mantle wedge can improve the fit of the dispersed waveform. Elevated attenuation in the low velocity layers can however be ruled out.

Garth, Tom; Rietbrock, Andreas

2014-09-01

43

Evidence for mantle metasomatism by hydrous silicic melts derived from subducted oceanic crust.  

OpenAIRE

The low concentrations of niobium, tantalum and titanium observed in island-arc basalts are thought to result from modification of the sub-arc mantle by a metasomatic agent, deficient in these elements, that originates from within the subducted oceanic crust 1. Whether this agent is an hydrous fluid 2 or a silica-rich melt 3 has been discussed using mainly a trace-element approach 4 and related to variable thermal regimes of subduction zones 5. Melting of basalt in the absence of fluid both r...

Prouteau, Gae?lle; Scaillet, Bruno; Pichavant, Michel; Maury, Rene?

2001-01-01

44

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.

45

Calcium carbonate veins in ocean crust record a threefold increase of seawater Mg/Ca in the past 30 million years  

Science.gov (United States)

Chemical (Sr, Mg) and isotopic (?18O, 87Sr/86Sr) compositions of calcium carbonate veins (CCV) in the oceanic basement were determined to reconstruct changes in Sr/Ca and Mg/Ca of seawater in the Cenozoic. We examined CCV from 10 basement drill sites in the Atlantic and Pacific, ranging in age between 165 and 2.3 Ma. Six of these sites are from cold ridge flanks in basement seawater composition. CCV of these young sites were dated, using the Sr isotopic evolution of seawater. For the other sites, temperature-corrections were applied to correct for seawater-basement exchange processes. The combined data show that a period of constant/low Sr/Ca (4.46-6.22 mmol/mol) and Mg/Ca (1.12-2.03 mol/mol) between 165 and 30 Ma was followed by a steady increase in Mg/Ca ratios by a factor of three to modern ocean composition. Mg/Ca-Sr/Ca relations suggest that variations in hydrothermal fluxes and riverine input are likely causes driving the seawater compositional changes. However, additional forcing may be involved in explaining the timing and magnitude of changes. A plausible scenario is intensified carbonate production due to increased alkalinity input to the oceans from silicate weathering, which in turn is a result of subduction-zone recycling of CO2 from pelagic carbonate formed after the Cretaceous slow-down in ocean crust production rate.

Rausch, Svenja; Böhm, Florian; Bach, Wolfgang; Klügel, Andreas; Eisenhauer, Anton

2013-01-01

46

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

Science.gov (United States)

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

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

2014-08-01

47

Vertical tectonics at a continental crust-oceanic plateau plate boundary zone: Fission track thermochronology of the Sierra Nevada de Santa Marta, Colombia  

Science.gov (United States)

The topographically prominent Sierra Nevada de Santa Marta forms part of a faulted block of continental crust located along the northern boundary of the South American Plate, hosts the highest elevation in the world (˜5.75 km) whose local base is at sea level, and juxtaposes oceanic plateau rocks of the Caribbean Plate. Quantification of the amount and timing of exhumation constrains interpretations of the history of the plate boundary, and the driving forces of rock uplift along the active margin. The Sierra Nevada Province of the southernmost Sierra Nevada de Santa Marta exhumed at elevated rates (?0.2 Km/My) during 65-58 Ma in response to the collision of the Caribbean Plateau with northwestern South America. A second pulse of exhumation (?0.32 Km/My) during 50-40 Ma was driven by underthrusting of the Caribbean Plate beneath northern South America. Subsequent exhumation at 40-25 Ma (?0.15 Km/My) is recorded proximal to the Santa Marta-Bucaramanga Fault. More northerly regions of the Sierra Nevada Province exhumed rapidly during 26-29 Ma (˜0.7 Km/My). Further northward, the Santa Marta Province exhumed at elevated rates during 30-25 Ma and 25-16 Ma. The highest exhumation rates within the Sierra Nevada de Santa Marta progressed toward the northwest via the propagation of NW verging thrusts. Exhumation is not recorded after ˜16 Ma, which is unexpected given the high elevation and high erosive power of the climate, implying that rock and surface uplift that gave rise to the current topography was very recent (i.e., ?1 Ma?), and there has been insufficient time to expose the fossil apatite partial annealing zone.

Villagómez, Diego; Spikings, Richard; Mora, AndréS.; GuzmáN, Georgina; Ojeda, GermáN.; CortéS, Elizabeth; van der Lelij, Roelant

2011-08-01

48

Positive geothermal anomalies in oceanic crust of Cretaceous age offshore Kamchatka  

Directory of Open Access Journals (Sweden)

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

49

Positive geothermal anomalies in oceanic crust of Cretaceous age offshore Kamchatka  

Directory of Open Access Journals (Sweden)

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

50

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)

51

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

52

Pillow basalts of the Angayucham terrane: Oceanic plateau and island crust accreted to the Brooks Range  

Science.gov (United States)

The Angayucham Mountains (north margin of the Yukon-Koyukuk province) are made up of an imbricate stack of four to eight east-west trending, steeply dipping, fault slabs composed of Paleozoic (Devonian to Mississippean), Middle to Late Triassic, and Early Jurassic oceanic upper crustal rocks (pillow basalt, subordinate diabase, basaltic tuff, and radiolarian chert). Field relations and geochemical characteristics of the basaltic rocks suggest that the fault slabs were derived from an oceanic plateau or island setting and were emplaced onto the Brooks Range continental margin. The basalts are variably metamorphosed to prehnite-pumpellyite and low-greenschist facies. Major element analyses suggest that many are hypersthene-normative olivine tholeiites. Classification based on immobile trace elements confirms the tholeiitic character of most of the basalts but suggests that some had primary compositions transitional to alkali basalt. Although field and petrographic features of the basalts are similar, trace element characteristics allow definition of geographically distinct suites. A central outcrop belt along the crest of the mountains is made up of basalt with relatively flat rare earth element (REE) patterns. This belt is flanked to the north and south by LREE (light rare earth element)-enriched basalts. Radiolarian and conodont ages from interpillow and interlayered chert and limestone indicate that the central belt of basalts is Triassic in age, the southern belt is Jurassic in age, and the northern belt contains a mixture of Paleozoic and Mesozoic ages. Data for most of the basalts cluster in the "within-plate basalt" fields of trace element discriminant diagrams; none have trace-element characteristics of island arc basalt. The Triassic and Jurassic basalts are geochemically most akin to modern oceanic plateau and island basalts. Field evidence also favors an oceanic plateau or island setting. The great composite thickness of pillow basalt probably resulted from obduction faulting, but the lack of fault slabs of gabbro or peridotite suggests that obduction faults did not penetrate below oceanic layer 2, a likely occurrence if layer 2 were anomalously thick, as in the vicinity of an oceanic island. The presence of basaltic tuff interbeds indicates proximity to an explosive basaltic eruptive center. The juxtaposition of submarine basalts of differing chemical affinity and age, adjacent to higher-grade Paleozoic metamorphic rocks of the Brooks Range to the north, may be explained by obduction of internally complex (thickened) oceanic crust formed in an ocean plateau setting. Emplacement and rotation of thrust plates to steep attitudes occurred during accretion of the Brooks Range passive margin, probably beginning in the Late to Middle Jurassic.

Pallister, John S.; Budahn, James R.; Murchey, Benita L.

1989-11-01

53

Variations of fluid pressure within the subducting oceanic crust :Fine-scale seismic structures correlating with slow earthquakes  

Science.gov (United States)

Recent studies of slow earthquakes point to the involvement of high-pressure fluids near the plate boundary in the occurrence of slow earthquakes. Here, we show fine-scale variations of seismic velocities and converted teleseismic waves that reveal the presence of zones of high-pressure fluids released by progressive metamorphic dehydration reactions in the subducting Philippine Sea plate in Tokai, Japan [Kato et al., 2010, GRL]. Megathrust earthquakes with Mw = 8 have occurred persistently in the Tokai district with time intervals of approximately 150 years. Down-dip from the locked zone, a long-term slow slip (LTSS), non-volcanic tremors, LFEs and short-term slow slips have been detected. The oceanic crust, of which the top and bottom (plate interface and oceanic Moho) are outlined by strong negative and positive amplitudes of the receiver functions, respectively, is subducting at a dip angle of approximately 15°. The subducting oceanic crust is on average characterized by relatively low-velocities and high Vp/Vs ratios, interpreted to indicate fluids released by progressive metamorphic dehydration reactions in the oceanic crust. Anomalously low Vp and Vs velocities with peak Vp/Vs ratio (?1.95) in the oceanic crust coincide with the centroid of the LTSS area. The combination of the anomalously low-velocities and high-Vp/Vs ratio with the paucity of intraslab earthquakes is consistent with the inference that overpressured fluids are trapped within the subducted ridge. At the greater depths of the LTSS area, well relocated LFEs align linearly along the plate interface at the onset of the contact between the serpentinized mantle wedge corner (i.e., the base of the land Moho as delineated by a positive amplitude in the receiver functions) and the top of the oceanic crust. The underlying oceanic crust is characterized by slightly reduced velocities and moderately high Vp/Vs ratio (?1.85), which implies that high-pressure fluids are still present. However, the amplitudes of the seismic anomalies are diminished compared with those of the subducted ridge (LTSS centroid area). We therefore infer that fluid pressures within the oceanic crust are somewhat reduced in the LFE zone. This fluid pressure reduction is likely to be caused by partial leakage of fluids into the overlying mantle wedge corner. Since the Philippine Sea plate is subducting beneath Kanto district and SW Japan, it is of crucial importance to reveal lateral variations of fluid pressures utilizing dense seismic networks such as MeSO-net (Metropolitan Seismic Observation network in Japan).

Kato, A.; Iidaka, T.; Ikuta, R.; Yoshida, Y.; Katsumata, K.; Iwasaki, T.; Sakai, S.; Thurber, C. H.; Tsumura, N.; Yamaoka, K.; Watanabe, T.; Kunitomo, T.; Yamazaki, F.; Okubo, M.; Suzuki, S.; Hirata, N.

2010-12-01

54

The tectonic history of southern Baffin Bay and Davis Strait - seismic refraction experiments and the evolution of oceanic crust  

Science.gov (United States)

Located between the Canadian Baffin Island and Greenland, Baffin Bay represents the northern extension of the rift system in Labrador Sea. Davis Strait is a bathymetric high, that connects Baffin Bay with the Labrador Sea and acts as a gateway for the water exchange between the two basins. The strait is dominated by the Ungava Fault Complex, a major transform fault, characterized by a series of positive gravity anomalies. It is generally agreed, that the extensional motion of the rift system in Labrador Sea was connected with the opening of Baffin Bay by the Ungava Fault Complex. Unlike in Labrador Sea, no magnetic spreading anomalies have clearly been identified in Baffin Bay, hence the nature of the crust remains a subject of discussion. In order to determine the character of the crust in southern Baffin Bay, a 710-km-long seismic refraction experiment was set up in two research cruises (MSM09/3 of RV Maria S. Merian in 2008 and ARK25/3 of RV Polarstern in 2010). P-wave velocity and density modeling display the following crustal units from southeast to northwest: transitional crust near Baffin Island, more than 300 km of oceanic crust with an extinct spreading center, 100 km of transitional crust of a volcanic type margin on the Greenland side of the profile, and Greenland continental crust. The nature of the crust in Davis Strait is also disputed. Both, blocks of continental crust or a thick pile of oceanic crust have been proposed. To identify the character of the crust, a 226-km-long seismic refraction line was set up across the strait (MSM09/3 expedition). The P-wave velocity and density models image continental blocks with deep reaching faults. The lower crust displays abnormally high P-wave velocities, which we attribute to intense intrusions of mafic material, most likely related to the arrival of the Iceland mantle plume beneath Greenland in the Paleocene. To set the crustal units along the two profiles into context with the tectonic history of the Labrador Sea and Baffin Bay region, we visualized the opening of these basins in GPlates with published poles of rotation. It became clear, that the existing tectonic map of this region needs modifications, which we present here. Furthermore, the reconstruction images the compression of previously rifted crust in Davis Strait which adds up to an overlap of 70 km.

Suckro, S. K.; Gohl, K.; Funck, T.; Heyde, I.; Schreckenberger, B.; Ehrhardt, A.; Gerlings, J.; Damm, V.; Jokat, W.

2011-12-01

55

Jurassic (~160 Ma) Lamprophyric Xenoliths from Southern Louisiana Salt Domes: A Unique Perspective on Gulf of Mexico Crust (Invited)  

Science.gov (United States)

No direct information about the age and composition of rift-related igneous activity associated with the Late Jurassic opening of the Gulf of Mexico exists because these rocks are buried beneath several kilometers of sediments. Salt diapirs from southern Louisiana bring to the surface samples of the alkalic igneous rock lamprophyre; these salt domes rise from the base of the sedimentary pile to the surface. Two salt domes (Avery and Weeks) bring up igneous rocks that we studied; a third (Jefferson) brings up igneous rocks but these samples are lost. All three salt diapirs rise from an isolated magnetic high, which may mark the position of an ancient mafic volcano or intrusion in what appears to have evolved as a magma-starved rifted margin. Three lamprophyre samples from Weeks and Avery salt domes were studied, but we cannot tell whether these were lavas or shallow intrusions. The lamprophyres are altered but preserve relict igneous minerals including Cr-rich spinel Cr# ( = 100Cr/Cr+Al) ranging from 36 to 42 rimmed with titanite strongly zoned clinopyroxene (diopside to Ti-augite). Diopside cores have two distinct compositions: one with a high Cr, Si and a second with low Cr, Si. Rims for both core types exhibit titanopyroxene (MgSi2 = TiAl2) and Ca-tschermaks (MgSi = AlviAliv) substitution and are identical to matrix diopside, indicating that rim compositions were in equilibrium with the final melt. Excellent 40Ar/39Ar plateau ages of 158.6±0.2 Ma and 160.1±0.7 Ma for lamprophyric Ti-rich biotite and kaersutite from two different salt domes are interpreted to date when the lamprophyre solidified. Alteration disturbed primary igneous compositions, especially Si, alkali metals, alkaline earths, Pb and H2O, but not high field strength elements (HFSE, e.g., Ti, Zr, Hf, Nb, Y). Elevated abundances of immobile incompatible trace elements are one to two orders of magnitude enriched relative to N-MORB. However, isotopic compositions of Nd and Hf (?Hf ~ +9, ?Nd ~ +7 at 160 Ma) indicate derivation from MORB-like depleted mantle. Trace element and isotopic data, considered together, imply that the lamprophyric magma was derived from low degree of partial melting (~2-3 %) of the mantle. This information supports the idea that crust beneath southern Louisiana formed as a magma-starved rifted margin on the northern flank of the Gulf of Mexico ~160 Ma.

Stern, R. J.; Anthony, E. Y.; Ren, M.; Kimura, J.; Lock, B.; Norton, I. O.

2010-12-01

56

Chlorine and Potassium Flux Into the Mantle via Subduction of Oceanic Crust: Constraints From Melt Inclusions in HIMU Lavas  

Science.gov (United States)

Previous studies have estimated the chlorine content of recycled oceanic crust as ranging from Lassiter et al., EPSL 202 (2002) 525-540. [2] Philippot et al., EPSL 161 (1998) 33-44. [3] Stroncik and Haase, Geology 32 (2004) 945-948. [4] Staudigel et al., EPSL 130 (1995) 169-185.

Szramek, L. A.; Lassiter, J. C.

2007-12-01

57

Rates of hydrothermal cooling of new oceanic upper crust derived from lithium-geospeedometry [rapid communication  

Science.gov (United States)

Episodic emplacement and cooling of lavas and dikes at mid-ocean ridges leads to large fluctuations in hydrothermal fluxes and biological activity. However, the processes operating beneath the seafloor during these transient events such as permeability creation and dike cooling are poorly understood. We have developed a new approach to determine the cooling rate of the sheeted dike complex based on the extent of diffusion of lithium from plagioclase into clinopyroxene during cooling. We have calibrated this Li-geospeedometer using new high-temperature experiments to determine both the temperature dependence of the partitioning of Li between plagioclase and clinopyroxene and the diffusion coefficient for Li in clinopyroxene. Application of this method to lavas and dikes from ODP Hole 504B shows that cooling rates vary dramatically with depth in the upper oceanic crust. Extremely rapid cooling rates (> 450 °C hr - 1 ) in the upper part of the sheeted dike complex are sufficient to power hydrothermal megaplume formation within the overlying water column.

Coogan, Laurence A.; Kasemann, Simone A.; Chakraborty, Sumit

2005-12-01

58

Lithium and Li-isotopes in young altered upper oceanic crust from the East Pacific Rise  

Science.gov (United States)

Lithium contents and Li-isotope ratios have been measured in a section of young oceanic upper crust formed at the East Pacific Rise and exposed in the Hess Deep rift. The Li contents of both lavas and sheeted dikes are lower than in the fresh protolith (by 43% and 74%, respectively) suggesting that axial hydrothermal systems leach Li from the entire upper crust. The Li-isotopic composition of the lavas (4.1 ± 1.7‰) is similar to the protolith (3.4 ± 1.4‰) whereas the dikes extend to both higher and lower isotopic compositions (?7Li from -11 to 14‰). Loss of Li from all lava samples was unexpected due to the petrographically fresh appearance of these rocks and the relatively cool fluids generally thought to exist within the lava pile. This Li loss is interpreted as indicating that warm (50-100 °C) fluids pervade the lava pile, at least episodically, leaching Li from the lavas without recrystallization of the primary igneous minerals. This hypothesis has been tested experimentally. Hydrothermal experiments at 125 °C, using synthetic basalts cooled at different rates to produce different starting material texture (glass to crystalline rock), demonstrate that Li is leached rapidly at this temperature and substantially more rapidly from crystalline rock than from glass. The natural and experimental data are consistent with Li diffusion out of plagioclase into warm hydrothermal fluids acting to leach Li from the lavas without any concomitant mineralogical changes. The wide range of ?7Li in the sheeted dike complex, and the lack of a systematic correlation of ?7Li with the concentration of Li, suggest that more than one process leads to Li depletion. A combination of mineral-fluid reactions that break-down igneous minerals and produce secondary minerals (principally amphibole, chlorite and secondary plagioclase), along with diffusive loss of Li from plagioclase into the fluid, can explain the Li-systematics of the sheeted dike complex at Hess Deep.

Brant, C.; Coogan, L. A.; Gillis, K. M.; Seyfried, W. E.; Pester, N. J.; Spence, J.

2012-11-01

59

Sub-seafloor hydrothermal alteration of oceanic crust of the Oman ophiolite - Interaction with global environmental change -  

Science.gov (United States)

The chemical change of rocks during hydrothermal alteration was investigated in a complete section through the Wadi Fizh oceanic crust in the Oman ophiolite was investigated in order to evaluate seawater-rock interaction through the oceanic crust. 20 elements analyzed in this study are classified into 6 groups: Group 1 (Ca, P, Al), Group 2 (Mg, Ni, Co, Cr), Group 3 (Fe, Ti, Na, Mn, Zn, Y), Group 4 (K, Rb, Ba), Group 5 (Li, Cd) and Group 6 (other elements such as Sr, Cu). The degree of alteration is assessed by 87Sr/86Sr ratios of rocks and the abundance of secondary minerals. Heavy metals (Cu, Zn, Ni, Cr, Co and Cd) and phosphorus were leached from the lower oceanic crust during amphibolite facies (> 450uC) alteration. Even if the scavenging and/or removal by hydrous Fe and Mn oxides is taken into consideration, the alteration of ophiolite complex, as a whole, works as a source of phosphorus to the ocean during the mid-Cretaceous. This period is characterized by deposition of black shales and oil generation caused by superplume activity. Although such carbon should have been supplied directly by mantle outgassing, nutrients such as phosphorus and silica required for the production of marine organic matter would have been supplied from enhanced hydrothermal activity.

Kawahata, H.; Nohara, M.

2003-12-01

60

In situ enrichment of ocean crust microbes on igneous minerals and glasses using an osmotic flow-through device  

Science.gov (United States)

The Integrated Ocean Drilling Program (IODP) Hole 1301A on the eastern flank of Juan de Fuca Ridge was used in the first long-term deployment of microbial enrichment flow cells using osmotically driven pumps in a subseafloor borehole. Three novel osmotically driven colonization systems with unidirectional flow were deployed in the borehole and incubated for 4 years to determine the microbial colonization preferences for 12 minerals and glasses present in igneous rocks. Following recovery of the colonization systems, we measured cell density on the minerals and glasses by fluorescent staining and direct counting and found some significant differences between mineral samples. We also determined the abundance of mesophilic and thermophilic culturable organotrophs grown on marine R2A medium and identified isolates by partial 16S or 18S rDNA sequencing. We found that nine distinct phylotypes of culturable mesophilic oligotrophs were present on the minerals and glasses and that eight of the nine can reduce nitrate and oxidize iron. Fe(II)-rich olivine minerals had the highest density of total countable cells and culturable organotrophic mesophiles, as well as the only culturable organotrophic thermophiles. These results suggest that olivine (a common igneous mineral) in seawater-recharged ocean crust is capable of supporting microbial communities, that iron oxidation and nitrate reduction may be important physiological characteristics of ocean crust microbes, and that heterogeneously distributed minerals in marine igneous rocks likely influence the distribution of microbial communities in the ocean crust.

Smith, Amy; Popa, Radu; Fisk, Martin; Nielsen, Mark; Wheat, C. Geoffrey; Jannasch, Hans W.; Fisher, Andrew T.; Becker, Keir; Sievert, Stefan M.; Flores, Gilberto

2011-06-01

61

Assimilation of sediments embedded in the oceanic arc crust: myth or reality?  

Science.gov (United States)

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

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

2014-06-01

62

The effect of recycled oceanic crust in the thermal evolution of the Galapagos Plume  

Science.gov (United States)

Current models suggest that the massive basaltic production responsible for the emplacement of Large Igneous Provinces (LIPS) during the Permian-Paleocene may represent the initial phases (plume heads) of some of the mantle plumes that feed the current ocean island basalts (OIB). In many cases, magmatism associated with the initiation of mantle plumes was so voluminous that produced global environmental impacts. The origin of these intra-plate magmatism is still debated but recent petrological, geochemical and geophysical studies of some of these localities like Samoa, Hawaii, Galapagos, provide evidence that melting is related to a true mantle plume, representing a geochemically heterogeneous, hot-buoyant domain that originates from a boundary layer beneath the upper mantle. Thus, plume-related magmas produced in OIB and LIPS and their connecting plume tracks are windows into the Earth's mantle, providing evidence on mantle temperature, size and composition of heterogeneities, and the deep earth geochemical cycles. Our preliminary petrological modeling suggests that mantle plumes for LIPS with Permian-Paleocene ages were generally hotter and melted more extensively than plumes of more modern oceanic islands. Although a lot of work has been done on LIPS and OIB, no complete record of the evolution of a mantle plume is available to this point, mostly due to the inaccessibility of the submerged sections of almost all plume tracks. Galapagos-related lavas provide a complete record of the evolution of a mantle plume since the plume's initial stages in the Cretaceous. In the case of the Galapagos, our work suggests a decrease from TP(max) of1650 C in the Cretaceous to 1500 C in the present day. Our recent work on the Galapagos Islands and the preliminary work on older Galapagos-related terranes suggest that this secular cooling is directly related with increasing amounts of recycled crust in the plume.

Gazel, E.; Herzberg, C. T.; Vidito, C. A.

2011-12-01

63

Dike intrusion controls on permeability and hydrothermal circulation of oceanic crust at IODP Hole 1256D  

Science.gov (United States)

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

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

2012-12-01

64

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

CERN Document Server

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 predict the signals expected for several other radioisotopes, which are independent of the supernova distance. Species likely to be present near or above background levels are 10Be, 26Al, 53Mn, 182Hf and 244Pu. Of these, 182Hf and 244Pu are nearly background-free, presenting the best opportunities to provide strong confirmation of the supernova origin of the 60Fe signal, and to demonstrate that at least some supernovae are the source for the r-process. The accuracies of our predictions are hampered by large uncertainties in the predicted 60...

Fields, B D; Ellis, Jonathan Richard

2005-01-01

65

Scattering beneath Western Pacific subduction zones: evidence for oceanic crust in the mid-mantle  

Science.gov (United States)

Small-scale heterogeneities in the mantle can give important insight into the dynamics and composition of the Earth's interior. Here, we analyse seismic energy found as precursors to PP, which is scattered off small-scale heterogeneities related to subduction zones in the upper and mid-mantle. We use data from shallow earthquakes (less than 100 km depth) in the epicentral distance range of 90°-110° and use array methods to study a 100 s window prior to the PP arrival. Our analysis focuses on energy arriving off the great circle path between source and receiver. We select coherent arrivals automatically, based on a semblance weighted beampower spectrum, maximizing the selection of weak amplitude arrivals. Assuming single P-to-P scattering and using the directivity information from array processing, we locate the scattering origin by ray tracing through a 1-D velocity model. Using data from the small-aperture Eielson Array (ILAR) in Alaska, we are able to image structure related to heterogeneities in western Pacific subduction zones. We find evidence for ˜300 small-scale heterogeneities in the region around the present-day Japan, Izu-Bonin, Mariana and West Philippine subduction zones. Most of the detected heterogeneities are located in the crust and upper mantle, but 6 per cent of scatterers are located deeper than 600 km. Scatterers in the transition zone correlate well with edges of fast features in tomographic images and subducted slab contours derived from slab seismicity. We locate deeper scatterers beneath the Izu-Bonin/Mariana subduction zones, which outline a steeply dipping pseudo-planar feature to 1480 km depth, and beneath the ancient (84-144 Ma) Indonesian subduction trench down to 1880 km depth. We image the remnants of subducted crustal material, likely the underside reflection of the subducted Moho. The presence of deep scatterers related to past and present subduction provides evidence that the subducted crust does descend into the lower mantle at least for these steeply dipping subduction zones. Applying the same technique to other source-receiver paths will increase our knowledge of the small-scale structure of the mantle and will provide further constraints on geodynamic models.

Bentham, H. L. M.; Rost, S.

2014-06-01

66

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

67

High Melt Porosity in the Lower Oceanic Crust Inferred from Phosphorus Zoning in Olivine  

Science.gov (United States)

The canonical view that the lower oceanic crust is composed of cumulates of fractional crystallization has been intensely debated in recent years. Migrating melts, reacting with previously crystallized minerals in the crystal mush, can modify the mineralogy and phase proportions inside the lower crust, as well as the composition of erupted MORB [1]. An extreme product of reactive melt migration was discovered during IODP Legs 304/305 at Atlantis Massif (MAR 30N). In this 1.5 km deep drillhole, there are several sequences of olivine-rich troctolite with ';textbook cumulate texture', which may be associated with the contact of a gabbroic pluton into peridotite [2,3]. While there is little ambiguity about the geological relationships, the exact mechanism for the in-situ reactive transformation of mantle peridotites into lower crustal gabbroic lithologies is still poorly understood. One widespread textural feature in support of the dominant role of reactive melt migration is the occurrence of interstitial and vermicular high-Mg# cpx, which form post-compaction at very low melt porosities. The rare screens of opx-bearing mantle peridotites in this drillhole also preserve evidence for low-porosity replacement of mantle opx by gabbroic cpx [4], with minimal volume change. However, we will show that a significant and possibly the main mass of the olivine crystals in the olivine-rich troctolites do not form at low melt porosities, but instead in a melt-rich local environment. Initially, olivines crystallize as rapidly grown dendrites, which is marked by distinct enrichments of the slowly diffusing element phosphorus. Subsequent slow growth produces the main mass of the otherwise P-free olivine crystal. Our observations on natural basalt-hosted and experimentally grown olivines indicate that strong undercooling in a crystal-poor environment is required for dendrite formation. By extrapolation, this would require a crystal-poor melt lens at the top of an evolving gabbroic intrusion into overlying lithospheric mantle peridotites. Disintegration and partial dissolution of the peridotite minerals can provide the conditions for compositional (rather than thermal) undercooling and promote the rapid growth of new olivines inside the deep melt lens. [1] Lissenberg, C.J. & Dick, H.J.B. (2008) EPSL 271, 311-325. [2] Suhr, G. et al. (2008) G3, doi: 10.1029/2008GC002012. [3] Drouin, M. et al. (2009) Chem. Geol. 264, 71-88. [4] von der Handt, A. & Hellebrand, E. (2010) AGU Fall Meeting abstract.

Hellebrand, E.; Welsch, B. T.; Hammer, J. E.

2013-12-01

68

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 (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

69

Near-axis heat flow measurements on the northern Juan De Fuca Ridge: Implications for fluid circulation in oceanic crust  

Science.gov (United States)

Present models of the cooling of oceanic crust suggest that convection of hydrothermal fluid is a major component of the process. In axial regions, abundant faults and open fissures are associated with the venting of high temperature hydrothermal fluid. In older crust, where the insulating sediment cover is thick, previous studies have shown that basement topography is the dominant forcing factor for within-crust fluid circulation. In the intermediate region, where young crust is lightly sedimented, heat flow data are difficult to obtain with traditional techniques. To determine whether topography or permeability is the dominant process controlling fluid circulation in the near-axis region, we conducted a profile of heat flow measurements using the submersible ALVIN, on the Endeavour Segment of the Juan de Fuca Ridge. Our data indicate that topographic forcing is responsible for the long wavelength variations, with high heat flow at the ridge summits, and low values in the inter-ridge valleys. The locations of the extreme values of heat flow taken within the context of subsurface faulting are consistent with a model where a ridge-valley topographic pair comprises a single circulation cell. This model predicts that the source area for the high temperature axial vents may be in the flanking inter-ridge valleys.

Johnson, H. Paul; Becker, Keir; Von Herzen, Richard

1993-09-01

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

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

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

S. L. Bazhenov

2001-01-01

72

Coupling of Oceanic and Continental Crust During Eocene Eclogite-Facies Metamorphism: Evidence From the Monte Rosa Nappe, Western Alps, Italy  

Science.gov (United States)

Subduction of continental crust to HP-UHP metamorphic conditions requires overcoming density contrasts that are unfavorable to deep burial, whereas exhumation of these rocks can be reasonably explained through buoyancy-assisted transport in the subduction channel to more shallow depths. In the western Alps, both continental and oceanic lithosphere has been subducted to eclogite-facies metamorphic conditions. The burial and exhumation histories of these sections of lithosphere bear directly on the dynamics of subduction and the stacking of units within the subduction channel. We address the burial history of the continental crust with high precision U-Pb rutile and Lu-Hf garnet geochronology of the eclogite-facies Monte Rosa nappe (MR), western Alps, Italy. U-Pb rutile ages from quartz-carbonate-white mica-rutile veins that are hosted within eclogite and schist of the MR, Gressoney Valley, Italy, indicate that it was at eclogite-facies metamorphic conditions at 42.6 +/- 0.6 Ma. The sample area (Indren glacier, Furgg zone; Dal Piaz, 2001) consists of eclogite boudins that are surrounded by micaceous schist. Associated with the eclogite and schist are quartz-carbonate-white mica-rutile veins that formed in tension cracks in the eclogite and along the contact between eclogite and surrounding schist. Intrusion of the veins occurred at eclogite-facies metamorphic conditions (480-570°C, >1.3-1.4 GPa) based on textural relations, oxygen isotope thermometry, and geothermobarometry. Lu-Hf geochronology of garnet from a chloritoid-talc-garnet-phengite-quartz-calcite-pyrite - chalcopyrite bearing boudin within talc-chloritoid whiteschists of the MR, Val d'Ayas, Italy (Chopin and Monie, 1984; Pawlig, 2001) yields an age of 40.54 +/- 0.36 Ma. The talc-chloritoid whiteschists from the area record pressures and temperatures of 1.6-2.4 GPa and 500-530°C (Chopin and Monie, 1984; Le Bayon et al., 2006) indicating near UHP metamorphic conditions. Based on the age, P-T, and textural data, the rutile age likely represents the prograde-leg of the eclogite-facies P-T path whereas the Lu-Hf garnet age likely represents higher grade metamorphic conditions. The timing of eclogite-facies metamorphism in the MR is within the same time interval as the duration of prograde metamorphism (~55-40) recorded in the structurally overlying Zermatt-Saas ophiolite (ZSO; e.g., Amato et al., 1999; Lapen et al., 2003; Mahlen et al., this meeting). In particular, the Lu-Hf garnet age from the MR is identical within error to a relatively young 40.8 +/- 1.8 Ma Lu-Hf garnet-whole rock-cpx age from a structurally low slice of the ZSO at Saas-Fee, Switzerland (Mahlen et al., this meeting). Not only do the ages of eclogite-facies metamorphism overlap between the MR and ZSO, but so do the P-T conditions (e.g., between 1.6-2.8 GPa; 500-600°C). These data, combined with the relative structural positions of the MR and ZSO in the western Alps, suggest that the MR and ZSO were likely juxtaposed within the subduction channel through underplating of the MR beneath the ZSO. The strong negative buoyancy of the MR has likely aided in the exhumation of sections of the ZSO. Therefore, coupling of continental and oceanic terranes in a subduction channel, perhaps a general feature in the western Alps, may be critical in preventing permanent loss of oceanic crust to the mantle.

Lapen, T. J.; Johnson, C. M.; Baumgartner, L. P.; Skora, S.; Mahlen, N. J.; Beard, B. L.

2006-12-01

73

Alteration of the upper oceanic crust, DSDP site 417: mineralogy and chemistry  

Science.gov (United States)

Basalts from DSDP Site 417 (109 Ma) exhibit the effects of several stages of alteration reflecting the evolution of seawater-derived solution compositions and control by the structure and permeability of the crust. Characteristic secondary mineral assemblages occur in often superimposed alteration zones within individual basalt fragments. By combining bulk rock and single phase chemical analyses with detailed mineralogic and petrographic studies, chemical changes have been determined for most of the alteration stages identified in the basalts. 1) Minor amounts of saponite, chlorite, and pyrite formed locally in coarse grained portions of massive units, possibly at high temperatures during initial cooling of the basalts. No chemical changes could be determined for this stage. 2) Possible mixing of cooled hydrothermal fluids with seawater resulted in the formation of celadonite-nontronite and Fe-hydroxide-rich black halos around cracks and pillow rims. Gains of K, Rb, H2O, increase of Fe3+/FeT, and possibly some losses of Ca and Mg occurred during this stage. 3a) Extensive circulation of oxygenated seawater resulted in the formation of various smectites, K-feldspar, and Fe-hydroxides in brown and light grey alteration zones around formerly exposed surfaces. K, Rb, H2O, and occasionally P were added to the rocks, Fe3+/FeT increased, and Ca, Mg, Si and occasionally Al and Na were lost. 3b) Anoxic alteration occurred during reaction of basalt with seawater at low water-rock ratios, or with seawater that had previously reacted with basalt. Saponite-rich dark grey alteration zones formed which exhibit very little chemical change: generally only slight increases in Fe3+/FeT and H2O occurred. 4) Zeolites and calcite formed from seawater-derived fluids modified by previous reactions with basalt. Chemical changes involved increases of Ca, Na, H2O, and CO2 in the rocks. 5) A late stage of anoxic conditions resulted in the formation of minor amounts of Mn-calcites and secondary sulfides in previously oxidized rocks. No chemical changes were determined for this stage. Recognition of such alteration sequences is important in understanding the evolution of submarine hydrothermal systems and in interpreting chemical exchange due to seawater-basalt reactions.

Alt, Jeffrey C.; Honnorez, Jose

1984-09-01

74

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

Science.gov (United States)

The major element relationships in ferromanganese (Fe-Mn) crusts from Afanasiy-Nikitin seamount (ANS), eastern equatorial Indian Ocean, appear to be atypical. High positive correlations (r = 0.99) between Mn/Co and Fe/Co ratios, and lack of correlation of those ratios with Co, Ce, and Ce/Co, indicate that the ANS Fe-Mn crusts are distinct from Pacific seamount Fe-Mn crusts, and reflect region-specific chemical characteristics. The platinum group elements (PGE: Ir, Ru, Rh, Pt, and Pd) and Au in ANS Fe-Mn crusts are derived from seawater and are mainly of terrestrial origin, with a minor cosmogenic component. The Ru/Rh (0.5-2) and Pt/Ru ratios (7-28) are closely comparable to ratios in continental basalts, whereas Pd/Ir ratios exhibit values ( 0.75) correlations between water depth and Mn/Co, Fe/Co, Ce/Co, Co, and the PGEs. Fractionation of the PGE-Au from seawater during colloidal precipitation of the major-oxide phases is indicated by well-defined linear positive correlations (r > 0.8) of Co and Ce with Ir, Ru, Rh, and Pt; Au/Co with Mn/Co; and by weak or no correlations of Pd with water depth, Co-normalized major-element ratios, and with the other PGE (r < 0.5). The strong enrichment of Pt (up to 1 ppm) relative to the other PGE and its positive correlations with Ce and Co demonstrate a common link for the high concentrations of all three elements, which likely involves an oxidation reaction on the Mn-oxide and Fe-oxyhydroxide surfaces. The documented fractionation of PGE-Au and their positive association with redox sensitive Co and Ce may have applications in reconstructing past-ocean redox conditions and water masses.

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

2007-01-01

75

Towards a complete in situ section of upper oceanic crust formed at a superfast spreading rate: Deep Drilling in Hole 1256D  

Science.gov (United States)

The Superfast Spreading Rate Crust mission is a multileg program to drill, for the first time, a complete section of the upper oceanic crust from extrusive lavas, through the dikes, and into the underlying gabbros. Hole 1256D was initiated on ODP Leg 206 in the eastern equatorial Pacific and is drilled into 15 Ma crust that formed at the East Pacific Rise during a period of superfast spreading (>200 mm/a). This site is chosen to exploit the inverse relationship between spreading rate and the depth to axial low velocity zones, thought to be magma chambers now frozen as gabbros, observed from seismic experiments. IODP Expedition 309 successfully deepened Hole 1256D to a total depth of 1255 meters below seafloor (mbsf, 1005 m sub-basement) having penetrated through >800 m of extrusive N-MOR basalts and entered a region dominated by intrusive rocks with numerous subvertical chilled margins. The uppermost crust at Site 1256 comprises a >74 m-thick ponded lava flow overlying massive, sheet and minor pillow flows, some of which exhibit inflation structures requiring eruption onto a subhorizontal surface. This suggests a total thickness of off-axis lavas of 284 m. Sheet and massive lava flows make up the remaining extrusive section (534 to 1004 mbsf) above subvertical cataclastic zones, intrusive contacts and spectacular mineralized breccias denoting a lithologic transition zone. The extrusive lavas are less hydrothermally altered than other basement sites (e.g., Sites 417/418, Holes 504B, 896A) and there is no systematic change with depth from oxidizing to reducing seawater alteration. Instead, oxidizing alteration occurs irregularly most commonly associated with steeply dipping vein networks. Below 1061 mbsf, sheeted intrusives are dominated by massive basalts, some with doleritic textures, cross-cut by numerous subvertical dikes commonly with brecciated and mineralized chilled margins. These rocks are altered under greenschist facies hydrothermal conditions, and have significantly higher thermal conductivities and p-wave velocities. IODP Expedition 309 exited Hole 1256D cleanly and the hole is in excellent condition and ready for deepening. At 1255 mbsf Hole1256D is tantalizingly close to the minimum estimated depth for the frozen axial magma chamber predicted to be 1275 to 1525 mbsf. IODP Expedition 312 will return to this site in late 2005 and despite the gruelling 15 m per day pace of advance and assuming further benign drilling conditions, is set to deepen Hole 1256D by a further 500 m. The total depth would then be well beyond where geophysical interpretations predict gabbros to occur.

Teagle, D. A.; Umino, S.; Alt, J. C.; Miyashita, S.; Wilson, D. S.; Banerjee, N.

2005-12-01

76

Ocean Circulation and Gateway Closures During the Late Miocene (~13-5 Ma)  

Science.gov (United States)

Long-term climate change is driven by tectonic influences, including changes in ocean circulation that are the result of ocean gateway closure. During the middle to late Miocene (~13-5 Ma), both tropical ocean circulation and deep water production were reorganized due to the increasing constriction of the Indonesian and Central American seaways. For example, the waters of the modern Pacific equatorial current system do not move freely into the Indian Ocean (i.e., via the Indonesian Throughflow, ITF) but instead pile up to form the Western Pacific Warm Pool (a thermal anomaly that greatly influences tropical Pacific climate and ocean circulation). Here we use a continuous record of multispecies stable isotope stratigraphy and foraminiferal assemblage counts from Ontong Java Plateau to demonstrate that during middle to late Miocene time, progressive restriction of the ITF, modulated by sea level fluctuations, resulted in the waxing and waning of a proto-warm pool in the western equatorial Pacific (WEP). The proto-warm pool profoundly affected the early late Miocene "carbonate crash" (an anomalous decrease of carbonate in deep sea sediments) and the late Miocene "biogenic bloom" (sharp increase in carbonate accumulation rates across the tropical Indo-Pacific). We hypothesize that El Niño/La Niña-like alternations of tropical carbonate preservation and productivity between the western and eastern equatorial Pacific during the late Miocene were the consequence of early warm pool development and decay. A proto-warm pool was formed ~12.1-10.6 Ma, which initiated a nutrient-rich Equatorial Undercurrent and/or increased Trade Wind strength. These La Niña-like conditions sustained carbonate productivity in the eastern equatorial Pacific (EEP) at a time when carbonate preservation sharply declined in the Caribbean. Proto-warm pool weakening and El Niño-like conditions ~10.6-8.8 Ma intensified a "carbonate crash" in the EEP, while resurgence of the warm pool and La Niña-like conditions after ~6.5 Ma spurred a "biogenic bloom". The production of deep water in the northern North Atlantic (i.e., Northern Component Water, NCW) may also have been modulated by sea level fluctuations as the Central American Seaway became increasingly constricted by the uplift of the Panama sill during the late Miocene. We suggest that the sea level fluctuations that facilitated the early development of a proto-warm pool in the WEP, particularly the Mi5 event at 11.4 Ma, also constricted flow through the Central American Seaway and controlled NCW production at this time in the North Atlantic.

Nathan, S. A.; Leckie, R. M.

2004-12-01

77

In-situ Carbonation of Magnesium Silicates: an Experimental Investigation of the Sequestration Potential of Oceanic Crust  

Science.gov (United States)

A promising solution to the problem of anthropogenic greenhouse carbon is through carbonation of ultramafic minerals. Investigations of the carbonation potential of magnesium silicate minerals in industrial settings have been conducted, but the process is energy intensive and expensive. Direct injection of carbon dioxide into seafloor hydrothermal systems that are rich in magnesium silicate minerals may provide an alternate, viable pathway for sequestering carbon dioxide. Experiments were conducted on seawater-oceanic crust-carbon dioxide systems to simulate injection of carbon dioxide into a seafloor hydrothermal system and determine the extent of fluid-rock reaction. The solid reactant was comprised of 71.4% olivine, 18.4% diopside, and 10.2% enstatite. This proxy for oceanic crust was reacted at 300 C and 500 bar in a synthetic seawater solution to approach steady state, then injected with supercritical carbon dioxide and reacted for 550 hours. Three mole percent of carbon dioxide relative to water was injected into the experimental system. The experimental pressure decreased 17 bars in three hours following carbon dioxide injection due to initial dissolution and mineralization of carbon dioxide. The pressure decreased an additional 20 bars due to mineralization during the remainder of the experiment. Approximately 20% of injected carbon dioxide was mineralized, and the remainder was dissolved into the aqueous fluid. Brine-rock reaction decreased pH from 7.4 to 5. Aqueous calcium was consumed following carbon dioxide injection, whereas magnesium and silica concentrations increased due to increased brine acidity. Mineral reactants dissolved, as indicated by surface pits and etching on mineral fragments and the increase of aqueous magnesium and silica concentration. Mineral reactants were serpentinized, and then reacted to talc and magnesite following carbon dioxide injection. Magnesite comprised approximately 30 weight % of the reaction products. Carbon dioxide dissolution and attendant magnesite precipitation indicates that direct injection of carbon dioxide into oceanic crust may be a viable means of sequestering anthropogenic carbon.

Carpenter, T. M.; Kaszuba, J. P.

2007-12-01

78

Density contrasts in the upper mantle and lower crust across the continent-ocean transition: constraints from 3-D gravity modelling at the Norwegian margin  

Science.gov (United States)

3-D gravity models that consider the highly complex density structure of the Norwegian passive volcanic continental margin require a significantly lower density in the mantle below the ocean than below the continent. This is especially pronounced on the crust-free residual gravity anomalies obtained by 3-D gravity stripping. This density contrast in the mantle indicates different physical properties either related to thermal or compositional conditions beneath the two domains. We assess the gravity response of thermal differences and find that a hotter oceanic mantle compared to a colder continental mantle could cause the density contrast. Accordingly, 3-D lithospheric-scale gravity models considering temperature-dependent mantle densities indicate that the transition between continental and oceanic lithosphere is rather sharp with a steep gradient in thickness beneath the continent-ocean transition zone. Furthermore, a mantle wedge of reduced density extends from the oceanic Jan Mayen Fracture Zone to the continental Jan Mayen Lineament and points to increased mantle temperatures and/or compositional changes beneath the lineament. The models further indicate the presence of high-density bodies within the lower crust at the continent-ocean transition that are continuous with the lowermost oceanic layer 3B in terms of geometry and density. This supports the concept of underplated lower crust beneath the western Norwegian margin. Finally, NE-SW trending linear zones of increased density are required in the lower crust beneath the eastern Vøring and Møre Basins to fit the short-wavelength gravity signal.

Maystrenko, Yuriy; Scheck-Wenderoth, Magdalena

2009-10-01

79

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)

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

80

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

81

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

Science.gov (United States)

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

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

2011-11-01

82

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

83

The Igneous Architecture of IODP Hole U1309D: Constructing Oceanic Crust from Multiple Sills  

Science.gov (United States)

Gabbroic rocks comprise a significant portion of the footwall of many oceanic core complexes. The decreasing age of these gabbros from the breakaway to the termination suggests that they are continuously accreted as the bounding detachment fault slips. But, questions remain; how are these large (>1 km diameter) gabbroic bodies constructed and at what scale and frequency is melt added to the system? Here we report a detailed lithologic analysis of IODP Hole U1309D drilled into the Atlantis Massif core complex (30° N, MAR). We present new, high resolution (1m scale) downhole lithological diagrams, compiled from observations of the archive core, refined using other available data including magnetic susceptibility. These data show that the thickness of the individual magmatic units is on the order of 1 to a few 10’s of m. Contacts between many of these units are sharp; many of the units are interpreted to result from small injections of melt as opposed to in-situ fractionation. However, these units are often disaggregated by later intrusive bodies and hence, it is likely that individual gabbro units were initially somewhat thicker. Downhole plots of magmatic fabric dip from both shipboard data and new electron backscatter diffraction (EBSD) data are consistent with the units having initially been intruded as sills, and subsequently rotated by ~40-50°, as constrained by paleomagnetic data (Morris et al., 2009). Using all of the available data, we propose the IODP Hole 1309D gabbro section is a composite body that grew episodically by relatively small (10’s m) repeated sill-like injections of melt. We conclude that the melt lens that formed these gabbros was relatively small at any one time. The EBSD data also provide textural constraints on the model for crustal accretion. Gabbroic samples analyzed so far show a moderate to weak plagioclase and clinopyroxene foliation, similar to those in continental mafic intrusions and are thus interpreted to be magmatic fabrics. J-indices for plagioclase a-axes range between 1.28-1.34, and between 1.26-1.29 for the b-axes. These values are comparable to those for the Dufek Intrusion, Antarctica (1.12-1.39 for plagioclase a-axes and 1.18-1.84 for plagioclase b-axes). There is little evidence for significant deformation, which might be expected if the sills were emplaced at one depth and then underwent gabbro-glacier flow to generate the several km thick gabbro crust. However, bent and kinked grains and deformation twins are commonly found in the coarser grained gabbros. These fabrics are likely produced as later sills intrude recently crystallized gabbros. Grimes et al. (2008) uses U-Pb zircon dating to show that the ~1.5km long borehole records a 200 ky emplacement history. This age range permits an estimate of the length-scale over which the sills were emplaced. Assuming the sills were intruded whilst the detachment fault footwall was moving (18mm/yr), then the present day distribution of the dated igneous units down the core implies that the intrusive activity occurred over a depth of ~3.6km. Therefore, we envision that the gabbros accreted as a series of multiple sills at 6-10km below the seafloor.

Christofferson, C. A.; John, B. E.; Cheadle, M. J.; Swapp, S. M.; Grimes, C. B.

2010-12-01

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

The association of continental crust rocks with ophiolites in the Northern Apennines (Italy): implications for the continent-ocean transition in the Western Tethys  

Science.gov (United States)

The Late Cretaceous sedimentary melanges from the External Liguride Units of the Northern Apennines include large slide-blocks of ophiolites and lower and upper continental crust rocks representative of a continent-ocean transition between the Internal Liguride oceanic domain and the thinned continental margin of the Adria plate. The slide-blocks preserve a record of the long-lived history of rifting which led to opening of the Jurassic Western Tethys Basin. The External Liguride ophiolites consist of: (1) undepleted spinel-peridoties, partly re-equilibrated under plagioclase-facies conditions, which were interpreted as unroofed subcontinental mantle; (2) rare gabbroic rocks (mainly troctolite to olivine-bearing gabbro) probably crystallised from N-MORB magmas; and (3) basalts with N- to T-MORB affinity covered by late Callovian-early Oxfordian radiolarian cherts. Both gabbroic rocks and basalts locally intrude the mantle peridotites and postdate their re-equilibration to plagioclase-facies conditions. The slide-blocks of lower continental crust are composed of gabbro-derived mafic granulites and felsic granulites. The latter include quartzo-feldspathic granulites and rare quartz-poor to quartz-free charnockitic rocks. In both mafic and felsic granulites, granulite-facies re-equilibration was followed by a retrograde metamorphic evolution to amphibolite-, greenschist- and subgreenschist-facies conditions. Retrogression is commonly accompanied by deformations progressively changing from plastic to brittle. The upper crustal rocks occurring as slide-blocks consist of Hercynian granitoids with orogenic affinity, mainly biotite-bearing granodiorites and peraluminous two-mica leucogranites. Locally, the granitoids are intruded by basaltic dykes or capped by basaltic flows and radiolarian cherts. The granitoids underwent polyphase brittle deformations under subgreenschist-facies conditions which predated the basalt emplacement. The tectono-metamorphic evolution recorded by the slide-blocks of the External Liguride Units started in the Late Carboniferous-Early Permian (about 290 Ma), with the emplacement at deep crustal levels of the gabbroic protoliths for the mafic granulites. The associated felsic granulites likely represent the remnants of the lower continental crust intruded by the gabbro-derived granulites. Mafic and felsic granulites subsequently underwent tectonic exhumation in Permo-Triassic times, as testified by the development of granulite- to amphibolite-facies ductile shear zones. The granulites were finally exhumed to shallow levels, probably in association with the subcontinental mantle, in Late Triassic-Middle Jurassic times. The latter period was most likely characterized by extensive brittle faulting at shallow crustal levels, thus giving rise to extensional allochthons formed by stretched slices of granitoids. The Western Tethys opening is finally testified by the basalt intrusion and effusion in the Late Jurassic, followed by deep-sea pelagic sedimentation. The External Liguride crustal stratigraphy can be regarded as a fossil example of the transitional realm at the continent-ocean boundary. This reconstruction fits well with the available data on the present-day continental margins derived from passive lithosphere stretching.

Marroni, M.; Molli, G.; Montanini, A.; Tribuzio, R.

1998-06-01

86

Reactive transport modeling of hydrothermal circulation in oceanic crust: effect of anhydrite precipitation on the dynamics of submarine hydrothermal systems  

Science.gov (United States)

Hydrothermal fluid circulation represents an extremely efficient mechanism for the exchange of heat and matter between seawater and oceanic crust. Precipitation and dissolution of minerals associated with hydrothermal flow at ridge axes can alter the crustal porosity and permeability and hence influence the dynamics of hydrothermal systems. In this study, a fully coupled fluid flow, heat transfer and reactive mass transport model was developed using TOUGHREACT to evaluate the role of mineral precipitation and dissolution on the evolution of hydrothermal flow systems, with a particular attention focused on anhydrite precipitation upon heating of seawater in recharge zones and the resultant change in the crustal porosity and permeability. A series of numerical case studies were carried out to assess the effect of temperature and aqueous phase inflow concentrations on the reactive geochemical system. The impact of chemically induced porosity and permeability changes on the dynamics of hydrothermal systems was also addressed.

Yang, J.

2009-12-01

87

Thermal modelling of a transform-divergent interaction zone, the Demerara Plateau, French Guiana margin: architecture of oceanic and continental crusts  

Science.gov (United States)

The crustal architecture of passive margins is a key to constrain their origin and subsequent evolution, as well as their thermal subsidence. The square shaped continental Demerara Plateau, French Guiana margin, surmounts Central and Equatorial Atlantic oceanic crusts surrounding it. Bounded to the northeast by a WNW-ESE-trending transform fault segment and to both the west and the east by N-S divergent fault segments, the Demerara Plateau is a complex transform-divergent interaction zone. The aim of this study is to refine the crustal architecture of this region as derived from gravity and seismic data, by thermal modelling, and by using surface heat flow data as an additional constraint. Previous studies show that the transform transition domain from continental to oceanic crust occurs across a region of approximately 70-km wide, where the Moho deepens abruptly from 25-27 km beneath the plateau (thinned continental crust), to 11-12 km in the abyssal oceanic domain (3-4 km thick oceanic crust). During the IGUANES cruise (onboard R/V L'Atalante in 2013) 10 surface heat flow measurements crossing the plateau have been carried out. These data are combined with borehole heat flows values around. Measures indicate that surface heat flow values range between 47 and 80 mW/m2 (with an uncertainty on the measurements of ~4mW/m2 on average), and slightly decreases in the continental domain toward the ocean. Preliminary 1D thermal modelling results indicate that these heat flow values are consistent with crustal and sediment thicknesses observed on the Plateau. Along the transform domain, at the transition towards the oceanic crust, heat flow values are lower than model results, if we consider an oceanic crust of more than hundred million years and with a thickness of around 3-4 km. We examine, using a 2D approach, whether this low heat flow could be reasonably accounted for by thermal exchange between oceanic and continental lithospheres.

Grall, Céline; Marcaillou, Boris; Loncke, Lies; Mercier de Lepinay, Marion; Basile, Christophe; Roest, Walter R.; A. M Van Wees, Jan Diederik; A. P. L Cloetingh, Sierd

2014-05-01

88

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

89

Carbonate formation in the ocean crust as a proxy for water-rock interactions  

OpenAIRE

The main objective of this thesis was to elucidate the authigenesis of carbonate minerals in modern and Devonian ocean-floor volcanic rocks and to demonstrate that Late Devonian (Frasnian) pillow basalts from the Saxothuringian zone once harbored microbial life. The ultramafic-hosted Logatchev hydrothermal field (LHF) at the Mid-Atlantic Ridge, the Arctic Gakkel Ridge (GR) and the Late Devonian Frankenwald feature carbonate precipitates (aragonite, calcite, dolomite) in voids and fractures of...

Eickmann, Benjamin

2009-01-01

90

Petrogenesis of alkaline basalts from Socorro Island, Mexico: Trace element evidence for contamination of ocean island basalt in the shallow ocean crust  

Science.gov (United States)

Postcaldera mafic volcanism on Socorro Island, Mexico, an island located in the eastern Pacific Ocean on a segment of a mid-ocean ridge spreading center abandoned at ˜3.5 Ma, dominantly comprises alkalic basalt, hawaiite, and mugearite with subordinate mildly alkalic-transitional basalt. Most major element data are consistent with differentiation of alkalic basalt to hawaiite and mugearite by up to 50% fractionation of plagioclase+clinopyroxene+olivine±Fe-Ti oxides. High-field strength element abundances are consistent with this interpretation, and the narrow range in Zr/Nb suggests that parental magmas formed by similar degrees of partial melting of a relatively homogeneous mantle source. Sr and Nd isotopic ratios exhibit relatively narrow ranges (0.7031-0.7032, 0.5128-0.5130, respectively). Despite this, a subset of the alkalic basalts has negative Ce anomalies and abundances of P2O5, Ba, Y, and some rare earth elements in excess of those predicted by fractional crystallization models; these chemical features require open-system processes. The observed heterogeneities are perhaps best explained by assimilation of ocean crustal components by basalt in a shallow magma reservoir. Regional mantle heterogeneity is evident from elemental and isotopic data for mildly alkalic-transitional basalt from Socorro Island, mugearite from San Benedicto Island, and submarine basalts from near Socorro Island. Evidence presented here indicates that compositional variations in ocean island basalts can be introduced by ocean crustal contamination and underscores the importance of considering this source of chemical heterogeneity in basalts before characterizing associated mantle.

Bohrson, Wendy A.; Reid, Mary R.

1995-12-01

91

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

92

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

Science.gov (United States)

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

93

Gabbroic xenoliths from La Palma, Tenerife and Lanzarote, Canary Islands: evidence for reactions between mafic alkaline Canary Islands melts and old oceanic crust  

Science.gov (United States)

Gabbroic and hornblendite xenoliths from La Palma, Tenerife and Lanzarote fall into three main groups based on petrography and chemistry. One group (comprising all xenoliths from Lanzarote and some from La Palma) consists of highly deformed orthopyroxene-bearing gabbroic rocks that show a strong affinity to N-MORB and oceanic gabbro cumulates in terms of mineral chemistry and REE relations. However, they show mild enrichment in the most incompatible elements (particularly Rb+Ba±K) relative to intermediate and heavy REE, and their Sr-Nd isotope ratios fall within or close to the N-MORB field. The second group (60% of the xenoliths from La Palma) are gabbroic cumulates with zoned clinopyroxenes (Ti-Al-poor cores, Ti-Al-rich rims) and reaction rims of hornblende, biotite and clinopyroxene on other phases. Their trace-element and Sr-Nd isotope relations are in general transitional between N-MORB cumulates and Canary Islands alkali basalts, but they show strong enrichment in Rb, Ba and K relative to other strongly incompatible elements. The third group (comprising some xenoliths from La Palma and all those from Tenerife) are undeformed gabbroic and hornblendite rocks in which hornblende and biotite appear to belong to the primary assemblage. These rocks show strong affinities to Canary Islands alkali basaltic magmas with respect to mineral, trace-element, and Sr-Nd isotope chemistry. The first two groups are interpreted as fragments of old oceanic crust which have been mildly to strongly metasomatized through reactions with Canary Islands alkaline magmas. The reaction process is a combination of enrichment in elements compatible with biotite (and hornblende), and simple mixing between N-MORB cumulates and trapped alkaline magmas. The third group represents intrusions/cumulates formed from mafic alkaline Canary Islands magmas. Modeling indicates that locally up to 50% new material has been added to the old oceanic crust through reactions with ocean island basalts. Reactions and formation of cumulates do not represent simple underplating at the mantle/crust boundary, but have taken place within the pre-existing oceanic crust, and are likely to have significantly thickened the old oceanic crust.

Neumann, E.-R.; Sørensen, V. B.; Simonsen, S. L.; Johnsen, K.

2000-12-01

94

Long-distance fluid and heat transport in the oceanic crust entering the Nankai subduction zone, NanTroSEIZE transect  

Science.gov (United States)

I examine the potential causes of anomalous seafloor heat flux on the oceanic plate in the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) transect offshore southern Japan. The most prominent anomaly is a ˜50 mW m change in heat flux between Integrated Ocean Drilling Program Sites C0011 and C0012 over a distance of thermal models to investigate the effects of hydrothermal circulation in the basaltic basement of the oceanic crust and variations in heat input from the mantle. The input from ?5 km depth would generate a >30 km wide transition at the seafloor. The observed surface heat flux pattern is indicative of hydrothermal circulation in the basement aquifer and advection of heat from the subducted crust into the aquifer on the incoming plate. For a 600 m thick aquifer, the permeability is likely ?7×10-11 m, and hydrothermal circulation transports at least 300 times more heat than conduction alone. The heat flux from the subduction zone seaward to the incoming plate is consistent with hydrothermal circulation in the subducting crust persisting to ˜100 km landward of the deformation front. Vigorous fluid circulation in the basaltic basement is consistent with both the seafloor thermal anomalies and geochemical anomalies near the sediment-basement interface.

Spinelli, Glenn A.

2014-03-01

95

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)

96

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

97

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

98

Reconstructing the carbonate compensation depth from 0 to 100 Ma using ocean ion concentrations and bathymetry models  

Science.gov (United States)

The oceans play an important part in regulating the carbon cycle and climate system, acting as a buffer between the carbon in the atmosphere and the deep earth. Of all dissolved inorganic carbon (DIC) in the ocean, only carbonate can exist in a solid state (mostly as calcite). In the near-surface ocean, calcite is supersaturated and thus precipitates. Deeper in the ocean, the solubility of calcite increases and all is entirely dissolved at the carbonate compensation depth (CCD), where the rate of falling carbonate equals the rate of dissolution. The CCD today is around 4.5 km depth, though previous work that looked at the composition of sediments on the ocean floor has suggested that CCD was different in the past (e.g. Pälike et al., 2012; Sclater et al., 1977). These studies mostly show the CCD decreasing to shallower depths through the Cenozoic and the Mesozoic. The deepening of the CCD through time is consistent with the decrease in atmospheric CO2 over time shown in the GEOCARB models (Berner, 1987; Berner and Kothavala, 2001; Berner, 2006); more carbon is being stored in the ocean as sediment. We look at the evolution of the CCD since 100 Ma by focusing on changes in the volume of the ocean basins. We combine recent advancements in determining palaeobathymetry into the Mesozoic from reconstructed ages of the ocean floor (Müller et al., 2008) in conjunction with a geochemical model by Boudreau et al. (2010) for the average CCD today, applying it from 0 to 100 Ma. A history of global ocean ion concentrations produced by Tyrrell and Zeebe (2004) was used. Various assumptions about productivity rates, solubility constants and other conditions in the past oceans were necessary to make a first order working model. The model was found to be very sensitive to even minor changes in the dissolved concentration of carbonate. In the reconstruction where the surface saturation state was decreased going back to 100 Ma, the CCD gradually deepens with time, consistent with other independent studies. Changes in CO2 concentrations likely influenced this, which would have affected the amount of silicate weathering from continents (Misra and Froelich, 2012). We will show maps of the extent of the global carbonate cover for the last 100 my, which suggest that the amount of sedimentary carbon being subducted has increased with time, despite an overall decrease in volcanic activity since the Mesozoic.

Davis, Joel; Lithgow-Bertelloni, Carolina

2014-05-01

99

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

Directory of Open Access Journals (Sweden)

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

S. Bhattacharya

2012-11-01

100

Relationship between depth and age in the North Pacific Ocean  

OpenAIRE

The North Pacific contains active mid-oceanic ridges and the oldest, Jurassic (166.8 ± 4 Ma), drilled oceanic crust. Its bathymetry is therefore critical to studies of the applicability of thermal contraction models (e.g., the infinite half-space and cooling plate) to the subsidence of seafloor with crustal age. The bathymetry, however, contains seamounts and oceanic islands (e.g., Mid-Pacific Mountains), oceanic plateaus (e.g., Hess, Magellan, and Shatsky), and midplate topographic swells (...

Hillier, Jk; Watts, Ab

2005-01-01

101

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

102

Formation of the lower ocean crust and the crystallization of gabbroic cumulates at a very slowly spreading ridge  

Science.gov (United States)

Ocean Drilling Program Hole 735B was extended to 1508 m below the sea floor during Leg 176, atop a shallow bank near Atlantis II Fracture Zone on the very slowly spreading Southwest Indian Ridge. All the drilling was in gabbro, and recovery averaged nearly 87%. The drill penetrated a series of stacked plutons consisting mostly of olivine gabbro, but some with troctolite. Each pluton is some 200-500 m thick, each has its own internally coherent stratigraphy, and each apparently represents an individual event of significant magma inflation and addition to the crust. The entire column was extensively deformed along inclined zones of distributed shear before it was completely frozen, this marking the onset of unroofing of the rocks and their ascent to high rift mountains. The deformation mobilized late-stage melts into flow patterns which led to concentration of ilmenite and magnetite in hundreds of seams of oxide gabbro along or near zones of strong crystal-plastic deformation, the highly differentiated melts overall being concentrated by buoyancy forces toward the top of the section, especially in one zone nearly 70 m thick. However, upward flow was ultimately blocked or deflected by zones of impermeable rock resulting either from downward freezing or grain-size reduction during shear. A melt lens probably did not form at the base of sheeted dikes, as it does at the East Pacific Rise. Despite this, the rocks are cumulates, and most are adcumulates, with very low residual melt porosities. Cumulate theory based on stratiform, layered intrusions does not entirely apply to these rocks. Instead, all gabbros, including the oxide gabbros, crystallized in a dense crystal mush in patterns dominated by fractures, channelized flow, and intergranular porous flow. Most gabbros are not layered; weak modal layering of uncertain origin is present in reversely stably magnetized at a consistent inclination, and it is rotated to the south, perhaps along a curving detachment surface, away from the ridge segment where it formed, by about 20°. It has an intensity of magnetization sufficient to account for the magnetic anomaly observed over the site. Drilling did not reach ultramafic rock, but dredging indicates that peridotite is probably within a few hundred meters of the bottom of the hole. Seismic Moho, however, is placed at 5 km beneath the summit of Atlantis Bank. Much of the rock between Moho and the bottom of Hole 735B must therefore be partially serpentinized peridotite.

Natland, James H.; Dick, Henry J. B.

2001-10-01

103

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

104

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

105

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

106

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

107

Center for Ocean Sciences Education Excellence Mid-Atlantic (COSEE-MA)  

Science.gov (United States)

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

108

Insights on the Formation and Evolution of the Upper Oceanic Crust from Deep Drilling at ODP/IODP Hole 1256D  

Science.gov (United States)

Deep drilling of Hole 1256D on ODP Leg 206 and IODP Expeditions 309/312 provides the first complete section of intact upper oceanic crust down to gabbros. Site 1256 is located on ocean crust of the Cocos Plate that formed at the East Pacific Rise (EPR) 15 million years ago during an episode of superfast rate ocean spreading in excess of 200 mm/yr. Past deep drilling of intact ocean crust has been fraught with difficulties due to the highly fractured nature of oceanic lavas. Site 1256 was specifically chosen because the observed relationship between spreading rate and the depth to axial seismic low velocity zones at modern mid-ocean ridges (thought to be magma chambers), suggests that gabbroic rocks should occur at the shallowest levels in ocean crust formed at the highest spreading rates. In line with pre-drilling predictions, gabbroic rocks were first encountered 1157 m into the basement. Hole 1256D penetrates 754 m of lavas, a 57-m thick transition zone and a thin (346 m) sheeted dike complex. The lower ~60 m of the sheeted dikes are contact metamorphosed to granoblastic textures. After encountering gabbros the hole was deepened a further 100 m before the cessation of drilling operations and the plutonic section comprises two gabbroic sills, 52 and 24 m-thick, intruded into a 24 m screen of granoblastic dikes. The gabbro sills have chilled margins and compositions similar to the overlying lavas and dikes, precluding formation of the cumulate lower oceanic crust from the melt lenses so far penetrated by Hole 1256D. A vertical seismic experiment conducted in Hole 1256D indicates that the bottom of the Hole is still within seismic layer 2 despite gabbroic rocks having been recovered. These data together with 1-D and imaging wire-line logs, have been used to construct a continuous volcano-stratigraphy for Site 1256. Comparison of this data with the recovered cores and the styles of eruption occurring at the modern EPR indicate that ~50% of lava sequences were formed within a few kilometres of the ridge axis, with a further 200 m of lavas that display inflation textures deposited at the base of the axial slope. The great thickness (>75 m) of the ponded lava that makes up the uppermost crust at Site 1256, and an absence of vertical fracturing within those rocks, supports the shipboard interpretation that this unit crystallized a significant distance (~5-10 km) off axis. Geochemical analyses of the Hole 1256D cores have been undertaken to evaluate the melting processes and mantle source heterogeneity under the superfast spreading ancient EPR. Hole 1256D cores have significantly lower incompatible element concentrations than present-day EPR lavas, a signature typically interpreted in terms of greater extents of mantle melting. However, similar crustal thicknesses between Site 1256 (5.5 km) and the EPR (5-7 km) challenge that view. This observation coupled with preliminary radiogenic isotope signatures indicate that the depleted mantle source variation might be caused by upwelling previously depleted Galapagos plume mantle at the paleo-Site 1256. Whole rock and mineral trace element analyses indicate that the gabbros sampled to date cannot be the cumulate rocks remaining after melt extraction. Instead, most of the gabbros represent slowly cooled equivalents of the overlying basaltic rocks. However, some patches of gabbro are magmatically highly evolved, and record late stage melts squeezed out from the crystallizing mush within the melt lens. Secondary mineralogy plus oxygen isotope analyses of vein minerals document the thermal structure of the fossil hydrothermal system penetrated by Hole 1256D, and record a complex history of repeated episodes of magmatism and hydrothermal alteration. The ~800 m volcanic section is partly altered to saponite and celadonite, typical of altered submarine basalts but with little oxidation. There is a stepwise increase in alteration grade across the lava-dike transition and the dikes are variably altered to chlorite and other greenschist minerals (250-350 deg C). The lower 60 m of granoblast

Teagle, D. A. H.

2009-04-01

109

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

110

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

111

A 5 Ma ocean history of the Bering Sea, observed through diatoms  

Science.gov (United States)

Until now, a paucity of marine records from the arctic and subarctic North Pacific has hindered a complete evaluation of the Pacific's role in abrupt climate events. The Bering Sea is a marginal sea in the North Pacific that has experienced, and is sensitive to, major climatic change. Although only insignificant amounts of North Pacific Intermediate Water (NPIW) forms in the Bering Sea today, the basin not only records, but is potentially involved in causing major climate changes. In 2009, IODP Exp. 323 recovered continuous cores from the Bering Sea extending back into the Pliocene. Diatoms are the dominant microfossil group in this region and reveal major ecological shifts throughout the 5 Ma record. Here we present fossil diatom abundance and assemblage records from core U1340A. The onset of Northern Hemisphere Glaciation is clearly depicted by a shift from warm, nutrient-rich, shade flora species Coscinodiscus marginatus and Lioloma pacificum, to the appearance of sea-ice species at ca. 2.65 Ma. Also at this time, Actinocyclus sp. appears and dominates the assemblage until the present during intergalcial periods. This species is correlated with low nutrient, stratified waters and suggests a reduction in nutrient availability in the subsurface waters. Increasing 15N of bulk organic matter values supports the idea that nutrients were less available and more fully utilized. Neodenticula sp., a species transported by the Alaskan Stream, has a strong presence during interglacial intervals and reflects ice-free open waters at the site. The overall decreasing trend of Neodenticula sp., particularly after 1.0 Ma until the Present, highlights increasing winter sea ice cover (glacial intensification) in the Bering Sea in line with the switch to 100 Ka cycles. A Uk'37-based SST record from this site, indicates about 7 degree C of cooling, in agreement with the idea that the flow of relatively warm Alaskan Stream water decreased since 5 Ma. Laminated sections in core 323-U1340 permit, for the first time, insight into Pliocene diatom seasonal dynamics. High resolution analysis, consisting of diatom counts, SEM, BSEI and X-ray imagery will be presented.

Stroynowski, Z.; Ravelo, C.; Caissie, B.

2011-12-01

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

2D Numerical Modeling of the Active Continental Margin High-Pressure Rocks Factory: Tectonic Mixing of Slices of Oceanic and Continental Crust With Hydrated Mantle Rocks  

Science.gov (United States)

Recent studies on high- and ultrahigh pressure metamorphic rocks exhumed from great depth at subduction zones have shown that: (1) these rocks have both continental and oceanic origin and (2) are often associated with hydrated peridototes, (3) the P-T record of these rocks appears to be highly variable, while (4) the time constraints are commonly narrow indicating that exhumation rates can be on the order of plate velocity. Our 2D numerical experiments including the simulation of metamorphic P-T-time paths suggest that one feasible model for the formation and exhumation of high-pressure metamorphic rocks at an active continental margin involves forced flow in a wedge-shaped subduction channel reaching down to depths of >100 km, with a geometry that develops due to progressive hydration of the mantle wedge. The shape of the P-T path of an individual HP metamorphic rock depends on its specific trajectory in the subduction channel. Both clockwise and counterclockwise P-T paths are possible for slices of oceanic and continental crust mixed with hydrated mantle rocks. Rapid near-isobaric cooling (with rates of up to 200 oC/Myr) is, for instance, predicted for both oceanic and continental material subducted at an early stage and accreted to the hangingwall. At a later stage, these rocks may be set free by progressing hydration of the mantle wedge and involved in the subduction channel circulation. An array of diverse, though interrelated, P-T paths rather then a single characteristic trajectory is expected to be characteristic for subduction related high-pressure metamorphic complexes. The proposed scenario reconciles with the metamorphic and structural record in many orogenic belts, where high- and ultrahigh-pressure metamorphic rocks are closely associated with hydrated mantle rocks. The results of our simulations suggest that the internal structure of orogenic belts with exposed high-pressure and ultrahigh-pressure metamorphic units, which feature different maximum P, T conditions and contrasting P-T-histories, may in fact partially represent material that has been extruded into the lower portion of the continental crust from a widening subduction channel to form an extending orogenic root beneath the forearc during ongoing subduction. Notably, our simulations suggest that a significant portion of incorporated continental material can be derived from the front of the upper plate by subduction erosion and need not indicate collision or accretion of continental terranes from the subducted plate. However, exhumation of the high-pressure metamorphic units and their exposure at the surface can be a consequence of later collision, with considerable post-HP-metamorphic deformation and obliteration of the earlier record.

Stoeckhert, B.; Gerya, T.

2002-12-01

114

Large scale obduction of preserved oceanic crust: linking the Lesser Caucasus and NE Anatolian ophiolites and implications for the formation of the Lesser Caucasus-Pontides Arc  

Science.gov (United States)

During the Mesozoic, the Southern margin of the Eurasian continent was involved in the closure of the Paleotethys and opening Neotethys Ocean. Later, from the Jurassic to the Eocene, subductions, obductions, micro-plate accretions, and finally continent-continent collision occurred between Eurasia and Arabia, and resulted in the closure of Neotethys. In the Lesser Caucasus and NE Anatolia three main domains are distinguished from South to North: (1) the South Armenian Block (SAB) and the Tauride-Anatolide Platform (TAP), Gondwanian-derived continental terranes; (2) scattered outcrops of ophiolite bodies, coming up against the Sevan-Akera and Ankara-Erzincan suture zones; and (3) the Eurasian plate, represented by the Eastern Pontides margin and the Somkheto-Karabagh Arc. The slivers of ophiolites are preserved non-metamorphic relics of the now disappeared Northern Neotethys oceanic domain overthrusting onto the continental South Armenian Block (SAB) as well as on the Tauride-Anatolide plateform from the north to the south. It is important to point out that the major part of this oceanic lithosphere disappeared by subduction under the Eurasian Margin to the north. In the Lesser Caucasus, works using geochemical whole-rock analyses, 40Ar/39Ar dating of basalts and gabbro amphiboles and paleontological dating have shown that the obducted oceanic domain originates from a back-arc setting formed throughout Middle Jurassic times. The comprehension of the geodynamic evolution of the Lesser Caucasus supports the presence of two north dipping subduction zones: (1) a subduction under the Eurasian margin and to the south by (2) an intra-oceanic subduction allowing the continental domain to subduct under the oceanic lithosphere, thus leading to ophiolite emplacement. To the West, the NE Anatolian ophiolites have been intensely studied with the aim to characterize the type of oceanic crust which they originated from. Geochemical analyses have shown similar rock types as in Armenia, Mid Ocean Ridge Basalt (MORB) to volcanic arc rocks and Intra-Plate Basalts (IPB). Lithostratigraphic comparisons have shown that the relations between the three units, well identified in the Lesser Caucasus, are similar to those found in NE Anatolia, including the emplacement of stratigraphically conform and discordant deposits. New field data has also shed light on an outcrop of low-grade metamorphic rocks of volcanic origin overthrusted by the ophiolites towards the south on the northern side of the Erzincan basin, along the North Anatolian Fault (NAF). We extend our model for the Lesser Caucasus to NE Anatolia and infer that the missing of the volcanic arc formed above the intra-plate subduction may be explained by its dragging under the obducting ophiolite with scaling by faulting and tectonic erosion. In this large scale model the blueschists of Stepanavan, the garnet amphibolites of Amasia and the metamorphic arc complex of Erzincan correspond to this missing volcanic arc. We propose that the ophiolites of these two zones originate from the same oceanic domain and were emplaced during the same obduction event. This reconstructed ophiolitic nappe represents a preserved non-metamorphic oceanic domain over-thrusting up to 200km of continental domain along more than 500km. Distal outcrops of this exceptional object were preserved from latter collision which was concentrated along the suture zones.

Hassig, Marc; Rolland, Yann; Sosson, Marc; Galoyan, Ghazar; Sahakyan, Lilit; Topuz, Gultelin; Farouk Çelik, Omer; Avagyan, Ara; Muller, Carla

2014-05-01

115

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

116

Dynamic melting of proterozoic upper mantle: Evidence from rare earth elements in oceanic crust of Eastern Newfoundland  

Science.gov (United States)

Rare Earth Element (REE) data confirm earlier suggestions from major and trace elements that the Proterozoic Burin Group in the southwestern Avalon Zone is similar to modern oceanic tholeiites, and also exhibit a systematic evolution through the sequence. The Group forms a 60-km long belt consisting of four formations of pillowed basalt, two of subaqueous volcaniclastic and minor stromatolitic sediments and pyroclastics, and a thick gabbro-quartz diorite sill, with a total thickness of about 5000 m. Basalts of the oldest formation are enriched in light (LREE) with a chondrite-normalized pattern similar to alkali basalts. REE patterns through the rest of the sequence can be matched by those of modern ocean basins, with a steady decrease in total REE and a distinct depletion in LREE at the top of the sequence. REE patterns of the gabbroic sill are similar to those of oceanic gabbros, with LREE depletion and a small positive Eu anomaly. This evolutionary pattern can be interpreted as the result of progressive ‘dynamic’ partial melting and depletion of a single mantle source region.

Strong, D. F.; Dostal, J.

1980-04-01

117

Thorium-derived dust fluxes to the tropical Pacific Ocean, 58 Ma  

Science.gov (United States)

Eolian dust in pelagic deep sea sediments can be used to reconstruct ancient wind patterns and paleoenvironmental response to climate change. Traditional methods to determine dust accumulation involve isolating the non-dissolvable aluminosilicate minerals from deep sea sediments through a series of chemical leaches, but cannot differentiate between minerals from eolian, authigenic and volcanogenic sources. Other geochemical proxies, such as sedimentary 232Th and crustal 4He content, have been used to construct high-resolution records of atmospheric dust fluxes to the deep sea during the Quaternary. Here we use sedimentary Th content as a proxy for terrigenous material (eolian dust) in ˜58 Myr-old sediments from the Shatsky Rise (ODP Site 1209) and compare our results with previous dust estimates generated using the traditional chemical extraction method and sedimentary 4Hecrustal concentrations. We find excellent agreement between Th-based dust estimates and those generated using the traditional method. In addition our results show a correlation between sedimentary Th and 4Hecrustal content, which suggests a source older than present day Asian loess supplied dust to the central subtropical Pacific Ocean during the early Paleogene.

Woodard, Stella C.; Thomas, Deborah J.; Marcantonio, Franco

2012-06-01

118

Progressive growth of the Earth's continental crust and depleted mantle: Geochemical constraints  

Science.gov (United States)

A three reservoir model, consisting of the continental crust, depleted mantle and a more primitive mantle reservoir is used as a basis to account for both the present-day as well as the evolving isotopic compositions of the Earth's crust and mantle. The rate of growth of the continental crust is used as an input parameter to constrain the concomitant growth and evolution of the depleted mantle reservoir. Recycling of large volumes of bulk continental crust into the mantle is not considered to be an important process, nor is the existence of an additional major enriched reservoir in the early Archean mantle. This relatively simple model of progressively growing continental crust extracted from an increasing volume of depleted mantle can account for the positive ?Nd values which characterise the Archean depleted mantle, the evolution of the strontium, neodymium, hafnium and lead isotopic systems as well as the budgets of a wide range of trace elements in the continental crust and depleted mantle; e.g., the nonprimitive Rb/Cs, Nb/U and Th/U observed in MORBs and OIBs as well as the Sm/Nd in the crust and mantle can be reproduced. The Re-Os isotopic system is most sensitive to the formation of basaltic crust in the early Archean and can potentially provide definitive limits on the volumes of stored mafic or ultramafic crust in the mantle. To account for the relatively radiogenic 206Pb /204Pb ratios of modern MORB it is necessary to assume that the overall efficiency of transfer of uranium from the mantle to the crust has decreased markedly since the Archean, a proposed consequence of slab dehydration rather than slab melting. In the post-Archean period, recycling of hydrothermally altered oceanic crust is thus likely to have had a significant influence on the lead isotopic systematics of the mantle. In the model described here it is assumed that the volume of depleted mantle increases in a stepwise manner which is arbitrarily linked to major episodes of rapid crustal formation. From observed crustal age distribution patterns, episodes of rapid crustal formation with high ?Nd values occur at ~3600 Ma, ~2700 Ma and 1800 Ma. Thus, in our first order calculations, the crust is modelled as being extracted from ~10% of the mass of the mantle (upper 220 km) from 4500 Ma to 3600 Ma, ~20% (upper 410 km) from 3600 Ma to 2700 Ma, ~30% (upper 660 km) from 2700 Ma to 1800 Ma and 40% to 50% (upper 800-1000 km) of the mantle from 1800 Ma to the present-day. This type of model with growing volumes of both continental crust and depleted mantle has the general effect of buffering the isotopic and trace element composition of the upper mantle through time to an approximately constant, but incompatible element depleted chemical composition.

McCulloch, Malcolm T.; Bennett, Victoria C.

1994-11-01

119

Evolution of the Indian Ocean Triple Junction between 65 and 49 Ma (anomalies 28 to 21)  

Science.gov (United States)

Reinterpretation of newly published geophysical data (Kamesh-Raju and Ramprasad, 1989) and older profiles of the Central Indian Basin, associated with similar studies of the Madagascar and Crozet basins, shows that the Indian Ocean Triple Junction trace on the Indian plate corresponds, at anomalies 23 and 22, to a N38°E offset of the magnetic lineations, oblique to both the Southeast Indian Ridge (SEIR) and Central Indian Ridge (CIR) spreading directions. The conjugate Triple Junction trace on the African plate identified in the Madagascar Basin is associated with a roughly north-south offset, parallel to the Southwest Indian Ridge (SWIR) fracture zones. In order to account for these observations and the velocity triangle of the Indian, African, and Antarctic plates close to the Triple Junction, a ridge-fault-fault mode is proposed, with a propagatorlike SEIR-CIR offset. The Triple Junction jumped between anomalies 24 and 23 and between anomalies 22 and 21, restoring a ridge-ridge-ridge configuration which immediately turned to a pseudo-ridge-ridge-fault and later to a true ridge-fault-fault configuration. After the Triple Junction jump at anomaly 21, the former SEIR-CIR offset was accommodated by a new CIR fracture zone. The lack of such a fracture zone prior to anomaly 21 suggests that either a pseudo-ridge-ridge-fault or an unstable ridge-ridge-ridge configuration prevailed before anomaly 24, in agreement with the velocity triangles which predict more unstable Triple Junction modes. Both modes support the creation of numerous SWIR fracture zones, presently observed between 52°30'E and 59°30'E, as a consequence of the Triple Junction evolution between anomalies 29 and 24. This result suggests that the physiography of the SWIR records the history of the Triple Junction.

Dyment, JéRôMe

1993-08-01

120

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

121

A geochemical traverse across the North Chilean Andes: Evidence for crust generation from the mantle wedge  

International Nuclear Information System (INIS)

Major and trace element and Sr- and Nd-isotope analyses are presented on 186-0 Ma magmatic rocks along an east-west traverse across North Chile at 220S. ?Sr ranges from -25 to +100 and ?Nd from +6 to -9, but the low ?Nd and high ?Sr values are in rocks generated in the last 15 Ma. It is argued that previous discussions of petrogenesis in North Chile have been hampered because the changes in magma chemistry in this area of unusually thick crust reflect not one, but two processes. One results in a progressive shift of ?Sr from -25 to +20 and ?Nd from +6 to -6 in Jurassic to Recent rocks, which is accompanied by increasing Ta/Sm and Sr decreasing Th/Ta. The second is largely confined to the younger rocks and it is characterised by ?Sr increasing up to +100 with increasing SiO2 and decreasing Sr, and it results in relatively shallow trends on an ?Nd-?Sr diagram. The preferred interpretation is that trend 1 is due to the mobilisation of old, late Proterozoic mantle lithosphere as magmatism migrated eastwards, and that trend 2 is due to crustal melting and contamination with differentiation in this area of thickened continental crust. It follows that the mantle wedge is the principal site of crust generation, and it is argued that <20% of the Sr in the recent northern Chile rocks is derived from the subducted ocean crust. (orig.)n crust. (orig.)

122

Subsurface structure of a submarine hydrothermal system in ocean crust formed at the East Pacific Rise, ODP/IODP Site 1256  

Science.gov (United States)

ODP/IODP Hole 1256D penetrates an in situ section of ocean crust formed at the East Pacific Rise, through lavas and sheeted dikes and ˜100 m into plutonic rocks. We use mineralogy, oxygen isotopes, and fluid inclusions to understand hydrothermal processes. The lavas are slightly altered at low temperatures (solutions in the lavas, also producing minor metal sulfide mineralization in the upper dikes. Alteration grade increases downward in the dikes, with increasing recrystallization to amphibole and loss of metals at higher temperatures (>350°C up to ˜600°C). Intrusion of gabbro bodies into the lower dikes resulted in contact metamorphism to granoblastic hornfels at 850°C-900°C, representing a thermal boundary layer between the axial melt lens and the overlying hydrothermal system. Downward penetration of hydrothermal fluids led to rehydration of granoblastic dikes and plutonic rocks at ˜800°C down to 450°C from hydrothermal fluids that were affected by supercritical phase separation. Fluids had variable salinities and were enriched in 18O (+0.4‰ to +3.5‰) relative to seawater, similar to seafloor vent fluids. Dike margins are brecciated and mineralized, suggesting hydrothermal activity coeval with magmatism. Anhydrite formed mainly in the upper dikes when partly reacted seawater fluids were heated as they penetrated deeper into the system. Low-temperature alteration of the volcanic section continued as cold seawater penetrated along fluid pathways, forming minor iron oxyhydroxides in the rocks. Hydrothermal processes at Site 1256 fit with current models whereby greenschist alteration of dikes at low water/rock ratios is overprinted by fracture-controlled alteration and mineralization by upwelling hydrothermal fluids, a conductive boundary layer above gabbroic intrusions, leaching of metals from dikes and gabbros in the deep "root zone," and stepped thermal and alteration gradients in the basement. The Site 1256 section, however, is intact and retains recharge effects (anhydrite), allowing an integrated view of processes in the subsurface.

Alt, Jeffrey C.; Laverne, Christine; Coggon, Rosalind M.; Teagle, Damon A. H.; Banerjee, Neil R.; Morgan, Sally; Smith-Duque, Christopher E.; Harris, Michelle; Galli, Laura

2010-10-01

123

Determination of the volcanostratigraphy of oceanic crust formed at superfast spreading ridge: Electrofacies analyses of ODP/IODP Hole 1256D  

Science.gov (United States)

The objective of this study is to construct a representative volcanostratigraphy of Ocean Drilling Program Hole 1256D, the first complete penetration of intact upper oceanic crust formed at a superfast spreading rate. An accurate knowledge of the volcanostratigraphy is vital to understand processes of crustal construction and submarine magmatism and to estimate chemical exchange with seawater, but this is rarely achieved due to very low recovery rates in most basement holes. We used two approaches to determine the rock types that form the wall rocks in the basement sections of Hole 1256D: (1) user guided interpretations of electrofacies acquired by imaging tools combined with other wireline tools; and (2) the use of an artificial neutral network to objectively classify the responses of all available logging information. Great availability of formation microscanner (FMS) images provided superior coverage of the borehole wall compared to previous attempts at core-log integration. This has resulted in more confident and detailed lithologic classifications, such as with the distinction between pillows and different styles of breciation. Ten lithology types are suggested for a volcanostratigraphy model: massive flows, ponded lava, fractured massive flows, fragmented flows, thin flows or thick pillows, pillows, breccias, dikes in dike complex, isolated dikes, and gabbros. Three major lithology types in the extrusive section are massive flows (both massive and fragmented massive flow, 22%), fragmented flows (32%), and breccias (19%). Pillow lavas make up only 1.9% of the volcanic section and are confined to a 100 m interval. Below the extrusive section, subvertical contacts interpreted to be dike margins are typically observed every 1 to 2 m with brecciated zones along the contacts. The dikes dip steeply to the northeast indicating slight rotation away from the ridge axis. We used an artificial neural network (ANN) approach to determine a quantitative lithostratigraphy. The ANN is most strongly influenced by porosity and alteration degrees and the resulting stratigraphy most closely resembles the above classifications when clustered by FMS texture as opposed to lithologic interpretation. The ANN thus provides a porosity-based stratigraphy of the basement rather than the traditional lithology-based stratigraphy.

Tominaga, Masako; Teagle, Damon A. H.; Alt, Jeffrey C.; Umino, Susumu

2009-01-01

124

Interaction between seawater and magma or very hot rock in the deep fast-spreading oceanic crust: Constraints from experimental petrology  

Science.gov (United States)

More and more publications on the dynamics of magmatic system at fast spreading ridges refer to deep interaction of seawater with magma or with just frozen rocks. Prominent models focus on deep hydrothermal circulation at magmatic temperatures [1], seawater involvement into magmatic processes due to ridge tectonics [2], or on stoping/assimilation of hydrothermally altered dikes at the top of the axial melt lens [3]. In order to understand the underlying hydrous MORB magmatism and the corresponding hydrous melting reactions at fast-spreading ocean ridges, we performed different series of crystallization and partial melting experiments in hydrous MORB-type systems at shallow pressures. (1) To understand the magmatic processes ongoing in the top of the axial melt lenses, we present here new phase diagrams for hydrous MORB systems at a pressure of 50 MPa, by extrapolating results of phase equilibria experiments in hydrous systems performed at shallow pressures in primitive to evolved tholeiitic, MORB-type systems. We applied our new phase diagrams to rocks from the sheeted dike/gabbro transition from the EPR crust (IODP Site 1256D) and from the Oman ophiolite, opening interesting perspectives to explain specific aspects of petrogenesis of these rocks. (2) Experimental hydrous partial melting of gabbro results not only in the production of oceanic plagiogranites, but also in the formation of characteristic interstitial residual parageneses like plagioclase with An-rich rims, orthopyroxene, and pargasitic amphibole rimming the primary phases. Such parageneses can be observed in gabbros from IODP Site 1256D and from the Oman ophiolite, documenting that hydrous partial melting proceeded. (3) Experimental partial melting of hydrated dikes results in the formation of plagiogranitic melts and of a typical residual granoblastic mineral paragenesis. Domains with a characteristic texture (microgranular wormy intergrowth of clino- and orthopyroxene, plagioclase, and oxides) can be observed rather often in Gabbros from IODP Site 1256D, implying that partial melting of stoped hydrated sheeted dikes was a major magmatic process which resulted in locally high water activities enabling primary amphibole crystallization. [1] Nicolas, A., and D. Mainprice (2005), Terra Nova, 17, 326-330. [2] Abily, B., G. Ceuleneer, and P. Launeau (2011), Geology, 39, 391-394. [3] Koepke, J., L. France, T. Müller, F. Faure, N. Goetze, W. Dziony, and B. Ildefonse (2011 accepted), Geochem. Geophys. Geosyst.

Koepke, J.; Botcharnikov, R. E.; Berndt, J.; Feig, S.; France, L.

2011-12-01

125

Tectonic implications of post-30 Ma Pacific and North American relative plate motions  

Science.gov (United States)

The Pacific plate moved northwest relative to North America since 42 Ma. The rapid half rate of Pacific-Farallon spreading allowed the ridge to approach the continent at about 29 Ma. Extinct spreading ridges that occur offshore along 65% of the margin document that fragments of the subducted Farallon slab became captured by the Pacific plate and assumed its motion proper to the actual subduction of the spreading ridge. This plate-capture process can be used to explain much of the post-29 Ma Cordilleran North America extension, strike slip, and the inland jump of oceanic spreading in the Gulf of California. Much of the post-29 Ma continental tectonism is the result of the strong traction imposed on the deep part of the continental crust by the gently inclined slab of subducted oceanic lithosphere as it moved to the northwest relative to the overlying continent. -from Authors

Bohannon, R.G.; Parsons, T.

1995-01-01

126

Thallium isotopes in ferromanganese crusts as a proxy for marine productivity  

Science.gov (United States)

We have constructed the first high resolution thallium (Tl) isotope records in two ferromanganese crusts (Fe- Mn crusts), CD29-2 and D11-1 from the central Pacific Ocean, which show pronounced systematic changes. Applying an osmium isotope age model both crusts display coherent virtually identical Tl isotope variations over the entire Cenozoic. The most prominent variations occur in the time interval between about 55 Ma and 45 Ma. The Tl isotope composition increases smoothly from ?205Tl = +6.0 at 55 Ma to ?205Tl = +10.5 at 45 Ma (?205Tl represents the deviation of the 205Tl/203Tl isotope ratio of a sample from NIST SRM 997 Tl in parts per 104). The remainder of the crusts displays relatively limited Tl isotope variations. These variations are unlikely to reflect diagenetic overprinting or changes in isotope fractionation between seawater and Fe-Mn crusts. Most probably, the Fe-Mn crusts track the Tl isotope composition of seawater over time. Using a simple box model we show that the Tl isotope composition of seawater depends almost exclusively on the ratio between the two principal output fluxes of marine Tl. These output fluxes are removal of Tl from seawater via scavenging by authigenic Fe-Mn oxyhydroxide precipitation and uptake of Tl during low temperature alteration of oceanic crust. However, the low Tl isotope composition that occurred during the Paleocene is consistent only with a more than four-fold higher sequestration of Tl by Fe-Mn oxyhydroxides compared with the present day. The seawater curves of Tl and S isotopes show close synchronicity through the entire Cenozoic and we conclude that both systems must have responded to the same change in the marine environment. We propose that this change was most likely a marked and permanent increase in marine productivity, which caused an increase in carbon export that led to higher pyrite burial rates and also significantly reduced the amount of Fe-Mn oxides precipitated because of biological uptake of Fe and Mn.

Nielsen, S.; Mar-Gerrison, S.; Larowe, D.; Klemm, V.; Halliday, A.; Burton, K.; Hein, J.

2008-12-01

127

ACEX Arctic Coring Expedition : paleoceanographic and tectonic evolution of the central Arctic Ocean  

OpenAIRE

The first scientific drilling expedition to the central Arctic Ocean was completed in late summer 2004. Integrated Ocean Drilling Program Expedition 302, Arctic Coring Expedition (ACEX), recovered sediment cores deeper than 400 meters below seafloor (mbsf) in water depths of ~1300 m at the top of the world, only 250 km from the North Pole. ACEX's destination was the Lomonosov Ridge, hypothesized to be a sliver of continental crust that broke away from the Eurasian plate at ~56 Ma. As the ...

Backman, Jan; Moran, Kathryn; Evans, Dan

2004-01-01

128

Seismic evidence for hyper-stretched crust and mantle exhumation offshore Vietnam.  

Science.gov (United States)

The Phu Khan basin is one of the sub-basins opened during the rifting of the South China Sea during the Eocene. The basin is located against the East-Vietnam Boundary Fault (EVBF) to the west and the oceanic crust to the east. Good quality seismic lines allow us to observe structures which highlight the rifting history of the South China Sea margin and the processes of crustal boudinage. A Moho rise is the prominent feature of the Central part of the basin. The mantle is shallowest in the center of the basin and at places is directly in contact with the sediments, via a large low-angle detachment fault which separates several crustal blocks. The axis of the Moho rise is roughly parallel to the South China Sea propagator direction. As a consequence, the upper and lower crusts are strongly extended by large crustal boudins. The network of normal fault is dense in the upper crust and propagates into the lower crust occasionally. However, the lower crust seems to be missing at some places. At the apex of the Moho rise, several indicators of fluids circulations have been observed, including volcanic edifices and gas escapes features. The seismic facies just above this Moho rise looks poorly stratified and might be affected by a certain degree of metamorphism. Three stages of extension are clearly identifiable, with age constrained by wells calibration of the horizons: the oldest rift sequence is identified from basement to Oligocene horizons; a second from Oligocene to Mid Miocene (15.5 Ma), and a third from Mid-Miocene to Upper Miocene (10.5 Ma). These three rifts have been formed with at least two directions of extension, a first which is North-South and a second NW - SE The well imaged 15.5 Ma horizon is tilted, as well as part of the subsequent in-filling which is fan-shaped. These are in turn sealed by the 10.5Ma unconformity. Although tectonic activity appears diachronous from north to south, we suggest that cessation of rifting and opening did not occur before 12 - 10.5 Ma, at variance with models derived from magnetic anomalies recognized over the oceanic portions of the South China Sea (15.5 to 20 Ma). keywords : South China Sea, Vietnam, Phu Khan Basin, mantle exhumation, polyphased rifting

Savva, D.; Meresse, F.; Pubellier, M.; Chamot-Rooke, N.; Franke, D.; Steuer, S.; Sapin, F.; Auxietre, J. L.

2012-04-01

129

The Western Sierras Pampeanas : protracted Grenville-age history (1330-1030 Ma) of intra-oceanic arcs, subduction-accretion at continental-edge and AMCG intraplate magmatism  

OpenAIRE

New U–Pb SHRIMP zircon ages combined with geochemical and isotope investigation in the Sierra de Maz and Sierra de Pie de Palo and a xenolith of the Precordillera basement (Ullún), provides insight into the identification of major Grenville-age tectonomagmatic events and their timing in the Western Sierras Pampeanas. The study reveals two contrasting scenarios that evolved separately during the 300 Ma long history: Sierra de Maz, which was always part of a continental crust, and the juveni...

Rapela, C. W.; Pankhurst, R. J.; Casquet, C.; Baldo, E.; Galindo, C.; Fanning, C. M.; Dahlquist, J. M.

2010-01-01

130

Preliminary Results from IODP Exp. 323 to the Bering Sea: the ocean history for the last 5 Ma  

Science.gov (United States)

The shift from the Late Pliocene to Pleistocene is an important transitional period from global warmth to the initiation of glacial-interglacial cycles on orbital time-scales. Little is understood about the processes responsible for this major shift in the Earth's climate, and until now, a lack of data in critical regions of the Pacific, such as the Bering Sea, has prevented an evaluation of the role of North Pacific processes in global climate change (Takahashi, 1999). The Bering Sea is a marginal sea in the North Pacific that has experienced, and is sensitive to, major climatic change. Here we present preliminary microfossil results from IODP Expedition 323 to the Bering Sea. Diatoms are the dominant microfossil group in this region and reveal major ecological shifts throughout the ~5 Ma record. The onset of Northern Hemisphere Glaciation is clearly depicted by a shift from warm, nutrient-rich, shade flora species Coscinodiscus marginatus and Pyxidicula horridus to the appearance of sea-ice species at ca. 2.5 Ma. A secondary shift is also observed at 1.0-0.9 Ma and coincides with the Middle Pleistocene Transition. The shift to sea-ice dominated assemblages in the northern sector of the Bering Sea, is further intensified at ca. 0.9 Ma and demonstrate a clear response to glacial/interglacial cycles. These observations are further supported by dinoflagellate, radiolarian, foraminifera and geochemical records. The persistence of C. marginatus beyond 2 Ma in the northern latitude sites suggests continued mixing and high nutrient supplies to this zone, and refutes dissolution issues. Increases in intermediate water formation radiolarian species after 1 Ma, together with diatom sea-ice related species suggest the development of NPIW during glacial times.

Stroynowski, Zuzia

2010-05-01

131

Shallow stratification prevailed for ?1700 to ?1300 Ma ocean: Evidence from organic carbon isotopes in the North China Craton  

Science.gov (United States)

The Late Paleoproterozoic to Early Mesoproterozoic (from ?1700 Ma to ?1300 Ma) was highlighted by the assembly of the Nuna supercontinent, expansion of euxinic marine environments and apparent stasis in the diversity of eukaryotes. The isotopic composition of carbonate carbon (?13Ccarb) was surprisingly constant during this interval, but little is known about the secular variation in the organic carbon isotopic composition (?13Corg). Here we report ?13Corg data from the latest Paleoproterozoic (>1650 Ma) to Early Mesoproterozoic (?1300 Ma) succession in North China. The ?13Corg values range from -25‰ to -34‰, and are dependent on sedimentary facies. In subtidal and deeper environments ?13Corg values are low and constant, ca. -32‰, but relatively enriched and more variable in shallower intertidal and supratidal environments. We attribute the facies-dependent variation in ?13Corg to the presence of a shallow chemocline. A probable result of a shallow chemocline is that it supported significant contributions of organic matter produced by chemoautotrophic and/or anaerobic photoautotrophic microbes in relatively deep environments from the latest Paleoproterozoic to Early Mesoproterozoic continental shelf of North China.

Luo, Genming; Junium, Christopher K.; Kump, Lee R.; Huang, Junhua; Li, Chao; Feng, Qinglai; Shi, Xiaoying; Bai, Xiao; Xie, Shucheng

2014-08-01

132

Evaluation of the Split-Ridge Hypothesis and Generation of Oceanic Crust along the Cleft Segment of the Southern Juan de Fuca Ridge (JdFR)  

Science.gov (United States)

Morphologic, structural and petrologic studies of the southern JdFR, suggest that it has evolved through cycles of accretion related to magmatic output. During the first cycle, which is dominated by magmatism and volcanic construction, the ridge axis has morphologic characteristics similar to a fast spreading ridge but as magmatism wanes tectonic extension dominates resulting in a more rugged, faulted morphology similar to a slow-spreading ridge ("split-ridge" hypothesis of Kappel and Ryan, 1986). Off-axis sampling and observations made with the ROV Tiburon provide a unique perspective on the crustal evolution along this typical moderate spreading-rate ridge. The rift valley walls are comprised of a series of major bounding faults, separated by blocks of oceanic crust that exhibit little or no deformation. Unlike the present axis that is dominated by sheetflows, these blocks are almost entirely comprised of unfaulted, constructional pillow ridges, mounds and hornitos. These blocks conform to the mound-like areas identified by Kappel and Ryan (1984) based on Sea MARC I images and our EM300 data. There is observational evidence of small volumes of off-axis volcanism along eruptive fissures and from point-sources that appear related to the formation of rift-bounding faults. Other volcanic constructs seem to be related to the formation of the first series of abyssal hills, consistent with "volcanic growth faults" draped with syntectonic lava flows. Off-axis samples are moderately to highly evolved N-MORB but also include ferro-andesites and a dacite with somewhat "transitional" chemical characteristics. The range of compositions is greater than that previously reported for the entire southern JdFR. In contrast, samples recovered from the south Cleft axis are moderately evolved N-MORB and exhibit little chemical variability. Lavas sampled in cross-axis traverses encompass the entire range of compositions with no readily discernable pattern of variation with distance from the axis nor are the chemical trends symmetric across the ridge crest; features which are inconsistent with the split-ridge hypothesis. Initial geological correlations suggest the less evolved flows form the outward-facing constructional mounds and hills, whereas the most evolved types (FeTi basalts) form the small hornitos and areas of extensive sheetflows and lava drainback that are perched on "benches" between faulted, pillowed walls. We suggest that the less evolved samples are related to axial volcanism while the more evolved samples were formed by post-axial or off-axis volcanism. The highly fractionated off-axis MORB could have been derived from the cooler, distal edges of a magma chamber. Most of the major and trace element chemical variability can be explained in terms of low-pressure fractional crystallization (up to 50%) of parental magmas with compositions similar to recently erupted axial MORB. Ferro-andesites recovered from an outcrop along the inner valley wall appear to be spatially related to the major axial bounding fault and to some extinct sulfide chimneys. These highly fractionated andesites (and the dacite from the RTI), require extreme amounts of fractional crystallization as well as additional processes (assimilation/mixing) to explain their unusual compositions.

Perfit, M. R.; Stakes, D.; Ridley, W. I.; Tivey, M.; Ramirez, T.; Kela, J.; Kulp, S.

2001-12-01

133

High-resolution grain size analysis and its significance for detecting ocean acidification at the onset of the Paleocene-Eocene Thermal Maximum (PETM; 55Ma) (Invited)  

Science.gov (United States)

The Paleocene-Eocene Thermal Maximum (PETM; 55Ma) is widely considered a close ancient analog to modern global warming. A host of recent investigations have elucidated the scale and nature of the climate forcing during the PETM, as well as the range of atmospheric, oceanographic and biotic impacts. Introduction of massive amounts of greenhouse gases into the ocean-atmosphere system at the onset of the event is known to have led to abrupt shoaling of the lysocline and calcite compensation depth in the oceans as observed at deep-sea locations by a marked increase in the dissolution of calcareous microfossils and correspondingly sharp lithologic changes. The occurrence of surface ocean acidification during the initial stages of the PETM is not documented largely because the potential evidence is overprinted by pervasive dissolution at the sea floor. We present detailed grain size analysis from a high-resolution sample set across the PETM at Ocean Drilling Program Sites 690 (Maud Rise, Southern Ocean), 1209 (Shatsky Rise, Pacific Ocean) and 1262 (Walvis Ridge, South Atlantic Ocean) and at the Wilson Lake drill hole from the New Jersey coastal plain. The Wilson Lake section is dominated by clastic material, thus samples were processed to obtain the grain size distribution of the carbonate fraction. Grain size data were collected using a Malvern Mastersizer, an instrument that optically measures particle size between 0.1 and 1000 micrometer in diameter. The results show dramatic differences is size trends between sites that are consistent with their depths with respect to the CCD and lysocline. At the same time, the base of the PETM is characterized by very sharp changes in grain size distribution at Site 1262, where dissolution is most severe and progressively less abrupt changes at Site 1209, Site 690 and Wilson Lake. This progression is consistent with known differences in the magnitude of the lysocline and CCD shoaling at these sites. Comparison of grain size, carbonate and stable isotope data produces more accurate estimates of the depth of carbonate “burn down” at Sites 1209 and 1262. At the other sites, comparison of nannofossil and benthic foraminiferal preservation across the base of the PETM allows us to evaluate whether there was a brief period of surface-water acidification prior to the onset of deep-water acidification. For all sites, grain size data provide more quantitative estimates of the changes in flux of planktonic foraminifera and nannoplankton during the course of the PETM. Nannoplankton dominate the carbonate flux at all sites except Site 690 where the event is marked by complex pattern of variation in foraminiferal flux.

Bralower, T. J.; Kump, L.; Eccles, L.; Smith, G. J.; Lindemann, T. L.; Bowen, G. J.; Schneider Mor, A.; Thomas, E.

2010-12-01

134

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.)

135

Chapter 50: Geology and tectonic development of the Amerasia and Canada Basins, Arctic Ocean  

Science.gov (United States)

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

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

2011-01-01

136

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

137

Anoxia and the nitrogen cycle during Cretaceous Oceanic Anoxic Event 1a (~120 Ma): a data-model comparison  

Science.gov (United States)

The Oceanic Anoxic Events (OAEs) of the Mesozoic are among the most dramatic examples of water column oxygen depletion in Earth's history. OAEs were likely characterized by high rates of denitrification and a profoundly different marine nitrogen cycle than that of today. High abundances of 2-methylhopane biomarkers commonly occur in OAE sediments, especially OAE 1a, and are often interpreted to reflect a major increase in the (relative) abundance of N2-fixing cyanobacteria (e.g., Kuypers et al., 2004). However the spatial extent of these postulated cyanobacterial blooms, control of water column anoxia on their occurrence, and impact on the nitrogen cycle is not well constrained and understood. Combining new data with an intermediate-complexity Earth system model (GENIE), we show that modeled reduced oxygenation of the water column is consistent with the majority of available data for OAE 1a. Increased nutrient availability likely caused widespread bottom water anoxia during OAE 1a, similar to OAE 2 (Monteiro et al., 2012). However, the different paleogeography appears to have prevented widespread expansion of euxinic conditions in the photic zone during OAE 1a, consistent with the general absence of isorenieratane in OAE 1a sediments. A compilation of newly generated and previously published 2-methylhopane biomarker data from the Tethys realm (Cismon core and three newly generated records from southern Spain) and Pacific Ocean (Shatsky Rise, DSDP Site 463, and new data from ODP Site 866) shows that relative abundances of 2-methylhopanes and temporal trends herein differ greatly during OAE 1a, even between proximal sites. Although modeled spatial distribution of nitrogen fixation for OAE1a bears similarity with 2-methylhopanes distributions, distinct differences are present. Altogether these results call into question the assumed effects of periods of anoxia on the nitrogen cycle and suggest that the response of the nitrogen cycle to large perturbations of the global carbon cycle and widespread anoxia is more complicated than previously assumed. As oxygen minimum zones are likely to expand in response to global warming (Keeling et al., 2009), these results are important for our understanding of future changes in the global nitrogen cycle. References: - Kuypers, M.M.M., et al., 2004. N2-fixing cyanobacteria supplied nutrient N for Cretaceous oceanic anoxic events. Geology 32 (10), 853-856. - Monteiro, F.M., et al., 2012. Nutrients as the dominant control on the spread of anoxia and euxinia across the Cenomanian-Turonian oceanic anoxic event (OAE2): Model-data comparison. Paleoceanography 27 (4), PA4209. - Keeling, R.F., et al., 2009. Ocean Deoxygenation in a Warming World. Annual Review of Marine Science 2 (1), 199-229.

Naafs, B. D.; Monteiro, F. M.; Froehner, S.; Lowson, C.; Quijano, M.; Castro, J.; Donnadieu, Y.; Schmidt, D. N.; Ridgwell, A. J.; Pancost, R. D.

2013-12-01

138

Petrology of Late Jurassic-Early Cretaceous ophiolites from Timpa delle Murge (Southern Apennines, Italy): insights from fragments of Tethyan oceanic crust  

OpenAIRE

On the Timpa delle Murge hill, located in the Lucania region close to Mt. Pollino (southern Apennines, Italy), Upper Jurassic – Lower Cretaceous metaigneous and metasedimentary rocks crop out, which are believed to represent fragments of Tethys oceanic cr

D Antonio, Massimo; Civetta, Lucia

2008-01-01

139

Evidence for hydrothermal venting in Fe isotope compositions of the deep Pacific Ocean through time  

Science.gov (United States)

Temporal variations in Fe isotope compositions at three locations in the Pacific Ocean over the last 10 Ma are inferred from high-resolution analyses of three hydrogenetic ferromanganese crusts. Iron pathways to the central deep Pacific Ocean appear to have remained constant over the past 10 Ma, reflected by a remarkably constant Fe isotope composition, despite large changes in the Fe delivery rates to the surface ocean via dust. These results suggest that the Fe cycle in the deep ocean is decoupled from that in surface waters. By contrast, one ferromanganese crust from the Izu-Bonin (IB) back-arc/marginal basin of the W. Pacific exhibits large ? 56Fe variations. In that crust, decreases in ? 56Fe values correlate with increases in Mn, Mg, Ni, Cu, Zn, Mo, and V contents, and consistent with periods of intense hydrothermal input and increased growth rates. A second crust located within 100 km of the first IB sample does not record any of these periods of enhanced hydrothermal input. This probably reflects the isolated pathways by which hydrothermally sourced Fe may have migrated in the back arc, highlighting the high degree of provinciality that Fe isotopes may have in the modern (oxic) oceans. Our results demonstrate that despite efficient removal at the source, hydrothermal Fe injected into the deep ocean could account for a significant fraction of the dissolved Fe pool in the deep ocean, and that hydrothermally sourced Fe fluxes to the open ocean may have lower ? 56Fe values than those measured so far in situ at hydrothermal vents. Correlation between ? 56Fe values and elements enriched in hydrothermal fluids may provide a means for distinguishing hydrothermal Fe from other low-? 56Fe sources to the oceans such as dissolved riverine Fe or porewaters in continental shelf sediments.

Chu, N.-C.; Johnson, C. M.; Beard, B. L.; German, C. R.; Nesbitt, R. W.; Frank, M.; Bohn, Marcel; Kubik, P. W.; Usui, A.; Graham, I.

2006-05-01

140

Has 7% of Continental Crust been Lost since Pangea Broke Up?  

Science.gov (United States)

After modern plate tectonics began, the net growth or loss of continental crust predominantly involved the mass balance at subduction zones (SZs) between the yin of adding mantle-sourced arc igneous rocks and the subtracting yang of recycling existing crust back into the mantle. Field observations suggest that during Mesozoic and Cenozoic time, a rough long-term balance existed at ocean-margin SZs (e.g., W. N. America, Andes). But a different picture, one of net loss, emerges when additions and losses at collisional or crust-suturing SZs (e.g., India-Tibet) are considered. GAINS AND LOSSES SINCE ~200 Ma Because Mesozoic and Cenozoic convergent margins can be field inspected, the net growth of continental crust after the breakup of Pangea at ~200 Ma can be estimated. Pangea breakup also marked the beginning of the present supercontinent cycle. Newly established (Eocene) ocean-margin SZs (e.g., IBM, Tonga-Kermadec) added juvenile arc crust for at least 10-15 Myr at rates ~10-15 times higher than later and elsewhere at long-established SZs (~30 km3/Myr/km). During the Cenozoic, at colliding SZs (e.g., Alps, India-Tibet, Arabia-Eurasia) tomographic and geological data document losses of subducted continental crust sustained for 15-50 Myr at rates ~15 times that typical from the upper plate at ocean-margin SZs (~70 km3/Myr/km). For additions, we considered that as the Atlantic opened in early Jurassic time, new, prodigiously productive SZs were initiated along the western margin of North and Middle America but not along western South America and the eastern margin of Eurasia. In the Cretaceous, new SZs formed along much of the northern margin of the Tethys, along western Sumatra and southern Java, and at the great arc of the Caribbean. In the early Eocene, in the offshore, a lengthy (~20,000 km) curtain of new, voluminously productive intra-oceanic SZs formed from the Aleutian Islands southward to the Kermadec Islands. For subtractions, we applied subduction losses (~70 km3/Myr/km) previously estimated for Cenozoic ocean-margin SZs. During the past 50 Myr, at Tethyan SZs from Gibraltar to western Australia, we estimate an average rate of lower plate continental crust recycling at ~900 km3/Myr/km. NEGATIVE CRUSTAL GROWTH AND IMPLICATIONS After the breakup of Pangea, the volume of continental crust recycled into the mantle is estimated to be ~500 x 10^6 km3 greater than that created (~400 x 10^6 km3). The net loss is ~7% of Earth's continental crust existing volume (7 x 10^9 km3), an estimate that does not include delamination losses. A large part of SZ losses (~40%) occurred during the past 50 Myr as a consequence of closing the Tethys, a process still underway. It seems likely that a long supercontinent cycle of breakup and dispersal of fragments favors positive crustal growth effected by elevated rates of magmatism at new SZs, whereas a shorter cycle involving rapid reassembly favors negative crustal growth effected by elevated rates of losses at crustal collision zones. Fragmentation of Pangea and the partial reassembly of its parts within 150 Myrs may be representative of a short, crustal destructive supercontinent cycle.

Scholl, D. W.; Stern, R. J.

2012-12-01

141

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

Science.gov (United States)

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 ˜25-30 km depth below mean sea level in southeastern coastal Norway and parts of Denmark, ˜35-45 km across the southern Scandes, and ˜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 (<35 km) underlies the Oslo Graben. Crustal Vp/Vs ratio measurements show trends that generally correlate with Moho depth; relatively high Vp/Vs occurs near the coast and areas affected by post-Caledonide rifting and lower Vp/Vs appears in older, unrifted crust across the southern Scandes. Our results indicate that most of the observed surface elevation in the southern Scandes is supported by an Airy-like crustal root and potentially thin mantle lithosphere. To the east, where thicker crust and mantle lithosphere underlie low elevations, the presence of dense mafic lower crust fits a Pratt-like model for isostatic compensation. Because the Scandes mountains occupy the location of the ancient Caledonian orogeny, which created presumably much thicker crust and lithosphere by ca. 400 Ma, much of the dense lower crust or mantle lithosphere that is expected to form beneath large mountain belts must have been removed sometime afterwards to instill the current lithospheric architecture that underlies the region.

Frassetto, Andrew; Thybo, Hans

2013-11-01

142

Combined147,146Sm-143,142Nd constraints on the longevity and residence time of early terrestrial crust  

Science.gov (United States)

silicate differentiation controlled the composition of Earth's oldest crust. Inherited 142Nd anomalies in Archean rocks are vestiges of the mantle-crust differentiation before ca. 4300 Ma. Here we report new whole-rock 147,146Sm-143,142Nd data for the Acasta Gneiss Complex (AGC; Northwest Territories, Canada). Our 147Sm-143Nd data combined with literature data define an age of 3371 ± 141 Ma (2 SD) and yield an initial ?143Nd of -5.6 ± 2.1. These results are at odds with the Acasta zircon U-Pb record, which comprises emplacement ages of 3920-3960 Ma. Ten of our thirteen samples show 142Nd deficits of -9.6 ± 4.8 ppm (2 SD) relative to the modern Earth. The discrepancy between 142Nd anomalies and a mid-Archean 147Sm-143Nd age can be reconciled with Nd isotope reequilibration of the AGC during metamorphic perturbations at ca. 3400 Ma. A model age of ca. 4310 Ma is derived for the early enrichment of the Acasta source. Two compositional end-members can be identified: a felsic component with 142Nd/144Nd identical to the modern Earth and a mafic component with 142Nd/144Nd as low as -14.1 ppm. The ca. 4310 Ma AGC source is ˜200 Myr younger than those estimated for Nuvvuagittuq (northern Québec) and Isua (Itsaq Gneiss Complex, West Greenland). The AGC does not have the same decoupled Nd-Hf isotope systematics as these other two terranes, which have been attributed to the crystallization of an early magma ocean. The Acasta signature rather is ascribed to the formation of Hadean crust that was preserved for several hundred Myr. Its longevity can be linked to 142Nd evolution in the mantle and does not require slow mantle stirring times nor modification of its convective mode.

Roth, Antoine S. G.; Bourdon, Bernard; Mojzsis, Stephen J.; Rudge, John F.; Guitreau, Martin; Blichert-Toft, Janne

2014-06-01

143

Distribution of thorium-232 in manganese nodules and crusts: Paleoceanographic implications  

Science.gov (United States)

Detailed depth profiles of 232Th in deep-sea ferromanganese nodules and encrustations often show a subsurface maximum followed by a drastic inward decrease to a certain depth before levelling out. Length scales of this feature vary among the profiles but are proportional to growth rates of the specimens, suggesting that the profiles record changes in oceanic 232Th flux with time. Consideration of the nodule chronology, the history of eolian accumulation recorded in sediment cores, and the marine geochemical behavior and budget of 232Th leads us to propose that 232Th in nodules/crusts is of eolian origin. The drastic inward decrease of 232Th is a reflection of increase in the wind borne dust flux induced by the Northern Hemisphere glaciation beginning 2-3 Ma. The subsurface 232Th peak may indicate the decline of eolian flux since 0.1-0.3 Ma.

Huh, C.-A.; Ku, T.-L.

1990-04-01

144

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

145

Remnants of the Rheic SSZ Oceanic Lithosphere (320 Ma) Within the Izmir-Ankara-Erzincan Suture Zone in NE Turkey: New Geochemical and Re-Os Isotope Data From the Rehafiye-Erzincan Ophiolite  

Science.gov (United States)

We report on new major-trace-REE and Re-Os isotope compositions and mineral chemistry data from upper mantle peridotites and ultramafic-mafic cumulate rocks in the Rehafiye-Erzincan ophiolite (REO) in NE Turkey, and discuss their siginificance for the tectonic evolution of various oceanic tracts in the eastern Mediterranean region. The REO is part of the Izmir-Ankara-Erzincan Suture Zone (IAESZ) between the Gondwana-derived Tauride-South Armenian ribbon continent to the south and the Rhodope-Pontide micro-continent to the north. It shows bidivergent thrusting along its southern and northern boundaries, resting tectonically on the margins of these continental masses. The IAESZ includes fragments of oceanic lithosphere with WPB, MORB, IAT-Boninite, OIB and LIP affinities that range in age from the Permo-Triassic to the latest Crecateous, although it is commonly interpreted as Neotethyan in origin. The REO consists of upper mantle peridotites including harzburgite with dunite bands/lenses and crosscutting dolerite dikes, ultramafic-mafic cumulate rocks making up a transitional Moho, isotropic gabbro, plagiogranites, and sheeted dikes. Extrusive rocks are missing in the ophiolite sequence but occur as blocks of pillow basalts in an ophiolitic mélange structurally beneath the REO. We have identified two types of upper mantle peridotites, abyssal and SSZ, in the REO. Less depleted, clinopyroxene-rich mantle harzburgites have higher concentrations of Al (1.75-2.12 wt.% Al2O3) and Ca (0.43-1.53 wt.% CaO) and contain spinel phases with Cr# ranging between 33.2 and 37.8. These abyssal peridotites represent a mantle residue of low degrees of partial melting of primitive upper mantle during MOR-type oceanic crust formation. Some peridotite samples, on the other hand, are highly depleted in clinopyroxene and display extremely low contents of Al (0.16-0.89 wt.% Al2O3) and Ca (0.07-0.77 wt.% CaO), characteristic of SSZ peridotites. Spinel phases in these samples have Cr# ranging between 57.0 and 73.5, indicating high degrees of partial melting (in a mantle wedge) of previously depleted peridotites. Both types of peridotites are characterized by low REE contents. Abyssal-type peridotites display a flat pattern from Lu to Tb and negative-flat pattern from Tb to La, although some samples show slight LREE enrichment. SSZ-type, depleted peridotite samples are characterized by nearly similar LREE concentrations, whereas their HREE and especially MREE concentrations are significantly depleted with respect to those of the abyssal peridotites. These rocks show U-shaped REE patterns, reminiscent of forearc peridotites. The PGE content of mantle tectonites in the REO, although low, are significantly higher than those presented by the cumulate gabbros. The 187Os/188Os isotope ratio of these peridotites (0.1195-0.1240) is typical of the depleted mantle and much lower than those of the cumulate gabbros (0.2074-0.5842). Whole-rock samples from the mantle tectonites and cumulate gabbros display a well-defined linear trend in a 187Re/188Os vs. 187Os/188Os isochron diagram. These samples collectively define an isochron with a best-fit age of about 320±16 Ma, suggesting that the melt evolution and crystallization of the SSZ ultramafic-mafic units of the REO may be as old as the late Carboniferous. This new finding of a late Carboniferous SSZ mantle lithosphere between the Gondwana and Eurasia continental masses in NE Turkey indicates: (1) the existence of Rheic Ocean relics farther away in the Tethyan realm of the eastern Mediterranean region than previously considered; (2) a highly heterogeneous character of the IAESZ, containing some lithospheric material that evolved in rift-drift, plume and subduction-accretion tectonic settings of the Rheic, Paleotethyan and Neotethyan Ocean basins. We discuss the geodynamic implications of these inferences.

Uysal, Ibrahim; Dilek, Yildirim; Sarifakioglu, Ender; Meisel, Thomas

2010-05-01

146

Seismological study of Lau back arc crust: Mantle water, magmatic differentiation, and a compositionally zoned basin  

Science.gov (United States)

On the basis of seismic tomography analyses, a structural model of crust formed along the Eastern Lau Spreading Center (ELSC) is presented as evidence for a transition from a “hydrous” type of oceanic crust to a more typical oceanic crust. The seismic data indicate that as the spreading center moved away from the active arc, the crust thinned from 8-9 km to ˜7 km, the lower crust changed from high P wave velocity values (7.2-7.4 km/s) to typical values for oceanic crust (7.0-7.2 km/s), and the upper-crustal volcanic layer changed from a thick low-velocity layer to a thinner layer with more typical wave speeds. The seismic results, in combination with other geophysical and geochemical data, suggest that crustal formation along the ELSC is strongly controlled by the influence of slab water. When a spreading center is near the active arc, water from the downgoing slab is entrained in the melting zone beneath the ridges where it enhances melting. Thereafter, the water enhances crustal differentiation within sub-ridge magma chambers. This creates an anomalous “hydrous” form of oceanic crust with a thick felsic volcanic layer and a mafic/ultramafic lower crust - features that are not typically observed in crust formed at mid-ocean ridges. The Lau basin has a zoned structure with an abrupt transition from this type of oceanic crust to more typical oceanic crust, which resulted from a rapid change in the influence of slab water as the ridge moved away from the arc. The abundance and high rate of production of the “hydrous” crust suggests that such crust may make up a significant proportion of the arc-like crust that forms continents.

Arai, Ryuta; Dunn, Robert A.

2014-03-01

147

Geochemical evidence for interaction between oceanic crust and lithospheric mantle in the origin of Cenozoic continental basalts in east-central China  

Science.gov (United States)

Cenozoic continental basalts from east-central China have been analysed for whole-rock major and trace elements, Sr-Nd-Hf isotopes, and mineral O isotopes. The basalts are alkalic to tholeiitic in composition, but are dominantly alkalic. They are characterized by OIB-like patterns of trace element distribution, with no depletion in Nb and Ta but with negative Pb anomalies on primitive mantle normalized diagrams. In addition, they have high Fe/Mn ratios and subchondritic Nb/Ta ratios. Sr-Nd-Hf isotope compositions indicate predominant contributions from young subcontinental lithospheric mantle (SCLM). Some phenocrysts of clinopyroxene and olivine have lower O isotope ratios than normal upper mantle, suggesting involvement of low ?18O melts derived from dehydrated oceanic basalt that experienced high-T seawater-hydrothermal alteration during MORB-type magma eruption. Such low ?18O melts are unlikely to be transported through the convective asthenosphere because of its rapid O isotope homogenization, but likely to directly metasomatize the overlying peridotite to form pyroxenite. Oxygen isotope disequilibrium between olivine and clinopyroxene suggests a maximum timescale of wedge to the juvenile SCLM beneath the thinned ancient SCLM. Partial melting of the dehydrated oceanic metabasalt and metasediment during the Late Mesozoic would generate adakitic and felsic melts, respectively, metasomatize the overlying juvenile SCLM peridotite to form isotopically depleted silica-deficient to enriched silica-excess pyroxenites. As a result, SCLM beneath the eastern China was stratified in the Cenozoic, with the juvenile SCLM underlying the ancient SCLM but overlying the pyroxenites. Lithosphere rifting during the Cenozoic would cause the melting of the pyroxenites, giving rise to the alkalic to tholeiitic basalts. Therefore, the Cenozoic continental basalts provide the petrological record of melt-peridotite reactions in response to the asthenosphere-lithosphere interaction during the Mesozoic thinning of continental lithosphere.

Zhang, Jun-Jun; Zheng, Yong-Fei; Zhao, Zi-Fu

2009-06-01

148

Icelandic-type crust.  

OpenAIRE

Numerous seismic studies, in particular using receiver functions and explosion seismology, have provided a detailed picture of the structure and thickness of the crust beneath the Iceland transverse ridge. We review the results and propose a structural model that is consistent with all the observations. The upper crust is typically 7 ± 1 km thick, heterogeneous and has high velocity gradients. The lower crust is typically 15–30 ± 5 km thick and begins where the velocity gradient decreases...

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

2003-01-01

149

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

150

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 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

151

Accreted oceanic materials in Japan  

Science.gov (United States)

The Phanerozoic circum-Pacific orogenic belts contain numerous ocean-derived materials accreted through plate converging processes. Japanese Islands, in particular, display various kinds of oceanic materials of different origins including fragments of seamounts, oceanic reef limestone, MORB-like rocks and oceanic mantle, and pelagic sediments. The compilation of these rocks in many subduction complexes of Late Permian to the present, led to following conclusions. Accretion processes work effectively only for materials primarily composing the upper portion of subducting oceanic crust, i.e. Layer 1 and Layer 2. Many fragments of seamount with alkali basalt (600), hot-spot seamount (26), oceanic reef limestone (291), MORB-like basalt (200), and numerous cherts (more than 1000) are recognized as ancient oceanic materials accreted to the Japanese Islands. However, gabbros and mantle materials of Layer 3 and lower parts of the oceanic lithosphere, scarcely occur in subduction-accretion complexes except for a few examples of back-arc basin or fore-arc origin. Accretion occurs episodically. In Southwest Japan, oceanic materials were accreted intermittently in (a) end-Permian, (b) Middle-Late Jurassic, (c) Late Cretaceous times, (d) at ca. 50 Ma, and (e) in Miocene times, while in Northeast Japan and Hokkaido this occurred in (b) Middle-Late Jurassic, (c) Late Cretaceous, and (f) Early Cretaceous times. In contrast to the general belief on accretion of younger oceanic plates, the majority of Japanese subduction-accretion complexes were formed during the subduction of plates, up to 160 Ma old. The accretionary events in end-Permian and Middle-Late Jurassic times coincide with northward collision of ancient island arcs, oceanic rises or seamount chains (of hot-spot origin) with the Asian continent. Accretion relevant to subduction of older plates may be controlled by the collision-subduction process of these topographic reliefs on an oceanic plate. In addition, the chronological coincidence with the continent collision-amalgamation between the Sino-Korean and Siberian platforms and between the Sino-Korean and Yangtze blocks, also implies collision-induced voluminous supply of elastics from back-arc regions and its contribution to the formation of huge accretionary complexes. Accreted fragments of ancient seamounts are much smaller than the average size of modern seamounts. This implies that most parts of a colliding seamount are not accreted but subducted together with the underlying oceanic crust to much deeper levels. With respect to the metamorphic grades for Japanese subduction complexes, oceanic materials have less than 1 vol.% in the zeolite facies, 15-20% in the prehnite-pumpellyite metagraywacke facies, and ca. 30% in the greenschist/glaucophane schist facies and albite-epidote amphibolite fades. This relationship indicates that the major process for landward accretion of oceanic materials is not off-scraping or sedimentary mixing at the trench, but underplating (subcretion) at much deeper levels of a subduction zone.

Isozaki, Y.; Maruyama, S.; Furuoka, F.

1990-09-01

152

Magnetic anomaly analysis of the Ionian Sea: Is it the oldest in-situ ocean fragment of the world?  

Science.gov (United States)

It is well known that the Ionian Sea is characterized by thin (8-11 km) crystalline crust, thick (5-7 km) sedimentary cover, and low heat flow, typical for a Mesozoic (at least) basin. Yet seismic data have not yielded univocal interpretations, and a debate has developed on the oceanic vs. 'thinned continental' nature of the Ionian basin. Here we analyze the magnetic anomaly pattern of the Ionian Sea, and compare it to synthetic fields produced by a geopotential field generator, considering realistic crust geometry. The Ionian basin is mostly characterized by slightly negative magnetic residuals, and by a prominent positive (150 nT at sea level) 'B' anomaly at the northwestern basin margin. We first test continental crust models, considering a homogeneous crystalline crust with K=1x10-3, then a 5 km thick deep crustal layer of serpentinite (K=1x10-1). First model yields insignificant anomalies, while the second gives an anomaly pattern anti-correlated with the observed residuals. We subsequently test oceanic crust models, considering a 2 km thick 2A basaltic layer with K=5x10-3, magnetic remanence of 5 A/m, and a unique magnetic polarity (no typical oceanic magnetic anomaly stripes are apparent in the observed data set). Magnetic remanence directions were derived from Pangean-African paleopoles in the 290-190 Ma age window. Only reverse-polarity models reproduce the B anomaly, and among them the 220-230 Ma models best approximate magnetic features observed on the abyssal plain and at the western basin boundary. The Ionian Sea turns out to be the oldest preserved oceanic floor known so far. Reference Speranza, F., L. Minelli, A. Pignatelli, and M. Chiappini (2012), The Ionian Sea: The oldest in situ ocean fragment of the world?, J. Geophys. Res., 117, B12101, doi:10.1029/2012JB009475.

Speranza, F.; Minelli, L.; Pignatelli, A.; Chiappini, M.

2013-12-01

153

A source for Icelandic magmas in remelted Iapetus crust  

Science.gov (United States)

The geochemistry and large melt volume in the Iceland region, along with the paucity of evidence for high, plume-like temperatures in the mantle source, are consistent with a source in the extensive remelting of subducted Iapetus crust. This may have been trapped in the Laurasian continental mantle lithosphere during continental collision in the Caledonian orogeny at ˜420-410 Ma, and recycled locally back into the asthenosphere beneath the mid-Atlantic ridge by lithospheric delamination when the north Atlantic opened. Fractional remelting of abyssal gabbro can explain the major-, trace- and rare-earth-element compositions, and the isotopic characteristics of primitive Icelandic tholeiite. An enriched component already present in the recycled crustal section in the form of enriched mid-ocean-ridge basalt, alkalic olivine basalt and/or related differentiates could contribute to the diversity of Icelandic basalts. Compositions ranging from ferrobasalt to olivine tholeiite are produced by various degrees of partial melting in eclogite, and the crystallization of ferrobasalt as oxide gabbro, i.e., containing the magmatic Fe-Ti oxide minerals, ilmenite and magnetite, may explain the anomalously high density of the Icelandic lower crust. The very high 3He/ 4He ratios observed in some Icelandic basalts may derive from old helium preserved in U+Th-poor residual Caledonian oceanic mantle lithosphere or olivine-rich cumulates in the crustal section. The persistence of anomalous volcanism at the mid-Atlantic ridge in the neighborhood of Iceland suggests that in the presence of lateral ridge migration, the shallow fertility anomaly must be oriented transverse to the mid-Atlantic ridge. The Greenland-Iceland-Faeroe ridge is co-linear with the western frontal thrust of the Caledonian collision zone, which may thus be associated with the fertility source. The fertile material beneath the Iceland region must lie at a steep angle or be thickened by deformation or imbrication to supply the large volumes of basalt required to build the thick crust there. "Hot spot" volcanism and large-igneous-province emplacement often occurs within or near to old suture zones and similar processes may thus explain anomalous magmatism elsewhere that is traditionally attributed to plumes.

Foulger, G. R.; Natland, J. H.; Anderson, D. L.

2005-03-01

154

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

2014-09-01

155

Structure and tectonic evolution of the Southern Eurasia Basin, Arctic Ocean  

Science.gov (United States)

Multichannel seismic reflection data acquired by Marine Arctic Geological Expedition (MAGE) of Murmansk, Russia in 1990 provide the first view of the geological structure of the Arctic region between 77-80°N and 115-133°E, where the Eurasia Basin of the Arctic Ocean adjoins the passive-transform continental margin of the Laptev Sea. South of 80°N, the oceanic basement of the Eurasia Basin and continental basement of the Laptev Sea outer margin are covered by 1.5 to 8 km of sediments. Two structural sequences are distinguished in the sedimentary cover within the Laptev Sea outer margin and at the continent/ocean crust transition: the lower rift sequence, including mostly Upper Cretaceous to Lower Paleocene deposits, and the upper post-rift sequence, consisting of Cenozoic sediments. In the adjoining Eurasia Basin of the Arctic Ocean, the Cenozoic post-rift sequence consists of a few sedimentary successions deposited by several submarine fans. Based on the multichannel seismic reflection data, the structural pattern was determined and an isopach map of the sedimentary cover and tectonic zoning map were constructed. A location of the continent/ocean crust transition is tentatively defined. A buried continuation of the mid-ocean Gakkel Ridge is also detected. This study suggests that south of 78.5°N there was the cessation in the tectonic activity of the Gakkel Ridge Rift from 33-30 until 3-1 Ma and there was no sea-floor spreading in the southernmost part of the Eurasia Basin during the last 30-33 m.y. South of 78.5°N all oceanic crust of the Eurasia Basin near the continental margin of the Laptev Sea was formed from 56 to 33-30 Ma.

Sekretov, Sergey B.

2002-07-01

156

An alternative early opening scenario for the Central Atlantic Ocean  

Science.gov (United States)

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

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

2009-12-01

157

Ocean basin volume constraints on global sea level since the Jurassic  

Science.gov (United States)

Changes in the volume of the ocean basins, predominately via changes in the age-area distribution of oceanic lithosphere, have been suggested as the main driver for long-term eustatic sea-level change. As ocean lithosphere cools and thickens, ocean depth increases. The balance between the abundance of hot and buoyant crust along mid ocean ridges relative to abyssal plains is the primary driving force of long-term sea level changes. The emplacement of volcanic plateaus and chains as well as sedimentation contribute to raising eustatic sea level. Quantifying the average ocean basin depth through time primarily relies on the present day preserved seafloor spreading record, an analysis of the spatio-temporal record of plate boundary processes recorded on the continental margins adjacent to ocean basins as well as a consideration of the rules of plate tectonics, to reconstruct the history of seafloor spreading in the oceanic basins through time. This approach has been successfully applied to predict the magnitude and pattern of eustatic sea-level change since the Cretaceous (Müller et. al. 2008) but uncertainties in reconstructing mid ocean ridges and flanks increase back through time, given that we mainly depend on information preserved in preserved ocean crust. We have reconstructed the age-area distribution of oceanic lithosphere and the plate boundary configurations back to the Jurassic (200 Ma) in order to assess long-term sea-level change from amalgamation to dispersal of Pangaea. We follow the methodology presented in Müller et. al. (2008) but incorporate a new absolute plate motion model derived from Steinberger and Torsvik (2008) prior to 100 Ma, a merged Wessel et. al. (2006) and Wessel and Kroenke (2008) fixed Pacific hotspot reference frame, and a revised model for the formation of Panthalassa and the Cretaceous Pacific. Importantly, we incorporate a model for the break-up of the Ontong Java-Manihiki-Hikurangi plateaus between 120-86 Ma. We extend a tentative model back to the Triassic by reconstructing the spreading history in Panthalassa and the Tethys and Mongol-Okhotsk Oceans. Even though the details of the mid-ocean ridge geometries in these ocean basins are largely unknown, we make conservative estimates, applying Occam's razor paired with the rules of plate tectonics as a guide to available geophysical and geological data, to estimate simple mid-ocean ridge geometries guided by what is know about Jurassic ocean crust in the Pacific. Our results suggest that old mid-ocean ridge flanks in the proto-Pacific and the Tethys oceans are gradually destroyed between 200 and 150 Myr ago, while new mid-ocean ridge systems are initiated in a stepwise fashion between the Late Jurassic and Early Cretaceous. This cycle leads to a substantial younging of the ocean basins from the Jurassic to the Cretaceous period, followed by a gradually ageing of the oceanic lithosphere from the Cretaceous seafloor spreading pulse to the present day. Our age-area distribution of oceanic lithosphere from 250 Ma to the present day produces a record of long-term sea level change that agrees well with independently computed estimates for eustasy over the same time period based on continental flooding and may be linked with the cycle of supercontinent formation and dispersal.

Seton, M.; Müller, R. D.

2011-12-01

158

Tectonic evolution of the Caribbean and northwestern South America: The case for accretion of two Late Cretaceous oceanic plateaus  

Science.gov (United States)

It is widely accepted that the thickened oceanic crust of the Caribbean plate, its basaltic accreted margins, and accreted mafic terranes in northwestern South America represent the remnants of a single ca. 90 Ma oceanic plateau. We review geologic, geochemical, and paleomagnetic evidence that suggests that the Caribbean-Colombian oceanic plateau in fact represents the remnants of two different oceanic plateaus, both dated as ca. 90 Ma. The first of these plateaus, the Caribbean Plateau, formed ca. 90 Ma in the vicinity of the present-day Galapagos hotspot. Northeastward movement of the Farallon plate meant that this plateau collided with the proto Caribbean arc and northwestern South America Sala y Gomez hotspot. Over the next ˜45 m.y., this plateau was carried progressively northeastward on the Farallon plate and collided in the middle Eocene with the proto Andean subduction zone in northwestern South America. The recognition of a second ca. 90 Ma Pacific oceanic plateau strengthens the link between plateau formation and global oceanic anoxic events.

Kerr, Andrew C.; Tarney, John

2005-04-01

159

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

160

Metasomatic overprint of CO2-rich fluids/melts from altered oceanic crust and subducted Triassic plume-influenced volcanics via shallow oceanic subduction of Cyprus arc, and remobilization of EM-II type mantle metasomes beneath Isparta Angle, SW Turkey  

Science.gov (United States)

Episodic extensional basins of Isparta Angle (SW Turkey) host similar post-collisional mafic (Si-poor and Si-rich) potassic rock types (e.g., lamproite, leucite-basalt, lamprophyre, absarokite, shoshonite) with contrasting geochemical signatures within the distinct geographical locations (e.g. orogenic-types at north, transitional-types at centre and anorogenic-types at centre and south) in a back-arc position. Such contrasting magmas producing these rock series can be evaluated into two groups from north to south: i) older (Early-Late Miocene) orogenic (Si-rich and Si-poor) mafic potassic magmas in the northern Kirka, Afyon, Sandikli extensional basins, and ii) younger (Late Miocene-Plio-Quaternary) transitional to anorogenic (Si-poor) mafic potassic magmas in the central Isparta and southern Bucak extensional basins. Accordingly, relative to the wide variation of 87-86Sr isotopic compositions in northern orogenic magmas, Nd-Pb isotopic variations reflect a gradual decrease in 207-208Pb with an increasing 206Pb and 143/144Nd ratios from north to south, relatively. Similarly, in contrast to the those of young transitional to anorogenic magmas with FOZO-like mantle signature, geochemical and isotopic variations of older orogenic magmas suggest a crust-contaminated, subduction-modified (metasomatised) EM-II type mantle source, interacting with asthenosphere. It is inferred that carbonate wall-rock assimilation for orogenic Si-poor potassic magmas and carbonate-free wall-rock assimilation for orogenic Si-rich potassic magmas played a significant role during their evolution, as well as source contamination. The striking geochemical and isotopic variations of overlapping post-collisional older orogenic and younger transitional to anorogenic potassic magmas also reflect the geochemical imprints of distinct metasomatic agents in mantle metasomes beneath northern orogenic, central transitional and southern anorogenic volcanic centres. We propose that the metasomatic enrichment mechanism of the mantle metasomes beneath the southern transitional to anorogenic volcanic centres can be explained by the remobilization and overprinting of CO2-rich FOZO-like metasomatic melts/fluids, released by partial melting of altered oceanic crust and subducted Triassic plume-influenced materials (e.g., Triassic-Early Cretaceous alkali basalts with HIMU signature), into the pre-existing EM-II type orogenic mantle metasomes during shallow oceanic subduction of Cyprus arc (between Late Miocene and Plio-Quaternary). Remelting of this remobilized mantle metasomes (producing ultrapotassic melts), followed by mixing with asthenospheric magmas, have produced transitional-anorogenic potassic magmas with FOZO-like mantle signature, under an extensional tectonic.

Çoban, H.; Hoang, N.; Flower, M. F. J.; Parlak, O.; Caran, ?.; Dallai, L.; Çak?r, Ö.; Erduran, M.

2012-04-01

161

New K-Ar ages from La Montagne massif, Réunion Island (Indian Ocean), supporting two geomagnetic events in the time period 2.2-2.0 Ma  

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We present new radiometric ages obtained at the type locality in La Réunion Island where palaeomagnetic records of the Réunion events have first been identified. Seven dated lava flows from the Rivière Saint Denis section, which recorded a reverse-to-normal transition, display ages from 2.12 +/- 0.03 to 2.17 +/- 0.03 Ma, with a mean age of 2.15 +/- 0.02 Ma. Two significantly younger flows from this section, interpreted here as valley fill flows from trace elements compositions and Pb isotopic data, have been dated at 2.06 +/- 0.03 and 2.08 +/- 0.03 Ma. Within the Grande Chaloupe section, where a normal-to-reverse transition is recorded, two coherent ages of 2.05 +/- 0.03 and 2.03 +/- 0.03 Ma have been obtained. When a direct comparison was possible, our new K-Ar ages performed on separated groundmass show a rather good coherence with previous ages from La Réunion Island. When considered with continuous palaeomagnetic sedimentary records in the 2.2-2.0 Ma interval, these new results suggest that two distinct events are recorded in La Montagne lava flows at La Réunion Island, with ages of 2.15 +/- 0.02 and 2.04 +/- 0.02 Ma. Following recent nomenclature, the former is the Réunion event s.s., while the latter can be related to the Huckleberry Ridge event. Globally distributed volcanic and sedimentary records show that the first (Réunion s.s.; RU-1) is associated with a large dipole intensity decrease at 2.15 +/- 0.02 Ma, and hence is recorded in many sequences. On the other hand, the dipole intensity decrease was not as pronounced at 2.04 +/- 0.02 Ma, when the Huckleberry Ridge (RU-2) palaeomagnetic event occurred. Consequently, it is not present as a full directional change in many sections worldwide, but rather appears as a geomagnetic excursion during an episode of increased secular variation. Finally, the use of the Réunion event for magnetostratigraphic studies is recommended, while the clear identification of the Huckleberry Ridge excursion might often be difficult, preventing its use as a reliable time marker.

Quidelleur, X.; Holt, J. W.; Salvany, T.; Bouquerel, H.

2010-08-01

162

Comment on "207Pb-206Pb single-zircon evaporation ages of some granitoid rocks reveal continent-oceanic island arc collision during the Cretaceous geodynamic evolution of the Central Anatolian crust, Turkey" - Boztug, D., Tichomirowa, M. & Bombach, K., 2007, JAES 31, 71-86  

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A continent-oceanic island arc collision model was proposed as a new geodynamic scenario for the evolution of the Cretaceous Central Anatolian granitoids in the Central Anatolian crystalline complex (CACC) by Boztug et al. (2007b) [Boztug, D., Tichomirowa, M., Bombach, K., 2007b. 207Pb-206Pb single-zircon evaporation ages of some granitoid rocks reveal continent-oceanic island arc collision during the Cretaceous geodynamic evolution of the central Anatolian crust, Turkey. Journal of Asian Earth Sciences 31, 71-86]. The key aspects of this model include an intra-oceanic subduction in the Neotethyan Izmir-Ankara Ocean, formation of an island arc and its subsequent collision with the northern margin of the Tauride-Anatolide Platform. The identical scenario was initially proposed by Göncüoglu et al. (1992) [Göncüoglu, M.C., Erler, A., Toprak, V., Yal?n?z, K., Olgun, E., Rojay, B., 1992. Geology of the western Central Anatolian Massif, Part II: Central Areas. TPAO Report No: 3155, 76 p] . Moreover, the weighted mean values of the reported 207Pb-206Pb single-zircon evaporation ages by Boztug et al. (2007b) [Boztug, D., Tichomirowa, M., Bombach, K., 2007b. 207Pb-206Pb single-zircon evaporation ages of some granitoid rocks reveal continent-oceanic island arc collision during the Cretaceous geodynamic evolution of the central Anatolian crust: Turkey. Journal of Asian Earth Sciences 31, 71-86] from A-type granitoids in the CACC seem to be miscalculated and contrast with the field data.

Göncüoglu, M. Cemal

163

R-modes of neutron stars with a solid crust  

OpenAIRE

We investigate the properties of $r$-mode oscillations of a slowly rotating neutron star with a solid crust, by taking account of the effects of the Coriolis force. For the modal analysis we employ three-component neutron star models that are composed of a fluid core, a solid crust and a surface fluid ocean. For the three-component models, we find that there exist two kinds of $r$-modes, that is, those confined in the surface fluid ocean and those confined in the fluid core,...

Yoshida, Shijun; Lee, Umin

2000-01-01

164

Is Tibetan Crust Hot?  

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Strong impedance contrasts, detected by seismic reflection profiles, have been interpreted as evidence for aqueous fluids or even partial melts below a depth of about 20 km in southern Tibet, suggesting unusually high temperatures in the crust which reaches a thickness of up to 75 km in places. A collection of recent, complementary seismic observations reveal that the northern edge of underthrust Indian mantle lithosphere ("Greater India") has advanced, en masse, beneath Tibet sub-horizontally some 600 km northward from the Himalayan collision front. This configuration of overlapping lithosphere, however, calls for a low geotherm in much of the lower Tibetan crust, because the progressive advance of the Archean (therefore cold) mantle keel provides an effective heat sink that cools the crust above. Here we present a thermal model for the Tibetan lithosphere that is consistent with the current tectonic configuration. In essence, the Tibetan crust is cooled from below by a horizontally advancing Greater India as the entire package of overlapping lithosphere adjusts itself thermally toward a new equilibrium state. Cooling is most pronounced in the first 10 My of collision; after 30 My, temperature in the lower crust and the uppermost mantle has become low. Even so, including in our model effects of deep circulation of groundwater along steeply inclined extensional fault zones can account for the common occurrence of hot springs in rift valleys of southern Tibet. Together with empirical results on the temperature of brittle-ductile transitions in crustal and mantle rocks, temperature profile in our model can also explain the bimodal distribution of seismicity beneath southern Tibet, i.e., pronounced seismicity in the upper crust down to depths of 15 to 20 km, general absence of seismicity from 20 to 80 km, and reappearance of seismicity in the uppermost mantle at about 80 to 100 km.

Wang, C.; Chen, W.

2011-12-01

165

Reconstructing the Quaternary evolution of the world's most active silicic volcanic system: insights from an ˜1.65 Ma deep ocean tephra record sourced from Taupo Volcanic Zone, New Zealand  

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The Taupo Volcanic Zone (TVZ), central North Island, New Zealand, is the most frequently active Quaternary rhyolitic system in the world. Silicic tephras recovered from Ocean Drilling Programme Site 1123 (41°47.16'S, 171°29.94'W; 3290 m water depth) in the southwest Pacific Ocean provide a well-dated record of explosive TVZ volcanism since ˜1.65 Ma. We present major, minor and trace element data for 70 Quaternary tephra layers from Site 1123 determined by electron probe microanalysis (1314 analyses) and laser ablation inductively coupled plasma mass spectrometry (654 analyses). Trace element data allow for the discrimination of different tephras with similar major element chemistries and the establishment of isochronous tie-lines between three sediment cores (1123A, 1123B and 1123C) recovered from Site 1123. These tephra tie-lines are used to evaluate the stratigraphy and orbitally tuned stable isotope age model of the Site 1123 composite record. Trace element fingerprinting of tephras identifies ˜4.5 m and ˜7.9 m thick sections of repeated sediments in 1123A (49.0-53.5 mbsf [metres below seafloor]) and 1123C (48.1-56.0 mbsf), respectively. These previously unrecognised repeated sections have resulted in significant errors in the Site 1123 composite stratigraphy and age model for the interval 1.15-1.38 Ma and can explain the poor correspondence between ?18O profiles for Site 1123 and Site 849 (equatorial Pacific) during this interval. The revised composite stratigraphy for Site 1123 shows that the 70 tephra layers, when correlated between cores, correspond to ˜37-38 individual eruptive events (tephras), 7 of which can be correlated to onshore TVZ deposits. The frequency of large-volume TVZ-derived silicic eruptions, as recorded by the deposition of tephras at Site 1123, has not been uniform through time. Rather it has been typified by short periods (25-50 ka) of intense activity bracketed by longer periods (100-130 ka) of quiescence. The most active period (at least 1 event per 7 ka) occurred between ˜1.53 and 1.66 Ma, corresponding to the first ˜130 ka of TVZ rhyolitic magmatism. Since 1.2 Ma, ˜80% of tephras preserved at Site 1123 and the more proximal Site 1124 were erupted and deposited during glacial periods. This feature may reflect either enhanced atmospheric transport of volcanic ash to these sites (up to 1000 km from source) during glacial conditions or, more speculatively, that these events are triggered by changes in crustal stress accumulation associated with large amplitude sea-level changes. Only 8 of the ˜37-38 Site 1123 tephra units (˜20%) can be found in all three cores, and 22 tephra units (˜60%) are only present in one of the three cores. Whether a tephra is preserved in all three cores does not have any direct relationship to eruptive volume. Instead it is postulated that tephra preservation at Site 1123 is 'patchy' and influenced by the vigorous nature of their deposition to the deep ocean floor as vertical density currents. At this site, at least 5 cores would need to have been drilled within a proximity of 10's to 100's of metres of each other to yield a >99% chance of recovering all the silicic tephras deposited on the ocean surface above it in the past 1.65 Ma.

Allan, Aidan S. R.; Baker, Joel A.; Carter, Lionel; Wysoczanksi, Richard J.

2008-12-01

166

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  

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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

167

Zircon Hf isotope signature of the depleted mantle in the Myanmar jadeitite: Implications for Mesozoic intra-oceanic subduction between the Eastern Indian Plate and the Burmese Platelet  

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This study systematically investigated the Lu-Hf isotope signatures of zircons in jadeitite from the North Myanmar, which was formed from jadeite-forming hydrothermal fluids and is always sourced in serpentinite as a mark for the subduction zone. Results show that the three group zircons have positive ?Hf( t) values. Group-I zircons (163.2 ± 3.3 Ma) have relatively higher 176Lu/ 177Hf ratios (> 0.0004) and the corresponding ?Hf( t) values range from 15.5 to 20.0 at t = 163 Ma. In contrast, Group-II zircons (146.5 ± 3.4 Ma) exhibit highly variable 176Lu/ 177Hf ratios from 0.000027 to 0.001398, but still have ?Hf( t) values of 15.6 to 18.5 at t = 147 Ma, which resemble to those of the Group-I zircons. The Group-III zircon (122.2 ± 4.8 Ma) has a 176Lu/ 177Hf ratio of 0.000578 and a ?Hf( t) value of 15.8 at t = 122 Ma. Such highly positive ?Hf( t) values for all jadeitite zircons indicate that they were derived from rapid reworking of the very juvenile crust. Therefore, zircon in jadeitite can be used as a valuable mineral to constrain the age of serpentinization/rodingitization, and even the age of formation of ultramafic rock within ophiolites. The results also suggest the presence of the Mesozoic intra-oceanic subduction within the Indo-Burman Range, and further suggest that the hydrothermal fluids were derived from dehydration of seawater-altered oceanic juvenile crust that had been hydrated during and/or after the formation of the oceanic crust, and additionally from serpentine minerals at greater depth. Consequently, the fluids carry the Hf isotope signature of the depleted mantle that can be later imparted to the jadeitite zircons.

Shi, Guang-Hai; Jiang, Neng; Liu, Yan; Wang, Xia; Zhang, Zhi-Yu; Xu, Yong-Jing

2009-10-01

168

Detrital footprint of the Mozambique ocean: U/Pb SHRIMP and Pb evaporation zircon geochronology of metasedimentary gneisses in eastern Madagascar  

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U-Pb Sensitive High-mass Resolution Microprobe (SHRIMP) and Pb evaporation analyses of detrital zircons from metasedimentary rocks in eastern Madagascar reveal that: 1) The protoliths of many of these rocks were deposited between ˜800 and 550 Ma; 2) these rocks are sourced from regions with rocks that date back to over 3400 Ma, with dominant age populations of 3200--3000 Ma, ˜2650 Ma, ˜2500 Ma, and 800--700 Ma. The Dharwar Craton of southern India is a potential source region for these sediments, as here rocks date back to over 3400 Ma and include abundant gneissic rocks with protoliths older than 3000 Ma, sedimentary rocks deposited at 3000--2600 Ma and granitoids that crystallised at 2513--2552 Ma. The 800-700 Ma zircons could potentially be sourced from elsewhere in India or from the Antananarivo Block of central Madagascar in the latter stages of closure of the Mozambique Ocean. The region of East Africa adjacent to Madagascar in Gondwana reconstructions (the Tanzania craton) is rejected as a potential source as there are no known rocks here older than 3000 Ma, and no detrital grains in our samples sourced from Mesoproterozoic and early Neoproterozoic rocks that are common throughout central east Africa. In contrast, coeval sediments 200 km west, in the Itremo sheet of central Madagascar, have detrital zircon age profiles consistent with a central East African source, suggesting that two late Neoproterozoic provenance fronts crop out in east Madagascar at approximately the position of the Betsimisaraka suture. These observations support an interpretation that the Betsimisaraka suture separates rocks that were derived from different locations within, or at the margins of, the Mozambique Ocean basin and therefore, that the suture is the site of subduction of a strand of Mozambique Ocean crust.

Collins, A. S.; Kröner, A.; Fitzsimons, I. C. W.; Razakamanana, T.; Windley, B. F.

2003-04-01

169

Melting of continental crust during subduction initiation: A case study from the Chaidanuo peraluminous granite in the North Qilian suture zone  

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

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

2014-05-01

170

Rapid crustal accretion and magma assimilation in the Oman-U.A.E. ophiolite: High precision U-Pb zircon geochronology of the gabbroic crust  

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New high-precision U/Pb zircon geochronology from the Oman-United Arab Emirates (U.A.E.) ophiolite provides insight into the timing and duration of magmatism and the tectonic setting during formation of the lower crust. The new data come from a well-preserved and exposed crustal section in the center of the Wadi Tayin massif. Single grain and grain fragment 206Pb/238U dates from upper-level gabbros, tonalites/trondhjemites and gabbroic pegmatites, corrected for initial Th exclusion, range from 112.55 ± 0.21 to 95.50 ± 0.17 Ma, with most data clustered between 96.40 ± 0.17 to 95.50 ± 0.17 Ma. Zircon dates from upper-level gabbros are most consistent with the ophiolite forming at a fast spreading ridge with half-rates of 50-100 km/Ma. Dates from tonalites/trondhjemites and from a gabbroic pegmatite associated with a wehrlite intrusion overlap with dates from adjacent upper-level gabbros, suggesting that any age differences between these three magmatic series are smaller than the analytical uncertainties or intrasample variability in the dates. Three of the dated upper-level gabbros and a single gabbroic pegmatite from the base of the crust have >1 Ma intrasample variability in single grain dates, suggesting assimilation of older crust during the formation or crystallization of the magmas. Whole rock ?Nd(t) of seven samples, including the upper-level gabbros with variable zircon dates, have tightly clustered initial values ranging from ?Nd(96 Ma) = 7.59 ± 0.23 to 8.28 ± 0.31. The ?Nd values are similar to those from other gabbros within the ophiolite, suggesting that any assimilated material had a similar isotopic composition to primitive basaltic magmas. The new dates suggest that the studied section formed at a fast spreading mid-ocean ridge between ˜96.4-95.5 Ma. The large intrasample variability in zircon dates in some samples is unexpected in this setting, and may be related to propagation of a younger ridge into older oceanic lithosphere.

Rioux, Matthew; Bowring, Samuel; Kelemen, Peter; Gordon, Stacia; DudáS, Frank; Miller, Robert

2012-07-01

171

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

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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

172

Relationship between depth and age in the North Pacific Ocean  

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The North Pacific contains active mid-oceanic ridges and the oldest, Jurassic (166.8 ± 4 Ma), drilled oceanic crust. Its bathymetry is therefore critical to studies of the applicability of thermal contraction models (e.g., the infinite half-space and cooling plate) to the subsidence of seafloor with crustal age. The bathymetry, however, contains seamounts and oceanic islands (e.g., Mid-Pacific Mountains), oceanic plateaus (e.g., Hess, Magellan, and Shatsky), and midplate topographic swells (e.g., Hawaii), which are unrelated to the current plate-scale thermal state of the oceanic lithosphere. We use here a regional-residual separation algorithm called MiMIC to remove these features and to isolate the depths associated with the subsidence of North Pacific oceanic crust. These depths, z (m), increase with time, t (Ma), as z = 3010 + 307? until 85 Ma. For greater ages the depths "flatten" and asymptotically approach ˜6.1 km and are well described by z = 6120 - 3010 exp(-0.026t). The flattening is not "abrupt" as recently described in z-t curves produced using the mean, median, and mode. As a result, the depths of both young and old seafloor are fit well (mean difference between and observed and calculated depths of 75 ± 54 m 1?) by a single cooling plate model. Using a thermal conductivity, k, of 3.138 mW m-2 as previous studies, however, gives a plate of similar thickness (i.e., thermal thickness, L, of ˜115 km) but one which is unreasonably hot (i.e., temperature at the base of the plate, Tb, of 1522 °C) and inexpansive (i.e., coefficient of thermal expansion, ?, of 2.57×10-5 °C-1). More reasonable values (i.e., Tb = 1363°C, k = 3.371 W m-1 °C-1, ? = 2.77×10-5 °C-1, and L = 120 km) are obtained if the crustal thickness is used to constrain Tb and a certain amount of the surface heat flow is allowed to be radiogenically generated within the oceanic lithosphere.

Hillier, J. K.; Watts, A. B.

2005-02-01

173

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

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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

174

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

175

Metamorphism and melting of picritic crust in the early Earth  

Science.gov (United States)

Partial melting experiments with models of Archean oceanic crust (MAOC; with 11, 13 and 15 wt.% MgO) are used to investigate the role of metamorphism and melting of primary picritic compositions in the formation of TTG-like melts and continental crust on the early Earth. The approach investigates the possibility that the average early crust composition was comparatively MgO-rich and evolved to lower magnesium content during the secular cooling of the Earth. High-pressure partial melting experiments indicate a transition of melt compositions from aluminous basaltic melts in MAOC 15 to predominantly tonalitic melts in MAOC 11 and higher melting temperatures with increasing magnesium in the bulk composition. Tonalitic melts were generated in MAOC 11 and 13 at pressures ? 12.5 kbar along with the residual phases garnet + clinopyroxene + plagioclase ± quartz (± orthopyroxene in the presence of quartz and at lower pressures) in the absence of amphibole. Basaltic melts were generated at pressures ? 15 kbar predominantly in the presence of granulite facies residues such as amphibole ± garnet ± plagioclase + orthopyroxene that lack quartz in all MAOC compositions. The tonalitic melts generated in MAOC 11 and 13 indicate that thicker oceanic crust with more magnesium than that of a modern MORB is a viable source for the generation of early Archean high-Si, low-MgO melts, and therefore TTG-like continental crust in the Archean. The favoured settings for the generation of these melts at pressures up to 15 kbar are the base of oceanic crust much thicker than today or melting of slabs in shallow subduction zones, both without interaction of the melts with the mantle during passage to the surface. Tonalitic melts may have formed in deeper subduction zones at 20 kbar beyond plagioclase stability but it is unlikely that such melts could migrate to shallower levels without further mantle interaction. This process may have become more important during the progressive cooling of the Earth.

Ziaja, Karen; Foley, Stephen F.; White, Richard W.; Buhre, Stephan

2014-02-01

176

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

177

Hawaiian submarine manganese-iron oxide crusts - A dating tool?  

Science.gov (United States)

Black manganese-iron oxide crusts form on most exposed rock on the ocean floor. Such crusts are well developed on the steep lava slopes of the Hawaiian Ridge and have been sampled during dredging and submersible dives. The crusts also occur on fragments detached from bedrock by mass wasting, on submerged coral reefs, and on poorly lithified sedimentary rocks. The thickness of the crusts was measured on samples collected since 1965 on the Hawaiian Ridge from 140 dive or dredge localities. Fifty-nine (42%) of the sites were collected in 2001 by remotely operated vehicles (ROVs). The thinner crusts on many samples apparently result from post-depositional breakage, landsliding, and intermittent burial of outcrops by sediment. The maximum crust thickness was selected from each dredge or dive site to best represent crusts on the original rock surface at that site. The measurements show an irregular progressive thickening of the crusts toward the northwest-i.e., progressive thickening toward the older volcanic features with increasing distance from the Hawaiian hotspot. Comparison of the maximum crust thickness with radiometric ages of related subaerial features supports previous studies that indicate a crust-growth rate of about 2.5 mm/m.y. The thickness information not only allows a comparison of the relative exposure ages of two or more features offshore from different volcanoes, but also provides specific age estimates of volcanic and landslide deposits. The data indicate that some of the landslide blocks within the south Kona landslide are the oldest exposed rock on Mauna Loa, Kilauea, or Loihi volcanoes. Crusts on the floors of submarine canyons off Kohala and East Molokai volcanoes indicate that these canyons are no longer serving as channelways for downslope, sediment-laden currents. Mahukona volcano was approximately synchronous with Hilo Ridge, both being younger than Hana Ridge. The Nuuanu landslide is considerably older than the Wailau landslide. The Waianae landslide southwest of Oahu has yielded samples with the greatest manganese-iron oxide crusts (9.5 mm thick) and therefore apparently represents the oldest submarine material yet found in the study area. The submarine volcanic field 100 km southwest of Oahu is apparently younger than the Waianae landslide. ?? 2004 Geological Society of America.

Moore, J.G.; Clague, D.A.

2004-01-01

178

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

179

The role of the mantle and lower crust in driving Transverse Range convergence  

Science.gov (United States)

Convergence in the Transverse Ranges is a consequence of lithosphere converging and sinking beneath the Transverse Ranges. By lying below the Pelona schist subduction complex, this lithosphere is not North America; it probably is a fragment of the Farallon oceanic plate. The schist outcrops on low-angle faults near the Transverse Ranges, suggesting that the aseismic lower crust is made largely of schist. The descending lithosphere pulls southern California crust toward the Transverse Ranges, maintaining the San Andreas as the dominant fault south of the Transverse Ranges. Lithospheric convergence is not matched by the crust, which avoids most convergence. This is a dynamic result of the mountains themselves, whose gravitational potential energy pushes back on the converging crust. Hence the upper crust and mantle are simultaneously dynamically coupled and kinematically decoupled by the viscous lower crust.

Humphreys, E.; Fay, N.

2004-12-01

180

A source for Icelandic magmas in remelted Iapetus crust.  

OpenAIRE

The geochemistry and large melt volume in the Iceland region, along with the paucity of evidence for high, plume-like temperatures in the mantle source, are consistent with a source in the extensive remelting of subducted Iapetus crust. This may have been trapped in the Laurasian continental mantle lithosphere during continental collision in the Caledonian orogeny at 420–410 Ma, and recycled locally back into the asthenosphere beneath the mid-Atlantic ridge by lithospheric delamination when...

Foulger, G. R.; Natland, J. H.; Anderson, D. L.

2005-01-01

181

Copper-nickel-rich, amalgamated ferromanganese crust-nodule deposits from Shatsky Rise, NW Pacific  

Science.gov (United States)

A unique set of ferromanganese crusts and nodules collected from Shatsky Rise (SR), NW Pacific, were analyzed for mineralogical and chemical compositions, and dated using Be isotopes and cobalt chronometry. The composition of these midlatitude, deep-water deposits is markedly different from northwest-equatorial Pacific (PCZ) crusts, where most studies have been conducted. Crusts and nodules on SR formed in close proximity and some nodule deposits were cemented and overgrown by crusts, forming amalgamated deposits. The deep-water SR crusts are high in Cu, Li, and Th and low in Co, Te, and Tl concentrations compared to PCZ crusts. Thorium concentrations (ppm) are especially striking with a high of 152 (mean 56), compared to PCZ crusts (mean 11). The deep-water SR crusts show a diagenetic chemical signal, but not a diagenetic mineralogy, which together constrain the redox conditions to early oxic diagenesis. Diagenetic input to crusts is rare, but unequivocal in these deep-water crusts. Copper, Ni, and Li are strongly enriched in SR deep-water deposits, but only in layers older than about 3.4 Ma. Diagenetic reactions in the sediment and dissolution of biogenic calcite in the water column are the likely sources of these metals. The highest concentrations of Li are in crust layers that formed near the calcite compensation depth. The onset of Ni, Cu, and Li enrichment in the middle Miocene and cessation at about 3.4 Ma were accompanied by changes in the deep-water environment, especially composition and flow rates of water masses, and location of the carbonate compensation depth.

Hein, J. R.; Conrad, T. A.; Frank, M.; Christl, M.; Sager, W. W.

2012-10-01

182

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

183

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

184

Generation of the Early Cenozoic adakitic volcanism by partial melting of mafic lower crust, Eastern Turkey: Implications for crustal thickening to delamination  

Science.gov (United States)

Early Cenozoic (48-50 Ma) adakitic volcanic rocks from the Eastern Pontides, NE Turkey, consist of calc-alkaline and high-K calc-alkaline andesite and dacite, with SiO 2 contents ranging from 56.01 to 65.44 wt.%. This is the first time that Early Eocene volcanism and adakites have been reported from the region. The rocks are composed of plagioclase, amphibole, quartz, and Mg-rich biotite. They have high and low-Mg# values ranging from 55 to 62 and 13 to 42, respectively. High-Mg# rocks have higher Ni and Co contents than low-Mg# samples. The rocks exhibit enrichments in large ion lithophile elements including the light rare earth elements, depletions in Nb, Ta and Ti and have high La/Yb and Sr/Y ratios. Their relative high ISr (0.70474-0.70640) and low ?Nd (50 Ma) values (- 2.3 to 0.8) are inconsistent with an origin as partial melts of a subducted oceanic slab. Combined major- and trace element and Sr-Nd isotope data suggest that the adakitic magmas are related to the unique tectonic setting of this region, where a transition from a collision to an extension stage has created thickening and delamination of the Pontide mafic lower crust at 50 Ma. The high-Mg adakitic magmas resulted from partial melting of the delaminated eclogitic mafic lower crust that sank into the relatively hot subcrustal mantle, and its subsequent interaction with the mantle peridotite during upward transport, leaving garnet as the residual phase, elevates the MgO content and Mg# of the magmas, whereas low-Mg# magmas formed by the melting of newly exposed lower crustal rocks caused by asthenospheric upwelling, which supplies heat flux to the lower crust. The data also suggest that the mafic lower continental crust beneath the region was thickened between the Late Cretaceous and the Late Paleocene and delaminated during Late Paleocene to Early Eocene time, which coincides with the initial stage of crustal thinning caused by crustal extensional events in the Eastern Pontides and rules out the possibility of an extensional regime before ~ 50 Ma in the region during the Late Mesozoic to Early Cenozoic.

Karsli, Orhan; Dokuz, Abdurrahman; Uysal, ?brahim; Aydin, Faruk; Kandemir, Raif; Wijbrans, Jan

2010-01-01

185

Spreading behaviour of the Pacific-Farallon ridge system between 83 and 28 Ma  

Science.gov (United States)

At 83 Ma, the roughly N-S oriented Pacific-Farallon ridge extended more than 10,000 km from 51° N to 43° S. Despite substantial shortening of the ridge system since ~55 Ma, this ridge and its remnants (e.g., the East Pacific Rise/EPR) have produced as much as 45% of all the reconstructable oceanic lithosphere created in the Late Cretaceous and Cenozoic. Accurately reconstructing the past spreading history of the Pacific-Farallon ridge is therefore of paramount importance for determining possible variations in global spreading rates over geological time, which are the basis of suggested interactions between mantle dynamics, surface tectonics, sea-level rise and climate in the past 100 Myr. However, attempts to accurately determine Pacific-Farallon spreading face the twin challenges of extensive subduction of Farallon crust - which precludes reconstruction by fitting conjugate magnetic anomaly and fracture zone traces - and the well-established asymmetric spreading behaviour of the EPR and its ancestor ridges for at least the past 51 Myr. We present improved rotation poles for the Pacific-Farallon spreading system between geomagnetic chrons 34y (83 Ma) and 10y (28.28 Ma), complete with uncertainties that allow easier combination into global plate circuits. These poles are derived by combining magnetic anomaly and fracture zone data from both the northern and southern Pacific plate, maximising the data distribution along the original ridge length to average out local variations in spreading behaviour. We have calculated best fit 'half'-stage poles for Pacific-Farallon spreading between nine Pacific plate magnetic anomalies (34y, 33y, 29o, 24.3o, 20o, 18.2o, 17.1y, 13y and 10y). For poles younger than chron 24.3o, full stage poles have been calculated by using anomaly picks from yet-to-be subducted Farallon/Nazca crust in the south Pacific to determine spreading asymmetry. Characterisation of the variation in spreading asymmetry in the past 50 Ma also allows bounds to be placed on the asymmetry between 50 and 83 Ma and full stage poles to be calculated for this period. Our new poles plot in a similar region to previously published Pacific-Farallon rotations, but synthetic flowlines produce a better match to Pacific fracture zone trends, with significantly less latitudinal drift. With the exception of the 18o-20o stage pole, the six stage poles between chrons 33y and 13y (74-33 Ma) all cluster tightly at 60-75° E, 60-68° N, which is consistent with the relatively constant orientation of the major Pacific fracture zones. This period of stability spans several episodes of Farallon plate fragmentation, as segments of the Pacific-Farallon ridge were subducted beneath the Americas. Therefore, these events do not appear to have particularly affected the position of the instantaneous rotation pole; in contrast, Farallon plate fragmentation events do appear to correlate to increases in spreading rate on the surviving ridge. Net Nazca:Pacific spreading asymmetry over the past 50 Myr is 56-58:42-44%, but over periods of less than 15 Myr there is considerably more variability. There are also quasi-periodic long-term variations in spreading asymmetry, with higher asymmetry between 15-25 Ma and 35-45 Ma, and more symmetric spreading in the past 10 Ma, between 25-35 Ma and possibly at around 50 Ma.

Rowan, C. J.; Rowley, D. B.

2012-12-01

186

Similarities Between a Sequence Stratigraphy Pattern Common to the Maldives and the Bahamas, and a Planktic Oxygen Isotope Record from the Equatorial Indian Ocean in the Miocene (19-9 Ma)  

Science.gov (United States)

A clear Miocene pattern emerges in the interpretation of a Shell 2-D seismic grid correlated to two industrial wells and three ODP sites in the Maldive Inner Sea and close vicinity. A basin-wide, spectacular, and synchronous aggradations of the Maldive carbonate bank margins occurred in the late part of the early Miocene (Burdigalian; 20.5-16.4 Ma) and earliest part of the middle Miocene (Langhian; 16.4-14.8 Ma). Then, following a systematic downward shift in the sedimentary deposition during the early part of the middle Miocene (early part of the Serravallian; 14.8-11.2 Ma), basin-wide synchronous progradations of the Maldive bank margins in the middle Miocene (Serravallian) are interpreted to represent five complete sea level cycles during an interval of falling sea level. The last cycle is characterized by unique massive gravity-flow deposits accumulated at the toe of the slope probably linked to a significant sea level drop at that time. The following late Miocene sequence (early Tortonian; 11.2-7.1 Ma) overlies the entire middle Miocene prograding complexes as well as the latest Burdigalian and Langhian bank margins. This sequence is interpreted as a major sea level transgression and flooding. This Miocene pattern observed in the Maldives is almost identical to the overall sequence stratigraphic signature observed in the interpretation of the Bahamas Western Line slightly modified from Eberli et al. (1987 & 1997). In the Bahamas, the carbonate bank margin can be interpreted as aggrading during the late early Miocene sequences "p" and "o". The middle Miocene sequences "n", "m", "l", and "k", likely corresponding to the middle Miocene prograding sequences 1 to 5 in the Maldives prograde just below the edge of the latest early Miocene carbonate margin. The first late Miocene sequence, "i" in the Bahamas is clearly overlying the middle Miocene prograding complex and the latest early Miocene carbonate bank top. Moreover, the Miocene sequence stratigraphy pattern, common to the Maldives and the Bahamas, mimics a planktic oxygen isotope record at Site 714 from the equatorial Indian Ocean in close vicinity of the Maldives where the variations of the sea surface temperatures were expected to be minimum. In this record, the lightest oxygen isotope values occurred at the very end of the Burdigalian, during the Langhian, and unexpectedly at the very beginning of the Tortonian. On the other hand, the Serravallian is characterized by a series of cycles that show a systematic increase of the heaviest values in each cycle, as generally observed in benthic foraminifer oxygen isotope records. The lightest values in the Serravallian cycles remain always heavier than the lightest values of the Langhian and early Tortonian. This observation contrasts with the benthic foraminifers oxygen isotope records in which values in the early Tortonian and during the following 10 my always remain heavier than the early middle Miocene values.

Droxler, A. W.; Belopolsky, A. V.; Vincent, E.

2001-05-01

187

Mars primordial crust: unique sites for investigating proto-biologic properties.  

Science.gov (United States)

The Martian meteorite collection suggests that intact outcrops or boulder-scale fragments of the 4.5 Ga Martian crust exist within tens of meters of the present day surface of Mars. Mars may be the only planet where such primordial crust samples, representing the first 100 Ma of a planet's environment, are available. The primordial crust has been destroyed on Earth by plate tectonics and other geological phenomena and is buried on the Moon under hundreds or thousands of meters of megaregoltih. Early Mars appears to have been remarkably similar to early Earth, and samples of rock from the first few Ma or first 100 Ma may reveal "missing link" proto-biological forms that could shed light on the transition from abiotic organic chemistry to living cells. Such organic snapshots of nascent life are unlikely to be found on Earth. PMID:17131091

Perry, Randall S; Hartmann, William K

2006-12-01

188

Lead isotopic evolution of Archean continental crust, Northern Tanzania  

Science.gov (United States)

The continental crust is stratified in composition; the upper crust is generally enriched in highly incompatible trace elements relative to the lower crust [1]. The Western Granulite section of the Mozambique Belt of Northern Tanzania yields Archean Nd model ages and has zircons with U-Pb ages of ~2.6 Ga [2,3], but was strongly re-worked during the Pan-African Orogeny, ca. 560 Ma [2,3,4]. Here we use time-integrated Pb isotopic modeling for lower and middle crustal xenoliths, as well as upper crustal granulites to determine the timing of, and degree of intra-crustal differentiation. The Pb isotopic compositions of most feldspars in the lower crustal samples, measured via LA-MC-ICPMS, fall on the trend defined by the Tanzanian Craton [5] and therefore, were most likely extracted from the mantle at a similar time, ca. 2.7 Ga. However, some xenoliths fall off this trend and show enrichment in 207Pb/204Pb, which we interpret as reflecting derivation from more heterogeneous mantle than that sampled in the Tanzanian Craton. In contrast to lower crustal xenoliths from the Tanzanian Craton [5], we see no single feldspar Pb-Pb isochrons, which indicates complete re-homogenization of the Pb isotopic composition of the feldspars in the lower crust of the Mozambique Belt during the Pan-African Orogeny, and heating to > 600°C [5]. Using time integrated Pb modeling, the upper crust of the Western Granulites is enriched in U by ˜ 2.5 relative to that of the lower crust, which must have taken place around the time of mantle extraction (ca. 2.7 Ga). In addition, these calculations are consistent with a Th/U ratio of ˜ 4 for the bulk lower crust and ˜ 3 for the bulk upper crust. The common Pb isotopic composition of a single middle crustal xenolith implies a Th/U of 20, but is unlikely to be generally representative of the middle crust. [1] Rudnick, R. L. and Gao, S. (2003). In the Crust, vol. 3, Treatise on Geochemistry:1-64. [2] Mansur, A. (2008) Masters Thesis, University of Maryland College-Park, http://hdl.handle.net/1903/8993 [3] Johnson, S. P et al. (2003). Tectonophysics, 375(1-4):125-145. [4] Blondes, M. S. et al. (2009). EOS Transactions, AGU, Fall Meeting(V13E-2073). [5] Bellucci, J. J. et al. (2010). Earth and Planetary Science Letters, in review.

Bellucci, J. J.; McDonough, W. F.; Rudnick, R. L.; Walker, R. J.

2010-12-01

189

Models of a partially hydrated Titan interior with clathrate crust  

Science.gov (United States)

We present an updated model of the interior evolution of Titan over time, assuming the silicate core was hydrated early in Titan's history and is dehydrating over time. The original model presented in Castillo-Rogez and Lunine (2010) was motivated by a Cassini-derived moment of inertia (Iess et al., 2010) for Titan too large to be accommodated by classical fully differentiated models in which an anhydrous silicate core was overlain by a water ice (with possible perched ocean) mantle. Our model consisted of a silicate core still in the process of dehydrating today, a situation made possible by the leaching of radiogenic potassium from the silicates into the liquid water ocean. The crust of Titan was assumed to be pure water ice I. The model was consistent with the moment of inertia of Titan, but neglected the presence of large amounts of methane in the upper crust invoked to explain methane's persistence at present and through geologic time (Tobie et al. 2006). We have updated our model with such a feature. We have also improved our modeling with a better physical model for the dehydration of antigorite and other hydrated minerals. In particular our modeling now simulates heat advection resulting from water circulation (e.g., Seipold and Schilling 2003), rather than the purely conductive heat transfer regime assumed in the first version of our model. The modeling proceeds as in Castillo-Rogez and Lunine (2010), with the thermal conductivity of the methane clathrate crust rather than that of ice I. The former is several times lower than that of the latter, and the two have rather different temperature dependences (English and Tse, 2009). The crust turns out to have essentially no bearing on the temperature of the silicate core and hence the timing of dehydration, but it profoundly affects the thickness of the high-pressure ice layer beneath the ocean. Indeed, with the insulating methane clathrate crust, there must be a liquid water ocean beneath the methane clathrate crust and in contact with the silicates beneath for most of Titan's history. Although a high-pressure ice layer is likely in place today, it is thin enough that plumes of hot water from the dehydrating core probably breach the high pressure ice layer maintaining contact between the ocean and the silicate core. Part of this work has been performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract to NASA. Government sponsorship acknowledged.

Lunine, J. I.; Castillo-Rogez, J.

2012-04-01

190

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

191

Issues of oxygen excess in the crust and upper mantle lithosphere  

Science.gov (United States)

Application of a new geochemical buffer, 'CeB' - Ce+4/Ce+3 for zircons, is promising for oxygen fugacity (FO2) estimation in crust and mantle. Absence of Ce+4 and Eu+2-enriched zircons are typical of the lower lithosphere. Reducing setting dominate in mantle rocks. Subduction adds oxidized substance for lithosphere into deeper mantle (Balashov ea, 2011-2012). The zircons in upper lithosphere are oxidized. Peridotites minerals show increased H2O and OH- preserves to 150-160 km at ?FMQ -1.4 - -0.1 (Babushkina et al, 2009) comparable with CeB 2.2 - 3.9. Increasing oceanic mass in the geological time controls water efflux and oxidation of upper the lithosphere. Oxygen source in crust and upper mantle is the most important, yet outstanding issues in geochemistry of Earth's upper shells. Oxygen excess in atmosphere correlating with long-term emergence and evolution of Earth's biosphere is an approach reflected in the schemes of cycle- and phase-wise biosphere evolution (Dobretsov et al, 2006; Sorokhtin et al, 2010). The both schemes demonstrate ideas for oxygen evolution of atmosphere, but are not confirmed by geochronology. Applying these outlines an actual picture FO2 evolution. Precambrian granitoids, detrital zircons and upper mantle lithosphere have similar CeB. The initial data include Australian Hadean and Archaean detrital zircons (Peck et al, 2001), CeB: 27.1 -1.96, and Eu+2/Eu+3: 0.015-0.12 (Balashov, Skublov, 2011). Greenland tonalities (3813 Ma) and granodiorite (3638 Ma) (Whitehouse, Kamber, 2002) CeB: 34 - 0.5. In oldest crust rocks dominated zircons with generation under high and heterogeneous FO2. Zircons in younger mantle-crustal rocks of S. American subduction zones (Ballard et al, 2002; Hoskin et al, 2000, etc.) show the same. Upper mantle lithosphere and crust represent continuously interacted with oxygen. If Progressively oxygen increase from Hadean to modern state (Dobretsov ea, 2006; Sorokhtin ea, 2010), contradicts with actual Archaean data. We believe in correlation of biosphere evolution with cyclic mantle and crustal magma activation (Balashov, Glaznev, 2006) reflecting variation of atmospheric volatiles. This corresponds to abrupt sulphur excess due to volcanogenic activation at the peak of the evolution fatally affected the biosphere state. However, volcanogenic epochs are relatively short-term not to contradict the synthesis of oxygen by the biosphere between them. This should ultimately result in significant oxygen heterogeneity in various rock types. Existence of a wide range of Ce+4/Ce+3 in all the surface systems of the Earth, and upper sequence of the mantle lithosphere is related to constant existence of exactly this heterogeneity. Alongside, various types of geological processes in the crust and mantle should have influence, or even define variation stages in the evolution of the biosphere itself. And, this has already been noted. Another constant oxygen source along the whole interval of the Earth's history should be considered solar wind. The continuous flow of the whole range of elements, which portion in the discharge of H, C, O, and other elements to the atmosphere in a proportion close to the composition of 1 (Anders, Grevesse, 1989), may be regarded as a quite competitive option with other sources of oxygen at the Earth's surface.

Balashov, Y. A.; Martynov, E. V.

2012-04-01

192

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 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

193

Nonradial Oscillations of Neutron Stars with a Solid Crust -- Analysis in the Relativistic Cowling Approximation--  

OpenAIRE

Nonradial oscillations of relativistic neutron stars with a solid crust are computed in the relativistic Cowling approximation, in which all metric perturbations are ignored. For the modal analysis, we employ three-component relativistic neutron star models with a solid crust, a fluid core, and a fluid ocean. As a measure for the relativistic effects on the oscillation modes, we calculate the relative frequency difference defined as $\\delta\\sigma/\\sigma\\equiv(\\sigma_{GR}-\\si...

Yoshida, Shijun; Lee, Umin

2002-01-01

194

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

195

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

196

147Sm? 143Nd, 146Sm? 142Nd systematics of early Archaean rocks and implications for crust-mantle evolution  

Science.gov (United States)

Combined high precision 147Sm? 143Nd, 146Sm? 142Nd data have been obtained for two early Archaean (pre-3.8 Ga) samples (a 4.0 ± 0.2 Ga meta-komatiite and an approximately 3.9 Ga orthogneiss) from the northern Labrador segment of the North Atlantic Craton. The samples have ?t143Nd values at 3.8 Ga of -4.0 and +1.4, respectively, however ?t142Nd142Nd for both samples is indistinguishable from the bulk earth value. For the sample with ?t143Nd - 4.0, ?t142Nd is + 0.03 ± 0.14, which suggests that it was derived from a source which differentiated from the primitive mantle at between 4.1 and 4.3 Ga. 14Sm? 143Nd and 146Sm? 142Nd data are also presented for a kyanite eclogite xenolith from a 350 Ma kimberlite from Yakutia, Siberia. 142Nd/ 144Nd for this sample is also within error of the bulk earth value. Eclogite xenoliths, such as those from kimberlite pipes, are, therefore, unlikely to be residues of crystallization of a pre-4.4 Ga magma ocean, but instead may represent fragments of post-4.3 Ga oceanic crust incorporated into the subcontinental lithospheric mantle. The absence of widespread variations in 142Nd/144Nd in early Archaean rocks supports models in which the volume of the depleted mantle has grown through time. Continuous growth of the depleted mantle and continental crust at the expense of the primordial mantle at between 4.56 and 4.0 Ga can generate variations in ?t143Nd and account for the absence of detectable variations in 142Nd/144Nd in most early Archaean rocks.

Regelous, Marcel; Collerson, Kenneth D.

1996-09-01

197

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

198

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

199

IODP Expedition 301 Installs Three Borehole Crustal Observatories, Prepares for Three-Dimensional, Cross-Hole Experiments in the Northeastern Pacific Ocean  

OpenAIRE

Introduction and Goals The basaltic upper oceanic crust comprises the largestaquifer on Earth, containing a volume of water about equalto that currently stored in ice sheets and glaciers. Annualfluid fluxes through the upper oceanic crust are at leastas large as the global river flux to the ocean. Much of theseafloor is hydrogeologically active, but the majority ofthe fluid flow within oceanic crust occurs on ridge flanks,regions located kilometers or more from active seafloorspreading cente...

Fisher, A. T.; Urabe, T.; Klaus, A.; Iodp, The Expedition Scientists

2005-01-01

200

R-modes of neutron stars with a solid crust  

CERN Document Server

We investigate the properties of $r$-mode oscillations of a slowly rotating neutron star with a solid crust, by taking account of the effects of the Coriolis force. For the modal analysis we employ three-component neutron star models that are composed of a fluid core, a solid crust and a surface fluid ocean. For the three-component models, we find that there exist two kinds of $r$-modes, that is, those confined in the surface fluid ocean and those confined in the fluid core, which are most important for the $r$-mode instability. The $r$-modes do not have any appreciable amplitudes in the solid crust if rotation rate of the star is sufficiently small. We find that the core $r$-modes are strongly affected by mode coupling with the crustal torsional (toroidal) modes and lose their simple properties of the eigenfunction and eigenfrequency as functions of the angular rotation velocity $\\Omega$. This indicates that the extrapolation formula, which is obtained in the limit of $\\Omega\\to 0$, cannot be used to examine...

Yoshida, S; Yoshida, Shijun; Lee, Umin

2001-01-01

201

"ma olen..." : [luuletused] / Kivisildnik  

Index Scriptorium Estoniae

Sisu: "ma olen..." ; "naine..." ; "ma olen viimne..." ; "asjad lähevad..." ; "oma taju..." ; "morss..." ; "jälgi..." ; "kui sinu..." ; "mats lehmaga..." ; "sellest peale..." ; "kuulan raadiost..." ; "pole kuulnud..." ; "laena kirvest..." ; "oled sa..." ; "harmsil on üks anekdoot..." ; "veel pole hilja..."

Kivisildnik, pseud., 1964-

2004-01-01

202

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

203

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

204

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

205

Folded continental and oceanic nappes on the southern side of Monte Rosa (western Alps, Italy): Anatomy of a double collision suture  

Science.gov (United States)

Previous work suggested a double collision suture, including ophiolites from two oceanic basins (Valais and Piemont-Liguria), on the southern side of Monte Rosa (Penninic Alps, northern Italy). This area was studied using field mapping, microstructural analysis, and neutron texture goniometry. After its formation and eclogite-facies metamorphism of continental and oceanic units, the suture was deformed by four successive folding and shearing events under greenschist-facies conditions, all of them taking place between 40 and 28 Ma. After fold retrodeformation, the following tectonostratigraphy results, from base to top: Monte Rosa gneiss (European margin), Balma serpentinite/eclogite unit (Cretaceous crust of Valais ocean), Stolemberg gneiss (Iberia-Briançonnais continent), Zermatt-Saas and Tsaté ophiolites (Jurassic crust of Piemont-Ligurian ocean), Sesia nappe (continental fragment off the Adria margin). The preservation of this lithological sequence suggests that deep-seated deformation during multiple continent collision produces heterogeneous strain and extreme thinning of nappes but their original stacking order can still be reconstructed using kinematic analysis and overprinting criteria. This is due to the ductile nature of the collisional deformation which retains the continuity of tectonic contacts.

Pleuger, Jan; Froitzheim, Nikolaus; Jansen, Ekkehard

2005-08-01

206

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

Science.gov (United States)

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

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

2014-05-01

207

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

Science.gov (United States)

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

208

Opening and Closing of a Mesoproterozoic Ocean along the SE Margin of India: Textural, Cathodoluminescence and SHRIMP Analyses of Zircon  

Science.gov (United States)

Alkaline rocks, especially nepheline syenites and carbonatites, are emplaced into extensional crust, hence continental rifts are considered to be favored tectonic regimes for such alkaline suites. Nepheline syenites and carbonatites mark continental rift margins from the Mesoproterozoic Gardar Province in Greenland to the Cenozoic East African Rift. These continental rifts finally culminate into oceanic margins in some cases, whereas in other cases they do not. In the former scenario, alkaline rocks are true petrologic indicators for the initiation of the Wilson cycle. The magmatic nepheline syenites and carbonatites so preserved along continental margins are intensely deformed and metamorphosed during oceanic consumption and subsequent continent-continent collision. Deformed alkaline suites therefore, can record the complete Wilson Cycle of opening and closing of an ocean. Deformed, metamorphosed nepheline syenites and carbonatites lie all along the craton-Eastern Ghats Mobile Belt (EGB) contact in SE India, representing two important segments of the Wilson cycle. The nepheline syenites are considered to have originally formed in a rift on thickened continental crust. Their subsequent deformation was caused by continent-continent collision between the EGB and juxtaposed cratons, thus the nepheline syenites characterize deep-seated tectonic remnants of fossil sutures. We have generated SHRIMP U-Pb age data on zircons of three nepheline syenite plutons from the southern segment of the EGB to illuminate the petrotectonic evolution of the SE margin of India. Three distinct age clusters are evident in the data. U-rich, texturally magmatic zircons have ages of approximately 1350 Ma, whereas U-poor, texturally metamorphic zircons possess ages around 1000 Ma. A few zircons with complex textures have ages around 550 Ma. Based on our new SHRIMP age data, aided by textural and CL images of these zircons, we suggest that a new ocean opened at about 1350 Ma along the SE margin of India and was later consumed around 1000 Ma. The Pan-African ages probably record growth of new (magmatic?) zircons in a fluid-rich environment and/or due to decompression melting of the older nepheline syenites in the underlying lithospheric mantle. The emplacement ages of nepheline syenites record fragmentation of the supercontinent Columbia, whereas the deformation ages indicate the assembly of the supercontinent Rodinia.

Kopparapu, V.; Ernst, W. G.; Chervela, L.

2011-12-01

209

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.

210

CRUST 5.1: A global crustal model at 5?? ?? 5??  

Science.gov (United States)

We present a new global model for the Earth's crust based on seismic refraction data published in the period 1948-1995 and a detailed compilation of ice and sediment thickness. An extensive compilation of seismic refraction measurements has been used to determine the crustal structure on continents and their margins. Oceanic crust is modeled with both a standard model for normal oceanic crust, and variants for nonstandard regions, such as oceanic plateaus. Our model (CRUST 5.1) consists of 2592 5?? ?? 5?? tiles in which the crust and uppermost mantle are described by eight layers: (1) ice, (2) water, (3) soft sediments, (4) hard sediments, (5) crystalline upper, (6) middle, (7) lower crust, and (8) uppermost mantle. Topography and bathymetry are adopted from a Mandard database (ETOPO-5). Compressional wave velocity in each layer is based on field measurements, and shear wave velocity and density are estimated using recently published empirical Vp-Vs and Vp -density relationships. The crustal model differs from previous models in that (1) the thickness and seismic/density structure of sedimentary basins is accounted for more completely, (2) the velocity structure of unmeasured regions is estimated using statistical averages that are based on a significantly larger database of crustal structure, (3) the compressional wave, shear wave, and density structure have been explicitly specified using newly available constraints from field and laboratory studies. Thus - this global crustal model is based on substantially more data than previous models and differs from them in many important respects. A new map of the thickness of the Earth's crust is presented, and we illustrate the application of this model by using it to provide the crustal correction for surface wave phase velocity maps. Love waves at 40 s are dominantly sensitive to crustal structure, and there is a very close correspondence between observed phase velocities at this period and those predicted by CRUST 5.1. We find that the application of crustal corrections to long-period (167 s) Rayleigh waves significantly increases the variance in the phase velocity maps and strengthens the upper mantle , velocity anomalies beneath stable continental regions. A simple calculation of crustal isostacy indicates significant lateral variations in upper mantle density. The model CRUST 5.1 provides a complete description of the physical properties of the Earth's crust at a scale of 5?? ?? 5?? and can be used for a wide range of seismological and nonseismological problems.

Mooney, W.D.; Laske, G.; Masters, T.G.

1998-01-01

211

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

Science.gov (United States)

Sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon ages of 24 samples from oceanic crust recovered in Integrated Ocean Drilling Program (IODP) Hole U1309D and from the surface of Atlantis Massif, Mid-Atlantic Ridge (MAR) (30??N) document a protracted history of accretion in the footwall to an oceanic detachment fault. Ages for 18 samples of evolved Fe-Ti oxide gabbro and felsic dikes collected 40-1415 m below seafloor in U1309D yield a weighted mean of 1.20 ?? 0.03 Ma (mean square of weighted deviates = 7.1). However, the ages range from 1.08 ?? 0.07 Ma and 1.28 ?? 0.05 Ma indicating crustal construction occurred over a minimum of 100-200 ka. The zircon ages, along with petrologic observations, indicate at least 2 major periods of intrusive activity with age peaks separated by 70 ka. The oldest ages are observed below 600 mbsf, an observation inconsistent with models requiring constant depth melt intrusion beneath a detachment fault. The data are most consistent with a "multiple sill" model whereby sills intrude at random depths below the ridge axis over a length scale greater than 1.4 km. Zircon ages from -broadly spaced samples collected along the southern ridge of Atlantis Massif yield a detachment fault slip rate of 28.7 ?? 6.7 mm/a and imply significant asymmetric plate spreading (up to 100% on the North American plate) for at least 200 ka during core complex formation. Copyright 2008 by the American Geophysical Union.

Grimes, C.B.; John, B.E.; Cheadle, M.J.; Wooden, J.L.

2008-01-01

212

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

Science.gov (United States)

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

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

2014-03-01

213

The Importance of Water to Oceanic Mantle Melting Regimes  

OpenAIRE

The formation of basaltic crust at mid-ocean ridges and ocean islands provides a window into the compositional and thermal state of the Earth's upper mantle. But the interpretation of geochemical and crustal-thickness data in terms of magma source parameters depends on our understanding of the melting, melt-extraction and differentiation processes that intervene between the magma source and the crust. Much of the quantitative theory developed to model these processes has neglected the role of...

Asimow, Paul D.; Langmuir, Charles

2003-01-01

214

Oceanic anoxic events and plankton evolution: Biotic response to tectonic forcing during the mid-Cretaceous  

Science.gov (United States)

Mid-Cretaceous (Barremian-Turonian) plankton preserved in deep-sea marl, organic-rich shale, and pelagic carbonate hold an important record of how the marine biosphere responded to short- and long-term changes in the ocean-climate system. Oceanic anoxic events (OAEs) were short-lived episodes of organic carbon burial that are distinguished by their widespread distribution as discrete beds of black shale and/or pronounced carbon isotopic excursions. OAE1a in the early Aptian (~120.5 Ma) and OAE2 at the Cenomanian/Turonian boundary (~93.5 Ma) were global in their distribution and associated with heightened marine productivity. OAE1b spans the Aptian/Albian boundary (~113-109 Ma) and represents a protracted interval of dysoxia with multiple discrete black shales across parts of Tethys (including Mexico), while OAE1d developed across eastern and western Tethys and in other locales during the latest Albian (~99.5 Ma). Mineralized plankton experienced accelerated rates of speciation and extinction at or near the major Cretaceous OAEs, and strontium isotopic evidence suggests a possible link to times of rapid oceanic plateau formation and/or increased rates of ridge crest volcanism. Elevated levels of trace metals in OAE1a and OAE2 strata suggest that marine productivity may have been facilitated by increased availability of dissolved iron. The association of plankton turnover and carbon isotopic excursions with each of the major OAEs, despite the variable geographic distribution of black shale accumulation, points to widespread changes in the ocean-climate system. Ocean crust production and hydrothermal activity increased in the late Aptian. Faster spreading rates [and/or increased ridge length] drove a long-term (Albian-early Turonian) rise in sea level and CO2-induced global warming. Changes in ocean circulation, water column stratification, and nutrient partitioning lead to a reorganization of plankton community structure and widespread carbonate (chalk) deposition during the Late Cretaceous. We conclude that there were important linkages between submarine volcanism, plankton evolution, and the cycling of carbon through the marine biosphere.

Leckie, R. Mark; Bralower, Timothy J.; Cashman, Richard

2002-08-01

215

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

216

Sink or swim? The fate of Archean primary crust and the generation of TTG magmas  

Science.gov (United States)

Petrological data and thermal models suggest ambient upper-mantle potential temperatures (Tp) in the Archean were > or >> 1500 °C, significantly hotter than the present day. Higher ambient Tp would have led to extensive melting of the mantle and the production of a thick (up to 45 km) MgO-rich primary crust that probably formed by over- and intra-accretion of magmas, akin to modern oceanic plateaux. The calculated composition of primary melts and their complementary residues may be used as a proxy for the changing bulk composition of primary crust and lithospheric mantle (LM) with time. Most Archean primary crust was picritic and underlain by a highly residual LM. However, the preserved volume of these picrites is low suggesting that much of the primary crust is missing. Here we model the equilibrium mineral assemblages developed within a wide range of metamorphosed (hydrated) primary crust compositions and their complementary residues, assuming a Moho temperature of 1000 °C. Although the density of the LM decreases slightly with increasing Tp, the increase in the density of the complementary primary crust is much more dramatic. Thick crust built from primary melts with MgO of 21-22 wt% would have become gravitationally unstable at ~1.5 GPa (~40-45 km depth), even when fully hydrated, so that any thickening beyond this should have caused the base of the crust to founder. The base of primary crust with MgO > 22 wt%, potentially produced at extreme Tp, would have foundered without the requirement for any thickening. Archean TTG magmas were derived from partial melting of (garnet-) amphibolite, which is only stable in low MgO 'MORB-like' compositions, and cannot have been produced directly from the high MgO primary crust expected in the Archean. Instead, the basaltic protoliths for TTG magmas generated in the early Archean (> 3 Ga) were probably formed from fractional crystallisation of ultramafic primary magmas and/or partial melting of ultramafic primary crust. Three possible tectonic scenarios to generate TTG magmas are suggested as follows. (1) Modest over-thickening of thicker (35-40 km thick) high MgO primary crust could have promoted foundering of the cumulate and/or residue rich lower crust to bring hydrated basaltic protoliths into direct contact with the hot LM and generate TTG magmas that would have risen to shallower levels to stabilize the differentiated crust. (2) Tectonic over-thickening of thinner (25-30 km thick) high MgO primary crust by collision with thicker plateau crust could have driven hydrothermally altered upper crust of the overridden lithosphere into the garnet-amphibolite stability field where it could have melted to produce TTG magmas that would have intruded the overriding plateau crust. (3) In thinner primary crust with lower MgO (TTG magmas that would have intruded the overriding plate.

Johnson, Tim; Brown, Michael; VanTongeren, Jill

2013-04-01

217

Iron Isotopic Fractionation in Early Planetary Crusts  

Science.gov (United States)

Differentiated meteorites (achondrites) derive from planetary bodies that experienced variable degrees of melting and silicate-metal segregation. The oldest achondrites, such as eucrites, angrites, brachinites and the oligoclase-rich meteorites Graves Nunataks 06128/06129 (GRA 06128/9), were formed ~2-5 Ma after the first Solar System solids. They represent the oldest differentiated silicate samples known in the Solar System and the study of these samples provides insight on the origins and conditions of formation of the first planetary crusts. Here, we present new high-precision data for the Fe isotopic compositions of eucrites, angrites, brachinites and GRA 06128/9 and interpret these results in terms of magmatism during formation of these samples. We find that most eucrites and brachinites are not fractionated compared to undifferentiated chondritic meteorites (?56Fe = 0.00±0.01, 2se), while the rare Stannern-trend eucrites are slightly enriched in the heavier isotopes of Fe. Angrites are also enriched in the heavier isotopes (?56Fe = 0.12±0.01, 2se), similar to what is observed for terrestrial basalts, reflecting the relatively high oxidation states of the angrite parent body(ies). Contrastingly to the 'basaltic' achondrites, GRA 06128/9 are enriched in light isotopes of Fe (?56Fe = -0.08±0.02, 2se). Evidence for light Fe isotope enrichments may be the consequence of the segregation of magma rich in sulphide (usually enriched in light isotopes of Fe compared to silicate and metal in undifferentiated meteorites). If correct, this result not only confirms that GRA 06128/9 represent products from <30% partial melting of an asteroidal body, prior to core formation, but also indicates complementary Fe isotope systematics between GRA 06128/9 and brachinites.

Wang, K.; Moynier, F.; Dauphas, N.; Barrat, J.; Day, J. M.; Sio, C.; Korotev, R. L.; Zeigler, R. A.

2012-12-01

218

Evolution and hydration of the Juan de Fuca crust and uppermost mantle: a plate-scale seismic investigation from ridge to trench  

Science.gov (United States)

The evolution of oceanic lithosphere involves incorporation of water into the physical and chemical structure of the crust and shallow mantle through fluid circulation, which initiates at the mid-ocean ridge and continues on the ridge flanks long after crustal formation. At subduction zones, water stored and transported with the descending plate is gradually released at depth, strongly influencing subduction zone processes. Cascadia is a young-lithosphere end member of the global subduction system where relatively little hydration of the downgoing Juan de Fuca (JdF) plate is expected due to its young age and presumed warm thermal state. However, numerous observations support the abundant presence of water within the subduction zone, suggesting that the JdF plate is significantly hydrated prior to subduction. Knowledge of the state of hydration of the JdF plate is limited, with few constraints on crustal and upper mantle structure. During the Cascadia Ridge-to-Trench experiment conducted in June-July 2012 over 4000 km of active source seismic data were acquired as part of a study of the evolution and state of hydration of the crust and shallow mantle of the JdF plate prior to subduction at the Cascadia margin. Coincident long-streamer (8 km) multi-channel seismic (MCS) and wide-angle ocean bottom seismometer (OBS) data were acquired in a two-ship program with the R/V Langseth (MGL1211), and R/V Oceanus (OC1206A). Our survey included two ridge-perpendicular transects across the full width of the JdF plate, a long trench-parallel line ~10 km seaward of the Cascadia deformation front, as well as three fan lines to study mantle anisotropy. The plate transects were chosen to provide reference sections of JdF plate evolution over the maximum range of JdF plate ages (8-9 Ma), offshore two contrasting regions of the Cascadia Subduction zone, and provide the first continuous ridge-to-trench images acquired at any oceanic plate. The trench-parallel line was designed to characterize variations in plate structure and hydration linked to JdF plate segmentation for over 450 km along the margin. Shipboard brute stacks of the MCS data reveal evidence for reactivation of abyssal hill faulting in the plate interior far from the trench. Ridgeward-dipping lower crustal reflectors are observed, similar to those observed in mature Pacific crust elsewhere, as well as conjugate reflectivity near the deformation front along the Oregon transect. Bright intracrustal reflectivity is also observed along the trench-parallel transect with marked changes in reflectivity along the Oregon and Washington margins. Initial inspection of the OBS record sections indicate good quality data with the expected oceanic crustal and upper mantle P-wave arrivals: Ps and Pg refractions through sedimentary and igneous layers, respectively, PmP wide-angle reflections from the crust-mantle transition zone, and Pn upper mantle refractions. The Pg-PmP-Pn triplication is typically observed at 40-50 km source-receiver offsets. Pn characteristics show evidence for upper mantle azimuthal anisotropic propagation: along the plate transects Pn is typically weaker and difficult to observe beyond ~80 km offsets, while along the trench-parallel transect Pn arrivals have higher amplitude and are easily observed up to source-receiver offsets of 160-180 km. An overview on the Cascadia Ridge to Trench data acquisition program and preliminary results will be presented.

Carbotte, S. M.; Canales, J.; Carton, H. D.; Nedimovic, M. R.; Han, S.; Marjanovic, M.; Gibson, J. C.; Janiszewski, H. A.; Horning, G.; Delescluse, M.; Watremez, L.; Farkas, A.; Biescas Gorriz, B.; Bornstein, G.; Childress, L. B.; Parker, B.

2012-12-01

219

42 CFR 495.202 - Identification of qualifying MA organizations, MA-EPs and MA-affiliated eligible hospitals.  

Science.gov (United States)

...qualifying MA organizations, MA-EPs...MA-affiliated eligible hospitals. 495.202...MA-affiliated eligible hospitals. (1) A qualifying MA organization, as part...MA-affiliated eligible hospitals for which the organization is seeking...

2010-10-01

220

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

Science.gov (United States)

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

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

2014-05-01

221

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

222

The Ninetyeast Ridge and its Relation to the Kerguelen, Amsterdam and St. Paul Mantle Plumes in the Indian Ocean  

Science.gov (United States)

The Ninetyeast Ridge, a 5000 km long north-south oriented submarine volcanic ridge in the eastern Indian Ocean, has been interpreted to have formed from magmatism associated with the deep-seated Kerguelen mantle plume as the Indian plate drifted rapidly northward during the Late Cretaceous. Samples recovered along the ridge have the characteristic Dupal geochemical signatures of Indian Ocean basalts, but debate concerning the nature and number of components in the mantle source contributing to the formation of these basalts persists. New MC-ICP-MS (Pb, Hf) and TIMS (Sr, Nd) isotopic analyses were determined for tholeiites representative of the ~180 m of basaltic basement recovered from three drill sites (758: 82 Ma; 757: 58 Ma, 756: 43 Ma) along the Ninetyeast Ridge during ODP Leg 121. These analyses provide substantially greater precision compared to earlier studies (ppm vs. % range) after removal of the effects of post-magmatic alteration by thorough acid leaching [1]. No systematic temporal or spatial isotopic variations are observed along the ridge, which is inconsistent with the hypothesis of an aging mantle plume origin for the ridge [2]. The Pb-Hf-Sr-Nd isotopic compositions of the Ninetyeast Ridge basalts are generally intermediate between those of the volcanic products of the Kerguelen and Amsterdam-St. Paul mantle plumes and define mixing trends between components with relatively enriched and depleted signatures. At least three, possibly four, source components are required to explain the observed isotopic variability along the Ninetyeast Ridge. The less radiogenic isotopic signatures of some Ninetyeast Ridge basalts (87Sr/86Sr = 0.70381-0.70438) are not consistent with mixing with shallow level Indian MORB and are instead consistent with the presence of a relatively depleted component in a deep mantle source. A similar source component is also identified in other Indian ocean island basalts (e.g., Crozet, Réunion [3, 4]) with typical EM-1-like compositions. The Pb-Hf-Sr-Nd isotopic compositions of the Ninetyeast Ridge basalts are consistent with provenance from a deep mantle source that has incorporated a mixture of recycled sediments and lower continental crust together with altered oceanic crust. This supports a deep origin for the EM-1-like Dupal signatures encountered in ocean island basalts. [1] Nobre Silva et al. (2009), G-cubed, 10, Q08012, doi:10.1029/2009GC002537. [2] Class et al. (1993), Nature, 362, 715-721. [3] Mahoney et al. (1996), Geology, 24, 7, 615-618. [4] Bosch et al. (2008), EPSL, 265, 748-768.

Nobre Silva, I. G.; Weis, D.; Scoates, J. S.

2011-12-01

223

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

OpenAIRE

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

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

2011-01-01

224

Plagioclase flotation and lunar crust formation  

Science.gov (United States)

Anorthitic plagioclase floats in liquids parental to the lunar highlands crust. The plagioclase enrichment that is characteristic of lunar highlands rocks can be the result of plagioclase flotation. Such rocks would form a gravitationally stable upper crust on their parental magma.

Walker, D.; Hays, J. F.

1977-01-01

225

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

226

Detrital footprint of the Mozambique ocean: U-Pb SHRIMP and Pb evaporation zircon geochronology of metasedimentary gneisses in eastern Madagascar  

Science.gov (United States)

The southern East African Orogen is a collisional belt where the identification of major suture zones has proved elusive. In this study, we apply U-Pb isotopic techniques to date detrital zircons from a key part of the East African Orogen, analyse their possible source region and discuss how this information can help in unravelling the orogen. U-Pb sensitive high-mass resolution ion microprobe (SHRIMP) and Pb evaporation analyses of detrital zircons from metasedimentary rocks in eastern Madagascar reveal that: (1) the protoliths of many of these rocks were deposited between ˜800 and 550 Ma; and (2) these rocks are sourced from regions with rocks that date back to over 3400 Ma, with dominant age populations of 3200-3000, ˜2650, ˜2500 and 800-700 Ma. The Dharwar Craton of southern India is a potential source region for these sediments, as here rocks date back to over 3400 Ma and include abundant gneissic rocks with protoliths older than 3000 Ma, sedimentary rocks deposited at 3000-2600 Ma and granitoids that crystallised at 2513-2552 Ma. The 800-700 Ma zircons could potentially be sourced from elsewhere in India or from the Antananarivo Block of central Madagascar in the latter stages of closure of the Mozambique Ocean. The region of East Africa adjacent to Madagascar in Gondwana reconstructions (the Tanzania craton) is rejected as a potential source as there are no known rocks here older than 3000 Ma, and no detrital grains in our samples sourced from Mesoproterozoic and early Neoproterozoic rocks that are common throughout central east Africa. In contrast, coeval sediments 200 km west, in the Itremo sheet of central Madagascar, have detrital zircon age profiles consistent with a central East African source, suggesting that two late Neoproterozoic provenance fronts pass through east Madagascar at approximately the position of the Betsimisaraka suture. These observations support an interpretation that the Betsimisaraka suture separates rocks that were derived from different locations within, or at the margins of, the Mozambique Ocean basin and therefore, that the suture is the site of subduction of a strand of Mozambique Ocean crust.

Collins, Alan S.; Kröner, Alfred; Fitzsimons, Ian C. W.; Razakamanana, Théodore

2003-11-01

227

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.

228

Late Paleozoic magmatism in South China: Oceanic subduction or intracontinental orogeny?  

Science.gov (United States)

The significant late Paleozoic magmatism has been widely recognized in the East Asian Blocks, which sheds a light on the assembly and break-up of the Pangea supercontinent. As one of major components in East Asia, however, the South China Block (SCB) does not have much late Paleozoic magmatism recognized. Here we report a gneissic granite intrusion in northeastern Fujian Province, eastern SCB. It is a S-type granite characterized by high K2O and Al2O3, and low SiO2 and Na2O with a high A/CNK ratio of 1.22. Zircons with stubby morphology from this gneissic granite yield 206Pb/238U ages ranging from 326 Ma to 301 Ma with a weighted average age of 313×4 Ma, and negative epsilonHf(t) values from -8.35 to -1.74 with two-stage Hf model ages of 1.43 to 1.84 Ga. This S-type granite was probably originated from late Paleoproterozoic crust during an intracontinental orogeny, not under oceanic subduction. Integrated with previous results on the paleogeographic reconstruction of the SCB, the nature of Paleozoic basins, Early Permian volcanism and U-Pb-Hf isotope of detrital zircons from the late Paleozoic to early Mesozoic sedimentary rocks, our data support a late Paleozoic orogeny in the SCB, which may have included Late Carboniferous (340-310 Ma) compressive episode and Early Permian (287-270 Ma) post-orogenic or intraplate extensive episode. Our interpretation is consistent with the late Paleozoic orogenic events recognized in other Pangea microcontinents, and thus provides a window for the reconstruction of Pangea. Acknowledgements: NSFC (41190070, 41190075)

Liu, Q.; Yu, J.; Zhao, G.

2013-12-01

229

Reconstructions of subducted ocean floor along the Andes: a framework for assessing Magmatic and Ore Deposit History  

Science.gov (United States)

The South American-Antarctic margin has been characterised by numerous episodes of volcanic arc activity and ore deposit formation throughout much of the Mesozoic and Cenozoic. Although its Cenozoic subduction history is relatively well known, placing the Mesozoic arc-related volcanics and the emplacement of ore bodies in their plate tectonic context remains poorly constrained. We use a merged moving hotspot (Late Cretaceous- present) and palaeomagnetic /fixed hotspot (Early Cretaceous) reference frame, coupled with reconstructed spreading histories of the Pacific, Phoenix and Farallon plates to understand the convergence history of the South American and Antarctic margins. We compute the age-area distribution of oceanic lithosphere through time, including subducting oceanic lithosphere and estimate convergence rates along the margin. Additionally, we map the location and migration of spreading ridges along the margin and relate this to processes on the overriding plate. The South American-Antarctic margin in the late Jurassic-early Cretaceous was dominated by rapid convergence, the subduction of relatively young oceanic lithosphere (Verdes" in southern South America. The speed of subduction increased again along the South American-Antarctic margin at ~105 Ma after another change in tectonic regime. Newly created crust from the Farallon-Phoenix ridge continued to be subducted along southern South America until the cessation of the Farallon-Phoenix ridge in the latest Cretaceous / beginning of the Cenozoic. The age of the subducting oceanic lithosphere along the South American-Antarctic margin has increased steadily through time.

Sdrolias, M.; Müller, R.

2006-05-01

230

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

231

Direct observations on the interaction between lower- and upper-crustal magmatic systems in a virtually complete section through 25 km of pre-Apline crust in the Sesia Valley, northwest Italy  

Science.gov (United States)

The linkage between "magmatic underplating" and continental silicic volcanism is uniquely exposed in the Sesia Valley, where a virtually complete section through the pre-Alpine crust is capped by a Permian bimodal volcanic field with a >15-km, rhyolitic caldera and floored by the famous deep-crustal section of the Ivrea-Verbano Zone (IVZ). Volcanic rocks are intruded by granophyre and fine-grained granite with miarolitic cavities that grade downward into coarse-grained granite "rooted" in paragneiss of the IVZ. Intruding the paragneiss at deeper crustal levels is a >8-km-thick, layered gabbronorite pluton referred to as the Mafic Complex (MC). SHRIMP zircon ages indicate that bimodal volcanism (288 ± 2 to 282 ± 3 Ma) and granitic plutonism (289 ± 3 to 275 ± 5 Ma) were coincident with intrusion of the MC in the lower crust (289 ± 3 to 286 ± 6 Ma). Palinspastic restoration of the Sesia section places the roof of the MC at a depth of 15 to 20 km, consistent with equilibration pressures in its contact aureole, and provides a petrologic model for interpretation of the seismic structure beneath large calderas and silicic volcanic fields analogous to that provided by ophiolites for the seismic structure of the oceanic crust. The interplay between the lower- and upper-crustal magmatic systems is recorded in field relations and geochemistry. Mafic enclaves and andesitic volcanism demonstrate that a mantle component reached the upper crust, contributing to its magmatic evolution, but peraluminous compositions and whole-rock ?18O (10-11.5) of the volumetrically more significant silicic volcanic and granitic rocks are consistent with crustal anatexis within a narrow (facies restite that should have been left behind by upward-migrating anatectic melts are notably scarce, but the geochemical signature of large volumes of granulite-facies restite is seen in the trace-element and isotopic geochemistry of the MC, which requires ~25% to 40% contamination by crustal material previously stripped of a granitic component. It appears that the roof of the MC behaved as a interface across which significant transfer of both mass and heat occurred. Uniformity of contamination, arcuate internal structure, and the distribution of strain and paragneiss inclusions are consistent with growth of the MC as a "gabbro glacier" during crustal extension, with focused delivery of mantle-derived melt to its roof and flow of contaminated crystal mush away from the resulting perched magma chamber. Thermal modeling indicates that (1) focused delivery of mantle melt was efficient in driving anatexis in overlying crustal rocks, (2) migration of anatectic melts to the upper crust was efficient in removing heat from the MC and restricting its thermal impact on the overlying crust, and (3) lower- and upper-crustal plutons were dominated by crystal mush rather than large magma chambers during their formation.

Quick, J. E.; Sinigoi, S.; Sbisa, A.; Demarchi, G.; Richards, I.

2011-12-01

232

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

233

Phantom Archean crust in Mangaia hotspot lavas and the meaning of heterogeneous mantle  

Science.gov (United States)

Lavas from Mangaia in the Cook-Austral island chain, Polynesia, define an HIMU (or high ?, where ?=U238/Pb204) global isotopic end-member among ocean island basalts (OIB) with the highest 206,207,208Pb/204Pb. This geochemical signature is interpreted to reflect a recycled oceanic crust component in the mantle source. Mass independently fractionated (MIF) sulfur isotopes indicate that Mangaia lavas sampled recycled Archean material that was once at the Earth's surface, likely hydrothermally-modified oceanic crust. Recent models have proposed that crust that is subducted and then returned to the surface in a mantle plume is expected to transform to pyroxenite/eclogite during transit through the mantle. Here we examine this hypothesis for Mangaia using high-precision electron microprobe analysis on olivine phenocrysts. Contrary to expectations of a crustal component and, hence pyroxenite, results show a mixed peridotite and pyroxenite source, with peridotite dominating. If the isotopic compositions were inherited from subduction of recycled oceanic crust, our work shows that this source has phantom-like properties in that it can have its lithological identity destroyed while its isotope ratios are preserved. This may occur by partial melting of the pyroxenite and injection of its silicic melts into the surrounding mantle peridotite, yielding a refertilized peridotite. Evidence from one sample reveals that not all pyroxenite in the melting region was destroyed. Identification of source lithology using olivine phenocryst chemistry can be further compromised by magma chamber fractional crystallization, recharge, and mixing. We conclude that the commonly used terms mantle “heterogeneities” and “streaks” are ambiguous, and distinction should be made of its lithological and isotopic properties.

Herzberg, C.; Cabral, R. A.; Jackson, M. G.; Vidito, C.; Day, J. M. D.; Hauri, E. H.

2014-06-01

234

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

235

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...

236

Kitoy sillimanite deposit (Eastern Siberia): an example of Neoarchaean-Paleoproterozoic paleoweathering crust  

Science.gov (United States)

We present the first reliable geochronological data on the age of the Kitoy sillimanite deposit (south-western Siberian craton) composed by andalusite-sillimanite, garnet-sillimanite and essentially sillimanite rocks intercalated with quartzite. These rocks experienced metamorphic reworking under amphibolite facies condition. Geochemical data indicates that hydrolysate clay rocks with kaolin, kaolinite and metalaterite or pyrophyllite have been a protholites of the high-alumina Kitoy rocks. Thus the Kitoy deposit high-alumina schist corresponds to mature paleoweathering crust (laterite and bauxite) with negligible amount of clastic material. U-Pb zircon geochronological investigations were undertaken for leucocratic biotite gneisses directly subjacent productive high-alumina suite as well as for migmatitic orthotektite vein cutting the andalusite-sillimanite rocks. Both samples contain typical magmatic zircon which was dated at 2578+/-16 Ma in the biotite gneisses and at 2483+/-4 Ma in the veined orthotektite. Pb-Pb age of sillimanite from high-alumina rocks is at 2536+/-27 Ma (Levchenkov et al., 2009) that is in good correspondence with our data. Results of our study allow to bracket the formation of the Kitoy deposit in the age interval 2536-2483 Ma and consider the high-alumina rocks of this deposit as example of the ancient paleoweathering crust recognized in the Eastern Siberia.

Salnikova, Ekaterina; Yakovleva, Sonya; Levitsky, Ivan; Kotov, Alexander; Levitsky, Valery; Reznitsky, Leonid; Anisimova, Irina

2010-05-01

237

Crust formation and its role during baking  

OpenAIRE

The final properties of the crumb and crust differ according to their heat-moisture dynamics. Compilations of heating and drying rates reported in the literature are discussed and will serve to validate future models of baking. Their impact on the structural elements in dough films and the porous network are discussed, highlighting the lack of data and the need to reproduce these dynamics inside the instrument of analysis. Some roles of the crust setting during the whole baking process are al...

Vanin, F.; Lucas, T.; Trystram, G.

2009-01-01

238

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

239

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

240

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

241

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

242

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

243

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

244

The Moho beneath western Tibet: Shear zones and eclogitization in the lower crust  

Science.gov (United States)

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

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

2014-12-01

245

Plio-Pleistocene denitrification in the eastern tropical North Pacific: Intensification at 2.1 Ma  

Science.gov (United States)

Global climate has changed substantially over the past ˜4 million years (Ma) toward, on average, colder conditions and higher amplitude oscillation between glacial and interglacial periods. Corresponding changes in marine biogeochemistry could have linked oceanic circulation changes to the global carbon cycle, but remain poorly understood. We report a high-resolution, 4-Ma-long ?15N record from Ocean Drilling Program Site 1012 on the California margin, which monitors denitrification intensity in one of the principal Oxygen Minimum Zones (OMZs) of the open ocean. The ?15N record demonstrates that eastern tropical North Pacific (ETNP) denitrification was weak for much of the late Pliocene. Though there was no apparent change associated with Northern Hemisphere glaciation (NHG) at ˜2.7 Ma, denitrification strengthened substantially (by ˜2‰ in ?15N) at ˜2.1 (Ma). Since 2.1 Ma, intense denitrification in the ETNP occurred during interglacial periods and was generally very climate-sensitive. We infer that two oceanic changes, the shoaling of the thermocline in the eastern Pacific (and thus the strengthened Walker circulation) and the development of a modern-like frontal system in high-latitude southern ocean around ˜2.1 Ma provided favorable conditions for intensification of ETNP denitrification that was also likely mirrored south of the equator. Intermediate water circulation changes likely created the poorly ventilated zones susceptible to the suboxic condition required by denitrification. 2.1 Ma thus marks a critical point for the reorganization of the ocean's nitrogen cycle in which open ocean denitrification became a significant loss term during interglacial phases of climate. Given linkage between the ocean's N and C cycle, ETNP denitrification changes probably contributed to Plio-Pleistocene climatic changes.

Liu, Zhonghui; Altabet, Mark A.; Herbert, Timothy D.

2008-11-01

246

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

247

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 (http://www.scec.org).

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

248

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

249

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

250

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

251

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

252

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

253

Diachronic and different metamorphic evolution in the fossil Variscan lower crust of Calabria  

Science.gov (United States)

Different P- T- t paths and Variscan tectonic evolution have been described for the lower crust of Calabria. New data have been collected through retrieval technique and construction of pseudosections to control the validity of the previous data and to check the appropriate model to describe the tectono-thermal evolution of the lower crust of the Serre (southern Calabria). The time-period from ~350 and ~270 Ma has been considered to depict the evolution from Variscan crustal thickening to exhumation as happens in the peri-Mediterranean blocks of south European Variscides and consistently with the available geochronological data. It results that: (1) P-peak at 0.9 and 1.03 GPa at the top and bottom, respectively, was reached earlier than T-peak, (2) crustal thickening developed likely earlier than 325 Ma within the stability field of kyanite, in agreement with previous studies, up to the P-peak along a geothermal gradient of about 21-22°C km-1, (3) the T-peak of 700 and 880°C at the top and bottom, respectively, was reached in the stability field of sillimanite after a nearly isobaric heating and (4) Variscan exhumation occurred under increasing T/depth ratio and stopped 270-280 Ma ago. The P-T paths for the upper and lower portions of the section, qualitatively comparable to the numerical simulation, reflect different styles of exhumation, cooling and, according to the available geochronological data, diachronic evolution.

Fornelli, A.; Pascazio, A.; Piccarreta, G.

2012-07-01

254

Accretion of arc and backarc crust to continental margins: Inferences from the Annieopsquotch accretionary tract, Newfoundland Appalachians  

Science.gov (United States)

The Ordovician Annieopsquotch accretionary tract (AAT) marks the easternmost peri-Laurentian realm along the Red Indian Line, the main Iapetus suture zone in the Northern Appalachians. It comprises a thrust stack of Lower to Middle Ordovician arc and backarc terranes. The AAT initially formed outboard of a peri-Laurentian Dashwoods following a collision along the Laurentian margin followed by subduction flip and formation of a nascent arc represented by supra-subduction zone Annieopsquotch ophiolite belt (c. 480 Ma). Continued development and rifting of this outboard arc is represented by the backarc Lloyd's River ophiolite belt and coeval Robert's Arm arc (c. 473 Ma). These terranes were accreted to the composite Laurentian margin of Iapetus within c. 5 Ma of their formation as a result initiation of subduction in the Lloyds River marginal basin. Hence, the AAT terranes occupied a lower plate setting with respect to composite Laurentian margin during accretion, analogous to modern collision of the Izu arc with the Ruykuy trench. Metamorphic mineral assemblages indicate that the terranes were underplated at depths ranging from ~ 3 km up to > 18 km. We infer the accretion of the terranes to be controlled by the brittle-ductile transition in the hydrated crust. The decoupling of brittle from ductile crust resulted in very high aspect ratios of the terranes, which comprise thin (< 5 km) but very large (up to 25 × 250 km) slabs of supracrustal arc rocks and ophiolite crust. Arc basement and ophiolitic mantle are not preserved and were either underplated at a greater depth or subducted and recycled back in the mantle. The accreted crust forms a reasonable approximation to bulk continental crust requiring little post-accretionary modification; hence, the accretion of arc-backarc complexes which occupy a lower plate setting can form an important mechanism for balancing crustal loss at convergent margins.

Zagorevski, Alexandre; van Staal, Cees R.; Lissenberg, Johan C.

2010-05-01

255

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

256

Seismic stratigraphy and sediment thickness of the Nansen Basin, Arctic Ocean  

Science.gov (United States)

A Norwegian expedition to the western Nansen Basin, Yermak Plateau and the Hinlopen margin in 2001 acquired about 1100 km of 2-D multichannel seismic profiles and 50 wide-angle sonobuoy record sections. Analysis of these data establishes a regional seismic stratigraphic framework for the western Nansen Basin integrating previously published stratigraphic schemes. P-wave velocities and sediment thickness were derived within 7-8 per cent uncertainty from 2-D seismic ray tracing models of each sonobuoy section. Sediment thickness reaches 2 km in the studied area and increases towards the depocentre of the giant Franz-Victoria fan on the Barents-Kara continental margin. High-relief oceanic crystalline crust with 3.7 km s-1 average near-top velocity is infilled by four seismic sediment units with typical velocities 2.4, 2.2, 2.0 and 1.8 km s-1. A prominent regional seismic horizon between units 2 and 3 is tentatively correlated by basement onlap and sedimentation rates to a Miocene (~10 Ma) palaeoceanographic event, possibly the opening of the Fram Strait. The youngest unit is correlated to prograding sequences on the margin and to the onset of major slope failure caused by intensified glacio-fluvial drainage and ice sheet erosion during Northern Hemisphere glaciations (2.6-0.01 Ma).

Engen, Øyvind; Gjengedal, Jakob Andreas; Faleide, Jan Inge; Kristoffersen, Yngve; Eldholm, Olav

2009-03-01

257

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

258

Guided wave observations and evidence for the low-velocity subducting crust beneath Hokkaido, northern Japan  

Science.gov (United States)

At the western side of the Hidaka Mountain range in Hokkaido, we identify a clear later phase in seismograms for earthquakes occurring at the uppermost part of the Pacific slab beneath the eastern Hokkaido. The later phase is observed after P-wave arrivals and has a larger amplitude than the P wave. In this study, we investigate the origin of the later phase from seismic wave observations and two-dimensional numerical modeling of wave fields and interpret it as a guided P wave propagating in the low-velocity subducting crust of the Pacific plate. In addition, the results of our numerical modeling suggest that the low-velocity subducting crust is in contact with a low-velocity material beneath the Hidaka Mountain range. Based on our interpretation for the later phase, we estimate P-wave velocity in the subducting crust beneath the eastern part of Hokkaido by using the differences in the later phase travel times and obtain velocities of 6.8 to 7.5 km/s at depths of 50 to 80 km. The obtained P-wave velocity is lower than the expected value based on fully hydrated mid-ocean ridge basalt (MORB) materials, suggesting that hydrous minerals are hosted in the subducting crust and aqueous fluids may co-exist down to depths of at least 80 km.

Shiina, Takahiro; Nakajima, Junichi; Toyokuni, Genti; Matsuzawa, Toru

2014-12-01

259

Postcollisional mafic igneous rocks record crust-mantle interaction during continental deep subduction.  

Science.gov (United States)

Findings of coesite and microdiamond in metamorphic rocks of supracrustal protolith led to the recognition of continental subduction to mantle depths. The crust-mantle interaction is expected to take place during subduction of the continental crust beneath the subcontinental lithospheric mantle wedge. This is recorded by postcollisional mafic igneous rocks in the Dabie-Sulu orogenic belt and its adjacent continental margin in the North China Block. These rocks exhibit the geochemical inheritance of whole-rock trace elements and Sr-Nd-Pb isotopes as well as zircon U-Pb ages and Hf-O isotopes from felsic melts derived from the subducted continental crust. Reaction of such melts with the overlying wedge peridotite would transfer the crustal signatures to the mantle sources for postcollisional mafic magmatism. Therefore, postcollisonal mafic igneous rocks above continental subduction zones are an analog to arc volcanics above oceanic subduction zones, providing an additional laboratory for the study of crust-mantle interaction at convergent plate margins. PMID:24301173

Zhao, Zi-Fu; Dai, Li-Qun; Zheng, Yong-Fei

2013-01-01

260

Composition and origin of Archean lower crust, Northern Tanzania  

Science.gov (United States)

Granulite-facies xenoliths from tuff cones erupted on the margin of the Tanzanian craton and within the adjacent Mozambique belt in northern Tanzania offer an opportunity to assess the role of lower crustal processes in the tectonic evolution of these two terranes. Both terranes are Archean, but record very different histories, starting in the Proterozoic and continuing today. Whereas the craton experienced little metamorphism or igneous activity following its stabilization around 2.8 Ga, Archean rocks of the Mozambique belt in the study area experienced at least one episode of high-grade metamorphism during the East African orogeny (ca. 640 Ma). Today, the East African rift exists at the contact between the Mozambique belt and the craton, implying a fundamental lithospheric weakness at this boundary. Granulite xenoliths come from Labait, on the craton margin, and Lashaine and Naibor Soito in the metamorphic belt. Most xenoliths are mafic and all are igneous in origin. Cratonic xenoliths (pl- opx±cpx±gt±hbl) are primarily anhydrous two-pyroxene granulites that likely originated as crystallized high-Ni, Archean basaltic melts. Xenoliths from the Mozambique belt are dominated by mafic granulites (pl-cpx-gt±opx) at Lashaine and banded, mafic to intermediate granulites at Naibor Soito. Positive Sr and Eu anomalies imply that the Lashaine granulites originated as plagioclase cumulates. The wide range in SiO2 (47-65 wt%) and correlation of Ni-MgO in the Naibor Soito xenoliths suggests they may have originated as igneous rocks that subsequently underwent partial melting to form the mafic (pl- opx±cpx±gt±hbl±bt) and felsic bands (pl-qtz-opx±kfs). U-Pb zircon ages for xenoliths from both terranes are Archean, as are most TDM ages, though younger TDM ages are seen in some Lashaine samples that were contaminated by rift magma. High pressures (up to 2.7GPa) are recorded by the Mozambique belt xenoliths, suggesting equilibration in thickened crust during the East African orogeny, but no igneous activity related to metamorphism has been detected and our samples suggest that the Tanzanian lower crust has persisted without significant chemical modification since the Archean. Proterozoic magmatism is also absent from the upper crust in this section of the Mozambique belt, raising the question of the heat source during metamorphism.

Mansur, A. T.; Manya, S.; Rudnick, R.

2008-12-01

261

The geological record of life 3500 Ma ago: Coping with the rigors of a young earth during late accretion  

Science.gov (United States)

Thin cherty sedimentary layers within the volcanic portions of the 3,500 to 3,300 Ma-old Onverwacht and Fig Tree Groups, Barberton Greenstone belt, South Africa, and Warrawoona Group, eastern Pilbara Block, Western Australia, contain an abundant record of early Archean life. Five principal types of organic and probably biogenic remains and or structures can be identifed: stromatolites, stromatolite detritus, carbonaceous laminite or flat stromalite, carbonaceous detrital particles, and microfossils. Early Archean stromatolites were reported from both the Barberton and eastern Pilbara greenstone belts. Systematic studies are lacking, but two main morphological types of stromatolites appear to be represented by these occurrences. Morphology of the stromalites is described. Preserved early Archean stromatolites and carbonaceous matter appear to reflect communities of photosynthetic cyanobacteria inhabiting shallow, probably marine environments developed over the surfaces of low-relief, rapidly subsiding, simatic volcanic platforms. The overall environmental and tectonic conditions were those that probably prevailed at Earth's surface since the simatic crust and oceans formed sometime before 3,800 Ma. Recent studies also suggest that these early Archean sequences contain layers of debris formed by large-body impacts on early Earth. If so, then these early bacterial communities had developed strategies for coping with the disruptive effects of possibly globe-encircling high-temperature impact vapor clouds, dust blankets, and impact-generated tsunamis. It is probable that these early Archean biogenic materials represent organic communities that evolved long before the beginning of the preserved geological record and were well adapted to the rigors of life on a young, volcanically active Earth during late bombardment. These conditions may have had parallels on Mars during its early evolution.

Lowe, Donald R.

1989-01-01

262

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

263

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)

264

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

265

“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

266

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 secondsbefore 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. I will also discuss shattering flares as electromagnetic counterparts to gravitational wave bursts during parabolic and elliptic encounters in dense star clusters.

Tsang, David; Read, Jocelyn; Piro, Anthony; Hinderer, Tanja

2014-08-01

267

Resonant Shattering of Neutron Star Crusts  

CERN Document Server

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.

Tsang, David; Hinderer, Tanja; Piro, Anthony L; Bondarescu, Ruxandra

2011-01-01

268

Iron isotope fractionation in planetary crusts  

Science.gov (United States)

We present new high precision iron isotope data (?56Fe vs. IRMM-014 in per mil) for four groups of achondrites: one lunar meteorite, 11 martian meteorites, 32 howardite-eucrite-diogenite meteorites (HEDs), and eight angrites. Angrite meteorites are the only planetary materials, other than Earth/Moon system, significantly enriched in the heavy isotopes of Fe compared to chondrites (by an average of +0.12‰ in ?56Fe). While the reason for such fractionation is not completely understood, it might be related to isotopic fractionation by volatilization during accretion or more likely magmatic differentiation in the angrite parent-body. We also report precise data on martian and HED meteorites, yielding an average ?56Fe of 0.00 ± 0.01‰. Stannern-trend eucrites are isotopically heavier by +0.05‰ in ?56Fe than other eucrites. We show that this difference can be ascribed to the enrichment of heavy iron isotopes in ilmenite during igneous differentiation. Preferential dissolution of isotopically heavy ilmenite during remelting of eucritic crust could have generated the heavy iron isotope composition of Stannern-trend eucrites. This supports the view that Stannern-trend eucrites are derived from main-group eucrite source magma by assimilation of previously formed asteroidal crust. These new results show that iron isotopes are not only fractionated in terrestrial and lunar basalts, but also in two other differentiated planetary crusts. We suggest that igneous processes might be responsible for the iron isotope variations documented in planetary crusts.

Wang, Kun; Moynier, Frédéric; Dauphas, Nicolas; Barrat, Jean-Alix; Craddock, Paul; Sio, Corliss K.

2012-07-01

269

Pulsar Glitches: The Crust may be Enough  

CERN Document Server

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

Piekarewicz, J; Horowitz, C J

2014-01-01

270

Actual timing of neodymium isotopic variations recorded by Fe-Mn crusts in the western North Atlantic  

Science.gov (United States)

Hydrogenetic ferromanganese (Fe-Mn) crusts from the western North Atlantic record variations in the Nd and Pb isotopic composition of Cenozoic deep water preserved during their growth. The timing and cause of the most striking change have been the subject of debate. Some have proposed that the shift took place after 4 Ma in response to the closure of the Panama gateway. Others have argued that the major change in isotope composition occurred as early as 8 Ma. This study presents high-resolution Nd isotope records for crusts previously dated using 10Be/9Be chronology. These data confirm that the shifts in Nd occurred after 4 Ma, consistent with a likely relationship with the closure of the Central American Isthmus and intensification of Northern Hemisphere Glaciation, and in accordance with changes seen in other physical and chemical records. These results illustrate the need for both a robust chronological framework and high-resolution records before a reliable paleoceanographic interpretation can be made of the variations recorded by Fe-Mn crusts.

Burton, K.W.; Lee, D.-C.; Christensen, J.N.; Halliday, A.N.; Hein, J.R.

1999-01-01

271

Recycled crust and the secular cooling of mantle plumes  

Science.gov (United States)

Current models suggest that the massive basaltic production responsible for the emplacement of Large Igneous Provinces (LIPS) during the Permian-Paleocene may represent the initial phases of some of the mantle plumes that feed the current ocean island basalts (OIB). In some cases this magmatism was so voluminous that it produced global environmental impacts. Recent petrological, geochemical and geophysical studies of some of these localities like Samoa, Hawaii, Galapagos provide evidence that melting is related to a true mantle plume that originates from a boundary layer beneath the upper mantle. Thus, plume-related magmas produced in OIB and LIPS and their connecting plume tracks provide evidence on mantle temperature, size and composition of heterogeneities, and deep geochemical cycles. Although a lot of work has been done on LIPS and OIB, no complete record of the evolution of a mantle plume is available to this point. Galapagos-related lavas provide a complete record of the evolution of a mantle plume since the plume's initial stages in the Cretaceous. In the case of the Galapagos, our work suggests a decrease from TP(max) of 1650 °C in the Cretaceous to 1500 °C in the present day. Our recent work on the Galapagos Islands and the preliminary work on older Galapagos-related terranes suggest that this secular cooling is related with increasing amounts of recycled crust in the plume. Detailed olivine chemistry shows that although peridotite is the dominant source lithology of the Galapagos Plume, a recycled pyroxenite component is also significant in both isotopically enriched and depleted domains of the archipelago. We suggest that this possibly represents two separate bodies of recycled crust within the Galapagos mantle plume.

Gazel Dondi, E.; Herzberg, C. T.; Vidito, C. A.

2012-12-01

272

Bimodal tholeiitic-dacitic magmatism and the Early Precambrian crust  

Science.gov (United States)

Interlayered plagioclase-quartz gneisses and amphibolites from 2.7 to more than 3.6 b.y. old form much of the basement underlying Precambrian greenstone belts of the world; they are especially well-developed and preserved in the Transvaal and Rhodesian cratons. We postulate that these basement rocks are largely a metamorphosed, volcanic, bimodal suite of tholeiite and high-silica low-potash dacite-compositionally similar to the 1.8-b.y.-old Twilight Gneiss - and partly intrusive equivalents injected into the lower parts of such volcanic piles. We speculate that magmatism in the Early Precambrian involved higher heat flow and more hydrous conditions than in the Phanerozoic. Specifically, we suggest that the early degassing of the Earth produced a basaltic crust and pyrolitic upper mantle that contained much amphibole, serpentine, and other hydrous minerals. Dehydration of the lower parts of a downgoing slab of such hydrous crust and upper mantle would release sufficient water to prohibit formation of andesitic liquid in the upper part of the slab. Instead, a dacitic liquid and a residuum of amphibole and other silica-poor phases would form, according to Green and Ringwood's experimental results. Higher temperatures farther down the slab would cause total melting of basalt and generation of the tholeiitic member of the suite. This type of magma generation and volcanism persisted until the early hydrous lithosphere was consumed. An implication of this hypothesis is that about half the present volume of the oceans formed before about 2.6 b.y. ago. ?? 1974.

Barker, F.; Peterman, Z.E.

1974-01-01

273

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)

274

Rapid vertical tectonics in ductile continental crust  

Science.gov (United States)

Research over the past two decades has shown that in regions of moderately high heat flow, the lower continental crust is ductile enough to flow on geological timescales. Vertical motions taking place within continental interiors produce localized features such as intracratonic basins and domes, and the results of this thesis indicate that ductile crustal flow can contribute significantly to the formation of these otherwise enigmatic features. A major goal of this thesis has been to analyze, quantitatively, the behaviour of loaded continental crust where a ductile layer is present. Specifically, I examine the long-term effects of sublithospheric heating events on crust with embedded density loads. Density anomalies within the crust can be initially supported by elastic stresses but sag appreciably if the elastic crust is thinned modestly. Beginning with a semi-analytic approach, I estimate the additional subsidence that would result from thermal reactivation, and introduce the previously unmodelled phenomenon of thermal annealing of stresses at the base of the elastic crust. In basins caused by intracrustal density loads, reactivated subsidence can be significant (of the order of 1 km, enough to account for about one quarter of the total Michigan basin subsidence). If the crust is sufficiently weakened, the long-term result is detachment of the load followed by rebound and inversion of the basin to form a dome. To model this phenomenon I use a full thermal and viscoelastic finite-element model, and find that such load detachment can occur for geologically reasonable load densities in high heat flow regions. Strikingly, the total upward displacement of material from depth during rebound can be as much as 10 km, enough to exhume the basin completely and expose basement rocks to some depth. Exhumation is rapid, lasting only about 5 to 10 million years. This raises the interesting question of what field evidence might support such a history for a dome: the results of my simulations are consistent with many of the features of metamorphic core complexes in the southern Basin and Range province, although an additional mechanism may be required to explain the exposure of rocks that originated at mid-crustal depths.

Pearse, Jillian

275

The Caledonian Bindal Batholith: episodic Ordovician-Silurian arc magmatism in nascent arc crust  

Science.gov (United States)

The Bindal Batholith (BB) is the largest Caledonian (s.l.) batholith in Norway and preserves evidence for at least 3 episodes of crustal magmatism and modification. The BB consists of plutons whose emplacement ages range from 478 to 424 Ma and in size from dikes to >500 km2 batholiths. The BB was emplaced in mid-crustal levels into amalgamated nappes of the Helgeland Nappe Complex. Some nappes record deposition, burial, metamorphism, and deformation in as little as 3 Ma before amalgamation (corresponding to at least 80 mm yr-1 burial rates) and three nappes underwent partial melting from 484-478 Ma, just 1-2 m.y. before amalgamation. Field relationships, pluton ages, and zircon inheritance data all indicate that the oldest BB magmas were emplaced during the waning stages of amalgamation. These oldest plutons are peraluminous and range from tourmaline-bearing two-mica granites to metasedimentary enclave-rich granodioritic plutons ("S-type") with initial 87Sr/86Sr from 0.7096 to 0.7469 and ?Nd -7.4 to -8.9. A probable source for the enclave-rich plutons is exposed in the structurally lowest Horta nappe, which contains migmatites that are lithologically and isotopically identical to the enclave-rich plutons. In addition, distinctive but minor basaltic enclaves and syn-magmatic dikes with high ?Nd and depleted LREE are present both in the largest "S-type" pluton and the Horta nappe migmatites. We therefore interpret initial magma emplacement as a continuation of widespread, pre-amalgamation crustal melting. Magmatism from 470-460 Ma was volumetrically small in terms of exposed plutons and was characterized by intense interaction of mafic magmas with the crust in the form of assimilation and crustal melting plus mixing. From ca. 450 Ma to ca. 424 Ma, primarily alkali-calcic and calc-alkalic magmas with a broad range of SiO2 contents were emplaced. These rocks have arc-like trace element signatures, with Sr and Nd isotope data suggesting mixing, either in the source or at the site of emplacement. Source mixing is preferred because many plutons of this age contain inherited zircons with ca. 460-470 Ma ages, which suggests wholesale modification of the nascent arc crust in the 460-470 Ma time range. Hf isotope (zircon) data support this interpretation, with ?Hf (t) becoming more uniform from oldest to youngest plutons. Thus, the BB began with an initial crustal melting event following nappe amalgamation with apparently minor MORB-like mafic magmatic input. Subsequent input of mafic magmas after 470 Ma facilitated continental arc evolution in which the lower crust was greatly reworked during two episodes (460-470 Ma and 450-424 Ma), all prior to Scandian collision of Baltica with Laurentia.

Barnes, C. G.; Nordgulen; Frost, C. D.; Andersen, T.; Marko, W. T.; Yoshinobu, A. S.; Prestvik, T.

2011-12-01

276

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

277

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

278

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

279

Reworking of Archaean and Palaeoproterozoic crust in the Mozambique belt of central Tanzania as documented by SHRIMP zircon geochronology  

Science.gov (United States)

New SHRIMP zircon ages for high-grade rocks from the Pan-African Mozambique belt (MB) of central Tanzania document reworking of Archaean-Palaeoproterozoic crust during the formation of this Neoproterozoic collisional orogen. Several gneisses and granulites from the Great Ruaha river area yielded late Archaean emplacement ages of 2575-2680 Ma for their magmatic precursors. Core-rim relationships in some zircons revealed new zircon growth during relatively short episodes of granitoid magmatism. Metamorphic zircons, dated at 1925 Ma, provide new evidence for high-grade metamorphism during formation of the Palaeoproterozoic Usagaran mobile belt. Metamorphic rims around magmatic cores in zircons extracted from granulite-facies garnet-amphibole bearing orthogneisses yielded an age of about 640 Ma, which is interpreted to reflect the peak of Pan-African regional high-grade metamorphism. Resorbed zircons extracted from migmatitic orthogneisses show a wide range of apparently concordant results, varying between ?500 and 700 Ma. This wide range reflects HT/HP conditions during peak metamorphism as well as subsequent strong decompression (ITC cooling path). Our data suggest that the MB in Tanzania consists largely of Archaean crust north of the Great Ruaha river and of Palaeoproterozoic crust south of this river. Both the Archaean and Palaeoproterozoic domains were strongly reworked during the formation of the MB and experienced high-grade metamorphism at ˜640 Ma. Our results imply that the Tanzania craton and Usagaran province originally extended much farther to the east and were structurally reworked, almost beyond recognition, during Pan-African ductile deformation and recrystallization.

Sommer, Holger; Kröner, Alfred; Hauzenberger, Christoph; Muhongo, Sospeter

2005-11-01

280

Spatial variations in effective elastic thickness in the Western Pacific Ocean and their implications for Mesozoic volcanism  

Science.gov (United States)

We have used free-air gravity anomaly and bathymetric data, together with a moving window admittance technique, to determine the spatial variation in oceanic elastic thickness, Te, in the Western Pacific ocean. Synthetic tests using representative seamounts show that Te can be recovered to an accuracy of ± 5 km for plates up to 30 km thick, with increased accuracy of ± 3 km for Te ? 20 km. The Western Pacific has a T e range of 0-50 km, with a mean of 9.4 km and a standard deviation of 6.8 km. The T e structure of the region is dominated by relatively high Te over the Hawaiian-Emperor Seamount Chain, intermediate values over the Marshall Islands, Gilbert Ridge, and Marcus-Wake Guyots, and low values over the Line Islands, Mid-Pacific Mountains, Caroline Islands, Shatsky Rise, Hess Rise, and Musician Seamounts. Plots of Te at sites with radiometric ages suggest that Te is to first order controlled by the age of the lithosphere at the time of loading. In areas that backtrack into the South Pacific Isotopic and Thermal Anomaly (SOPITA), Te may be as low as the depth to the 180 ± 120 °C isotherm at least locally. In the northern part of the study area including the Hawaiian-Emperor Seamount Chain, Te correlates with the depth to 310 ± 120 °C. These best-fitting isotherms imply peak rates of volcanism during 100-120 Ma (Early Cretaceous) and 140-150 Ma (Late Jurassic). The corresponding addition of 8 × 10 6 km 3 and 4 × 10 6 km 3 of volcanic material to the surface of the oceanic crust would result in long-term sea-level rises of 20 m and 10 m respectively. The Late Jurassic volcanic event, like the later Early Cretaceous event, appears to have influenced the tectonic evolution of the Pacific plate convergent boundaries, resulting in increased volcanism and orogenesis.

Kalnins, L. M.; Watts, A. B.

2009-08-01

281

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)

282

Aragonite crusts and pisolites beneath dolomitic tepees, Lake MacLeod evaporate basin, Western Australia  

Energy Technology Data Exchange (ETDEWEB)

Research currently being conducted by the Sedimentology and Marine Geology Group, under Brian W. Logan at the University of Western Australia, has recently concentrated on Lake MacLeod, a 2000 km/sup 2/ (770 mi/sup 2/) coastal salina on the western coast of Australia. This work has shown that this evaporite basin, which is 3 to 4 m (10 to 13 ft) below sea level, is separated from the Indian Ocean by a topographic barrier, but seawater under hydrostatic head, seeps freely through the barrier and discharges from several vents and springs in a carbonate mud flat at the north end of the basin. From there, seawater flows slowly across the basin, evaporating and depositing carbonate, gypsum, and ephemeral halite. About 10 to 12 m (33 to 39 ft) of evaporites have been deposited in the past 5300 years. In July 1982, the authors visited the carbonate mud flats and discovered abundant aragonite pisolites and botryoidal-mammillary crusts of fibrous aragonite cement beneath lily-pad tepee slabs of cemented protodolomite. Thick aragonite crusts cover both the undersides of lily-pad slabs and the lithified floors of tepees. Crusts covering the floors are more botryoidal and consist of both aragonite nubs and mounds (0.2 to 2.5 cm, 0.08 to 1 in., in diameter), and a few scattered, loose pisolites, several millimeters in diameter. The manner in which crusts, pisolites, and tepees occur at Lake MacLeod raises the possibility that they and their ancient counterparts from the Permian basin share a common origin. Perhaps Permian pisolites and aragonite crusts formed beneath cemented slabs of peritidal sediments in tepees bathed by marine water which seeped across exposed portions of the shelf crest.

Handford, C.R.; Kendall, A.C.; Dunham, J.B.; Logan, B.W.

1983-03-01

283

Ocean World  

Science.gov (United States)

This ocean-science Web site allows users to learn about many important ocean processes, linked to teaching material and sources of real-time ocean data. The site also has complete college-level and graduate courses and textbook (which can be freely downloaded) in oceanography and physical oceanography.

2009-01-01

284

Models for Interpreting Tungsten Isotope Anomalies in the Earth's Crust  

Science.gov (United States)

There have been several reports of positive tungsten isotope anomalies of about +15 ppm in rocks from Nuvvuagittuq (4.3 Ga), Isua (3.8 Ga) and Kostomuksha (2.8 Ga) that challenge models of differentiation and mantle mixing. Here, we employ constraints from experimental partitioning of W between metal and silicate, and from partial melting models, to evaluate the production and preservation of these W isotope anomalies in the Earth's earliest crust. We will also provide a revised interpretation of the Kostomuksha W isotope anomalies based on flow differentiation and metamorphism of komatiites. Two sets of models are produced. Model Set 1: Because D(metal-silicate) for W diminishes with increasing depth, the deep mantle has a higher W abundance, and a lower Hf/W ratio and consequently evolves a negative anomaly in W while the upper mantle evolves a complementary positive anomaly. Subsequent solid-state convection (4.55-2.8 Ga) mixes away the complementary W isotope anomalies to yield the modern mantle null value. This set of models predicts that the complementary negative anomalies in W should eventually be discovered in ancient magmatic rocks of deep mantle origin such as komatiites. Model Set 2: Tungsten is significantly more incompatible (like U, Th and Ba) than Hf, the latter being similar in compatibility to Sm. Our results show that extraction of low-degree partial melts (Hadean depleted mantle that can have Sm/Nd~20% higher than chondrites also creates a f(Hf/W)~2-3, sufficient to generate the anomalies observed in Archean rocks. These models increase Hf/W and Sm/Nd ratios in a correlated fashion explaining the tendency of positive W isotope anomalies to occur in rocks with positive Nd isotope anomalies. Recycling of the complementary Hadean crust would result in negative anomalies in later plume lavas, while partitioning of W into an enriched "hidden reservoir" would not. Nd isotope anomalies indicate a melting event around 35-75 Ma after solar system formation, the upper end of which is consistent with our models of Hf/W fractionation, that also yield a depleted mantle composition consistent with DMM. Production of the anomalies is accompanied by the need to preserve the anomalies. We argue that the most effective means of preserving the W isotope anomalies is by crustal storage, and we hypothesize that W is efficiently recycled back to juvenile crust during subduction zone processing. Eventually, the addition of juvenile W from the mantle will dilute the positive W isotope anomalies stored in the early crust. The amount of juvenile W required to be added is about a factor of 4 or more, and it is likely that this occurred during the late Archean crustal growth phase (2.5 Ga), so that positive W isotope anomalies are not likely to occur in much younger rocks. A secular decline in crustal W isotope anomalies is predicted.

Humayun, M.; Brandon, A. D.; Righter, K.

2012-12-01

285

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 (left-skewed curve with a mode occurring at approximately 70 wt.% SiO2. Samples from the northern region (n=107) demonstrate a tighter distribution of silica content (60.1-78.7 wt.% SiO2 with a median of 72.2 wt.% SiO2) compared to samples from the central region (n=115, 55.4-74.2 wt.% SiO2 with a median of 67.1 wt.% SiO2). The least evolved samples come from the Tiribi Formation in the Valle Central and are chemically distinct from rocks in the 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

286

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

287

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

288

Crusted Demodicosis in an Immunocompetent Pediatric Patient  

OpenAIRE

Demodicosis refers to the infestation by Demodex spp., a saprophytic mite of the pilosebaceous unit. Demodex proliferation can result in a number of cutaneous disorders including pustular folliculitis, pityriasis folliculorum, papulopustular, and granulomatous rosacea, among others. We report the case of a 7-year-old female presenting with pruritic grayish crusted lesions over her nose and cheeks, along with facial erythema, papules, and pustules. The father referred chronic use of topical st...

Guillermo Antonio Guerrero-González; Maira Elizabeth Herz-Ruelas; Xf Mez-flores, Minerva G.; Jorge Ocampo-Candiani

2014-01-01

289

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

290

Black crusts in the European built environment  

OpenAIRE

Buildings and monuments act as repositories of airborne organic pollutants, which accumulate at the surfaces in zones frequently soaked by rainwater but are not washed out. In these areas thick black crust deposits can be found, which contribute to soiling of stone surfaces. The exposed building materials act as a non-selective surface, passively entrapping all deposited airborne particulate matter and organic compounds, which obviously modifies the composition of the materials present in the...

Sa?iz-jime?nez, Cesa?reo; Hermosi?n, Bernardo

2004-01-01

291

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

292

Towards a metallurgy of neutron star crusts  

OpenAIRE

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...

Kobyakov, Dmitry; Pethick, Christopher J.

2013-01-01

293

Topological characterization of neutron star crusts  

CERN Document Server

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; López, J A

2012-01-01

294

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

295

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

296

Lithosphere dynamics and evolution of continental crust sixth Australian geological convention  

Energy Technology Data Exchange (ETDEWEB)

Abstracts of papers presented are given. Sessions included: recent plate movements and deformations in the Australasian region; lithosphere transect studies of the Australian continent; evolution of fold belts; metamorphism and geothermal gradients; palaeomagnetism and lithosphere dynamics, evolution of Precambrian terrains; tectonics of crators and platform covers; continental margin evolution; oceanic lithosphere studies; intraplate igneous activity; magmatism and crustal evolution; chemical and isotopic constraints on the evolution of continental crust; palaeoenvironments and lithosphere dynamics; surficial mantle of crators; lithosphere dynamics, biogeography and faunal and floral provinciality; geological time-scale; metallogeny and crustal evolution; lithosphere dynamics and the accumulation of coal, oil shale and fluid hydrocarbons; and kimberlites and carbonatites.

1983-01-01

297

Deformations of Accreting Neutron Star Crusts and Gravitational Wave Emission  

CERN Document Server

Motivated by the narrow range of spin frequencies of nearly 20 accreting neutron stars, Bildsten (1998) conjectured that their spin-up had been halted by the emission of gravitational waves. He also pointed out that small nonaxisymmetric temperature variations in the accreted crust will lead to "wavy" electron capture layers, whose horizontal density variations naturally create a mass quadrupole moment. We present a full calculation of the crust's elastic adjustment to these density perturbations and find that the elastic response of the crust reduces Bildsten's original estimate of the quadrupole moment in the thin outer crust by a factor of 20-50. However, this basic picture, when applied to capture layers in the deep inner crust, can generate quadrupoles in the necessary range as long as there are ~5% lateral temperature variations in the inner crust. By calculating the thermal flow throughout the core and the crust, we find that temperature gradients this large are easily maintained by asymmetric heat sou...

Ushomirsky, G; Bildsten, L; Ushomirsky, Greg; Cutler, Curt; Bildsten, Lars

2000-01-01

298

Tectonic evolution of the Perth Abyssal Plain's Quiet Zone, Southeast Indian Ocean  

Science.gov (United States)

During the Late Jurassic period, the Greater-Indian plate was torn away from Australia, dissociating East Gondwanaland. The Perth Abyssal Plain (PAP) is the southernmost rift segment along the western Australian margin, and has an onset age of ~136 Ma. New marine magnetic and swath bathymetry data, crossing the entire PAP, were acquired recently on geophysical cruise ss2011v06 aboard the R/V Southern Surveyor. These have lead to the outline of conjugate Indian and Australian M-series isochrons in the east and west PAP, respectively [1]. Yet, most of the PAP was created during the Cretaceous Normal Superchron (CNS, 121-83 Ma), a period of no geomagnetic field reversals, hence no comprehensive tectonic model for the PAP exists . Here we present preliminary findings of an analytic bathymetric and magnetic investigation aimed at elucidating the PAP's quiet zone. Recent discoveries regarding the evolution of the geomagnetic field during the CNS [2] provide new time markers that can be utilized to date the oceanic crust. The magnetic anomaly data exhibit the Q2 anomaly marker (~108 Ma), further constraining the spreading history of the PAP. Together with the ridgelet transform method [3] for automated abyssal hill delineation, we present new constraints on the development of crustal construction processes (spreading location, direction and rates) that took place along the PAP spreading center. References: [1] S.E. Williams, J.M. Whittaker, R. Granot, R.D. Muller (in preparation), New constraints on the seafloor spreading history in the Perth Abyssal Plain. [2] Granot, R., J. Dyment, and Y. Gallet (2012), Geomagnetic field variability during the Cretaceous Normal Superchron, Nature Geoscience, 5(3), 220-223. [3] Downey, N. J. and R. W. Clayton (2007), A ridgelet transform method for constraining tectonic models via abyssal-hill morphology, Geochemistry Geophysics Geosystems, 8, Q03004, doi: 10.1029/2006GC001440.

Ehrlich, Zohar Louis; Granot, Roi; Williams, Simon E.

2013-04-01

299

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

300

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

301

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

OpenAIRE

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 p...

Frey, Fa; Coffin, Mf; Wallace, Pj; Weis, D.; Zhao, X.; Wise Jr, Sw; Wa?hnert, V.; Teagle, Dah; Saccocia, Pj; Reusch, Dn; Pringle, Ms; Nicolaysen, Ke; Neal, Cr; Mu?ller, Rd; Moore, Cl

2000-01-01

302

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 (207Pb/206Pb age of 4127 ± 4?Ma) shows normal oscillatory zonation and constant oxygen isotope ratios (?18O = 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 ?18O (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

303

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 (207Pb/206Pb age of 4127 +/- 4 Ma) shows normal oscillatory zonation and constant oxygen isotope ratios (?18O = 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 ?18O (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.

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

2014-06-01

304

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

305

Paleomagnetism continents and oceans  

CERN Document Server

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

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

1999-01-01

306

Research Finds Life 1,000 Feet Beneath Ocean Floor  

Science.gov (United States)

This Oregon State University news article provides a brief overview of the discovery of microbes in a spreading ridge off the Oregon coast. The microbes were found in fractured basalts 1000 meters below the ocean floor and may hold evidence regarding what microorganisms exist within the Earth?s crust.

David Stauth

307

Geochemical constraints on the composition of Archaean lower continental crust: Partial melting in the Lewisian granulites  

Science.gov (United States)

New geochemical data for the Lewisian granulites of NW Scotland show that the protolith to the neoArchaean Scourie granulites was a tonalite containing a mafic hydrous phase, most probably hornblende. This observation when combined with recent thermobarometric calculations and new partial melting studies on the Lewisian makes it very probable that the Lewisian granulites experienced fluid-absent melting. Partial melting calculations based upon a proxy for the unmelted granulite protolith demonstrate that Scourie granulite protolith was depleted in both LILE (Rb, Th, U) and HFSE (Ta) relative to unmelted amphibolite facies Lewisian gneisses of the same age. Both granulite facies and amphibolite facies gneisses are members of the tonalite-trondhjemite-granidiorite (TTG) magmatic suite. Nb/Ta ratios in the TTG granulite protolith are suprachondritic and can be attributed to the partial melting of a basaltic source with a rutile eclogite residue, whereas the amphibolite facies TTG gneisses possess Nb/Ta ratios which are subchondritic and can be attributed to the partial melting of a basaltic source with a garnet amphibolite residue. However, these differences do not account for the differences in LILE between the two suites implying that in addition the two basaltic precursors were different. It is proposed that the depleted character of the tonalitic protolith to the Scourie granulites was inherited from ultra-depleted basalt with very low concentrations of Rb, Th, U and Ta. Felsic melts formed during partial melting are no longer present in the granulite terrane and were probably removed to the upper crust, now removed by erosion. Thus partial melting of the Lewisian granulites contributed to the process of crustal fractionation. The process of fluid-absent melting in the Lewisian took place ca 200 Ma after crust formation. This long time interval indicates that the stabilisation and differentiation of the crust was probably a separate event from that of crust formation.

Rollinson, Hugh

2012-10-01

308

Oceanization of the northern Neotethys: Geochemical evidence from ophiolitic melange basalts within the ?zmir-Ankara suture belt, NW Turkey  

Science.gov (United States)

The remnants of the Neotethyan Izmir-Ankara Ocean, the main branch of Neotethys in the eastern Mediterranean are represented by the Dagküplü Melange Complex in Central Sakarya, NW Turkey. It comprises several blocks or tectonic slices of pillow lavas, some of which include mudstones and radiolarian cherts as intra-pillow-fillings or interlayers. In the Igdecik area, a huge basaltic block has been studied in detail. Geochemical data reveal three distinct basalt types separated by sheared contacts. The first of these groups is an enriched mid-oceanic ridge basalt (E-MORB) type which is enriched in the most incompatible trace elements relative to normal MORB (N-NORB) in addition to having heavy rare earth elements (HREE) depletion, suggesting the influence of residual garnet in their mantle source region. The second is back-arc basin basalt (BABB) type with relatively depleted trace element compositions with respect to N-MORB together with a negative Nb anomaly, suggesting generation above an intra-oceanic subduction zone where partial melts are derived from a depleted (MORB-like) mantle. The final group is island-arc tholeiite (IAT) type, displaying the most depleted trace element abundances among the studied groups in addition to marked Nb depletion, reflecting intra-oceanic supra-subduction zone (SSZ) signatures similar to the BABB-type but requiring a depleted mantle source which has experienced a previous melt extraction. Combined with a previously ascribed Late Triassic age of Tekin et al. (2002) (221 Ma, Late Carnian; based on the radiolarian fauna found in a chert layer alternating with mudstones), the associated basalts with E-MORB-type geochemical signatures, suggest formation of oceanic crust as early as Late Carnian. This age is the oldest thus far obtained from the basalts of the Izmir-Ankara Ocean. This new data provides constraints on tectonic models for the opening the Izmir-Ankara Ocean and its relationship to other branches of the Neotethyan ocean in the Eastern Mediterranean area.

Göncüoglu, M. Cemal; Sayit, Kaan; Tekin, U. Kagan

2010-04-01

309

Hydrothermal cooling of the oceanic lithosphere and the square root age law  

Science.gov (United States)

In geodynamic models of mid-ocean ridges and cooling lithosphere hydrothermal convection processes are important to control the temperature and thus the rheological behavior of the crust. However, the characteristic time scale of hydrothermal convection is considerably shorter than that of cooling of the oceanic lithosphere and can hardly be addressed in a conjoined model. To overcome this problem we present an approach to mimic hydrothermal cooling by an equivalent, increased thermal conductivity. First the temperature and pressure dependence of crack related porosity and permeability are derived based on composite theory. A characteristic pore closure depth as a function of pressure, temperature and pore aspect ratio is defined. Two-dimensional porous convection models are used to derive scaling laws for parameterized convection including a Rayleigh-Nusselt number relation for a permeability exponentially decreasing with depth. These relations are used to derive an equivalent thermal conductivity to account for consistently evolving hydrothermal heat transport in thermally evolving systems. We apply our approach to a 1D model for cooling of the oceanic lithosphere. Within the context of our modeling parameters we found a pronounced effect for young lithosphere (younger than 10 Ma) down to about 20 km. Significant deviations of the heat flux versus age from the 1/square root t - law may occur due to hydrothermal convection. For the bathymetry versus age curves slopes steeper than 1/square root t - slopes already occur for very young lithosphere. Hydrothermal convection leads to an increase of the total heat flux and heat loss with respect to the classical purely conductive cooling model. Comparison of the total heat flow and its conductive contribution with observations confirm previous suggestions that for young lithosphere heat flow measurements represent only the conductive part, while at older ages the total heat flow is observed. Within their scatter and uncertainties heat flow and bathymetry data are in general agreement with our hydrothermally enforced cooling model suggesting that hydrothermal convection may be important even up to high ages.

Schmeling, Harro; Marquart, Gabriele

2014-05-01

310

Geochemical Characteristics of Intra-oceanic Plateaus and Recognition of Ancient Plateau Remnants on the Continents  

Science.gov (United States)

Early geochemical work on Cretaceous oceanic plateaus suggested that tholeiitic plateau basalts might have a distinctive isotopic and trace element signature that could be diagnostic for identifying fragments of ancient plateau crust now incorporated in the continents [e.g. Mahoney, 1987]. Subsequent work has demonstrated that basalts of intra-oceanic plateaus formed far from continental lithosphere indeed have general similarities in Nd-Sr-Pb-Hf isotope ratios and alteration-resistant incompatible elements [e.g. Neal et al., 1997; Kerr et al., 1997; Fitton et al., 2004; Hoernle et al., 2004]. These characteristics differ from those of most N-MORB and island arc basalts. For example, most basalts of the ca. 120 Ma Ontong Java Plateau have relatively flat primitive-mantle-normalized element patterns from Nb to Lu and a small range of age-corrected isotope ratios (e.g., epsilonNd = +5.4 to +6.5) distinct from those of present-day and pre-120 Ma Pacific N-MORB. Similar signatures are found in several accreted mafic terranes interpreted to be Mesozoic plateau fragments [e.g. Lassiter et al., 1995; Kerr et al. 2002], and in Precambrian examples that may have a plateau origin [e.g. Abouchami et al., 1990; Puchtel et al., 1998; Polat et al., 1998]. However, many E- (or T-) MORB have broadly similar isotopic and incompatible-element characteristics, and recent sampling has revealed that such MORB are common, even dominant, along some segments of the ocean-ridge system far from hotspots. Data for the central Southeast Indian Ridge, for example, overlap with those for the Ontong Java Plateau. The E-MORB and plateau basalts differ when major-element indicators of partial melting are combined with isotope and trace element data. However, major-element compositions can be modified significantly by alteration and even relatively low-grade metamorphism. Disregarding possible long-term temporal changes in plateau and/or ocean-ridge composition, these effects make it difficult to prove a plateau origin on geochemical grounds alone, particularly for smaller and/or poorly preserved terranes. Other evidence (on thickness and extent of lava flows, total thickness of basalt lava section, etc.) is required in addition to geochemical data.

Mahoney, J. J.

2005-12-01

311

Evolution of high Arctic ocean basins and continental margins  

Energy Technology Data Exchange (ETDEWEB)

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

Engen, Oeyvind

2005-08-01

312

Contamination of basaltic magma by mafic crust at Amboy and Pisgah Craters, Mojave Desert, California  

Science.gov (United States)

Quaternary alkali basalts from Pisgah Crater and Amboy Crater in southern California exhibit unusual chemical and isotopic variations which probably result from assimilation of mafic crust. Although lavas from both volcanoes are alkali basalts and hawaiites with isotopic and chemical characteristics that are similar to ocean island basalts (OIB) (e.g., ?Nd=2.6 - 5.9, 87Sr/86Sr = 0.7038 - 0.7049, Hf/Ba=0.014 - 0.017), they display highly correlated and unusual variations in their chemical and isotopic compositions. At each volcano, MgO decreased during the eruptive sequence from ?8.5 wt % to ?4.5 wt %. Incompatible elements are positively correlated with MgO and therefore also decreased during the eruptive sequence. Nd, Sr, and Pb isotope ratios correlate strongly with MgO. Compositional and isotopic data cannot be explained by any combination of closed-system fractionation, partial melting of the mantle, or silicic contamination. These data indicate that the basalts represent mixing between a high-MgO, high-?Nd component, common to both Pisgah and Amboy craters, with lower-MgO and -?Nd components that are unique to each center. The high-MgO component most likely is a primitive mantle-derived magma, based in part on its similarity to nearby xenolith-bearing Quaternary basalts. The low-MgO components are interpreted to be partial melts of mafic crust. If our model is correct, then both volcanoes evolved from eruption of nearly pure mantle melts early in their history to eruption of nearly pure remelted mafic crust late in their history. The crustal source could not have been underplated Mesozoic or younger oceanic crust, but its age is otherwise unconstrained; nearby Mesozoic gabbros and Proterozoic diabases have appropriate isotopic compositions. The basalt data provide no evidence that ancient enriched lithospheric mantle currently underlies the Mojave Desert. If such mantle was present at any time beneath this region, it must have been removed during one or more of the many tectonic events that affected the Mojave Desert during the Phanerozoic. Regional variability of isotope ratios in basalts is commonly interpreted to reflect variability of the underlying mantle. Data from this study raise the possibility that some of this variability may result from cryptic contamination of OIB-like basalts by mafic crust.

Glazner, Allen F.; Farmer, G. Lang; Hughes, William T.; Wooden, Joseph. L.; Pickthorn, William

1991-07-01

313

Garnet Sm-Nd and Zircon U-Pb Ages Track Pluton Emplacement, Granulite Facies Metamorphism, Partial Melting, and Extension in the Lower Crust, Fiordland New Zealand  

Science.gov (United States)

Extensional collapse of over thickened magmatic arc crust depends on crustal thickness variations and architecture. The structural architecture of the lower- and mid-crustal at the onset of extension may be complex due to lithologic variation resulting from structural juxtaposition of pre-arc lithologies, intrusion of plutons, and local partial melting. Additional complexity is introduced by the dynamic nature of arcs. Thus, robust ages for deformation, metamorphism, intrusion, and partial melting are essential for unraveling arc evolution and discerning the role of arc plutonism and metamorphism in strain localization during both contraction and extension. Eclogite and granulite facies metamorphic minerals indicate that Mesozoic arc crust in Fiordland was ? 50 km thick ca. 130 Ma prior to extensional collapse. This mid- to lower-crust records a history of mafic to intermediate magmatism, high-grade metamorphism, lower crustal melting, and the formation of extensional detachments that border eclogite- and granulite-cored gneiss domes. U-Pb zircon and Sm-Nd garnet ages indicate that intrusion of voluminous plutons, including the Western Fiordland Orthogneiss (WFO), and subsequent metamorphism occurred sequentially from north to south. Pluton emplacement occurred at 0.6-1.1 GPa in the north to 1.0-1.2 GPa in the south. In northern Fiordland [Milford Sound], intrusion of 135-128 Ma gabbroic magma was followed by 0.6-1.1 GPa 2-pyroxene granulite metamorphism at 126-135 Ma, and then 1.2-1.4 GPa garnet granulite metamorphism and partial melting ca. 126-123 Ma. To the south, WFO plutons have a similar history from north to south: the low-P 125-120.1 Ma Worsley was metamorphosed to garnet granulite at 1.2-1.4 GPa, ca. 115 Ma; the low-P ca. 120 Ma Misty was metamorphosed to garnet granulite at 1.2 GPa, ca. 115 Ma; the high-P 117.8-113.2 Ma Malaspina was metamorphosed to garnet granulite at 1.0-1.4 GPa, ca. 113 Ma along Doubtful Sound and 111.9±1.6 Ma to the south on Resolution Island. New ages, and metamorphic and structural studies link periods of magmatism and metamorphism to specific spatial and temporal patterns of extensional deformation. The Malaspina pluton in central Fiordland underwent garnet granulite metamorphism <3.5 Myr after emplacement and then in part cooled to amphibolite facies conditions ca. 110 Ma. Fabrics record a progression from magmatic flow to high-T extensional deformation at garnet granulite & eclogite (700-800°C, 1.2-1.8 GPa) facies to lower-T extensional deformation at upper amphibolite facies (550-650°C, 0.7-0.9 GPa). Locally, the lower crust cooled thru 550-650°C without significant exhumation by 111 Ma. Elsewhere garnet ages indicate hot lower crust at 111 Ma. Within a 3.5 Myr timeframe the pluton cooled & crystallized, and extensional shear zones underwent a rapid (<3 Ma) transition from distributed melt-assisted deformation to cooler, localized upper amphibolite facies deformation at pluton margins. These results indicate that rheological variations and structural transitions linked to magmatism and partial melting may be more spatially heterogeneous and short-lived than previously believed.

Stowell, H. H.; Klepeis, K. A.; Odom Parker, K.

2011-12-01

314

Regional variations of the shear-wave polarization anisotropy in the crust and mantle wedge beneath the Tohoku district  

Science.gov (United States)

We investigated the regional variations of the shear-wave polarization anisotropy in the upper crust, lower crust, and mantle wedge beneath the Tohoku district in the northeast Japan arc. The shear-wave splitting parameters (fast polarization direction (FPD) and split time) for the upper and lower crusts are estimated by shear-wave splitting analysis of the Ps phases, which are generated by the conversion of a P wave into an S wave at the Conrad and Moho discontinuity interfaces. The splitting parameters for the mantle wedge are determined by splitting analysis of the slab Ps phase converted at the oceanic crust on the subducting Pacific slab and the direct S waves from deep focus earthquakes located just below seismic stations. The Ps phases are identified on the P-wave receiver functions constructed from the teleseismic records. To accurately determine the splitting parameters for the lower crust from the Moho Ps phase, the Ps phase should be corrected for the shear-wave splitting effect in the upper crust. Similarly, to estimate the mantle wedge anisotropy, the direct S wave and slab Ps phase must be corrected for the splitting effect in the crust. The anisotropies of the crust and mantle wedge are estimated from the corrected Ps phases and direct S waves. The anisotropy of the upper crust exhibits a regional variation where FPD of the split shear wave is predominant in the N-S direction in the Pacific coast area and in the E-W direction in the rest of the Tohoku district. The split time is less than 0.2 s. The upper crustal anisotropy is attributed to the alignment of vertical cracks induced in the upper crust by tectonic stress. In the lower crust, FPD is predominant in the E-W direction with a split time similar to that in the upper crust, and the anisotropy of the lower crust is due to the lattice preferred orientation of rock-forming minerals. In the mantle wedge, FPD is predominant in the N-S direction (trench-parallel) in the fore-arc side of the volcanic front, but in the E-W direction (trench-perpendicular) in the back-arc side. The split time in the mantle wedge is similar to that of the upper crust. The most likely cause for the trench-perpendicular anisotropy in the back-arc mantle wedge is the lattice preferred orientation of A-type olivine along flows of the secondary mantle convection. For trench-parallel anisotropy in the fore-arc mantle wedge, we considered two possible causes: the preferred orientation of B-type olivine along the secondary mantle convection and that of dry olivine along the trench-parallel flow produced by the along-strike dip variation of the Pacific slab. However, we could not determine which one is the origin.

Watanabe, Mitsumi; Oda, Hitoshi

2014-10-01

315

Provenance And Tectonomagmatic Setting Of The Santa Marta Schists, Northern Colombia Caribbean Region: Insights On The Styles Of Growth And Approach Of Caribbean Intra- Oceanic Domains To The Continental Margin  

Science.gov (United States)

The life cycle of an intra-oceanic terranemincludes different phases and styles of magmatic growth, accretion with other terranes and translation before reaching a continental margin. In order to unveil the nature of these phases in crystalline rocks from northern Colombia, U/Pb LA-MC-ICP-MS detrital geochronology and whole rock geochemical data were obtained from stacks of intercalated metavolcanic-sedimentary rocks of the Santa Marta Schists in the Sierra Nevada de Santa Marta. Immobile elements whole rock geochemistry from greenschist to amphibolite facies units are characterized by low to moderate LREE/HREE, variable Th enrichment and weakly negative Nb and Ti anomalies, which are similar to island arc and MORB signatures. The intercalated metasedimentary rocks show a REE pattern similar to the PAAS and high Zr/Sc vs Th/Sc ratios, which suggest a felsic and highly diferentiated upper crust sources for the protoliths. Detrital zircons from three different units were obtained, The maximum depositional age for the northwestern unit is limited to the late Cretaceous, with a major peak of 83 Ma. Variable input of older crustal sources with Jurassic (153 Ma), Permo-Triassic (250-290 Ma), Cambrian to Late Neoproterozoic (520-560 Ma) and Middle Mesoproterozic (1000-1500 Ma) ages which are clearly recognized in older units of the Sierra Nevada de Santa Marta massif and the northern South American basement are also recorded. This type of volcano-sedimentary record within an intra-oceanic arc bears strong similarities with the modern Lesser-Antilles and the Tonga-Kermadec arcs, where continentally derived sediments can be transported houndred of kilometers to the fore-arc, back-arc or the accretionary prism of the active intra-oceanic arc. This record also suggests that this arc has an intra-Americas position, near to its final accretionary stop. Although the metamorphic overprint has obliterated the stratigraphic relations, apparent variations of the LREE/HREE in the metavolcanics and the preservation of continentally derived sediments, can be ascribed to variation in the sediment subduction vs accretion sediment budget in an evolving back-arc basin. Similar lithostratigraphic associations within the Caribbean metamorphosed and unmetamorphosed magmatic and sedimentary units can be related to a similar detrital continental input, whereas, the existence of coherent metamorphosed pre-Mesozoic crustal elements within subduction-accretion complexes, may reflect the existence of continental islands that were detached during the initial proto-Caribbean rifting phases and mixed within intra-oceanic accretionary prisms.

Cardona, A.; Jaramillo, C.; Ojeda, G.; Ruiz, J.; Valencia, V.; Weber, M.

2007-12-01

316

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

Science.gov (United States)

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

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

2012-07-01

317

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

Science.gov (United States)

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