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1

Oceanic and Continental Crust  

Science.gov (United States)

This site contains 25 questions on the topic of oceanic and continental crust, which covers the physical properties and features of the two crust types. This is part of the Principles of Earth Science course at the University of South Dakota. Users submit their answers and are provided immediate verification.

Heaton, Timothy

2

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

DEFF Research Database (Denmark)

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

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

2006-01-01

3

Future Scientific Drilling of Oceanic Crust  

Science.gov (United States)

Processes that occur within and across the oceanic crust-in particular along mid-ocean ridges and oceanic spreading centersplay a huge role in the dynamics of the Earth. The largest fluxes of heat and material between the Earth's mantle, crust, and seawater occur via magmatic, tectonic, and hydrothermal processes along oceanic spreading centers and their vast flanks. Roughly two thirds of the Earth's surface is accreted through magmatic and tectonic processes along mid-ocean ridges, and subduction of this ocean crust in turn influences mantle compositions. Exchange of elements between ocean crust and seawater strongly influences seawater compositions and leaves a geologic record of fluid-rock reactions in altered ocean crust. Some of these reactions contribute energy to microbial activity of a largely unexplored biosphere. The dynamics of ridge and ocean crustal processes therefore have enormous implications for thermal, chemical, and biological exchanges between the solid Earth and the hydrosphere.

Hayman, Nicholas W.; Bach, Wolfgang; Blackman, Donna; Christeson, Gail L.; Edwards, Katrina; Haymon, Rachel; Ildefonse, Benoit; Schulte, Mitch; Teagle, Damon; White, Scott

2010-04-01

4

Why Does Calcium Carbonate Precipitate in the Ocean Crust?  

Science.gov (United States)

Calcium carbonate (CaCO3) precipitation in extant ocean crust may provide lessons for industrial carbon dioxide (CO2) sequestration to mitigate against future climate change. The formation and alteration of the ocean crust during mid-ocean ridge spreading plays an integral role in the geological carbon (C) cycle. Magma-degassing during mid-ocean ridge volcanism releases large volumes of CO2 to the oceans and atmosphere, whereas the precipitation of CaCO3 minerals in rocks and veins during hydrothermal alteration of the ocean crust is a major mechanism of natural CO2 sequestration. However, the physical and chemical conditions that control the nature, extent and rate of calcium carbonate precipitation in the ocean crust are not well known, and the magnitudes of the carbon fluxes associated with the formation and alteration of the ocean crust remain poorly quantified. Mesozoic ocean crust contains significantly more hydrothermal CaCO3 than young ocean crust, but the past ocean conditions responsible for this enhanced C-uptake by the ocean crust are yet to be determined. The thermal, hydrogeologic, and chemical characteristics of active, low temperature seawater-basalt interaction have been investigated by a series of integrated studies in young ocean crust (<3.6 Ma) on the eastern flank of the Juan de Fuca ridge (JdFR). This has included sampling by scientific ocean drilling of CaCO3 veins, their parent fluids, and the basalts that host the veins. This area provides a unique opportunity to investigate the conditions of in situ hydrothermal CaCO3 precipitation on short geologic timescales. Fluid and vein analyses provide boundary conditions for geochemical modeling using 'Phreeqc' to determine why basement fluids are saturated with respect to CaCO3. We evaluate the controlling effects of key parameters including fluid dissolved inorganic carbon (DIC), Mg and Ca concentrations, alkalinity, temperature, and pH on CaCO3 precipitation. Our results provide insights into how differing past conditions (e.g. ocean temperature and pCO2) affected the ocean crustal C-reservoir, and enable better quantification of the magnitude of this reservoir and how it has varied over geological time. This information is directly applicable to the development of industrial scale CO2 sequestration using methods that mimic but accelerate natural CO2-drawdown processes.

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

2012-12-01

5

Uranium in the oceanic crust  

Energy Technology Data Exchange (ETDEWEB)

Thermal-neutron induced fission of uranium-235 recorded in appropriate nuclear-track detectors and visualized by chemical etching offers a unique means for measuring the concentration of uranium and mapping its spatial distribution within a rock. The accuracy in measuring uranium concentrations as low as 10/sup -10/ g/g and the high local resolution of the uranium atom sites within only a few microns makes this method extremely sensitive. Uranium mapping was applied to tholeiitic basalts coming from various parts of the world-wide mid-oceanic rift system. The results led to the following findings: - on a micro-scale, the analysis of the main constituent phases of the basalts (glass, minerals, matrix) makes it possible to follow the mode of uranium enrichment in function of the degree of sea-water alteration and to determine the pristine uranium content of a tholeiite. - On a regional scale, the pristine uranium contents of the samples provide a means to discriminate between different generations of basalts flows. - On a global scale, differences in bulk uranium contents among tholeiites from the various parts of the mid-oceanic rifts might be related to the interaction of three processes; the degree of partial melting of a chemically homogeneous upper mantle, the subsequent modification of the melts by fractional crystallization, and their local contamination by ''hot spot'' type material through lateral flow along the ridges. 7 ref.; 13 tabl.; 20 refs.

Selo, M.; Storzer, D.

1987-05-29

6

Uranium in the oceanic crust  

International Nuclear Information System (INIS)

Thermal-neutron induced fission of uranium-235 recorded in appropriate nuclear-track detectors and visualized by chemical etching offers a unique means for measuring the concentration of uranium and mapping its spatial distribution within a rock. The accuracy in measuring uranium concentrations as low as 10-10 g/g and the high local resolution of the uranium atom sites within only a few microns makes this method extremely sensitive. Uranium mapping was applied to tholeiitic basalts coming from various parts of the world-wide mid-oceanic rift system. The results led to the following findings: - on a micro-scale, the analysis of the main constituent phases of the basalts (glass, minerals, matrix) makes it possible to follow the mode of uranium enrichment in function of the degree of sea-water alteration and to determine the pristine uranium content of a tholeiite. - On a regional scale, the pristine uranium contents of the samples provide a means to discriminate between different generations of basalts flows. - On a global scale, differences in bulk uranium contents among tholeiites from the various parts of the mid-oceanic rifts might be related to the interaction of three processes; the degree of partial melting of a chemically homogeneous upper mantle, the subsequent modification of the melts by fractional crystallization, and their local contamination by ''hot spot'' type material through lateral flow along the ridges. 7 ref.; 13 tabl.; 20 refs

1987-05-29

7

Magnetization of the oceanic crust: TRM or CRM?  

Science.gov (United States)

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

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

1987-01-01

8

Chemical remanent magnetization of oceanic crust  

Energy Technology Data Exchange (ETDEWEB)

The effects of chemical remanent magnetization (CRM) of oceanic crust on the anomalous skewness of sea-floor spreading magnetic anomalies are investigated. Considering a realistic constraint that the actual magnetization at anomaly M0 is reversed, the CRM of layer 2A basalts fails to explain the anomalous skewness of the magnetic anomalies. The CRM of the deeper layers does contribute to the anomalous skewness of anomalies 33/34, but the major contribution comes from thermal remanent magnetization.

Verhoef, J. (Bedford Institute of Oceanography, Dartmouth, Nova Scotia (Canada)); Arkani-Hamed, J. (McGill University, Montreal, Quebec (Canada))

1990-10-01

9

New Images Suggest Oceanic Crust Generated from Several Magma Sources  

Science.gov (United States)

... Physics Press Release 05-149New Images Suggest Oceanic Crust Generated from Several Magma ... August 25, 2005 Some of the highest quality images ever taken of the Earth's lower crust reveal that ...

10

Effect of thicker oceanic crust in the Archaean on the growth of continental crust through time  

International Nuclear Information System (INIS)

Present crustal evolution models fail to account for the generation of the large volume of continental crust in the required time intervals. All Archaean plate tectonic models, whether invoking faster spreading rates, similar to today's spreading rates, or longer ridge lengths, essentially propose that continental crust has grown by island arc accretion due to the subduction of oceanic crust. The petrological differences that characterize the Archaean from later terrains result from the subduction of hotter oceanic crust into a hotter mantle. If the oceanic crust was appreciably thicker in the Archaean, as geothermal models would indicate, this thicker crust is surely going to have an effect on tectonic processes. A more valid approach is to compare the possible styles of convergence of thick oceanic crust with modern convergence zones. The best modern analog occurs where thick continental crust is colliding with thick continental crust. Oceanic crustal collision on the scale of the present-day Himalayan continental collision zone may have been a frequent occurrence in the Archaean, resulting in extensive partial melting of the hydrous underthrust oceanic crust to produce voluminous tonalite melts, leaving a depleted stabilized basic residuum. Present-day island arc accretion may not have been the dominant mechanism for the growth of the early Archaean crust

1987-07-13

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Continental Crust Growth as a Result of Continental Collision: Ocean Crust Melting and Melt Preservation  

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 tectonic settings, volcanic arc rocks associated with oceanic lithosphere 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 genetic 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 40 years. However, it has been recognized also that this island-arc model has several difficulties. These include (1) bulk arc crust (AC) is basaltic, whereas the bulk CC is andesitic [1]; (2) AC has a variably large Sr excess whereas the CC is Sr deficient [2]; and (3) AC production is mass-balanced by subduction-erosion and sediment recycling, thus contributing no new mass to CC growth, at least in the Phanerozoic [3,4]. Our data on magmatic rocks (both volcanic and intrusive) formed during the India-Asia continental collision (~65 - ~45Ma) [5] show a remarkable compositional similarity to the bulk CC with the typical arc-like signature [6]. Also, these syncollisional felsic rocks exhibit strong mantle isotopic signatures, implying that they were recently derived from a mantle source. The petrology and geochemistry of these syncollisional felsic rocks is most consistent with an origin via partial melting of upper oceanic crust (i.e., last fragments of underthrusting oceanic crust) 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 continental growth. References: [1] Gill, Orogenic andesites and plate tectonics, Springer-Verlag, New York., 390 pp, 1981; [2] Niu & OHara, Lithos, 112, 1-17, 2009; [3] von Huene & Scholl, Rev. Geophys., 29, 279-316, 1991; [4] Clift & Vannucchi, Rev. Geophys., 42, RG2001., 2004; [5] Mo et al., Chem. Geol., 250, 49-67, 2008; [6] Rudnick & Gao, Treat. Geochem., 3, 1-64, 2003.

Niu, Y.; Zhao, Z.; Zhou, S.; Zhu, D.; Dong, G.; Mo, X.; Xie, G.; Dong, X.

2010-12-01

12

Constraining the Extent and Duration of Low Temperature Alteration in Pacific Ocean Crust  

Science.gov (United States)

The generation, alteration, and subduction of oceanic lithosphere is one of the primary chemical cycles that occur on Earth. The circulation of seawater through ocean crust facilitates the exchange of elements and chemical compounds between the hydrosphere and lithosphere, ultimately having a profound effect on both the chemical composition of seawater and ocean crust. Our understanding of the chemical cycles that result from ocean crustal alteration and subsequent subduction is largely based on core recovered by the Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP). Basement holes drilled in the Pacific Ocean basin however, poorly represent the span of crustal ages that exist in fast-spreading Pacific Ocean crust. Thus, several pressing questions concerning the evolution of seawater-rock interactions and the extent of chemical exchange remain unanswered. The 46 Ma igneous basement that was drilled during ODP Leg 200 at Hole 1224F fills a key age gap in the drill core collection that falls between Holes 597C ( 29 Ma) and 843B ( 110 Ma) enabling a better understanding of how ocean crustal alteration proceeds. The secondary mineral assemblages identified in Hole 1224 cores are typical of low temperature alteration, consisting of Fe-oxyhydroxides, saponite, celadonite, carbonate, minor pyrite and quartz, and rare phillipsite. When compared to adjacent fresh rock, haloed rock samples have increases in Si, Fe and Cr and minor increases in Ca, CO2, K, Mg, Mn, P, Na and Al. A significant finding thus far is that the extent of primary mineral replacement and quantity of secondary minerals filling voids is surprisingly low; much less than that found in younger crust (e.g. 6.9 Ma, Sites 504, 896). This suggests that crustal age may not be a controlling factor in the extent of crustal alteration. Hole 1224F also provides a critical constraint for understanding the controls on carbonate uptake in the Pacific upper ocean crust through time. Preliminary calculations show that the carbon content of Hole 1224F cores is less than that contained in younger crust, suggesting that age is not a dominant factor in regulating carbon uptake by the ocean crust.

Paul, H. J.; Gillis, K. M.

2003-12-01

13

Deep-ocean ferromanganese crusts and nodules  

Science.gov (United States)

Ferromanganese crusts and nodules may provide a future resource for a large variety of metals, including many that are essential for emerging high- and green-technology applications. A brief review of nodules and crusts provides a setting for a discussion on the latest (past 10 years) research related to the geochemistry of sequestration of metals from seawater. Special attention is given to cobalt, nickel, titanium, rare earth elements and yttrium, bismuth, platinum, tungsten, tantalum, hafnium, tellurium, molybdenum, niobium, zirconium, and lithium. Sequestration from seawater by sorption, surface oxidation, substitution, and precipitation of discrete phases is discussed. Mechanisms of metal enrichment reflect modes of formation of the crusts and nodules, such as hydrogenetic (from seawater), diagenetic (from porewaters), and mixed diagenetichydrogenetic processes.

Hein, James R.; Koschinsky, Andrea

2013-01-01

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Strontium and Oxygen Isotopic Profiles Through a Complete Sequence of Oceanic Crust Exposed on Macquarie Island  

Science.gov (United States)

Macquarie Island, approximately 1000 km south of New Zealand, is a thin sliver of ocean crust exposed by recent transpression along the Pacific - Australian plate boundary. The crust formed 11 Ma-ago most probably on a short spreading segment of a slow spreading mid-ocean ridge. Uplift and erosion on Macquarie Island has exposed unique sections through the ocean crust and an opportunity to describe and sample regions of the ocean basement that have yet to be sampled in situ despite 30 years of scientific ocean drilling. Here we present the first strontium and oxygen isotope profiles for a complete ocean basement stratigraphy from a single location. Although there are local complications, the isotope profiles yield general trends of decreasing seawater Sr and increasing temperatures of alteration with depth. 87Sr/86Sr ratios of the uppermost lavas are highly elevated ( 0.703-0.704) compared to the primary ratios determined from fresh glass (87Sr/86Sr 0.70255-0.70275) due to reaction with seawater. These lavas yield heavy oxygen isotopic compositions (? 18O 8-10 ) indicative of low temperature exchange with seawater. Relatively radiogenic 87Sr/86Sr of prehnites and chlorite bearing samples with elevated ? 18O indicate that there may be a lower temperature overprint of some samples. Dikes from deeper in the crust yield a more restricted range of 87Sr/86Sr ( 0.703-0.7035). Oxygen isotopic compositions of these samples bracket the primary mantle value and show a trend of decreasing ? 18O with depth representing greater extents of exchange at higher temperatures. Sr isotope ratios of gabbros, dikes and layered gabbros from the lower Macquarie crust are slightly elevated from primary igneous values (87Sr/86Sr 0.7025-0.703) indicating that there is minor penetration of seawater-derived strontium to the lowermost regions of the crust. These samples consistently yield relatively light oxygen isotopic compositions (? 18O 3-6 ) requiring significant exchange with hydrothermal fluids at high temperatures. Two peridotite samples from near the crust-mantle boundary have near seawater 87Sr/86Sr ratios ( 0.7085-0.7088) but visible late stage carbonate may be responsible for these highly elevated values. Heavy oxygen isotope ratios for some peridotites may indicate serpentinization at low temperatures although this occurred with little exchange of seawater strontium. The Macquarie Island section reveals similar trends in strontium and oxygen isotopic compositions with depth to those from the uppermost crust drilled into in situ ocean crust (ODP Holes 504B and 896A) although the rocks are less altered than samples from the major supra-subduction zone ophiolites (Troodos and Oman).

Teagle, D. A.; Coggon, R. M.; Alt, J. C.; Davidson, G. J.

2003-12-01

15

Tectonic evolution and continental crust growth of Northern Xinjiang in northwestern China: Remnant ocean model  

Science.gov (United States)

The Northern Xinjiang region is located in the southwestern part of the Central Asian Orogenic Belt (CAOB, also known as the Altaid Tectonic Collage). Despite extensive research aimed at understanding the crustal growth of the CAOB and the evolution of the Paleo-Asian Ocean, the tectonic evolution mechanism of continental crust growth in Northern Xinjiang remains controversial. The geology of Northern Xinjiang is characterized by widespread ophiolites, granitoids, intermediate-basic dikes. Most of the ophiolites were generated in the early Paleozoic. The ophiolites are widely spread around the Junggar Basin, but their distribution does not indicate a well-defined band. Their outcrops are generally related to various faults. The basic rocks widespread in Northern Xinjiang are grouped into two categories: (i) gabbros, diabases basalts of the ophiolites and (ii) basic dikes that intrude into the Paleozoic strata granite plutons. The basic rocks associated with the early Paleozoic ophiolites were reworked by later geothermal events with a peak 40Ar/39Ar age of 310-290 Ma. The basic dikes intruded into Paleozoic strata and granite plutons during the Carboniferous-Jurassic, displaying three peaks of emplacement at 260-250 Ma, 220 Ma, and 200-190 Ma. These two types of basic rocks and the documented Variscan magmatic rocks were derived from the same source. Their isotope geochemical characteristics and widespread distribution suggest that since the Paleozoic, a large geochemical province has existed in Northern Xinjiang with an affinity to mid-ocean ridge basalts (MORB) and ocean island basalts (OIB), which is related to a long-lived remnant ocean and the underlying early Paleozoic oceanic crust. The existence of remnant oceanic crust in Northern Xinjiang was confirmed by seismic, gravity and aeromagnetic data. Therefore, we propose the following remnant ocean model for the Paleozoic tectonic evolution of Northern Xinjiang. It consists of three stages: 1) oceanic crust formation and deposition of the overlying volcanic-sedimentary rocks during the early Paleozoic; 2) retaining of the remnant ocean with marine sediments deposited during the early stage of the late Paleozoic; and 3) widespread and pervasive emplacement of Variscan granites, intermediate-basic dikes, and their volcanic equivalents during the Late Carboniferous and Early Permian, and termination of marine sedimentation at the end of the Early Permian. The tectonic evolution of Northern Xinjiang has been in a state of intracontinental deformation since the Mesozoic. The Variscan granitoids and basic dikes of Northern Xinjiang originated from the partial melting of the remnant oceanic crust formed in the early Paleozoic. These Variscan intrusive rocks represent the production of continental crust transferred from the basic crust. The Phanerozoic continental growth of Northern Xinjiang was completed by mass transfer from the early Paleozoic remnant oceanic crust; this approach may considerably change our views of continental growth.

Xu, Qin-Qin; Ji, Jian-Qing; Zhao, Lei; Gong, Jun-Feng; Zhou, Jing; He, Guo-Qi; Zhong, Da-Lai; Wang, Jin-Duo; Griffiths, Lee

2013-11-01

16

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; Frster, Hans-Jrgen; Al Safarjalani, Abdulrahman; Nasir, Sobhi; Whitehouse, Martin J.; Leybourne, Matthew I.; Romer, Rolf L.

2014-05-01

17

Anorthositic oceanic crust in the Archean Earth  

Science.gov (United States)

Ultrapure minerals separated from eclogite inclusions in kimberlites were analyzed for Sm, Nd, Sr, and oxygen isotopes and for major and trace elements. Clinopyroxene (cpx) and garnet (gnt) are the only primary mineral phases in these rocks, and mineral phases and their alteration products. The WR sub calc. is the reconstructed bulk composition excluding all the contamination influences. Two groups of eclogites: are distinguished: (1) type A Noritic-anorthositic eclogites; and (2) type B Ti-ferrogabbroic eclogites. The oxygen isotopes are primary mantle-derived features of these rocks and are not caused by posteruption processes, as they were measured on unaltered, clean mineral separates and show a correlation with REE pattern and Sr and Nd isotopes. It is suggested that the variation of the oxygen isotopes are caused by crustal-level fluid-rock interaction at relatively low temperature. It is shown that oxygen isotopes variation in MORB basalts caused by the hydrothermal system are in the same range as the observed oxygen isotope variation in eclogites. A model to explain the new set of data is proposed. It is thought that some of these eclogites might be emplaced into the upper lithosphere or lower crust at the time corresponding to their internal isochron age. The calculated WR composition was used to estimate model ages for these rocks.

Jagoutz, E.; Dawson, J. B.; Hoernes, S.; Spettel, B.; Waenke, H.

1985-01-01

18

Continental crust beneath the Agulhas Plateau, Southwest Indian Ocean  

Energy Technology Data Exchange (ETDEWEB)

The Agulhas Plateau lies 500 km off the Cape of Good Hope in the southwestern Indian Ocean. Acoustic basement beneath the northern one third of this large, aseismic structural high has rugged morphology, but basement in the south is anomalously smooth, excepting a 30- to 90-km-wide zone with irregular relief that trends south-southwest through the center of the plateau. Seismic refraction profiles across the southern plateau indicate that the zone of irregular acoustic basement overlies thickened oceanic crust and that continental crust, locally thinned and intruded by basalts, underlies several regions of smooth acoustic basement. Recovery of quartzo-feldspathic gneisses in dredge hauls confirms the presence of continental crust. The smoothness of acoustic basement probably results from erosion (perhaps initially subaerial) of topographic highs with depositions and cementation of debris in ponds to form high-velocity beds. Basalt flows and sills also may contribute locally to form smooth basement. The rugged basement of the northern plateau appears to be of oceanic origin. A plate reconstruction to the time of initial opening of the South Atlantic places the continental part of the southern plateau adjacent to the southern edge of the Falkland Plateau, and both abut the western Mozambique Ridge. Both the Agulhas and Falkland plateaus were displaced westward during initial rifting in the Early Cretaceous. Formation of an RRR triple junction at the northern edge of the Agulhas continental fragment during middle Cretaceous time may explain the origin of the rugged, thickened oceanic crust beneath plateau as well as the apparent extension of the continental crust and intrusion of basaltic magmas beneath the southern plateau.

Tucholke, B.E.; Houtz, R.E.; Barrett, D.M.

1981-05-10

19

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

20

Seismic anisotropy observed in upper oceanic crust  

Energy Technology Data Exchange (ETDEWEB)

Seismic anisotropy in the upper 150Om. of oceanic basement has been observed by polarization analysis of three-component bore-hole seismometer records. The most convincing evidence for the anisotropy is shear wave splitting for explosive sources at four azimuths. Compressional wave particle motion deviations suggestive of anisotropy are also observed but they may be caused by lateral inhomogeneities. The anisotropy was not resolved by travel-time analysis. The observed velocities and particle motions in the horizontal plane can be modelled to within a standard deviation by assuming a perfectly elastic, homogeneous, anisotropic layer 2 with hexagonal symmetry and a horizontal symmetry axis. The most probable cause of the anisotropy is preferred crack orientation.

Stephen, R.A.

1981-08-01

 
 
 
 
21

U Mineral Hosts and Enrichment Processes in Altered Oceanic Crust  

Science.gov (United States)

The U-Pb-Th isotopic system is a primary tool for understanding mantle and continental evolution and for quantifying the flow of mass and heat through the Earth's reservoirs. One of the major sites of U-Pb-Th fractionation is the oceanic crust, which is a sink for seawater U. For example, the upper, oxidized oceanic crust (U ~0.4ppm) may be as much as 4x enriched over pristine igneous values (U ~0.09ppm) with a minor net change in Pb and Th. Little, however, is understood about the mechanisms controlling uranium enrichment, its mineral hosts, or the timing of the process. We have used laser ablation inductively coupled mass spectrometry (LA-ICP-MS) and electron microprobe data to study the distribution of U in the oldest sampled crust in the Pacific, Jurassic mid-ocean ridge basalts (MORB) from ODP Site 801C (1000 km seaward of the Mariana trench), formed at fast spreading rates. Seventeen thin sections, 8 with the highest whole rock U content (HUC) (0.61-1.7ppm) and 9 with low U content (LUC) (U celadonite and carbonates were also analyzed. Initial analysis suggests high U ( ~1.8ppm) is associated with Fe-oxides, halos immediately surrounding the mixed Fe-oxide veins, or redox fronts further into the basaltic hosts (up to 0.5mm from the oxide veins). Thus, U in altered MORB is associated with three types of alteration phases: 1) carbonate veins, 2) halos surrounding mixed Fe-oxide veins, and 3) Fe-oxide phases. Of the excess U in the upper 500 m at 801C, roughly 2/3 is carbonate-hosted. Given the variety of hosts, we might predict variable release of U into the mantle as these different phases breakdown in the subduction zone. These results also point to a need for more data on carbonate and oxide fluid partitioning at high pressure.

Farr, L. C.; Plank, T.; Kelley, K.; Alt, J. C.

2001-12-01

22

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

23

Os-187/Os-186 in oceanic island basalts: tracing oceanic crust recycling in the mantle  

Science.gov (United States)

We measured Os-187/Os-186, Re and Os concentrations in 14 lavas from different oceanic islands using negative thermal ionization mass spectrometry. The counting precision (2 sigma) on Os-187/Os-186 is 0.1-0.6%. Uncertainties introduced by blank and age corrections can increase the total error up to 3%. Os-187/Os-186 corrected for blank and Re-187 in situ decay range from 1.082 +/- 0.001 (Reunion) to 1.485 +/- 0.045 (Mangaia). Re and Os concentrations can be explained by the chalcophile behaviour of these elements during magmatic processes. A weak correlation between Os-187/Os-186 and Pb-206/Pb-204 was found. Principal component analysis in Os-Sr-Nd-Pb isotopic ratio space shows that the scattering of the data is related to a partial decoupling between Os-187/Os-186 and Pb-206/Pb-204. Modeling of oceanic crust, terrigenous and pelagic sediments recycling in the mantle shows that mixing of old oceanic crust in OIB sources can account for the highest Os-187/Os-186 (Mangaia, Tubuai, Comores). Small quantities of sediments (greater than 1%) subducted with the oceanic crust can explain the lack of good correlation between Os-187/Os-186 and Pb-206/Pb-204. Since Os-187/Os-186 is much less sensitive to sediment subduction than Pb-206/Pb-204, it could be a more suitable tool for studying the recycling of old oceanic crust in the mantle. However, even if the assimilation of altered material by the lavas during their ascent through the oceanic crust is not a dominant process, it can significantly affect the Os-187/Os-186 of some Os-poor lavas and, therefore, it contributes to the Os-Pb decoupling.

Roy-Barman, Matthieu; Allegre, Claude J.

1995-01-01

24

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

Science.gov (United States)

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

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

2014-05-01

25

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("1"1B/"1"0B 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 21"0 and 13"0N have B contents of 0.39 +- 0.03 and 0.46 +-0.03 ppm (about one order of magnitude lower than previous estimates) and delta"1"1B 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 21"0 and 13"0N (EPR) have variable B enrichments, relative to seawater (416 ?moles/kg), of between 8 and 32% and have "1"1B 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 10"9 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 "1"8O, and increases with degree of alteration. Altered basalts (n = 7) containing between 8.9 and 69 ppm B have "1"1B 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 "1"1B 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 10"9 moles/yr. The maximum diffusive flux of B into the crust from sediment pore waters is 0.8 x 10"9 moles/yr with a "1"1B less than 43 per mil. (author)

1987-01-01

26

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

Digital Repository Infrastructure Vision for European Research (DRIVER)

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...

2013-01-01

27

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

Digital Repository Infrastructure Vision for European Research (DRIVER)

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 Cu-0. We propose that both hydrothermally-released Cu and seawater were the sources of Cu at these sites. Phase stability diagra...

2013-01-01

28

Astronaut Scott Carpenter being recovered from Ocean after MA-7 flight  

Science.gov (United States)

Astronaut M. Scott Carpenter, prime pilot for the Mercury-Atlas 7 (MA-7) mission, is seen being recovered from Atlantic Ocean after MA-7 flight. A diver helps Carpenter into a life raft while the capsule floats nearby.

1962-01-01

29

Low-temperature alteration of mesozoic oceanic crust, Ocean Drilling Program Leg 185  

Science.gov (United States)

Basalts drilled from the Mesozoic crust at Ocean Drilling Program (ODP) Sites 801 and 1149 during Leg 185 have been affected by a succession of submarine, variably oxidative alteration processes by cold seawater. The low-temperature alteration resulted in various combinations of secondary minerals both as replacements of igneous phases, and as void and crack fillings. Mg-smectite, calcite, celadonite and Fe-oxyhydroxide are the dominant secondary minerals. Quartz, chalcedony and phillipsite are always very minor phases that fill open spaces. The rocks generally exhibit concentric haloes, 0.5 to 1 cm in thickness, around gray inner cores and usually adjacent to exposed surfaces. Color zonations result from the abundance of various secondary mineral assemblages present in each zone. The green rocks are dominated by celadonitic phyllosilicates, whereas the dusky red haloes are characterized by abundant iddingsite and Fe-oxyhydroxides replacing the fine clinopyroxene. Finally Mg-smectite calcite, are the most common secondary phases in the inner gray cores. The most unusual effects of the low-temperature alteration of the study rocks are the pervasive, pale green to gray green color and the relatively abundant dusky red haloes surrounding inner reddish gray cores. Alteration of Mesozoic crust cored during Leg 185 is comparable with low-temperature alteration of 310 m thick uppermost volcanic section of the 5.9 My old crust of Hole 504B. Except the renewed precipitation of calcite in veins, the aging of oceanic crust does not depend on time probably because it occurs during the first few millions of years after the accretion, and ceases when the crust is sealed off.

Talbi, El Hassan; Honnorez, Jos

2003-05-01

30

Investigating the Interaction Between Long-lived Compositional Reservoirs and Subducted Oceanic Crust. (Invited)  

Science.gov (United States)

Seismic tomography has revealed the presence of two large, low shear-velocity provinces (LLSVPs) in the lower mantle beneath Africa and the Pacific. Additional seismic studies provide evidence that these regions are compositionally-distinct from the surrounding, background mantle. It has been hypothesized that the LLSVPs are caused by long-lived compositional reservoirs in a mantle that is undergoing large-scale thermochemical convection. Several hypotheses exist as to the cause and formation of these reservoirs, each of which having significantly different consequences for heat and chemical transport within the Earth. One possibility is that the LLSVPs are caused by the accumulation of subducted oceanic crust. We explore this hypothesis geodynamically, finding that oceanic crust is too thin to survive viscous stirring and accumulate into larger structures in the lower mantle. Therefore, a more promising hypothesis is that LLSVPs are caused by older, more-primordial reservoirs that formed earlier in Earth's history. If so, it is critical to discover how these reservoirs interact with subducted oceanic crust because this process has important implications for geochemical evolution and small-scale lower mantle structure. We perform geodynamical calculations that include more-primordial reservoirs and subducted oceanic crust. We find that a significant fraction of oceanic crust is expected to be entrained into primordial reservoirs along the top cusps of the reservoirs, where mantle plumes are rooted. Within the reservoirs, entrained oceanic crust produces considerable heterogeneity, possibly explaining the seismic heterogeneity observed within LLSVP regions. Furthermore, some of this crust is later entrained back out of the reservoir into mantle plumes. In fact, these experiments predict that mantle plumes should, at a minimum, contain the following components: background mantle (e.g., MORB source), primordial reservoir material, relatively young subducted oceanic crust, and older, more-evolved oceanic crust. These results help to explain the variability in ocean island basalt (OIB) chemistry observed at hotspots.

McNamara, A. K.; Li, M.; Williams, C.; Garnero, E. J.

2013-12-01

31

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

International Nuclear Information System (INIS)

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

2009-08-23

32

Transition from oceanic to continental lithosphere subduction in southern Tibet: Evidence from the Late CretaceousEarly Oligocene (~ 9130 Ma) intrusive rocks in the ChanangZedong area, southern Gangdese  

Science.gov (United States)

Little is known about the detailed processes associated with the transition from oceanic to continental lithosphere subduction in the Gangdese Belt of southern Tibet (GBST). Here, we report zircon UPb age, major and trace element and SrNdHf isotopic data for Late CretaceousEarly Oligocene (~ 9130 Ma) intermediate-acid intrusive rocks in the ChanangZedong area immediately north of the YarlungTsangpo suture zone. These rocks represent five magmatic episodes at ~ 91, ~ 77, ~ 62, ~ 48, and ~ 30 Ma, respectively. The 9148 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: 10065 Ma roll-back of subducted Neo-Tethyan oceanic lithosphere; 6560 Ma initial collision between Indian and Asian continents; 6040 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

33

Investigating the Causes of Variability in the Intensity of Hydrothermal Alteration of Ocean Crust Formed at Intermediate to Fast Spreading Rates  

Science.gov (United States)

The hydrothermal alteration of the oceanic crust at the ridge axis and on the ridge flanks has a profound influence on the oceans and atmosphere and through subduction of altered crust, the chemistry of the upper mantle. Hydrothermal inputs play a fundamental role in controlling seawater composition and influence many of the records (e.g., Sr, S or O) we use to reconstruct past climate, erosion and other major other Earth processes. If possible, quantifying the chemical and isotopic exchange fluxes from the alteration of the ocean crust will provide a solid foundation for assessing changes in the more complex and variable systems draining the continents. At least in terms of seismic structure, ocean crust formed at intermediate to fast spreading rates is relatively uniform and should most closely relate to the ideal "Penrose" stratigraphy. Consequently, observations from deep drill hole into fast spread crust can be reasonably extrapolated to describe the seawater-basalt exchange over significant portions of the ocean basins. Here we compare lithologic, wire-line, alteration, mineralogic, isotopic (87Sr/86Sr, ?18O), and trace element profiles from DSDP/ODP Sites 504, 801, 843, 896, 1149, 1179, 1224, 1243, and 1256, combined with regional geophysical data, to determine the geologic factors that influence the variability and intensity of alteration in intermediate to fast spread ocean crust. These holes sample ocean crust formed between 6.9 and 170 Ma, but are constructed from contrasting eruptive lava styles (e.g., pillow lavas, sheet flows, massive flows, breccias), and have endured very different tectonic and sediment burial histories. Basement topography, the nature of the uppermost volcanic rocks, and the rapidity and thickness of sediment cover strongly influence the intensity of chemical and isotopic exchange. Unfortunately, progress in establishing these exchange budgets is greatly hampered by poor core recovery during hard rock drilling and a paucity of comprehensive wireline logging and imagery.

Teagle, D. A.; Smith-Duque, C. E.; Cooper, M. J.; Coggon, R. A.; Alt, J. C.; Banerjee, N. R.; Gillis, K. M.; Revillon, S.; Harris, M.; Foley, L.

2008-12-01

34

Methane deep in ocean crust could feed chemical-hungry microorganisms  

Science.gov (United States)

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.

Hines, Sandra; Information, Uw O.

35

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

Science.gov (United States)

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

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

2008-01-01

36

The Red Sea analog for the early Gulf of Mexico: Salt basins on oceanic crust  

Energy Technology Data Exchange (ETDEWEB)

New geophysical data from the Red Sea and the Gulf of Mexico support the concept that the early Gulf closely resembled the modern Red Sea. Oceanic crust like that now forming along the axis of the Red Sea basin may underlie much of the continental slope offshore Louisiana and Texas. Original depositional thicknesses greater than 4 km characterize both salt depocenters. The thickest salt overlies oceanic crust, probably for isostatic reasons. Deep crustal detachment faulting in a simple shear model with ductile flow below 15 km and narrow zones (up to 50 km) of severely extended crust on the hanging wall characterizes the early tectonic development. The landward edge of thick (> 2--4 km) salt generally follows the edge of oceanic crust, but the seaward edge is localized by depositional factors, modified by subsequent gravity spreading.

Hall, D.J. [Excalibur Interpretation Co., Houston, TX (United States)

1996-12-31

37

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

Science.gov (United States)

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

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

2007-05-01

38

Jurassic ophiolites within the Valais domain of the Western and Central Alps: geochronological evidence for re-rifting of oceanic crust  

Science.gov (United States)

Metabasic rocks from different parts of the Antrona ophiolites, Western Alps, as well as from the Misox zone, Central Alps, were dated using ion microprobe (SHRIMP) U-Pb analyses of zircon, in association with cathodoluminescence (CL) imaging. HP metamorphism must have affected at least the major part of the Antrona ophiolites, although HP relics are rarely preserved, probably due to the Lepontine metamorphic overprint. HP metamorphism has affected also the area of the Misox zone. The origin of the Antrona ophiolites is arguable. They were interpreted as part of both the Piemont Ligurian (PL) and the Valais ocean, the two main oceans in the area of the Alps before Alpine convergence. SHRIMP-analyses of co-magmatic zircon domains from the Antrona ophiolites (Guggilihorn, Passo del Mottone and Quarata areas) yielded identical (within uncertainty) weighted mean 206 Pb/238U ages of 155.21.6 Ma, 15817 Ma (or 163.12.4 Ma: one analysis; 1? error) and 155.62.1 Ma, respectively, interpreted as the time of crystallization of the magmatic protoliths. These Late Jurassic ages fit well to the time span considered for the formation of Piemont Ligurian oceanic crust. The metagabbro of the Misox zone (Hinterrhein area), for which a Valaisan origin is generally accepted, gave also a Late Jurassic, PL protolith age of 161.03.9 Ma. The metamorphic zircon domains from the amphibolitized eclogite of Mottone yielded an age of 38.50.7 Ma, interpreted as the time of HP metamorphism. This age is in good agreement with the time of metamorphism reported from previous zircon SHRIMP-data for eclogites and amphibolites of other parts in the Valais domain. In order to bring in line the PL protolith ages with the Valaisan metamorphic ages, we suggest a scenario involving emplacement of part of the PL oceanic crust to the north of the newly formed Brianonnais peninsula, inside the Valais geotectonic domain. This paleotectonic configuration was probably established when younger Valaisan oceanic crust formed by spreading and re-rifting, partly within PL oceanic crust.

Liati, Anthi; Froitzheim, Nikolaus; Fanning, C. Mark

2005-06-01

39

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

Science.gov (United States)

The goal of Integrated Ocean Drilling Expedition 330, Louisville Seamount Trail, was to understand the motion of the Louisville hotspot during 50-80 Ma. As such, >1 km of volcanic basement was collected from five sites on four seamounts, providing an excellent chance to study how microbial populations are effected by different lithologies, different seamounts and age of basement rock along the Louisville Seamount Chain (LSC). Analysis of bacteria growing in enrichment incubations that targeted oligotrophs (with 1% or 10% Marine Broth 2216 diluted with 3% NaCl) and sulfur oxidizers reveals the presence of a diverse array of bacteria, including ?-proteobacteria closely related to Sulfurimonas autotrophica, ?-proteobacterial methylotrophs, ?-proteobacteria and Bacteroidetes most closely related to organisms cultured from sediments. Many of these sequences are <97% similar to the closest cultured relatives, indicating the subsurface LSC may be home to unique new species. One isolate recently obtained in pure culture from the original enrichments is 99% similar to Halomonas sulfidaeris str. Esulfude1, a bacterium originally isolated from a hydrothermal sulfide chimney. A second isolate may be a new species of Bacillus. Initial molecular analysis of bacterial communities by pyrosequencing of the 16S rRNA gene as part of the Census of Deep Life (CoDL) supports the data from the culturing work; in one sample collected 174 meters below seafloor, the most abundant bacteria detected include species from the genera Pseudomonas, Sulfurimonas, Methyloversatilis and Desulfocapsa. More CoDL samples will be analyzed in the near future. We will describe results to date on subsurface microbial diversity along the Louisville Seamount Chain from the culturing work and CoDL project and draw comparisons to data derived from younger crustal sites to try to understand how the LSC ecosystem fits into our global picture of life in ocean crust.

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

2012-12-01

40

Compositional variation and genesis of ferromanganese crusts of the AfanasiyNikitin Seamount, Equatorial Indian Ocean  

Science.gov (United States)

Eight ferromanganese crusts (Fe-Mn crusts) with igneous and sedimentary substrates collected at different water depths from the Afanasiy-Nikitin Seamount are studied for their bulk major, minor and rare earth element composition. The Mn/Fe ratios < 1.5 indicate the hydrogenetic accretion of the Fe-Mn hydroxides. These Fe-Mn crusts are enriched in Co (up to 0.9%, average 0.5%) and Ce. The Ce-content is the highest reported so far (up to 3763 ppm, average 2250 ppm) for global ocean seamount Fe-Mn crusts. In spite of general similarity in the range of major, minor, and strictly trivalent rare earth element composition, the dissimilarity between the present Fe-Mn crusts and the Pacific seamount Fe-Mn crusts in Co and Ce associations with major mineral phases indicates inter-oceanic heterogeneity and region-specific conditions responsible for their enrichment. The decrease in Ce-anomaly (from 8 to 1.5) with increasing water depth (from 1.7 km to 3.2 km) might suggest that the modern intermediate depth low oxygen layer was shifted and sustained at a deeper depth for a long period in the past.

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

2005-02-01

 
 
 
 
41

Structure and Geochemistry of the Continental-Oceanic Crust Boundary of the Red Sea and the Rifted Margin of Western Arabia  

Science.gov (United States)

The continental-oceanic crust boundary and an incipient oceanic crust of the Red Sea opening are exposed within the Arabian plate along a narrow zone of the Tihama Asir coastal plain in SW Saudi Arabia. Dike swarms, layered gabbros, granophyres and basalts of the 22 Ma Tihama Asir (TA) continental margin ophiolite represent products of magmatic differentiation formed during the initial stages of rifting between the African and Arabian plates. Nearly 4-km-wide zone of NW-trending sheeted dikes are the first products of mafic magmatism associated with incipient oceanic crust formation following the initial continental breakup. Gabbro intrusions are composed of cpx-ol-gabbro, cpx-gabbro, and norite/troctolite, and are crosscut by fine-grained basaltic dikes. Granophyre bodies intrude the sheeted dike swarms and are locally intrusive into the gabbros. Regional Bouger gravity anomalies suggest that the Miocene mafic crust represented by the TA complex extends westward beneath the coastal plain sedimentary rocks and the main trough of the Red Sea. The TA complex marks an incipient Red Sea oceanic crust that was accreted to the NE side of the newly formed continental rift in the earliest stages of seafloor spreading. Its basaltic to trachyandesitic lavas and dikes straddle the subalkaline-mildly alkaline boundary. Incompatible trace element relationships (e.g. Zr-Ti, Zr-P) indicate two distinct populations. The REE concentrations show an overall enrichment compared to N-MORB; light REEs are enriched over the heavy ones ((La/Yb)n > 1), pointing to an E-MORB influence. Nd-isotope data show ?Nd values ranging from +4 to +8, supporting an E-MORB melt source. The relatively large variations in ?Nd values also suggest various degrees of involvement of continental crust during ascent and emplacement, or by mixing of another mantle source.

Dilek, Y.; Furnes, H.; Schoenberg, R.

2009-12-01

42

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

Science.gov (United States)

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 570C, >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

43

Recycled oceanic crust observed in 'ghost plagioclase' within the source of Mauna Loa lavas  

Science.gov (United States)

The hypothesis that mantle plumes contain recycled oceanic crust is now widely accepted. Some specific source components of the Hawaiian plume have been inferred to represent recycled oceanic basalts, pelagic sediments or oceanic gabbros. Bulk lava compositions, however, retain the specific trace-element fingerprint of the original crustal component in only a highly attenuated form. Here we report the discovery of exotic, strontium-enriched melt inclusions in Mauna Loa olivines. Their complete trace-element patterns strongly resemble those of layered gabbros found in ophiolites, which are characterized by cumulus plagioclase with very high strontium abundances. The major-element compositions of these melts indicate that their composition cannot be the result of the assimilation of present-day oceanic crust through which the melts have travelled. Instead, the gabbro has been transformed into a (high-pressure) eclogite by subduction and recycling, and this eclogite has then been incorporated into the Hawaiian mantle plume. The trace-element signature of the original plagioclase is present only as a 'ghost' signature, which permits specific identification of the recycled rock type. The 'ghost plagioclase' trace-element signature demonstrates that the former gabbro can retain much of its original chemical identity through the convective cycle without completely mixing with other portions of the former oceanic crust. PMID:10801125

Sobolev; Hofmann; Nikogosian

2000-04-27

44

Tectonic Windows Throw Light on the Processes That Create the Upper Ocean Crust  

Science.gov (United States)

Several of the most critical problems of the creation of oceanic crust at mid-ocean ridges cannot be answered by observations of the ocean floor alone because of the lack of a third dimension. Models abound, but need testing. Examples of the problems are: What is the width of the zone of crustal construction, including the distribution of dike intrusion about the spreading axis and the role of off-axis eruptions in creating the volcanic section? What is the nature of the hydrothermal reaction zones below the mid-ocean ridges in which seawater is transformed to black smoker fluid? Are rotations of the upper crust produced by loading during the construction of the lava pile, or in the hanging wall of near-axis faults? What proportion of the crustal extension by spreading takes place by faulting as opposed to dyke intrusion? Results from ocean drilling act as a partial constraint, but generalisation from these one-dimensional samples is not easy. Ophiolites can begin to bridge the gap, but both their tectonic environment of formation and their spreading rate remain controversial. In addition no intact ophiolite is known that contains a magnetic reversal, and that turns out to be an important limitation. Tectonic windows allow integration of research on a section through mature upper ocean crust with studies of the adjacent sea floor, and can provide sections tens of kilometres long that comfortably span a magnetic reversal. Especially fruitful have been wide-ranging projects that include petrology, palaeomagnetism, geochemistry and fluid inclusion studies as well as seafloor imagery at visual and sidescan scales and magnetic field surveys. Results from studies of tectonic windows published to date show significant progress in answering some of the questions posed above. For example, rotations in the upper crust have been shown to be towards the spreading centre rather than away from it as one of the faulting models would predict. Other questions will be harder to tackle, especially the last, for which individual faults must be tracked down into the crust to determine their geometry and the extent to which they have been active during volcanism and hence have a throw that increases with depth. Further observations through tectonic windows will help clarify and generalise the results of those already made, and will significantly improve our understanding of the processes that create the ocean crust.

Cann, J.

2005-12-01

45

Constancy of oceanic deposition of "1"0Be as recorded in manganese crusts  

International Nuclear Information System (INIS)

Measurements of "1"0Be and "9Be in two ferromanganese oxide crusts from the sea floor of the equatorial Atlantic and the North Pacific, are reported, that indicate deposition of "1"0Be and "1"0Be/"9Be during the past 7-9 Myr has been constant. Averaged over time intervals of approximately 1 Myr, the variation is within +- 6%. Before that time, both crusts show similar significant deviations. It is considered that the 7-9 Myr demarcation may be related to the reported late Miocene global abyssal circulation change in the ocean. (U.K.)

1982-09-16

46

Eruptive hummocks : building blocks of the upper ocean crust.  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The spreading axis at many slow-spreading mid-ocean ridges is marked by an axial volcanic ridge. In this study, we use a combination of high-resolution remote sensing methods to elucidate the detailed nature of volcanoes in such a ridge. We find that the hummocks described in previous sidescan sonar studies are dome- or cone-shaped edifices, 5150 m high with diameters of 30330 m. We estimate they form quickly, in single eruptions, each of which may produce several hummocks. Hummoc...

2012-01-01

47

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

Science.gov (United States)

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

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

1996-01-01

48

DEFORMATION and RUPTURE of the OCEANIC CRUST MAY CONTROL GROWTH of HAWAIIAN VOLCANOES  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Hawaiian volcanoes are formed by the eruption of large quantities of basaltic magma related to hot- spot activity below the Pacific Plate(1,2). Despite the apparent simplicity of the parent process emission of magma onto the oceanic crust - the resulting edifices display some topographic complexity(3-5). 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...

Got, Jean-luc; Monteiller, Vadim; Monteux, Julien; Hassani, Riad; Okubo, Paul

2008-01-01

49

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

Digital Repository Infrastructure Vision for European Research (DRIVER)

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

Swann, George E. A.

2010-01-01

50

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, Lydric; Ildefonse, Benoit; Koepke, Juergen

2013-08-01

51

Carbonate veins as recorders of seawater evolution, CO2 uptake by the ocean crust, and seawater-crust interaction during low-temperature alteration  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Carbonates precipitate in fractures, vesicles and matrix of breccias within the basaltic basement of oceanic ridge flanks. They form during low-temperature alteration and can provide insights into processes related to seawater circulation and seawater-basement interaction. It has been proposed that the aging ocean crust is a major sink for CO2 and plays a major role for the global carbon cycle. However, detailed knowledge about the abundance and composition of carbonate is necessary to fu...

Rausch, Svenja

2012-01-01

52

The Impact of Microbes on the Composition of the Ocean Crust and Seawater  

Science.gov (United States)

Microbes attack volcanic glass and primary igneous minerals in aqueous environments and they participate in the transformation of glass and minerals into secondary minerals such as clay. This transformation also proceeds in the absence of microbes. In either case some elements go into solution and others are sequestered in secondary minerals. Our hypothesis is that in the ocean crust secondary minerals produced by abiotic reactions are chemically distinct from those produced by microbial attack. If this is true, then microbial attack on volcanic rocks could affect the composition of the ocean crust and sea water. The best source of deep sea volcanic rocks to test this hypotheses is the Ocean Drilling Program's archived collection. We selected several samples from this collection and used an electron microprobe to measure the composition of secondary clays that were deemed to be produced either bioticly or abioticly. Abundance measurements were obtained for eleven elements (Na, Mg, Al, Si, P, Cl, K, Ca, Ti, Mn, and Fe). We used Principal Component Analysis to extract three factors that were linear combinations of the eleven elements and that accounted for more than 80% of the data variance. These three factors were used as inputs to a stochastic, non-linear, Artificial Neural Network that demonstrates that abiotic secondary minerals are chemically distinct from biotic secondary minerals. Significant differences in clay compositions were found for MgO (0.2), MnO (0.3), K2O (3), CaO (2), TiO2 (3), and possibly FeO (1.2). (Numbers in parentheses are biotic:abiotic oxide wt. % ratios.) This result suggests that biotic alteration of the ocean crust will release more MgO and MnO from the rocks and sequester more K2O, CaO, TiO2, and possibly FeO in clay than abiotic alteration. The impact of microbial alteration on the composition of the crust therefore depends on the fraction of basalt alteration that is caused by microbes. The impact of microbial alteration on the composition of sea water, also depends on this fraction, and whether released elements Mg and K are sequestered elsewhere in the crust or are vented to the ocean.

Fisk, M.; Josef, J.; Storrie-Lombardi, M.

2004-12-01

53

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

Science.gov (United States)

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

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

2011-12-01

54

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

55

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

56

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

Science.gov (United States)

IODP Expedition 345 drilled the first holes in the lower plutonic crust at a fast-spreading ridge, recovering primitive layered gabbros (Gillis et al 2014). Alteration can be subdivided into two series: 1) a largely static pseudomorphic alteration affecting predominantly olivine. This began in the amphibolite facies with minor secondary cinopyroxene and hornblendic amphibole replacing primary pyroxene, and sporadically developed corona textures with tremolite and chlorite replacing olivine and plagioclase respectively, but was predominantly in the greenschist and sub-greenschist facies with talc, serpentine, clay minerals,oxides andsulphides replacing olivine, and prehnite and locally other calcsilicates replacing plagioclase, commonly in micro-vein networks. Albitic plagioclase is sporadically developed, and locally zeolite and carbonate. 2) An overprinting metasomatic alteration under sub-greenschist or perhaps lowermost greenschist conditions(200 C. Preliminary modelling using Comsol Multiphysics suggests that the temperatures of the overprinting alteration could be achieved in a permeable fault slot cutting through crust 0.5 to 1 m.y. old. Our study reveals a low temperature alteration assemblage dominated by prehnite and chlorite that is not normally associated with the lower oceanic crust. Yet it is likely to be common in any location where faults intersect the Moho off-axis, including transform faults, near axis normal faults at slow spreading ridges, and bending faults at subduction zones, and would be accompanied by serpentinites in upper mantle rocks, as seen at ODP site 895 in Hess Deep. This prehnite + chlorite assemblage may therefore be significant in the release of volatiles in subduction zones. Gillis, K.M., Snow J. E. and Shipboard Science Party (2014) Primitive layered gabbros from fast-spreading lower oceanic crust. Nature, 505,204-207, doi: 10.1038/nature12778

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

2014-05-01

57

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 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; Bhm, Florian; Bach, Wolfgang; Klgel, Andreas; Eisenhauer, Anton

2013-01-01

58

Elasticity of glaucophane, seismic velocities and anisotropy of the subducted oceanic crust  

Science.gov (United States)

Upon subduction, the oceanic crust transforms to blueschists and eclogites, with seismic properties that gradually become similar to those of the surrounding mantle. In order to evaluate the anisotropy of blueschists and glaucophane-bearing eclogites, the elastic constants of glaucophane single-crystal plates from the Sesia-Lanzo Zone (Aosta Valley, Western Alps) were measured using Brillouin spectroscopy at ambient conditions. The mean P- and S-wave velocities are 7.8 and 4.6 km s -1 respectively, and the anisotropy is high (38.1% (AV P) and 27.3% (AV S)). Glaucophane develops strong LPO, characterized by the [001]-axes concentrated sub-parallel to the lineation, and the {110} poles concentrated sub-perpendicular to the foliation in both blueschist and eclogite rocks. The measured LPO is in good agreement with viscoplastic self-consistent numerical models. Seismic properties of glaucophane-bearing blueschist and eclogite are calculated by combining measured LPO and the single-crystal elastic moduli of glaucophane with the other main mineral constituents of the rock: mostly epidote for blueschist, and omphacite and garnet for eclogite. Blueschists present stronger anisotropies (AV P = 16.1% and AV S = 10.3%) than eclogites (AV P = 2.9% and AV S = 1.7%). The shear-wave splitting and resulting delay times for a 7-km thick layer of eclogite or blueschist are low for the eclogite (< 0.03 s), but significant for blueschist (0.16 s). Application to the subducted oceanic crust yields a decrease of velocity contrast with the surrounding mantle and of anisotropy at depth depending on the temperature of the slab. The details of the velocity variations in subducted oceanic crust are difficult to reconcile with the blueschist-eclogite transition as probed by exhumed metamorphic rocks and may require additional phases such as lawsonite or chemical variations such as oxidation state.

Bezacier, L.; Reynard, B.; Bass, J. D.; Wang, J.; Mainprice, D.

2010-11-01

59

Early (pre-8 Ma) Fault Activity and Temporal Strain Accumulation in the Central Indian Ocean  

Science.gov (United States)

About 8.0-7.5 Ma, large-scale lithospheric deformation started within the central Indian Ocean, with widespread development of reverse faults at 5-10 km spacing and of long (100-300 km) wavelength folds. The onset of this deformation is commonly regarded as a key far-field indicator for the start of major uplift of the Himalayas and Tibet, some 4000 km further to the north, due to increased deviatoric stresses within the wider India-Asia area. The Himalayan uplift history is thought to have led to strengthening of the monsoon and associated profound changes in regional climate. Here we analyse fault displacement data from seismic reflection profiles within the central Indian Ocean, which demonstrates that compressional activity started much earlier, at around 15.4-13.9 Ma. We reconstruct that 12% of the total reverse fault population had been activated, and 14% of the total strain accumulated, prior to a sharp increase in the deformation rate at 8.0-7.5 Ma. This much earlier onset of deformation would imply considerable Himalayan uplift before 8.0 Ma, which complicates our understanding of the tectono-climatic history of the region.

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

2008-12-01

60

North Atlantic Deep Water export to the Southern Ocean over the past 14 Myr: Evidence from Nd and Pb isotopes in ferromanganese crusts  

Science.gov (United States)

The intensity of North Atlantic Deep Water (NADW) production has been one of the most important parameters controlling the global thermohaline ocean circulation system and climate. Here we present a new approach to reconstruct the overall strength of NADW export from the North Atlantic to the Southern Ocean over the past 14 Myr applying the deep water Nd and Pb isotope composition as recorded by ferromanganese crusts and nodules. We present the first long-term Nd and Pb isotope time series for deep Southern Ocean water masses, which are compared with previously published time series for NADW from the NW Atlantic Ocean. These data suggest a continuous and strong export of NADW, or a precursor of it, into the Southern Ocean between 14 and 3 Ma. An increasing difference in Nd and Pb isotope compositions between the NW Atlantic and the Southern Ocean over the past 3 Myr gives evidence for a progressive overall reduction of NADW export since the onset of Northern Hemisphere glaciation (NHG). The Nd isotope data allow us to assess at least semiquantitatively that the amount of this reduction has been in the range between 14 and 37% depending on location.

Frank, M.; Whiteley, N.; Kasten, S.; Hein, J. R.; O'Nions, K.

2002-01-01

 
 
 
 
61

Viscous magnetization at 300 K in a profile through Troodos type oceanic crust  

Science.gov (United States)

This study examines the question of the relative importance of viscous magnetism, (VM), at 300 K, to the total magnetization of older oceanic crust. A section through the Troodos, Cyprus, ophiolite, now recognized as a good proxy for in-situ oceanic crust, has been used as the source of samples. The section extends downwards continuously from little altered submarine extrusives through greenschist facies sheeted dikes to mafic and variously serpentinized ultramafic cumulate intrusives. The principal result of the investigation is that VM is not expected to be the dominant magnetization, and is often relatively negligible, at all crustal levels. VM acquisition varies irregularly with depth, with predicted maximum values equivalent to about one-third of the total magnetization, in the extrusives and sheeted dikes. The contribution of VM in two samples of serpentinized ultramafics is insignificant. VM acquisition does not show any simple relationships with primary lithology, alteration history, or magnetic properties or history. The occurrence of two-stage VM growth mechanisms is widespread. The second stage typically has an onset time of about 10 3 min and an acquisition rate three times that of the initial growth mechanism. The physical origin of this two-stage mechanism, and its significance in predicting VM growth over geologic time intervals, are considered to be important problems for future work.

Hall, James M.; Walls, Charles C.; Lata Hall, S.

1995-03-01

62

Intraterrestrial life in igneous ocean crust: advances, technologies, and the future (Invited)  

Science.gov (United States)

The next frontier of scientific investigation in the deep sub-seafloor microbial biosphere lies in a realm that has been a completely unexplored until just the past decade: the igneous oceanic crust. Problems that have hampered exploration of the hard rock marine deep biosphere have revolved around sample access (hard rock drilling is technologically complex), contamination (a major hurdle), momentum (why take on this challenge when the relatively easier marine muds also have been a frontier) and suspicion that microbes in more readily accessed using (simpler) non-drilling technologies - like vents - are truly are endemic of subsurface clades/activities. Since the late 1990s, however, technologies and resultant studies on microbes in the igneous ocean crust deep biosphere have risen sharply, and offer a new and distinct view on this biome. Moreover, microbiologists are now taking leading roles in technological developments that are critically required to address this biosphere - interfacing and collaborating closely with engineers, genomic biologists, geologists, seismologists, and geochemists to accomplish logistically complex and long-term studies that bring observatory research to this deep realm. The future of this field for the least decade is rich - opportunities abound for microbiologists to play new roles in how we study microbiology in the deep subsurface in an oceanographic and Earth system science perspective.

Edwards, K. J.; Wheat, C. G.

2010-12-01

63

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

64

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. d98Mo values also fall well below modern equivalents (0.99 ± 0.13? versus ~ 2.35? today). These signatures are consistent with model predictions where sulfidic waters cover ~ 1??4% of the global continental shelf area, corresponding to a ~ 400??800 fold increase compared to the modern ocean. Therefore, our results suggest globally expansive sulfidic water masses in mid-Neoproterozoic oceans, bridging a nearly 700 million-year gap in previous Mo data. We propose that anoxic and sulfidic (euxinic) conditions governed Mo cycling in the oceans even as ferruginous subsurface waters re-appeared 800??750 Ma, and we interpret this anoxic ocean state to reflect a markedly lower atmospheric and oceanic O2 level, consistent with the hypothesis that pO2 acted as an evolutionary barrier to the emergence of large motile bilaterian animals prior to the Ediacaran Period.

Dahl, Tais Wittchen; Canfield, Donald Eugene

2011-01-01

65

Constructional features of Troodos type oceanic crust: Relationships between dike density, alteration, magnetization, and ore body distribution and their implications for in situ oceanic crust  

Science.gov (United States)

Quantitative relationships are demonstrated between dike density and hydrothermal alteration, magnetization, location of volcanic massive sulfide (VMS) ore bodies and a number of other properties in a 40 by 15 km segment of the Extrusive Series of the Troodos Ophiolite. Low-temperature alteration extends from the sediment-extrusive interface to close to the 25% dike density surface, while the onset of greenstone type alteration is closely associated with the 50% dike density surface. These associations are explained in terms of the regional scale variations in the dike density contour surfaces following the form of the top of the gabbroic layer of the ophiolite. An upper magnetic zone, characterized by stable high remanence, terminates just below the 25% dike density surface. A deeper magnetic zone, characterized by high induced magnetization, straddles the top of the Sheeted Complex. Volcanic massive sulfide bodies are concentrated in a narrow depth range centered about 0.1 km above the 25% dike density surface. A model for the postridge crest evolution of ore bodies accounts for this optimum depth range for preservation. Physical property changes with dike density, and the depth distribution of VMS ore bodies and prospects for the ophiolite as a whole support this model. Similar relationships appear to be present in the Samail, Macquarie Island, Newfoundland and Chilean ophiolites and Icelandic crust. As a result of relationships found in the Troodos ophiolite, an alternative is proposed to the current lithologic profile for Ocean Drilling Program Hole 504B.

Hall, James M.; Yang, Jing-Sui

1995-10-01

66

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

67

Depleted Peridotites of Macquarie Island, an Uplifted Section of In-situ Oceanic Crust  

Science.gov (United States)

Macquarie Island, located 1500 km southeast of southernmost Australia, is thought be the sole complete section of ocean crust uplifted in the ocean basin in which it formed. It is an exposure of the Macquarie Ridge complex, which marks the modern Australian-Pacific plate boundary. The oceanic crust of the island formed in the final stages of spreading, ~6 mya, as indicated by Ar-Ar plateau ages of basaltic glass. Geometries of marine faults on the island suggest that it formed near the intersection of a ridge and a transform. At this latitude, the plate boundary evolved from a spreading ridge to a transpressional boundary between ~33 and ~6 mya, thus the rocks of the island record an interesting tectonic history and may provide clues to the mantle process during a major plate motion re-organization. Residual, plagioclase-free mantle peridotite samples were collected along transects through all of the mantle sections on the island, with an average of 100 meter spacing between samples. Orthopyroxenes, clinopyroxenes and chrome spinels were analysed by electron microprobe. Spinel chrome numbers (Cr-nr) ranged from 0.39 to 0.46 (n=23), which corresponds to 15-16% fractional melting applying the empirical melting equation of Hellebrand et al (2001). Their low Ti contents (0.02-0.07) attest to the residual nature of the Macquarie Island peridotites. Cpx is preserved in only 7 samples (alteration, depletion), and occurs mainly as small interstitial grains or as exsolved blebs in opx porphyroclasts. Cpx titanium (0.00 - 0.04 wt% TiO2) and sodium (0.00 - 0.05 wt% Na2O) contents are extremely low, confirming the high depletion and supporting highly efficient melt extraction. Opx porphyroclast cores have very high Mg-nr (0.92 on average). Spreading rates at the time of formation of the Macquarie Island crust have been calculated to be 30mm/yr (full) which is considered "slow". However, the levels of depletion indicated by the spinel Cr-nr and Ti and Na contents of cpx of the Macquarie Island peridotites are more similar to those seen at fast spreading centers or ophiolites. This depletion could be caused by the progressively changing spreading direction disrupts mixing in the mantle, causing repeated melting of the same mantle source or biased sampling in the existing abyssal peridotite database. Further analyses of peridotites and associated basalts will test which model is most likely. Hellebrand et al., (2001) Nature 410, 677-681.

Wertz, K.; Snow, J. E.; Hellebrand, E.; von der Handt, A.; Mosher, S.

2002-12-01

68

Tectonic evolution of Macquarie Island: extensional structures and block rotations in oceanic crust  

Science.gov (United States)

Three distinct tectonic periods are recognized on Macquarie Island. D1 is a protracted period of palaeo-N-S extension, encompassing initial crust formation at the Proto-Macquarie Spreading Ridge (PMSR), over-printing by late-stage dykes, and a wide range of extensional and dilational structures formed in the near- to off-axis environment. Extensional structures in the lower-crust include fractures, serpentine veinlets, brittle faults, semi-ductile shear zones and ductile mylonites. All were formed in a stress regime with sub-vertical ?1 and sub-horizontal N-trending ?3 and are consistent with pure extension in a spreading ridge environment. A minor set of orthogonal fractures and serpentine veinlets suggest a component of along-axis extension during D1. Early D1 tilting of 20-58 around horizontal axes parallel to the ridge axis accompanied growth faults and major differential block uplifts in the near-axis environment. Superimposed on, and in the waning stages of D1, was a period of palaeo-NE-SW extension ( D2) with dolerite dyke emplacement. D2 occurred in a transtensional regime, during transition from extension at the spreading ridge to a transcurrent plate margin ( D3). D3 constitutes dextral transcurrent movements at the NNE-trending Indo-Australian/Pacific plate margin, active from approximately 10.5 Ma to the present day. Both palaeo-stress analysis of D3 faults and neotectonic fault scarp geometry indicate dominantly strike-slip and rare thrust events, with NE-trending ?1 compatible with dextral transpression. Sharp angular discordances in the palaeo-seafloor fabric ( D1 dyke trend), document clockwise rotation of km-scale crustal blocks around vertical axes during D3, entirely consistent with dextral transpression at the plate margin.

Goscombe, Ben D.; Everard, J. L.

2001-04-01

69

Geological storage of CO2 within the oceanic crust by gravitational trapping  

Science.gov (United States)

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

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

2013-12-01

70

Spatial and temporal variability in crystallization of celadonites within the Troodos ophiolite, Cyprus: Implications for low-temperature alteration of the oceanic crust  

Science.gov (United States)

Celadonite, a low-temperature (less than 30 deg C) hydrothermal alteration mineral, is a predominant fracture and void-filling phase within the volcanic rocks of the Troodos ophiolite. The combined chemical and structural properties of celadonite (i.e., high K2O and Ar retentive), along with its common occurrence, provide a valuable tracer for studying the temporal and spatial variablility of low-temperature hydrothermal fluid circulation and alteration within this ancient oceanic crust. Some 54 new K/Ar age determinations of celadonites from various geographic and stratigraphic locations within the extrusive rocks of Troodos yield crystallization ages ranging from 90.9 +/- 1.0 to 49.8 +/- 0.5 Ma, with the oldest in close agreement with the estimated 91-92 Ma crystallization age of Troodos igneous rocks. The youngest age indicates that low-temperature water/rock chemical exchange continued for at least 40 m.y. after crustal formation. This represents a 100% increase over previous estimates, based on limited numbers of K/Ar dates of celadonite, of the duration of low temperature mineral precipitation in Troodos. Correlation between celadonite ages and field relationships suggests the following: (1) There is no apparent relationship between celadonite crystallization and stratigraphic depth. (2) Celadonite crystallization occurs homogeneously on an outcrop scale, with adjacent samples from a single flow unit yielding equivalent K/Ar ages. (3) Flow units with relatively high primary permeabilities, e.g., pillows and breccias, yield celadonites with younger K/Ar ages. (4) Celadonite precipitation at distinct locations occurred rapidly, with little or no age difference between rim and core sections of single large deposits. The above correlations suggest that low-temperature hydrothermal fluid circulation and secondary mineral precipitation are controlled by the local alteration conditions, such as intrinsic permeability, degree of fracturing, and water/rock ratios, and are independent of time and space. Integration of these local heterogeneities over the entire extrusive sequence suggests that the upper oceanic crust undergoes relatively homogeneous closure to hydrothermal fluid circulation. No evidence is found to support a progressive upward sealing of the oceanic crust.

Gallahan, William E.; Duncan, Robert A.

1994-02-01

71

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

Science.gov (United States)

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

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

1996-01-01

72

Slow-Spreading Oceanic Crust Formed By Steady-State Axial Volcanic Ridges  

Science.gov (United States)

Oceanic crust originates at mid-ocean spreading ridges (MORs), covers almost three quarters of the earth's surface and dominates the global magmatic flux. Axial volcanic ridges (AVRs) are almost ubiquitous features of orthogonal slow-spreading ridges, which account for three quarters of the global mid-ocean spreading ridge system today. Typically 3-6 km wide, 200-500 m high and 10-20 km long, AVRs are the loci of recent volcanic activity and form the most prominent topography rising above the otherwise flat-lying Median Valley floor. Previous studies indicate that AVRs, and their related crustal magma reservoirs are episodic, on a time scale of 150-300 ka. Yet their near ubiquitous occurrence at slow-spreading ridge segments provides us with a paradox: if AVRs have a life cycle of formation and degradation, does their near ubiquitous presence at slow spreading ridges imply their life-cycles are synchronised? In this contribution, we report the findings from a high-resolution study of a well-developed axial volcanic ridge (AVR) at 45N on the Mid-Atlantic Ridge (MAR). Here, the MAR is typical of most slow-spreading ridges: it spreads generally symmetrically and orthogonally, at a full rate of 23.6 mm per year, has second and third-order segmentation, and contains a typical AVR. Using a combination of detailed micro-bathymetry, sidescan sonar, visual surveying and petrology, we suggest that the AVR is the product of quasi-steady state volcanotectonic processes. Small volume lava flows, originating at or near the crest and with short run-out lengths, form ~60 m high hummocky pillow-lava mounds that dominate the construction of the AVR. The lavas are the product of moderate degrees of mantle melting that are typical for normal mid-ocean ridge basalt. Synchronous with these eruptions the flanks of the AVR subside forming a structural horst. Subsidence is partially accommodated by a series of outward-facing volcanic growth faults that step-down and away from the AVR crest and towards the Median Valley floor. Here, much larger volume, yet less frequent, effusions of massive lava flows erupt rapidly from large flat-topped seamounts, found almost exclusively outside of the AVR. The sheet-flows have run-out lengths of up to several kilometres, a combined thickness sufficient to bury the hummocky topography of the AVR flanks, producing smooth flat-lying seafloor typical of the Median Valley floor and its uplifted flanks. These lavas are relatively enriched geochemically and are characteristic of small melt fractions from the mantle. Thus it appears that the volcanic crust at slow-spreading ridges is formed through a continuous process of small volcanic eruptions along AVRs that evolve through syn-volcanic subsidence and episodic burial by large volume massive lava eruptions. From this, we conclude that AVRs have neither a particular life cycle nor are they synchronised along the global mid-ocean ridge system. Rather, they approximate steady-state features in which subsidence plays as large a part in their origin as volcanic construction.

Murton, B. J.; Schroth, N.; LeBas, T.; Van Calsteren, P. W.; Yeo, I. A.; Achenbach, K. L.; Searle, R. C.

2012-12-01

73

Petrology and geochemistry of primitive lower oceanic crust from Pito Deep: Implications for the accretion of the lower crust at the Southern East Pacific Rise  

Science.gov (United States)

A suite of samples collected from the uppermost part of the plutonic section of the oceanic crust formed at the southern East Pacific Rise and exposed at the Pito Deep has been examined. These rocks were sampled in situ by ROV and lie beneath a complete upper crustal section providing geological context. This is only the second area (after the Hess Deep) in which a substantial depth into the plutonic complex formed at the East Pacific Rise has been sampled in situ and reveals significant spatial heterogeneity in the plutonic complex. In contrast to the uppermost plutonic rocks at Hess Deep, the rocks studied here are generally primitive with olivine forsterite contents mainly between 85 and 88 and including many troctolites. The melt that the majority of the samples crystallized from was aggregated normal mid-ocean ridge basalt (MORB). Despite this high Mg# clinopyroxene is common despite model predictions that clinopyroxene should not reach the liquidus early during low-pressure crystallization of MORB. Stochastic modeling of melt crystallisation at various levels in the crust suggests that it is unlikely that a significant melt mass crystallized in the deeper crust (for example in sills) because this would lead to more evolved shallow level plutonic rocks. Similar to the upper plutonic section at Hess Deep, and in the Oman ophiolite, many samples show a steeply dipping, axis-parallel, magmatic fabric. This suggests that vertical magmatic flow is an important process in the upper part of the seismic low velocity zone beneath fast-spreading ridges. We suggest that both temporal and spatial (along-axis) variability in the magmatic and hydrothermal systems can explain the differences observed between the Hess Deep and Pito Deep plutonics. ?? Springer-Verlag 2007.

Perk, N. W.; Coogan, L. A.; Karson, J. A.; Klein, E. M.; Hanna, H. D.

2007-01-01

74

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.

2014-04-01

75

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

Science.gov (United States)

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

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

1985-01-01

76

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

77

Detachment Fault Initiation and Control by Partially Molten Zones in the Lower Ocean Crust  

Science.gov (United States)

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

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

2012-12-01

78

Locating hyperextended passive margins based on plate reconstructions and limits of oceanic crust derived from potential fields data.  

Science.gov (United States)

Recent advances in understanding of passive margins have emphasized that there is a spectrum of margin styles, ranging from volcanic to hyperextended. All extensional margins will eventually develop sea floor spreading if continental separation continues long enough; the differing margin styles reflect local response of the lithosphere and asthenosphere to ongoing extension. Hyperextended margins can be viewed as extensional systems where continental separation has progressed to a point where there is no more continental crust left, but the asthenosphere has not reached the melting conditions necessary for creation of oceanic crust. The result is that the lithosphere starts to delaminate and mantle is exhumed. The trend then is to form hyperextended margins with exhumation where there is a large amount of continental separation before sea floor spreading begins. One documented area is the Iberia-Newfoundland system, where plate reconstructions suggest that separation between Iberia and Newfoundland was more than 500 km before onset of sea floor spreading. In the South Atlantic, seismic data suggests that mantle exhumation did occur in some areas of the salt basins. Plate reconstructions here suggest more than 300 km of movement between South America and Africa before sea floor spreading was able to start. To try and predict other passive margins where exhumed mantle may underlie significant portions of the margins we have compared extension amounts calculated from plate reconstructions to widths of the margins. This requires mapping of the limits of oceanic crust in areas with inadequate seismic or difficult seismic imaging of deep crustal structure. A regional tool for mapping this boundary is the Bouguer gravity anomaly, with the horizontal gradient of the Bouguer anomaly being a refinement of the method. Studies of the Gulf of Mexico, Gulf of Aden and the Australia - Antarctica conjugate margins are presented here. These studies show that it is possible to predict areas of mantle exhumation based on plate reconstructions and mapping of the limits of oceanic crust.

Norton, I.; Lawver, L.; Gahagan, L.

2008-12-01

79

First Paleoproterozoic ophiolite from Gondwana: Geochronologic-geochemical documentation of ancient oceanic crust from Kandra, SE India  

Science.gov (United States)

SHRIMP-RG zircon U-Pb ages confirm the 1.85 Ga age of oceanic crust generated along the SE margin of India. This Paleoproterozoic ophiolite was accreted along a NE-trending suture that juxtaposes the outboard Proterozoic Eastern Ghats Granulite Belt (EGGB) against the inboard Archean Nellore Schist Belt of the Dharwar craton. Collision between the EGGB arc crust and India apparently was highly oblique, involving SW thrusting of oceanic crust and a trailing arc onto the Dharwar craton. Although deformed and dismembered, the original lithologic sequence of the Kandra Ophiolite Complex (KOC) has been largely retained. From SW to NE, the complex consists of layered + isotropic gabbros, sheeted dolerite dikes and amygdaloidal pillow basalts. Ultramafic units are intercalated within the gabbroic rocks, and plagiogranite veins + patches occur within the dolerites. Metacherty layers cap the basalts. The KOC exhibits EMORB geochemistry overprinted by subduction-zone metasomatism. Mafic magmas show high LILE/HFSE, positive Ba and Pb anomalies and negative anomalies for Nb, Zr and Hf in spidergrams typical of a suprasubduction-zone setting. Its structure and geochemistry suggest that the KOC represents a Chilean-type continental backarc ophiolite. This is the first unequivocal Paleoproterozoic ophiolite reported from India, and probably the first from Gondwana. The 1.85 Ga KOC represents an important Gondwana example in the cascade of arc-continent collisions that assembled the Paleoproterozoic supercontinent Columbia.

Vijaya Kumar, K.; Ernst, W. G.; Leelanandam, C.; Wooden, J. L.; Grove, M. J.

2010-05-01

80

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 (480-570C, >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 500-530C (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; 500-600C). 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

 
 
 
 
81

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

82

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

83

Evidence for biogenic processes during formation of ferromanganese crusts from the Pacific Ocean: implications of biologically induced mineralization.  

Science.gov (United States)

Ferromanganese [Fe/Mn] crusts formed on basaltic seamounts, gain considerable economic importance due to their high content of Co, Ni, Cu, Zn and Pt. The deposits are predominantly found in the Pacific Ocean in depths of over 1000m. They are formed in the mixing layer between the upper oxygen-minimum zone and the lower oxygen-rich bottom zone. At present an almost exclusive abiogenic origin of crust formation is considered. We present evidence that the upper layers of the crusts from the Magellan Seamount cluster are very rich in coccoliths/coccolithophores (calcareous phytoplankton) belonging to different taxa. Rarely intact skeletons of these unicellular algae are found, while most of them are disintegrated into their composing prisms or crystals. Studies on the chemical composition of crust samples by high resolution SEM combined with an electron probe microanalyzer (EPMA) revealed that they are built of distinct stacked piles of individual compartments. In the center of such piles Mn is the dominant element, while the rims of the piles are rich in Fe (mineralization aspect). The compartments contain coccospheres usually at the basal part. Energy dispersive X-ray spectroscopy (EDX) analyses showed that those coccospheres contain, as expected, CaCO3 but also Mn-oxide. Detailed analysis displayed on the surface of the coccolithophores a high level of CaCO3 while the concentration of Mn-oxide is relatively small. With increasing distance from the coccolithophores the concentration of Mn-oxide increases on the expense of residual CaCO3. We conclude that coccoliths/coccolithophores are crucial for the seed/nucleation phase of crust formation (biomineralization aspect). Subsequently, after the biologically induced mineralization phase Mn-oxide deposition proceeds "auto"catalytically. PMID:19443230

Wang, Xiao-Hong; Schlossmacher, Ute; Natalio, Filipe; Schrder, Heinz C; Wolf, Stephan E; Tremel, Wolfgang; Mller, Werner E G

2009-01-01

84

Insights into C and H storage in the altered oceanic crust: Results from ODP/IODP Hole 1256D  

Science.gov (United States)

Carbon and hydrogen concentrations and isotopic compositions were measured in 19 samples from altered oceanic crust cored in ODP/IODP Hole 1256D through lavas, dikes down to the gabbroic rocks. Bulk water content varies from 0.32 to 2.14 wt% with ?D values from -64 to -25. All samples are enriched in water relative to fresh basalts. The ?D values are interpreted in terms of mixing between magmatic water and another source that can be either secondary hydrous minerals and/or H contained in organic compounds such as hydrocarbons. Total CO 2, extracted by step-heating technique, ranges between 564 and 2823 ppm with ? 13C values from -14.9 to -26.6. As for water, these altered samples are enriched in carbon relative to fresh basalts. The carbon isotope compositions are interpreted in terms of a mixing between two components: (1) a carbonate with ? 13C = -4.5 and (2) an organic compound with ? 13C = -26.6. A mixing model calculation indicates that, for most samples (17 of 19), more than 75% of the total C occurs as organic compounds while carbonates represent less than 25%. This result is also supported by independent estimates of carbonate content from CO 2 yield after H 3PO 4 attack. A comparison between the carbon concentration in our samples, seawater DIC (Dissolved Inorganic Carbon) and DOC (Dissolved Organic Carbon), and hydrothermal fluids suggests that CO 2 degassed from magmatic reservoirs is the main source of organic C addition to the crust during the alteration process. A reduction step of dissolved CO 2 is thus required, and can be either biologically mediated or not. Abiotic processes are necessary for the deeper part of the crust (>1000 mbsf) because alteration temperatures are greater than any hyperthermophilic living organism (i.e. T > 110 C). Even if not required, we cannot rule out the contribution of microbial activity in the low-temperature alteration zones. We propose a two-step model for carbon cycling during crustal alteration: (1) when "fresh" oceanic crust forms at or close to ridge axis, alteration starts with hot hydrothermal fluids enriched in magmatic CO 2, leading to the formation of organic compounds during Fischer-Tropsch-type reactions; (2) when the crust moves away from the ridge axis, these interactions with hot hydrothermal fluids decrease and are replaced by seawater interactions with carbonate precipitation in fractures. Taking into account this organic carbon, we estimate C isotope composition of mean altered oceanic crust at -4.7, similar to the ? 13C of the C degassed from the mantle at ridge axis, and discuss the global carbon budget. The total flux of C stored in the altered oceanic crust, as carbonate and organic compound, is 2.9 0.4 10 12 molC/yr.

Shilobreeva, S.; Martinez, I.; Busigny, V.; Agrinier, P.; Laverne, C.

2011-05-01

85

Distribution and sources of pre-anthropogenic lead isotopes in deep ocean water from Fe-Mn crusts  

Science.gov (United States)

The lead isotope composition of ocean water is not well constrained due to contamination by anthropogenic lead. Here the global distribution of lead isotopes in deep ocean water is presented as derived from dated (ca. 100 ka) surface layers of hydrogenetic Fe-Mn crusts. The results indicate that the radiogenic lead in North Atlantic deep water is probably supplied from the continents by river particulates, and that lead in Pacific deep water is similar to that characteristic of island and continental volcanic arcs. Despite a short residence time in deep water (80-100 a), the isotopes of lead appear to be exceedingly well mixed in the Pacific basin. There is no evidence for the import of North Atlantic deep water-derived lead into the Pacific ocean, nor into the North Indian Ocean. This implies that the short residence time of lead in deep water prohibits advection over such long distances. Consequently, any climate-induced changes in deep-water flow are not expected to result in major changes in the seawater Pb-isotope record of the Pacific Ocean.

Von Blanckenburg, F.; O'Nions, R. K.; Hein, J. R.

1996-01-01

86

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

International Nuclear Information System (INIS)

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

1987-07-13

87

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

Science.gov (United States)

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

Taylor, Lawrence A.; Neal, Clive R.

1988-01-01

88

Significance of serpentinization of lower crust in deep-sea hydrothermal biosphere -case study of gabbroic rocks from accreted oceanic plateau (Mikabu high P/T rocks, Japan)-  

Science.gov (United States)

Hydrothermal activity in the Archean-Ridge system has been considered to play a major role to maintain the oldest biosphere in early Earth. In the present ridge-system, hydrogen production in the serpentinized peridotite layer, is considered as major energy source. However, low temperature hydrothermal zone in the lower crust layer in the ridge has been recognized as hydrogen producing zone. Thickness of oceanic crust is less then 10 km in the present Earth. However, the thickness of Archean oceanic crust has been estimated as 50 km. That is, hydration process of oceanic crust in the Archean-ridge is significantly important. Hydration rate of the peridotite layer in the Archean ridge is less extensive than Phanerozoic because thicker oceanic crust prevents hydration in the peridotite layer. Lower crustal rocks of accreted oceanic plateau is one of the best sample to describe hydration process due to deep-sea-hydrothermal alteration because it is easy to observe huge outcrops and collect samples systematically in whole section. We have collected gabbroic rocks from Mikabu high P/T rocks in Toba area and from Ootoyo area, Japan because there are large scale trench cliffs in the mine. Serpentinization of olivine gabbro and troctolite and hydrogen production rate will be shown in the present poster.

Ando, Y.; Ishimori, C.; Fukumura, S.; Okamoto, K.

2013-12-01

89

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

90

Hydrothermal Alteration of the Ocean Crust: Constraints From O and Sr Isotopic Compositions of Whole Rocks and Secondary Minerals From Macquarie Island  

Science.gov (United States)

Hydrothermal circulation is a fundamental process in the formation and aging of the ocean crust, influencing its structure, physical and chemical properties, and the composition of the oceans and the mantle. The impact of hydrothermal circulation on mid-ocean ridge processes depends on the composition and volume of circulating hydrothermal fluids, and the extent of partitioning between high temperature axial- and low temperature ridge flank- systems, but these processes remain poorly constrained. Macquarie Island, approximately 1500 km south of New Zealand, is a unique sub-aerial exposure of a complete section of ocean crust in the ocean basin in which it formed. The crust formed during a phase of slow spreading along a short segment of mid-ocean ridge approximately 11 Myr ago and was uplifted during recent transpression along the Pacific-Australian plate boundary. Oxygen and Strontium isotopic compositions of whole-rock and secondary minerals from Macquarie Island provide a time-integrated record of fluid-rock exchange and fluid evolution during the hydrothermal cooling of the crust. Sr and O isotope analyses, combined with stratigraphic reconstructions of the island provide the first isotopic profiles through a complete section of normal ocean crust. The oxygen isotope profile is similar to profiles through intact ocean crust and the Troodos and Semail ophiolites, indicating that O-exchange during hydrothermal circulation is controlled by the same processes in different spreading environments. However, the Macquarie crust is on average enriched in 18O relative to fresh MORB (?18O = 7.1 0.5 and 5.9 0.1 , respectively) indicating that Macquarie crust was a net sink for 18O from the oceans. The Macquarie crust also exchanged Sr with the oceans. The lavas are on average enriched in 87Sr (87Sr /86Sr = 0.7028 to 0.7035) relative to fresh MORB glasses (0.7025 to 0.70275) with the 87Sr /86Sr ratio increasing to 0.704 at the lava-dike transition, below which the average ratio decreases with depth through the sheeted dikes and gabbros towards primary magmatic compositions. However, a significant variation in whole-rock 87Sr /86Sr ratios at a given depth in the lower crust (with an average range of 0.0005) indicates that fluids were channelled. High primary Sr contents resulted in more rock-dominated fluid-rock Sr-exchange in the Macquarie Island crust, compared to other crustal sections. Consequently the Sr-isotopic compositions of the upwelling `black smoker' hydrothermal fluids, as recorded by epidote veins, are relatively rock- dominated (0.7031 0.0003). Tracer transport mass balance calculations indicate that a time-integrated fluid flux of 4 1 106 kg/m2 is required to produce the observed shift in Sr-isotopic composition. This can be supported by the available mid-ocean ridge magmatic heat and is similar to estimates for sections of intact ocean crust, but a factor of 10 lower than estimates for ophiolites. The Macquarie crust is relatively thin (3 - 4 km), due to its formation on a short segment of a slow spreading ridge. This indicates that hydrothermal circulation removes a greater proportion of the available magmatic heat at slower spreading ridges.

Coggon, R. M.; Teagle, D. A.; Alt, J. C.; Davidson, G. J.

2006-12-01

91

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

Energy Technology Data Exchange (ETDEWEB)

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. Celadonites show U and Pb contents and Pb isotopic compositions little changed from their basalt precursors, while Th contents are significantly lower. Celadonites thus have unusually high alkali/U,Th ratios and low Th/U ratios. If this celadonite alteration signature is significantly imprinted on oceanic crust as a whole, it will lead to very distinctive Pb isotope signatures for any hot spot magmas which contain a component of aged subducted recycled oceanic crust. Initial Sr isotope ratios of ocean crust vein minerals (smectite, celadonite, zeolite, calcite) are intermediate between primary basalt values and contemporary sea water values, and indicate formation under seawater-dominated systems with effective water/rock ratios of 20-200.

Hart, S.R.; Staudigel, H.

1986-12-01

92

Ocean-continent transition and tectonic framework of the oceanic crust at the continental margin off NE Brazil: Results of LEPLAC project  

Science.gov (United States)

In 1992, Brazilian Navy and PETROBRAS carried out a geophysical survey along the continental margin off northeastern Brazil, as part of a governmental plan to delineate the "Legal Continental Shelf" according to the international Law of the Sea. This data set is leading to a better understanding of the crustal transition processes and on the evolution of the oceanic crust over that part of the Brazilian continental margin. On our seismic transects, we show a rifted marginal plateau (Pernambuco Plateau) where crustal extension was controlled by detachment faulting, possibly in a non-volcanic margin setting. Farther north, dealing with the ocean-continent transition nearby a major transform margin, we found a normal passive margin-style transition zone instead of transform-related structures. With the support of multichannel seismic profiles and gravity data derived from GEOSAT altimetry, several well-known oceanic fracture zones and structural lineaments were properly located and correlated. The relationship of these structures with volcanic ridges and extensional, compressive and strike-slip tectonic reactivations suggests that fracture zones at this area behaved either as zones of weakness or as locked transform fault scars. Striking lithospheric flexural deformation is also related to FZs in this region. In the surroundings of the Fernando de Noronha Ridge, lithospheric flexure represents an isostatic response to volcanic loading, while bending across Ascension FZ is likely to have been caused by differential subsidence in crustal segments of contrasting ages. We also correlate some other deformation of the oceanic crust with changes in spreading directions that possibly took place at the Upper Cretaceous.

Gomes, Paulo Otvio; Gomes, Benedito S.; Palma, Jorge J. C.; Jinno, Koji; de Souza, Jairo M.

93

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

94

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

95

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

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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

96

Banda-Celebes-Sulu basin - a trapped Cretaceous-Eocene oceanic crust  

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The Banda, Celebes and Sulu Basins are three poorly understood marginal seas that are located at the junction of the Eurasian, Indian-Australian, Pacific and Philippine Sea plates. The incomplete data sets from each of these three marginal basins, the complex geological arrangement of the surrounding islands, and the compound late Cenozoic evolutions involving subduction, rifting, transform faulting and island arc collision have complicated any tectonic interpretations of this region. On the bases of marine geophysical data and on-land geology, the authors propose that the Banda, Celebes and Sulu Basins are the remanents of a once-continuous Cretaceous to Eocene ocean basin. Magnetic anomalies from the Banda, Celebes and Sulu Basins show the similar trends of about N70/sup 0/E, N60/sup 0/E and N55/sup 0/E respectively. Their best fit to the reversal models are as follows: (1) anomalies M1-M11 in the Banda Basin, (2) anomalies 30-33 in the Celebes Basin and (3) anomalies 17-20 in the Sulu Basin. The heat flow data from each of these basins is consistent with the relationship of our assigned magnetic ages. The on-land geology of this region is complicated by numerous land masses which dissect this old oceanic basin into the present configuration of marginal seas. The authors argue that each of these land masses arrived at its present location by either late Tertiary tectonic movements or has been built in place upon this older oceanic basement.

Lee, C.S.; McCabe, R.

1985-01-01

97

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

Science.gov (United States)

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

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

2013-12-01

98

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

Science.gov (United States)

Cation exchange experiments (ammonium acetate and cation resin) on celadonite-smectite vein minerals from three DSDP holes demonstrate selective removal of common Sr relative to Rb and radiogenic Sr. This technique increases the Rb/Sr ratio by factors of 2.3 to 22 without significantly altering the age of the minerals, allowing easier and more precise dating of such vein minerals. The ages determined by this technique (site 261 - 121.4 1.6m.y.; site 462A - 105.1 2.8m.y.; site 516F - 69.9 2.4m.y.) are 34, 54 and 18 m.y. younger, respectively, than the age of crust formation at the site; in the case of site 462A, the young age is clearly related to off-ridge emplacement of a massive sill/flow complex. At the other sites, either the hydrothermal circulation systems persisted longer than for normal crust (10-15 m.y.), or were reactivated by off-ridge igneous activity. Celadonites show U and Pb contents and Pb isotopic compositions little changed from their basalt precursors, while Th contents are significantly lower. Celadonites thus have unusually high alkali/U,Th ratios and low Th/U ratios. If this celadonite alteration signature is significantly imprinted on oceanic crust as a whole, it will lead to very distinctive Pb isotope signatures for any hot spot magmas which contain a component of aged subducted recycled oceanic crust. Initial Sr isotope ratios of ocean crust vein minerals (smectite, celadonite, zeolite, calcite) are intermediate between primary basalt values and contemporary sea water values and indicate formation under seawaterdominated systems with effective water/rock ratios of 20-200.

Hart, S. R.; Staudigel, H.

1986-12-01

99

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

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

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

2012-12-01

100

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 regio no oceano Atlntico Sul, centrada em 48W e 35S, foi observada pela primeira vez devido integrao de dados de gravimetria marinha convencionais e dados de gravidade derivados de altimetria por satlite, adquiridos pela misso [...] GEOSAT/ERM. O limite norte desta anomalia coincide com o Lineamento Chu e o limite sul indica outro lineamento, que uma extenso da Zona de Fratura Meteoro. A anomalia tem direo NE-SW, sua largura de 400 km e seu comprimento de 600 km. Foi utilizada uma metodologia de inverso linear bidimensional, com vnculos relativos e absolutos, para calcular a distribuio de densidades ao longo de trs perfis paralelos ao eixo principal da anomalia. O resultado sugere que a espessura de sedimentos na parte mais profunda da bacia de no mnimo 3,0 km onde a batimetria ocenica de 4.800 m. Esta feio tectnica, um semi-grben assimtrico formado entre dois lineamentos, provavelmente situa-se sobre uma crosta ocenica. O volume de sedimentos estimado para esta bacia de cerca de 50% do volume de sedimentos ps-Mioceno depositados no Rio Grande Cone, onde hidratos de gs 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 48W and 35S. 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 Chu 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.

Leite, Emilson Pereira; Ussami, Naomi.

 
 
 
 
101

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)

1986-01-01

102

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., Selln, 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

103

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

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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

104

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

Science.gov (United States)

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

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

2012-11-01

105

Motion between the Indian, Capricorn and Somalian plates since 20 Ma: implications for the timing and magnitude of distributed lithospheric deformation in the equatorial Indian ocean  

Science.gov (United States)

Approximately 2200 magnetic anomaly crossings and 800 fracture zone crossings flanking the Carlsberg ridge and Central Indian ridge are used to estimate the rotations of the Indian and Capricorn plates relative to the Somalian Plate for 20 distinct points in time since 20 Ma. The data are further used to place limits on the locations of the northern edge of the rigid Capricorn Plate and of the southern edge of the rigid Indian Plate along the Central Indian ridge. Data south of and including fracture zone N (the fracture zone immediately south of the Vema fracture zone), which intersects the Central Indian ridge near 10S, are well fit assuming rigid Capricorn and Somalian plates, while data north of fracture zone N are not, in agreement with prior results. Data north of fracture zone H, which intersects the Central Indian ridge near 3.2S, are well fit assuming rigid Indian and Somalian plates, while data south of and including fracture zone H are not, resulting in a smaller rigid Indian Plate and a wider diffuse oceanic plate boundary than found before. The data are consistent with Capricorn-Somalia motion about a fixed pole since ~8 Ma, but require rotation about a pole 15 farther away from the Central Indian ridge from 20 to ~8 Ma. The post-8-Ma pole also indicates Capricorn-Somalia displacement directions that are 7 clockwise of those indicated by the pre-8-Ma stage pole. In contrast, India-Somalia anomaly and fracture crossings are well fit by a single fixed pole of rotation for the past 20 Ma. India-Somalia motion has changed little during the past 20 Myr. Nonetheless, astronomically calibrated ages for reversals younger than 12.9 Ma allow resolution of the following small but significant changes in spreading rate: India-Somalia spreading slowed from 31 to 28 mm yr-1 near 7.9 Ma and later sped up to 31 mm yr-1 near 3.6 Ma; Capricorn-Somalia spreading slowed from 40 to 36 mm yr-1 near 11.0 Ma, later sped up to 38 mm yr-1 near 5.1 Ma and further sped up to 40 mm yr-1 near 2.6 Ma. The motion between the Indian and Capricorn plates is estimated by differencing India-Somalia and Capricorn-Somalia rotations, which differ significantly for all 20 pairs of reconstructions. India has rotated relative to the Capricorn Plate since at least ~20 Ma. If about a pole located near 4S, 75E, the rate of rotation was slow, 0.11+/- 0.01 Myr-1 (95 per cent confidence limits), from 20 to 8 Ma, but increased to 0.28+/- 0.01 Myr-1 (95 per cent confidence limits) at ~8 Ma. The onset of more rapid rotation coincides, within uncertainty, with the inferred onset at 7-8 Ma of widespread thrust faulting in the Central Indian basin, and with the hypothesized attainment of maximum elevation and initiation of collapse of the Tibetan plateau at ~8 Ma. The plate kinematic data are consistent with steady India-Capricorn motion since 8 Ma and provide no evidence for previously hypothesized episodic motions during that interval. The convergence since 8 Ma between the Indian and Capricorn plates significantly exceeds (by 13 to 20 km) the convergence estimated from three north-south marine seismic profiles in the Central Indian basin. Where and how the additional convergence was accommodated is unclear.

DeMets, Charles; Gordon, Richard G.; Royer, Jean-Yves

2005-05-01

106

Sulfur geochemistry and microbial sulfate reduction during low-temperature alteration of uplifted lower oceanic crust: Insights from ODP Hole 735B  

Science.gov (United States)

Sulfide petrography plus whole rock contents and isotope ratios of sulfur were measured in a 1.5 km section of oceanic gabbros in order to understand the geochemistry of sulfur cycling during low-temperature seawater alteration of the lower oceanic crust, and to test whether microbial effects may be present. Most samples have low SO4/?S values (? 0.15), have retained igneous globules of pyrrhotite chalcopyrite pentlandite, and host secondary aggregates of pyrrhotite and pyrite laths in smectite iron-oxyhydroxide magnetite calcite pseudomorphs of olivine and clinopyroxene. Compared to fresh gabbro containing 1001800 ppm sulfur our data indicate an overall addition of sulfide to the lower crust. Selection of samples altered only at temperatures ? 110 C constrains microbial sulfate reduction as the only viable mechanism for the observed sulfide addition, which may have been enabled by the production of H2 from oxidation of associated olivine and pyroxene. The wide range in ?34Ssulfide values (? 1.5 to + 16.3) and variable additions of sulfide are explained by variable ?sulfate-sulfide under open system pathways, with a possible progression into closed system pathways. Some samples underwent oxidation related to seawater penetration along permeable fault horizons and have lost sulfur, have high SO4/?S (? 0.46) and variable ?34Ssulfide (0.7 to 16.9). Negative ?34Ssulfate?34Ssulfide values for the majority of samples indicate kinetic isotope fractionation during oxidation of sulfide minerals. Depth trends in sulfidesulfur contents and sulfide mineral assemblages indicate a late-stage downward penetration of seawater into the lower 1 km of Hole 735B. Our results show that under appropriate temperature conditions, a subsurface biosphere can persist in the lower oceanic crust and alter its geochemistry.

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

2011-01-01

107

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

Science.gov (United States)

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

Dutta, R. K.; Sideras-Haddad, E.; Connell, S. H.

2001-07-01

108

BATOLITO DE SABANALARGA, PLUTONISMO DE ARCO EN LA ZONA DE SUTURA ENTRE LAS CORTEZAS OCENICA 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 Formacin Barroso y las Diabasas de San Jos de Urama y en el borde oriental las rocas metamrficas del Complejo Cajamarca. El Batol [...] ito de Sabanalarga est formado por al menos dos pulsos magmticos: uno inicial representado por gabros y dioritas de afinidad subalcalina toletica y un segundo pulso constituido por cuarzodioritas y tonalitas de afinidad subalcalina de la serie calcoalcalina baja en K. El magmatismo se gener en un ambiente localizado por encima de la zona de subduccin, en un arco volcnico plutnico localizado en el borde de sutura entre la corteza continental y la corteza ocenica, afectando ambas cortezas. Hace parte del arco las rocas volcnicas de la Formacin Barroso. La edad del Batolito y del arco en general, en concordancia con los datos radiomtricos, las relaciones intrusivas y el registro fsil, ocurri dentro del rango comprendido entre el Cenomaniano-Aptiano superior, localizndose 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 Antioqueo, es contemporneo con el arco que gener el Batolito de Sabanalarga, pudiendo ser parte del mismo evento magmtico. 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, RODRGUEZ G.; MARA ISABEL, ARANGO; JOS GILBERTO, BERMDEZ.

109

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

110

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

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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

111

Isotopic composition of gypsum in the Macquarie Island ophiolite: Implications for the sulfur cycle and the subsurface biosphere in oceanic crust  

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The O, S, and Sr isotope compositions were determined for 17 samples of gypsum that replaced anhydrite in the sheeted-dike complex of the Macquarie Island ophiolite. Elevated ?34S (26.2 29.0) and ?18O values (12.5 14.4) of gypsum compared to those of seawater sulfate are the result of microbial sulfate reduction. Low organic carbon contents and little sulfate reduction in sediments, plus a large basaltic Sr component in the gypsum (87Sr/86Sr = 0.70446 0.70524), indicate that the sulfate source was not pore waters in the overlying sediment. Low ?34S values of sulfide in basalt lavas are consistent with microbial reduction of seawater sulfate within the volcanic rocks. Tectonic activity at the slow-spreading ridge allowed evolved formation waters to enter hot sheeted-dike complex basement, resulting in heating and precipitation of anhydrite. Results show that microbes can leave geochemical tracers of their activity in oceanic basement and that anhydrite can be preserved in oceanic crust and may be of significance for the oceanic sulfur budget.

Alt, Jeffrey C.; Davidson, Garry J.; Teagle, Damon A. H.; Karson, Jeffrey A.

2003-06-01

112

Oceanic crust and island arc formation in Central Asia during late Neoproterozoic times - evidence from petrological and geochemical studies  

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Die vorliegende Arbeit behandelt die Entwicklung des 570 Ma alten, neoproterozoischen Agardagh - Tes-Chem Ophioliths (ATCO) in Zentralasien. Dieser Ophiolith liegt sdwestlich des Baikalsees (50.5 N, 95 E) und wurde im frhen Stadium der Akkretion des Zentralasiatischen Mobilgrtels auf den nordwestlichen Rand des Tuvinisch-Mongolischen Mikrokontinentes aufgeschoben. Bei dem Zentralasiatische Mobilgrtel handelt es sich um einen riesigen Akkretions-Subduktionskomplex, der heute das gr...

Pfa?nder, Jo?rg A.

2002-01-01

113

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

International Nuclear Information System (INIS)

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

2013-03-01

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  

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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 elik, Omer; Avagyan, Ara; Muller, Carla

2014-05-01

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

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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

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Preliminary Results from a Gas Tracer Injection Experiment in the Upper Oceanic Crust on the Eastern Flank of the Juan de Fuca Ridge  

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We present the first results from a gas tracer injection experiment in the ocean crust on the eastern flank of the Juan de Fuca Ridge, in an area of vigorous hydrothermal circulation. A mixture of tracers was injected in Hole 1362B in 2010, during IODP Expedition 327, as part of a 24-hour pumping experiment. Fluid samples were subsequently collected from this hole and three additional holes (1026B, 1362A, and 1301A), located 300 to 500 m away. The array of holes is located on 3.5 M.y. old seafloor, and oriented N20E, subparallel to the Endeavor Segment of Juan de Fuca Ridge, 100 km to the west. Sulfur hexafluoride (SF6) was injected at a concentration of 0.0192 mol/min, with fluid pumping rate of 6.7 L/s for 20.2 h, resulting in a mean concentration of 47.6 ?M and 23.3 mol of SF6 being added to crustal fluids. Borehole fluid samples were collected in copper coils using osmotic pumps attached to the wellheads of several long-term, subseafloor observatories (CORKs). These samples were recovered from the seafloor using a remotely-operated vehicle in 2011 and 2013. Analyses of SF6 concentrations in samples recovered in 2011 indicate the first arrival of SF6 in Hole 1301A, 550 m south of the injection Hole 1362B, ~265 days after injection. This suggests that the most rapid lateral transport of gas (at the leading edge of the plume) occurred at ~2 m/day. Samples recovered in 2013 should provide a more complete breakthrough curve, allowing assessment of the mean lateral transport rate. Additional insights will come from analysis of metal salts and particle tracers injected contemporaneously with the SF6, the cross-hole pressure response to injection and a two-year fluid discharge experiment. Additional wellhead samples will be collected in Summer 2014, as will downhole osmosamplers deployed in perforated casing within the upper ocean crust in Holes 1362A and 1362B.

Neira, N. M.; Clark, J. F.; Fisher, A. T.; Wheat, C. G.

2013-12-01

117

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

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

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

2003-01-01

118

Evidence of upper-mantle processes related to continental rifting versus oceanic crust in the Gulf of California  

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Receiver functions from teleseismic events, recorded by stations around the Gulf of California, are used to map the upper-mantle seismic discontinuities. We observe a mean transition zone thickness comparable to the global average for most of the region. A low-velocity layer is detected above the 410 discontinuity that varies in thickness along the Gulf of California. The 660 discontinuity shows complex waveforms south of latitude 30N as a result of the phase change of garnet to perovskite. Within the transition zone, a complex behaviour of the receiver functions is observed mainly at the southern end of the Gulf. The north-south variations of this zone are likely associated with a slab window at the northern Gulf, resulting from the cessation of subduction of the Farallon plate 12 Ma, and the subduction of the Guadalupe and Magdalena microplates at the southern end, resulting in a hydrated upper mantle. Our results suggest that change in rifting styles occurring along the Gulf of California mirrors deeper processes in the upper mantle.

Prez-Campos, Xyoli; Clayton, Robert W.

2013-08-01

119

Borehole-to-borehole hydrologic response across 2.4 km in the upper oceanic crust: Implications for crustal-scale properties  

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Subseafloor hydrologic observatories (CORKs) were installed in four boreholes in young seafloor on the eastern flank of the Juan de Fuca ridge to evaluate the hydrogeology of the upper oceanic crust. Two CORKs installed at Site 1301 were incompletely sealed, allowing cold bottom water to flow into basement at 2-5 L/s and causing a pressure perturbation in a preexisting sealed CORK at Site 1027, which was 2.4 km away. The pressure perturbation at Site 1027 is analyzed using conventional aquifer test methods, yielding transmissivity of T = 0.5 to 1.2 10-2 m2/s and bulk permeability of k = 0.7 to 2 10-12 m2 in the upper 300 m of basement. Storativity (a parameter that includes fluid and aquifer compressibilities, porosity, and layer thickness) is S = 1 to 3 10-3, corresponding to a crustal aquifer (matrix) compressibility of ?m = 3 to 9 10-10 Pa-1. The inferred basement permeability is consistent with, but at the low end of, permeabilities calculated from single-hole packer experiments at this and other young oceanic crustal sites; it is much less than values estimated from numerical models, analyses of formation response to tidal pressure oscillations, or pressure responses to coseismic strain events. The relatively low permeability indicated by the cross-hole response may result from the basement aquifer in this area being azimuthally anisotropic, with a preferential flow direction oriented subparallel to the abyssal hill topography and tectonic structural fabric created at the ridge axis; this hypothesis will be tested during future experiments.

Fisher, A. T.; Davis, E. E.; Becker, K.

2008-07-01

120

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

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Full Text Available Zircon age peaks at 21001650 and 12001000 Ma correlate with craton collisions in the growth of supercontinents Nuna and Rodinia, respectively, with a time interval between collisions mostly <50 Myr (range 0250 Myr. Collisional orogens are two types: those with subduction durations <500 Myr and those ?500 Myr. The latter group comprises orogens with long-lived accretionary stages between Nuna and Rodinia assemblies. Neither orogen age nor duration of either subduction or collision correlates with the volume of orogen preserved. Most rocks preserved date to the pre-collisional, subduction (ocean-basin closing stage and not to the collisional stage. The most widely preserved tectonic setting in Proterozoic orogens is the continental arc (10%90%, mean 60%, with oceanic tectonic settings (oceanic crust, arcs, islands and plateaus, serpentinites, pelagic sediments comprising <20% and mostly <10%. Reworked components comprise 20%80% (mean 32% and microcratons comprise a minor but poorly known fraction. Nd and Hf isotopic data indicate that Proterozoic orogens contain from 10% to 60% of juvenile crust (mean 36% and 40%75% reworked crust (mean 64%. Neither the fraction nor the rate of preservation of juvenile crust is related to the collision age nor to the duration of subduction. Regardless of the duration of subduction, the amount of juvenile crust preserved reaches a maximum of about 60%, and 37% of the volume of juvenile continental crust preserved between 2000 and 1000 Ma was produced in the Great Proterozoic Accretionary Orogen (GPAO. Pronounced minima occur in frequency of zircon ages of rocks preserved in the GPAO; with minima at 16001500 Ma in Laurentia; 17001600 Ma in Amazonia; and 17501700 Ma in Baltica. If these minima are due to subduction erosion and delamination as in the Andes in the last 250 Myr; approximately one third of the volume of the Laurentian part of the GPAO could have been recycled into the mantle between 1500 and 1250 Ma. This may have enriched the mantle wedge in incompatible elements and water leading to the production of felsic magmas responsible for the widespread granite-rhyolite province of this age. A rapid decrease in global Nd and in detrital zircon Hf model ages between about 1600 and 1250 Ma could reflect an increase in recycling rate of juvenile crust into the mantle; possibly in response to partial fragmentation of Nuna.

Kent C. Condie

2013-05-01

 
 
 
 
121

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

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The great majority of continental flood basalts (CFBs) have a marked lithospheric geochemical signature, suggesting derivation from the continental lithosphere, or contamination by it. Here we present new Pb and Os isotopic data and review previously published major element, trace element, mineral chemical, and Sr and Nd isotopic data for geochemically unusual mafic and ultramafic dikes located in the Antarctic segment (Ahlmannryggen, western Dronning Maud Land) of the Karoo CFB province. Some of the dikes show evidence of minor contamination with continental crust, but the least contaminated dikes exhibit depleted mantle like initial ?Nd (+9) and 187Os/188Os (0.12440.1251) at 180 Ma. In contrast, their initial Sr and Pb isotopic compositions (87Sr/86Sr = 0.70350.7062, 206Pb/204Pb = 18.218.4, 207Pb/204Pb = 15.4915.52, 208Pb/204Pb = 37.737.9 at 180 Ma) are more enriched than expected for depleted mantle, and the major element and mineral chemical evidence indicate contribution from (recycled) pyroxenite sources. Our Sr, Nd, Pb, and Os isotopic and trace element modeling indicate mixed peridotitepyroxenite sources that contain 1030% of seawater-altered and subduction-modified MORB with a recycling age of less than 1.0 Ga entrained in a depleted Os-rich peridotite matrix. Such a source would explain the unusual combination of elevated initial 87Sr/86Sr and Pb isotopic ratios and relative depletion in LILE, U, Th, Pb and LREE, high initial ?Nd, and low initial 187Os/188Os. Although the sources of the dikes probably did not play a major part in the generation of the Karoo CFBs in general, different kind of recycled source components (e.g., sediment-influenced) would be more difficult to distinguish from lithospheric CFB geochemical signatures. In addition to underlying continental lithosphere, the involvement of recycled sources in causing the apparent lithospheric geochemical affinity of CFBs should thus be carefully assessed in every case.

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

2014-05-01

122

The European margin of the Jurassic Tethys in Corsica: dating of Balagne trondhjemites and evidence to support a continental crust beneath the Balagne-Ligurian domain  

International Nuclear Information System (INIS)

Vein trondhjemite in gabbro of the Carnispola Bridge has been dated to 169 3 Ma (U-Pb on zircon). This date indicates that E-MORB-type ophiolites were emplaced in the marginal Balagne part of the Ligurian Jurassic basin about some 10 Ma before the emplacement of N-MORB ophiolites in the most central part of the ocean. In addition, the presence of inherited zircons with Ordovician (431 8 Ma) and Archean (2693 12 Ma) ages reveals that the Balagne ophiolites were emplaced on a thinned continental crust. Finally, the 298 4 Ma age of zircons from eclogitised meta-arkoses in the eclogitic Morteda-Farinole unit ('Schistes lustres' zone) confirms the attribution of these rocks to a paleogeographic area that laid between continent and ocean, along the edge of the Hercynian granite batholith in Corsica. (authors)

2002-05-01

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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 (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 (>350C up to 600C). Intrusion of gabbro bodies into the lower dikes resulted in contact metamorphism to granoblastic hornfels at 850C-900C, 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 800C down to 450C 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

124

Magmatic evolution of the South Shetland Islands, Antarctica, and implications for continental crust formation  

Science.gov (United States)

Lavas from the South Shetland Islands volcanic arc (northern Antarctic Peninsula) have been investigated in order to determine the age, petrogenesis and compositional evolution of a long-lived volcanic arc constructed on 32-km-thick crust, a thickness comparable with average continental crust. New 40Ar-39Ar ages for the volcanism range between 135 and 47 Ma and, together with published younger ages, confirm a broad geographical trend of decreasing ages for the volcanism from southwest to northeast. The migration pattern breaks down in Palaeogene time, with Eocene magmatism present on both Livingston and King George islands, which may be due to a change in both subduction direction and velocity after c. 60 Ma. The lavas range from tholeiitic to calc-alkaline, but there is no systematic change with age or geographic location. The compositions of lavas from the north-eastern islands indicate magma generation in a depleted mantle wedge with relatively low Sr and high Nd isotopic compositions and low U/Nb, Th/Nd and Ba/Nb ratios that was metasomatized by hydrous fluids from subducted basaltic oceanic crust. Lavas from the south-western islands show an additional sedimentary influence most likely due to fluid release from subducted sediments into the mantle wedge. Although magmatic activity in the South Shetland arc extended over c. 100 m.y., there is no evolution towards more enriched or evolved magmas with time. Few South Shetland arc lavas are sufficiently enriched with incompatible elements to provide a potential protolith for the generation of average continental crust. We conclude that even long-established subduction zones with magmatic systems founded on relatively thick crust do not necessarily form continental crustal building blocks. They probably represent only the juvenile stages of continental crust formation, and additional re-working, for example during subsequent arc-continental margin collision, is required before they can evolve into average continental crust.

Haase, K. M.; Beier, C.; Fretzdorff, S.; Smellie, J. L.; Garbe-Schnberg, D.

2012-06-01

125

Deep seismic reflection images of the Wharton Basin oceanic crust and uppermost mantle offshore Northern Sumatra: Relation with active and past deformation  

Science.gov (United States)

present deep seismic reflection images along two profiles collected in 2006 in the Wharton Basin offshore Northern Sumatra. The main profile is located subparallel to the Sumatran trench at a distance of 32-66 km. Faulting of the entire sedimentary section (strike-slip deformation sometimes accompanied by a dip-slip component) is imaged over two fracture zones of the extinct Wharton Spreading Center that prior studies have shown to be reactivated as left-lateral faults. The western fracture zone is associated with a wide region of strong basement topography, a difference in crustal thickness of ~1.5 km, and an age offset of 9 Ma. The epicenters of the 11 April 2012 Mw 8.6 great strike-slip earthquake, its Mw 7.2 foreshock, and Mw 8.2 aftershock align along this major structure > 100 km south of the profile intersection. Our high-quality long-offset seismic reflection data also reveal bright dipping reflections extending down to a maximum of ~24 km into the oceanic mantle (~37 km below sea level). Apparent dips are mostly 25-35, corresponding to 30-55 along either N-S to NNE-SSW or E-W to WNW-ESE directions, which encompass the directions of plate fabric and nodal planes of the Mw 8.6 event. We suggest that these enigmatic reflections arise from presently inactive dip-slip fault planes reaching for the deepest ones to the base of the brittle layer. Possible origins include extension related to plate bending or an episode of now inactive thrust-type deformation reactivating paleonormal faults, similar to that taking place in the Central Indian Basin.

Carton, Hlne; Singh, Satish C.; Hananto, Nugroho D.; Martin, James; Djajadihardja, Yusuf S.; Udrekh; Franke, Dieter; Gaedicke, Christoph

2014-01-01

126

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

127

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 mlange 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 32016 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

128

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.)

1989-01-01

129

Genesis and evolution of the upper oceanic crust (ODP-IODP site 1256, East Pacific Rise): inferences from structure and composition of late magmatic veins in a lava pond  

Science.gov (United States)

A complete intact "in situ" section of upper oceanic crust, from extrusive lavas, through dikes into gabbros has been recently drilled for the first time in a 15 Ma old crust that formed at the East Pacific Rise with a full spreading rate of >200 mm/yr. The study area is ODP-IODP Site 1256 (644.2N, 9156.1W, Pacific Ocean). Holes 1256C and 1256D have been drilled into the basaltic basement during ODP Leg 206, IODP Expeditions 309 and 312. Hole 1256D has been deepened to a depth of ca. 1500 meters below seafloor (mbsf). The upper section of the igneous basement consists of thin (3m). The massive flows include a ponded lava flow, located near the top of both Hole 1256C and 1256D, where it has a thickness of 32m and 74m, respectively. The lava pond is interpreted as a thick lava flow delivered either on-axis or off-axis and accumulated in a topographic depression. Although very close (ca. 30m), the two holes record different structural patterns of the lava pond, probably related to different steps of the lava flow emplacement. In the lava pond, both igneous (magmatic foliation, flow-related folds, late-magmatic veins) and post-magmatic (joints, veins, shear veins, and microfaults) structures were recognized. Late magmatic veins (LMVs), which were identified as primary features typical of the lava pond, are the main goal of this work. Mm-thick LMVs are mainly clustered in the middle (290-300 mbsf in hole 1256C and 282-297 mbsf in hole 1256D) and bottom (312-313 mbsf in hole 1256C and 311-328 mbsf in hole 1256D) parts of the lava pond. Structural measurements on cores suggest that they are mostly gently dipping structures, but we also observed sub-vertical LMVs. At the bottom of the lava pond in hole 1256C, late magmatic veins are often arranged in en echelon arrays and sigmoidal pull aparts, suggesting a syn/post-magmatic shear component. Thin-section observations show that basalt including LMVs consists of plagioclase, clinopyroxene, ilmenite, and spinel. LMVs cutting basalt are filled with quartz, quartz + plagioclase intergrowth showing a granophyric texture, clinopyroxene, ilmenite, spinel, and apatite. Rarely we observed pyrite crystals at the LMV core that cut plagioclase + quartz intergrowth. Quartz + plagioclase intergrowth (with apatite) are also present in the basalt as mm-size interstitial domains or rimming plagioclase (IDs = intergrowth domains). Rare samples display IDs with interstitial K-feldspar growing around plagioclase. LMVs often show sharp contacts with basalt. Plagioclase or pyroxene crystals of the basalt may be fragmented at the contact with LMVs (brittle rheology of basalt). Differently, IDs commonly corrode plagioclase crystals, without fragmentation (ductile rheology). The composition of basalt plagioclase ranges from Ab37 to Ab62, with a main concentration around Ab50. On the contrary, plagioclase in the LMVs intergrowth as well as that in the mm-sized IDs are Na-rich (Ab64-Ab98). Mineral analyses also highlight homogeneous clinopyroxene, spinel and ilmenite, without variations in the LMVs and IDs. Clinopyroxene usually shows a Ca-poor core (mainly augite or pigeonite) and a diopsidic rim. Opaque minerals often exhibit ilmenite-ulvospinel lamellae intergrowths. EDS mapping of IDs and LMVs cutting basalt supports the previous observations. LMVs and IDs have higher Si, Na and lower Al, Ca values than basalt. This distribution is only due to albitic plagioclase concentration in LMVs and IDs. K has low and homogeneous concentrations: rarely IDs are characterized by interstitial K enrichment (K-feldspar). Incompatible (Zr, Rb, Sr, Ba), hydrothermal elements (Cu, Cl), and F are undetectable or absent. We infer that LMVs and IDs likely crystallized from a pure Si-Al-Na-(Ca) melt. K, rarely noticed in the IDs, may be related to late magmatic fluids differentiation or to subsequent hydrothermal fluids. Core description, microstructural observations, mineral compositions and EDS mapping of the stud

Panseri, M.

2009-04-01

130

Distribution of crust in deep eastern Gulf of Mexico  

Energy Technology Data Exchange (ETDEWEB)

Most current models for the evolution of the Gulf of Mexico suggest that Upper Jurassic oceanic crust underlies the deep part of the basin, surrounded by older, attenuated continental crust (thin transitional crust). Based on a detailed analysis of existing University of Texas Institute for Geophysics multifold seismic reflection data and a recently available grid of industry data, they have redefined the oceanic/transitional crust boundary as well as characterized and mapped basement trends in areas of oceanic crust in the deep eastern gulf. Along the eastern part of the study area, the crust boundary is defined where Middle Jurassic(.) salt and equivalent strata, deposited on transitional crust prior to the emplacement of oceanic crust, onlap and pinch out basinward onto a basement block. In the northern and southern parts of the study area, the boundary is obscured by low-relief salt tongues. Regional basement highs and lows associated with oceanic crust within the central part of the deep eastern Gulf trend east-northeast-west-southwest. Deep marine rocks overlying oceanic crust appear to onlap both north and south onto a median high, suggesting that this linear feature may represent a segment of an extinct mid-ocean spreading ridge. A north-northwest-striking trough perpendicular to and bisecting the median ridge could represent an extinct fracture zone. These trends indicate that in the central gulf, sea-floor spreading was probably in a north-northwest-south-southeast direction. Apparent lack of regional structural trends in the eastern part of the study area, however, suggest a different origin for this part of the oceanic crust. These boundaries are structural trends provide important constraints for Gulf of Mexico reconstructions.

Rosenthal, D.B.; Buffler, R.T.; Corso, W.P.; Weimer, P.

1987-05-01

131

Isotopic constraints on the collision age from the Kohistan-Ladakh Arc crust  

Science.gov (United States)

In the Himalayas, despite intensive research, the age of collision between India and Eurasia is still debated, with estimates ranging from 70 to 35 Ma. In the Western Himalaya, separated from the Eurasian plate to the north by the Shyok suture, and from the Indian plate to the south by the Indus suture, lies the Kohistan-Ladakh Arc (KLA). This geological entity is recognized as a fully preserved oceanic arc, formed in the Tethys Ocean and now incorporated in the Himalayan collision system. The age collision estimate between the Arc and Eurasia range from Mid-Creataceous to Oligocene, whereas the India-Arc collision ranges from Early Paleocene to Late-Eocene. The middle to upper crustal part of the KLA is made of plutonic rocks ranging from diorite to granites, together with Volcano-sedimentary units. The crust-forming granitoids records a magmatic activity between >120 and 30 Ma, covering the range of estimated India-Arc and Arc-Eurasia collisions. The limited available data in the literature for the KLA indicates that Jurassic/Cretaceous rocks are juvenile in composition, reflecting a subduction setting whereas lower Eocene rocks have a distinctively different isotopic composition involving evolved crust. Here, we present a detailed geochronological and isotopic study of the KLA granitoids in order to: (1) pinpoint the exact timing of the change in isotopic composition; and (2) to constrain the different possible sources accounting for those differences. We present U-Pb and Hf isotopic data on zircon coupled with Nd isotopic data on whole rocks from the KLA granitoids. Our results indicate a pronounced and abrupt shift in isotopic composition from typical juvenile oceanic arc isotopic signature (?Nd(i) ? +1; ?Hf(i) ? +10) during the Jurassic/Early Paleocene to a sudden abrupt change to more evolved but highly variable crustal like composition in the Eocene/Oligocene (-10 < ?Nd(i) , -4; -15 < ?Hf(i) < 0). Inherited Paleozoic zircon crystals in the Eocene/Oligocene rocks indicate the participation of old evolved continental crust like the Indian continental crust in their formation. Therefore we speculate that the Indian crust was underplated below the juvenile KLA since the Eocene, contributing to the build up of the post-collisional KLA crust. The change in isotopic composition can thus be used to give a minimum constraint on the India-KLA collision.

Bouilhol, P.; Jagoutz, O. E.; Hanchar, J. M.

2010-12-01

132

The European margin of the Jurassic Tethys in Corsica: dating of Balagne trondhjemites and evidence to support a continental crust beneath the Balagne-Ligurian domain; La marge europeenne de la Tethys jurassique en Corse: datation de trondhjemites de Balagne et indices de croute continentale sous le domaine Balano-Ligure  

Energy Technology Data Exchange (ETDEWEB)

Vein trondhjemite in gabbro of the Carnispola Bridge has been dated to 169 {+-} 3 Ma (U-Pb on zircon). This date indicates that E-MORB-type ophiolites were emplaced in the marginal Balagne part of the Ligurian Jurassic basin about some 10 Ma before the emplacement of N-MORB ophiolites in the most central part of the ocean. In addition, the presence of inherited zircons with Ordovician (431 {+-} 8 Ma) and Archean (2693 {+-} 12 Ma) ages reveals that the Balagne ophiolites were emplaced on a thinned continental crust. Finally, the 298 {+-} 4 Ma age of zircons from eclogitised meta-arkoses in the eclogitic Morteda-Farinole unit ('Schistes lustres' zone) confirms the attribution of these rocks to a paleogeographic area that laid between continent and ocean, along the edge of the Hercynian granite batholith in Corsica. (authors)

Rossi, Ph.; Cocherie, A.; Lahondere, D. [Bureau de Recherches Geologiques et Minieres (BRGM), 75 - Paris (France); Mark Fanning, C. [Research School of Earth Sciences, ANU, Canberra (Australia)

2002-05-01

133

Hafnium Isotope Constraints on the Longevity and Distribution of the Indian Ocean Mantle Signature  

Science.gov (United States)

Hf isotope and trace element concentration data are presented for a suite of oceanic crustal basalts from 20 deep sea drilling sites in the Indian Ocean, located on 55 Ma to 155 Ma oceanic crust. Sr, Nd and Pb isotope data for these samples have been presented previously by other authors (Weis and Frey, JGR, 1996; Mahoney et al., J. Petrol., 1998; Zhang et al., J.Petrol., 2005). Old Indian Ocean (OIO) crustal basalts measured thus far span a range in epsilon Hf (eHf) values of +5.5 to +21.3 (corresponding eNd range = +2.7 to +10.4). Similar to most Indian Ocean MORB, nearly all of the of the OIO basalts lie on or above the Nd-Hf isotope "mantle array" line (i.e., the regression through all oceanic basalt Nd-Hf isotope data), with the most extreme samples lying 7.7 eHf units above and 1.7 eHf units below this line (these vertical departures are termed delta epsilon Hf or DeHf). Apart from samples that are products of enriched sources (i.e., eNd Hanan et al., Nature, 2004). Thus, our data indicate that distinct "Indian Ocean-type" Nd-Hf isotopic signatures have been present in the upper mantle beneath most of the Indian Ocean basin for at least the last 100 Myr. Samples of Tethyan oceanic crust (> 150 Ma) from Sites 261 and 765 lie near the mantle array in Nd-Hf isotope space (-2Ocean-type source component. This preliminary finding contrasts with Sr-Nd-Pb isotopic evidence requiring a major contribution from an Indian Ocean-type source for nearly all Tethyan oceanic crust, including that from Sites 261 and 765 (Zhang et al., J. Petrol., 2005). Further Hf isotope measurements of Tethyan ocean crust and ophiolite samples should help to clarify the time at which the Nd-Hf isotope signature of the Indian Ocean upper mantle developed.

Janney, P. E.

2005-12-01

134

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

Science.gov (United States)

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

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

2005-01-01

135

Earth's partial pressure of CO2 over the past 120 Ma; evidence from Ce anomalies in the deep (greater than 600 m) Pacific Ocean, 1  

Science.gov (United States)

It was found that Ce serves as a chemical tracer of paleo-oceanic redox conditions. It was shown that the unoxidized and soluble Ce(3+) in modern seawater exhibits a negative anomaly relative to the other soluble REE(3+). An expression of soluble Ce(3+) in seawater that was approximately 1900X greater than the average observed in Ce in 600-5000 m Pacific seawater was derived. Since Ce(CO3)(+) and Ce(CO3)2(-) complexes greatly exceed the Ce(PO4) complexes in seawater, the formulations of using carbonate complexes were followed and it was found that the calculated Ce and observed concentrations in the deep 600-5000 m Pacific Ocean agree within the uncertainties of the thermodynamic data. As expected, the calculated Ce concentrations are a strong function of pH and found to be lesser functions of CO3(2-) activities.

Liu, Y.-G; Schmitt, R. A.

1993-01-01

136

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

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

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

2012-01-01

137

Structural expression of oblique seafloor spreading in the Macquarie Island ophiolite, Southern Ocean  

Science.gov (United States)

The Macquarie Island ophiolite is an uplifted block of oceanic crust formed at the Australia-Pacific spreading center between 12 and 9 Ma. The sense of motion and geological processes across this plate boundary reflect an evolution from orthogonal spreading through progressively more oblique spreading to the present-day transpressional regime. The crust that makes up the island was formed during an interval of oblique spreading along east-trending spreading segments punctuated by a series of northwest-trending discontinuities. The discontinuities are accommodation zones marked by oblique-slip dextral-normal faults, localized dikes and lava flows, and extensive hydrothermal alteration, indicating that these zones were active near the spreading axis. These features provide a window into the internal structure of oceanic crust generated by oblique spreading.

Rivizzigno, Peter A.; Karson, Jeffrey A.

2004-02-01

138

Antipodal Hotspots on the Earth: Vestiges of Major Oceanic Bolide Impacts?  

Science.gov (United States)

The distribution of hotspots on the Earth has a distinct antipodal character, which has previously been shown to be statistically significant (pComores, 10 Ma) was antipodal to the proposed Ewing impact structure ( 7-11 Ma) in the Pacific Ocean. Many antipodal hotspots have similar ages (e.g. Galpagos, 85 Ma; Nikitin, 80 Ma), and no contradictions to this prediction have been found. Only two primary hotspots (Hawai`i, Louisville) have continental antipodal sites where no volcanic features exist. These two hotspots, however, are among the oldest (>100 Myr) and could have drifted quite far from antipodality with their opposite hotspots. The remaining 9 primary hotspots have antipodes in the Pacific Ocean where submerged hotspots or impact structures could yet be identified. All hotspots antipodal to those associated with flood basalt provinces or formed in continental crust are, or were, in oceanic crust suggesting links between major deep-ocean impacts, greater impact/seismic efficiency, and the creation of antipodal hotspot pairs. In general, `spotless' areas [5] occur opposite to continental masses, and no hotspot volcanism is found at or antipodal to known continental impact structures. [1] Rampino and Caldeira, GRL (1992) 2011; [2] Schultz and Gault, The Moon (1975) 159; [3] Boslough et al., GSA Spec. Pap. 307 (1996) 541; [4] Roddy et al., Int. J. Impact Eng. (1987) 525; [5] Vogt, JGR (1981) 950.

Hagstrum, J. T.

2003-12-01

139

Precollisional, multistage exhumation of subducted continental crust: The Sesia Zone, western Alps  

Science.gov (United States)

The Sesia Zone within the Tertiary arc of the western Alps is a relic of the subducted part of the Adriatic continental margin along the SE border of the Tethyan ocean. The Sesia Zone comprises three basement nappes which individuated during Late Cretaceous (65-80 Ma) subduction to different depths at high-pressure (HP, blueschist, eclogite facies) conditions (peak pressures of 1.0-1.2, 1.0-1.5, and 1.5-2.0 GPa). The thrusts bounding these nappes developed where the crust was previously thinned during Jurassic rifting. Crustal-scale shear zones partly overprinted these early thrusts and exhumed coherent slices of crust containing HP rocks. Initial exhumation of the internal part of the accreted margin involved thrusting (D1) and transpressional shearing (D2) along a subvertical, E-W trending mylonitic shear zone under retrograde blueschist- to greenschist-facies conditions. This exhumation was nearly isothermal to a depth of about 25 km, where the basement nappes were juxtaposed. Subsequent exhumation of these nappes to a common depth of about 15-20 km occurred in the footwall of a greenschist-facies, top-SE extensional shear zone (D3) preserved in some of the highest mountain peaks of the Sesia Zone. New Rb-Sr mineral ages constrain D2 to have occurred at about 60-65 Ma and D3 at about 45-55 Ma. Thus top-SE extensional exhumation was broadly coeval with Eocene, SE directed subduction of the Liguro-Piemont oceanic lithosphere beneath the Adriatic margin. Slow cooling and erosional denudation of the Sesia Zone from 45 to 30 Ma occurred in the hanging wall of the Gressoney extensional shear zone (D4), which itself contributed to the exhumation of Eocene HP and ultra-HP oceanic rocks in its footwall. By 30 Ma, HP rocks of the Sesia Zone were intruded by shallow granitic plutons which were eroded and redeposited within volcanoclastic sediments. Oligo-Miocene Insubric backfolding and thrusting (D5) only exhumed northeastern parts of the Sesia Zone, where HP metamorphism is absent or was overprinted by Tertiary temperature-dominated metamorphism. Most exhumation of continental HP rocks in the Sesia Zone therefore preceded Tertiary Alpine collision and coincided with Late Cretaceous to Early Tertiary subduction of the Adriatic and Tethyan lithosphere. The transition from D2 to D3 in the Sesia Zone is interpreted to mark a change from high-stress, oblique SE directed subduction and accretion of the distal Adriatic continental margin to NW retreating, low-stress subduction of the Liguro-Piemont oceanic lithosphere.

Babist, J.; Handy, M. R.; Konrad-Schmolke, M.; Hammerschmidt, K.

2006-12-01

140

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

Science.gov (United States)

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

Busigny, Vincent; Laverne, Christine; Bonifacie, Magali

2005-12-01

 
 
 
 
141

Mars Crust: Made of Basalt  

Science.gov (United States)

By combining data from several sources, Harry Y. (Hap) McSween (University of Tennessee), G. Jeffrey Taylor (University of Hawaii) and Michael B. Wyatt (Brown University) show that the surface of Mars is composed mostly of basalt not unlike those that make up the Earth's oceanic crust. McSween and his colleagues used data from Martian meteorites, analyses of soils and rocks at robotic landing sites, and chemical and mineralogical information from orbiting spacecraft. The data show that Mars is composed mostly of rocks similar to terrestrial basalts called tholeiites, which make up most oceanic islands, mid-ocean ridges, and the seafloor beneath sediments. The Martian samples differ in some respects that reflect differences in the compositions of the Martian and terrestrial interiors, but in general are a lot like Earth basalts. Cosmochemistst have used the compositions of Martian meteorites to discriminate bulk properties of Mars and Earth, but McSween and coworkers' synthesis shows that the meteorites differ from most of the Martian crust (the meteorites have lower aluminum, for example), calling into question how diagnostic the meteorites are for understanding the Martian interior.

Taylor, G. J.

2009-05-01

142

Thinned continental crust below northern Evoikos Gulf, central Greece, detected from deep seismic soundings  

Science.gov (United States)

In March 1996, a wide aperture reflection/refraction profiling (WARRP) seismic survey was carried out across the Maliakos-Sporades and northern Evoikos basins, central Greece. Two onshore/offshore seismic lines were recorded, using 14 ocean bottom seismographs (OBS) and 10 land stations. As an energy source, we used one sleevegun of 60 l volume, operating at 120-bar pressure. The results acquired by kinematic, two-point raytracing modelling of the time sections provided evidence of a thinned, stretched continental crust, of only 20 km thickness, below the central part of the northern Evoikos Basin. This basin was developed by the separation of the island of Eviawith continental crust of approximately 30 km thicknessfrom the Greek mainland, by transtension and stretching of the crust, forming the north Evia and the Sperchios valleys. Along the Maliakos-north Sporades Basin, the crust thins from 34 km at the mainland to 22 km below the Sporades Basin. The crust at the northern part of the island of Evia and the Trikeri Straits is 30-32 km thick and of normal continental structure; it is separated by an intercrustal discontinuity between an upper and lower crust. The sediments thicken significantly towards the Sporades Basin, where they obtain maximum thicknesses of the order of 8-10 km. The igneous and metamorphic crust thins significantly in the basin maintaining however its continental character along the entire section. The transition from the Maliakos-Trikeri Straits to the Sporades Basin is controlled by a steep listric fault that downthrows the basin crust by more than 6-7 km. The crustal transition of the thinned Sporades Basin to the 30-km thick crust of the northern Evia and the Maliakos Straits is poorly understood. Simple or pure shear stretching cannot have developed this margin, and we favour the assumption that it denotes the limit of two different crustal domains; these were merged together during a compressive tectonic episode, prior to the present-day extension and stretching of the Aegean Sea that was activated 5 Ma ago.

Makris, J.; Papoulia, J.; Papanikolaou, D.; Stavrakakis, G.

2001-11-01

143

Stromatolites, ooid dunes, hardgrounds, and crusted mud beds, all products of marine cementation and microbial mats in subtidal oceanic mixing zone on eastern margin of Great Bahama Bank  

Energy Technology Data Exchange (ETDEWEB)

The interisland channels along the eastern margin of the Great Bahamas Bank contain lithified structures that owe their origin to recent marine cementation. This cementation appears to be commonly associated with a complex microbial community of plants and microorganisms living within a bank-margin oceanographic mixing zone. In this region, reversing tidal and wind-driven currents flow up to 3 knots (150 cm/sec) three hours out of each six-hour tidal period. Here, marine-cement crusted, carbonate mud beds are found interbedded within migrating ooid sand bars and dunes and are associated with growing, lithified stromatolites up to 2 m in height. These laminated mud beds are found with thicknesses of up to 1 m in subtidal depths of 4 to 8 m (12 to 25 ft). The muds appear to be homogeneous, but closer examination by SEM and under a microscope reveals they are composed of pelletoid aggregates of needle-shaped aragonite crystals with diameters of up to 50 ..mu... The size of these soft pellets is similar to the smaller grains of ooid sands that are abundant in the area. This size similarity could explain why both the mud beds are found in similar high-energy hydraulic regimes as the ooid sands, but does not suggest how or why the aggregates of pure aragonite needles form. A high production of ooid sand within this bank margin environment permits the formation of natural levees along the margins of tidal channels. The back sides of these levees are being lithified by marine cements to form hardgrounds. Skeletal and ooid sand dunes stabilized by Thallasia in channel bottoms also are becoming lithified. Grapestones form at the distributaries of flood tidal deltas of ooid sand. All of these features have a common attribute: they are continually in contact with the turbulent mixing-zone waters.

Dill, R.F.; Kendall, C.S.C.G.; Steinen, R.P.

1989-03-01

144

Structure and evolution of the continental crust in the Baikal Fold Region  

Science.gov (United States)

A summary of original Nd isotopic data on granitoids, silicic volcanics, and metasediments of the Baikal Fold Region is presented. The available Nd isotopic data, in combination with new geological and geochronological evidence, allowed recognition of the Early Baikalian (1000 100 to 720 20 Ma) and Late Baikalian (700 10 to 590 5 Ma) tectonic cycles in the geological evolution. The tectonic stacking, deformation, metamorphism, and granite formation are related to orogenic events that occurred 0.80-0.78 Ga and 0.61-0.59 Ga ago. The crust-forming events dated at 1.0-0.8 Ga and 0.70-0.62 Ga pertain to each cycle. The Early Baikalian crust formation developed largely in the relatively narrow and spatially separated Kichera and Param-Shamansky zones of troughs in the Baikal-Muya Belt. The formation and reworking of the Late Baikalian continental crust played the leading role in the Karalon-Mamakan, Yana, and Kater-Uakit zones and in the Svetlinsky Subzone of the Anamakit-Muya Zone in the Baikal-Muya Belt. In general, three large historical periods are recognized in the evolution of the Baikal Fold Region. The Early Baikalian period was characterized by prevalence of reworking of the older continental crust. The Late Baikalian-Early Caledonian period is distinguished by more extensive formation and transformation of the juvenile crust. The third, Late Paleozoic period was marked by reworking of the continental crust with juxtaposition of all older crustal protoliths. Two models of paleogeodynamic evolution of the Baikalian fold complexes are considered: (1) the autochthonous model that corresponds to the formation of suboceanic crust in rift-related basins of the Red Sea type and its subsequent reworking in the course of collision-related squeezing of paleorifts and intertrough basins and (2) the allochthonous model that implies the formation of fragments of the Baikal-Muya Belt at the shelf of the Rodinia supercontinent, their subsequent participation in the evolution of the Paleoasian ocean, and their eventual juxtaposition during Late Baikalian and Early Caledonian events in the structure of the Caledonian Siberian Superterrane of the Central Asian Foldbelt.

Rytsk, E. Yu.; Kovach, V. P.; Kovalenko, V. I.; Yarmolyuk, V. V.

2007-11-01

145

Mass and Composition of the Continental Crust Estimated Using the CRUST2.0 Model  

Science.gov (United States)

The mass, age, and chemical composition of the continental crust are fundamental data for understanding Earth differentiation. The inaccessibility of most of the volume of the crust requires that inferences be made about geochemistry using seismic and heat flow data, with additional constraints provided by scarce lower crustal samples (Rudnick and Fountain, Rev. Geophys., 1995; Rudnick and Gao, Treatise on Geochem., 2003). The global crustal seismic database CRUST2.0 (Bassin, et al., EOS, 2000; Mooney, et al., JGR, 1998; hereafter C2) provides a useful template with which the size and composition of the continents can be assessed, and may be a useful vehicle to organize and analyze diverse geochemical data. We have used C2 to evaluate the modern mass and composition of the continental crust and their uncertainties, and explored our results in the context of global mass balances, such as continents versus depleted mantle. The major source of uncertainty comes from the definition of "continent." The ultimate constraint is the total mass of Earth's crust (oceanic + continental), which, from C2, is 2.77 (in units of 1022 kg). Using crustal thickness as a definition of continent, the mass of continental crust (CC) is 2.195 if the minimum thickness is 12-18km, 2.085 for 22.5km, 2.002 for 25km, and 1.860 for 30km. These numbers include all sediment as continental crust. Using C2 definitions to distinguish oceanic and continental crust (and including oceanic plateaus which contain some continental crust), we calculate the CC mass as 2.171. To estimate chemical composition, we use the C2 reservoir masses. For minimum thickness of 22.5km, C2 yields the proportions 0.016 oceanic sediment, 0.038 continental sediment, 0.321 upper crust, 0.326 middle crust, 0.299 lower crust. Upper, middle, and lower crust are assigned compositions from Rudnick and Gao (2003), continental sediments are assigned upper crust composition, and oceanic sediments are assigned GLOSS composition (Plank and Langmuir, Chem. Geology, 1998). Because the C2 model gives a larger proportion of upper versus lower crust, the resulting CC composition is enriched in P2O5, K2O, Rb, Pb, Th, U, Hf and LREE, and depleted in MgO and CaO, relative to Rudnick and Gao (2003). The mass and chemical composition of CC figure prominently in calculations of the mass of depleted mantle (DM) that is required to balance the crustal enrichments in incompatible elements. However, because thin crust also tends to be young crust, and is less enriched in incompatible elements, uncertainties in the mass of CC have little effect on the calculated size of DM, which must be considerably smaller than the whole mantle. Using different approaches to estimating crustal composition, and adding age provinces as well as other types of geochemical and geochronological data, could make CRUST2.0 an important geochemical resource.

Peterson, B. T.; Depaolo, D. J.

2007-12-01

146

When did the subduction first initiate in the southern Paleo-Asian Ocean: New constraints from a Cambrian intra-oceanic arc system in West Junggar, NW China  

Science.gov (United States)

We report here a Cambrian southward-subducting intra-oceanic arc system in the southern West Junggar, NW China, where an immature arc occurred initially on SSZ-type ophiolites and finally evolved into a more mature one. The immature arc is dominantly represented by Early-Middle Cambrian (510 Ma) low-K tholeiitic felsic rocks, whereas the mature arc is characterized by Late Cambrian (495 Ma) medium- and high-K calc-alkaline felsic and mafic rocks. The SSZ-type ophiolites show remarkable depletion of Nb and Ta and contain high-Cr spinel (Cr# > 0.6), resembling these formed in the forearc. Altogether, they record the initiation of subduction and transformation of crust during early subduction of the Paleo-Asian Ocean in the southern part of the Central Asian Orogenic Belt (CAOB). The subduction initiation might occur in the Early Cambrian (>515 Ma), as constrained by both the SSZ-type ophiolites (516 Ma) and the oldest arc plutons (515-509 Ma) that crosscut the ophiolites. The immature felsic plutons have high SiO2 (>72 wt%) contents and variable MgO (0.42-1.49 wt%) and Mg# values (22-62). Crustal anatexis may be responsible for the genesis of those plutons and thus the transformation from oceanic to continental crust. These results, combined with regional data, convincingly indicate that it is one of the oldest intra-oceanic arc systems in the southern CAOB, which may mark the initial subduction of the Paleo-Asian Ocean in its southern part, much later than those reported in the north. An archipelago-type model is proposed for the evolution of the southern West Junggar and has implications for the development of the southern part of the CAOB.

Ren, Rong; Han, Bao-Fu; Xu, Zhao; Zhou, Yin-Zhang; Liu, Bo; Zhang, Lei; Chen, Jia-Fu; Su, Li; Li, Jiao; Li, Xian-Hua; Li, Qiu-Li

2014-02-01

147

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.

Program, Ocean D.; Texas A&M University

148

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

149

Coupling, decoupling and metasomatism: Evolution of crust-mantle relationships beneath NW Spitsbergen  

Science.gov (United States)

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

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

2012-09-01

150

Does subduction zone magmatism produce average continental crust  

International Nuclear Information System (INIS)

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

1987-07-13

151

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

International Nuclear Information System (INIS)

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

1983-01-01

152

ConcepTest: Age of Red Sea Crust  

Science.gov (United States)

The floor of the Atlantic Ocean is approximately 20 times wider than the Red Sea basin. Predict the approximate age of the oldest oceanic crust below the Red Sea. a. 500,000 years b. 10 million years c. 50 million ...

153

Forward modelling of petrological crust-forming processes on the early Earth  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Tonalite-trondhjemite-granodiorite (TTG) gneisses form up to two-thirds of the preserved Archean continental crust and there is considerable debate regarding the primary magmatic processes of the generation of these rocks. The popular theories indicate that these rocks were formed by partial melting of basaltic oceanic crust which was previously metamorphosed to garnet-amphibolite and/or eclogite facies conditions either at the base of thick oceanic crust or by subduction processes.rnThis stu...

Ziaja, Karen

2012-01-01

154

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

International Nuclear Information System (INIS)

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

1986-01-01

155

Petrological studies on the mantle peridotites recovered from the ocean floor in the western Pacific  

Science.gov (United States)

Geological and geophysical models for the various oceanic crusts (or lithosphere) have been proposed on the basis of the combined studies between seismic observation for the oceanic crusts and petrological models of the onland ophiolites, which have been assumed as fossil of oceanic crusts. It is very important to collect basement rocks constituting various oceanic crusts and to characterize those petrological features. Ocean floor is commonly covered by effusive volcanic rocks, however occasionally hypabyssal and plutonic rocks are observed among the unique geological environments in the Western Pacific as partly shown in the followings. VOLCANIC DIATREME(?): Very unique volcanic knolls have been recently discovered by N. Hirano at the typical oceanic crust in the Northwestern Pacific, off Tohoku of Northeastern Japan. The constituting rocks for the main volcanic edifice are porous alkaline lavas with 1-5Ma age containing abundant lithic fragments including gabbros as well as mantle peridotites. They are assumed as a volcanic diatreme induced in the Cretaceous oceanic lithosphere . Geological and petrological analyses on those volcano and volcanic rocks can make clear the geological cross (or columnar) section of the typical oceanic lithosphere including crust as well as upper mantle down to 100 km deep asthenospheric mantle. PARECE VERA BASIN: The Parece Vela Basin (PVB) is an extinct backarc basin in the Philippine Sea. The NNE extending escarpments and depressions (maximum depth 7500 m) are fossil fracture zones and extinct segmented spreading axes (first-order segments), respectively. Oceanic core complexes (OCCs), or megamullions, develop at each first-order segment. Recently discovered OCCs at slow-spreading ridges have been interpreted as exhumed footwalls of oceanic detachment faults in magma-starved ridge environments. Godzilla Mullion, one of the OCC in the PVR, is the worlds largest OCC, 10 times larger in area than the normal OCCs in the Mid-Atlantic Ridge (Ohara et al., 2001). Various plutonic rocks including mantle peridotites were recovered from the megamullion. TONGA FOREARC: The serpentine seamounts have never been observed along the forearc, on the other hand, geological cross section is recognized along the Tonga Trench inner wall including fresh mantle peridotite. IZU-OGASAWARA(BONIN)-MARIANA FOREARC: Many topographic highs are recognized along the Izu- Ogasawara-Mariana forearc. A number of igneous rocks including lavas, gabbros and serpentinized depleted peridotites; so called ophiolitic rocks were dredged from those seamounts by several investigators, who concluded that these seamounts originated from serpentinite diapirs derived from the upper parts of the mantle wedge. Remnant mantle diapir is assumed to be the depleted source peridotite of diapiric serpentinite seamount. Tonga forearc and Izu-Ogasawara-Mariana forearc may be assumed as modern analogue of the Oman ophiolite and the Trodos ophiolite, respectively.

Ishii, T.; Hirano, N.; Ohara, Y.; Bloomer, S.

2006-12-01

156

The Continental Crust: A Geophysical Approach  

Science.gov (United States)

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

Christensen, Nikolas I.

157

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 <15 Myr, assuming fPACs of 0.5 and 0.36, respectively. Synthetic flowlines generated from our new stage rotation produce a better match to Pacific FZ trends than previously published rotations. With the exception of the chron 18o-20o rotation, the six stage poles for rotations between chrons 33y and 13y (74-33 Ma) all cluster tightly at 60-75E, 60-68N, consistent with the relatively constant trend of the major Pacific FZs. This stability spans at least one episode of Farallon Plate fragmentation caused by subduction of PFR segments beneath the Americas, at 55-48 Ma, which appears to have greatly accelerated divergence on the surviving ridge without significantly affecting the location of the instantaneous rotation pole. Together with quasi-periodic 15-20 Myr variations in the degree of spreading asymmetry that also appear to correlate with changes in spreading rate, this indicates that forces other than slab pull may be a factor in determining Pacific-Farallon Plate motions.

Rowan, Christopher J.; Rowley, David B.

2014-04-01

158

Corium crust strength measurements.  

Energy Technology Data Exchange (ETDEWEB)

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

Lomperski, S.; Nuclear Engineering Division

2009-11-01

159

Corium crust strength measurements  

Energy Technology Data Exchange (ETDEWEB)

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

Lomperski, S. [Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439-4840 (United States)], E-mail: lomperski@anl.gov; Farmer, M.T. [Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439-4840 (United States)], E-mail: farmer@anl.gov

2009-11-15

160

Corium crust strength measurements  

International Nuclear Information System (INIS)

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

2009-11-01

 
 
 
 
161

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  

Science.gov (United States)

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

162

Review of MA cavity  

Energy Technology Data Exchange (ETDEWEB)

We have developed several different types of Magnetic Alloy (MA) loaded cavities. Characteristics and performances of the cavities are reviewed. One of the MA-loaded cavities was developed for Barrier Bucket experiment. The beam loading and wave form problem are also reported. (author)

Ohmori, Chihiro [High Energy Accelerator Research Organization, Tsukuba, Ibaraki (Japan)

2003-03-01

163

Uranium-lead zircon geochronology, hafnium isotope and trace element geochemistry of a unique lower crustal - upper mantle section of a dying slow-spreading mid-ocean ridge (Macquarie Island, Southern Ocean)  

Science.gov (United States)

Macquarie Island, located in the Southern Ocean, is a section of subaerially exposed oceanic crust formed at the now extinct proto-Macquarie, slow-spreading, mid-ocean ridge. Macquarie Island is unique among oceanic ophiolite sequences in that it is still located in the basin in which it formed. The northernmost part of Macquarie Island is composed primarily of lower crustal gabbro and upper mantle peridotite, and therefore provides a unique window into lower oceanic crust. Here, we report integrated Pb/U zircon ages and Lu-Hf isotopic and trace element data from six samples of the lower crust-upper mantle sequence. Samples consist of two lower crustal gabbros, and three mantle-hosted gabbro dikes/dikelettes and one phlogopite-bearing vein from the upper mantle sequence. 206Pb/238U SHRIMP-RG zircon error weighted average ages for the lower crustal gabbros are 8.7 0.3 Ma (N = 9) and 9.0 0.2 Ma (N = 13), whereas the mantle-hosted gabbro dikes/dikelettes yield overlapping error-weighted average ages of 8.7 0.2 Ma (N = 9), 8.7 0.3 Ma (N = 8), and 8.9 0.2 Ma (N = 11) (all errors 2?). The phlogopite vein yielded a slightly younger error weighted average age of 8.5 0.1 Ma (N = 11). Initial epsilon Hf results for zircons from the same samples show a broad distribution ranging from +9.5 to +13.3 for the lower crustal gabbro (N = 28), +7.0 to +16.4 for the gabbro dikes/dikelettes (N = 24), and +8.4 to +12.2 for the phlogopite vein (N = 12). The wide range in values (particularly from the gabbro dikes/dikelettes) is consistent with a heterogeneous source region composed of depleted- and enriched mantle sources. Zircon trace element concentrations also support a heterogeneous source, displaying enrichment in U/Yb relative to N-MORB zircons from the Mid-Atlantic and Southwest Indian Ridge systems. No pattern of enrichment with time is observed within the resolution of the U-Pb zircon dates. We interpret these results to indicate that magmatic construction of Macquarie Island occurred between 8.7 and 9.0 Ma and involved sampling of at least two distinct mantle sources.

Jeffcoat, Charles Ryan

164

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

Science.gov (United States)

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; DudS, Frank; Miller, Robert

2012-07-01

165

The End of Tethys: Opening and Closing of Oceans between Australia, India and SE Asia  

Science.gov (United States)

SE Asia has grown by closure of Tethyan oceans south of Asia, principally by addition of fragments rifted from the Gondwana margins, resulting in a mosaic of continental crust and arc/ophiolite sutures. A new reconstruction identifies the blocks rifted from West and NW Australia in the Late Jurassic. They are now in Borneo, Java and Sulawesi, not West Burma as often assumed. Rifting in the Banda and Argo regions began at about 160 Ma, possibly due to south-directed subduction at the north Gondwana margin. Greater India is proposed to have extended north to the northern edge of the Exmouth Plateau and began to separate from Australia at about 140 Ma. The Banda and Argo blocks collided with the SE Asian margin between 110 and 90 Ma. At 90 Ma the Woyla intra-oceanic arc also collided with the Sumatra margin. This terminated subduction beneath Sundaland. The Indian and Australian plates were separated by a leaky transform from about 90 to 75 Ma which became a slightly convergent transform from about 75 to 55 Ma. This transform boundary is considered the eastern end of Tethys from the mid Cretaceous. There was a completely different history of subduction north of India compared to that north of Australia. The subduction history is recorded in the deep mantle by distinctive velocity anomalies which change from east to west abruptly at about 110E. Between 90 and 45 Ma, India moved rapidly north with north-directed subduction within Tethys and at the Asian margin. It collided with an intra-oceanic arc at about 57 Ma, west of Sumatra, but continued to move north. The first contact of India with Asia was probably about 45 Ma, an estimate dependent on the shape of Greater India and the Asian margin; final ocean closure was later. North of Australia, between 90 and 45 Ma, there was no subduction beneath Sumatra and Java. During this interval south Sundaland was a mainly passive margin with some strike-slip deformation and extension. At 45 Ma Australia began to move north and subduction resumed beneath Indonesia. This was a time of major changes in lengths of subduction boundaries which may be of global importance. Subduction has continued to the present. The structure of the now-subducted ocean floor south of Indonesia, and the rifted NW Australian margin, subsequently influenced the Cenozoic development of SE Asia.

Hall, R.

2008-12-01

166

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

167

Biogenic Sedimentation in the eastern equatorial Pacific, 0-18 Ma: XRF scanning on Site U1338, IODP Expedition 320/321  

Science.gov (United States)

The equatorial Pacific is a pelagic region with high primary productivity (~200 gC/m2/yr), the bulk of whose sediments are the remains of plankton. The equatorial Pacific is thus an important region to study how productivity and the carbon cycle evolved through the Cenozoic. We have XRF-scanned the 0-18 Ma sediment sequence from Site U1338, Pacific Equatorial Age Transect (PEAT, IODP exp 320/321). Site U1338 is now located at 230N, 117 58W. Ocean crust beneath Site U1338 was formed at the East Pacific Rise at a latitude of about 1S and has been carried to the northwest by Pacific plate movements. Site U1338 crossed the equator at about 12.5 Ma and left the equatorial zone (within 2 of the equator) at about 3 Ma. It thus records sedimentation within the equatorial zone between 18 Ma and 3 Ma. We were able to core and reassemble an essentially complete sedimentary section for the 0-18 Ma time interval, with an average sedimentation rate of 22.5 m/m.y., ranging between 11 and 37 m/m.y. The XRF-scan chemical information (Al, Si, K, Ca, Ti, Fe, Mn, Ba) defines two major dissolution intervals between 3-4.5 Ma and 9.5-11 Ma, marked by multiple events with low CaCO3 and high clay-associated elements. These carbonate dissolution events have been traced throughout much of the eastern equatorial Pacific in other drillsites with a Miocene carbonate section. Poorer preservation of both CaCO3 and bio-SiO2 can also be found in the top of the U1338 sedimentary section, 3000 km, and thus exhibit both the coherent response and huge length scales of the equatorial Pacific productivity zone.

Lyle, M. W.; Olivarez Lyle, A.; Baldauf, J.; Backman, J.; Shipboard Scientific Party, E.

2010-12-01

168

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

Science.gov (United States)

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

Akinin, V. V.

2013-12-01

169

Crustal Thickness and Oceanic Lithosphere Distribution in the Eastern Mediterranean from Satellite Gravity Anomaly Inversion  

Science.gov (United States)

The distribution of oceanic and continental lithosphere in the eastern Mediterranean is not well understood. Gravity inversion, incorporating a lithosphere thermal gravity anomaly correction, has been used to map Moho depth, crustal thickness and continental lithosphere thinning factor for the eastern Mediterranean in order to determine the distribution of oceanic and continental lithosphere and the ocean-continent transition location. Data used in the gravity inversion are bathymetry, free-air gravity and sediment thickness data from Smith and Sandwell (1997), Sandwell and Smith (2009) and Laske and Masters (1997) respectively. Moho depths from the gravity inversion are dependent on the age of oceanic lithosphere and continental breakup because of the lithosphere thermal gravity correction; however, these ages are uncertain for the eastern Mediterranean. Gravity inversion sensitivities to break-up ages of 225Ma (late Triassic) and 100Ma (early Cretaceous) have been examined. Gravity inversion results show thin crust (5 - 10km thickness) for the Ionian Sea and the Herodotus Basin of the eastern Mediterranean consistent with these basins being underlain by oceanic or highly thinned continental crust. Predicted Moho depths from the gravity inversion are in agreement with published Ionian Sea ESP results (Voogd et al, 1992) and suggest a gravity inversion reference Moho depth increasing to the north, which we attribute to subduction dynamic subsidence. Calibration of gravity inversion Moho against ESP results show a trade-off between break-up age and reference Moho depth; a Cretaceous age ocean requires a larger Moho reference depth than a Triassic age ocean. Lithosphere thinning factor maps from gravity inversion for Africa do not show continuity between the Cretaceous African rift system (Benue Trough, Chad, CASZ and Sudan basins) and eastern Mediterranean basins. If the Ionian Sea is of Cretaceous age then it more probably links to Cretaceous rifting and sea-floor spreading to its north and north-west.

Cowie, L.; Kusznir, N. J.

2010-12-01

170

Microenvironments and microscale productivity of cyanobacterial desert crusts  

Science.gov (United States)

We used microsensors to characterize physicochemical microenvironments and photosynthesis occurring immediately after water saturation in two desert soil crusts from southeastern Utah, which were formed by the cyanobacteria Microcoleus vaginatus Gomont, Nostoc spp., and Scytonema sp. The light fields within the crusts presented steep vertical gradients in magnitude and spectral composition. Near-surface light-trapping zones were formed due to the scattering nature of the sand particles, but strong light attenuation resulted in euphotic zones only ca. 1 mm deep, which were progressively enriched in longer wavelengths with depth. Rates of gross photosynthesis (3.4a??9.4 mmol O2A?ma??2A?ha??1) and dark respiration (0.81a??3.1 mmol Oa??2A?ma??2A?ha??1) occurring within 1 to several mm from the surface were high enough to drive the formation of marked oxygen microenvironments that ranged from oxygen supersaturation to anoxia. The photosynthetic activity also resulted in localized pH values in excess of 10, 2a??3 units above the soil pH. Differences in metabolic parameters and community structure between two types of crusts were consistent with a successional pattern, which could be partially explained on the basis of the microenvironments. We discuss the significance of high metabolic rates and the formation of microenvironments for the ecology of desert crusts, as well as the advantages and limitations of microsensor-based methods for crust investigation.

Garcia-Pichel, F.; Belnap, Jayne

1996-01-01

171

Nature and evolution of the lower crust in the eastern North China craton: A review  

Science.gov (United States)

In this paper, published data for granulite terrain rocks exposed at the surface, lower crustal xenoliths, and Mesozoic intermediate-felsic igneous rocks from the eastern North China craton (NCC) are integrated to constrain the nature and evolution of the lower crust in this area. U-Pb zircon dating shows that the protolith ages for most of the granulite terrain rocks are 2500 to 2600 Ma and that many of them experienced 1800-1900 Ma metamorphism. Lower crustal xenoliths entrained in volcanic rocks with ages varying from ~ 460 to ~ 10 Ma suggest that the lower crust is dominated by Neoarchean rocks, although there may be minor rocks with ages of Meso- to Paleoarchean (> 3000 Ma), ~ 45 Ma and possibly ~ 1900 Ma locally. The Mesozoic intrusive rocks, although varying from diorite to granite and spanning from Triassic to Cretaceous, contain ~ 2500 Ma inherited zircons and have magmatic zircons with Hf crust model ages (TDMHf, C) ages of 2500-2700 Ma and whole-rock Sr-Nd isotopic compositions falling within the field of the granulite terrain rocks, pointing to their derivation by the melting of Neoarchean lower crust. The combined data for the granulite terrain rocks, lower crustal xenoliths and Mesozoic intermediate-felsic igneous rocks indicate that the present lower crust is dominated by rocks with Neoarchean ages and is intermediate to mafic in composition (i.e., SiO2 < 62%). The (87Sr/86Sr)i, ?Nd (t) and ?Hf (t) of the lower crust at 130 Ma are considered to be 0.705 to 0.716, - 10 to - 28 and - 13 to - 28, respectively. The ?Nd (t) range is very different from that proposed previously (- 32 to - 44). The large range of ?Hf (t) for the lower crust implies that significant ?Hf (t) variations for magmatic zircons from the Mesozoic intermediate-felsic igneous rocks do not necessarily reflect mixing of mantle- and crustal-magmas as commonly thought, instead they may reflect heterogeneity in the ancient lower crust. Given that the voluminous Mesozoic intermediate to felsic igneous rocks in the eastern NCC are derived dominantly by partial melting of the Archean lower crust, it requires that a large amount of Archean lower crust be restitic. A restite origin can explain some of the Hannuoba granulite xenoliths having higher Mg# than the granulite terrains. It may be applicable to other parts of the world.

Jiang, Neng; Guo, Jinghui; Chang, Guohu

172

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 4177 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 4136 Ma to 4427 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 2645 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

173

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

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Preservation of ultrahigh-pressure (UHP) minerals formed at depths of 90125 km require unusual conditions. Our subduction model involves underflow of a salient (250 150 km wide, 90125 km long) of continental crust embedded in cold, largely oceanic crust-capped lithosphere; loss of leading portions of the high-density oceanic lithosphere by slab break-off, as increasing volumes of microcontinental material enter the subduction zone; buoyancy-driven return toward midcrustal levels of a...

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

1997-01-01

174

The nature of the crust under Cayman Trough from gravity  

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Considerable crustal thickness variations are inferred along Cayman Trough, a slow-spreading ocean basin in the Caribbean Sea, from modeling of the gravity field. The crust to a distance of 50 km from the spreading center is only 2-3 km thick in agreement with dredge and dive results. Crustal thickness increases to ???5.5 km at distances between 100 and 430 km west of the spreading center and to 3.5-6 km at distances between 60 and 370 km east of the spreading center. The increase in thickness is interpreted to represent serpentinization of the uppermost mantle lithosphere, rather than a true increase in the volume of accreted ocean crust. Serpentinized peridotite rocks have indeed been dredged from the base of escarpments of oceanic crust rocks in Cayman Trough. Laboratory-measured density and P-wave speed of peridotite with 40-50% serpentine are similar to the observed speed in published refraction results and to the inferred density from the model. Crustal thickness gradually increases to 7-8 at the far ends of the trough partially in areas where sea floor magnetic anomalies were identified. Basement depth becomes gradually shallower starting 250 km west of the rise and 340 km east of the rise, in contrast to the predicted trend of increasing depth to basement from cooling models of the oceanic lithosphere. The gradual increase in apparent crustal thickness and the shallowing trend of basement depth are interpreted to indicate that the deep distal parts of Cayman Trough are underlain by highly attenuated crust, not by a continuously accreted oceanic crust. Published by Elsevier Science Ltd.

ten, Brink, U. S.; Coleman, D. F.; Dillon, William, P.

2002-01-01

175

Breakup of Australia and Antarctica estimated as mid-Cretaceous (95 5 Ma) from magnetic and seismic data at the continental margin  

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A positive magnetic anomaly marks the seaward edge of the magnetic quiet zone along the southern margin of Australia eastward between 114 and 131E and along the conjugate Antarctic margin between 105 and 132E. This anomaly was originally interpreted as the oldest seafloor-spreading anomalyA22, revised by Cande and Mutter to A34in the Southeast Indian Ocean, but is better modelled as the edge effect at the continent-ocean boundary (COB) constrained by seismic data. Continental crust abuts the oceanic sequence of normal and reversed spreading blocks, truncated within the Cretaceous normal interval at an extrapolated age of 96 Ma, rounded to 95 5 Ma to take into account the uncertainty of the initial spreading rate and of the location of the COB. The occurrence of the anomaly on both margins defines this as the age of breakup. Farther east between 131 and 139E on the Australian margin, the COB anomaly is modelled as due to the same kind of effect but with successively younger ages of truncation to 49 Ma, interpreted as indicating the most recent ridge-crest jumping to the Australian COB. The magnetic data from the conjugate sector of Antarctica, albeit scanty, are consistent with this interpretation. The 95 5 Ma age of breakup coincides with that of the breakup unconformity in southern Australia, expressed by a short mid-Cretaceous lacuna in the Otway Basin between faulted Early Cretaceous rift-valley sediments of the Otway Group and the overlying Late Cretaceous Sherbrook Group, and by an unconformity of similar age in the Great Australian Bight Basin.

Veevers, J. J.

1986-02-01

176

The crust of neutron stars  

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

Chamel, Nicolas

2006-01-01

177

Diverse Manifestations of Convective Upwelling Beneath the North Atlantic Ocean  

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

White, Nicky; Parnell-Turner, Ross

2013-04-01

178

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

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

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

2012-12-01

179

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

Science.gov (United States)

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

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

2013-04-01

180

ConcepTest: Oceans by South America #6  

Science.gov (United States)

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

 
 
 
 
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.

2012-01-01

182

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

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

Guillermo E. Alvarado

2013-08-01

183

Lead isotopic evolution of Archean continental crust, Northern Tanzania  

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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 > 600C [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

184

Nonvolcanic tectonic islands in ancient and modern oceans  

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Most oceanic islands are due to excess volcanism caused by thermal and/or compositional mantle melting anomalies. We call attention here to another class of oceanic islands, due not to volcanism but to vertical motions of blocks of oceanic lithosphere related to transform tectonics. Sunken tectonic islands capped by carbonate platforms have been previously identified along the Vema and Romanche transforms in the equatorial Atlantic. We reprocessed seismic reflection lines, did new facies analyses and 87Sr/86Sr dating of carbonate samples from the carbonate platforms. A 50 km long narrow paleoisland flanking the Vema transform, underwent subsidence, erosion, and truncation at sea level; it was then capped by a 500 m thick carbonate platform dated by 87Sr/86Sr at 11-10 Ma. Three former islands on the crest of the Romanche transverse ridge are now at 900 m bsl; they show horizontal truncated surfaces of oceanic crust capped by 300 m thick carbonate platforms, with 10-6 Ma Sr isotopic ages. These sunken islands formed due to vertical tectonics related to transtension/transpression along long-offset slow-slip transforms. Another tectonic sunken island is Atlantis Bank, an uplifted gabbroic block along the Atlantis II transform (SW Indian Ridge) 700 m bsl. A modern tectonic island is St. Peter and St. Paul Rocks, a rising slab of upper mantle located at the St. Paul transform (equatorial Atlantic). "Cold" tectonic islands contrast with "hot" volcanic islands related to mantle thermal and/or compositional anomalies along accretionary boundaries and within oceanic plates, or to supra-subduction mantle melting that gives rise to islands arcs.

Palmiotto, Camilla; Corda, Laura; Ligi, Marco; Cipriani, Anna; Dick, Henry J. B.; Douville, Eric; Gasperini, Luca; Montagna, Paolo; Thil, Franois; Borsetti, Anna Maria; Balestra, Barbara; Bonatti, Enrico

2013-10-01

185

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

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

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

2012-04-01

186

Crust Formation in Aluminum Cells  

Science.gov (United States)

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

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

1985-11-01

187

Deep inside a neoproterozoic intra-oceanic arc: growth, differentiation and exhumation of the Amalaoulaou complex (Gourma, Mali)  

Science.gov (United States)

We show here that the Amalaoulaou complex, in the Pan-African belt of West Africa (Gourma, Mali), corresponds to the lower and middle sections of a Neoproterozoic intra-oceanic arc. This complex records a 90-130-Ma-long evolution of magmatic inputs and differentiation above a subducting oceanic slab. Early c. 793 Ma-old metagabbros crystallised at lower crustal or uppermost mantle depths (25-30 km) and have geochemical characteristic of high-alumina basalts extracted from a depleted mantle source slightly enriched by slab-derived sedimentary components ((La/Sm)N leucosomes with garnet-clinopyroxene-rutile residues. Slightly after the granulitic event, the arc root was subject to strong HT shearing during partial exhumation (detachment faults/rifting or thrusting), coeval with the emplacement of spinel- and garnet-pyroxenite dykes crystallised from a high-Mg andesitic parental magma. Quartz and hornblende-gabbros (700-660 Ma) with composition typical of hydrous volcanic rocks from mature arcs ((La/Sm)N: 0.9-1.8; ?Nd: +4.6 to +5.2; 87Sr/86Sr: 0.7028-0.7031) were subsequently emplaced at mid-arc crust levels (~15 km). Trace element and isotopic data indicate that magmas tapped a depleted mantle source significantly more enriched in oceanic sedimentary components (0.2%). Exhumation occurred either in two stages (700-660 and 623 Ma) or in one stage (623 Ma) with a final exhumation of the arc root along cold P-T path (550C, 6-9 kbar; epidote-amphibolite and greenschist facies conditions) during the main Pan-African collision event (620-580 Ma). The composition of magmas forming the Cryogenian Amalaoulaou arc and the processes leading to intra-arc differentiation are strikingly comparable to those observed in the deep section of exposed Mezosoic oceanic arcs, namely the Kohistan and Talkeetna complex. This evolution of the Amalaoulaou oceanic arc and its accretion towards the West African craton belong to the life and closure of the Pharusian Ocean that eventually led to the formation of the Greater Gondwana supercontinent, a similar story having occurred on the other side of the Sahara with the Mozambique Ocean.

Berger, Julien; Caby, Renaud; Ligeois, Jean-Paul; Mercier, Jean-Claude C.; Demaiffe, Daniel

2011-10-01

188

U-Pb Zircon geochronology, Hf isotope and trace element geochemistry of a unique lower crustal - upper mantle section of a dying slow-spreading mid-ocean ridge (Macquarie Island, Southern Ocean)  

Science.gov (United States)

Macquarie Island, located in the Southern Ocean, is a section of oceanic crust formed at the now extinct proto-Macquarie, slow-spreading, mid-ocean ridge. The northernmost part of Macquarie Island is composed primarily of lower crustal gabbro and upper mantle peridotite. The mantle sequence consists of harzburgite intruded by a variety of gabbro dikes/dikelettes and zircon and phlogopite-bearing veins. Here, we report integrated Pb/U zircon ages and Hf isotopic and trace element data from six samples of the lower crust-upper mantle sequence. Samples consist of two lower crustal gabbros, and three gabbro dikes/dikelettes and one phlogopite-bearing vein from the upper mantle sequence. 206Pb/238U SHRIMP-RG zircon ages for the gabbros range from 8.7 0.3 Ma to 9.0 0.2 Ma, whereas gabbro dikes/dikelettes yield overlapping ages of 8.7 0.2 Ma, 8.7 0.3 Ma, and 8.9 0.2 Ma (all errors 2s). The phlogopitic vein yielded a slightly younger age of 8.5 0.1 Ma. Initial epsilon Hf results for zircons from the same samples show a broad distribution ranging from +9.5 to +13.3 for the lower crustal gabbro (N=28), +7.0 to +16.4 for the gabbro dikes/diklettes (N=24), and +8.4 to +12.2 for the phlogopite vein (N=12). The wide range in values (particularly from the gabbro dikes/diklettes) is consistent with a heterogeneous source region composed of a range of relatively long-lived depleted- and enriched mantle sources. Zircon trace element concentrations also support a heterogeneous source, displaying enrichment in U/Yb relative to N-MORB zircons from the Mid-Atlantic and Southwest Indian Ridge systems. We interpret these results to indicate that magmatic construction of Macquarie Island occurred from 8.7 to 9.0 Ma and involved sampling of at least two distinct mantle sources.

Jeffcoat, C. R.; Schwartz, J. J.; Wooden, J. L.; Mueller, P. A.; Saal, A. E.

2011-12-01

189

Precambrrian continental crust evolution of southeastern Sao Paulo state-Brazil: based on isotopico evidences  

International Nuclear Information System (INIS)

The focussed area comprises five major different tectonic terranes separated by faults, which are named Alto Rio Grande Belt, Socorro-Guaxupe Nappe, Sao Roque, Embu and Costeiro Domains. The geological and geochronological history of these terranes show that the metamorphic episodes of crust-forming occurred involving both mantle-derived magmas and reworking of continental material since 3.4 Ga until 600 Ma. The post-tectonic granitic activities occurred within 1000-500 Ma range and in general, the rocks are progressively younger from the Socorro-Guaxupe Nappe (1000-850 Ma) in the NW towards the Costeiro Domain (550 Ma) in the SE. The Sr and Pb isotopic evidences, together with geological and geophysical informations, suggest that the proportions of the rock-forming processes through the geological time are: Archean, 10%; Lower Proterozoic, 10%; Middle Proterozoic, 38%; Late Proterozaic, 42%. Although the Mid and Late Proterozoic time were a period of a large amount of rocks were formed, they were not a major crustforming period, because these rocks are mainly constituted by recycled continental crust material. In our view, at end of the Early Proterozoic time, at least 85% of continetal crust, in this area, has accreted and differentiate. During the Middle and Late Proterozoic the continental crust grew at small rate. (author)

1988-01-01

190

Complexity, depth, and rapidity of processes that formed the lunar crust  

International Nuclear Information System (INIS)

From its birth the moon had a large-scale, complex magma system. The evidence is the massive differentiation of the moon that was partially preserved. The system might be a magma ocean or a magmasphere; even in the former case it was superposed by smaller but also very complex magma systems. The main episode produced a plagioclase-rich crust including genuine anorthosites; it was over by about 4.35 b.y. ago, although magmatism continues. The processes of crust-building remain in serious dispute. Lunar crusts; massive differentiation; crustal material composition; processes which produced crustal materials; and differences from earth are briefly discussed

1987-07-13

191

Project IBM: Towards a Comprehensive Understanding of Arc Evolution and Continental Crust Formation  

Science.gov (United States)

A complex drilling project proposal (Project IBM), including non-riser/riser IODP expeditions and petrological/geochemical/geophysical studies, is designed to achieve one major goal of the IODP Initial Science Plan, i.e., a comprehensive understanding of intra-oceanic arc evolution and continental crust formation. We focus on the Izu-Bonin-Mariana (IBM) arc-trench system, which is undoubtedly one of the best- surveyed intra-oceanic arcs and, more importantly, is characterized by the presence of 6.0-6.3 km/s Vp middle crust possibly having intermediate compositions similar to the average continental crust. We propose four linked thematic operations of IBM complex drilling to sample volcaniclastic sediments and upper and middle crustal rocks: Expedition 1 -Characterization of pre-arc crust and mantle, decoding the nature of pre-existing crust/mantle and understanding the tectonic setting of IBM arc formation by drilling at the Amami Sankaku Basin, the eastern margin of the West Philippine Basin; Expedition 2 Characterization of initial IBM magmatism, extending our understanding of the nature and tectonic setting of initial IBM arc magmatism and crust formation by deepening ODP Site 786B; Expedition 3 -Documentation of temporal variation of rear-arc magmatism, investigating volcaniclastic sediments for a temporal record of rear-arc magmatism, which may be distinct from that along the volcanic front, by drilling at the site located between two rear-arc volcanic chains; and Expedition 4 -Ultra-deep drilling to the middle crust, documenting the characteristics of the entire IBM upper and middle crust and understanding the formation process of distinctive middle crust and its role in continental crust formation and solid Earth evolution by riser-drilling at ODP Site 792. These drilling experiments, together with non-drilling elements including extensive geophysical and petrological efforts, should provide a landmark not only in IODP research, but also in the study on Earth evolution.

Tatsumi, Y.; Stern, R.

2006-12-01

192

Crust and Mantle Structure Beneath the Samoan Islands  

Science.gov (United States)

We used teleseismic receiver functions to map the seismic structure under the Samoan Islands in the southern Pacific Ocean. We acquired seismograms for the permanent seismic station, AFI, and for five temporary stations located across the island chain from the Samoan Lithospheric Integrated Seismic Experiment (SLISE). We used multiple-taper correlation and Markov chain Monte Carlo algorithms to calculate receiver functions for events with epicentral distance of 30 to 95 and examined the results in a frequency range of 1.0 - 5.0 Hz for crustal structure and 0.1 - 2.0 Hz for mantle structure. We identify complex crustal layering, including the interface between volcanic rocks and the ocean crust and a substantial underplated layer beneath the normal ocean crust. We find that the crust thins with decreasing age across the Samoan Islands and correlates with previous observations from gravity data (Workman, 2005). We additionally identify a velocity increase in the range of 50-100 km depth, potentially the Hales discontinuity. Deeper in the mantle, we observe transition zone thickness of 245-250 km across the island chain, which is within the margin of error for globally observed transition zone thickness. When migrated with IASP, transition zone discontinuity depths do appear deeper beneath the youngest island, indicating slower velocities and/or deeper discontinuity depths relative to the older islands in the system. We will provide improved constraints on transition zone discontinuity depths from ScS reverberations for all stations, and will place the crust and mantle results into a multi-disciplinary context, with comparisons to geochemical and surface observations. Workman, R., 2005. Geochemical characterization of endmember mantle components, Doctoral dissertation, Massachusetts Institute of Technology, http://dspace.mit/edu/handle/1721.1/33721.

Browning, J. M.; Courtier, A. M.; Jackson, M. G.; Lekic, V.; Hart, S. R.; Collins, J. A.

2013-12-01

193

Re-melting of Vesta's crust with 26Al?  

Science.gov (United States)

Eucrites are pigeonite-plagioclase basalts and gabbros, most probably from the asteroid 4-Vesta. Along with diogenites and howardites, these rocks show evidence for a complex thermal history of the HED parent body, with relatively rapid emplacement of basaltic crust, extensive crustal metamorphism, possible re-melting, and maybe even serial magmatism involving Mg-rich cumulates. Understanding these complexities requires identification of appropriate heat sources, and modeling of associated thermal structures as a function of time and position within the parent body. Geochemical analyses of various chondrite and achondrite meteorite groups are consistent with the idea that the principal energy source driving differentiation in the early solar system is the short-lived isotope 26Al. Within the framework of this scenario, basaltic eucrites are liquids produced directly or indirectly by the melting of chondritic precursors. In detail, REE contents of primitive basaltic eucrites are consistent with two endmember scenarios: either such samples are the result of ~20% direct partial melting, or they represent the liquid produced by 80% crystallization of a global magma ocean. Each of these scenarios puts a constraint on the latest possible time of accretion relative to condensation of calcium-aluminum inclusions (CAI) for which the 26Al /27Al is considered known. For example, to completely melt Vesta (magma ocean) accretion must have occurred less 1.4 Myr after CAI condensation, whereas, if only 20% melting occurred, accretion could have taken place as late as 1.9 Myr after CAI condensation. While several studies have considered these global-scale considerations, it is less widely appreciated that once plagioclase has been eliminated from the mantle residue (at ~20% partial melting) almost all available aluminum will be concentrated in the crust. Depending on exactly when the crust is emplaced, there is thus a possibility that sufficient 26Al will remain to reheat and possibly re-melt the crust, and maybe even reheat underlying magma-ocean cumulates that are Al-free. This situation has been numerically modeled for a Vesta sized body of H-chondrite bulk composition, beginning with an endmember set of parameters that represent the pessimistic situation of emplacement of a 'cold' crust (290K) that is ~15km thick. The results of these models show that if the crust is emplaced less than 5.5 Myr after CAI condensation, the lower crust may be completely re-melted and underlying cumulates partially molten by the available 26Al. On the other hand, for crust emplacement later than ~6 Myr after CAI condensation, no partial melting of either the crust or the mantle is predicted for this scenario. More realistic scenarios for thermal profiles at the time of crust emplacement are being investigated to constrain the time and depth windows over which partial melting of HED lithologies may occur.

Mizzon, H.; Monnereau, M.; Toplis, M. J.; Barrat, J. A.; Forni, O.

2012-04-01

194

Differentiation of Vesta: Implications for a shallow magma ocean  

Science.gov (United States)

The Dawn mission confirms earlier predictions that the asteroid 4 Vesta is differentiated with an iron-rich core, a silicate mantle and a basaltic crust, and supports the conjecture of Vesta being the parent body of the HED meteorites. To better understand its early evolution, we perform numerical calculations of the thermo-chemical evolution adopting new data obtained by the Dawn mission such as mass, bulk density and size of the asteroid. We have expanded the thermo-chemical evolution model of Neumann et al. (2012) that includes accretion, compaction, melting and the associated changes of the material properties and the partitioning of incompatible elements such as the radioactive heat sources, advective heat transport, and differentiation by porous flow, to further consider convection and the associated effective cooling in a potential magma ocean. Depending on the melt fraction, the heat transport by melt segregation is modelled either by assuming melt flow in a porous medium or by simulating vigorous convection and heat flux of a magma ocean with a high effective thermal conductivity. Our results show that partitioning of 26Al and its transport with the silicate melt is crucial for the formation of a global and deep magma ocean. Due to the enrichment of 26Al in the liquid phase and its accumulation in the sub-surface (for formation times t0approximately 100 Ma, supporting the idea of an early magnetic field to explain the remnant magnetisation observed in some HED meteorites.

Neumann, Wladimir; Breuer, Doris; Spohn, Tilman

2014-06-01

195

Cyanobacteria and the Cryptobiotic Crust  

Science.gov (United States)

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

Deacon, J. W.

2010-02-25

196

Ma - UK Energy Research Centre  

Genomics. +. Traits. Pre-commercial Trials,. Production and Sales. (1 year). \\Finding the. Winners ... Disease Resistant and High Yielding. Types Identified in \\Brazil. New disease tolerant very sweet ... Highly correlated with biomass trait. \\Ma. X...

197

Oceanic-type accretion may begin before complete continental break-up  

Science.gov (United States)

Oceanic accretion is thought to be the process of oceanic crust (and lithosphere) edification through adiabatic melting of shallow convecting mantle at oceanic spreading ridges. It is usually considered as a post-breakup diagnostic process following continents rupturing. However, this is not always correct. The structure of volcanic passive margins (representing more than 50% of passive continental margins) outlines that the continental lithosphere is progressively changed into oceanic-type lithosphere during the stage of continental extension. This is clear at least, at crustal level. The continental crust is 'changed' from the earliest stages of extension into a typical -however thicker- oceanic crust with the typical oceanic magmatic layers (from top to bottom: lava flows/tuffs, sheeted dyke complexes, dominantly (sill-like) mafic intrusions in the lower crust). The Q-rich continental crust is highly extended and increases in volume (due to the magma) during the extensional process. At the continent-ocean transition there is, finally, no seismic difference between this highly transformed continental crust and the oceanic crust. Using a large range of data (including deep seismic reflection profiles), we discuss the mantle mechanisms that governs the process of mantle-assisted continental extension. We outline the large similarity between those mantle processes and those acting at purely-oceanic spreading axis and discuss the effects of the inherited continental lithosphere in the pattern of new mafic crust edification.

Geoffroy, L.; Zalan, P. V.; Viana, A. R.

2011-12-01

198

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

Science.gov (United States)

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

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

2013-12-01

199

Geochemical evolution of Ngorongoro Caldera, Northern Tanzania: Implications for crust magma interaction  

Science.gov (United States)

Ngorongoro Caldera is the largest and best-preserved of nine Plio-Pleistocene volcanoes that make-up the Ngorongoro Volcanic Highlands (NVH) complex situated at the southern bifurcation of Gregory Rift, part of the East African Rift system of northern Tanzania. We report here, major and trace element abundances, Sr-Nd-Pb isotope analyses and 40Ar/ 39Ar laser incremental-heating and total fusion ages on lava and tephra sampled from stratigraphic sections exposed within the Ngorongoro Caldera. Major and trace elements measured on samples collected from the Ngorongoro Caldera wall indicate a stratified magma chamber whose silicic top and basaltic bottom was inverted by sequential eruptions. Samples from the lower part of the exposed Ngorongoro Caldera wall are high in silica, alkalis and HFSE (High Field Strength Elements). The Zr, Nb and Hf concentrations are highly correlated with each other and decrease up-section, indicative of the extent of magma evolution. Modeling of major, trace as well as Sr, Nd and Pb isotope data suggests that assimilation fractional crystallization processes were essential in producing the observed geochemical variations. The Sr and Nd isotope ratios from the Ngorongoro samples are widely dispersed ( 87Sr/ 86Sr = 0.70405 to 0.70801, 143Nd/ 144Nd = 0.512205 to 0.512623) and Pb isotope ratios are consistent with previous studies: 206Pb/ 204Pb = 18.73 to 19.37, 207Pb/ 204Pb = 15.64 to 15.69, 208Pb/ 204Pb = 39.52 to 39.55. Although Sr isotopic ratios are similar to Oceanic Island Basalt (OIB), the more radiogenic samples ( 87Sr/ 86Sr > 0.705) from the lower part of the section suggest crust-magma interaction; this is supported by Ce/Pb ratios (< 25 5) which are lower than those expected from primary mantle derived fluids. Moderate normalized Tb/Yb ratios (2.7-2.1) of these samples suggest lithospheric mantle melting, a finding consistent with other studies in the region. Laser incremental-heating of basalt samples and single grain total fusion of anorthoclase from tephra samples collected from the Ngorongoro Caldera wall section yield 40Ar/ 39Ar ages of 2.25 0.02 Ma to 2.01 0.02 Ma, constraining a duration of volcanism of the order of ~ 240 kyr. These ages suggest correlation of a normal to reverse geomagnetic polarity transition measured in the Ngorongoro Caldera wall section with the ~ 2.14 Ma C2r.1n-1r (normal to reverse) Runion-Matuyama boundary.

Mollel, Godwin F.; Swisher, Carl C.; Feigenson, Mark D.; Carr, Michael J.

2008-07-01

200

Seasonal Methane Oxidation Potential in Manure Crusts  

Digital Repository Infrastructure Vision for European Research (DRIVER)

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

 
 
 
 
201

Crust formation in cryolite based baths.  

Science.gov (United States)

In this work crust samples were made in the laboratory. In the experimental part of this work it is shown that the crust formation is a heat transfer controlled process. Hence, the rate of penetration of bath into the loose alumina during crust formation ...

K. Rye

1992-01-01

202

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 (Tepl-Barrandian and Moldanubian domains). The deep burial of this mostly refractory felsic metaigneous material is evidenced by the presence of coesite/diamond (Massonne 2001; Kotkov 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 (Vrna 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. Kotkov, 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. Vrna, S. 2013. Journal of Geosciences 58, 347-378. Zheng, Y. F. 2012. Chemical Geology 328, 5-48.

Janouek, Vojt?ch; Schulmann, Karel; Lexa, Ondrej; Holub, Frantiek; Fran?k, Jan; Vrna, Stanislav

2014-05-01

203

Earthquakes in stable continental crust  

Energy Technology Data Exchange (ETDEWEB)

Earthquakes can strike even in stable crust, well away from the familiar earthquake zones at the edges of tectonic plates, but their mere occurrence is both a source of concern in planning critical facilities such as nuclear power plants. The authors sought answers to two major questions: Just how much seismic activity does take place within the stable parts of continents And are there specific geologic features that make some areas of stable crust particularly susceptible to earthquakes They began by studying North America alone, but it soon became clear that the fairly short record of these rare events on a single continent would not provide enough data for reliable analysis. Hence, they decided to substitute space for time--to survey earthquake frequency and distribution in stable continental areas worldwide. This paper discusses their findings.

Johnson, A.C.; Kanter, L.R. (Memphis State Univ., TN (USA))

1990-03-01

204

Intra-Oceanic Subduction Systems: Tectonic and Magmatic Processes  

Science.gov (United States)

Subduction zones are locations where oceanic lithosphere, capped with altered crust and sediments, is recycled into Earth's interior. Dehydration of the subducted slab causes the generation of arc magmas with unique geochemical characteristics. The total length of active margins is about 43,500 km, and most of this length is situated on continental crust and marked by spectacular volcanoes as in the Andes and Japan. Some 17,000 km (~40%) of volcanic arcs, are situated on oceanic crust. These intra-oceanic arcs are much less studied than their continental counterparts. The reason is that they are located in some of the world's most remote regions and their volcanoes are typically submerged below sea level. Because intra-oceanic arcs are situated on thin, mafic oceanic crust, contamination of magmas by high silica crust normally does not occur, allowing for a more straightforward investigation of mantle processes than in continental arcs. Intra-oceanic arcs are also thought to represent locations where basaltic crust is modified to form continental crust, a still poorly understood process.

Fischer, Tobias

2004-10-01

205

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

Science.gov (United States)

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

Stein, Carol A.; Stein, Seth

1994-01-01

206

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

Science.gov (United States)

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

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

2008-01-01

207

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

208

Accretion tectionics of the Japanese islands and evolution of continental crust  

Science.gov (United States)

Revised interpretation of the basement geology of the Japanese island arcs which has emerged in the last 20 years or so, indicates that they are mostly composed of two geological belts: volcanics (greenstone)-granitoid belt (VGB) and turbidite-granitoid belt (TGB). The VGB exhibits thrust-bounded thick sequences of basic volcanics and associated granitoid plutons and was formed by arc-arc collision process. The TGB is composed of turbidite and melange units and was formed by progressive growth of trench accretionary prism and later intrusion of granitoids. Both belts represent a style of crustal growth in a convergent margin. In these orogenic belts, involvement ot many oceanic island arcs and formation of the VGB were a major mechanism of juvenile crust, in addition to the continental crust. The formation of the TGB played a major role in the reworking and recycling of the continental crust which resulted in a long-term secular compositional change of the upper crust.

Taira, Asahiko; Kiyokawa, Shoichi; Aoike, Kan; Saito, Saneatue

1997-10-01

209

Evolution of the earth's crust in regions of active continental riftogenesis  

Energy Technology Data Exchange (ETDEWEB)

Destruction of the continental crust whose extreme manifestation is leading to the formation of an oceanic type crust is taking place in the regions of active continental riftogenesis of the Afar type such as the articulation zones of the Red Sea, Aden, and Ethiopian rifts. The destruction is found to occur through the crushing, stretching, and saturation of continental crust by base and ultra-base rock of mantle origin. The similarity between the evolution of volcanism - the basic indicator of lithospheric plutonic dynamics - of the Afar and mobile oceanic structures, makes it possible to relate the singularity of the destructive process in the earth's history which results in a better understanding of the mechanism that underlies the formation of peripheral and interior seas as well as their analogs in the geological past. The specificity of Afar type structures is manifested in the scattered nature of the endogenic conditions. 42 references, 2 figures.

Razvalyaev, A.V.; Ponikarov, V.P.

1980-05-01

210

Scaling of X pinches from 1 MA to 6 MA  

International Nuclear Information System (INIS)

This final report for Project 117863 summarizes progress made toward understanding how X-pinch load designs scale to high currents. The X-pinch load geometry was conceived in 1982 as a method to study the formation and properties of bright x-ray spots in z-pinch plasmas. X-pinch plasmas driven by 0.2 MA currents were found to have source sizes of 1 micron, temperatures >1 keV, lifetimes of 10-100 ps, and densities >0.1 times solid density. These conditions are believed to result from the direct magnetic compression of matter. Physical models that capture the behavior of 0.2 MA X pinches predict more extreme parameters at currents >1 MA. This project developed load designs for up to 6 MA on the SATURN facility and attempted to measure the resulting plasma parameters. Source sizes of 5-8 microns were observed in some cases along with evidence for high temperatures (several keV) and short time durations (<500 ps).

2010-01-01

211

Scaling of X pinches from 1 MA to 6 MA.  

Energy Technology Data Exchange (ETDEWEB)

This final report for Project 117863 summarizes progress made toward understanding how X-pinch load designs scale to high currents. The X-pinch load geometry was conceived in 1982 as a method to study the formation and properties of bright x-ray spots in z-pinch plasmas. X-pinch plasmas driven by 0.2 MA currents were found to have source sizes of 1 micron, temperatures >1 keV, lifetimes of 10-100 ps, and densities >0.1 times solid density. These conditions are believed to result from the direct magnetic compression of matter. Physical models that capture the behavior of 0.2 MA X pinches predict more extreme parameters at currents >1 MA. This project developed load designs for up to 6 MA on the SATURN facility and attempted to measure the resulting plasma parameters. Source sizes of 5-8 microns were observed in some cases along with evidence for high temperatures (several keV) and short time durations (<500 ps).

Bland, Simon Nicholas (Imperial College, London, United Kingdom); McBride, Ryan D.; Wenger, David Franklin; Sinars, Daniel Brian; Chittenden, Jeremy Paul (imperial College, London, United Kingdom); Pikuz, Sergei A. (Cornell University, Ithaca, NY); Harding, Eric; Jennings, Christopher A.; Ampleford, David J.; Yu, Edmund P.; Cuneo, Michael Edward; Shelkovenko, Tatiana A. (Cornell University, Ithaca, NY); Hansen, Stephanie B.

2010-09-01

212

Early Paleozoic (ca. 465 Ma) eclogites from Beishan (NW China) and their bearing on the tectonic evolution of the southern Central Asian Orogenic Belt  

Science.gov (United States)

The Beishan orogen is situated in the southern margin of the Central Asian Orogenic Belt (CAOB). It is connected with the Tianshan orogen to the west and the Mongolia-Xing'anling orogen to the east. This orogen exhibits a well-preserved Neoproterozoic to Late Paleozoic sequence containing ophiolites, accretionary complexes and eclogites. In this paper we provide new geochronological data for eclogites and their country paragneiss to constrain the tectonic evolution of the Beishan orogen. U-Pb zircon dating of eclogites yielded two concordant ages, 886 4 Ma for the cores and 465 10 Ma for the rims. The zircon cores display bright luminescence with homogeneous or planar/sector zoning, intermediate Th/U ratios (0.17-0.47), and HREE-enriched patterns with negative Eu anomalies (Eu/Eu * = 0.16-0.58). These features, coupled with their formation temperatures of 680-710 C, suggest that the core age represents the time of an earlier upper amphibolite facies metamorphic episode. By contrast, the zircon rims have dark luminescence with homogeneous or fir-tree zoning, low Th/U ratios (12 kbar for the eclogite facies metamorphism. On the other hand, zircon dating of a paragneiss near the eclogite produced two major groups of ages, ca. 1450 Ma for a magmatic event and ca. 900 Ma for a metamorphic event. It seems that eclogites and their country rocks have suffered the same earlier metamorphic episode. Based on the available age data obtained for eclogites, ophiolites and granitoids from the Beishan area, a tectonic scenario can be constructed as follows. The ca. 1000 Ma oceanic crust representing by protoliths of the eclogites was involved in Early Neoproterozoic orogenesis to have formed a microcontinental block. During the Early Paleozoic, island arcs and microcontinental blocks were progressively accreted to the northern margin of the Tarim craton (Dunhuang block). This was followed by the closure of Paleoasian Ocean and final amalgamation between the Tarim craton and Kazakhstan block in the Late Paleozoic.

Liu, Xiaochun; Chen, Bolin; Jahn, Bor-ming; Wu, Ganguo; Liu, Yongsheng

2011-09-01

213

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

Science.gov (United States)

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

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

2013-12-01

214

Two contemporaneous magma series on Mayotte Island, Comores Archipelago, Indian Ocean  

Science.gov (United States)

The Comores archipelago is comprised of four islands all of which are characterised by alkaline lavas. On Mayotte, two clearly separate magma series can be defined. The origin of such contemporaneous but compositionally different series in several oceanic islands is still a subject of a debate. The mineralogy and geographically locations allow to identify three different lava groups. (1) The north lava group composed of relatively homogeneous alkaline basalts. (2) The north-east group comprised of tephrites to trachy-phonolites series and (3) the south lava group is made up of a nephelinite to phonolite series. Ol+cpx+plag are the main phenocryst phases in the north and north-east lavas, whereas the south lava series have ol+cpx+nepheline as phenocrysts. Composition of clinopyroxene is variable with diopside compositions in the north alkaline basalts and tephrites, and diopside to aegerine compositions in the south nephelinite lavas. A regular enrichment of Na content in plagioclases phenocrysts and groundmass crystals from the lavas of the north group to those from the north-east is observed. In contrast, alkali-feldspars are present in the south lavas. The crystallisation of clinopyroxene instead of plagioclase after olivine fractionation in the north lavas suggests that fractionation occurred at pressure between 0.4 and 0.9 GPa, which is consistent with the presence of Na-rich cpx-cores in all lava groups. The major element composition of lavas from Mayotte allow to define two distinct magma series: a moderately undersaturated and a highly undersaturated series. The moderately undersaturated series is composed of the north alkali basalt and the north-east tephrite lavas, whereas the south nephlinites represent the highly undersaturated series. Compilation of published age determinations and new Ar/Ar datings suggest that the north lavas erupted from 7.7 Ma to 4.4 Ma, followed by the north-east lavas erupted from 4.7 Ma to 1.4 Ma. The south lavas erupted contemporaneously from 7.7 Ma to 2.7 Ma. Basic volcanic activity resumed in the North between 2.9 Ma and 1.2 Ma, and from 2 Ma to 1.5 Ma in the South. Taken together, these preliminary petrological and geochemical results suggest that Mayotte island was constructed by two volcanoes. These volcanoes were active at the same time producing two distinct magma series. Migration of the activity of the northern volcano to the east occurred with emission of increasingly differentiated lavas with time. Depth of crystallisation can be evaluated at more than 15 km for alkaline basalt and tephrite lavas, which corresponds to the mantle-crust interface.

Debeuf, D.; Bachlery, P.; Sigmarsson, O.

2003-04-01

215

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

International Nuclear Information System (INIS)

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

1987-01-01

216

Ocean drilling surveys planned  

Science.gov (United States)

As a continuation of the International Phase of Ocean Drilling (IPOD), the Glomar Challenger is slated to drill in the Pacific and North Atlantic oceans during 1982-83. In preparation for the drilling, the Joint Oceanographic Institutions (JOI), Inc. will manage the site survey program during 1981-82. These site surveys will be focused to support four programs: a hydrogeology study on the equatorial East Pacific Rise flank; a study of Mesozoic sediments in the western Pacific; a study in sedimentation of the equatorial Pacific basin; and a study of the geochemistry of the North Atlantic ocean crust.JOI has issued a request for proposals for the United States site survey program. Proposal deadline is March 5. For additional information, contact JOI, Inc., 2600 Virginia Avenue, N.W., Suite 512, Washington, D.C. 20037.

217

The return of subducted continental crust in Samoan lavas  

Science.gov (United States)

Substantial quantities of terrigenous sediments are known to enter the mantle at subduction zones, but little is known about their fate in the mantle. Subducted sediment may be entrained in buoyantly upwelling plumes and returned to the Earth's surface at hotspots, but the proportion of recycled sediment in the mantle is small, and clear examples of recycled sediment in hotspot lavas are rare. Here we report remarkably enriched 87Sr/86Sr and 143Nd/144Nd isotope signatures in Samoan lavas from three dredge locations on the underwater flanks of Savai'i island, Western Samoa. The submarine Savai'i lavas represent the most extreme 87Sr/86Sr isotope compositions reported for ocean island basalts to date. The data are consistent with the presence of a recycled sediment component (with a composition similar to the upper continental crust) in the Samoan mantle. Trace-element data show affinities similar to those of the upper continental crust-including exceptionally low Ce/Pb and Nb/U ratios-that complement the enriched 87Sr/86Sr and 143Nd/144Nd isotope signatures. The geochemical evidence from these Samoan lavas significantly redefines the composition of the EM2 (enriched mantle 2; ref. 9) mantle endmember, and points to the presence of an ancient recycled upper continental crust component in the Samoan mantle plume.

Jackson, Matthew G.; Hart, Stanley R.; Koppers, Anthony A. P.; Staudigel, Hubert; Konter, Jasper; Blusztajn, Jerzy; Kurz, Mark; Russell, Jamie A.

2007-08-01

218

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

Science.gov (United States)

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

Neal, Clive R.

1988-01-01

219

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

International Nuclear Information System (INIS)

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

1987-07-13

220

Paleozoic to Triassic ocean opening and closure preserved in Central Iran: Constraints from the geochemistry of meta-igneous rocks of the Anarak area  

Science.gov (United States)

The Anarak area belongs to an ophiolitic belt along the northern border of the Central-East Iranian Microcontinent, and is thought to contain fragments of the former Paleotethys and Neotethys oceans. A wide range of meta-igneous rocks from the Late Paleozoic to Triassic Anarak Metamorphic Complex (AMC) and nearby Meraji area have been studied to constrain the origins and modes of emplacement of oceanic remnants in Central Iran. Our samples occur as layers and lenses embedded in extensive sequences of deformed meta-sediments and smaller bodies of serpentinized ultramafic rocks. Petrographical and geochemical data combined with field and satellite observations allow recognition of seven types of meta-igneous rocks preserved from low grade to blueschist facies conditions. Their origins based on relative abundances of immobile trace elements include subduction zone, mid-ocean ridge, ocean intraplate, and continental rift settings. These data and existing geochronological constraints show the AMC formed an accretionary complex formed/exhumed incrementally during the Carboniferous, Permo-triassic and Triassic. Igneous rocks from Meraji formed in the Early Devonian due to opening of the Paleotethys, and belong to a rift sequence extending over 300 km along the edge of the Central-East Iranian Microcontinent. The AMC and nearby rock associations record the evolution of the Paleotethys during a complete Wilson Cycle between ca. 450 and 225 Ma, with implications for: (1) continental rifting; (2) ocean opening; (3) subduction initiation; (4) ocean intraplate and continued mid-ocean volcanism; (5) ridge subduction; and (6) final closure of the ocean during continent-continent collision. Alternate interpretations of the Anarak metabasites are possible, but require radical departures from the widely accepted model for tectonic evolution of the Paleotethys, with the existence of Paleotethyan backarc basin(s) and Permian or earlier collision of continental blocks in Central Iran. In any case, our results show accretionary complexes preserved along suture zones contain an important record of the evolution of oceanic crust from ancient ocean basins.

Buchs, David M.; Bagheri, Sasan; Martin, Laure; Hermann, Joerg; Arculus, Richard

2013-07-01

 
 
 
 
221

The Future of Deep-Ocean Drilling  

Science.gov (United States)

Describes the scientific accomplishments of the International Program of Ocean Drilling (IPOD) during its first decade. Notable are the scientific contributions to understanding the sea floor. Critical decisions for the second decade include economic and social implications. (MA)

Heirtzler, J. R.; Maxwell, A. E.

1978-01-01

222

Seismic characters of the crust for a back-arc opening of the northern Japan Sea deduced from the seismic survey  

Science.gov (United States)

The Japan Sea is a back-arc basin in the northwestern Pacific. Based on geophysical, geological, and petrological results, it is suggested that the opening of the Japan Sea was initiated by crustal rifting and the separation of Japan Island Arcs from the Asian continent in the Early Oligocene, followed by the ocean floor spreading in the Late Oligocene (e.g., Tamaki et al., 1992). The Japan Basin, which is located in the northern Japan Sea, has an oceanic crust formed by the seafloor spreading (e.g., Hirata et al., 1992). On the other hand, in the Yamato Basin located from the northern to southeastern Japan Sea, the crustal structure is neither a typical oceanic nor continental crust (e.g., Nishizawa and Asada, 1996). However, seismic crustal models from the Japan and Yamato Basins to the marginal area, which presumed to show the transition of the structure from the basin toward the island arc, have been inadequate to elucidate the process from the rifting to the seafloor spreading in the Japan Sea. For this study, to understand this process in the Sea, we present seismic velocity structure models in the crust from the southern tip of the Japan Basin and the northern Yamato Basin to the marginal area in the eastern margin of the Japan Sea deduced from the seismic refraction/wide-angle reflection survey using ocean bottom seismographs (OBSs). In the southern tip of the Japan Basin, the crustal thickness is about 10 km. The P-wave velocity and thickness of the crust in the southern tip of the Japan Basin resembles those of a typical oceanic crust (White et al., 1992). On the other hand, the crust in the marginal area is thicker than a typical oceanic crust although the P-wave velocity is similar. Moreover, the Vp/Vs ratio in the crust of the southern tip of the Japan Basin and the marginal area is 1.78-1.82. This ratio implies that the nature of the crust in the Basin and the marginal area has an oceanic origin. The crustal structures in the southern tip of the Japan Basin and in the marginal area are suggested as the oceanic crust and the thicker oceanic crust, respectively. Therefore, the oceanic crust formed by the opening of the Japan Sea may propagate to the southern tip of the Japan Basin and the marginal area. On the other hand, the crust in the northern Yamato Basin is the thickness of 16 km. The crustal structure of the northern Yamato Basin has the character of the thicker oceanic crust because the distribution of the P-wave velocity is similar to that of a typical oceanic crust, the crust is thinner than that of the Korean Peninsula, and there is a little part of the P-wave velocity of 5.4-6.0 km/s which corresponds to the continental upper crust.

Sato, T.; No, T.; Takahashi, N.; Kodaira, S.; Kaneda, Y.

2013-12-01

223

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

Digital Repository Infrastructure Vision for European Research (DRIVER)

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

2012-01-01

224

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

Digital Repository Infrastructure Vision for European Research (DRIVER)

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

225

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.

226

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 3134 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

227

Mid-to-Lower-level Plutonic Rocks From Crust of the Southern Mariana Forearc: Implications for Growth of Continental Crust  

Science.gov (United States)

Tonalitic plutonic rocks dredged from the southern Mariana forearc are similar in terms of major element composition to tonalitic plutonic rocks of the Tanzawa Mountains on the Izu Peninsula of Japan. The tonalites of the Tanzawa Mountains have been interpreted to represent mid-lower crustal plutonic rocks that make up the 6.0 to 6.3 km/s layer identified in seismic velocity profiles of the Izu arc at 32N. The tonalities of the southern Mariana forearc may be analogous to the Tanzawa tonalities in terms of lithology and presumably seismic velocities, but have distinctive trace element and isotopic compositions. The exposure of these rocks on the southern Mariana forearc in a location where it is narrower by up to 80 km than elsewhere along its strike indicates a truncation of the arc lithosphere by tectonic erosion in the southern Mariana forearc. If tectonic processes in the forearc have exposed silicic plutonic rock of the arc lithosphere within 150 km of the volcanic front, then the structure of the Mariana arc and forearc is likely similar to that of the Izu arc, where seismic velocity structure suggests 25% of the arc/forearc lithosphere is comprised of a mid-crustal level tonalitic plutonic complex. The trace element and Sr isotopic compositions of the tonalities dredged from the Mariana forearc links them to a suprasubduction-zone environment. The Pb isotopic compositions, however, are consistent with crystallization ages that may be as old as Cretaceous. The compositions of these tonalites differ markedly from those of silicic volcanic rocks that have erupted throughout the history of the IBM arc and suggest that they represent a minor component of the arc. Nevertheless, the presence of Cretaceous tonalites in the Mariana forearc suggests that a portion of its crust may predate subduction initiation. The presence of silicic mid-to-lower crustal level plutonics beneath the Mariana arc as well as Eocene rhyolites on Saipan indicate that average major element composition of the arc crust may be comparable with average continental crust. This is consistent with estimates of the average composition of the Izu arc crust from seismic velocity studies and petrologic studies of exposures of the Izu arc crust in southern Japan's Izu peninsula. These data imply that the island arc that developed along the entire margin of the Philippine Sea plate may have had a generally similar structure and composition. Most components of the IBM arc crust, however, have relatively flat rare-earth patterns and low rare-earth concentrations compared with average continental crust. The averaged composition of the IBM crust, as a whole, differs markedly from that suggested by studies of the velocity structure of the central Aleutian arc. If the continental crust was generated in oceanic island arc settings throughout the history of the Earth, then its sources were significantly more enriched in LREE than the sources for the Cenozoic IBM arcs.

Fryer, P.; Reagan, M.

2006-12-01

228

Low Temperature Alteration of Basalts During the Last 9 Ma at 14o15'S on the South East Pacific Rise  

Science.gov (United States)

Little is known about the influence of low temperature alteration on the mineralogical and chemical composition of the oceanic crust once it leaves the spreading axis. Yet this crust is one of the major inputs to subduction zones, its composition is therefore highly relevant for the evolution of mantle, crust and biosphere. In addition, any secondary minerals contain a large amount of H20 (i.e. clays), in consequence they play an important role as a water depot and therefore affect the deydration reactions during subduction. For a better comprehension of the alteration effects it is important to understand the effects of the initial low temperature alteration. To determine these features, electron microprobe, X-ray diffraction and ICP-MS analyses have been carried out on basalts from the eastern flank of the East Pacific Rise at 14o15'S. The investigation concentrates on the products of a single spreading segment in a corridor perpendicular to the SEPR over a period from 0 to 9 Ma. Fresh rocks were sampled at the ridge axis (0-0.015 Ma), whereas off-axis basalts contain features of seawater generated alteration, which increases in intensity with time. Celadonite is the main alteration component in 0.12-4.6 Ma old rocks, whereas phillipsite is more abundant in rocks older than 4.6 Ma. The secondary minerals show evidence for a slight change in redox condition with time, from oxidizing, water-dominated to more reducing, rock-dominated environment. Iron-oxyhydroxide and celadonite are the first alteration products filling voids and veins and replacing olivine, partly replaced by saponite under more reducing conditions. The Fe necessary for the formation of these minerals is furnished by the dissolution of glass and the breakdown of olivine. Phillipsite is present in fractures and veins in rocks older than 1 Ma. Analyses also indicate an illite-smectite mixed layer which is believed to be an intermediate between saponite and celadonite and small amounts of a chlorite/smectite mixed layer. The presence of calcite is due to contamination with foraminifera and/or sediment. All samples are characterized by the lack of hydrothermal related minerals. We conclude that the alteration took place under seawater-dominated conditions at low temperature. A comparison of ICP-MS and XRF analyses from altered whole rock samples and their appendant fresh glass chips provide a record of element flow during alteration. Off-axis basalts show a significant increase of Rb, Cs and Ba which are supplied by seawater and incorporated in or on secondary minerals. The enrichment of U is connected to the oxidative conditions. K2O is also gained in all altered off-axis basalts, which is believed to be the result of the formation of celadonite.

Schramm, B.; Devey, C. W.; Gillis, K. M.; Lackschewitz, K.

2002-12-01

229

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

Directory of Open Access Journals (Sweden)

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

Robert P. Dziak

2012-03-01

230

MoMA: Cindy Sherman  

Science.gov (United States)

Everyone's abuzz about Cindy Sherman - her current retrospective at MoMA was reviewed in the Arts sections of the "New York Times" on February 23rd, she's in the February 27th "New Yorker," and was even mentioned in the "Wall Street Journal" on March 5, in an article by Pia Catton, who admits to being skeptical of Sherman's elevated status in the art world. So it's a good thing that we can all use the online version of MoMA's exhibition to do a reality check. All of the characters in Sherman's photographs are the artist herself, in different guises. Sherman has gone large for the first time in this exhibition - creating larger-than-life, floor-to-ceiling murals for the lofty museum space. Another new thing is her use of Photoshop to digitally alter her features in addition to the tricks of hair and makeup she has always employed. At the website, visitors can view works chronologically or by Gallery. There's also a set of videos, My Favorite Cindy Sherman, that consists of art critics, curators, and other artists commenting on Sherman's work.

Sherman, Cindy

2012-03-16

231

Neoproterozoic oceanic remnants in eastern Brazil: Further evidence and refutation of an exclusively ensialic evolution for the Araua West Congo orogen  

Science.gov (United States)

The Araua (eastern Brazil) and West Congo (southwestern Africa) belts are counterparts of the same Neoproterozoic orogen located between the So Francisco and Congo cratons. The Macabas Group represents a major passive margin sequence and is a key unit for interpreting the evolution of that orogen. The Salinas Formation is the distal rock assemblage of the Macabas Group and consists of a deep-sea sand-mud sequence, and a volcanic-sedimentary unit called the Ribeiro da Folha facies. The latter includes metamorphosed volcanic-exhalative sediments associated with ocean-floor basalts (amphibolites). The magmatic protoliths of these amphibolites crystallized at about 816 72 Ma (Sm-Nd whole-rock isochron, ?Nd(t) =+3.8 0.2). Regional metamorphism reached the amphibolite facies at about 630 Ma (Rb-Sr whole-rock isochron), when slabs of ultramafic rocks were tectonically emplaced over the Ribeiro da Folha facies. We consider this volcanic-sedimentary facies and the coeval slabs of ultramafic rocks to be remnants of a branch of the Adamastor-Brazilide ocean. The extensive occurrence of syntectonic to late tectonic calc-alkalic granitoids along the internal domain of the Araua belt implies that a reasonably large amount of ocean crust was consumed, via an east-dipping subduction zone, during formation of the Araua West Congo orogen.

Pedrosa-Soares, Antnio Carlos; Vidal, Philippe; Leonardos, Othon Henry; Bley de Brito Neves, Benjamin

1998-06-01

232

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

Science.gov (United States)

The Afanasy Nikitin seamount (ANS) is a major structural feature in the Central Indian Basin. An understanding of its evolution using multibeam bathymetry, magnetic and seismic reflection data provides new insights on growth of the seamount through time, emplacement of the 85E Ridge in the Bay of Bengal and deformation of the lithosphere in the equatorial Indian Ocean. The seafloor morphology and internal structure of the ANS show that the seamount consists of extensive plateaus extending from 215' to 530'S in water depths of 3000-4500 m, numerous elevated features (seamount highs) pierce through the northern part of the seamount plateau reaching up to 1600 m water depth and faulted blocks up to 1.0 s TWT throw in the southern part of the seamount plateau. Model studies of magnetic profiles suggest that the main plateau of the seamount was emplaced during the normal magnetisation period between the formation of seafloor spreading anomalies 33-32n.2 (79-73 Ma) and that the seamount high was formed in a reverse magnetisation period later than the main seamount plateau formation. Integrated geophysical results clearly demonstrate that the ANS was constructed in two phases, initially coeval with the formation of the oceanic crust during 79-73 Ma, and later at about 55 Ma in an intraplate setting. Based on present geophysical results and published plate reconstruction results of the Indian Ocean from Late Cretaceous to Early Cenozoic, we believe that the Conrad hotspot has emplaced the main plateau of the ANS and Conrad Rise during the period 79-73 Ma in an on-ridge setting, after which the hotspot has continued its activity in Antarctica plate leaving the main plateau of the ANS as an isolated feature on the Indian plate. Subsequently another hotspot that formed the 85E Ridge and buried hills in the Bay of Bengal has rebuilt the existing main plateau of the ANS at around 55 Ma and eventually the hotspot became defunct in the vicinity of the ANS.

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

2009-04-01

233

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

Digital Repository Infrastructure Vision for European Research (DRIVER)

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

2007-01-01

234

Growth of the Afanasy Nikitin seamount and its relationship with the 85E Ridge, northeastern Indian Ocean  

Science.gov (United States)

The Afanasy Nikitin seamount (ANS) is a major structural feature (400 km-long and 150 km-wide) in the Central Indian Basin, situated at the southern end of the so-called 85E Ridge. Combined analyses of new multibeam bathymetric, seismic reflection and geochronological data together with previously described magnetic data provide new insights into the growth of the ANS through time, and its relationship with the 85E Ridge. The ANS comprises a main plateau, rising 1200 m above the surrounding ocean floor (4800 m), and secondary elevated seamount highs, two of which (lie at 1600 and 2050 m water depths) have the morphology of a guyot, suggesting that they were formed above or close to sea-level. An unbroken sequence of spreading anomalies 34 through 32n.1 identified over the ANS reveal that the main plateau of the ANS was formed at 80-73 Ma, at around the same time as that of the underlying oceanic crust. The 40Ar/39Ar dates for two basalt samples dredged from the seamount highs are consistent, within error, at 67 Ma. These results, together with published results of late Cretaceous to early Cenozoic Indian Ocean plate reconstructions, indicate that the Conrad Rise hotspot emplaced both the main plateau of the ANS and Conrad Rise (including the Marion Dufresne, Ob and Lena seamounts) at 80-73 Ma, close to the India-Antarctica Ridge system. Subsequently, the seamount highs were formed by late-stage volcanism c. 6-13 Myr after the main constructional phase of the seamount plateau. Flexural analysis indicates that the main plateau and seamount highs of the ANS are consistent with Airy-type isostatic compensation, which suggest emplacement of the entire seamount in a near spreading-center setting. This is contrary to the flexural compensation of the 85E Ridge further north, which is interpreted as being emplaced in an intraplate setting, i.e., 25-35 Myr later than the underlying oceanic crust. Therefore, we suggest that the ANS and the 85E Ridge appear to be unrelated as they were formed by different mantle sources, and that the proximity of the southern end of the 85E Ridge to the ANS is coincidental.

Krishna, K. S.; Bull, J. M.; Ishizuka, O.; Scrutton, R. A.; Jaishankar, S.; Banakar, V. K.

2014-02-01

235

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 Linds 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, Hkon

2014-05-01

236

Macquarie island and the cause of oceanic linear magnetic anomalies.  

Science.gov (United States)

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

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

1969-10-10

237

42 CFR 495.204 - Incentive payments to qualifying MA organizations for MA-EPs and MA-affiliated eligible hospitals.  

Science.gov (United States)

...for MA-EPs and MA-affiliated eligible hospitals. 495.204 Section 495.204 Public Health CENTERS FOR MEDICARE & MEDICAID...Specific to Medicare Advantage (MA) Organizations § 495.204 Incentive payments to qualifying MA...

2010-10-01

238

Superfluidity and entrainment in neutron star crusts  

Digital Repository Infrastructure Vision for European Research (DRIVER)

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

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

2013-01-01

239

Biogenic crust dynamics on sand dunes  

CERN Document Server

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

Kinast, Shai; Yizhaq, Hezi; Ashkenazy, Yosef

2012-01-01

240

33 CFR 80.120 - Cape Ann, MA to Marblehead Neck, MA.  

Science.gov (United States)

...2009-07-01 2009-07-01 false Cape Ann, MA to Marblehead Neck, MA. 80.120...LINES Atlantic Coast § 80.120 Cape Ann, MA to Marblehead Neck, MA. ...Massachusetts from Halibut Point at Cape Ann to Marblehead Neck. (b) A line...

2009-07-01

 
 
 
 
241

Dredged bedrock samples from the Amerasia Basin, Arctic Ocean  

Science.gov (United States)

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

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

2013-12-01

242

Component geochronology of the ca. 3920 Ma Acasta Gneiss  

Science.gov (United States)

Compiled U-Pb zircon ages of the oldest parts of the Acasta Gneiss Complex (AGC) in the Northwest Territories (Canada) span about 4050-3850 Ma (Stern and Bleeker, 1998); yet older 4200 Ma xenocrystic U-Pb zircon ages have also been reported for this terrane (Iizuka et al., 2006). The AGC has at least 50 km2 of outcrop exposure, but only a small subset has been documented in the detail required to investigate a complex history. To better understand this history, ion microprobe zircon geochronology was combined with whole-rock and zircon rare earth element compositions (+Y; [REE+Y]zirc) and Ti-in-zircon thermometry (Tixln) from a sub-divided ~60 cm2 slab of Acasta banded gneiss, and compared to other nearby variably deformed AGC granitoid gneiss samples. Micro-sampling by this method reveals components with distinctive [Th/U]zirc vs. Tixln and [REE+Y]zirc that are correlative with separate 235,238U-207,206Pb zircon age populations and whole-rock compositions, but not with 147Sm-143Nd isotope systematics. Lattice-strain theory used to model [REE+Y] reconciles U-Pb zircon geochronology for the individual components, which also preserve strong positive Eu* anomalies. Modeling shows that the magmas that gave rise to the oldest domains formed at contemporary oxygen fugacities. The AGC preserves a legacy older than about 4000 Ma, but this derives from incomplete assimilation of older crust. Magmatic emplacement at ca. 3920 Ma is contemporaneous with the Late Heavy Bombardment (LHB) of the Moon. Later superimposed Eoarchean events (3850-3720 Ma) are reminiscent of formation times for the Itsaq Gneiss Complex in West Greenland (Nutman et al., 1996), Nuvvuagittuq Supracrustal Belt in northern Qubec (Cates et al. 2013), and Manfred Complex in Western Australia (Kinny et al., 1990). Equilibration of Sm-Nd occurred at the scale of individual components over the course of one or more of these events.

Cates, Nicole L.; Mojzsis, Stephen J.; Caro, Guillaume; Hopkins, Michelle D.; Abramov, Oleg; Trail, Dustin; Bleeker, Wouter; Guitreau, Martin; Blichert-Toft, Janne

2013-04-01

243

A Seafloor Microbial Biome Hosted within Incipient Ferromanganese Crusts  

Energy Technology Data Exchange (ETDEWEB)

Unsedimented volcanic rocks exposed on the seafloor at ridge systems and Seamounts host complex, abundant and diverse microbial communities that are relatively cosmopolitan in distribution (Lysnes, Thorseth et al. 2004; Mason, Stingl et al. 2007; Santelli, Orcutt et al. 2008). The most commonly held hypothesis is that the energy released by the hydration, dissolution and oxidative alteration of volcanic glasses in seawater drives the formation of an ocean crust biosphere (Thorseth, Furnes et al. 1992; Fisk, Giovannoni et al. 1998; Furnes and Staudigel 1999). The combined thermodynamically favorable weathering reactions could theoretically support anywhere from 105 to 109 cells/gram of rock depending upon the metabolisms utilized and cellular growth rates and turnover (Bach and Edwards 2003; Santelli, Orcutt et al. 2008). Yet microbially-mediated basalt alteration and energy conservation has not been directly demonstrated on the seafloor. By using synchrotron-based x-ray microprobe mapping, x-ray absorption spectroscopy and high-resolution scanning and transmission electron microscopy observations of young volcanic glasses recovered from the outer flanks of Loihi Seamount, we intended to identify the initial rates and mechanisms of microbial basalt colonization and bioalteration. Instead, here we show that microbial biofilms are intimately associated with ferromanganese crusts precipitating onto basalt surfaces from cold seawater. Thus we hypothesize that microbial communities colonizing seafloor rocks are established and sustained by external inputs of potential energy sources, such as dissolved and particulate Fe(II), Mn(II) and organic matter, rather than rock dissolution.

Templeton, Alexis S.; Knowles, A. S.; Eldridge, D. L.; Arey, Bruce W.; Dohnalkova, Alice; Webb, Samuel M.; Bailey, B. E.; Tebo, Bradley M.; Staudigel, Hubert

2009-11-15

244

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

245

Shear modulus of neutron star crust  

Digital Repository Infrastructure Vision for European Research (DRIVER)

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

246

Consequences of the low density of the lunar primary crust on its magmatic history (Invited)  

Science.gov (United States)

The lunar highlands are very old, with ages covering a timespan between 4.5 to 4.2 Gyr, and probably formed by flotation of light plagioclase minerals on top of the lunar magma ocean. The lunar crust provides thus an invaluable evidence of the geological and magmatic processes occurring in the first times of the terrestrial planets history. According to the last estimates from the GRAIL mission, the lunar primary crust is particularly light and relatively thick. This low-density crust acted as a barrier for the dense primary mantle melts. This is particularly evident in the fact that subsequent mare basalts erupted primarily within large impact basins: at least part of the crust must have been removed for the magma to reach the surface. However, the trajectory of the magma from the mantle to the surface is unknown. Here, we provide evidence of intrusions within the crust of the Moon as surface deformations in the form of low-slope lunar domes and floor-fractured craters. All these geological features have morphologies consistent with models of magma spreading at depth and deforming an overlying elastic layer. Furthermore, at floor-fractured craters, the deformation is contained within the crater interior, suggesting that the overpressure at the origin of magma ascent and intrusion was less than the pressure due to the weight of the crust removed by impact. The pressure release due to material removal by impact is significant over a depth equivalent to the crater radius. Because many of these floor-fractured craters are relatively small, i.e. less than 20 to 30 km in radius, this observation suggests that the magma at the origin of the intrusion was already stored within or just below the crust, in deeper intrusions. Thus, a large fraction of the mantle melt might have stored at depth below or within the light primary crust before reaching shallower layers. And hence, magma intrusions must have had a large influence on the thermal and geological evolution of the lunar primary crust and could have induced a prolonged heating of the crust.

Michaut, C.; Thorey, C.

2013-12-01

247

Is Macquarie Island a Section of Slow-spread Crust?  

Science.gov (United States)

Peridotite melting at mid-ocean ridges is believed to be affected by spreading rate, with faster spreading rates leading to higher degrees of melting. Macquarie Island, located 1500 km southeast of southernmost Australia, is thought be the sole complete section of ocean crust uplifted in the ocean basin in which it formed; moreover it has been proposed that it formed during slow spreading (30mm/yr full rate, calculated from geophysical studies). The oceanic crust of the island formed in the final stages of spreading, 6 mya, as indicated by Ar-Ar plateau ages of basaltic glass. At this latitude, the plate boundary evolved from a spreading ridge to a transpressional boundary between 33 and 6 mya, thus the rocks of the island record an interesting tectonic history and may provide clues to the mantle processes during a major plate motion re-organization, and slow spreading. Residual, plag-free peridotites were collected along transects through all of the mantle sections on the island, with an average of 100 meter spacing between samples. Spinel chrome numbers (Cr#) ranged from 0.39 to 0.46 (n=23), which corresponds to 15-16% fractional melting [1]. Their low Ti contents (0.02-0.07) attest to the residual nature of the Macquarie Island peridotites. Cpx is preserved in only 7 samples (alteration, depletion), and occurs mainly as small interstitial grains or as exsolved blebs in opx porphyroclasts. Cpx titanium (0.00-0.04 wt% TiO_2) and sodium (0.00-0.05 wt% Na_2O) contents are extremely low, confirming the high depletion and supporting highly efficient melt extraction. Opx porphyroclast cores have very high Mg# (0.92 on average). Trace element analysis (SIMS) revealed a strong depletion of HREE, and enrichment of LREE, as well as a positive Sr anomaly in some samples. Exsolved and matrix grains have identical compositions. The levels of depletion indicated by the spinel Cr# and HREE contents of cpx of the Macquarie Island peridotites are more similar to those seen at fast spreading centers or ophiolites, not at most slow spreading centers. This depletion could be caused by the progressively changing spreading direction disrupting mixing in the mantle, causing repeated melting of the same mantle source. Alternately, this depletion may be caused by melting enhanced by the presence of fluids, possibly introduced by some limited (temporal?) subduction in the region. This could also explain the LREE- and fluid mobile element enrichment. Further analyses of associated gabbros and basalts will test which model is most likely. [1] Hellebrand et al., (2001) Nature 410, 677-681.

Wertz, K.; Hellebrand, E.; Snow, J. E.; von der Handt, A.; Mosher, S.

2003-04-01

248

Ma Ying-jeous Presidential Discourse  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Despite the substantial advances made in cross-Strait relations during Ma Ying-jeous (Ma Yingjiu) first term, the ROC presidents rhetoric varied considerably as he grappled with the difficult reality of implementing campaign and inauguration pledges to establish better relations with China while striving to maintain national respect and sovereignty. In this article, we put forward a framework for measuring, analysing and explaining this variation in President Mas first-term discourse...

2012-01-01

249

Ma Ying-jeous Presidential Discourse  

Digital Repository Infrastructure Vision for European Research (DRIVER)

"Despite the substantial advances made in cross-Strait relations during Ma Ying-jeou's (Ma Yingjiu) first term, the ROC president's rhetoric varied considerably as he grappled with the difficult reality of implementing campaign and inauguration pledges to establish better relations with China while striving to maintain national respect and sovereignty. In this article, the authors put forward a framework for measuring, analyzing and explaining this variation in President Ma's first-term disco...

Jonathan Sullivan; Sapir, Eliyahu V.

2012-01-01

250

Ma Ying-jeou's presidential discourse  

Digital Repository Infrastructure Vision for European Research (DRIVER)

"Despite the substantial advances made in cross-Strait relations during Ma Ying-jeou's (Ma Yingjiu) first term, the ROC president's rhetoric varied considerably as he grappled with the difficult reality of implementing campaign and inauguration pledges to establish better relations with China while striving to maintain national respect and sovereignty. In this article, the authors put forward a framework for measuring, analyzing and explaining this variation in President Ma's first-term disco...

Sullivan, Jonathan; Sapir, Eliyahu V.

2012-01-01

251

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

Science.gov (United States)

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

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

2013-01-01

252

Comparative Study on the Electrical Properties of the Oceanic Mantle Beneath the Northwest Pacific Ocean  

Science.gov (United States)

We have been conducting long-term seafloor electromagnetic (EM) observations at two sites in the northwest Pacific since 2001. The older site was established at the deep seafloor (~5600m) on the northwest Pacific basin (Site NWP), while the new one was installed on the west Philippine basin (Site WPB) in 2006 at the slightly deeper (~5700m) seafloor. The ages of the oceanic basins at those sites are approximately 129 Ma for Site NWP (Shipboard Scientific Party of ODP Leg 191, 2000) and 49 Ma for Site WPB (Salisbury et al., 2006), respectively. The EM instruments deployed at those sites are seafloor EM stations (SFEMS; Toh et al., 2004 and 2006) and capable of measuring vector EM fields at the seafloor for as long as one year or more with other physical quantities such as the instruments' attitude, orientation and temperature. One of the objectives of the seafloor long-term EM observations by SFEMSs is to make a comparative study of the oceanic mantle with and without influence of the so-called 'stagnant slabs' in terms of their electrical conductivity. It is anticipated that the mantle transition zone under the influence of the stagnant slab has a higher electrical conductivity because the transition zone there could be wetter than that in the absence of the stagnant slab. In this context, the mantle transition zone beneath Site WPB can be said to have influence by the stagnant slab, while that beneath Site NWP does not. It, therefore, is basically possible to estimate how much water is present in each transition zone by comparison of the electrical conductivity profiles of the two. The one-dimensional electrical profile beneath Site NWP has been derived so far using the magnetotelluric (MT) and geomagnetic depth sounding (GDS) methods with significant jumps in the electrical property at 410 and 660km discontinuities. The jumps are approximately factors of 10 and 2, respectively (Ichiki et al., 2009). Here we show a profile beneath Site WPB using both MT and GDS responses as well. It, however, should be also noted here that the penetration depth beneath Site WPB is significantly smaller than that beneath Site NWP because the solar activity has been very low since 2006. References Ichiki, M., K. Baba, H. Toh and K. Fuji-ta, An overview of electrical conductivity structures of the crust and upper mantle beneath the northwestern Pacific, the Japanese Islands, and continental East Asia, Gondwana Research, 16, 545?562, doi:10.1016/j.gr.2009.04.007, 2009. Salisbury MH et al (2006) 2. Leg 195 Synthesis: Site 1201?A geological and geophysical section in the West Philippine Basin from the 660-km discontinuity to the mudline. Proc. Ocean Drilling Program, Scientific Reports 195:27. Shipboard Scientific Party of ODP Leg 191 (2000) Northwest Pacific seismic observatory and hammer drill tests, Proc. Ocean Drilling Program, Initial Reports 191. Toh, H., Y. Hamano and M. Ichiki, Long-term seafloor geomagnetic station in the northwest Pacific: A possible candidate for a seafloor geomagnetic observatory, Earth Planets Space, 58, 697-705, 2006. Toh, H., Y. Hamano, M. Ichiki and H. Utada, Geomagnetic observatory operates at the seafloor in the Northwest Pacific Ocean, Eos, Trans. Am. Geophys. Union, 85, 467/473, DOI: 10.1029/2004EO450003, 2004.

Toh, H.

2013-12-01

253

Nuclei in Strongly Magnetised Neutron Star Crusts  

CERN Multimedia

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

Nandi, Rana

2012-01-01

254

Growth of the lower continental crust  

International Nuclear Information System (INIS)

One of the largest uncertainties in crustal composition and growth models is the nature of the lower continental crust. Specifically, by what processes is it formed and modified, and when is it formed, particularly in reference to the upper crust? The main reason for this lack of information is the scarcity of lower crustal rock samples. These are restricted to two types: rocks which outcrop in granulite facies terrains and granulite facies xenoliths which are transported to the earth's surface by young volcanics. The important conclusions arising from the xenolith studies are: the majority of mafic lower crustal xenoliths formed through cumulate process, resitic xenoliths are rare; and formation and metamorphism of the deep crust is intimately linked to igneous activity and/or orogeny which are manifest in one form or another at the earth's surface. Therefore, estimates of crustal growth based on surface exposures is representative, although the proportion of remobilized pre-existing crust may be significantly greater at the surface than in the deep crust

1987-07-13

255

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: 202 Ma). b- Puchenque-Atravesadas Intrusive Complex (PAIC), composed by dykes and stocks ranging from diorites to granodiorites (age: 12.51 Ma). c- Arroyo Serrucho Stock (SAS), an epizonal and zoned stock, with four facies, with K/Ar and Ar/Ar dates of 101 and 9.50.5 Ma. d- Portezuelo de los Cerros Bayos (PCB), that includes porphyritic rocks of rhyolitic composition, of 7.50.5 Ma. e- Cerro Bayo Vitrophyres (CBV), with andesitic sills and dykes (age: 4.80.2 Ma). We present in this paper new Sr and Pb isotopes data that constrain the evolution of the PAIB (au)

2001-10-01

256

Software Aspects of PuMa-II  

Science.gov (United States)

The Pulsar Machine II (PuMa-II) is a state of the art pulsar machine-installed at the Westerbork Synthesis Radio Telescope (WSRT), in December 2005. PuMa-II is a flexible instrument and is designed around an ensemble of 44 high-performance computers running the Linux operating system. Much of the flexibility of PuMa-II comes from the software that is being developed for this instrument. The radio signals reaching the telescope undergo several stages of electronic and software processing before a scientifically useful data product is generated. The electronic processing of signals includes the usual RF to IF conversion, analogue to digital conversion and telescope dependent electronic digital delay compensation that happen in the signal chain of WSRT. Within PuMa-II, this data is acquired, stored and suitably processed. In this poster we present various aspects of PuMa-II software and illustrate its pulsar signal processing capabilities.

Karuppusamy, R.; Stappers, B.; Stappers, B.

2006-08-01

257

Detecting deeply subducted crust from the elasticity of hollandite  

Science.gov (United States)

Subduction of differentiated continental and oceanic crusts through sediments and basalt to the deep mantle has been shown to be a likely source for the geochemical signature of ocean island basalts that are enriched in large ion lithophile elements such as K, Na, Rb, and Sr. At high pressure such a lithology will consist of stishovite, majorite and hollandite, where hollandite (KAlSi 3O 8) can readily host the large ion lithophile elements, and is hence a geochemically important phase. Here we study the elasticity of hollandite up to lower mantle pressure by electronic structure simulations and attempt to constrain the volume percent of hollandite in a subduction zone environment. In agreement with experiments we predict a phase transition from a low pressure tetragonal phase to a high pressure monoclinic phase at 33 GPa. The phase transition has significant effects on the elastic properties of hollandite, with an increase in shear modulus of 10%. Based on the computed reflection coefficient across the transition and observed reflectance for mid-mantle seismic scatterers (920 km discontinuity) we constrain the maximum volume of hollandite to be around 5% in a subduction zone environment.

Mookherjee, Mainak; Steinle-Neumann, Gerd

2009-11-01

258

Widespread, Off-axis Magmatism at a Young Oceanic Rift, the Sedimented Guaymas Basin Spreading Center  

Science.gov (United States)

A thick layer of sediment commonly blankets spreading centers within young rifted margins such as the Guaymas Basin within the Gulf of California. The shallow oceanic crust in these environments differs significantly from deep-water, unsedimented ridges in that there is little to no extrusive volcanism, and crust is accreted by the intrusion of magmatic sills into sediments. From initial observations at the seafloor and through drill holes in the S. Guaymas Basin, a model of magmatic accretion similar to that of deep-water mid-ocean ridges was proposed wherein sills are intruded at the rift axis and subsequently buried by sediments as they are rafted off axis. Seismic reflection data collected throughout the N. Guaymas basin in 2002, however, found that sills do not deepen with off-axis distance suggesting that sill intrusion to shallow levels within the sediment pile occurs throughout the basin out to 50 km from the plate boundary (spreading age of 2 Ma). This suggests that magmatic accretion within the shallow crust is active over a very wide area (10-20 times larger than at deep-water mid-ocean ridges) independent of spreading age. During a cruise to the Guaymas Basin in 2009, we collected deep-towed sidescan sonar, sub-bottom imaging, multibeam bathymetry, near-bottom photographs, and bottom water samples across the N. Guaymas Basin to test this hypothesis. Acoustic backscatter imagery revealed nearly 100 localized, acoustically bright seafloor reflectors scattered throughout the survey area. Some of these backscatter anomalies were investigated with a deep-towed camera system and found to contain authigenic carbonate, tubeworms, clams, bacterial mats, and indurated sediment outcrops. Some sites showed small thermal anomalies in near-bottom waters, methane concentrations well in excess of background, and high 3He anomalies. Where coverage overlaps, these sites correlate with the position of seismically imaged subsurface sills. In this presentation, we present this evidence in support of widespread magmatic accretion by intrusion of sills into sediments and explore the possibility that such a process is inherently related to a thick sedimentary blanket and as such common to young rifted margins and the implications of widespread, off-axis magmatic accretion in the Guaymas Basin.

Soule, S.; Lizarralde, D.; Seewald, J.; Proskurowski, G.

2010-12-01

259

The crust of Iceland- a reassessment  

Science.gov (United States)

The evolving knowledge is at variance with the expectations build upon the idea of an island in making, around Iceland. Shallow thick crusted Shetland-Greenland ridge, extensive distribution of old and continental rocks along Mid Atlantic Ridge, granitic and dolomitic xenoliths in Quaternary Icelandic lava, rhyolitic to dacitic central volcanoes, voluminous pumice drifted onto eastern shores of Atlantic are a few among the valid reasons to consider that the Iceland bears a hidden continental crust. In present study, gravity, seismic and magnetic data over Iceland were scrutinized to pick up continental characteristics. To test the hypothesis here, Iceland is considered as remnant continent, which failed to be eaten up by mantle during Cenozoic basification. It denies any chance for lithospheric spreading centered to Iceland and looks at crustal- mantle hybridization processes resulting in basalt and its derivatives ( crustal basification) as alternative explanation to the exotic ( in terms of plate tectonics) geological and geophysical behaviour of Icelandic crust.

Longhinos, Biju

2014-05-01

260

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, Dbora P; Providncia, Constana; Alloy, Marcelo D

2011-01-01

 
 
 
 
261

Geophysical investigations of crust-scale structural model of the Qiongdongnan Basin, Northern South China Sea  

Science.gov (United States)

Rifting of the Qiongdongnan Basin was initiated in the Cenozoic above a pre-Cenozoic basement, which was overprinted by extensional tectonics and soon after the basin became part of the rifted passive continental margin of the South China Sea. We have integrated available grids of sedimentary horizons, wells, seismic reflection data, and the observed gravity field into the first crust-scale structural model of the Qiongdongnan Basin. Many characteristics of this model reflect the tectonostratigraphic history of the basin. The structure and isopach maps of the basin allow us to reconstruct the history of the basin comprising: (a) The sediments of central depression are about 10 km thicker than on the northern and southern sides; (b) The sediments in the western part of the basin are about 6 km thicker than that in the eastern part; (c) a dominant structural trend of gradually shifting depocentres from the Paleogene sequence (45-23.3 Ma) to the Neogene to Quaternary sequence (23.3 Ma-present) towards the west or southwest. The present-day configuration of the basin reveals that the Cenozoic sediments are thinner towards the east. By integrating several reflection seismic profiles, interval velocity and performing gravity modeling, we model the sub-sedimentary basement of the Qiongdongnan Basin. There are about 2-4 km thick high-velocity bodies horizontal extended for a about 40-70 km in the lower crust (v > 7.0 km/s) and most probably these are underplated to the lower stretched continental crust during the final rifting and early spreading phase. The crystalline continental crust spans from the weakly stretched domains (about 25 km thick) near the continental shelf to the extremely thinned domains (7.0), while that on northern and southern sides is lower (passive margin and the evolution of the thermal field of the basin.

Qiu, Ning; Wang, Zhenfeng; Xie, Hui; Sun, Zhipeng; Wang, Zhangwen; Sun, Zhen; Zhou, Di

2013-12-01

262

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 (28-33S), 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 27-28S. Well constrained reconstructions of the margin near 26-28S 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 26S to 34S and that the active arc extrapolates through the amagmatic flat-slab region (28-33S) at 300 km from the trench, accelerated forearc removal could be inferred from ~34S to 26S 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 27S, 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 19.5S 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

263

Nuclei in Strongly Magnetised Neutron Star Crusts  

Science.gov (United States)

We discuss the ground state properties of matter in outer and inner crusts of neutron stars under the influence of strong magnetic fields. In particular, we demonstrate the effects of Landau quantization of electrons on compositions of neutron star crusts. First we revisit the sequence of nuclei and the equation of state of the outer crust adopting the Baym, Pethick and Sutherland (BPS) model in the presence of strong magnetic fields and most recent versions of the theoretical and experimental nuclear mass tables. Next we deal with nuclei in the inner crust. Nuclei which are arranged in a lattice, are immersed in a nucleonic gas as well as a uniform background of electrons in the inner crust. The Wigner-Seitz approximation is adopted in this calculation and each lattice volume is replaced by a spherical cell. The coexistence of two phases of nuclear matterliquid and gas, is considered in this case. We obtain the equilibrium nucleus corresponding to each baryon density by minimizing the free energy of the cell. We perform this calculation using Skyrme nucleon-nucleon interaction with different parameter sets. We find nuclei with larger mass and charge numbers in the inner crust in the presence of strong magnetic fields than those of the zero field case for all nucleon-nucleon interactions considered here. However, SLy4 interaction has dramatic effects on the proton fraction as well as masses and charges of nuclei. This may be attributed to the behaviour of symmetry energy with density in the sub-saturation density regime. Further we discuss the implications of our results to shear mode oscillations of magnetars.

Nandi, Rana; Bandyopadhyay, Debades

264

Fluid Flow in the Deep Crust  

Science.gov (United States)

The heating and burial of rock masses during mountain building drives chemical reactions that liberate volatile fluid species (Figure 1). These volatiles, including H2O, CO2, and CH4, are much less dense and viscous than the surrounding rock and will, therefore, have a strong tendency to migrate along grain boundaries or fractures through the Earth's crust. Fluids released in the deep crust interact geochemically with their surroundings (Rye et al., 1976) as they ascend to shallow levels where they invade hydrothermal and groundwater systems and, ultimately, interact with the hydrosphere and atmosphere. This flux of fluid from active mountain belts to the surface is a major contributor to planetary volatile cycling and is estimated to be currently in excess of 1017 kg Myr-1 (based on Kerrick and Caldeira, 1998; Wallmann, 2001a, b). (13K)Figure 1. Diagram of crustal fluid cycling. The deep crust is composed largely of metamorphic rock (cf. Rudnick and Fountain, 1995; Wedepohl, 1995; see Chapter 3.01). Fluids and magmas are the primary agents of chemical mass transport through the deep crust; fluid flow dominates at temperatures rock rheology is critical for determining the geochemical and petrological evolution of the crust. Moreover, metamorphic fluids impact directly many problems of societal relevance, including ore deposit formation (see Chapter 3.12), global release of greenhouse gases, seismic hazards, and arc magma genesis (see Chapter 3.18) and the associated volcanic hazards. This chapter first examines basic fluid flow, mass transfer, and reaction concepts. This discussion is followed by a review of selected natural examples of fluid transport during active metamorphism. The focus is on deeper levels of the crust (>15 km depth), although many of the concepts discussed are general and also apply to shallower levels.

Ague, J. J.

2003-12-01

265

Sea surface temperature and salinity in the south Atlantic subtropical gyre over the last 4 Ma  

Science.gov (United States)

The early Pliocene (3-5 Ma) is the most recent time in the Earth's history when climate was significantly warmer than today. Even though atmospheric pCO2 (360-410 ppm) was similar to today, the average global temperature was 3-4C warmer, ice sheets were smaller, and sea level was approximately 35 m higher than today. Recent studies show that reduced zonal and meridional sea surface temperature (SST) gradients played a role in sustaining Pliocene warmth and increasing SST gradients coincides with Pliocene-Pleistocene climate cooling. Ocean circulation is another important factor in climate transitions. Changes in shallow overturning circulation (ventilated thermocline) and ocean heat transport are hypothesized to be responsible for increasing Earth's sensitivity to orbital forcing and the onset of Pleistocene glaciations. However, most early Pliocene SST records are focused in the northern hemisphere and tropical upwelling regions, while data from the subtropical southern hemisphere is scarce. We present G.sacculifer Mg/Ca and ?18O records from ODP site 1264 (28.53S; 2.85E, 2505 m water depth), located on the Walvis Ridge in the south Atlantic subtropical gyre and within the influence of the Agulhas leakage. The Mg/Ca SST record displays no long-term trend over the past 4 Ma. The ?18O record indicates that sea surface salinity (SSS) gradually decreased from the early Pliocene to today. In the modern ocean the Agulhas Current is a western boundary current in the Indian Ocean that sheds rings of salty Indian Ocean waters into the SE Atlantic. The current is driven by a high air pressure system (the downward portion of the Hadley cell) over the south Indian Ocean and therefore responds to climatic shifts. Previous work indicates that weaker Hadley circulation in the early Pliocene lead to expanded subtropical gyres. This would have enhanced inter-ocean flow from the Indian to the Atlantic Ocean, resulting in higher SSS. Contraction of the south Indian Ocean subtropical gyre due to strengthening Hadley circulation from the early Pliocene to today would have resulted in a decreasing influence of the Agulhas Current, and therefore reduced SSS in the SE Atlantic. Increasing foraminifera abundance and decreasing nannofossil content from 4 Ma to today at ODP site 1264 implies a change from turbulent conditions in the Pliocene to calm gyre-like conditions in the modern ocean, consistent with a reduced Agulhas current influence. We conclude that the SST and SST records at ODP site 1264 reflect a decreasing influence of the Agulhas Current on the south-eastern Atlantic over the last 4 Ma.

Wojcieszek, D. E.; Dekens, P. S.

2011-12-01

266

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

267

The Wet, Oxidizing Crust of Mars  

Science.gov (United States)

Studies have inferred that the oxidation state of Martian basaltic meteorites (the shergottites) is correlated with diagnostic geochemical parameters. For example, Meenakshi Wadhwa (Field Museum, Chicago) showed that as the ratio of triply-charged europium to doubly-charged europium increases in a group of shergotties, the ratio of strontium-87 to strontium-86 also increases, and neodynmium-143 to neodynmium-144 decreases. [See PSRD article: Gullies and Canyons, Rocks and Experiments: The Mystery of Water on Mars]. Eu3+/Eu2+ is a measure of the oxidation state and can be used to infer the availability of oxygen to react chemically, a property called the oxygen fugacity. Christopher Herd, Lars Borg, and Jim Papike (University of New Mexico) and John Jones (Johnson Space Center) decided to measure the oxygen fugacity more directly by making very careful and painstaking analyses of oxide minerals in Martian meteorites. Herd and his co-workers find that as oxygen fugacity increases in a group of shergottites, the ratios of strontium-87 to strontium-86 and of lanthanum (La) to ytterbium (Yb) also increase, while neodynmium-143 to neodynmium-144 decreases. They suggest these trends indicate that, compared to the Martian mantle, the crust is more oxidizing, has higher strontium-87 to strontium-86 and La/Yb, and lower neodynmium-143 to neodynmium-144. Magmas formed in the mantle would have low oxygen fugacity. As magmas rose through the crust, they reacted with the surrounding rocks to varying extents, producing the observed chemical trends. How did the crust become more oxidizing than the mantle? They suggest that circulating hot water oxidized the crust. Alternatively, water-rich magmas might have crystallized in the crust, forming deposits of hydrous minerals. Subsequent magmas could react with the hydrated minerals to become more oxidizing. Whatever the details, the work by Herd and colleagues indicates that the mantle and crust differ significantly, that the crust has significant deposits of water, and many pristine magmas are modified by interaction with the crust.

Taylor, G. J.

2002-08-01

268

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

Science.gov (United States)

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

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

2014-05-01

269

Ocean climate  

Energy Technology Data Exchange (ETDEWEB)

Various aspects of the problem of ocean climate are briefly reviewed. First, the concept of ocean climate is defined and compared with that of atmospheric climate. Attention is then given to the global characteristics of ocean climate, vertical profiles, zonal profiles, nonzonal inhomogeneities, and periodic fluctuations (diurnal, synoptic, seasonal, annual, and secular). In particular, the discussion covers the role of the atmosphere-ocean interaction, ocean stratification, the El Nino-Southern Oscillation phenomenon, and small-scale processes in the ocean. 15 refs.

Monin, A.S. (RAN, Inst. Okeanologii, Moscow (Russian Federation))

1993-01-01

270

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

271

The worship of Bh?ma  

Digital Repository Infrastructure Vision for European Research (DRIVER)

This study deals with stone representations of Bh?ma, one of the protagonists of the Hindu epic Mah?bh?rata. This epic which originates from India, is already known on Java in the tenth century. The Bh?ma representations which include statues and reliefs appeared during the Majapahit Period (1296-1527 C.E.) and were mainly found on mountain sanctuaries in present East and Central Java. Given the number of statues found, there was a worship of Bh?ma going on during the Majapahit Perio...

Duijker, Marijke

2010-01-01

272

Enhanced mantle-to-crust rhenium transfer in undegassed arc magmas.  

Science.gov (United States)

Variations in the 187Os/188Os isotopic signature of mantle and mantle-derived rocks have been thought to provide a powerful chemical tracer of deep Earth structure. Many studies have inferred from such data that a long-lived, high-rhenium component exists in the deep mantle (187Re is the parent isotope decaying to 187Os, with a half-life of approximately 42 billion years), and that this reservoir probably consists of subducted oceanic crust. The interpretation of these isotopic signatures is, however, dependent on accurate estimates of rhenium and osmium concentrations in all of the main geochemical reservoirs, and the crust has generally been considered to be a minor contributor to such global budgets. In contrast, we here present observations of high rhenium concentrations and low Yb/Re ratios in arc-type melt inclusions. These results indicate strong enrichment of rhenium in undegassed arc rocks, and consequently the continental crust, which results in a crustal estimate of 2 p.p.b. rhenium, as compared to previous estimates of 0.4-0.2 p.p.b. (refs 4, 5). Previous determinations of rhenium in arc materials, which were largely measured on subaerially erupted samples, are likely to be in error owing to rhenium loss during degassing. High mantle-to-crust rhenium fluxes, as observed here, require a revaluation of geochemical models based on the 187Re-187Os decay system. PMID:12646918

Sun, Weidong; Bennett, Vickie C; Eggins, Stephen M; Kamenetsky, Vadim S; Arculus, Richard J

2003-03-20

273

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

274

Norwegian crusted scabies: an unusual case presentation.  

Science.gov (United States)

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

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

2014-01-01

275

Collective excitations in neutron-star crusts  

CERN Multimedia

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

Chamel, N; Reddy, S

2013-01-01

276

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 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)

2001-10-01

277

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

278

Salatoimikud : ma tahan uskuda / Mart Rummo  

Index Scriptorium Estoniae

USA sarjale "The X-Files" phinev teine jrjefilm "Salatoimikud: Ma tahan uskuda" ("The X-Files: I Want to Believe") : reissr Chris Carter : peaosades David Duchovny, Gillian Anderson : Ameerika hendriigid - Kanada 2008

Rummo, Mart

2008-01-01

279

COOL: Crust of the Oman Ophiolite and its Lithosphere - a passive seismic experiment  

Science.gov (United States)

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

Weidle, Christian; Agard, Philippe; Ducassou, Cline; El-Hussain, Issa; Prigent, Ccile; Meier, Thomas

2014-05-01

280

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, zgr; Dehant, Vronique

2014-05-01

 
 
 
 
281

Timing and duration of garnet granulite metamorphism in magmatic arc crust, Fiordland, New Zealand  

Science.gov (United States)

Pembroke Granulite from Fiordland, New Zealand provides a window into the mid- to lower crust of magmatic arcs. Garnet Sm-Nd and zircon U-Pb ages constrain the timing and duration of high-P partial melting that produced trondhjemitic high Sr/Y magma. Trace element zoning in large, euhedral garnet is compatible with little post growth modification and supports the interpretation that garnet Sm-Nd ages of 126.1??2.0 and 122.6??2.0. Ma date crystal growth. Integration of the garnet ages with U-Pb zircon ages elucidates a history of intrusion(?) and a protracted period of high-temperature metamorphism and partial melting. The oldest zircon ages of 163 to 150. Ma reflect inheritance or intrusion and a cluster of zircon ages ca. 134. Ma date orthopyroxene-bearing mineral assemblages that may be magmatic or metamorphic in origin. Zircon and garnet ages from unmelted gneiss and garnet reaction zones record garnet granulite facies metamorphism at 128 to 126. Ma. Peritectic garnet and additional zircon ages from trondhjemite veins and garnet reaction zones indicate that garnet growth and partial melting lasted until ca. 123. Ma. Two single fraction garnet ages and young zircon ages suggest continued high-temperature re-equilibration until ca. 95. Ma. Phase diagram sections constrain orthopyroxene assemblages to <0.6 GPa @ 650??C, peak garnet granulite facies metamorphic conditions to 680-815??C @ 1.1-1.4. GPa, and a P-T path with a P increase of???0.5. GPa. These sections are compatible with water contents???0.28wt.%, local dehydration during garnet granulite metamorphism, and <0.3. GPa P increases during garnet growth. Results demonstrate the utility of integrated U-Pb zircon and Sm-Nd garnet ages, and phase diagram sections for understanding the nature, duration, and conditions of deep crustal metamorphism and melting. Geochronologic and thermobarometric data for garnet granulite indicate that thickening of arc crust, which caused high-pressure metamorphism in northern Fiordland, must have occurred prior to 126. Ma, that loading occurred at a rate of ca. 0.06. GPa/m.y., and that garnet granulite metamorphism lasted 3-7m.y. Locally-derived partial melts formed and crystallized in considerably less than 10 and perhaps as little as 3m.y. ?? 2010 Elsevier B.V.

Stowell, H.; Tulloch, A.; Zuluaga, C.; Koenig, A.

2010-01-01

282

Improving Durbin's method to estimate MA processes  

Digital Repository Infrastructure Vision for European Research (DRIVER)

This paper provides a simple method to estimate both univariate and multivariate MA processes. Similar to Durbin's method, it rests on the recursive relation between the parameters of the MA process and those of its AR representation. This recursive relation is shown to be valid both for invertible / stable and non invertible / unstable processes under the assumption that the process has no constant and started from zero. This makes the method suitable for unit root processe...

Ludwig, Maximilian

2013-01-01

283

Ocean drilling program with 15 cruises completed  

Energy Technology Data Exchange (ETDEWEB)

The Ocean Drilling Program (ODP) soon commences its campaign of scientific ocean drilling in the Indian Ocean aboard leg 115 of D.V. Sedco/BP471 (JOIDES Resolution). Many of ODP's scientific and technical objectives have been addressed during their initial 15 cruises (approximately 2.5 years). For example, high-latitude drilling, initiated in the northern polar latitudes in 1985 (leg 104-Norwegian Sea; leg 105-Labrador Sea/Baffin Bay), continued in 1987 in the southern polar latitudes of the Weddell Sea and southern South Atlantic Ocean (legs 113 and 114). Another important thrust requiring a major commitment in engineering development has been in drilling and coring the oceanic crust - in particular, drilling both supported and unsupported holes in zero-age crust in regions with little or no sediment cover (legs 106 and 109). The establishment of natural sea-floor laboratories has also been further enhanced by continued operations at former deep DSDP crustal holes (legs 102, 109, and 111). This paper focuses on the operational and scientific successes of the past ODP cruises and identifies areas of future studies in the Indian Ocean as outlined by JOIDES.

Rabinowitz, P.D.; Meyer, A.; Baldauf, J.

1987-05-01

284

Compositional Freeze-Out of Neutron Star Crusts  

Digital Repository Infrastructure Vision for European Research (DRIVER)

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

285

Diffuse degassing through magmatic arc crust (Invited)  

Science.gov (United States)

The crust of magmatic arcs plays an important role in the volatile cycle at convergent margins. The fluxes of subduction- and arc-related volatiles such as H2O, C, Cl, S are poorly known. It is commonly believed that gases emitted from volcanoes account nearly quantitatively for the volatiles that cross the Moho beneath the volcanic front. This volcanic degassing may occur during eruption, emission from summit fumaroles and hot springs, or more 'diffuse' delivery to volcano flanks. However, several observations suggest that volatiles also transit arc crust by even more diffuse pathways, which could account for significant volatile loss on long time and length scales. Active metamorphism of arc crust produces crustal-scale permeability that is sufficient to transport a large volume of subducted volatiles (Ingebritsen and Manning, 2002, PNAS, 99, 9113). Arc magmas may reach volatile saturation deeper than the maximum depths recorded by melt inclusions (e.g., Blundy et al., 2010, EPSL, 290, 289), and exhumed sections of magmatic arc crust typically record voluminous plutons reflecting magma crystallization and volatile loss at depths well below the volcanic edifice. At shallower depths, topographically driven meteoric groundwater systems can absorb magmatic volatiles and transport them laterally by tens of km (e.g., James et al., 1999, Geology, 27, 823; Evans et al., 2002, JVGR, 114, 291). Hydrothermal ore deposits formed at subvolcanic depths sequester vast amounts of volatiles, especially sulfur, that are only returned to the surface on the time scale of exhumation and/or erosion. Water-rich metamorphic fluids throughout the crust can readily carry exsolved volcanic gases because the solubilities of volatile bearing minerals such as calcite, anhydrite, and fluorite are quite high at elevated pressure and temperature (e.g., Newton and Manning, 2002, Am Min, 87, 1401; 2005, J Pet, 46, 701; Tropper and Manning, 2007, Chem Geol, 242, 299). Taken together, these considerations dictate that volatile entrainment in the metamorphic/meteoric fluid-flow system represents a highly diffuse pathway for degassing through arc crust which must be taken into account in models of volatile cycling at convergent margins.

Manning, C. E.; Ingebritsen, S.

2013-12-01

286

Ocean drilling program: Recent results and future drilling plans  

Energy Technology Data Exchange (ETDEWEB)

The Ocean Drilling Program (ODP) has completed 48 internationally-staffed expeditions of scientific ocean drilling in search of answers relating to the evolution of passive and active continental margins, evolution of oceanic crust, origin and evolution of marine sedimentary sequences, and paleoceanography. During the past year of drilling operations, ODP expeditions cored Cretaceous reef-bearing guyots of the Western Pacific, with the objective of using them as monitors of relative sea-level changes and thereby of the combined effects of the tectonic subsidence (and uplift) history of the seamounts and of global fluctuations of sea level (Legs 143 and 144); studied high-resolution variations of surface and deep-water circulation and chemistry during the Neogene, the late Cretaceous and Cenozoic history of atmospheric circulation, ocean chemistry, and continental climate, and the age and nature of the seafloor in the North Pacific (Leg 145); studied the relationship between fluid flow and tectonics in the accretionary wedge formed at the Cascadia convergent plate boundary off Vancouver and Oregon (Leg 146); drilled in Hess Deep to understand igneous, tectonic and metamorphic evolution of fast spreading oceanic crust and to understand the processes of rifting in young ocean crust (Leg 147); and continued efforts at Hole 504B at 2,000 mbsf, the deepest hole they have beneath seafloor (Leg 148). After Leg 148 (March 1993), the JOIDES Resolution will commence an Atlantic Ocean drilling campaign.

Rabinowitz, P.D.; Francis, T.J.G.; Baldauf, J.G.; Allan, J.F.; Heise, E.A.; Seymour, J.C. (Texas A and M Univ., College Station, TX (United States))

1993-02-01

287

Paleo movement of continents since 300 Ma, mantle dynamics and large wander of the rotational pole  

Science.gov (United States)

Apparent polar wander (APW) is known to be mainly linked to internal mass distribution changes and in particular to changes in subduction and large-scale upwellings in the mantle. We investigate plate motions during the last 410 million years in a reference frame where Africa is fixed. Indeed, Africa has remained a central plate from which most continents diverged since the break-up of Pangea. The exact amount of subduction is unknown prior to 120 Ma. We propose an approach, based on one hand on the study of the past subduction volcanism to locate ancient subduction activity, and on the other hand microplate motion history in the Tethyan area derived from geology and paleomagnetism. The peri-Pacific subductions seem to be a quasi-permanent feature of the Earth's history at least since the Paleozoic, with however localized interruptions. The "Tethyan" subductions have a complex history with successive collisions of continental blocs (Hercynian, Indo-Sinian, Alpine and Himalayan) and episodical rebirth of E-W subduction trending zones. Assuming that subducted slabs sink vertically into the mantle and taking into account large-scale upwellings derived from present-day tomography and intra-plate volcanism in the past, we compute the time variation of mantle density heterogeneities since 280 Ma. Due to conservation of the angular momentum of the Earth, the temporal evolution of the rotational axis is computed in a mantle reference frame where the Africa plate is fixed, and compared to the apparent polar wander (APW) observed by paleomagnetism since 280 Ma. We find that a major trend of both paleomagnetic and computed APW are successive oscillatory clockwise or counter-clockwise motions, with tracks separated by abrupt cusps (around 230 Ma, 190 Ma and 140-110 Ma). We find that cusps result from earlier major geodynamic events: the 230 Ma cusp is related to the end of active subduction due to the closure of the Rheic Ocean basin after the Hercynian continental collision (340-300 Ma) and to renewed subduction zone West of Laurentia, whereas the 190 Ma cusp results from the Indo-Sinian collision (270-230 Ma) and the subsequent end of the Neo-Tethys ocean subduction.

Greff-Lefftz, Marianne; Besse, Jean

2012-09-01

288

Subduction geodynamics in Archean and formation of diamond-bearing lithospheric keels and early continental crust of cratons  

Science.gov (United States)

The diamond-bearing mantle keels underlying Archean cratons are a unique phenomenon of Early Precambrian geology. The common stable assemblage of the Archean TTG early continental crust and underlying subcontinental lithospheric mantle clearly shows their coupled tectogenesis, which was not repeated in younger geological epochs. One of the least studied aspects of this phenomenon is concerned with the eclogitic xenoliths carried up by kimberlite pipes together with mantle-derived nodules. The eclogitic xenoliths reveal evidence for their subduction-related origin, but the Archean crustal counterparts of such xenoliths remained unknown for a long time, and the question of their crustal source and relationships to the formation of early continental crust remained open. The Archean crustal eclogites recently found in the Belomorian Belt of the Baltic Shield are compared in this paper with eclogitic xenoliths from kimberlites in the context of the formation of both Archean subcontinental lithospheric mantle (SCLM) and early continental crust. The crustal eclogites from the Belomorian Belt are identical in mineral and chemical compositions to the eclogite nodules (group B), including their diamond-bearing varieties. The eclogite protoliths are comparable in composition with the primary melts of the Meso- and Neoarchean oceanic crust, which was formed at a potential temperature of the upper mantle which exceeded its present-day temperature by 150-250 K. The reconstructed pathways of the Archean oceanic crust plunging in the upper mantle suggest that the Archean mantle was hotter than in the modern convergence settings. The proposed geodynamic model assumes coupled formation of the Archean diamond-bearing SCLM and growth of early continental crust as a phenomenon related to the specific geodynamics of that time controlled by a higher terrestrial heat flow.

Shchipansky, A. A.

2012-03-01

289

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

Science.gov (United States)

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

Volk, Tyler

1989-01-01

290

Towards a metallurgy of neutron star crusts  

CERN Document Server

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

Kobyakov, D

2013-01-01

291

Molecular controls on Cu and Zn isotopic fractionation in Fe-Mn crusts  

Science.gov (United States)

The isotopic systems of the transition metals are increasingly being developed as oceanic tracers, due to their tendency to be fractionated by biological and/or redox-related processes. However, for many of these promising isotope systems the molecular level controls on their isotopic fractionations are only just beginning to be explored. Here we investigate the relative roles of abiotic and biotic fractionation processes in controlling modern seawater Cu and Zn isotopic compositions. Scavenging to Fe-Mn oxides represents the principal output for Cu and Zn to sediments deposited under normal marine (oxic) conditions. Using Fe-Mn crusts as an analogue for these dispersed phases, we investigate the phase association and crystal chemistry of Cu and Zn in such sediments. We present the results of an EXAFS study that demonstrate unequivocally that Cu and Zn are predominantly associated with the birnessite (?-MnO2) phase in Fe-Mn crusts, as previously predicted from sequential leaching experiments (e.g., Koschinsky and Hein, 2003). The crystal chemistry of Cu and Zn in the crusts implies a reduction in coordination number in the sorbed phase relative to the free metal ion in seawater. Thus, theory would predict equilibrium fractionations that enrich the heavy isotope in the sorbed phase (e.g., Schauble, 2004). In natural samples, Fe-Mn crusts and nodules are indeed isotopically heavy in Zn isotopes (at ?1) compared to deep seawater (at ?0.5), consistent with the predicted direction of equilibrium isotopic fractionation based on our observations of the coordination environment of sorbed Zn. Further, ?50% of inorganic Zn? is chloro-complexed (the other ?50% is present as the free Zn2+ ion), and complexation by Cl- is also predicted to favour equilibrium partitioning of light Zn isotopes into the dissolved phase. The heavy Zn isotopic composition of Fe-Mn crusts and nodules relative to seawater can therefore be explained by an inorganic fractionation during uptake. However, Cu in Fe-Mn crusts is isotopically light (at ?0.3 to 0.5) compared to the dissolved phase in seawater (at ?0.9). We suggest that this is because dissolved Cu in the oceans is overwhelmingly complexed to strong organic ligands, which are better competitors for the heavy isotope.

Little, S. H.; Sherman, D. M.; Vance, D.; Hein, J. R.

2014-06-01

292

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)

2004-01-01

293

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

294

Ma Ying-jeous Presidential Discourse  

Directory of Open Access Journals (Sweden)

Full Text Available Despite the substantial advances made in cross-Strait relations during Ma Ying-jeous (Ma Yingjiu first term, the ROC presidents rhetoric varied considerably as he grappled with the difficult reality of implementing campaign and inauguration pledges to establish better relations with China while striving to maintain national respect and sovereignty. In this article, we put forward a framework for measuring, analysing and explaining this variation in President Mas first-term discourse. Analysing a very large number of Mas speeches, addresses, etc., we provide empirical assessments of how the content of Mas public pronouncements has developed over time, how his rhetoric varies according to the strategic context and timing of a speech, and how his discourse compares to that of his predecessor, Chen Shui-bian (Chen Shuibian. In addressing these questions, the article contributes a quantitative perspective to existing work on political discourse in Taiwan and to the growing methodological and applied literature on how to systematically analyse Chinese political text.

Jonathan Sullivan

2012-01-01

295

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

2006-11-14

296

Critical Metals In Western Arctic Ocean Ferromanganese Mineral Deposits  

Science.gov (United States)

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

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

2013-12-01

297

Pyrolysis of waste plastic crusts of televisions.  

Science.gov (United States)

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

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

2012-09-01

298

Microprobe monazite constraints for and early (ca. 790 Ma) Braziliano orogeny: The Embu Terrane, southeastern Brazil  

International Nuclear Information System (INIS)

The evolution of the Mantiqueira Orogenetic System, Southeastern Brazil, comprises discrete episodes of tectonic collage and docking of remnants of Rodinia break-up in the borders of the Sao Francisco Craton. This system is related to the closure of the Adamastor ocean and assemblage of the western Gondwana super-continent during Neoproterozoic times (ca. 610-530 Ma, Brito Neves et al., 1999; Campos Neto, 2000). This report presents monazite microprobe dating results for metassediments from the Embu Complex, an important lithological unit from the Ribeira Belt, currently included in the Juiz de Fora terrane, a unit added to the Sao Francisco Craton at ca. 600-580 Ma. (Campos Neto, 2000). The age results unravel a main metamorphic episode and related orogeny at ca. 790 Ma and bring new insights concerning the agglutination of Gondwana in this region during the Neoproterozoic (au)

2001-10-01

299

Metabolism of dodecyldimethylamine by Pseudomonas MA3.  

Science.gov (United States)

Pseudomonas MA3 was isolated from activated sludge on the basis of its capacity to use dodecyldimethylamine as a sole carbon (C) and energy source. Dodecylamine, dodecanal, dodecanoic acid and acetic acid also supported growth of Pseudomonas MA3. Dodecyldimethylamine-grown cells oxidized a wide range of alkylamine derivatives, dodecanal, dodecanoic acid and acetic acid. Degradation of the alkyl chain of dodecyldimethylamine by Pseudomonas MA3 appeared from the stoichiometric liberation of dimethylamine. A dehydrogenase catalysed the cleavage of the Calkyl-N bond. The first intermediate of the proposed degradation pathway, dodecanal, accumulated in the presence of decanal used as a competitive inhibitor. The second intermediate, dodecanoic acid, was formed in the presence of acrylic acid during the degradation of dodecyldimethylamine. Dodecanal was converted into dodecanoic acid by a dehydrogenase and dodecanoic acid was then degraded via the beta oxidation pathway. PMID:7765813

Kroon, A G; Pomper, M A; van Ginkel, C G

1994-10-01

300

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

 
 
 
 
301

M&A information technology best practices  

CERN Document Server

Add value to your organization via the mergers & acquisitions IT function As part of Deloitte Consulting, one of the largest mergers and acquisitions (M&A) consulting practice in the world, author Janice Roehl-Anderson reveals in M&A Information Technology Best Practices how companies can effectively and efficiently address the IT aspects of mergers, acquisitions, and divestitures. Filled with best practices for implementing and maintaining systems, this book helps financial and technology executives in every field to add value to their mergers, acquisitions, and/or divestitures via the IT

Roehl-Anderson, Janice M

2013-01-01

302

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 Dagkpl 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.

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

2010-04-01

303

Fabrication technology for ODS Alloy MA957  

Energy Technology Data Exchange (ETDEWEB)

A successful fabrication schedule has been developed at Carpenter Technology Corporation for the production of MA957 fuel and blanket cladding. Difficulties with gun drilling, plug drawing and recrystallization were overcome to produce a pilot lot of tubing. This report documents the fabrication efforts of two qualified vendors and the support studies performed at WHC to develop the fabrication-schedule.

ML Hamilton; DS Gelles; RJ Lobsinger; MM Paxton; WF Brown

2000-03-16

304

Fabrication technology for ODS Alloy MA957  

International Nuclear Information System (INIS)

A successful fabrication schedule has been developed at Carpenter Technology Corporation for the production of MA957 fuel and blanket cladding. Difficulties with gun drilling, plug drawing and recrystallization were overcome to produce a pilot lot of tubing. This report documents the fabrication efforts of two qualified vendors and the support studies performed at WHC to develop the fabrication-schedule

2000-01-01

305

How does the continental crust thin in a hyper-extended rifted margin: insights from the Iberia margin  

Science.gov (United States)

During the last twenty years the knowledge of rifted margins was strongly challenged by new observations in particular related to ODP drilling and seismic refraction surveys. The discovery of hyper-thinned continental crust and exhumed mantle in deep water rifted margins results in two fundamental questions: How does the crust thin and what controls extreme crustal thinning and mantle exhumation? Reflection and refraction seismic lines indicate that distinct domains in rifted margins are separated by structures that cut decoupling levels in the crust and eventually transfer deformation to mantle levels. This evolution from decoupled to coupled deformation on a crustal level may explain the crustal architecture of magma-poor rifted margins. A comparison of seismic sections across the Iberia rifted margin shows that decoupled extension was distributed in the North and more localized in the South. In contrast coupled extension resulted in similar structures and width of the extended domain along the whole margin. Based on drill hole data the transition from decoupled to coupled deformation occurred during the Tithonian (145Ma). A delay or retardation of subsidence observed over the hyper-extended coupled domain may indicate that crustal thinning had to occur simultaneously with lithospheric necking and may be controlled by deeper mantle processes. Based on our observations we suggest that the decoupled deformation affecting the crust seems to be mainly controlled by crustal structure and rheology, both of which are dependent on inheritance. In contrast the coupled deformation cuts the entire crust and occurs in a completely embrittled crust and is therefore much less dependent on the initial structure of the margin.

Sutra, E.; Manatschal, G.

2011-12-01

306

Titan's internal ocean: evolution, exchange processes and geophysical signatures. (Invited)  

Science.gov (United States)

For many years, a variety of theoretical models have predicted the presence of liquid water oceans inside Titan and the large icy moons of Jupiter. Magnetic data returned by the Galileo spacecraft seems to confirm the presence of an ocean underneath the icy crust of Europa, Ganymede and Callisto. On Titan, the Cassini-Huygens mission also provide indirect evidences for an internal liquid layer (change in spin rate, long-wavelength topography, Schumann resonance). However, the physical and chemical characteristics of these internal ocean remain very uncertain. It is likely that such water-rich oceans have been generated in the primordial stages of the satellite evolution, probably as a consequence of the accretional heat. On Titan, it is even envisaged that the water ocean was stable up to the surface during several tens of millions years, because the massive primitive atmosphere limited the cooling of the moon. The formation of a solid crust at the surface, which strongly reduces the efficiency of heat release, further limits the cooling and solidification of the ocean and may explain the presence of an internal liquid layer 4.5 billions after the satellite formation. Interactions through the icy crust between the ocean and the atmosphere have certainly occurred via various cryovolcanic processes all along Titan's evolution, thus partly controlling the chemical evolution of the atmosphere. Owing to differences in initial composition, size and orbital configuration, Titan and the Galilean moons have followed different evolutionary paths. Despite these differences, surprisingly, they all seem to have preserved an internal ocean. After reviewing the main characteristics of these different moons, we will detail the possible evolution of Titan's ocean and its differences with the other moons liquid layers. Expected exchanges with Titans surface and atmosphere will also be discussed. Finally, key geophysical measurements needed to constrain the physical characteristics of the present-day ocean will be presented.

Tobie, G.; Grasset, O.

2010-12-01

307

Upper and lower crust recycling in the source of CAMP basaltic dykes from southeastern North America  

Science.gov (United States)

The densest dykes swarm of the Central Atlantic magmatic province (CAMP) occur in southeastern North America (SENA) and were intruded between 202 and 195 Ma during Pangea break-up. New combined geochemical data (major and trace elements, SrNdPbOs isotopes) constrain the mantle source of these magmatic bodies and their evolution path. While SrNd isotopic compositions for SENA rocks (87Sr/86Sr200Ma 0.704380.70880 and 143Nd/144Nd200Ma 0.512510.51204) fall within the low-Ti CAMP field, PbPb isotopes (206Pb/204Pb200Ma 17.4618.85, 207Pb/204Pb200Ma 15.5415.65, 208Pb/204Pb200Ma 37.4738.76) are peculiar to this area of the CAMP and cover a considerable span of compositions, especially in 206Pb/204Pb200Ma. Given the generally unradiogenic Os isotopic compositions (187Os/188Os200Ma 0.1270.144) observed and the lack of correlation between these and other geochemical markers, crustal contamination during the evolution of SENA dykes must have been limited (less than 10%). Thus the isotopic variation is interpreted to reside primarily within the mantle source. These observations, coupled with typical continental signatures in trace elements (positive anomaly in Pb and negative anomalies in Ti and Nb), require another means of conveying a continental flavor to these magmas, which is here hypothesized to be the shallow recycling within the upper mantle of subducted lower and upper crustal materials. Pseudo-ternary mixing models show that a maximum of 10% recycled crust is enough to explain their trace element patterns as well as their isotopic heterogeneity. Looking at the larger picture of the origin of the CAMP, the thermal contribution of a mantle plume cannot be ruled out due to the relatively high mantle potential temperatures (14301480?C) calculated for high-Fo SENA olivines. Nevertheless, our results suggest that the chemical involvement of a mantle plume is negligible (less than 5%) if either a C- or an EM-flavored plume is considered. Rather, the possibility of a PREMA-flavored mantle plume, enriched by 520% recycled crustal material, remains a possible, though less plausible, source for these tholeiites.

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

2013-08-01

308

Geological Models for the Uppermost Martian Crust  

Science.gov (United States)

Prototype cross-sections through the uppermost 100 m of the Martian crust are attempted for several distinct terrains: (a) young and uncratered (northern lowlands); (b) young and cratered (northern lowlands); (c) older and cratered (southern highlands) and (d) older and uncratered (southern highlands). Polar regions are also considered. The cross-sections are built from four main materials (1) uncemented sediment (i.e., dust and aeolian deposits); (2) cemented sediment (e.g., evaporites, sediments consolidated by diagenesis); (3) igneous rock (e.g., basaltic lavas and related hypabyssal intrusions, impact melt); and (4) megaregolith (i.e., impact-bombarded and impact-mixed material derived from 1-3 above). Megaregolith constitutes the foundation component, given that the entire crust had probably been impact processed by the end of the heavy bombardment period. The cross-sections have been constructed primarily in order to optimize the design of an orbiting synthetic aperture radar (SAR)/Sounder system for Mars. The cross-sections are also intended for use in mission planning (i.e., site selection, rover design and equipment selection). Understanding the composition and structure of the uppermost 100 m of the Martian crust is important for future missions. We need to estimate the likely substructure for landing sites so that we can optimize mission design. This is particularly important for rover-based drilling, ground-penetrating radar technology, sampling for evidence of life, and accessing H2O. Constructing cross-sections is an iterative process, largely based on existing remote sensing data (Mariner, Viking, MGS, Odyssey), combined with analogies with other terrestrial planets, especially Earth and the Moon. In this respect, Mars shows similarities with both the Moon (e.g., in megaregolith development and its preservation) and Earth (e.g., recent volcanism, presence of sedimentary deposits).

Spray, J.

2004-05-01

309

Magma flow directions in the sheeted dike complex at superfast spreading mid-ocean ridges: Insights from IODP Hole 1256D, Eastern Pacific  

Science.gov (United States)

Integrated Ocean Drilling Program (IODP) Hole 1256D successfully sampled a complete section of an intact oceanic crustal sheeted dike complex (SDC) (from 1061 to 1320 meters below seafloor; mbsf) on a 15 Ma old Cocos Plate. A series of rock magnetic measurements were carried out to understand the magmatic processes that accreted this end-member, superfast-spread (200 mm/yr full rate) oceanic crust. Results indicate that main ferromagnetic minerals are predominantly pseudo single-domain (titano)magnetite crystals, responsible for both anisotropy of magnetic susceptibility (AMS) and magnetic remanence signals. AMS fabrics were reoriented into a geographic reference frame using magnetic remanence data, and corrected for a counterclockwise rotation of the Cocos Plate relative to the East Pacific Rise (EPR) ca. 15 Ma. Corrected AMS fabrics were then compared with the orientations of chilled margins previously obtained from Formation MicroScanner (FMS) images of the SDC at Hole 1256D. For some samples taken from close to dike margins, a dike-normal orientation of the minimum AMS axes (Kmin) of prolate AMS ellipsoids mean that the long axis (Kmax) can be used to infer magma flow directions. Subvertical Kmin orientations in the interior of the dikes, however, may have required settling or compaction of the magma shortly after intrusion, thus rearranging the AMS fabric. Despite this orientation of Kmin axes, orientation of Kmax axes indicate a rather constant subhorizontal paleo-flow direction, suggesting that magmas most probably traveled to the surface considerable distances from source regions within the EPR system.

Veloso, Eugenio E.; Hayman, Nicholas W.; Anma, Ryo; Tominaga, Masako; Gonzlez, Rodrigo T.; Yamazaki, Toshitsugu; Astudillo, Natalia

2014-04-01

310

Development of the early continental crust. III  

International Nuclear Information System (INIS)

New analyses and critical re-evaluation of older data permit another estimate of the abundances of some 42 elements in the silicate shells of the Earth, excluding the core. These data and estimations of volcanic rock production lead to a series of estimates of the mass-fractions of depleted and undepleted mantle. Most likely estimates of the latter lie in the range 0.2-0.4. The best estimates are obtained for the most incompatible elements K, Rb, Sr, Ba, Nb, Th, U. The development of the continental crust and the change in tectonic processes over time is consistent with these interpretations. (Auth.)

1980-02-01

311

Antarctic and Southern Ocean Crustal Thickness and Ocean-Continent Transition Location from Gravity Inversion  

Science.gov (United States)

Using gravity anomaly inversion, we have produced the first comprehensive regional maps of crustal thickness and oceanic lithosphere distribution for Antarctica and the Southern Ocean. We determine Moho depth, crustal basement thickness, continental lithosphere thinning (1-1/?) and ocean-continent transition location using a 3D spectral domain gravity inversion method (Chappell and Kusznir 2008) which incorporates a lithosphere thermal gravity anomaly correction. Lithosphere thermal model re-equilibration (cooling) times, used to calculate the lithosphere thermal gravity anomaly correction, are conditioned by ocean isochron information, and continental rifting and breakup ages. The continental lithosphere thinning distribution, used to define the initial thermal model temperature perturbation are derived from the gravity inversion and use no a priori isochron information; as a consequence the gravity inversion method provides a prediction of OCT location which is independent of ocean isochron information. The gravity anomaly contribution from ice thickness is included in the gravity inversion, as is the contribution from sediments which assumes a compaction controlled sediment density increase with depth. A correction to the predicted continental lithospheric thinning derived from gravity inversion is made for the addition of volcanic material produced by decompression melting during continental rifting, breakup lithosphere thinning and seafloor spreading. Data used in the gravity inversion are elevation and bathymetry, free-air gravity anomaly, sediment and ice thickness from Smith and Sandwell (2008), Sandwell and Smith (2008) and Laske and Masters (1997) respectively, supplemented by Bedmap2 data south of 60 degrees south. Our gravity inversion study predicts thick crust (> 45 km) under interior East Antarctica penetrated by continent scale rifts. Intermediate crustal thickness with a pronounced rift fabric is predicted under Coates Land. An extensive region of either thick oceanic crust or highly thinned continental crust is predicted offshore Oates Land and north Victoria Land. Thin crust is predicted under the West Antarctica Rift System and the Ross Sea. Crustal thickness and lithosphere thinning derived from gravity inversion allows the determination of circum-Antarctic ocean-continent transition structure and the mapping of continent-ocean boundary location. Superposition of illuminated satellite gravity data onto crustal thickness maps from gravity inversion provides improved determination of Southern Ocean rift orientation, pre-breakup rifted margin conjugacy and continental breakup trajectory.

Kusznir, Nick J.; Alvey, Andy; Vaughan, Alan P. M.; Ferraccioli, Fausto; Jordan, Tom A. R. M.; Roberts, Alan M.

2013-04-01

312

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

Directory of Open Access Journals (Sweden)

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

Christopher J. MacLeod

2006-09-01

313

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

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

2014-05-01

314

The Neoproterozoic Malani magmatism of the northwestern Indian shield: implications for crust-building processes  

Science.gov (United States)

Malani is the largest event of anorogenic felsic magmatism (covering 50, 000 km2) in India. This magmatic activity took place at 750 Ma post-dating the Erinpura granite (850 Ma) and ended prior to Marwar Supergroup (680 Ma) sedimentation. Malani eruptions occurred mostly on land, but locally sub-aqueous conditions are shown by the presence of conglomerate, grits and pillow lava. The Malani rocks do not show any type of regional deformation effects. The Malanis are characterised by bimodal volcanism with a dominant felsic component, followed by granitic plutonism and a terminal dyke phase. An angular unconformity between Malani lavas and basement is observed, with the presence of conglomerate at Sindreth, Diri, and Kankani. This indicates that the crust was quite stable and peneplained prior to the Malani activity. Similarly, the absence of any thrust zone, tectonic mlange and tectonised contact of the Malanis with the basement goes against a plate subduction setting for their genesis. After the closure of orogenic cycles in the Aravalli craton of the northwestern shield, this anorogenic intraplate magmatic activity took place in a cratonic rift setting under an extensional tectonic regime.

Sharma, Kamal K.

2004-12-01

315

When and How Did Continental Crust Form?  

Science.gov (United States)

Given the extensive literature on the composition and evolution of continental crust there are a number of teaching strategies that can be employed to encourage active learning by students. A critical reading of this collection of articles will provide students with a good opportunity to evaluate the chemical isotopic and physical evidence that has led to the development of these models of continental crustal growth. These instructional approaches build on recommendations from Project 2061, Science for all Americans: 1) Start with questions about nature. 2) Engage students actively. 3) Concentrate onthe collection and use of evidence. 4) Provide historical perspectives. 5) Use a team approach. 6) Do not separate knowing from finding out. A compilation from the primary literature has been provided (see the reference list at the end of this web page: http://serc.carleton.edu/NAGTWorkshops/earlyearth/questions/crust.html), along with guiding questions for deeper exploration and discovery. Recommended instructional methods include: jigsaw method, role playing or debates (have each student play the role of Richard Armstrong, Ross Taylor, William Fyfe...), reading the primary literature, or problem-based learning (which is purposefully ambiguous and addresses questions that require independent discovery).

Mogk, Dave

316

Longitudinal photosynthetic gradient in crust lichens' thalli.  

Science.gov (United States)

In order to evaluate the self-shading protection for inner photobionts, the photosynthetic activities of three crust lichens were detected using Microscope-Imaging-PAM. The false color images showed that longitudinal photosynthetic gradient was found in both the green algal lichen Placidium sp. and the cyanolichen Peltula sp. In longitudinal direction, all the four chlorophyll fluorescence parameters Fv/Fm, Yield, qP, and rETR gradually decreased with depth in the thalli of both of these two lichens. In Placidium sp., qN values decreased with depth, whereas an opposite trend was found in Peltula sp. However, no such photosynthetic heterogeneity was found in the thalli of Collema sp. in longitudinal direction. Microscope observation showed that photobiont cells are compactly arranged in Placidium sp. and Peltula sp. while loosely distributed in Collema sp. It was considered that the longitudinal photosynthetic heterogeneity was ascribed to the result of gradual decrease of incidence caused by the compact arrangement of photobiont cells in the thalli. The results indicate a good protection from the self-shading for the inner photobionts against high radiation in crust lichens. PMID:24477924

Wu, Li; Zhang, Gaoke; Lan, Shubin; Zhang, Delu; Hu, Chunxiang

2014-05-01

317

Control of crust permeability and crispness retention in crispy breads  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Crispness of bread crust is rapidly lost after baking. It is known that the speed of this loss is influenced by the water vapor permeability of the crust. A high water vapor permeability benefits crispness retention but could lead to crumb dryness. In this paper we aimed to determine the water vapor permeability that is optimal for both crispness retention and crumb softness retention. The water vapor permeability of crust was changed by creating different quantities and sizes of channels...

Hirte, A.; Hamer, R. J.; Meinders, M. B. J.; Hoek, K.; Primo Martin, C.

2012-01-01

318

MoMA: Ron Arad: No Discipline  

Science.gov (United States)

Who is Ron Arad? He happens to be an influential designer known for his "daredevil approach to form, structure, technology, and materials in work that spans the disciplines of industrial design, sculpture, architecture, and mixed-medium installation." In the fall of 2009, the Museum of Modern Art (MoMA) featured a show by Arad that brought together a selection of his works. This site was created to complement the onsite show, and visitors can view some of his art right here. Users will want to click on over to the "Works" area to get a sense of Arad's oeuvre, which includes wavy chairs, elaborate steel sculptures, and delicately arranged boxes. Moving on, "The Show" area features highlights from his show at MoMA, complete with some time-lapse videos of the installation process.

319

PuMa-ECR ion source operation  

International Nuclear Information System (INIS)

The PuMa (Pulsed Magnetic field)-ECR ion source uses a pulsed solenoid coil to improve the peak current by opening the magnetic bottle along the beam axis. After demonstration of the principle of the pulsed magnetic extraction, the ion source was tested with different gases. We got promising results from helium up to krypton. For xenon the enhancement of the analyzed current was only in the same order as the enhancement of the afterglow. The influence of the current in the pulsed coil on the analyzed ion current was measured. With increased current levels in the pulsed coil the pulse height of the PuMa-pulse increases within the given pulse length of the coil. By using the pulsed coil the maximum of the charge state distribution can be shifted to higher charge states. (author)

1995-09-01

320

Manipulating multiple sequence alignments via MaM and WebMaM  

Digital Repository Infrastructure Vision for European Research (DRIVER)

MaM is a software tool that processes and manipulates multiple alignments of genomic sequence. MaM computes the exact location of common repeat elements, exons and unique regions within aligned genomics sequences using a variety of user identified programs, databases and/or tables. The program can extract subalignments, corresponding to these various regions of DNA to be analyzed independently or in conjunction with other elements of genomic DNA. Graphical displays further allow an assessment...

2005-01-01