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Sample records for crust geochemical evidence

  1. Geochemical and Sr-Nd-Pb isotopic evidence for ancient lower continental crust beneath the Xi Ujimqin area of NE China

    NASA Astrophysics Data System (ADS)

    Gao, Xiaofeng; Guo, Feng; Xiao, Peixi; Kang, Lei; Xi, Rengang

    2016-05-01

    The Central Asian Orogenic Belt (CAOB) is the largest Phanerozoic accretionary orogen on Earth. The role that Precambrian continental microblocks played in its formation, however, remains a highly controversial topic. New zircon U-Pb age data and whole-rock geochemical and Sr-Nd-Pb isotopic studies on Permian (253-251 Ma) andesites from the Xi Ujimqin area provide the first evidence for the existence of a continental lower mafic crust in the eastern segment of the CAOB. These Permian lavas generally have chemical compositions similar to experimental melts of garnet pyroxenites. Based on Sr-Nd-Pb isotopic compositional differences, they can be further subdivided into two groups. Group 1 has moderately radiogenic Sr (87Sr/86Sr(i) = 0.7060-0.7062) and nonradiogenic Nd (εNd(t) = - 9.0-8.3) and Pb (e.g., 206Pb/204Pb = 17.18-17.23) isotopic compositions similar to the ancient lower mafic crust beneath the North China Craton (NCC). Compared with Group 1, Group 2 has less radiogenic Sr (87Sr/86Sr(i) = 0.7051-0.7055), and more radiogenic Nd (εNd(t) = - 0.2-+1.4) and Pb (e.g., 206Pb/204Pb = 18.04-18.20) isotopic compositions as observed in the Phanerozoic granitoids and felsic lavas of the CAOB. The combined geochemical and isotopic data indicate that Group 1 was derived from ancient lower mafic crust of the NCC affinity, with a residual assemblage of pyroxene + plagioclase + amphibole. The source for Group 2 was a mixture of ancient lower mafic crust and a juvenile crustal component, and melting left a residue of orthopyroxene + clinopyroxene + plagioclase + garnet + amphibole. Generation of these two types of late Permian andesites favors a model whereby breakoff of a subducted slab and subsequent lithospheric extension triggered extensive asthenospheric upwelling and melting of the continental mafic lower crust of the eastern CAOB. The discovery of ancient lower continental crust of the NCC affinity in the CAOB implies that the NCC experienced continental breakup during

  2. Experimental and geochemical evidence for derivation of the El Capitan Granite, California, by partial melting of hydrous gabbroic lower crust

    USGS Publications Warehouse

    Ratajeski, K.; Sisson, T.W.; Glazner, A.F.

    2005-01-01

    Partial melting of mafic intrusions recently emplaced into the lower crust can produce voluminous silicic magmas with isotopic ratios similar to their mafic sources. Low-temperature (825 and 850??C) partial melts synthesized at 700 MPa in biotite-hornblende gabbros from the central Sierra Nevada batholith (Sisson et al. in Contrib Mineral Petrol 148:635-661, 2005) have major-element and modeled trace-element (REE, Rb, Ba, Sr, Th, U) compositions matching those of the Cretaceous El Capitan Granite, a prominent granite and silicic granodiorite pluton in the central part of the Sierra Nevada batholith (Yosemite, CA, USA) locally mingled with coeval, isotopically similar quartz diorite through gabbro intrusions (Ratajeski et al. in Geol Soc Am Bull 113:1486-1502, 2001). These results are evidence that the El Capitan Granite, and perhaps similar intrusions in the Sierra Nevada batholith with lithospheric-mantle-like isotopic values, were extracted from LILE-enriched, hydrous (hornblende-bearing) gabbroic rocks in the Sierran lower crust. Granitic partial melts derived by this process may also be silicic end members for mixing events leading to large-volume intermediate composition Sierran plutons such as the Cretaceous Lamarck Granodiorite. Voluminous gabbroic residues of partial melting may be lost to the mantle by their conversion to garnet-pyroxene assemblages during batholithic magmatic crustal thickening. ?? Springer-Verlag 2005.

  3. Small scale heterogeneity of Phanerozoic lower crust: evidence from isotopic and geochemical systematics of mid-Cretaceous granulite gneisses, San Gabriel Mountains, southern California

    USGS Publications Warehouse

    Barth, A.P.; Wooden, J.L.; May, D.J.

    1992-01-01

    An elongate belt of mid-Cretaceous, compositionally banded gneisses and granulites is exposed in Cucamonga terrane, in the southeastern foothills of the San Gabriel Mountains of southern California. Banded gneisses include mafic granulites of two geochemical types: type 1 rocks are similar to high Al arc basalts and andesites but have higher HFSE (high-field-strength-element) abundances and extremely variable LILE (largeion-lithophile-element) abundances, while type 2 rocks are relatively low in Al and similar to alkali rich MOR (midocean-ridge) or intraplate basalts. Intercalated with mafic granulites are paragneisses which include felsic granulites, aluminous gneisses, marble, and calc-silicate gneisses. Type 1 mafic granulites and calcic trondhjemitic pegmatites also oceur as cross-cutting, synmetamorphic dikes or small plutons. Small-scale heterogeneity of deep continental crust is indicated by the lithologic and isotopic diversity of intercalated ortho-and paragneisses exposed in Cucamonga terrane. Geochemical and isotopic data indicate that K, Rb, and U depletion and Sm/Nd fractionation were associated with biotite +/- muscovite dehydration reactions in type 1 mafic granulites and aluminous gneisses during high-grade metamorphism. Field relations and model initial isotopic ratios imply a wide range of protolith ages, ranging from Early Proterozoic to Phanerozoic. ?? 1992 Springer-Verlag.

  4. Archean crust-mantle geochemical differentiation

    NASA Technical Reports Server (NTRS)

    Tilton, G. R.

    1983-01-01

    Isotope measurements on carbonatite complexes and komatiites can provide information on the geochemical character and geochemical evolution of the mantle, including the sub-continental mantle. Measurements on young samples establish the validity of the method. These are based on Sr, Nd and Pb data from the Tertiary-Mesozoic Gorgona komatiite and Sr and Pb data from the Cretaceous Oka carbonatite complex. In both cases the data describe a LIL element-depleted source similar to that observed presently in MORB. Carbonatite data have been used to study the mantle beneath the Superior Province of the Canadian Shield one billion years (1 AE) ago. The framework for this investigation was established by Bell et al., who showed that large areas of the province appear to be underlain by LIL element-depleted mantle (Sr-85/Sr-86=0.7028) at 1 AE ago. Additionally Bell et al. found four complexes to have higher initial Sr ratios (Sr-87/Sr-86=0.7038), which they correlated with less depleted (bulk earth?) mantle sources, or possibly crustal contamination. Pb isotope relationships in four of the complexes have been studied by Bell et al.

  5. Geochemical evidence for the tectonic setting of the Coast Range ophiolite: A composite island arc oceanic crust terrane in western California

    NASA Astrophysics Data System (ADS)

    Shervais, John W.; Kimbrough, David L.

    1985-01-01

    The Middle to Late Jurassic age Coast Range ophiolite (CRO) of California contains two geochemically distinct volcanic rock associations that formed in different tectonic settings. Volcanic rocks from the southern CRO (Point Sal, Cuesta Ridge, Stanley Mountain, Llanada, Quinto Creek, and Del Puerto) and parts of the northern CRO (Healdsburg, Elder Creek) are similar to low-K tholeiites and calc-alkaline rocks of the island-arc suite. The thin volcanic sections of these ophiolite remnants suggest formation by intra-arc rifting. In contrast, volcanic rocks from Stonyford seamount and Paskenta in the northern CRO are transitional subalkaline metabasalts with geochemical characteristics similar to enriched mid-ocean ridge basalts or ocean-island tholeiites. These rocks are associated with Tithonian radiolarian cherts and may be part of the Franciscan Complex. Alternatively, they may represent a change in tectonic setting within the CRO during the Late Jurassic. Regardless, the CRO as currently conceived cannot be considered a single terrane with one mode of origin.

  6. Shallow origin for South Atlantic Dupal Anomaly from lower continental crust: Geochemical evidence from the Mid-Atlantic Ridge at 26°S

    NASA Astrophysics Data System (ADS)

    Regelous, Marcel; Niu, Yaoling; Abouchami, Wafa; Castillo, Pat R.

    2009-09-01

    We measured trace element concentrations and Pb isotope compositions of fresh volcanic glass samples from the Mid-Atlantic Ridge at 26°S, and from nearby off-axis seamounts. The samples have previously been studied for major elements and Sr-Nd-He isotopes. All samples are depleted MORB, and include some of the most incompatible trace element depleted lavas yet reported from the Atlantic. The seamount lavas are more depleted in highly incompatible elements than the axial lavas, but have high Sr, Pb and Eu concentrations, relative to REE of similar incompatibility. The lavas with the highest Sr/Nd, Pb/Ce and Eu/Eu* have the highest 3He/4He (up to 11.0 R/RA) ratios and the lowest incompatible trace element concentrations. They also have the highest 87Sr/86Sr (up to 0.7036) and 208Pb/204Pb for a given 206Pb/204Pb ratio, which are characteristics of lavas from the Dupal Anomaly in the South Atlantic, and of many EM-1 type intraplate lavas generally. Our data place constraints on the origin of the Dupal Anomaly. The enrichments in Sr, Pb and Eu, together with the low Ca/Al ratios of the seamount lavas indicate that their mantle source consists of material that at one time contained plagioclase, and must therefore have resided at crustal pressures. We argue that the trace element and isotopic compositions of the seamount lavas are best explained by derivation from a mantle source contaminated with lower continental crust, which was introduced into the upper mantle during continental rifting and breakup in the South Atlantic. Our results support previous suggestions that the Dupal Anomaly in the South Atlantic has a relatively recent, shallow origin in lower continental crust and continental lithospheric mantle, rather than in recycled material supplied from the deeper mantle by plumes. Plate reconstructions place the Parana-Etendeka flood basalt province over the central part of the Dupal Anomaly at the time of rifting of South America and Africa at 134 Ma. The flood

  7. Geochemical, oxygen, and neodymium isotope compositions of metasediments from the Abitibi greenstone belt and Pontiac Subprovince, Canada: Evidence for ancient crust and Archean terrane juxtaposition

    SciTech Connect

    Feng, R.; Kerrich, R. ); Maas, R. )

    1993-02-01

    The Abitibi greenstone belt (AGB) and Pontiac Subprovince (PS) in the southwestern Superior Province are adjacent greenstone-plutonic and metasedimentary-dominated terranes, respectively, separated by a major fault zone. Metasediments from these two contrasting terranes are compared in terms of major- and trace-element and O- and Nd-isotope compositions, and detrital zircon ages. The following two compositional populations of metasediments are present in the low-grade, Abitibi southern volcanic zone: (1) a mafic-element-enriched population (MEP) characterized by flat, depleted REE patterns; enhanced Mg, Cr, Co, Ni, and Sc; low-incompatible-element contents; and minor or absent normalized negative troughs at Nb, Ta, and Ti; and (2) a low-mafic-element population (LMEP) featuring LREE-enriched patterns; enhanced Rb, Cs, Ba, Th, and U contents; and pronounced normalized negative troughs at Nb, Ta, and Ti. These geochemical features are interpreted to indicate that the MEP sediments were derived from an ultramafic- and mafic-dominated oceanic provenance, whereas the LMEP sediments represent mixtures of mafic and felsic are source rocks. The PS metasediments are essentially indistinguishable from Abitibi LMEP on the basis of major-element and transition metal abundances, suggesting comparable types of source rocks and degrees of maturity, but are distinct in terms of some trace elements and O-isotope compositions. The Pontiac metasediments are depleted in [sup 18]O and enriched in Cs, Ba, Pb, Th, U, Nb, Ta, Hf, Zr, and total REE and also have higher ratios of Rb/K, Cs/Rb, Ba/Rb, Ta/Nb, Th/La, and Ba/La relative to the Abitibi LMEP. Two subtypes of REE patterns have been identified in PS metasediments. The first subtype is interpreted to be derived from provenances of mixed mafic and felsic volcanic rocks, whereas the Eu-depleted type has features that are typical of post-Archean sediments or Archean K-rich granites and volcanic equivalents. 100 refs., 9 figs., 4 tabs.

  8. Isotope-geochemical Nd-Sr evidence of Palaeoproterozoic plume magmatism in Fennoscandia and mantle-crust interaction on stages of layered intrusions formation

    NASA Astrophysics Data System (ADS)

    Serov, Pavel; Bayanova, Tamara; Kunakkuzin, Evgeniy; Steshenko, Ekaterina

    2016-04-01

    Palaeoproterozoic Fennoscandian layered intrusions belong to the pyroxenite-gabbronorite-anorthosite formation and spread on a vast area within the Baltic Shield. Based on isotope U-Pb, Sm-Nd, Rb-Sr and Re-Os data the duration of this formation can be to 100-130 Ma (2.53-2.40 Ga) [Serov et. al., 2008; Bayanova et. al., 2009]. We have studied rocks of layered PGE-bearing Fedorovo-Pansky, Monchetundra, Burakovsky, Olanga group intrusions and Penikat intrusion. According to recent and new complex Nd-Sr-REE data magma source of the vast majority of these intrusions was a mantle reservoir with unusual characteristics: negative values of ɛNd (from 0 to -4) and ISr = 0.702-0.706, flat spectra of REE (value of (La/Yb)N ~ 1.0-5.8) with positive Eu-anomalies [Bayanova et. al., 2009; Bayanova et. al., 2014]. However, the distribution of REE for ore-bearing gabbronorite intrusions Penikat (Sm-Nd age is 2426 ± 38 Ma [Ekimova et. al., 2011]) has a negative Eu-anomalies. This may be due to the formation of plagioclase and its removal from the magma chamber. One of the aims of isotope geochemical investigations is to establish the contribution of mantle components in the formation of layered intrusions rocks and the degrees of contamination of the magma source by crustal material. To calculate the proportion of mantle component model binary mixture was used [Jahn et. al., 2000]. As the mantle components we used data for CHUR: ɛNd = 0, [Nd] = 1.324 [Palm, O'Neil, 2003] and for crustal components were used host-rocks Nd-data. The proportion of mantle component for the studied intrusions was 77-99%. Also, data were obtained for the Monchetundra dike complex and amphibolized gabbro, for which the proportion of mantle material was 20-40%. For these rocks a significant crustal contamination is most likely. This process resulted in low values of ɛNd, a direct relationship between ɛNd and Nd concentration, and significant differences between the U-Pb and Sm-Nd model ages. A

  9. Hydrothermal hydration of Martian crust: illustration via geochemical model calculations.

    PubMed

    Griffith, L L; Shock, E L

    1997-04-25

    If hydrothermal Systems existed on Mars, hydration of crustal rocks may have had the potential to affect the water budget of the planet. We have conducted geochemical model calculations to investigate the relative roles of host rock composition, temperature, water-to-rock ratio, and initial fluid oxygen fugacity on the mineralogy of hydrothermal alteration assemblages, as well as the effectiveness of alteration to store water in the crust as hydrous minerals. In order to place calculations for Mars in perspective, models of hydrothermal alteration of three genetically related Icelandic volcanics (a basalt, andesite, and rhyolite) are presented, together with results for compositions based on SNC meteorite samples (Shergotty and Chassigny). Temperatures from 150 degrees C to 250 degrees C, water-to-rock ratios from 0.1 to 1000, and two initial fluid oxygen fugacities are considered in the models. Model results for water-to-rock ratios less than 10 are emphasized because they are likely to be more applicable to Mars. In accord with studies of low-grade alteration of terrestrial rocks, we find that the major controls on hydrous mineral production are host rock composition and temperature. Over the range of conditions considered, the alteration of Shergotty shows the greatest potential for storing water as hydrous minerals, and the alteration of Icelandic rhyolite has the lowest potential. PMID:11541456

  10. Hydrothermal hydration of Martian crust: illustration via geochemical model calculations

    NASA Technical Reports Server (NTRS)

    Griffith, L. L.; Shock, E. L.

    1997-01-01

    If hydrothermal Systems existed on Mars, hydration of crustal rocks may have had the potential to affect the water budget of the planet. We have conducted geochemical model calculations to investigate the relative roles of host rock composition, temperature, water-to-rock ratio, and initial fluid oxygen fugacity on the mineralogy of hydrothermal alteration assemblages, as well as the effectiveness of alteration to store water in the crust as hydrous minerals. In order to place calculations for Mars in perspective, models of hydrothermal alteration of three genetically related Icelandic volcanics (a basalt, andesite, and rhyolite) are presented, together with results for compositions based on SNC meteorite samples (Shergotty and Chassigny). Temperatures from 150 degrees C to 250 degrees C, water-to-rock ratios from 0.1 to 1000, and two initial fluid oxygen fugacities are considered in the models. Model results for water-to-rock ratios less than 10 are emphasized because they are likely to be more applicable to Mars. In accord with studies of low-grade alteration of terrestrial rocks, we find that the major controls on hydrous mineral production are host rock composition and temperature. Over the range of conditions considered, the alteration of Shergotty shows the greatest potential for storing water as hydrous minerals, and the alteration of Icelandic rhyolite has the lowest potential.

  11. Hydrothermal hydration of Martian crust: illustration via geochemical model calculations.

    PubMed

    Griffith, L L; Shock, E L

    1997-04-25

    If hydrothermal Systems existed on Mars, hydration of crustal rocks may have had the potential to affect the water budget of the planet. We have conducted geochemical model calculations to investigate the relative roles of host rock composition, temperature, water-to-rock ratio, and initial fluid oxygen fugacity on the mineralogy of hydrothermal alteration assemblages, as well as the effectiveness of alteration to store water in the crust as hydrous minerals. In order to place calculations for Mars in perspective, models of hydrothermal alteration of three genetically related Icelandic volcanics (a basalt, andesite, and rhyolite) are presented, together with results for compositions based on SNC meteorite samples (Shergotty and Chassigny). Temperatures from 150 degrees C to 250 degrees C, water-to-rock ratios from 0.1 to 1000, and two initial fluid oxygen fugacities are considered in the models. Model results for water-to-rock ratios less than 10 are emphasized because they are likely to be more applicable to Mars. In accord with studies of low-grade alteration of terrestrial rocks, we find that the major controls on hydrous mineral production are host rock composition and temperature. Over the range of conditions considered, the alteration of Shergotty shows the greatest potential for storing water as hydrous minerals, and the alteration of Icelandic rhyolite has the lowest potential.

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

    NASA Technical Reports Server (NTRS)

    Lowman, P. D., Jr.

    1973-01-01

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

  13. Effects of phosphatization on the geochemical and mineralogical composition of marine ferromanganese crusts

    NASA Astrophysics Data System (ADS)

    Koschinsky, Andrea; Stascheit, Annemarie; Bau, Michael; Halbach, Peter

    1997-10-01

    Most hydrogenetic ferromanganese crusts in the central Pacific consist of two growth generations: a phosphatized older growth generation that is impregnated with carbonate fluorapatite (CFA), and a younger non-phosphatized growth generation. Geochemical and mineralogical investigations of non-phosphatized and phosphatized crust layers revealed significant differences that are not simply dilution effects by CFA. Certain elements are depleted in old crusts compared to young crusts in the order Si > Fe > Al > Ti ≥ Co > Mn ≥ Pb; others like Ni, Zn, Cu, Y, and REEs (rare earth elements) are mostly enriched. Suboxic conditions in the phosphate-rich water of the oxygen-minimum-zone that infiltrated the crusts caused a partial redissolution of crust phases, resulting in mobilization of associated elements, a partial recrystallization of more stable phases, and a secondary input of nutrient-type elements. The frequent occurrence of todorokite in phosphatized crust layers indicates that Mn mobilized from the vernadite crust phase was partially recrystallized as todorokite, which is more stable under suboxic conditions, incorporating Ni, Cu, and Zn into the crystal lattice, but rejecting Co. Other metals like Pb, Y, and REEs probably form stable phosphate phases in phosphatized crusts. Our investigations emphasize that under the influence of phosphatization, diagenetic remobilization and reorganization in FeMn crusts took place. In contrast to non-phosphatized crusts, the older phosphatized crust generation no longer represents a complete primary crust precipitate. Therefore, young non-phosphatized and old phosphatized crust generations should be considered as two different sample types with respect to geochemical composition, mineralogy, and evolution. We also suggest that most methods for age determination like the 87Sr/86Sr isotopic dating method or Co chronometry cannot be used for phosphatized crusts without caution.

  14. Geochemical Evidence for a Terrestrial Magma Ocean

    NASA Technical Reports Server (NTRS)

    Agee, Carl B.

    1999-01-01

    The aftermath of phase separation and crystal-liquid fractionation in a magma ocean should leave a planet geochemically differentiated. Subsequent convective and other mixing processes may operate over time to obscure geochemical evidence of magma ocean differentiation. On the other hand, core formation is probably the most permanent, irreversible part of planetary differentiation. Hence the geochemical traces of core separation should be the most distinct remnants left behind in the mantle and crust, In the case of the Earth, core formation apparently coincided with a magma ocean that extended to a depth of approximately 1000 km. Evidence for this is found in high pressure element partitioning behavior of Ni and Co between liquid silicate and liquid iron alloy, and with the Ni-Co ratio and the abundance of Ni and Co in the Earth's upper mantle. A terrestrial magma ocean with a depth of 1000 km will solidify from the bottom up and first crystallize in the perovskite stability field. The largest effect of perovskite fractionation on major element distribution is to decrease the Si-Mg ratio in the silicate liquid and increase the Si-Mg ratio in the crystalline cumulate. Therefore, if a magma ocean with perovskite fractionation existed, then one could expect to observe an upper mantle with a lower than chondritic Si-Mg ratio. This is indeed observed in modern upper mantle peridotites. Although more experimental work is needed to fully understand the high-pressure behavior of trace element partitioning, it is likely that Hf is more compatible than Lu in perovskite-silicate liquid pairs. Thus, perovskite fractionation produces a molten mantle with a higher than chondritic Lu-Hf ratio. Arndt and Blichert-Toft measured Hf isotope compositions of Barberton komatiites that seem to require a source region with a long-lived, high Lu-Hf ratio. It is plausible that that these Barberton komatiites were generated within the majorite stability field by remelting a perovskite

  15. Petrological, geochemical and isotopic investigations on a carbonate-dyke and enclosed pyroxenite xenoliths from Val Mastallone (Ivrea-Verbano Zone): evidence of a cumulate carbonatite in the lower crust?

    NASA Astrophysics Data System (ADS)

    Galli, Andrea; Grassi, Daniele; Burg, Jean-Pierre; Schwab, Leo; Rickli, Jörg; Gianola, Omar

    2016-04-01

    The Ivrea-Verbano Zone (Italy/Switzerland) represents one of the best exposed mantle-crust sections worldwide. Its geological evolution has been governed by the Permian underplating of mantle-derived basic magmas („Mafic Complex") into the high-grade basement of the Southern Alps. In the Ivrea-Verbano Zone, marbles occur as concordant bodies or partly discordant carbonate-dykes. Generally, these dykes are constituted of calcite, diopside, scapolite, contain enclave of the host rocks and display sharp contacts to the host lithologies without evidences of alteration zones. In Val Mastallone, an up to 40 m thick carbonate-dyke with different characteristics occurs within mafic granulites. This dyke is composed of calcite, clinopyroxene and subordinate allanite and zircon. No scapolite is observed. The contacts to the host granulites are characterized by alteration zones composed of actinolite, chlorite, clinozoisite, plagioclase and calcite. The carbonate-dyke bears enclave of phlogopite-amphibole-apatite-rutile-ilmenite ± garnet or spinel clinopyroxenites. These rock type is not outcropping elsewhere in the proximity of the dyke, suggesting a significant transport. Host mafic granulite enclave are found exclusively at the margin of the dyke. Calcite dykelets rich in zircon, baddeleyite and other Ba, U, Th, REE-rich phases cut across the enclave. The carbonate-dyke shows an enrichment of LREE over HREE ((La/Yb)N = 14), with a Σ REE = 338 and Y/Ho = 27. On the chondrite-normalized REE abundances diagram, no Eu anomaly is observed. Mantle-normalized pattern shows strong negative anomalies at Cs, Rb, K, Pb, P, Zr, Hf, Ti and positive Ba, Th, Sr, Nd anomalies, similarly to the "world average carbonatites". Measured absolute trace element concentrations are lower than average carbonatites but significantly higher than typical limestones and similar to cumulate carbonatites found elsewhere in the world (e.g. India, China, Brazil). Grt-bearing clinopyroxenite enclave

  16. No evidence for Hadean continental crust within Earth's oldest evolved rock unit

    NASA Astrophysics Data System (ADS)

    Reimink, J. R.; Davies, J. H. F. L.; Chacko, T.; Stern, R. A.; Heaman, L. M.; Sarkar, C.; Schaltegger, U.; Creaser, R. A.; Pearson, D. G.

    2016-10-01

    Due to the acute scarcity of very ancient rocks, the composition of Earth's embryonic crust during the Hadean eon (>4.0 billion years ago) is a critical unknown in our search to understand how the earliest continents evolved. Whether the Hadean Earth was dominated by mafic-composition crust, similar to today's oceanic crust, or included significant amounts of continental crust remains an unsolved question that carries major implications for the earliest atmosphere, the origin of life, and the geochemical evolution of the crust-mantle system. Here we present new U-Pb and Hf isotope data on zircons from the only precisely dated Hadean rock unit on Earth--a 4,019.6 +/- 1.8 Myr tonalitic gneiss unit in the Acasta Gneiss Complex, Canada. Combined zircon and whole-rock geochemical data from this ancient unit shows no indication of derivation from, or interaction with, older Hadean continental crust. Instead, the data provide the first direct evidence that the oldest known evolved crust on Earth was generated from an older ultramafic or mafic reservoir that probably surfaced the early Earth.

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

    NASA Astrophysics Data System (ADS)

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

    2009-04-01

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

  18. Geochemical characteristics and metal element enrichment in crusts from seamounts of the Western Pacific

    NASA Astrophysics Data System (ADS)

    Zhang, Xiaoyu; Zhu, Kechao; Du, Yong; Zhang, Fuyuan; Zhang, Weiyan; Ren, Xiangwen; Jiang, Binbin; Huang, Dasong

    2016-03-01

    Elemental geochemistry is an essential part of understanding mineralization mechanisms. In this paper, a data set of 544 cobalt crust samples from seamounts of the Western Pacific are used to study the enrichment characteristics of metal elements. REE normalization is utilized to reveal the origin of the crusts; effects of water depth on Co enrichment and impacts of phosphatization on mineral quality are discussed to obtain the evolution of these marine mineral deposits, which gives support to further resource assessment. Conclusions are reached as follows: 1) Elemental abundances, inter-element relationships, and shale-normalized REE patterns for phosphate-poor crusts from different locations reflect hydrogenetic origin of the crusts. EFs (enrichment coefficients) of REE exhibit exponential increase from surface sediments to phosphorite to polymetallic nodules to crusts, suggesting that the improved degree of hydrogeneous origin induces the enrichment of REE. 2) The crusts in the Western Pacific, formed through hotspot produced guyots trails, have relatively lower REE than those in the Mid-Pacific. The latter could be attributed to the peculiar submarine topography of seamounts formed by intraplate volcanism. 3) The non-phosphatized younger crust layers have 40% higher Co than the phosphatized older layers. This indicates the modification of the elemental composition in these crusts by phosphatization. A general depletion of hydroxide-dominated elements such as Co, Ni, and Mn and enrichment of P, Ca, Ba, and Sr is evident in phosphatized crusts, whereas non-phosphatized younger generation crusts are rich in terrigenous aluminosilicate detrital matter. 4) Co increases above the carbonate compensation depth (CCD) from less than 0.53% to over 0.65% in seamount regions with water depth of less than 2,500 m, suggesting the significance of the dissolution of carbonate in the sea water column to the growth and composition of crusts.

  19. Geochemical study of black crusts as a diagnostic tool in cultural heritage

    NASA Astrophysics Data System (ADS)

    La Russa, Mauro F.; Belfiore, Cristina M.; Comite, Valeria; Barca, Donatella; Bonazza, Alessandra; Ruffolo, Silvestro A.; Crisci, Gino M.; Pezzino, Antonino

    2013-12-01

    This contribution focuses on spectrometric analyses carried out on crust samples covering the stone surface of the boundary walls of the Tower of London. The main goal of this research is to investigate the degradation processes related to the environmental impact on cultural heritage. Specifically, the chemical contamination of stone substrate in the Tower of London due to the crust formation was examined through laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). This technique allowed us to achieve a complete characterization of the damage layers in terms of trace elements. In addition, optical microscopy (OM), scanning electron microscopy coupled with energy-dispersive X-ray spectrometry (SEM-EDS) and infrared spectroscopic techniques (FT-IR) were also used for an exhaustive characterization of the examined samples. Results obtained demonstrated that such a geochemical approach represents a powerful diagnostic tool in the study of black crusts, since it represents a reliable indicator of the environmental pollution. The higher concentrations of most heavy metals in black crusts with respect to the underlying stone suggest that crusts were greatly influenced by atmospheric inputs in their formation, mainly represented by mobile combustion sources. In addition, the possibility of analyzing in some samples the portion of altered substrate allowed us to hypothesize that some specific heavy metals tend to migrate from the crust to the unaltered substrate over time, becoming catalysts for the formation of new crust. Therefore, this research focuses on the role of diagnostics in order to plan suitable cleaning and consolidation intervention aimed at a better protection of the monument.

  20. In situ evidence for continental crust on early Mars

    NASA Astrophysics Data System (ADS)

    Sautter, V.; Toplis, M. J.; Wiens, R. C.; Cousin, A.; Fabre, C.; Gasnault, O.; Maurice, S.; Forni, O.; Lasue, J.; Ollila, A.; Bridges, J. C.; Mangold, N.; Le Mouélic, S.; Fisk, M.; Meslin, P.-Y.; Beck, P.; Pinet, P.; Le Deit, L.; Rapin, W.; Stolper, E. M.; Newsom, H.; Dyar, D.; Lanza, N.; Vaniman, D.; Clegg, S.; Wray, J. J.

    2015-08-01

    Understanding of the geologic evolution of Mars has been greatly improved by recent orbital, in situ and meteorite data, but insights into the earliest period of Martian magmatism (4.1 to 3.7 billion years ago) remain scarce. The landing site of NASA’s Curiosity rover, Gale crater, which formed 3.61 billion years ago within older terrain, provides a window into this earliest igneous history. Along its traverse, Curiosity has discovered light-toned rocks that contrast with basaltic samples found in younger regions. Here we present geochemical data and images of 22 specimens analysed by Curiosity that demonstrate that these light-toned materials are feldspar-rich magmatic rocks. The rocks belong to two distinct geochemical types: alkaline compositions containing up to 67 wt% SiO2 and 14 wt% total alkalis (Na2O + K2O) with fine-grained to porphyritic textures on the one hand, and coarser-grained textures consistent with quartz diorite and granodiorite on the other hand. Our analysis reveals unexpected magmatic diversity and the widespread presence of silica- and feldspar-rich materials in the vicinity of the landing site at Gale crater. Combined with the identification of feldspar-rich rocks elsewhere and the low average density of the crust in the Martian southern hemisphere, we conclude that silica-rich magmatic rocks may constitute a significant fraction of ancient Martian crust and may be analogous to the earliest continental crust on Earth.

  1. A Detailed Geochemical Study of Island Arc Crust: The Talkeetna Arc Section, South-central Alaska

    NASA Astrophysics Data System (ADS)

    Greene, A. R.; Debari, S. M.; Kelemen, P. B.; Clift, P. D.; Blusztajn, J.

    2002-12-01

    The Talkeetna arc section in south-central Alaska is recognized as the exposed upper mantle and crust of an accreted, Late Triassic to Middle Jurassic island arc. Detailed geochemical studies of layered gabbronorite from the middle and lower crust of this arc and a diverse suite of volcanic and plutonic rocks from the middle and upper crust provide crucial data for understanding arc magma evolution. We also present new data on parental magma compositions for the arc. The deepest level of the arc section consists of residual mantle and ultramafic cumulates adjacent to garnet gabbro and basal gabbronorite interlayered with pyroxenite. The middle crust is primarily layered gabbronorite, ranging from anorthosite to pyroxenite in composition, and is the most widespread plutonic lithology. The upper mid crust is a heterogenous assemblage of dioritic to tonalitic rocks mixed with gabbro and intruded by abundant mafic dikes and chilled pillows. The upper crust of the arc is comprised of volcanic rocks of the Talkeetna Formation ranging from basalt to rhyolite. Most of these volcanic rocks have evolved compositions (<5% MgO, Mg# <60) and overlap the composition of intermediate to felsic plutonic rocks (<3.5% MgO, Mg# <45). However, several chilled mafic rocks and one basalt have primitive characteristics (>8% MgO, Mg# >60). Ion microprobe analyses of clinopyroxene in mid-crustal layered gabbronorites have parallel REE patterns with positive-sloping LREE segments (La/Sm(N)=0.05-0.17; mean 0.11) and flat HREE segments (5-25xchondrite; mean 10xchondrite). Liquids in REE equilibrium with the clinopyroxene in these gabbronorite cumulates were calculated in order to constrain parental magmas. These calculated liquids(La/Sm(N)=0.77-1.83; mean 1.26) all fall within the range of dike and volcanic rock(La/Sm(N)=0.78-2.12; mean 1.23) compositions. However, three lavas out of the 44 we have analyzed show strong HREE depletion, which is not observed in any of the liquid compositions

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

    PubMed

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

    2011-10-01

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

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

    PubMed

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

    2011-10-01

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

  4. Assessment of the effects of air pollution on european monuments through a geochemical characterization of black crusts

    NASA Astrophysics Data System (ADS)

    Comite, Valeria; Barca, Donatella; Belfiore, Cristina Maria; Bonazza, Alessandra; La Russa, Mauro Francesco; Ruffolo, SIlvestro Antonio; Pezzino, Antonino; Sabbioni, Cristina

    2014-05-01

    This contribution focuses on spectrometric analyses carried out on black crust samples, collected from buildings and churches belonging to the European built Heritage, i.e., the Corner Palace in Venice (Italy), the Cathedral of St. Rombouts in Mechelen (Belgium), the Church of St. Eustache in Paris (France) and the Tower of London (United Kingdom). Such monuments, all built in carbonate stones, were selected for their historic and artistic relevance, as well as for their location in different urban environments. For an exhaustive account of the sampled black crusts, an approach integrating complementary techniques was used, including OM, SEM-EDS, FT-IR and LA-ICP-MS. The complete characterization of the damage layers provided information on their chemical composition, the state of conservation of the underlying substrates and the interactions between crusts and stones. In particular, the geochemical study in terms of trace elements revealed that all crusts are enriched in heavy metals (As, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, Pb, Sb, Sn, Ti, V, and Zn) compared to substrates. The different concentrations of such elements in all analyzed crust samples can be ascribed to several factors, such as: height of sampling, morphology of the sampled surfaces (vertical or horizontal), exposure to atmospheric agents as well as to direct (road or boat traffic) or indirect (industries) sources of pollution, accumulation time of pollutants on the surface, wash out and particulate air pollution. Specifically, the crusts collected at lower heights (Corner Palace, Cathedral of St. Rombouts, Tower of London) resulted to be mainly influenced by mobile sources of pollution (vehicular or boat traffic), while samples taken at higher heights (Church of St. Eustache and Corner Palace) are generally mostly affected by stationary combustion sources. In some cases, the detailed analysis of multilayered crusts (Palazzo Corner) contributed to recognize the variation of combustion sources responsible

  5. Do Two Deep Drill Holes Into the Upper Ocean Crust Quantify the Hydrothermal Contribution to Global Geochemical Cycles?

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

    Vigorous circulation of seawater at the ocean ridges is required to cool and crystallize magma to form new ocean crust. Axial and ridge flank hydrothermal fluid circulation is accompanied by seawater-basalt exchanges over a spectrum of temperatures that buffer the chemistry of seawater, provide unique microbial niches, alter the chemistry and mineralogy of the ocean crust, and through subduction return surface-derived geochemical tracers to the interior of our planet. In many models of axial and ridge flank hydrothermal circulation, most fluid-rock interaction occurs in the upper oceanic crust. Hence inventories of seawater exchange should be captured by relatively shallow (<2 km) boreholes. However, after 45+ years of ocean drilling we have just two deep drill holes that sample the lava and dike layers of intact upper oceanic crust. DSDP Hole 504B on 6.9 Ma ocean crust produced at the intermediate spreading rate Costa Rica Rift penetrates 1836 m into basement through a complete sequence of lavas to near the base of the sheeted dike complex. In isolation, Hole 504B became the 'reference section' for upper oceanic crust from which hydrothermal contributions to global geochemical cycles were determined. The recent drilling of Hole 1256D in 15 Ma superfast spreading rate Pacific crust penetrated through the complete volcanic and sheeted dike layers into the underlying gabbroic rocks in intact ocean crust for the first time. These boreholes are complemented by observations from seafloor tectonic windows, fracture zones, and ophiolites, but these are imperfect analogs. Although Holes 504B and 1256D formed at different spreading rates, crust from both sites is expected to conform to textbook Penrose-type layering, albeit with different thicknesses of lavas and dikes. However, what was not anticipated was the contrasting distribution and nature of elemental and isotopic hydrothermal exchanges. Differences reflect the influence of local crustal structure, such as lava

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

    USGS Publications Warehouse

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

    2001-01-01

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

  7. The geochemical fingerprint of serpentinite- and crust-dominated plate-interface settings: some tectonic implications

    NASA Astrophysics Data System (ADS)

    Cannaò, Enrico; Scambelluri, Marco; Agostini, Samuele; Tonarini, Sonia

    2014-05-01

    garnet peridotite and harzburgite from Gagnone have low B (up to 9 ppm), low B/Nb (<100) and high Pb and Sr isotopic ratios (206Pb/204Pb up to 18.84; 87Sr/86Sr 0.7124). Eclogite and HP metarodingite in the Gagnone peridotite show comparable values. The host metasediments and gneiss show higher B (6-16 ppm), 206Pb/204Pb (up to 18.98) and 87Sr/86Sr (0.7275). than peridotites and mafic rocks. All the Gagnone rocks have negative delta11B (ultramafic and mafic rocks = 0 to -10 per mil; country rocks = -3 to -12 per mil). The Gagnone peridotites reveal geochemical mixing between ultramafic and host crustal reservoirs. Considering that these peridotites derive from serpentinized protoliths, we expect that the initial high 11B of serpentinites was modified by two combined processes: (1) serpentine dehydration, releasing heavy B to fluids, and (2) exchange between ultramafic rocks and sediment-derived subduction fluids during burial and exhumation. The geochemical signature of the Voltri serpentinites is indicative of interaction with slab fluids enriched in heavy B-rich and in crust-derived components, such as in mantle rocks which evolved atop of the subducting slab. This implies that the slices of the downgoing slab are emplaced early during their burial history atop of the subducting plate. The geochemical signature of peridotites and host metasediments from Gagnone, points to significant exchange between ultramafic bodies and host rocks during prograde subduction prior to peak metamorphism. This again indicates accretion to the plate interface of slab and wedge materials during an early stage of subduction. Moreover, Voltri and Gagnone represent distinct reservoirs, showing positive versus negative delta11B. Serpentinite-dominated settings, like Voltri, produce high B and 11B fluids which can explain 11B-enrichment of much Pacific arcs. Differently, the sediment- and gneiss-dominated Gagnone mélange shows high B, negative δ11B, high radiogenic Pb and Sr: fluids released

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

    NASA Astrophysics Data System (ADS)

    Shimoda, G.; Kogiso, T.

    2013-12-01

    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-μ) 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

  9. Geochemical investigation of Gabbroic Xenoliths from Hualalai Volcano: Implications for lower oceanic crust accretion and Hualalai Volcano magma storage system

    NASA Astrophysics Data System (ADS)

    Gao, Ruohan; Lassiter, John C.; Barnes, Jaime D.; Clague, David A.; Bohrson, Wendy A.

    2016-05-01

    The patterns of axial hydrothermal circulation at mid-ocean ridges both affect and are influenced by the styles of magma plumbing. Therefore, the intensity and distribution of hydrothermal alteration in the lower oceanic crust (LOC) can provide constraints on LOC accretion models (e.g., "gabbro glacier" vs. "multiple sills"). Gabbroic xenoliths from Hualalai Volcano, Hawaii include rare fragments of in situ Pacific lower oceanic crust. Oxygen and strontium isotope compositions of 16 LOC-derived Hualalai gabbros are primarily within the range of fresh MORB, indicating minimal hydrothermal alteration of the in situ Pacific LOC, in contrast to pervasive alteration recorded in LOC xenoliths from the Canary Islands. This difference may reflect less hydrothermal alteration of LOC formed at fast ridges than at slow ridges. Mid-ocean ridge magmas from slow ridges also pond on average at greater and more variable depths and undergo less homogenization than those from fast ridges. These features are consistent with LOC accretion resembling the "multiple sills" model at slow ridges. In contrast, shallow magma ponding and limited hydrothermal alteration in LOC at fast ridges are consistent with the presence of a long-lived shallow magma lens, which limits the penetration of hydrothermal circulation into the LOC. Most Hualalai gabbros have geochemical and petrologic characteristics indicating derivation from Hualalai shield-stage and post-shield-stage cumulates. These xenoliths provide information on the evolution of Hawaiian magmas and magma storage systems. MELTS modeling and equilibration temperatures constrain the crystallization pressures of 7 Hualalai shield-stage-related gabbros to be ∼2.5-5 kbar, generally consistent with inferred local LOC depth. Therefore a deep magma reservoir existed within or at the base of the LOC during the shield stage of Hualalai Volcano. Melt-crust interaction between Hawaiian melts and in situ Pacific crust during magma storage partially

  10. Carbonation and Serpentinization of the Martian Crust inputs from by Geochemical Modelling

    NASA Astrophysics Data System (ADS)

    Bultel, B.; Klein, F.; Andréani, M.; Quantin-Nataf, C.

    2015-10-01

    The investigation of the CRISM data indicates that the serpentinization and the carbonation have to be considered as significant processes of alteration of the early Mars. We report here a tentative of characterization of these processes by geochemical modelling with EQ3/6 programs.

  11. Evolution of continental crust and mantle heterogeneity: Evidence from Hf isotopes

    USGS Publications Warehouse

    Jonathan, Patchett P.; Kouvo, O.; Hedge, C.E.; Tatsumoto, M.

    1982-01-01

    We present initial 176Hf/177 Hf ratios for many samples of continental crust 3.7-0.3 Gy old. Results are based chiefly on zircons (1% Hf) and whole rocks: zircons are shown to be reliable carriers of essentially the initial Hf itself when properly chosen on the basis of U-Pb studies. Pre-3.0 Gy gneisses were apparently derived from an unfractionated mantle, but both depleted and undepleted mantle are evident as magma sources from 2.9 Gy to present. This mantle was sampled mainly from major crustal growth episodes 2.8, 1.8 and 0.7 Gy ago, all of which show gross heterogeneity of 176Hf/177Hf in magma sources from ??Hf=0 to +14, or about 60% of the variability of the present mantle. The approximate ??Hf=2??Nd relationship in ancient and modern igneous rocks shows that 176Lu/177Hf fractionates in general twice as much as 147Sm/144Nd in mantle melting processes. This allows an estimation of the relative value of the unknown bulk solid/liquid distribution coefficient for Hf. DLu/DHf=??? 2.3 holds for most mantle source regions. For garnet to be an important residual mantle phase, it must hold Hf strongly in order to preserve Hf-Nd isotopic relationships. The ancient Hf initials are consistent with only a small proportion of recycled older cratons in new continental crust, and with quasi-continuous, episodic growth of the continental crust with time. However, recycling of crust less than 150 My old cannot realistically be detected using Hf initials. The mantle shows clearly the general positive ??Hf resulting from a residual geochemical state at least back to 2.9 Gy ago, and seems to have repeatedly possessed a similar degree of heterogeneity, rather than a continuously-developing depletion. This is consistent with a complex dynamic disequilibrium model for the creation, maintenance and destruction of heterogeneity in the mantle. ?? 1981 Springer-Verlag.

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

    USGS Publications Warehouse

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

    2009-01-01

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

  13. Pb isotopic evidence for early Archaean crust in South Greenland

    NASA Technical Reports Server (NTRS)

    Taylor, P. N.; Kalsbeek, F.

    1986-01-01

    The results of an isotopic remote sensing study focussed on delineating the extent of Early Archean crust north and south of the Nuuk area and in south Greenland is presented. Contamination of the Late Archean Nuk gneisses and equivalents by unradiogenic Pb uniquely characteristic of Amitsoq gneiss was detected as far south as Sermilik about 70 km south of Nuuk and only as far north as the mouth of Godthabsfjord. This study was extended to the southern part of the Archean craton and the adjoining Early Proterozoic Ketilidian orogenic belt where the Pb isotopes suggest several episodes of reworking of older uranium depleted continental crust. The technique of using the Pb isotope character of younger felsic rocks, in this case Late Archean and Early Proterozoic gneisses and granites to sense the age and isotopic character of older components, is a particularly powerful tool for reconstructing the evolutionary growth and development of continental crust.

  14. Plutonic xenoliths from Martinique, Lesser Antilles: evidence for open system processes and reactive melt flow in island arc crust

    NASA Astrophysics Data System (ADS)

    Cooper, George F.; Davidson, Jon P.; Blundy, Jon D.

    2016-10-01

    The Lesser Antilles Volcanic Arc is remarkable for the abundance and variety of erupted plutonic xenoliths. These samples provide a window into the deeper crust and record a more protracted crystallisation history than is observed from lavas alone. We present a detailed petrological and in situ geochemical study of xenoliths from Martinique in order to establish their petrogenesis, pre-eruptive storage conditions and their contribution to construction of the sub-volcanic arc crust. The lavas from Martinique are controlled by crystal-liquid differentiation. Amphibole is rarely present in the erupted lavas, but it is a very common component in plutonic xenoliths, allowing us to directly test the involvement of amphibole in the petrogenesis of arc magmas. The plutonic xenoliths provide both textural and geochemical evidence of open system processes and crystal `cargos'. All xenoliths are plagioclase-bearing, with variable proportions of olivine, spinel, clinopyroxene, orthopyroxene and amphibole, commonly with interstitial melt. In Martinique, the sequence of crystallisation varies in sample type and differs from other islands of the Lesser Antilles arc. The compositional offset between plagioclase (~An90) and olivine (~Fo75), suggests crystallisation under high water contents and low pressures from an already fractionated liquid. Texturally, amphibole is either equant (crystallising early in the sequence) or interstitial (crystallising late). Interstitial amphibole is enriched in Ba and LREE compared with early crystallised amphibole and does not follow typical fractionation trends. Modelling of melt compositions indicates that a water-rich, plagioclase-undersaturated reactive melt or fluid percolated through a crystal mush, accompanied by the breakdown of clinopyroxene, and the crystallisation of amphibole. Geothermobarometry estimates and comparisons with experimental studies imply the majority of xenoliths formed in the mid-crust. Martinique cumulate xenoliths are

  15. Geochemical Evidence of Microbially-Mediated Subglacial Mineral Weathering

    NASA Astrophysics Data System (ADS)

    Montross, S. N.; Skidmore, M. L.

    2006-12-01

    Interactions between dilute meltwater and fine-grained, freshly comminuted debris at the bed of temperate glaciers liberate significant solute. The proportions of solute produced in the subglacial environment via biotic and abiotic processes remains unknown, however, this work suggests the biotic contribution is substantial. Laboratory analyses of microbiological and geochemical properties of sediment and meltwater from the Haut Glacier d'Arolla (HGA) indicates that a metabolically active microbial community exists in water-saturated sediments at the ice-bedrock interface. Basal sediment slurries and meltwater were incubated in the laboratory for 100 days under near in situ subglacial conditions. Relative proportions of solute produced via abiotic v. biotic mineral weathering were analyzed by comparing the evolved aqueous chemistry of biologically active "live" sediment slurries with sterilized controls. Aqueous chemical analyses indicate an increase in solute produced from mineral weathering coupled with nitrate depletion in the biologically active slurries compared with the killed controls. These results infer that microbial activity at HGA is likely an important contributor to chemical weathering associated solute fluxes from the glaciated catchment. Due to the magnitude of past glaciations throughout geologic time (e.g., Neoproterozoic and Late-Pleistocene), and evidence that subglacial microbial activity impacts mineral weathering, greater consideration needs to be given to cold temperature biogeochemical weathering and its impact on global geochemical cycles.

  16. Effect of subduction components on production of basalts from Tateshina volcano, central Japan: geochemical calculation of dehydration of subducting oceanic crust and partial melting of overlying sediments, and subsequent fluid-mantle interaction

    NASA Astrophysics Data System (ADS)

    Katoh, Masayasu; Shuto, Kenji

    Effect of subduction components on production of basalts from Tateshina volcano, central Japan: geochemical calculation of dehydration of subducting oceanic crust and partial melting of overlying sediments, and subsequent fluid-mantle interaction

  17. Magnetism of the oceanic crust: Evidence from ophiolite complexes

    SciTech Connect

    Banerjee, S.K.

    1980-07-10

    The magnetic properties of six ophiolite complexes from around the world, ranging in age from Jurassic to Miocene, are presented. An emphasis is placed in our study on the petrologic and isotopic data from these ophiolite complexes in order to determine first whether the rock samples presently available represent the pristine ocean crust or whether they have been altered subaerially since their formation. Five of the ophiolites are found to be acceptable, and the conclusion is overwhelmingly in favor of a marine magnetic source layer that includes not only the pillow lavas but also the underlying dikes and gabbro. At the moment, however, our observations do not suggest that the magnetic contributions of the basaltic dikes should be overlooked in favor of gabbro. A second important conclusion is that nearly pure magnetite could indeed be a magnetic carrier which contributes to marine magnetic anomanies. It only awaits discovery by deeper ocean crustal penetration by future Deep Sea Drilling Project legs.

  18. Evidence for oceanic crust in the Herodotus Basin

    NASA Astrophysics Data System (ADS)

    Granot, Roi

    2016-04-01

    Some of the fundamental tectonic problems of the Eastern Mediterranean remain unresolved due to the extremely thick sedimentary cover (10 to 15 km) and the lack of accurate magnetic anomaly data. I have collected 7,000 km of marine magnetic profiles (2012-2014) across the Herodotus and Levant Basins, Eastern Mediterranean, to study the nature and age of the underlying igneous crust. The towed magnetometer array consisted of two Overhauser sensors recording the total magnetic anomaly field in a longitudinal gradiometer mode, and a fully oriented vector magnetometer. The total field data from the Herodotus Basin reveal a newly detected short sequence of long-wavelength NE-SW lineated anomalies that straddle the entire basin suggesting a deep two-dimensional magnetic source layer. The three components of the magnetic vector data indicate that an abrupt transition from a 2D to 3D magnetic structure occurs east of the Herodotus Basin, along where a prominent NE-SW gravity feature is found. Altogether, these new findings confirm that the Herodotus Basin preserves remnants of oceanic crust that formed along the Neotethyan mid-ocean ridge system. The continuous northward and counterclockwise motion of the African Plate during the Paleozoic and Mesozoic allow predicting the evolution of remanent magnetization directions, which in-turn dictate that shape of the anomalies. The shape of the Herodotus anomalies best fit Late Carboniferous to Early Permian (300±20 Myr old) magnetization directions. Finally, I will discuss the implications of these results on the tectonic architecture of the region as well as on various geodynamic processes.

  19. Geochemical Evidence of Cryptic Sulfur Cycling in Salt Marsh Sediments

    NASA Astrophysics Data System (ADS)

    Mills, J. V.; Antler, G.; Turchyn, A. V.

    2014-12-01

    In modern marine and marginal marine sediments, bacterial sulfate reduction dominates the subsurface oxidation of organic carbon due to the abundance of sulfate in many surface environments. While bacterial sulfate reduction may control anaerobic organic carbon oxidation, there is increasing evidence that iron redox chemistry may be intimately linked to sulfur redox chemistry in the anoxic subsurface, with iron species acting as catalysts or electron shuttles for the microbial use of sulfur, and vice versa. We use stable isotope and geochemical techniques to explore the coupling of the iron and sulfur cycles in salt marsh sediments in North Norfolk, UK. Unique among previously studied environments, these sediments contain high concentrations of both sulfate (20-40mM) and ferrous iron (1-3mM). High ferrous iron concentrations require extended regions of bacterial iron reduction. Within these zones of iron reduction we would predict no sulfate reduction, and lack of change in sulfur isotopes and no loss of sulfate suggest that there is no net sulfate reduction in this zone. However, coincident with the increase in ferrous iron concentrations, the δ18Osulfate exhibits significant increases of up to 5‰. The decoupling of the sulfur and oxygen isotopes of sulfate is suggestive of a cryptic sulfur cycle in which sulfate is reduced to an intermediate valence state sulfur species and subsequently reoxidized to sulfate; this cycle must by quasi-quantitative to produce the suite of geochemical observations. We further explore the nature of this cycling through a series of batch reactor incubation experiments. When sediments are incubated in 18O-enriched water, significant shifts (>15‰) in the δ18Osulfate are observed with no corresponding shift in sulfur isotopes. This provides direct evidence that microbial assemblages in these salt marsh sediments facilitate a cryptic cycling of sulfur, potentially mediated by iron species in the zone of iron reduction. We contrast

  20. Sources of continental crust: neodymium isotope evidence from the sierra nevada and peninsular ranges.

    PubMed

    Depaolo, D J

    1980-08-01

    Granitic rocks from batholiths of the Sierra Nevada and Peninsular Ranges exhibit initial (143)Nd/(144)Nd ratios that vary over a large range and correlate with (87)Sr/(86)Sr ratios. The data suggest that the batholiths represent mixtures of materials derived from (i) chemically depleted mantle identical to the source of island arcs and (ii) old continental crust, probably sediments or metasediments with a provenance age of approximately 1.6 x 10(9) years. These conclusions are consistent with a model for continental growth whereby new crustal additions are repeatedly extracted from the same limited volume of the upper mantle, which has consequently become depleted in elements that are enriched in the crust. There is little evidence that hydrothermally altered, subducted oceanic crust is a primary source of the magmas. PMID:17821189

  1. Sources of continental crust: neodymium isotope evidence from the Sierra Nevada and Peninsular ranges

    SciTech Connect

    DePaolo, D.J.

    1980-08-08

    Granitic rocks from batholiths of the Sierra Nevada and Peninsular Ranges exhibit initial /sup 143/Nd//sup 144/Nd ratios that vary over a large range and correlate with /sup 87/Sr//sup 86/Sr ratios. The data suggest that the batholiths represent mixtures of materials derived from (i) chemically depleted mantle identical to the source of island arcs and (ii) old continental crust, probably sediments or metasediments with a provenance age of approx. 1.6 x 10/sup 9/ years. These conclusions are consistent with a model for continental growth whereby new crustal additions are repeatedly extracted from the same limited volume of the upper mantle, which has consequently become depleted in elements that are enriched in the crust. There is little evidence that hydrothermally altered, subducted oceanic crust is a primary source of the magmas.

  2. Geophysical and geochemical nature of relaminated arc-derived lower crust underneath oceanic domain in southern Mongolia

    NASA Astrophysics Data System (ADS)

    Guy, Alexandra; Schulmann, Karel; Janoušek, Vojtech; Štípská, Pavla; Armstrong, Robin; Belousova, Elena; Dolgopolova, Alla; Seltmann, Reimar; Lexa, Ondrej; Jiang, Yingde; Hanžl, Pavel

    2016-04-01

    The Central Asian Orogenic Belt (CAOB) in southern Mongolia consists of E-W trending Neoproterozoic cratons and Silurian-Devonian oceanic tectonic zones. Previous study revealed that the Early Paleozoic accretionary wedge and the oceanic tectonic zone are underlain by a layer giving a homogeneous gravity signal. Forward gravity modelling suggests that this layer is not formed of high-density material typical of lower oceanic crust but is composed of low- to intermediate-density rocks resembling continental crust. The nature of this lower crust is constrained by the whole-rock geochemistry and zircon Hf isotopic signature of abundant Late Carboniferous high-K calc-alkaline and Early Permian A-type granitoids intruding the two Early Paleozoic domains. It is possible to explain the genesis of these granitoids by anatexis of juvenile, metaigneous (tonalitic-gabbroic) rocks of Late Cambrian age, the source of which is presumed to lie in the "Khantaishir" arc (520-495Ma) further north. In order to test this hypothesis, the likely modal composition and density of Khantaishir arc-like protoliths are thermodynamically modelled at granulite- and higher amphibolite-facies conditions. It is shown that the current average density of the lower crust inferred by gravity modelling (2730 ±20kg/m3) matches best metamorphosed leucotonalite to diorite. Based on these results, it is now proposed that Mongolian CAOB has an architecture in which the accretionary wedge and oceanic upper crust is underlain by allochthonous lower crust that originated in a Cambrian arc. A tectonic model explaining relamination of allochthonous felsic to intermediate lower crust beneath mafic upper crust is proposed.

  3. Geophysical and geochemical nature of relaminated arc-derived lower crust underneath oceanic domain in southern Mongolia

    NASA Astrophysics Data System (ADS)

    Guy, Alexandra; Schulmann, Karel; Janoušek, Vojtěch; Å típská, Pavla; Armstrong, Robin; Belousova, Elena; Dolgopolova, Alla; Seltmann, Reimar; Lexa, Ondrej; Jiang, Yingde; Hanžl, Pavel

    2015-05-01

    The Central Asian Orogenic Belt (CAOB) in southern Mongolia consists of E-W trending Neoproterozoic cratons and Silurian-Devonian oceanic tectonic zones. Previous study revealed that the Early Paleozoic accretionary wedge and the oceanic tectonic zone are underlain by a layer giving a homogeneous gravity signal. Forward gravity modelling suggests that this layer is not formed of high-density material typical of lower oceanic crust but is composed of low- to intermediate-density rocks resembling continental crust. The nature of this lower crust is constrained by the whole-rock geochemistry and zircon Hf isotopic signature of abundant Late Carboniferous high-K calc-alkaline and Early Permian A-type granitoids intruding the two Early Paleozoic domains. It is possible to explain the genesis of these granitoids by anatexis of juvenile, metaigneous (tonalitic-gabbroic) rocks of Late Cambrian age, the source of which is presumed to lie in the "Khantaishir" arc (520-495 Ma) further north. In order to test this hypothesis, the likely modal composition and density of Khantaishir arc-like protoliths are thermodynamically modelled at granulite- and higher amphibolite-facies conditions. It is shown that the current average density of the lower crust inferred by gravity modelling (2730 ± 20 kg/m3) matches best metamorphosed leucotonalite to diorite. Based on these results, it is now proposed that Mongolian CAOB has an architecture in which the accretionary wedge and oceanic upper crust is underlain by allochthonous lower crust that originated in a Cambrian arc. A tectonic model explaining relamination of allochthonous felsic to intermediate lower crust beneath mafic upper crust is proposed.

  4. Fiskenaesset Anorthosite Complex: Stable isotope evidence for shallow emplacement into Archean ocean crust

    SciTech Connect

    Peck, W.H.; Valley, J.W.

    1996-06-01

    Oxygen and hydrogen isotope ratios indicate that unusual rocks at the upper contact of the Archean Fiskenaesset Anorthosite Complex at Fiskenaesset Harbor (southwest Greenland) are the products of hydrothermal alteration by seawater at the time of anorthosite intrusion. Subsequent granulite-facies metamorphism of these Ca-poor and Al- and Mg-rich rocks produced sapphirine- and kornerupine-bearing assemblages. Because large amounts of surface waters cannot penetrate to depths of 30 km during granulite-facies metamorphism, the isotopic signature of the contact rocks must have been obtained prior to regional metamorphism. The stable isotope and geochemical characteristics of the contact rocks support a model of shallow emplacement into Archean ocean crust for the Fiskenaesset Anorthosite Complex. 45 refs., 3 figs., 2 tabs.

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

    USGS Publications Warehouse

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

    2008-01-01

    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.

  6. Ancient oceanic crust in island arc lower crust: Evidence from oxygen isotopes in zircons from the Tanzawa Tonalitic Pluton

    NASA Astrophysics Data System (ADS)

    Suzuki, Kazue; Kitajima, Kouki; Sawaki, Yusuke; Hattori, Kentaro; Hirata, Takafumi; Maruyama, Shigenori

    2015-07-01

    Knowledge of the lithological variability and genesis of island arc crust is important for understanding continental growth. Although the volcanic architecture of island arcs is comparatively well known, the nature of island arc middle- and lower-crust remains uncertain owing to limited exposure. One of the best targets for deciphering the evolution of an island arc system is the Tanzawa Tonalites (4-9 Ma), in the intra-oceanic Izu-Bonin-Mariana arc. These tonalities which occupied a mid-crustal position were generated by partial melting of lower crust. To constrain protoliths of the plutonic rocks in the island arc lower crust, in-situ O-isotopic analysis using an IMS-1280 Secondary Ion Mass Spectrometer was carried out on 202 zircon grains separated from 4 plutons in the Tanzawa Tonalite. δ18O value of the zircons ranges from 4.1‰ to 5.5‰ and some zircons have δ18O slightly lower than the mantle range. The low zircon δ18O values from the Tanzawa Tonalite suggest that their protoliths involved materials with lower δ18O values than those of the mantle. Hydrothermally altered gabbros in the lower oceanic crust often have lower δ18O values than mantle and can be primary components of arc lower crust. The Tanzawa Tonalite is interpreted to have been formed by partial melting of island arc lower crust. Thus the low δ18O values in zircons from the Tanzawa Tonalites may originate by melting of the hydrothermally altered gabbro. Ancient oceanic crustal material was likely present in the Izu-Bonin-Mariana arc lower crust, at the time of formation of the Tanzawa Tonalites.

  7. Geochemical evidence from the Sudbury structure for crustal redistribution by large bolide impacts.

    PubMed

    Mungall, James E; Ames, Doreen E; Hanley, Jacob J

    2004-06-01

    Deformation and melting of the crust during the formation of large impact craters must have been important during the Earth's early evolution, but such processes remain poorly understood. The 1.8-billion-year-old Sudbury structure in Ontario, Canada, is greater than 200 km in diameter and preserves a complete impact section, including shocked basement rocks, an impact melt sheet and fallback material. It has generally been thought that the most voluminous impact melts represent the average composition of the continental crust, but here we show that the melt sheet now preserved as the Sudbury Igneous Complex is derived predominantly from the lower crust. We therefore infer that the hypervelocity impact caused a partial inversion of the compositional layering of the continental crust. Using geochemical data, including platinum-group-element abundances, we also show that the matrix of the overlying clast-laden Onaping Formation represents a mixture of the original surficial sedimentary strata, shock-melted lower crust and the impactor itself. PMID:15175748

  8. Geochemical evidence from the Sudbury structure for crustal redistribution by large bolide impacts.

    PubMed

    Mungall, James E; Ames, Doreen E; Hanley, Jacob J

    2004-06-01

    Deformation and melting of the crust during the formation of large impact craters must have been important during the Earth's early evolution, but such processes remain poorly understood. The 1.8-billion-year-old Sudbury structure in Ontario, Canada, is greater than 200 km in diameter and preserves a complete impact section, including shocked basement rocks, an impact melt sheet and fallback material. It has generally been thought that the most voluminous impact melts represent the average composition of the continental crust, but here we show that the melt sheet now preserved as the Sudbury Igneous Complex is derived predominantly from the lower crust. We therefore infer that the hypervelocity impact caused a partial inversion of the compositional layering of the continental crust. Using geochemical data, including platinum-group-element abundances, we also show that the matrix of the overlying clast-laden Onaping Formation represents a mixture of the original surficial sedimentary strata, shock-melted lower crust and the impactor itself.

  9. Southeastward lower crustal channel flow beneath southern Tibet: geochemical evidences from Miocene adakitic rocks

    NASA Astrophysics Data System (ADS)

    Xu, J.; Chen, J.; Zhao, W.; Dong, Y.; Wang, B.; Kang, Z.; Tethyan Tectonic Evolution

    2010-12-01

    It is generally believed that the Miocene adakitic rocks in the Lhasa Block (Terrane), southern Tibet, were derived from a thickened lower crust (e.g. Chung et al., 2003; Hou et al., 2004; Guo et al., 2007), and that their compositions will therefore reflect the geochemical characteristics of their lower crustal source(s). Here we present new geochemical and dating data for the adakitic rocks in eastern and western part of the Lhasa Block, which, when combined with previous geochemical data on south sides of the Indus-Yalu suture (IYS), allow us to assess different possible sources between western and eastern part of the Lhasa block as well as on both sides of the IYS. These adakitic rocks in the Lhasa block and on the south side of the IYS were emplaced in Miocene (12-26 Ma), but they show different geochemical characteristics. Adakitic rocks in the western part of the Lhasa Block have geochemical and Sr-Nd isotopic compositions that clearly differ from those in the east. For example, the western rocks are more enriched in Nd-Sr isotopes and have older model ages than the eastern rocks, hinting that the compositions of the lower crust under western and eastern Lhasa also differ. In addition, it should be noted that adakitic rocks on the south side of the IYS have the same geochemical characteristics as adakitic rocks in western Lhasa, but differ from those of eastern Lhasa. Moreover, geochemical and Sr-Nd isotopic compositions of the adakitic rocks on the south side of the IYS indicate that they were probably not generated by partial melting of crust beneath the Himalayas. We suggest that the lower crustal material beneath western Lhasa extends to the southern side of the IYS, and that its presence there can be explained by channel flow. Previous study (King et al., 2007) proposed southward ductile flow of Asian crust below eastern part of the Lhasa Block in southern Tibet, but our data lead us to believe the southeastward-moving of lower crustal material from

  10. Geochemical evidence for a brooks range mineral belt, Alaska

    USGS Publications Warehouse

    Marsh, S.P.; Cathrall, J.B.

    1981-01-01

    Geochemical studies in the central Brooks Range, Alaska, delineate a regional, structurally controlled mineral belt in east-west-trending metamorphic rocks and adjacent metasedimentary rocks. The mineral belt extends eastward from the Ambler River quadrangle to the Chandalar and Philip Smith quadrangles, Alaska, from 147?? to 156??W. longitude, a distance of more than 375 km, and spans a width from 67?? to 69??N. latitude, a distance of more than 222 km. Within this belt are several occurrences of copper and molybdenum mineralization associated with meta-igneous, metasedimentary, and metavolcanic rocks; the geochemical study delineates target areas for additional occurrences. A total of 4677 stream-sediment and 2286 panned-concentrate samples were collected in the central Brooks Range, Alaska, from 1975 to 1979. The -80 mesh ( 2.86) nonmagnetic fraction of the panned concentrates from stream sediment were analyzed by semiquantitative spectrographic methods. Two geochemical suites were recognized in this investigation; a base-metal suite of copper-lead-zinc and a molybdenum suite of molybdenum-tin-tungsten. These suites suggest several types of mineralization within the metamorphic belt. Anomalies in molybdenum with associated Cu and W suggest a potential porphyry molybdenum system associated with meta-igneous rocks. This regional study indicates that areas of metaigneous rocks in the central metamorphic belt are target areas for potential mineralized porphyry systems and that areas of metavolcanic rocks are target areas for potential massive sulfide mineralization. ?? 1981.

  11. Geochemical Evidence for Gas Hydrates in the Indian Ocean

    NASA Astrophysics Data System (ADS)

    Kastner, M.; Solomon, E.; Torres, M.; Spivack, A. J.; Borole, D. V.; Hangsterfer, A.; Das, H. C.; Robertson, G. A.

    2007-12-01

    Geochemical analyses of pore fluids collected from both non-pressurized and pressurized cores provide important constraints on the presence and distribution of gas hydrates in the Indian Ocean. Cores were recovered from two deepwater (900-1170 meter) basins offshore southeast India, the Krishna-Godavari (KG) (10 sites) and Mahanadi Basins (2 sites), and from one site in the Andaman Sea (1600 meters water depth). A bottom simulating reflector (BSR) (i.e., possible base of methane hydrate stability) was present at most of the sites cored and evidence for methane hydrate was obtained at each of the sites. The gas hydrates in most of the conventional cores decomposed prior to sampling. The former presence and distribution of methane hydrate in some of the conventional cores was inferred by indirect methods: from infra-red imaging of cold core temperatures that correlated with pore fluids having lower salinity and chloride concentrations and occasionally mousse-like textures. Chloride concentrations and salinity anomalies, assuming dilution by water released from decomposed methane hydrate, suggest pore volume occupancies on the order of, <1% to a maximum of ~61% at two of the sites in the KG Basin, and <1% to a maximum of ~76% at the site in the Andaman Sea. In the KG Basin, the highest methane hydrate concentrations were associated with fracture zones in clay/silt sediments or in some coarser grained horizons, although gas hydrate was not present in all coarse- grained sediment horizons. Overall, the % occupancies based on pressure core methane concentrations and the chloride concentrations in conventional cores were similar. The pressure core samples used for these determinations were 1 m long, and the conventional core samples were 10-30 cm whole round core samples, chosen via IR imaging. Variations in sulfate gradients were observed; the steepest gradient with the sulfate/methane interface (SMI) at 8 mbsf was observed in the KG Basin. The deepest SMI obtained, at

  12. A geochemical study of the Riddle Peaks Gabbro, North Cascades: Implications for amphibole accumulation in magmatic differentiation processes in the mid crust of an arc

    NASA Astrophysics Data System (ADS)

    Cota, A. C.; DeBari, S. M.

    2012-12-01

    The Riddle Peaks Pluton (~77 Ma) is an amphibole-rich layered gabbro situated within the Cretaceous and Paleogene North Cascades Crystalline Core (NCCC), emplaced at mid-crustal pressures (6 kbar). This pluton provides an opportunity to constrain the geochemical signature produced during amphibole fractionation in the mid-crust of an ancient arc. Amphibole fractionation is hypothesized to produce a distinctive geochemical signature in arc magmas and continental crust (e.g..high Mg- #s, low Dy/Yb and Nb/Ta ratios). Geochemical characterization of the Riddle Peaks cumulates supports these hypotheses. The Riddle Peaks Gabbro has 40.7-47.2 wt.% SiO2 and Mg-# (100xMg/Mg+Fe) 33-67. These low Mg#s contrast with commonly described primitive pyroxene cumulates, which have Mg# > 85. Fractionation of these low-Mg# amphibole cumulates may be responsible for producing high-Mg residual magmas - which have been noted in the NCCC and average continental crust. Garnet fractionation is commonly thought to produce high-Mg magmas, but crystallization of amphibole cumulates may also generate these magmas. Plots of whole rock chondrite-normalized Rare Earth Element (REE) abundances show patterns consistent with amphibole accumulation, with relative enrichment of moderately heavy REE to either light or heavy REE. REE patterns of cumulate amphibole crystals show similar patterns but have higher abundances. REE concentrations of modeled liquids produced by crystallization of these gabbros closely resemble estimates for average continental crust and arc compositions. REE concentrations in calculated liquids were also compared to compositions from the dominantly tonalitic Cardinal Peak Pluton (72 Ma), which intrudes the Riddle Peaks on its Eastern side, and the tonalitic Ten Peak pluton, chosen to represent an average NCCC pluton. A range of compositions representing mafic and felsic end-members are found in each pluton. Calculated liquids match REE concentrations of the mafic diorites and

  13. Neodymium and lead isotope evidence for enriched early Archean crust in North America

    NASA Technical Reports Server (NTRS)

    Bowring, Samuel A.; Housh, Todd B.; Isachsen, Clark E.; Podosek, Frank A.; King, Janet E.

    1989-01-01

    Neodymium and lead isotope measurements and uranium-lead zircon geochronology from Archaean gneisses of the Slave Province in the Northwest Territories of Canada are reported. The gneisses contain zircons with cores older than 3.842 Gyr and an epsilon(Nd) (3.7 Gyr) of - 4.8. This is the oldest reported chondritic model age for a terrestrial sample and provides evidence for strongly enriched pre-3.8-Gyr crust, a reservoir complementary to the depleted mantle already in existence by 3.8 Gyr before the present.

  14. Geochronologic and isotopic evidence for early Proterozoic crust in the eastern Arabian Shield

    SciTech Connect

    Stacey, J.S.; Hedge, C.E.

    1984-05-01

    The authors report zircon U-Pb, feldspar common Pb, whole-rock Sm-Nd, and Rb-Sr data from sample Z-103, a fine-grained granodiorite from the Jabal Khida region of the Saudi Arabian Shield (lat 21/sup 0/19'N; long 44/sup 0/50'W). The measurements yield conclusive evidence for continental crust of early Proterozoic age (approx.1630 Ma) at that locality. Furthermore, lead-isotope data indicate an even earlier, perhaps Archean, crustal history for the source of the lower Proterozoic rocks. 17 references, 4 figures, 1 table.

  15. Mantle CO2 degassing through the Icelandic crust: Evidence from carbon isotopes in groundwater

    NASA Astrophysics Data System (ADS)

    Stefánsson, Andri; Sveinbjörnsdóttir, Árný E.; Heinemeier, Jan; Arnórsson, Stefán; Kjartansdóttir, Ríkey; Kristmannsdóttir, Hrefna

    2016-10-01

    Carbon isotopes of groundwater in Iceland were studied in order to determine the source and reactions of carbon at divergent plate boundaries not associated with active volcanic systems. All the waters were of meteoric origin, with temperatures of 1-130 °C, pH of ∼4.5-10.5 and dissolved inorganic carbon (∑CO2) between 1.8 and 4100 ppm. The measured range of δ13CO2 and 14CO2 in these waters was large, -27.4 to +2.0‰ and 0.6-118 pMC, respectively. The sources and reactions of dissolved inorganic carbon were studied by comparing the measured chemical and isotope composition with those simulated using isotope geochemical models. Three major sources of CO2 were identified: (1) dissolution of partially degassed basaltic rocks formed at the surface or shallow depths, (2) atmospheric CO2 through air-water exchange at surface, and (3) input of gas at depth into the groundwater systems that has similar carbon and isotope composition as the pre-erupted melt of the upper mantle and lower crust beneath Iceland. In the groundwater systems the CO2 chemistry and isotope content are modified due to carbonate mineral precipitation and changes in aqueous species distribution upon progressive water-rock interaction; these changes needed to be quantified in order to reveal the various CO2 sources. The CO2 flux of the Icelandic crust was estimated to be ∼5-10 · 1010 mol/yr with as high as 50% of the flux not associated with active volcanic centers but placed off-axis where a significant proportion of the CO2 may originate from the mantle. The mantle input of the groundwater off-axis corresponds to CO2 partial pressures of ∼10-6-1 bar and to a mantle CO2 flux of <5 · 105 mol/km2/yr for most areas and up to 125 · 105 and 1600 · 105 for the Southern Lowlands and Snæfellsnes Peninsula, respectively. The CO2 flux from active volcanic geothermal systems in Iceland was estimated to be ∼500-3000 · 105 mol CO2/km2/yr, considerably greater than the highest values observed off-axis.

  16. Seismic evidence for overpressured subducted oceanic crust and megathrust fault sealing.

    PubMed

    Audet, Pascal; Bostock, Michael G; Christensen, Nikolas I; Peacock, Simon M

    2009-01-01

    Water and hydrous minerals play a key part in geodynamic processes at subduction zones by weakening the plate boundary, aiding slip and permitting subduction-and indeed plate tectonics-to occur. The seismological signature of water within the forearc mantle wedge is evident in anomalies with low seismic shear velocity marking serpentinization. However, seismological observations bearing on the presence of water within the subducting plate itself are less well documented. Here we use converted teleseismic waves to obtain observations of anomalously high Poisson's ratios within the subducted oceanic crust from the Cascadia continental margin to its intersection with forearc mantle. On the basis of pressure, temperature and compositional considerations, the elevated Poisson's ratios indicate that water is pervasively present in fluid form at pore pressures near lithostatic values. Combined with observations of a strong negative velocity contrast at the top of the oceanic crust, our results imply that the megathrust is a low-permeability boundary. The transition from a low- to high-permeability plate interface downdip into the mantle wedge is explained by hydrofracturing of the seal by volume changes across the interface caused by the onset of crustal eclogitization and mantle serpentinization. These results may have important implications for our understanding of seismogenesis, subduction zone structure and the mechanism of episodic tremor and slip.

  17. Freshly brewed continental crust

    NASA Astrophysics Data System (ADS)

    Gazel, E.; Hayes, J. L.; Caddick, M. J.; Madrigal, P.

    2015-12-01

    Earth's crust is the life-sustaining interface between our planet's deep interior and surface. Basaltic crusts similar to Earth's oceanic crust characterize terrestrial planets in the solar system while the continental masses, areas of buoyant, thick silicic crust, are a unique characteristic of Earth. Therefore, understanding the processes responsible for the formation of continents is fundamental to reconstructing the evolution of our planet. We use geochemical and geophysical data to reconstruct the evolution of the Central American Land Bridge (Costa Rica and Panama) over the last 70 Ma. We also include new preliminary data from a key turning point (~12-6 Ma) from the evolution from an oceanic arc depleted in incompatible elements to a juvenile continental mass in order to evaluate current models of continental crust formation. We also discovered that seismic P-waves (body waves) travel through the crust at velocities closer to the ones observed in continental crust worldwide. Based on global statistical analyses of all magmas produced today in oceanic arcs compared to the global average composition of continental crust we developed a continental index. Our goal was to quantitatively correlate geochemical composition with the average P-wave velocity of arc crust. We suggest that although the formation and evolution of continents may involve many processes, melting enriched oceanic crust within a subduction zone, a process probably more common in the Achaean where most continental landmasses formed, can produce the starting material necessary for juvenile continental crust formation.

  18. Geochemical evidence for a comet shower in the late Eocene.

    PubMed

    Farley, K A; Montanari, A; Shoemaker, E M; Shoemaker, C S

    1998-05-22

    Analyses of pelagic limestones indicate that the flux of extraterrestrial helium-3 to Earth was increased for a 2.5-million year (My) period in the late Eocene. The enhancement began approximately 1 My before and ended approximately 1.5 My after the major impact events that produced the large Popigai and Chesapeake Bay craters approximately 36 million years ago. The correlation between increased concentrations of helium-3, a tracer of fine-grained interplanetary dust, and large impacts indicates that the abundance of Earth-crossing objects and dustiness in the inner solar system were simultaneously but only briefly enhanced. These observations provide evidence for a comet shower triggered by an impulsive perturbation of the Oort cloud. PMID:9596575

  19. Geochemical evidence for a comet shower in the late Eocene.

    PubMed

    Farley, K A; Montanari, A; Shoemaker, E M; Shoemaker, C S

    1998-05-22

    Analyses of pelagic limestones indicate that the flux of extraterrestrial helium-3 to Earth was increased for a 2.5-million year (My) period in the late Eocene. The enhancement began approximately 1 My before and ended approximately 1.5 My after the major impact events that produced the large Popigai and Chesapeake Bay craters approximately 36 million years ago. The correlation between increased concentrations of helium-3, a tracer of fine-grained interplanetary dust, and large impacts indicates that the abundance of Earth-crossing objects and dustiness in the inner solar system were simultaneously but only briefly enhanced. These observations provide evidence for a comet shower triggered by an impulsive perturbation of the Oort cloud.

  20. Geochemical evidence for a comet shower in the late Eocene

    USGS Publications Warehouse

    Farley, K.A.; Montanari, A.; Shoemaker, E.M.; Shoemaker, C.S.

    1998-01-01

    Analyses of pelagic limestones indicate that the flux of extraterrestrial helium-3 to Earth was increased for a 2.5-million year (My) period in the late Eocene. The enhancement began ~1 My before and ended ~1.5 My after the major impact events that produced the large Popigai and Chesapeake Bay craters ~36 million years ago. The correlation between increased concentrations of helium-3, a tracer of fine-grained interplanetary dust, and large impacts indicates that the abundance of Earth-crossing objects and dustiness in the inner solar system were simultaneously but only briefly enhanced. These observations provide evidence for a comet shower triggered by an impulsive perturbation of the Oort cloud.

  1. Source and magma mixing processes in continental subduction factory: Geochemical evidence from postcollisional mafic igneous rocks in the Dabie orogen

    NASA Astrophysics Data System (ADS)

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

    2015-03-01

    Postcollisional mafic igneous rocks commonly exhibit petrological and geochemical heterogeneities, but their origin still remains enigmatic. While source mixing is substantial due to the crust-mantle interaction during continental collision, magma mixing is also significant during postcollisional magmatism. The two processes are illustrated by Early Cretaceous mafic igneous rocks in the Dabie orogen. These mafic rocks show arc-like trace element distribution patterns and enriched Sr-Nd-Pb isotope compositions, indicating their origination from enriched mantle sources. They have variable whole-rock ɛNd(t) values of -17.6 to -5.2 and zircon ɛHf(t) values of -29.0 to -7.7, pointing to source heterogeneities. Such whole-rock geochemical features are interpreted by the source mixing through melt-peridotite reaction in the continental subduction channel. Clinopyroxene and plagioclase megacrystals show complex textural and compositional variations, recording three stages of mineral crystallization during magma evolution. Cpx-1 core has low Cr and Ni but high Ba, Rb and K, indicating its crystallization from a mafic melt (Melt 1) derived from partial melting of hydrous peridotite rich in phlogopite. Cpx-1 mantle and Cpx-2 exhibit significantly high Cr, Ni and Al2O3 but low Rb and Ba, suggesting their crystallization from pyroxenite-derived mafic melt (Melt 2). Whole-rock initial 87Sr/86Sr ratios of gabbro lies between those of Pl-1core (crystallized from Melt 1) and Pl-1 mantle and Pl-2 core (crystallized from Melt 2), providing isotopic evidence for magma mixing between Melt 1 and Melt 2. Taken together, a heterogeneously enriched mantle source would be generated by the source mixing due to reaction of the overlying subcontinental lithospheric mantle wedge peridotite with felsic melts derived from partial melting of different rocks of the deeply subducted continental crust during the continental collision. The magma mixing would occur between mafic melts that were

  2. Geochemical evidence for the melting of subducting oceanic lithosphere at plate edges.

    PubMed

    Yogodzinski, G M; Lees, J M; Churikova, T G; Dorendorf, F; Wöerner, G; Volynets, O N

    2001-01-25

    Most island-arc magmatism appears to result from the lowering of the melting point of peridotite within the wedge of mantle above subducting slabs owing to the introduction of fluids from the dehydration of subducting oceanic crust. Volcanic rocks interpreted to contain a component of melt (not just a fluid) from the subducting slab itself are uncommon, but possible examples have been recognized in the Aleutian islands, Baja California, Patagonia and elsewhere. The geochemically distinctive rocks from these areas, termed 'adakites, are often associated with subducting plates that are young and warm, and therefore thought to be more prone to melting. But the subducting lithosphere in some adakite locations (such as the Aleutian islands) appears to be too old and hence too cold to melt. This implies either that our interpretation of adakite geochemistry is incorrect, or that our understanding of the tectonic context of adakites is incomplete. Here we present geochemical data from the Kamchatka peninsula and the Aleutian islands that reaffirms the slab-melt interpretation of adakites, but in the tectonic context of the exposure to mantle flow around the edge of a torn subducting plate. We conclude that adakites are likely to form whenever the edge of a subducting plate is warmed or ablated by mantle flow. The use of adakites as tracers for such plate geometry may improve our understanding of magma genesis and thermal structure in a variety of subduction-zone environments. PMID:11206543

  3. Geochemical evidence for the melting of subducting oceanic lithosphere at plate edges.

    PubMed

    Yogodzinski, G M; Lees, J M; Churikova, T G; Dorendorf, F; Wöerner, G; Volynets, O N

    2001-01-25

    Most island-arc magmatism appears to result from the lowering of the melting point of peridotite within the wedge of mantle above subducting slabs owing to the introduction of fluids from the dehydration of subducting oceanic crust. Volcanic rocks interpreted to contain a component of melt (not just a fluid) from the subducting slab itself are uncommon, but possible examples have been recognized in the Aleutian islands, Baja California, Patagonia and elsewhere. The geochemically distinctive rocks from these areas, termed 'adakites, are often associated with subducting plates that are young and warm, and therefore thought to be more prone to melting. But the subducting lithosphere in some adakite locations (such as the Aleutian islands) appears to be too old and hence too cold to melt. This implies either that our interpretation of adakite geochemistry is incorrect, or that our understanding of the tectonic context of adakites is incomplete. Here we present geochemical data from the Kamchatka peninsula and the Aleutian islands that reaffirms the slab-melt interpretation of adakites, but in the tectonic context of the exposure to mantle flow around the edge of a torn subducting plate. We conclude that adakites are likely to form whenever the edge of a subducting plate is warmed or ablated by mantle flow. The use of adakites as tracers for such plate geometry may improve our understanding of magma genesis and thermal structure in a variety of subduction-zone environments.

  4. Geochemical evidence for widespread euxinia in the later Cambrian ocean.

    PubMed

    Gill, Benjamin C; Lyons, Timothy W; Young, Seth A; Kump, Lee R; Knoll, Andrew H; Saltzman, Matthew R

    2011-01-01

    Widespread anoxia in the ocean is frequently invoked as a primary driver of mass extinction as well as a long-term inhibitor of evolutionary radiation on early Earth. In recent biogeochemical studies it has been hypothesized that oxygen deficiency was widespread in subsurface water masses of later Cambrian oceans, possibly influencing evolutionary events during this time. Physical evidence of widespread anoxia in Cambrian oceans has remained elusive and thus its potential relationship to the palaeontological record remains largely unexplored. Here we present sulphur isotope records from six globally distributed stratigraphic sections of later Cambrian marine rocks (about 499 million years old). We find a positive sulphur isotope excursion in phase with the Steptoean Positive Carbon Isotope Excursion (SPICE), a large and rapid excursion in the marine carbon isotope record, which is thought to be indicative of a global carbon cycle perturbation. Numerical box modelling of the paired carbon sulphur isotope data indicates that these isotope shifts reflect transient increases in the burial of organic carbon and pyrite sulphur in sediments deposited under large-scale anoxic and sulphidic (euxinic) conditions. Independently, molybdenum abundances in a coeval black shale point convincingly to the transient spread of anoxia. These results identify the SPICE interval as the best characterized ocean anoxic event in the pre-Mesozoic ocean and an extreme example of oxygen deficiency in the later Cambrian ocean. Thus, a redox structure similar to those in Proterozoic oceans may have persisted or returned in the oceans of the early Phanerozoic eon. Indeed, the environmental challenges presented by widespread anoxia may have been a prevalent if not dominant influence on animal evolution in Cambrian oceans. PMID:21209662

  5. Evidence for Anisotropic Crust Within the Tibetan Plateau From Teleseismic Receiver Functions

    NASA Astrophysics Data System (ADS)

    Folsom, H.; Zandt, G.

    2001-12-01

    synthetics that provide a good match to observed data in many cases. The amount of anisotropy is expressed as a percent deviation of the fast axis velocity from the slow axis velocity; observed transverse component amplitudes can be mimicked with models having values of 10-15 percent, which are reasonable based on experimental studies of anisotropic rocks and common rock-forming minerals. Though the forward modeling discussed here is non-unique, even the simplistic nature of the anisotropy in these models fits more aspects of the data than any isotropic model. In addition, dipping layer models do not seem to adequately explain the observed high amplitudes of transverse receiver functions without unrealistically large velocity contrasts between steeply dipping layers. While reasonable models for many stations share the aforementioned anisotropy characteristics, the stations are too far apart to correlate individual layers, and velocities and thicknesses of layers in models for different stations vary slightly. In addition, stations in the southern portion of the Plateau seem to require more complex models, with more layers, than stations in the north. Despite some structural variations among stations, these basic observations provide strong evidence for anisotropic crust in the Tibetan Plateau. This anisotropy could be a result of past or present tectonic processes involved in the development of the Plateau, and further constraints on its orientation and spatial distribution could provide insight into deformational mechanisms currently or formerly active within the Plateau.

  6. Andesite Magmas are Produced along Oceanic Arcs where the Crust is Thin: Evidence from Nishinoshima Volcano, Ogasawara Arc, Japan

    NASA Astrophysics Data System (ADS)

    Tamura, Y.; Ishizuka, O.; Sato, T.; Nichols, A. R.

    2015-12-01

    The incentive for this study is the ongoing explosive eruption of Nishinoshima volcano, located about 1,000 km south of Tokyo along the Ogasawara (Bonin) Arc. The straightforward but unexpected relationship presented here relates crustal thickness and magma type in the Izu-Ogasawara Oceanic Arc. Volcanoes along the Ogasawara segment of the arc, which include Nishinoshima, are underlain by thin crust (16-21 km)—in contrast to those along the Izu segment, where the crust is ~35 km thick. Interestingly, andesite magmas are dominant products from the former volcanoes and mostly basaltic lavas erupt from the latter. Why and how do volcanoes on the thin crust erupt andesite magmas? An introductory petrology textbook might answer this question by suggesting that, under decreasing pressure and hydrous conditions, the liquidus field of forsterite expands relative to that of enstatite, with the result that, at some point, enstatite melts incongruently to produce primary andesite melt. According to the hypothesis presented here, however, rising mantle diapirs stall near the base of the oceanic arc crust at depths controlled by the thickness of the overlying crust. Where the crust is thin, as along the Ogasawara segment of the arc, pressures are relatively low, and magmas produced in the mantle wedge tend to be andesitic. Where the crust is thick, as along the Izu segment, pressures are greater, and only basaltic magmas tend to be produced. To examine this hypothesis, JAMSTEC cruise NT15-E02 on the R/V Natsushima took place from 11 June to 21 June 2015 to Nishinoshima. It's present island has an elevation of only ~150 m, but its submarine flanks extend to ocean depths of 2,000-3,000 m, so the great bulk of the volcano is submarine and yet-to-be explored. We present the new hypothesis and its evidence from Nishinoshima based on the primitive lavas collected from the submarine parts of the volcano.

  7. The Agoudal (High Atlas Mountains, Morocco) Shattered Limestone: Petrographical and Geochemical Studies and Additional Evidence of Impact

    NASA Astrophysics Data System (ADS)

    El Kerni, H.; Chennaoui Aoudjehane, H.; Marjanac, T.

    2016-08-01

    Agoudal impact structure shattered limestone and breccia are well studied and described using petrographical observations and geochemical analyses, and a new discovery of the magnesiwustite mineral as a further evidence of impact event.

  8. Pervasive Layering in the Lunar Highland Crust: Evidence from Apollos 15, 16,and 17

    NASA Technical Reports Server (NTRS)

    Lowman, Paul D., Jr.; Yang, Tiffany

    2005-01-01

    This paper presents results of a photogeologic reconnaissance of 70 mm photographs taken on the lunar surface during the Apollo 15, 16, and 17 missions, whose primary objective was to investigate the lunar highland crust. Photographs at all three sites, notably the Apennine Front, show pervasive layered structure. These layers are easily distinguished from lighting artifacts, and are considered genuine crustal structures. Their number, thickness, and extent implies that they are lava flows, not ejecta blankets or intrusive features. They appear to be the upper part of the earliest lunar crust, possibly forming a layer tens of kilometers thick. Remote sensing studies (X-ray fluorescence and reflectance spectroscopy), indicate that the highland crust is dominantly a feldspathic basalt. It is concluded that the highland layers represent a global crust formed by eruptions of high-alumina basalt in the first few hundred million years of the Moon's history.

  9. Geochemical evidence for invasion of Kilauea's plumbing system by Mauna Loa magma

    USGS Publications Warehouse

    Rhodes, J.M.; Wenz, K.P.; Neal, C.A.; Sparks, J.W.; Lockwood, J.P.

    1989-01-01

    From the beginning of the study of Hawaiian volcanism there has been controversy over possible relationships between the neighbouring active volcanoes Mauna Loa and Kilauea1-5. Seismic activity, thought to reflect upward migration of magma, reveals that the magmatic plumbing systems apparently converge at depth to form a broad funnel within the mantle6. Although on rare occasions they have erupted concurrently, the brief historical eruptive record appears to show that when Kilauea is most active, Mauna Loa is in repose and vice versa, suggesting that they may be competing for the same magma supply5,7. Petrological, geochemical and isotope data, however, require a diametrically opposite conclusion. Distinct differences in major-element, trace-element and isotope compositions of lavas are regarded as compelling evidence that the two volcanoes have separate magmatic plumbing systems, supplied by parental magmas from physically and geochemically distinct mantle sources8-13. Here we present preliminary geochemical data which show that in the past 2,000 years Kilauea has erupted a spectrum of lava compositions resembling historical Kilauea lavas at one end and Mauna Loa lavas at the other. We discuss the cause of this diversity, and speculate that magma from Mauna Loa may have invaded Kilauea's 'high-level' magmatic plumbing system. ?? 1989 Nature Publishing Group.

  10. Evidence for micronutrient limitation of biological soil crusts: Importance to arid-lands restoration

    USGS Publications Warehouse

    Bowker, M.A.; Belnap, J.; Davidson, D.W.; Phillips, S.L.

    2005-01-01

    Desertification is a global problem, costly to national economies and human societies. Restoration of biological soil crusts (BSCs) may have an important role to play in the reversal of desertification due to their ability to decrease erosion and enhance soil fertility. To determine if there is evidence that lower fertility may hinder BSC recolonization, we investigated the hypothesis that BSC abundance is driven by soil nutrient concentrations. At a regional scale (north and central Colorado Plateau, USA), moss and lichen cover and richness are correlated with a complex water-nutrient availability gradient and have approximately six-fold higher cover and approximately two-fold higher species richness on sandy soils than on shale-derived soils. At a microscale, mosses and lichens are overrepresented in microhabitats under the north sides of shrub canopies, where water and nutrients are more available. At two spatial scales, and at the individual species and community levels, our data are consistent with the hypothesis that distributions of BSC organisms are determined largely by soil fertility. The micronutrients Mn and Zn figured prominently and consistently in the various analyses, strongly suggesting that these elements are previously unstudied limiting factors in BSC development. Structural-equation modeling of our data is most consistent with the hypothesis of causal relationships between the availability of micronutrients and the abundance of the two major nitrogen (N) fixers of BSCs. Specifically, higher Mn availability may determine greater Collema tenax abundance, and both Mn and Zn may limit Collema coccophorum; alternative causal hypotheses were less consistent with the data. We propose experimental trials of micronutrient addition to promote the restoration of BSC function on disturbed lands. Arid lands, where BSCs are most prevalent, cover ???40% of the terrestrial surface of the earth; thus the information gathered in this study is potentially useful

  11. Geochemical evidence of multiple isolated magma batches in the paired Mamaku and Ohakuri eruption, Taupo Volcanic Zone, New Zealand

    NASA Astrophysics Data System (ADS)

    Bégué, F.; Deering, C. D.; Gravley, D.; kennedy, B.; Chambefort, I. S.

    2012-12-01

    same type pumice fragments), suggesting that these magma batches did not interact with each other. Based on our comprehensive geochemical analysis, the pre-eruptive geometry of the magmatic reservoirs can be deduced and a possible origin for triggering of the eruption proposed. First, the region most likely included five different magma batches, extracted from the same source region (i.e. a continuous intermediate mush zone beneath the Mamaku-Ohakuri region). These magma batches were juxtaposed and isolated from each other in the upper crust and evolved separately until eruption. Minor geochemical differences in the batches are likely to be generated because of different extraction conditions of the rhyolites, from a slight heterogeneous mush. Second, the lack of evidence in melt compositions for recharge prior to eruption, and from quartz-cathodoluminescence imaging, suggests that a magmatic input is unlikely to be an eruption trigger. However, tectonic activity, already suggested as a potential trigger by previous authors, could be an efficient way to activate these isolated magma batches. The evacuation of one magma batch could adjust regional tectonics sufficiently to trigger earthquakes and erupt the other isolated batches. Collateral subsidence features between the calderas may indicate lateral magma withdrawal of one or more of the batches.

  12. Geochemical evidence for anoxic deep water in the Arabian Sea during the last glaciation

    SciTech Connect

    Sarkar, A.; Bhattacharya, S.K.; Sarin, M.M. )

    1993-03-01

    Various paleoceanographic studies have indicated that the deep ocean was probably depleted in dissolved oxygen during the last glacial period ([approximately]18 kyr B.P.; [delta][sup 18]O, stage 2) compared to present time. However, direct evidence of low oxygen content in the deep waters has been lacking. Here, the authors report geochemical evidence of near anoxic conditions in the deep Arabian Sea during the entire last glacial cycle ([delta][sup 18]O; stages 2, 3, and 4). Anoxia is inferred from the concomitant enrichment of organic carbon and authigenic uranium in the glacial sections of a core from the deep eastern Arabian Sea. The anoxic conditions during the last glacial period, probably caused by a change in deep water circulation, evidently enhanced preservation of organic matter and simultaneous removal of uranium from seawater. 57 refs., 3 figs., 2 tabs.

  13. Geochemical Evidence for Subduction in the Early Archaean from Quartz-Carbonate-Fuchsite Mineralization, Isua Supracrustal Belt, West Greenland

    NASA Astrophysics Data System (ADS)

    Pope, E. C.; Rosing, M. T.; Bird, D. K.

    2011-12-01

    Quartz, carbonate and fuchsite (chromian muscovite) is a common metasomatic assemblage observed in orogenic gold systems, both in Phanerozoic convergent margin settings, and within supracrustal and greenstone belts of Precambrian rocks. Geologic and geochemical observations in younger orogenic systems suggest that ore-forming metasomatic fluids are derived from subduction-related devolitilization reactions, implying that orogenic Au-deposits in Archaean and Proterozoic supracrustal rock suites are related to subduction-style plate tectonics beginning early in Earth history. Justification of this metasomatic-tectonic relationship requires that 1) Phanerozoic orogenic Au-deposits form in subduction-zone environments, and 2) the geochemical similarity of Precambrian orogenic deposits to their younger counterparts is the result of having the same petro-genetic origin. Hydrogen and oxygen isotope compositions of fuchsite and quartz from auriferous mineralization in the ca. 3.8 Ga Isua Supracrustal Belt (ISB) in West Greenland, in conjunction with elevated concentrations of CO2, Cr, Al, K and silica relative to protolith assemblages, suggest that this mineralization shares a common petro-tectonic origin with Phanerozoic orogenic deposits and that this type of metasomatism is a unique result of subduction-related processes. Fuchsite from the ISB has a δ18O and δD of +7.7 to +17.9% and -115 to -61%, respectively. δ18O of quartz from the same rocks is between +10.3 and +18.6%. Muscovite-quartz oxygen isotope thermometry indicates that the mineralization occurred at 560 ± 90oC, from fluids with a δD of -73 to -49% and δ18O of +8.8 to +17.2%. Calculation of isotopic fractionation during fluid-rock reactions along hypothetical fluid pathways demonstrates that these values, as well as those in younger orogenic deposits, are the result of seawater-derived fluids liberated from subducting lithosphere interacting with ultramafic rocks in the mantle wedge and lower crust

  14. Along and across arc geochemical variations in NW Central America: Evidence for involvement of lithospheric pyroxenite

    NASA Astrophysics Data System (ADS)

    Heydolph, Ken; Hoernle, Kaj; Hauff, Folkmar; Bogaard, Paul van den; Portnyagin, Maxim; Bindeman, Ilya; Garbe-Schönberg, Dieter

    2012-05-01

    The Central American Volcanic Arc (CAVA) has been the subject of intensive research over the past few years, leading to a variety of distinct models for the origin of CAVA lavas with various source components. We present a new model for the NW Central American Volcanic Arc based on a comprehensive new geochemical data set (major and trace element and Sr-Nd-Pb-Hf-O isotope ratios) of mafic volcanic front (VF), behind the volcanic front (BVF) and back-arc (BA) lava and tephra samples from NW Nicaragua, Honduras, El Salvador and Guatemala. Additionally we present data on subducting Cocos Plate sediments (from DSDP Leg 67 Sites 495 and 499) and igneous oceanic crust (from DSDP Leg 67 Site 495), and Guatemalan (Chortis Block) granitic and metamorphic continental basement. We observe systematic variations in trace element and isotopic compositions both along and across the arc. The data require at least three different endmembers for the volcanism in NW Central America. (1) The NW Nicaragua VF lavas require an endmember with very high Ba/(La, Th) and U/Th, relatively radiogenic Sr, Nd and Hf but unradiogenic Pb and low δ18O, reflecting a largely serpentinite-derived fluid/hydrous melt flux from the subducting slab into a depleted N-MORB type of mantle wedge. (2) The Guatemala VF and BVF mafic lavas require an enriched endmember with low Ba/(La, Th), U/Th, high δ18O and radiogenic Sr and Pb but unradiogenic Nd and Hf isotope ratios. Correlations of Hf with both Nd and Pb isotopic compositions are not consistent with this endmember being subducted sediments. Granitic samples from the Chiquimula Plutonic Complex in Guatemala have the appropriate isotopic composition to serve as this endmember, but the large amounts of assimilation required to explain the isotope data are not consistent with the basaltic compositions of the volcanic rocks. In addition, mixing regressions on Nd vs. Hf and the Sr and O isotope plots do not go through the data. Therefore, we propose that this

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

    PubMed

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

    2009-01-01

    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

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

    PubMed

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

    2009-01-01

    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.

  17. Evidence for gas accumulation beneath the surface crust driving cyclic rise and fall of the lava surface at Halema`uma`u, Kilauea Volcano

    NASA Astrophysics Data System (ADS)

    Patrick, M. R.; Orr, T. R.; Wilson, D.; Sutton, A. J.; Elias, T.; Fee, D.; Nadeau, P. A.

    2010-12-01

    The ongoing eruption in Halema`uma`u crater, at the summit of Kilauea Volcano, has surpassed the two-year mark and is characterized by lava lake activity in the vent. As of August 2010, the lava lake is about 70 m in diameter and 180 m below the rim of a narrow vent cavity. Although the explosive events that typified the first year of activity have abated, episodic rise and fall of the lava surface remains common. Cycles of rise and fall range from several minutes to eight hours in duration and are characterized by a quiescent rise phase and violent, gas-charged fall, spanning a height change of 20-30 m. Several models have been proposed to explain the cyclic rise and fall of lava surfaces at basaltic volcanoes, which in some cases is referred to as “gas pistoning”. In one model, episodic rise and fall is driven by the ascent of gas slugs from depth. In another, the cyclic behavior is driven by gas accumulation beneath the surface crust, with each cycle terminated by an abrupt failure of the crust, resulting in gas release. Seismic and infrasound data, as well as gas and webcam monitoring, at Halema`uma`u over the past two years strongly support the gas accumulation model, based on several lines of evidence. First, gas emission rates drop significantly below background levels during the rise phase, and increase dramatically during the fall phase, suggesting a process of gas buildup and release as opposed to slug flow. Second, the rise phases can last several hours, which is longer than reasonable slug ascent times. Third, the rise rate decreases over time, and in many cases plateaus, as the lava reaches its high stand, which is contrary to the exponential increase expected for gas slugs. Fourth, webcam video has captured numerous instances where rockfalls piercing the surface crust initiate gas release and lava level drop, suggestive of gas accumulation at shallow levels. Lastly, FTIR (Fourier Transform Infrared Spectroscopy) data reveal changes in gas

  18. Remote sensing evidence for an ancient carbon-bearing crust on Mercury

    NASA Astrophysics Data System (ADS)

    Peplowski, Patrick N.; Klima, Rachel L.; Lawrence, David J.; Ernst, Carolyn M.; Denevi, Brett W.; Frank, Elizabeth A.; Goldsten, John O.; Murchie, Scott L.; Nittler, Larry R.; Solomon, Sean C.

    2016-04-01

    Mercury’s global surface is markedly darker than predicted from its measured elemental composition. The darkening agent, which has not been previously identified, is most concentrated within Mercury’s lowest-reflectance spectral unit, the low-reflectance material. This low-reflectance material is generally found in large impact craters and their ejecta, which suggests a mid-to-lower crustal origin. Here we present neutron spectroscopy measurements of Mercury’s surface from the MESSENGER spacecraft that reveal increases in thermal-neutron count rates that correlate spatially with deposits of low-reflectance material. The only element consistent with both the neutron measurements and visible to near-infrared spectra of low-reflectance material is carbon, at an abundance that is 1-3 wt% greater than surrounding, higher-reflectance material. We infer that carbon is the primary darkening agent on Mercury and that the low-reflectance material samples carbon-bearing deposits within the planet’s crust. Our findings are consistent with the formation of a graphite flotation crust from an early magma ocean, and we propose that the heavily disrupted remnants of this ancient layer persist beneath the present upper crust. Under this scenario, Mercury’s globally low reflectance results from mixing of the ancient graphite-rich crust with overlying volcanic materials via impact processes or assimilation of carbon into rising magmas during secondary crustal formation.

  19. Geochemical evidence for airborne dust additions to soils in Channel Islands National Park, California

    USGS Publications Warehouse

    Muhs, D.R.; Budahn, J.R.; Johnson, D.L.; Reheis, M.; Beann, J.; Skipp, G.; Fisher, E.; Jones, J.A.

    2008-01-01

    There is an increasing awareness that dust plays important roles in climate change, biogeochemical cycles, nutrient supply to ecosystems, and soil formation. In Channel Islands National Park, California, soils are clay-rich Vertisols or Alfisols and Mollisols with vertic properties. The soils are overlain by silt-rich mantles that contrast sharply with the underlying clay-rich horizons. Silt mantles contain minerals that are rare or absent in the volcanic rocks that dominate these islands. Immobile trace elements (Sc-Th-La and Ta-Nd-Cr) and rare-earth elements show that the basalt and andesite on the islands have a composition intermediate between upper-continental crust and oceanic crust. In contrast, the silt fractions and, to a lesser extent, clay fractions of the silt mantle have compositions closer to average upper-continental crust and very similar to Mojave Desert dust. Island shelves, exposed during the last glacial period, could have provided a source of eolian sediment for the silt mantles, but this is not supported by mineralogical data. We hypothesize that a more likely source for the silt-rich mantles is airborne dust from mainland California and Baja California, either from the Mojave Desert or from the continental shelf during glacial low stands of sea. Although average winds are from the northwest in coastal California, easterly winds occur numerous times of the year when "Santa Ana" conditions prevail, caused by a high-pressure cell centered over the Great Basin. The eolian silt mantles constitute an important medium of plant growth and provide evidence that abundant eolian silt and clay may be delivered to the eastern Pacific Ocean from inland desert sources. ?? 2007 Geological Society of America.

  20. Crustal structure of Hubei Province of China from teleseismic receiver functions: Evidence for lower crust delamination

    NASA Astrophysics Data System (ADS)

    Huang, Rong; Zhu, Lupei; Xu, Yixian

    2014-12-01

    Western Hubei Province is at the southern end of the 3000-km-long north-south-oriented Xing'anling-Taihangshan-Wulingshan topographic step in China, which separates high-rising plateaus and mountain ranges in the west from low-elevation plains in the east. We calculated teleseismic P receiver functions of 32 permanent broadband seismic stations in Hubei Province and estimated crustal thicknesses under them using the H-κ method. We also obtained detailed crustal structural images along three profiles using the CCP stacking method. The results show an east-west crustal thickness increase in the study area from 30-35 km to 45-50 km in less than 20 km of horizontal distance, most likely in a step-wise fashion. The thin crust beneath the Nanxiang and Jianghan basins in eastern Hubei extends into the interior of the Wuling Uplift and the Huangling Massif in western Hubei. The lack of mirror symmetry between the Moho and surface topography suggests that part of the mountain ranges in western Hubei is either compensated by non-Airy-type isostasy models or is not in isostatic equilibrium but supported by the strength of the lithosphere. The brittle or localized ductile deformation in the lower crust/uppermost mantle as indicated by the abrupt Moho steps seems to be decoupled with brittle deformation in the upper crust. The CCP images also reveal an apparent double Moho beneath the Wudang Mts. which is interpreted to be due to a partially eclogitized lower crust after the original cratonic mantle lithosphere was replaced by warm and hydrated mantle materials in eastern China in the Late Mesozoic. The Moho steps were formed when a segment of eclogitized lower crust became gravitationally unstable and foundered into the mantle.

  1. Geochemical Evidence for Recent Hydrothermal Alteration of Marine Sediments in Mid-Okinawa Trough, Southwest Japan

    NASA Astrophysics Data System (ADS)

    Tanaka, A.; Abe, G.; Yamaguchi, K. E.

    2014-12-01

    Recent studies have shown that submarine hydrothermal system supports diverse microbial life. Bio-essential metals supporting such microbial communities were released from basalts by high-temperature water-rock interaction in deeper part of the oceanic crust and carried by submarine fluid flow. Its total quantity in global hydrothermal settings has been estimated to be on the order of ~1019 g/yr, which is surprisingly on the same order of the total river flows (Urabe et al., 2011). Therefore, it is important to explore how submarine river system works, i.e., to understand mechanism and extent of elemental transport, which should lead to understanding of the roles of hydrothermal circulation in oceanic crust in controlling elemental budget in the global ocean and geochemical conditions to support deep hot biosphere.  We performed REE analysis of marine sediments influenced by submarine hydrothermal activity in Mid-Okinawa Trough. The sediment samples used in this study are from IODP site at Iheya North region and JADE site at Izena region. The samples show alternation between volcanic and clastic sediments. Hydrothermal fluids of this area contain elevated concentrations of volatile components such as H2, CO2, CH4, NH4+, and H2S, supporting diverse chemoautotrophic microbial community (Nakagawa et al., 2005). The purpose of this study is to examine the effect of hydrothermal activity on the REE signature of the sediments. Chondrite-normalized REE patterns of the samples show relative enrichment of light over heavy REEs, weak positive Ce anomalies, and variable degrees of negative Eu anomalies. The REE patterns suggest the sediments source was mainly basalt, suggesting insignificant input of continental materials. Negative Eu anomalies found in the IODP site become more pronounced with increasing depth, suggesting progressive increase of hydrothermal alteration where Eu was reductively dissolved into fluids by decomposition of feldspars. Contrary, at the JADE site

  2. Detrital zircon evidence for Hf isotopic evolution of granitoid crust and continental growth

    NASA Astrophysics Data System (ADS)

    Iizuka, Tsuyoshi; Komiya, Tsuyoshi; Rino, Shuji; Maruyama, Shigenori; Hirata, Takafumi

    2010-04-01

    We have determined U-Pb ages, trace element abundances and Hf isotopic compositions of approximately 1000 detrital zircon grains from the Mississippi, Congo, Yangtze and Amazon Rivers. The U-Pb isotopic data reveal the lack of >3.3 Ga zircons in the river sands, and distinct peaks at 2.7-2.5, 2.2-1.9, 1.7-1.6, 1.2-1.0, 0.9-0.4, and <0.3 Ga in the accumulated age distribution. These peaks correspond well with the timing of supercontinent assembly. The Hf isotopic data indicate that many zircons, even those having Archean U-Pb ages, crystallized from magmas involving an older crustal component, suggesting that granitoid magmatism has been the primary agent of differentiation of the continental crust since the Archean era. We calculated Hf isotopic model ages for the zircons to estimate the mean mantle-extraction ages of their source materials. The oldest zircon Hf model ages of about 3.7 Ga for the river sands suggest that some crust generation had taken place by 3.7 Ga, and that it was subsequently reworked into <3.3 Ga granitoid continental crust. The accumulated model age distribution shows peaks at 3.3-3.0, 2.9-2.4, and 2.0-0.9 Ga. The striking attribute of our new data set is the non-uniformitarian secular change in Hf isotopes of granitoid crusts; Hf isotopic compositions of granitoid crusts deviate from the mantle evolution line from about 3.3 to 2.0 Ga, the deviation declines between 2.0 and 1.3 Ga and again increases afterwards. Consideration of mantle-crust mixing models for granitoid genesis suggests that the noted isotopic trends are best explained if the rate of crust generation globally increased in two stages at around (or before) 3.3 and 1.3 Ga, whereas crustal differentiation was important in the evolution of the continental crust at 2.3-2.2 Ga and after 0.6 Ga. Reconciling the isotopic secular change in granitoid crust with that in sedimentary rocks suggests that sedimentary recycling has essentially taken place in continental settings rather than

  3. Hydrological behaviour of microbiotic crusts on sand dunes of NW China: Experimental evidences and numerical simulations

    NASA Astrophysics Data System (ADS)

    Wang, Xin Ping; Tedeschi, Anna; Orefice, Nadia; de Mascellis, Roberto; Menenti, Massimo

    2010-05-01

    Large ecological engineering projects were established to reduce and combat the hazards of sandstorms and desertification in northern China. An experiment to evaluate the effects of dunes stabilization by vegetation was carried out at Shapotou in Ningxia Hui Autonomous Region at the southeast edge of the Tengger Desert using xerophyte shrubs (Caragana korshinskii, Hedysarum scoparium and Artemisia ordosica) planted in straw checkerboard plots in 1956, 1964, 1981, 1987, 1998, and 2002. The fixed sand surface led to the formation of biotic soil crusts. Biotic crusts formed at the soil surface in the interspaces between shrubs and contribute to stabilization of soil surfaces. Previous results on the area have showed that: i) straw checkerboards enhance the capacity of the dune system to trap dust, leading to the accumulation of soil organic matter and nutrients; ii) the longer the period of dune stabilization, the greater the soil clay content in the shallow soil profile (0-5 cm), and greater the fractal dimension of soil particle size distribution. Benefit apart, one should be aware that the formation of a crusted layer at the soil surface is generally characterized by an altered pore-size distribution, with a frequent decrease of hydraulic conductivity which can induce changes of the water regime of the whole soil profile. Accordingly, the main objective of the paper is to evaluate the equivalent (from a hydraulic point of view) geometry of the crusted layer and to verify if the specific characteristics of the crusted soil layer, although local by nature, affect the hydrological behaviour of the whole soil profile. In fact, it is expected that, due to the formation of an upper, impeding soil layer, the lower soil layers do not reach saturation. Such behaviour has important consequences on both water flow and storages in soils. The final aim will be to understand how the crust at the surface of the artificially stabilized sand dune affects the infiltration capacity

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

    NASA Astrophysics Data System (ADS)

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

    2013-11-01

    We study the evolution of the Eocene-Recent Phu Khanh Basin opened during the rifting of the South China Sea (SCS). This sub-basin formed when continental crust ruptured along the East-Vietnam Boundary Fault (EVBF) at the western edge of the SCS. Using high quality long-streamer seismic lines we interpret structures that highlight the different phases of the SCS rifting and processes related to crustal boudinage. Extreme crustal thinning and mantle uplift that sometimes places sediments in contact with the Moho discontinuity mark the central part of the basin. The mantle is shallowest there and marks the final rupture of the continental crust during an intense phase of mantle upwelling. There, a low-angle detachment fault separates several crustal blocks from the Moho. The cylindrical axis of the Moho rise is roughly parallel to the trend of the South China Sea propagator. Above the mantle, the upper and lower crusts form large crustal boudins. The network of normal faults is dense in the upper crust and occasionally propagates into the lower crust. However, the lower crust is missing at some places. The seismic facies above the Moho rise is poorly stratified and might have been affected by a certain degree of metamorphism. At the apex of mantle uplift, there are frequent indications of fluid circulations, including volcanic edifices and gas escapes features. Three stages of extension are clearly identifiable, with ages of the two youngest constrained by well calibration: the first and oldest rift sequence is situated between the tilted pre-rift basement and the Oligocene horizons (32 Ma); the second is delimited by the Oligocene to the Mid Miocene (15.5 Ma) horizons, and the third is bound by the Mid Miocene and the Upper Miocene (before 10.5 Ma) horizons. These three rift episodes formed in at least two extension directions, the first N-S and the second NW-SE. The distinct Mid Miocene (15.5 Ma) horizon is tilted and the above layers show a diverging reflection

  5. The geochemical signature of Neogene Eastern Mediterranean sediment: evidence for temporal and spatial variations in provenance

    NASA Astrophysics Data System (ADS)

    Klaver, Martijn; Vroon, Pieter; Wijbrans, Jan

    2014-05-01

    Subducted sediment is one of the main geochemical components in arc magmas. Although an east-west gradient in Eastern Mediterranean Sea (EMS) sediment composition has been evoked to explain the along-arc geochemical variations in Aegean arc magmas [1], solid evidence for E-W changes in EMS sediment composition is lacking. The EMS sediment dataset is limited to Holocene samples, while the sediment currently underneath the volcanic arc has an age of at least 6 Ma. In order to characterise EMS sediment, we have used a combination of thermogravimetric and geochemical analyses of 45 Neogene DSDP and ODP drill core samples. Thermogravimetric dissociation curves provide a rapid way to determine carbonate-content and relative abundance of clay mineral groups in mixed sediment samples. Clear clay mineral distribution patterns are observed in the EMS: smectite is dominant in Nile sediment, aeolian dust consist mainly of kaolinite while illite is present in coarser (shelf) sediment. Four distinct provenance areas can be recognised on the basis of radiogenic isotope and trace element ratios of the EMS sediment samples. In line with previous studies [e.g. 2], we conclude that Sahara dust and Nile sediment are main constituents of EMS sediment. However, we recognize two additional source areas. Sediment derived from Cyprus and/or SW Turkey, characterised by high Ni/Nb ratios, is an important component in Quaternary EMS sediment. Sediment from the Aegean region has an arc signature (e.g. high La/Nb) and is distinguishable only in the Hellenic Trench. During the Neogene, an increase in aeolian dust input to the EMS is observed, which is consistent with the progressive aridification of the Sahara region. The geochemical characterisation of Neogene EMS sediment and the recognition of distinct provenance areas can be used to assess the contribution of subducted sediment to magmas in the Aeolian and Aegean volcanic arcs. In addition, it allows further high-resolution investigation of

  6. Composition and origin of ferromanganese crusts from equatorial western Pacific seamounts

    NASA Astrophysics Data System (ADS)

    Wang, Guozhi; Jansa, Luba; Chu, Fengyou; Zou, Can; Sun, Guosheng

    2015-04-01

    In the equatorial western Pacific, iron-manganese oxyhydroxide crusts (Fe-Mn crusts) and nodules form on basaltic seamounts and on the top of drowned carbonate platform guyots that have been swept free of pelagic sediments. To date, the Fe-Mn crusts have been considered to be almost exclusively of abiotic origin. However, it has recently been suggested that these crusts may be a result of biomineralization. Although the Fe-Mn crust textures in the equatorial western Pacific are similar to those constructed by bacteria and algae, and biomarkers also document the existence of bacteria and algae dispersed within the Fe-Mn crusts, the precipitation, accumulation and distribution of elements, such as Fe, Mn, Ni and Co in Fe-Mn crusts are not controlled by microbial activity. Bacteria and algae are only physically incorporated into the crusts when dead plankton settle on the ocean floor and are trapped on the crust surface. Geochemical evidence suggests a hydrogenous origin of Fe-Mn crusts in the equatorial western Pacific, thus verifying a process for Fe-Mn crusts that involves the precipitation of colloidal phases from seawater followed by extensive scavenging of dissolved trace metals into the mineral phase during crust formation.

  7. Pinch and swell structures: evidence for brittle-viscous behaviour in the middle crust

    NASA Astrophysics Data System (ADS)

    Gardner, R.; Piazolo, S.; Daczko, N.

    2015-04-01

    The flow properties of middle to lower crustal rocks are commonly represented by viscous flow. However, examples of pinch and swell structures found in a mid-crustal high strain zone at St. Anne Point (Fiordland, New Zealand) suggest pinch and swell structures are initiated by brittle failure of the more competent layer in conjunction with material softening. On this basis we develop a flexible numerical model using brittle-viscous flow where Mohr-Coulomb failure is utilised to initiate pinch and swell structure development. Results show that pinch and swell structures develop in a competent layer in both Newtonian and non-Newtonian flow provided the competent layer has enough viscosity contrast and initially fails brittlely. The degree of material softening after initial failure is shown to impact pinch and swell characteristics with high rates of material softening causing the formation of thick necks between swells by limiting the successful localisation of strain. The flow regime and yielding characteristics of the matrix do not impact pinch and swell structure formation itself, so long as the matrix is less competent. To aid analysis of the structures and help derive the flow properties of rocks in the field, we define three stages of pinch and swell development and offer suggestions for measurements to be made in the field. Our study suggests that Mohr-Coulomb behaviour combined with viscous flow is an appropriate way to represent the heterogeneous rocks of the middle to lower crust. This type of mid-crustal rheological behaviour has significant influence on the localization of strain at all scales. For example, inclusion of Mohr-Coulomb brittle failure with viscous flow in just some mid-crustal layers within a crustal scale model will result in strain localisation throughout the whole crustal section allowing the development of through-going high strain structures from the upper crust into the middle and lower crust. This localization then has a significant

  8. Evidence from the lamarck granodiorite for rapid late cretaceous crust formation in California

    USGS Publications Warehouse

    Coleman, D.S.; Frost, T.P.; Glazner, A.F.

    1992-01-01

    Strontium and neodymium isotopic data for rocks from the voluminous 90-million-year-old Lamarck intrusive suite in the Sierra Nevada batholith, California, show little variation across a compositional range from gabbro to granite. Data for three different gabbro intrusions within the suite are identical within analytical error and are consistent with derivation from an enriched mantle source. Recognition of local involvement of enriched mantle during generation of the Sierran batholith modifies estimates of crustal growth rates in the United States. These data indicate that parts of the Sierra Nevada batholith may consist almost entirely of juvenile crust added during Cretaceous magmatism.

  9. Evidence from the lamarck granodiorite for rapid late cretaceous crust formation in california.

    PubMed

    Coleman, D S; Glazner, A F; Frost, T P

    1992-12-18

    Strontium and neodymium isotopic data for rocks from the voluminous 90-million-year-old Lamarck intrusive suite in the Sierra Nevada batholith, California, show little variation across a compositional range from gabbro to granite. Data for three different gabbro intrusions within the suite are identical within analytical error and are consistent with derivation from an enriched mantle source. Recognition of local involvement of enriched mantle during generation of the Sierran batholith modifies estimates of crustal growth rates in the United States. These data indicate that parts of the Sierra Nevada batholith may consist almost entirely of juvenile crust added during Cretaceous magmatism.

  10. Evidence from the lamarck granodiorite for rapid late cretaceous crust formation in california.

    PubMed

    Coleman, D S; Glazner, A F; Frost, T P

    1992-12-18

    Strontium and neodymium isotopic data for rocks from the voluminous 90-million-year-old Lamarck intrusive suite in the Sierra Nevada batholith, California, show little variation across a compositional range from gabbro to granite. Data for three different gabbro intrusions within the suite are identical within analytical error and are consistent with derivation from an enriched mantle source. Recognition of local involvement of enriched mantle during generation of the Sierran batholith modifies estimates of crustal growth rates in the United States. These data indicate that parts of the Sierra Nevada batholith may consist almost entirely of juvenile crust added during Cretaceous magmatism. PMID:17836186

  11. Thorium isotope evidence for melting of the mafic oceanic crust beneath the Izu arc

    NASA Astrophysics Data System (ADS)

    Freymuth, Heye; Ivko, Ben; Gill, James B.; Tamura, Yoshihiko; Elliott, Tim

    2016-08-01

    We address the question of whether melting of the mafic oceanic crust occurs beneath ordinary volcanic arcs using constraints from U-Series (238U/232Th, 230Th/232Th and 226Ra/230Th) measurements. Alteration of the top few hundred meters of the mafic crust leads to strong U enrichment. Via decay of 238U to 230Th, this results in elevated (230Th/232Th) (where brackets indicate activity ratios) over time-scales of ∼350 ka. This process leads to the high (230Th/232Th), between 2.6 and 11.0 in the mafic altered oceanic crust (AOC) sampled at ODP Sites 801 and 1149 near the Izu-Bonin-Mariana arc. Th activity ratios in the Izu arc lavas range from (230Th/232Th) = 1.2-2.0. These values are substantially higher than those in bulk sediment subducting at the Izu trench and also extend to higher values than in mid-ocean ridge basalts and the Mariana arc. We show that the range in Th isotope ratios in the Izu arc lavas is consistent with the presence of a slab melt from a mixed source consisting of AOC and subducted sediments with an AOC mass fraction of up to approximately 80 wt.% in the component added to the arc lava source. The oceanic plate subducting at the Izu arc is comparatively cold which therefore indicates that temperatures high enough for fluid-saturated melting of the AOC are commonly achieved beneath volcanic arcs. The high ratio of AOC/sediments of the slab melt component suggested for the Izu arc lavas requires preferential melting of the AOC. This can be achieved when fluid-saturated melting of the slab is triggered by fluids derived from underlying subducted serpentinites. Dehydration of serpentinites and migration of the fluid into the overlying crust causes melting to start within the AOC. The absence of a significant sediment melt component suggests there was insufficient water to flux both AOC and overlying sediments.

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

    NASA Astrophysics Data System (ADS)

    Xu, Yi-Gang

    2014-10-01

    end of this stagnant slab does not go beyond the NNE-trending NSGL (Huang and Zhao, 2006; Xu, 2007). Given the subduction of Pacific plate underneath eastern Asian continent, the slab-derived materials are expected to be involved in the sources of the Mesozoic-Cenozoic magmas in this region. Recent studies have shown the ubiquitous presence of subduction-related components in late Cenozoic basalts in eastern China (Zhang et al., 2009; Xu et al., 2012b; Sakuyama et al., 2013). However, it remains unclear whether similar recycled oceanic components are present in earlier basalts (i.e., those emplaced during 90-40 Ma, Fig. 1), for which high quality geochemical data are not available until very recently (Zhang et al., 2008; Kuang et al., 2012; Xu et al., 2012a). In addition, the provenance of recycled oceanic components, if any, is highly relevant to the proposal of the Pacific subduction as one of the possible triggers of the destruction of the NCC. The timing of the first appearance of oceanic components in magmas will provide constraints on the role of the Pacific subduction on the evolution of the NCC.The objective of this study is to review and compile major, trace elements and Sr-Nd-Pb isotopic compositions of mafic magmas emplaced since 90 Ma in North and Northeastern China, and to use these data to elaborate their petrogenesis. We will demonstrate the ubiquitous involvement of subduction-related components in the magma sources. Furthermore, temporal variation in geochemical features suggests that different parts of the recycled oceanic crust are preferentially sampled at different time. In collaborating with melting solidus temperature and the melting column concept, this is interpreted as differential melting of upwelling heterogeneous mantle as a result of lithospheric thinning. The peculiar isotopic compositions of these oceanic crust components suggests a link with the subducted Pacific slab, which currently stagnates at the mantle transition zone beneath the

  13. Paleoproterozoic gabbro-diorite-granite magmatism of the Batomga Rise (NE Aldan Shield): Sm-Nd isotope geochemical evidence

    NASA Astrophysics Data System (ADS)

    Kuzmin, V. K.; Bogomolov, E. S.; Glebovitskii, V. A.

    2016-02-01

    The geochemical similarity and almost simultaneous (2055-2060 Ma) formation of Utakachan gabbro-amphibolite, Jagdakin granodiorite-diorite, Khoyunda granitoid, and Tygymyt leucogranite complexes, which inruded metamorphic formations of the Batomga Group are evidence of their formaton from unified magmatic source. All this makes it possibble to combine aforementioned complexes into the unified Early Proterozoic diferentiated gabbro-diorite-granite complex.

  14. Geochemical evidence for possible natural migration of Marcellus Formation brine to shallow aquifers in Pennsylvania.

    PubMed

    Warner, Nathaniel R; Jackson, Robert B; Darrah, Thomas H; Osborn, Stephen G; Down, Adrian; Zhao, Kaiguang; White, Alissa; Vengosh, Avner

    2012-07-24

    The debate surrounding the safety of shale gas development in the Appalachian Basin has generated increased awareness of drinking water quality in rural communities. Concerns include the potential for migration of stray gas, metal-rich formation brines, and hydraulic fracturing and/or flowback fluids to drinking water aquifers. A critical question common to these environmental risks is the hydraulic connectivity between the shale gas formations and the overlying shallow drinking water aquifers. We present geochemical evidence from northeastern Pennsylvania showing that pathways, unrelated to recent drilling activities, exist in some locations between deep underlying formations and shallow drinking water aquifers. Integration of chemical data (Br, Cl, Na, Ba, Sr, and Li) and isotopic ratios ((87)Sr/(86)Sr, (2)H/H, (18)O/(16)O, and (228)Ra/(226)Ra) from this and previous studies in 426 shallow groundwater samples and 83 northern Appalachian brine samples suggest that mixing relationships between shallow ground water and a deep formation brine causes groundwater salinization in some locations. The strong geochemical fingerprint in the salinized (Cl > 20 mg/L) groundwater sampled from the Alluvium, Catskill, and Lock Haven aquifers suggests possible migration of Marcellus brine through naturally occurring pathways. The occurrences of saline water do not correlate with the location of shale-gas wells and are consistent with reported data before rapid shale-gas development in the region; however, the presence of these fluids suggests conductive pathways and specific geostructural and/or hydrodynamic regimes in northeastern Pennsylvania that are at increased risk for contamination of shallow drinking water resources, particularly by fugitive gases, because of natural hydraulic connections to deeper formations.

  15. Geochemical evidence for possible natural migration of Marcellus Formation brine to shallow aquifers in Pennsylvania

    PubMed Central

    Warner, Nathaniel R.; Jackson, Robert B.; Darrah, Thomas H.; Osborn, Stephen G.; Down, Adrian; Zhao, Kaiguang; White, Alissa; Vengosh, Avner

    2012-01-01

    The debate surrounding the safety of shale gas development in the Appalachian Basin has generated increased awareness of drinking water quality in rural communities. Concerns include the potential for migration of stray gas, metal-rich formation brines, and hydraulic fracturing and/or flowback fluids to drinking water aquifers. A critical question common to these environmental risks is the hydraulic connectivity between the shale gas formations and the overlying shallow drinking water aquifers. We present geochemical evidence from northeastern Pennsylvania showing that pathways, unrelated to recent drilling activities, exist in some locations between deep underlying formations and shallow drinking water aquifers. Integration of chemical data (Br, Cl, Na, Ba, Sr, and Li) and isotopic ratios (87Sr/86Sr, 2H/H, 18O/16O, and 228Ra/226Ra) from this and previous studies in 426 shallow groundwater samples and 83 northern Appalachian brine samples suggest that mixing relationships between shallow ground water and a deep formation brine causes groundwater salinization in some locations. The strong geochemical fingerprint in the salinized (Cl > 20 mg/L) groundwater sampled from the Alluvium, Catskill, and Lock Haven aquifers suggests possible migration of Marcellus brine through naturally occurring pathways. The occurrences of saline water do not correlate with the location of shale-gas wells and are consistent with reported data before rapid shale-gas development in the region; however, the presence of these fluids suggests conductive pathways and specific geostructural and/or hydrodynamic regimes in northeastern Pennsylvania that are at increased risk for contamination of shallow drinking water resources, particularly by fugitive gases, because of natural hydraulic connections to deeper formations. PMID:22778445

  16. Recycling of oceanic crust from a stagnant slab in the mantle transition zone: Evidence from Cenozoic continental basalts in Zhejiang Province, SE China

    NASA Astrophysics Data System (ADS)

    Li, Yan-Qing; Ma, Chang-Qian; Robinson, Paul T.; Zhou, Qin; Liu, Ming-Liang

    2015-08-01

    Cenozoic continental basalts from Zhejiang Province, southeast China are tholeiitic to weakly alkalic in composition, with moderate MgO contents (6-11 wt.%) and an average Mg# of 62. They display typical OIB-like trace element features, including enrichment in most incompatible elements, both LILE and LREE, and negative K, Pb, Zr, Hf anomalies. In particular, they are characterized by high Fe/Mn (73 ± 5), La/Yb (19 ± 6) and Nb/Ta (18.8 ± 0.4) ratios, which can be attributed to the presence of residual clinopyroxene, garnet and rutile in the mantle source. Based on these minerals, the following hybrid source rocks are hypothesized: garnet pyroxenite/eclogite and peridotite. Clinopyroxene-liquid thermobarometry indicates clinopyroxene crystallization temperatures of > 1257 °C. This is higher than the assumed temperature at the base of the sub-continental lithospheric mantle (SCLM) (~ 1220 °C) beneath Zhejiang, thus the magmas were presumably derived from the asthenosphere. Some typical geochemical features such as negative K, Pb anomalies, positive Ba, Sr, Nb, Ta anomalies and the extremely high Os isotopic signatures, suggest participation of EM-like mantle sources, indicative of ancient subducted oceanic crust. (87Sr/86Sr)i (0.7037-0.7046) and 143Nd/144Nd (0.512832-0.512990) isotope ratios point to the presence of mixed components in the source region, i.e., DMM, EM1 and EM2. Recent seismic tomographic images of the mantle beneath Zhejiang suggest the presence of a subducted slab of oceanic lithosphere in the transition zone. Based on the combined geophysical and geochemical evidence, we propose that the major source of the Zhejiang basaltic magmas was the ancient subducted oceanic slab in the transition zone with an EM-like signature. The other magma sources include depleted asthenospheric peridotite possessing a DMM-like signature. The dynamics of this upwelling hybrid magma was apparently related to westward subduction of the Pacific plate underneath the

  17. Partial melting of the South Qinling orogenic crust, China: Evidence from Triassic migmatites and diorites of the Foping dome

    NASA Astrophysics Data System (ADS)

    Zhang, He; Ye, Ri-Sheng; Liu, Bing-Xiang; Wang, Yan; Zhang, Yuan-Shuo; Siebel, Wolfgang; Chen, Fukun

    2016-09-01

    The Qinling orogen was ultimately formed by suturing of the South Qinling and Yangtze blocks, but the exact timing of the final amalgamation of the two blocks has not been well established so far. Partial melting of the Qinling orogenic continental crust resulted in the generation of migmatites, and such rocks may help to decipher the chronology of such event. In this paper, we report U-Pb ages, trace element, and Hf isotopic compositions of zircons from migmatites and diorite gneisses of the Foping dome, South Qinling. Zircons from migmatites form anhedral grains of variable sizes that are characterized by complex trace element compositions. Based on zircon U-Pb ages, the migmatites can be subdivided into two groups: Group 1 migmatites mainly retain Triassic zircons with U-Pb ages of 214-211 Ma and Hf model ages of ~ 1.46 Ga in core and rim domains; zircons from Group 2 migmatites record both Triassic (~ 210 Ma) and Neoproterozoic U-Pb ages, analogous to igneous rocks of the Wudang and Yaolinghe Groups exposed in South Qinling. Zircons from the diorite gneisses yield U-Pb ages of 216-210 Ma with Hf isotopic composition (TDM2 ages of ~ 1.46 Ga) similar to the migmatites. Evidence from whole-rock Nd isotopic analyses also points to a similar genesis between migmatites and diorite gneisses. It is proposed that Group 1 migmatites were derived by melting of Triassic diorites, while Group 2 migmatites were derived from Neoproterozoic igneous rocks, a major basement lithology of South Qinling. Partial melting of the orogenic crust took place at ~ 214-210 Ma, approximately consistent with the retrograde metamorphism of granulites exposed along the suture zone between the South Qinling and Yangtze blocks. We suggest that the collision of these two blocks occurred prior to ~ 215 Ma and that the Foping dome resulted from rapid collapse of an overthickened crust followed by partial melting enhanced by asthenospheric influx.

  18. Mineralogical Evidence for the Bulk Transformation of Continental Crust to Ultrahigh-Pressure Conditions in Subduction Zones

    NASA Astrophysics Data System (ADS)

    Peterman, E. M.; Hacker, B. R.; Kylander-Clark, A. R.

    2005-12-01

    Evidence for (ultra)high-pressure --(U)HP-- metamorphism in modern orogenic belts and the preservation of exhumed (U)HP terranes around the world suggest that subduction and exhumation of continental crust plays an important role in Phanerozoic plate tectonics. The Western Gneiss region (WGR) of Norway, a major (U)HP province extending over 60,000 km2, provides an excellent opportunity to study how subduction to depths >100 km affects continental crust. By studying a ~60 km wide transect bounded to the north by Vartdalsfjorden and Rovdefjorden and the south by the Möre og Romsdal county boundary, we are able to examine mineralogical changes that occurred during subduction and exhumation within a rock composed predominantly of orthogneiss and variably transformed mafic bodies, which indicate the depths to which these rocks were subducted. Previous studies (e.g. Hacker et al., 2005) have suggested that Caledonian deformation in WGR host gneisses is primarily limited to brittle-ductile fabrics characterized by greenschist to lower-amphibolite facies metamorphism; the majority of the deformation in the rocks, including the pervasive foliation and foliation-parallel isoclinal folds, occurred between 1200 and 900 Ma. On the northern half of our study area, however, locally occurring neoblastic garnet crosscuts the foliation in the gneiss. The boundary of this garnet zone coincides with the local HP-UHP boundary, as determined by the presence of coesite in eclogite. Because garnet can retain information about changes in pressure and temperature, as well as the availability of water within the crust to catalyze chemical reactions, our findings suggest that 1) portions of the orthogneiss did transform at high pressures, 2) the presence of garnet within the orthogneiss may indicate conditions that approximate UHP and can therefore be useful in defining the boundaries between UHP and HP conditions, and 3) the growth of garnet during (U)HP metamorphism may be controlled by

  19. Geochemical Evidence of the Seasonality, Affinity and Pigmenation of Solenopora jurassica.

    PubMed

    Barden, Holly E; Behnsen, Julia; Bergmann, Uwe; Leng, Melanie J; Manning, Phillip L; Withers, Philip J; Wogelius, Roy A; van Dongen, Bart E

    2015-01-01

    Solenopora jurassica is a fossil calcareous alga that functioned as an important reef-building organism during the Palaeozoic. It is of significant palaeobiological interest due to its distinctive but poorly understood pink and white banding. Though widely accepted as an alga there is still debate over its taxonomic affinity, with recent work arguing that it should be reclassified as a chaetetid sponge. The banding is thought to be seasonal, but there is no conclusive evidence for this. Other recent work has, however demonstrated the presence of a unique organic boron-containing pink/red pigment in the pink bands of S. jurassica. We present new geochemical evidence concerning the seasonality and pigmentation of S. jurassica. Seasonal growth cycles are demonstrated by X-ray radiography, which shows differences in calcite density, and by varying δ13C composition of the bands. Temperature variation in the bands is difficult to constrain accurately due to conflicting patterns arising from Mg/Ca molar ratios and δ18O data. Fluctuating chlorine levels indicate increased salinity in the white bands, when combined with the isotope data this suggests more suggestive of marine conditions during formation of the white band and a greater freshwater component (lower chlorinity) during pink band precipitation (δ18O). Increased photosynthesis is inferred within the pink bands in comparison to the white, based on δ13C. Pyrolysis Gas Chromatography Mass Spectrometry (Py-GCMS) and Fourier Transform Infrared Spectroscopy (FTIR) show the presence of tetramethyl pyrrole, protein moieties and carboxylic acid groups, suggestive of the presence of the red algal pigment phycoerythrin. This is consistent with the pink colour of S. jurassica. As phycoerythrin is only known to occur in algae and cyanobacteria, and no biomarker evidence of bacteria or sponges was detected we conclude S. jurassica is most likely an alga. Pigment analysis may be a reliable classification method for fossil

  20. Geochemical Evidence of the Seasonality, Affinity and Pigmenation of Solenopora jurassica.

    PubMed

    Barden, Holly E; Behnsen, Julia; Bergmann, Uwe; Leng, Melanie J; Manning, Phillip L; Withers, Philip J; Wogelius, Roy A; van Dongen, Bart E

    2015-01-01

    Solenopora jurassica is a fossil calcareous alga that functioned as an important reef-building organism during the Palaeozoic. It is of significant palaeobiological interest due to its distinctive but poorly understood pink and white banding. Though widely accepted as an alga there is still debate over its taxonomic affinity, with recent work arguing that it should be reclassified as a chaetetid sponge. The banding is thought to be seasonal, but there is no conclusive evidence for this. Other recent work has, however demonstrated the presence of a unique organic boron-containing pink/red pigment in the pink bands of S. jurassica. We present new geochemical evidence concerning the seasonality and pigmentation of S. jurassica. Seasonal growth cycles are demonstrated by X-ray radiography, which shows differences in calcite density, and by varying δ13C composition of the bands. Temperature variation in the bands is difficult to constrain accurately due to conflicting patterns arising from Mg/Ca molar ratios and δ18O data. Fluctuating chlorine levels indicate increased salinity in the white bands, when combined with the isotope data this suggests more suggestive of marine conditions during formation of the white band and a greater freshwater component (lower chlorinity) during pink band precipitation (δ18O). Increased photosynthesis is inferred within the pink bands in comparison to the white, based on δ13C. Pyrolysis Gas Chromatography Mass Spectrometry (Py-GCMS) and Fourier Transform Infrared Spectroscopy (FTIR) show the presence of tetramethyl pyrrole, protein moieties and carboxylic acid groups, suggestive of the presence of the red algal pigment phycoerythrin. This is consistent with the pink colour of S. jurassica. As phycoerythrin is only known to occur in algae and cyanobacteria, and no biomarker evidence of bacteria or sponges was detected we conclude S. jurassica is most likely an alga. Pigment analysis may be a reliable classification method for fossil

  1. Geochemical Evidence of the Seasonality, Affinity and Pigmenation of Solenopora jurassica

    SciTech Connect

    Barden, Holly E.; Behnsen, Julia; Bergmann, Uwe; Leng, Melanie J.; Manning, Phillip L.; Withers, Philip J.; Wogelius, Roy A.; van Dongen, Bart E.; Silva, Lucas C. R.

    2015-09-14

    Solenopora jurassica is a fossil calcareous alga that functioned as an important reef-building organism during the Palaeozoic. It is of significant palaeobiological interest due to its distinctive but poorly understood pink and white banding. Though widely accepted as an alga there is still debate over its taxonomic affinity, with recent work arguing that it should be reclassified as a chaetetid sponge. The banding is thought to be seasonal, but there is no conclusive evidence for this. Other recent work has, however demonstrated the presence of a unique organic boron-containing pink/red pigment in the pink bands of S. jurassica. We present new geochemical evidence concerning the seasonality and pigmentation of S. jurassica. Seasonal growth cycles are demonstrated by X-ray radiography, which shows differences in calcite density, and by varying δ13C composition of the bands. Temperature variation in the bands is difficult to constrain accurately due to conflicting patterns arising from Mg/Ca molar ratios and δ18O data. Fluctuating chlorine levels indicate increased salinity in the white bands, when combined with the isotope data this suggests more suggestive of marine conditions during formation of the white band and a greater freshwater component (lower chlorinity) during pink band precipitation (δ18O). Increased photosynthesis is inferred within the pink bands in comparison to the white, based on δ13C. Pyrolysis Gas Chromatography Mass Spectrometry (Py-GCMS) and Fourier Transform Infrared Spectroscopy (FTIR) show the presence of tetramethyl pyrrole, protein moieties and carboxylic acid groups, suggestive of the presence of the red algal pigment phycoerythrin. This is consistent with the pink colour of S. jurassica. As phycoerythrin is only known to occur in algae and cyanobacteria, and no biomarker evidence of bacteria or sponges was detected we conclude S. jurassica is most likely an alga. Pigment analysis may be a

  2. Geochemical Evidence of the Seasonality, Affinity and Pigmenation of Solenopora jurassica

    DOE PAGES

    Barden, Holly E.; Behnsen, Julia; Bergmann, Uwe; Leng, Melanie J.; Manning, Phillip L.; Withers, Philip J.; Wogelius, Roy A.; van Dongen, Bart E.; Silva, Lucas C. R.

    2015-09-14

    Solenopora jurassica is a fossil calcareous alga that functioned as an important reef-building organism during the Palaeozoic. It is of significant palaeobiological interest due to its distinctive but poorly understood pink and white banding. Though widely accepted as an alga there is still debate over its taxonomic affinity, with recent work arguing that it should be reclassified as a chaetetid sponge. The banding is thought to be seasonal, but there is no conclusive evidence for this. Other recent work has, however demonstrated the presence of a unique organic boron-containing pink/red pigment in the pink bands of S. jurassica. We presentmore » new geochemical evidence concerning the seasonality and pigmentation of S. jurassica. Seasonal growth cycles are demonstrated by X-ray radiography, which shows differences in calcite density, and by varying δ13C composition of the bands. Temperature variation in the bands is difficult to constrain accurately due to conflicting patterns arising from Mg/Ca molar ratios and δ18O data. Fluctuating chlorine levels indicate increased salinity in the white bands, when combined with the isotope data this suggests more suggestive of marine conditions during formation of the white band and a greater freshwater component (lower chlorinity) during pink band precipitation (δ18O). Increased photosynthesis is inferred within the pink bands in comparison to the white, based on δ13C. Pyrolysis Gas Chromatography Mass Spectrometry (Py-GCMS) and Fourier Transform Infrared Spectroscopy (FTIR) show the presence of tetramethyl pyrrole, protein moieties and carboxylic acid groups, suggestive of the presence of the red algal pigment phycoerythrin. This is consistent with the pink colour of S. jurassica. As phycoerythrin is only known to occur in algae and cyanobacteria, and no biomarker evidence of bacteria or sponges was detected we conclude S. jurassica is most likely an alga. Pigment analysis may be a reliable classification method for

  3. Geochemical Evidence of the Seasonality, Affinity and Pigmenation of Solenopora jurassica

    PubMed Central

    Barden, Holly E.; Behnsen, Julia; Bergmann, Uwe; Leng, Melanie J.; Manning, Phillip L.; Withers, Philip J.; Wogelius, Roy A.; van Dongen, Bart E.

    2015-01-01

    Solenopora jurassica is a fossil calcareous alga that functioned as an important reef-building organism during the Palaeozoic. It is of significant palaeobiological interest due to its distinctive but poorly understood pink and white banding. Though widely accepted as an alga there is still debate over its taxonomic affinity, with recent work arguing that it should be reclassified as a chaetetid sponge. The banding is thought to be seasonal, but there is no conclusive evidence for this. Other recent work has, however demonstrated the presence of a unique organic boron-containing pink/red pigment in the pink bands of S. jurassica. We present new geochemical evidence concerning the seasonality and pigmentation of S. jurassica. Seasonal growth cycles are demonstrated by X-ray radiography, which shows differences in calcite density, and by varying δ13C composition of the bands. Temperature variation in the bands is difficult to constrain accurately due to conflicting patterns arising from Mg/Ca molar ratios and δ18O data. Fluctuating chlorine levels indicate increased salinity in the white bands, when combined with the isotope data this suggests more suggestive of marine conditions during formation of the white band and a greater freshwater component (lower chlorinity) during pink band precipitation (δ18O). Increased photosynthesis is inferred within the pink bands in comparison to the white, based on δ13C. Pyrolysis Gas Chromatography Mass Spectrometry (Py-GCMS) and Fourier Transform Infrared Spectroscopy (FTIR) show the presence of tetramethyl pyrrole, protein moieties and carboxylic acid groups, suggestive of the presence of the red algal pigment phycoerythrin. This is consistent with the pink colour of S. jurassica. As phycoerythrin is only known to occur in algae and cyanobacteria, and no biomarker evidence of bacteria or sponges was detected we conclude S. jurassica is most likely an alga. Pigment analysis may be a reliable classification method for fossil

  4. Zircon U-Pb dating, geochemical and Sr-Nd-Hf isotopic characteristics of the Jintonghu monzonitic rocks in western Fujian Province, South China: Implication for Cretaceous crust-mantle interactions and lithospheric extension

    NASA Astrophysics Data System (ADS)

    Li, Bin; Jiang, Shao-Yong; Lu, An-Huai; Zhao, Hai-Xiang; Yang, Tang-Li; Hou, Ming-Lan

    2016-09-01

    Comprehensive petrological, in situ zircon U-Pb dating, Ti-in-zircon temperature and Hf isotopic compositions, whole rock geochemical and Sr-Nd isotopic data are reported for the Jintonghu monzonitic intrusions in the western Fujian Province (Interior Cathaysia Block), South China. The Jintonghu monzonitic intrusions were intruded at 95-96 Ma. Their Sr-Nd-Hf isotopic compositions are similar to the coeval and nearby enriched lithospheric mantle-derived mafic and syenitic rocks, indicating that the Jintonghu monzonitic rocks were likely derived from partial melting of the enriched mantle sources. Their high Nb/Ta ratios (average 21.6) suggest that the metasomatically enriched mantle components were involved, which was attributed to the modification of slab-derived fluid and melt by the subduction of the paleo-Pacific Plate. The presence of mafic xenoliths, together with geochemical and isotopic features indicates a mafic-felsic magma mixing. Furthermore, the Jintonghu intrusions may have experienced orthopyroxene-, biotite- and plagioclase-dominated crystallization. Crust-mantle interaction can be identified as two stages, including that the Early Cretaceous mantle metasomatism and lithospheric extension resulted from the paleo-Pacific slab subduction coupled with slab rollback, and the Late Cretaceous crustal activation and enhanced extension induced by dip-angle subduction and the underplating of mantle-derived mafic magma.

  5. Origin of enriched components in the South Atlantic: Evidence from 40 Ma geochemical zonation of the Discovery Seamounts

    NASA Astrophysics Data System (ADS)

    Schwindrofska, Antje; Hoernle, Kaj; Hauff, Folkmar; van den Bogaard, Paul; Werner, Reinhard; Garbe-Schönberg, Dieter

    2016-05-01

    Spatial geochemical zonation is being increasingly recognized in Pacific and Atlantic hotspot tracks and is believed to reflect zonation within plumes upwelling from the margins of the Large Low Shear Velocity Provinces (LLSVPs) at the base of Earth's mantle. We present new 40Ar/39Ar age data for the Discovery Rise (South Atlantic Ocean) that show an age progression in the direction of plate motion from 23 Ma in the southwest to 40 Ma in the northeast of the Rise, consistent with formation of the Rise above a mantle plume. The lavas have incompatible element and Sr-Nd-Pb-Hf radiogenic isotope characteristics similar to the enriched DUPAL anomaly occurring in the southern hemisphere. The northern chain of seamounts is compositionally similar to the adjacent Gough subtrack of the bilaterally-zoned Tristan-Gough hotspot track, whereas the southern chain has some of the most extreme DUPAL compositions found in South Atlantic intraplate lavas thus far. The nearby southern Mid-Atlantic Ridge, believed to interact with the Discovery hotspot, shows a similar spatial geochemical distribution, consistent with the Discovery hotspot being zoned over its entire 40 Ma history. Our study implies a deep origin for the DUPAL anomaly, suggesting recycling of subcontinental lithospheric mantle (± lower crust) and oceanic crust through the lower mantle. The presence of an additional (Southern Discovery) DUPAL-like component, in addition to the Tristan and Gough/Northern Discovery components, in long-term zoned South Atlantic hotspots, points to the presence of a third lower mantle reservoir and thus is not consistent with the simple model that bilaterally-zoned plumes sample a chemically distinct LLSVP and the ambient mantle outside of the LLSVP.

  6. Featureless spectra on the Moon as evidence of residual lunar primordial crust

    NASA Astrophysics Data System (ADS)

    Yamamoto, S.; Nakamura, R.; Matsunaga, T.; Ogawa, Y.; Ishihara, Y.; Morota, T.; Hirata, N.; Ohtake, M.; Hiroi, T.; Yokota, Y.; Haruyama, J.

    2015-12-01

    We report the global distribution of areas exhibiting no absorption features (featureless or FL) on the lunar surface, based on the reflectance spectral data set obtained by the Spectral Profiler onboard Kaguya/SELENE. We found that FL sites are located in impact basins and large impact craters in the Feldspathic Highlands Terrane, while there are no FL sites in the Procellarum regions nor the South Pole-Aitken basin. FL sites in each impact basin/crater are mainly found at the peak rings or rims, where the purest anorthosite (PAN) sites are also found. At the local scale, most of the FL and PAN points are associated with impact craters and peaks. Most of the FL spectra show a steeper (redder) continuum than the PAN spectra, suggesting the occurrence of space weathering effects. We propose that most of the material exhibiting a FL spectrum originate from space weathered PAN. Taking into account all the occurrence trends of FL sites on the Moon, we propose that both the FL and PAN materials were excavated from the primordial lunar crust during ancient basin formations below the megaregolith in the highlands. Since the FL and PAN sites are widely distributed over the lunar surface, our new data may support the existence of a massive PAN layer below the lunar surface.

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

    NASA Technical Reports Server (NTRS)

    Taylor, Lawrence A.; Neal, Clive R.

    1988-01-01

    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.

  8. Erosion by flowing lava: Geochemical evidence in the Cave Basalt, Mount St. Helens, Washington

    USGS Publications Warehouse

    Williams, D.A.; Kadel, S.D.; Greeley, R.; Lesher, C.M.; Clynne, M.A.

    2004-01-01

    We sampled basaltic lava flows and underlying dacitic tuff deposits in or near lava tubes of the Cave Basalt, Mount St. Helens, Washington to determine whether the Cave Basalt lavas contain geochemical evidence of substrate contamination by lava erosion. The samples were analyzed using a combination of wavelength-dispersive X-ray fluorescence spectrometry and inductively-coupled plasma mass spectrometry. The results indicate that the oldest, outer lava tube linings in direct contact with the dacitic substrate are contaminated, whereas the younger, inner lava tube linings are uncontaminated and apparently either more evolved or enriched in residual liquid. The most heavily contaminated lavas occur closer to the vent and in steeper parts of the tube system, and the amount of contamination decreases with increasing distance downstream. These results suggest that erosion by lava and contamination were limited to only the initially emplaced flows and that erosion was localized and enhanced by vigorous laminar flow over steeper slopes. After cooling, the initial Cave Basalt lava flows formed an insulating lining within the tubes that prevented further erosion by later flows. This interpretation is consistent with models of lava erosion that predict higher erosion rates closer to sources and over steeper slopes. A greater abundance of xenoliths and xenocrysts relative to xenomelts in hand samples indicates that mechanical erosion rather than thermal erosion was the dominant erosional process in the Cave Basalt, but further sampling and petrographic analyses must be performed to verify this hypothesis. ?? Springer-Verlag 2003.

  9. Geochemical evidence for an Eolian sand dam across the North and South Platte rivers in Nebraska

    USGS Publications Warehouse

    Muhs, Daniel R.; Swinehart, James B.; Loope, David B.; Been, Josh; Mahan, Shannon; Bush, Charles A.

    2000-01-01

    Geochemical and geomorphic data from dune fields in southwestern Nebraska provide new evidence that the Nebraska Sand Hills once migrated across the North and South Platte rivers and dammed the largest tributary system to the Missouri River. The Lincoln County and Imperial dune fields, which lie downwind of the South Platte River, have compositions intermediate between the Nebraska Sand Hills (quartz-rich) and northeastern Colorado dunes (K-feldspar-rich). The most likely explanation for the intermediate composition is that the Lincoln County and Imperial dunes are derived in part from the Nebraska Sand Hills and in part from the South Platte River. The only mechanism by which the Nebraska Sand Hills could have migrated this far south is by complete infilling of what were probably perennially dry North Platte and South Platte river valleys. Such a series of events would have required an extended drought, both for activation of eolian sand and decreased discharges in the Platte River system. A nearby major tributary of the North Platte River is postulated to have been blocked by eolian sand about 12,000 14C yr B.P. We propose that an eolian sand dam across the Plattes was constructed at about this same time.

  10. Geochemical Variation of Subducting Pacific Crust Along the Izu-Bonin Arc System and its Implications on the Generation of Arc Magmas

    NASA Astrophysics Data System (ADS)

    Durkin, K.; Castillo, P.; Abe, N.; Kaneko, R.; Straub, S. M.; Garcia, E. S. M.; Yan, Q.; Tamura, Y.

    2015-12-01

    Subduction zone magmatism primarily occurs due to flux melting of the mantle wedge that has been metasomatized by the slab component. The latter is enriched in volatiles and fluid-mobile elements and derived mainly from subducted sediments and altered oceanic crust (AOC). Subduction input has been linked to arc output in many studies, but this relationship is especially well documented in sedimented arc-trench systems. However, the Izu-Bonin system is sediment-poor, therefore the compositional and latitudinal variations (especially in Pb isotopes) of its arc magmas must be sourced from the subduction component originating primarily from the AOC. Pb is a very good tracer of recycled AOC that may contribute 50% or more of arc magma Pb. Izu-Bonin arc chemistry suggests a subduction influx of Indian-type crust, but the subducting crust sampled at ODP Site 1149 is Pacific-type. The discrepancy between subduction input and arc output calls into question the importance of the AOC as a source of the subduction component, and raises major concerns with our understanding of slab input. During the R/V Revelle 1412 cruise in late 2014, we successfully dredged vertical fault scarps at several sites from 27.5 N to 34.5 N, spanning a range of crustal ages that include a suggested compositional change at ~125 Ma. Major element data show an alkali enrichment towards the north of the study transect. Preliminary incompatible trace element data (e.g. Ba, Zr and Sr) data support this enrichment trend. Detailed mass balance calculations supported by Sr, Nd, Hf and especially Pb isotope analyses will be performed to evaluate whether the AOC controls the Pb isotope chemistry of the Izu-Bonin volcanic arc.

  11. Granitoid formation is ineffective in isotopically homogenizing continental crust: Evidence from archean rocks of the Wind River Mountains, Wyoming

    SciTech Connect

    Frost, C.D. ); Hulsebosch, T.P. ); Chamberlain, K.R.; Frost, B.R. )

    1992-01-01

    The Archean core of the Laramide Wind River uplift records evidence of at least three major granitoid-forming episodes. The oldest, the Dry Creek gneiss (DCG), was emplaced by 2.8 Ga and occupies the northeastern part of the range. Mafic, pelitic and ultramafic inclusions occur in the DCG. Elsewhere in the Wind River Mountains there is evidence for crustal components as old as 3.8 Ga. The Bridger batholith (BB), intruded at 2.67 Ga, is found in the west-central Wind River Mountains. The Wind River batholith (WRB) refers to the youngest Late Archean granodiorites and granites which are found throughout the range and includes granitoids previously name the Louis Lake, Bears Ears, Popo Agie, and Middle Mountain intrusions. Although granitoids of the Wind River batholith have been dated at 2.63 and 2.55 Ga, they are considered together here because there is a complete gradation in rock type and because definite intrusive contacts are scarce. The DCG, BB, and WRB each span the metaluminous/peraluminous boundary and are indistinguishable on Harker diagrams. Each has variable trace element and isotopic characteristics which do not correlate with silica content. Although the isotopic characteristics of these granitoids may be explained by mixing of variable amounts of preexisting continental crust and contemporary depleted mantle, this hypothesis is difficult to reconcile with the evolved nature of even those samples with the most mantle-like isotopic signatures. The authors suggest that each of these granitoid batholiths was formed primarily by remelting of pre-existing heterogeneous continental crust, and that the granite-forming process was not effective in obliterating these trace element and isotopic heterogeneities. Isotopic homogeneity in granitoid batholiths may reflect the isotopic homogeneity of their sources rather than an effective magmatic mixing process.

  12. Hafnium isotope evidence from Archean granitic rocks for deep-mantle origin of continental crust

    NASA Astrophysics Data System (ADS)

    Guitreau, Martin; Blichert-Toft, Janne; Martin, Hervé; Mojzsis, Stephen J.; Albarède, Francis

    2012-07-01

    Combined whole-rock and zircon MC-ICP-MS Lu-Hf isotope data are reported for a large collection of Archean granitoids belonging to typical tonalite-trondhjemite-granodiorite (TTG) suites. Our data demonstrate that the time-integrated Lu/Hf of the mantle source of TTGs has not significantly changed over the last 4 Gy. Continents therefore most likely grew from nearly primordial unfractionated material extracted from the deep mantle via rising plumes that left a depleted melt residue in the upper mantle. The deep mantle could retain its primitive relative element abundances over time because sinking plates are largely stripped barren of their oceanic and continental crust components at subduction zones; this process results in only small proportions (<15-25%) of present-day continental mass getting recycled to great depths. Zircon populations extracted from the analyzed TTGs have Hf isotopic compositions broadly consistent with those of their host whole-rocks, whereas the U-Pb system in the same grains is often disturbed, causing a discrepancy that creates spurious initial ɛHf values. This problem is endemic to the Archean detrital zircon record and consistent with experimental results bearing on the relative retentivity of Hf vs. U and Pb in zircon. We argue that this behavior biases the Archean zircon record toward negative ɛHf values, which are at odds with the present TTG data set. If Hadean Jack Hills zircons are considered in light of these results, the mantle source of continents has remained unchanged for the last 4.3 Gy.

  13. Geochemical evidence for a Cretaceous oil sand (Bima oil sand) in the Chad Basin, Nigeria

    NASA Astrophysics Data System (ADS)

    Bata, Timothy; Parnell, John; Samaila, Nuhu K.; Abubakar, M. B.; Maigari, A. S.

    2015-11-01

    Paleogeographic studies have shown that Earth was covered with more water during the Cretaceous than it is today, as the global sea level was significantly higher. The Cretaceous witnessed one of the greatest marine transgressions in Earth's history, represented by widespread deposition of sands directly on underlying basement. These sand bodies hold much of the world's heavy oil. Here, we present for the first time, geochemical evidence of a Cretaceous oil sand (Bima oil sand) in the Chad Basin, Nigeria. Bima oil sand is similar to other Cretaceous oil sands, predominantly occurring at shallow depths on basin flanks and generally lacking a seal cover, making the oil susceptible to biodegradation. The bulk properties and distribution of molecular features in oils from the Bima oil sand suggest that they are biodegraded. Sterane maturity parameters and the trisnorhopane thermal indicator for the oils suggest thermal maturities consistent with oils generated as conventional light oils, which later degraded into heavy oils. These oils also show no evidence of 25-norhopane, strongly suggesting that biodegradation occurred at shallow depths, consistent with the shallow depth of occurrence of the Bima Formation at the study locality. Low diasterane/sterane ratios and C29H/C30H ratios greater than 1 suggest a carbonate source rock for the studied oil. The Sterane distribution further suggests that the oils were sourced from marine carbonate rocks. The C32 homohopane isomerization ratios for the Bima oil sand are 0.59-0.60, implying that the source rock has surpassed the main oil generation phase, consistent with burial depths of the Fika and Gongila Formations, which are both possible petroleum source rocks in the basin.

  14. Composition of the ancient north american crust.

    PubMed

    Condie, K C

    1967-02-24

    Geochemical studies of Wyoming Precambrian graywackes derived from continental crust older than 3.2 x 10(9) years indicate that their source area was at least as highly differentiated as most younger Precambrian crust. The composition of this early crust (approximately that of calcium-rich granite) is not unlike that of the 2.5 to 3.2 x 10(9) year old North American crust. Limited geochemical data suggest that the composition of North America may not have changed significantly during the last 3.0 to 3.5 x 10(9) years.

  15. Geochemical evidence for the formation of the Moon by impact induced fission of the proto-Earth

    NASA Technical Reports Server (NTRS)

    Waenke, H.; Dreibus, G.

    1984-01-01

    Geochemical evidence is discussed which advocates the theory that the Moon was formed by impact induced fission of the Earth. The Earth's mantle exhibits a number of geochemical peculiarities which make our planet a unique object in the solar system. Terrestrial basalts are compared with those from the Eucrite parent body and the Shergotty parent body. Also the Moon's composition is very close to the Earth's in all details except the lower FeO content which is explained. Evidence is discussed for the plausible physical process of formation of the Moon by impact induced fission. Also the theory that impact induced fission occurred at the moment at which accretion of the Earth was not totally complete is briefly discussed.

  16. Geochemical characterisation of gases along the dead sea rift: Evidences of mantle-co2 degassing

    NASA Astrophysics Data System (ADS)

    Inguaggiato, C.; Censi, P.; D'Alessandro, W.; Zuddas, P.

    2016-06-01

    The Dead Sea Transform (DST) fault system, where a lateral displacement between the African and Arabian plates occurs, is characterised by anomalous heat flux in the Israeli area close to the border with Syria and Jordan. The concentration of He and CO2, and isotopic composition of He and total dissolved inorganic carbon were studied in cold and thermal waters collected along the DST, in order to investigate the source of volatiles and their relationship with the tectonic framework of the DST. The waters with higher temperature (up to 57.2 °C) are characterised by higher amounts of CO2 and helium (up to 55.72 and 1.91 ∗ 10- 2 cc l- 1, respectively). Helium isotopic data (R/Ra from 0.11 to 2.14) and 4He/20Ne ratios (0.41-106.86) show the presence of deep-deriving fluids consisting of a variable mixture of mantle and crust end-members, with the former reaching up to 35%. Carbon isotope signature of total dissolved carbon from hot waters falls within the range of magmatic values, suggesting the delivery of deep-seated CO2. The geographical distribution of helium isotopic data and isotopic carbon (CO2) values coupled with (CO2/3He ratios) indicate a larger contribution of mantle-derived fluids affecting the northern part of the investigated area, where the waters reach the highest temperature. These evidences suggest the occurrence of a favourable tectonic framework, including a Moho discontinuity up-rise and/or the presence of a deep fault system coupled with the recent magmatic activity recognised in the northern part of Israel.

  17. Geochemical evidence for waning magmatism and polycyclic volcanism at Crater Flat, Nevada

    SciTech Connect

    Perry, F.V.; Crowe, B.M.

    1991-12-31

    Petrologic and geochemical studies of basaltic rocks in the Yucca Mountain region are currently focused on understanding the evolution of volcanism in the Crater Flat volcanic field and the mechanisms of polycyclic volcanism at the Lathrop Wells volcanic center, the youngest center in the Crater Flat volcanic field. Geochemical and petrologic data indicate that the magma chambers which supplied the volcanic centers in Crater Flat became situated at greater crustal depths as the field evolved. Deep magma chambers may be related to a waning magma flux that was unable to sustain upper crystal magma conduits and chambers. Geochemical data from the Lathrop Wells volcanic center indicate that eruptive units identified from field and geomorphic relationships are geochemically distinct. The geochemical variations cannot be explained by fractional crystallization of a single magma batch, indicating that several magma batches were involved in the formation of the Lathrop Wells center. Considering the low magma flux in the Yucca Mountain region in the Quaternary, the probability of several magma batches erupting essentially simultaneously at Lathrop Wells in considered remote. It is more likely that the Lathrop Wells center was formed by a series of eruptions that took place over many thousands of years. The geochemical data from Lathrop Wells is consistent with the concept of a complex, polycyclic volcano, which was originally proposed based on geomorphic and soil-development data.

  18. Geochemical evidence for waning magmatism and polycyclic volcanism at Crater Flat, Nevada

    NASA Astrophysics Data System (ADS)

    Perry, F. V.; Crowe, B. M.

    Petrologic and geochemical studies of basaltic rocks in the Yucca Mountain region are currently focused on understanding the evolution of volcanism in the Crater Flat volcanic field and the mechanisms of polycyclic volcanism at the Lathrop Wells volcanic center, the youngest center in the Crater Flat volcanic field. Geochemical and petrologic data indicate that the magma chambers which supplied the volcanic centers in Crater Flat became situated at greater crustal depths as the field evolved. Deep magma chambers may be related to a waning magma flux that was unable to sustain upper crystal magma conduits and chambers. Geochemical data from the Lathrop Wells volcanic center indicate that eruptive units identified from field and geomorphic relationships are geochemically distinct. The geochemical variations cannot be explained by fractional crystallization of a single magma batch, indicating that several magma batches were involved in the formation of the Lathrop Wells center. Considering the low magma flux in the Yucca Mountain region in the Quaternary, the probability of several magma batches erupting essentially simultaneously at Lathrop Wells in considered remote. It is more likely that the Lathrop Wells center was formed by a series of eruptions that took place over many thousands of years. The geochemical data from Lathrop Wells is consistent with the concept of a complex, polycyclic volcano, which was originally proposed based on geomorphic and soil-development data.

  19. Geochemical characteristics of fast-spreading lower oceanic crust: an example of troctolites at the Hess Deep Rift (IODP Expedition 345)

    NASA Astrophysics Data System (ADS)

    Akizawa, N.; Godard, M.; Ildefonse, B.; Arai, S.

    2014-12-01

    Troctolites were recovered during IODP Expedition 345 (Dec 2012 - Feb 2013), which targeted plutonic rocks from fast-spread lower oceanic crust at the Hess Deep Rift. The troctolites are divided into three groups based on textural diversity; "skeletal olivine-bearing", "fine-grained" and "coarse-grained" troctolites. For major- and trace-element compositions of olivine and plagioclase, the skeletal olivine-bearing and fine-grained troctolites are more evolved (olivine Fo and NiO contents, 83 to 86 and 0.08 to 0.2 wt.%, respectively, and plagioclase An content, 77 to 84) than coarse-grained ones (olivine Fo and NiO contents, 87to 89 and 0.2 to 0.3 wt.%, respectively, and plagioclase An content, 85 to 90). Clinopyroxenes show scattered chemical compositions in the skeletal olivine-bearing and fine-grained troctolites, down to the scale of a single thin section. Clinopyroxenes, some of which show no Eu anomaly, are clearly zoned in the coarse-grained troctolites: the rims are chemically more evolved than the cores. The skeletal olivine-bearing and fine-grained troctolites record signs of melt invasion, which caused the chemical heterogeneity of the clinopyroxenes. In contrast, the coarse-grained troctolites show no such sign, and contain the zoned clinopyroxenes, which were co-precipitated with plagioclases. MORB (mid-ocean ridge basalt) melts experience multi-stage evolution, including fractional crystallization and melt/troctolite interaction during migration. Such various melt migration processes in the lower oceanic crust possibly cause regional diversity of MORB chemistry.

  20. Geochronological and isotopic evidence for early Proterozoic crust in the eastern Arabian Shield.

    USGS Publications Warehouse

    Stacey, J.S.; Hedge, C.E.

    1984-01-01

    Zircon U/Pb, feldspar common Pb, whole-rock Sm/Nd, and Rb/Sr data indicate that the fine-grained granodiorite (Z103) has yielded conclusive evidence for rocks of early Proterozoic age in the eastern Arabian Shield (21o19' N, 44o50' W). Z103 may have been emplaced approx 1630 m.y. ago and subsequently was severely deformed or perhaps even remobilized at approx 660 m.y. Furthermore, lead isotope data, along with other evidence, show that the 1630 m.y. crustal rocks inherited material from an older, probably Archaean, source at the time of their formation. At that time addition of mantle material considerably modified the Rb-Sr and Sm-Nd systems so that they now yield similar, or only slightly older apparent ages (1600-1800 m.y.).-L.diH.

  1. Xenoliths in ultrapotassic volcanic rocks in the Lhasa block: direct evidence for crust-mantle mixing and metamorphism in the deep crust

    NASA Astrophysics Data System (ADS)

    Wang, Rui; Collins, William J.; Weinberg, Roberto F.; Li, Jin-xiang; Li, Qiu-yun; He, Wen-yan; Richards, Jeremy P.; Hou, Zengqian; Zhou, Li-min; Stern, Richard A.

    2016-07-01

    δ18O (+6 to 7.5 ‰), intermediate (δ18O +8.5 to 9.0 ‰), and high δ18O (+11.0 to 12.0 ‰). The fourth is almost pure andradite with δ18O 10-12 ‰. Both the low and intermediate δ18O groups show significant variation in Fe content, whereas the two high δ18O groups are compositionally homogeneous. We interpret these features to indicate that the low and intermediate δ18O group garnets grew in separate fractionating magmas that were brought together through magma mixing, whereas the high δ18O groups formed under high-grade metamorphic conditions accompanied by metasomatic exchange. The garnets record complex, open-system magmatic and metamorphic processes in a single rock. Based on these features, we consider that ultrapotassic magmas interacted with juvenile 35-20 Ma crust after they intruded in the deep crust (>50 km) at ~13 Ma to form hybridized Miocene granitoid magmas, leaving a refractory residue. The ~13 Ma zircons retain the original, evolved isotopic character of the ultrapotassic magmas, and the garnets record successive stages of the melting and mixing process, along with subsequent high-grade metamorphism followed by low-temperature alteration and brecciation during entrainment and ascent in a late UPV dyke. This is an excellent example of in situ crust-mantle hybridization in the deep Tibetan crust.

  2. Mantle-crust interactions in a paleosubduction zone: Evidence from highly siderophile element systematics of eclogite and related rocks

    NASA Astrophysics Data System (ADS)

    Penniston-Dorland, Sarah C.; Walker, Richard J.; Pitcher, Lynnette; Sorensen, Sorena S.

    2012-02-01

    Substantial differences in 187Os/188Os and absolute and relative abundances of highly siderophile elements (HSE: Os, Ir, Ru, Pt, Pd, Re) in mantle peridotites compared to oceanic crust indicate that this suite of elements may prove useful in assessing mechanisms and pathways of mantle and slab mass transfer within the subduction channel. Currently, however, information regarding the mobility of the HSE in subduction zone environments is limited. To better understand the systematics of highly siderophile elements in subduction settings, we measured Os isotopic compositions and HSE abundances of cores and rinds of meter-scale blocks of eclogite, blueschist and garnet amphibolite from subduction-related mélanges within 1) the Franciscan Complex, CA, and a related terrane in Oregon; 2) the Catalina Schist, CA; and 3) the Samana Metamorphic Complex, Samana Peninsula, Dominican Republic. Rinds composed of amphibole, chlorite, ± phengite, talc, titanite and rutile partly enclose blocks at each location. Mineralogic, petrologic and geochemical data suggest that these rinds are metasomatic features that formed by fluid-mediated interaction between mafic blocks and an ultramafic matrix. The cores of high-grade blocks are characterized by high (Pt + Pd)/(Os + Ir + Ru), and variably radiogenic Os; initial 187Os/188Os ranges from 0.197 to 4.30. These characteristics are consistent with the HSE compositions of subducted basalts. In contrast, (Pt + Pd)/(Os + Ir + Ru) of rinds are generally much lower than cores, approaching values typical of mantle peridotites. The initial 187Os/188Os of most rinds are also typical of upper mantle peridotites; values for most rinds fall between 0.125 and 0.14. The similarities of the 187Os/188Os and HSE abundances between rinds and mantle peridotites indicate that the HSE contained within rinds reflect a contribution by mantle peridotite, whether by fluid-mediated transfer or through mechanical processes. If the HSE signatures of the rinds were

  3. Zircon Lu-Hf isotopes and granite geochemistry of the Murchison Domain of the Yilgarn Craton: Evidence for reworking of Eoarchean crust during Meso-Neoarchean plume-driven magmatism

    NASA Astrophysics Data System (ADS)

    Ivanic, Timothy J.; Van Kranendonk, Martin J.; Kirkland, Christopher L.; Wyche, Stephen; Wingate, Michael T. D.; Belousova, Elena A.

    2012-09-01

    New in situ Lu-Hf data on zircons from GSWA geochronology samples has provided a unique isotopic dataset with a high temporal resolution for the Murchison Domain of the Yilgarn Craton in Western Australia. These data identify extended periods of juvenile mantle input (positive ɛHf values) into the crust firstly at c. 2980 Ma and then from c. 2820 Ma to c. 2640 Ma with significant pulses of crustal recycling at c. 2750 Ma and c. 2620 Ma (highly negative ɛHf values). Geochemical data from well-characterised granitic suites of the Murchison Domain provide additional constraints on the crustal evolution of the area and indicate a prolonged period of crustal melting and remelting at progressively shallower depths from c. 2750 to c. 2600 Ma. At c. 2760-2753 Ma, widespread calc-alkaline, intermediate to silicic volcanic rocks of the Polelle Group were erupted, accompanied by intrusion of felsic to intermediate melts derived from a variety of crustal sources that likely formed by partial mixing with basaltic melts. The intrusive rocks include a wide geochemical array of rocks in the Cullculli and Eelya suites that were sourced over a wide range of crustal depths. At this time a major departure to negative ɛHf values (<-5) occurred, indicating sampling of c. 3.80 Ga model aged source rocks as well as continued juvenile input. Post-volcanic granitic rocks emplaced between c. 2710 and c. 2600 Ma show geochemical evidence for progressive fractionation through time and derivation from an evolving crustal source. We interpret the driving force for this protracted history of mantle and crustal melting to be two mantle plumes at 2.81 and 2.72 Ga. These data document the process of cratonization through progressive melt depletion of the lower crust, progressively fractionating and shallower melts, culminating with a final phase of crustal recycling (ɛHf < - 5) and the cessation of juvenile input at c. 2630-2600 Ma during intrusion of the Bald Rock Supersuite, resulting in

  4. A hybrid origin for two Cretaceous monzonitic plutons in eastern Zhejiang Province, Southeast China: Geochronological, geochemical, and Sr-Nd-Hf isotopic evidence

    NASA Astrophysics Data System (ADS)

    Liu, Liang; Qiu, Jian-Sheng; Zhao, Jiao-Long

    2016-01-01

    Monzonites can provide important information about the nature of the mantle sources and the mechanism of crust-mantle interactions. However, details on the origin of Late Mesozoic monzonites in the Southeastern China remain poorly constrained. This paper presents whole-rock geochemical, Sr-Nd isotopic and zircon U-Pb and Hf isotopic data for two monzonitic plutons (Huangtanyang and Kanggu) in eastern Zhejiang Province, with the aim of elucidating their petrogenesis, and providing important insights into the process of crust-mantle interaction. LA-ICP-MS zircon U-Pb dating results imply that the Huangtanyang and Kanggu quartz monzonites were emplaced in Cretaceous (104-109 Ma). All quartz monzonites are intermediate to acidic, metaluminous to weakly peraluminous, subalkaline, and K-rich in composition. They are enriched in large ion lithophile (e.g., Rb, Ba and Pb) and light rare earth elements, depleted in high-field strength elements (e.g., Nb, Ta, and Ti), and show weakly negative or no Eu anomalies (δEu = 0.78-1.02). All quartz monzonites have homogeneous initial ISr values (0.7084-0.7090) and εNd(t) values (-7.50 to -6.84). They are characterised by highly variable zircon Hf isotopic compositions, with εHf(t) values ranging from -13.3 to -5.7. The combined geochemical evidences (such as high Mg# values, low Nb/U and Ta/U ratios, and variable zircon Hf isotopic compositions) suggests that both depleted asthenospheric and metasomatically enriched mantle components were involved in the formation of the monzonites. The existence of some zircons with unusually low εHf(t) values (low to -13.3) and Palaeoproterozoic two-stage Hf model ages from the Huangtanyang and Kanggu quartz monzonites also argues strongly for Palaeoproterozoic crustal involvement. Magma mixing played a dominated role in the genesis of these monzonites, as indicated by their wide range in zircon Hf isotopic compositions and the occurrence of mafic microgranular enclaves (MMEs). The MMEs show

  5. Caribbean affinities of mafic crust from northern Colombia: preliminary geochemical results from basaltic rocks of the Sinu-San Jacinto belt

    NASA Astrophysics Data System (ADS)

    Bustamante, C.; Cardona, A.; Valencia, V.; Weber, M.; Guzman, G.; Montes, C.; Ibañez, M.; Lara, M.; Toro, M.

    2009-12-01

    The petrotectonic characterization of accreted mafic remnants within the northern Andes and the Caribbean yield major insights on the growth and evolution of oceanic plates, as well as in the identification of the role of terrane accretion within the northern Andes orogeny. Within the northern termination of the Andes, in northern Colombia, several exposures of mafic and ultramafic rocks have been identified. However, extensive sedimentary cover and difficulties in field access have left the petogenetic analysis and tectonic implications of this rocks scarcely studied. Preliminary geochemical constrains from volcanic rocks obtained in outcrops and as clasts from a Paleocene-Eocene conglomerate indicate that the mafic rocks are mainly andesitic in composition, with well defined enrichment in Th and Ce and depletion in Nb and flat to weakly enriched LREE. These features suggest a relatively immature intra-oceanic volcanic arc setting for the formation of these rocks. Hornblende-dioritic dikes in peridotites also attest to the role of water in the magmatic evolution, and the affinity to a subduction related setting. The tectonic implications of this arc remnants and the relation between these rocks and other oceanic domains in the northern Andes suggest that the compositional and tectonic setting on the different accreted margins of the Caribbean plate are heterogeneous.

  6. Magma Mixing at Pulang Porphyry Copper Deposit, NW China: Petrographic, Geochemical and Geochronological evidence

    NASA Astrophysics Data System (ADS)

    Cao, D.; Wang, A.

    2009-12-01

    Pulang porphyry copper deposit, located in Zhongdian arc, NW China, was formed by the subduction process of eastern Paleo-Tethys Oceanic crust. Plenty of mafic microgranular enclaves (MME) were found in the granodiorite porphyries, which intruded in quartz monzonite porphyries, the wall rocks of copper orebodies. The enclaves are diorite in composition, and represent blobs of mafic magma injected into a felsic host magma. The MME have a mineral assemblage (plagioclase + amphibole + biotite ± quartz ± K-feldspar) almost identical to that of host granodiorite porphyries, but with different mineral proportions. The MME are strip-shaped with grouped and directional alignment, and some of them contact gradually with the host rocks. The characteristic petrographic features of the MME are the presence of mingling and quench textures, such as acicular apatites, quartz ocelli rimmed by mafic minerals, and K-feldspar poikilitically enclosing mafic minerals, etc. These evidences show magma mingling or mixing. For the MME and host rocks, main oxides or elements with SiO2, or Al2O3/MgO with SiO2/MgO display significant linear correlations. Characteristics of spidergrams and REE patterns of MME and host rocks are similar, the MME present negative Eu and Sr anomalies, and the host rocks show weak Eu and obvious Sr positive anomalies. According to these, we can conclude that elements transferred between the MME and host rocks, which indicates the existence of magma mixing. The SHRIMP U-Pb concordia ages of the MME, granodiorite porphyries and quartz monzonite porphyries are 213±1Ma, 211±1Ma and 213.1±1.7Ma respectively. Molybdenite Re-Os isochron age is 213±3.8Ma. All these ages indicated the events of magma mixing and copper mineralization were at the same period. Phenomena of magma mixing have been found at many large-scale porphyry copper deposits around the world. Pulang porphyry copper deposit will be a new object to study the role of mafic magmas in the generation of

  7. Origin of Eclogites from the Sanbagawa Metamorphic Belt, Southwestern Japan: Geochemical and Sr-Nd Isotopic Evidence

    NASA Astrophysics Data System (ADS)

    Utsunomiya, Atsushi; Jahn, Bor-Ming; Okamoto, Kazuaki; Ota, Tsutomu; Shinjoe, Hironao

    2010-05-01

    The Sanbagawa belt is a celebrated Cretaceous high-P/T metamorphic belt occurring in SW Japan. Although most metamorphic rocks from the Sanbagawa belt are non-eclogitic, several eclogite-facies metagabbro bodies are exposed in the Besshi area of the Shikoku island. These bodies were earlier recognized as tectonic blocks derived from the lower crust of the former hanging wall (= mantle wedge) in the subduction zone (Takasu et al., 1994). However, more recent studies of field occurrence and metamorphic petrology led to a suggestion that these bodies represent subducted oceanic materials along with their surrounding schists, but not tectonic blocks from mantle wedge (Ota et al., 2004; Terabayashi et al., 2005; Aoya et al., 2006). Furthermore, the protoliths of these eclogites were considered to have formed in a seamount (Aoya et al., 2006) or an oceanic plateau (Terabayashi et al., 2005). In this study, we aim to resolve the controversy about the protoliths and tectonic setting of eclogites and associated rocks using geochemical and Sr-Nd isotopic tracer techniques. All samples were collected from the Iratsu body, the best exposure of eclogitic rocks in the Besshi area. Eclogites, garnet clinopyroxenites and garnet amphibolites from the Iratsu body have flat to LREE-enriched REE patterns, and show Nb and Zr-Hf depletion in the conventional spidergrams. They have Sr-Nd isotopic characteristics of OIB (epsilon Nd (t) = -1 - +4), hence are different from the surrounding mafic schists which are akin to MORB. The overall geochemical and isotopic signatures suggest that the protoliths of the Iratsu body formed in a subduction setting and were produced by melting of mixed sources between a depleted and an enriched mantle components. The geochemical feature of garnet clinopyroxenites (strong Zr-Hf depletion and Ti enrichment) may be explained by accumulation of clinopyroxene + plagioclase + magnetite. A relevant experimental study indicates that the mineral assemblage

  8. Geochemical evidence concerning the nature of the source region to the Middle Proterozoic Granite-Rhyolite Province

    SciTech Connect

    Shuster, R.D. . Dept. Geography Mueller, P.A.; Heatherington, A.L. . Dept. Geology)

    1992-01-01

    The mostly buried 1.5--1.3 Ga old Granite-Rhyolite Province of the midcontinent of North America, is characterized by extensive, undeformed silicic volcanic rocks and related epizonal granitic plutons. Thirty-three previously dated samples from a wide geographic range (Michigan to Colorado) have been analyzed to determine their chemical and Nd and Pb isotopic compositions in order to constrain source regions and processes involved in the formation of these rocks. Major and trace element analyses of these rocks indicate their anorogenic nature, with relatively high Ce/Nb and Y/Nb ratios, as well as relatively high Ga/Al ratios. Geochemically, these rocks are similar to the A2 granites of Eby (1992), which are thought to be generated from the melting of crust which has experienced at least one cycle of subduction-related magmatism. Rare earth element and Pb isotopic data suggest melting at middle to shallow depths. The isotopic data (Nd and Pb) indicate little to no contribution of Archean crust to the source of these rocks. Initial Pb isotopic ratios (208Pb/204Pb) suggest a low Th/U ratio in the source, which contrasts strongly with high Th/U ratios of the Wyoming Province. The Pb isotopic ratios for these rocks are variable, but cluster about the orogene plumbotectonics curve. The variability in the data suggest sources which are variable in their U/Pb ratios and/or ages. The isotopic data are consistent with the existence of a proposed lithospheric boundary which trend NE-SW through the Granite-Rhyolite Province and separates 1.65 Ga old lithosphere (to the NW) from 1.5 Ga old lithosphere (to the SE). Samples analyzed from either side of this boundary have different isotopic signatures. Many of the samples appear to be derived from sources which are only slightly older than the crystallization ages of the granites themselves.

  9. Magmatic Processes in Monogenetic Eruptions, Procida Island, Campi Flegrei, Italy: Geochemical Evidence From Melt Inclusions

    NASA Astrophysics Data System (ADS)

    Severs, M. J.; Fedele, L.; Esposito, R.; Bodnar, R.; Petrosino, P.; Lima, A.; de Vivo, B.; Shimizu, N.

    2008-12-01

    Campi Flegrei is an active volcanic complex located in the greater Naples area, which has produced more than 50 eruptions over the past 60,000 years. These have ranged from small eruptions such as Monte Nuovo eruption of 1538 CE to extremely large eruptions such as the Campanian Ignimbrite (150-200 DRE; Barbieri et al., 1978). The volcanic field includes the mainland area located to the west of Naples and also the two islands of Ischia and Procida. The volcanic products range from basalts to shoshonitic phonolites and trachytes, with the more evolved magmas being more abundant. Three eruptive units from Procida Island have been studied to observe geochemical trends over time within a small area and to better understand magmatic processes between monogenetic eruptions. Juvenile samples from Pozzo Vecchio, Breccia Museo, and Solchiara were collected to examine the geochemistry of the mineral phases present and melt inclusions (MIs) found within the phenocrysts. Solchiara contained phenocrysts of olivine and clinopyroxene, whereas Breccia Museo and Pozzo Vecchio samples contained clinopyroxene and sanidine as the dominant phenocryst phases. Melt inclusions from Solchiara have narrow compositional ranges in major and trace elements (i.e., CaO, TiO2, Zr, Dy, La) over a large range in SiO2 contents (47 to 55 wt%) while MI from the Breccia Museo have a limited range of SiO2 contents (57 to 61 wt%) with a wider range for major and trace elements (i.e., FeO, Al2O3, CaO, La, Th, Rb). Pozzo Vecchio MI from clinopyroxene and sanidine define different chemical compositions, but petrographic evidence does not suggest a xenocrystic origin for either mineral phase. This suggests that Pozzo Vecchio is the result of magma mixing. Modeling of fractional crystallization of olivine, clinopyroxene, and sanidine are capable of producing most of the trends in major and trace elements between the most primitive samples to the most evolved samples. Volatile concentrations between the

  10. Primary carbonatite melt from deeply subducted oceanic crust.

    PubMed

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

    2008-07-31

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

  11. Primary carbonatite melt from deeply subducted oceanic crust

    SciTech Connect

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

    2008-07-01

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

  12. Geochemical evidence for atmospheric pollution derived from prehistoric copper mining at Copa Hill, Cwmystwyth, mid-Wales, UK.

    PubMed

    Mighall, T M; Abrahams, P W; Grattan, J P; Hayes, D; Timberlake, S; Forsyth, S

    2002-06-20

    This paper presents geochemical data from a blanket peat located close to a Bronze Age copper mine on the northern slopes of the Ystwyth valley, Ceredigion, mid-Wales, UK. The research objective was to explore the possibility that the peat contained a geochemical record of the pollution generateD by mining activity. Four peat monoliths were extracted from the blanket peat to reconstruct the pollution history of the prehistoric mine. Three different geochemical measurement techniques were employed and four copper profiles have been reconstructed, two of which are radiocarbon-dated. The radiocarbon dates at one profile located close to the mine confirm that copper enrichment occurs in the peat during the known period of prehistoric mining. Similar enrichment of copper concentrations is shown in one adjacent profile and a profile within 30 m away. In contrast, copper was not enriched in the other radiocarbon-dated monolith, collected approximately 1.35 km to the north of the mine. Whilst other possible explanations to explain the copper concentrations are discussed, it is argued that the high copper concentrations represent evidence of localised atmospheric pollution caused by Bronze Age copper mining in the British Isles. The results of this study suggest that copper may be immobile in blanket peat and such deposits can usefully be used to reconstruct atmospheric pollution histories in former copper mining areas.

  13. Melt extraction and enrichment processes in the New Caledonia lherzolites: Evidence from geochemical and Sr-Nd isotope data

    NASA Astrophysics Data System (ADS)

    Secchiari, Arianna; Montanini, Alessandra; Bosch, Delphine; Macera, Patrizia; Cluzel, Dominique

    2016-09-01

    asthenospheric mantle source that experienced a recent MORB-producing depletion event. This evolution was most likely accomplished in a spreading ridge. However, geochemical trace element modelling and Nd isotopes do not support a genetic mantle-crust link between the lherzolites and enriched-MOR-type basalts from the Poya Terrane.

  14. Shear Wave Velocity Structure of Southern African Crust: Evidence for Compositional Heterogeneity within Archaean and Proterozoic Terrains

    SciTech Connect

    Kgaswane, E M; Nyblade, A A; Julia, J; Dirks, P H H M; Durrheim, R J; Pasyanos, M E

    2008-11-11

    Crustal structure in southern Africa has been investigated by jointly inverting receiver functions and Rayleigh wave group velocities for 89 broadband seismic stations spanning much of the Precambrian shield of southern Africa. 1-D shear wave velocity profiles obtained from the inversion yield Moho depths that are similar to those reported in previous studies and show considerable variability in the shear wave velocity structure of the lower part of the crust between some terrains. For many of the Archaean and Proterozoic terrains in the shield, S velocities reach 4.0 km/s or higher over a substantial part of the lower crust. However, for most of the Kimberley terrain and adjacent parts of the Kheis Province and Witwatersrand terrain, as well as for the western part of the Tokwe terrain, mean shear wave velocities of {le} 3.9 km/s characterize the lower part of the crust along with slightly ({approx}5 km) thinner crust. These findings indicate that the lower crust across much of the shield has a predominantly mafic composition, except for the southwest portion of the Kaapvaal Craton and western portion of the Zimbabwe Craton, where the lower crust is intermediate-to-felsic in composition. The parts of the Kaapvaal Craton underlain by intermediate-to-felsic lower crust coincide with regions where Ventersdorp rocks have been preserved, and thus we suggest that the intermediate-to-felsic composition of the lower crust and the shallower Moho may have resulted from crustal melting during the Ventersdorp tectonomagmatic event at c. 2.7 Ga and concomitant crustal thinning caused by rifting.

  15. Further Evidence for Geochemical Diversity, and Possible Bimodality, Among Cumulate Eucrites

    NASA Astrophysics Data System (ADS)

    Warren, P. H.; Kallemeyn, G. W.

    1992-07-01

    We have used INAA, RNAA, and fused-bead analysis to determine the bulk compositions of numerous Antarctic eucrites (and also the LEW88516 SNC meteorite). Only a few of the most unusual eucrites can be discussed in the limited space here. Takeda et al. (1988) noted that Y791195 is a slowly cooled eucrite, with an equant, medium-grained texture, and pyroxene exsolution lamellae up to 10 micrometers across. In Y791195,81-3, we find lamellae up to 14 micrometers across. In this respect, Y791195 resembles RKPA80224, in which exsolution lamellae up to 12 micrometers across. We have previously discussed the evidence that RKPA80224 is a mildly accumulative rock that formed from an unusually low-mg parent melt. Our second analysis of RKPA80224 only partly confirms the unusually low incompatible trace element (ITE) content, but the Ce anomaly is consistently small (Ce/La = 0.90-1.02 x CI), and based on a weighted mean composition the implied parent melt is still unlike any noncumulate eucrite (see Fig. 1, which shows results from mass balance calculations modeling the sample as a mixture of cumulus px and plag, plus trapped melt). A parent melt similar to an extreme low-mg, variant of the "Nuevo Laredo Trend" would plausibly account for RKPA80224. The spectrum of possible parents for Y791195 is similar, even though its "true" Sm content is slightly obscured by weathering (Ce/La = 1.4 x CI). The [Sm] used in the figure is scaled to the highest CI-normalized REE concentration. Data of Mittlefehldt and Lindstrom (1991) indicate that except for exterior samples "showing extreme rustiness," Sm even in weathered eucrites is generally not altered beyond a few tens of pct. relative (sample size seems to account for more of the variation in [Sm] among interior, non-rusty samples). Even assuming a Sm content twice that assumed in the figure, the parent melt still must be well to the low-MgO/FeO, low-Sm side of all known eucrites. The LEW87002 eucrite is brecciated, but probably

  16. Ancient recycled crust beneath the Ontong Java Plateau: Isotopic evidence from the garnet clinopyroxenite xenoliths, Malaita, Solomon Islands

    NASA Astrophysics Data System (ADS)

    Ishikawa, Akira; Kuritani, Takeshi; Makishima, Akio; Nakamura, Eizo

    2007-07-01

    We present a Sr, Nd, Hf and Pb isotope investigation of a set of garnet clinopyroxenite xenoliths from Malaita, Solomon Islands in order to constrain crustal recycling in the Pacific mantle. Geological, thermobarometric and petrochemical evidence from previous studies strongly support an origin as a series of high-pressure (> 3 GPa) melting residues of basaltic material incorporated in peridotite, which was derived from Pacific convective mantle related to the Ontong Java Plateau magmatism. The present study reveals isotopic variations in the pyroxenites that are best explained by different extents of chemical reaction with ambient peridotite in the context of a melting of composite source mantle. Isotopic compositions of bimineralic garnet clinopyroxenites affected by ambient peridotite fall within the oceanic basalt array, similar to those of Ontong Java Plateau lavas. In contrast, a quartz-garnet clinopyroxenite, whose major element compositions remain intact, has lower 206Pb/ 204Pb- 143Nd/ 144Nd and higher 87Sr/ 86Sr- 207Pb/ 204Pb ratios than most oceanic basalts. These isotopic signatures show some affinity with proposed recycled sources such as the so-called EM-1 or DUPAL types. Constraints from major and trace element characteristics of the quartz-garnet clinopyroxenite, the large extent of Hf-Nd isotopic decoupling and the good coincidence of Pb isotopes to the Stacey-Kramers curve, all indicate that pollution of southern Pacific mantle occurred by the subduction or delamination of Neoproterozoic granulitic lower crust (0.5-1 Ga). This crustal recycling could have taken place around the suture of Rodinia supercontinent, a part of which resurfaced during mantle upwelling responsible for creating the Cretaceous Ontong Java Plateau.

  17. Neodymium isotopic evidence for decreasing crustal contributions to cenozoic ignimbrites of the western United States. Implications for the thermal evolution of the Cordilleran crust

    SciTech Connect

    Perry, F.V. ); DePaolo, D.J. ); Baldridge, W.S. )

    1993-07-01

    We have estimated the crustal contributions to 12 early Oligocene to Pleistocene rhyolite systems located throughout the Cordillera. We have determined that (1) crustal contributions to large-volume rhyolite systems decrease from the Oligocene to the Miocene, and (2) rhyolite systems younger than 20 Ma are dominated by mantle components. The crustal contributions to rhyolite systems may be controlled by system size and duration, crustal thickness, tectonic setting, crustal composition, crustal density, and crustal temperature. We conclude that regional cooling of the lower crust, which progressively limited the amount of crustal wallrock assimilated by rhyolite systems, is the only parameter that is consistent with geologic and geochemical data for rhyolite systems and the geologic evolution of the Cordillera. A quantitative model that relates the amount of crustal contribution to assimilation/recharge rates and the temperature of the crust indicates that lower-crustal temperatures would have to decrease about 300[degrees]C between early Oligocene and early Miocene time to account for the decrease in crustal contributions. 57 refs., 7 figs., 1 tab.

  18. Isotopic evidence for the dependence of recurrent felsic magmatism on new crust formation: An example from the Georgetown region of Northeastern Australia

    SciTech Connect

    Black, L.P. ); McCulloch, M.T. )

    1990-01-01

    U-Pb zircon, Sm-Nd, and Rb-Sr isotopic data, together with previously accumulated geological and chemical evidence show that the Georgetown inlier of northeast Queensland and its immediate environs were subjected to three widespread, temporally discrete episodes of felsic magmatism. The earliest of these, at about 1550 Ma, produced widespread anatexis within the metasedimentary rocks of the inlier, which have a poorly constrained depositional age, but which were derived by the degradation of ca. 2000-2500 Ma crust. Contemporaneous I-type magmas contained more radiogenic Nd and are thought to have formed from mixing of newly formed crustal material with the igneous precursors of the metasediments. The data are best explained in terms of successive addition from below of new crustal material (via underplating or emplacement into the lower crust) at about 1550 Ma, 420 Ma, and 300 Ma. The model requires that such newly accreted material does not necessarily melt and mobilize the preceeding underplate. Often it is a still earlier underplate that is activated. These regions of the lower crust can remain dormant for well over a billion years before they produce widespread magmatism destined for the upper crust.

  19. Shear wave velocity structure of the lower crust in southern Africa: Evidence for compositional heterogeneity within Archaean and Proterozoic terrains

    NASA Astrophysics Data System (ADS)

    Kgaswane, Eldridge M.; Nyblade, Andrew A.; Juliã, Jordi; Dirks, Paul H. G. M.; Durrheim, Raymond J.; Pasyanos, Michael E.

    2009-12-01

    The nature of the lower crust across the southern African shield has been investigated by jointly inverting receiver functions and Rayleigh wave group velocities for 89 broadband seismic stations located in Botswana, South Africa and Zimbabwe. For large parts of both Archaean and Proterozoic terrains, the velocity models obtained from the inversions show shear wave velocities ≥4.0 km/s below ˜20-30 km depth, indicating a predominantly mafic lower crust. However, for much of the Kimberley terrain and adjacent parts of the Kheis Province and Witwatersrand terrain in South Africa, as well as for the western part of the Tokwe terrain in Zimbabwe, shear wave velocities of ≤3.9 km/s are found below ˜20-30 km depth, indicating an intermediate-to-felsic lower crust. The areas of intermediate-to-felsic lower crust in South Africa coincide with regions where Ventersdorp rocks have been preserved, suggesting that the more evolved composition of the lower crust may have resulted from crustal reworking and extension during the Ventersdorp tectonomagmatic event at c. 2.7 Ga.

  20. The growth and contamination mechanism of the Cana Brava layered mafic-ultramafic complex: new field and geochemical evidences

    NASA Astrophysics Data System (ADS)

    Giovanardi, Tommaso; Girardi, Vicente A. V.; Correia, Ciro T.; Sinigoi, Silvano; Tassinari, Colombo C. G.; Mazzucchelli, Maurizio

    2016-09-01

    The Cana Brava complex is the northernmost of three layered complexes outcropping in the Goiás state (central Brasil). New field and geochemical evidences suggest that Cana Brava underwent hyper- to subsolidus deformation during its growth, acquiring a high-temperature foliation that is generally interpreted as the result of a granulite-facies metamorphic event. The increase along the stratigraphy of the incompatible elements abundances (LREE, Rb, Ba) and of the Sr isotopic composition, coupled with a decrease in ɛNd(790), indicate that the complex was contaminated by the embedded xenoliths from the Palmeirópolis Sequence. The geochemical data suggest that the contamination occurred along the entire magma column during the crystallization of the Upper Mafic Zone, with in situ variations determined by the abundance and composition of the xenoliths. These features of the Cana Brava complex point to an extremely similarity with the Lower Sequence of the most known Niquelândia intrusion (the central of the three complexes). This, together with the evidences that the two complexes have the same age (c.a. 790 Ma) and their thickness and units decrease northwards suggests that Cana Brava and Niquelândia are part of a single giant Brasilia body grown through several melt impulses.

  1. Termination time of peak decratonization in North China: Geochemical evidence from mafic igneous rocks

    NASA Astrophysics Data System (ADS)

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

    2016-01-01

    Geophysical and petrological data indicate destruction of the cratonic lithosphere in North China in the Mesozoic, resulting in replacement of the ancient subcontinental lithospheric mantle (SCLM) by the juvenile SCLM. However, it remains to be answered when the craton destruction would have been terminated in the Mesozoic. This question is resolved by studying the two types of mafic igneous rocks with contrasting geochemical compositions from North China. The first type of mafic igneous rock shows arc-like trace element distribution patterns and enriched radiogenic Sr-Nd isotope compositions, with emplacement ages spanning from the Triassic to Early Cretaceous. The mafic magmatism is absent in a period from ~ 200 Ma to ~ 135 Ma, recording the thinning of cratonic lithosphere due to the westward flat subduction of the Paleo-Pacific slab beneath the North China Craton. In contrast, the second type of mafic igneous rocks exhibits oceanic island basalts (OIB)-like trace element distribution patterns and relatively depleted radiogenic Sr-Nd isotope compositions, with emplacement ages spanning from the Early Cretaceous to Cenozoic. Zircon U-Pb dating yields an age of ~ 121 Ma for the geochemical transformation between the two types of mafic igneous rocks. This age marks a dramatic demarcation in the composition of their mantle sources. As such, the nature of mantle lithosphere in North China was changed from the ancient SCLM to the juvenile SCLM at ~ 121 Ma. Thus, this age not only signifies the tectonic transition from the enriched mantle to the depleted mantle in the Early Cretaceous, but also dates the termination of peak decratonization in North China. Therefore, the craton destruction in the Early Cretaceous is temporally and spatially associated with the dramatic changes in the geochemical composition of mantle lithosphere.

  2. Geochemical evidence for diversity of dust sources in the southwestern United States

    USGS Publications Warehouse

    Reheis, M.C.; Budahn, J.R.; Lamothe, P.J.

    2002-01-01

    Several potential dust sources, including generic sources of sparsely vegetated alluvium, playa deposits, and anthropogenic emissions, as well as the area around Owens Lake, California, affect the composition of modern dust in the southwestern United States. A comparison of geochemical analyses of modern and old (a few thousand years) dust with samples of potential local sources suggests that dusts reflect four primary sources: (1) alluvial sediments (represented by Hf, K, Rb, Zr, and rare-earth elements, (2) playas, most of which produce calcareous dust (Sr, associated with Ca), (3) the area of Owens (dry) Lake, a human-induced playa (As, Ba, Li, Pb, Sb, and Sr), and (4) anthropogenic and/or volcanic emissions (As, Cr, Ni, and Sb). A comparison of dust and source samples with previous analyses shows that Owens (dry) Lake and mining wastes from the adjacent Cerro Gordo mining district are the primary sources of As, Ba, Li, and Pb in dusts from Owens Valley. Decreases in dust contents of As, Ba, and Sb with distance from Owens Valley suggest that dust from southern Owens Valley is being transported at least 400 km to the east. Samples of old dust that accumulated before European settlement are distinctly lower in As, Ba, and Sb abundances relative to modern dust, likely due to modern transport of dust from Owens Valley. Thus, southern Owens Valley appears to be an important, geochemically distinct, point source for regional dust in the southwestern United States. Copyright ?? 2002 Elsevier Science Ltd.

  3. Geochemical evidence for active tropical serpentinization in the Santa Elena Ophiolite, Costa Rica: An analog of a humid early Earth?

    NASA Astrophysics Data System (ADS)

    Sánchez-Murillo, Ricardo; Gazel, Esteban; Schwarzenbach, Esther M.; Crespo-Medina, Melitza; Schrenk, Matthew O.; Boll, Jan; Gill, Ben C.

    2014-05-01

    is a planetary process that has important consequences on geochemical cycles, supporting microbial activity through the formation of H2 and CH4 and having the potential to sequester atmospheric CO2. We present geochemical evidence of active serpentinization in the Santa Elena Ophiolite, Costa Rica which is sustained by peridotites with a degree of serpentinization less than 50% with no evidence of an internal heat source. Average spring water temperatures are 29.1°C. Two hyperalkaline spring systems were discovered, with a spring fluid pH up to 11.18. The fluids are characterized by low Mg (1.0-5.9 mg/L) and K (1.0-5.5 mg/L) and relative high Ca (29-167 mg/L), Na (16-27 mg/L), Cl (26-29 mg/L), hydroxide (41-63 mg/L), and carbonate (31-49 mg/L). Active CH4 (24.3% v/v) vents coupled with carbonate deposits (δ13CCO2 =-27 to -14‰; δ18OCO2 =-17 to - 6‰) also provide evidence for active serpentinization and carbonation. Isotope ratios of the alkaline fluids (δ18O = -7.9‰, δ2H = -51.4‰) and groundwater (δ18O = -7.6‰; δ2H = -48.0‰) suggests that, during base flow recession, springs are fed by groundwater circulation. Methanogenic Archaea, which comprises a relatively high percentage of the 16S rRNA gene tag sequences, suggests that biological methanogenesis may play a significant role in the system. Santa Elena's extreme varying weather results in a scenario that could be of significant importance for (a) improving the knowledge of conditions on a humid early Earth or Mars that had periodic changes in water supply, (b) revealing new insights on serpentinizing solute transport, and (c) modeling hydrogeochemical responses as a function of recharge.

  4. Evidence for an intimate geochemical factor in the etiology of esophageal cancer.

    PubMed

    Kibblewhite, M G; Van Rensburg, S J; Laker, M C; Rose, E F

    1984-04-01

    Epidemiological data for esophageal cancer in the Butterworth District, Transkei, was used to calculate incidence contours which confirmed large variations within short distances (less than 5 km). High- and low-risk zones were demarcated, and a close relationship with underlying geology observed. The low-incidence zones in the study regions were underlain by dolerite intrusions, whereas higher-risk regions were on sedimentary strata. Analysis of rocks indicated that those from the higher-risk regions contain less copper, cobalt, and manganese. Soil samples were analyzed for boron, cobalt, copper, manganese, molybdenum, nickel, sodium, lead, vanadium, and zinc; the results also indicated a strong geochemical association with the disease. The concentrations of copper (P = 0.001), nickel (P = 0.001), and boron were markedly lower in the high-risk zones. Manganese, zinc, and molybdenum levels in soils also tended to be substantially lower in the high-risk zone.

  5. SEISMIC AND GEOCHEMICAL EVIDENCE FOR SHALLOW GAS IN SEDIMENT ON NAVARIN CONTINENTAL MARGIN, BERING SEA.

    USGS Publications Warehouse

    Carlson, Paul R.; Golan-Bac, Margaret; Karl, Herman A.; Kvenvolden, Keith A.

    1985-01-01

    Marine sesmic studies coupled with geochemical investigations demonstrate tha hydrocarbon gases are ubiquitous in the near-surface sediment of the Navarin continental margin in the northern Bering Sea. Three types of acoustic anomalies appear to be related to the presence of gas in the sediment. These anomalies are most prevalent in the northern half of the Navarin basin. Acoustic anomalies attributed to gas hydrates and to diagenetic boundaries are present on seismic records of the lower slope between Navarinsky and Zhemchug Canyons. Hydrocarbon gases, methane through butanes, are common in the surface sediment of the Navarin continental margin. The source of methane is mainly biogenic, but the hydrocarbon gas compositions in 17 of 141 cores suggest the presence of thermogenic gas. No direct correlation could be found between acoustic anomalies and gas concentrations in the sediment. Refs.

  6. Geochemical evidence for mixing of three components in martian orthopyroxenite ALH 84001. [Abstract only

    NASA Technical Reports Server (NTRS)

    Mittlefehldt, D. W.; Lindstrom, M. M.

    1994-01-01

    ALH 84001, a ferroan martian orthopyroxenite, originally consisted of three petrographically defined components: a cumulus assemblage of orthopyroxene + chromite, a trapped melt assemblage of orthopyroxene(?) + chromite + maskelynite + apatite + augite +/- pyrite, and a metasomatic assemblage of carbonate +/- pyrite. We present the results of Instrumental Neutron Activation Analysis (INAA) study of five bulk samples of ALH 84001, combined with Scanning Ion Mass Spectrometer (SIMS) data on the orthopyroxene, in order to attempt to set limits on the geochemical characteristics of the latter two components, and therefore on the petrogenesis of ALH 84001. The INAA data support the petrographic observations, suggesting that there are at least three components in ALH 84001. We will assume that each of the three geochemically required components can be equated with one of the petrographically observed components. Both trapped melt and metasomatic components in ALH 84001 have higher Na than orthopyroxene based on compositions of maskelynite, apatite, and carbonate. For the metasomatic component, we will assume its Na content is that of carbonate, while for a trapped melt component, we will use a typical Na content inferred for martian meteorite parent melts, approximately 1 wt% Na2O. Under these assumptions, we can set limits on the Light Rare Earth Elements/Heavy Rare Earth Elements (LREE/HREE) ratios of the components, and use this information to compare the petrogenesis of ALH 84001 with other martian meteorites. The above calculations assume that the bulk samples are representative of different portions of ALH 84001. We will also evaluate the possible heterogeneous distribution of mineral phases in the bulk samples as the cause of compositional heterogeneity in our samples.

  7. Icelandic-type crust

    USGS Publications Warehouse

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

    2003-01-01

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

  8. Evidence for Archean inheritance in the pre-Panafrican crust of Central Cameroon: Insight from zircon internal structure and LA-MC-ICP-MS Usbnd Pb ages

    NASA Astrophysics Data System (ADS)

    Ganwa, Alembert Alexandre; Klötzli, Urs Stephan; Hauzenberger, Christoph

    2016-08-01

    contributing sources. It is likely that erosion, transport and deposition took place between 2116 and 821 Ma. Geochemical data show that the REE, Y, Yb, Sr/Y of some samples are similar to the known Archean craton formations (depletion in REE, Y ≤ 10 ppm, Yb ≤ 1 ppm, Sr/Y ≥ 30). These characteristics are known as specific for the Archean TTG (Tonalite-Trondhjemite-Granodiorite). It means that: i) Archean TTG contribute significantly to the detritus of the sedimentary basin, ii) The depositional basin and the source rock were close and the detritus was immature. Our results show that the Pre-Panafrican history of central Cameroon includes Meso- to Neo-Archean crustal accretion and associated magmatism prior to the Paleoproterozoic event of the West Central African Belt. In respect to this new insight, any evolutionary reconstruction of the area should integrate the presence of Archean crust.

  9. Evidence for Archean inheritance in the pre-Panafrican crust of Central Cameroon: Insight from zircon internal structure and LA-MC-ICP-MS Usbnd Pb ages

    NASA Astrophysics Data System (ADS)

    Ganwa, Alembert Alexandre; Klötzli, Urs Stephan; Hauzenberger, Christoph

    2016-08-01

    sources. It is likely that erosion, transport and deposition took place between 2116 and 821 Ma. Geochemical data show that the REE, Y, Yb, Sr/Y of some samples are similar to the known Archean craton formations (depletion in REE, Y ≤ 10 ppm, Yb ≤ 1 ppm, Sr/Y ≥ 30). These characteristics are known as specific for the Archean TTG (Tonalite-Trondhjemite-Granodiorite). It means that: i) Archean TTG contribute significantly to the detritus of the sedimentary basin, ii) The depositional basin and the source rock were close and the detritus was immature. Our results show that the Pre-Panafrican history of central Cameroon includes Meso- to Neo-Archean crustal accretion and associated magmatism prior to the Paleoproterozoic event of the West Central African Belt. In respect to this new insight, any evolutionary reconstruction of the area should integrate the presence of Archean crust.

  10. Dynamics of the Precambrian Continental Crust

    NASA Astrophysics Data System (ADS)

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

    2003-04-01

    The Precambrian continental crust is mainly composed of (1) granite greenstone belts (GGB) and (2) granulite facies complexes (GFC). The GFC are often separated from GGB by inward dipping crustal scale shear zones with characteristic sense of movements reflecting thrusting of GFC onto cratonic rocks. The isotope age of the shear zones is identical to GFC, while the latter are always younger than the granite greenstone belts. The dynamics relationships between these two geological units strongly determine tectonic evolution of the Precambrian continental crust. Numerous thermobarometric studies of magmatic and metamorphic rocks show that the Archaean to Early Protorozoic crust as well as the Mantle were hot and therefore relatively soft. Such geothermal regimes may limit separation and movement of micro continents, limiting collisional mechanisms in evolution of the Precambrian crust. The goal of this paper is to show evidence for an alterative model that is based on the mechanism of gravitational redistribution of rocks within the Precambrian continental crust, which might be initiated by a fluid/heat flow related to mantle plumes. The model is tested on the basis of geological, geochemical, geophysical and petrologic data for many paired GFT GGB complexes around the word. Studied granulite complexes are located in between Archaean GGB from which they are separated by inward dipping crustal scale shear zones with reverse sense of movements. The most important evidence for this mechanism is: (i) the near isobaric cooling (IC) and (ii) decompression cooling (DC) shapes of the retrograde P T paths recorded in GFC, while rocks from the juxtaposed GGB in footwalls of the bounding shear zones record P T loops. The Pmax of the loops corresponds to the Pmin, recorded in GFC. Thus the GGB P T loop reflects the burial and ascending of the juxtaposed GGB while the GFC P T path records the exhumation only. The identical isotopic age of GFC and contacting rocks from the shear

  11. Faulting and hydration of the upper crust of the SW Okinawa Trough during continental rifting: Evidence from seafloor compliance inversion

    NASA Astrophysics Data System (ADS)

    Kuo, Ban-Yuan; Crawford, Wayne C.; Webb, Spahr C.; Lin, Ching-Ren; Yu, Tai-Chieh; Chen, Liwen

    2015-06-01

    The elastic response of seafloor to ocean gravity wave loading, or seafloor compliance, provides a constraint on the elastic properties of the crust. We measured seafloor compliance at three ocean bottom seismometer (OBS) sites around Taiwan—two in the southwestern (SW) Okinawa Trough and one on the Ryukyu arc—and performed inversion for crustal structures beneath them. Models best fitting the data demonstrate a decrease in upper crustal shear velocity and an increase in the compressional/shear velocity ratio from the arc site to the trough sites with increasing amount of back-arc extension. This variation suggests that the upper continental crust is highly faulted and hydrated during rifting of the Eurasian lithosphere.

  12. Mid-Holocene Climate Variability and Coastal Upwelling: Geochemical Evidence From Mytilus californianus

    NASA Astrophysics Data System (ADS)

    Dyck, K.; Koch, P.; Schellenberg, S.; Ford, H.

    2007-12-01

    Oxygen isotope values in carbonate shells from the coastal zone are influenced by temperature and by the δ18O value of water, which could be 18O-depleted relative to average ocean water due to input of freshwater runoff. If Mg/Ca ratios are a reliable independent proxy for past sea-surface temperature (SST), we can reconstruct the δ18O value of coastal waters from the δ18O value of biogenic calcite as a proxy for coastal precipitation and runoff. We conducted geochemical studies (δ18O, Mg/Ca, and 14C) on shells of Mytilus californianus from archaeological middens near Pt. Año Nuevo, on the central California coast, to study shifts in upwelling and moisture regimes from ~1,000 to ~5,500 BP. The dated specimens were sectioned along the growth axis and the prismatic calcite layer sampled down the length of visible growth bands with samples split for Mg/Ca and stable isotope analysis. Seven shells have been analyzed for δ18O so far and six of these have also been analyzed for Mg/Ca ratios. Shells typically contain 3-7 years of accretionary growth that can be resolved at the monthly scale with our sampling approach. Under normal Walker circulation in central California, conditions are characterized by strong summer upwelling, relatively cool summer SSTs, and rainy winters. Interruption of normal Walker circulation due to a weak North Pacific high pressure system and therefore reduced offshore Ekman transport is reflected by warmer summer and winter SSTs, suppressed summer upwelling and even higher winter precipitation. Overall biogenic calcite Mg/Ca ratios show a negative seasonal covariation with δ18O values, as expected due to shifts in ocean temperature. Several specimens show geochemical values consistent with a breakdown in Walker circulation during several winters. With additional coupled Mg/Ca and δ18O analyses we will calculate the frequency of these events through time and will calculate the δ18O value of coastal water as a proxy for precipitation and

  13. Geochemical evidence of mantle reservoir evolution during progressive rifting along the western Afar margin

    NASA Astrophysics Data System (ADS)

    Rooney, Tyrone O.; Mohr, Paul; Dosso, Laure; Hall, Chris

    2013-02-01

    The Afar triple junction, where the Red Sea, Gulf of Aden and African Rift System extension zones converge, is a pivotal domain for the study of continental-to-oceanic rift evolution. The western margin of Afar forms the southernmost sector of the western margin of the Red Sea rift where that margin enters the Ethiopian flood basalt province. Tectonism and volcanism at the triple junction had commenced by ˜31 Ma with crustal fissuring, diking and voluminous eruption of the Ethiopian-Yemen flood basalt pile. The dikes which fed the Oligocene-Quaternary lava sequence covering the western Afar rift margin provide an opportunity to probe the geochemical reservoirs associated with the evolution of a still active continental margin. 40Ar/39Ar geochronology reveals that the western Afar margin dikes span the entire history of rift evolution from the initial Oligocene flood basalt event to the development of focused zones of intrusion in rift marginal basins. Major element, trace element and isotopic (Sr-Nd-Pb-Hf) data demonstrate temporal geochemical heterogeneities resulting from variable contributions from the Afar plume, depleted asthenospheric mantle, and African lithosphere. The various dikes erupted between 31 Ma and 22 Ma all share isotopic signatures attesting to a contribution from the Afar plume, indicating this initial period in the evolution of the Afar margin was one of magma-assisted weakening of the lithosphere. From 22 Ma to 12 Ma, however, diffuse diking during continued evolution of the rift margin facilitated ascent of magmas in which depleted mantle and lithospheric sources predominated, though contributions from the Afar plume persisted. After 10 Ma, magmatic intrusion migrated eastwards towards the Afar rift floor, with an increasing fraction of the magmas derived from depleted mantle with less of a lithospheric signature. The dikes of the western Afar margin reveal that magma generation processes during the evolution of this continental rift margin

  14. Geochemical evidence for magmatic water within Mars from pyroxenes in the Shergotty meteorite.

    PubMed

    McSween, H Y; Grove, T L; Lentz, R C; Dann, J C; Holzheid, A H; Riciputi, L R; Ryan, J G

    2001-01-25

    Observations of martian surface morphology have been used to argue that an ancient ocean once existed on Mars. It has been thought that significant quantities of such water could have been supplied to the martian surface through volcanic outgassing, but this suggestion is contradicted by the low magmatic water content that is generally inferred from chemical analyses of igneous martian meteorites. Here, however, we report the distributions of trace elements within pyroxenes of the Shergotty meteorite--a basalt body ejected 175 million years ago from Mars--as well as hydrous and anhydrous crystallization experiments that, together, imply that water contents of pre-eruptive magma on Mars could have been up to 1.8%. We found that in the Shergotty meteorite, the inner cores of pyroxene minerals (which formed at depth in the martian crust) are enriched in soluble trace elements when compared to the outer rims (which crystallized on or near to the martian surface). This implies that water was present in pyroxenes at depth but was largely lost as pyroxenes were carried to the surface during magma ascent. We conclude that ascending magmas possibly delivered significant quantities of water to the martian surface in recent times, reconciling geologic and petrologic constraints on the outgassing history of Mars.

  15. The crust-mantle interaction in continental subduction channels: Zircon evidence from orogenic peridotite in the Sulu orogen

    NASA Astrophysics Data System (ADS)

    Li, Hai-Yong; Chen, Ren-Xu; Zheng, Yong-Fei; Hu, Zhaochu

    2016-02-01

    A combined secondary ion mass spectrometer and laser ablation-(multicollector)-inductively coupled plasma mass spectrometer study of zircon U-Pb ages, trace elements, and O and Hf isotopes was carried out for orogenic peridotite and its host gneiss in the Sulu orogen. Newly grown zircon domains exhibit weak zoning or no zoning, relatively low Th/U ratios (<0.1), low heavy rare earth element (HREE) contents, steep middle rare earth element-HREE patterns, negative Eu anomalies, and negative to low δ18O values of -11.3 to 0.9‰ and U-Pb ages of 220 ± 2 to 231 ± 4 Ma. Thus, these zircons would have grown from metasomatic fluids during the early exhumation of deeply subducted continental crust. The infiltration of metasomatic fluids into the peridotite is also indicated by the occurrence of hydrous minerals such as amphibole, serpentine, and chlorite. In contrast, relict zircon domains exhibit magmatic zircon characteristics. Their U-Pb ages and trace element and Hf-O isotope compositions are similar to those for protolith zircons from ultrahigh-pressure metamorphic rocks in the Dabie-Sulu orogenic belt. Thus, these relict magmatic zircons would be physically transported into the peridotite by metasomatic fluids originated from the deeply subducted continental crust. Therefore, the peridotite underwent metasomatism by aqueous solutions derived from dehydration of the deeply subducted continental crust during the early exhumation. It is these crustally derived fluids that would have brought not only such chemical components as Zr and Si but also tiny zircon grains from the deeply subducted crustal rocks into the peridotite at the slab-mantle interface in continental subduction channels. As such, the orogenic peridotite records the crust-mantle interaction at the deep continental subduction zone.

  16. Continental crust

    USGS Publications Warehouse

    Pakiser, L.C.

    1964-01-01

    The structure of the Earth’s crust (the outer shell of the earth above the M-discontinuity) has been intensively studied in many places by use of geophysical methods. The velocity of seismic compressional waves in the crust and in the upper mantle varies from place to place in the conterminous United States. The average crust is thick in the eastern two-thirds of the United States, in which the crustal and upper-mantle velocities tend to be high. The average crust is thinner in the western one-third of the United States, in which these velocities tend to be low. The concept of eastern and western superprovinces can be used to classify these differences. Crustal and upper-mantle densities probably vary directly with compressional-wave velocity, leading to the conclusion that isostasy is accomplished by the variation in densities of crustal and upper-mantle rocks as well as in crustal thickness, and that there is no single, generally valid isostatic model. The nature of the M-discontinuity is still speculative.

  17. Evidence for partial melt in the crust beneath Mt. Paektu (Changbaishan), Democratic People's Republic of Korea and China.

    PubMed

    Kyong-Song, Ri; Hammond, James O S; Chol-Nam, Ko; Hyok, Kim; Yong-Gun, Yun; Gil-Jong, Pak; Chong-Song, Ri; Oppenheimer, Clive; Liu, Kosima W; Iacovino, Kayla; Kum-Ran, Ryu

    2016-04-01

    Mt. Paektu (also known as Changbaishan) is an enigmatic volcano on the border between the Democratic People's Republic of Korea (DPRK) and China. Despite being responsible for one of the largest eruptions in history, comparatively little is known about its magmatic evolution, geochronology, or underlying structure. We present receiver function results from an unprecedented seismic deployment in the DPRK. These are the first estimates of the crustal structure on the DPRK side of the volcano and, indeed, for anywhere beneath the DPRK. The crust 60 km from the volcano has a thickness of 35 km and a bulk V P/V S of 1.76, similar to that of the Sino-Korean craton. The V P/V S ratio increases ~20 km from the volcano, rising to >1.87 directly beneath the volcano. This shows that a large region of the crust has been modified by magmatism associated with the volcanism. Such high values of V P/V S suggest that partial melt is present in the crust beneath Mt. Paektu. This region of melt represents a potential source for magmas erupted in the last few thousand years and may be associated with an episode of volcanic unrest observed between 2002 and 2005. PMID:27152343

  18. Evidence for partial melt in the crust beneath Mt. Paektu (Changbaishan), Democratic People’s Republic of Korea and China

    PubMed Central

    Kyong-Song, Ri; Hammond, James O. S.; Chol-Nam, Ko; Hyok, Kim; Yong-Gun, Yun; Gil-Jong, Pak; Chong-Song, Ri; Oppenheimer, Clive; Liu, Kosima W.; Iacovino, Kayla; Kum-Ran, Ryu

    2016-01-01

    Mt. Paektu (also known as Changbaishan) is an enigmatic volcano on the border between the Democratic People’s Republic of Korea (DPRK) and China. Despite being responsible for one of the largest eruptions in history, comparatively little is known about its magmatic evolution, geochronology, or underlying structure. We present receiver function results from an unprecedented seismic deployment in the DPRK. These are the first estimates of the crustal structure on the DPRK side of the volcano and, indeed, for anywhere beneath the DPRK. The crust 60 km from the volcano has a thickness of 35 km and a bulk VP/VS of 1.76, similar to that of the Sino-Korean craton. The VP/VS ratio increases ~20 km from the volcano, rising to >1.87 directly beneath the volcano. This shows that a large region of the crust has been modified by magmatism associated with the volcanism. Such high values of VP/VS suggest that partial melt is present in the crust beneath Mt. Paektu. This region of melt represents a potential source for magmas erupted in the last few thousand years and may be associated with an episode of volcanic unrest observed between 2002 and 2005. PMID:27152343

  19. Evidence for partial melt in the crust beneath Mt. Paektu (Changbaishan), Democratic People's Republic of Korea and China.

    PubMed

    Kyong-Song, Ri; Hammond, James O S; Chol-Nam, Ko; Hyok, Kim; Yong-Gun, Yun; Gil-Jong, Pak; Chong-Song, Ri; Oppenheimer, Clive; Liu, Kosima W; Iacovino, Kayla; Kum-Ran, Ryu

    2016-04-01

    Mt. Paektu (also known as Changbaishan) is an enigmatic volcano on the border between the Democratic People's Republic of Korea (DPRK) and China. Despite being responsible for one of the largest eruptions in history, comparatively little is known about its magmatic evolution, geochronology, or underlying structure. We present receiver function results from an unprecedented seismic deployment in the DPRK. These are the first estimates of the crustal structure on the DPRK side of the volcano and, indeed, for anywhere beneath the DPRK. The crust 60 km from the volcano has a thickness of 35 km and a bulk V P/V S of 1.76, similar to that of the Sino-Korean craton. The V P/V S ratio increases ~20 km from the volcano, rising to >1.87 directly beneath the volcano. This shows that a large region of the crust has been modified by magmatism associated with the volcanism. Such high values of V P/V S suggest that partial melt is present in the crust beneath Mt. Paektu. This region of melt represents a potential source for magmas erupted in the last few thousand years and may be associated with an episode of volcanic unrest observed between 2002 and 2005.

  20. Isotopic evidence from the eastern Canadian shield for geochemical discontinuity in the proterozoic mantle

    USGS Publications Warehouse

    Ashwal, L.D.; Wooden, J.L.

    1983-01-01

    Most workers agree that Proterozoic anorthosite massifs represent the crystallization products of mantle-derived magmas1,2, although the composition of the parental melts is a major unsolved petrological problem 3. As mantle-derived rocks, the massifs can be used as geochemical probes of their late Precambrian upper mantle sources. We report here Nd and Sr isotopic compositions of anorthosites and related rocks from the Grenville and Nain Provinces of the eastern Canadian shield. Here 75% of the Earth's known anorthosite is found in a 1,600-km belt from the Adirondack Mountains of northern New York State to the eastern coast of Labrador4 (Fig. 1). The results indicate that the massifs were derived from at least two distinct mantle source regions which were established before 1,650 Myr ago, and were episodically involved in magmatism over ???500 Myr. One reservoir, below the Grenville Province, and probably below much of the eastern Superior Province, was isotopically similar to the depleted, modern-day mid-ocean ridge basalt (MORB) source. The other reservoir was chondritic to moderately enriched, and is most easily identified in the Nain Province, but may have occurred scattered throughout the Superior Province. ?? 1983 Nature Publishing Group.

  1. Emplacement history of Granophyre dikes in the Vredefort Impact Structure, South Africa, inferred from geochemical evidence

    NASA Astrophysics Data System (ADS)

    Lieger, Daniel; Riller, Ulrich

    2012-05-01

    The central Vredefort Impact Structure is characterised by impact melt rocks, known as the Vredefort Granophyre dikes, the mode of emplacement of which is not well known. Whole-rock and petrographic analyses of two dikes were conducted and compared to published geochemical data to elucidate the mode and timing of dike formation. The dikes are characterised by compositional and textural heterogeneity between, and within, individual dikes. Specifically, central dike portions are felsic and rich in wall rock fragments, whereas marginal dike phases are more mafic and fragment-poor. Collectively, this suggests that melt was derived from compositionally different parental melts and emplaced in at least two pulses. In addition, the chemical heterogeneity between fragment-rich and fragment-poor dike zones can be explained by variable assimilation of a mafic component, notably Ventersdorp basalt, at the base of the impact melt sheet, from which melt of the Granophyre dikes is derived. This scenario accounts for the mafic and fragment-poor character of melt emplaced first in the dikes and the more felsic and fragment-rich nature of melts of the following emplacement pulse, i.e., when the impact melt was less hot and thus less capable of digesting large quantities of (mafic) wall rock fragments. Differences in geometrical, textural, chemical and fragment characteristics between the Granophyre dikes and pseudotachylite bodies can be explained by the same process, i.e., impact melt drainage, but operating at different times after impact.

  2. Granitic rocks and metasediments in Archean crust, Rainy Lake area, Ontario: ND isotope evidence for mantle-like SM/ND sources

    NASA Technical Reports Server (NTRS)

    Shirey, S. B.; Hanson, G. N.

    1983-01-01

    Granitoids, felsic volcanic rocks and clastic metasediments are typical rocks in Archean granite-greenstone belts that could have formed from preexisting continentasl crust. The petrogenesis of such rocks is assessed to determine the relative roles of new crust formation or old crust formation or old crust recycling in the formation of granite-greenstone belts.

  3. Evidence for a long-term geochemical zonation of the Tristan-Gough Hotspot

    NASA Astrophysics Data System (ADS)

    Deppe, Joana; Hauff, Folkmar; Hoernle, Kaj; Werner, Reinhard; Garbe-Schönberg, Dieter; O`Connor, John; Jokat, Wilfried

    2010-05-01

    The Walvis aseismic ridge and associated Guyot Province in the South Atlantic connect the Etendeka continental flood basalts (CFB) in Namibia with the volcanically active islands of Tristan da Cunha and Gough. Available age data indicate age progressive magmatism along this volcanic lineament consistent with a hotspot track origin (O'Connor and Duncan 1990). The Walvis aseismic ridge and guyots may also serve as a textbook example for the life cycle of a mantle plume: from vigorous, widespread CFB volcanism during the initial plume-head stage to generation of an aseismic ridge above the plume stem during the hotspot stage to production of discrete volcanic edifices during melting of a diffuse upwelling (possibly small blobs) during the final stages. Here we report new geochemical data for volcanic rocks from the Guyot Province where two distinct and spatially separated seamount chains can be identified - the Tristan and the Gough tracks. A total of 22 samples from these volcanic subtracks and the Walvis Ridge have been analyzed for major and trace elements and Sr-Nd-Pb isotopic ratios in order to map out possible compositional differences between the two seamount chains. Notably trace element and Sr-Nd-Pb isotopic data indicate distinct source compositions for the two seamount tracks, suggesting a long-term spatial zonation of the Tristan-Gough mantle source. While the Gough source seems to be most strongly influenced by the EMI mantle endmember and possibly by EMII and FOZO, the Tristan source shows a stronger influence of the FOZO-type component. The lavas from the Walvis ridge at DSDP Site 525 commonly serve as the Atlantic EMI-type endmember, a component that continues to be present within the Gough track. Furthermore, the Gough source seems to melt to a lower degree and/or is generally more trace element or garnet enriched than the Tristan source. These different geochemical signatures can be traced back for at least the past 60 Ma, the time when volcanism

  4. Geochemical evidence for seasonal controls on the transportation of Holocene loess, Matanuska Valley, southern Alaska, USA

    NASA Astrophysics Data System (ADS)

    Muhs, Daniel R.; Budahn, James R.; Skipp, Gary L.; McGeehin, John P.

    2016-06-01

    Loess is a widespread Quaternary deposit in Alaska and loess accretion occurs today in some regions, such as the Matanuska Valley. The source of loess in the Matanuska Valley has been debated for more than seven decades, with the Knik River and the Matanuska River, both to the east, being the leading candidates and the Susitna River, to the west, as a less favorable source. We report here new stratigraphic, mineralogic, and geochemical data that test the competing hypotheses of these river sources. Loess thickness data are consistent with previous studies that show that a source or sources lay to the east, which rules out the Susitna River as a source. Knik and Matanuska River silts can be distinguished using Sc-Th-La, LaN/YbN vs. Eu/Eu∗, Cr/Sc, and As/Sb. Matanuska Valley loess falls clearly within the range of values for these ratios found in Matanuska River silt. Dust storms from the Matanuska River are most common in autumn, when river discharge is at a minimum and silt-rich point bars are exposed, wind speed from the north is beginning to increase after a low-velocity period in summer, snow depth is still minimal, and soil temperatures are still above freezing. Thus, seasonal changes in climate and hydrology emerge as critical factors in the timing of aeolian silt transport in southern Alaska. These findings could be applicable to understanding seasonal controls on Pleistocene loess accretion in Europe, New Zealand, South America, and elsewhere in North America.

  5. Geochemical evidence for the hydrology of a Tamarack-peat bog, Brimfield Township, Portage County, Ohio

    SciTech Connect

    Wilson, T.P.; Miller, L.A. . Dept. of Geology and Water Resources)

    1992-01-01

    Peat Bogs and wetlands represent unique environmental settings what are increasingly subjected to anthropogenic stresses involving inputs of water and chemicals. This study used geochemical and hydrologic monitoring to determine the inputs and fates of elements of the Kent-Brimfield bog located in Portage County, Ohio. Based on physical and chemical information collected over one year, a model is proposed here describing the hydrologic connection between a bog and shallow ground water surrounding the bog. The chemical composition of precipitation, soil water and ground water in the bog vicinity were monitored for one year. Field measurements included water levels, pH, Eh, alkalinity and temperature. Trace metal content of the peat, the pore waters, soil water and ground waters were determined by GFAA, ICP and LIC methods. This bog was found to function as part of a perched water table aquifer. Water in the upper 3 m of the bog is found to be chemically similar to precipitation, but modified by reactions involving dissolution of mineral matter and biologic processes. The chemistry of water deeper in the bog (> 3m) resembles shallow ground water surrounding the bog, modified by weathering of underlying geologic materials and sulfate reduction. This similarity, along with ground water elevations within and outside of the bog, supports that shallow ground water interacts with, and helps maintain water levels in the upper surface of the bog. From these results, a model is proposed for the seasonal variations in hydrologic processes operating in the wetland and surrounding basin, and describes how wetlands may change seasonally from being influent to effluent systems.

  6. Geochemical evidence for seasonal controls on the transportation of Holocene loess, Matanuska Valley, southern Alaska, USA

    USGS Publications Warehouse

    Muhs, Daniel; Budahn, James R.; Skipp, Gary L.; McGeehin, John

    2016-01-01

    Loess is a widespread Quaternary deposit in Alaska and loess accretion occurs today in some regions, such as the Matanuska Valley. The source of loess in the Matanuska Valley has been debated for more than seven decades, with the Knik River and the Matanuska River, both to the east, being the leading candidates and the Susitna River, to the west, as a less favorable source. We report here new stratigraphic, mineralogic, and geochemical data that test the competing hypotheses of these river sources. Loess thickness data are consistent with previous studies that show that a source or sources lay to the east, which rules out the Susitna River as a source. Knik and Matanuska River silts can be distinguished using Sc–Th–La, LaN/YbN vs. Eu/Eu∗, Cr/Sc, and As/Sb. Matanuska Valley loess falls clearly within the range of values for these ratios found in Matanuska River silt. Dust storms from the Matanuska River are most common in autumn, when river discharge is at a minimum and silt-rich point bars are exposed, wind speed from the north is beginning to increase after a low-velocity period in summer, snow depth is still minimal, and soil temperatures are still above freezing. Thus, seasonal changes in climate and hydrology emerge as critical factors in the timing of aeolian silt transport in southern Alaska. These findings could be applicable to understanding seasonal controls on Pleistocene loess accretion in Europe, New Zealand, South America, and elsewhere in North America.

  7. The geochemical proxies for the eutrophic and hypoxia in the Changjiang estuary: evidence from sedimentary records

    NASA Astrophysics Data System (ADS)

    Xuwen, F.

    2013-12-01

    Three cores were selected in the Changjiang Estuary to study potential hundrend-years eutrophication and hypoxia. The sediment record in the Changjiang Estuary mud area (CEMA) within the region of pronounced hypoxia showed that an increase in TOC (21%), biomarkers (141%) and δ13 Corg (1.6‰PDB ) occurred since 1950s and a marked increase since 1970s. Some redox sensitive elements (RSEs) have been enriched significantly since the late 1960s to 1970s, the rates of Mo/Al, Cd/Al and As/Al increased about 83%, 73% and 50% respectively. And the contents of some biogenic elements also increased since the late 1960s, e.g. Ca(129%), Sr(65%) and P(38%) respectively. For the core sediment in the Cheju Island mud area (SCIMA) outside the hypoxia region, the organic geochemical indicators (TOC, biomarkers and δ13Corg ) increased in difference degrees before 1950s~1970s and then were almost the constant. The RSEs were controlled by 'grain size effects' which indicated no hypoxia occurred. For the core sediment in the Zhejiang coastal mud area (ZCMA) within the region of milder hypoxia, the distribution of biomarkers is highly similar to the CEMA, but the other indictactors such as δ13 Corg et al.were different from the above two cores. Productivity in the SCIMA have been mainly influenced by climate ocean circulation changes over the last 100 years. Productivities in the hypoxia areas were corresponding with the fertilizer consumption and high nutrient inputs from the Changjiang River, which stimulated the algae (e g. brassicasterol, dinosterol) blooming and resulted an enrichment of organic matter. Hypoxia invoked organic matter preserved in the sediment. This study concluded that biomarkers in sediment could be as the eutrophic proxies in the Changjiang Estuary and its adjacent region, and δ13 Corg, RSEs and biogenic elements could be as the proxies to trace or reconstruct history of eutrophication and hypoxia in the CEMA.

  8. Geochemical and 3He/ 4He evidence for mantle and crustal contributions to geothermal fluids in the western Canadian continental margin

    NASA Astrophysics Data System (ADS)

    Clark, I. D.; Phillips, R. J.

    2000-12-01

    Isotopic and geochemical evidence together with helium isotopes are used to identify contributions of deep crustal to upper mantle volatile components in thermal waters at three sites across the western North American plate margin: (1) the low heat-flow forearc of the Cascadia subduction zone; (2) the high heat-flow volcanic arc; and (3) the interior crystalline terrain of the ancestral continental margin. Western continental margin hotsprings issue 50°C, low salinity Na-Cl water and N 2 gas with 0.25% CH 4. Stable isotopes and 14C indicate local meteoric recharge during the early Holocene. Redox is buffered by sulphate reduction, suggesting that the methane originates from a deeper source. The waters have high helium contents (He/He air=30) and a 3He-excess ( R/ Rair=0.27; 3He/ 3Heair=8), representing a mixture of radiogenic 4He production in crystalline rock with >10% He from subducted oceanic crust. Geophysical data indicate fluid-filled discontinuities in the subduction zone that may provide a pathway for He, and possibly for CH 4 and a saline fluid component from depth. In the Garibaldi belt of Quaternary arc-volcanism, 60°C Na-Cl hotsprings and 200°C geothermal well waters discharge from fractures in the basement rocks. δ18O and δ2H show the thermal waters to be a mixture of local recharge with up to 8% "andesitic" water from the upper mantle. He isotopes indicate a mantle origin ( R/ Rair=6.0), with a minor crustal signature, consistent with observations in the Cascadia range to the south and at other circum-Pacific volcanic arc settings. High PCO 2, an enriched δ13C DIC, elevated 3He/CO 2 ratios and elevated Cl - are likely to be derived from subducted Juan de Fuca plate sediments and pore waters. Thermal Na-SO 4 waters (up to 58°C) from the Omineca Crystalline Belt are locally recharged and have an unusually rapid circulation time of less than 40 years. This contrasts with their high radiogenic He content (176×10 -7 cc/g) with minimal mantle input

  9. The Khida terrane - Geochronological and isotopic evidence for Paleoproterozoic and Archean crust in the eastern Arabian Shield of Saudi Arabia

    USGS Publications Warehouse

    Whitehouse, M.J.; Stoeser, D.B.; Stacey, J.S.

    2001-01-01

    The Khida terrane of the eastern Arabian Shield of Saudi Arabia has been proposed as being underlain by Paleoproterozoic to Archean continental crust (Stoeser and Stacey, 1988). Detailed geological aspects of the Khida terrane, particularly resulting from new fieldwork during 1999, are discussed in a companion abstract (Stoeser et al., this volume). We present conventional and ion- microprobe U-Pb zircon geoenronology, Nd whole-rock, and feldspar Pb isotopic data that further elucidate the pre-Pan-African evolution of the Khida terrane. Locations for the Muhayil samples described below are shown in figure 2 of Stoeser et al. (this volume). 

  10. Geochemical evidence for Late Cretaceous marginal arc-to-backarc transition in the Sabzevar ophiolitic extrusive sequence, northeast Iran

    NASA Astrophysics Data System (ADS)

    Khalatbari Jafari, Morteza; Babaie, Hassan A.; Gani, Moslem

    2013-07-01

    The ophiolitic extrusive sequence, exposed in an area north of Sabzevar, has three major parts: a lower part, with abundant breccia, hyaloclastic tuff, and sheet flow, a middle part with vesicular, aphyric pillow lava, and an upper part with a sequence of lava and volcanic-sedimentary rocks. Pelagic limestone interlayers contain Late Cretaceous (Maastrichtian-Late Maastrichtian) microfauna. The supra-ophiolitic series includes a sequence of turbidititic and volcanic-sedimentary rocks with lava flow, aphyric and phyric lava, and interlayers of pelagic limestone and radiolarian chert. Paleontological investigation of the pelagic limestone and radiolarite interlayers in this series gives a Late Cretaceous age, supporting the idea that the supra-ophiolitic series formed in a trough, synchronous with the Sabzevar oceanic crust during the Late Cretaceous. Geochemical data indicate a relationship between lava in the upper part of the extrusive sequence and lava in the supra-ophiolitic series. These lavas have a calc-alkaline to almost alkaline characteristic, and show a clear depletion in Nb and definite depletions in Zr and Ti in spider diagrams. Data from these rocks plot in the subduction zone field in tectonomagmatic diagrams. The concentration and position of the heavy rare earth elements in the spider diagrams, and their slight variation, can be attributed to partial melting of the depleted mantle wedge above the subducted slab, and enrichment in the LILE can be attributed to subduction components (fluid, melt) released from the subducting slab. In comparison, the sheet flow and pillow lava of the lower and middle parts of the extrusive sequence show OIB characteristics and high potassium magmatic and shoshonitic trends, and their spider diagram patterns show Nb, Zr, and Ti depletions. The enrichment in the LILE in the spider diagram patterns suggest a low rate of partial melting of an enriched, garnet-bearing mantle. It seems that the marginal arc basin, in which

  11. Crusts: biological

    USGS Publications Warehouse

    Belnap, Jayne; Elias, Scott A.

    2013-01-01

    Biological soil crusts, a community of cyanobacteria, lichens, mosses, and fungi, are an essential part of dryland ecosystems. They are critical in the stabilization of soils, protecting them from wind and water erosion. Similarly, these soil surface communities also stabilized soils on early Earth, allowing vascular plants to establish. They contribute nitrogen and carbon to otherwise relatively infertile dryland soils, and have a strong influence on hydrologic cycles. Their presence can also influence vascular plant establishment and nutrition.

  12. Geochemical Evidence for Gas Hydrates at Atwater Valley and Keathley Canyon, Gulf of Mexico

    NASA Astrophysics Data System (ADS)

    Kastner, M.; Claypool, G.; Robertson, G.; Schultheiss, P.; Holland, M.

    2005-12-01

    Geochemical analyses of pore fluids of both pressurized and non-pressurized cores provide important constraints on the main objective of the 2005 Gulf of Mexico (GOM) Joint Industry Program (JIP) cruise: to calibrate geophysical estimates of methane hydrate distribution and concentration with downhole logs and measurements on cores. Cores were recovered in two deepwater (1300 meter) lease areas of the GOM (Atwater Valley 13/14 and Keathley Canyon 151). Selected intervals were cored to a maximum depth of 390 meters beneath the seafloor, as indicated in prior logging while drilling boreholes. Methane hydrate is associated with sediment intrusions and seafloor mounds at Atwater Valley 13/14, and with a bottom simulating reflection (BSR) (i.e., possible base of methane hydrate stability) at KC 151. Pore fluid samples were analyzed from both conventional cores and pressure cores; the hydrates in the conventional cores decomposed prior to sampling. The former presence and distribution of methane hydrates in the some of the conventional cores was inferred by indirect methods: from infra-red scanning of cold core temperatures that correlated with pore fluids having lower salinity and chlorinity and mouse-like textures. Cores that were successfully collected under in situ pressure retained methane quantities consistent with 1-5% methane hydrate in the sediment pore volume. Overall, similar proportions were indicated by pore water salinity/chlorinity anomalies, assuming dilution by fresh water released from decompose methane hydrate. In some specific horizons, however, at the Atwater Valley 14 mound sites, the pore fluid salinity/chlorinity data suggest up to 7-9% methane hydrate in the sediment pore volume. Significant variations in sulfate gradients were observed. The steepest gradients, with the sulfate/methane interface (SMI) at or just below the seafloor, were found at the Atwater Valley 14 mound sites. Away from the mound the SMI occurs at a depth of about 7-10 m below

  13. Seismic-reflection evidence that the hayward fault extends into the lower crust of the San Francisco Bay Area, California

    USGS Publications Warehouse

    Parsons, T.

    1998-01-01

    This article presents deep seismic-reflection data from an experiment across San Francisco Peninsula in 1995 using large (125 to 500 kg) explosive sources. Shot gathers show a mostly nonreflective upper crust in both the Franciscan and Salinian terranes (juxtaposed across the San Andreas fault), an onset of weak lower-crustal reflectivity beginning at about 6-sec two-way travel time (TWTT) and bright southwest-dipping reflections between 11 and 13 sec TWTT. Previous studies have shown that the Moho in this area is no deeper than 25 km (~8 to 9 sec TWTT). Three-dimensional reflection travel-time modeling of the 11 to 13 sec events from the shot gathers indicates that the bright events may be explained by reflectors 15 to 20 km into the upper mantle, northeast of the San Andreas fault. However, upper mantle reflections from these depths were not observed on marine-reflection profiles collected in San Francisco Bay, nor were they reported from a refraction profile on San Francisco Peninsula. The most consistent interpretation of these events from 2D raytracing and 3D travel-time modeling is that they are out-of-plane reflections from a high-angle (dipping ~70??to the southwest) impedance contrast in the lower crust that corresponds with the surface trace of the Hayward fault. These results suggest that the Hayward fault truncates the horizontal detachment fault suggested to be active beneath San Francisco Bay.

  14. Crustal contamination and crystal entrapment during polybaric magma evolution at Mt. Somma-Vesuvius volcano, Italy: Geochemical and Sr isotope evidence

    USGS Publications Warehouse

    Piochi, M.; Ayuso, R.A.; de Vivo, B.; Somma, R.

    2006-01-01

    New major and trace element analyses and Sr-isotope determinations of rocks from Mt. Somma-Vesuvius volcano produced from 25 ky BP to 1944 AD are part of an extensive database documenting the geochemical evolution of this classic region. Volcanic rocks include silica undersaturated, potassic and ultrapotassic lavas and tephras characterized by variable mineralogy and different crystal abundance, as well as by wide ranges of trace element contents and a wide span of initial Sr-isotopic compositions. Both the degree of undersaturation in silica and the crystal content increase through time, being higher in rocks produced after the eruption at 472 AD (Pollena eruption). Compositional variations have been generally thought to reflect contributions from diverse types of mantle and crust. Magma mixing is commonly invoked as a fundamental process affecting the magmas, in addition to crystal fractionation. Our assessment of geochemical and Sr-isotopic data indicates that compositional variability also reflects the influence of crustal contamination during magma evolution during upward migration to shallow crustal levels and/or by entrapment of crystal mush generated during previous magma storage in the crust. Using a variant of the assimilation fractional crystallization model (Energy Conservation-Assimilation Fractional Crystallization; [Spera and Bohrson, 2001. Energy-constrained open-system magmatic processes I: General model and energy-constrained assimilation and fractional crystallization (EC-AFC) formulation. J. Petrol. 999-1018]; [Bohrson, W.A. and Spera, F.J., 2001. Energy-constrained open-system magmatic process II: application of energy-constrained assimilation-fractional crystallization (EC-AFC) model to magmatic systems. J. Petrol. 1019-1041]) we estimated the contributions from the crust and suggest that contamination by carbonate rocks that underlie the volcano (2 km down to 9-10 km) is a fundamental process controlling magma compositions at Mt. Somma

  15. Interactions between fracturing and pressure solution creep in the upper crust: evidence from natural observations and experiments

    NASA Astrophysics Data System (ADS)

    Gratier, Jean-Pierre

    2014-05-01

    Fracturing and pressure solution creep processes are very often associated in the upper crust deformation. One can see for example that mineral grains and pebbles are both fractured and dissolved under stress, that stylolites are bounded by fractures or that pressure solution spaced cleavage is associated with veins. The question is thus how pressure solution creep and fracturing processes interact in the upper crust to make it behave in both a brittle and viscous manner? Dynamic indenting experiments allow understanding how fracturing and pressure solution creep processes interact. Such experimental technique includes the static loading of a sample by an indenter and its dynamic loading by episodic shocks. The experimental results show that fracturing and comminuting processes induced by the shock accelerate the pressure solution creep rate by reducing the distance of mass transfer. However, it is also observed that after each shock the creep rate progressively decreases due to healing and sealing processes. The main healing processes, in non-porous material such as halite crystal, are crack healing and grain compaction. In porous aggregates such as plaster, the main healing processes are grain sliding, grain dissolution indenting and redeposition. In both cases, when pressure solution is the creep mechanism, fracturing and comminuting weaken the rock whereas compaction, healing and sealing strengthen it. This leads to a non-steady state creep process. This also leads to a chemical segregation of the rocks which is amplified by lithological effect: all the conditions of the deformation being the same, pressure solution creep rate is faster for a mixture of soluble and insoluble minerals than for monomineralic soluble material. Consequently in natural deformation, starting from the common mixture of soluble (quartz, calcite, feldspars…) and insoluble (oxides, phyllosilicates) minerals, successive fracturing and creep-healing events develop a segregation of the

  16. Early Archean crust in the northern Wyoming province Evidence from U-Pb ages of detrital zircons

    USGS Publications Warehouse

    Mueller, P.A.; Wooden, J.L.; Nutman, A.P.; Mogk, D.W.

    1998-01-01

    U-Pb ages of individual detrital and metamorphic zircons from 12 Archean metasedimentary rocks, including quartzites, from the Beartooth, Ruby, and Tobacco Root uplifts of the northern Wyoming province indicate that they were deposited between 2.7 and 3.2 Ga. Younger, metamorphic zircons are found as overgrowths and new grains in some samples, and yield ages between 2.7 and 1.9 Ga. They are, however, much less abundant than detrital grains, which constitute >75% of the 355 grains analyzed. The majority of the detrital grains have ages between 3.2 and 3.4 Ga; none are younger than 2.9 Ga. Grains with 207Pb/206Pb ages between 3.4 and 4.0 Ga constituted approximately 15% of all grains with analyses within 10% of concordia, but are concentrated in samples from the eastern Beartooth Mountains. Comparison of the average of the Pb-Pb ages of individual zircons within 10% of concordia with previously published Lu-Hf chondritic model ages for some individual samples suggests that the age distribution recorded by the U-Pb system in these zircons has not been significantly disturbed by pre- or post-depositional Pb-loss. Collectively, these data suggest that the individual metasedimentary rocks did not completely share a common provenance and that a major crust-forming cycle occurred 3.2 to 3.4 Ga. In conjunction with previously published U-Th-Pb whole-rock data, these results suggest that rocks with a relatively high proportion of > 3.4 Ga grains may have had crust of comparable age in their provenance. ?? 1998 Elsevier Science B.V.

  17. Geochemical evidence for combustion of hydrocarbons during the K-T impact event

    PubMed Central

    Belcher, Claire M.; Finch, Paul; Collinson, Margaret E.; Scott, Andrew C.; Grassineau, Nathalie V.

    2009-01-01

    It has been proposed that extensive wildfires occurred after the Cretaceous–Tertiary (K-T) impact event. An abundance of soot and pyrosynthetic polycyclic aromatic hydrocarbons (pPAHs) in marine K-T boundary impact rocks (BIRs) have been considered support for this hypothesis. However, nonmarine K-T BIRs, from across North America, contain only rare occurrences of charcoal yet abundant noncharred plant remains. pPAHs and soot can be formed from a variety of sources, including partial combustion of vegetation and hydrocarbons whereby modern pPAH signatures are traceable to their source. We present results from multiple nonmarine K-T boundary sites from North America and reveal that the K-T BIRs have a pPAH signature consistent with the combustion of hydrocarbons and not living plant biomass, providing further evidence against K-T wildfires and compelling evidence that a significant volume of hydrocarbons was combusted during the K-T impact event. PMID:19251660

  18. Petrographic, geochemical and isotopic evidence of crustal assimilation processes in the Ponte Nova alkaline mafic-ultramafic massif, SE Brazil

    NASA Astrophysics Data System (ADS)

    Azzone, Rogério Guitarrari; Montecinos Munoz, Patricio; Enrich, Gaston Eduardo Rojas; Alves, Adriana; Ruberti, Excelso; Gomes, Celsode Barros

    2016-09-01

    Crustal assimilation plus crystal fractionation processes of different basanite magma batches control the evolution of the Ponte Nova cretaceous alkaline mafic-ultramafic massif in SE Brazil. This massif is composed of several intrusions, the main ones with a cumulate character. Disequilibrium features in the early-crystallized phases (e.g., corrosion and sieve textures in cores of clinopyroxene crystals, spongy-cellular-textured plagioclase crystals, gulf corrosion texture in olivine crystals) and classical hybridization textures (e.g., blade biotite and acicular apatite crystals) provide strong evidence of open-system behavior. All samples are olivine- and nepheline-normative rocks with basic-ultrabasic and potassic characters and variable incompatible element enrichments. The wide ranges of whole-rock 87Sr/86Sri and 143Nd/144Ndi ratios (0.70432-0.70641 and 0.512216-0.512555, respectively) are indicative of crustal contribution from the Precambrian basement host rocks. Plagioclase and apatite 87Sr/86Sr ratios (0.70422-0.70927) obtained for the most primitive samples of each intrusion indicate disequilibrium conditions from early- to principal-crystallization stages. Isotope mixing-model curves between the least contaminated alkaline basic magma and heterogeneous local crustal components indicate that each intrusion of the massif is differentiated from the others by varied degrees of crustal contribution. The primary mechanisms of crustal contribution to the Ponte Nova massif involve the assimilation of host rock xenoliths during the development of the chamber environment and the assimilation of partial melts from the surrounding host rocks. Thermodynamic models using the melts algorithm indicate that parental alkaline basic magmas can be strongly affected by contamination processes subsequently to their initial stages of crystallization when there is sufficient energy to assimilate partial melts of crustal host rocks. The assimilation processes are considered to

  19. Geochemical and Sr Nd Pb isotopic evidence for a combined assimilation and fractional crystallisation process for volcanic rocks from the Huichapan caldera, Hidalgo, Mexico

    NASA Astrophysics Data System (ADS)

    Verma, Surendra P.

    2001-03-01

    This study reports new geochemical and Sr-Nd-Pb isotopic data for Miocene to Quaternary basaltic to andesitic, dacitic, and rhyolitic volcanic rocks from the Huichapan caldera, located in the central part of the Mexican Volcanic Belt (MVB). The initial Sr and Nd isotopic ratios, except for one rhyolite, range as follows: 87Sr/ 86Sr 0.70357-0.70498 and 143Nd/ 144Nd 0.51265-0.51282. The Sr-Nd-Pb isotopic ratios are generally similar to those for volcanic rocks from other areas of the central and eastern parts of the MVB. The isotopic ratios of one older pre-caldera rhyolite (HP30) from the Huichapan area, particularly its high 87Sr/ 86Sr, are significantly different from rhyolitic rocks from this and other areas of the MVB, but are isotopically similar to some felsic rocks from the neighbouring geological province of Sierra Madre Occidental (SMO), implying an origin as a partial melt of the underlying crust. The evolved andesitic to rhyolitic magmas could have originated from a basaltic magma through a combined assimilation and fractional crystallisation (AFC) process. Different compositions, representing lower crust (LC) and upper crust (UC) as well as a hypothetical crust similar to the source of high 87Sr/ 86Sr rhyolite HP30, were tested as plausible assimilants for the AFC process. The results show that the UC represented by granitic rocks from a nearby Los Humeros area or by Cretaceous limestone (L) rocks outcropping in the northern part of the study area, and the LC represented by granulitic xenoliths from a nearby San Luis Potosı´ (SLP) area are not possible assimilants for Huichapan magmas, whereas a hypothetical crust (HA) similar in isotopic compositions to rhyolite HP30 could be considered a possible assimilant for the AFC process. Chemical composition of assimilant HA, although not well constrained at present, was inferred under the assumption that HP30 type partial melts could be generated from its partial melting. These data were then used to evaluate

  20. Mechanical and statistical evidence of the causality of human-made mass shifts on the Earth's upper crust and the occurrence of earthquakes

    NASA Astrophysics Data System (ADS)

    Klose, Christian D.

    2013-01-01

    A global catalog of small- to large-sized earthquakes was systematically analyzed to identify causality and correlatives between human-made mass shifts in the upper Earth's crust and the occurrence of earthquakes. The mass shifts, ranging between 1 kt and 1 Tt, result from large-scale geoengineering operations, including mining, water reservoirs, hydrocarbon production, fluid injection/extractions, deep geothermal energy production and coastal management. This article shows evidence that geomechanical relationships exist with statistical significance between (a) seismic moment magnitudes M of observed earthquakes, (b) lateral distances of the earthquake hypocenters to the geoengineering "operation points" and (c) mass removals or accumulations on the Earth's crust. Statistical findings depend on uncertainties, in particular, of source parameter estimations of seismic events before instrumental recoding. Statistical observations, however, indicate that every second, seismic event tends to occur after a decade. The chance of an earthquake to nucleate after 2 or 20 years near an area with a significant mass shift is 25 or 75 %, respectively. Moreover, causative effects of seismic activities highly depend on the tectonic stress regime in which the operations take place (i.e., extensive, transverse or compressive). Results are summarized as follows: First, seismic moment magnitudes increase the more mass is locally shifted on the Earth's crust. Second, seismic moment magnitudes increase the larger the area in the crust is geomechanically polluted. Third, reverse faults tend to be more trigger-sensitive than normal faults due to a stronger alteration of the minimum vertical principal stress component. Pure strike-slip faults seem to rupture randomly and independently from the magnitude of the mass changes. Finally, mainly due to high estimation uncertainties of source parameters and, in particular, of shallow seismic events (<10 km), it remains still very difficult to

  1. The French Guyana doleritic dykes: geochemical evidence of three populations and new data for the Jurassic Central Atlantic Magmatic Province

    NASA Astrophysics Data System (ADS)

    Nomade, S.; Pouclet, A.; Chen, Y.

    2002-12-01

    A petrographic and geochemical study of 15 Early Jurassic and 7 Proterozoic dolerites of French Guyana, and of one Jurassic dolerite from Ivory-Coast were carried out. The Early Jurassic SSW-NNE trending dykes have doleritic aphyric or gabbroic phyric texture. Their chemical compositions, slightly under-saturated to over-saturated, show moderate to low Mg-ratios (63-36), high TiO 2 contents (1.85-3.56 wt.%), weak rare earth element fractionation [1.8<(La/Yb) n <4.6], negative Sr-anomalies (0.41geochemical signatures and recent isotopic data of Bertrand et al. [Bertrand, H., Liegeois, J.P., Deckart, K., Féraud, G., 1999. High-Ti tholeiites in Guinea and Their Connection with the Central Atlantic CFB Province: Elemental and Nd-Sr-Pb Isotopic Evidence for Preferential Zone of Mantle Upwelling in Cause of Rifting. AGU spring meeting (Abst. p 317)] suggest that their magmatic source is different from that of the other basalts of the Central Atlantic Magmatic Province (CAMP). Such signatures are restricted to a central zone coinciding with the Panafrican Rokelides suture. We propose a model of sub-lithospheric preferential channelling of an asthenospheric ascent in this zone. Two other groups of dykes were identified in French Guyana. Compared to the Jurassic ones the Proterozoic dykes have NNW-SSE and E-W trending direction, more important LILE enrichment, low TiO 2 contents (<2 wt%) and Nb-Ta negative anomalies. Their calc-alkaline signature could be the result of a previous subduction and may be related to the 1800 Ma Venturi-Tapajós event, which contaminated the mantle source.

  2. Geochemical and mineralogical evidence from eolian sediments for northwesterly mid-Holocene paleowinds, central Kansas, USA

    USGS Publications Warehouse

    Arbogast, A.F.; Muhs, D.R.

    2000-01-01

    A prominent (4500 km2) dune field in the Great Plains is the Great Bend Sand Prairie of south-central Kansas. Dunes here overlie late Quaternary alluvium and were reactivated extensively in the late Holocene. Geomorphic and soil evidence suggests that the most likely eolian sand source is the Arkansas River valley to the northwest. Nevertheless, orientations of stabilized dunes indicate that the most recent dune-forming winds came from the south or southwest, in agreement with modern wind data. Mineralogy and trace element concentrations in eolian sands of the Great Bend Sand Prairie are similar to those to the Arkansas River, which permits the Arkansas River as a sediment source. Ca and Sr abundances, which reflect small amounts of carbonate minerals, are higher in Arkansas River sand compared to eolian sands and show a systematic depletion away from the Arkansas River to the southeast. These trends are likely due to carbonate mineral depletion downwind from abrasion and size reduction. Thus, paleowinds probably were northwesterly during initial deposition. Northwesterly winds occur today when dry, Pacific-derived air is dominant. We hypothesize that the residence time of this air mass was much greater while dunes initially formed, possibly during a warmer and drier mid-Holocene period.

  3. Palaeoproterozoic high-pressure granulite overprint of the Archaean continental crust: evidence for homogeneous crustal thickening (Man Rise, Ivory Coast)

    NASA Astrophysics Data System (ADS)

    Pitra, Pavel; Kouamelan, Alain N.; Ballèvre, Michel; Peucat, Jean-Jacques

    2010-05-01

    The character of mountain building processes in the Palaeoproterozoic times is subject to much debate. The local observation of Barrovian-type assemblages and high-pressure granulite relics in the Man Rise (Côte d'Ivoire), led some authors to argue that Eburnean (Palaeoproterozoic) reworking of the Archaean basement was achieved by modern-style thrust-dominated tectonics (e.g., Feybesse & Milési, 1994). However, it has been suggested that crustal thickening and subsequent exhumation of high-pressure crustal rocks can be achieved by virtue of homogeneous, fold-dominated deformation of hot crustal domains even in Phanerozoic orogenic belts (e.g., Schulmann et al., 2002; 2008). We describe a mafic granulite of the Kouibli area (Archaean part of the Man Rise, western Ivory Coast) that displays a primary assemblage (M1) containing garnet, diopsidic clinopyroxene, red-brown pargasitic amphibole, plagioclase (andesine), rutile, ilmenite and quartz. This assemblage is associated with a subvertical regional foliation. Symplectites that develop at the expense of the M1 assemblage contain orthopyroxene, clinopyroxene, plagioclase (bytownite), green pargasitic amphibole, ilmenite and magnetite (M2). Multiequilibrium thermobarometric calculations and P-T pseudosections calculated with THERMOCALC suggest granulite-facies conditions of ca. 13 kbar, 850°C and <7 kbar, 700-800°C for M1 and M2, respectively. In agreement with the qualitative information obtained from reaction textures and chemical zoning of minerals, this suggests an evolution dominated by decompression accompanied by moderate cooling. A Sm-Nd garnet - whole-rock age of 2.03 Ga determined on this sample indicates that this evolution occurred during the Palaeoproterozoic. We argue that from the geodynamic point of view the observed features are best explained by homogeneous thickening of the margin of the Archaean craton, re-heated and softened due to the accretion of hot, juvenile Palaeoproterozoic crust, as

  4. Geochemical and Textural Evidence for Transitions in Degassing Regimes at Merapi Volcano, Java, Indonesia

    NASA Astrophysics Data System (ADS)

    Genareau, K. D.; Cronin, S. J.; Lube, G.

    2013-12-01

    Transitions in degassing regimes at erupting volcanoes can cause significant changes in magma rheology and the intensity of eruptive activity. The October/November 2010 events at Merapi Volcano, Java, Indonesia began with an intrusive stage (Phase 1), followed by an explosion on October 26th (Phase 2) that removed the pre-existing dome causing deadly pyroclastic density currents (PDCs). This led to rapid ascent of juvenile magma that progressively degassed and crystallized until a large-scale collapse (Phase 3) of the new lava dome on November 5th and associated explosions produced even larger PDCs, killed more citizens, and destroyed several more distal villages. PDC deposits from the two main surge events on October 26th and November 5th, in addition to tephras generated by intermediate explosions, were sampled from numerous locations on the edifice. Secondary ion mass spectrometer (SIMS) depth profiling analyses were performed on free feldspar phenocrysts from the PDC deposits to examine the behavior of volatile elements during the eruptive events of 2010. Phenocrysts were mounted in indium and depth profiled with an O2+ primary ion beam for 2-14 hours. Profiles encounter glass and groundmass on the surface of the crystals before the feldspar is reached, determined from the point at which the B/Si and F/Si signals fall below detection limits. For all samples, the H/Si signal decreases from the glass into the crystal, but the Li/Si signal behaves differently between samples from Phase 2 and Phase 3. In Phase 2 samples, Li/Si ratios are higher in the glass compared to the crystal, revealing a build-up of Li in the groundmass not observed in Phase 3 samples. These observations suggest that Li was able to successfully diffuse out of both the feldspar and the surrounding groundmass in the Phase 3 products, but remained trapped in the groundmass of the Phase 2 products. The build-up of Li in the groundmass, coupled with textural evidence of a late-stage vesiculation

  5. Geochemical Interpretation of Collision Volcanism

    NASA Astrophysics Data System (ADS)

    Pearce, Julian

    2014-05-01

    Collision volcanism can be defined as volcanism that takes place during an orogeny from the moment that continental subduction starts to the end of orogenic collapse. Its importance in the Geological Record is greatly underestimated as collision volcanics are easily misinterpreted as being of volcanic arc, extensional or mantle plume origin. There are many types of collision volcanic province: continent-island arc collision (e.g. Banda arc); continent-active margin collision (e.g. Tibet, Turkey-Iran); continent-rear-arc collision (e.g. Bolivia); continent-continent collision (e.g. Tuscany); and island arc-island arc collision (e.g. Taiwan). Superimposed on this variability is the fact that every orogeny is different in detail. Nonetheless, there is a general theme of cyclicity on different time scales. This starts with syn-collision volcanism resulting from the subduction of an ocean-continent transition and continental lithosphere, and continues through post-collision volcanism. The latter can be subdivided into orogenic volcanism, which is related to thickened crust, and post-orogenic, which is related to orogenic collapse. Typically, but not always, collision volcanism is preceded by normal arc volcanism and followed by normal intraplate volcanism. Identification and interpretation of collision volcanism in the Geologic Record is greatly facilitated if a dated stratigraphic sequence is present so that the petrogenic evolution can be traced. In any case, the basis of fingerprinting collision terranes is to use geochemical proxies for mantle and subduction fluxes, slab temperatures, and depths and degrees of melting. For example, syn-collision volcanism is characterized by a high subduction flux relative to mantle flux because of the high input flux of fusible sediment and crust coupled with limited mantle flow, and because of high slab temperatures resulting from the decrease in subduction rate. The resulting geochemical patterns are similar regardless of

  6. Further X-ray observations of EXO 0748-676 in quiescence: evidence for a cooling neutron star crust

    NASA Astrophysics Data System (ADS)

    Degenaar, N.; Wolff, M. T.; Ray, P. S.; Wood, K. S.; Homan, J.; Lewin, W. H. G.; Jonker, P. G.; Cackett, E. M.; Miller, J. M.; Brown, E. F.; Wijnands, R.

    2011-04-01

    In late 2008, the quasi-persistent neutron star X-ray transient and eclipsing binary EXO 0748-676 started a transition from outburst to quiescence, after it actively accreted for more than 24 yr. In a previous work, we discussed Chandra and Swift observations obtained during the first 5 months of this transition. Here, we report on further X-ray observations of EXO 0748-676, extending the quiescent monitoring to 1.6 yr. Chandra and XMM-Newton data reveal quiescent X-ray spectra composed of a soft, thermal component that is well fitted by a neutron star atmosphere model. An additional hard power-law tail is detected that changes non-monotonically over time, contributing between 4 and 20 per cent to the total unabsorbed 0.5-10 keV flux. The combined set of Chandra, XMM-Newton and Swift data reveals that the thermal bolometric luminosity fades from ˜ 1 × 1034 to 6 × 1033 (D/7.4 kpc)2 erg s -1, whereas the inferred neutron star effective temperature decreases from ˜124 to 109 eV. We interpret the observed decay as cooling of the neutron star crust and show that the fractional quiescent temperature change of EXO 0748-676 is markedly smaller than observed for three other neutron star X-ray binaries that underwent prolonged accretion outbursts.

  7. Reactive flow as dominant evolution process in the lowermost oceanic crust: evidence from olivine of the Pineto ophiolite (Corsica)

    NASA Astrophysics Data System (ADS)

    Sanfilippo, Alessio; Tribuzio, Riccardo; Tiepolo, Massimo; Berno, Davide

    2015-10-01

    The Jurassic Pineto ophiolite from Corsica exposes a ~1-km-thick troctolite-olivine-gabbro sequence, interpreted to represent a lowermost sector of the gabbroic oceanic crust from a (ultra-)slow spreading system. To constrain the petrogenesis of the olivine-gabbros, minor and trace element analyses of olivine (forsterite = 84-82 mol%) were carried out. Olivine from the olivine-gabbros is depleted in incompatible trace elements (Sc, V, Ti, Y, Zr and heavy rare earth elements) with respect to olivines from associated troctolites. Depleted incompatible element compositions are also shown by olivine (forsterite = 86 mol%) from a clinopyroxene-rich troctolite. The incompatible element compositions of olivine argue against a petrogenetic process entirely driven by fractional crystallization. We propose that melts migrating through an olivine-plagioclase crystal mush chemically evolved by reaction with the existing minerals, changing in composition as it flowed upward. The melt residual from these interactions led to partial dissolution of preexisting olivine and to crystallization of clinopyroxene, generating olivine-gabbro bodies within a troctolite matrix. Reactive flow was the major evolution process active in the ~1-km crustal transect exposed at the Pineto ophiolite, producing lithological variations classically attributed to fractional crystallization processes.

  8. Geochemical evidence in the northeast Lau Basin for subduction of the Cook-Austral volcanic chain in the Tonga Trench

    NASA Astrophysics Data System (ADS)

    Price, Allison A.; Jackson, Matthew G.; Blichert-Toft, Janne; Blusztajn, Jerzy; Conatser, Christopher S.; Konter, Jasper G.; Koppers, Anthony A. P.; Kurz, Mark D.

    2016-05-01

    Lau Basin basalts host an array of geochemical signatures that suggest incorporation of enriched mantle source material often associated with intraplate hotspots, but the origin of these signatures remain uncertain. Geochemical signatures associated with mantle material entrained from the nearby Samoan hotspot are present in northwest Lau Basin lavas, and subducted seamounts from the Louisville hotspot track may contribute geochemical signatures to the Tonga Arc. However, lavas in the northeast Lau Basin (NELB) have unique enriched geochemical signatures that cannot be related to these hotspots, but can be attributed to the subduction of seamounts associated with the Cook-Austral volcanic lineament. Here we present geochemical data on a new suite of NELB lavas—ranging in 40Ar/39Ar age from 1.3 Ma to 0.365 ka—that have extreme signatures of geochemical enrichment, including lavas with the highest 206Pb/204Pb (19.580) and among the lowest 143Nd/144Nd (0.512697) encountered in the Lau Basin to date. These signatures are linked to the canonical EM1 (enriched mantle 1) and HIMU (high-μ = 238U/204Pb) mantle end-members, respectively. Using a plate reconstruction model, we show that older portions of the traces of two of the Cook-Austral hotspots that contributed volcanism to the Cook-Austral volcanic lineament—the Rarotonga and Rurutu hotspots—were potentially subducted in the Tonga Trench beneath the NELB. The geochemical signatures of the Rarotonga, Rurutu, and Samoan hotspots provide a compelling match to the extreme geochemical components observed in the new NELB lavas.

  9. Generation of Palaeoproterozoic tonalites and associated high-K granites in southwestern Tanzania by partial melting of underplated mafic crust in an intracontinental setting: Constraints from geochemical and isotopic data

    NASA Astrophysics Data System (ADS)

    Manya, Shukrani; Maboko, Makenya A. H.

    2016-09-01

    The southwestern part of the 2.0-1.8 Ga Palaeoproterozoic Usagaran Belt in the Njombe area of SW Tanzania is intruded by two types of synchronous granitic rocks with different chemical and petrological characteristics. The first type consists of hornblende-rich tonalites that have major element compositions similar to those of Archaean TTG but differ significantly in their trace element composition. The tonalites are spatially and closely associated with felsic, high-K, I-type granites, some of which are gneissic and/or aplitic. U-Pb zircon geochronology shows that the emplacement of tonalites at 1887 ± 11 Ma was largely contemporaneous with emplacement of high-K granitic gneisses at 1877 ± 15 Ma and aplitic granites at 1857 ± 19 Ma. The data also reveal the presence of Archaean crust of 2648 ± 25 Ma in the zircon cores of some samples in the otherwise Palaeoproterozoic terrane. The tonalites are characterized by MgO contents of 1.60-4.11 wt.% at a SiO2 range of 58.1-67.9 wt.%, the Mg# of 34-55, lower Sr contents (220-462 ppm) and less fractionated REE patterns (La/YbCN = 3.55-12.9) compared to Archaean TTG (Sr > 500 ppm, La/YbCN > 20). These features, coupled with the εNd (1887 Ma) values of + 0.37 to - 0.66 as well as the associated mafic enclaves are suggestive of derivation of the tonalites by low pressure (below the garnet stability) partial melting of a mantle-derived mafic underplate that was subsequently contaminated with small amounts of pre-existing igneous crustal rocks. The evolved nature of the high-K granites (MgO = 0.20-1.30 wt.%, SiO2 = 65.5-73.9 wt.%, Mg# = 25-42, εNd = - 3.20 to - 4.75) coupled with old TDM ages which are 200-1000 Ma older than their emplacement age requires a higher degree of assimilation of older crustal material by the magma derived from partial melting of the underplated mafic crust which was subsequently followed by crystal fractionation involving plagioclase, pyroxene and amphibole. The close spatial and temporal

  10. Silica-rich lavas in the oceanic crust: experimental evidence for fractional crystallization under low water activity

    NASA Astrophysics Data System (ADS)

    Erdmann, Martin; Koepke, Jürgen

    2016-10-01

    We experimentally investigated phase relations and phase compositions as well as the influence of water activity ( aH2O) and redox conditions on the equilibrium crystallization path within an oceanic dacitic potassium-depleted system at shallow pressure (200 MPa). Moreover, we measured the partitioning of trace elements between melt and plagioclase via secondary ion mass spectrometry for a highly evolved experiment (SiO2 = 74.6 wt%). As starting material, we used a dacitic glass dredged at the Pacific-Antarctic Rise. Phase assemblages in natural high-silica systems reported from different locations of fast-spreading oceanic crust could be experimentally reproduced only in a relatively small range of temperature and melt-water content ( T ~950 °C; melt H2O < 1.5 wt%) at redox conditions slightly below the quartz-fayalite-magnetite buffer. The relatively low water content is remarkable, because distinct hydrothermal influence is generally regarded as key for producing silica-rich rocks in an oceanic environment. However, our conclusion is also supported by mineral and melt chemistry of natural evolved rocks; these rocks are only congruent to the composition of those experimental phases that are produced under low aH2O. Low FeO contents under water-saturated conditions and the characteristic enrichment of Al2O3 in high aH2O experiments, in particular, contradict natural observations, while experiments with low aH2O match the natural trend. Moreover, the observation that highly evolved experimental melts remain H2O-poor while they are relatively enriched in chlorine implies a decoupling between these two volatiles during crustal contamination.

  11. Formation of iddingsite veins in the martian crust by centripetal replacement of olivine: Evidence from the nakhlite meteorite Lafayette

    NASA Astrophysics Data System (ADS)

    Lee, M. R.; Tomkinson, T.; Hallis, L. J.; Mark, D. F.

    2015-04-01

    common microstructural control on water-mineral interaction between Mars and Earth, and indicates that prior shock deformation was not a prerequisite for aqueous alteration of the martian crust.

  12. Silicon Isotope Geochemistry of Ocean Island Basalts: Search for Deep Mantle Heterogeneities and Evidence for Recycled Altered Oceanic Crust

    NASA Astrophysics Data System (ADS)

    Pringle, E. A.; Savage, P. S.; Jackson, M. G.; Moreira, M. A.; Day, J. M.; Moynier, F.

    2014-12-01

    recycled altered oceanic crust in the plume source. However, the sampling of a primitive reservoir enriched in the light isotopes of Si, as suggested by [4], cannot be ruled out as a potential source of Si isotope variations in OIBs. References: [1] Ziegler et al., GCA 2005 [2] Savage et al., GCA 2011 [3] Savage et al., EPSL 2010 [4] Huang et al., GCA 2014

  13. Evidence for chondritic Lu/Hf in the early crust - mantle system from Antarctic and Western Australian Eoarchean zircon

    NASA Astrophysics Data System (ADS)

    Hiess, J.; Bennett, V. C.; Black, L.; Eggins, S. M.

    2009-12-01

    The timing and extent of early continental crust formation and mantle differentiation remain contentious issues in Earth Sciences. Here we report new, SHRIMP U-Pb zircon geochronology (n = 142 spot analyses on 69 grains), combined with LA-MC-ICPMS 176Hf determinations (n = 110 spot analyses on 59 grains), in three well-characterized orthogneiss samples of the Napier Complex (Mt Sones and Gage Ridge), East Antarctica and the Narryer Complex, West Australia. Prior to analysis all crystals were extensively documented by cathodoluminesence, reflected and transmitted light imaging to guide beam placement and to identify zones of magmatic oscillatory growth from metamorphic recrystallisation textures. Each sample records complex 207Pb/206Pb age structures that can extend from Hadean through to the Neoarchean, while displaying concordant, reverse-discordant and normal-discordant behaviour. U-Pb systematics within individual grains can be correlated with distinct variations in their measured Lu and Hf isotopic compositions. These features lead to the presence of multiple Lu/Hf arrays within each rock sample evolving towards progressively un-radiogenic values through time. Highly concordant populations of Eoarchean zircon from each rock represent the intrusive age of their igneous protolith at approximately 3.88, 3.85 and 3.73 Ga. Across all three samples these zircon populations typically record initial ɛHf values that lie within error of the Earth’s chondritic uniform reservoir (calculated using λ176Lu of 1.867×10-11 yr-1 and CHUR parameters of Bouvier et al., 2008, EPSL 273: 48-57). These near-chondritic results are consistent with recent LA-MC-ICPMS 176Hf work on primitive Eoarchean TTG zircon from other cratons (e.g. the Itsaq Complex of Greenland, Hiess et al., 2009, GCA 73: 4489-4516) and together argue against voluminous continental crustal growth or significant mantle Lu/Hf differentiation during the Hadean or Eoarchean on a global scale. Two grains of

  14. Long-term geochemical evolution of the near field repository: insights from reactive transport modelling and experimental evidences.

    PubMed

    Arcos, David; Grandia, Fidel; Domènech, Cristina; Fernández, Ana M; Villar, María V; Muurinen, Arto; Carlsson, Torbjörn; Sellin, Patrik; Hernán, Pedro

    2008-12-12

    The KBS-3 underground nuclear waste repository concept designed by the Swedish Nuclear Fuel and Waste Management Co. (SKB) includes a bentonite buffer barrier surrounding the copper canisters and the iron insert where spent nuclear fuel will be placed. Bentonite is also part of the backfill material used to seal the access and deposition tunnels of the repository. The bentonite barrier has three main safety functions: to ensure the physical stability of the canister, to retard the intrusion of groundwater to the canisters, and in case of canister failure, to retard the migration of radionuclides to the geosphere. Laboratory experiments (< 10 years long) have provided evidence of the control exerted by accessory minerals and clay surfaces on the pore water chemistry. The evolution of the pore water chemistry will be a primordial factor on the long-term stability of the bentonite barrier, which is a key issue in the safety assessments of the KBS-3 concept. In this work we aim to study the long-term geochemical evolution of bentonite and its pore water in the evolving geochemical environment due to climate change. In order to do this, reactive transport simulations are used to predict the interaction between groundwater and bentonite which is simulated following two different pathways: (1) groundwater flow through the backfill in the deposition tunnels, eventually reaching the top of the deposition hole, and (2) direct connection between groundwater and bentonite rings through fractures in the granite crosscutting the deposition hole. The influence of changes in climate has been tested using three different waters interacting with the bentonite: present-day groundwater, water derived from ice melting, and deep-seated brine. Two commercial bentonites have been considered as buffer material, MX-80 and Deponit CA-N, and one natural clay (Friedland type) for the backfill. They show differences in the composition of the exchangeable cations and in the accessory mineral

  15. Long-term geochemical evolution of the near field repository: insights from reactive transport modelling and experimental evidences.

    PubMed

    Arcos, David; Grandia, Fidel; Domènech, Cristina; Fernández, Ana M; Villar, María V; Muurinen, Arto; Carlsson, Torbjörn; Sellin, Patrik; Hernán, Pedro

    2008-12-12

    The KBS-3 underground nuclear waste repository concept designed by the Swedish Nuclear Fuel and Waste Management Co. (SKB) includes a bentonite buffer barrier surrounding the copper canisters and the iron insert where spent nuclear fuel will be placed. Bentonite is also part of the backfill material used to seal the access and deposition tunnels of the repository. The bentonite barrier has three main safety functions: to ensure the physical stability of the canister, to retard the intrusion of groundwater to the canisters, and in case of canister failure, to retard the migration of radionuclides to the geosphere. Laboratory experiments (< 10 years long) have provided evidence of the control exerted by accessory minerals and clay surfaces on the pore water chemistry. The evolution of the pore water chemistry will be a primordial factor on the long-term stability of the bentonite barrier, which is a key issue in the safety assessments of the KBS-3 concept. In this work we aim to study the long-term geochemical evolution of bentonite and its pore water in the evolving geochemical environment due to climate change. In order to do this, reactive transport simulations are used to predict the interaction between groundwater and bentonite which is simulated following two different pathways: (1) groundwater flow through the backfill in the deposition tunnels, eventually reaching the top of the deposition hole, and (2) direct connection between groundwater and bentonite rings through fractures in the granite crosscutting the deposition hole. The influence of changes in climate has been tested using three different waters interacting with the bentonite: present-day groundwater, water derived from ice melting, and deep-seated brine. Two commercial bentonites have been considered as buffer material, MX-80 and Deponit CA-N, and one natural clay (Friedland type) for the backfill. They show differences in the composition of the exchangeable cations and in the accessory mineral

  16. Stable isotope geochemical study of Pamukkale travertines: New evidences of low-temperature non-equilibrium calcite-water fractionation

    NASA Astrophysics Data System (ADS)

    Kele, Sándor; Özkul, Mehmet; Fórizs, István; Gökgöz, Ali; Baykara, Mehmet Oruç; Alçiçek, Mehmet Cihat; Németh, Tibor

    2011-06-01

    , besides the detailed geochemical analyses along downstream sections, we present new evidences of non-equilibrium calcite-water fractionation in lower temperature range (13.3 to 51.3 °C). Our measurements and calculations on natural hot water travertine precipitations at Pamukkale and Egerszalók revealed that the δ 18O travertine is equal with the δ 18O HCO3 at the orifice of the thermal springs, which means that practically there is no oxygen isotope fractionation between these two phases. High rate of CO 2 degassing with rapid precipitation of carbonate could be responsible for this as it was theoretically supposed by O'Neil et al. (1969). Thus, for the determination of the deposition temperature of a fossil travertine deposit we propose to use the water-bicarbonate oxygen isotope equilibrium fractionation instead of the water-travertine fractionation, which can result 8-9 °C difference in the calculated values. Our study is the first detailed empirical proof of O'Neil's hypothesis on a natural carbonate depositing system. The presented observations can be used to identify more precisely the deposition temperature of fossil travertines during paleoclimate studies.

  17. Origin of paleosubduction-modified mantle for Silurian gabbro in the Cathaysia Block: Geochronological and geochemical evidence

    NASA Astrophysics Data System (ADS)

    Wang, Yuejun; Zhang, Aimei; Fan, Weiming; Zhang, Yanhua; Zhang, Yuzhi

    2013-02-01

    In the eastern South China Block (SCB), the presence of middle Paleozoic volcanics and mafic rocks is poor in spite of the extensive occurrence of granites in response to the synchronous (Kwangsian) intracontinental orogen event. The nature of the lithosphere and its associated orogenic process are poorly known for such a major orogen. In this paper, a set of new U-Pb zircon geochronological, elemental and Sr-Nd isotopic data are presented for several gabbroic plutons, which were recently identified in the Cathaysia Block. The representative samples of the Longhugang, Xinchuan and Xinsi plutons yielded the weighted mean 206Pb/238U ages of 423 ± 8 Ma, 434 ± 6 Ma and 420 ± 3 Ma, respectively, and their gneissoid country-rocks are dated at 442-466 Ma. This evidences the presence of Silurian mafic rocks in the Cathaysia Block. These gabbroic rocks can be geochemically classified into three groups. Group 1 from the western Longhugang pluton exhibits flat REE patterns represented by (La/Yb)cn = 1.1-1.7, (Gd/Yb)cn = 0.99-1.15, and δEu = 1.29-1.51 with negative Nb-Ta and P and markedly positive Sr anomalies. It shows enrichment in LILEs and 87Sr/86Sr(t) ratios of 0.70412-0.70727 and ɛNd(t) values of - 2.6 to - 0.6. Group 2 from the Xinchuan, Zhouya and Yunlu plutons is characterized by high MgO (11.4-15.8 wt.%), mg-number (66-74) and Cr and Ni contents. It is marked by significantly negative Nb-Ta, Zr-Hf and P-Ti anomalies with high 87Sr/86Sr(t) (0.70920-0.71072) and low ɛNd(t) values (- 3.9 to - 7.9). Group 3 from the Xinsi and eastern Longhugang plutons has ɛNd(t) values from - 2.8 to - 5.7 and shows high Nb content of 7.05-9.89 ppm with pronounced enrichment in LILE and weak depletion in HFSE, resembling Nb-enriched arc basalt in HFSE composition. A synthesis of these geochemical data points to the following petrogenesis: Groups 1 and 2 derived from a spinel-bearing, plagioclase-rich source and a garnet-bearing, orthopyroxene-rich source, respectively

  18. Long-term geochemical evolution of the near field repository: Insights from reactive transport modelling and experimental evidences

    NASA Astrophysics Data System (ADS)

    Arcos, David; Grandia, Fidel; Domènech, Cristina; Fernández, Ana M.; Villar, María V.; Muurinen, Arto; Carlsson, Torbjörn; Sellin, Patrik; Hernán, Pedro

    2008-12-01

    The KBS-3 underground nuclear waste repository concept designed by the Swedish Nuclear Fuel and Waste Management Co. (SKB) includes a bentonite buffer barrier surrounding the copper canisters and the iron insert where spent nuclear fuel will be placed. Bentonite is also part of the backfill material used to seal the access and deposition tunnels of the repository. The bentonite barrier has three main safety functions: to ensure the physical stability of the canister, to retard the intrusion of groundwater to the canisters, and in case of canister failure, to retard the migration of radionuclides to the geosphere. Laboratory experiments (< 10 years long) have provided evidence of the control exerted by accessory minerals and clay surfaces on the pore water chemistry. The evolution of the pore water chemistry will be a primordial factor on the long-term stability of the bentonite barrier, which is a key issue in the safety assessments of the KBS-3 concept. In this work we aim to study the long-term geochemical evolution of bentonite and its pore water in the evolving geochemical environment due to climate change. In order to do this, reactive transport simulations are used to predict the interaction between groundwater and bentonite which is simulated following two different pathways: (1) groundwater flow through the backfill in the deposition tunnels, eventually reaching the top of the deposition hole, and (2) direct connection between groundwater and bentonite rings through fractures in the granite crosscutting the deposition hole. The influence of changes in climate has been tested using three different waters interacting with the bentonite: present-day groundwater, water derived from ice melting, and deep-seated brine. Two commercial bentonites have been considered as buffer material, MX-80 and Deponit CA-N, and one natural clay (Friedland type) for the backfill. They show differences in the composition of the exchangeable cations and in the accessory mineral

  19. Subduction-related cryptic metasomatism in fore-arc to nascent fore-arc Neoproterozoic mantle peridotites beneath the Eastern Desert of Egypt: mineral chemical and geochemical evidences

    NASA Astrophysics Data System (ADS)

    Hamdy, Mohamed; Salam Abu El-Ela, Abdel; Hassan, Adel; Kill, Youngwoo; Gamal El Dien, Hamed

    2013-04-01

    Mantle spinel peridotites beneath the Arabian Nubian Shield (ANS) in the Eastern Desert (ED) of Egypt were formed in arc stage in different tectonic setting. Thus they might subject to exchange with the crustal material derived from recycling subducting oceanic lithosphere. This caused metasomatism enriching the rocks in incompatible elements and forming non-residual minerals. Herein, we present mineral chemical and geochemical data of four ophiolitic mantle slice serpentinized peridotites (W. Mubarak, G. El-Maiyit, W. Um El Saneyat and W. Atalla) widely distributed in the ED. These rocks are highly serpentinized, except some samples from W. Mubarak and Um El-Saneyat, which contain primary olivine (Fo# = 90-92 mol %) and orthopyroxene (En# = 86-92 mol %) relics. They have harzburgite composition. Based on the Cr# and Mg# of the unaltered spinel cores, all rocks formed in oceanic mantle wedge in the fore-arc setting, except those from W. Atalla formed in nascent fore-arc. This implies that the polarity of the subduction during the arc stage was from the west to the east. These rocks are restites formed after partial melting between 16.58 in W. Atalla to 24 % in G-El Maiyit. Melt extraction occurred under oxidizing conditions in peridotites from W. Mubarak and W. Atalla and under reducing conditions in peridotites from G. El-Maiyit and Um El-Saneyat. Cryptic metasomatism in the studied mantle slice peridotites is evident. This includes enrichment in incompatible elements in minerals and whole rocks if compared with the primitive mantle (PM) composition and the trend of the depletion in melt. In opx the Mg# doesn't correlate with TiO2, CaO, MnO, NiO and Cr2O3concentrations. In addition, in serpentinites from W. Mubarak and W. Atalla, the TiO2spinel is positively correlated with the TiO2 whole-rock, proposing enrichment by the infiltration of Ti-rich melts, while in G. El- Maiyit and Um El-Saneyat serpentinites they are negatively correlated pointing to the reaction

  20. Phylogenetic Diversity of Young Ocean Crust at the East Pacific Rise 9° N

    NASA Astrophysics Data System (ADS)

    Santelli, C. M.; Bach, W.; Rogers, D. R.; Edwards, K. J.

    2004-12-01

    Numerous studies show increasing evidence for a significant biosphere in oceanic lithosphere. Geochemical modeling suggests that most biological activity at or below the seafloor occurs in young crust (<10 Ma) on mid-ocean ridge flanks where low-temperature fluid circulation is substantial. In this environment, oxygenated seawater reacts with basalt and releases chemical energy that could support the growth of microorganisms. Fluid fluxes rapidly decrease further off-axis in older, more altered crust likely leading to a sharp decline in biological activity. To date, most evidence in support of a deep biosphere relies on anomalous textural features and geochemical signatures in aged basalt glass. In order to unambiguously attribute these alteration features to microbial activity, molecular microbiological data is required to corroborate these morphological and chemical observations. The application of molecular techniques to old ocean crust, however, can be difficult because of issues such as low cell density, contamination, and sluggish activity. Hence, studies on young ocean crust may provide insight and constraints on processes that could also apply to older crust. In this study, we have investigated the initial colonization of very young mid-ocean ridge basalt by endolithic microorganisms, and the changes in microbial diversity as a result of increasing rock alteration. Seafloor basalt samples were collected during RV Atlantis cruise AT11-7 in February 2004, from the East Pacific Rise (EPR) between 9° 28'N and 9° 50'N. Samples representing various flow morphologies, glass contents, and ages (up to ˜20 kyrs) were collected by DSV Alvin and brought to the surface in bioboxes. All basalts contain glass that ranges from very fresh to slightly altered with Fe-oxidation rims and/or Mn-oxide crusts. Total community DNA was successfully extracted from glass samples representative of a variety of alteration states. Clone libraries were constructed from PCR products of

  1. Platinum-group element abundances and Re-Os isotopic systematics of the upper continental crust through time: Evidence from glacial diamictites

    NASA Astrophysics Data System (ADS)

    Chen, Kang; Walker, Richard J.; Rudnick, Roberta L.; Gao, Shan; Gaschnig, Richard M.; Puchtel, Igor S.; Tang, Ming; Hu, Zhao-Chu

    2016-10-01

    The fine-grained matrix of glacial diamictites, deposited periodically by continental ice sheets over much of Earth history, provides insights into the average composition and chemical evolution of the upper continental crust (UCC) (Gaschnig et al., 2016, and references therein). The concentrations of platinum-group elements (PGEs, including Os, Ir, Ru, Pt and Pd) and the geochemically related Re, as well as 187Re/188Os and 187Os/188Os ratios, are reported here for globally-distributed glacial diamictites that were deposited during the Mesoarchean, Paleoproterozoic, Neoproterozoic and Paleozoic eras. The medians and averages of PGE concentrations of these diamictite composites decrease from the Mesoarchean to the Neoproterozoic, mimicking decreases in the concentrations of first-row transition elements (Sc, V, Cr, Co and Ni). By contrast, Re concentrations are highly variable with no discernable trend, owing to its high solubility. Assuming these diamictites are representative of average UCC through time, the new data are fully consistent with the previous inference that the Archean UCC contained a greater proportion of mafic-ultramafic rocks relative to younger UCC. Linear regressions of PGEs versus Cr and Ni concentrations in all the diamictite composites from the four time periods are used to estimate the following concentrations of the PGEs in the present-day UCC: 0.059 ± 0.016 ng/g Os, 0.036 ± 0.008 ng/g Ir, 0.079 ± 0.026 ng/g Ru, 0.80 ± 0.22 ng/g Pt and 0.80 ± 0.26 ng/g Pd (2σ of 10,000 bootstrapping regression results). These PGE estimates are slightly higher than the estimates obtained from loess samples. We suggest this probably results from loess preferentially sampling younger UCC rocks that have lower PGE concentrations, or PGEs being fractionated during loess formation. A Re concentration of 0.25 ± 0.12 ng/g (2σ) is obtained from a regression of Re versus Mo. From this, time-integrated 187Re/188Os and 187Os/188Os ratios for the UCC are

  2. U-Pb zircon and geochemical evidence for bimodal mid-Paleozoic magmatism and syngenetic base-metal mineralization in the Yukon-Tanana terrane, Alaska

    USGS Publications Warehouse

    Dusel-Bacon, C.; Wooden, J.L.; Hopkins, M.J.

    2004-01-01

    New SHRIMP (sensitive, high-resolution ion microprobe) U-Pb zircon ages and trace element geochemical data for mafic and felsic metaigneous rocks of the pericratonic Yukon-Tanana terrane in east-central Alaska help define the tectonic setting of mid-Paleozoic magmatism and syngenetic hydrothermal Zn-Pb-Ag mineralization along the ancient Pacific margin of North America. We compare data from similar greenschist-facies sequences of bimodal volcanic and subvolcanic rocks associated with carbonaceous and siliciclastic marine sedimentary rocks, in the Wood River area of the Alaska Range and the Salcha River area of the Yukon-Tanana Upland, and from amphibolite-facies augen gneiss and mafic gneiss (amphibolite) in the Goodpaster River area of the upland. Allowing for analytical uncertainties, igneous crystallization age ranges of 376-353 Ma, 378-346 Ma, and 374-358 Ma are indicated by 13 new SHRIMP U-Pb dates for the Wood River, Salcha River, and Goodpaster River areas, respectively. Bimodal magmatism is indicated by Late Devonian crystallization ages for both augen gneiss (371 ?? 3 and 362 ?? 4 Ma) and associated orthoamphibolite (369 ?? 3 Ma) in the upland and by stratigraphic interleaving of mafic and felsic rocks in the Alaska Range. Metabasites in all three study areas have elevated HFSE (high field strength element) and REE (rare earth element) contents indicative of generation in a within-plate (extensional) tectonic setting. Within-plate trace element signatures also are indicated for peralkaline metarhyolites that host the largest volcanogenic massive sulfide deposits of the Bonnifield district in the Wood River area and for metarhyolite tuff interlayered with the carbonaceous Nasina assemblage, which hosts sedimentary exhalative sulfide occurrences in the Salcha River area. Most of the other felsic metaigneous samples from the Alaska Range and the Yukon-Tanana Upland have geochemical signatures that are similar to those of both average upper continental crust

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

    NASA Astrophysics Data System (ADS)

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

    2010-12-01

    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

  4. Petrologic and Geochemical Evidence for Biologic Contributions to Carbonate Formation in the Panoche-Tumey Hills Paleoseep, California

    NASA Astrophysics Data System (ADS)

    Sample, J.; Schwartz, H.; Moore, C.

    2004-12-01

    The Panoche-Tumey Hills (PTH) paleoseep is a Paleocene seep system that extends greater than 20 km nearly continuously along strike along the western edge of the Great Valley. The seep fauna is dominated by tubeworms but includes bivalves, gastropods, and solitary corals. The entire system, including feeder sandstone intrusions, is 800 m thick. Carbonate deposits occur in the upper 150 m, and are most concentrated in the upper 50 m. Authigenic carbonates include several generations of cements and veins. The carbonates are mainly calcite, but there is evidence for calcite replacing aragonite in pipe structures. Textures indicate the seep system evolved from an open, relatively permeable sedimentary matrix to one of increasingly focused flow. Late stages of cementation included formation of pipe structures with flow velocities vigorous enough to allow precipitation of carbonate relatively free of silt and sand. \\delta13CPDB values as low as -45 permil are consistent with a component of biogenic methane during carbonate formation. Pronounced laminated structures in the carbonates suggest microorganisms played an important role in carbonate formation, by binding cements and/or varying the PCO2 in distinct microenvironments. Biomarker analysis will help to constrain the importance of bacterially mediated carbonate precipitation. During the fluid evolution of the seep organic and inorganic structures served as conduits, and isolated fluid reservoirs at small scales. Carbon isotopes differ outside and inside tubeworms and pipe structures by 18 permil and 45 permil, respectively, but in opposite directions. Clearly fluid sources, as characterized by carbon isotope signatures, can vary substantially over short distances and presumably over short time scales. The geochemical data along with the cement textures indicate that individual seep sites within the system were doomed by their own carbonate- precipitating success. As one seep site became clogged and less permeable

  5. Cambrian rift-related magmatism in the Ossa-Morena Zone (Iberian Massif): Geochemical and geophysical evidence of Gondwana break-up

    NASA Astrophysics Data System (ADS)

    Sarrionandia, F.; Carracedo Sánchez, M.; Eguiluz, L.; Ábalos, B.; Rodríguez, J.; Pin, C.; Gil Ibarguchi, J. I.

    2012-10-01

    Volcanic rocks of Cambrian age from Zafra (Ossa-Morena Zone, Iberian Massif) are the result of rift processes that affected Cadomian arc units accreted to the NW edge of Gondwana during the Neoproterozoic-Early Cambrian transition. Tephrite to rhyolite volcanics define an alkaline transitional association (Coombs type). Basic-ultrabasic rocks exhibit typical alkaline REE-patterns, strongly enriched in LREE with respect to HREE. Two parental magmas are identified, one with a mantle signature, lack of Nb negative anomaly and εNd500Ma + 3.8 to + 4.2; another with crustal contribution, minor Nb negative anomaly and εNd500Ma + 0.8 to + 1.8. Intermediate-acid rocks show variable REE fractionation and share geochemical characteristics of both basic-ultrabasic groups with restricted εNd500Ma + 2.2 to 3.1 and general absence of Nb negative anomaly. Basic-ultrabasic melts resulted from different amounts of partial melting of a homogeneous source and segregation at the garnet-spinel transition zone. We argue that the "Hales transition" recently recognized in reflection seismic experiments of SW Iberia might image such a source region. Mantle-derived magmas ponded at the base of the crust and weakly interacted with crustal rocks/melts, whilst intermediate-acid rocks were generated by plagioclase ± clinopyroxene ± amphibole fractionation. Melt ascent took place through fractures, with limited crustal interaction. Based upon the new geochemical results and complementary cartographic and geophysical data, a model is presented for the Cambrian break-up of North Gondwana due to magma ascent from the mantle.

  6. The Continental Crust.

    ERIC Educational Resources Information Center

    Burchfiel, B. Clark

    1983-01-01

    Continental crust underlies the continents, their margins, and also small shallow regions in oceans. The nature of the crust (much older than oceanic crust) and its dynamics are discussed. Research related to and effects of tectonics, volcanism, erosion, and sedimentation on the crust are considered. (JN)

  7. Evidence for Niche Partitioning Revealed by the Distribution of Sulfur Oxidation Genes Collected from Areas of a Terrestrial Sulfidic Spring with Differing Geochemical Conditions

    PubMed Central

    Engel, Annette Summers

    2013-01-01

    The diversity and phylogenetic significance of bacterial genes in the environment has been well studied, but comparatively little attention has been devoted to understanding the functional significance of different variations of the same metabolic gene that occur in the same environment. We analyzed the geographic distribution of 16S rRNA pyrosequences and soxB genes along a geochemical gradient in a terrestrial sulfidic spring to identify how different taxonomic variations of the soxB gene were naturally distributed within the spring outflow channel and to identify possible evidence for altered SoxB enzyme function in nature. Distinct compositional differences between bacteria that utilize their SoxB enzyme in the Paracoccus sulfide oxidation pathway (e.g., Bradyrhizobium, Paracoccus, and Rhodovulum) and bacteria that utilize their SoxB enzyme in the branched pathway (e.g., Chlorobium, Thiothrix, Thiobacillus, Halothiobacillus, and Thiomonas) were identified. Different variations of the soxB genes were present at different locations within the spring outflow channel in a manner that significantly corresponded to geochemical conditions. The distribution of the different soxB gene sequence variations suggests that the enzymes encoded by these genes are functionally different and could be optimized to specific geochemical conditions that define niche space for bacteria capable of oxidizing reduced sulfur compounds. PMID:23220955

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

    USGS Publications Warehouse

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

    2002-01-01

    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.

  9. Evidence from Olivine-Hosted Melt Inclusions that the Martian Mantle has a Chondritic D/H Ratio and that Some Young Basalts have Assimilated Old Crust

    NASA Technical Reports Server (NTRS)

    Usui, Tomohiro; Alexander, O'D.; Wang, J.; Simon, J. I.; Jones, J. H.

    2012-01-01

    Magmatic degassing of volatile elements affects the climate and near-surface environment of Mars. Telescopic and meteorite studies have revealed that the Martian atmosphere and near-surface materials have D/H ratios 5-6 times terrestrial values [e.g., 1, 2]. Such high D/H ratios are interpreted to result from the preferential loss of H relative to heavier D from the Martian atmosphere, assuming that the original Martian water inventory had a D/H ratio similar to terrestrial values and to H in primitive meteorites [e.g., 1, 3]. However, the primordial Martian D/H ratio has, until now, not been well constrained. The uncertainty over the Martian primordial D/H ratio has arisen both from the scarcity of primitive Martian meteorites and as a result of contamination by terrestrial and, perhaps, Martian surface waters that obscure the signature of the Martian mantle. This study reports a comprehensive dataset of magmatic volatiles and D/H ratios in Martian primary magmas based on low-contamination, in situ ion microprobe analyses of olivine-hosted melt inclusions from both depleted [Yamato 980459 (Y98)] and enriched [Larkman Nunatak 06319 (LAR06)] Martian basaltic meteorites. Analyses of these primitive melts provide definitive evidence that the Martian mantle has retained a primordial D/H ratio and that young Martian basalts have assimilated old Martian crust.

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

    NASA Astrophysics Data System (ADS)

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

    2007-12-01

    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

  11. Double-layer structure of the crust beneath the Zhongdian arc, SW China: U-Pb geochronology and Hf isotope evidence

    NASA Astrophysics Data System (ADS)

    Cao, Kang; Xu, Ji-Feng; Chen, Jian-Lin; Huang, Xiao-Xiao; Ren, Jiang-Bo; Zhao, Xiang-Dong; Liu, Zhen-Xing

    2016-01-01

    U-Pb ages and Hf isotopes of zircons in Late Triassic and Cretaceous intrusive rocks from the Zhongdian arc, SW China, are used to decipher the tectonic, magmatic, and metallogenic processes that occurred during this period. New U-Pb dating of zircons from Late Triassic porphyries yielded ages of ca. 216 Ma and εHf(t) values of -2.1 to +6.1. Combined with previous results, the data indicate that these Late Triassic rocks were most likely derived from a juvenile mafic lower-crust with minor old crust material. However, the Cretaceous granites (∼80 Ma) have lower εHf(t) values (-7.6 to -2.4) than the Late Triassic rocks, indicating that the former originated from old crust. Based on the new data and previous studies of Mesozoic magmatic activity, a plausible model for the tectono-magmatism and metallogenesis of the Zhongdian arc is proposed. The westwards subduction of the Ganzi-Litang oceanic crust began before ∼230 Ma, resulting in the formation of a juvenile lower crust beneath the Zhongdian arc due to the underplating of mafic arc magmas during ca. 230-216 Ma. At ca. 216 Ma, break-off or slab-tearing of the west-dipping Ganzi-Litang oceanic slab led to partial melting of the juvenile lower crust, which gave rise to Cu-bearing porphyries. In the Late Cretaceous, the Zhongdian arc probably underwent post-collision extension, triggering the partial melting of the old middle-upper crustal materials and producing various granites and related Mo-Cu deposits. According to this model, the crust beneath the Zhongdian arc probably has a double-layer structure, with older crust at shallow levels and juvenile crust at deeper levels.

  12. Petrographic and geochemical evidence for the formation of primary, bacterially induced lacustrine dolomite: La Roda 'white earth' (Pliocene, Central Spain)

    USGS Publications Warehouse

    Garcia, Del; Cura, M.A.; Calvo, J.P.; Ordonez, S.; Jones, B.F.; Canaveras, J.C.

    2001-01-01

    low and negative (r = -0??05), whereas it is higher and positive (r = 0??47) for calcite. The lacustrine dolomite deposit from La Roda is interpreted mainly as a result of primary precipitation of dolomite in a shallow, hydrologically closed perennial lake. The lake was supplied by highly saturated HCO3-/CO32- groundwater that leached dolomitic Mesozoic formations. Precipitation of dolomite from alkaline lake waters took place under a semi-arid to arid climate. However, according to our isotopic data, strong evaporative conditions were not required for the formation of the La Roda dolomite. A significant contribution by bacteria to the formation of the dolomites is assumed in view of both petrographic and geochemical evidence.

  13. Molecular and Geochemical Evidence of in situ Denitrification at a Dairy Field Site in the Central Valley of California

    NASA Astrophysics Data System (ADS)

    Esser, B. K.; Letain, T. E.; Singleton, M. J.; Beller, H. R.; Kane, S. R.; Balser, L. M.; Moran, J. E.

    2005-12-01

    Rising nitrate concentrations in California groundwater threaten an already strained water supply. Under certain conditions, however, intrinsic microbial denitrification can mitigate this problem. We present results from a field study at a central California dairy that document saturated-zone denitrification using a combination of molecular and geochemical methods. Geochemical measurements to assess denitrification included nitrate concentration, dissolved oxygen (DO) concentration, dissolved excess N2, and stable isotope composition of nitrate. Sharp decreases in nitrate concentrations with depth corresponded to sharp decreases in DO concentrations and decreasing redox potential. Nitrate in groundwater from this study had δ15N values (5 to 60 ‰) and δ18O values (-4 to 25 ‰) that plotted with a δ18O/δ15N slope of 0.5, consistent with denitrification. Dissolved N2 was found at concentrations well above Ar-normalized concentrations predicted for atmospheric N2, consistent with reduction of nitrate to N2. in situ denitrification was further documented by increased populations of denitrifying bacteria in zones with geochemical signatures of denitrification. Real-time, quantitative, Polymerase Chain Reaction (qPCR) analysis was used to determine denitrifying bacterial cell populations present in aquifer sediment samples by measuring the abundance of genes encoding nitrite reductase, a central enzyme involved in denitrification. Real-time qPCR primers and probes allowing for universal detection of both the nirS (Fe-containing nitrite reductase) and nirK (Cu-containing nitrite reductase) genes in environmental samples were designed based on multiple alignments of over 30 nirS and nirK gene sequences available in GenBank. Trends in total eubacterial populations were also monitored by real-time qPCR analysis. Although geochemical measurements alone can sometimes convincingly indicate denitrification, the real-time qPCR analysis used in this study provides additional

  14. Devonian Nb-enriched basalts and andesites of north-central Tibet: Evidence for the early subduction of the Paleo-Tethyan oceanic crust beneath the North Qiangtang Block

    NASA Astrophysics Data System (ADS)

    Zhang, Hongrui; Yang, Tiannan; Hou, Zengqian; Bian, Yeke

    2016-07-01

    The early evolution of the Tethyan Ocean in north-central Tibet is currently poorly constrained. A sequence of volcanic rocks ranging from basic to intermediate in composition has been identified in the Zaduo area of the North Qiangtang Block. SHRIMP U-Pb dating of zircons from a sample of Zaduo andesite suggests an eruption age of Late Devonian (~ 380 Ma). The Zaduo volcanic rocks exhibit geochemical characteristics similar to those of typical Nb-enriched basalts, with relatively high Nb, Ta, and Zr contents, resulting in high Nb/La ratios (0.70-1.08) and Nb/U ratios (10.57-34.37). The relative enrichment in high field strength elements, together with positive εNd(t) values of + 4.6 to + 5.8 and low (87Sr/86Sr)i ratios of 0.70367-0.70532, indicates the Zaduo volcanic rocks were derived from a depleted mantle source metasomatized by silicate melts of a subducted oceanic slab. The occurrence of Nb-enriched volcanic rocks in the North Qiangtang Block suggests that the subduction of Paleo-Tethyan oceanic crust was initiated in the Late Devonian. Available geochronological data from ophiolites surrounding the North Qiangtang Block suggest that the subducted slab is most likely the Longmucuo-Shuanghu Paleo-Tethyan oceanic crust.

  15. Translation and docking of an arc terrane: geological and geochemical evidence from the southern Zambales Ophiolite Complex, Philippines

    NASA Astrophysics Data System (ADS)

    Yumul, G. P.; Dimalanta, C. B.; Faustino, D. V.; De Jesus, J. V.

    1998-08-01

    The Zambales Ophiolite Complex is made up of three massifs: the Masinloc, Cabangan and San Antonio Massifs. Field, petrographic and geochemical analyses show that the Cabangan and San Antonio Massifs are genetically related to the Coto (transitional mid-ocean ridge-island arc) and Acoje (island arc) blocks of the Masinloc Massif, respectively. The Subic Bay Fault Zone, a left-lateral fault zone, separates the San Antonio Massif island arc terrane from the transitional mid-ocean ridge-island arc-like sheeted diabase dikes-pillow basalts of the Cabangan Massif. The San Antonio Massif is a rifted terrane from the Acoje block which was translated southward to its present position through the West Luzon Shear-Subic Bay Fault Zone. Tectonized clinopyroxenite and gabbronorite hills, which mimic the physical and geochemical characteristics of the Acoje block and the San Antonio Massif ultramafic-mafic cumulate rocks, were left behind along the western side of the Cabangan Massif during the translation of the arc massif southward. This scenario can account for the present-day configuration of the Zambales Ophiolite Complex.

  16. Sources of organic pollution in particulate matter and soil of Silesian Agglomeration (Poland): evidence from geochemical markers.

    PubMed

    Fabiańska, Monika J; Kozielska, Barbara; Konieczyński, Jan; Kowalski, Adam

    2016-06-01

    The exact input of particular sources to polycyclic aromatic hydrocarbons (PAHs) concentrations in urban and industrial agglomerations is still not well recognized. The major breakthrough is possible using geochemical markers. In the air aerosol and soil samples from areas located in the direct influence of industry/traffic in Silesian Agglomeration (Poland), PAHs and other organic compounds were analyzed, including geochemical markers (biomarkers) and polar compounds from fossil fuels and biomass. Gas chromatography (GC-FID) and gas chromatography-mass spectrometry (GC-MS) were applied to investigate the composition of particulate matter and soil extracts. The results suggest that the predominant source of PAHs is fossil fuel. The presence and distribution of steranes, pentacyclic triterpenoids (i.e., hopanes and moretanes) and alkyl PAHs point to traffic emissions and fossil fuel combustion, mainly bituminous coal for power and heat purposes, as the main source of PAHs in the region. Moreover, the presence of fossil fuel biomarker in particulate matter and soil extracts from a rural site, previously considered to be free of organic pollution, requires a cautious interpretation for PAHs results. Apart from the fossil fuel, also other sources of contamination were identified in particulate matter extracts by their markers: phenols and levoglucosan for biomass and diisopropylnaphthalenes for printing materials combustion. The absence of polar biomass combustion indicators in soil extracts might be related to their higher reactivity. PMID:26362678

  17. Elemental composition of the Martian crust.

    PubMed

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

    2009-05-01

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

  18. Elemental composition of the Martian crust.

    PubMed

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

    2009-05-01

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

  19. Thermomagmatic evolution of Mesoproterozoic crust in the Blue Ridge of SW Virginia and NW North Carolina: Evidence from U-Pb geochronology and zircon geothermometry

    USGS Publications Warehouse

    Tollo, Richard P.; Aleinikoff, John N.; Wooden, Joseph L.; Mazdab, Frank K.; Southworth, Scott; Fanning, Mark C.

    2010-01-01

    New geologic mapping, petrology, and U-Pb geochronology indicate that Mesoproterozoic crust near Mount Rogers consists of felsic to mafic meta-igneous rocks emplaced over 260 m.y. The oldest rocks are compositionally diverse and migmatitic, whereas younger granitoids are porphyritic to porphyroclastic. Cathodoluminescence imaging indicates that zircon from four representative units preserves textural evidence of multiple episodes of growth, including domains of igneous, metamorphic, and inherited origin. Sensitive high-resolution ion microprobe (SHRIMP) trace-element analyses indicate that metamorphic zircon is characterized by lower Th/U, higher Yb/Gd, and lower overall rare earth element (REE) concentrations than igneous zircon. SHRIMP U-Pb isotopic analyses of zircon define three episodes of magmatism: 1327 ± 7 Ma, 1180–1155 Ma, and 1061 ± 5 Ma. Crustal recycling is recorded by inherited igneous cores of 1.33–1.29 Ga age in 1161 ± 7 Ma meta-monzogranite. Overlapping ages of igneous and metamorphic crystallization indicate that plutons of ca. 1170 and 1060 Ma age were emplaced during episodes of regional heating. Local development of hornblende + plagioclase + quartz ± clinopyroxene indicates that prograde metamorphism at 1170–1145 Ma and 1060–1020 Ma reached upper-amphibolite-facies conditions, with temperatures estimated using Ti-in-zircon geothermometry at ~740 ± 40 °C during both episodes. The chemical composition of 1327 ± 7 Ma orthogranofels from migmatite preserves the first evidence of arc-generated rocks in the Blue Ridge, indicating a subduction-related environment that may have been comparable to similar-age systems in inliers of the Northern Appalachians and the Composite Arc belt of Canada. Granitic magmatism at 1180–1155 Ma and ca. 1060 Ma near Mount Rogers was contemporaneous with anorthosite-mangerite-charnockite-granite (AMCG) plutonism in the Northern Appalachian inliers and Canadian Grenville Province. Metamorphism at ca. 1160

  20. Reviewing the circulation and mixing of Antarctic Intermediate Water in the South Pacific using evidence from geochemical tracers and Argo float trajectories

    NASA Astrophysics Data System (ADS)

    Bostock, Helen C.; Sutton, Phil J.; Williams, Michael J. M.; Opdyke, Bradley N.

    2013-03-01

    Evidence from physical and geochemical tracers measured during the World Ocean Circulation Experiment (WOCE) shows that there are four sub-types of Antarctic Intermediate Water (AAIW) in the South Pacific. The main formation region of AAIW is the southeast Pacific, where fresh, cold, high oxygen, low nutrient, intermediate waters are created. This AAIW is transported north and mixes with Equatorial Pacific Intermediate Waters (EqPIW), themselves a combination of AAIW and nutrient rich, old North Pacific deep waters. 'Tasman' AAIW found in the Coral and Tasman Seas is more saline and warmer than the main subtropical gyre, and appears to have formed from mixing of AAIW with thermocline waters in the Tasman Gyre. Tasman AAIW leaks out of the Tasman basin to the north of New Zealand and along Chatham Rise, and also in the South Tasman Sea via the Tasman Leakage. Another source of relatively fresh, high oxygen, low nutrient, young AAIW comes directly from the Southern Ocean, flowing into the southwest and central South Pacific Basin, west of the East Pacific Rise. This 'Southern Ocean' (SO) AAIW is most likely a mixture of AAIW formed locally at the Subantarctic Front (SAF), and AAIW formed along the SAF in the southeast Pacific or Indian oceans and transported by the Antarctic Circumpolar Current (ACC). Interpreting physical and geochemical tracers, combined with velocity estimates from Argo floats, and previous research, has allowed us to refine the detailed circulation pattern of AAIW in the South Pacific, especially in the topographically complex southwest Pacific.

  1. A Geochemical Transect Across the Lau and North Fiji Basins: New Evidence for the Distribution of Multiple Mantle Plume Components

    NASA Astrophysics Data System (ADS)

    Price, A. A.; Jackson, M. G.; Blichert-Toft, J.; Arculus, R. J.; Conatser, C. S.; Konter, J. G.; Koppers, A. A. P.; Blusztajn, J.

    2014-12-01

    The Lau and North Fiji backarc basins are located in a tectonically complex region of the South Pacific, where the upper mantle may have been modified by up to five hotspots (Samoa, Rurutu, Rarotonga, Macdonald, and Louisville), each with distinct geochemical fingerprints. We present new Hf, Nd, Sr, and Pb isotopic data for basaltic samples dredged from seven areas along an east-west transect spanning the Lau and North Fiji basins to determine the possible influence and distribution of these various hotspot sources. We find that the isotope ratios of nearly all samples can be explained by mixing a depleted mantle component, which is ubiquitous in the Lau Basin, with a component similar to that found in Samoan shield (EMII) and/or rejuvenated (EMI) lavas. Lavas as far southwest as the Fiji Triple Junction (North Fiji Basin) show enriched geochemical signatures (87Sr/86Sr and 206Pb/204Pb up to 0.7037 and 18.635 respectively, and 143Nd/144Nd and 176Hf/177Hf down to 0.51285 and 0.283023, respectively) trending toward Samoa. This observation extends the range of Samoan influence into the North Fiji Basin 400 km south of its previous observed extent at South Pandora Ridge. The few samples that cannot be explained solely by incorporation of Samoan material are from the northeastern Lau Basin (Falloon et al., 2007) and host a dilute HIMU component that may relate to the incorporation of material from the Rurutu hotspot. This component is not observed further to the west in the Lau and North Fiji basins. A ubiquitous EMI signature in the region may be linked to the Rarotonga hotspot. New dredges from the northeast Lau Basin may give clearer signals that will reveal the identity of the enriched plume component.

  2. Petrographic and geochemical evidence for an allochthonous, possibly impact melt, origin of pseudotachylite from the Vredefort Dome, South Africa

    NASA Astrophysics Data System (ADS)

    Lieger, Daniel; Riller, Ulrich; Gibson, Roger L.

    2011-08-01

    Hypotheses proposed to explain the origin of pseudotachylite bodies formed during impact cratering include: (1) frictional heating, (2) shock loading, (3) decompression or (4) drainage of impact melt into target rocks. In order to differentiate among these processes, we conducted detailed geochemical and petrographic analysis of the matrices in pseudotachylitic veins and dikes and of their respective wall rocks. Our analyses indicate that the chemical compositions of matrices locally deviate significantly from their immediate wall rocks and that assimilation of wall rock has substantially modified the pseudotachylite matrix compositions in places. Variable magnitudes of assimilation can be explained by the surface area of wall rock or its fragments in contact with melt, as well as the initial temperature and cooling rate of the pseudotachylitic melt. Chemical trends observed can be explained either by admixture of an exotic melt component with immediate wall rock or by mixing of melts derived from local lithologies. Trends in the compositional deviation of centimetre to metre-wide pseudotachylite dikes from their immediate wall rocks are consistent with the presence of a primary melt component having granitoid composition akin to the average composition of Vredefort Granophyre dikes. Within veins, melt transport can be geochemically and petrographically traced for distances of centimetres to metres, with the direction of melt transport from larger pseudotachylite veins toward smaller ones and into apophyses. Sulphide and silicate mineralogy indicates that the initial temperature of pseudotachylitic melt must have been at least 1200-1700 °C. Collectively, these characteristics point to an allochthonous origin of pseudotachylitic melt . We advocate the possibility that impact melt from the initially superheated impact melt sheet contributed to the formation of pseudotachylite bodies at Vredefort.

  3. Ferromanganese crusts as indicators for paleoceanographic events in the NE Atlantic

    USGS Publications Warehouse

    Koschinsky, A.; Halbach, P.; Hein, J.R.; Mangini, A.

    1996-01-01

    Hydrogenetic ferromanganese crusts reflect the chemical conditions of the seawater from which they formed. Fine-scale geochemical analysis of crust layers in combination with age determinations can therefore be used to investigate paleoceanographic changes which are recorded in geochemical gradients in the crusts. At Tropic seamount (off northwest Africa), uniform crust growth influenced by terrigenous input from the African continent occurred during approximately the past 12 Ma. Phosphatization of these crusts is minor. In contrast, crusts from Lion seamount, located between Madeira and the Portuguese coast, display a much more variable growth history. A pronounced increase in Ni, Cu, and Zn is observed in some intervals of the crusts, which probably reflects increased surface productivity. A thick older phosphatized generation occurs in many samples. Hydrographic profiles indicate that Mediterranean outflow water (MOW) may play an important role in the composition of these crusts. 10Be dating of one sample confirms that the interruption of the MOW during the Messinian salinity crisis (6.2-5 Ma ago) resulted in changes in element composition. Sr-isotope dating of the apatite phase of the old crust generation has been carried out to obtain a minimum age for the older generation of Atlantic crusts and to determine whether crust phosphatization in the Atlantic can be related to phosphatization episodes recorded in Pacific crusts. The preliminary data show that the old phosphatized crust generation might be as old as approximately 30-40 Ma.

  4. Ferromanganese crusts as indicators for paleoceanographic events in the NE Atlantic

    NASA Astrophysics Data System (ADS)

    Koschinsky, A.; Halbach, P.; Hein, J. R.; Mangini, A.

    Hydrogenetic ferromanganese crusts reflect the chemical conditions of the seawater from which they formed. Fine-scale geochemical analysis of crust layers in combination with age determinations can therefore be used to investigate paleoceanographic changes which are recorded in geochemical gradients in the crusts. At Tropic seamount (off northwest Africa), uniform crust growth influenced by terrigenous input from the African continent occurred during approximately the past 12Ma. Phosphatization of these crusts is minor. In contrast, crusts from Lion seamount, located between Madeira and the Portuguese coast, display a much more variable growth history. A pronounced increase in Ni, Cu, and Zn is observed in some intervals of the crusts, which probably reflects increased surface productivity. A thick older phosphatized generation occurs in many samples. Hydrographic profiles indicate that Mediterranean outflow water (MOW) may play an important role in the composition of these crusts. 10Be dating of one sample confirms that the interruption of the MOW during the Messinian salinity crisis (6.2-5Ma ago) resulted in changes in element composition. Sr-isotope dating of the apatite phase of the old crust generation has been carried out to obtain a minimum age for the older generation of Atlantic crusts and to determine whether crust phosphatization in the Atlantic can be related to phosphatization episodes recorded in Pacific crusts. The preliminary data show that the old phosphatized crust generation might be as old as approximately 30-40Ma.

  5. Generation of continental crust in intra-oceanic arcs

    NASA Astrophysics Data System (ADS)

    Gazel, E.; Hayes, J. L.; Kelemen, P. B.; Everson, E. D.; Holbrook, W. S.; Vance, E.

    2014-12-01

    The origin of continental crust is still an unsolved mystery in the evolution of our planet. Although the best candidates to produce juvenile continental crust are intra-oceanic arcs these systems are dominated by basaltic lavas, and when silicic magmas are produced, the incompatible-element compositions are generally too depleted to be a good match for continental crust estimates. Others, such as the W. Aleutians, are dominated by andesitic melts with trace element compositions similar to average continental crust. In order to evaluate which intra-oceanic arcs produced modern continental crust, we developed a geochemical continental index (CI) through a statistical analysis that compared all available data from modern intra-oceanic arcs with global estimates of continental crust. Our results suggest that magmas from Costa Rica (<10 Ma) have a CI <50, closer to the CI (~20) computed from available average continental crust estimates. Transitional CI values of 50-100 were found in the Aleutians, the Iwo-Jima segment of Izu-Bonin, the L. Antilles, Panama, Nicaragua, and Vanuatu. The geochemical signature of the Costa Rican lavas is controlled by melts from the subducting Galapagos tracks. Iwo-Jima and Vanuatu are in a similar tectonic scenario with subducting intraplate seamounts. Melts from the subducting oceanic crust are thought to significantly control the geochemical signature in the W. Aleutians and Panama. In the L. Antilles and E. Aleutians the continental signature may reflect recycling of a component derived from subducting continental sediments. Most of Izu-Bonin, Marianas, S. Scotia and Tonga arcs with a CI >100 have the least continent-like geochemical signatures. In these arcs the subducting plate is old (>100 Ma), not overprinted by enriched intraplate volcanism and the geochemistry may be dominated by slab-derived, aqueous fluids. We also found a strong correlation between the CI and average crustal P-wave velocity, validating the geochemical index

  6. New geochemical and combined zircon U-Pb and Lu-Hf isotopic data of orthogneisses in the northern Altyn Tagh, northern margin of the Tibetan plateau: Implication for Archean evolution of the Dunhuang Block and crust formation in NW China

    NASA Astrophysics Data System (ADS)

    Long, Xiaoping; Yuan, Chao; Sun, Min; Kröner, Alfred; Zhao, Guochun

    2014-07-01

    Dunhuang Block is previously considered to be an eastern part of the Tarim Craton, but now it is proposed to be the western extension of the Alxa Block of the NCC as a result of displacement along the Altyn Tagh fault. The oldest basement rocks of the Dunhuang Block, named Aketashitage Complex, were mainly exposed in the northern Altyn Tagh. Migmatites of the Aketashitage Complex are sodic and subalkaline in composition. Zircon U-Pb dating of the tonalitic and monzogranitic migmatites indicates that these rocks were produced by strong deformation of earlier formed tonalites with innumerous granitic veins. The tonalitic melanosome of the migmatites was generated approximately at 2.7-2.8 Ga, whereas the granitic leucosome was likely produced by partial melting of the earlier tonalitic melanosome at 2.51-2.53 Ga. Zircon SHRIMP U-Pb dating of metadiorites exposed in the Aketashitage Complex yielded a crystallization age of 2498 ± 10 Ma. The metadiorites are characterized by high Sr and Sr/Y and low Y, but their low SiO2 and high Cr, Co, and Ni indicate similar geochemical characters of modern low-Si adakites. Their high MgO, Cr, Co, Ni, and Mg# and high εHf(t) suggest that the metadiorites were probably produced by partial melting of slab-melt metasomatized mantle peridotites. In combination with previously published age data, it is concluded that the orthogneisses in the Aketashitage Complex were generated during three periods, namely at 2.83, 2.71-2.77 and 2.50-2.57 Ga. Except for the metamorphic rims, zircons from the Aketashitage tonalitic and monzogranitic orthogneisses have TDMc model ages ranging from 3.1 Ga to 3.5 Ga and suggest that their formation involved partly the recycling of Paleoarchean to Mesoarchean crust. The younger TDMc model ages of 2.7-2.9 for the metadioritic rocks suggest another period of formation of juvenile crust in the Neoarchean. The similarity of crustal formation and growth between the Dunhuang Block and the northern Tarim Craton

  7. Growth of early continental crust by partial melting of eclogite.

    PubMed

    Rapp, Robert P; Shimizu, Nobumichi; Norman, Marc D

    2003-10-01

    The tectonic setting in which the first continental crust formed, and the extent to which modern processes of arc magmatism at convergent plate margins were operative on the early Earth, are matters of debate. Geochemical studies have shown that felsic rocks in both Archaean high-grade metamorphic ('grey gneiss') and low-grade granite-greenstone terranes are comprised dominantly of sodium-rich granitoids of the tonalite-trondhjemite-granodiorite (TTG) suite of rocks. Here we present direct experimental evidence showing that partial melting of hydrous basalt in the eclogite facies produces granitoid liquids with major- and trace-element compositions equivalent to Archaean TTG, including the low Nb/Ta and high Zr/Sm ratios of 'average' Archaean TTG, but from a source with initially subchondritic Nb/Ta. In modern environments, basalts with low Nb/Ta form by partial melting of subduction-modified depleted mantle, notably in intraoceanic arc settings in the forearc and back-arc regimes. These observations suggest that TTG magmatism may have taken place beneath granite-greenstone complexes developing along Archaean intraoceanic island arcs by imbricate thrust-stacking and tectonic accretion of a diversity of subduction-related terranes. Partial melting accompanying dehydration of these generally basaltic source materials at the base of thickened, 'arc-like' crust would produce compositionally appropriate TTG granitoids in equilibrium with eclogite residues.

  8. Constrained potential field modeling of the crustal architecture of the Musgrave Province in central Australia: Evidence for lithospheric strengthening due to crust-mantle boundary uplift

    NASA Astrophysics Data System (ADS)

    Aitken, Alan R. A.; Betts, Peter G.; Weinberg, Roberto F.; Gray, Daniel

    2009-12-01

    We image the crustal architecture of the Musgrave Province with petrophysically constrained forward models of new potential field data. These models image divergent shallow-dipping crustal scale thrusts that, at depth, link with an axial zone defined by steeper, lithospheric scale transpressional shear zones. They also show that to permit a near-surface density distribution that is consistent with petrophysical and geological observations, approximately 15-20 km of crust-mantle boundary uplift is necessary beneath the axial zone. The long-term preservation of this crust-mantle boundary offset implies a change from relatively weak lithosphere to relatively strong lithosphere during the intraplate Petermann Orogeny. To explain this, we propose a model in which uplift of the axial zone of the orogen leads to local lithospheric strengthening as a result of the uplift of mantle rocks into the lower crust, coupled with long-term lithospheric cooling due to the erosion of a radioactive upper crust. Brace-Goetze lithospheric strength models suggest that these processes may have increased the integrated strength of the lithosphere by a factor of 1.4-2.8. Because of this strengthening, this system is self-limiting, and activity will cease when lithospheric strength is sufficient to resist external forces and support isostatic imbalances. A simple force-balance model demonstrates that the force required to uplift the axial zone is tectonically reasonable and that the system can subsequently withstand significant tensional forces. This example shows that crust-mantle boundary uplift coupled with reduced crustal heat production can profoundly affect the long-term strength of the continental lithosphere and may be a critical process in the tectonic stabilization of intraplate regions.

  9. Continued Evidence for Input of Chlorine into the Martian Crust from Degassing of Chlorine-Rich Martian Magmas with Implications for Potential Habitability

    NASA Astrophysics Data System (ADS)

    Filiberto, J.; Gross, J.

    2014-12-01

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

  10. Evidence for Milankovitch periodicities in Cenomanian-Turonian lithologic and geochemical cycles, western interior U.S.A.

    USGS Publications Warehouse

    Sageman, B.B.; Rich, J.; Arthur, M.A.; Birchfield, G.E.; Dean, W.E.

    1997-01-01

    The limestone/marlstone bedding couplets of the Bridge Creek Limestone Member, Cenomanian-Turonian Greenhorn Formation, were analyzed by applying spectral techniques to high-resolution lithologic and geochemical data from a core. The results suggest that the Bridge Creek contains a complex record of orbital cyclicity. The dominant signal appears to be obliquity, but signals corresponding to precession and eccentricity were also observed. The development of the bedding couplets is interpreted to have resulted from a combination of factors, including insolation-controlled changes in higher-latitude precipitation leading to dilution/redox cycles, and in lower-latitude evaporation, leading to changes in surface water conditions and productivity cycles in the calcareous plankton. The data interpreted to reflect redox cycles appear to be more strongly influenced by obliquity, and show a weak precessional signal. In contrast, trends in the carbonate record show the opposite response. The complex bedding pattern observed in the Bridge Creek Limestone is interpreted to result from the competing influences of different orbital cycles expressed through different pathways of the depositional system, and was also affected by changes in sedimentation rates related to relative sea level fluctuations, aperiodic dilution by volcanic ash, and changes in organic-matter production and redox conditions related to a global "oceanic anoxic event". These factors complicate cycle analysis in the lower part of the member but leave a relatively undisturbed record in the upper Bridge Creek Limestone. Copyright ?? 1997, SEPM (Society for Sedimentary Geology).

  11. Geochemical and detrital mode evidence for two sources of Early Proterozoic sedimentary rocks from the Tonto Basin Supergroup, central Arizona

    USGS Publications Warehouse

    Condie, K.C.; Noll, P.D., Jr.; Conway, C.M.

    1992-01-01

    The Tonto Basin Supergroup includes up to 6.5 km of Early Proterozoic sedimentary and volcanic rocks that were deposited in a relatively short period of time at about 1.7 Ga in central Arizona. Moderate correlations of rare earth elements (REE) and Ti with Al2O3 and REE distributions in detrital sediments of this supergroup suggest that these elements are contained chiefly in clay-mica and/or zircon fractions. REE distributions, including negative Eu anomalies in most Tonto Basin sediments, are similar to those in Phanerozoic shales. Weak to moderate correlations of Fe, Sc, Ni, and Co to Al2O3 also suggest a clay-mica control of these elements. Detrital modes and geochemical characteristics of sediments indicate two dominant sources for sedimentary rocks of the Tonto Basin Supergroup: a granitoid source and a volcanic source. The granitoid source was important during deposition of the upper part of the succession (the Mazatzal Group) as shown by increases in K2O, Al2O3, and Th in pelites with stratigraphic height, and increases in Zr and Hf and decreases in Eu/Eu*, Cr, and Ni in in pelites of the Maverick Shale. Sediment provenance characteristics and paleocurrent indicators are consistent with deposition of the supergroup in a continental-margin back-arc basin. The granitoid sediment source appears to have been the North American craton on the north, and the volcanic source a more local source from an arc on the south. ?? 1992.

  12. Late Devonian Kellwasser Event mass-extinction horizon in Germany: no geochemical evidence for a large-body impact

    SciTech Connect

    McGhee, G.R. Jr.; Orth, C.J.; Quintana, L.R.; Gilmore, J.S.; Olsen, E.J.

    1986-09-01

    The hypothesis that the Late Devonian (Frasnian-Famennian) mass extinction was triggered by an asteroidal impact has received renewed attention with the discovery of a Late Devonian Ir anomaly in Australia. In Europe, the mass-extinction event corresponds stratigraphically to the geographically widespread Kellwasser black-shale and bituminous limestone units, and the biological crisis itself has been alternatively designated the Kellwasser Event. The authors report here the results of an extensive geochemical analysis of the Kellwasser stratigraphic interval in a section with exceptional conodont zonal control in the Federal Republic of Germany. No Ir anomaly was found, neither at the biological crisis horizon recognized in Europe nor at the conodont horizon that corresponds to the Ir anomaly zone reported in Australia. No shock-metamorphosed quartz, sanidine spherules, or siderophile-rich magnetic spherules were found, which might have been indicative of a cometary impact. Oxygen-isotope ratios show little variation across the mass-extinction horizon, though carbon-isotope data suggest a sudden increase in phytoplankton activity. They further note that the Australian Ir anomaly (1) is most likely not associated with a large-body impact because no equivalent Ir signature occurs in Europe and (2) is stratigraphically above the European biological crisis horizon, thus postdating the Kellwasser mass-extinction event.

  13. Geochemical Evidence of a Near Surface History for the Source Rocks of the Central Coast Mountains Batholith, British Columbia

    NASA Astrophysics Data System (ADS)

    Stremtan, C. C.; Wetmore, P. H.; Ducea, M. N.

    2009-04-01

    Major and trace elemental concentrations as well as Sr, Pb and O isotopic data, completed on 41 plutonic samples from the Coast Mountains Batholith (CMB) ranging in age from ~108 Ma to ~50 Ma indicate that the source regions for these rocks were relatively uniform and typical for island arcs around the Pacific. The studied rocks are mineralogically and chemically metaluminous to weakly peraluminous and are mainly calc-alkaline with a few samples (mostly from the eastern part of the Coastal Shear Zone (CSZ) and three samples from the western part) showing a high-K calc-alkaline feature. Trace elements, especially REE, suggest a mafic source, probably oceanic plateau or island arc in origin, buried to different depths in the crust and that underwent various degrees of partial melting. Initial whole-rock 87Sr/86Sr range from 0.7035 up to 0.7053, whereas lead isotopic data range from 18.586 to 19.078 for 206Pb/204Pb, 15.545 to 15.634 for 207Pb/204Pb, and 37.115 to 38.661 for 208Pb/204Pb. In contrast to these fairly primitive isotopic data, δ18O values for quartz separates determined for 19 of the samples range from 6.8 up to 10.0 . Such δ18O values exclude the possibility that these melts were solely generated from the Mesozoic mantle wedge of this continental arc, just as the Sr and Pb data preclude significant involvement of an old (Precambrian) crustal/mantle lithospheric source. We interpret the high δ18O component to represent materials that had a multi-stage crustal evolution. They were originally mafic rocks derived from a circum-Pacific juvenile mantle wedge that experienced a period of near surface residence after initial crystallization. During this interval these primitive rocks interacted with meteoric waters at low temperatures, as indicated by the high δ18O values. Subsequently, these materials were buried to lower crustal depths where they re-melted to form the high δ18O component of the CMB. We estimate that, based on REE ratios and the presence of

  14. Anorogenic nature of magmatism in the Northern Baikal volcanic belt: Evidence from geochemical, geochronological (U-Pb), and isotopic (Pb, Nd) data

    USGS Publications Warehouse

    Neymark, L.A.; Larin, A.M.; Nemchin, A.A.; Ovchinnikova, G.V.; Rytsk, E. Yu

    1998-01-01

    The Northern Baikal volcanic belt has an age of 1.82-1.87 Ga and extends along the boundary between the Siberian Platform and the Baikal foldbelt. The volcanic belt is composed of volcanics of the Akitkan Group and granitic rocks of the Irel and Primorsk complexes. The geochemistry of the rocks points to the intraplate anorogenic nature of the belt. U-Pb zircon dating of the Chuya granitoids revealed that they are older (2020-2060 Ma) than the Northern Baikal volcanic belt and, thus, cannot be regarded as its component. Data on the Pb isotopic system of feldspars from the granitoids confirm the contemporaneity of all volcanic rocks of the belt except the volcanics of the upper portion of the Akitkan Group (Chaya Formation). Our data suggest its possibly younger (???1.3 Ga) age. The isotopic Nd and Pb compositions of the acid volcanic rocks provide evidence of the heterogeneity of their crustal protoliths. The volcanics of the Malaya Kosa Formation have ??Nd(T) = -6.1, ??2 = 9.36, and were most probably produced with the participation of the U-depleted lower continental crust of Archean age. Other rocks of the complex show ??Nd(T) from -0.1 to -2.4, ??2 = 9.78, and could have been formed by the recycling of the juvenile crust. The depletion of the Malaya Kosa volcanics in most LILEs and HFSEs compared with other acid igneous rocks of the belt possibly reflects compositional differences between the Late Archean and Early Proterozoic crustal sources. The basaltic rocks of the Malaya Kosa Formation (??Nd varies from -4.6 to -5.4) were produced by either the melting of the enriched lithospheric mantle or the contamination of derivatives of the depleted mantle by Early Archean lower crustal rocks, which are not exposed within the area. Copyright ?? 1998 by MAEe Cyrillic signK Hay??a/Interperiodica Publishing.

  15. Three-dimensional conductivity model of crust and uppermost mantle at the northern Trans North China Orogen: Evidence for a mantle source of Datong volcanoes

    NASA Astrophysics Data System (ADS)

    Zhang, Huiqian; Huang, Qinghua; Zhao, Guoze; Guo, Zhen; Chen, Y. John

    2016-11-01

    While the Eastern Block of North China Craton (NCC) had experienced significant lithospheric destruction in the Mesozoic, the Western Block of NCC and the Trans North China Orogen (TNCO) have undergone localized lithospheric modification since the Cenozoic. The northern TNCO is highlighted by the Cenozoic magmatic activities including Hannuoba basalts and Datong volcanoes and is a seismically active region. In this study 3-D electrical conductivity model of the crust and uppermost mantle is derived by the 3-D inversion technique using data from 72 broadband magnetotelluric (MT) stations. The final model shows that a 15 km thick resistive layer of about 3000 Ω m dominates the upper crust, which may represent the intact Archean and Paleoproterozoic terrains. Whereas in the mid-crust there are marked high conductivity anomalies of about 10 Ω m beneath Shanxi rifting basin, which may result from the interconnected saline fluid of 0.2% to 6% volume fraction. The most important finding is that one significant conductor extended into the mantle is located between Hannuoba field and Datong volcanoes and it connects with the mid-crust conductor beneath the Datong volcanoes. We suggest that this could be the mantle source (partial melting region) for the Quaternary volcanic activities of Datong volcanoes and the melt fraction is estimated as 6.6%. Its location inside the Western Block suggests that the volcanic activities at Datong volcanoes are irrelevant to the tectonic process to the east of TNCO. It is likely to be related to the mantle flows from the Tibetan Plateau around the Ordos block which converges at the northeastern corner of the Ordos block and local upward flow along the slope of the thinning lithosphere resulted in decompression partial melting and the melt percolated upward through the crust to feed the lava eruptions at the Datong volcanoes to the east. Finally, large crustal earthquakes in this region are generally located in resistive zones with high

  16. Meteorite fusion crust variability.

    NASA Astrophysics Data System (ADS)

    Thaisen, Kevin G.; Taylor, Lawrence A.

    2009-06-01

    Two assumptions commonly employed in meteorite interpretation are that fusion crust compositions represent the bulk-rock chemistry of the interior meteorite and that the vesicles within the fusion crust result from the release of implanted solar wind volatiles. Electron microprobe analyses of thin sections from lunar meteorite Miller Range (MIL) 05035 and eucrite Bates Nunataks (BTN) 00300 were performed to determine if the chemical compositions of the fusion crust varied and/or represented the published bulk rock composition. It was determined that fusion crust compositions are significantly influenced by the incorporation of fragments from the substrate, and by the composition and grain size of those minerals. Because of compositional heterogeneities throughout the meteorite, one cannot assume that fusion crust composition represents the bulk rock composition. If the compositional variability within the fusion crust and mineralogical differences among thin sections goes unnoticed, then the perceived composition and petrogenetic models of formation will be incorrect. The formation of vesicles within these fusion crusts were also compared to current theories attributing vesicles to a solar wind origin. Previous work from the STONE-5 experiment, where terrestrial rocks were exposed on the exterior of a spacecraft heatshield, produced a vesicular fusion crust without prolonged exposure to solar wind suggesting that the high temperatures experienced by a meteorite during passage through the Earth's atmosphere are sufficient to cause boiling of the melt. Therefore, the assumption that all vesicles found within a fusion crust are due to the release of implanted volatiles of solar wind may not be justified.

  17. Late Quaternary Provenance and Flow Regime Reconstruction through Sedimentologic and Geochemical Evidence from the Bering/Chukchi Seas

    NASA Astrophysics Data System (ADS)

    Pelto, B. M.; Brigham-Grette, J.; Kocis, J. J.; Petsch, S.

    2013-12-01

    The last 20 kyr have been marked by great changes in the Arctic, as the Laurentide Ice Sheet melted and led to the submergence of the Bering Land Bridge and the re-opening of the Bering Strait (BS). The BS is a narrow connection (about 85 km wide) between the Arctic and Pacific Oceans averaging less than 50 m in depth, with present-day flow of seawater northward through the BS, from the Pacific to the Arctic. This flow is of vital importance to global ocean circulation through its role in formation and stability of North Atlantic Deep Water (NADW). An open BS is believed to speed dispersal of North Atlantic freshwater anomalies, both by keeping thermohaline circulation strong, and through reversals of flow through the BS when the North Atlantic is hosed with freshwater. When the BS is closed, these anomalies cannot efficiently dissipate and thermohaline circulation is weakened, which is considered a factor in climate perturbations outside of orbital forcing. Given the period of flux and transition in the Arctic following the Last Glacial Maximum (LGM), the paleoceanographic history of the Bering and Chukchi Seas post-LGM, is important to an understanding of Arctic Ocean circulation, and consequent climate impacts. Today the Arctic is in a period of rapid change, multi-year sea ice is disappearing, and the continuation of climatic stability of the Holocene appears to be at an end. Comprehension of the functioning of the Arctic as a dynamic system is essential to predict future response of the system to change, such as seawater salinity-density changes, lowered sea and land albedo, and rising temperatures. Changes in BS throughflow intensity and direction during deglaciation and submergence of the Bering Land Bridge are proposed and supported in modeling simulations, and are thought to occur during millennial-scale climate changes. We have conducted a coupled sedimentological and geochemical investigation of a suite of marine sediment cores from the Bering and

  18. Geochemical and mineralogical evidence for relative and eustatic sea levels changes in Eocene to Oligocene carbonates in Alabama

    SciTech Connect

    Baum, J.S.; Humphrey, J.D. ); Baum, G.R. ); Thompson, P.R. )

    1990-05-01

    Eocene to Oligocene marine shelf carbonates cored in Baldwin County, Alabama, were subdivided into a sequence stratigraphic framework and sampled at closely spaced intervals to determine characteristic geochemical and mineralogical responses to fluctuations in eustatic and relative sea level. Except for the Eocene/Oligocene boundary interval, the carbonates were deposited landward of their respective depositional shelf edges. Sequence boundaries, transgressive surfaces, and surfaces of maximum starvation were delineated by lithologic and textural variations and gamma logs. Comparison of the Baldwin County core data with correlative Deep Sea Drilling Project/Ocean Drilling Program (DSDWODP) records reveal more clearly defined stable isotopic data trends due to greater sampling density per unit time. In the sequence stratigraphic framework the {delta}{sup 13}C signature mimics relative sea level changes (paleobathymetry) and changes in the O{sub 2} minimum; whereas, the {delta}{sup 18}O signature records the eustatic sea level history. Because the stable isotopes are recording different aspects of sea level history, it is necessary to evaluate isotopic signatures independently. Although minor diagenetic perturbations in the stable isotopic signals are found, the depositional signal clearly dominates the diagenetic signal. The paleogeographic position of this core gives a more accurate sea level record than either more landward sections presently exposed in outcrop that have undergone intense subaerial alteration and erosion or DSDP/ODP records that may be so condensed that the record appears incomplete in conventional sampling schemes . Authigenic mineralization related to sea level changes can be used to characterize surfaces within the depositional sequence framework. Trace elemental data shows that hydrologic flow, porosity, and cementation trends are subsequently controlled by the sequence framework.

  19. Early Variscan magmatism along the southern margin of Laurasia: geochemical and geochronological evidence from the Biga Peninsula, NW Turkey

    NASA Astrophysics Data System (ADS)

    Şengün, Fırat; Koralay, O. Ersin

    2016-05-01

    Massive, fine-grained metavolcanic rocks of the Çamlıca metamorphic unit exposed in the Biga Peninsula, northwestern Anatolia, have provided new Carboniferous ages and arc-related calc-alkaline petrogenesis constraints, suggesting that the Biga Peninsula was possibly involved in the Variscan orogeny. The metavolcanic rocks are mainly composed of metalava and metatuff and have the composition of andesite. Chondrite-normalized REE patterns from these rocks are fractionated (LaN/YbN ~ 2.2 to 8.9). Europium anomalies are slightly variable (Eu/Eu* = 0.6 to 0.7) and generally negative (average Eu/Eu* = 0.68). The metavolcanic rocks have a distinct negative Nb anomaly and negative Sr, Hf, Ba, and Zr anomalies. These large negative anomalies indicate crustal involvement in their derivation. Tectonic discrimination diagrams show that all metavolcanic rocks formed within a volcanic arc setting. Zircon ages (LA-ICP-MS) of two samples yield 333.5 ± 2.7 and 334.0 ± 4.8 Ma. These ages are interpreted to be the time of protolith crystallization. This volcanic episode in the Biga Peninsula correlates with other Variscan age and style of magmatism and, by association with a collisional event leading to the amalgamation of tectonic units during the Variscan contractional orogenic event. Carboniferous calc-alkaline magmatism in the Sakarya Zone is ascribed to arc-magmatism as a result of northward subduction of Paleo-Tethys under the Laurasian margin. Geochemical and U-Pb zircon data indicate that the Sakarya Zone is strikingly similar to that of the Armorican terranes in central Europe. The Biga Peninsula shows a connection between the Sakarya Zone and the Armorican terranes.

  20. Geochemical gradients in the Topopah Spring Member of the Paintbrush Tuff: Evidence for eruption across a magmatic interface

    SciTech Connect

    Schuraytz, B.C.; Vogel, T.A.; Younker, L.W.

    1986-06-01

    The Topopah Spring Member of the Paintbrush Tuff in southern Nevada is a classic example of a compositionally zoned ash-flow sheet that is inferred to have resulted from eruption of a compositionally zoned magma body. Geochemical and petrographic analyses of whole-rock tuff samples indicate that the base of the ash-flow sheet and the dominant volume of erupted material are composed of crystal-poor high-silica rhyolite, with a gradational transition into overlying crystal-rich quartz latite at the top of the sequence. These compositional variations are consistent with a model of progressive eruption of a stratified magma body in which relatively cooler, crystal-poor high-silica rhyolitic magma overlay hotter, crystal-rich quartz latitic magma. Major and trace element chemical analyses of whole glassy pumices and analyses of coexisting ilmenite and magnetite phenocrysts from within the pumices provide closer approximations to the chemical and thermal gradients within the inferred magma body. The magmatic gradients inferred from these data indicate that the transition from high-silica rhyolitic to quartz latitic magma within the chamber was abrupt rather than gradational, with a distinct liquid-liquid interface separating the two contrasting magmas. Throughout the ash-flow sheet, individual pumice lumps with distinct and variable textural characteristics are present within outcrop, hand-sample, and thin-section scale. Within the lower portion of the ash-flow sheet, the individual pumices are all high-silica rhyolites with relatively small variations in trace-element composition and estimated quench temperatures, and thus are chemically similar to their associated whole-rock tuff composites. In contrast, the chemical variability among pumices within the uppermost quartz latite is as great as that of the entire ash-flow sheet.

  1. Geochemical Evidence for Periods of Increased Mineral Dust Deposition in Patagonian Peat Bogs Since the Last Deglaciation

    NASA Astrophysics Data System (ADS)

    Vanneste, H.; De Vleeschouwer, F.; Mattielli, N. D.; Vanderstraeten, A.; von Scheffer, C.; Piotrowska, N.; Coronato, A.; Le Roux, G.

    2013-12-01

    Atmospheric mineral dust plays an important role in the earth's climate system, influencing atmospheric parameters such as cloud condensation as well as biogeochemical cycles, affecting atmospheric CO2 levels. Antarctic ice core records show that mineral dust deposition has varied in the Southern Hemisphere over glacial-interglacial stages, suggesting major changes in atmospheric circulation. Nevertheless, to make predictions for the near future possible, a better understanding of atmospheric dust load and transport variability in the recent past, is essential. Ombrotrophic peat bogs have proven to provide excellent records of atmospheric dust deposition for the Holocene as their accumulation rates are higher than any other archive. Hence two ombrotrophic peat bogs, located southwest (Karukinka) and southeast (Harberton) on Isla Grande de Tierra del Fuego, were sampled to investigate dust-palaeoclimatic interactions in southern South America since the last deglaciation. Here we present a detailed geochemical (major, trace elements and Nd isotopes) record for both sites. The base of the peat sequences in Karukinka and Harberton were dated by 14C at ca. 8,000 cal yr BP and ca. 16,500 cal yr BP, respectively. The distribution of trace elemental (Sc, REE) concentrations within the cores indicates, besides tephra layers, episodes of increased mineral dust deposition at Harberton and Karukinka. The glacial-interglacial transition can be observed in the Harberton record (at ca. 11,500 cal yr BP), marked by a drop in the dust flux from 102 g/m2/yr to 10 g/m2/yr. The most significant episode of mineral dust deposition at Karukinka is concentrated around 1,600 cal yr BP with a maximum dust flux of 108 g/m2/yr. Its neodymium isotopic signature of -1 suggests crustal admixing, compared to the ɛNd values of ˜2, for both tephra layers.

  2. Variations in the Pb isotope composition in polyformational magmatic rocks of the Ketkap-Yuna igneous province of the Aldan Shield: Evidence for mantle-crust interaction

    NASA Astrophysics Data System (ADS)

    Polin, V. F.; Dril, S. I.; Khanchuk, A. I.; Velivetskaya, T. A.; Vladimirova, T. A.; Il'ina, N. N.

    2016-06-01

    The Pb isotope composition of polyformational Mesozoic igneous rocks of the Ketkap-Yuna igneous province (KYIP) and lower crustal metamorphic rocks of the Batomga granite-greenstone area (the complex of the KYIP basement) of the Aldan Shield was studied for the first time. Based on the data obtained, several types of material sources participating in petrogenetic processes were distinguished. The mantle source identified as PREMA is registered in most of the igneous formations and predominates in mafic alkaline rocks. According to the isotope characteristics, the upper crustal source corresponds to a source of the "Orogen" type by the model of "plumbotectonics" or to the average composition of the continental crust by the Stacey-Kramers model. The lower crust is the third material source; however, the type of lower crustal protolith involved in the igneous process is still not defined, which makes difficult to estimate its role in the petrogenetic processes.

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

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

    The formation and evolution of the continental crust during the Precambrian, and in particular during the Archaean eon (4.0-2.5 Ga), is still a matter of debate. In particular, it is not yet clear in which tectonic environment the genesis of crust took place and how the large volume of granitoid rocks that form ~70% of the Archaean crust were extracted from the mantle. Many studies highlighted that radiogenic isotope systems, especially Lu-Hf and Sm-Nd, are powerful tools to unravel the respective extent of crustal growth and recycling in Archaean terranes. This work presents coupled Hf and Nd isotope data (analyzed both in situ in accessory minerals and in whole rock samples) of Meso- to Neoarchaean granitoids, applied to unravel the processes of crust formation and evolution of the Pietersburg crustal block in South Africa. This crustal segment, the northermost one of the Archaean Kaapvaal Craton, is separated from older crust (3.65-3.10 Ga) by a large-scale suture zone, and the processes related to amalgamation of both blocks and their subsequent evolution are still unclear. The Pietersburg block is made up of a wide range of Archaean granitoid rocks, including tonalite-trondhjemite-granodiorite (TTG) series, high-K monzogranites as well as (grano)diorites belonging to the so-called "sanukitoid" group [1], all intruded by late Paleoproterozoic alkaline complexes. Age determinations highlighted two stages of granitoid formation: (1) TTG magmatism took place episodically over >400 Ma between 3.34 and 2.89 Ga, with a major pulse at 2.97-2.90 Ga; while (2) all the other (high-K) granitoid types emplaced subsequently between 2.84 and 2.69 Ga before a long magmatic shutdown until the intrusion of alkaline complexes at ~2.00 Ga [2-3]. Isotope systematics reveal that these two stages are related to juvenile crust formation and crust reworking, respectively. Indeed, all Hf-Nd isotope data from TTG gneisses are suprachondritic, pointing to a juvenile origin and precluding

  4. Geochemical evidence of groundwater flow paths and the fate and transport of constituents of concern in the alluvial aquifer at Fort Wingate Depot Activity, New Mexico, 2009

    USGS Publications Warehouse

    Robertson, Andrew J.; Henry, David W.; Langman, Jeffery B.

    2013-01-01

    As part of an environmental investigation at Fort Wingate Depot Activity, New Mexico, the U.S. Geological Survey, in cooperation with the U.S. Army Corps of Engineers, interpreted aqueous geochemical concentrations to better understand the groundwater flow paths and the fate and transport of constituents of concern in the alluvial aquifer underlying the study area. The fine-grained nature of the alluvial matrix creates a highly heterogeneous environment, which adds to the difficulty of characterizing the flow of groundwater and the fate of aqueous constituents of concern. The analysis of the groundwater geochemical data collected in October 2009 provides evidence that is used to identify four groundwater flow paths and their extent in the aquifer and indicates the dominant attenuation processes for the constituents of concern. The extent and interaction of groundwater flow paths were delineated by the major ion concentrations and their relations to each other. Four areas of groundwater recharge to the study area were identified based on groundwater elevations, hydrogeologic characteristics, and geochemical and isotopic evidence. One source of recharge enters the study area from the saturated alluvial deposits underlying the South Fork of the Puerco River to the north of the study area. A second source of recharge is shown to originate from a leaky cistern containing production water from the San Andres-Glorieta aquifer. The other two sources of recharge are shown to enter the study area from the south: one from an arroyo valley draining an area to the south and one from hill-front recharge that passes under the reported release of perchlorate and explosive constituents. The spatial extent and interaction of groundwater originating from these various sources along identified flow paths affect the persistence and attenuation of constituents of concern. It was determined that groundwater originating in the area of a former explosives’ wash-out operation and an

  5. First findings of Paleo- and Mesoarchean zircons in the rocks from the Central Arctic province of oceanic rises as an evidence of the ancient continental crust

    NASA Astrophysics Data System (ADS)

    Sergeev, S. A.; Presnyakov, S. L.; Antonov, A. V.; Belyatsky, B. V.; Rodionov, N. V.; Shevchenko, S. S.

    2015-07-01

    This report presents the results of local U-Pb zircon dating (SIMS SHRIMP II) for a sample of migmatite gneiss dredged on the western slope of Alpha Ridge in the Arctic Ocean in the course of the "Arktika-2012" Russian polar expedition. The distribution of U-Pb ages of the examined zircon points to the Early Precambrian origin of this gneiss, for the bulk of the zircon was crystallized at least 3450 Ma ago from a magmatic melt under acidic volcanism at the primary crust formation. Zircon of the second generation was crystallized 3300 Ma ago under the remelting of acid volcanics and appearance of migmatite gneisses under the amphibolite facies of metamorphism. Most likely, a partial recrystallization of zircon and formation of microfolded structures and foliation took place 3000 Ma ago at the stage of rocks deformation. The latest zircon was formed 1900 Ma ago from the crust fluid or melt under the low-gradient metamorphism. In view of the possibility of the appearance of the treated clastogenic gneiss fragment under current oceanic erosion, the obtained results allow one to affirm that the occurrence of a fragment of the most ancient sialic continental crust formed at least 3450 Ma ago is possible at the submarine rises of the Arctic Ocean (Alpha Ridge and the Mendeleev Rise).

  6. Geochronological and Geochemical evidence of amphibolite from the Hualong Group, northwest China: Implication for the early Paleozoic accretionary tectonics of the Central Qilian belt

    NASA Astrophysics Data System (ADS)

    Wang, Tao; Wang, Zongqi; Yan, Zhen; Ma, Zhenhui; He, Shengfei; Fu, Changlei; Wang, Dongsheng

    2016-04-01

    The Hualong Group, located in the Central Qilian belt, northwest China, consists mainly of schist, amphibolite, quartzite, and marble, ranging from greenschist to amphibolite facies metamorphism. On the basis of the medium-grade metamorphism, the group has been considered to comprise Proterozoic basement rocks. In this study, geochemical, Sr-Nd isotopic, and zircon U-Pb geochronological analyses were performed on lentoid amphibolites from the Hualong Group, to characterize their age, petrogensis, and tectonic setting. Uranium-lead zircon dating of amphibolite revealed a formation age of 456 ± 2 Ma and a metamorphic age of 440 ± 1 Ma. Major, trace, and rare earth element data indicate that the amphibolites are predominantly basaltic-andesitic to andesitic rocks, with island arc affinities. The trace element patterns show enrichment in large-ion lithophile elements and depletion in high field strength elements relative to the N-MORB which confirm their island arc signatures. Obviously enriched light REEs ((La/Yb)N = 2.5-16.9) to chondrite normalized REE patterns further support this interpretation. The εNd(t) values for the amphibolites range from 4.6 to + 2.1, indicating subducted sediments as a larger endmember in the source. Geochemical data for these rocks suggest an island arc setting, and the rocks were derived from the depleted mantle that was enriched by melts of subducted sediments in an active continental margin setting at ca. 456 Ma. Together with regional evidence it suggests that the Hualong Group is an accretionary complex that was incorporated into the Central Qilian belt during the 440-400 Ma orogenic event.

  7. Petrogenesis of coeval silica-saturated and silica-undersaturated alkaline rocks: Mineralogical and geochemical evidence from the Saima alkaline complex, NE China

    NASA Astrophysics Data System (ADS)

    Zhu, Yu-Sheng; Yang, Jin-Hui; Sun, Jin-Feng; Zhang, Ji-Heng; Wu, Fu-Yuan

    2016-03-01

    A combined study of zircon U-Pb ages, mineral chemistry, whole-rock elements and Sr-Nd-Hf isotopes was carried out for the Saima alkaline complex in the northeastern China, in order to investigate the source and petrogenesis of coeval silica-saturated and silica-undersaturated alkaline rocks. The Saima alkaline complex consists of nepheline syenites, quartz-bearing syenites and alkaline volcanic rocks (i.e., phonolite and trachyte), with minor mafic dikes and carbonatitic veins. Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) and secondary ion mass spectrometry (SIMS) zircon U-Pb dating gives consistent ages of 230-224 Ma for these rocks, suggesting that they are coeval. All alkaline rocks in the Saima complex are enriched in large ion lithophile elements (LILEs) and light rare earth elements (LREEs), and depleted in high field strength elements (HFSEs) with significant negative Nb, Ta and Ti anomalies. Geochemical data and Sr-Nd-Hf isotopic compositions indicate that the various alkaline rocks were all derived from partial melting of an ancient, re-enriched lithospheric mantle in the garnet stability field, but experienced variable siliceous- or carbonate-rich crustal contamination. Based on petrographic evidence, mineral compositions, and whole-rock geochemical data, two distinct magmatic evolutionary trends are proposed to explain the coeval emplacement of the various rock types within the Saima alkaline complex. The silica-undersaturated rocks (nepheline syenites and phonolites) result from alkali feldspar + apatite + titanite crystal fractionation of an alkaline mafic parental melt combined with assimilation of marine carbonate host rocks. In contrast, the generation of silica-saturated rocks (quartz-bearing syenites and trachytes) may be attributed to subsequent and continued clinopyroxene + apatite + biotite crystal fractionation coupled with assimilation of siliceous sediments.

  8. Mesoproterozoic continental arc magmatism and crustal growth in the eastern Central Tianshan Arc Terrane of the southern Central Asian Orogenic Belt: Geochronological and geochemical evidence

    NASA Astrophysics Data System (ADS)

    He, Zhen-Yu; Klemd, Reiner; Zhang, Ze-Ming; Zong, Ke-Qing; Sun, Li-Xin; Tian, Zuo-Lin; Huang, Bo-Tao

    2015-11-01

    Numerous microcontinents are known to occur in the Central Asian Orogenic Belt (CAOB), one of the largest accretionary orogens and the most significant area of Paleozoic crustal growth in the world. The evolution of the Precambrian crust in these microcontinents is central to understanding the accretionary and collisional tectonics of the CAOB. Here, we present systematic zircon U-Pb dating and Hf isotope studies of Mesoproterozoic gneissic granitoids from the eastern Central Tianshan Arc Terrane (CTA) of the southern CAOB. The investigated intermediate to felsic (SiO2 = 60.48-78.92 wt.%) granitoids belong to the calcic- to calc-alkaline series and usually have pronounced negative Nb, Ta and Ti anomalies, relative enrichments of light rare earth elements (LREEs) and large ion lithophile elements (LILEs) while heavy rare earth elements (HREEs) and high field strength elements (HFSEs) are depleted, revealing typical active continental margin magmatic arc geochemical characteristics. These spatially-distant rocks show consistent zircon U-Pb crystallization ages from ca. 1.45 to 1.40 Ga and thus constitute a previously unknown Mesoproterozoic continental magmatic arc covering hundreds of kilometers in the eastern segment of the CTA. Furthermore the high and mainly positive zircon εHf(t) values between - 1.0 and + 8.6 and the zircon Hf model ages of 1.95 to 1.55 Ga, which are slightly older than their crystallization ages, suggest that they were mainly derived from rapid reworking of juvenile material with a limited input of an ancient crustal component. Therefore, the formation of these granitoids defines an extensive Mesoproterozoic intermediate to felsic, subduction-related intrusive magmatic arc activity that was active from at least 1.45 to 1.40 Ga, involving significant juvenile continental growth in the eastern segment of the CTA. Furthermore the zircon U-Pb and Hf isotopic data challenge the common belief that the CTA was part of the Tarim Craton during Paleo

  9. Paleozoic subduction erosion involving accretionary wedge sediments in the South Tianshan Orogen: Evidence from geochronological and geochemical studies on eclogites and their host metasediments

    NASA Astrophysics Data System (ADS)

    Liu, Xin; Su, Wen; Gao, Jun; Li, Jilei; Jiang, Tuo; Zhang, Xi; Ge, Xiaomei

    2014-12-01

    Geochronological and geochemical data regarding eclogites and their metasedimentary host rocks exposed in two localities of the South Tianshan (U)HP-LT metamorphic belt are presented to reveal the protolith of the eclogites and the provenance of the metasediments. The rim domains of zircon grains from the eclogites contain omphacite, phengite and rutile inclusions and give a U-Pb Concordia age of 321.4 ± 2.4 Ma, representing the peak of eclogite-facies metamorphism. The core domains of zircon grains with magmatic oscillatory zoning yield a U-Pb Concordia age of 453.9 ± 9.4 Ma, suggesting a Late Ordovician age for the eclogites' protolith. Furthermore, the inherited cores of some zircon grains have apparent U-Pb ages between 609 Ma and 2305 Ma, implying the involvement of the Precambrian basement in the formation of the eclogites' protolith. The depletion of high field strength elements and the trace element ratios indicate the eclogite protolith's continental arc affinity. The zircon U-Pb age data of the high-pressure micaschists yield seven age groups ranging from 401 Ma to 3201 Ma and cluster at a pronounced peak of ~ 445 Ma. The major and trace element compositions of the micaschists overlap those of the average upper continental crust. The protolith of the micaschist seems to have formed at an accretionary wedge, which is predominantly composed of sediments derived from Ordovician-Silurian arc-type magmatic rocks and Precambrian basement rocks in an active continental margin. The basic blocks represented by the protolith of the eclogites were most likely scraped from the basement of a continental arc by basal erosion during the subduction of the South Tianshan Ocean in Late Paleozoic. At the same time, the fragments composing the micaschists' protolith are believed to have been dragged into the subduction channel by the frontal erosion of the accretionary prism. Both the basic blocks and the sediment fragments were forced into the subduction channel, mingled

  10. Geophysical and Geochemical Evidence For Methane Venting at Large Gas Blowouts Along the US Mid-Atlantic Shelf Edge

    NASA Astrophysics Data System (ADS)

    Newman, K. K.; Cormier, M.; Driscoll, N.; Hill, J.; Kastner, M.; Singh, H.; Weissel, J.

    2005-12-01

    Kilometer-scale, elongate gas blowouts are present at the edge of the North Carolina/Virginia continental shelf. We conducted a detailed survey in July 2004 to determine if fluids are venting at the blowouts site and to understand their origin. The Autonomous Underwater Vehicle (AUV) SeaBED collected underway data 3 m above the seafloor using a METS methane sensor, CTD and color digital camera. In addition, piston cores and hydrocasts were acquired for geochemical analysis of pore waters and the water column. Based on the AUV data, salinity and temperature exhibit a negative correlation with dissolved methane concentration. However, the raw METS measurements of dissolved methane lag behind the salinity and temperature anomalies, progressively ramping up or down compared to the impulse signal recorded for the salinity and temperature anomalies. This type of response is consistent with that expected for diffusion across a membrane, which is a characteristic of the METS sensor. Using the assumption that diffusion is responsible for the observed lag we calculated the time constant of the system to be approximately 11 minutes and used that to correct the instrument response function for the METS sensor. The corrected dissolved methane measurements show concentrations of 50-200 nM, values well above that of normal seawater (2-4 nM). Hydrocast water samples indicate methane maxima between 100 and 130 m with concentrations up to 43 nM. The positive anomalies, both from the AUV and hydrocast data, are concentrated on the upper parts of the blowout walls, extend westward onto the shelf and are observed up to 70 m depth in the water column. Methane anomalies are not generally present in the axes of the blowouts, suggesting that methane presently discharges along the blowout walls rather than through the floors. To determine if density driven stratification is present, and assuming that salinity and temperature are good proxies for methane concentration, we examined vertical

  11. Geochemical and isotope characterization of geothermal spring waters in Sri Lanka: Evidence for steeper than expected geothermal gradients

    NASA Astrophysics Data System (ADS)

    Chandrajith, Rohana; Barth, Johannes A. C.; Subasinghe, N. D.; Merten, Dirk; Dissanayake, C. B.

    2013-01-01

    SummarySeven geothermal springs from the Precambrian high-grade metamorphic terrain of Sri Lanka were investigated to assess their formation processes and to determine reservoir temperatures based on their chemical compositions. Silica-based geothermometric calculations for the Marangala and Nelumwewa springs showed the highest average reservoir temperatures of 122 °C and 121 °C, respectively. Samples of low temperature (<35 °C) groundwater from nearby springs, piezometers and open wells were also collected for comparison. All samples were analyzed for their major and trace element compositions as well as stable isotope ratios 2H/1H and 18O/16O (expressed as δ2HH2O and δ18OH2O). Discharge temperatures of the thermal waters varied from 39-62 °C. These waters showed low concentrations of selected trace elements (Fe < 0.09; Mn < 0.04; Cu < 0.01; Cr < 0.01; As < 0.025 mg/L)) and were also comparable to that of non-geothermal groundwaters. Stable isotope compositions of geothermal waters ranged from -6.5 to -5.0‰ for δ18OH2O and between -39 ‰ to -28 ‰ for δ2HH2O. In the non-geothermal waters, the isotope values were almost identical within the analytical uncertainties of 0.1‰ and 1‰ for δ2HH2O and δ18OH2O, respectively. In addition, all isotope ratios of geothermal and non-geothermal water samples scattered around the local meteoric water lines for the dry and intermediate climatic regions of Sri Lanka, thus indicating origin from precipitation without further influences of evaporation or water rock interaction. This similarity to the local meteoric water lines also makes influences of seawater an unlikely factor. Close matches of geochemical and isotope data from geothermal and corresponding non-geothermal waters confirm the hypothesis of a common source. The proposed model for Sri Lanka subsurface waters is that rainfall from the dry and/or intermediate climatic zones percolates with little time delay downward through structurally weaker zones in

  12. The Oceanic Crust.

    ERIC Educational Resources Information Center

    Francheteau, Jean

    1983-01-01

    The earth's oceanic crust is created and destroyed in a flow outward from midocean ridges to subduction zones, where it plunges back into the mantle. The nature and dynamics of the crust, instrumentation used in investigations of this earth feature, and research efforts/findings are discussed. (JN)

  13. Lower-crustal xenoliths from Jurassic kimberlite diatremes, upper Michigan (USA): Evidence for Proterozoic orogenesis and plume magmatism in the lower crust of the southern Superior Province

    USGS Publications Warehouse

    Zartman, Robert E.; Kempton, Pamela D.; Paces, James B.; Downes, Hilary; Williams, Ian S.; Dobosi, Gábor; Futa, Kiyoto

    2013-01-01

    Jurassic kimberlites in the southern Superior Province in northern Michigan contain a variety of possible lower-crustal xenoliths, including mafic garnet granulites, rare garnet-free granulites, amphibolites and eclogites. Whole-rock major-element data for the granulites suggest affinities with tholeiitic basalts. P–T estimates for granulites indicate peak temperatures of 690–730°C and pressures of 9–12 kbar, consistent with seismic estimates of crustal thickness in the region. The granulites can be divided into two groups based on trace-element characteristics. Group 1 granulites have trace-element signatures similar to average Archean lower crust; they are light rare earth element (LREE)-enriched, with high La/Nb ratios and positive Pb anomalies. Most plot to the left of the geochron on a 206Pb/€204Pb vs 207Pb/€204Pb diagram, and there was probably widespread incorporation of Proterozoic to Archean components into the magmatic protoliths of these rocks. Although the age of the Group 1 granulites is not well constrained, their protoliths appear to be have been emplaced during the Mesoproterozoic and to be older than those for Group 2 granulites. Group 2 granulites are also LREE-enriched, but have strong positive Nb and Ta anomalies and low La/Nb ratios, suggesting intraplate magmatic affinities. They have trace-element characteristics similar to those of some Mid-Continent Rift (Keweenawan) basalts. They yield a Sm–Nd whole-rock errorchron age of 1046 ± 140 Ma, similar to that of Mid-Continent Rift plume magmatism. These granulites have unusually radiogenic Pb isotope compositions that plot above the 207Pb/€204Pb vs 206Pb/€204Pb growth curve and to the right of the 4·55 Ga geochron, and closely resemble the Pb isotope array defined by Mid-Continent Rift basalts. These Pb isotope data indicate that ancient continental lower crust is not uniformly depleted in U (and Th) relative to Pb. One granulite xenolith, S69-5, contains quartz, and has a

  14. Evidence for Thin Oceanic Crust on the Extinct Aegir Ridge, Norwegian Basin, N.E. Atlantic Derived from Satellite Gravity Inversion

    NASA Astrophysics Data System (ADS)

    Greenhalgh, E. E.; Kusznir, N. J.

    2006-12-01

    Satellite gravity inversion incorporating a lithosphere thermal gravity correction has been used to map crustal thickness and lithosphere thinning factor for the N.E. Atlantic. The inversion of gravity data to determine crustal thickness incorporates a lithosphere thermal gravity anomaly correction for both oceanic and continental margin lithosphere. Predicted crustal thicknesses in the Norwegian Basin are between 7 and 4 km on the extinct Aegir oceanic ridge which ceased sea-floor spreading in the Oligocene. Crustal thickness estimates do not include a correction for sediment thickness and are upper bounds. Crustal thicknesses determined by gravity inversion for the Aegir Ridge are consistent with recent estimates derived using refraction seismology by Breivik et al. (2006). Failure to incorporate a lithosphere thermal gravity anomaly correction produces an over-estimate of crustal thickness. Oceanic crustal thicknesses within the Norwegian Basin are predicted by the gravity inversion to increase to 9-10 km eastwards towards the Norwegian (Moere) and westwards towards the Jan Mayen micro-continent, consistent with volcanic margin continental breakup at the end of the Palaeocene. The observation (from gravity inversion and seismic refraction studies) of thin oceanic crust produced by the Aegir ocean ridge in the Oligocene has implications for the temporal evolution of asthenosphere temperature under the N.E. Atlantic during the Tertiary. Thin Oligocene oceanic crust may imply cool (normal) asthenosphere temperatures during the Oligocene in contrast to elevated asthenosphere temperatures in the Palaeocene and Miocene-Recent as indicated by volcanic margin formation and the formation of Iceland respectively. Gravity inversion also predicts a region of thin oceanic crust to the west of the northern part of the Jan Mayen micro-continent and to the east of the thicker oceanic crust currently being formed at the Kolbeinsey Ridge. Thicker crust (c.f. ocean basins) is

  15. Diapirs and cantaloupes: Layering and overturn of Triton's crust

    NASA Technical Reports Server (NTRS)

    Schenk, P.; Jackson, M. P. A.

    1993-01-01

    It has recently been proposed that cantaloupe terrain formed as a result of instability and overturn (i.e., diapirism) of Triton's crust. Morphologic evidence implicates compositional layering within Triton's crust as the driving mechanism for the overturn. Here, we review the morphologic evidence for this origin and evaluate some of the implications.

  16. Absence of geochemical evidence for an impact event at the Bølling–Allerød/Younger Dryas transition

    PubMed Central

    Paquay, François S.; Goderis, Steven; Ravizza, Greg; Vanhaeck, Frank; Boyd, Matthew; Surovell, Todd A.; Holliday, Vance T.; Haynes, C. Vance; Claeys, Philippe

    2009-01-01

    High concentrations of iridium have been reported in terrestrial sediments dated at 12.9 ka and are interpreted to support an extraterrestrial impact event as the cause of the observed extinction in the Rancholabrean fauna, changes in the Paleoindian cultures, and the onset of the Younger Dryas cooling [Firestone RB, et al. (2007) Proc Natl Acad Sci USA 104:16016–16021]. Here, we report platinum group element (PGE: Os, Ir, Ru, Rh, Pt, Pd), gold (Au) concentrations, and 187Os/188Os ratios in time-equivalent terrestrial, lacustrine, and marine sections to seek robust evidence of an extraterrestrial contribution. First, our results do not reproduce the previously reported elevated Ir concentrations. Second, 187Os/188Os isotopic ratios in the sediment layers investigated are similar to average crustal values, indicating the absence of a significant meteoritic Os contribution to these sediments. Third, no PGE anomalies distinct from crustal signatures are present in the marine record in either the Gulf of California (DSDP 480, Guaymas Basin) or the Cariaco Basin (ODP 1002C). Our data show no evidence of an extraterrestrial (ET)-PGE enrichment anomaly in any of the investigated depositional settings investigated across North America and in one section in Belgium. The lack of a clear ET-PGE signature in this sample suite is inconsistent with the impact of a large chondritic projectile at the Bølling–Allerød/Younger Dryas transition. PMID:20007789

  17. Mineralogical and geochemical evidence for hydrothermal activity at the west wall of 12°50′N core complex (Mid-Atlantic ridge): a new ultramafic-hosted seafloor hydrothermal deposit?

    USGS Publications Warehouse

    Dekov, Vesselin; Boycheva, Tanya; Halenius, Ulf; Billstrom, Kjell; Kamenov, George D.; Shanks, Wayne C.; Stummeyer, Jens

    2011-01-01

    Dredging along the west wall of the core complex at 12°50′N Mid-Atlantic Ridge sampled a number of black oxyhydroxide crusts and breccias cemented by black and dark brown oxyhydroxide matrix. Black crusts found on top of basalt clasts (rubble) are mainly composed of Mn-oxides (birnessite, 10-Å manganates) with thin films of nontronite and X-ray amorphous FeOOH on their surfaces. Their chemical composition (low trace- and rare earth-element contents, high Li and Ag concentrations, rare earth element distribution patterns with negative both Ce and Eu anomalies), Sr–Nd–Pb-isotope systematic and O-isotope data suggest low-temperature (~ 20 °C) hydrothermal deposition from a diffuse vent area on the seafloor. Mineralogical, petrographic and geochemical investigations of the breccias showed the rock clasts were hydrothermally altered fragments of MORBs. Despite the substantial mineralogical changes caused by the alteration the Sr–Nd–Pb-isotope ratios have not been significantly affected by this process. The basalt clasts are cemented by dark brown and black matrix. Dark brown cement exhibits geochemical features (very low trace- and rare earth- element contents, high U concentration, rare earth element distribution pattern with high positive Eu anomaly) and Nd–Pb-isotope systematics (similar to that of MORB) suggesting that the precursor was a primary, high-temperature Fe-sulfide, which was eventually altered to goethite at ambient seawater conditions. The data presented in this work points towards the possible existence of high- and low-temperature hydrothermal activity at the west wall of the core complex at 12°50′N Mid-Atlantic Ridge. Tectonic setting at the site implies that the proposed hydrothermal field is possibly ultramafic-hosted.

  18. Evidence from meimechites and other low-degree mantle melts for redox controls on mantle-crust fractionation of platinum-group elements

    PubMed Central

    Mungall, James E.; Hanley, Jacob J.; Arndt, Nicholas T.; Debecdelievre, Anne

    2006-01-01

    Understanding of the geochemistry of the chalcophile elements [i.e., Os, Ir, Ru, Pt, Pd (platinum-group elements), and Au, Cu, Ni] has been informed for at least 20 years by the common assumption that when crust-forming partial melts are extracted from the upper mantle, sulfide liquid in the restite sequesters chalcophile elements until the extent of partial melting exceeds ≈25% and all of the sulfide has been dissolved in silicate melt [Hamlyn, P. R. & Keays, R. R. (1985) Geochim. Cosmochim. Acta 49, 1797–1811]. Here we document very high, unfractionated, chalcophile element concentrations in small-degree partial melts from the mantle that cannot be reconciled with the canonical residual sulfide assumption. We show that the observed high, unfractionated platinum-group element concentrations in small-degree partial melts can be attained if the melting takes place at moderately high oxygen fugacity, which will reduce the amount of sulfide due to the formation of sulfate and will also destabilize residual monosulfide solid solution by driving sulfide melts into the spinel-liquid divariant field. Magmas formed at high oxygen fugacity by small degrees of mantle melting can be important agents for the transfer of chalcophile elements from the upper mantle to the crust and may be progenitors of significant ore deposits of Pt, Pd, and Au. PMID:16908861

  19. Impact of recent coastal development and human activities on Nha Trang Bay, Vietnam: evidence from a Porites lutea geochemical record

    NASA Astrophysics Data System (ADS)

    Nguyen, A. D.; Zhao, J.-x.; Feng, Y.-x.; Hu, W.-p.; Yu, K.-f.; Gasparon, M.; Pham, T. B.; Clark, T. R.

    2013-03-01

    Nha Trang Bay (NTB) is located on the Central Vietnam coast, western South China Sea. Recent coastal development of Nha Trang City has raised public concern over an increasing level of pollution within the bay and degradation of nearby coral reefs. In this study, multiple proxies (e.g., trace metals, rare earth elements (REEs), and Y/Ho) recorded in a massive Porites lutea coral colony were used to reconstruct changes in seawater conditions in the NTB from 1995 to 2009. A 14-year record of REEs and other trace metals revealed that the concentrations of terrestrial trace metals have increased dramatically in response to an increase in coastal development projects such as road, port, and resort constructions, port and river dredging, and dumping activities since 2000. The effects of such developmental processes are also evident in changes in REE patterns and Y/Ho ratios through time, suggesting that both parameters are critical proxies for marine pollution.

  20. Cretaceous crust-mantle interaction and tectonic evolution of Cathaysia Block in South China: Evidence from pulsed mafic rocks and related magmatism

    NASA Astrophysics Data System (ADS)

    Li, Bin; Jiang, Shao-Yong; Zhang, Qian; Zhao, Hai-Xiang; Zhao, Kui-Dong

    2015-10-01

    Cretaceous tectono-magmatic evolution of the Cathaysia Block in South China is important but their mechanism and geodynamics remain highly disputed. In this study we carried out a detailed geochemical study on the recently found Kuokeng mafic dikes in the western Fujian Province (the Interior Cathaysia Block) to reveal the petrogenesis and geodynamics of the Cretaceous magmatism. Kuokeng mafic dikes were emplaced in three principal episodes: ~ 129 Ma (monzogabbro), ~ 107 Ma (monzodiorite), and ~ 97 Ma (gabbro). Geochemical characteristics indicate that the monzogabbros were derived from the unmodified mantle source, while gabbros were likely derived from metasomatized mantle by subducted slab (fluids and sediments). Sr-Nd isotope compositions indicate that the parental magmas of the monzodiorites were generated by mixing of enriched, mantle-derived, mafic magmas and felsic melts produced by partial melting of crustal materials. Until the Early Cretaceous (~ 123 Ma), the dominant ancient Interior Cathaysia lithospheric mantle exhibited insignificant subduction signature, indicating the melting of asthenospheric mantle and the consequent back-arc extension, producing large-scale partial melting of the crustal materials under the forward subduction regime of the paleo-Pacific plate. The monzodiorites and gabbros appear to be associated with northwestward subduction of Pacific plate under an enhanced lithospheric extensional setting, accompanying with mantle modification, which triggered shallower subduction-related metasomatically enriched lithospheric mantle to melt partially. After ca. 110 Ma, the coastal magmatic belts formed due to a retreat and rollback of the subducting Pacific Plate underneath SE China in the continental margin arc system.

  1. Formation of lower continental crust by relamination of buoyant arc lavas and plutons

    NASA Astrophysics Data System (ADS)

    Kelemen, Peter B.; Behn, Mark D.

    2016-03-01

    The formation of the Earth's continents is enigmatic. Volcanic arc magmas generated above subduction zones have geochemical compositions that are similar to continental crust, implying that arc magmatic processes played a central role in generating continental crust. Yet the deep crust within volcanic arcs has a very different composition from crust at similar depths beneath the continents. It is therefore unclear how arc crust is transformed into continental crust. The densest parts of arc lower crust may delaminate and become recycled into the underlying mantle. Here we show, however, that even after delamination, arc lower crust still has significantly different trace element contents from continental lower crust. We suggest that it is not delamination that determines the composition of continental crust, but relamination. In our conceptual model, buoyant magmatic rocks generated at arcs are subducted. Then, upon heating at depth, they ascend and are relaminated at the base of the overlying crust. A review of the average compositions of buoyant magmatic rocks -- lavas and plutons -- sampled from the Aleutians, Izu-Bonin-Marianas, Kohistan and Talkeetna arcs reveals that they fall within the range of estimated major and trace elements in lower continental crust. Relamination may thus provide an efficient process for generating lower continental crust.

  2. Geochemical evidences for two chondritic-like cometary or asteroidal impacts before and at the K/T boundary

    NASA Technical Reports Server (NTRS)

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

    1993-01-01

    A number of geological and palaeontological evidences support multiple impacts of cometary showers within a short time (approximately 1-3 Ma) and their connection with mass extinctions. Observations include clustered crater ages, stratigraphic horizons of impact ejecta closely spaced in time, and evidence for stepwise mass extinctions spanning intervals of 1-3 Ma. For the K/T boundary, three candidates, Popigai, Manson, and Yucatan, have been proposed as impact craters. Two distinct strata at the K/T boundary in western North America have been interpreted as evidence for two sequential impacts. If multiple impacts occurred within a time span of about 1 Ma then multiple Ir enrichments should be observed. DSDP Hole 577B on the Shatsky Plateau in the northern Pacific at K/T time is the first site. Samples contain approximately greater than 97 percent CaCO3, which exhibit clear chemical signals associated with asteroidal/cometary impact. Ir, Fe, and Cr data are presented. From the Th-normalized data, two satellite peaks below the major peak at 78 cm and 81 cm of 577B-1-4 are clearly shown. The major Ir peak (K/T boundary) is at 72 cm. Fe and Cr, from C1-like impactor ejecta fallout, also show two peaks at the same positions. For hole 738C on the southern Kerguelen Plateau, Ir values reach a peak concentration of 18 ppb in the clay layer at 96.0-96.2 cm in section 20R-5, and gradually tail off. In the sample 115 cm above the boundary, Ir concentrations have still not reached background levels. From the Ir peak downward to the lowermost sample analyzed at 102 cm, the Ir concentration is still as high as 1.7 ppb. From the Th-normalized data, we observe a small Ir/Th peak at 100-101 cm. Though this peak is within the error margin, the trend is clear. Fe and Cr exhibit the same pattern. The third case is Hole 690C on the Queen Maud Ridge. Again, the Ir/Th plot indicates the strong possibility of satellite peaks at approximately 52 cm. The main peak is at 39-40 cm. For the

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

    NASA Astrophysics Data System (ADS)

    Xu, Yi-Gang

    2014-10-01

    Major, trace element and Sr-Nd-Pb isotopic data of basalts emplaced during 90-40 Ma in the North and Northeast China are compiled in this review, with aims of constraining their petrogenesis, and by inference the evolution of the North China Craton during the late Cretaceous and early Cenozoic. Three major components are identified in magma source, including depleted component I and II, and an enriched component. The depleted component I, which is characterized by relatively low 87Sr/86Sr (<0.7030), moderate 206Pb/204Pb (18.2), moderately high εNd (∼4), high Eu/Eu∗ (>1.1) and HIMU-like trace element characteristics, is most likely derived from gabbroic cumulate of the oceanic crust. The depleted component II, which distinguishes itself by its high εNd (∼8) and moderate 87Sr/86Sr (∼0.7038), is probably derived from a sub-lithospheric ambient mantle. The enriched component has low εNd (2-3), high 87Sr/86Sr (>0.7065), low 206Pb/204Pb (17), excess Sr, Rb, Ba and a deficiency of Zr and Hf relative to the REE. This component is likely from the basaltic portion of the oceanic crust, which is variably altered by seawater and contains minor sediments. Comparison with experimental melts and trace element modeling suggest that these recycled oceanic components may be in form of garnet pyroxenite/eclogite. These components are young (<0.5 Ga) and show an Indian-MORB isotopic character. Given the share of this isotopic affinity by the extinct Izanaghi-Pacific plate, currently stagnated within the mantle transition zone, we propose that it ultimately comes from the subducted Pacific slab. Eu/Eu∗ and 87Sr/86Sr of the 90-40 Ma magmas increases and decreases, respectively, with decreasing emplacement age, mirroring a change in magma source from upper to lower parts of subducted oceanic crust. Such secular trends are created by dynamic melting of a heterogeneous mantle containing recycled oceanic crust. Due to different melting temperature of the upper and lower ocean

  4. Geochemical evidence for Se mobilization by the weathering of pyritic shale, San Joaquin Valley, California, U.S.A.

    USGS Publications Warehouse

    Presser, T.S.; Swain, W.C.

    1990-01-01

    Acidic (pH 4) seeps issue from the weathered Upper Cretaceous-Paleocene marine sedimentary shales of the Moreno Formation in the semi-arid Coast Ranges of California. The chemistry of the acidic solutions is believed to be evidence of current reactions ultimately yielding hydrous sodium and magnesium sulfate salts, e.g. mirabilite and bloedite, from the oxidation of primary pyrite. The selenate form of Se is concentrated in these soluble salts, which act as temporary geological sinks. Theoretically, the open lattice structures of these hydrous minerals could incorporate the selenate (SeO4-2) anion in the sulfate (SO4-2) space. When coupled with a semi-arid to arid climate, fractional crystallization and evaporative concentration can occur creating a sodium-sulfate fluid that exceeds the U.S. Environmental Protection Agency limit of 1000 ??g l-1 for a toxic Se waste. The oxidative alkaline conditions necessary to ensure the concentration of soluble selenate are provided in the accompanying marine sandstones of the Panoche and Lodo Formations and the eugeosynclinal Franciscan assemblage. Runoff and extensive mass wasting in the area reflect these processes and provide the mechanisms which transport Se to the farmlands of the west-central San Joaquin Valley. Subsurface drainage from these soils consequently transports Se to refuge areas in amounts elevated to cause a threat to wildlife. ?? 1990.

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

    The formation of the most recent flood basalt province on Earth, the Columbia River Flood Basalts (CRBs) of the northwestern USA, was accompanied by eruptions of several thousand km3 of rhyolite in a short time window from 16.7 to 15 Ma. These rhyolites span from low (+1‰) to high (+11‰) in δ18O values as recorded by major phenocrysts, and alteration-resistant zircons within each rhyolite commonly display diversity of up to 6‰ δ18O, indicative of batch assembly prior to eruption. Significant variation in ɛHf also exists in zircons, ranging from -39 to 0 in rhyolites erupted through the North American cratonic crust, and from -1 to +9 in rhyolites erupted through accreted oceanic terranes to the east of the Sr87/86Sr = 0.706 line. This isotopic diversity cannot be accounted for by fractionation of a CRB-like parent magma, demonstrating that the syn-CRB rhyolites must have been derived from melting of the crust. Abundant low-δ18Omelt values among syn-CRB rhyolites further constrains this crustal melting to shallow depths of 5-10 km, due to the shallow depths of the necessary hydrothermal alteration of the protolith. By contrast, high-δ18O rhyolites must have been formed by remelting of sedimentary or metasedimentary rocks. Low-δ18O rhyolites are also most common in the vicinity of the crustal suture between the thick lithosphere of the Archean craton and the thin lithosphere of the accreted terranes. Thermomechanical modeling suggests that this contrast concentrates crustal heating and deformation, creating pathways for meteoric water to penetrate the crust and cause extensive hydrothermal alteration less than 1 Ma before those same rocks remelt to form low-δ18O rhyolites. Finally, we suggest that this extensive crustal hydrothermal alteration and melting may be typical of continental flood basalt provinces world wide, and particularly when there is syn-volcanic extension.

  6. Geochemical evidence of past earthquakes in sediments of the Reloncaví fjord (Chilean Patagonia) during the last ˜ 1000 years

    NASA Astrophysics Data System (ADS)

    Rebolledo, Lorena; Lange, Carina; Muñoz, Práxedes; Salamanca, Marco

    2014-05-01

    The Chilean fjords are excellent archives of paleoearthquakes, tsunamis and landslides (St-Onge et al., 2012 in Sedimentary Geology 243-244: 89-107). Here we report on new sedimentological and geochemical evidence of past earthquakes in sediments of the Reloncavi fjord, Northern Patagonia (41° S, 72° W), during the last ~1000 years. We recovered four sediment cores from the Reloncaví fjord (RH-5B, RH-5C, RH-6B, RH7B, water depth range = 90-260 m; core length range = 45-75 cm). Age models were based on 210Pb, AMS-14C and the first appearance of the diatom Rhizosolenia setigera cf. pungens in the fossil record as statigraphic marker. The cores span the last ~122 to 800 years of sedimentation with sedimentation rates ranging between 0.1 and 0.24 cm yr-1. The cores revealed evidence of turbidites associated with the historical earthquakes of 1960, 1837, 1737 and 1575 AD, and an earlier period for which there is no historical information, 1200-1400 AD. The turbidites exhibit a grading-up pattern with sand layers, and are characterized by a decrease in organic carbon and biogenic opal, an increase in the C/N molar ratio, negative values of δ13Corg(average -27),and an increase in the relative abundance of Paralia sulcata, a diatom associated with sandy environments, being the turbite layers mainly freshwater in origen. We suggest that these turbidite layers were triggered by past earthquakes that produced movement of land from the cliff areas that surround the Reloncaví fjord. Funding: Project FONDECYT # 11110103 and COPAS Sur-Austral project PFB-31.

  7. Geochemical evidence for a magmatic CO2 degassing event at Mammoth Mountain, California, September-December 1997

    USGS Publications Warehouse

    McGee, K.A.; Gerlach, T.M.; Kessler, R.; Doukas, M.P.

    2000-01-01

    Recent time series soil CO2 concentration data from monitoring stations in the vicinity of Mammoth Mountain, California, reveal strong evidence for a magmatic degassing event during the fall of 1997 lasting more than 2 months. Two sensors at Horseshoe Lake first recorded the episode on September 23, 1997, followed 10 days later by a sensor on the north flank of Mammoth Mountain. Direct degassing from shallow intruding magma seems an implausible cause of the degassing event, since the gas released at Horseshoe Lake continued to be cold and barren of other magmatic gases, except for He. We suggest that an increase in compressional strain on the area south of Mammoth Mountain driven by movement of major fault blocks in Long Valley caldera may have triggered an episode of increased degassing by squeezing additional accumulated CO2 from a shallow gas reservoir to the surface along faults and other structures where it could be detected by the CO2 monitoring network. Recharge of the gas reservoir by CO2 emanating from the deep intrusions that probably triggered deep long-period earthquakes may also have contributed to the degassing event. The nature of CO2 discharge at the soil-air interface is influenced by the porous character of High Sierra soils and by meteorological processes. Solar insolation is the primary source of energy for the Earth atmosphere and plays a significant role in most diurnal processes at the Earth surface. Data from this study suggest that external forcing due largely to local orographic winds influences the fine structure of the recorded CO2 signals.

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

    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

  9. Geochemical and sedimentological evidence of varying intense hurricane activity and precipitation patterns from the Caribbean during the late Holocene

    NASA Astrophysics Data System (ADS)

    Woodruff, J. D.; Donnelly, J. P.; Tierney, J.; Scileppi, E.; Giosan, L.

    2005-12-01

    Coastal lagoons are relatively unique systems because they have the potential for recording both marine and terrestrial paleo-climate signals. An analysis of sediment cores collected from a backbarrier lagoon in the Northeastern Caribbean (Laguna Playa Grande, Vieques, Puerto Rico) reveal paleoclimatic records of intense hurricane activity, sea-level elevation, and shifts in regional precipitation and vegetation. Coarse-grain laminae preserved within the 5000 year fine-grained sedimentary record at the site consist of beach and nearshore sediments and provide evidence of past intense hurricane strikes. Sediment ages based on C-14 dating indicate a relatively constant long-term sedimentation rate in the basin appears to be in steady state with sea-level rise. X-ray fluorescences (XRF) analyses of surficial terrestrial and barrier-beach samples show that they are distinct in their elemental composition with terrestrial samples containing high concentrations of Fe, Ti and Mn, while Ca and Sr dominate the composition for samples collected from the barrier beach. XRF scans of cores collected from the lagoon indicate that the observed coarse-grain strata contain high concentrations of Ca and Sr confirming the seaward origin of this material, while fine-grained sediments are high in Fe, Ti and Mn indicating its terrestrial source. A transition from high to low intense hurricane activity occurred at the site approximately 1250 yr BP. XRF results reveal a dramatic increase in the concentration of terrestrial elements (Ti, Fe and Mn) throughout the basin after this 1250 yr BP horizon indicating an abrupt transition from a more arid to a more humid local climate. An additional increase in Ti, Fe and Mn concentrations is observed within sediments deposited in the 18th century A.D. and is most likely associated with anthropogenic land-use changes. The earlier 1250 yr BP transition from low to high precipitation is consistent with records from other sites within the Caribbean

  10. Multielement geochemical investigations by SRXRF microprobe studies on tectite material: Evidence from the NE-Mexican Cretaceous/Tertiary record

    NASA Astrophysics Data System (ADS)

    Harting, M.; Rickers, K.; Kramar, U.; Simon, R.; Staub, S.; Schulte, P.

    2002-12-01

    excavated by the Chicxulub impact. There is no evidence at the moment that there is a homogeneous origin in the sample material or distribution in the investigated sections. The enrichment of Ce in spherules from the Mesa-Juan Perez section indicates a possible origin from the Yucatan carbonate platform generated by the Chicxulub impact event near the K/T-boundary. Area scans from tektite material of the Bochil section show a clearly zonation in the inner part, dominated by Ba and Sr as well as a alteration margin dominated by secondary CaCO3. Glassy material of the Beloc (Haiti) section is characterised by a homogeneous trace element distribution but shows characteristic differences between Ca-rich and Ca-poor glass. Moreover there is no similarity to material from other sections investigated. A clear differentiation between alteration rims, non-alterated material and mixing of different source materials can be shown by space resolved trace element determination in æm scale of schlieren structures and inclusions.(see also Schulte et al. this volume)

  11. An organic geochemical multi-proxy study of the Paleocene-Eocene Thermal Maximum: Evidence from northern Spain

    NASA Astrophysics Data System (ADS)

    Manners, H. R.; Grimes, S.; Sutton, P.; Domingo, L.; Pancost, R. D.; Leng, M. J.; Twitchett, R. J.; Hart, M. B.; Taylor, K. W.

    2012-12-01

    , profile shape, and onset of the CIE in a localised system. Preliminary results for Claret suggest that the continental δ13CTOC records a lower magnitude excursion than δ13Cn-alkane data, where excursions of up to 8‰ are recorded - one of the largest continental excursions recorded to date. In comparison, there is good agreement between the δ13CTOC and δ13Cn-alkane records for the marine section of Zumaia, where an excursion of ca. 4‰ occurs in both proxies. The apparent enhancement in magnitude of the continental CIE is particularly significant, because unlike other sites4, there is no biomarker evidence for vegetation change, e.g. the average chain length of the n-alkanes, in either the Claret or Zumaia sections. Higher plant-derived biomarker and palynological analysis will serve to further test these results, which will provide the first data of this type in northern Spain, and could have implications for our understanding of the PETM. References 1. Bains, S. et al. 1999. Science, 285, (5428) 724-727. 2. Bowen, G. J. et al. 2001. University of Michigan Papers on Paleontology, 33, 73 - 88. 3. Schmitz, B. and Pujalte, V. 2003. Geology. 689-692. 4. Smith, F. A. et al. 2007. Earth and Planetary Science Letters, 262, (1-2) 50-65. 5. Schouten, S. et al. 2007. Earth and Planetary Science Letters, 258, (3-4) 581 - 592.

  12. Corium crust strength measurements.

    SciTech Connect

    Lomperski, S.; Nuclear Engineering Division

    2009-11-01

    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.

  13. Frozen magma lenses below the oceanic crust.

    PubMed

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

    2005-08-25

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

  14. Oxygen consumption in subseafloor basaltic crust

    NASA Astrophysics Data System (ADS)

    Orcutt, B. N.; Wheat, C. G.; Hulme, S.; Edwards, K. J.; Bach, W.

    2012-12-01

    Oceanic crust is the largest potential habitat for life on Earth and may contain a significant fraction of Earth's total microbial biomass, yet little is known about the form and function of life in this vast subseafloor realm that covers nearly two-thirds of the Earth's surface. A deep biosphere hosted in subseafloor basalts has been suggested from several lines of evidence; yet, empirical analysis of metabolic reaction rates in basaltic crust is lacking. Here we report the first measure of oxygen consumption in young (~ 8 Ma) and cool (<25 degrees C) basaltic crust, calculated from modeling oxygen and strontium profiles in basal sediments collected during Integrated Ocean Drilling Program (IODP) Expedition 336 to 'North Pond', a sediment 'pond' on the western flank of the Mid-Atlantic Ridge (MAR), where vigorous fluid circulation within basaltic crust occurs. Dissolved oxygen concentrations increased towards the sediment-basement interface, indicating an upward diffusional supply from oxic fluids circulating within the crust. A parametric reaction-transport model suggests oxygen consumption rates on the order of 0.5-500 nmol per cubic centimeter fluid per day in young and cool basaltic crust, providing sufficient energy to support a subsurface crustal biosphere.

  15. Hydrotaxis of cyanobacteria in desert crusts.

    PubMed

    Pringault, O; Garcia-Pichel, F

    2004-05-01

    We studied the migration of cyanobacteria in desert crusts from Las Bárdenas Reales (Spain). The crusts were almost exclusively colonized by the filamentous cyanobacterium Oscillatoria, which formed a dense layer approximately 600 microm thick located between 1.5 and 2.1 mm deep. Laboratory and field experiments showed that saturation of the crust with liquid water induced a migration of the cyanobacteria leading to a significant greening of the surface within a few minutes. Under light and rapid evaporation, the green color rapidly disappeared and the crust surface was completely devoid of filaments within 60 min. In contrast, 260 min was required to recover the original white color of the crust when slow evaporation was experimentally imposed. The up and down migration following wetting and drying occurred also in the dark. This demonstrates that light was not a required stimulus. Addition of ATP synthesis inhibitors prevented the cyanobacterium from migrating down into the crust, with filaments remaining on the surface. Therefore, the disappearance of the green color observed during desiccation can only be attributed to an active cyanobacterial motility response to the decrease in the water content. The simplest explanation that can account for the evidence gathered is the presence of a mechanism that links, directly or indirectly, these motility responses to gradients in water content, namely a form of hydrotaxis. PMID:14605777

  16. Early formation of evolved asteroidal crust.

    PubMed

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

    2009-01-01

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

  17. Early formation of evolved asteroidal crust

    NASA Astrophysics Data System (ADS)

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

    2009-01-01

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

  18. Early formation of evolved asteroidal crust.

    PubMed

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

    2009-01-01

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

  19. Frozen magma lenses below the oceanic crust.

    PubMed

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

    2005-08-25

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

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

    NASA Astrophysics Data System (ADS)

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

    2006-12-01

    Subduction of continental crust to HP-UHP metamorphic conditions requires overcoming density contrasts that are unfavorable to deep burial, whereas exhumation of these rocks can be reasonably explained through buoyancy-assisted transport in the subduction channel to more shallow depths. In the western Alps, both continental and oceanic lithosphere has been subducted to eclogite-facies metamorphic conditions. The burial and exhumation histories of these sections of lithosphere bear directly on the dynamics of subduction and the stacking of units within the subduction channel. We address the burial history of the continental crust with high precision U-Pb rutile and Lu-Hf garnet geochronology of the eclogite-facies Monte Rosa nappe (MR), western Alps, Italy. U-Pb rutile ages from quartz-carbonate-white mica-rutile veins that are hosted within eclogite and schist of the MR, Gressoney Valley, Italy, indicate that it was at eclogite-facies metamorphic conditions at 42.6 +/- 0.6 Ma. The sample area (Indren glacier, Furgg zone; Dal Piaz, 2001) consists of eclogite boudins that are surrounded by micaceous schist. Associated with the eclogite and schist are quartz-carbonate-white mica-rutile veins that formed in tension cracks in the eclogite and along the contact between eclogite and surrounding schist. Intrusion of the veins occurred at eclogite-facies metamorphic conditions (480-570°C, >1.3-1.4 GPa) based on textural relations, oxygen isotope thermometry, and geothermobarometry. Lu-Hf geochronology of garnet from a chloritoid-talc-garnet-phengite-quartz-calcite-pyrite - chalcopyrite bearing boudin within talc-chloritoid whiteschists of the MR, Val d'Ayas, Italy (Chopin and Monie, 1984; Pawlig, 2001) yields an age of 40.54 +/- 0.36 Ma. The talc-chloritoid whiteschists from the area record pressures and temperatures of 1.6-2.4 GPa and 500-530°C (Chopin and Monie, 1984; Le Bayon et al., 2006) indicating near UHP metamorphic conditions. Based on the age, P-T, and textural

  1. Evolution of the Archaean crust by delamination and shallow subduction.

    PubMed

    Foley, Stephen F; Buhre, Stephan; Jacob, Dorrit E

    2003-01-16

    The Archaean oceanic crust was probably thicker than present-day oceanic crust owing to higher heat flow and thus higher degrees of melting at mid-ocean ridges. These conditions would also have led to a different bulk composition of oceanic crust in the early Archaean, that would probably have consisted of magnesium-rich picrite (with variably differentiated portions made up of basalt, gabbro, ultramafic cumulates and picrite). It is unclear whether these differences would have influenced crustal subduction and recycling processes, as experiments that have investigated the metamorphic reactions that take place during subduction have to date considered only modern mid-ocean-ridge basalts. Here we present data from high-pressure experiments that show that metamorphism of ultramafic cumulates and picrites produces pyroxenites, which we infer would have delaminated and melted to produce basaltic rocks, rather than continental crust as has previously been thought. Instead, the formation of continental crust requires subduction and melting of garnet-amphibolite--formed only in the upper regions of oceanic crust--which is thought to have first occurred on a large scale during subduction in the late Archaean. We deduce from this that shallow subduction and recycling of oceanic crust took place in the early Archaean, and that this would have resulted in strong depletion of only a thin layer of the uppermost mantle. The misfit between geochemical depletion models and geophysical models for mantle convection (which include deep subduction) might therefore be explained by continuous deepening of this depleted layer through geological time.

  2. The oceanic crust as a bioreactor

    NASA Astrophysics Data System (ADS)

    Staudigel, Hubert; Tebo, Bradley; Yayanos, Art; Furnes, Harald; Kelley, Katie; Plank, Terry; Muehlenbachs, Karlis

    Various lines of evidence suggest that large portions of hydrothermal systems in the oceanic crust acts like a giant bioreactor that mediates water-rock exchange and buffers the chemical composition of seawater. We review the current literature and present new chemical, biological and petrographic data on microbially mediated alteration of glass in the oceanic crust. Microbial alteration of glass displays characteristic bioalteration features, in particular in the alteration fronts around residual glass in the oceanic crust, suggesting that microbes take on an active role in its dissolution. Such features are found throughout the oceanic crust ranging in age from a few million to 170 Ma and they are found down to crustal depths of 500 m, possibly defining a Deep Oceanic Biosphere (DOB) that covers up to 2/3 of the earth's surface area. Microbial glass alteration substantially increases the active surface area of dissolving glass particles thereby enhancing the reaction rates during microbially aided dissolution. Microbially mediated glass alteration involves the establishment of two types of diffusion barriers including hydration rinds and biofilms that play an important role in mediating glass alteration. In particular biofilms may be very active by creating a localized chemical environment conducive to glass alteration, and by sequestering dissolved chemical inventory from solution. When compared with abiotic alteration of the oceanic crust, bio-alteration causes more rapid deposition of biotic and abiotic reaction products in the oceanic crust that result in a more effective removal of elements from seawater and a more rapid sealing of the oceanic crust. Thus, it is likely that microbial activity increases the fluxes of seawater components into the crust, while reducing the low—temperature flux of basalt components into seawater. However, much about the microbial activity and its relationships to the chemistry of hydrothermal systems still remains to be

  3. Weathering crusts on peridotite

    NASA Astrophysics Data System (ADS)

    Bucher, Kurt; Stober, Ingrid; Müller-Sigmund, Hiltrud

    2015-05-01

    Chemical weathering of dark-green massive peridotite, including partly serpentinized peridotite, produces a distinct and remarkable brown weathering rind when exposed to the atmosphere long enough. The structure and mineral composition of crusts on rocks from the Ronda peridotite, Spain, have been studied in some detail. The generic overall weathering reaction serpentinized peridotite + rainwater = weathering rind + runoff water describes the crust-forming process. This hydration reaction depends on water supply from the outcrop surface to the reaction front separating green peridotite from the brown crust. The reaction pauses after drying and resumes at the front after wetting. The overall net reaction transforms olivine to serpentine in a volume-conserving replacement reaction. The crust formation can be viewed as secondary serpentinization of peridotite that has been strongly altered by primary hydrothermal serpentinization. The reaction stoichiometry of the crust-related serpentinization is preserved and reflected by the composition of runoff waters in the peridotite massif. The brown color of the rind is caused by amorphous Fe(III) hydroxide, a side product from the oxidation of Fe(II) released by the dissolution of fayalite component in olivine.

  4. Raindrop induced crust formation

    NASA Astrophysics Data System (ADS)

    Szabó, Judit Alexandra; Jakab, Gergely; Józsa, Sándor; Németh, Tibor; Kovács, Ivett; Szalai, Zoltán

    2016-04-01

    Rainfall simulators are wildly used to study soil erosion because all parts of the erosion process can be simulated with them. Small-scale laboratory rainfall simulator was used to examine the detachment phase of the erosion and study the redistribution trend of the organic and mineral components of the soil. Splash erosion often creates crust on the soil surface that decreases porosity and infiltration. Crusts have crucial role in physical soil degradation processes, erosion and crop production fall. Intensive rainfall on a recently tilled Regosol and a Cambisol plots detached the aggregates and the occurred runoff scattered the individual particles on the surface. Oriented thin sections from the various morphological types of surface crusts were made similar as a thin section from any rock but during the preparation the samples were saturated often with dilute two-component adhesive to solidify the soil to preserve the crust. Raman spectroscopy and XRD analysis measurements are in progress in order to identify spatial changes in organic matter and mineralogical composition among the crust layers. Preliminary results suggest the separation of the mineral and organic soil components. The lighter organic matter seems to be enriched in the soil loss while the heavier minerals are deposited and stratified in the deeper micromorphological positions of the surface. The understanding of this selectivity is necessary in soil loss estimation.

  5. Osmium isotope evidence for Early to Middle Proterozoic mantle lithosphere stabilization and concomitant production of juvenile crust in Dish Hill, CA peridotite xenoliths

    NASA Astrophysics Data System (ADS)

    Armytage, Rosalind M. G.; Brandon, Alan D.; Peslier, Anne H.; Lapen, Thomas J.

    2014-07-01

    The 187Os/188Os compositions in peridotite samples from the sub-continental lithospheric mantle (SCLM) can be used to constrain the timing of melt extraction and potentially test the link between large-scale mantle melting and juvenile crust production. The SCLM has often experienced a complex history such that some lithophile elements such as REEs (rare earth elements) in these rocks typically record overprinting during metasomatism. New 187Os/188Os, major and trace element compositional data were obtained on sixteen Dish Hill peridotite xenoliths (California, USA) and are used to examine these issues. The samples show strong correlations between 187Os/188Os and indicators of melt depletion such as Lu abundance in clinopyroxene, modal abundance of clinopyroxene, bulk rock Al2O3 and the Cr# (Cr/(Cr + Al) in spinel. These relationships indicate that metasomatism did not compromise the 187Os/188Os systematics. The data appear to form two melt depletion trends consistent with Re depletion model ages (TRD) obtained from the two Al2O3 versus 187Os/188Os trends are 2.1 ± 0.5 Ga and 1.3 ± 0.3 Ga (±95% conf.). It has been suggested that the SCLM under Dish Hill may be fragments of oceanic lithosphere emplaced as the result of Farallon plate subduction during the Late Cretaceous (Luffi et al., 2009). However, the strong melt depletion trends, major element compositions and Re-depletion ages are not consistent with the interpretation of this suite of xenoliths having an oceanic lithospheric origin. Rather, the 2.1 Ga age coincides with Nd model ages of 2-2.3 Ga (Bennett and DePaolo, 1987; Rämö and Calzia, 1998) for the overlying Mojavia crustal province. The 1.3 Ga age is consistent with large-scale A-type magmatism in the nearby region at this time that is purported to be the result of mantle plume melting processes. Therefore, data from this study point to the SCLM under Dish Hill being formed by two ancient mantle-melting events, which could be the result of

  6. Grenville age of basement rocks in Cape May NJ well: New evidence for Laurentian crust in U.S. Atlantic Coastal Plain basement Chesapeake terrane

    USGS Publications Warehouse

    Sheridan, R.E.; Maguire, T.J.; Feigenson, M.D.; Patino, L.C.; Volkert, R.A.

    1999-01-01

    The Chesapeake terrane of the U.S. mid-Atlantic Coastal Plain basement is bounded on the northwest by the Salisbury positive gravity and magnetic anomaly and extends to the southeast as far as the Atlantic coast. It underlies the Coastal Plain of Virginia, Maryland, Delaware and southern New Jersey. Rubidium/Strontium dating of the Chesapeake terrane basement yields an age of 1.025 ?? 0.036 Ga. This age is typical of Grenville province rocks of the Middle to Late Proterozoic Laurentian continent. The basement lithologies are similar to some exposed Grenville-age rocks of the Appalachians. The TiO2 and Zr/P2O5 composition of the metagabbro from the Chesapeake terrane basement is overlapped by those of the Proterozoic mafic dikes in the New Jersey Highlands. These new findings support the interpretation that Laurentian basement extends southeast as far as the continental shelf in the U.S. mid-Atlantic region. The subcrop of Laurentian crust under the mid-Atlantic Coastal Plain implies unroofing by erosion of the younger Carolina (Avalon) supracrustal terrane. Dextral-transpression fault duplexes may have caused excessive uplift in the Salisbury Embayment area during the Alleghanian orogeny. This extra uplift in the Salisbury area may have caused the subsequent greater subsidence of the Coastal Plain basement in the embayment.

  7. Ophiolites and oceanic crust

    USGS Publications Warehouse

    Moores, E.M.; Jackson, E.D.

    1974-01-01

    OPHIOLITES consist of a pseudostratiform sequence, of harzburgite, tectonite, ultramafic and mafic cumulates sometimes including gabbro and quartz diorite (plagiogranite) intrusions, dolerite dyke swarms, pillow lava 1, and deep-sea sediments2-4. This assemblage occurs in all Phanerozoic mountain systems and is interpreted as fossil oceanic crust and uppermost mantle5-10. Outstanding problems include differences between the chemical properties of Ophiolites and rocks thought to represent present-day oceanic crust11,12, the lack in some complexes of recognised dyke swarms or cumulates, and the relative thinness of ophiolite mafic rocks compared with standard oceanic crustal sections5,8,13. ?? 1974 Nature Publishing Group.

  8. Magmatic intrusions in the lunar crust

    NASA Astrophysics Data System (ADS)

    Michaut, C.; Thorey, C.

    2015-10-01

    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 [1] 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 basin: 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. Using a model of magma emplacement below an elastic overlying layer with a flexural wavelength Λ, we characterize the surface deformations induced by the presence of shallow magmatic intrusions. We demonstrate that, depending on its size, the intrusion can show two different shapes: a bell shape when its radius is smaller than 4 times Λ or a flat top with small bended edges if its radius is larger than 4 times Λ[2]. These characteristic shapes for the intrusion result in characteristic deformations at the surface that also depend on the topography of the layer overlying the intrusion [3].Using this model we provide evidence of the presence 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. Further more,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

  9. A billion years of crustal evolution recorded in the Nuvvuagittuq greenstone belt: Pb-Hf evidence for Eoarchean TTGs produced from melting of Hadean mafic crust

    NASA Astrophysics Data System (ADS)

    O'Neil, J.; Boyet, M. M.; Carlson, R. W.; Paquette, J.

    2012-12-01

    Investigation of Earth's primitive crust is limited by the scarcity of Eoarchean/Hadean terrains. Most of these terrains are dominated by felsic Tonalite-Trondhjemite-Granodiorite (TTG) rocks. These felsic rocks however, cannot be directly produced from melting of the mantle but must instead have been derived from the melting of an older mafic precursor. Obtaining accurate ages on old terrestrial mafic rocks is challenging. The geochronology constraints on Archean mafic rocks commonly come from long-lived radiogenic isotopic systems that can be affected by younger metamorphic/metasomatic events. The short-lived 146Sm-142Nd isotopic system is less susceptible to partial resetting because 146Sm became extinct prior to ~4 Ga. The mafic rocks from the Nuvvuagittuq greenstone belt (NGB), called the Ujaraaluk unit, have 146Sm-142Nd systematics consistent with them being formed in the Hadean, between 4.3 and 4.4 Ga. This age has been challenged because the oldest U-Pb ages on zircons obtained in the NGB are ~3.8 Ga. The ~3.8 Ga zircons, however, are from trondhjemetic bands intruding the NGB and thus provide only a minimum age for the mafic rocks. The NGB is surrounded by 3.65 Ga tonalites having deficits in 142Nd suggesting derivation from an Hadean precursor such as the Ujaraaluk unit. We have now identified two additional tonalite generations dated at 3.75 Ga and 3.4-3.5 Ga suggesting a more complex thermal history for the NGB. In order to better constrain the geologic relationship between the mafic and the felsic rocks and the evolution of the NGB through time, we present whole-rock Lu-Hf data for the Nuvvuagittuq rocks as well as combined Pb-Hf analyses in zircons from a series of surrounding TTGs dated at 3.35 Ga, 3.4-3.5 Ga, 3.65 Ga and 3.75 Ga. The Lu-Hf isotopic compositions of the NGB mafic rocks have been partially reset by a Neoarchean metamorphic/metasomatic event, consistent with what is observed for the long-lived 147Sm-143Nd system. Zircons from the 3

  10. Oxidation of the ocean crust: When does it happen?

    NASA Astrophysics Data System (ADS)

    Rutter, Jennifer; Harris, Michelle; Teagle, Damon; Coggon, Rosalind; Alt, Jeff; Smith-Duque, Christopher

    2016-04-01

    Hydrothermal circulation on the ridge flanks is responsible for removing over two thirds of the global hydrothermal heat flux and given the large volumes of fluids involved it has the potential to impart significant geochemical signatures into the ocean crust and oceans. Despite this global significance, two of the key parameters that will influence the heat and geochemical fluxes of ridge flank circulation, the distribution and timing, are poorly constrained. Ridge flank circulation is recorded by the ocean crust through the formation of secondary hydrothermal minerals that form in response to seawater/rock interaction. In the upper oceanic crust this alteration is characterised into secondary minerals formed under either oxidising or reducing conditions, where oxidising conditions suggest relatively open circulation and reducing conditions restricted circulation. These prevailing conditions are intimately linked to fluid pathways and their distribution in the crust and result in variations in the oxidation of the crust from primary values. The oxidation state of whole rock samples combined with the secondary minerals present therefore preserve a record of the prevailing conditions of hydrothermal alteration and can be used to investigate ridge flank fluid/rock reaction. In this study we use new and literature whole rock Fe3+/FeTOT ratios from a global sample set of ocean crust sampled by DSDP/ODP/IODP to investigate the timing and distribution of oxidation of the ocean crust. This data set represents the most comprehensive sampling of the ocean crust (0.3 - 170 Ma), and for each site accounts for variation in flow types, alteration type and crustal depth, allowing robust estimates for the overall oxidation state of each crustal site to be made and global trends to be deciphered. The vast majority of the dataset has a range in oxidation state of 18-60% Fe3+/FeTOT, where 18% represents the general pre-alteration value of the crust, indicating that nearly all

  11. Direct dating of paleomagnetic results from Precambrian sediments in the Amazon craton: Evidence for Grenvillian emplacement of exotic crust in SE Appalachians of North America

    NASA Astrophysics Data System (ADS)

    D'Agrella-Filho, Manoel S.; Tohver, Eric; Santos, João O. S.; Elming, Sten-Åke; Trindade, Ricardo I. F.; Pacca, Igor I. G.; Geraldes, Mauro C.

    2008-03-01

    We apply a new diagenetic dating technique to determine the age of magnetization for Precambrian sedimentary rocks in the SW Amazon craton. Two new paleomagnetic poles are reported from the rocks of the Aguapeí Gp.: red beds of the Fortuna Fm. (Plat = 59.8°N, Plon = 155.9°E, A95 = 9.5, K = 14, 18 sites, N/n 128/115, Q = 5) and the reverse-polarity mudstones of the overlying Vale da Promissão Formation (Plat = 49.5°N, Plon = 89.3°E, A95 = 12.5, K = 30, 6 sites, N/n = 94/80, Q = 4). The Fortuna Fm. magnetization is hosted by massive, interstitial hematite cement and constitutes a post-depositional remanence. The age of diagenesis of the red beds is well-constrained by the 1149 ± 7 Ma U-Pb age of authigenic xenotime rims on detrital zircons determined by SHRIMP analysis. The magnetite-hosted remanence of the Vale da Promissão Fm. may be detrital in origin, but the age of deposition is poorly constrained. The reliable and precisely-dated Fortuna Fm. paleomagnetic pole fixes the paleogeographic position of the Amazon craton near the SE Appalachians portion of North America at 1.15 Ga. These data demonstrate a mobile Grenvillian link between these two cratons, and support the recent identification of Amazon crust in the Blue Ridge province region of North America.

  12. A relatively reduced Hadean continental crust

    NASA Astrophysics Data System (ADS)

    Yang, Xiaozhi; Gaillard, Fabrice; Scaillet, Bruno

    2014-05-01

    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

  13. Growth of the continental crust: A planetary-mantle perspective

    NASA Technical Reports Server (NTRS)

    Warren, Paul H.

    1988-01-01

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

  14. Does subduction zone magmatism produce average continental crust

    NASA Technical Reports Server (NTRS)

    Ellam, R. M.; Hawkesworth, C. J.

    1988-01-01

    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.

  15. Lu-Hf systematics of the ultra-high temperature Napier Complex, East Antarctica: evidence for the early Archean formation of continental crust

    NASA Astrophysics Data System (ADS)

    Choi, S.; Mukasa, S. B.; Andronikov, A. V.; Osanai, Y.; Harley, S. L.; Kelly, N. M.

    2009-12-01

    The Napier Complex in East Antarctica comprises some of the oldest rocks on earth (~3.8 billion years old), overprinted by an ultra-high temperature (UHT) metamorphic event near the Archean-Proterozoic boundary. Garnet, orthopyroxene, sapphirine, osumilite, rutile and a whole rock representing an equilibrated assemblage from this belt yield a Lu-Hf isochron age of 2,403 ± 43 Ma. Preservation of the UHT mineral assemblage in the rock analyzed suggests rapid cooling with closure likely to have occurred for the Lu-Hf system at post-peak UHT conditions near a temperature of ~800C. Individual zircon grains from Gage Ridge within the Napier Complex yielded a remarkably uniform range of 176Hf/177Hf values between 0.280433 ± 7 and 0.280505 ± 10, corresponding to ɛHf > +5.6 at 3.85 Ga relative to the chondritic uniform reservoir (CHUR). Because of their exceedingly low Lu/Hf values (<0.001), the grains are effectively recording the initial Hf isotope composition of the magmatic systems from which the gneiss protoliths crystallized. These results indicate that (1) the source of the crustal materials that formed the Napier Complex at 3.85 Ga were depleted relative to the CHUR. The extent of depletion involved is higher than has been predicted by extrapolation from the Lu-Hf isotopic evolution inferred for the source of Proterozoic and Phanerozoic basalts, judging from an fLu/Hf value of 0.51, (2) the depleted mantle reservoir has been in existence since very early in Earth’s history, in agreement with the early differentiation of the Earth that the latest core formation models require, and (3) an extremely depleted source also mean that the bulk of continental crust was extracted from the mantle by ~3.8 Ga. Moreover, the results demonstrate that even the oldest silicic rocks in the complex are not likely to have formed from remobilized older crustal materials, but were instead juvenile products of mantle melting. In addition, zircons with metamorphic rims have a similar

  16. Fault zone architecture, San Jacinto fault zone, southern California: evidence for focused fluid flow and heat transfer in the shallow crust

    NASA Astrophysics Data System (ADS)

    Morton, N.; Girty, G. H.; Rockwell, T. K.

    2011-12-01

    We report results of a new study of the San Jacinto fault zone architecture in Horse Canyon, SW of Anza, California, where stream incision has exposed a near-continuous outcrop of the fault zone at ~0.4 km depth. The fault zone at this location consists of a fault core, transition zone, damage zone, and lithologically similar wall rocks. We collected and analyzed samples for their bulk and grain density, geochemical data, clay mineralogy, and textural and modal mineralogy. Progressive deformation within the fault zone is characterized by mode I cracking, subsequent shearing of already fractured rock, and cataclastic flow. Grain comminution advances towards the strongly indurated cataclasite fault core. Damage progression towards the core is accompanied by a decrease in bulk and grain density, and an increase in porosity and dilational volumetric strain. Palygorskite and mixed-layer illite/smectite clay minerals are present in the damage and transition zones and are the result of hydrolysis reactions. The estimated percentage of illite in illite/smectite increases towards the fault core where the illite/smectite to illite conversion is complete, suggesting elevated temperatures that may have reached 150°C. Chemical alteration and elemental mass changes are observed throughout the fault zone and are most pronounced in the fault core. We conclude that the observed chemical and mineralogical changes can only be produced by the interaction of fractured wall rocks and chemically active fluids that are mobilized through the fault zone by thermo-pressurization during and after seismic events. Based on the high element mobility and absence of illite/smectite in the fault core, we expect that greatest water/rock ratios occur within the fault core. These results indicate that hot pore fluids circulate upwards through the fractured fault core and into the surrounding damage zone. Though difficult to constrain, the site studied during this investigation may represent the top

  17. Composition and origin of the Dewar geochemical anomaly

    USGS Publications Warehouse

    Lawrence, S.J.; Hawke, B.R.; Gillis-Davis, J. J.; Taylor, G.J.; Lawrence, D.J.; Cahill, J.T.; Hagerty, J.J.; Lucey, P.G.; Smith, G.A.; Keil, Klaus

    2008-01-01

    Dewar crater is a 50-km diameter impact structure located in the highlands northwest of the South Pole-Aitken basin on the lunar farside. A low-albedo area with enhanced Th and Sm values is centered east-oortheast of Dewar crater. This area also exhibits elevated FeO abundances (9.0-16.6 wt %) and TiO2 values (0.6-2 wt %). The range of FeO and TiO2 abundances determined for the darkest portions of the geochemical anomaly overlap the range of FeO and TiO2 values determined for nearside mare basalt deposits. Analysis of Clementine spectra obtained from the darkest portions of the Dewar geochemical anomaly indicates that the low-albedo materials contain large amounts of high-Ca clinopyroxene consistent with the presence of major amounts of mare basalt. Cryptomare deposits have played an important role in the formation of the Dewar geochemical anomaly. The evidence indicates that buried basalt, or cryptomare, was excavated from depth during impact events that formed dark-haloed craters in the region. We show that an early Imbrian- or Nectarian-age, low-TiO2 mare basalt deposit with enhanced Th concentrations (6-7 ??g/g) exists in the Dewar region. This ancient mare unit was buried by ejecta from Dewar crater, creating a cryptomare. Although most mare units on the central farside of the Moon exhibit low Th abundances, the enhanced Th values associated with the Dewar cryptomare deposit indicate that at least some portions of the underlying lunar interior (mantle and crust) on the farside of the Moon were not Th poor. Copyright 2008 by the American Geophysical Union.

  18. Earthquakes in Stable Continental Crust.

    ERIC Educational Resources Information Center

    Johnston, Arch C.; Kanter, Lisa R.

    1990-01-01

    Discussed are some of the reasons for earthquakes which occur in stable crust away from familiar zones at the ends of tectonic plates. Crust stability and the reactivation of old faults are described using examples from India and Australia. (CW)

  19. No evidence for anoxia during the Valanginian carbon isotope event—An organic-geochemical study from the Vocontian Basin, SE France

    NASA Astrophysics Data System (ADS)

    Kujau, Ariane; Heimhofer, Ulrich; Ostertag-Henning, Christian; Gréselle, Benjamin; Mutterlose, Jörg

    2012-07-01

    The Valanginian time interval (Early Cretaceous) is characterized by a positive carbon isotope excursion (CIE) which represents the first of several prominent Cretaceous δ13C anomalies. A combined chemostratigraphic and organic-geochemical approach has been chosen to investigate the composition and distribution of sedimentary organic matter (OM) deposited before the Valanginian CIE, during its onset and plateau-phase. This was done to test whether this CIE is accompanied by changes in marine primary production and/or OM preservation. Biostratigraphically well-calibrated deposits from two hemipelagic sections located in the Vocontian Basin of SE France are used as sedimentary archives. A newly established high-resolution δ13C record covering the composite succession shows a characteristic Valanginian pattern and enables a detailed correlation with existing carbon isotope curves from the northern Tethyan margin. The analyzed solvent extractable fraction of the sedimentary OM is mainly composed of a marine origin with an admixture of land plant material. Variations in specific biomarkers for cyanobacteria (2α-methyl-hopanes), dinoflagellates (dinosterane or 4-desmethyl-23,24-dimethyl steranes) and terrigenous plant-derived OM (odd-numbered long-chain n-alkanes) as well as the sterane/hopane ratio, the C35 hopane index and the isoprenoids pristane and phytane were investigated. In contrast to the well-studied mid-Cretaceous Oceanic Anoxic Events (OAEs), neither significant OM enrichment nor prominent fluctuations in the selected biomarker abundances can be observed during the build-up phase of the Valanginian CIE. This points to relatively stable marine paleoenvironmental conditions with well-oxygenated bottom waters. Prior to the CIE, four cm-thick, finely laminated, dark layers (known as Barrande layers) with total organic carbon content reaching up to 4% show an exception from the generally stable biomarker pattern. Sedimentological and biomarker evidence support

  20. Origin of the Sinai-Negev erg, Egypt and Israel: mineralogical and geochemical evidence for the importance of the Nile and sea level history

    USGS Publications Warehouse

    Muhs, Daniel R.; Roskin, Joel; Tsoar, Haim; Skipp, Gary; Budahn, James R.; Sneh, Amihai; Porat, Naomi; Stanley, Jean-Daniel; Katra, Itzhak; Blumberg, Dan G.

    2013-01-01

    The Sinai–Negev erg occupies an area of 13,000 km2 in the deserts of Egypt and Israel. Aeolian sand of this erg has been proposed to be derived from the Nile Delta, but empirical data supporting this view are lacking. An alternative source sediment is sand from the large Wadi El Arish drainage system in central and northern Sinai. Mineralogy of the Negev and Sinai dunes shows that they are high in quartz, with much smaller amounts of K-feldspar and plagioclase. Both Nile Delta sands and Sinai wadi sands, upstream of the dunes, also have high amounts of quartz relative to K-feldspar and plagioclase. However, Sinai wadi sands have abundant calcite, whereas Nile Delta sands have little or no calcite. Overall, the mineralogical data suggest that the dunes are derived dominantly from the Nile Delta, with Sinai wadi sands being a minor contributor. Geochemical data that proxy for both the light mineral fraction (SiO2/10–Al2O3 + Na2O + K2O–CaO) and heavy mineral fraction (Fe2O3–MgO–TiO2) also indicate a dominant Nile Delta source for the dunes. Thus, we report here the first empirical evidence that the Sinai–Negev dunes are derived dominantly from the Nile Delta. Linkage of the Sinai–Negev erg to the Nile Delta as a source is consistent with the distribution of OSL ages of Negev dunes in recent studies. Stratigraphic studies show that during the Last Glacial period, when dune incursions in the Sinai–Negev erg began, what is now the Nile Delta area was characterized by a broad, sandy, minimally vegetated plain, with seasonally dry anastomosing channels. Such conditions were ideal for providing a ready source of sand for aeolian transport under what were probably much stronger glacial-age winds. With the post-glacial rise in sea level, the Nile River began to aggrade. Post-glacial sedimentation has been dominated by fine-grained silts and clays. Thus, sea level, along with favorable climatic conditions, emerges as a major influence on the timing of dune

  1. Argon, oxygen, and boron isotopic evidence documenting 40ArE accumulation in phengite during water-rich high-pressure subduction metasomatism of continental crust

    NASA Astrophysics Data System (ADS)

    Menold, Carrie A.; Grove, Marty; Sievers, Natalie E.; Manning, Craig E.; Yin, An; Young, Edward D.; Ziegler, Karen

    2016-07-01

    were even older, exceeding the time of eclogite formation by a factor of 1.7. In contrast, lower pressure retrograde muscovite present within the host gneiss and in discrete shear zones cutting the selvage yield 40Ar/39Ar ages that were younger than the time of HP metamorphism and consistent with regional cooling age patterns. Our observation of high 40ArE concentrations in phengite from schistose rocks infiltrated by regionally extensive fluids at HP conditions runs contrary to widely held expectations. Conventional wisdom dictates that low phengite/fluid partition coefficients for argon (Dphg/fluid Ar =10-3to10-5) coupled with the dry, closed systems conditions that are widely reported to characterize HP metamorphism of continental crust explains why high concentrations of 40ArE partitions are able to accumulate within phengite. We alternatively propose that phengite/fluid partition coefficients for argon increase linearly with pressure to values as high as 10-2 to allow phengites to accumulate large amounts of 40ArE from aqueous fluids under HP to UHP conditions.

  2. Geochemical processes in landfills

    NASA Astrophysics Data System (ADS)

    Förstner, Ulrich; Kersten, Michael; Wienberg, Reinhard

    The present review focusses on the qualitative long-term perspectives of processes and mechanisms controlling the interactions of critical pollutants with organic and inorganic substrates both in "reactor landfills" and in deposits, which already consist of rock-like material ("final storage quality"). The behavior of pollutants in landfills is determined by the chemistry of interstitial solutions, i.e. by pH and redox conditions, and concentration of inorganic and organic ligands; in "reactor landfills" these conditions are widely variable as a result of biochemical reactions, while "final storage quality" implies less variations of chemical interactions. In both alternatives, however, prediction of short- and long-term effects on groundwater quality should be based on the proportion of "active species" of compounds ("mobility concept"). Qualitative assessment of potentially mobile pollutants may involve a controlled significative intensivation of important parameters such as pH-values. Using sequential extraction rearrangements of specific solid "phases" can be evaluated prior to the actual remobilisation of the pollutant into the dissolved phase. From a geochemical point of view the "reactor landfill" is characterized by labile conditions during the initial aerobic and acid anaerobic phases, the former mainly due to uncontrolled interactions with organic solutes. On the other hand, final storage quality, which is defined by the composition of earth crust material, in most cases is not attained by simple incineration of municipal waste, i.e. by reduction of organic fractions only. There is, in particular, the problem of easily soluble minerals, such as chlorides. Nonetheless the type of inorganic residue deposits will increasingly receive prevalence as a method of final storage for municipal wastes in the future.

  3. Neutron star crusts

    NASA Technical Reports Server (NTRS)

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

    1993-01-01

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

  4. Psoriasis or crusted scabies.

    PubMed

    Goyal, N N; Wong, G A

    2008-03-01

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

  5. Petrology and geochronology of crustal xenoliths from the Bering Strait region: Linking deep and shallow processes in extending continental crust

    USGS Publications Warehouse

    Akinin, V.V.; Miller, E.L.; Wooden, J.L.

    2009-01-01

    Petrologic, geochemical, and metamorphic data on gneissic xenoliths derived from the middle and lower crust in the Neogene Bering Sea basalt province, coupled with U-Pb geochronology of their zircons using sensitive high-resolution ion microprobe-reverse geometry (SHRIMP-RG), yield a detailed comparison between the P-T-t and magmatic history of the lower crust and magmatic, metamorphic, and deformational history of the upper crust. Our results provide unique insights into the nature of lithospheric processes that accompany the extension of continental crust. The gneissic, mostly maficxenoliths (constituting less than two percent of the total xenolith population) from lavas in the Enmelen, RU, St. Lawrence, Nunivak, and Seward Peninsula fields most likely originated through magmatic fractionation processes with continued residence at granulite-facies conditions. Zircon single-grain ages (n ??? 125) are interpreted as both magmatic and metamorphic and are entirely Cretaceous to Paleocene in age (ca. 138-60 Ma). Their age distributions correspond to the main ages of magmatism in two belts of supracrustal volcanic and plutonic rocks in the Bering Sea region. Oscillatory-zoned igneous zircons, Late Cretaceous to Paleocene metamorphic zircons and overgrowths, and lack of any older inheritance in zircons from the xenoliths provide strong evidence for juvenile addition of material to the crust at this time. Surface exposures of Precambrian and Paleozoic rocks locally reached upper amphibolite-facies (sillimanite grade) to granulite-facies conditions within a series of extension-related metamorphic culminations or gneiss domes, which developed within the Cretaceous magmatic belt. Metamorphic gradients and inferred geotherms (??30-50 ??C/km) from both the gneiss domes and xenoliths aretoo high to be explained by crustal thickening alone. Magmatic heat input from the mantle is necessary to explain both the petrology of the magmas and elevated metamorphic temperatures. Deep

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

    NASA Astrophysics Data System (ADS)

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

    2015-08-01

    Eruptions of voluminous 18O-depleted rhyolite provide the best evidence that the extreme conditions required to produce and accumulate huge volumes of felsic magma can occur in the upper 10 km of the crust. Mesoproterozoic bimodal volcanic sequences from the Talbot Sub-basin in central Australia contain possibly the world's most voluminous accumulation of 18O-depleted rhyolite. This volcanic system differs from the better known, but geochemically similar, Miocene Snake River Plain - Yellowstone Plateau of North America. Both systems witnessed 'super' sized eruptions from shallow crustal chambers, and produced 18O-depleted rhyolite. The Talbot system, however, accumulated over a much longer period (>30 Ma), at a single depositional centre, and from a magma with mantle-like isotopic compositions that contrast strongly with the isotopically evolved basement and country-rock compositions. Nevertheless, although the Talbot rhyolites are exclusively 18O-depleted, the unavoidable inference of an 18O-undepleted precursor requires high-temperature rejuvenation of crust in an upper-crustal chamber, and in this respect the evolution of the Talbot rhyolites and 18O-depleted rhyolites of the Snake River Plain - Yellowstone Plateau is very similar. However, instead of older crustal material, the primary upper-crustal source recycled into Talbot rhyolites was comagmatic (or nearly so) felsic rock itself derived from a contemporaneous juvenile basement hot-zone. Whereas giant low δ18O volcanic systems show that voluminous melting of upper crust can occur, our studies indicate that felsic magmas generated at lower crustal depths can also contribute significantly to the thermal and material budget of these systems. The requirement that very high-temperatures be achieved and sustained in the upper crust means that voluminous low δ18O magmatism is rare, primarily restricted to bimodal tholeiitic, high-K rhyolite (A-type) magmatic associations in highly attenuated lithosphere. In the

  7. SEDIMENT GEOCHEMICAL MODEL

    EPA Science Inventory

    Until recently, sediment geochemical models (diagenetic models) have been only able to explain sedimentary flux and concentration profiles for a few simplified geochemical cycles (e.g., nitrogen, carbon and sulfur). However with advances in numerical methods, increased accuracy ...

  8. The Mafic Lower Crust of Neoproterozoic age beneath Western Arabia: Implications for Understanding African Lower Crust

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

    We review evidence that the lower crust of Arabia - and by implication, that beneath much of Africa was formed at the same time as the upper crust, rather than being a product of Cenozoic magmatic underplating. Arabia is a recent orphan of Africa, separated by opening of the Red Sea ~20 Ma, so our understanding of its lower crust provides insights into that of Africa. Arabian Shield (exposed in W. Arabia) is mostly Neoproterozoic (880-540 Ma) reflecting a 300-million year process of continental crustal growth due to amalgamated juvenile magmatic arcs welded together by granitoid intrusions that make up as much as 50% of the Shield's surface. Seismic refraction studies of SW Arabia (Mooney et al., 1985) reveal two layers, each ~20 km thick, separated by a well-defined Conrad discontinuity. The upper crust has average Vp ~6.3 km/sec whereas the lower crust has average Vp ~7.0 km/sec, corresponding to a granitic upper crust and gabbroic lower crust. Neogene (<30 ma) lava fields in Arabia (harrats) extend over 2500 km, from Yemen to Syria. Many of these lavas contain xenoliths, providing a remarkable glimpse of the lower-crustal and upper-mantle lithosphere beneath W. Arabia. Lower crustal xenoliths brought up in 8 harrats in Saudi Arabia, Jordan, and Syria are mostly 2-pyroxene granulites of igneous (gabbroic, anorthositic, and dioritic) origin. They contain plagioclase, orthopyroxene, and clinopyroxene, and a few contain garnet and rare amphibole and yield mineral-equilibrium temperatures of 700-900°C. Pyroxene-rich and plagioclase-rich suites have mean Al2O3 contents of 13% and 19%, respectively: otherwise the two groups have similar elemental compositions, with ~50% SiO2 and ~1% TiO2, with low K2O (<0.5%) and Na2O (1-3%). Both groups show tholeiitic affinities, unrelated to their alkali basalt hosts. Mean pyroxene-rich and plagioclase-rich suites show distinct mean MgO contents (11% vs. 7%), Mg# (67 vs. 55), and contents of compatible elements Ni (169 vs. 66 ppm

  9. The cordierite- to spinel-cataclasite transition - Structure of the lunar crust

    NASA Technical Reports Server (NTRS)

    Herzberg, C. T.; Baker, M. B.

    1980-01-01

    A two-layer lunar crust model is proposed in light of geochemical relationships between spinel cataclasites and anorthosites, discussing the relative abundances of these rocks. The uppermost stratigraphic unit of this model consists of members of the anorthositic series, and is estimated to be 12-20 km thick. The lower, Mg-rich unit consists of rocks with cotectic mineralogical proportions, and may constitute the greatest volume of the crust.

  10. Seismotectonics of thin- and thick-skinned deformation in the Andean foreland from local network data - Evidence for a seismogenic lower crust

    NASA Technical Reports Server (NTRS)

    Smalley, Robert, Jr.; Isacks, Bryan L.

    1990-01-01

    Local network data from San Juan, Argentina, provides new information about crustal seismicity in the Andean foreland above a horizontal segment of the subducted Nazca Plate. Two areas of foreland seismicity are found, one associated with the Sierras Pampeanas basement uplifts, and the other beneath, but not within, the Precordillera foreland fold-thrust belt. The Precordillera seismicity provides direct evidence for basement deformation beneath the sediments of the thrust belt and supports the idea that its eastern part is significantly modified by underlying basement deformation. In both areas, events are concentrated between 15 and 35 km depth and have volumetric, rather than planar, faultlike distributions. The depth distribution is unusually deep for intraplate earthquakes and suggests a brittle-ductile transition near 30-35 km.

  11. Aleutian basin oceanic crust

    USGS Publications Warehouse

    Christeson, Gail L.; Barth, Ginger A.

    2015-01-01

    We present two-dimensional P-wave velocity structure along two wide-angle ocean bottom seismometer profiles from the Aleutian basin in the Bering Sea. The basement here is commonly considered to be trapped oceanic crust, yet there is a change in orientation of magnetic lineations and gravity features within the basin that might reflect later processes. Line 1 extends ∼225 km from southwest to northeast, while Line 2 extends ∼225 km from northwest to southeast and crosses the observed change in magnetic lineation orientation. Velocities of the sediment layer increase from 2.0 km/s at the seafloor to 3.0–3.4 km/s just above basement, crustal velocities increase from 5.1–5.6 km/s at the top of basement to 7.0–7.1 km/s at the base of the crust, and upper mantle velocities are 8.1–8.2 km/s. Average sediment thickness is 3.8–3.9 km for both profiles. Crustal thickness varies from 6.2 to 9.6 km, with average thickness of 7.2 km on Line 1 and 8.8 km on Line 2. There is no clear change in crustal structure associated with a change in orientation of magnetic lineations and gravity features. The velocity structure is consistent with that of normal or thickened oceanic crust. The observed increase in crustal thickness from west to east is interpreted as reflecting an increase in melt supply during crustal formation.

  12. [Crusted scabies: A review].

    PubMed

    Jouret, G; Bounemeur, R; Presle, A; Takin, R

    2016-04-01

    Crusted scabies is a rare and severe form of infestation by Sarcoptes scabies var. hominis. It is characterized by profuse hyperkeratosis containing over 4000 mites per gram of skin, with treatment being long and difficult. The condition is both direct and indirectly contagious. It has a central role in epidemic cycles of scabies, the incidence of which is on the rise in economically stable countries. Recent discoveries concerning the biology of mites, the pathophysiology of hyperkeratosis and the key role of IL-17 in this severe form open up new therapeutic perspectives.

  13. Intermittency and lifetime of the 625 Hz quasi-periodic oscillation in the 2004 hyperflare from the magnetar SGR 1806-20 as evidence for magnetic coupling between the crust and the core

    SciTech Connect

    Huppenkothen, Daniela; Watts, Anna L.; Levin, Yuri

    2014-10-01

    Quasi-periodic oscillations (QPOs) detected in the 2004 giant flare from SGR 1806-20 are often interpreted as global magneto-elastic oscillations of the neutron star. There is, however, a large discrepancy between theoretical models, which predict that the highest frequency oscillations should die out rapidly, and the observations, which suggested that the highest-frequency signals persisted for ∼100 s in X-ray data from two different spacecraft. This discrepancy is particularly important for the high-frequency QPO at ∼625 Hz. However, previous analyses did not systematically test whether the signal could also be present in much shorter data segments, more consistent with the theoretical predictions. Here, we test for the presence of the high-frequency QPO at 625 Hz in data from both the Rossi X-ray Timing Explorer (RXTE) and the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) systematically both in individual rotational cycles of the neutron star, as well as averaged over multiple successive rotational cycles at the same phase. We find that the QPO in the RXTE data is consistent with being only present in a single cycle, for a short duration of ∼0.5 s, whereas the RHESSI data are as consistent with a short-lived signal that appears and disappears as with a long-lived QPO. Taken together, this data provides evidence for strong magnetic interaction between the crust and the core.

  14. Geochemical cycles of atmospheric gases

    NASA Technical Reports Server (NTRS)

    Walker, J. C. G.; Drever, J. I.

    1988-01-01

    The processes that control the atmosphere and atmospheric changes are reviewed. The geochemical cycles of water vapor, nitrogen, carbon dioxide, oxygen, and minor atmospheric constituents are examined. Changes in atmospheric chemistry with time are discussed using evidence from the rock record and analysis of the present atmosphere. The role of biological evolution in the history of the atmosphere and projected changes in the future atmosphere are considered.

  15. Physical constraints on dolomite crust formation, Ambergris Cay Belize

    SciTech Connect

    Birdwell, B.A.; Bischoff, W.D.; Mazzullo, S.J. )

    1990-05-01

    Dolomitic crusts forming on a peritidal flat on Ambergris Cay, Belize, occur beneath surface sediment adjacent to, but not within, small saline (60-90 ppt) ponds. Upper crusts, 2-12 cm thick forming at or slightly below the water table (approximately equivalent to lagoon water level) are areally restricted by (1) ponds where sediment lies below 20-50 cm of water, (2) high and relatively dry areas where sediment accumulation of more than 15 cm above water level supports diverse vegetation, and (3) low areas affected by mangrove encroachment where preexisting crusts are perforated by roots and displaced. The lower crusts occur immediately above the Pleistocene in lows beneath the Holocene sediment and on exposed Pleistocene surfaces. Estimates from x-ray diffraction analysis indicate 80-100% dolomite content within the upper crusts and 50-60% dolomite content in the lower crusts. Unlithified sediment above and below the upper crust contain up to 80% dolomite. Compositions range from Ca{sub 56}, Mg{sub 44} in the upper crusts to Ca{sub 60} Mg{sub 40} in the lower crusts. There is no correlation between stoichiometry and ordering in the dolomites; all are poorly ordered as indicated by very weak (015) and (021) superstructure peaks. Where crusts are not 100% dolomite, the dolomite is evident as euhedral cements within pores, especially within foraminiferal tests, and as micrite along algal laminations and walls of burrows. However, preliminary examinations with scanning electron microscopy and energy dispersive x-ray mapping show that magnesium enrichment is pervasive within these crusts and may represent Mg-enrichment of calcite as an intermediate stage in dolomite formation.

  16. Reactive overprint of the Central Indian Ridge mantle and formation of hybrid troctolites: reassessing the significance of bulk oceanic crust

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  17. Diffusive transfer of oxygen from seamount basaltic crust into overlying sediments: An example from the Clarion-Clipperton Fracture Zone

    NASA Astrophysics Data System (ADS)

    Mewes, K.; Mogollón, J. M.; Picard, A.; Rühlemann, C.; Eisenhauer, A.; Kuhn, T.; Ziebis, W.; Kasten, S.

    2016-01-01

    The Clarion-Clipperton Fracture Zone (CCFZ) in the Pacific Ocean is characterized by organic carbon-starved sediments and meter-scale oxygen penetration into the sediment. Furthermore, numerous seamounts occur throughout its deep-sea plain, which may serve as conduits for low-temperature hydrothermal seawater circulation through the oceanic crust. Recent studies in deep-sea environments of the Pacific and Atlantic Oceans have suggested and presented evidence of dissolved constituent exchange between the seawater flowing in the basaltic crust and the pore water of the overlying sediments. Through high-resolution pore-water oxygen and nutrient measurements, we examined fluxes and geochemical interactions between the seamount basaltic basement and pore waters of the overlying sediments at three sites located on a radial transect from the foot of Teddy Bare, a small seamount in the CCFZ. At three sites, located 1000, 700 and 400 m away from the foot of the seamount, we found that oxygen concentrations initially decrease with sediment depth but start to increase at depths of 3 and 7 m toward the basaltic basement. Nitrate (NO3-) concentrations mirror the oxygen concentration profiles, as they increase with sediment depth but decrease towards the basement. These profiles suggest an upward diffusion of oxygen from seawater circulating within the seamount crust into the overlying basal sediments and a downward diffusion of NO32- from sediment pore water into the basaltic crust. At one site, we determined that the 87Sr/86Sr ratios of the bottom water and of the deep sediment near the basaltic crust are similar, further supporting diffusive exchange between basaltic crust fluids and sediment pore water. Transport-reaction modeling performed at two of the study sites revealed that (1) the diffusive flux of oxygen from the basaltic basement outpaces the oxygen consumption through organic matter oxidation and nitrification in the basal sediments and (2) the nutrient exchange

  18. Isobaric heating and cooling path of the lower crust of a Variscan exotic unit: evidences from P -T estimates in NW Iberian metapelitic granulites

    NASA Astrophysics Data System (ADS)

    Alampi, A.; Gomez Barreiro, J.; Alvarez Valero, A.; Castiñeiras, P.

    2012-12-01

    Allochthonous complexes (AC) in NW Iberia consist of a pile of exotic units characterized by distinct tectonothermal evolution and lithological association, and separated from each other by tectonic contacts, either thrust or extensional detachments. In the Órdenes AC, three groups of units are recognized from bottom to top in the pile: Basal, Ophiolitic and Upper units. Upper units comprise an ensemble of arc-related rocks with a lower section that underwent a high-P and high-T (HP-HT) evolution, and an upper section with an intermediate - pressure (IP) evolution. Extensional detachments have been commonly identifying at the boundary between HP-HT and IP Upper units, like the Fornás and Corredoiras detachments. Pelitic granulites from one IP upper unit, the O Pino unit, have been investigated. Petrologic studies reveal an isobaric tectono-metamorphic crustal evolution throughout a multidisciplinary integration of: (i) detailed microstructural analysis; (ii) EMP mineral chemistry; (iii) mass-balance of the key and representative chemical reactions observed in the microstructures and subsequent interpretation of the reaction sequence; (iv) P-T estimates and paths from phase diagram modeling. Results in the NCKFMASHT system describe an isobaric (c. 7 kbar) continuous heating (and later cooling) evolution ranging from c. 620 to 680 C crossing into the melt-bearing stability fields. These achieved anatectic conditions are evidenced by the presence of both leucosomes (quartz, plagioclase and muscovite) and glass inclusions, mainly in local garnet cores through the dehydration/melting reactions of muscovite first and biotite later. Chemically distinct generations of garnet and plagioclase are evident and stoichiometrically balance each other in continuous reactions. They are consistent with the observations of both a garnet overgrowth and large patches of plagioclase which host euhedral, smaller garnets. In the cooling episode, the newly-grown garnet reacted with melt

  19. Evolution of the Archaean crust by delamination and shallow subduction.

    PubMed

    Foley, Stephen F; Buhre, Stephan; Jacob, Dorrit E

    2003-01-16

    The Archaean oceanic crust was probably thicker than present-day oceanic crust owing to higher heat flow and thus higher degrees of melting at mid-ocean ridges. These conditions would also have led to a different bulk composition of oceanic crust in the early Archaean, that would probably have consisted of magnesium-rich picrite (with variably differentiated portions made up of basalt, gabbro, ultramafic cumulates and picrite). It is unclear whether these differences would have influenced crustal subduction and recycling processes, as experiments that have investigated the metamorphic reactions that take place during subduction have to date considered only modern mid-ocean-ridge basalts. Here we present data from high-pressure experiments that show that metamorphism of ultramafic cumulates and picrites produces pyroxenites, which we infer would have delaminated and melted to produce basaltic rocks, rather than continental crust as has previously been thought. Instead, the formation of continental crust requires subduction and melting of garnet-amphibolite--formed only in the upper regions of oceanic crust--which is thought to have first occurred on a large scale during subduction in the late Archaean. We deduce from this that shallow subduction and recycling of oceanic crust took place in the early Archaean, and that this would have resulted in strong depletion of only a thin layer of the uppermost mantle. The misfit between geochemical depletion models and geophysical models for mantle convection (which include deep subduction) might therefore be explained by continuous deepening of this depleted layer through geological time. PMID:12529633

  20. Origin and age of the earliest Martian crust from meteorite NWA 7533.

    PubMed

    Humayun, M; Nemchin, A; Zanda, B; Hewins, R H; Grange, M; Kennedy, A; Lorand, J-P; Göpel, C; Fieni, C; Pont, S; Deldicque, D

    2013-11-28

    The ancient cratered terrain of the southern highlands of Mars is thought to hold clues to the planet's early differentiation, but until now no meteoritic regolith breccias have been recovered from Mars. Here we show that the meteorite Northwest Africa (NWA) 7533 (paired with meteorite NWA 7034) is a polymict breccia consisting of a fine-grained interclast matrix containing clasts of igneous-textured rocks and fine-grained clast-laden impact melt rocks. High abundances of meteoritic siderophiles (for example nickel and iridium) found throughout the rock reach a level in the fine-grained portions equivalent to 5 per cent CI chondritic input, which is comparable to the highest levels found in lunar breccias. Furthermore, analyses of three leucocratic monzonite clasts show a correlation between nickel, iridium and magnesium consistent with differentiation from impact melts. Compositionally, all the fine-grained material is alkalic basalt, chemically identical (except for sulphur, chlorine and zinc) to soils from Gusev crater. Thus, we propose that NWA 7533 is a Martian regolith breccia. It contains zircons for which we measured an age of 4,428 ± 25 million years, which were later disturbed 1,712 ± 85 million years ago. This evidence for early crustal differentiation implies that the Martian crust, and its volatile inventory, formed in about the first 100 million years of Martian history, coeval with earliest crust formation on the Moon and the Earth. In addition, incompatible element abundances in clast-laden impact melt rocks and interclast matrix provide a geochemical estimate of the average thickness of the Martian crust (50 kilometres) comparable to that estimated geophysically. PMID:24256724

  1. Origin and age of the earliest Martian crust from meteorite NWA 7533.

    PubMed

    Humayun, M; Nemchin, A; Zanda, B; Hewins, R H; Grange, M; Kennedy, A; Lorand, J-P; Göpel, C; Fieni, C; Pont, S; Deldicque, D

    2013-11-28

    The ancient cratered terrain of the southern highlands of Mars is thought to hold clues to the planet's early differentiation, but until now no meteoritic regolith breccias have been recovered from Mars. Here we show that the meteorite Northwest Africa (NWA) 7533 (paired with meteorite NWA 7034) is a polymict breccia consisting of a fine-grained interclast matrix containing clasts of igneous-textured rocks and fine-grained clast-laden impact melt rocks. High abundances of meteoritic siderophiles (for example nickel and iridium) found throughout the rock reach a level in the fine-grained portions equivalent to 5 per cent CI chondritic input, which is comparable to the highest levels found in lunar breccias. Furthermore, analyses of three leucocratic monzonite clasts show a correlation between nickel, iridium and magnesium consistent with differentiation from impact melts. Compositionally, all the fine-grained material is alkalic basalt, chemically identical (except for sulphur, chlorine and zinc) to soils from Gusev crater. Thus, we propose that NWA 7533 is a Martian regolith breccia. It contains zircons for which we measured an age of 4,428 ± 25 million years, which were later disturbed 1,712 ± 85 million years ago. This evidence for early crustal differentiation implies that the Martian crust, and its volatile inventory, formed in about the first 100 million years of Martian history, coeval with earliest crust formation on the Moon and the Earth. In addition, incompatible element abundances in clast-laden impact melt rocks and interclast matrix provide a geochemical estimate of the average thickness of the Martian crust (50 kilometres) comparable to that estimated geophysically.

  2. The fate of mafic and ultramafic intrusions in the continental crust

    NASA Astrophysics Data System (ADS)

    Roman, Alberto; Jaupart, Claude

    2016-11-01

    Geochemical and petrological data indicate that the bulk continental crust results from the fractionation of basaltic magmas followed by the foundering of residual mafic cumulates. Structural and geological evidence for foundering has been elusive and it is argued that it lies in the shapes of mafic intrusions that have been preserved in the crust. Numerical calculations of visco-elasto-plastic deformation induced by a dense intrusive body in continental crust have been carried out for a wide range of physical conditions. Three regimes are defined on the basis of the amount of dense material that remains at the original emplacement level as well as on the shape of the residual body. With strong encasing rocks, the intrusion deforms weakly in a sagging regime characterized by downwarping of the floor. At the other extreme, the intrusion sinks through weak surroundings, leaving behind a very small volume of material. In an intermediate regime, the intrusion does not sink wholesale and undergoes a dramatic change of shape. A residual body is preserved with a shape that depends on the aspect ratio of the initial intrusion. For aspect ratios of order one, the residual body is funnel-shaped above a thin and deep vertical extension. For the small aspect ratios that typify large igneous complexes such as the Bushveld, South Africa, the residual body is characterized by thick peripheral lobes with inward-dipping igneous layers and a thinner central area that has lost some of the basal cumulates. The transitions between these regimes depend on the rheology and temperature of encasing rocks.

  3. Geochemistry and Geochronology of the Lower Crust Beneath Central Mongolia

    NASA Astrophysics Data System (ADS)

    Ancuta, L. D.; Carlson, R. W.; Ionov, D. A.; Zeitler, P. K.

    2014-12-01

    Two-pyroxene granulite xenoliths recovered from the Shavaryn-Tsaram Quaternary basaltic breccia pipe near Tariat Mongolia provide a snapshot of the modern lower crust beneath the elevated Hangay Dome in central Mongolia. Two-pyroxene thermometry indicates the xenoliths equilibrated at temperatures of 840 ± 30ºC. Previous studies using the Grt-Opx-Pl barometer indicated an upper limit on pressures between 12.5 and 15.5 kbar for samples collected from the same locality (Stosch et al., 1995). Whole-rock trace-element data for the xenoliths show similar trends to arc-derived magmatic rocks, with enrichments in the large-ion lithophile elements and rare earth elements compared to depletion in the high field-strength elements. The arc-like geochemical signature of the lower crust suggests it did not form through underplating during the most recent phase of Cenozoic volcanism, which has geochemical characteristics typical of intraplate volcanism. Instead, the lower crust beneath the Hangay dome may have formed during the accretion of the Central Asian Orogenic Belt (CAOB) in the Late Paleozoic to Early Mesozoic. Alternatively, it could include remnants of older Precambrian basement accreted during the formation of the CAOB. Abundant zircon observed in the samples will be dated by U-Pb laser ablation ICP-MS and ID-TIMS to better determine the timing of lower-crust formation and metamorphism. To the extent that a crustal root supports the high elevation of Hangay region, constraining the age of the root will help place constraints on the timing of crustal thickening and the attainment of high elevation. The post-orogenic, long-term thermal evolution of the lower crust can be assessed using U-Pb analyses of a number of trace phases, including monazite and apatite. Results from these measurements will address the timing and evolution of the high topography in central Mongolia.

  4. Primitive layered gabbros from fast-spreading lower oceanic crust

    NASA Astrophysics Data System (ADS)

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

    2014-01-01

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

  5. Primitive layered gabbros from fast-spreading lower oceanic crust.

    PubMed

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

    2014-01-01

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

  6. Geochemical and isotopic evidence for the petrogenesis and emplacement tectonics of the Serra dos Órgãos batholith in the Ribeira belt, Rio de Janeiro, Brazil

    NASA Astrophysics Data System (ADS)

    Machado, Rômulo; Philipp, Ruy Paulo; McReath, Ian; Peucat, Jean Jacques

    2016-07-01

    The Serra dos Órgãos batholith in the State of Rio de Janeiro (Brazil) is a NE-SW-trending elongated body that occupies ca. 5000 km2 in plan view. It is a foliated intrusion, especially at its borders and is crosscut by syn-magmatic shear zones, with foliations that are moderately-to steeply-dipping to the northwest and moderately-to shallow-dipping in the center and to the southeast, in a configuration of a large laccolith. It was emplaced between 560 and 570 Ma, during an extensional episode that was part of a series of events that comprise the Brasiliano Orogeny in SE Brazil, and which includes deformation, metamorphism and granite intrusion during the interval between 630 and 480 Ma. The two main rock types in the batholith are biotite-hornblende monzogranite, and biotite leucogranite, with subordinate tonalite, granodiorite, diorite, quartz diorite (enclaves), aplite and pegmatite. Harker-type diagrams help show two rock groups with similar trends of evolution: a dioritic and a granitic. The first one is tholeiitic, whereas the second is calc-alkaline, with medium-to high-K calc-alkaline affinity and metaluminous to slightly peraluminous character. In both groups strong decrease in Al2O3, MgO, FeOT and CaO relative to silica contents are observed, which is compatible with trends of fractional crystallization involving clinopyroxene and/or hornblende, plagioclase, opaque minerals, apatite, microcline and biotite. The Sr and Nd isotopic data suggest recycling of a Paleoproterozoic crust as an important petrological process to generate the batholith rocks. Geothermometry (amphibole composition) and geobarometry (saturation in zircon and apatite) indicate that most of the batholith solidified at mid to lower crustal levels at about 750 °C and between 5 and 5.5 kbar. We consider that Serra dos Órgãos crustal protoliths underwent melting caused by the interaction with hotter mafic magma at the base of the crust. These two magmas, with distinct initial

  7. The supra-ophiolitic sedimentary cover of the Asbestos ophiolite, Québec, Canada: First geochemical evidence of transition from oceanic to continental sediment flux

    NASA Astrophysics Data System (ADS)

    Bédard, É.; Hébert, R.; Guilmette, C.; Dostal, J.

    2008-10-01

    The 473 + 5/- 3 Myr Asbestos ophiolite complex of the Quebec Appalachians was formed in a forearc basin and obducted on a margin of Laurentia ~ 460 Myr ago. The complex together with its sedimentary cover is well exposed at Burbank Hill (~ 130 km SW of Québec City) where eight distinct lithologies have been identified: 1) pyroxenites and wehrlites with minor dunitic layers, (2) fractured gabbros, (3) breccia containing gabbro and diabase fragments, (4) polygenic conglomerates, (5) red mudstone/chert/sandstone succession, (6) tuffs with intercalated grey chert, (7) greenish grey mudstones and (8) slates and sandstones of the Saint-Daniel Mélange. The gabbroic and mafic volcanic rocks show the characteristics of arc tholeiites (TiO 2 ~ 0.8-0.85 wt.%) and boninites (TiO 2 < wt. 0.35% and Mg# > 45; (La/Sm) N ~ 1.9-3.3). These rock-types also occur as mafic fragments in the polygenic conglomerate. The mafic fragments (crystals and rocks) within the conglomerate and the ophiolitic sandstones also show the same greenschist facies metamorphism as the mafic igneous rocks of the complex. The conglomerate was probably formed after the forearc crust was fragmented by pre-obduction normal faults. These fault scarps would have promoted the erosion of the oceanic crust, leading to the accumulation of diverse detritus in grabens. The sedimentary lithologies which overlie the conglomerate exhibit a gradual increase in continent-derived material ((La/Sm) N ~ 5 and La/Yb ~ 20) and a decrease in ophiolitic material moving upward in the stratigraphic sequence. The sedimentary rocks at the bottom of the sequence (group 3) are mostly composed of ophiolite material with only 20% of crustal material. They also have a high content of Cr and MgO with values of 350-480 ppm and 5.7-8%, respectively. The rocks of the uppermost part of the pile (group 1) which contain up to 80% crustal material have low abundances of Cr and MgO (30-100 ppm and 2.7-4.5 wt.%). Continental detritus on the top of

  8. The geodynamic province of transitional crust adjacent to magma-poor continental margins

    NASA Astrophysics Data System (ADS)

    Sibuet, J.; Tucholke, B. E.

    2011-12-01

    Two types of 'transitional crust' have been documented along magma-poor rifted margins. One consists of apparently sub-continental mantle that has been exhumed and serpentinized in a regime of brittle deformation during late stages of rifting. A second is highly thinned continental crust, which in some cases is known to have been supported near sea level until very late in the rift history and thus is interpreted to reflect depth-dependent extension. In both cases it is typically assumed that formation of oceanic crust occurs shortly after the breakup of brittle continental crust and thus that the transitional crust has relatively limited width. We here examine two representative cases of transitional crust, one in the Newfoundland-Iberia rift (exhumed mantle) and one off the Angola-Gabon margin (highly thinned continental crust). Considering the geological and geophysical evidence, we propose that depth-dependent extension (riftward flow of weak lower/middle continental crust and/or upper mantle) may be a common phenomenon on magma-poor margins and that this can result in a much broader zone of transitional crust than has hitherto been assumed. Transitional crust in this extended zone may consist of sub-continental mantle, lower to middle continental crust, or some combination thereof, depending on the strength profile of the pre-rift continental lithosphere. Transitional crust ceases to be emplaced (i.e., final 'breakup' occurs) only when emplacement of heat and melt from the rising asthenosphere becomes dominant over lateral flow of the weak lower lithosphere. This model implies a two-stage breakup: first the rupture of the brittle upper crust and second, the eventual emplacement of oceanic crust. Well-defined magnetic anomalies can form in transitional crust consisting of highly serpentinized, exhumed mantle, and they therefore are not diagnostic of oceanic crust. Where present in transitional crust, these anomalies can be helpful in interpreting the rifting

  9. Rapid vertical tectonics in ductile continental crust

    NASA Astrophysics Data System (ADS)

    Pearse, Jillian

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

  10. Tectonomagmatic evolution of the Earth: from the primordial crust to plate tectonics

    NASA Astrophysics Data System (ADS)

    Sharkov, E. V.; Bogatikov, O. A.

    2009-12-01

    There are two dominating hypotheses about composition of the primordial Earth’s crust now: basic or sialic. Both models require a global melting of primary chondritic material, and final result would depend on degree of melt differentiation during hardening of global magmatic ocean. Such a solidification, due to difference in adiabatic and melting point gradients proceeded in bottom-top direction and resulted in accumulation of low-temperature derivates in outer shell of the planet. Geological data, namely granite-dominated Archean crust, composed mainly by tonalite-trondhjemite-granodiorite (TTG) rocks, and Hadean detrital zircons from Australia with U-Pb age 4.4-4.2 Ga supports the primordial-sialic crust hypothesis. Formation of the sialic crust was responsible for the depletion of the upper mantle matter. The early Precambrian (Archean, Early Paleoproterozoic) tectonomagmatic activity was rather different from the Phanerozoic: the major structures were granite-greenstone terranes and their separating granulite belts; high-Mg melts (komatiite-basaltic and boninite-like), derived from a depleted source of the first generation mantle superplumes, predominated. Situation can be described in terms of plume-tectonics. Cardinal change of tectonomagmatic processes occurred in the period of 2.3 to 2.0 Ga, which was characterized by voluminous eruption of Fe-Ti picrites and basalts similar to the Phanerozoic within-plate magmas, derived from geochemical-enriched mantle source. Simultaneously, important compositional changes occurred in the atmosphere, hydrosphere and biosphere (Melezhik et al., 2005). The first Phanerozoic-type orogens (Svecofennian of the Baltic Shield, Trans-Hudson and others of the Canadian Shield, etc.) appeared ca. 2 Ga. Since then, subduction of the ancient sialic continental crust (together with newly-formed oceanic crust) is a permanent process and the crustal material has stored in the “slab graveyard”, estimated in the mantle by seismic

  11. Continental crust: a geophysical approach

    SciTech Connect

    Meissner, R.

    1986-01-01

    This book develops an integrated and balanced picture of present knowledge of the continental crust. Crust and lithosphere are first defined, and the formation of crusts as a general planetary phenomenon is described. The background and methods of geophysical studies of the earth's crust and the collection of related geophysical parameters are examined. Creep and friction experiments and the various methods of radiometric age dating are addressed, and geophysical and geological investigations of the crustal structure in various age provinces of the continents are studied. Specific tectonic structures such as rifts, continental margins, and geothermal areas are discussed. Finally, an attempt is made to give a comprehensive view of the evolution of the continental crust and to collect and develop arguments for crustal accretion and recycling. 647 references.

  12. Mars Geochemical Instrument (MarGI): An instrument for the analysis of the Martian surface and the search for evidence of life

    NASA Technical Reports Server (NTRS)

    Kojiro, Daniel R.; Mancinelli, Rocco; Martin, Joe; Holland, Paul M.; Stimac, Robert M.; Kaye, William J.

    2005-01-01

    The Mars Geochemical Instrument, MarGI, was developed to provide a comprehensive analysis of the rocks and surface material on Mars. The instrument combines Differential Thermal Analysis (DTA) with miniature Gas Chromatography-Ion Mobility Spectrometry (GC-IMS) to identify minerals, the presence and state of water, and organic compounds. Miniature pyrolysis ovens are used to both, conduct DTA analysis of soil or crushed rocks samples, and pyrolyze the samples at temperatures up to 1000 degrees C for GC-IMS analysis of the released gases. This combination of analytical processes and techniques, which can characterize the mineralogy of the rocks and soil, and identify and quantify volatiles released during pyrolysis, has applications across a wide range of target sites including comets, planets, asteroids, and moons such as Titan and Europa. The MarGI analytical approach evolved from the Cometary Ice and Dust Experiment (CIDEX) selected to fly on the Comet Rendezvous Asteroid Flyby Mission (CRAF).

  13. Apollo 15 Geochemical X-ray Fluorescence Experiment: Preliminary Report.

    PubMed

    Adler, I; Trombka, J; Gerard, J; Lowman, P; Schmadebeck, R; Blodget, H; Eller, E; Yin, L; Lamothe, R; Gorenstein, P; Bjorkholm, P

    1972-01-28

    Although only part of the information from the x-ray fluorescence geochemical experiment has been analyzed, it is clear that the experiment was highly successful. Significant compositional differences among and possibly within the maria and highlands have been detected. When viewed in the light of analyzed lunar rocks and soil samples, and the data from other lunar orbital experiments (in particular, the Apollo 15 gamma-ray spectroscopy experiment), the results indicate the existence of a differential lunar highland crust, probably feldspathic. This crust appears to be related to the plagioclase-rich materials previously found in the samples from Apollo 11, Apollo 12, Apollo 14, Apollo 15, and Luna 16.

  14. Avanavero mafic magmatism, a late Paleoproterozoic LIP in the Guiana Shield, Amazonian Craton: U-Pb ID-TIMS baddeleyite, geochemical and paleomagnetic evidence

    NASA Astrophysics Data System (ADS)

    Reis, Nelson Joaquim; Teixeira, Wilson; Hamilton, Mike A.; Bispo-Santos, Franklin; Almeida, Marcelo Esteves; D'Agrella-Filho, Manoel Souza

    2013-08-01

    The Avanavero Large Igneous Province (LIP) constitutes the most important Paleoproterozoic mafic magmatism event in the Guiana Shield, northern Amazonian Craton. It comprises voluminous dykes and sills, the latter intruded into regional sedimentary cover successions such as the Roraima Supergroup and Urupi Formation. Roughly contemporary mafic magmatism such as the Crepori Dolerite occurs in the southern part of the Amazonian Craton (Central Brazil Shield). This study reports new geochemical data for samples from the Avanavero Dolerite and the coeval Quarenta Ilhas Dolerite, as well as reassessing published information on roughly contemporaneous mafic dykes in the shield to address issues related with the tectonic significance of such an intraplate igneous event and paleogeographic reconstructions. The Avanavero magmatism is tholeiitic and is geochemically similar to E-MORB and subcontinental lithospheric mantle basalts. New U-Pb baddeleyite ages of 1795 ± 2 Ma and 1793 ± 1 Ma, respectively, from occurrences in both the Pakaraima and Urupi Blocks at the north and south portions of the Guiana Shield confirm that they belong to the Avanavero LIP. These two ages are within error of a U-Pb age of 1794 ± 4 Ma for an Avanavero dyke in Guyana. Slightly younger published U-Pb ages range from about 1780-1787 Ma, including the Crepori Dolerite in Central Brazil Shield (southern Amazonian Craton). This may indicate that two pulses of magmatism could be associated with the Avanavero event. The paleomagnetic data favour a Laurentia/Baltica/Amazonia link at 1.79 Ga, and this large landmass may have constituted the core of the Columbia supercontinent during Late Proterozoic times.

  15. Tectonic significance of the Dongqiao ophiolite in the north-central Tibetan plateau: Evidence from zircon dating, petrological, geochemical and Sr-Nd-Hf isotopic characterization

    NASA Astrophysics Data System (ADS)

    Liu, Tong; Zhai, Qing-guo; Wang, Jun; Bao, Pei-sheng; Qiangba, Zhaxi; Tang, Suo-han; Tang, Yue

    2016-02-01

    The Dongqiao ophiolite occurs in the central segment of the Bangong-Nujiang suture zone, in north-central Tibet, China. It is still debated on the tectonic setting of the Dongqiao ophiolite despite after more than 30 years' studies. The Dongqiao ophiolite has a complete section of a typical ophiolite, composed of harzburgite, dunite, layered and isotropic gabbros, pillow and massive basalts, as well as radiolarian chert. Whole-rock geochemical analyses show that harzburgite displays a broad U-shaped REE pattern and has a fore-arc affinity, whereas basalts show affinities of E-MORB, OIB and IAB. The basalts were probably formed in different tectonic settings, that is, mid-ocean ridge, oceanic island and island arc. The gabbros and basalts are characterized by positive εNd(t) (+1.6 to +6.7) and εHf(t) (+8.1 to +13.9) values. Zircon U-Pb dating yielded ages of 188 ± 1 Ma for the layered gabbro and 181 ± 1 Ma for the amphibole gabbro. The new ages and the published age data of the Dingqing and Dong Co ophiolites led us to conclude that the Bangong-Nujiang Ocean existed from the Late Triassic to Early Cretaceous. The new geochemical data also suggested that the Dongqiao ophiolite was a typical SSZ-type ophiolite formed in an initial fore-arc oceanic basin. Fore-arc ophiolites are probably widely distributed along the Bangong-Nujiang suture zone. If so, the Tethys Ocean of the Bangong-Nujiang area probably existed as a fore-arc oceanic basin during the Late Triassic to Early Jurassic.

  16. Assimilation of High 18O/16O Crust by Shergottite-Nakhlite-Chassigny (SNC) Magmas on Mars

    NASA Astrophysics Data System (ADS)

    Day, J. M.; Taylor, L. A.; Valley, J. W.; Spicuzza, M. J.

    2005-12-01

    There is significant geochemical evidence for assimilation of crustal material into sub-aerial, mantle-derived, terrestrial basaltic magmas. Some of the most powerful constraints on crustal assimilation come from oxygen isotope studies, because supracrustal rocks often have distinct 18O/16O ratios resulting from interaction with Earth's hydrosphere. From a planetary perspective, studies of carbonate concretions from meteorite ALH84001 have yielded evidence for low-temperature crustal interaction at or near the surface of its putative parent body, Mars. This finding raises the possibility that crustal assimilation processes may be tracked using oxygen isotopes in combination with geochemical data of other reputed martian (SNC) meteorites. The whole-rock oxygen isotope ratios (Laser fluorination δ18O = +4.21 to +5.85‰ VSMOW) of SNC meteorites, correlate with aspects of their incompatible element chemistry. Some of the oxygen isotope variability may be explained by post-magmatic alteration on Mars or Earth; however, it appears, based on petrographic and geochemical observations, that a number of SNC meteorites, especially Shergottites, retain the original whole-rock oxygen isotope values of their magmas prior to crystallisation. Correlations between oxygen isotopes and incompatible element geochemistry are consistent with assimilation of a high-18O/16O, incompatible-element rich, oxidizing crustal component by hot, mantle-derived magmas (δ18O = ~~4.2‰). A crustal component has previously been recognized from Sr-Nd-Os isotope systematics and oxygen fugacity measurements of SNC meteorites. Oxygen isotope evidence from SNC meteorites suggests high-18O/16O crustal contaminants on Mars result from low temperature (< 300°C) interaction with martian hydrosphere. The extent of apparent crustal contamination tracked by oxygen isotopes in SNC meteorites implies that the majority of martian crust may have undergone such interactions. Evidence for assimilation of

  17. Hornblendite delineates zones of mass transfer through the lower crust.

    PubMed

    Daczko, Nathan R; Piazolo, Sandra; Meek, Uvana; Stuart, Catherine A; Elliott, Victoria

    2016-01-01

    Geochemical signatures throughout the layered Earth require significant mass transfer through the lower crust, yet geological pathways are under-recognized. Elongate bodies of basic to ultrabasic rocks are ubiquitous in exposures of the lower crust. Ultrabasic hornblendite bodies hosted within granulite facies gabbroic gneiss of the Pembroke Valley, Fiordland, New Zealand, are typical occurrences usually reported as igneous cumulate hornblendite. Their igneous features contrast with the metamorphic character of their host gabbroic gneiss. Both rock types have a common parent; field relationships are consistent with modification of host gabbroic gneiss into hornblendite. This precludes any interpretation involving cumulate processes in forming the hornblendite; these bodies are imposter cumulates. Instead, replacement of the host gabbroic gneiss formed hornblendite as a result of channeled high melt flux through the lower crust. High melt/rock ratios and disequilibrium between the migrating magma (granodiorite) and its host gabbroic gneiss induced dissolution (grain-scale magmatic assimilation) of gneiss and crystallization of mainly hornblende from the migrating magma. The extent of this reaction-replacement mechanism indicates that such hornblendite bodies delineate significant melt conduits. Accordingly, many of the ubiquitous basic to ultrabasic elongate bodies of the lower crust likely map the 'missing' mass transfer zones. PMID:27546342

  18. Hornblendite delineates zones of mass transfer through the lower crust.

    PubMed

    Daczko, Nathan R; Piazolo, Sandra; Meek, Uvana; Stuart, Catherine A; Elliott, Victoria

    2016-08-22

    Geochemical signatures throughout the layered Earth require significant mass transfer through the lower crust, yet geological pathways are under-recognized. Elongate bodies of basic to ultrabasic rocks are ubiquitous in exposures of the lower crust. Ultrabasic hornblendite bodies hosted within granulite facies gabbroic gneiss of the Pembroke Valley, Fiordland, New Zealand, are typical occurrences usually reported as igneous cumulate hornblendite. Their igneous features contrast with the metamorphic character of their host gabbroic gneiss. Both rock types have a common parent; field relationships are consistent with modification of host gabbroic gneiss into hornblendite. This precludes any interpretation involving cumulate processes in forming the hornblendite; these bodies are imposter cumulates. Instead, replacement of the host gabbroic gneiss formed hornblendite as a result of channeled high melt flux through the lower crust. High melt/rock ratios and disequilibrium between the migrating magma (granodiorite) and its host gabbroic gneiss induced dissolution (grain-scale magmatic assimilation) of gneiss and crystallization of mainly hornblende from the migrating magma. The extent of this reaction-replacement mechanism indicates that such hornblendite bodies delineate significant melt conduits. Accordingly, many of the ubiquitous basic to ultrabasic elongate bodies of the lower crust likely map the 'missing' mass transfer zones.

  19. Hornblendite delineates zones of mass transfer through the lower crust

    NASA Astrophysics Data System (ADS)

    Daczko, Nathan R.; Piazolo, Sandra; Meek, Uvana; Stuart, Catherine A.; Elliott, Victoria

    2016-08-01

    Geochemical signatures throughout the layered Earth require significant mass transfer through the lower crust, yet geological pathways are under-recognized. Elongate bodies of basic to ultrabasic rocks are ubiquitous in exposures of the lower crust. Ultrabasic hornblendite bodies hosted within granulite facies gabbroic gneiss of the Pembroke Valley, Fiordland, New Zealand, are typical occurrences usually reported as igneous cumulate hornblendite. Their igneous features contrast with the metamorphic character of their host gabbroic gneiss. Both rock types have a common parent; field relationships are consistent with modification of host gabbroic gneiss into hornblendite. This precludes any interpretation involving cumulate processes in forming the hornblendite; these bodies are imposter cumulates. Instead, replacement of the host gabbroic gneiss formed hornblendite as a result of channeled high melt flux through the lower crust. High melt/rock ratios and disequilibrium between the migrating magma (granodiorite) and its host gabbroic gneiss induced dissolution (grain-scale magmatic assimilation) of gneiss and crystallization of mainly hornblende from the migrating magma. The extent of this reaction-replacement mechanism indicates that such hornblendite bodies delineate significant melt conduits. Accordingly, many of the ubiquitous basic to ultrabasic elongate bodies of the lower crust likely map the ‘missing’ mass transfer zones.

  20. Hornblendite delineates zones of mass transfer through the lower crust

    PubMed Central

    Daczko, Nathan R.; Piazolo, Sandra; Meek, Uvana; Stuart, Catherine A.; Elliott, Victoria

    2016-01-01

    Geochemical signatures throughout the layered Earth require significant mass transfer through the lower crust, yet geological pathways are under-recognized. Elongate bodies of basic to ultrabasic rocks are ubiquitous in exposures of the lower crust. Ultrabasic hornblendite bodies hosted within granulite facies gabbroic gneiss of the Pembroke Valley, Fiordland, New Zealand, are typical occurrences usually reported as igneous cumulate hornblendite. Their igneous features contrast with the metamorphic character of their host gabbroic gneiss. Both rock types have a common parent; field relationships are consistent with modification of host gabbroic gneiss into hornblendite. This precludes any interpretation involving cumulate processes in forming the hornblendite; these bodies are imposter cumulates. Instead, replacement of the host gabbroic gneiss formed hornblendite as a result of channeled high melt flux through the lower crust. High melt/rock ratios and disequilibrium between the migrating magma (granodiorite) and its host gabbroic gneiss induced dissolution (grain-scale magmatic assimilation) of gneiss and crystallization of mainly hornblende from the migrating magma. The extent of this reaction-replacement mechanism indicates that such hornblendite bodies delineate significant melt conduits. Accordingly, many of the ubiquitous basic to ultrabasic elongate bodies of the lower crust likely map the ‘missing’ mass transfer zones. PMID:27546342

  1. Geochemical and Sr, Ca, U Isotopic Evidence of Recent changes of Weathering Reactions in a Forested Granitic Catchment (Strengbach Watershed, France)

    NASA Astrophysics Data System (ADS)

    Chabaux, F. J.; Pierret, M.; Stille, P.; Prunier, J.

    2009-12-01

    The characterization of the present-day weathering processes controlling the chemical composition of waters and soils and of their past evolution in natural ecosystems is important to predict and hence to model the response of the ecosystem to recent environmental changes. The geochemical analysis of the various hydro-geochemical compartments of experimental watersheds is well adapted to answer such questions. Here, we present the elemental and isotopic composition of soil solutions collected within two soil profiles located in the small granitic watershed of the Strengbach creek in the Vosges Mountain (France). The depth variation of elemental concentration confirms the important influence of the vegetation cycling on the budget of Ca, K, Rb and Sr, whereas Mg and Si budget in soil solutions are quasi exclusively controlled by weathering processes. Variation of Sr, Ca and U isotopic ratios with depth also demonstrates that the sources and biogeochemical processes controlling the Sr budget of soil solutions is different in the uppermost soil horizons and in the deeper ones. In the deeper part (>60cm) of the soil profile, dissolution of alumino-silicates is the main parameter accounting for the Sr budget of the soil solutions. The comparison of elemental concentrations and Sr isotope ratios in recent soil solutions with data obtained for soil solutions over the last 15 years, demonstrate that the source of elements in soil solutions has changed over this time period. Such a variation cannot be related to diminution of dissolution processes nor to declining of atmospheric input. It is related to a modification of the weathering reactions occurring within the weathering profile, including, most probably, an increasing dissolution of alumino-silicate minerals over 15 yrs. All our results demonstrate that the Strengbach watershed is in a transient state of weathering - with an important loss of nutriments such as Ca in soils solutions since at least 15yrs, associated

  2. Geochemical variations within a laminated evaporite deposit: evidence for brine composition during formation of the Permian Castile Formation, Texas and New Mexico, USA

    NASA Astrophysics Data System (ADS)

    Leslie, Alick B.; Harwood, Gill M.; Kendall, Alan C.

    1997-06-01

    The Upper Permian Castile Formation of the Delaware Basin in northwest Texas and New Mexico consists of up to 600 m of evaporites and is subdivided into units of anhydrite overlain by halite. The Castile Formation has commonly been interpreted as a deep-water, deep-basin deposit in which sediments were laid down in several hundred metres of water or brine. Recent textural observations within anhydrite units, in which the thick-bedded anhydrite horizons have been interpreted as being of shallow-water origin, have challenged this assumption. This geochemical study of the oldest anhydrite unit in the Castile Formation (the Anhydrite 1 Member) attempts to resolve some of the problems regarding brine depth and evolution in the basin. The Anhydrite 1 Member has been subdivided into five major cycles on the basis of the distribution of stratigraphic units of thick-bedded anhydrite. Stable isotopic analyses of sulphur from anhydrite, and oxygen and carbon from calcite show that the basin waters were chemically homogeneous during precipitation of anhydrite, and do not indicate any significant input of meteoric, continental-derived waters. Throughout the section studied progressive enrichment of 18O upwards within cored intervals indicates continuous evaporation of the water body. Carbon isotopes appear to indicate fluctuations in organic activity within the cycles. Trace elemental analyses of Fe, Mg, Sr, Mn, Al, Ba, Zn, Pb and Cu from the sulphate fraction of the samples show a very high variability. There is a distinct increase in trace elemental abundances at the tops of cycles which may indicate variations in precipitation kinetics. Analyses of texturally defined cycles show that up-core trends for many of the trace elements correlate with changes in δ18O, indicating a progressive increase in the influence of evaporation. In addition, cyclical variations in trace elemental composition indicate changes in basin conditions with around a 350-year cyclicity. These changes

  3. The continental crust: Its composition and evolution

    SciTech Connect

    Taylor, S.R.; McLennan, S.M.

    1985-01-01

    This book describes the composition of the present upper crust, and deals with possible compositions for the total crust and the inferred composition of the lower crust. The question of the uniformity of crustal composition throughout geological time is discussed. It describes the Archean crust and models for crustal evolution in Archean and Post-Archean time. The rate of growth of the crust through time is assessed, and the effects of the extraction of the crust on mantle compositions. The question of early pre-geological crusts on the Earth is discussed and comparisons are given with crusts on the Moon, Mercury, Mars, Venus and the Galilean Satellites.

  4. Crust and Mantle Structure Beneath the Samoan Islands

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

    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.

  5. The evolution of Mercury's crust: a global perspective from MESSENGER.

    PubMed

    Denevi, Brett W; Robinson, Mark S; Solomon, Sean C; Murchie, Scott L; Blewett, David T; Domingue, Deborah L; McCoy, Timothy J; Ernst, Carolyn M; Head, James W; Watters, Thomas R; Chabot, Nancy L

    2009-05-01

    Mapping the distribution and extent of major terrain types on a planet's surface helps to constrain the origin and evolution of its crust. Together, MESSENGER and Mariner 10 observations of Mercury now provide a near-global look at the planet, revealing lateral and vertical heterogeneities in the color and thus composition of Mercury's crust. Smooth plains cover approximately 40% of the surface, and evidence for the volcanic origin of large expanses of plains suggests that a substantial portion of the crust originated volcanically. A low-reflectance, relatively blue component affects at least 15% of the surface and is concentrated in crater and basin ejecta. Its spectral characteristics and likely origin at depth are consistent with its apparent excavation from a lower crust or upper mantle enriched in iron- and titanium-bearing oxides. PMID:19407196

  6. Geochemical and C, O, Sr, and U-series isotopic evidence for the meteoric origin of calcrete at Solitario Wash, Crater Flat, Nevada, USA

    USGS Publications Warehouse

    Neymark, L.A.; Paces, J.B.; Marshall, B.D.; Peterman, Z.E.; Whelan, J.F.

    2005-01-01

    Calcite-rich soils (calcrete) in alluvium and colluvium at Solitario Wash, Crater Flat, Nevada, USA, contain pedogenic calcite and opaline silica similar to soils present elsewhere in the semi-arid southwestern United States. Nevertheless, a ground-water discharge origin for the Solitario Wash soil deposits was proposed in a series of publications proposing elevation-dependent variations of carbon and oxygen isotopes in calcrete samples. Discharge of ground water in the past would raise the possibility of future flooding in the unsaturated zone at Yucca Mountain, Nevada, site of a proposed high-level nuclear waste repository. New geochemical and carbon, oxygen, strontium, and uranium-series isotopic data disprove the presence of systematic elevation-isotopic composition relations, which are the main justification given for a proposed ground-water discharge origin of the calcrete deposits at Solitario Wash. Values of ??13C (-4.1 to -7.8 per mil [???]), ??18O (23.8-17.2???), 87Sr/ 86Sr (0.71270-0.71146), and initial 234U/238U activity ratios of about 1.6 in the new calcrete samples are within ranges previously observed in pedogenic carbonate deposits at Yucca Mountain and are incompatible with a ground-water origin for the calcrete. Variations in carbon and oxygen isotopes in Solitario Wash calcrete likely are caused by pedogenic deposition from meteoric water under varying Quaternary climatic conditions over hundreds of thousands of years. ?? Springer-Verlag 2005.

  7. Geochemical and C, O, Sr, and U-series isotopic evidence for the meteoric origin of calcrete at Solitario Wash, Crater Flat, Nevada, USA

    NASA Astrophysics Data System (ADS)

    Neymark, L. A.; Paces, J. B.; Marshall, B. D.; Peterman, Z. E.; Whelan, J. F.

    2005-08-01

    Calcite-rich soils (calcrete) in alluvium and colluvium at Solitario Wash, Crater Flat, Nevada, USA, contain pedogenic calcite and opaline silica similar to soils present elsewhere in the semi-arid southwestern United States. Nevertheless, a ground-water discharge origin for the Solitario Wash soil deposits was proposed in a series of publications proposing elevation-dependent variations of carbon and oxygen isotopes in calcrete samples. Discharge of ground water in the past would raise the possibility of future flooding in the unsaturated zone at Yucca Mountain, Nevada, site of a proposed high-level nuclear waste repository. New geochemical and carbon, oxygen, strontium, and uranium-series isotopic data disprove the presence of systematic elevation-isotopic composition relations, which are the main justification given for a proposed ground-water discharge origin of the calcrete deposits at Solitario Wash. Values of δ13C (-4.1 to -7.8 per mil [‰]), δ18O (23.8-17.2‰), 87Sr/86Sr (0.71270-0.71146), and initial 234U/238U activity ratios of about 1.6 in the new calcrete samples are within ranges previously observed in pedogenic carbonate deposits at Yucca Mountain and are incompatible with a ground-water origin for the calcrete. Variations in carbon and oxygen isotopes in Solitario Wash calcrete likely are caused by pedogenic deposition from meteoric water under varying Quaternary climatic conditions over hundreds of thousands of years.

  8. Heat flow studies: Constraints on the distribution of uranium, thorium and potassium in the continental crust

    NASA Astrophysics Data System (ADS)

    Jaupart, Claude; Sclater, John G.; Simmons, Gene

    1981-02-01

    radioactivity data are compatible with this model. Our analysis and numerical results are supported by data from deep boreholes and by geochemical evidence, such as detailed investigations of plutonic series and studies of U-Th-Pb systematics.

  9. Ordovician appinites in the Wugongshan Domain of the Cathaysia Block, South China: Geochronological and geochemical evidence for intrusion into a local extensional zone within an intracontinental regime

    NASA Astrophysics Data System (ADS)

    Zhong, Yufang; Ma, Changqian; Liu, Lei; Zhao, Junhong; Zheng, Jianping; Nong, Junnian; Zhang, Zejun

    2014-06-01

    Palaeozoic mafic igneous rocks are potentially significant in constraining the tectonic nature and evolution of the Kwangsian Orogeny in the eastern South China Block, yet they have received little attention because of their limited outcrop. Geochemistry and geochronology was carried out on newly identified Ordovician ultramafic-mafic appinites in the Wugongshan Domain of the Cathaysia Block. Seven appinite samples yielded 206Pb/238U crystallisation ages ranging from 452 ± 4 Ma to 473 ± 3 Ma. Abundant 480-500 Ma zircon xenocrysts and/or inherited zircons were found in the appinites, possibly indicating an earlier magmatism episode in the early Palaeozoic period. The Wugongshan appinites are ultramafic to mafic in composition, and the ultramafic rocks display features of cumulates (high concentrations of MgO, Fe2O3t, Cr, Ni, and low concentrations of total alkali and total rare earth elements [REE]). The appinite geochemistry displays: relatively flat chondrite normalised REE patterns with slight enrichment in light REE and weak negative Eu anomalies; enrichment in large-ion lithophile elements (such as Rb, K), and weak depletion in Nb-Ta in primitive mantle normalised trace element patterns. We suggest that the Wugongshan appinites likely originated from an ancient metasomatised mantle, and that crustal assimilation, fractional crystallisation (AFC), magma mingling and hydration were involved in the petrogenetic process, based on the combination of geochemistry, crust-like bulk Sr, Nd and zircon Hf isotopic compositions (εNd (t) = - 8.2 to - 3.2, initial 87Sr/86Sr ratios of 0.7067-0.7144, zircon εHf (t) values peaking at - 9 to - 3) and regional geological data. Further considering the alignment and chronology of the appinites, we suggest that the appinitic magmas probably were emplaced along the Jiangshan-Shaoxing Fault in a local extensional zone in an intracontinental regime in the early Palaeozoic.

  10. Paleoproterozoic Kimozero kimberlite (Karelian Craton): Geological setting and geochemical typing

    NASA Astrophysics Data System (ADS)

    Kargin, A. V.; Babarina, I. I.; Bogatikov, O. A.; Yutkina, E. V.; Kondrashov, I. A.

    2015-11-01

    Geological and structural mapping of Paleoproterozoic Kimozero kimberlite with account for lithological facies and geochemical specialization provides evidence for the multiphase structure of the kimberlite pipe, which underwent fragmentation as a result of shear-faulting deformations. Two geochemical types of kimberlite (magnesium and carbonate) are distinguished.

  11. New Geochemical and Isotopic Evidence for Igneous Activity at the Triassic-Jurassic Boundary: the Effects of Volcanism in the Central Atlantic Magmatic Province

    NASA Astrophysics Data System (ADS)

    Cohen, A. S.; Coe, A. L.

    2001-12-01

    Although the Triassic-Jurassic (T-J) boundary marks one of the `big five' extinction events of the Phanerozoic, the processes driving global change at that time remain obscure. The main contenders include substantial volcanic activity, large meteorite impacts, and major tectonic realignment. Recent results from high-precision Ar-Ar and U-Pb dating suggest that a major phase of volcanic activity, associated with the breakup of Pangea, started ~200 Ma ago in the so-called Central Atlantic magmatic province (Marzoli et al., Science 284, p. 616, 1999). However, it is often hard to accurately assess the global impact of this volcanic activity because of the difficulties in correlating igneous ages with the changes in the sedimentary successions which in practice define the position of the T-J boundary, and because of the difficulties in estimating the volume and extent of volcanic activity. In this study, we have adopted a new approach by determining the Mo, Re and platinum group element (PGE) abundances, and Os isotope compositions, of a suite of fully marine organic-rich mudrocks from three T-J boundary sections in the U.K. One of these sections (St. Audrie's Bay, Somerset) has been proposed as a candidate GSSP for the T-J boundary. The underlying rationale is that organic-rich mudrocks concentrate these elements from seawater, and reflect the particular geochemical and isotopic characteristics of seawater on a global scale at the time of mudrock deposition. Because the Re and PGE signatures of chondritic meteorites and terrestrial volcanism are distinctive, as are the signatures they impart to seawater, the patterns of these elements in well-preserved mudrock samples should help to define both the timing and nature of environmental change at the T-J boundary. Our new results show that Os abundances in marine mudrocks increased more than five-fold in the latest Triassic; Re abundances started to rise at the same time and had increased by up to 2 orders of magnitude in

  12. U-Pb Geochronologic, Nd Isotopic, and Geochemical Evidence for the Correlation of the Chopawamsic and Milton Terranes, Piedmont Zone, Southern Appalachian Orogen.

    PubMed

    Coler; Wortman; Samson; Hibbard; Stern

    2000-07-01

    continental crust. PMID:10856010

  13. U-Pb Geochronologic, Nd Isotopic, and Geochemical Evidence for the Correlation of the Chopawamsic and Milton Terranes, Piedmont Zone, Southern Appalachian Orogen.

    PubMed

    Coler; Wortman; Samson; Hibbard; Stern

    2000-07-01

    continental crust.

  14. Laboratory simulation of organic geochemical processes.

    NASA Technical Reports Server (NTRS)

    Eglinton, G.

    1972-01-01

    Discussion of laboratory simulations that are important to organic geochemistry in that they provide direct evidence relating to geochemical cycles involving carbon. Reviewed processes and experiments include reactions occurring in the geosphere, particularly, short-term diagenesis of biolipids and organochlorine pesticides in estuarine muds, as well as maturation of organic matter in ancient sediments.

  15. Geochemical and isotopic composition of Pan-African metabasalts from southwestern Gondwana: Evidence of Cretaceous South Atlantic opening along a Neoproterozoic back-arc

    NASA Astrophysics Data System (ADS)

    Will, Thomas M.; Frimmel, Hartwig E.; Gaucher, Claudio; Bossi, Jorge

    2014-08-01

    A lithogeochemical and Sr-Nd-Pb isotope study of former oceanic crustal rocks from the Cuchilla Dionisio Terrane in the southern Dom Feliciano Belt, Uruguay (La Tuna amphibolites) and metabasites in the Chameis Subterrane of the Marmora Terrane in the Gariep Belt, Namibia/South Africa shows that these rocks are compositionally very similar and probably represent the same unit on opposite sides of the modern South Atlantic. The mafic rocks from both terranes are tholeiitic metabasalts and -andesites and have depleted rare earth element patterns, generally low TiO2 (< 1.5 wt.%), very low Th/Nb ratios and lack negative Nb-Ta anomalies, all features that are typical of ‘normal' mid-ocean ridge basalts (N-MORB) and/or back-arc basin basalts (BABB). In addition, both rock suites have extremely depleted Nd isotope compositions (εNd630 Ma = 6.7-9.4), superchondritic 147Sm/144Nd ratios, and low 206Pb/204Pb and 207Pb/204Pb initial ratios. The 87Sr/86Sr initial ratios of the La Tuna mafic rocks are low, whereas the Chameis metagabbro samples have higher, possibly alteration-related ratios. The geochemical and isotopic signatures are consistent with the formation of both rock suites in the same mature Neoproterozoic back-arc basin (Marmora Basin), supporting conclusions drawn from earlier provenance studies of metasedimentary units from these terranes. Other mafic rocks from the Marmora Terrane are interpreted as ocean island basalts that formed in a within-plate setting. A corollary of the conclusion that the mafic rocks in the Cuchilla Dionisio and Marmora Terranes formed in the same back-arc basin is (1) that the main Pan-African suture between the Río de la Plata Craton and the Kalahari Craton lies to the west of the Dom Feliciano Belt in South America, and (2) that the opening of the modern South Atlantic did not occur along that suture but along the axis of the Neoproterozoic Marmora back-arc basin.

  16. Genesis of andesitic boninitic magmas at mid-ocean ridges by melting of hydrated peridotites: Geochemical evidence from DSDP Site 334 gabbronorites

    NASA Astrophysics Data System (ADS)

    Nonnotte, Philippe; Ceuleneer, Georges; Benoit, Mathieu

    2005-08-01

    The gabbronoritic cumulates drilled at DSDP Site 334 (Mid-Atlantic Ridge off the FAMOUS area) are neither crystallization products of the associated basalts, nor from any MORB composition documented along ocean ridges. Their parent melts are richer in SiO 2 than MORB at a given MgO content, as attested by the crystallization sequence starting with an olivine + calcic and sub-calcic pyroxene assemblages. These melts are issued from a source highly depleted in incompatible elements, likely residual peridotite left after MORB extraction. To understand the role of water in the genesis of these lithologies whose occurrence in a mid-ocean ridge setting is rather puzzling, we performed a geochemical study on clinopyroxene separates following an analytical protocol able to remove the effects of water rock interactions post-dating their crystallization. Accordingly, the measured isotopic signatures can be used to trace magma sources. We find that Site 334 clinopyroxenes depart from the global mantle correlation: normal MORB values for the 143Nd / 144Nd ratio (0.51307-0.51315) are associated to highly radiogenic 87Sr / 86Sr (0.7034-0.7067) ratios. This indicates that the parent melts of Site 334 cumulates are issued from a MORB source but that seawater contamination occurred at some stage of their genesis. The extent of contamination, traced by the Sr isotopic signature, is variable within all cumulates but more developed for gabbronorites sensus stricto, suggesting that seawater introduction was a continuous process during all the magmatic evolution of the system, from partial melting to fractional crystallization. Simple masse balance calculations are consistent with a contaminating agent having the characters of a highly hydrated (possibly water saturated) silica-rich melt depleted in almost all incompatible major, minor and trace elements relative to MORB. Mixing in various proportions of contaminated melts similar to the parent melts of Site 334 cumulates with MORB can

  17. Hot N Sour Mantle Soup on Indian Plate During Cretaceous- Evidence from Clumped Isotope and Geochemical Studies of Sung Valley Carbonatite, India

    NASA Astrophysics Data System (ADS)

    Ghosh, P.; Banerjee, Y.; Tiwari, A.; Srivastava, R. K.

    2015-12-01

    based data (1), suggesting involvement of water during calcite crystallization. Ref: 1.Romanchev 1972, Geochem Intern; 2.Gaillard et al., 2008, Science; 3. Ghosh et al.,2006, GCA ; 4. Dennis and Schrag, 2010, GCA; 5. Srivastava et al., 2005. Lithos ; 6.Taylor et al.,1967, GCA; 7. Kluge et al.,2015, GCA, 8. Watkinson and Willey, 1971, Jour. Of Petrology

  18. Genesis and formation conditions of deposits in the unique Strel'tsovka Molybdenum-Uranium ore field: New mineralogical, geochemical, and physicochemical evidence

    NASA Astrophysics Data System (ADS)

    Aleshin, A. P.; Velichkin, V. I.; Krylova, T. L.

    2007-10-01

    The ambiguity of genetic interpretations of uranium ore formation at Mo-U deposits of the Strel’tsovka ore field led us to perform additional geochemical, mineralogical, and thermobarogeochemical studies. As a result, it has been established that closely related U and F were progressively gained in the Late Mesozoic volcanic rocks from the older basic volcanics (170 Ma) to the younger silicic igneous rocks (140 Ma). The Early Cretaceous postmagmatic hydrothermal epoch (140-125 Ma) is subdivided into preore, uranium ore, and first and second postore stages. The primary brannerite-pitchblende ore was formed in association with fluorite. At the first postore stage, this assemblage was replaced by a U-Si metagel, which was previously identified as coffinite. The metagel shows a wide compositional variation; its fine structure has been studied. The preore metasomatic alteration and related veined mineralization were formed under the effect of sodium (bicarbonate)-chloride solution at a temperature of 250-200°C. The uranium ore formation began with albitization and hematitization of rocks affected by supercritical fluid at 530-500°C; brannerite and pitchblende precipitated at 350-300°C. The chondrite-normalized REE patterns of pitchblende hosted in trachybasalt, trachydacite, and granite demonstrate a pronounced Sm-Nd discontinuity and a statistically significant tetrad effect of W type. These attributes were not established in REE patterns of rhyolites derived from the upper crustal magma chamber. This circumstance and a chronological gap of 5 Ma between silicic volcanism and ore formation do not allow us to suggest that uranium was derived from this magma chamber. According to the proposed model, the evolved silicic Li-F magma was a source of uranium. U4+, together with REE, was fractionated into the fluid phase as complex fluoride compounds. The uranium mineralization was deposited at a temperature barrier. It is suggested that hydromica alteration and the

  19. Geochemical Redox Indices and microfacies of the Cenomanian-Turonian Agua Nueva/Eagle Ford Fm, Mexico, Evidence for Anoxia Related to OAE2

    NASA Astrophysics Data System (ADS)

    Maurrasse, F. J.; Sanchez-hernandez, Y.; Blanco, A.

    2013-05-01

    Widespread occurrence of black, C-organic-rich sediments within the time of the Cenomanian/Turonian boundary attests to the occurrence of a major global event affecting the carbon cycle coined OAE 2. Intense carbon sequestration in sediments associated with the development of anoxic waters in the deep-ocean and epicontinental seas also led to enhanced export of trace elements as organo-metallic compounds, hence their subsequent enrichment in oxygen-deficient to anoxic sediments. In some areas, stratification of the water column coupled with controlling local factors affected microbial productivity leading to TOC-enriched sediments developed under suboxic/anoxic conditions, in others microbial communities led to high TOC values. We integrate geochemical redox indicators and microfacies characterization to assess oxygenic conditions in the Cenomanian-Turonian C-org-rich deposit of the Agua Nueva Formation and the coeval Eagle Ford Fm/ Boquillas Fm. We studied laminated samples of the Agua Nueva from Xilitla, San Luis Potosi State; San Eugenio (type locality of the Formation), Tamaulipas State; and the Eagle Ford at Quarry Los Temporales, northern Coahuila State). Microfacies at all localities reveal the prevalence of coccoid cyanobacteria, some filamentous morphotypes and degraded shell fragments, as the primary components, regardless of TOC values. Planktonic foraminifera constitute 15 to 20 % of the microfossils reaching highest abundance at Los Temporales, including macro-organisms (crustaceans). Absence of benthic foraminifera, and parallel alignment of all components attest to the absence of bioturbation, thus oxygen-deficient bottom waters. Eagle Ford samples are low in TOC, whereas the Agua Nueva samples are enriched in OM as brown amorphous macerals with bacterial coccospheres in lamination attributed to sustained microbial blooms. TE concentrations (V, Ni, U) and redox indices (V/(V+Ni), Ni/Co, V/Cr and U/Th) from the three localities confirm that these

  20. Geochemical evidence of chemical and physical weathering of mine waste downriver from the New Idria Mercury Mine, San Benito County, California

    NASA Astrophysics Data System (ADS)

    Sharma, R. K.; Weinman, B.

    2014-12-01

    Soil, river bank, and sediment samples were collected from Panoache Creek's mine tailings and its drainages in the Mendota Pool area of California's Central Valley. The samples were collected in order to understand the transport mechanisms of mercury and other heavy metals from the abandoned New Idria Mercury Mine (NIMM) in San Banito County, CA. It is generally thought that materials weathered from the NIMM site flow down gradient into the San Carlos Creek, which then joins Silver Creek and Panoche Creek, before finally ending up in the Valley's Mendota pool and San Joaquin River (SJR). While we know that factors like geology, anthropogenic activities, and weathering can accelerate heavy metal accumulation at downgradient reaches (Chakravarty and Patgiri, 2009), it is unclear how this part of the SJR has responded to the mine's abandonment since the 1970s. To investigate how mercury and other heavy metals are weathering and being transported through this portion of the SJR drainage, gains and losses using "enrichment factors" (EF) were calculated and compared along a gradient downstream. Overall, EF of fine and bank sediments show Hg is being enriched and stored within bank sediments. For example, Hg in banks sediments are up to 5% enriched compared to the bed sediments. There is also an enrichment gain trending downstream, as sediments settling in the Mendota pool have comparatively higher EF for Hg (0.94 ppm to 6.91 ppm) relative to background concentrations. Along with other geochemical indices, which can be used to more highly resolve exactly how mine contaminants like Hg are chemically and physically being weathered, (i.e., Igeo, PLI, and CIA) the overall enrichment trend is interpreted to be the physical transport of erosion material during runoff events from the stream banks of SJR tributaries. This interpretation is also supported by depleted Sr and enriched Rb/Sr ratios, which further support physical transport as a dominating factor in contaminant

  1. Estimation of lower crust magnetization form satellite derived anomaly field

    NASA Technical Reports Server (NTRS)

    Schnetzler, C. C.; Allenby, R. J.

    1983-01-01

    Various lines of evidence point to the lower crust as the source of the long-wavelength magnetic anomaly field measured by the POGO and Magsat satellites. Using seismically determined lower crust thicknesses and equivalent source inversion of the satellite anomaly data, magnetization for the lower crust for much of the United States has been calculated. The average magnetization for two hundred sixty-six 150 x 150 km areas is 3.5 A/m with a standard deviation of 1.1 A/m. These values are consistent with laboratory measurements of mafic-ultramafic rocks expected in the lower crust, and in agreement with previous estimates of lower crust magnetization based on long-wavelength aeromagnetic data. Average lower crust thickness for the same areas is 18.2 km (sigma = 6.4). Thus, over large regions, it appears that variation in magnetization and variation in magnetic layer thickness contribute almost equally in causing the anomaly field variation at satellite altitude.

  2. Fe-XANES analyses of Reykjanes Ridge basalts: Implications for oceanic crust's role in the solid Earth oxygen cycle

    NASA Astrophysics Data System (ADS)

    Shorttle, Oliver; Moussallam, Yves; Hartley, Margaret E.; Maclennan, John; Edmonds, Marie; Murton, Bramley J.

    2015-10-01

    The cycling of material from Earth's surface environment into its interior can couple mantle oxidation state to the evolution of the oceans and atmosphere. A major uncertainty in this exchange is whether altered oceanic crust entering subduction zones can carry the oxidised signal it inherits during alteration at the ridge into the deep mantle for long-term storage. Recycled oceanic crust may be entrained into mantle upwellings and melt under ocean islands, creating the potential for basalt chemistry to constrain solid Earth-hydrosphere redox coupling. Numerous independent observations suggest that Iceland contains a significant recycled oceanic crustal component, making it an ideal locality to investigate links between redox proxies and geochemical indices of enrichment. We have interrogated the elemental, isotope and redox geochemistry of basalts from the Reykjanes Ridge, which forms a 700 km transect of the Iceland plume. Over this distance, geophysical and geochemical tracers of plume influence vary dramatically, with the basalts recording both long- and short-wavelength heterogeneity in the Iceland plume. We present new high-precision Fe-XANES measurements of Fe3+ / ∑ Fe on a suite of 64 basalt glasses from the Reykjanes Ridge. These basalts exhibit positive correlations between Fe3+ / ∑ Fe and trace element and isotopic signals of enrichment, and become progressively oxidised towards Iceland: fractionation-corrected Fe3+ / ∑ Fe increases by ∼0.015 and ΔQFM by ∼0.2 log units. We rule out a role for sulfur degassing in creating this trend, and by considering various redox melting processes and metasomatic source enrichment mechanisms, conclude that an intrinsically oxidised component within the Icelandic mantle is required. Given the previous evidence for entrained oceanic crustal material within the Iceland plume, we consider this the most plausible carrier of the oxidised signal. To determine the ferric iron content of the recycled component ([Fe2O

  3. Magma Emplacement and Mafic-Felsic Magma Hybridisation: Structural, Microstructural and Geochemical Evidences From the Pan-African Negash Pluton, Northern Ethiopia

    NASA Astrophysics Data System (ADS)

    ASRAT, A.; BARBEY, P.; GLEIZES, G.; LUDDEN, J.

    2002-05-01

    The Negash pluton (50 sq. km) consists of late Pan-African, high-K, calc-alkaline granitoids intruded into low-grade metavolcanics and metasediments. This almost circular massif consists of monzogranites, granodiorites, monzodiorites, monzogabbros, and hybrid quartz monzodiorites. The rocks are enriched in LIL-elements, depleted in HFS-elements, have fractionated REE patterns, low 87Sr/86Sri (0.702344 - 0.703553) and 143Nd/144Ndi (0.512031 - 0.512133) ratios, positive ɛ Nd values (3.46 to 5.40), and Pan-African model Nd ages (0.83 to 1.08 Ga). These data, along with single zircon U-Pb dating, show that the pluton was emplaced at 608 Ma from primitive source (underplated material or juvenile island arc crust) with contamination by the country rocks. The pluton shows widespread mafic-felsic magma interactions of two types: (i) homogeneous and heterogeneous hybrid monzodiorites at the northwestern part; and (ii) mingled interfaces at the diorite-granodiorite contact zones in the Eastern and Southeastern parts. Detailed structural (using the method of the anisotropy of magnetic susceptibility) and microstructural studies have been applied to understand how these interactions occurred with respect to pluton construction. The monzodiorites constituting the northwestern part of the pluton, which are composed of complexly zoned plagioclases or unzoned plagioclase laths, euhedral hornblende with biotite cores and acicular apatites, are characterized by abundant net veining, synplutonic dikes, microgranitoid enclaves, and juxtaposed series of discrete mafic-felsic pulses of hybrid nature with vertical syn-emplacement structures. The mingled interfaces between the diorites and granodiorites, on the other hand, are characterized by lobate contacts with interfingering of diorites into granodiorites at the decametric scale, abundant inclined to horizontal granitic pipes, breccia dykes and veins, which are strongly enriched in megacrysts of K-feldspars, and numerous swarms of

  4. Lava crusts and flow dynamics

    NASA Technical Reports Server (NTRS)

    Kilburn, C. R. J.

    1993-01-01

    Lava flows can be considered as hot viscous cores within thinner, solidified crusts. Interaction between crust and core determines a flow's morphological and dynamical evolution. When the lava core dominates, flow advance approaches a steady state. When crusts are the limiting factor, advance is more irregular. These two conditions can be distinguished by a timescale ratio comparing rates of core deformation and crustal formation. Aa and budding pahoehoe lavas are used as examples of core- and crustal-dominated flows, respectively. A simple model describes the transition between pahoehoe and aa flow in terms of lava discharge rate, underlying slope, and either the thickness or velocity of the flow front. The model shows that aa morphologies are characterized by higher discharge rates and frontal velocities and yields good quantitative agreement with empirical relations distinguishing pahoehoe and aa emplacement on Hawaii.

  5. Incorporation of crust at the Lesser Antilles arc

    NASA Astrophysics Data System (ADS)

    Davidson, J. P.; Bezard, R. C.

    2012-12-01

    Most convergent margin magmas exhibit geochemical characteristics of continental crust, incorporated via subduction of continental sediment into the arc source (mantle wedge) or via assimilation of continental crust by arc magmas en route to surface. Resolving which of these processes dominate at a given arc is important in avoiding the circularity of the question of the origin of the continental crust. The Lesser Antilles is built on oceanic lithosphere so in principle any crustal signature has been introduced via sediment subduction. Geochemical variations in magmas along the arc have been matched with the variations displayed in sediments outboard of the trench 1 . At about the same time, similarly comprehensive data sets were produced from along the Lesser Antilles, arguing that much of the geochemical diversity reflected crustal contamination rather than source contamination 2. These claims were based on; 1) correlations between isotopic ratios and indices of differentiation, 2) high delta18O, which argues for extensive interaction with material that has interacted with water at low T and finally the observation that the highest Pb isotope ratios in the lavas actually exceed the highest seen in the sediments. The latter problem has now been solved since a wider range of sediments have now been examined, with a section of black shales exhibiting remarkably radiogenic Pb isotopes 3 . We have re-examined the origin of geochemical variations by comparing two specific volcanoes, Mt Pelee in the centre of the arc and The Quill in the north 4. The idea is to explore differentiation trends at a given volcano, and back project them to reasonable primitive magma compositions. In that way we can account for geochemical effects resulting from differentiation, and focus on source variations (contributions from slab to wedge along the Antilles). From this we conclude that 1) both suites differentiate largely by amphibole-plag fractionation, along with contamination by the

  6. Thermal diffusion of the lunar magma ocean and the formation of the lunar crust

    NASA Astrophysics Data System (ADS)

    Zhu, D.; Wang, S.

    2010-12-01

    The magma ocean hypothesis is consistent with several lines of evidence including planet formation, core-mantle differentiation and geochemical observations, and it is proved as an inevitable stage in the early evolution of planets. The magma ocean is assumed to be homogeneous in previous models during solidification or crystallization[1]. Based on the recent advance and our new data in experimental igneous petrology[2], we question this assumption and propose that an gabbrotic melt, from which the anorthositic lunar crust crystallized, can be produced by thermal diffusion, rather than by magma fractionation. This novel model can provide explanations for the absence of the advection in lunar magma ocean[3] and the old age of the anorthositic lunar crust[4-5]. 1. Solomatov, V., Magma Oceans and Primordial Mantle Differentiation, in Treatise on Geophysics, S. Gerald, Editor. 2007, Elsevier: Amsterdam. p. 91-119. 2. Huang, F., et al., Chemical and isotopic fractionation of wet andesite in a temperature gradient: Experiments and models suggesting a new mechanism of magma differentiation. Geochimica Et Cosmochimica Acta, 2009. 73(3): p. 729-749. 3. Turcotte, D.L. and L.H. Kellogg, Implications of isotope data for the origin of the Moon, in Origin of the Moon, W.K. Hartmann, R.J. Phillips, and G.J. Taylor, Editors. 1986, Lunar and Planet. Inst.: Houston, TX. p. 311-329. 4. Alibert, C., M.D. Norman, and M.T. McCulloch, An ancient Sm-Nd age for a ferroan noritic anorthosite clast from lunar breccia 67016. Geochimica Et Cosmochimica Acta, 1994. 58(13): p. 2921-2926. 5. Touboul, M., et al., Tungsten isotopes in ferroan anorthosites: Implications for the age of the Moon and lifetime of its magma ocean. Icarus, 2009. 199(2): p. 245-249.

  7. Crystallization Age and Impact Resetting of Ancient Lunar Crust from the Descartes Terrane

    NASA Technical Reports Server (NTRS)

    Norman, M. D.; Borg, L. E.; Nyquist, L. E.; Bogard, D. D.

    2002-01-01

    Lunar ferroan anorthosites (FANs) are relics of an ancient, primary feldspathic crust that is widely believed to have crystallized from a global magma ocean. Compositions and ages of FANs provide fundamental information about the origin and magmatic evolution of the Moon, while the petrology and thermal history of lunar FANs illustrate the structure and impact history of the lunar crust. Here we report petrologic, geochemical, and isotopic (Nd-Sr-Ar) studies of a ferroan noritic anorthosite clast from lunar breccia 67215 to improve our understanding of the composition, age, and thermal history of the Moon.

  8. Sm-nd and rb-sr chronology of continental crust formation.

    PubMed

    McCulloch, M T; Wasserburg, G J

    1978-06-01

    Samarium-neodymium and rubidium-strontium isotopic systematics together with plausible assumptions regarding the geochemical evlution of continental crust material, have been used to ascertain the times at which segments of continental crust were formed. Analyses of composites from the Canadian Shield representing portions of the Superior, Slave, and Churchill structural provinces indicate that these provinces were all formed within the period 2.5 to 2.7 aeons. It has been possible to determine the mean age of sediment provenances, as studies of sedimentary rocks suggest that the samarium-neodymium isotopic system is not substantially disturbed during sedimentation or diagenesis. PMID:17740673

  9. Sm-nd and rb-sr chronology of continental crust formation.

    PubMed

    McCulloch, M T; Wasserburg, G J

    1978-06-01

    Samarium-neodymium and rubidium-strontium isotopic systematics together with plausible assumptions regarding the geochemical evlution of continental crust material, have been used to ascertain the times at which segments of continental crust were formed. Analyses of composites from the Canadian Shield representing portions of the Superior, Slave, and Churchill structural provinces indicate that these provinces were all formed within the period 2.5 to 2.7 aeons. It has been possible to determine the mean age of sediment provenances, as studies of sedimentary rocks suggest that the samarium-neodymium isotopic system is not substantially disturbed during sedimentation or diagenesis.

  10. Pliocene granodioritic knoll with continental crust affinities discovered in the intra-oceanic Izu-Bonin-Mariana Arc: Syntectonic granitic crust formation during back-arc rifting

    NASA Astrophysics Data System (ADS)

    Tani, Kenichiro; Dunkley, Daniel J.; Chang, Qing; Nichols, Alexander R. L.; Shukuno, Hiroshi; Hirahara, Yuka; Ishizuka, Osamu; Arima, Makoto; Tatsumi, Yoshiyuki

    2015-08-01

    A widely held hypothesis is that modern continental crust of an intermediate (i.e. andesitic) bulk composition forms at intra-oceanic arcs through subduction zone magmatism. However, there is a critical paradox in this hypothesis: to date, the dominant granitic rocks discovered in these arcs are tonalite, rocks that are significantly depleted in incompatible (i.e. magma-preferred) elements and do not geochemically and petrographically represent those of the continents. Here we describe the discovery of a submarine knoll, the Daisan-West Sumisu Knoll, situated in the rear-arc region of the intra-oceanic Izu-Bonin-Mariana Arc. Remotely-operated vehicle surveys reveal that this knoll is made up entirely of a 2.6 million year old porphyritic to equigranular granodiorite intrusion with a geochemical signature typical of continental crust. We present a model of granodiorite magma formation that involves partial remelting of enriched mafic rear-arc crust during the initial phase of back-arc rifting, which is supported by the preservation of relic cores inherited from initial rear-arc source rocks within magmatic zircon crystals. The strong extensional tectonic regime at the time of intrusion may have allowed the granodioritic magma to be emplaced at an extremely shallow level, with later erosion of sediment and volcanic covers exposing the internal plutonic body. These findings suggest that rear-arc regions could be the potential sites of continental crust formation in intra-oceanic convergent margins.

  11. Lunar crust: structure and composition.

    PubMed

    Toksöz, M N; Press, F; Anderson, K; Dainty, A; Latham, G; Ewing, M; Dorman, J; Lammlein, D; Sutton, G; Duennebier, F; Nakamura, Y

    1972-06-01

    Lunar seismic data from artificial impacts recorded at three Apollo seismometers are interpreted to determine the structure of the moon's interior to a depth of about 100 kilomneters. In the Fra Mauro region of Oceanus Procellarum, the moon has a layered crust 65 kilometers thick. The seismic velocities in the upper 25 kilometers are consistent with those in lunar basalts. Between 25 and 65 kilometers, the nearly constant velocity (6.8 kilometers per second) corresponds to velocities in gabbroic and anorthositic rocks. The apparent velocity is high (about 9 kilometers per second) in the lunar mantle immediately below the crust.

  12. Palaeomagnetism and the continental crust

    SciTech Connect

    Piper, J.D.A.

    1987-01-01

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

  13. The Paleocene-Eocene Thermal Maximum (PETM) in the Dababiya Quarry Section, Egypt: New evidence for environmental changes from mineralogical and geochemical data

    NASA Astrophysics Data System (ADS)

    Schulte, P.; Scheibner, C.; Speijer, R. P.

    2009-04-01

    In the Dababiya Quarry section, the Paleocene-Eocene Thermal Maximum (PETM) consists of a succession of five characteristic beds that can be traced throughout eastern Egypt. The base of these beds defines the Global boundary Stratotype Section and and Point (GSSP) of the Eocene. Previous studies of mineralogical and geochemical proxies have suggested a period of euxinic conditions from the onset of the PETM up to the beginning of the recovery phase (Aubry et al., 2007). Dupuis et al. (2003) described prominent mineralogical changes (increase of illite and chlorite-smectite mixed layers) that occurred contemporaneous to the maximum negative carbon isotope values. A sea-level fall immediately preceding the onset of the PETM, followed by a sea-level rise and enhanced upwelling during the PETM is postulated in Egypt (Speijer and Wagner, 2002). However, a detailed study of the Dababiya Quarry beds, and specifically their element geochemistry, is currently lacking. Therefore, we investigated the Dababiya Quarry section by X-ray diffractometry (XRD; bulk rock and clay mineralogy) as well as by X-ray fluorescence analysis (XRF; major and trace elements and rare earth elements, REE) to detail the succession of environmental events during the PETM. (i) The absence of carbonate (as low as <2 wt%) in the basal event bed 1 indicates severe carbonate dissolution. A sharp short-lived increase in siliciclastic detritus (PETM) as well as an increase of chlorite and illite as well as well-crystallized smectite suggest deposition during low sea-level and increased weathering rates. Event bed 1 is also strongly deprived in REEs and shows high Zr/Rb ratios, indicative for input of coarse siliciclastic detritus. (ii) Subsequently, during the peak phase of the PETM, i.e. during the maximum negative shift of the Carbon Isotope Excursion ("CIE"), a short-lived period of pronounced anoxic sedimentary conditions is indicated by sediment lamination, absence of benthic life, elevated TOC

  14. Petrologic and chemical changes in ductile shear zones as a function of depth in the continental crust

    NASA Astrophysics Data System (ADS)

    Yang, Xin-Yue

    Petrologic and geochemical changes in ductile shear zones are important for understanding deformational and geochemical processes of the continental crust. This study examines three shear zones that formed under conditions varying from lower greenschist facies to upper amphibolite facies in order to document the petrologic and geochemical changes of deformed rocks at various metamorphic grades. The studied shear zones include two greenschist facies shear zones in the southern Appalachians and an upper amphibolite facies shear zone in southern Ontario. The mylonitic gneisses and mylonites in the Roses Mill shear zone of central Virginia are derived from a ferrodiorite protolith and characterized by a lower greenschist facies mineral assemblage. Both pressure solution and recrystallization were operative deformation mechanisms during mylonitization in this shear zone. Strain-driven dissolution and solution transfer played an important role in the mobilization of felsic components (Si, Al, K, Na, and Ca). During mylonitization, 17% to 32% bulk rock volume losses of mylonites are mainly attributed to removal of these mobile felsic components by a fluid phase. Mafic components (Fe, Mg, Ti, Mn and P) and trace elements, REE, Y, V and Sc, were immobile. At Rosman, North Carolina, the Brevard shear zone (BSZ) shows a deformational transition from the coarse-grained Henderson augen gneiss (HAG) to proto-mylonite, mylonite and ultra-mylonite. The mylonites contain a retrograde mineral assemblage as a product of fluid-assisted chemical breakdown of K-feldspar and biotite at higher greenschist facies conditions. Recrystallization and intra-crystalline plastic deformation are major deformation mechanisms in the BSZ. Fluid-assisted mylonitization in the BSZ led to 6% to 23% bulk volume losses in mylonites. During mylonitization, both major felsic and mafic elements and trace elements, Rb, Sr, Zr, V, Sc, and LREE were mobile; however, the HREEs were likely immobile. A shear zone

  15. Genetic aspects of a gold deposit in high grade Cambro-Ordovician metasediments, Nova Scotia: geological, mineral, geochemical and isotopic evidence

    SciTech Connect

    Smith, P.K.

    1985-01-01

    The Cochrane Hill gold deposit is hosted by amphibolite grade turbidite metasediments of the Cambro-Ordovician Goldenville Formation. Interbedded pelitic, semipelitic and psammite host sediments were polydeformed, metamorphosed and intruded by granitoid plutons during the Devonian Acadian Orogeny. Six distinct types of quartz veins are recognized from the 30m wide mineralized zone. Textural evidence suggests that vein emplacement commenced prior to deformation and ceased after the termination of the Acadian Orogeny. Essential mineralogy of the veins is quartz, plagioclase, phyllo-silicates, aluminosilicates, amphiboles and garnet. Arsenopyrite, pyrrhotite, loellingite, pyrite, marcasite, sphalerite, galena, chalcopyrite, Fe-Ti oxides and gold are the ore minerals. FeO/MgO, Na/sub 2/O/K/sub 2/O and Al/sub 2/O/sub 3//K/sub 2/O ratios in biotite and muscovite increase in the ore zone. Whole rock geochemistry of pelitic lithologies shows marked increases in TiO/sub 2//MgO, TiO/sub 2//Fe/sub 3/O/sub 3/, TiO/sub 2//P/sub 2/O/sub 5/ and Na O/K/sub 2/O ratios across the ore horizon. In the psammitic units TiO/sub 2//P/sub 2/O/sub 5/ shows marked increase whereas CaO/MgO and Na/sub 2/O/K/sub 2/O ratios decrease slightly in the auriferous zone. Fluid inclusion temperatures from quartz vary from 260 to +450/sup 0/C. Preliminary lead isotope data on galenas from Cochrane Hill are substantially more radiogenic than whole rock leads from the Goldenville Formation. These lead isotopic data, which are similar to those from granite-hosted mineral deposits together with other lines of evidence, support a granite related epigenetic gold models.

  16. Statistics of Magnetar Crusts Magnetoemission

    NASA Astrophysics Data System (ADS)

    Kondratyev, V. N.; Korovina, Yu. V.

    2016-05-01

    Soft repeating gamma-ray (SGR) bursts are considered as magnetoemission of crusts of magnetars (ultranamagnetized neutron stars). It is shown that all the SGR burst observations can be described and systematized within randomly jumping interacting moments model including quantum fluctuations and internuclear magnetic interaction in an inhomogeneous crusty nuclear matter.

  17. Geochemical constraints on adakites of different origins and copper mineralization

    USGS Publications Warehouse

    Sun, W.-D.; Ling, M.-X.; Chung, S.-L.; Ding, X.; Yang, X.-Y.; Liang, H.-Y.; Fan, W.-M.; Goldfarb, R.; Yin, Q.-Z.

    2012-01-01

    The petrogenesis of adakites holds important clues to the formation of the continental crust and copper ?? gold porphyry mineralization. However, it remains highly debated as to whether adakites form by slab melting, by partial melting of the lower continental crust, or by fractional crystallization of normal arc magmas. Here, we show that to form adakitic signature, partial melting of a subducting oceanic slab would require high pressure at depths of >50 km, whereas partial melting of the lower continental crust would require the presence of plagioclase and thus shallower depths and additional water. These two types of adakites can be discriminated using geochemical indexes. Compiled data show that adakites from circum-Pacific regions, which have close affinity to subduction of young hot oceanic plate, can be clearly discriminated from adakites from the Dabie Mountains and the Tibetan Plateau, which have been attributed to partial melting of continental crust, in Sr/Y-versus-La/Yb diagram. Given that oceanic crust has copper concentrations about two times higher than those in the continental crust, whereas the high oxygen fugacity in the subduction environment promotes the release of copper during partial melting, slab melting provides the most efficient mechanism to concentrate copper and gold; slab melts would be more than two times greater in copper (and also gold) concentrations than lower continental crust melts and normal arc magmas. Thus, identification of slab melt adakites is important for predicting exploration targets for copper- and gold-porphyry ore deposits. This explains the close association of ridge subduction with large porphyry copper deposits because ridge subduction is the most favorable place for slab melting. ?? 2012 by The University of Chicago.

  18. Evolving morphology of thermochemical piles caused by accumulation of subducted oceanic crust

    NASA Astrophysics Data System (ADS)

    Li, M.; McNamara, A. K.

    2015-12-01

    Seismic tomography results have shown two large low shear velocity provinces (LLSVPs) in the lowermost mantle beneath Africa and Pacific. The LLSVPs have been hypothesized to be caused by large-scale compositional heterogeneity. Two hypotheses have been proposed for the origin of this compositional heterogeneity: (1) primordial material formed during Earth's early differentiation, and (2) accumulations of subducted oceanic crust on the core-mantle boundary (CMB). Previous geodynamical calculations often show that stable thermochemical piles caused by primordial material have sharp boundaries. So, if the accumulation of subducted oceanic crust has different morphology than that of piles caused by primordial material, we may be able to constrain the origin of compositional heterogeneity from high resolution seismic observations of the boundaries of LLSVPs.Here, we performed geodynamic calculations to investigate the morphology of accumulation of subducted oceanic crust on the CMB. We found that the ability of subducted oceanic crust to accumulate on the CMB and the sharpness of the boundaries of the accumulations both strongly depends on the crustal thickness. A thick (e.g., ~30 km) oceanic crust produced from the early hot mantle can form into large-scale accumulations on the CMB, but with fuzzy and diffuse top boundaries. However, as the oceanic crust becomes thinner, it becomes more difficult to accumulate on the CMB, and the top boundaries of the accumulations of subducted oceanic crust also gradually become sharp, more like that of piles caused by primordial material. Thus, a sharp top boundaries of LLSVPs in the present-day Earth does not guarantee that they are caused by piles of primordial material. In addition, as the oceanic crust becomes thinner, more subducted oceanic crust is entrained and recycled to shallow depth, which may have important implications for geochemical observations on Earth's surface.

  19. Sequestration of volatiles in the martian crust through hydrated minerals

    NASA Astrophysics Data System (ADS)

    Mustard, J. F.; Ehlmann, B. L.; Poulet, F.; Fraeman, A. A.; Carter, J.

    2011-12-01

    The magnitude and history of volatile reservoirs is a key question in understanding Mars' evolution. The volumes of reservoirs for water through time have been estimated on the basis of morphology (e.g. Carr 1996) and modeling while the volume of active identifiable modern reservoirs such as the polar caps, the near-surface cryosphere, and the atmosphere are reasonably well known. One reservoir that has been hypothesized but not examined is the crust where water would be in the form of hydrous minerals. The OMEGA and CRISM experiments on Mars Express and Mars Reconnaissance Orbiter respectively, have shown that phyllosilicate minerals are commonly observed in the Noachian crust of Mars. Modeling has shown that depending on the location the abundance of clays and phyllosilicates can exceed 50% but more typically is less or absent, particularly in the Hesperian and younger terrains (Poulet 2007). Phyllosilicate-bearing outcrops have been observed in the deepest wall exposures of Valles Marineris (8 km below the rim) and in the central peaks of impact craters as large of 100 km. Modeling suggests that the porosity of the crust in maintained to approximate 8-10 km depth permitting the circulation of water to this depth and formation of phyllosilicate and other hydrated minerals. Based on these and other observations it is evident that at least the top 10 km of the crust can be considered to contain hydrated silicate minerals. These observations also show that phyllosilicates are globally present in Noachian crust. We use altered oceanic crust as an analog for the amount of alteration on Mars. Analyses show that the average volume fraction of hydrous phases in the lower oceanic crust is 10%. Simple calculations show this results in a water content of between 1 - 3%. If the upper 10 km of the martian crust is altered to this extent then a global equivalent thickness (GET) of water of 0.3 to 0.9 km is stored in the crust due to alteration minerals. This is comparable to

  20. Open geochemical database

    NASA Astrophysics Data System (ADS)

    Zhilin, Denis; Ilyin, Vladimir; Bashev, Anton

    2010-05-01

    We regard "geochemical data" as data on chemical parameters of the environment, linked with the geographical position of the corresponding point. Boosting development of global positioning system (GPS) and measuring instruments allows fast collecting of huge amounts of geochemical data. Presently they are published in scientific journals in text format, that hampers searching for information about particular places and meta-analysis of the data, collected by different researchers. Part of the information is never published. To make the data available and easy to find, it seems reasonable to elaborate an open database of geochemical information, accessible via Internet. It also seems reasonable to link the data with maps or space images, for example, from GoogleEarth service. For this purpose an open geochemical database is being elaborating (http://maps.sch192.ru). Any user after registration can upload geochemical data (position, type of parameter and value of the parameter) and edit them. Every user (including unregistered) can (a) extract the values of parameters, fulfilling desired conditions and (b) see the points, linked to GoogleEarth space image, colored according to a value of selected parameter. Then he can treat extracted values any way he likes. There are the following data types in the database: authors, points, seasons and parameters. Author is a person, who publishes the data. Every author can declare his own profile. A point is characterized by its geographical position and type of the object (i.e. river, lake etc). Value of parameters are linked to a point, an author and a season, when they were obtained. A user can choose a parameter to place on GoogleEarth space image and a scale to color the points on the image according to the value of a parameter. Currently (December, 2009) the database is under construction, but several functions (uploading data on pH and electrical conductivity and placing colored points onto GoogleEarth space image) are

  1. Geochemical evidence for African dust and volcanic ash inputs to terra rossa soils on carbonate reef terraces, northern Jamaica, West Indies

    USGS Publications Warehouse

    Muhs, D.R.; Budahn, J.R.

    2009-01-01

    The origin of red or reddish-brown, clay-rich, "terra rossa" soils on limestone has been debated for decades. A traditional qualitative explanation for their formation has been the accumulation of insoluble residues as the limestone is progressively dissolved over time. However, this mode of formation often requires unrealistic or impossible amounts of carbonate dissolution. Therefore, where this mechanism is not viable and where local fluvial or colluvial inputs can be ruled out, an external source or sources must be involved in soil formation. On the north coast of the Caribbean island of Jamaica, we studied a sequence of terra rossa soils developed on emergent limestones thought to be of Quaternary age. The soils become progressively thicker, redder, more Fe- and Al-rich and Si-poor with elevation. Furthermore, although kaolinite is found in all the soils, the highest and oldest soils also contain boehmite. Major and trace element geochemistry shows that the host limestones and local igneous rocks are not likely source materials for the soils. Other trace elements, including the rare earth elements (REE), show that tephra from Central American volcanoes is not a likely source either. However, trace element geochemistry shows that airborne dust from Africa plus tephra from the Lesser Antilles island arc are possible source materials for the clay-rich soils. A third, as yet unidentified, source may also contribute to the soils. We hypothesize that older, more chemically mature Jamaican bauxites may have had a similar origin. The results add to the growing body of evidence of the importance of multiple parent materials, including far-traveled dust, to soil genesis.

  2. Crusted scabies in a healthy infant.

    PubMed

    Baysal, V; Yildirim, M; Türkman, C; Aridogan, B; Aydin, G

    2004-03-01

    Crusted scabies is generally seen in mentally retarded and immunosuppressed individuals. We report the case of a patient with crusted scabies with neither systemic disease nor immunosuppression; all the lesions were cured with applications of 5% permethrin lotion.

  3. Repeated episodes of magmatic fluid injections into the hydrothermal system of Campi Flegrei. Geochemical evidences and thermo-fluid-dynamic simulations

    NASA Astrophysics Data System (ADS)

    Chiodini, G.; Caliro, S.; De Martino, P.; Avino, R.; Cardellini, C.; Gherardi, F.

    2012-04-01

    Campi Flegrei caldera subsided for the twenty years following the last large crisis of 1982-1984. The subsidence was interrupted by 3 minor uplift episodes each accompanied by swarms of low energy earthquakes and by a peak of the concentration of magmatic fluids discharged by the fumaroles. Since 2000 the behavior of the system changed: the magmatic component of fumaroles started to increase almost continuously, swarms of earthquakes became more frequent and, after a decrease in the subsidence rate, the ground started a general uplifting trend. Contemporaneously the temperature of one of the biggest fumaroles increased, new vents formed and the deeply derived CO2 released by the soils changed degassing pattern and locally increased. In order to investigate these phenomena, long time series of fluid pressure and temperature, estimated on the base of CO2-H2O-H2-CO gas equilibria, were considered. The fumarole external to Solfatara crater (Pisciarelli) shows an annual cycle of CO contents indicating the occurrence of shallow secondary processes which mask the deep signals. On the contrary the fumaroles located inside the crater (BG and BN) do not show evidences of secondary processes and the compositional variations are linked to T-P changes within the hydrothermal system, indicating a pressurization of the upper part of the hydrothermal system. Furthermore, the CO2/CH4 ratio of the fumaroles, a good tracer of the input of magmatic fluids into the hydrothermal system, displayed a general increase with numerous peaks well correlated in the time with pulsed episodes of ground uplifts and seismic swarm suggesting the occurrence of repeated episodes of injections of deep magmatic gases with high CO2 contents. The process was modeled by means of a geothermal simulator which was able to reproduce the H2S/CO2 fumarolic ratios and the PCO2 independently estimated for the fumaroles. Total injected fluid in the simulated events are in the range of fluids emitted during small

  4. Tracking abrupt climate shifts with stable isotopes: geochemical evidence for dynamic temperature, precipitation and seasonality regimes during the last deglaciation (8 to 15ka BP)

    NASA Astrophysics Data System (ADS)

    Candy, I.; Blockley, S.; Matthews, I.; Palmer, A.; Darvill, C.

    2012-12-01

    The interval between the Last Glacial Maximum (LGM) and the stabilisation of climatic conditions during the Holocene (ca 8ka BP) is well-known to have been punctuated with abrupt climatic shifts on a range of different time scales. This is clearly seen in the oxygen and deuterium isotope signal of the Greenland ice core records (e.g. NGRIP). How the magnitude and duration of these events translates across a region, such as western Europe, is however, unclear, primarily because many traditional proxies do not respond rapidly enough to provide a clear expression of abrupt climatic events. In this paper, we present a range of new oxygen and carbon isotopic records from lacustrine carbonate sequences, and a review of existing datasets, spanning the interval 8 to 15ka BP. These records lie on W-E and N-S transects across the British Isles allowing the spatial variability of the structure and magnitude of abrupt climatic events to be investigated. The oxygen isotopic signal is primarily driven by temperature and indicates that the climatic structure of abrupt events, such as the Lateglacial interstadial, is highly variable over relatively short spatial scales. Records from eastern Britain suggest patterns of warming and cooling in the Lateglacial interstadial (GI-1) comparable to those observed in Greenland, where a temperature maxima occurs early in GI-1. However records in western Britain provide evidence for more subdued climatic oscillations with peak temperatures occurring later in GI-1. Spatial variability in the isotopic expression of abrupt climatic events allows the role of different factors such as the position of the polar front, the influence of the thermohaline conveyer and the influence of maritime versus continental climates, to be investigated. This study also indicates that a major depletion event occurs in the δ18O value of lake carbonates during the early Holocene which affects all currently known sequences. This depletion event cannot be explained by

  5. Tracking abrupt climate shifts with stable isotopes in lacustrine sediments: geochemical evidence for dynamic temperature, precipitation and seasonality regimes during the last deglaciation (8 to 15ka BP)

    NASA Astrophysics Data System (ADS)

    Candy, Ian; Blockley, Simon; Matthews, Ian; Palmer, Adrian; Darvill, Chris

    2013-04-01

    The interval between the Last Glacial Maximum (L.G.M.) and the stabilisation of climatic conditions during the Holocene (ca 8ka BP) is well-known to have been punctuated with abrupt climatic shifts on a range of time scales. This is clearly seen in the oxygen and deuterium isotope signal of the Greenland ice core records (e.g. NGRIP). How the magnitude and duration of these events translates across a region, such as western Europe, is however, unclear, primarily because many traditional proxies do not respond rapidly enough to provide a clear expression of abrupt climatic events. In this paper we present a range of new oxygen and carbon isotopic records, and a review of existing datasets, from lacustrine carbonate sequences spanning the interval 8 to 15ka BP. These records lie on W-E and N-S transects across the British Isles allowing the spatial variability of the structure and magnitude of abrupt climatic events to be investigated. The oxygen isotopic signal is primarily driven by temperature and indicates that the climatic structure of abrupt events in this time interval, the Lateglacial interstadial for example, is highly variable over relatively small distances. Records from the east of Britain suggest patterns of warming and cooling in the Lateglacial interstadial comparable to that observable in Greenland (e.g. a temperature maximum occurring early in the interstadial), whilst record in the west provide evidence for more subdued climatic oscillations with peak temperatures occurring late in the interstadial. Spatial variability in the isotopic expression of abrupt climatic events allows the role of different factors; i.e. the position of the polar front, the influence of the thermohaline conveyer and the influence of maritime versus continental climates, to be investigated. This work also indicates that a major depletion event occurs in the δ18O value of lake carbonates during the early Holocene which effects all currently known sequences. This depletion

  6. Petrogenesis of the flood basalts from the Early Permian Panjal Traps, Kashmir, India: Geochemical evidence for shallow melting of the mantle

    NASA Astrophysics Data System (ADS)

    Shellnutt, J. Gregory; Bhat, Ghulam M.; Wang, Kuo-Lung; Brookfield, Michael E.; Jahn, Bor-Ming; Dostal, Jaroslav

    2014-09-01

    The Early Permian Panjal Traps of northern India represent a significant eruption of volcanic rocks which occurred during the opening of the Neotethys Ocean. Basaltic, basaltic-andesites, dacitic and rhyolitic rocks collected from Guryal Ravine and Pahalgam show evidence for subaerial and subaqueous eruptions indicating that they are contemporaneous with the formation of a shallow marine basin. The major and trace element geochemistry of the basalts is consistent with a within-plate setting and there are basalts which have high-Ti (TiO2 > 2.0 wt.%) and low-Ti (TiO2 < 1.8 wt.%) compositions. The ‘high-Ti’ basalts are similar to OIB whereas the ‘low-Ti’ basalts are similar to continental tholeiites. The identification of ‘high- and low-Ti’ basalts within the Panjal Traps is analogous to other large igneous provinces (e.g. Karoo, Deccan, Parana, Emeishan). The Sr-Nd isotopic values (εNd(T) = - 5.3 to + 1.3; ISr = 0.70432 to 0.71168) of both types of basalts overlap indicating that the rocks may have originated from the same ancient subcontinental lithospheric (i.e. EMII-like) mantle source (TDM = ~ 2000 Ma). The two groups of basalts can be modeled by using a primitive mantle source and different degrees of partial melting where the high-Ti rocks are produced by ~ 1% partial melting of a spinel peridotite source whereas the low-Ti rocks are produced by ~ 8% partial melting. Trace elemental and isotope modeling indicates that some of the basalts assimilated ≤ 10% crustal material. In contrast, the basaltic-andesites are likely formed by mixing between basaltic magmas and crustal melts which produced rocks with higher SiO2 (~ 55 wt.%) content and enriched isotopic signatures (εNd(T) = - 6.1; ISr = 0.70992). The Panjal Trap volcanism was likely due to partial melting of the SCLM within a passive extensional setting related to the rifting of Cimmeria from Gondwana. Contemporaneous volcanic and plutonic granitic rocks throughout the Himalaya are probably

  7. Magmatic processes that generate chemically distinct silicic magmas in NW Costa Rica and the evolution of juvenile continental crust in oceanic arcs

    NASA Astrophysics Data System (ADS)

    Deering, Chad D.; Vogel, Thomas A.; Patino, Lina C.; Szymanski, David W.; Alvarado, Guillermo E.

    2012-02-01

    Northwestern Costa Rica is built upon an oceanic plateau that has developed chemical and geophysical characteristics of the upper continental crust. A major factor in converting the oceanic plateau to continental crust is the production, evolution, and emplacement of silicic magmas. In Costa Rica, the Caribbean Large Igneous Province (CLIP) forms the overriding plate in the subduction of the Cocos Plate—a process that has occurred for at least the last 25 my. Igneous rocks in Costa Rica older than about 8 Ma have chemical compositions typical of ocean island basalts and intra-oceanic arcs. In contrast, younger igneous deposits contain abundant silicic rocks, which are significantly enriched in SiO2, alkalis, and light rare-earth elements and are geochemically similar to the average upper continental crust. Geophysical evidence (high Vp seismic velocities) also indicates a relatively thick (~40 km), addition of evolved igneous rocks to the CLIP. The silicic deposits of NW Costa Rica occur in two major compositional groups: a high-Ti and a low-Ti group with no overlap between the two. The major and trace element characteristics of these groups are consistent with these magmas being derived from liquids that were extracted from crystal mushes—either produced by crystallization or by partial melting of plutons near their solidi. In relative terms, the high-Ti silicic liquids were extracted from a hot, dry crystal mush with low oxygen fugacity, where plagioclase and pyroxene were the dominant phases crystallizing, along with lesser amounts of hornblende. In contrast, the low-Ti silicic liquids were extracted from a cool, wet crystal mush with high oxygen fugacity, where plagioclase and amphibole were the dominant phases crystallizing. The hot-dry-reducing magmas dominate the older sequence, but the youngest sequence contains only magmas from the cold-wet-oxidized group. Silicic volcanic deposits from other oceanic arcs (e.g., Izu-Bonin, Marianas) have chemical

  8. Controls on OIB and MORB Geochemical Variabilty

    NASA Astrophysics Data System (ADS)

    Shorttle, O.; Maclennan, J.

    2014-12-01

    The geochemical variability preserved in Ocean Island and Mid-ocean ridge basalt (MORB) is a key tracer of the magmatic storage and transport processes they experience during their ascent through the mantle and crust. The effect of these processes is to collapse the huge diversity of melt compositions predicted to form during polybaric fractional melting of a lithologically heterogeneous mantle, into the narrow range we see expressed in most ocean island and mid-ocean ridge settings. Magma mixing can therefore be seen as contaminating the variance structure of primitive mantle melts, akin to the way in which wall-rock assimilation contaminates melts by chemical addition. The key observation from the melt inclusion and whole-rock records from ocean islands such as Iceland, is that as crystallisation proceeds mixing in magma chambers progressively reduces geochemical variability, until by ~5wt% MgO almost all primary chemical diversity has been lost. These chemical systematics allow us to extend the observations made at ocean islands to make predictions about how mixing processes should operate in MORB generally and the key factors controlling mixing efficiency: melt flow out of the mantle, crustal thickness, magma supply rate, and by extension spreading rate, and mantle potential temperature. However, with its low sampling density, the global MORB database does not easily allow testing of these hypotheses. We have developed a novel geospatial statistical analysis to bridge the gap between observations made on a small scale - at single ocean islands and ridge segments - to the entire global dataset of MORB chemistry. By analysing the geochemical variance in MORB over a range of bandwidths we have captured the ~200km lengthscale at which the simple relationships between geochemical variability and MgO appear. Our results demonstrate that on short lengthscales mantle chemical structure and magmatic processes operate coherently in destruction of geochemical variability

  9. The petrogenesis of felsic calc-alkaline magmas from the southernmost Cascades, California: origin by partial melting of basaltic lower crust

    USGS Publications Warehouse

    Borg, L.E.; Clynne, M.A.

    1998-01-01

    The majority offelsic rocks from composite centers in teh southernmost Cascades have geochemical and Sr, Nd and Pb isotopic ratios that suggest derivation by partial melting of lower crust that is compositionally similar to cale-alkaline basalts observed in the region. Only a few felsic rocks have ???18O and Pb isotopic compositions that indicate interaction with the upper crust. Mineralogical and geochemical differences among the felsic magmas results primarily from melting under variable f(H2O) and lower temperature conditions leaves an amphibole-rich residuum, and produced magmas that have amphibole ?? biotite phenocrysts, relatively high silica contents, and pronounced middle rare earch element depletions. These conclusions are consistent with published thermal models that suggest that reasonable volumes of basaltic magma emplaced beneath large composite centers in the southernmost Cascades can serve as the eat source for melting of the lower crust. Melting of the lower crust under varible f(H2O contents of these basaltic magmas.

  10. Continental crust beneath southeast Iceland.

    PubMed

    Torsvik, Trond H; Amundsen, Hans E F; Trønnes, Reidar G; Doubrovine, Pavel V; Gaina, Carmen; Kusznir, Nick J; Steinberger, Bernhard; Corfu, Fernando; Ashwal, Lewis D; Griffin, William L; Werner, Stephanie C; Jamtveit, Bjørn

    2015-04-14

    The magmatic activity (0-16 Ma) in Iceland is linked to a deep mantle plume that has been active for the past 62 My. Icelandic and northeast Atlantic basalts contain variable proportions of two enriched components, interpreted as recycled oceanic crust supplied by the plume, and subcontinental lithospheric mantle derived from the nearby continental margins. A restricted area in southeast Iceland--and especially the Öræfajökull volcano--is characterized by a unique enriched-mantle component (EM2-like) with elevated (87)Sr/(86)Sr and (207)Pb/(204)Pb. Here, we demonstrate through modeling of Sr-Nd-Pb abundances and isotope ratios that the primitive Öræfajökull melts could have assimilated 2-6% of underlying continental crust before differentiating to more evolved melts. From inversion of gravity anomaly data (crustal thickness), analysis of regional magnetic data, and plate reconstructions, we propose that continental crust beneath southeast Iceland is part of ∼350-km-long and 70-km-wide extension of the Jan Mayen Microcontinent (JMM). The extended JMM was marginal to East Greenland but detached in the Early Eocene (between 52 and 47 Mya); by the Oligocene (27 Mya), all parts of the JMM permanently became part of the Eurasian plate following a westward ridge jump in the direction of the Iceland plume. PMID:25825769

  11. Continental crust beneath southeast Iceland.

    PubMed

    Torsvik, Trond H; Amundsen, Hans E F; Trønnes, Reidar G; Doubrovine, Pavel V; Gaina, Carmen; Kusznir, Nick J; Steinberger, Bernhard; Corfu, Fernando; Ashwal, Lewis D; Griffin, William L; Werner, Stephanie C; Jamtveit, Bjørn

    2015-04-14

    The magmatic activity (0-16 Ma) in Iceland is linked to a deep mantle plume that has been active for the past 62 My. Icelandic and northeast Atlantic basalts contain variable proportions of two enriched components, interpreted as recycled oceanic crust supplied by the plume, and subcontinental lithospheric mantle derived from the nearby continental margins. A restricted area in southeast Iceland--and especially the Öræfajökull volcano--is characterized by a unique enriched-mantle component (EM2-like) with elevated (87)Sr/(86)Sr and (207)Pb/(204)Pb. Here, we demonstrate through modeling of Sr-Nd-Pb abundances and isotope ratios that the primitive Öræfajökull melts could have assimilated 2-6% of underlying continental crust before differentiating to more evolved melts. From inversion of gravity anomaly data (crustal thickness), analysis of regional magnetic data, and plate reconstructions, we propose that continental crust beneath southeast Iceland is part of ∼350-km-long and 70-km-wide extension of the Jan Mayen Microcontinent (JMM). The extended JMM was marginal to East Greenland but detached in the Early Eocene (between 52 and 47 Mya); by the Oligocene (27 Mya), all parts of the JMM permanently became part of the Eurasian plate following a westward ridge jump in the direction of the Iceland plume.

  12. Magnetization of the Lunar Crust

    NASA Technical Reports Server (NTRS)

    Carley, R. A.; Whaler, K. A.; Purucker, M. E.; Halekas, J. S.

    2012-01-01

    Magnetic fields measured by the satellite Lunar Prospector show large scale features resulting from remanently magnetized crust. Vector data synthesized at satellite altitude from a spherical harmonic model of the lunar crustal field, and the radial component of the magnetometer data, have been used to produce spatially continuous global magnetization models for the lunar crust. The magnetization is expressed in terms of localized basis functions, with a magnetization solution selected having the smallest root-mean square magnetization for a given fit to the data, controlled by a damping parameter. Suites of magnetization models for layers with thicknesses between 10 and 50 km are able to reproduce much of the input data, with global misfits of less than 0.5 nT (within the uncertainties of the data), and some surface field estimates. The magnetization distributions show robust magnitudes for a range of model thicknesses and damping parameters, however the magnetization direction is unconstrained. These global models suggest that magnetized sources of the lunar crust can be represented by a 30 km thick magnetized layer. Average magnetization values in magnetized regions are 30-40 mA/m, similar to the measured magnetizations of the Apollo samples and significantly weaker than crustal magnetizations for Mars and the Earth. These are the first global magnetization models for the Moon, providing lower bounds on the magnitude of lunar crustal magnetization in the absence of multiple sample returns, and can be used to predict the crustal contribution to the lunar magnetic field at a particular location.

  13. Continental crust beneath southeast Iceland

    PubMed Central

    Torsvik, Trond H.; Amundsen, Hans E. F.; Trønnes, Reidar G.; Doubrovine, Pavel V.; Gaina, Carmen; Kusznir, Nick J.; Steinberger, Bernhard; Corfu, Fernando; Ashwal, Lewis D.; Griffin, William L.; Werner, Stephanie C.; Jamtveit, Bjørn

    2015-01-01

    The magmatic activity (0–16 Ma) in Iceland is linked to a deep mantle plume that has been active for the past 62 My. Icelandic and northeast Atlantic basalts contain variable proportions of two enriched components, interpreted as recycled oceanic crust supplied by the plume, and subcontinental lithospheric mantle derived from the nearby continental margins. A restricted area in southeast Iceland—and especially the Öræfajökull volcano—is characterized by a unique enriched-mantle component (EM2-like) with elevated 87Sr/86Sr and 207Pb/204Pb. Here, we demonstrate through modeling of Sr–Nd–Pb abundances and isotope ratios that the primitive Öræfajökull melts could have assimilated 2–6% of underlying continental crust before differentiating to more evolved melts. From inversion of gravity anomaly data (crustal thickness), analysis of regional magnetic data, and plate reconstructions, we propose that continental crust beneath southeast Iceland is part of ∼350-km-long and 70-km-wide extension of the Jan Mayen Microcontinent (JMM). The extended JMM was marginal to East Greenland but detached in the Early Eocene (between 52 and 47 Mya); by the Oligocene (27 Mya), all parts of the JMM permanently became part of the Eurasian plate following a westward ridge jump in the direction of the Iceland plume. PMID:25825769

  14. Overlapping Sr-Nd-Hf-O isotopic compositions in Permian mafic enclaves and host granitoids in Alxa Block, NW China: Evidence for crust-mantle interaction and implications for the generation of silicic igneous provinces

    NASA Astrophysics Data System (ADS)

    Dan, Wei; Wang, Qiang; Wang, Xuan-Ce; Liu, Yu; Wyman, Derek A.; Liu, Yong-Sheng

    2015-08-01

    In general, the mantle provides heat and/or material for the generation of the silicic igneous provinces (SIPs). The rarity of mafic microgranular enclaves (MMEs), however, hampers understanding of the mantle's role in generating SIPs and the process of crust-mantle interaction. The widespread distributed MMEs in the newly reported Alxa SIP provide an opportunity to study these processes. This study integrates in situ zircon U-Pb age and Hf-O isotope analyses, whole-rock geochemistry and Sr-Nd isotope results for the MMEs and host granitoids in the Alxa Block. SIMS zircon U-Pb dating reveals that there are two generations of MMEs and host granitoids. The MMEs in the Bayannuoergong batholith were formed at ca. 278 Ma, similar to the age (280 Ma) of host granitoids, and the MMEs and host granitoids in the Yamaitu pluton were formed at ca. 272-270 Ma. All MMEs have relatively low SiO2 (50.7-61.4 wt.%) and Th (0.8-2.8 ppm), but relatively high MgO (2.6-4.9 wt.%), Cr (23-146 ppm) and Ni (6-38 ppm) contents compared to the host granitoids, with SiO2 (63.6-77.5 wt.%), Th (5.2-41 ppm), MgO (0.23-2.1 wt.%), Cr (10-38 ppm) and Ni (5-14 ppm). All MMEs have whole rock Sr-Nd and zircon Hf-O isotope compositions similar to their corresponding host granitoids. The 280 Ma MMEs have lower whole rock εNd(t) (- 13.5) and higher initial 87Sr/86Sr values (0.7095) and zircon δ18O values (6.3‰) compared to the εNd(t) (- 11.5), initial 87Sr/86Sr values (0.7070) and zircon δ18O values (5.6‰) of the 270 Ma MMEs. The occurrences of quartz xenocrysts, K-feldspar megacrysts, corroded feldspars and acicular apatites indicate that the MMEs are the products of the mixing between mantle- and crust-derived magmas. The striking similarities in the zircon Hf-O isotopic compositions in both MME-host granitoid pairs indicate that the granitoids and MMEs have similar sources. The granitoids are proposed to be mainly sourced from magmas generated by remelting of newly formed mafic rocks, which

  15. Early evolution of the continental crust, the oxygenated atmosphere and oceans, and the heterogeneous mantle

    NASA Astrophysics Data System (ADS)

    Ohmoto, H.

    2011-12-01

    The current paradigm for the evolution of early Earth is that, only since ~2.5 Ga ago, the Earth began to: (a) form a large granitic continental crust; (b) form an oxygenated atmosphere; (c) operate oxidative weathering of rocks on land; (d) form Fe-poor, but S-, U- and Mo-rich, oceans; (e) operate large-scale transfers of elements between oceans and oceanic crust at MORs; (f) subduct the altered oceanic crust; (g) create the mantle heterogeneity, especially in the concentrations and isotopic compositions of Fe(III), Fe(II), U, Pb, alkali elements, C, S, REEs, and many other elements; (h) create chemical and isotopic variations among OIB-, OPB-, and MORB magmas, and between I- and S-type granitoid magmas; and (i) create variations in the chemical and isotopic compositions of volcanic gas. Submarine hydrothermal fluids have typically developed from seawater-rock interactions during deep (>2 km) circulation of seawater through underlying hot volcanic rocks. When the heated hydrothermal fluids ascend toward the seafloor, they mix with local bottom seawater to precipitate a variety of minerals on and beneath the seafloor. Thus, the mineralogy and geochemistry of submarine hydrothermal deposits and associated volcanic rocks can be used to decipher the chemistry of the contemporaneous seawater, which in turn indicate the chemistry of the atmosphere and the compositions and size of the continental crust. The results of mineralogical and geochemical investigations by our and other research groups on submarine hydrothermal deposits (VMS and BIF) and hydrothermally-altered submarine volcanic rocks in Australia, South Africa, and Canada, ~3.5-2.5 Ga in ages, suggest that the above processes (a)-(i) had began by ~3.5 Ga ago. Supportive evidence includes, but not restricted to, the similarities between Archean submarine rocks and modern ones in: (1) the abundance of ferric oxides; (2) the Fe(III)/Fe(I) ratios; (3) the abundance of barite; (4) the increased Li contents; (5) the

  16. Geochemical modeling: a review

    SciTech Connect

    Jenne, E.A.

    1981-06-01

    Two general families of geochemical models presently exist. The ion speciation-solubility group of geochemical models contain submodels to first calculate a distribution of aqueous species and to secondly test the hypothesis that the water is near equilibrium with particular solid phases. These models may or may not calculate the adsorption of dissolved constituents and simulate the dissolution and precipitation (mass transfer) of solid phases. Another family of geochemical models, the reaction path models, simulates the stepwise precipitation of solid phases as a result of reacting specified amounts of water and rock. Reaction path models first perform an aqueous speciation of the dissolved constituents of the water, test solubility hypotheses, then perform the reaction path modeling. Certain improvements in the present versions of these models would enhance their value and usefulness to applications in nuclear-waste isolation, etc. Mass-transfer calculations of limited extent are certainly within the capabilities of state-of-the-art models. However, the reaction path models require an expansion of their thermodynamic data bases and systematic validation before they are generally accepted.

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

    NASA Astrophysics Data System (ADS)

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

    2015-10-01

    In our paper, we suggested that the Junan granulite xenoliths and xenocrysts record evolution of the Precambrian lower crust beneath the southeastern North China Craton (NCC). Yuan and Xia (2015) disagree with us. However, they have not fully considered the evolutional histories of the NCC lithosphere, and geochemical and isotopic compositions of the Junan xenoliths. We also contend that they have misinterpreted the available geophysical data. Synthesizing the geochronological characteristics of the NCC lower crust, nature of the Junan granulite xenoliths, and reinterpretation of the resistivity profile, we again emphasize that the Junan granulite xenoliths are tectonically affiliated to the NCC lower crust, and the Junan zircon data could reflect the complex evolution of the lower crust beneath the southeastern NCC.

  18. Geochemical evidence of present-day serpentinization

    USGS Publications Warehouse

    Barnes, I.; LaMarche, Valmore C.; Himmelberg, G.

    1967-01-01

    Ultrabasic (pH > 11) water issues from some fresh ultramafic bodies. The properties of the ultrabasic solutions are believed to be due to current reactions yielding serpentine from primary olivines and pyroxenes. The low concentrations of divalent iron, divalent magnesium, and dissolved silica from the serpentinization require an increase in rock volume.

  19. Mineralogical and geochemical evidence of Quachita provenance

    SciTech Connect

    Sutton, S.J. . Dept. of Earth Resources); Land, L.S.; Hutson, F. . Dept. of Geological Science); Awwiller, D.N. . Dept. of Geology)

    1994-03-01

    Provenance of the extremely thick Ouachita flysch has not been specifically identified, despite several recent studies. Detrital mineral compositions, Sm-Nd model ages, and single zircon dates strongly suggest that several sources contributed sediment that became well-mixed before deposition. Sm-Nd model ages are nearly uniform for flysch samples, regardless of geographic position or exact stratigraphic position. These ages appear to rule out significant contribution by a Paleozoic island arc, and suggest a single source, multiple sources with similar model ages, or thorough mixing of sources with different model ages. In contrast, single zircon ages are variable, including Archean, Proterozoic, and Paleozoic ages. These ages indicate multiple sediment sources, including some external to North America. The zircon ages, however, may not be generally representative of Ouachita sediment sources because they were obtained on zircons far larger than typical Ouachita detrital zircons. The Sm-Nd model ages and single zircon ages can both be explained if sediment entering the Ouachita Trough came from multiple sources, but was already thoroughly mixed before entering the trough. Such mixing is also suggested by detrital mineral analyses. Muscovite, garnet, and tourmaline have been analyzed.

  20. Origin of water and mantle-crust interactions on Mars inferred from hydrogen isotopes and volatile element abundances of olivine-hosted melt inclusions of primitive shergottites

    NASA Astrophysics Data System (ADS)

    Usui, Tomohiro; Alexander, Conel M. O'D.; Wang, Jianhua; Simon, Justin I.; Jones, John H.

    2012-12-01

    Volatile elements have influenced the differentiation and eruptive behavior of Martian magmas and played an important role in the evolution of Martian climate and near-surface environments. However, the abundances of volatiles, and in particular the amount of water in the Martian interior, are disputed. A record of volatile reservoirs is contained in primitive Martian basalts (shergottites). Olivine-hosted melt inclusions from a geochemically depleted shergottite (Yamato 980459, representing a very primitive Martian melt) possess undegassed water with a chondritic and Earth-like D/H ratio (δD≤275‰). Based on volatile measurements in these inclusions, the water content of the depleted shergottite mantle is calculated to be 15-47 ppm, which is consistent with the dry mantle hypothesis. In contrast to D/H in the depleted shergottite, melt from an enriched shergottite (Larkman Nunatak 06319), which either formed by melting of an enriched mantle or by assimilation of crust, exhibits an extreme δD of ˜5000‰, indicative of a surface reservoir (e.g., the Martian atmosphere or crustal hydrosphere). These data provide strong evidence that the Martian mantle had retained the primordial low-δD component until at least the time of shergottite formation, and that young Martian basalts assimilated old Martian crust.

  1. History of the earth's crust

    SciTech Connect

    Eicher, D.L.; Mcalester, A.L.; Rottman, M.L.

    1984-01-01

    The history of the earth's crust since its formation 4.6 Gyr ago is traced in an introductory textbook, with consideration of the global climate and the general outline of biological evolution. The methodology of paleogeology is introduced, and the origin of the solar system, the accumulation and differentiation of the earth, the beginnings of life, and the history of the moon are examined. Separate chapters are then devoted to the Precambrian, Paleozoic, Mesozoic, and Cenozoic earth. Photographs, maps, diagrams, and drawings are provided. 49 references.

  2. Implications for the evolution of continental crust from Hf isotope systematics of Archean detrital zircons

    NASA Technical Reports Server (NTRS)

    Stevenson, Ross K.; Patchett, P. Jonathan

    1990-01-01

    Results from the fractionation of zircon by sedimentary processes into continental margin sandstone yield information on the preservation of preexisting continental crust in the form of zircon, making it possible to distinguish between the contrasting theories of gradual growth versus constant volume of continental crust over geologic time. In this work, Hf-176/Hf-177 ratios were determined for detrital zircon fractions from 2.0-2.5, 2.6-3.0, and pre-3.0 Gyr old sandstones from the Canadian-Shield, the North-Atlantic, the Wyoming, and the Kaapvaal Cratons. Results pointed to small amounts of continental crust prior to 3.0 Gyr ago and a rapid addition of continental crust between 2.5 and 3.0 Gyr ago, consistent with the gradual growth of continental crust, and giving evidence against no-growth histories.

  3. Seismic evidence for a mantle source for mid-Proterozoic anorthosites and implications for models of crustal growth

    USGS Publications Warehouse

    Musacchio, G.; Mooney, W.D.

    2002-01-01

    Voluminous anorthosite intrusions are common in mid-Proterozoic crust. Historically, two end-member models have been proposed for the origin of these anorthosites. In the first model anorthosites derive from fractionation of a mantle source leaving a residue of metagabbro in the lower crust; in the second model anorthosites are the product of partial melting of the lower crust with residual pyroxene and high-grade minerals (i.e. a pyroxenitic and/or metapelitic lower crust). Although a general consensus has developed that the first model provides the best fit to petrological and geochemical constraints, the sparse evidence for mafic and ultramafic counterparts to the anorthosites leaves the issue still unresolved. We use the absolute P-wave velocity and the ratio between P- and S-wave velocities (VP/VS) to infer the composition of the lower crust beneath the Marcy Anorthosite (New York State, USA). Seismic refraction data reveal a lower crust 20 km thick, where VP and VP/VS range from top to bottom between 7.0 km s-1 and 7.2 ?? 0.1 and 1.84 km s-1 and 1.81 ?? 0.02, respectively. Laboratory measurements on rock samples indicate that these seismic properties are typical of plagioclase-rich rocks. Magmatic underplating of basaltic melts is a mechanism to form plagioclase-rich bulk composition for the Grenville crust. At the bottom of the lower crust, increase of P-wave velocity, slight decrease of VP/VS ratios and the presence of a low-reflective seismic Moho are additional observations supporting crust-mantle interactions related to magmatic underplating. High P-wave velocity (8.6 km s-1) in the upper mantle may indicate that the ultramafic portion (e.g. pyroxenites) of the underplated magma has become eclogite. High average P-wave velocity (6.7 km s-1) and VP/VS (1.81), and the exceptional abundance of anorthosites-norites-troctolites among the rocks exposed at the surface, indicate that the Grenville Proterozoic crust may have a unique plagioclase-rich bulk

  4. Late Hadean-Eoarchean transitions in crustal evolution from Hf isotopic evidence in the Jack Hills zircons

    NASA Astrophysics Data System (ADS)

    Bell, E. A.; Harrison, M.; Kohl, I. E.; Young, E. D.

    2013-12-01

    The evolution of the Earth's earliest crust remains largely unknown due to the dearth of Hadean (>4 Ga) rocks, with most observational evidence of the planet's first few hundred million years deriving from geochemical studies of 4.4-4.0 Ga detrital zircons from Jack Hills (Narryer Gneiss Complex, Yilgarn craton). Previous Lu-Hf investigations of the zircons have suggested that continental-like (low Lu/Hf) crust formation began by ~4.4-4.5 Ga and may have continued for several hundred million years. The most ancient crust represented in the Jack Hills population was preserved until at least ~4 Ga. However, evidence for the involvement of Hadean materials in later crustal evolution is sparse, and even at Jack Hills the most unradiogenic, ancient materials represented by some Hadean zircons have not been identified in the younger rock and zircon record. We present new Lu-Hf results from <4 Ga Jack Hills zircons that indicate an important transition in Yilgarn crustal evolution between 4.0 and 3.6 Ga. Although Hadean samples are permissive of crustal extraction from the mantle up to ~4 Ga, crust in the Jack Hills source evolves dominantly by internal reworking 4.0-3.8 Ga, and both the most ancient and most juvenile components of the crust are lost from the zircon record after ~4 Ga. New juvenile additions to the crust at ~3.8-3.7 Ga are accompanied by the disappearance of crust with model ages >4.3 Ga. These new data indicate a tectonic regime in the Eoarchean (4.0-3.6 Ga) Yilgarn characterized by internal crustal reworking punctuated by one relatively short juvenile crust extraction event. The coupling in time of juvenile crust formation with the loss of ancient crust is best explained by a mechanism similar to subduction, which accomplishes both processes on the modern Earth. We interpret these data as consistent with the action of destructive plate boundary interactions by Eoarchean times.

  5. Recycling lower continental crust in the North China craton.

    PubMed

    Gao, Shan; Rudnick, Roberta L; Yuan, Hong-Ling; Liu, Xiao-Ming; Liu, Yong-Sheng; Xu, Wen-Liang; Ling, Wen-Li; Ayers, John; Wang, Xuan-Che; Wang, Qing-Hai

    2004-12-16

    Foundering of mafic lower continental crust into underlying convecting mantle has been proposed as one means to explain the unusually evolved chemical composition of Earth's continental crust, yet direct evidence of this process has been scarce. Here we report that Late Jurassic high-magnesium andesites, dacites and adakites (siliceous lavas with high strontium and low heavy-rare-earth element and yttrium contents) from the North China craton have chemical and petrographic features consistent with their origin as partial melts of eclogite that subsequently interacted with mantle peridotite. Similar features observed in adakites and some Archaean sodium-rich granitoids of the tonalite-trondhjemite-granodiorite series have been interpreted to result from interaction of slab melts with the mantle wedge. Unlike their arc-related counterparts, however, the Chinese magmas carry inherited Archaean zircons and have neodymium and strontium isotopic compositions overlapping those of eclogite xenoliths derived from the lower crust of the North China craton. Such features cannot be produced by crustal assimilation of slab melts, given the high Mg#, nickel and chromium contents of the lavas. We infer that the Chinese lavas derive from ancient mafic lower crust that foundered into the convecting mantle and subsequently melted and interacted with peridotite. We suggest that lower crustal foundering occurred within the North China craton during the Late Jurassic, and thus provides constraints on the timing of lithosphere removal beneath the North China craton. PMID:15602559

  6. Recycling lower continental crust in the North China craton.

    PubMed

    Gao, Shan; Rudnick, Roberta L; Yuan, Hong-Ling; Liu, Xiao-Ming; Liu, Yong-Sheng; Xu, Wen-Liang; Ling, Wen-Li; Ayers, John; Wang, Xuan-Che; Wang, Qing-Hai

    2004-12-16

    Foundering of mafic lower continental crust into underlying convecting mantle has been proposed as one means to explain the unusually evolved chemical composition of Earth's continental crust, yet direct evidence of this process has been scarce. Here we report that Late Jurassic high-magnesium andesites, dacites and adakites (siliceous lavas with high strontium and low heavy-rare-earth element and yttrium contents) from the North China craton have chemical and petrographic features consistent with their origin as partial melts of eclogite that subsequently interacted with mantle peridotite. Similar features observed in adakites and some Archaean sodium-rich granitoids of the tonalite-trondhjemite-granodiorite series have been interpreted to result from interaction of slab melts with the mantle wedge. Unlike their arc-related counterparts, however, the Chinese magmas carry inherited Archaean zircons and have neodymium and strontium isotopic compositions overlapping those of eclogite xenoliths derived from the lower crust of the North China craton. Such features cannot be produced by crustal assimilation of slab melts, given the high Mg#, nickel and chromium contents of the lavas. We infer that the Chinese lavas derive from ancient mafic lower crust that foundered into the convecting mantle and subsequently melted and interacted with peridotite. We suggest that lower crustal foundering occurred within the North China craton during the Late Jurassic, and thus provides constraints on the timing of lithosphere removal beneath the North China craton.

  7. Pacific ferromanganese crust geology and geochemistry

    SciTech Connect

    Andreev, S.I.; Vanstein, B.G.; Anikeeva, L.I. )

    1990-06-01

    Cobaltiferous ferromanganese crusts form part of a large series of oceanic ferromanganese oxide deposits. The crusts show high cobalt (commonly over 0.4%), low nickel and copper sum (0.4-0.8%), considerably high manganese (18-20%), and iron (14-18%). Less abundant elements in crusts are represented by molybdenum and vanadium; the rare-earth elements cerium, lanthenum, and yttrium; and the noble metals platinum and rhodium. Co-rich crusts form at water depths of 600 to 2,500 m. Crust thicknesses range from millimeters to 15-17 cm, averaging 2-6 cm. The most favorable conditions for 4-10 cm thick crusts to occur is at water depths of 1,200-2,200 m. The crusts formed on basaltic, calcareous, siliceous, and breccia bedrock surfaces provided there were conditions preventing bottom sedimentation at them. If the sedimentation takes place, it may be accompanied by nodules similar in composition to the crusts. The most favorable topography for extensive crust formation is considered to be subdued (up to 20{degree}) slopes and summit platforms of conical seamounts, frequently near faults and their intersection zones. Subhorizontal guyot summits do not usually favor crust growth. Crust geochemistry is primarily defined by mineralogy and manganese hydroxides (vernadite)/iron ratio. The first associated group of compounds includes cobalt, nickel, molybdenum, vanadium, cerium, and titanium; the other is strontium, yttrium, cerium, and cadmium. The aluminosilicate phase is associated with titanium, iron, chromium, and vanadium; phosphate biogenic phase includes copper, nickel, zinc, lead, and barium. The crucial point in cobaltiferous crust formation is their growth rate on which is dependent the degree of ferromanganese matrix sorption saturation with cobalt. The optimum for cobalt-rich ferromanganese ores is the conditions facilitating long-term and continuous hydrogenic processes.

  8. Pulsar glitches: the crust is not enough.

    PubMed

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

    2012-12-14

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

  9. Pulsar glitches: the crust is not enough.

    PubMed

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

    2012-12-14

    Pulsar glitches are traditionally viewed as a manifestation of vortex dynamics associated with a neutron superfluid reservoir confined to the inner crust of the star. In this Letter we show that the nondissipative entrainment coupling between the neutron superfluid and the nuclear lattice leads to a less mobile crust superfluid, effectively reducing the moment of inertia associated with the angular momentum reservoir. Combining the latest observational data for prolific glitching pulsars with theoretical results for the crust entrainment, we find that the required superfluid reservoir exceeds that available in the crust. This challenges our understanding of the glitch phenomenon, and we discuss possible resolutions to the problem. PMID:23368300

  10. The role of soil surface crusting in desertification and strategies to reduce crusting.

    PubMed

    Fox, D M; Bryan, R B; Price, A G

    2004-12-01

    Soil crusting decreases infiltration, increases erosion, and impedes vegetation establishment, so reducing the impact of crusting is of major importance in combating desertification. Although surface crusting has been the subject of considerable research over the past 50 years or more, the practical management of soil crusts remains a challenge for many dryland communities. Crusting occurs in two steps, an initial aggregate breakdown period that occurs under rainfall and a subsequent hardening phase during drying. Several factors influence crust development, but the single most important one is soil aggregate stability. Strategies to reduce crusting can be based either on protecting the surface from raindrop impact or improving aggregate stability, or a combination of both. However, crust control is labor and/or capital intensive and must be thought out clearly in terms of the benefits to be achieved.

  11. Man as an object of geochemical and geophysical influences

    NASA Astrophysics Data System (ADS)

    Stoilova, Irina

    There are an increasing number of papers in the last years that evidence of a correlation between geochemical and geophysical factors and human health parameters and human behaviour. The basic factors that could affect human health and behaviour are: the geochemical composition of the geographical environment; the tectonic processes; the geomagnetic field variations (GMV), the climatic changes and the changes of the solar activity as well as the fact that all of them could influence mutually each other. The subject of this paper is the theoretical basis of the geochemical and geophysical influences on human health. The biological mechanisms according to which the geomagnetic field influences the psychological and behavioural reactions of people are not highlighted or identified yet. We present some of the existing suggestions and theories trying to explain these mechanisms. The studies performed in this area and the obtained results will be very useful in developing measures to protect man from the harmful influence of geochemical and geophysical factors.

  12. Evolution of the Chilka Lake granulite complex, northern Eastern Ghats Belt, India: First evidence of ~ 780 Ma decompression of the deep crust and its implication on the India-Antarctica correlation

    NASA Astrophysics Data System (ADS)

    Bose, S.; Das, K.; Torimoto, J.; Arima, M.; Dunkley, D. J.

    2016-10-01

    High-grade para- and orthogneissic rocks near the Chilka Lake granulite complex, northern part of the Eastern Ghats Belt show complex structural and petrological history. Based on field and petrographic characters, five (M1-M5) metamorphic events could be identified. The earliest metamorphic event (M1) produced amphibolite grade mineral assemblage which produced the peak granulite (M2) assemblages at 900-950 °C, 8.5-9.0 kbar. The third metamorphic event caused decompression of the deeper crust up to 700-800 °C, 6.0-6.5 kbar. This was followed by cooling (M4) and subsequent thermal overprinting (M5). Fluid-composition during M3 was dominated by high-density CO2 and changed to low-density mixed CO2-H2O during the M3. Zircon U-Pb SHRIMP data suggest 781 ± 9 Ma age for M3 event. Texturally constrained monazite U-Th-Pb EPMA data, on the other hand, yield a group age of 988 ± 23 Ma from grain interior, which can signifies the age of M2 event. Few spots with younger dates in the range of 550-500 Ma are also noted. This interpretation changes the existing tectonothermal history of northern Eastern Ghats Belt. Our data show that the two adjacent crustal domains of the Eastern Ghats Belt show distinctly contrasting Neoproterozoic histories. While the central Domain 2 evolved through early anticlockwise P-T path culminating in ultrahigh temperature, the northern Domain 3 evolved through a clockwise P-T path. It appears that the Domain 3 was contiguous to East Antarctica and became part of the Eastern Ghats Belt during the assembly of Gondwana. The ca. 780 Ma decompression event in the northern Eastern Ghats Belt opens up new possibilities for interpreting the breakup of Rodinia.

  13. High-temperature fracturing and subsequent grain-size-sensitive creep in lower crustal gabbros: Evidence for coseismic loading followed by creep during decaying stress in the lower crust?

    NASA Astrophysics Data System (ADS)

    Okudaira, Takamoto; Jeřábek, Petr; Stünitz, Holger; Fusseis, Florian

    2015-05-01

    The mechanism of shear zone formation in lower crustal, relatively "dry" rocks is still poorly understood. We have studied the high-temperature deformation of the Hasvik gabbro (northern Norway) which commences by fracturing. The 10-20 µm wide fractures show little displacement. The fine-grained plagioclase and orthopyroxene in the fractures lack a crystallographic preferred orientation (CPO) or a systematic crystallographic orientation with respect to the host grains. Fractures grade into narrow shear zones, which are composed of fine (10-20 µm), equant grains of recrystallized plagioclase, amphibole, and pyroxene. Recrystallized plagioclase and pyroxene have compositions different from the magmatic grains, suggesting that they have formed by nucleation and growth. Based on conventional plagioclase-amphibole thermobarometry, the shear zones have formed at temperatures and pressures of 700-750°C and 0.5-0.6 GPa. The observed primary minerals cut by fractures suggest high-temperature fracturing in the absence of high pore pressures, which implies a high strength of the lower crustal gabbros and high stresses at fracturing. The shear zones are characterized by the lack of CPO and a small grain size, suggesting that the mechanism of deformation of the fine-grained plagioclase and orthopyroxene has been grain boundary sliding accommodated by diffusive mass transfer. The amphibole grains have strong CPOs, which most likely result from oriented growth and/or rigid body rotations during deformation. The process that initiated the fracturing and subsequent viscous creep in the Hasvik gabbro may have resulted from a process of coseismic loading followed by creep during decaying stress in the lower crust.

  14. Crustal radiogenic heat production and the selective survival of ancient continental crust

    NASA Technical Reports Server (NTRS)

    Morgan, P.

    1985-01-01

    It is pointed out that the oldest terrestrial rocks have so far revealed no evidence of the impact phase of earth evolution. This observation suggests that processes other than impact were dominant at the time of stabilization of these units. However, a use of the oldest terrestrial rocks as a sample of the early terrestrial crust makes it necessary to consider the possibility that these rocks may represent a biased sample. In the present study, the global continental heat flow data set is used to provide further evidence that potassium, uranium, and thorium abundances are, on the average, low in surviving Archean crust relative to younger continental crust. An investigation is conducted of the implications of relatively low crustal radiogenic heat production to the stabilization of early continental crust, and possible Archean crustal stabilization models are discussed.

  15. Crustal radiogenic heat production and the selective survival of ancient continental crust

    NASA Technical Reports Server (NTRS)

    Morgan, P.

    1985-01-01

    It is pointed out that the oldest terrestrial rocks have so far revealed no evidence of the impact phase of Earth evolution. This observation suggests that processes other than impact were dominant at the time of stabilization of these units. However, a use of the oldest terrestrial rocks as a sample of the early terrestrial crust makes it necessary to consider the possibility that these rocks may represent a biased sample. In the present study, the global continental heat flow data set is used to provide further evidence that potassium, uranium, and thorium abundances are, on the average, low in surviving Archean crust relative to younger continental crust. An investigation is conducted of the implications of relatively low crustal radiogenic heat production to the stabilization of early continental crust, and possible Archean crustal stabilization models are discussed.

  16. The extent of continental crust beneath the Seychelles

    NASA Astrophysics Data System (ADS)

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

    2013-11-01

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

  17. Global geochemical problems

    NASA Technical Reports Server (NTRS)

    Harriss, R. C.

    1980-01-01

    Application of remote sensing techniques to the solution of geochemical problems is considered with emphasis on the 'carbon-cycle'. The problem of carbon dioxide sinks and the areal extent of coral reefs are treated. In order to assess the problems cited it is suggested that remote sensing techniques be utilized to: (1)monitor globally the carbonate and bicarbonate concentrations in surface waters of the world ocean; (2)monitor the freshwater and oceanic biomass and associated dissolved organic carbon; (3) inventory the coral reef areas and types and the associated oceanographic climatic conditions; and (4)measure the heavy metal fluxes from forested and vegetated areas, from volcanos, from different types of crustal rocks, from soils, and from sea surfaces.

  18. Juvenile accretion (2360-2330 Ma) in the São Francisco Craton, and implications for the Columbia supercontinent: evidence from U-Pb zircon ages, Sr-Nd-Hf and geochemical constraints

    NASA Astrophysics Data System (ADS)

    Teixeira, W.; Ávila, C.

    2012-12-01

    The Mineiro and the Itabuna-Salvador-Curaçá belts are segments of an Early Proterozoic orogen, in the São Francisco/West Congo-North Gabon craton. The latter segment includes island-arc rocks with preserved portions of the accretionary prism and back-arc basins, developed between 2.4 and 2.0 Ga. The Mineiro belt evolved marginally to the Minas passive margin basin (<2.55 to 2.35 Ga). It contains mainly granitoid rocks with ages between 2.25-2.20 Ga and 2.12-2.08 Ga, along with coeval back arc sequences. The overall framework includes regional metamorphism and related faults and shear zones across both belts. Similar tectonic features are portrayed by the West Central African belt (of Eburnean age) by considering the early contiguous African counterpart. We present an integrated geochronologic and geochemical study for the Resende Costa orthogneiss (Mineiro belt): the gneissic rocks are slightly metaluminous to peraluminous, subalkaline, show varied SiO2 (69 to 73wt.%) contents, and low K2O and high- Na2O +CaO ones. Chemically, they are compatible with high Al2O3 trondhjemites. They also show weak positive Eu/Eu* anomalies, low Rb (24 to 70ppm), Ba (500 to 1000ppm), Th (2.1 to 8.5ppm) contents, very high Sr/Y ratios (75 to 158) and variable LREE and low HREE patterns (Yb < 1.23 ppm). The Resende Costa pluton yields two U-Pb (LA-ICPMS) zircon crystallization ages (2358±10 Ma and 2356±12 Ma), while the zircon rims yield 2133±32 Ma, interpreted as the age of metamorphism. The Sm/Nd TDM whole rock model ages are between 2.35-2.50 Ga, whereas the ɛNd(t) values range from +1.2 to +3.0, ɛSr(t) from +10 to -6, and ɛHf(t) in zircon between -3 to +6. The nearby Ramos gneissic pluton gives U-Pb zircon age of 2331±17 Ma, TDM age of 2.4 Ga, ɛNd(t) +2.2, ɛHf(t) (-9/+9) and ɛSr(t) +40 values. The overall signature implies to short crustal residence for the protholiths with minor contamination during the petrogenesis. Published data reveal that the nearby Lagoa Dourada

  19. Geochemical Speciation Mass Transfer

    1985-12-01

    PHREEQC is designed to model geochemical reactions. Based on an ion association aqueous model, PHREEQC can calculate pH, redox potential, and mass transfer as a function of reaction progress. It can be used to describe geochemical processes for both far-field and near-field performance assessment and to evaluate data acquisition needs and test data. It can also calculate the composition of solutions in equilibrium with multiple phases. The data base, including elements, aqueous species, and mineralmore » phases, is independent of the program and is completely user-definable. PHREEQC requires thermodynamic data for each solid, gaseous, or dissolved chemical species being modeled. The two data bases, PREPHR and DEQPAK7, supplied with PHREEQC are for testing purposes only and should not be applied to real problems without first being carefully examined. The conceptual model embodied in PHREEQC is the ion-association model of Pearson and Noronha. In this model a set of mass action equations are established for each ion pair (and controlling solid phases when making mass transfer calculations) along with a set of mass balance equations for each element considered. These sets of equations are coupled using activity coefficient values for each aqueous species and solved using a continued fraction approach for the mass balances combined with a modified Newton-Raphson technique for all other equations. The activity coefficient expressions in PHREEQC include the extended Debye-Huckel, WATEQ Debye-Huckel, and Davies equations from the original United States Geological Survey version of the program. The auxiliary preprocessor program PHTL, which is derived from EQTL, converts EQ3/6 thermodynamic data to PHREEQC format so that the two programs can be compared. PHREEQC can be used to determine solubility limits on the radionuclides present in the waste form. These solubility constraints may be input to the WAPPA leach model.« less

  20. [Microbial geochemical calcium cycle].

    PubMed

    Zavarzin, G A

    2002-01-01

    The participation of microorganisms in the geochemical calcium cycle is the most important factor maintaining neutral conditions on the Earth. This cycle has profound influence on the fate of inorganic carbon, and, thereby, on the removal of CO2 from the atmosphere. The major part of calcium deposits was formed in the Precambrian, when prokaryotic biosphere predominated. After that, calcium recycling based on biogenic deposition by skeletal organisms became the main process. Among prokaryotes, only a few representatives, e.g., cyanobacteria, exhibit a special calcium function. The geochemical calcium cycle is made possible by the universal features of bacteria involved in biologically mediated reactions and is determined by the activities of microbial communities. In the prokaryotic system, the calcium cycle begins with the leaching of igneous rock predominantly through the action of the community of organotrophic organisms. The release of carbon dioxide to the soil air by organotrophic aerobes leads to leaching with carbonic acid and soda salinization. Under anoxic conditions, of major importance is the organic acid production by primary anaerobes (fermentative microorganisms). Calcium carbonate is precipitated by secondary anaerobes (sulfate reducers) and to a smaller degree by methanogens. The role of the cyanobacterial community in carbonate deposition is exposed by stromatolites, which are the most common organo-sedimentary Precambrian structures. Deposition of carbonates in cyanobacterial mats as a consequence of photoassimilation of CO2 does not appear to be a significant process. It is argued that carbonates were deposited at the boundary between the "soda continent", which emerged as a result of subaerial leaching with carbonic acid, and the ocean containing Ca2+. Such ecotones provided favorable conditions for the development of the benthic cyanobacterial community, which was a precursor of stromatolites.

  1. Plagioclase flotation and lunar crust formation

    NASA Technical Reports Server (NTRS)

    Walker, D.; Hays, J. F.

    1977-01-01

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

  2. Secular evolution of the continental crust: Implications for crust evolution models

    NASA Astrophysics Data System (ADS)

    Rollinson, Hugh

    2008-12-01

    The present-day flux from mantle to crust is basaltic and yet the average composition of the continental crust is andesitic. This is the crust composition paradox. A new solution to this paradox is proposed whereby the secular evolution in the composition of the continental crust reflects a changing flux from mantle to crust over time. Thus it is proposed that the present-day composition of the continental crust is a time-integrated average. Crustal growth curves show that 48-54% of the continental crust was formed by the end of the Archaean. A mass balance model based upon a tonalite-trondhjemite-granodiorite compositional model for the Archaean continental crust shows that the post-Archaean mantle to crust flux was predominantly basaltic and likely a mix of arc-plume basalts. Trace element modeling, however, reveals that additional processes also contributed to the average crust composition. Balancing Y, Ho, and Yb concentrations requires a garnetiferous mafic granulite lower Archaean crust, which in turn drives the post-Archaean flux toward a high mg # andesite. This suggests that there was a slab melt contribution to the continents, in addition to basalt. An excess of fluid mobile elements in the continental crust can be explained either by the addition of a slab melt or small fraction melts. A deficiency in Sr requires that the post-Archaean crustal composition has been modified by erosion. Both Archaean and post-Archaean continental crust contain contributions from basalt and a slab melt. In the Archaean crust the slab melt contribution is dominant. In the post-Archaean crust the basaltic contribution is dominant.

  3. Reactivation of Paleoproterozoic crust in southern Finland based on paleomagnetic studies of shear zones

    NASA Astrophysics Data System (ADS)

    Preeden, U.; Mertanen, S.; Plado, J.

    2009-04-01

    Paleomagnetic and mineralogical studies have been carried out on Paleoproterozoic crystalline rocks in shear and fault zones in southern Finland. That was done to obtain further evidence on Proterozoic-Paleozoic reactivation of the crust due to juvenile fluid activity. Locating in the central part of Baltica, the area has been tectonically relatively quiet since Precambrian, but previous paleomagnetic studies from the same region have shown presence of remagnetization events. The oldest identified remanence component represents primary magnetization, referring to the late stages of Svecofennian orogeny at ~1.85 Ga. According to the SEM and rock magnetic studies the remanence resides in (titano)magnetite. Younger Proterozoic component is related to the ~1.6 Ga rapakivi intrusions. The most prevalent remanence represents Permian remagnetization, carried by fine-grained hematite. We presume this component reflects reactivation of the Svecofennian crust either due to (i) spreading of basinal fluids caused by tectonic processes at the edges of the plate (Caledonian, Hercynian and Uralian orogens) or (ii) extensive erosion/regression that allowed subsurface meteoric fluids to circulate in the already existing fault systems. The likely source for iron in these fluids could be the processes of alteration and dissolution of micas, epidote and/or earlier iron-titanium oxides. Additional iron could also have been transported into the system by externally derived fluids. Solving the exact origin of fluids still needs geochemical analysis of fluid inclusions and studies on isotopic compositions. In a wider view all these tectonic events can be related to the formation and break-up of supercontinent Pangea. Worldwide, there exist hundreds of documented indications of a late Paleozoic secondary magnetization in different rock types. Similar secondary magnetizations have been found also in the Fennoscandian region, for instance in the crystalline rocks in eastern and northern

  4. Lithium isotope constraints on crust-mantle interactions and surface processes on Mars

    NASA Astrophysics Data System (ADS)

    Magna, Tomáš; Day, James M. D.; Mezger, Klaus; Fehr, Manuela A.; Dohmen, Ralf; Aoudjehane, Hasnaa Chennaoui; Agee, Carl B.

    2015-08-01

    Lithium abundances and isotope compositions are reported for a suite of martian meteorites that span the range of petrological and geochemical types recognized to date for Mars. Samples include twenty-one bulk-rock enriched, intermediate and depleted shergottites, six nakhlites, two chassignites, the orthopyroxenite Allan Hills (ALH) 84001 and the polymict breccia Northwest Africa (NWA) 7034. Shergottites unaffected by terrestrial weathering exhibit a range in δ7Li from 2.1 to 6.2‰, similar to that reported for pristine terrestrial peridotites and unaltered mid-ocean ridge and ocean island basalts. Two chassignites have δ7Li values (4.0‰) intermediate to the shergottite range, and combined, these meteorites provide the most robust current constraints on δ7Li of the martian mantle. The polymict breccia NWA 7034 has the lowest δ7Li (-0.2‰) of all terrestrially unaltered martian meteorites measured to date and may represent an isotopically light surface end-member. The new data for NWA 7034 imply that martian crustal surface materials had both a lighter Li isotope composition and elevated Li abundance compared with their associated mantle. These findings are supported by Li data for olivine-phyric shergotitte NWA 1068, a black glass phase isolated from the Tissint meteorite fall, and some nakhlites, which all show evidence for assimilation of a low-δ7Li crustal component. The range in δ7Li for nakhlites (1.8 to 5.2‰), and co-variations with chlorine abundance, suggests crustal contamination by Cl-rich brines. The differences in Li isotope composition and abundance between the martian mantle and estimated crust are not as large as the fractionations observed for terrestrial continental crust and mantle, suggesting a difference in the styles of alteration and weathering between water-dominated processes on Earth versus possibly Cl-S-rich brines on Mars. Using high-MgO shergottites (>15 wt.% MgO) it is possible to estimate the δ7Li of Bulk Silicate Mars

  5. Europium mass balance in polymict samples and implications for plutonic rocks of the lunar crust

    SciTech Connect

    Korotev, R.L.; Haskin, L.A. )

    1988-07-01

    From correlations of SM concentration and Sm/Eu ratio with Th concentration for a large number of polymict samples from various locations in the lunar highlands and the value of 0.91 {mu}g/g for the mean Th concentration of the highlands surface crust obtained by the orbiting gamma-ray experiments. The authors estimate the mean concentrations of Sm and Eu in the lunar surface crust to be between 2 and 3 {mu}g/g Sm and 0.7 and 1.2 {mu}g/g Eu. The compositional trends indicate that there is no significant enrichment or depletion of Eu, on the average, compared to Sm relative to chondritic abundances, i.e., there is no significant Eu anomaly in average upper crust. Although rich in plagioclase ({approximately}70%), the upper crust does not offer evidence for a gross vertical separation of plagioclase from the final liquid from which it crystallized. This and the chondritic ratio of Eu/Al in average highlands material imply that the net effect of the processes that led to formation of the lunar crust was to put most of the Al and incompatible elements in the crust. Among plutonic rocks, only plagioclase in rocks from the magnesian suite can supply the excess Eu in the polymict rocks. Owing to the intermediate value of the mean Mg/Fe ratio of the crust, a significant fraction of the mafic rocks of the lunar highlands must have lower Mg/Fe ratios than the norites and troctolites of the magnesian-suite of plutonic rocks. A large fraction of the plagioclase in the lunar crust is associated not with ferroan anorthosite, but with more mafic rocks. There is little evidence in the Eu data that the lunar crust ever consisted of a thick shell of nearly pure plagioclase, as envisioned in some formulations of the magma ocean model of its formation.

  6. Sulfur and metal fertilization of the lower continental crust

    NASA Astrophysics Data System (ADS)

    Locmelis, Marek; Fiorentini, Marco L.; Rushmer, Tracy; Arevalo, Ricardo; Adam, John; Denyszyn, Steven W.

    2016-02-01

    Mantle-derived melts and metasomatic fluids are considered to be important in the transport and distribution of trace elements in the subcontinental lithospheric mantle. However, the mechanisms that facilitate sulfur and metal transfer from the upper mantle into the lower continental crust are poorly constrained. This study addresses this knowledge gap by examining a series of sulfide- and hydrous mineral-rich alkaline mafic-ultramafic pipes that intruded the lower continental crust of the Ivrea-Verbano Zone in the Italian Western Alps. The pipes are relatively small (< 300 m diameter) and primarily composed of a matrix of subhedral to anhedral amphibole (pargasite), phlogopite and orthopyroxene that enclose sub-centimeter-sized grains of olivine. The 1 to 5 m wide rim portions of the pipes locally contain significant blebby and disseminated Fe-Ni-Cu-PGE sulfide mineralization. Stratigraphic relationships, mineral chemistry, geochemical modeling and phase equilibria suggest that the pipes represent open-ended conduits within a large magmatic plumbing system. The earliest formed pipe rocks were olivine-rich cumulates that reacted with hydrous melts to produce orthopyroxene, amphibole and phlogopite. Sulfides precipitated as immiscible liquid droplets that were retained within a matrix of silicate crystals and scavenged metals from the percolating hydrous melt. New high-precision chemical abrasion TIMS U-Pb dating of zircons from one of the pipes indicates that these pipes were emplaced at 249.1 ± 0.2 Ma, following partial melting of lithospheric mantle pods that were metasomatized during the Eo-Variscan oceanic to continental subduction (~ 420-310 Ma). The thermal energy required to generate partial melting of the metasomatized mantle was most likely derived from crustal extension, lithospheric decompression and subsequent asthenospheric rise during the orogenic collapse of the Variscan belt (< 300 Ma). Unlike previous models, outcomes from this study suggest a

  7. Vertical movements of crust, uplift of lithosphere, and isostatic unroofing: case histories from the Ozark dome and northern Appalachians

    SciTech Connect

    Friedman, G.M.

    1987-05-01

    Evidence of former deep burial of Ordovician to Devonian strata of the Ozark dome and northern Appalachians has been obtained from petrographic and geochemical studies of carbonates and coal-bearing rocks. In diagenetic minerals of the carbonate rocks, fluid inclusion homogenization temperatures and delta/sup 18/O values indicate paleotemperatures of 100 to 200/sup 0/C. The geothermometers used also include vitrinite reflectance, level of organic metamorphism (LOM), Staplin kerogen alteration index, and conodont alteration index (CAI). Maximum depths of burial were calculated from the estimated paleotemperatures assuming a geothermal gradient of about 25/sup 0/C/km. Strata of the Silurian of the northern Appalachian basin and of the Ordovician of the Ozark dome are interpreted to have reached maximum burial depths of 5 and 4.3 km, respectively; Devonian strata in the Catskill Mountains of New York had former burial depths of about 6.5 km; Lower Ordovician carbonate sequences of the northern Appalachian basin were buried to more than 7 km; Middle Ordovician strata from the same basin had paleodepths of approximately 5 km, and Devonian strata, 4.5 to 5 km. If these strata were formerly buried much more deeply than previously thought, then unexpectedly large amounts of uplift and erosion, ranging from 4.3 to 7 km, must also have occurred to bring these strata to the present land surface. The occurrence of such large-scale vertical movements of the crust and lithosphere needs to be recognized in paleogeographic reconstructions.

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

    NASA Astrophysics Data System (ADS)

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

    2010-12-01

    Investigation of Earth’s primitive crust is biased towards felsic rocks because they contain zircons that provide robust geochronological constraints. Felsic rocks, however, cannot be derived directly from the mantle thus the first crust had to be mafic in composition. Obtaining precise ages on old mafic rocks is however difficult due to their lack of zircon and the metamorphic overprinting they have suffered. 143Nd and 142Nd analyses on the Nuvvuagittuq greenstone belt suggests that its mafic components formed more than 4 billion years ago and that the dominant lithology of the belt, known as the “faux-amphibolite”, represents the only relict of Hadean crust formed at ~4.3 Ga. Its protolith is interpreted to be mafic volcanic rocks and volcanic pyroclastic deposits. The faux-amphibolite can be divided into three distinct geochemical group stratigraphically superimposed. The faux-amphibolite at the base of the sequence is tholeiitic with a composition indicating derivation of primary melts from an undepleted mantle and fractionation under dry conditions. Towards the top of the volcanic sequence, the faux-amphibolites are characterized by higher Al/Ti ratios. They appear to have fractionated under elevated water pressure and are geochemically similar to modern boninite and calc-alkaline volcanic rocks. A new series of faux-amphibolite was analysed for 142Nd isotopic composition. 21 samples have deficits in 142Nd ranging from -7 to -18 ppm compared to the terrestrial standard. These deficits have now been confirmed by measurements of the same samples at ETH (Roth et al., GCA, A886, 2010). A 146Sm-142Nd isochron constructed for all faux-amphibolite yields an age of 4368 +72-142 Ma (n=30). A line fit only to the faux-amphibolite compositional group that shows the widest range in LREE enrichment, including corresponding co-genetic ultramafic sills gives a 146Sm-142Nd age of 4381 +67-123 Ma (n=21). The Hadean age for the faux-amphibolite is supported by a 4079

  9. Evolution of the continental crust as recorded in accessory minerals

    NASA Astrophysics Data System (ADS)

    Iizuka, Tsuyoshi

    2013-04-01

    Recent developments in precise in situ isotopic analysis by LA-ICPMS and SIMS allow correlating multiple isotopic systems within single grains of accessory minerals such as zircon and monazite. The combined isotope systematics have provided valuable insights into the evolution of the continental crust. Zircon, a common accessory phase in granitoids, can be precisely dated by the U-Pb system. Zircon Lu-Hf isotopic composition is a function of crustal residence time of the magmatic protolith, whereas the O isotopic composition is a sensitive record of reworking of mature sediments such as pelite. An integration of U-Pb, Lu-Hf and O isotopic data for detrital zircons from modern large rivers indicates that: (1) the preserved continental crust dominantly formed between 3.6 and 1.0 Ga, (2) the major mode of crustal development would change during the supercontinent cycle, i.e., the generation of juvenile crust during supercontinent fragmentation versus the stabilization of the generated crust via crustal remelting during supercontinent fragmentation, and (3) reworking of mature sediments increased abruptly at ca. 2.1 Ga. No granitoids are known to have survived since 4.03 Ga. Yet evidence of an even older evolved crust is provided by detrital zircons with ages up to 4.4 Ga from Mt. Narryer and Jack Hills metasedimentary rocks in the Yilgarn Craton, Western Australia. Recently, such Hadean zircons have been found from outside the Yilgarn Craton, indicating that the young Earth had widespread granitoid crust. In addition, another accessory phase, monazite, in the Mt. Narryer and Jack Hills metasedimentary rocks offers an unique opportunity to advance our knowledge of early crustal evolution. Monazite, a light rare earth element phosphate mineral, occurs as an igneous accessory phase particularly in low-Ca granitoids, in contrast to the occurrence of igneous zircon in a wide range of granitoids. U-Pb and Sm-Nd isotope systematic of monazite are analogous to U-Pb and Lu

  10. Forearc oceanic crust in the Izu-Bonin arc - new insights from active-source seismic survey -

    NASA Astrophysics Data System (ADS)

    Kodaira, S.; Noguchi, N.; Takahashi, N.; Ishizuka, O.; Kaneda, Y.

    2009-12-01

    Petrological studies have suggested that oceanic crust is formed in forearc areas during the initial stage of subduction. However, there is little geophysical evidence for the formation of oceanic crust in those regions. In order to examine crustal formation process associated with a subduction initiation process, we conducted an active-source seismic survey at a forearc region in the Izu-Bonin intra-oceanic arc. The resultant seismic image shows a remarkably thin crust (less than 10 km) at the northern half of the Bonin ridge (at the north of the Chichi-jima) and abrupt thickening the crust (~ 20 km thick) toward the south (at the Haha-jima). Comparison of velocity-depth profiles of the thin forearc crust of the Bonin ridge with those of typical oceanic crusts showed them to be seismologically identical. The observed structural variation also well corresponds to magmatic activities along the forearc. Boninitic magmatism is evident in the area of thin crust and tholeiitic-calcalkaline andesitic volcanism in the area of thick crust. Based on high precision dating studies of those volcanic rocks, we interpreted that the oceanic-type thin crust associated with boninitic volcanism has been created soon after the initiation of subduction (45-48 Ma) and and that the nonoceanic thick crust was created by tholeiitic-calcalkaline andesitic magmatism after the boninitic magmatism was ceased. The above seismological evidences strongly support the idea of forearc oceanic crust (or phiolite) created by forearc spreading in the initial stage of subduction along the intra-oceanic arc.

  11. Geodynamic investigation of the processes that control Lu-Hf isotopic differences between different mantle domains and the crust

    NASA Astrophysics Data System (ADS)

    Jones, Rosie; van Keken, Peter; Hauri, Erik; Vervoort, Jeff; Ballentine, Chris J.

    2016-04-01

    The chemical and isotopic composition of both the Earth's mantle and the continental crust are greatly influenced by subduction zone processes, such as the formation of continental crust through arc volcanism and the recycling of surface material into the deep mantle. Here we use a combined geodynamical-geochemical approach to investigate the long term role of subduction on the Lu-Hf isotopic evolution of the mantle and the continental crust. We apply the geodynamic model developed by Brandenburg et al., 2008. This model satisfies the geophysical constraints of oceanic heat flow and average plate velocities, as well as geochemical observations such as 40Ar in the atmosphere, and reproduces the geochemical distributions observed in multiple isotope systems which define the HIMU, MORB and EM1 mantle endmembers. We extend this application to investigate the detail of terrestrial Lu-Hf isotope distribution and evolution, and specifically to investigate the role of sediment recycling in the generation of EM2 mantle compositions. The model has been updated to produce higher resolution results and to include a self-consistent reorganisation of the plates with regions of up-/down-wellings. The model assumes that subduction is initiated at 4.5 Ga and that a transition from 'dry' to 'wet' subduction occurred at 2.5 Ga. The modelling suggests that the epsilon Hf evolution of the upper mantle can be generated through the extraction and recycling of the oceanic crust, and that the formation of continental crust plays a lesser role. Our future intention is to utilise the model presented here to investigate the differences observed in the noble gas compositions (e.g., 40Ar/36Ar, 3He/4He) of MORB and OIB. Brandenburg, J.P., Hauri, E.H., van Keken, P.E., Ballentine, C.J., 2008. Earth and Planetary Science Letters 276, 1-13.

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

    PubMed

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

    2014-09-01

    Ocean acidification by atmospheric carbon dioxide has increased almost continuously since the last glacial maximum (LGM), 21,000 years ago. It is expected to impair tropical reef development, but effects on reefs at the present day and in the recent past have proved difficult to evaluate. We present evidence that acidification has already significantly reduced the formation of calcified bacterial crusts in tropical reefs. Unlike major reef builders such as coralline algae and corals that more closely control their calcification, bacterial calcification is very sensitive to ambient changes in carbonate chemistry. Bacterial crusts in reef cavities have declined in thickness over the past 14,000 years with largest reduction occurring 12,000-10,000 years ago. We interpret this as an early effect of deglacial ocean acidification on reef calcification and infer that similar crusts were likely to have been thicker when seawater carbonate saturation was increased during earlier glacial intervals, and thinner during interglacials. These changes in crust thickness could have substantially affected reef development over glacial cycles, as rigid crusts significantly strengthen framework and their reduction would have increased the susceptibility of reefs to biological and physical erosion. Bacterial crust decline reveals previously unrecognized millennial-scale acidification effects on tropical reefs. This directs attention to the role of crusts in reef formation and the ability of bioinduced calcification to reflect changes in seawater chemistry. It also provides a long-term context for assessing anticipated anthropogenic effects. PMID:25040070

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

    PubMed

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

    2014-09-01

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

  14. Thickened juvenile lower crust-derived ~ 90 Ma adakitic rocks in the central Lhasa terrane, Tibet

    NASA Astrophysics Data System (ADS)

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

    2015-05-01

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

  15. Microbial Inventory of Deeply Buried Oceanic Crust from a Young Ridge Flank.

    PubMed

    Jørgensen, Steffen L; Zhao, Rui

    2016-01-01

    The deep marine biosphere has over the past decades been exposed as an immense habitat for microorganisms with wide-reaching implications for our understanding of life on Earth. Recent advances in knowledge concerning this biosphere have been achieved mainly through extensive microbial and geochemical studies of deep marine sediments. However, the oceanic crust buried beneath the sediments, is still largely unexplored with respect to even the most fundamental questions related to microbial life. Here, we present quantitative and qualitative data related to the microbial inventory from 33 deeply buried basaltic rocks collected at two different locations, penetrating 300 vertical meters into the upper oceanic crust on the west flank of the Mid-Atlantic spreading ridge. We use quantitative PCR and sequencing of 16S rRNA gene amplicons to estimate cell abundances and to profile the community structure. Our data suggest that the number of cells is relatively stable at ~10(4) per gram of rock irrespectively of sampling site and depth. Further, we show that Proteobacteria, especially Gammaproteobacteria dominate the microbial assemblage across all investigated samples, with Archaea, in general, represented by < 1% of the community. In addition, we show that the communities within the crust are distinct from the overlying sediment. However, many of their respective microbial inhabitants are shared between the two biomes, but with markedly different relative distributions. Our study provides fundamental information with respect to abundance, distribution, and identity of microorganisms in the upper oceanic crust. PMID:27303398

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

    PubMed

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

    2013-01-01

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

  17. Microbial Inventory of Deeply Buried Oceanic Crust from a Young Ridge Flank.

    PubMed

    Jørgensen, Steffen L; Zhao, Rui

    2016-01-01

    The deep marine biosphere has over the past decades been exposed as an immense habitat for microorganisms with wide-reaching implications for our understanding of life on Earth. Recent advances in knowledge concerning this biosphere have been achieved mainly through extensive microbial and geochemical studies of deep marine sediments. However, the oceanic crust buried beneath the sediments, is still largely unexplored with respect to even the most fundamental questions related to microbial life. Here, we present quantitative and qualitative data related to the microbial inventory from 33 deeply buried basaltic rocks collected at two different locations, penetrating 300 vertical meters into the upper oceanic crust on the west flank of the Mid-Atlantic spreading ridge. We use quantitative PCR and sequencing of 16S rRNA gene amplicons to estimate cell abundances and to profile the community structure. Our data suggest that the number of cells is relatively stable at ~10(4) per gram of rock irrespectively of sampling site and depth. Further, we show that Proteobacteria, especially Gammaproteobacteria dominate the microbial assemblage across all investigated samples, with Archaea, in general, represented by < 1% of the community. In addition, we show that the communities within the crust are distinct from the overlying sediment. However, many of their respective microbial inhabitants are shared between the two biomes, but with markedly different relative distributions. Our study provides fundamental information with respect to abundance, distribution, and identity of microorganisms in the upper oceanic crust.

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

    PubMed Central

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

    2013-01-01

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

  19. Microbial Inventory of Deeply Buried Oceanic Crust from a Young Ridge Flank

    PubMed Central

    Jørgensen, Steffen L.; Zhao, Rui

    2016-01-01

    The deep marine biosphere has over the past decades been exposed as an immense habitat for microorganisms with wide-reaching implications for our understanding of life on Earth. Recent advances in knowledge concerning this biosphere have been achieved mainly through extensive microbial and geochemical studies of deep marine sediments. However, the oceanic crust buried beneath the sediments, is still largely unexplored with respect to even the most fundamental questions related to microbial life. Here, we present quantitative and qualitative data related to the microbial inventory from 33 deeply buried basaltic rocks collected at two different locations, penetrating 300 vertical meters into the upper oceanic crust on the west flank of the Mid-Atlantic spreading ridge. We use quantitative PCR and sequencing of 16S rRNA gene amplicons to estimate cell abundances and to profile the community structure. Our data suggest that the number of cells is relatively stable at ~104 per gram of rock irrespectively of sampling site and depth. Further, we show that Proteobacteria, especially Gammaproteobacteria dominate the microbial assemblage across all investigated samples, with Archaea, in general, represented by < 1% of the community. In addition, we show that the communities within the crust are distinct from the overlying sediment. However, many of their respective microbial inhabitants are shared between the two biomes, but with markedly different relative distributions. Our study provides fundamental information with respect to abundance, distribution, and identity of microorganisms in the upper oceanic crust. PMID:27303398

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

    PubMed

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

    2013-12-04

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

  1. Biogenic crust dynamics on sand dunes.

    PubMed

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

    2013-02-01

    Sand dunes are often covered by vegetation and biogenic crusts. Despite their significant role in dune stabilization, biogenic crusts have rarely been considered in model 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 crossover between two different forms of desertification.

  2. Microphytic crusts: 'topsoil' of the desert

    USGS Publications Warehouse

    Belnap, Jayne

    1990-01-01

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

  3. Crust rheology, slab detachment and topography

    NASA Astrophysics Data System (ADS)

    Duretz, T.; Gerya, T. V.

    2012-04-01

    The collision between continents following the closure of an ocean can lead to the subduction of continental crust. The introduction of buoyant crust within subduction zones triggers the development of extensional stresses in slabs which eventually result in their detachment. The dynamic consequences of slab detachment affects the development of topography, the exhumation of high-pressure rocks and the geodynamic evolution of collision zones. We employ two-dimensional thermo-mechanical modelling in order to study the importance of crustal rheology on the evolution of spontaneous subduction-collision systems and the occurrence of slab detachment. The modelling results indicate that varying the rheological structure of the crust can results in a broad range of collisional evolutions involving slab detachment, delamination (associated to slab rollback), or the combination of both mechanisms. By enhancing mechanical coupling at the Moho, a strong crust leads to the deep subduction of the crust (180 km). These collisions are subjected to slab detachment and subsequent coherent exhumation of the crust accommodated by eduction (inversion of subduction sense) and thrusting. In these conditions, slab detachment promotes the development of a high (> 4.5 km) and narrow (< 200 km) topographic plateau located in the vicinity of the suture. A contrasting style of collision is obtained by employing a weak crustal rheology. The weak