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Sample records for crust geochemical evidence

  1. Recycled oceanic crust in the Hawaiian Plume: evidence from temporal geochemical variations within the Koolau Shield

    NASA Astrophysics Data System (ADS)

    Huang, Shichun; Frey, Frederick A.

    2005-07-01

    The subaerial surface of Koolau volcano is composed of lavas that define the distinctive endmember composition for Hawaiian shield lavas, known as the Koolau component, now designated as the Makapuu-stage. The geochemical characteristics of lavas recovered by the Koolau Scientific Drilling Project (KSDP) show that this distinctive composition forms a <300-m thick veneer. Below this veneer, from ~300m to 470 m below sea level, Koolau shield lavas transition to a composition similar to Mauna Loa lavas, now designated as the Kalihi-stage. This transition was gradual, occurring over >80 ka; therefore it was not caused by an abrupt event, such as a landslide. Among all Koolau shield lavas, there are correlations between radiogenic isotopic ratios of Sr, Nd and Pb and compositional characteristics, such as SiO2 content (adjusted to be in equilibrium with Fo90 olivine), Sr/Nb, La/Nb and Th/La. These long-term compositional and isotopic trends show that as the shield aged, there was an increasing role for an ancient recycled marine sediment component (<3% of the source) accompanied by up to 20% SiO2-rich dacitic melt. This melt was generated by partial melting of garnet pyroxenite, probably kilometers in size, that formed from recycled basaltic oceanic crust. In detail, time series analyses of depth profiles of Al2O3/CaO, Sr/Nb, La/Nb and Th/La in the KSDP drill core show correlations among these ratios indicating that recycled oceanic crust contributed episodically, ~29 ka period, to the magma source during the prolonged transition from Kalihi- to Makapuu-stage lava compositions. The long-term geochemical trends show that recycled oceanic crust was increasingly important as the Koolau shield moved away from the plume and encountered lower temperature.

  2. 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

  3. 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.

  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. The geochemical composition of the terrestrial surface (without soils) and comparison with the upper continental crust

    NASA Astrophysics Data System (ADS)

    Hartmann, Jens; Dürr, Hans H.; Moosdorf, Nils; Meybeck, Michel; Kempe, Stephan

    2012-01-01

    The terrestrial surface, the "skin of the earth", is an important interface for global (geochemical) material fluxes between major reservoirs of the Earth system: continental and oceanic crust, ocean and atmosphere. Because of a lack in knowledge of the geochemical composition of the terrestrial surface, it is not well understood how the geochemical evolution of the Earth's crust is impacted by its properties. Therefore, here a first estimate of the geochemical composition of the terrestrial surface is provided, which can be used for further analysis. The geochemical average compositions of distinct lithological classes are calculated based on a literature review and applied to a global lithological map. Comparison with the bulk composition of the upper continental crust shows that the geochemical composition of the terrestrial surface (below the soil horizons) is significantly different from the assumed average of the upper continental crust. Specifically, the elements Ca, S, C, Cl and Mg are enriched at the terrestrial surface, while Na is depleted (and probably K). Analysis of these results provide further evidence that chemical weathering, chemical alteration of minerals in marine settings, biogeochemical processes (e.g. sulphate reduction in sediments and biomineralization) and evaporite deposition are important for the geochemical composition of the terrestrial surface on geological time scales. The movement of significant amounts of carbonate to the terrestrial surface is identified as the major process for observed Ca-differences. Because abrupt and significant changes of the carbonate abundance on the terrestrial surface are likely influencing CO2-consumption rates by chemical weathering on geological time scales and thus the carbon cycle, refined, spatially resolved analysis is suggested. This should include the recognition of the geochemical composition of the shelf areas, now being below sea level.

  6. Geochemical Evidence from the Kohistan Complex for Differentiation of Garnet Granulitic lower Crust in Island Arcs by Dehydration Melting of Amphibole-bearing Plutonics: Implications for the Andesite Model of Continental Crustal Growth

    NASA Astrophysics Data System (ADS)

    Garrido, C. J.; Bodinier, J.; Burg, J.; Zeilinger, G.; Hussain, S. S.; Dawood, H.; Gervilla, F.

    2005-12-01

    We report a geochemical study of the Jijal and Sarangar complexes constituting the lower crust of the Mesozoic Kohistan paleo-island arc (N. Pakistan). The Jijal complex is composed of basal peridotites topped by a gabbroic section made up of mafic garnet granulite-with minor lenses of garnet hornblendite and granite-grading up section to hornblende gabbronorite. The Sarangar complex is constituted by metagabbro. Sarangar gabbro and Jijal hornblende gabbronorite have melt-like, LREE-enriched REE patterns similar to those of island arc basalts. These rocks and Jijal garnet granulite define altogether negative covariations of LaN, YbN and (La/Sm)N with Eu* (=2xEuN/SmN+GdN; N= chondrite normalized), and positive covariations of (Yb/Gd)N with Eu*. REE modeling indicates that these covariations cannot be accounted for by high-pressure crystal fractionation of hydrous primitive or derivative andesites. They are consistent with formation of garnet granulites as plagioclase-garnet assemblages with variable trapped melt fractions via either high-pressure crystallization of primitive island arc basalts or dehydration melting of hornblende gabbronorite, providing that the amount of segregated or restitic garnet was low (< 5 wt.%). Field, petrographic, geochemical and experimental evidence is more consistent with formation of Jijal garnet granulite by dehydration melting of Jijal hornblende gabbronorite. Similarly, Jijal garnet-bearing hornblendite lenses were most likely generated by coeval dehydration melting of hornblendites. Furthermore, melting models and geochronological data point to intrusive leucogranites in the overlying Metaplutonic complex as the melts generated by dehydration melting of the plutonic protoliths of Jijal garnet-bearing restites. Consistently with the metamorphic evolution of the Kohistan lower arc crust, dehydration melting occurred at the mature stage of this island arc when shallower hornblende-bearing plutonics were buried to depth exceeding 25

  7. 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

  8. 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

  9. 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.

  10. 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.

  11. 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

  12. 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

  13. 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.

  14. 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.

  15. 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.

  16. 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.

  17. 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

  18. 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

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

    NASA Astrophysics Data System (ADS)

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

    2014-09-01

    In contrast to igneous systems, the geochemical twins Zr and Hf are decoupled from each other in seawater, and specific Zr/Hf ratios appear to be characteristic of individual marine water masses. Hydrogenetic marine ferromanganese (Fe-Mn) crusts which accumulate trace metals from seawater may be an archive of Zr/Hf ratios that reveal changes in oceanic paleocirculation over millions of years. To verify whether Fe-Mn crusts truly reflect the Zr-Hf distribution in seawater, we studied these particle-reactive elements together with Nb and Ta (another geochemical twin pair) in bulk Fe-Mn crusts and their surface layers from different locations in the Atlantic and Pacific oceans. Zirconium (400-1000 mg kg-1), Hf (5-18 mg kg-1), Nb (42-83 mg kg-1) and Ta (0.5-1.5 mg kg-1) are significantly enriched in Fe-Mn crusts relative to the average continental crust, and their Zr/Hf and Nb/Ta ratios are super-chondritic (57-87 and 35-96, respectively), whereas the continental crust shows ratios close to those of chondrites. We emphasize that neither bulk Fe-Mn crusts nor their surface layers match the Zr/Hf or Nb/Ta ratios of modern deep seawater, but are lower and higher, respectively. The presence of aluminosilicate detritus cannot explain the different Zr/Hf ratios of crusts and ambient seawater, as potential detritus has much lower Zr and Hf concentrations. Consequently, these geochemical twins must be fractionated during their removal from seawater and their incorporation into Fe-Mn (oxyhydr)oxides. Hafnium is preferentially scavenged as shown by Zr/Hf ratios of crust surface layers (75-100) that are always below those of modern deep seawater (150-300). The decoupled behavior of geochemical twins during sorption, which is also observed for Nb-Ta, can be related to differences in the electron structures of these elements. Iron-normalized concentrations of Zr, Hf, Nb, and Ta increase with increasing size of the positive Ce anomaly (known to increase with decreasing growth rate

  20. 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

  1. 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

  2. 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.

  3. 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

  4. Granulite xenoliths from Cenozoic Basalts in SE China provide geochemical fingerprints to distinguish lower crust terranes from the North and South China tectonic blocks

    NASA Astrophysics Data System (ADS)

    Yu, Jin-Hai; Xu, Xisheng; O'Reilly, Suzanne Y.; Griffin, W. L.; Zhang, Ming

    2003-03-01

    A large suite of lower crustal xenoliths from Cenozoic basaltic rocks from three widely spaced localities in the eastern part of the South China Block (SCB; Xilong, Qilin and Leizhou) and two localities (Nushan and Guizishan) close to the suture between the North and South China Blocks in Southeast China has been studied in detail. They are used to define the nature of the lower crust beneath these regions and to define two distinct types of lithospheric domains resulting from different tectonic episodes. Lower crustal xenoliths from the Nushan area have distinctive petrologic and geochemical (including isotopic) characteristics indicating formation by underplating of continental basaltic magmas around the crust-mantle boundary. Geochemistry of these granulites demonstrates that an isotopically enriched old source component had played a significant role in the formation of the Nushan xenoliths. The lower crustal xenoliths from the South China Block reveal different origins and source compositions from those from the Nushan area. The South China Block xenoliths are considered to be derived from the underplating of continental arc-type basaltic magmas, but also show evidence of significant assimilation and fractional crystallisation (AFC) that produced abundant cumulates and fractionated liquids with strong crustal contamination signatures. They are geochemically and isotopically similar to the outcropping Late Mesozoic gabbros and basalts in the same region and are inferred to be their underplated high-pressure analogues and products of AFC with the lower crustal sources parental to the more silicic Mesozoic outcropping magmatic rocks. These Mesozoic basaltic magmas from the South China Block are geochemically distinct from the Cenozoic basaltic volcanics in this region, indicating a secular source change from a continental arc setting to an intraplate extensional regime. This significant change from Mesozoic to Cenozoic may indicate eastward movement of the

  5. 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

  6. 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.

  7. 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.

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

    NASA Astrophysics Data System (ADS)

    Taylor, P. N.; Kalsbeek, F.

    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.

  9. 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.

  10. The Origin of Andesite in the Deep Crust: Evidence from the West-Central Mexican Arc

    NASA Astrophysics Data System (ADS)

    Lange, R. A.; Ownby, S.; Delgado Granados, H.; Hall, C. M.

    2011-12-01

    by the crystal-poor examples, and (2) minimal plagioclase accumulation affects the bulk composition of the crystal-rich andesites, which is consistent with the absence of any Eu anomalies. No dacite or rhyolite has erupted, and there is thus no evidence of crystal fractionation of andesite, including segregation of interstitial liquid from crystal-rich andesites in the upper crustal chamber feeding V. Tancítaro. Magma mixing between dacite/rhyolite and basalt/basaltic andesite to make andesite can be ruled out on two counts: (1) the absence of any dacite or rhyolite as a requisite end-member for mixing, and (2) such mixing could not produce crystal-poor (1-5%) andesite. Although hornblende is absent from the phenocryst assemblage of the basalts and basaltic andesites, rare-earth element patterns among the andesites, basaltic andesites, and basalts show that crystal-liquid separation of up to 40% hornblende is required to form the andesites from the basalts. Therefore, the primary mechanism to form crystal-poor andesite in this arc segment appears to be partial melting of hornblende-rich (~40%) gabbronorite in the lowermost crust. No other mechanisms satisfy all the geochemical and petrological constraints.

  11. 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.

  12. 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.

  13. 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

  14. Geochemical evidence of mantle reservoir evolution during progressive rifting

    NASA Astrophysics Data System (ADS)

    Rooney, T. O.; Mohr, P.; Dosso, L.; Hall, C. M.

    2010-12-01

    The Afar region in East Africa, which represents the triple junction of three well-exposed Cenozoic rift systems, is a pivotal domain in the study of rift evolution. The western margin of Afar, defined by a wide transitional region from plateau to rift floor, developed in response to the rifting of the Red Sea commencing shortly after the eruption of the ~31-29 Ma Ethiopian-Yemen flood basalts. The Oligocene lava sequence which covers this rift margin was fed from intensive diking. The dikes and the block-faulting and monoclinal warping that followed provide an opportunity to probe the geochemical reservoirs preserved in the magmatic record and the development of the rifting processes. Argon geochronology reveals that dikes along the western Afar margin 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 and trace element, and isotopic results (Sr-Nd-Pb-Hf) from these dikes demonstrate temporal geochemical heterogeneity defined by variable contributions from the Afar plume, depleted mantle and African lithosphere, consistent with studies of Quaternary basalts from the Ethiopian Rift. On a broader scale our results show that as the western Afar margin matures, the initially significant contribution from the Afar plume wanes in favor of shallow asthenospheric and lithospheric reservoirs. The early dikes, which are coincident with the initial weakening of the lithosphere in a magma-assisted rifting model, geochemically resemble the widespread plume-derived flood basalts and shields that constitute the Ethiopian Plateau. Subsequent diking is characterized by a lesser role for the Afar plume and greater contributions from the African lithosphere and depleted mantle. During the terminal stage of dike emplacement, where focused magmatic intrusion accommodated extension, a more significant fraction is derived from the depleted mantle and less of a

  15. 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

  16. Evidence for a thick oceanic crust adjacent to the Norwegian Margin

    NASA Astrophysics Data System (ADS)

    Mutter, John C.; Talwani, Manik; Stoffa, Paul L.

    1984-01-01

    The oceanic crust created during this first few million years of accretion in the Norwegian-Greenland Sea lies at an unusually shallow depth for its age, has a smooth upper surface, and in many places the results of multichannel seismic reflection profiling reveal that its upper layers comprise a remarkable sequence of arcuate, seaward-dipping reflectors. These have been attributed to lava flows generated during a brief period of subaerial seafloor spreading. We describe the results of inversions of digitally recorded sonobuoy measurements and two-ship expanded spread profiles collected over the oceanic crust adjacent to the Norwegian passive margin. We find that the crust of the deep Lofoten Basin is indistinguishable from normal oceanic crust in thickness and structure. Closer to the margin we observe up to a four times expansion in thickness of layers with velocities equal to those of oceanic layer 2, while the layer 3 region retains approximately the same thickness. The area over which the seaward-dipping reflectors can be observed on reflection profiles corresponds to the region of greatest expansion in "Layer 2" thickness. In the very oldest crust immediately adjacent to an escarpment that probably marks the continent-ocean boundary, we see evidence for a low velocity zone overlying an indistinct reflector that may mark the dyke-lava interface in the thick crust. Comparing the structure of the thick crust to that of eastern Iceland, we find a strong resemblance, especially in the expansion in thickness of material with layer 2 velocities. These results support the suggestion that during the earliest stages of spreading extrusive volcanism at the ridge crest was unusually voluminous, building a thick pile of lavas erupted from a subaerial spreading center.

  17. 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

  18. Granulite xenoliths from Cenozoic basalts in SE China provide geochemical fingerprints to distinguish lower crust terranes from the North and South China tectonic blocks: comment

    NASA Astrophysics Data System (ADS)

    Zhang, Kai-Jun

    2004-03-01

    A careful examination of the geochemical data set for SE China granulite xenoliths in Cenozoic basalts shows differences between the magmatic and cumulate granulite xenoliths, but no distinction between the Nushan and the other South China magmatic granulite xenoliths. Nushan granulite xenoliths with Archean Nd model ages were most likely derived from the Archean basement of the Yangtze craton itself and overprinted by a Paleoproterozoic to Mesoproterozoic tectonothermal event that occurred in the South China block, including the northern margin of the Yangtze craton. The granulate xenoliths therefore cannot be used to distinguish the North China and South China lower crust. Further, the discovery of the UHP eclogite xenoliths west of the Tanlu fault zone, along with recent paleomagnetic, seismic profiling, and other geochemical studies, favors a deep-seated, Tibetan-type, continental subduction of the Yangtze craton beneath North China along the Tanlu belt.

  19. 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.

  20. 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.

  1. 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.

  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. 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.

  4. 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

  5. Geochemical evidences of magma dynamics at Campi Flegrei (Italy)

    NASA Astrophysics Data System (ADS)

    Caliro, S.; Chiodini, G.; Paonita, A.

    2014-05-01

    Campi Flegrei caldera, within the Neapolitan area of Italy, is potentially one of the most dangerous volcanoes in the world, and during the last decade it has shown clear signs of reactivation, marked by the onset of uplift and changes in the geochemistry of gas emissions. We describe a 30-year-long data set of the CO2-He-Ar-N2 compositions of fumarolic emissions from La Solfatara crater, which is located in the center of the caldera. The data display continuous decreases in both the N2/He and N2/CO2 ratios since 1985, paralleled by an increase in He/CO2. These variations cannot be explained by either processes of boiling/condensation in the local hydrothermal system or with changes in the mixing proportions between a magmatic vapor and hydrothermal fluids. We applied the magma degassing model of Nuccio and Paonita (2001, Earth Planet. Sci. Lett. 193, 467-481) using the most recent inert-gas solubilities in order to interpret these peculiar features in accordance with petrologic constraints derived from the ranges of the melt compositions and reservoir pressures at Campi Flegrei. The model simulations for mafic melts (trachybasalt and shoshonite) show a remarkably good agreement with the measured data. Both decompressive degassing of an ascending magma and mixing between magmatic fluids exsolved at various levels along the ascent path can explain the long-term geochemical changes. Recalling that (i) a sill-like reservoir of gases at a depth of 3-4 km seems to be the main source of ground inflation and (ii) there is petrologic and geophysical evidence for a reservoir of magma at about 8 km below Campi Flegrei, we suggest that the most-intense episodes of inflation occur when the gas supply to the sill-like reservoir comes from the 8 km-deep magma, although fluids exsolved by magma bodies at shallower depths also contribute to the gas budget. Our work highlights that, in caldera systems where the presence of hydrothermal aquifers commonly masks the magmatic signature

  6. 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.

  7. 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.

  8. The Earth's crust deep structure of eastern Eurasia on evidence derived from deep seismic investigation

    NASA Astrophysics Data System (ADS)

    Salnikov, A.; Efimov, A.; Soloviev, V. M.

    2013-05-01

    Interest in investigations of the deep structure of the Earth's crust has quickened in the past few years. However, deep seismic sounding performed with the use of boreholes, big shots and less-than-conditioned spreads is connected with its technological inefficiency, high cost, risk and in many cases with impossibility of performance for reasons of ecology. We present an up-to-date DSS technology that is free from negative limitations, inherent in classical technologies, and so allows much room for its application under any conditions, including industrial estates, reserves, hydroelectric power stations, atomic power-stations, and so on. The technology is based on detailed deep seismic multiwave studies using multiple overlapping spreads and powerful seismic vibrators, which generate seismic waves of high radiation stability, and also remote self-contained recorders. Powerful 40-60 ton vibrators provide vibroseis records at 300 to 350 km that compare well with those of powerful 3 to 5 ton trotyl explosions in wells and basins. This technology is being profitably integrated into exploration practice. It was for the first time in the world that mobile powerful 40-60 t vibrators were used to measure four 300 km seismic lines and one 5 000 km lines in the Altai-Sayan fold area and Okhotsk-Kolyma region, respectively. In the Altai-Sayan area we measured the Novosibirsk-Novokuznetsk line more than 300 km in extent with 30 vibrators spaced at intervals of 5 to 10 km. We studied in details a wave field of the uphole vibrator at distances of 0 to 300 km, obtained fresh evidence on elastic P-wave and S-wave characteristics in the Earth's crust and upper mantle, and revealed the main structural elements of the region (Tom-Kolyvan folded zone, Salair Ridge, Kuznetsk trough). We also detected elastic anisotropy on the Moho and its almost complete absence in the upper Earth's crust. Eastern Eurasia remains worst studied as to deep seismic investigations. By the nature of

  9. Blocks in the Europan Crust Provide More Evidence of Subterranean Ocean

    NASA Technical Reports Server (NTRS)

    2000-01-01

    The image on the left shows a region of Europa's crust made up of blocks which are thought to have broken apart and 'rafted' into new positions. These features are the best geologic evidence to date that Europa may have had a subsurface ocean at some time in its past. Combined with the geologic data, the presence of a magnetic field leads scientists to believe an ocean is most likely present at Europa today. In this false color image, reddish-brown areas represent non-ice material resulting from geologic activity. White areas are rays of material ejected during the formation of the 25-km diameter impact crater Pwyll (see global view). Icy plains are shown in blue tones to distinguish possibly coarse-grained ice (dark blue) from fine-grained ice (light blue). Long, dark lines are ridges and fractures in the crust, some of which are more than 3,000 kilometers (1,850 miles) long. These images were obtained by NASA's Galileo spacecraft during September 7, 1996, December 1996, and February 1997 at a distance of 677,000 kilometers (417,489 miles).

  10. 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. PMID:19122639

  11. 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.

  12. Geological and geochemical characteristics in the paleo-weathering crust sedimentary type REE deposits, western Guizhou, China

    NASA Astrophysics Data System (ADS)

    Zhou, Lingjie; Zhang, Zhengwei; Li, Yujiao; You, Fuhua; Wu, Chengquan; Zheng, Chaofei

    2013-09-01

    A supergene REE deposit closely interrelated with the weathering of the Emeishan basalt formation was produced in the Xuanwei formation, the overlying stratum of the late Permian Emeishan basalt formation in West Guizhou, China. The host strata consist primarily of offwhite kaolinite clay rock and/or grayish black carbonaceous shale. Mineralogical analyses reveal that kaolinites are the major minerals in REE ores with small amounts of smectite, illite, boehmite, hornblende, pyrophyllite, calcite, dolomite and/or iron-bearing minerals, with a certain proportion of feldspar, quartz crystal debris and noncrystal debris. Geochemical analyses reveal high enrichment of trace elements like Cu, Nb, Ta, Zr and Hf. The host strata feature considerable lithological variability, close interrelation of the REE grade with the lithology and uneven spatial distribution of the REE ores, which are mostly found in Lufang, Maojiaping and Zhangsigou profiles of Weining County and can be as thick as 20 m. Of the five stratigraphic profiles, 48% have their whole-rock ∑REE higher than 1000 ppm. The REE in this framework consists primarily of ion adsorbed phases and REE-rich residual independent mineral phases. Comprehensive analyses suggest that the source may not only include the Emeishan basalt, but the intermediate acid volcanic rocks evolved from the Emeishan basalt in the later periods; the hydrothermal alteration subsequently imposed on the host strata might have boosted the mineralization of the rare earth. The preliminary genetic model should have been: the denudation product from the weathering of the parent rock was migrated to the sea-continental margin at the continent side carrying huge quantities of REE with it and was preserved by the quick marine transgression. The host strata consist primarily of kaolinite clay rock and/or carbonaceous shale, which are so far believed to be a sedimentary type REE deposit closely interrelated with weathering effect.

  13. 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.

  14. 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

  15. Lithium in Jack Hills zircons: Evidence for extensive weathering of Earth's earliest crust

    NASA Astrophysics Data System (ADS)

    Ushikubo, Takayuki; Kita, Noriko T.; Cavosie, Aaron J.; Wilde, Simon A.; Rudnick, Roberta L.; Valley, John W.

    2008-08-01

    In situ Li analyses of 4348 to 3362 Ma detrital zircons from the Jack Hills, Western Australia by SIMS reveal that the Li abundances (typically 10 to 60 ppm) are commonly over 10,000 times higher than in zircons crystallized from mantle-derived magmas and in mantle-derived zircon megacrysts (typically < 2 ppb). High Li concentrations in zircons (10 to 250 ppm) have also been found in igneous zircons from three continental parent rocks: granites, Li-rich pegmatites, and migmatites in pelitic metasediment. The substitution of trivalent cations (REEs and Y) in zircon correlates with Li + 1 and P + 5 , suggesting that an interstitial site for Li, as well as the xenotime substitution for P, provides charge balance for REEs. Li is thus fixed in the zircon structure by coupled substitutions, and diffusive changes in [Li] composition are rate-limited by slow diffusion of REEs. The Jack Hills zircons also have fractionated lithium isotope ratios ( δ7Li = - 19 to + 13‰) about five times more variable than those recorded in primitive ocean floor basalts (2 to 8‰), but similar to continental crust and its weathering products. Values of δ7Li below - 10‰ are found in zircons that formed as early as 4300 Ma. The high Li compositions indicate that primitive magmas were not the source of Jack Hills zircons and the fractionated values of δ7Li suggest that highly weathered regolith was sampled by these early Archean magmas. These new Li data provide evidence that the parent magmas of ancient zircons from Jack Hills incorporated materials from the surface of the Earth that interacted at low temperature with liquid water. These data support the hypothesis that continental-type crust and oceans existed by 4300 Ma, within 250 million years of the formation of Earth and the low values of δ7Li suggest that weathering was extensive in the early Archean.

  16. Geochemical evidence for relict degassing pathways preserved in andesite

    NASA Astrophysics Data System (ADS)

    Plail, M. J.; Humphreys, M.; Edmonds, M.; Barclay, J.; Herd, R.

    2012-12-01

    The andesitic Soufrière Hills Volcano (SHV) active since 1995 emits large fluxes of volcanic gases, even during eruptive pauses lasting > 1 year. It has been observed that the flux of gas is largely decoupled from the flux of magma to the surface, indicating efficient magma-vapour segregation, followed by vapour transport to the surface. Evidence for vapour transport is very rarely preserved in the erupted rocks, perhaps due to overprinting during eruption, or perhaps because the transport zones themselves are not usually erupted. The deposits of two recent eruptive events from SHV, the 3rd January 2009 vulcanian explosion products, and the 11th February 2010 dome collapse deposits, contained narrow zones of sheared material. These sheared zones are up to 2m in length and 2 to 10cm in width within blocks of andesite, and have the appearance of alternating darker fine-grained and lighter coarser bands. The dark, fine-grained bands (~30-70μm) have very low porosity (~1%) with abundant zones of oxides (<8% vol), cristobalite microlites, quartz, cordierite and sieved plagioclase. The light, coarse bands (~100 - 350 μm) consist of predominantly broken plagioclase crystals, orthopyroxene, clinopyroxene and rarely large amphibole crystals. Porosity ranges from 7 to 19 % vol with reduced abundance of oxides (<2.4% vol) in comparison to the darker low-porosity bands. Glass is rare to absent in both types of band. Mineral compositions in both of these sheared zones are identical to the established mineral compositions in the andesitic and mafic at SHV. However, bulk XRF analyses indicate that some metal concentrations are greatly enhanced relative to the surrounding andesite. Copper concentrations (236ppm) for example are up to four times higher than the SHV andesites and the mafic enclaves. We find that copper is typically hosted as copper iron sulphide inclusions in Ti-magnetites and plagioclase phenocrysts. Tin oxides are also present and these are typically associated

  17. 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

  18. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2014-06-01

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

  20. 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.

  1. 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.

  2. Within-plate magmatism under condition of abnormally thick sialic crust: Evidence for Proterozoic anorthosite-rapakivi granite complexes of the East-European Craton

    NASA Astrophysics Data System (ADS)

    Sharkov, Evgenii

    2010-05-01

    Mid-Proterozoic (1.8-1.5 Ga) large bimodal multistage anorthosite-rapakivi granite complexes (ARGCs) are distinct magmatic assemblages in central part of the East European Craton. ARGCs formation commenced after stabilization of the Svecofennian orogen and relics of its abnormally thick (up to 50-60 km now) crust survived here in many places. Such massifs are practically absent at the eastern part of the craton (Kola-Karelian, Volga-Urals, etc. domains) with normal thickness (~40 km) of the crust. The ARGCs formation was accompanied by emplacement of diabase, quartz porphyry and complex dike swarms. Intra-plutonic diabase dikes (Fe-Ti basalts plume-related type), intruding the rapakivi granites, are often crossed in turn by later portions of granites; injections of basaltic melt into granitic magma chambers resulted in magma mingling. It indicates that melted out occurred simultaneously in mantle and crust during ARGC formation. Geochemical peculiarities of the ARGC rocks are enrichment in alkali (especially in K), Ti, Zn, Pb, and Zr, relatively high concentrations of Be, Sn, In, Y, Nb, Rb, F, Cu, W and Mo, and sometimes - Li and U. ɛNd value, ranges from -1.2 to +1.6, and relative high Th and Zn contents, most frequently observable in anorthosites, imply that the mafic magmas were considerably contaminated by crustal components. According to geophysical data, ARGCs represent upper parts of large transcrustal systems, composed by alternation of basic and silicic rocks, which located above rises of the mantle up to 10-20 km high. Such localization of ARGCs, probably evidence that such protuberances were mantle plume heads in time, where melting of their material occurred due to adiabatic decompression. Newly-formed basaltic melts (apparently Fe-Ti basalts, similar in composition to intra-plutonic dike rocks) intruded at different depths into abnormally thick sialic crust of stabilized by then Svecofennian orogen in form of large sills and caused melting of crustal

  3. 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.

  4. 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.

  5. Heterogeneous Hadean hafnium: evidence of continental crust at 4.4 to 4.5 ga.

    PubMed

    Harrison, T M; Blichert-Toft, J; Müller, W; Albarede, F; Holden, P; Mojzsis, S J

    2005-12-23

    The long-favored paradigm for the development of continental crust is one of progressive growth beginning at approximately 4 billion years ago (Ga). To test this hypothesis, we measured initial 176Hf/177Hf values of 4.01- to 4.37-Ga detrital zircons from Jack Hills, Western Australia. epsilonHf (deviations of 176Hf/177Hf from bulk Earth in parts per 10(4)) values show large positive and negative deviations from those of the bulk Earth. Negative values indicate the development of a Lu/Hf reservoir that is consistent with the formation of continental crust (Lu/Hf approximately 0.01), perhaps as early as 4.5 Ga. Positive epsilon(Hf) deviations require early and likely widespread depletion of the upper mantle. These results support the view that continental crust had formed by 4.4 to 4.5 Ga and was rapidly recycled into the mantle. PMID:16293721

  6. 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

  7. Lead isotopic evidence for evolutionary changes in magma-crust interaction, Central Andes, southern Peru

    NASA Astrophysics Data System (ADS)

    Barreiro, Barbara A.; Clark, Alan H.

    1984-07-01

    Lead isotopic measurements were made on Andean igneous rocks of Jurassic to Recent age in Moquegua and Tacna Departments, southernmost Peru, to clarify the petrogenesis of the rocks and, in particular, to investigate the effect of crustal thickness on rock composition. This location in the Cordillera Occidental is ideal for such a study because the ca. 2 Ga Precambrian basement rocks (Arequipa massif) have a distinct Pb isotopic signature which is an excellent tracer of crustal interaction, and because geomorphological research has shown that the continental crust was here thickened drastically in the later Tertiary. Seven samples of quartz diorites and granodiorites from the Ilo and Toquepala intrusive complexes, and seven samples of Toquepala Group subaerial volcanics were analyzed for Pb isotopic compositions. The plutonic rocks range in age from Jurassic to Eocene; the volcanic rocks are all Late Cretaceous to Eocene. With one exception, the Pb isotopic ratios are in the ranges 206Pb/ 204Pb= 18.52-18.75, 207Pb/ 204Pb= 15.58-15.65, and 208Pb/ 204Pb= 38.53-38.74. The data reflect very little or no interaction with old continental material of the Arequipa massif type. Lead from four Miocene Huaylillas Formation ash-flow tuffs, two Pliocene Capillune Formation andesites and five Quaternary Barroso Group andesites has lower 206Pb/ 204Pb than that in the pre-Miocene rocks, but relatively high 207Pb/ 204Pb and 208Pb/ 204Pb ( 206Pb/ 204Pb= 18.16-18.30, 207Pb/ 204Pb= 15.55-15.63, 208Pb/ 204Pb= 38.45-38.90). Tilton and Barreiro [9] have shown that contamination by Arequipa massif granulites can explain the isotopic composition of the Barosso Group lavas, and the new data demonstrate that this effect is evident, to varying degrees, in all the analysed Neogene volcanic rocks. The initial incorporation of such basement material into the magma coincided with the Early Miocene uplift of this segment of the Cordillera Occidental [32], and thus with the creation of a thick

  8. 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.

  9. 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.

  10. 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.

  11. 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

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

    NASA Astrophysics Data System (ADS)

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

    2014-07-01

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

  13. 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

  14. 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

  15. 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

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

    PubMed

    Zhou, Huaiyang; Dick, Henry J B

    2013-02-14

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

  17. Revisiting diagenesis on the Ontong Java Plateau: Evidence for authigenic crust precipitation in Globorotalia tumida

    NASA Astrophysics Data System (ADS)

    Branson, Oscar; Read, Elizabeth; Redfern, Simon A. T.; Rau, Christoph; Elderfield, Henry

    2015-11-01

    The calcite tests of foraminifera lie in marine sediments for thousands to millions of years, before being analyzed to generate trace element and isotope paleoproxy records. These sediments constitute a distinct physio-chemical environment from the conditions in which the tests formed. Storage in sediments can modify the trace element and isotopic content of foraminiferal calcite through diagenetic alteration, which has the potential to confound their paleoceanographic interpretation. A previous study of Globorotalia tumida from the Ontong Java Plateau, western equatorial Pacific, found that preferential dissolution of higher-Mg chamber calcite and the preservation of a low-Mg crust on the tests significantly reduced whole-test Mg/Ca and Sr/Ca. Here we revisit specimens with a combination of synchrotron X-ray computed tomography (sXCT) and electron probe microanalyses to reevaluate the nature of their diagenetic alteration. The dissolution of higher-Mg calcite with depth was directly observed in the sXCT data, confirming the inference of the previous study. The sXCT data further reveal a thickening of the chemically and structurally distinct calcite crust with depth. We propose that these crusts have a diagenetic origin, driven by the simultaneous dissolution of high-Mg chamber calcite and precipitation of low-Mg crust from the resulting modified pore water solution. While the breadth of the study is limited by the nature of the techniques, the observation of both dissolution and reprecipitation of foraminiferal calcite serves to demonstrate the action of two simultaneous diagenetic alteration processes, with significant impacts on the resulting paleoproxy signals.

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

    NASA Astrophysics Data System (ADS)

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

    2012-04-01

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

  19. Melt/peridotite interaction in the Southern Lanzo peridotite: Field, textural and geochemical evidence

    NASA Astrophysics Data System (ADS)

    Piccardo, G. B.; Zanetti, A.; Müntener, O.

    2007-03-01

    This paper presents field, petrographic-structural and geochemical data on spinel and plagioclase peridotites from the southern domain of the Lanzo ophiolitic peridotite massif (Western Alps). Spinel lherzolites, harzburgites and dunites crop out at Mt. Arpone and Mt. Musinè. Field evidence indicates that pristine porphyroclastic spinel lherzolites are transformed to coarse granular spinel harzburgites, which are in turn overprinted by plagioclase peridotites, while strongly depleted spinel harzburgite and dunite bands and bodies replace the plagioclase peridotites. On the northern flank of Mt. Arpone, deformed, porphyroclastic (lithospheric) lherzolites, with diffuse pyroxenite banding, represent the oldest spinel-facies rocks. They show microstructures of a composite subsolidus evolution, suggesting provenance from deeper (asthenospheric) mantle levels and accretion to the lithosphere. These protoliths are locally transformed to coarse granular (reactive) spinel harzburgites and dunites, which show textures reminiscent of melt/rock reaction and geochemical characteristics suggesting that they are products of peridotite interaction with reactively percolating melts. Geochemical data and modelling suggest that <1-5% fractional melting of spinel-facies DMM produced the injected melts. Plagioclase peridotites are hybrid rocks resulting from pre-existing spinel peridotites and variable enrichment of plagioclase and micro-gabbroic material by percolating melts. The impregnating melts attained silica-saturation, as testified by widespread orthopyroxene replacement of olivine, during open system migration in the lithosphere. At Mt. Musinè, coarse granular spinel harzburgite and dunite bodies replace the plagioclase peridotites. Most of these replacive, refractory peridotites have interstitial magmatic clinopyroxene with trace element compositions in equilibrium with MORB, while some Cpx have REE-depleted patterns suggesting transient geochemical features of the

  20. 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

  1. 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.

  2. Evidence for the Buried "Pre-Noachian" Crust Pre-Dating the Oldest Observed Surface Units on Mars

    NASA Technical Reports Server (NTRS)

    Frey, H. V.; Frey, E. L.; Hartmann, W. K.; Tanaka, K. L. T.

    2003-01-01

    MOLA gridded data shows clear evidence for Quasi-Circular Depressions not visible on images in Early Noachian (EN) terrain units on Mars. We suggest these are buried impact basins that pre-date the superimposed craters whose high density makes these EN units the oldest visible at the surface of Mars. There is crust older than the oldest visible terrain units on Mars, and these EN units cannot date from 4.6 BYA. These and other Noa-chian units have similar total (visible + buried) crater retention ages, suggesting a common "pre-Noachian" crustal age OR crater saturation beyond which we cannot see.

  3. Geochemical evidence for Paleozoic oil in Lower Cretaceous O Sandstone, northern Denver basin

    USGS Publications Warehouse

    Clayton, J.L.

    1989-01-01

    Organic geochemical properties of the oil produced from the Lower Cretaceous O sandstone on the eastern flank of the Denver basin indicate that this oil has been derived from a different source rock than other Cretaceous oils in the basin. O sandstone oil is characterized by low pristane/phytane ratio, high isoprenoid/n-alkane ratios, high asphaltene content, high sulfur content, and slight predominance of even-carbon numbered n-alkanes in the C25+ fraction. These features are evidence of a Paleozoic source and indicate a carbonate rock is the likely source. Preliminary source rock evaluation and correlation data suggest that calcareous black shales and marls of Middle Pennsylvanian (Desmoinesian) age are the source of the O sandstone oil. This is the first reported occurrence of oil from Paleozoic source rocks in a Cretaceous reservoir in the Denver basin. -from Author

  4. 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

  5. Deucalionis Regio, Mars: Evidence for a unique mineralogic endmember and a crusted surface

    NASA Astrophysics Data System (ADS)

    Merenyi, E.; Edgett, K. S.; Singer, R. B.

    1993-03-01

    A small equatorial region south of Sinus Meridiani, Deucalionis Regio, has been found spectrally distinct from other regions as seen in a high spectral resolution telescopic image of the meridian hemisphere of Mars. Analysis of Viking IRTM and other related data suggest that Deucalionis Regio has a crusted surface. The crust-bonding minerals may contribute to the spectral uniqueness of this region. Two independent analyses of spectral images, linear spectral mixing and supervised classification based on the spectral shapes, showed that in addition to the well-known spectral endmember regions in this image (western Arabia, south Acidalia, and Sinus Meridiani), Deucalionis Regio has spectral properties that are unique enough to make it a principle endmember unit. In those earlier works, Deucalionis Regio was referred to as 'Meridiani Border.' Analysis of thermal inertia, rock abundance, and albedo information derived from Viking images and Infrared Thermal Mapper (IRTM) data obtained 1977-80 also indicate that Deucalionis Regio has a surface of distinctly different physical properties when compared to Arabia, Sinus Meridiani, and Acidalia. Deucalionis Regio has a thermal inertia equivalent to the Martian average, a low rock abundance (less than 5 percent), and an intermediate albedo and color. Considerable effort by previous investigators has revealed a consistent model for the surface (upper few cm) properties of the endmember reigons Arabia, Sinus Meridiani, and Acidalia. Compared with these regions, we consider that Deucalionis Regio is not a region of either (1) unconsolidated, fine bright dust like Arabia, (2) considerable windblown unconsolidated sand like Sinus Meridiani, or (3) a rocky-and-sandy surface like Acidalia. Thus, we are forced to consider that either the surface of Deucalionis Regio is made of unconsolidated fine to medium sand (about 250 microns) of an unusual and previously unreported color and albedo, or that the surface is crusted, fine

  6. 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.

  7. 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

  8. 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.

  9. 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

  10. 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.

  11. Two stage melt-rock interaction in the lower oceanic crust of the Parece Vela basin (Philippine sea), evidence from the primitive troctolites from the Godzilla Megamullion

    NASA Astrophysics Data System (ADS)

    Sanfilippo, A.; Dick, H. J.; Ohara, Y.

    2011-12-01

    Godzilla Megamullion is a giant oceanic core complex exposed in an extinct slow- to intermediate-spreading segment of the Parece Vela Basin (Philippine sea) [1; 2]. It exposes lower crust and mantle rocks on the sea-floor, offering a unique opportunity to unravel the architecture and the composition of the lower oceanic lithosphere of an extinct back arc basin. Here we present data on primitive troctolites and associated olivine-gabbros from the breakaway area of the Godzilla Megamullion. On the basis of the olivine/plagioclase volume ratio, the troctolites are subdivided into Ol-troctolites (Ol/Pl >1) and Pl-troctolites (Ol/Pl<1), which show evident textural differences. Ol-troctolites have rounded to polygonal olivine, subhedral plagioclase, and poikilitic clinopyroxene. This texture suggests chemical disequilibrium between the olivine and a melt crystallizing plagioclase and clinopyroxene. We interpret these rocks as reaction products of a dunite matrix with transient basaltic melts [e.g. 3; 4]. Pl-troctolites have euhedral plagioclase and poikilitic olivine and clinopyroxene. Irregular shapes and inverse zoning of the plagioclase chadacrysts within the olivine indicate disequilibrium between existing plagioclase and an olivine-clinopyroxene saturated melt. The occurrence of plagioclase chadacrysts within clinopyroxene ranging from irregular to euhedral in shape suggests crystallization of new lower-Na plagioclase with the clinopyroxene. Olivine oikocrysts in the Pl-troctolites have low-NiO olivine in equilibrium with a high-MgO melt. The Pl-troctolites, then, may be the product of reaction between a plagioclase cumulate and a basaltic melt produced by mixing the high-MgO melt residual to the formation of the Ol-troctolites with new magma. The effect of melt-rock reaction in the Pl- and Ol- troctolites explains the sharp decrease in plagioclase An with respect to Mg# in clinopyroxene and olivine. Furthermore, the melt is shifted towards lower Na, which is

  12. 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

  13. Processes of lithosphere evolution: New evidence on the structure of the continental crust and uppermost mantle

    USGS Publications Warehouse

    Artemieva, I.M.; Mooney, W.D.; Perchuc, E.; Thybo, H.

    2002-01-01

    We discuss the structure of the continental lithosphere, its physical properties, and the mechanisms that formed and modified it since the early Archean. The structure of the upper mantle and the crust is derived primarily from global and regional seismic tomography studies of Eurasia and from global and regional data on seismic anisotropy. These data as documented in the papers of this special issue of Tectonophysics are used to illustrate the role of different tectonic processes in the lithospheric evolution since Archean to present. These include, but are not limited to, cratonization, terrane accretion and collision, continental rifting (both passive and active), subduction, and lithospheric basal erosion due to a relative motion of cratonic keels and the convective mantle. ?? 2002 Elsevier Science B.V. All rights reserved.

  14. 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

  15. 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. PMID:22778445

  16. 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.

  17. Acidic Plutonism in the Izu-Ogasawara (Bonin)-Mariana (IBM) Arc and Growth of Arc Crust: Petrological and Geochemical Characteristics of the Tonalite at the Komahashi-Daini Seamount and Difference From the Tanzawa Plutonic Complex

    NASA Astrophysics Data System (ADS)

    Haraguchi, S.; Ishii, T.

    2003-12-01

    Recent seismic refraction and reflection data suggest that the continents are underlain by mafic lower crust and felsic middle crust. Petrogenesis of granitic middle crust layers is important for understanding the formation and evolution of continental crust. In modern tectonic regimes, tonalitic rocks and chemically equivalent volcanic rocks occur in island arcs and active continental margins. Thus, the petrogenesis of tonalite and related rocks in intra-oceanic arc settings is of great importance in understanding the processes of both recent island arc and continental crust formation. The Komahashi-Daini Seamount, in the northern Kyushu-Palau Ridge in the northern Philippine Sea plate, was investigated by the Japanese Geodynamics Project (GDP) cruises in the 1970's, and by the R/V Tansei-maru (Ocean Research Institute, University of Tokyo) in the 1990's. Plutonic rocks were dredged from the seamount, and have great importance for understanding the processes of island arc and continental crust formation. The petrographical and geochemical characteristics of the Komahashi-Daini Seamount tonalite are summarized as follows: (1) These tonalites are classified into biotite-hornblende tonalite and hornblende tonalite. Phenocrysts, especially plagioclase, show common lamellar twins and oscillatory zoning patterns; (2) This tonalite show low content of bulk LILE, and classified into low-K calc-alkaline, 1 to 8 wt.% MgO with 55 to 75 wt.% SiO2; (3) This tonalite shows roughly parallel and increasing total REE content with increasing SiO2 content, except for increasingly strong negative Eu anomaly at higher SiO2. These factors indicate that the Komahashi-Daini Seamount tonalite was produced by fractional crystallization. The parent magma of this tonalite is considered lower than 56 wt.% SiO2. Based on this relationship, we concluded that the source for the parental magma was arc mantle peridotite. We compared these tonalites with typical tonalite, i.e., Tanzawa Complex

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

    NASA Astrophysics Data System (ADS)

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

    2013-09-01

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

  19. 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

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

    NASA Astrophysics Data System (ADS)

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

    2012-11-01

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

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

    NASA Astrophysics Data System (ADS)

    Piochi, Monica; Ayuso, Robert A.; De Vivo, Benedetto; Somma, Renato

    2006-02-01

    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

  2. Southward shift of the Intertropical Convergence Zone in the western Pacific during the late Tertiary: Evidence from ferromanganese crusts on seamounts west of the Marshall Islands

    NASA Astrophysics Data System (ADS)

    Kim, Jonguk; Hyeong, Kiseong; Jung, Hoi-Soo; Moon, Jai-Woon; Kim, Ki-Hyune; Lee, Insung

    2006-12-01

    Hydrogenetic ferromanganese crusts were dredged from four seamounts in the western Pacific, OSM7, OSM2, Lomilik, and Lemkein, aligned in a NW-SE direction parallel to Pacific Plate movement. The crusts consist of four well-defined layers with distinct textural and geochemical properties. The topmost layer 1 is relatively enriched in Mn, Co, Ni, and Mo compared to the underlying layer 2, which is relatively enriched in Al, Ti, K, and Rb and Cu, Zn, and excess Ba. Textural and geochemical properties of layer 2 suggest growth conditions under high biogenic and detrital flux. Such conditions are met in the equatorial Pacific (i.e., between the Intertropical Convergence Zone (ITCZ) and equatorial high-productivity zone). Layer 2 likely formed when each seamount was beneath the equatorial Pacific along its back track path. On the other hand, layer 1 probably started to grow after seamounts moved northwest from the ITCZ. This interpretation is consistent with the thickness of layer 1 across the four crusts, which increases to the northwest. Ages of the layer 1-layer 2 boundary in each crust, a potential proxy for northern margin of the ITCZ, also increase to the northwest at 17, 11, 8, and 5 Ma for OSM7, OSM2, Lomilik, and Lemkein, respectively. Assuming Pacific Plate motion of 0.3°/Myr, the seamounts were located at 12°N, 11°N, 9°N, and 8°N at the time of boundary formation. This result suggests that the north edge of the ITCZ has shifted south since the middle Miocene in the western Pacific, which agrees with information from the eastern Pacific.

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

    DOE PAGESBeta

    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

  4. 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

  5. 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

  6. 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

  7. 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.

  8. 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.

  9. 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.

  10. 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.

  11. 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.

  12. 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.

  13. 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.

  14. 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.

  15. 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.

  16. Geochemical and Sr-Nd isotope evidences of the suprasubduction nature of mesozoic magmatism in the Mongol-Okhotsk Sector of the Pacific Fold Belt

    NASA Astrophysics Data System (ADS)

    Derbeko, I. M.; Chugaev, A. V.; Oleinikova, T. I.; Bortnikov, N. S.

    2016-02-01

    In this article we present geochemical and isotope characteristics of rocks of the Unerikan, Selitkan and Aezop-Yamalin volcano-plutonic zones of the eastern termination of the Mongol-Okhotsk Orogenic Belt. The obtained data demonstrate that the Mesozoic igneous rocks of the Mongol-Okhotsk sector of the Pacific Folded Belt were formed due to the melting of the continental crust in a tectonic setting corresponding to a suprasubduction one.

  17. 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.

  18. 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.

  19. 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.

  20. 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.

  1. 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.

  2. 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.

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

    SciTech Connect

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

    1992-11-01

    This paper reports that 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 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 at 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 crustal 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 is considered remote.

  5. A comparative Rb-Sr, Sm-Nd, and K-Ar study of shocked norite 78236 - Evidence of slow cooling in the lunar crust

    NASA Technical Reports Server (NTRS)

    Nyquist, L. E.; Bogard, D. D.; Wooden, J. L.; Bansal, B. M.; Wiesmann, H.; Shih, C.-Y.; Reimold, W. U.

    1982-01-01

    The sample 78236 was chipped from the top of a norite boulder at Station 8 by the Apollo 17 landing team. Jackson et al. (1975) concluded that this rock formed at a depth of 8-30 km in the lunar crust and suggested that it was excavated by a large basin-forming impact event. A petrographic description of the boulder is provided, and isotopic analyses are discussed. Attention is given to a chronology for 78236 which seems to be most consistent with radiometric and other evidence. It is proposed that cumulate norite 78236 formed deep in the lunar crust approximately 4.4 AE ago. The rock cooled slowly in the crust until it was excavated by a major basin-forming event. Excavation may have occurred about 4.2 AE ago, but the time of this event is not well constrained.

  6. The Cool Early Earth: Oxygen Isotope Evidence for Continental Crust and Oceans on Earth at 4.4 Ga

    NASA Astrophysics Data System (ADS)

    Valley, J. W.; King, E. M.; Peck, W. H.; Graham, C. M.; Wilde, S. A.

    2001-05-01

    oceans at that time. The evidence for liquid water and possibly oceans at 4.4 Ga suggests a Cool Early Earth. This contrasts with the Hot Early Earth and global magma oceans envisioned at 4.5-4.3 Ga based on: an impact origin of the Moon (4.45-4.50 Ga), core formation, higher Hadean radioactive heat production, and intense early meteorite bombardment. Magma on the surface of the Earth cools quickly by radiation to form a crust, but a magma ocean caused by these processes might persist beneath the initially thin crust for up to 400 m.y. and might erupt as massive flood basalts in response to major meteorite impacts, boiling surface waters. The thermal contrasts presented by these lines of evidence are minimized if the Moon and core formed earlier (\\sim4.5 Ga), if the Moon formed by a process not involving a Mars-size impactor, or if the early meteorite bombardment was less intense or irregular in timing. It is possible that periods of Cool vs. Hot Early Earth alternated, with boiling of early oceans after major impact events followed by periods of cooler surface conditions. If life evolved in these seas, multiple extinctions before 3.9 Ga are suggested.

  7. 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

  8. Geochemical and Isotopic Evidences of the Magmatic Sources in the Eastern Sector of the Trans-Mexican Volcanic Belt: Xihuingo-Chichicuautla Volcanic Field

    NASA Astrophysics Data System (ADS)

    Valadez, S.; Martinez-serrano, R.; Juarez-Lopez, K.; Solis-Pichardo, G.; Perez-Arvizu, O.

    2011-12-01

    derived from the subduction processes. These magmas probably suffered fractional crystallization and minor assimilation in the continental crust. Sr, Nd isotopic compositions for this first group display the most radiogenic values (87Sr/86Sr from 0.7046 to 0.7047 and ɛNd from 2.2 to 2.8). The second source for the basaltic-andesite and basalt could be associated with an enriched mantle. These rocks present a minor LILE enrichment with respect to HSFE, and Sr and Nd isotopic values less radiogenic than the felsic rocks of the first group (87Sr/86Sr from 0.7040 to 0.7045 and ɛNd from 3.1 to 4.8). According to these evidences we can establish that the magmas emplaced in the study area were produced from a heterogeneous mantle source with complex magmatic processes combined with different interaction degrees between the magmas and continental crust.

  9. Partial melting of thickened continental crust in central Tibet: Evidence from geochemistry and geochronology of Eocene adakitic rhyolites in the northern Qiangtang Terrane

    NASA Astrophysics Data System (ADS)

    Long, Xiaoping; Wilde, Simon A.; Wang, Qiang; Yuan, Chao; Wang, Xuan-Ce; Li, Jie; Jiang, Ziqi; Dan, Wei

    2015-03-01

    The composition of the deep crust is a key to understanding the formation of the low-velocity zone in the middle to lower crust of the Tibetan Plateau. The Suyingdi rhyolites exposed in the northern Qiangtang Terrane have high Sr (296-384 ppm) and low Y (5.81-7.93 ppm), with therefore high Sr/Y ratios (42-56), showing geochemical features of adakitic rocks. Zircon U-Pb dating yields an eruption age of 38.2 ± 0.8 Ma (MSWD = 0.78). These adakitic rhyolites are high-K calc-alkaline in composition, displaying a weakly peraluminous character. They have low MgO content (0.20-0.70 wt.%) and Mg# values (24-39), as well as low Sc (2.25-2.76 ppm), Cr (8-14 ppm), Co (1.6-3.5 ppm) and Ni (2-3 ppm) concentrations. The rocks are LREE-enriched ((La/Yb)N = 50-62) and display weakly negative Eu anomalies (Eu/Eu* = 0.82-0.95) and pronounced negative Nb and Ta anomalies. They have low initial (87Sr/86Sr)i ratios (0.707860 to 0.708342) and enriched Nd isotopic compositions with εNd (t) values ranging from -8.4 to -5.0, which are indistinguishable from those of Cenozoic potassic and ultra-potassic lavas exposed in northern Tibet. Their much higher SiO2 and lower Fe2O3 contents, yet similar MgO, Cr, Co, Ni, and Mg# values to the potassic and ultra-potassic lavas, however, indicate that the rhyolites are unlikely to have formed by fractional crystallization of these lavas. Because of their low Nb/Ta ratios and similar Sr-Nd isotopic compositions to granulite xenoliths within the Cenozoic potassic rocks, we infer that the Suyingdi adakitic rhyolites were most likely produced by partial melting of a thickened lower crust in the garnet stability field. The magma source is most likely dominated by granulite facies metabasalts and clay-poor metamorphosed sedimentary rocks which indicate that the lower crust of northern Tibet is heterogeneous. In combination with data from previously-reported peraluminous and metaluminous adakitic rocks in the same region, the age and petrogenesis of the

  10. 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.

  11. 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

  12. 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.

  13. 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

  14. 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.

  15. Anisotropic low-velocity lower crust beneath the northeastern margin of Tibetan Plateau: Evidence for crustal channel flow

    NASA Astrophysics Data System (ADS)

    Shen, Xuzhang; Yuan, Xiaohui; Ren, Junsheng

    2015-12-01

    Detailed seismic structure in the crust beneath the northeastern margin of Tibetan Plateau was revealed by receiver functions of a regional permanent seismic network. At most stations, negative P-to-S converted phases can be detected in the radial receiver functions, prior to the Moho phases, indicating low velocities in the midlower crust. Prominent azimuthal variations in the transverse receiver functions with polarity reversal suggest azimuthal anisotropy in the crust. We used time variations of the P-to-S converted phases at the Moho in the radial receiver functions and the azimuth-weighted stacking of transverse receiver functions to determine the fast direction and magnitude of anisotropy. The low-velocity midlower crust with the coherent azimuthal anisotropy in the northeastern margin of Tibetan Plateau is consistent with the lower crustal channel flow model.

  16. Steady State Growth of Continental Crust?

    NASA Astrophysics Data System (ADS)

    Bowring, S. A.; Bauer, A.; Dudas, F. O.; Schoene, B.; McLean, N. M.

    2012-12-01

    any age. If one accepts that the probability of preserving old crust decreases with increasing age, the few exposures of rocks older than 3.5 Ga should not be surprising. The thickness and compositional differences between Archean and younger lithospheric mantle are not fully understood nor is the role of thicker buoyant mantle in preserving continental crust; these lead to the question of whether the preserved rock record is representative of what formed. It is notable that the oldest known rocks, the ca. 4.0 Ga Acasta Gneisses, are tonalities-granodiorites-granites with evidence for the involvement of even older crust and that the oldest detrital zircons from Australia (ca. 4.0-4.4 Ga) are thought to have been derived from granitoid sources. The global Hf and Nd isotope databases are compatible with both depleted and enriched sources being present from at least 4.0 Ga to the present and that the lack of evolution of the MORB source or depleted mantle is due to recycling of continental crust throughout earth history. Using examples from the Slave Province and southern Africa, we argue that Armstrong's concept of steady state crustal growth and recycling via plate tectonics still best explains the modern geological and geochemical data.

  17. 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.

  18. 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.

  19. Paleoproterozoic accretionary and collisional processes and the build-up of the Borborema Province (NE Brazil): Geochronological and geochemical evidence from the Central Domain

    NASA Astrophysics Data System (ADS)

    Neves, Sérgio P.; Lages, Geysson A.; Brasilino, Roberta G.; Miranda, Alan W. A.

    2015-03-01

    Several Brasiliano-Pan-African belts consist of large areas of reworked Paleoproterozoic rocks. Characterization of these rocks is needed to place better controls on Precambrian paleogeographic reconstructions. The Borborema Province, northeastern Brazil, occupies a central position in West Gondwana configuration, and knowledge of its geological evolution is crucial to infer relationships between Paleoproterozoic units in South America and Africa. Here, we report U-Pb ages, major- and trace-elements analyses and Sm-Nd isotopic data for orthogneisses in the eastern portion of Central Domain. The dominant basement units in the study area are banded gneisses of intermediate composition and relatively juvenile character, and migmatitic gneisses of granitic composition with Archean Nd TDM model ages. One sample of the banded gneiss yielded a weighted 207Pb/206Pb age of 2096 ± 23 Ma and an upper intercept age of 2044 ± 27 Ma, which we interpret, respectively, as ages of crystallization and metamorphism. Two large units of migmatitic gneiss in the southern and central parts of the area gave ages of, respectively, 2057 ± 20 Ma and 2055 ± 23 Ma; an orthoamphibolite associated with the latter yielded crystallization age of 2042 ± 11 Ma and metamorphic age of 1996 ± 13 Ma. All these rocks have geochemical signatures typical of subduction zone-related magmas. Combined with evidence provided by previous studies, we suggest that the evolution of the study area starts with island arc construction around 2.2 Ga, leading to an expressive volcanic arc edifice by 2.13-2.10 Ga. By 2.06 Ga, the crust had evolved enough to become intruded by magmas formed at the mantle wedge of the now largely continental magmatic arc, which continued to be intruded by mantle melts until at least 2.04 Ga. An augen gneiss in the northern part of the area, with an age of 2109 ± 15 Ma, and a migmatitic gneiss with a much older age (2183 ± 9 Ma), both of which have geochemical characteristics akin

  20. 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.

  1. Deep depleted and shallow enriched mantle sources of Karoo CFBs: geochemical evidence from Antarctica

    NASA Astrophysics Data System (ADS)

    Luttinen, A. V.; Heinonen, J. S.

    2011-12-01

    The Karoo continental flood basalts and associated intrusive rocks are typified by a great diversity of geochemical types that are most readily identified based on incompatible element and isotopic characteristics. The principal mantle source in the Karoo province has been frequently ascribed to lithospheric mantle, possibly affected by previous subduction-related fluids and/or melts; only a few rock types show unambiguous affinity to significant asthenospheric source components. Most recently, uncontaminated dyke rocks with depleted mantle geochemical characteristics (e.g. initial epsilon Nd values of up to +8) and strong garnet signatures have been discovered in Vestfjella, in the Antarctic extension of the Karoo CFB province. Our geochemical modeling implicates that a rich variety of low-Ti and high-Ti daughter magma types, notably similar to Karoo CFB types in Vestfjella, can be generated by contamination of a single depleted mantle-derived parental magma type with lithospheric material: We generalize that the deep depleted mantle source of the Vestfjella dykes may also have been the principal source of the numerous low-Ti and high-Ti magma types that are found within or adjacent to the Kaapvaal-Grunehogna craton in the Karoo province. In contrast, the low-Ti magma types that lack a garnet signature and are found outside the craton were derived from shallow, possibly subduction-contaminated mantle sources.

  2. 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.

  3. 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.

  4. Evidence for silicic crust formation in an incipient stage of intra-oceanic subduction zone: discovery of deep crustal sections in Izu-Bonin forearc

    NASA Astrophysics Data System (ADS)

    Tani, K.; Shukuno, H.; Hirahara, Y.; Chang, Q.; Kimura, J.; Nichols, A. R.; Ishii, T.; Tatsumi, Y.; Dunkley, D. J.

    2009-12-01

    Recent research cruises surveying forearc slopes of Izu-Bonin-Mariana (IBM) arc have discovered outcrops representing the deep crustal section of the early IBM arc. Ongoing geochemcial, petrological, and geochronological studies of recovered rocks are providing new insights into the magmatism and development of arc crust during the inception of an intra-oceanic subduction system. ROV traverses, conducted at the northern Izu-Bonin forearc, discovered peridotite exposures from deep (~7000 mbsl) sections, and observed a drastic shift in lithofacies towards the upper sections (~5000 mbsl), from gabbro, through dolerite, porphyrite, tonalite, and finally volcanic breccia and sedimentary rocks. This indicates that the traverses covered a full arc crust section from uppermost mantle to upper crust. The gabbroic and doleritic rocks show geochemical signatures (e.g. LREE-depletion and low-Ba/La) similar to those of N-MORB, with minimum arc signatures (e.g. Nb-depletion and LILE-enrichment). The results from zircon and titanite U-Pb geochronology show that this MORB-like basaltic magmatism was episodically active ~52 Ma, predating the boninitic magmatism broadly exposed in the uppermost section of forearc slope, which began ~49 Ma (Ishizuka et al., 2006) and previously considered to be the first magmatism in IBM arc. The collected peridotite samples were mostly dunite and harzburgite, and show variable degrees of serpentinization. Compositions of Cr-spinel and olivine, and the calculated oxygen fugacity indicate that these peridotite samples probably coexisted with MORB-type magma rather than the boninitic or island-arc basaltic magmas. Massive outcrops of tonalitic rocks and associated dacitic-rhyolitic prophyrites were discovered in one of the surveyed area, underlain by gabbro and dolerite with MORB-like geochemical signatures. Tonalitic rocks, weakly-foliated hornblende tonalites, are continuously exposed in a ~300 m-high wall in the middle section of the forearc slope

  5. 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.

  6. 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.

  7. 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.

  8. 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.

  9. Geochemical and geothermal evidence for fluid migration in the Barbados Accretionary Prism (ODP leg 110)

    NASA Astrophysics Data System (ADS)

    Vrolijk, Peter; Fisher, Andrew; Gieskes, Joris

    Measured geochemical and geothermal effects of fluid migration in the northern Barbados accretionary prism indicate that: (1) fluid flows laterally along low-angle active faults; (2) the entire length of the Leg 110 transect is a fluid discharge zone; (3) migration results in heating of sediments across the transect but most strongly at the deformation front; and (4) migration results in dilution of pore water-Cl and 18O enrichment more in the arcward than the seaward side of the prism, suggesting that the Cl-poor, 18O-rich fluids that intrude the prism are derived from smectiteillite reactions occurring arcward. These conclusions have strengthened those derived from the initial appraisal of shipboard data, modified ideas about where fluids are heated most and where the geochemistry is strongly altered, and provided new ideas about the sources of fluids and their relative contributions.

  10. 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.

  11. Geochemical evidence for seasonal variations in potential loess sources of the western Chinese Loess Plateau

    NASA Astrophysics Data System (ADS)

    Wang, Xunming; Lang, Lili; Li, Hui; Hua, Ting; Wang, Guangtao; Zhou, Na; Jiao, Linlin

    2015-11-01

    This paper aims to characterize seasonal variations in the potential sources of loess deposited on the western Chinese Loess Plateau (CLP). Fallout was sampled every five days between March 2013 and August 2014 at Jiuzhoutai, the site with the largest loess deposits in northwest China. A total of 45 macro and trace elements, as well as rare earth elements, were analyzed in the samples. The results show that, at present, the potential sources of loess are mixed with pollutants. After exclusion of the pollutants, principal component analysis (PCA) showed significant seasonal variations in the potential loess sources on the western CLP. During the spring, summer-autumn, and winter periods, there are possibly six, four, and two potential loess sources, respectively, with crustal provenance at Jiuzhoutai. No significant differences in fallout amounts were found on the western CLP during periods with and without dust events, which suggests that the frequency of dust events may not be an effective index for deposition rates in areas of loess formation. The relatively high proportion of coarser material (>50 μm in diameter) in the fallout indicates that at least part of the loess in the fallout samples originated from adjacent deserts and associated systems, and that both dry and wet deposition contributed equally to the fallout. Based on the significant seasonal and spatial variations in the loess sources, combined with variations in the link between the different particle size classes and the geochemical characteristics of potential components of the loess on the western CLP, the use of the geochemical characteristics of loess stratigraphic sequences as a proxy for regional paleoclimatic and environmental reconstructions should be carefully appraised.

  12. 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

  13. 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.

  14. 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.

  15. 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

  16. 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

  17. Calcium isotopic evidence for rapid recrystallization of bulk marine carbonates and implications for geochemical proxies

    NASA Astrophysics Data System (ADS)

    Fantle, Matthew S.

    2015-01-01

    Strontium and calcium isotopic data for bulk carbonate solids and pore fluids from ODP Sites 1170 and 1171 are presented. The data suggest that bulk carbonate sediments actively exchange with coexisting pore fluids over tens of millions of year time scales. Recrystallization rates constrained by Sr isotopes and Sr elemental data are ∼3% per Ma at 1170A and ∼7% per Ma at 1171A. The pore fluid chemistries at both sites are affected by advection, which occurs in the downwards direction at 1170 (∼-25 m/Ma) and upwards at 1171A (∼250 m/Ma). Both the direction and the rate of advection are reflected by the width of the diffusive boundary layer for Sr at both 1170A (∼300 m) and 1171A (∼50 m), compared to ODP Site 807A (∼150 m) where no chemically-detectable advection is occurring. Recrystallization is supported not only by interpretations of pore fluid data, but also by the alteration of the bulk solid. This is especially true at 1171A, where advection drives significant alteration of Sr/Ca, Mg/Ca, and 87Sr/86Sr. Numerical simulations of pore fluid geochemical and isotopic evolution over tens of millions of years, conducted with a depositional, time-dependent reactive transport model, suggest that recrystallization rates in the upper tens of meters of the sedimentary section at both sites are more rapid than suggested by the Sr geochemical data. When the Sr-constrained rates are applied to the pore fluid Ca isotope data, the model does not predict pore fluid δ44Ca within analytical uncertainty. The simulations indicate rates that are initially ∼20% to 40% per Ma in young, <1 Ma sediments. The Ca isotope data cannot be explained by either inaccurate diffusion coefficients, inaccurate temporal evolution of pore fluid Ca concentrations, or upwards advection. Ultimately, such high rates in young sediments can impact paleoclimate and paleoenvironmental proxies used by geoscientists to study the past. Diagenetic effects due to rapid recrystallization

  18. 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

  19. 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

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

    NASA Astrophysics Data System (ADS)

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

    2011-11-01

    We investigate the crustal structure of the SW Iberian margin along a 340 km-long refraction and wide-angle reflection seismic profile crossing from the central Gulf of Cadiz to the Variscan continental margin in the Algarve, Southern Portugal. The seismic velocity and crustal geometry model obtained by joint refraction and reflection travel-time inversion reveal three distinct crustal domains: the 28-30 km-thick Variscan crust in the north, a 60 km-wide transition zone offshore, where the crust abruptly thins ~ 20 km, and finally a ~ 7 km-thick and ~ 150 km-wide crustal section that appears to be oceanic in nature. The oceanic crust is overlain by a 1-3 km-thick section of Mesozoic to Eocene sediments, with an additional 3-4 km of low-velocity, unconsolidated sediments on top belonging to the Miocene age, Gulf of Cadiz imbricated wedge. The sharp transition between continental and oceanic crust is best explained by an initial rifting setting as a transform margin during the Early Jurassic that followed the continental break-up in the Central Atlantic. The narrow oceanic basin would have formed during an oblique rifting and seafloor spreading episode between Iberia and Africa that started shortly thereafter (Bajocian) and lasted up to the initiation of oceanic spreading in the North Atlantic at the Tithonian (late Jurassic-earliest Cretaceous). The velocity model displays four wide, prominent, south-dipping low-velocity anomalies, which seem to be related with the presence of crustal-scale faults previously identified in the area, some of which could well be extensional faults generated during this rifting episode. We propose that this oceanic plate segment is the last remnant of an oceanic corridor that once connected the Alpine-Tethys with the Atlantic ocean, so it is, in turn, one of the oldest oceanic crustal fragments currently preserved on Earth. The presence of oceanic crust in the central Gulf of Cadiz is consistent with geodynamic models suggesting the

  1. 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. PMID:11206539

  2. 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.

  3. 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.

  4. The nature and location of the suture zone in the Rokelide orogen, Sierra Leone: Geochemical evidence

    NASA Astrophysics Data System (ADS)

    Lytwyn, Jennifer; Burke, Kevin; Culver, Stephen

    2006-12-01

    The boundaries of the West African Craton mark the location of a continuous suture zone that records Neoproterozoic to Early Cambrian oceanic closure. The western part of the circum-West African suture zone extends through the line of outcrop of the Mauritanide, Bassaride and Rokelide mountain belts. Our geochemical analyses are consistent with the idea that igneous and metamorphic rocks of the Rokelide and Southern Mauritanide mountain belts of West Africa occupy a suture zone that records the closing of a Neoproterozoic to Early Cambrian ocean basin during the Pan-African orogeny and final assembly of Gondwana. The closing of that basin was marked by the collision between Archean rocks of the Leo massif of the West African Craton and reactivated Archean and Paleoproterozoic rocks that now outcrop nearer to the coast of Africa in Sierra Leone and Liberia. Within the Rokelides, the geochemistry of the Kasewe Hills volcanic rocks and Marampa amphibolite indicate that remnants of an arc system are caught up in the suture zone. The geochemistry of Guingan schists that outcrop along strike of the Rokelides is compatible with the idea that the metamorphosed equivalents of the Marampa and Kasewe Hills arc volcanic rocks extend through the Bassarides and into the Southern Mauritanides.

  5. 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.

  6. First Geochemical Evidences for Existence of Slow-Spreading Ridges in the Pacific Ocean

    NASA Astrophysics Data System (ADS)

    Krasnova, E. A.; Portnyagin, M.; Silantyev, S.; Werner, R.; Hoernle, K.

    2012-12-01

    Stalemate Fracture Zone (SFZ) is a 500 km long SE-NW trending transverse ridge between the northernmost Emperor Seamounts and the Aleutian Trench which originated by flexural uplift of Cretaceous (?) oceanic lithosphere along a transform fault at the Kula-Pacific plate boundary [1]. Sampling at the SFZ and the fossil Kula-Pacific Rift valley was carried out during the R/V SONNE cruise SO201 Leg 1b in July 2009. These rocks are thought to represent a complete section of oceanic lithosphere formed at the fossil Kula-Pacific Spreading Center. A broad spectrum of mantle peridotites ranging from spinel lherzolites to dunites were dredged at station DR37 at the northern bend of SFZ. Spinel in lherzolites has Mg#=0.65-0.68, NiO=0.26-0.34 wt%, Cr#=0.26-0.33, Fe3+#=0.021-0.030 and TiO2=0.04-0.09 wt%. Clinopyroxene has Mg#=91.7-92.4, Cr#=0.12-0.16, TiO2=0.06- 0.15 wt%, Na2O=0.19-0.41 wt%, NiO=0.06-0.09 wt% and is moderately depleted in HREE and extremely depleted in MREE and Zr (C1-normalized YbN=4.0- 5.6, [Sm/Yb]N=0.05-0.14, [Zr/Y]N=0.001-0.009) [2,3]. In terms of spinel and clinopyroxene Cr# and absolute concentrations of HREE, Ti and Na, these compositions are less depleted than those from the Hess Deep peridotites [4] formed at the fast spreading East-Pacific Rise. The SFZ peridotites are more similar to abyssal peridotites from slow-spreading ridges [e.g., 5]. Geochemical modeling suggests that the SFZ peridotites can be formed by 10-12% of near-fractional partial melting of depleted MORB mantle. We used the correlation between degree of partial mantle melting and full spreading rate [6] to estimate the spreading rate of 4-5 cm/year at the formation of the SFZ residual lherzolites (Fig.1). These results agree well with paleomagnetic data [1] suggesting asymmetric spreading at the ancient Kula-Pacific Center with the full rate of ~6 cm/year. Thus both geochemical and paleomagnetic data suggest the existence of slow-spreading ridges in the Pacific Basin during the Old

  7. 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

  8. Pinch and swell structures: evidence for strain localisation by brittle-viscous behaviour in the middle crust

    NASA Astrophysics Data System (ADS)

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

    2015-09-01

    The flow properties of middle crustal rocks are commonly represented by viscous flow. Examples of pinch and swell structures found in a high strain zone at St. Anne Point (Fiordland, New Zealand) and Wongwibinda (N.S.W., Australia) suggest pinch and swell structures may be initiated by brittle failure of the more competent layer in conjunction with subsequent material softening. On this basis we develop a numerical model where Mohr-Coulomb constitutive strain localising behaviour 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 sufficient viscosity contrast and can localise strain to form shear bands. The flow regime and strain localising characteristics of the surrounding country rock appear not to impact pinch and swell structure formation. The degree of material softening after the initial strain localising behaviour is shown to impact pinch and swell characteristics, while extensive material softening causes the formation of thick necks between swells by limiting the focused localisation of strain into shear bands. 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 strain localising behaviour combined with viscous flow is a viable alternative representation of the heterogeneous rheological behaviour of rocks seen in the middle crust. This type of mid-crustal rheological behaviour can have significant influence on the localisation of strain at all scales. For example, inclusion of Mohr-Coulomb strain localising behaviour with viscous flow in just some mid-crustal layers within a crustal-scale model can result in significant strain localisation, extending from the upper crust into the middle crust. This

  9. 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.

  10. 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.

  11. 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.

  12. Seismic velocity structure at Deep Sea Drilling Project site 504B, Panama Basin: Evidence for thin oceanic crust

    NASA Astrophysics Data System (ADS)

    Collins, John A.; Purdy, Michael G.; Brocher, Thomas M.

    1989-07-01

    We present an analysis of wide-angle reflection/refraction data collected in the immediate vicinity of Deep Sea Drilling Project hole 504B in the Panama Basin, currently the deepest drill hole (1.288 km) into oceanic crust. The data were acquired with a 1785 inch3 air gun array and fixed-gain sonobuoy receivers and consist of four intersecting profiles shot along three different azimuths. Near-normal-incidence, multichannel seismic (MCS) reflection data were acquired simultaneously. Observed P and S wave arrivals out to maximum ranges of 30 km provide constraints on the velocity structure of the middle and lower crust and on total crustal thickness. Comparison of the travel times and amplitudes of the P and S wave arrivals on all four profiles revealed important similarities which were modeled using the reflectivity synthetic seismogram method. Forward modeling shows that in contrast to standard oceanic velocity models, a velocity-depth profile that better explains the observed data is characterized by high-velocity gradients (up to 0.6 km/s/km) in the middle crust, a 1.8-km-thick low-velocity zone (Vp = 7.1-6.7 km/s) immediately above Moho, and a total crustal thickness of only 5 km. Interpretation of the high-velocity gradients in the middle crust is constrained by the observation of P wave amplitude focusing at ranges of 16-19 km. Although not as well developed in comparison to the P wave arrivals, S wave arrivals show similar focusing. Total crustal thickness is constrained by the combined interpretation of a P wave, wide-angle reflection event observed at a range of 16-28 km and an MCS reflection event with a crustal travel time of 1.4-1.5 s. Although these events cannot be directly correlated, their travel times are consistent with the assumption that both have a common origin. Amplitude modeling of the wide-angle event demonstrates that these events are generated at the Moho.

  13. 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). 

  14. Geochemical, sedimentary and micropaleontological evidence for a Late Maastrichtian oceanic seamount within the Pindos ocean (Arvi Unit, S Crete, Greece)

    NASA Astrophysics Data System (ADS)

    Palamakumbura, Romesh N.; Robertson, Alastair H. F.; Dixon, John E.

    2013-06-01

    We test the model of Bonneau (1984) who hypothesised that the Arvi Unit in southern Crete represents Upper Cretaceous oceanic crust of a Pindos oceanic basin. The Arvi Unit is dominated by basaltic lava flows, pelagic carbonates and terrigenous sandstone turbidites. The "enriched" within-plate-type geochemistry of the basaltic lavas is consistent with a seamount setting. The subaqueous lava structures and associated pelagic carbonates further justify a seamount origin. Peperites composed of lava-pelagic carbonate mixtures date the Arvi Unit as Late Maastrichtian using diagnostic planktic foraminifera. The lavas are overlain by pelagic carbonates, also of Late Maastrichtian age, that then pass gradationally upwards into sand to pebble-grade gravity flows. The clastic sediments contain grains derived from several sources, namely continental (metamorphic and plutonic), ophiolite-related (e.g. serpentinite, gabbro, diabase), deep-sea (e.g. chert, pelagic carbonate) and shallow-marine (e.g. shell fragments). The terrigenous detritus is inferred to have come from the Pelagonian microcontinent unit (~ Asteroussia nappe) then to the northeast where ophiolites and deep-sea sediments were obducted during Late Jurassic time. The inferred Arvi seamount was accreted at the southeasterly-subducting active margin of the Pelagonian microcontinent after Maastrichtian time, related to closure of the Pindos ocean. The new evidence from the Arvi Unit provides additional evidence for the existence of the Pindos ocean between the Apulian and Pelagonian continental units in the Greece-Albania region.

  15. 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

  16. Dyke Swarms in Southeastern British Columbia: Mineralogical and Geochemical Evidence for Emplacement of Multiple Magma Types During Orogenic Collapse

    NASA Astrophysics Data System (ADS)

    Freeman, M.; Owen, J. P.; Hoskin, P. W.

    2009-05-01

    Eocene dyke swarms in southeastern British Columbia provide an important record of the tectonic and magmatic history of the Cordillera following orogenic collapse. New field mapping, petrographic, and geochemical data is presented for a swarm of more than thirty dykes located near the mining town of Trail, B.C. Detailed field mapping revealed that individual dykes are highly diverse, both in composition and morphology. As a group, the dykes trend northwest (average strike of 338 degrees) and dip steeply to the southwest. Their average thickness is approximately 1.5m, with a range from 4.5m to less than 1cm. Three sub-parallel dykes were mapped for a length of 2km, and exhibit irregularities in their form such as branching and offshoots that follow fractures in the country rock. Thin-section analysis shows a wide variety of rock types within the swarm, including: micro-quartz syenite, micro-syenite, micro-monzonite, latite, basalt, basaltic andesite, and lamprophyre. Texturally, these samples are consistently porphyritic and partially altered to chlorite and sericite. This alteration commonly occurs in concentric rims around phenocrysts. The samples are typically intergranular, although some display trachytic texture. Whole-rock geochemistry shows that the dykes have a wide range in composition, with SiO2 between 76.45 wt.% and 45.15 wt.% and MgO between 0.13 wt.% and 13.16 wt.%. The results also revealed that one dyke has very high values of Ni (430 ppm), Cr (1420 ppm), and Co (50 ppm), giving it a fairly primitive composition. Harker diagrams and trace element plots show three distinct groups: mafic calc-alkaline dykes, felsic calc- alkaline dykes, and minette lamprophyres. The felsic dykes are characterized by negative Eu and Sr anomalies suggesting fractionation of plagioclase feldspar, as well as pronounced negative P and Ti anomalies. The minettes are enriched in LILE and depleted in HSFE relative to the mafic dykes. The three groups do not appear to be

  17. Geochemical evidence of water source characterization and hydrodynamic responses in a karst aquifer

    NASA Astrophysics Data System (ADS)

    Caetano Bicalho, C.; Batiot-Guilhe, C.; Seidel, J. L.; Van Exter, S.; Jourde, H.

    2012-07-01

    SummaryThe Lez karst spring, the main perennial outlet of the Lez karst system in southern France, plays an important role in supplying drinking water to the Montpellier metropolitan region. In order to investigate the origin of groundwater, its circulation patterns, and to understand the connectivity and compartmentalization of a karst system, a multi-tracer approach was used to describe the hydrogeology of the Lez karst system. Groundwater samples were collected from Lez karst during a range of hydrologic conditions (between March 2006 and August 2009) and analyzed for major and trace elements, total organic carbon, fecal, and total coliform. During the first recharge event of autumn, highly-mineralized water was observed at Lez Spring during the studied years. Multiple parameters of water during this rise were monitored with a fine time-step in 2008. Discriminate Factorial Analyses revealed the existence of different water-types discharging at Lez Spring. During high stage periods, highly mineralized water initially discharges from the spring, followed by rapid infiltration water. This behavior suggests that hydrodynamics affect groundwater circulation by soliciting different endmembers. These characteristics were observed on a larger scale when monitoring three intermittent springs connected to Lez Spring. A detailed analysis using bivariate diagrams of major, trace elements and elemental ratios provided insight into different water origins, associated lithologies, and mineral-solution reactions related to hydrodynamic responses. From the five identified water-types, the two more contrasting ones are emphasized: the first one corresponds to the most geochemically evolved waters, issued from deep layers where evaporite chemical fingerprinting has been identified. They are characterized by high mineralization and high concentrations in Cl, Na, Mg, Li, B and Br elements, and high Sr/Ca, Mg/Ca and Cl/Br ratios. The second water-type corresponds to the most diluted

  18. Geochemical evidence for biogenic methane production and consumption in the shallow sediments of the SE Mediterranean shelf (Israel)

    NASA Astrophysics Data System (ADS)

    Sela-Adler, Michal; Herut, Barak; Bar-Or, Itay; Antler, Gilad; Eliani-Russak, Efrat; Levy, Elan; Makovsky, Yizhaq; Sivan, Orit

    2015-06-01

    This study presents geochemical evidence for biogenic methane formation (methanogenesis) in the shallow sediments of the oligotrophic SE Mediterranean continental shelf at water depths between 46 and 88 m. Depth-profiles of methane concentrations and related chemical parameters such as dissolved sulfate, dissolved inorganic carbon (DIC), and the stable carbon isotope composition of DIC and methane (δ13CDIC, δ13CCH4, respectively) were measured in six sediment cores (each 4.2-5.4 m long) in order to characterize the processes that involve methane production and decomposition. All the sediment cores reached the consumption depth of the entire sulfate pool and the in-situ microbial methane production (methanogenesis) zone. Methane concentrations reached saturation levels in one of the cores, but not in the others, probably because the zone of maximum methanogenesis was at a greater depth. Although the sediments exhibit a low TOC content of ~1%, the biogenic methane formation indicates a relatively high organic carbon lability capable of sustaining all redox microbial activity potential. Anaerobic oxidation of methane (AOM) was also evident in the sulfate-methane transition zone, showing a distinct isotope signature in diffusion limited conditions.

  19. Geochemical evidence for groundwater mixing in the western Great Artesian Basin and recognition of deep inputs in continental-scale flow systems

    NASA Astrophysics Data System (ADS)

    Crossey, L. J.; Karlstrom, K. E.; Love, A.; Priestley, S.; Shand, P.

    2010-12-01

    Mound springs of the western Great Artesian Basin (GAB), Australia, represent a significant proportion of the discharge of the continental-scale confined aquifers of the region. They also provide unique ecological niches, and they are important historical and cultural sites in an austere landscape. Fed by confined aquifers within the GAB, these spring systems are at risk due to anthropogenic drawdown and increasing demand on scarce hydrologic resources. New water and gas geochemical data indicate that they record hydrologic mixing and complex, fault-influenced flow paths within the western GAB. Elevated 3He/4He gas values, termed “xenowhiffs”, with RA up to 0.09 (Bubbler Spring) provide evidence for mantle-derived fluids introduced through fault conduits into the groundwater system in the last several million years and hence an active mantle-to-groundwater fluid linkage. We apply multiple tracers to understand mixing. Major and trace element data show distinctly different water chemistries for Dalhousie versus southern mound springs suggesting different flow paths and mixing proportions. The source of the C for the CO2 -rich springs is evaluated using water chemistry and C-isotope data. Carbon isotope values range from -9 (Bubbler) to -16 (Strangways). Mixing models allow us to distinguish contributions from dissolution of carbonate in the aquifer (Ccarb=Ca+Mg-SO4 and δ13C= 0), from biological/organic sources (δ13C= -28), and from endogenic sources (deeply derived; δ13C= -3). Results show that all of the springs contain appreciable (many > 50%) endogenic CO2, with Dalhousie showing less endogenic CO2 than the southern mound springs and Paralana hot spring system. CO2/3He values of 4 to 8 x 109 (Bubbler and Jersey Springs) are close to MORB end member values of 2 x 109 whereas other springs have values strongly enriched in CO2 (up to 1013 at Elizabeth Spring). Elevated but highly variable 87Sr/86Sr values up to 0.718 at Dalhousie and up to 0.76 at Paralana

  20. 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

  1. Chemical and isotopic composition of the lower crust beneath the Meguma Lithotectonic Zone, Nova Scotia: evidence from granulite facies xenoliths

    NASA Astrophysics Data System (ADS)

    Eberz, G. W.; Clarke, D. B.; Chatterjee, A. K.; Giles, P. S.

    1991-03-01

    Granulite-facies xenoliths from an Upper Devonian lamprophyre dyke near Tangier, Nova Scotia, provide new information about the lower crust in the Meguma Zone. Two mineralogically and chemically distinet groups of xenoliths occur. Both groups contain quartz+feldspar+biotite+Fe-Ti oxides+sulfides. In addition, group A contains garnet+graphite±[aluminosilicates+spinel±sapphirine (hight Al2O3 subgroups A1 and A2)] or [clinopyroxene+sphene+apatite (high CaO sub-group A3)]. Group B has highly variable proportions of orthopyroxene (B1), clinopyroxene (B2), and amphibole (B3). Trace-element contents of the highly aluminous xenoliths compare closely with average to upper crustal model compositions and are similar in many aspects to other “undepleted” granulite-facies rocks. Low P-T sedimentary assemblages of quartz-chlorite-clay minerals-calcite-albite or paragonite can account for the compositions of group A xenoliths. Within group B, a high-MgO (MgO>13 wt%) subgroup with high transition-metal contents, and low-MgO (MgO<9 wt%) sub-groups with higher LIL (large-ion-lithophile) element contents exist. Although the rare-earth and high-field-strength elements indicate a tholeiitic or low-K calc-alkaline chemistry, the LIL elements are as high as those from high-K calc-alkaline volcanic are rocks. Isotopically, group A ranges from ɛNdt=-2.56 to-0.80 and87Sr/86Sr t =0.7046 to 0.7182 for t=370 Ma. For group B these values are +1.45 to +5.33 and 0.7028 to 0.7048, respectively. Model ages (TCHUR) are correspondingly low and are tightly constrained (544±52 Ma). These “young” ages contrast with the middle Proterozoic Nd model ages of the overlying Meguma Zone low-grade flysch. This time-inverted stratigraphy appears to be the product of a tectonic break between a younger autochthonous Tangier lower crust (≡Avalon), and an older allochthonous Meguma Group upper crust.

  2. The Andesites of the Tancítaro Volcanic Field, Michoacán Mexico: Evidence for their Formation in the Lower Crust.

    NASA Astrophysics Data System (ADS)

    Ownby, S. E.; Lange, R. A.

    2006-12-01

    that the andesites of the Tancítaro volcanic field are generated in the lower crust and are emplaced into the upper crust as hydrous, crystal-poor liquids. There is no evidence to support their origin through magma mixing of basalt and rhyolite. Degassing appears to be the primary drive for crystallization, and there is no evidence of any differentiation of these andesites toward dacite and/or rhyolite in an upper crustal chamber. Nor do the andesites appear to be directly related to one another by crystal fractionation processes in the upper crust, as there is no correlation between their silica content and 40Ar/39Ar eruption age (from numerous scattered and isolated vents peripheral to Volcán Tancítaro). Therefore, the primary staging area for the compositional diversity among the andesites in the Tancítaro volcanic field is within the lower crust and is not a result of magmatic processes in the upper crust. The variation in their phenocryst assemblages reflects different initial water concentrations and ascent rates toward the surface.

  3. 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.

  4. 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

  5. Magma mixing and crust-mantle interaction in Southeast China during the Early Cretaceous: Evidence from the Furongshan granite porphyry and mafic microgranular enclaves

    NASA Astrophysics Data System (ADS)

    Wang, Hong-Zuo; Chen, Pei-Rong; Sun, Li-Qiang; Ling, Hong-Fei; Zhao, You-Dong; Lan, Hong-Feng

    2015-11-01

    The petrogenesis and tectonic setting of Early Cretaceous granitoids and their enclaves emplaced in the Gan-Hang Tectonic Belt are still controversial. Here, we investigate mafic microgranular enclaves (MMEs) and their host granite porphyry from the Furongshan caldera to elucidate magma mixing and crust-mantle interaction in the Gan-Hang Tectonic Belt. The Furongshan granite porphyry is characterized by enrichments of alkalis, REE, Zr + Nb + Ce + Y contents (averaging 377 ppm), and high zircon saturation temperatures (793-843 °C), suggesting A-type granitic affinities. The granite porphyry can be further classified as an A2 subtype granite based on high Y/Nb ratios (averaging 1.37). Zircon cores from the Furongshan MMEs exhibit the same εHf(t) values (-10.0 to -3.0) and U-Pb ages (127-129 Ma) as zircons form the granite porphyry, implying that they were captured from the felsic magma as xenocrysts. Petrological and mineralogical characteristics (such as needle-like apatite and disequilibrium feldspar xenocryst) suggest that the Furongshan MMEs and host granite porphyry were formed by magma mixing rather than restite, xenolith or fractional crystallization of mafic magma. The Furongshan granite porphyry samples have initial 87Sr/86Sr ratios of 0.7073-0.7099 and εNd(t) values of -3.7 to -3.3, which are similar to those of the MMEs (0.7068-0.7077 and -3.2 to -2.9, respectively). Similar trace element and Sr-Nd isotopic compositions imply a high degree of geochemical equilibration between the granite porphyry and its MMEs, and hence intense magma mixing, although some element contents and zircons εHf(t) values differ due to high zircon closure temperature and rapid cooling of commingled magmas. A binary mixing model based on Sr-Nd isotopes indicates a contribution of ∼50% basaltic melt to the hybrid magma of the Furongshan granite porphyry. A compilation of Sr-Nd-Hf isotopic data of the granitoids and MMEs from the Xiangshan, Furongshan and Muchen areas suggest

  6. 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

  7. 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

  8. Pyroxenite and granulite xenoliths from beneath the Scottish Northern Highlands Terrane: evidence for lower-crust/upper-mantle relationships

    NASA Astrophysics Data System (ADS)

    Upton, B. G. J.; Aspen, P.; Hinton, R. W.

    2001-08-01

    Xenolith suites from Permian host rocks in Orkney and the extreme NE of the Scottish mainland (Duncansby Ness) are described and compared to those from elsewhere in the Northern Highlands Terrane. Those from the Tingwall dyke, Orkney, comprise roughly equal proportions of ultramafic rocks (wehrlites, clinopyroxenites, websterites, hornblendites) and mafic to felsic rocks (gabbroic, noritic and dioritic granulites, with subordinate tonalites and trondhjemites). Those from Duncansby (45 km to the south) are dominantly olivine-poor ultramafic rocks (clinopyroxenites, pargasite pyroxenites, biotite-pyroxenites), together with granulites grading from gabbroic through to tonalites and trondhjemites. Most of the granulites are meta-igneous, comprising plagioclase and one- or two-pyroxene species with equilibration temperatures of 810-710 °C, and are regarded as samples of the lower crust. Absence of garnet and olivine, together with the association of relatively sodic plagioclase and aluminous pyroxenes, is consistent with derivation from depths corresponding to 5-10 kbar. Positive Eu anomalies in the granulites imply that most originated as plagioclase-rich cumulates from basaltic magmas. Scarce peraluminous quartzo-feldspathic xenoliths, such as a garnet-sillimanite-bearing sample from Duncansby, are regarded as metasedimentary in origin. Pyroxenes (and biotites) in the ultramafic xenoliths tend to have higher mg numbers than those of the granulites, reflecting higher temperatures of formation. Whereas the pyroxene-rich ultramafic rocks may be partly interleaved with the granulites in the lower crust, it is concluded that they also constitute a zone of substantial thickness at or around Moho level, separating the granulites from underlying peridotites, and that they originated as cumulates cognate to the granulites. They have, however, been variably metasomatised with formation of amphibole. This zone may constitute a density trap at which melt fractions, rich in K, Fe

  9. 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.

  10. The metamorphic sole of New Caledonia ophiolite: 40Ar/39Ar, U-Pb, and geochemical evidence for subduction inception at a spreading ridge

    NASA Astrophysics Data System (ADS)

    Cluzel, Dominique; Jourdan, Fred; Meffre, SéBastien; Maurizot, Pierre; Lesimple, StéPhane

    2012-06-01

    Amphibolite lenses that locally crop out below the serpentinite sole at the base of the ophiolite of New Caledonia (termed Peridotite Nappe) recrystallized in the high-temperature amphibolite facies and thus sharply contrast with blueschists and eclogites of the Eocene metamorphic complex. Amphibolites mostly display the geochemical features of MORB with a slight Nb depletion and thus are similar to the youngest (Late Paleocene-Eocene) BABB components of the allochthonous Poya Terrane. Thermochronological data from hornblende (40Ar/39Ar), zircon, and sphene (U-Pb) suggest that these mafic rocks recrystallized at ˜56 Ma. Using various geothermobarometers provides a rough estimate of peak recrystallization conditions of ˜0.5 GPa at ˜800-950°C. The thermal gradient inferred from the metamorphic assemblage (˜60°C km-1), geometrical relationships, and geochemical similarity suggest that these mafic rocks belong to the oceanic crust of the lower plate of the subduction/obduction system and recrystallized when they subducted below young and hot oceanic lithosphere. They were detached from the down-going plate and finally thrust onto unmetamorphosed Poya Terrane basalts. This and the occurrence of slab melts at ˜53 Ma suggest that subduction inception occurred at or near to the spreading ridge of the South Loyalty Basin at ˜56 Ma.

  11. 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

  12. Three-dimensional electrical structure of the crust and upper mantle in Ordos Block and adjacent area: Evidence of regional lithospheric modification

    NASA Astrophysics Data System (ADS)

    Dong, Hao; Wei, Wenbo; Ye, Gaofeng; Jin, Sheng; Jones, Alan G.; Jing, Jianen; Zhang, Letian; Xie, Chengliang; Zhang, Fan; Wang, Hui

    2014-06-01

    magnetotelluric (MT) data from project SINOPROBE were acquired and modeled, using three-dimensional (3D) MT inversion, to study the electrical structure of Ordos Block, a component of the North China Craton. For the first time, a high-resolution 3D resistivity model of the lithosphere is defined for the region. Contrary to what would be expected for a stable cratonic block, a prominent lithospheric conductive complex is revealed extending from the upper mantle to the mid-to-lower crust beneath the northern part of Ordos. Correlating well with results of seismic studies, the evidence from our independent magnetotelluric data supports regional modification of the lithosphere under the north Ordos and lithosphere thinning beneath Hetao Graben. The abnormally conductive structure may result from upwelling of mantle material in mid-to-late Mesozoic beneath the northern margin of the Ordos block.

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

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

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

  14. 207Pb-206Pb zircon ages of eastern and western Dharwar craton, southern India : Evidence for contemporaneous Archaean crust

    NASA Astrophysics Data System (ADS)

    Maibam, B.; Goswami, J. N.; Srinivasan, R.

    2009-04-01

    Dharwar craton is one of the major Archaean crustal blocks in the Indian subcontinent. The craton is comprised of two blocks, western and eastern. The western domain is underlain by orthogneisses and granodiorites (ca. 2.9-3.3 Ga) collectively termed as Peninsular Gneiss [e.g., 1] interspersed with older tracts of metasedimentary and metamorphosed igneous suites (Sargur Group and Dharwar Group; [2]). The eastern part of the craton is dominated by Late Archaean (2.50-2.75 Ga) granitoids and their gneissic equivalents. They are interspersed with schist belts (also of Sargur Group and Dharwar Group), which are lithologically similar to the Dharwar Supergroup in the western block, but are in different metamorphic dress. Here we report 207Pb-206Pb age of zircons separated from the metasedimentary and gneissic samples from the two blocks to constrain the evolution of the Dharwar craton during the early Archaean. Detrital zircons of the metasedimentary rocks from both the blocks show a wide range of overlapping ages between ~2.9 to >3.5 Ga. Zircon ages of the orthogneisses from the two blocks showed that most of the analysed grains of the eastern Dharwar block are found to be of the age as old as the western Dharwar gneisses. Imprints of younger events could be discerned from the presence of overgrowths in zircons from the studied samples throughout the craton. Our data suggest that crust forming cycles in the two blocks of the Dharwar craton occurred contemporaneously during the Archaean. References [1] Beckinsale, R.D., Drury, S.A., Holt, R.W. (1980) Nature 283, 469-470. [2] Swami Nath J., Ramakrishnan M., Viswanatha M.N. (1976) Rec. Geol. Surv. Ind., 107, 149-175.

  15. 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.

  16. 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

  17. 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

  18. 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.

  19. Geochemical and Isotopic Evidence for Melting and Erosion of Wyoming Craton Mantle Lithosphere Prior to 48 Ma

    NASA Astrophysics Data System (ADS)

    Duke, G. I.; Carlson, R. W.; Frost, C. D.

    2010-12-01

    Trace-element geochemistry of Cretaceous-Tertiary Great Plains igneous rocks supports isotopic data that reveal a sequence of digestion of lithospheric mantle followed by intrusion of dominantly asthenospheric magmas. Multiple Archean, Proterozoic, and Phanerozoic subduction events beneath the Wyoming craton concentrated Ba and K within the underlying mantle lithosphere, resulting in earliest Cretaceous-Tertiary lithospheric melts with fingerprints of high K, high Ba/Nb and negative epsilon-Nd, but low U, Th, total REE, and less extreme values of LREE/HREE. Youngest (Eocene-Oligocene) magmas were kimberlite and carbonatite, with high U, Th, LREE, extremely high LREE/HREE, and positive epsilon-Nd, but with high-T xenoliths from depths of only 150 km (Carlson et al., 1999). Importantly, in the entire Wyoming craton, the Homestead kimberlite is the only one of K-T age that has transported a diamond—a single micro-diamond discovered. The shallow low-T to high-T xenolith transition, lack of diamonds, and changing magma geochemistry, suggest that a significant portion of the mantle lithosphere beneath the Wyoming Archean craton must have been consumed prior to the ≤48 Ma kimberlite eruptions. In contrast, the earliest phase of Cretaceous magmatism in Arkansas was explosive diamond-containing lamproite (~102 Ma) with a Proterozoic lithospheric isotopic signature (Lambert et al., 1995). In Arkansas, there was no earlier subalkalic magmatism, and no evidence of slow digestion of the mantle lithosphere, although later magmatism trended toward higher positive epsilon-Nd values (i.e. larger asthenospheric component). Removal by melting of a significant portion of the Wyoming mantle lithosphere during late Cretaceous-early Tertiary magmatism, along with heating, may have helped promote lithospheric “relaxation” related to extension further west between 53 Ma and 49 Ma, followed by more facile penetration by asthenospheric magmas, an idea proposed to explain the time

  20. 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.

  1. 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

  2. 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

  3. 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

  4. 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

  5. Multiple sources for the origin of granites: Geochemical and Nd/Sr isotopic evidence from the Gudaoling granite and its mafic enclaves, northeast China

    NASA Astrophysics Data System (ADS)

    Yang, Jin-Hui; Wu, Fu-Yuan; Chung, Sun-Lin; Wilde, Simon A.; Chu, Mei-Fei

    2004-11-01

    Geochemical and Sr- and Nd-isotopic data have been determined for mafic to intermediate microgranular enclaves and host granitoids from the Early Cretaceous Gudaoling batholith in the Liaodong Peninsula, NE China. The rocks include monzogranite, porphyric granodiorite and quartz diorite. Monzogranites have relatively high 87Rb/ 86Sr ratios (0.672-0.853), low initial 87Sr/ 86Sr ratios (0.7052-0.7086) and ɛ Nd(t) values (-18.5 to -20.9) indicating that they were mainly derived from a newly underplated crustal source with a short crustal residence time. Quartz diorites have high initial 87Sr/ 86Sr ratios (0.7118-0.7120) and negative ɛ Nd(t) values (-13.2 to -18.1) coupled with high Al 2O 3 and MgO contents, indicating they were derived from enriched lithospheric mantle with contributions of radiogenic Sr from plagioclase-rich metagreywackes or meta-igneous rocks, i.e., ancient lower crust. Two groups of enclaves with igneous textures and abundant acicular apatites are distinguished: dioritic enclaves and biotite monzonitic enclaves. Dioritic enclaves have low Al 2O 3 (13.5-16.4 wt%) and high MgO (Mg# = ˜72.3) concentrations, low initial 87Sr/ 86Sr ratios (0.7058-0.7073) and negative ɛ Nd(t) values (˜-7.2), and are enriched in LILEs and LREEs and depleted in HFSEs, suggesting they were derived from an enriched lithospheric mantle source. Biotite monzonitic enclaves have Sr and Nd isotopic compositions similar to the monzogranites, indicating they were crystal cumulates of the parental magmas of these monzogranites. Granodiorites have transitional geochemistry and Nd- and Sr-isotopic compositions, intermediate between the monzogranites, quartz diorites and the enclaves. Geochemical and Sr- and Nd-isotopic compositions rule-out simple crystal-liquid fractionation or restite unmixing as the major genetic link between enclaves and host rocks. Instead, magma mixing of mafic mantle-derived and juvenile crustal-derived magmas, coupled with crystal fractionation and

  6. 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.

  7. 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)

  8. Permeability within basaltic oceanic crust

    NASA Astrophysics Data System (ADS)

    Fisher, Andrew T.

    1998-05-01

    Water-rock interactions within the seafloor are responsible for significant energy and solute fluxes between basaltic oceanic crust and the overlying ocean. Permeability is the primary hydrologic property controlling the form, intensity, and duration of seafloor fluid circulation, but after several decades of characterizing shallow oceanic basement, we are still learning how permeability is created and distributed and how it changes as the crust ages. Core-scale measurements of basaltic oceanic crust yield permeabilities that are quite low (generally 10-22 to 10-17 m²), while in situ measurements in boreholes suggest an overlapping range of values extending several orders of magnitude higher (10-18 to 10-13 m²). Additional indirect estimates include calculations made from borehole temperature and flow meter logs (10-16 to 10-11 m²), numerical models of coupled heat and fluid flow at the ridge crest and within ridge flanks (10-16 to 10-9 m²), and several other methods. Qualitative indications of permeability within the basaltic oceanic crust come from an improved understanding of crustal stratigraphy and patterns of alteration and tectonic modification seen in ophiolites, seafloor samples and boreholes. Difficulties in reconciling the wide range of estimated permeabilities arise from differences in experimental scale and critical assumptions regarding the nature and distribution of fluid flow. Many observations and experimental and modeling results are consistent with permeability varying with depth into basement and with primary basement lithology. Permeability also seems to be highly heterogeneous and anisotropic throughout much of the basaltic crust, as within crystalline rocks in general. A series of focused experiments is required to resolve permeability in shallow oceanic basement and to directly couple upper crustal hydrogeology to magmatic, tectonic, and geochemical crustal evolution.

  9. Across and along arc geochemical variations in altered volcanic rocks: Evidence from mineral chemistry of Jurassic lavas in northern Chile, and tectonic implications

    NASA Astrophysics Data System (ADS)

    Rossel, Pablo; Oliveros, Verónica; Ducea, Mihai N.; Hernandez, Laura

    2015-12-01

    Postmagmatic processes mask the original whole-rock chemistry of most Mesozoic igneous rocks from the Andean arc and back-arc units preserved in Chile. Mineral assemblages corresponding to subgreenschist metamorphic facies and/or propylitic hydrothermal alteration are ubiquitous in volcanic and plutonic rocks, suggesting element mobility at macroscopic and microscopic scale. However, fresh primary phenocrysts of clinopyroxene and plagioclase do occur in some of the altered rocks. We use major and trace element chemistry of such mineral phases to infer the geochemical variations of four Jurassic arc and four back-arc units from northern Chile. Clinopyroxene belonging to rocks of the main arc and two units of the bark-arc are augites with low contents of HFSE and REE; they originated from melting of an asthenospheric mantle source. Clinopyroxenes from a third back-arc unit show typical OIB affinities, with high Ti and trace element contents and low Si. Trace elemental variations in clinopyroxenes from these arc and back-arc units suggest that olivine and clinopyroxene were the main fractionating phases during early stages of magma evolution. The last back-arc unit shows a broad spectrum of clinopyroxene compositions that includes depleted arc-like augite, high Al and high Sr-Ca diopside (adakite-like signature). The origin of these lavas is the result of melting of a mixture of depleted mantle plus Sr-rich sediments and subsequent high pressure fractionation of garnet. Thermobarometric calculations suggest that the Jurassic arc and back-arc magmatism had at least one crustal stagnation level where crystallization and fractionation took place, located at ca. ~ 8-15 km. The depth of this stagnation level is consistent with lower-middle crust boundary in extensional settings. Crystallization conditions calculated for high Al diopsides suggest a deeper stagnation level that is not consistent with a thinned back-arc continental crust. Thus minor garnet fractionation

  10. Geochemical evidence for origin of magma mixing for the Triassic monzonitic granite and its enclaves at Mishuling in the Qinling orogen (central China)

    NASA Astrophysics Data System (ADS)

    Qin, Jiangfeng; Lai, Shaocong; Grapes, Rodney; Diwu, Chunrong; Ju, Yinjuan; Li, Yongfei

    2009-10-01

    Whole-rock major-trace elements and Sr-Nd isotopes, mineral chemistry, and zircon U-Pb and Lu-Hf isotope compositions are reported for I-type monzogranite and its mafic microgranular enclaves from the Triassic Mishuling granite in the Qinling orogen, central China. Zircon U-Pb dating yields an age of 212 ± 5 Ma for the mafic enclaves and 213 ± 3 Ma for the host monzogranite. Whole-rock major-trace elements, Sr-Nd isotopes and zircon Hf isotopes preclude simple crystal fractionation or restite unmixing for a genetic link between the enclaves and the monzogranite. Mixing between mafic and felsic magmas is compatible with the geochemical data. The monzogranite displays an enriched LILE and LREE pattern, high ( 87Sr/ 86Sr) i ratios of 0.7068 to 0.7071 and low ɛNd( t) values of - 9.2 to - 5.7, suggesting a crustal origin. Its high Mg# (47.6 to 50.7), Cr and Ni contents clearly indicate its derivation from mafic refractory lower crust. Most of the zircons in the monzogranite have negative ɛHf( t) values of - 12.5 to - 0.1 and two-stage Hf model ages of 1049 to 1683 Ma, and some zircons in the host monzogranite have positive ɛHf( t) values of 0.3 to 4.8 and single-stage Hf model ages of 665 to 845 Ma. This indicates that the monzogranite is most likely to be formed by partial melting of a Proterozoic lower continental crust. The mafic enclaves have fine-grained igneous textures and contain abundant acicular apatite that indicates rapid cooling. They also have a high ( 87Sr/ 86Sr) i ratio of 0.7071 and a low ɛNd( t) value of - 6.5, suggesting a derivation from ancient subcontinental lithosphere mantle (SCLM). Zircons in the mafic microgranular enclaves have ɛHf( t) values of - 1.6 to 5.1 and single-stage Hf model ages of 700 to 910 Ma, suggesting a possible origin from partial melting of the Neoproterozoic SCLM that may form in association with contemporaneously supercontinental rifting in the South China Block. Therefore, the Mishuling I-type granite may form by

  11. 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

  12. 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.

  13. 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.

  14. 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.

  15. 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

  16. Generation of felsic crust in the Archean: a geodynamic modeling perspective

    NASA Astrophysics Data System (ADS)

    Sizova, Elena; Gerya, Taras; Stüwe, Kurt; Brown, Michael

    2015-04-01

    The relevance of contemporary tectonics to the formation of the Archean terrains is a matter of vigorous debate. Higher mantle temperatures and higher radiogenic heat production in the past would have impacted on the thickness and composition of the oceanic and continental crust. As a consequence of secular cooling, there is generally no modern analog to assist in understanding the tectonic style that may have operated in the Archean. For this reason, well-constrained numerical modeling, based on the fragmentary evidence preserved in the geological record, is the most appropriate tool to evaluate hypotheses of Archean crust formation. The main lithology of Archean terrains is the sodic tonalite-trondhjemite-granodiorite (TTG) suite. Melting of hydrated basalt at garnet-amphibolite to eclogite facies conditions is considered to be the dominant process for the generation of the Archean TTG crust. Taking into account geochemical signatures of possible mantle contributions to some TTGs, models proposed for the formation of Archean crust include subduction, melting at the bottom of thickened continental crust and fractional crystallization of mantle-derived melts under water-saturated conditions. We evaluated these hypotheses using a 2D coupled petrological-thermomechanical numerical model with initial conditions appropriate to the Eoarchean-Mesoarchean. As a result, we identified three tectonic settings in which intermediate to felsic melts are generated by melting of hydrated primitive basaltic crust: 1) delamination and dripping of the lower primitive basaltic crust into the mantle; 2) local thickening of the primitive basaltic crust; and, 3) small-scale crustal overturns. In addition, we consider remelting of the fractionated products derived from underplated dry basalts as an alternative mechanism for the formation of some Archean granitoids. In the context of a stagnant lid tectonic regime which is intermittently terminated by short-lived subduction, we identified

  17. 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.

  18. 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

  19. Loss of primary texture and geochemical signatures in speleothems due to diagenesis: Evidences from Castañar Cave, Spain

    NASA Astrophysics Data System (ADS)

    Martín-García, Rebeca; Alonso-Zarza, Ana M.; Martín-Pérez, Andrea

    2009-11-01

    Geochemical signals from speleothems are commonly used in the investigation of palaeoenvironments. In most cases, however, little attention is paid to whether or not these signals are primary or altered by diagenesis. The speleothems of the Castañar Cave (Cáceres, Spain), which are initially formed of calcite or aragonite, have undergone a variety of meteoric diagenetic processes such as micritization and neomorphism (inversion), that collectively modify their primary features (textures, mineralogy, geochemical signals). The mean δ13C and δ18O values of the aragonites in the cave are -8.66 and -4.64 respectively, whereas the primary calcites have mean δ13C and δ18O values of -9.99 and -5.77, respectively. Following the diagenetic process of micritization, the aragonite isotopic signals averaged -7.63 δ13C and -4.74 δ18O and the calcite micrite signals -9.53 δ13C and -5.21 δ18O. Where inversion took place, some secondary calcites after the aragonite show preserved aragonite, whereas others do not. The secondary calcites without aragonite relics show isotopic values slightly higher than those of the primary calcite due to the inheritance of the aragonite signal. Where aragonite relics are preserved, the isotopic signatures are very similar to those of the aragonite micrite. In addition, the stable isotopic values and Sr and Mg contents of the speleothems became also modified by micritization and/or inversion. These diagenetic processes were driven by the changes in composition of the cave waters over time and space, but also, in the case of aragonite, by its initial unstable mineralogy. The present results highlight how important diagenesis is in caves and how the initial features of cave minerals may be lost. These changes alter the geochemical signals shown by speleothems, which may have an impact on the interpretation of the results obtained in palaeoenvironmental studies.

  20. 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.

  1. Geochemical heterogeneities and dynamics of magmas inside the plumbing system of a persistently active volcano: evidences from Stromboli

    NASA Astrophysics Data System (ADS)

    Pompilio, Massimo; Bertagnini, Antonella; Métrich, Nicole; Belhadj, Oulfa

    2010-05-01

    Shallow processes such as degassing, crystallization and magma drain-back commonly operate in the upper parts of the plumbing systems of open-conduit basaltic volcanoes, often hindering the identification of potentially important geochemical changes in the volcano systems. Stromboli, known for its long-lived persistent activity over the last 18 centuries, is a suitable subject of study for addressing this issue, since basaltic magmas presently erupting at in this volcano record both deep and shallow processes. We report petrological and geochemical data on magmas erupted by Stromboli since the beginning of the persistent activity, in order to find a correlation between magma composition and the dynamics of magma in the plumbing system. Geochemical data on deep-derived magmas erupted as pumice during paroxysmal eruptions allowed us to identify two distinct parental melts (1944- and 2003-Type). These magmas, in which geochemical differences are linked to source processes rather than crystal fractionation, have alternately fed the deep reservoir in the last two millennia several times. The chemical heterogeneities recorded in lava flows and the products of Strombolian activity testify to the extent of homogenization after magma recharges at shallow depths. Persistent heterogeneities in the shallow plumbing system have important implications for magma residence times calculated on the basis of time-series analysis. These models are based on the assumptions that the reservoir is well stirred and chemically homogeneous and that the time for the re-homogenization after recharge (or mixing) is shorter than the residence time. We argue that these models do not apply to present-day activity at Stromboli and may not apply to other open-conduit, persistently degassing basaltic volcanoes. Thus compositional variations within the shallow magma bodies provide only a biased signal of ongoing changes within the plumbing system. We conclude that source changes responsible for

  2. 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

  3. 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

  4. 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

  5. Crust-mantle interaction in the central North China Craton during the Mesozoic: Evidence from zircon U-Pb chronology, Hf isotope and geochemistry of syenitic-monzonitic intrusions from Shanxi province

    NASA Astrophysics Data System (ADS)

    Ying, Ji-Feng; Zhang, Hong-Fu; Tang, Yan-Jie

    2011-07-01

    In-situ zircon U-Pb ages, Hf isotopic compositions and whole rock geochemical and Sr-Nd isotopic compositions are presented for three Mesozoic syenitic-monzonitic intrusions from Shanxi province, central North China Craton. Zircons from these intrusions all show core-rim structures in that the oscillatory rims recorded their intrusive ages, whereas the cores are interpreted as xenocrysts. The U-Pb age data reveal that the northernmost Dishuiyan monzonite was emplaced at 241 Ma, while the Huyanshan and Erfengshan syenitic-monzonitic complexes were emplaced at 130 Ma and 128-134 Ma, respectively. The Dishuiyan monzonite is petrologically and geochemically uniform, it shows LREE enrichment and HFSE depletion and exhibits enriched Sr and Nd isotopic compositions with (87Sr/86Sr)i = 0.7065 and εNd(t) = - 8.3--7.9. The zircon rims in the Dishuiyan monzonite show highly varied Hf isotopic compositions with εHf(t) = - 27.8--6.4. The varied Hf isotopic compositions and enriched Sr-Nd isotopic compositions, together with the ubiquitous xenocrystic zircon cores, suggest the Dishuiyan monzonite was produced by the mixing of melts from enriched lithospheric mantle and lower crust. The monzonite and syenite from the Huyanshan complexes exhibit different geochemical features. The εNd(t) values of syenite, which are higher than those of monzonite resemble the enriched lithospheric mantle, and together with the absence of zircon in the syenite, we propose that it was originated by partial melting of enriched lithospheric mantle. Monzonites from Huyanshan and Erfengshan share similar petrological and geochemical characteristics, being enriched in LREE and depleted in HFSE, and they show low εNd(t) values between - 18.2 and - 13.9. Similar to zircon rims in the Dishuiyan monzonite, those in the Huyanshan and Erfengshan monzonites also exhibit highly varied Hf isotopic compositions. The consistent ages between the xenocrystic zircons in monzonites and the lower crustal basement

  6. Geochemical and geomorphological evidence for the provenance of aeolian deposits in the Badain Jaran Desert, northwestern China

    NASA Astrophysics Data System (ADS)

    Hu, Fangen; Yang, Xiaoping

    2016-01-01

    Identifying provenance of aeolian deposits in the mid-latitude deserts of Asia is essential for understanding formation and changes of Earth surface processes due to palaeoclimatic fluctuations. While some earlier studies focused on the interpretation of palaeoenvironments on the basis of aeolian deposits mainly in the desert margins and inter-dune lacustrine sediments, research on provenance of desert sands in the vast Asian mid-latitude deserts is still rare. In this paper, we present new geochemical data which provide insight to the provenance of dune sands in the Badain Jaran Desert, northwestern China, an important part of this desert belt. We sampled aeolian and lacustrine sediments in various parts of the Badain Jaran Desert, and examined their major, trace and rare earth elements (REE) in bulk samples, coarse and fine fractions, respectively. In addition, we took and analyzed samples from a rarely known dune field with red sands, northeast of the Badain Jaran. Our results show that the sands from the Badain Jaran Desert are generally different from those in the red sand dune field in terms of REE pattern and geochemical characteristics, suggesting different sediment origins. Geochemical composition of the aeolian sand samples indicates these sediments should be mainly derived from mixed source rocks of granite, granitoids and granodiorite. Comparing the immobile trace elements and REE ratios of the samples from the Badain Jaran Desert, red sand dune field with rocks of granite, granitoids in their potential source areas, we conclude that: (1) The aeolian deposits in the Badain Jaran Desert are predominantly derived from the Qilian Mountains, northeastern Tibetan Plateau initially via fluvial processes; (2) The Altay Mountains and Mongolian Gobi are the ultimate source areas for the red sand dune field; (3) The Altai Mountains and Mongolian Gobi in the northwest, that could produce massive amounts of materials via intensive deflation and alluvial process

  7. 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.

  8. Geochemical and mineralogical characteristics of fault gouge in the Median Tectonic Line, Japan: evidence for earthquake slip

    NASA Astrophysics Data System (ADS)

    Ishikawa, Tsuyoshi; Hirono, Tetsuro; Matsuta, Noriko; Kawamoto, Kazuro; Fujimoto, Koichiro; Kameda, Jun; Nishio, Yoshiro; Maekawa, Yuka; Honda, Go

    2014-12-01

    We carried out geochemical and mineralogical analyses on fault-zone rocks from the Anko section of the Median Tectonic Line in Nagano Prefecture, Japan, to investigate coseismic physicochemical processes in the fault zone. The latest fault zone in the Anko section contains cataclasite, fault breccia, and fault gouge of granitic composition, and brecciated basic schist. Protoliths of the granitic composition are from the Ryoke metamorphic belt and those of the basic schist from the Sambagawa metamorphic belt. X-ray diffraction analyses show a selective decrease of clay minerals coupled with an increase of amorphous phase in an intensely deformed layer of black gouge (5- to 10-cm thick). SEM observation reveals that the black gouge is characterized by a drastic reduction of grain size and abundant ultrafine particles of submicrometer to several tens of nanometers with well-rounded spheroidal shapes. These observations for the black gouge are indicative of strong mineral lattice distortion and granulation associated with earthquake slip. Geochemically, the black gouge is characterized by distinctly higher Li content and 87Sr/86Sr isotope ratio than surrounding cataclasites, breccias, and gouges, which have similar major element compositions. Model analysis reveals that the trace element composition of the black gouge is consistent with high-temperature (up to 250°C) coseismic fluid-rock interactions. Thermal and kinetic constraints indicate that there have been repeated slips on the fault at moderate depths (e.g., 600 m), although the tectonic process by which the fault zone has been uplifted and exposed in this area is not well understood.

  9. 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.

  10. 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.

  11. 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.

  12. Miocene to Late Quaternary Patagonian basalts (46 47°S): Geochronometric and geochemical evidence for slab tearing due to active spreading ridge subduction

    NASA Astrophysics Data System (ADS)

    Guivel, Christèle; Morata, Diego; Pelleter, Ewan; Espinoza, Felipe; Maury, René C.; Lagabrielle, Yves; Polvé, Mireille; Bellon, Hervé; Cotten, Joseph; Benoit, Mathieu; Suárez, Manuel; de la Cruz, Rita

    2006-01-01

    + 5.4). These features are consistent with their derivation from an enriched mantle source contaminated by ca. 10% rutile-bearing restite of altered oceanic crust. The petrogenesis of the studied Mio-Pliocene basalts from MLBA and MCC is consistent with contributions of the subslab asthenosphere, the South American subcontinental lithospheric mantle and the subducted Pacific oceanic crust to their sources. However, their chronology of emplacement is not consistent with an ascent through an asthenospheric window opened as a consequence of the subduction of segment SCR-1, which entered the trench at 6 Ma. Indeed, magmatic activity was already important between 12 and 8 Ma in MLBA and MCC as well as in southernmost plateaus, i.e., 6 Ma before the subduction of the SCR-1 segment. We propose a geodynamic model in which OIB and intermediate magmas derived from deep subslab asthenospheric mantle did uprise through a tear-in-the-slab, which formed when the southernmost segments of the SCR collided with the Chile Trench around 15 Ma. During their ascent, they interacted with the Patagonian supraslab mantle and, locally, with slivers of subducted Pacific oceanic crust that contributed to the geochemical signature of the intermediate basalts.

  13. Geochemical evidence for fluid flow in the upper and subducting plates of the Costa Rica margin: Results from CRISP drilling during Exp. 334 and 344 (Invited)

    NASA Astrophysics Data System (ADS)

    Torres, M. E.; Solomon, E. A.; Kastner, M.; Harris, R. N.; Formolo, M.; Choi, J.; Berg, R. D.; Nuzzo, M.

    2013-12-01

    lithologic Units I and II that defines the interface between the slope sediment cover and deeper upper plate material. The Cl, Li, and hydrocarbon concentration data indicate that this fluid originated from a source temperature greater than 90°C. Within the well-cemented sediments below the slope cover, there is a more pervasive non-foccussed transport of a fluid, fresher than seawater, having a strong signature of ash/basalt interaction (high Ca, low Mg), and a marked increase in C1/C2+ ratio, indicating either a more biogenic signature of the gases or/and an extensive migration effect on the gas composition. Geochemical data at two sites drilled on the incoming plate, reveal fluid flow within the permeable upper oceanic crust. At Site U1381, the sulfate concentration profile shows a reversal below ~40 mbsf, with a steady increase in concentration with depth. This observation is similar to that previously reported at the nearby incoming sediment section offshore the Nicoya Peninsula, and reflect diffusional communication with a fluid with seawater-like chemistry in the igneous basement. The sulfate concentration profile at Site U1414 is unusual in that it displays a second minimum at 330 mbsf, which corresponds to a sharp minimum in calcium and a maximum in barium concentrations. These data suggest lateral flow of fluid originating landward of Site U1414 where microbial oxidation of methane and/or other organic carbon sources has depleted dissolved sulfate.

  14. 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

  15. Seismic evidence of bending and unbending of subducting oceanic crust and the presence of mantle megathrust in the 2004 Great Sumatra earthquake rupture zone

    NASA Astrophysics Data System (ADS)

    Singh, Satish C.; Chauhan, Ajay P. S.; Calvert, Andrew J.; Hananto, Nugroho D.; Ghosal, Dibakar; Rai, Abhishek; Carton, Helene

    2012-03-01

    In subduction zones the plate interface (megathrust) is typically poorly imaged at depths > 12 km, however its precise geometry and nature as well as the positions of updip and downdip limits of the seismogenic zone are important elements to understand the generation of megathrust earthquakes. Using deep marine seismic reflection and refraction data, we observed discontinuous reflections off the top of the subducting oceanic crust down to 60 km depth in the 2004 great Sumatra-Andaman earthquake rupture zone. We find that the top of the downgoing plate does not dip gently into the subduction zone but instead displays a staircase geometry with three successive, 5-15 km vertical steps, spaced ~ 50 km apart. Micro-earthquake data indicate that most of the seismicity lies below this interface, suggesting that the oceanic plate is deforming actively. Along part of the profile, we also image a second reflector located 8-10 km below the top of the oceanic crust. The forward modelling of the gravity data along the profile supports the presence of a high-density material above this reflector. The presence of a staircase shape for the top of the oceanic crust, together with constraints from gravity data and earthquake data, require that the megathrust goes through this second reflector. This leads us to conclude that the megathrust is at least partly located in the oceanic mantle and that underplating of oceanic crust beneath the wedge and underplating of upper mantle beneath the forearc basin are taking place in this region.

  16. 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. PMID:14534583

  17. 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.

  18. Ultrahigh-pressure metamorphism: tracing continental crust into the mantle

    NASA Astrophysics Data System (ADS)

    Chopin, Christian

    2003-07-01

    velocities (1-2 cm/yr), especially during early stages of exhumation, and bear no relation to normal erosion rates. Important observations are that: (i) as a result of strain partitioning and fluid channelling, significant volumes of subducted crust may remain unreacted (i.e. metastable) even at conditions as high as 700°C and 3 GPa - with implications as to geophysical modeling; (ii) subducted continental crust shows no isotopic or geochemical evidence of interaction with mantle material. An unknown proportion of subducted continental crust must have escaped exhumation and effectively recycled into the mantle, with geochemical implications still to be explored, bearing in mind the above inefficiency of mixing. The repeated occurrence of UHP metamorphism, hence of continental subduction, through time and space since at least the late Proterozoic shows that it must be considered a common process, inherent to continental collision. Evidence of older, Precambrian UHP metamorphism is to be sought in high-pressure granulite-facies terranes.

  19. Evidence for Milankovitch periodicities in Cenomanian-Turonian lithologic and geochemical cycles, Western Interior U.S.A.

    SciTech Connect

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

    1997-03-01

    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.

  20. 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.

  1. 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).

  2. 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.

  3. 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.

  4. 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

  5. Receiver function probing of the crust and the upper mantle underneath north German basin, Scandinavia and the Baltic shield: evidence of ancient collision zones

    NASA Astrophysics Data System (ADS)

    Alinaghi, A.; Kind, R.; Bock, G.

    2002-05-01

    Teleseismic earthquakes (distance: 30-90 degrees)recorded by TOR (passive-source seismic experiment, north Germany and Scandinavia, 1996-1997), GRSN (German Regional Seismic Network, 1992-1999), and SVEKALAPKO (seismic experiment, Finland, 1998-1999) have been used for calculating receiver functions. Source and epicentral distance equalized receiver functions were utilized to probe into the crust and upper mantle beneath the recording stations. Detailed images of the crust-mantle boundary across the studied regions show a pronounced increase of the crustal thickness at the junction between northern Europe Paleozoic and Precambrian provinces known as TESZ( TransEuropean Suture Zone). This is accompanied by some north dipping intracrustal structures which run deep into the lower crust and are believed to be remnants of an ancient subduction zone. Another significance of the TESZ is its effect on the two major upper mantle discontinuities and thereby on the upper mantle transition zone. The deepening of the moho is associated with early arrival of 410 km discontinuity compared to predicted time by IASP91 velocity model, while the 660 km discontinuity has been affected to a lesser degree. The crust, underneath the Baltic shield, is thicker (on average 45-50 km) than beneath the north German Basin( 30-35 km on average ), and the moho shows a complex topography in sharp contrast to rather flat surface topography. Beneath SVEKALAPKO, both in stacked time domain and migrated receiver function sections the 410 km and 660 km upper mantle discontinuities arrive earlier than predicted by IASP91 velocity model. This is an indication of higher shear velocity of upper mantle in the Baltic shield compared to that of the Paleozoic northern Europe. We found no sign of the lithosphere-asthenosphere in our study which might be explained by the masking effect of the moho multiples and/or lack of strong Ps conversions due to gradual lithosphere-asthenosphere boundary

  6. 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

  7. 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).

  8. 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.

  9. 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.

  10. 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.

  11. 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.

  12. 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

  13. 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.

  14. 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.

  15. Evolution of Palaeoproterozoic mafic intrusions located within the thermal aureole of the Sudbury Igneous Complex, Canada: Isotopic, geochronological and geochemical evidence

    NASA Astrophysics Data System (ADS)

    Prevec, Stephen A.; Baadsgaard, Halfdan

    2005-07-01

    Impact cratering and their resultant geological phenomena are recognised as significant factors in the lithological and biologic evolution of the earth. Age-dating of impact events is critical in correlating cause and effects for these catastrophic processes. The Falconbridge and Drury Township (Twp) intrusions were emplaced at the contact between Neoarchaean basement and Palaeoproterozoic volcanosedimentary rocks, and also lie at the southeast and southwest edges of the Sudbury Igneous Complex (SIC), within its thermal contact aureole. The Falconbridge Twp intrusion is dated at 2441 ± 3 Ma by U-Pb zircon, with evidence of Archaean inheritance from its host granitoids. Granitoids from the southernmost Abitibi Subprovince are dated here between 2670 ± 11 Ma for an undeformed Algoman granite, and 2696 ± 18 Ma for a foliated granitoid, consistent with existing data from the Abitibi Greenstone Belt and from the Wawa Subprovince. Major and trace element geochemical evidence, common-Pb isotopic compositions, and ɛNd2440 values between 0 and -1 are all consistent with a Palaeoproterozoic origin for the Falconbridge Twp intrusion, and support inclusion in the East Bull Lake-type suite of leucogabbroic plutons and sills. In contrast, the Drury Twp intrusion gives a U-Pb zircon age of 1859 ± 13 Ma, coincident with the date of SIC-emplacement. While the major and trace element compositions are comparable to the Falconbridge data, the Drury displays significant heterogeneity in ɛNd2440, with values ranging from +3.7 to -0.1, and contains more radiogenic Pb isotopic compositions. Field, geochemical and isotopic evidence clearly distinguishes this intrusion from constituents of the SIC itself, and indicates that the Drury too is a Palaeoproterozoic intrusion. This requires that apparently unshocked, undeformed magmatic-looking zircon has been grown or reset in a postmagmatic setting. This has significant implications for the identification of mantle-derived magmas and

  16. The Crusts of Mars and Earth

    NASA Astrophysics Data System (ADS)

    McLennan, S. M.; Taylor, S. R.; Hahn, B. C.

    2007-05-01

    The differentiation of terrestrial planets and large moons results in crusts with compositions differing greatly from primitive mantles. Typically, large fractions of incompatible elements, including heat-producing elements, are transferred into the crust. Mechanisms and timing of this process differ greatly from planet to planet. Accordingly, in order to understand planetary evolution, it is necessary to understand the composition and evolution of planetary crusts. Crustal evolution on Earth is perhaps the least representative of the terrestrial planets and large moons of the solar system. Although Earth substantially melted after the giant impact that resulted in the Moon, there is little evidence for the existence of a primary crust suggesting that such crust was recycled and mixed into the mantle during the Hadean. Instead, Earth has a very young, continually recycled basaltic secondary (oceanic) crust and an andesitic tertiary (continental) crust, unique in the solar system, that grew episodically over 4 Gyr, but with an average age of about 2 Gyr. The continental - oceanic crust dichotomy, temporal changes in continental crust composition, role of plume volcanism and continental growth are largely consequences of evolving plate- tectonic processes. Mars provides a valuable comparison to Earth because it is a planet that is, in many ways, intermediate between Earth and planetary bodies, such as the Moon and Mercury, that completed crustal development by about 3 Gyr and have been dormant since. Martian crust is mostly ancient (>3.5 Gyr) but volcanism has persisted, possibly episodically, to 200 Myr or younger. Proposals of early plate tectonics persist, but the weight of evidence suggests Mars is a one-plate planet. The 50 km thick crust constitutes 3.2% of the mass of the planet and, even with modest levels of LILE enrichment (K=0.33%), has had well in excess of 50% of incompatible elements removed from the mantle during early differentiation that likely

  17. 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)

  18. 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

  19. 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

  20. 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

  1. 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

  2. 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.

  3. The tectonic emplacement of Sumba in the Sunda-Banda Arc: paleomagnetic and geochemical evidence from the early Miocene Jawila volcanics

    NASA Astrophysics Data System (ADS)

    Wensink, Hans; van Bergen, Manfred J.

    1995-11-01

    The island of Sumba is a continental fragment in the fore-arc region near the transition between the Sunda Arc and Banda Arc in southeastern Indonesia. Paleomagnetic and geochemical evidence from the early Miocene volcanics of the Jawila Formation in western Sumba constrain the final drift stage and tectonic emplacement of the island. The lavas range from predominantly andesites to dacites, and display textural evidence for a weak metamorphism. Rock magnetic and mineral chemical data point to pseudo-single- to multi-domain (titano)magnetite (Fe 2.5-3Ti 0.5-0O 3), with grain sizes up to 10 μm, as the main carrier of remanence. The Jawila Formation reveals a ChRM direction with declination = 4.6°, inclination = - 19.2°, α95 = 9.9° and a paleolatitude of 9.9°S, which corroborates earlier results (Chamalaun and Sunata, 1982). Taking paleomagnetic evidence from other formations on the island into account, we conclude that the Sumba fragment has occupied approximately its present position since the Miocene. The calc-alkaline affinity and trace-element signatures of the lavas point to an origin in an arc environment. This occurrence of subduction-related volcanic activity in the early Miocene on Sumba implies that a volcanic arc existed south of the present-day East Sunda Arc, or that the island was located within the latter arc between Sumbawa and eastern Flores, and still had a minor southward drift to cover.

  4. Oceanic crust in the mid-mantle beneath Central-West Pacific subduction zones: Evidence from S-to-P converted waveforms

    NASA Astrophysics Data System (ADS)

    He, X.

    2015-12-01

    The fate of subducted slabs is enigmatic, yet intriguing. We analyze seismic arrivals at ~20-50 s after the direct P wave in an array in northeast China (NECESSArray) recordings of four deep earthquakes occurring beneath the west-central Pacific subduction zones (from the eastern Indonesia to Tonga region). We employ the array analyzing techniques of 4th root vespagram and beam-form analysis to constrain the slowness and back azimuth of later arrivals. Our analyses reveal that these arrivals have a slightly lower slowness value than the direct P wave and the back azimuth deviates slightly from the great-circle direction. Along with calculation of one-dimensional synthetic seismograms, we conclude that the later arrival is corresponding to an energy of S-to-P converted at a scatterer below the sources. Total five scatterers are detected at depths varying from ~700 to 1110 km in the study region. The past subducted oceanic crust most likely accounts for the seismic scatterers trapped in the mid-mantle beneath the west-central subduction zones. Our observation in turn reflects that oceanic crust at least partly separated from subducted oceanic lithosphere and may be trapped substantially in the mid-mantle surrounding subduction zones, in particular in the western Pacific subduction zones.

  5. Oceanic crust in the mid-mantle beneath west-central Pacific subduction zones: evidence from S to P converted waveforms

    NASA Astrophysics Data System (ADS)

    Yang, Zhongtao; He, Xiaobo

    2015-10-01

    The fate of subducted slabs is enigmatic, yet intriguing. We analyse seismic arrivals at ˜20-50 s after the direct P wave in an array in northeast China (NECESSArray) recordings of four deep earthquakes occurring beneath the west-central Pacific subduction zones (from the eastern Indonesia to Tonga region). We employ the array analysing techniques of fourth root vespagram and beam-forming analysis to constrain the slowness and backazimuth of later arrivals. Our analyses reveal that these arrivals have a slightly lower slowness value than the direct P wave and the backazimuth deviates slightly from the great circle direction. Along with calculation of 1-D synthetic seismograms, we conclude that the later arrival is corresponding to an energy of S-to-P converted at a scatterer below the sources. Total five scatterers are detected at depths varying from ˜700 to 1110 km in the study region. The past subducted oceanic crust most likely accounts for the seismic scatterers trapped in the mid-mantle beneath the west-central subduction zones. Our observation in turn reflects that oceanic crust at least partly separated from subducted oceanic lithosphere and may be trapped substantially in the mid-mantle surrounding subduction zones, in particular in the western Pacific subduction zones.

  6. 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

  7. 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

  8. 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.

  9. 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.

  10. Geochemical Uniformity over 30 Million Years of Volcanic Activity in the Caribbean Large Igneous Province: Evidence from Curacao and Haiti

    NASA Astrophysics Data System (ADS)

    Loewen, M. W.; Kent, A. J.; Duncan, R. A.; Krawl, K.; Michael, P. J.; Graham, D. W.

    2012-12-01

    New 40Ar/39Ar age determinations from Caribbean Large Igneous Province (CLIP) lavas, dikes, and sills from Curacao and Haiti record almost 30 million years of volcanism, beginning at ~93 Ma and continuing until ~63 Ma, with peak activity at 93-90, 86-85, 80-76 and 66-63 Ma. A variety of rock types are apparent. Despite the significant age range evident in our sample set, which includes picritic to tholeitic pillow lavas, thick hyaloclastite sequences, and poikolitic sills, compositions show only subtle compositional differences between groups of different age. Most whole rock samples appear to derive from a similar mantle source peridotite and to have undergone a common set differentiation processes (primarily partial melting followed by fractionation of olivine, clinopyroxene, and plagioclase). The recognition of this range of ages and compositional similarities poses important questions for the extent and cause of CLIP magmatism. We present new 40Ar-39Ar ages and major and trace elements for whole rock, minerals and glass samples from throughout the exposed volcanic sections at the two locations. Hyaloclastite glasses have also been analyzed by FTIR for volatile abundances. Unlike major element compositions of whole rock samples, major element and volatile analyses of hyaloclastite glasses reveal the presence of at least three distinct magma series. In addition, variations in Cl and Cl/K suggest that differences exist in the degree to which magmatic systems interact with seawater-derived components. One He-isotopic analysis from a Haiti picrite (3He/4He = 12.3 Ra) is consistent with other isotopic evidence for a significant mantle plume contribution to CLIP construction. Future work will focus on trace element modeling to further constrain magma sources and extents of melting, and on expanding the number of samples for which we have age control.

  11. 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

  12. Geochemical and isotopic (Sr, C, O) data from the alkaline complex of Grønnedal-ĺka (South Greenland): evidence for unmixing and crustal contamination

    NASA Astrophysics Data System (ADS)

    Taubald, H.; Morteani, G.; Satir, M.

    The alkaline intrusion of Grønnedal-ĺka (South Greenland) is the oldest of the ten major rift-related plutonic complexes of southern Greenland that intruded during the Gardar period between 1330 and 1150 Ma into the 2.6-Ga-old gneisses and metasediments of the Ketilidian basement. The Grønnedal-ĺka alkaline intrusion consists of carbonatites, silicocarbonatites, transitional carbonatites and nepheline-bearing syenites. The silicocarbonatites exhibit locally ocellar textures that are typical for immiscibility processes. A 87Sr/86Sr initial ratio of about 0.703184 major and trace element compositions-including REE and C-, and O-isotope data from 15 carbonatite, 12 silicocarbonatite, 10 transitional carbonatite and 8 syenite and samples-provide evidence for minor crustal contamination of the mantle-derived magma that generated by unmixing carbonatites, silicocarbonatites and syenites. A scatter in major and trace element contents and isotope ratios is related to late- to post-magmatic alteration processes. The Grønnedal-ĺka silicocarbonatites are one of the rather rare cases in which unmixing of a highly alkaline mantle-derived magma into an alkalisilicate and a carbonatitic magma-fraction under plutonic conditions is well documented by textural and geochemical data.

  13. The syncollisional granitoid magmatism and continental crust growth in the West Kunlun Orogen, China - Evidence from geochronology and geochemistry of the Arkarz pluton

    NASA Astrophysics Data System (ADS)

    Zhang, Yu; Niu, Yaoling; Hu, Yan; Liu, Jinju; Ye, Lei; Kong, Juanjuan; Duan, Meng

    2016-02-01

    The West Kunlun orogenic belt (WKOB) at the northwest margin of the Greater Tibetan Plateau records seafloor subduction, ocean basin closing and continental collision with abundant syncollisional granitoids in response to the evolution of the Proto- and Paleo-Tethys Oceans from the early-Paleozoic to the Triassic. Here we present a combined study of detailed zircon U-Pb geochronology, whole-rock major and trace elements and Sr-Nd-Hf isotopic geochemistry on the syncollisional Arkarz (AKAZ) pluton with mafic magmatic enclaves (MMEs) exposed north of the Mazha-Kangxiwa suture (MKS) zone. The granitoid host rocks and MMEs of the AKAZ pluton give the same late Triassic age of ~ 225 Ma. The granitoid host rocks are metaluminous granodiorite and monzogranite. They have initial 87Sr/86Sr of 0.70818 to 0.70930, εNd(225 Ma) = - 4.61 to - 3.91 and εHf(225 Ma) = - 3.01 to 0.74. The MMEs are more mafic than the host with varying SiO2 (51.00-63.24 wt.%) and relatively low K2O (1.24-3.02 wt.%), but have similar Sr-Nd-Hf isotope compositions to the host ((87Sr/86Sr)i = 0.70830-0.70955, εNd(225 Ma) = - 4.88 to - 4.29, εHf(225 Ma) = - 2.57 to 0.25). Both the host and MMEs have rare earth element (REE) and trace element patterns resembling those of bulk continental crust (BCC). The MMEs most likely represent cumulate formed from common magmas parental to the granitoid host. The granitoid magmatism is best explained as resulting from melting of amphibolite of MORB protolith during continental collision, which produces andesitic melts with a remarkable compositional similarity to the BCC and the inherited mantle-like isotopic compositions. Simple isotopic mixing calculations suggest that ~ 80% ocean crust and ~ 20% continental materials contribute to the source of the AKAZ pluton. Thus, the hypothesis "continental collision zones as primary sites for net continental crust growth" is applicable in the WKOB as shown by studies in southern Tibet, East Kunlun and Qilian orogens. In

  14. 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.

  15. Late Triassic melting of a thickened crust in southeastern China: Evidence for flat-slab subduction of the Paleo-Pacific plate

    NASA Astrophysics Data System (ADS)

    Zhu, Kong-Yang; Li, Zheng-Xiang; Xu, Xi-Sheng; Wilde, Simon A.

    2013-09-01

    The Dashuang complex in Zhejiang Province of southeast China is composed of two distinct lithologies: syenite in the west and quartz monzonite in the east. They record similar zircon U-Pb ages of 224 ± 3 Ma (syenite), and 226 ± 2 Ma and 227 ± 1 Ma (quartz monzonite), respectively, but are notably different in petrography, magnetic susceptibility, whole-rock chemistry, zircon Hf isotope and zircon trace element characteristics. The west Dashuang syenitic pluton (the west body) has high modal alkali feldspar, high zircon saturation temperatures, high whole-rock and zircon MREE/HREE ratios, low Fe-Mg-Ti contents, and is depleted in Ba, Sr and Eu. It also has low magnetic susceptibilities, belongs to the ilmenite-series, and is a peraluminous and ferroan granitoid. The east Dashuang quartz monzonitic pluton (the east body) has abundant K-feldspar megacrysts, with hornblende, titanite and biotite being the major ferromagnesian minerals. In contrast to the west body, the east body has lower zircon saturation temperatures, lower whole-rock and zircon MREE/HREE ratios, higher Fe-Mg-Ti contents, and shows no depletion in Ba, Sr or Eu. The east body has higher magnetite contents, high magnetic susceptibilities and belongs to the magnetite-series. It is a metaluminous and magnesian granitoid of arc-affinity. Zircon Hf isotopic data reveal that both bodies were derived from partial melting of Paleoproterozoic igneous protoliths in the lower crust, but the east body possibly incorporated subducted terrigenous sediments. Both bodies have higher melting temperatures and pressures than adjacent Cretaceous granitoids, reflecting their origin in a thickened, hotter lower crust. The most feasible model to explain their differences is variations in water content during crustal melting, resulting in different melting and crystallization behaviors. Such melting in a Triassic thickened crust with variable water involvement, followed by Cretaceous magmatism in an extensional setting

  16. 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.

  17. 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

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

    NASA Astrophysics Data System (ADS)

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

    1993-03-01

    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.

  19. Interaction between ultrapotassic magmas and carbonate rocks: Evidence from geochemical and isotopic (Sr, Nd, O) compositions of granular lithic clasts from the Alban Hills Volcano, Central Italy

    NASA Astrophysics Data System (ADS)

    Peccerillo, Angelo; Federico, Marcella; Barbieri, Mario; Brilli, Mauro; Wu, Tsai-Wan

    2010-05-01

    Magma-carbonate rock interaction is investigated through a geochemical and Sr-Nd-O isotope study of granular lithic clasts ( ejecta) from the Alban Hills ultrapotassic volcano, Central Italy. Some samples (Group-1) basically represent intrusive equivalents of Alban Hills magmas. A few samples (Group-2) are ultramafic, have high MgO (˜30 to 40 wt%) and δ 18O‰, and originated by accumulation of mafic phases that crystallised from ultrapotassic melts during assimilation of dolomitic rocks. Group-3 ejecta consist of dominant K-feldspar, and show major element compositions similar to phonolites, which, however, are absent among the Alban Hills volcanics. Finally, another group (Group-4) contains corroded K-feldspars, surrounded by a microgranular to porphyritic matrix, made of igneous minerals (K-feldspar, foids, clinopyroxene, phlogopite) plus wollastonite, garnet, and some cuspidine. Group-4 ejecta are depleted in SiO 2 and enriched in CaO with respect to Group-3. The analysed ejecta have similar 143Nd/ 144Nd (0.51204-0.51217) as the Alban Hills lavas, whereas 87Sr/ 86Sr (0.70900-0.71067) is similar to lower. Whole rocks δ 18O‰ ranges from +7.0 to +13.2, reaching maximum values in ultramafic samples. A positive correlation with CaO is observed in single rock groups. Large Ion Lithophile Element (LILE) abundances and REE fractionation are generally high, and extreme values of Th, U and LREE are found in some Group-3 and Group-4 rocks. Mineralogical, petrological and geochemical data reveal extensive interaction between magma and carbonate wall rocks, involving both dolostones and limestones. These processes had dramatic effects on magma compositions, especially on phonolites, which were transformed to foidites. Evidence of such a process is found in Group-4 samples, in which K-feldspar is observed to react with a matrix that represents strongly undersaturated melts formed by interaction between silicate magma and carbonates. Trace element data also testify to a

  20. 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

  1. Geological and geochemical evidence for vertical migration of Upper Cretaceous sourced oils into tertiary reservoirs: Winn and Grant Parishes, Louisiana

    SciTech Connect

    Echols, J.B.; Zimmerman, R.K.; Goddard, D.G.

    1995-10-01

    Analyses of three Wilcox oils taken from the Colgrade Field (Carrizo sand reservoir), Bulger Creek Field (middle Wilcox sand reservoir), both in Winn Parish, and the Blue Lick Field (middle Wilcox sand reservoir) Grant Parish, Louisiana, correlate with other Gulf Coast oils sourced from the Upper Cretaceous (probably Eagle Ford or Tuscaloosa). All three samples are mature and highly biodegraded. The oils from Blue Lick and Bulger Fields contain bisnorhopane (BNH), a Wilcox and younger biomarker in central Louisiana and southwest Mississippi. BNH was identified previously by the authors in chalk oil from Clark Creek Field, Wilkinson Co., Mississippi. Previous studies project northeast-southwest wrench faulting (Boeuf River Fault) into the oil sample area. This faulting, and associated extensional fracture/fault and salt tectonism cause the conduits for vertical migration which emplaced the sampled oils into Wilcox reservoirs. The oils probably migrated from both the Eagle Ford/Tuscaloosa source beds and from fractured Austin chalk reservoirs breached by faulting. The oils provide additional evidence that Wilcox and younger oil reservoirs of central Louisiana and southwest Mississippi were filled by vertical migration from Mesozoic source beds in contrast to long range lateral migration proposed by earlier workers. The Central Louisiana Fracture Zone is described as an extensional fracture zone generated by wrench faulting. The trend of the zone coincides closely with that of the LaSalle Arch and is important in concentrating hydrocarbons along the arch. Exploration targets arise in all traps above Eagle Ford/Tuscaloosa source beds in the area.

  2. 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.

  3. 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.

  4. Receiver Function Images of Crust and Upper Mantle: Evidence For Depp-seated Differences Between Precambrian and Paleozoic Provinces In Northern Europe

    NASA Astrophysics Data System (ADS)

    Alinaghi, A.; Bock, G.; Kind, R.; Tor Working Group; Svekalapko Working Group

    The crust-mantle boundary and upper mantle discontinuities underneath the North German Basin, the Tornquist Zone and the Baltic Shield have been detected and inves- tigated by receiver functions. Teleseismic earthquakes recorded by the TOR seismic profile and the SVEKALAPKO seismic network have been used to calculate distance and source equalized receiver functions. To enhance converted P-to-S amplitudes and to enable a consistent mapping of discontinuities, receiver functions were stacked and the arrival times of Ps were measured and plotted over the study area. The resulting delay time map of Moho is accompanied by crustal thickness measurement made us- ing Zhu &Kanamori Method so that both maps show considerable thickening of crust across the TESZ from 30 km in the German Basin to over 50 km in the Scandinavian Shield. In the Blatic Shield across SVEKALAPKO seismic network Moho takes on a complex shape in sharp contrast to the flat surface topography. Moho depths ranging from 40 to 60 km are observed under the SVEKALAPKO stations . The two major 410 km and 660 km upper mantle discontinuities are clearly observed both under the TOR profile and underneath the SVEKALAPKO network. Maps depicting the arrival times of Ps phases stemming from each of those discontinuities are presented along with a Map of the time difference between the arrival times of the aforementioned phases. While the arrival times of Ps from 410 km and 660 km discontinuities un- dergo changes across the TESZ, the difference between the arrival times of P410s and P660s phases, being a measure of the thickness of the transition zone, grows larger . This is an indication of cooler upper mantle underneath the Baltic Shield( Precam- brian) than that of the North German Basin(Paleozoic). Across the SVEKALAPKO profile the thickness of the transition zone shows signs of increase in the eastern part while traces of some local anomalies can be found in the central parts.

  5. 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.

  6. 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)

  7. 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.

  8. 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

  9. 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.

  10. 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

  11. 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

  12. A mantle- and a lower crust-derived bimodal suite in the Yusufeli (Artvin) area, NE Turkey: trace element and REE evidence for subduction-related rift origin of Early Jurassic Demirkent intrusive complex

    NASA Astrophysics Data System (ADS)

    Dokuz, Abdurrahman; Tanyolu, Erkan; Genç, Salim

    2006-06-01

    The Yusufeli area, in the Eastern Black Sea Region of Turkey, contains a crystalline complex that intruded into the Carboniferous metamorphic basement and is composed of two intrusive bodies: a gabbro-diorite and a tonalite-trondhjemite. The mafic body (45 57 wt% SiO2) displays a broad lithological spectrum ranging from plagioclase-cumulate to quartz diorite. Primitive varieties of the body have Mg-number, MgO and Cr contents that are close to those expected for partial melts from mantle peridotite. Data are consistent with the magma generation in an underlying mantle wedge that was depleted in Zr, Nb and Ti, and enriched in large ion lithophile elements (K, Rb, Ba, Th). However, high Al2O3, CaO and generally low Ni (<65 ppm) contents are not in agreement with the unfractionated mantle-derived primitive magmas and require some Al2O3- and CaO-poor mafic phases, in particular, olivine and orthopyroxene. Absence of orthopyroxene in crystallization sequence, uralitization, and a common appearance of clinopyroxene surrounded by hornblende imply an anhydrous phase fractionated from highly hydrous (5 6%) parent. Geochemical modelling suggests derivation by 15 20% melting of a depleted-lherzolitic mantle. The tonalite-trondhjemite body (58 76 wt% SiO2) ranges in composition from quartz diorite to granodiorite with a low-K calc-alkaline trend. Although LILE- and LREE- enriched characteristics of the primitive samples imply a metasomatic sub-arc mantle for their source region, low MgO, Ni and Cr concentrations rule out direct derivation from the mantle wedge. Also, lack of negative Eu anomalies suggests an unfractionated magma and precludes a differentiation from the diorites of mafic body, which show negative Eu anomalies. Their Na enrichments relative to Ca and K are similar to those of Archean tonalites, trondhjemites and granodiorites and Cenozoic adakites. However, they exhibit important geochemical differences from them, including low-Al (<15 wt%) contents

  13. 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.

  14. 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.

  15. Strange Quark Star Crusts

    SciTech Connect

    Steiner, Andrew W.

    2007-02-27

    If strange quark matter is absolutely stable, some neutron stars may be strange quark stars. Strange quark stars are usually assumed to have a simple liquid surface. We show that if the surface tension of droplets of quark matter in the vacuum is sufficiently small, droplets of quark matter on the surface of a strange quark star may form a solid crust on top of the strange quark star. This solid crust can significantly modify the predictions for the photon emission for the surface in an observable way.

  16. 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

  17. Recycled oceanic crust and marine sediment in the source of alkali basalts in Shandong, eastern China: Evidence from magma water content and oxygen isotopes

    NASA Astrophysics Data System (ADS)

    Liu, Jia; Xia, Qun-Ke; Deloule, Etienne; Chen, Huan; Feng, Min

    2015-12-01

    The magma water contents and cpx δ18O values in alkali basalts from the Fuyanyshan (FYS) volcano in Shandong, eastern China, were investigated by an inverse calculation based on the water content of clinopyroxene (cpx) phenocrysts, the ivAlcpx-dependent water partitioning coefficient Dwatercpx>/melt, and secondary ion mass spectrometer, respectively. The calculated water content (H2O wt.) of magma ranges from 0.58% to 3.89%. It positively correlates with heavy rare earth element concentrations and bulk rock 87Sr/86Sr ratios, and it negatively correlates with Nb/U ratios. However, it is not correlated with bulk Mg# (Mg# = 100 × Mg / (Mg + Fe)) and (La/Yb)n (n represents primitive mantle normalization). Combined with the rather homogenous distribution of water content within cpx grains, these correlations indicate that the water variations among different samples represent the original magma signature, rather than results of a shallow process, such as degassing and diffusion. The δ18O of cpx phenocrysts varies from 3.6‰ to 6.3‰ (±0.5‰, 2SD), which may be best explained by the involvement of components from the lower and upper oceanic crust with marine sediments within the mantle source. The H2O/Ce ratios of the calculated melts range from 113 to 696 and form a positive trend with bulk rock 87Sr/86Sr, which cannot be explained by the recycled Sulu eclogite or by the metasomatized lithospheric mantle. Our modeling calculation shows that the decoupling of ɛHf and ɛNd could be caused by the involvement of marine sediments. Combing the high Ba/Th ratios, positive Sr spikes, and low Ce/Pb ratios for the Fuyanshan basalts, we suggest that the hydrous nature of the FYS basalts was derived from the hydrous mantle transition zone with ancient sediments.

  18. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2014-06-01

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

  20. 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.

  1. 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

  2. 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)

  3. A revised stratigraphy and petrogenetic interpretation of the Cordilleran basement in the Mazatlan, Sinaloa region (NW Mexico): Evidence from new geochemical and geochronological data

    NASA Astrophysics Data System (ADS)

    Arrieta, G. F.; Schaaf, P. E.; Maldonado, R.; Solis, G.; Weber, B.; Rodriguez, L.

    2013-12-01

    The age, composition, and thermal history of the cordilleran basement in NW Mexico are still poorly known. We focus here on the Mazatlan, Sinaloa region where previous works identified the following sequences: 158 Ma orthogneisses(U-Pb) are covered by metamorphosed clastic sediments of supposed Jurassic age which are intruded by a mafic complex mainly represented by 134-139 Ma gabbros (K-Ar hornblende ages). Both rock series are considered as parts of the basement in this region. The remaining stratigraphy is characterized by a batholitic complex formed by two major magmatic events. The first one, around 100 Ma (U-Pb and K-Ar), is tonalitic in composition with some signs of regional deformation associated to a syntectonic emplacement. The second one, between 46 and 83 Ma (K-Ar), varies from diorite to granodiorite. The latter is the most widely distributed event, hosts the important ore deposits in the region and is interpreted to be emplaced postectonically, and. Our new U-Pb geochronological data include the identification of two additional plutonic events. The first occurred at 136 Ma and was obtained from the Concordia granodiorite 30 km E of Mazatlan. This unit was formerly dated at 54 Ma (hornblend, biotite; K-Ar). A fourth plutonic unit (Copala granodiorite) was dated at 28 Ma and can be correlated to the Upper Sierra Madre Supergroup. Additionally we obtained detrital zircon ages from a paragneiss covering the 158 Ma orthogneisses with peaks at 256 Ma, 492 Ma, and 1014 Ma. Our field observations indicate a reset of the 134-139 Ma gabbro ages by the close Concordia granodiorite, whose biotites were in turn rehomogenized by the 46 to 83 Ma plutonic event. Until now very few geochemical and Sr-Nd isotopic studies with petrogenetic interpretation of the units mentioned above are published. The results of our new investigations will be presented at the meeting as well as a new dataset for the coastal Quaternary Piaxtla basalt and its peridotitic xenoliths. These

  4. 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

  5. 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

  6. 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.

  7. Electrical signature of modern and ancient tectonic processes in the crust of the Atlas mountains of Morocco

    NASA Astrophysics Data System (ADS)

    Ledo, Juanjo; Jones, Alan G.; Siniscalchi, Agata; Campanyà, Joan; Kiyan, Duygu; Romano, Gerardo; Rouai, Mohamed; TopoMed MT Team

    2011-04-01

    The Atlas Mountains in Morocco are considered as type examples of intracontinental mountain chains, with high topography that contrasts with moderate crustal shortening and thickening. Whereas recent geological studies and geodynamic modelling suggest the existence of dynamic topography to explain this apparent contradiction, there is a lack of modern geophysical data at the crustal scale to corroborate this hypothesis. To address this deficiency, magnetotelluric data were recently acquired that image the electrical resistivity distribution of the crust from the Middle Atlas to the Anti-Atlas, crossing the tabular Moulouya plain and the High Atlas. All tectonic units show different, distinct and unique electrical signatures throughout the crust reflecting the tectonic history of development of each one. In the upper crust, electrical resistivity values and geometries can be associated to sediment sequences in the Moulouya and Anti-Atlas and to crustal scale fault systems in the High Atlas developed likely during Cenozoic times. In the lower crust, the low resistivity anomaly found below the Moulouya plain, together with other geophysical (low velocity anomaly, lack of earthquakes and minimum Bouguer anomaly) and geochemical (Neogene-Quaternary intraplate alkaline volcanic fields) evidences, infer the existence of a small degree of partial melt at the base of the crust. Resistivity values suggest a partial melt fraction of the order of 2-8%. The low resistivity anomaly found below the Anti-Atlas may be associated with a relict subduction of Precambrian oceanic sediments, or to precipitated minerals during the release of fluids from the mantle during the accretion of the Anti-Atlas to the West African Supercontinent during the Panafrican orogeny (ca. 685 Ma).

  8. 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.

  9. 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

  10. 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

  11. 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

  12. 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.

  13. 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 ...

  14. Geochemical and isotopic (Sr, O, C, S) evidence for multiple fluid sources for the Muþ barite deposits, SE Anatolia, Turkey

    NASA Astrophysics Data System (ADS)

    Kumral, M.; Baran, A.; Budakoglu, M.

    2009-05-01

    Barite mineralizations are observed in recrystalized and dolomitic limestones relating to Devonian aged Ground Formation, and in forms of veins, clearance filling ; and they are mostly hydrothermal characterized. It was defined by the results of field observations and analysis that barite mineralization is accompanied by the elements like copper, lead and zinc. That 18O and 87Sr/86Sr isotope values of limestones and barites which are wall rocks are very close reminds same origin formation. According to the relations of ore and wall rock, the most natural source that may cause mineralization is Devonian sea water, but mineralization has higher trace element and rare earth element concentrations. That Eu characterizing continental climate from rare earth elements was found high and Ce, characteristic of sea water, was negative preoccupy that mineralization in the area did not create any formation. In addition, considering that high 34S (‰ 17-44) and 18O values (‰ 12-20) and 87Sr/86Sr isotopes characterize the continental crust, a complicate formation mechanism is seen. It is thought that the solution causing mineralization is the Devonian sea water affected from continental crust. 34S ve 18O are enriched when Devonian sea water, in continental crust, enters a hydrothermal cycle which is permanently heatened by granitic intrusion and in consequence of bacterial activities occured during this process. It dissolves and incorporates Ba, Sr, Pb, Cu and Zn during this cycle. Eventually, mineralization occured through impregnation of these elements into piling in parts sedimentation continued.

  15. 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.

  16. Stable isotope and Ar/Ar evidence of prolonged multiscale fluid flow during exhumation of orogenic crust: Example from the Mont Blanc and Aar Massifs (NW Alps)

    NASA Astrophysics Data System (ADS)

    Rossi, M.; Rolland, Y.

    2014-09-01

    The spatial and temporal scales and the geometry of fluid pathways in a collisional orogen are investigated using stable isotope analysis (O, C, and H) and 40Ar/39Ar dating of vein minerals formed at circa 11-16 Ma in the Mont Blanc and the Aar External Crystalline Massifs. In both massifs 40Ar/39Ar dating of veins adularia provides evidence for progressive crystallization from 16 to 9 Ma, and mainly at 11-12 Ma following veins opening during shear zone activity. The fluid flow duration thus ranges from 4 to 5 Ma in the two massifs. The δ18O values of vein quartz and calcite are similar to those of undeformed crystalline and sedimentary host rocks, suggesting rock buffering, while carbon isotope ratios of vein calcites fall into three compositional groups. A-type veins have δ13C values that are buffered by the Helvetic metasediments, which suggests that these veins formed in a closed system from a locally derived CO2-rich fluid. The fluid in equilibrium with C-type veins has depleted δ13C values similar to mantle-CO2, while the intermediate δ13C values of B-type veins suggest mixing between the A-type and C-type fluids. These results are in agreement with crustal- to lithosphere-scale upward vertical fluid flow along vertical shear zones related to the strike-slip system bounding the Adriatic block since 16-20 Ma, connecting a deep-seated fluid to some downward flow in the sedimentary cover of External Crystalline Massifs.

  17. [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. PMID:26948093

  18. 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.

  19. 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

    NASA Astrophysics Data System (ADS)

    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.

  20. Early Formation of Terrestrial Crust

    NASA Astrophysics Data System (ADS)

    Harrison, T. M.; Schmitt, A. K.; McCulloch, M. T.; Lovera, O. M.

    2007-12-01

    indicate essentially continuous derivation of crust from the mantle from 4.5 to 4.2 Ga, concurrent with recycling into the mantle and internal crustal re-working. These results represent further evidence that by 4.35 Ga, portions of the crust had taken on continental characteristics.

  1. 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.

  2. 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

  3. 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

  4. 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

  5. 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.

  6. 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

  7. Origin and age of the earliest Martian crust from meteorite NWA 7533

    NASA Astrophysics Data System (ADS)

    Humayun, M.; Nemchin, A.; Zanda, B.; Hewins, R. H.; Grange, M.; Kennedy, A.; Lorand, J.-P.; Göpel, C.; Fieni, C.; Pont, S.; Deldicque, D.

    2013-11-01

    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.

  8. Geochemical provenance analyses on ANDRILL SMS (AND-2A) core: Evidence for Miocene ice-flow directions in the Ross embayment, Antarctica

    NASA Astrophysics Data System (ADS)

    Hoffmann, S.; von Eynatten, H.; Kuhn, G.

    2009-12-01

    ANDRILL (ANtarctic geological DRILLing) completed in 2007 the AND-2A sediment drillcore from the Southern McMurdo Sound Project (SMS) in the Ross Sea, Antarctica. The main target of the Project was to recover an ice-proximal drillcore, which contain the transition from a warming period during the Middle Miocene Climate Optimum (MMO, 15-17 Ma) to the today permanent ice sheet. After recovering, the AND-2A drillcore comprises an almost undisturbed ~600 m thick section of Miocene strata (800-223 m). Particular, the Middle Miocene strata in the core has the potential to understand the behaviour of the Ross Ice system and the adjacent outlet glacier of the East Antarctic Ice Sheet (EAIS) during a pronounced warm period (MMO). Bulk rock geochemical analyses on diamictites and silt-, mudstone are the appropriate method to determine the source rocks from the surrounding area and as result the ice-flow direction in high frequency downcore. Geochemical analyses of the AND-2A drillcore were carried out with an Avaatech XRF Core Scanner (XRF-CS) and on discrete samples. The geochemical composition of the sediment was analysed by the XRF-CS with a downcore resolution of least 10 cm. Discrete samples for bulk rock analysis were taken from about each meter of the core and were analyses by conventional XRF. According to multivariate statistic (Principal Component Analysis, PCA) on the geochemical dataset, the AND-2A core is mainly controlled by a (i) Ce, Nb, La, Y, Zr component and a (ii) Cr, Ni component. The first (i) component could be related to heavy minerals (REE-Phosphates, Zircon etc.) from felsic source rocks. Especially, the high content on Zr (up to 550 ppm) and Ce (up to 140 ppm) can be a result of weathering and enrichment of the corresponding heavy minerals in the sediment. The second component (ii) seems to be reflecting a mafic source from the McMurdo Volcanic Group and/or dolerites from the Ferrar Supergroup. Moreover, by applying cluster analysis on diamictites

  9. 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

  10. 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. PMID:24291793

  11. Multiple sources for the origin of Late Jurassic Linglong adakitic granite in the Shandong Peninsula, eastern China: Zircon U-Pb geochronological, geochemical and Sr-Nd-Hf isotopic evidence

    NASA Astrophysics Data System (ADS)

    Ma, Liang; Jiang, Shao-Yong; Dai, Bao-Zhang; Jiang, Yao-Hui; Hou, Ming-Lan; Pu, Wei; Xu, Bin

    2013-03-01

    The Linglong granite is one of the most important Mesozoic plutons in the Shandong Peninsula, eastern China, and its petrogenesis has long been controversial, particularly with regard to the nature of source region and geodynamic setting. Our new precise zircon U-Pb dating results reveal that the Linglong granite was emplaced in the Late Jurassic (157-160 Ma). In addition, abundant inherited zircons are identified in the granite with four groups of age peaked at ~ 208, ~ 750, ~ 1800 and ~ 2450 Ma. Geochemical studies indicate that the Linglong granite is weakly peraluminous I-type granite, and is characterized by high SiO2, Sr and La, but low MgO, Y and Yb contents, strongly fractionated REE pattern and high Sr/Y and La/Yb ratios. It also exhibits high initial 87Sr/86Sr ratios (0.7097 to 0.7125), low ɛNd(t) (- 17.7 to - 20.3) and variable zircon ɛHf(t) (- 22.2 to - 8.7) values. Calculation of the zircon saturation temperature (TZr) reveals that the magma temperatures are 760 ± 20 °C, and the lowest TZr value of 740 °C may be close to initial magma temperature of this inheritance-rich rock. Interpretation of the elemental and isotopic data suggests that the Linglong granite has some affinities with the adakite, and was most likely derived from partial melting of thickened lower crust without any significant contribution of mantle components. The presence of a large number of inherited zircons and variable Sr-Nd-Hf isotopic compositions reveal that the Linglong granite probably has multiple sources consisting of the lower crust of both South China Block and North China Block, as well as the collision-related alkaline rocks and UHP metamorphic rocks. The continental arc-rifting related to the Izanagi plate subduction was the most likely geodynamic force for formation of the Jurassic Linglong adakatic granite in the Shandong Peninsula.

  12. New evidence of effusive and explosive volcanism in the Lower Carboniferous formations of the Moroccan Central Hercynian Massif: Geochemical data and geodynamic significance

    NASA Astrophysics Data System (ADS)

    Ntarmouchant, A.; Smaili, H.; Bento dos Santos, T.; Dahire, M.; Sabri, K.; Ribeiro, M. L.; Driouch, Y.; Santos, R.; Calvo, R.

    2016-03-01

    The Azrou-Khénifra basin, located in the SE sector of the Moroccan Central Hercynian Massif of the Western Meseta of Morocco comprises volcanic and volcanoclastic rocks where two magmatic sequences can be distinguished: i) the Dhar Lahmar Sequence, composed of Upper Visean basaltic lava flows and pyroclastic deposits; and ii) the Kef Al Asri Sequence, composed of Visean - Serpukhovian intermediate to acid rocks. A continuous spatial and temporal evolution between the two volcano-sedimentary sequences was observed during the detailed geological work performed in the studied area. Petrography and geochemical studies additionally suggest a continuous compositional evolution from the more basic magmatic rocks to the intermediate/acid rocks, which implies a cogenetic magmatic differentiation controlled by crystal fractionation (with minor crustal assimilation) of a calc-alkaline trend magmatic suite. The inferred magmatic evolution is consistent with a geodynamic environment of an orogenic zone within an active continental margin setting. This partly explosive Visean - Serpukhovian volcanism, identified for the first time in the Western Meseta of Morocco, displays very similar petrographic and geochemical characteristics to its Eastern Meseta analogues, which implies that the emplacement of these magmatic rocks must have occurred in similar collisional geodynamic settings for both major geological domains, further constraining the evolution of this major crustal segment within the Carboniferous events that shaped the Hercynian Orogeny.

  13. Geochemical evidence of multistage retrogressive failure during the 160,000ka Icod landslide from turbidite facies analysis: multidisciplinary investigative approaches using destructive and non-destructive methodologies

    NASA Astrophysics Data System (ADS)

    Hunt, James; Wynn, Russell; Masson, Doug; Croudace, Ian

    2010-05-01

    The study of modern deep-sea systems through targeted piston coring has enabled detailed investigations into the process mechanics of turbidity currents. In complex systems such as the Moroccan Turbidite System the derivation of provenance is of vital importance, since flows from different sources in this system have been found to behave differently. Early provenance studies in the Madeira Abyssal Plain found that bulk sand-fraction geochemical analysis through ICP-AES could enable successful attribution of provenance to specific turbidites alongside electron microprobe analysis (de Lange, Jarvis & Kuijpers, 1987; Pearce & Jarvis, 1992). These sources including the Moroccan siliclastic shelf, Tenerife, Las Palma, El Hierro and Madeira. ICP-AES, MC-ICP-MS and XRF have been utilised here, however these present destructive methodologies, using 0.1-5g of material >63µm. Deep-sea piston cores are also expensive to collect, and often there is not enough material to remove for analysis without compromising the core. Furthermore, routine sampling, preparation and analysis using the destructive methods stated above are undertaken at considerable cost and analytical time. The successful use of non-destructive instruments to yield quantitative geochemical has become paramount at the NOC. This presentation serves to show the successful application of the TM-1000 tabletop SEM EDS analyser, ITRAX micro-XRF analyser and the GEOTEK XYZ logger, in coincidence with traditional destructive methods. These instruments can only supply semi-quantitative data, unless correct calibration can be achieved, and will be shown here. The 160,000ka Icod landslide from Tenerife generated a 150km3 debris avalanche with a runout of 105km and a >180km3 turbidity, which will form the case study for application of these instruments. The vertically stacked subunit facies of the Icod turbidite has been attributed to generation from a multistage retrogressive failure (Wynn & Masson, 2003). Here there

  14. Neodymium-142 evidence for Hadean mafic crust.

    PubMed

    O'Neil, Jonathan; Carlson, Richard W; Francis, Don; Stevenson, Ross K

    2008-09-26

    Neodymium-142 data for rocks from the Nuvvuagittuq greenstone belt in northern Quebec, Canada, show that some rock types have lower 142Nd/144Nd ratios than the terrestrial standard (epsilon142Nd = -0.07 to -0.15). Within a mafic amphibolite unit, 142Nd/144Nd ratios correlate positively with Sm/Nd ratios and produce a 146Sm-142Nd isochron with an age of 4280(-81)(+53) million years. These rocks thus sample incompatible-element-enriched material formed shortly after Earth formation and may represent the oldest preserved crustal section on Earth. PMID:18818357

  15. 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

  16. Geochemical and Tectonic Evidence for the Role of Crustal Thickening and Forearc Subduction Erosion in Miocene to Recent Andean Southern Volcanic Zone Magmas

    NASA Astrophysics Data System (ADS)

    Kay, S. M.; Godoy, E.

    2002-12-01

    Key to understanding temporal and spatial variations in Andean-type margin magmas is identifying their mantle and crustal sources in well constrained tectonic settings. A study of Andean Southern Volcanic Zone (SVZ) Early Miocene to Recent magmas in an west to east transect near 34° S points to a linked role for crustal thickening and forearc subduction erosion. The whole rock and isotopic chemistry of these magmas show a trend from Early Miocene tholeiitic sequences with low pressure pyroxene-bearing residual mineral assemblages and more depleted isotopic signatures (87Sr/^{86}Sr ~ 0.7036; ɛ Nd ~ +6 to +4; 206Pb/^{204}Pb ~ 18.5) to Pliocene/Recent high-K calc-alkaline sequences with high pressure garnet-bearing residual mineral assemblages and enriched isotopic signatures (87Sr/^{86}Sr ~ 0.7042; ɛ Nd ~ +1 to 0; 206Pb/^{204}Pb ~ 18.6). Relatively abrupt breaks in this sequence at ~ 19 to 15 Ma and ~ 7 to 4 Ma coincide with an early Miocene extensional to compressional deformational switch and major Late Miocene out-of-sequence thrusting that accompany eastward shifts of ~ 35 and ~ 50 km of the arc front and peaks in uplift history. These changes are consistent with peaks in forearc subduction erosion that introduce continental crust into subcrustal magma sources and in backarc shortening that enhance MASH processes in a thickening crust. Shifts to higher pressure chemical signatures and isotopic enrichment as frontal arc magmatism wanes in the west and migrates eastward fit with continental crust entering subcrustal magma sources. The projections of extinct arc fronts near 34° S into chemically equivalent units to the south indicate an ~ 35 km shift of the arc front north of 36.5° S at ~ 19 to 15 Ma and another ~ 50 km shift north of 34.5° S between ~ 7 to 4 Ma. These SVZ shifts coincide with major tectonic changes along the Andean margin that are best related to variations in plate convergence parameters.

  17. 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

  18. 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

  19. Petrogenesis and geodynamic setting of Neoproterozoic and Late Paleozoic magmatism in the Manzhouli-Erguna area of Inner Mongolia, China: Geochronological, geochemical and Hf isotopic evidence

    NASA Astrophysics Data System (ADS)

    Gou, Jun; Sun, De-You; Ren, Yun-Sheng; Liu, Yong-Jiang; Zhang, Shu-Yi; Fu, Chang-Liang; Wang, Tian-Hao; Wu, Peng-Fei; Liu, Xiao-Ming

    2013-05-01

    U-Pb dating and Hf isotopic analyses of zircons from various granitoids, combined with major and trace element analyses, were undertaken to determine the petrogenesis and geodynamic setting of Neoproterozoic and Late Paleozoic magmatism in the Manzhouli-Erguna area of Inner Mongolia, China. The Neoproterozoic granitoids are mainly biotite monzogranites with zircon U-Pb ages of 894 ± 13 Ma and 880 ± 10 Ma, and they are characterised by enrichment in large ion lithophile elements (LILEs; e.g., Rb, Ba, K) and light rare earth elements (LREEs), depletion in high field strength elements (HFSEs; e.g., Nb, Ta, Ti) and heavy rare earth elements (HREEs). The Late Devonian granitoids are dominantly syenogranites and mylonitised syenogranites with zircon U-Pb ages of 360 ± 4 Ma, and they form a bimodal magmatic association with subordinate gabbroic rocks of the same age. The Late Devonian syenogranites have A-type characteristics including high total alkalis, Zr, Nb, Ce and Y contents, and high FeOt/MgO, Ga/Al and Rb/Sr ratios. The Carboniferous granitoids are mainly tonalites, granodiorites and monzogranites with U-Pb ages varying from 319 to 306 Ma, and they show very strong adakitic characteristics such as high La/Yb and Sr/Y ratios but low Y and Yb contents. The Late Permian granitoids are dominated by monzogranites and syenogranites with zircon U-Pb ages ranging between 257 and 251 Ma. Isotopically, the ɛHf(t) values of the Neoproterozoic granitoids range from +4.3 to +8.3, and the two-stage model ages (TDM2) from 1.2 to 1.5 Ga. The Late Devonian granitoids are less radiogenic [ɛHf(t) from +12.0 to +12.8 and TDM2 from 545 to 598 Ma] than the Carboniferous [ɛHf(t) from +6.8 to +9.5 and TDM2 from 722 to 894 Ma] and Late Permian granitoids [ɛHf(t) from +6.1 to +9.4 and TDM2 in the range of 680-895 Ma]. These data indicate (1) the Neoproterozoic granitoids may have been generated by melting of a juvenile crust extracted from the mantle during the Mesoproterozoic

  20. 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.

  1. 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).

  2. 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.

  3. 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.

  4. Habitability Of Europa's Crust

    NASA Astrophysics Data System (ADS)

    Greenberg, R.; Tufts, B. R.; Geissler, P.; Hoppa, G.

    Physical characterization of Europa's crust shows it to be rich in potentially habitable niches, with several timescales for change that would allow stability for organisms to prosper and still require and drive evolution and adaptation. Studies of tectonics on Europa indicate that tidal stress causes much of the surface cracking, that cracks pen- etrate through to liquid water (so the ice must be thin), and that cracks continue to be worked by tidal stress. Thus a global ocean is (or was until recently) well linked to the surface. Daily tidal flow (period~days) transports substances up and down through the active cracks, mixing surface oxidants and fuels (cometary material) with the oceanic reservoir of endogenic and exogenic substances. Organisms moving with the flow or anchored to the walls could exploit the disequilibrium chemistry, and those within a few meters of the surface could photosynthesize. Cracks remain active for at least ~10,000 yr, but deactivate as nonsynchronous rotation moves them to different stress regimes in less than a million yr. Thus, to survive, organisms squeezed into the ocean must migrate to new cracks, and those frozen in place must hibernate. Most sites remelt and would release captive organisms within about a million yr based on the prevalence of chaotic terrain, which covers nearly half of Europa. Linkage of the ocean to the surface also could help sustain life in the ocean by delivering oxidants and fuels. Suboceanic volcanism (if any) could provide additional sites and support for life, but is not necessary. Recent results support this model. We further constrain the non-synchronous rotation rate, demonstrate the plausibility of episodic melt-through, show that characteristics of pits and uplift features do not imply thick ice, and demonstrate polar wander, i.e. that the ice crust is detached from the solid interior and has slipped as a unit relative to the spin axis. Thus Europa's biosphere (habitable if not inhabited) likely

  5. Seismic structure of crust formed by back-arc spreading

    NASA Astrophysics Data System (ADS)

    Grevemeyer, I.; Ranero, C. R.

    2012-12-01

    intermediate- to fast-spreading crust, although in practice most data from MOR are interpreted following a "Penrose" model derivative. Yet, the geodynamic origin of ophiolite assemblages remains an unresolved research subject. Ophiolite geochemistry has been long interpreted to support a supra-subduction-zone arc or back-arc setting, but ophiolite's considerable structural and geochemical variability are interpreted to indicate that at least some of them may have a MOR origin. This uncertainty is fueled by a lack of systematics on back-arc crustal structure due to sparse seismic experiments, contrasting to the wealth of studies of present-day MOR. Here we present the first extensive seismic study of back-arc oceanic crust providing constrains on accretion processes and ophiolite origin. The depth-velocity distribution of back-arc crust resembles MOR layered structure, but velocities are systematically different to MOR crust formed at any spreading rate. In particular, Layer 3 display lower velocities, indicating either a considerable different rock composition or deformation process. Taken with results from other back-arc basins, the substantial range of crustal velocities may explain ophiolite structural and geochemical variability. In sum, the seismic structure of the crust indicates that back-arc spreading represents a class in its own.

  6. 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

  7. Episodic Entrainment of Subducted Oceanic Crust into Primordial Reservoirs of the Lower Mantle

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

    The origin and dynamic nature of proposed large-scale compositional heterogeneity in Earth's lowermost mantle is not well understood. The preservation of primordial dense materials in the lowermost mantle has long been proposed by both geochemical and geophysical studies. As is shown in geodynamic models, primordial reservoirs can be passively swept to upwelling regions by mantle flow and form stable piles. In addition, oceanic crust can be subducted into the lowermost mantle, providing an additional source of compositional heterogeneity. We performed high resolution 2D calculations to study the interaction between primordial reservoirs and subducted oceanic crust that is compositionally different from the reservoirs, and its implications for deep mantle structure, dynamics, and chemical evolution. We find that oceanic crust subducted to the lowermost mantle is viscously dragged toward upwelling regions, where primordial reservoirs are hypothesized to exist. While most oceanic crust avoids interaction with a reservoir itself and is entrained into thermal plumes that form along the tops of reservoirs, some crust accumulates at the base of the plumes, along reservoir tops. Eventually the volume of accumulated crust at the base of a plume gains enough negative buoyancy such that it is flushed into the reservoir below, through the reservoir's top margin. Crust that is flushed into the reservoirs may explain some of the internal heterogeneity within LLSVPs observed by seismic studies. Experiments and theoretical calculations have suggested a reduction in viscosity within cold regions due to the post-perovskite phase transition. We find that this viscosity drop may act to increase the amount of crust that is entrained into primordial reservoirs. Furthermore, plumes that originate from reservoir tops will exhibit a time-dependent variation in chemistry, containing all three components (background mantle, primordial reservoir, subducted oceanic crust), perhaps explaining

  8. 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

  9. 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

  10. Oceanic crust recycling and the formation of lower mantle heterogeneity

    NASA Astrophysics Data System (ADS)

    van Keken, Peter E.; Ritsema, Jeroen; Haugland, Sam; Goes, Saskia; Kaneshima, Satoshi

    2016-04-01

    The Earth's lower mantle is heterogeneous at multiple scales as demonstrated for example by the degree-2 distribution of LLSVPs seen in global tomography and widespread distribution of small scale heterogeneity as seen in seismic scattering. The origin of this heterogeneity is generally attributed to leftovers from Earth's formation, the recycling of oceanic crust, or a combination thereof. Here we will explore the consequences of long-term oceanic crust extraction and recycling by plate tectonics. We use geodynamical models of mantle convection that simulate plates in an energetically consistent manner. The recycling of oceanic crust over the age of the Earth produces persistent lower mantle heterogeneity while the upper mantle tends to be significantly more homogeneous. We quantitatively compare the predicted heterogeneity to that of the present day Earth by tomographic filtering of the geodynamical models and comparison with S40RTS. We also predict the scattering characteristics from S-P conversions and compare these to global scattering observations. The geophysical comparison shows that lower mantle heterogeneity is likely dominated by long-term oceanic crust recycling. The models also demonstrate reasonable agreement with the geochemically observed spread between HIMU-EM1-DMM in ocean island basalts as well as the long-term gradual depletion of the upper mantle as observed in Lu-Hf systematics.

  11. 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

  12. 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.

  13. 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.

  14. 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.

  15. Interplay between fluid flow and fault-fracture mesh generation within underthrust sediments: Geochemical evidence from the Chrystalls Beach Complex, New Zealand

    NASA Astrophysics Data System (ADS)

    Fagereng, Å.; Harris, C.

    2014-02-01

    The Chrystalls Beach Complex, in the Otago Schist on the South Island of New Zealand, is a mélange comprising sheared trench-fill sediments and fragments of oceanic crust. It represents an exhumed analogue for underthrust sediments actively deforming along modern subduction thrust interfaces. The mélange is cross-cut by a fault-fracture mesh, comprising subvertical extension veins and subhorizontal slickenfibre-coated shear surfaces. Both shear and extension veins have a ‘crack-seal’ microstructure indicating episodic growth. Shear veins are associated with pressure solution selvages along the shear surface, whereas wall rock alteration is not observed adjacent to extension veins. Electron microprobe analyses of selvage seams indicate dissolution of silica from the immediate surroundings of slickenfibre shear veins, and therefore these slickenfibres probably grew by local dissolution-precipitation of silica. On the contrary, no depletion or addition of silica is detected around extension veins, indicating these veins grew by precipitation from advecting fluids. Oxygen isotope ratios measured in vein quartz show that shear and extension veins both precipitated from an aqueous fluid with 7 %° < δ18O < 10 %°, consistent with a fluid derived from low-grade metamorphic dehydration reactions. Fluid pressure therefore probably increased as fluids were introduced to a relatively impermeable mélange with increasing metamorphic grade and decreasing porosity. Fault-fracture mesh generation therefore involved localized shear assisted by dissolution-precipitation creep and concomitant extension fracturing. This led in turn to transient permeability associated with a fluid pressure drop, allowing episodic vein growth. This process may be analogous to geophysically observed episodic tremor and slow slip, which also involves a mixture of deformation styles that, put together, achieve shear slip along the subduction thrust interface.

  16. 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.

  17. 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

  18. Geochemical evidence for Holocene millennial-scale climatic and environmental changes in the south-eastern Mu Us Desert, northern China

    NASA Astrophysics Data System (ADS)

    Liu, Bing; Jin, Heling; Sun, Liangying; Sun, Zhong; Zhao, Shuang

    2015-10-01

    Deserts and sandylands that are located in the semi-arid and arid regions in the middle latitudes of the Northern Hemisphere are usually regarded as environmentally sensitive change belts which respond to global climatic change. In northern China, activation or immobilization of sand dunes is mainly influenced by humid and dry variation and is related to stronger or weaker Asian monsoons. In this paper, the history of Holocene millennial-scale climatic and environmental change is reconstructed by the systematic analysis of the geochemical element contents and parameters, along with the OSL and 14C chronologies, from the different lithologies of the palaeosol-aeolian sand sequence in the south-eastern Mu Us Desert, northern China. Our results indicate that the region was dominated by a dry climate with intensive aeolian activity before 7.2 ka BP, and there was an optimal humid climate and fixed desert in 7.2-4.6 ka. Afterwards, the dune fields became mobile again as the effective humidity declined. Additionally, six dry events were discovered with times of ~7.2, 7.0-6.8, 6.6-5.7, 4.6-4.1, 3.7-3.5, and 3.3-2.5 ka, which were not only coincident with the intervals of millennial-scale weaker Asian summer monsoons, but also accordant with the cold events evidenced in the ice cores and deep-sea deposits of the high latitudes in the Northern Hemisphere. In general, the Holocene climatic and environmental changes had the characteristics of the "monsoonal mode" and "abrupt millennial-scale oscillation" in the Mu Us Desert.

  19. 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

  20. 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.

  1. 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.

  2. 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

  3. 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

  4. 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.

  5. 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

  6. 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

  7. Geochemical and paleoenvironmental record of the early to early late Aptian major episodes of accelerated change: Evidence from Sierra del Rosario, Northeast Mexico

    NASA Astrophysics Data System (ADS)

    Núñez-Useche, Fernando; Barragán, Ricardo; Moreno-Bedmar, Josep Anton; Canet, Carles

    2015-07-01

    The lower to lower-upper Aptian succession of northern Mexico documents the drowning of the shallow-water Cupido/Cupidito carbonate platform system followed by deposition of the deep-water sediments of the La Peña Formation. Using δ13C stratigraphy, geochemical and mineralogical information coupled with previous microfacies, paleontological and total organic carbon (TOC) data from a stratigraphic section, which includes such lithological change, this study identifies distinctive episodes of accelerated global environmental change, and determines the paleoenvironmental conditions conducive to the deposition of TOC-rich intervals. Within the Cupidito unit, the Oceanic Anoxic Event 1a (OAE 1a) is recorded near the base of the section and the Intra-Furcata Negative Excursion in the topmost beds of the unit. The upper part of the section, within the La Peña Formation, is correlatable with the Noire level. Organic-carbon rich intervals occur in the lower and middle part of the OAE 1a, upper part of the Cupidito unit, base of the La Peña Formation, and in the Noire level equivalent. Reducing conditions within the sediment and oxic-dysoxic at the seafloor, locally controlled, persisted both before and during OAE 1a interval in the Cupidito lagoon. Oxygen-depleted conditions (dysoxic-anoxic) were more permanent and stronger during the deposition of the base of the la Peña Formation and the Noire equivalent level. It is proposed here that deposition of the lower-middle part of the OAE 1a and the base of the La Peña Formation was influenced by climate-controlled increases in detrital and accompanying nutrient influx that supplied especially biolimiting nutrients (Fe, P), fostering marine productivity and TOC burial. Upwelling of nutrient-rich deeper waters and minor arrival of nutrients from runoff, both account for the domination of radiolaria and organic-carbon burial during the Noire level equivalent. Record of the OAE 1a time-equivalent facies in the Cupidito

  8. 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

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

    NASA Astrophysics Data System (ADS)

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

    2013-04-01

    A number of deep sea mud volcanoes (>4700 m water depth) were discovered during a recent expedition with the German research vessel Meteor along a prominent WSW-ENE trending strike-slip fault (SWIM 1; Zitellini et al., 2009) in the western extension of the Gulf of Cadiz (NE Atlantic). Mud volcanism was unambiguously related to tectonic activity along the fault and fluids expelled at these sites show a very distinct geochemical composition that has not been reported from any other mud volcano to date. In previous studies on deep-water mud volcanoes in the Western Gulf of Cadiz accretionary wedge it was hypothesized that the discharge fluids were affected by alteration processes occurring in the old (>140 Ma) and deeply buried (>4 km) oceanic crust (Scholz et al., 2009; Sallarès et al, 2011). This hypothesis is supported by recent findings at the mud volcanoes located to the west of the realm of tectonic deformation driven by the accretionary wedge of the Gulf of Cadiz. Pore water geochemical analyses revealed fluid sources from oceanic crust and oldest sedimentary strata. Regardless of the ultimate source, these findings suggest that large strike-slip faults may play a significant, yet unrecognized role in terms of fluid circulation and element redistribution. To date, hot vents and cold seeps occurring at active spreading centers and forearcs of subduction zones have been pinpointed as hotspots of fluid activity. However, bearing in mind that transform-type plate boundaries are equal in length compared to other types of plate boundaries, fluid exchange at this type of plate boundary may provide a similarly important pathway for water and element exchange between the lithosphere and ocean. Sallarès V., Gailler A., Gutscher M.-A., Graindorge D., Bartolomé R., Gràcia E., Díaz J., Dañobeitia J.J. and Zitellini N. (2011) Seismic evidence for the presence of Jurassic oceanic crust in the central Gulf of Cadiz (SW Iberian margin), Earth and Planetary Science Letters

  10. Oceanic crust deep seismic survey

    NASA Astrophysics Data System (ADS)

    McBride, J. H.; White, R. S.

    In September 1991, the British Institutions Reflection Profiling Syndicate (BIRPS) collected 578 km of deep seismic reflection profiles over the oceanic crust beneath the Cape Verde abssyal plain in approximately 4900 m of water (Fig. 1). The survey, under the direction of J. H. McBride, was undertaken in response to a proposal made by R. S. White at the 1990 BIRPS open syndicate meeting in Birmingham, England, and was acquired using GECO-PRAKLA'S M/V Bin Hai 511. The survey consisted of two strike lines parallel to magnetic sea-floor lineations and nine orthogonal crossing lines oriented parallel to the spreading direction (Fig. 2). Adjacent lines are spaced at 4 km. For the first time, this provides the ability to map oceanic crust in “3D,” since the line spacing is less than or equal to the Fresnel-zone diameter for the lower crust.

  11. 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.

  12. 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

  13. Profiling planktonic foraminiferal crust formation

    NASA Astrophysics Data System (ADS)

    Steinhardt, Juliane; de Nooijer, Lennart L. J.; Brummer, Geert-Jan; Reichart, Gert-Jan

    2015-07-01

    Planktonic foraminifera migrate vertically through the water column during their life, thereby growing and calcifying over a range of depth-associated conditions. Some species form a calcite veneer, crust, or cortex at the end of their lifecycle. This additional calcite layer may vary in structure, composition, and thickness, potentially accounting for most of their total shell mass and thereby dominating the element and isotope signature of the whole shell. Here we apply laser ablation ICP-MS depth profiling to assess variability in thickness and Mg/Ca composition of shell walls of three encrusting species derived from sediment traps. Compositionally, Mg/Ca is significantly lower in the crusts of Neogloboquadrina dutertrei and Globorotalia scitula, as well as in the cortex of Pulleniatina obliquiloculata, independent of the species-specific Mg/Ca of their lamellar calcite shell. Wall thickness accounts for nearly half of the total thickness in both crustal species and nearly a third in cortical P. obliquiloculata, regardless of their initial shell wall thickness. Crust thickness and crustal Mg/Ca decreases toward the younger chambers in N. dutertrei and to a lesser extent, also in G. scitula. In contrast, the cortex of P. obliquiloculata shows a nearly constant thickness and uniform Mg/Ca through the complete chamber wall. Patterns in thickness and Mg/Ca of the crust indicate that temperature is not the dominant factor controlling crust formation. Instead, we present a depth-resolved model explaining compositional differences within individuals and between successive chambers as well as compositional heterogeneity of the crust and lamellar calcite in all three species studied here.

  14. 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

  15. 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.

  16. 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.

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

    NASA Astrophysics Data System (ADS)

    Sharkov, Evgenii

    2010-05-01

    rocks suggests an important assimilation of the Archean lower-crustal rocks. We assume that origin of the SHMS magmas was linked with floating up of magma chambers of high-temperature mantle-derived ultramafic melts through the crust according to zone refinement principle, i.e. by melting of roof accompanied by crystallization at bottom. It suggests that the Early Precambrian tectonomagmatic activity was linked with ascending of the first generation mantle superplumes, composed by depleted ultramafic material Cardinal change of tectonomagmatic processes occurred in the period of 2.35 to 2.0 Ga, which was characterized by voluminous eruption of Fe-Ti picrites and basalts similar to the Phanerozoic within-plate magmas, derived from geochemical-enriched mantle source. Simultaneously, important compositional changes occurred in the atmosphere, hydrosphere and biosphere (Melezhik et al., 2005). The first Phanerozoic-type orogens (Svecofennian of the Baltic Shield, Trans-Hudson and others of the Canadian Shield, etc.) appeared ca. 2 Ga. Since then, subduction of the ancient sialic continental crust (together with newly-formed oceanic crust) is a permanent process and the crustal material has stored in the "slab graveyard", revealed in the mantle by seismic tomography. We believe that the ascending of the second generation mantle plumes (thermochemical), enriched in Fe, Ti, P, LREE, etc., was responsible for those changes. Those plumes were generated at the core-mantle boundary and this process is active so far. The thermochemical plume matter possessed less density and could reach shallower depths; triggering plate tectonics processes. So, previously absent geochemical-enriched material started to involve from ~2.3 Ga in the Earth's tectonomagmatic processes. Where such material was "conserved" and how it was activated? The established succession of events could be provided by a combination of two independent factors: (1) the Earth originally was heterogeneous, and (2) the

  18. Mantle-related carbonados? Geochemical insights from diamonds from the Dachine komatiite (French Guiana)

    NASA Astrophysics Data System (ADS)

    Cartigny, P.

    2010-08-01

    Carbonado is a unique type of polycrystalline diamond characterised, among others, by 13C-depleted isotope compositions (δ 13C ˜ -25‰ vs. PDB), little advanced nitrogen aggregation (Ib-IaA) and sintered (ceramic-like) diamond grains. Its origin remains an enigma, with models proposing a formation either in the Earth's crust or even within an exploding super-nova. The possibility that carbonado formed in the Earth's mantle is often rejected because diamond with carbonado-like geochemical features has never been found in rocks, such as kimberlites, that carry diamonds from the mantle. In this study, it is shown that the C- and N- stable isotope compositions, nitrogen contents and nitrogen speciation of diamonds from the Dachine komatiite (French Guyana) exhibit unambiguous similarities with carbonados. These include C-isotopes (from -32.6 to + 0.15‰, mode at ˜-27‰), N-aggregation (only Ib-IaA diamonds, from 2 to 76% of N-pairs) and N-isotopes (from -4.1 to + 6.9‰, average ˜ + 2.1 ± 2.9‰), which all strikingly match the carbonado data. This evidence illustrates that the main geochemical arguments usually called to reject a mantle origin of carbonado are no longer valid. A model linking carbonado crystallisation from komatiite volatiles is developed. In this model, the sintering is produced by the high temperature of the komatiite magma thus accounting for their absence in colder kimberlites. The low δ 13C compositions of carbonados would be inherited from the transition zone (> 300 km depths), which is known to yield diamonds with distinct C-isotope distributions compared to most lithospheric diamonds (150-300 km depths). This model can account for most available observations of carbonados, including their large size, sintering, photoluminescense/cathodoluminescence features and geochemical characteristics.

  19. 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

  20. 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.

  1. 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.

  2. Bioalteration of basaltic glass in the oceanic crust

    NASA Astrophysics Data System (ADS)

    Furnes, Harald; Staudigel, Hubert; Thorseth, Ingunn H.; Torsvik, Terje; Muehlenbachs, Karlis; Tumyr, Ole

    2001-08-01

    Bioalteration of Quaternary to Early Cretaceous basaltic glass from pillow lavas of the upper oceanic crust can be documented in Deep Sea Drilling Project/Ocean Drilling Program (DSDP/ODP) samples from shallow to deep drill holes from the north to central Atlantic Ocean, Lau Basin, and Costa Rica Rift, a wide range of marine settings. Biogenerated textures are rooted in fractures and occur as two main types, a granular type and a tubular type. The granular type, common at all depths within the volcanic pile, appears as solid bands, semicircles or irregular patches of individual and/or coalesced spherical bodies, mostly 0.2-0.6 μm in diameter, with irregular protrusions into the fresh glass. The tubular type is more common at deeper levels in the crust and consists of thin tubes, sometime branching bodies, mostly 20-30 μm long and are more common at deeper levels. The upper crust displays a large variability in the relative importance of biotic to abiotic alteration, and the degree of bioalteration appears to decrease with depth. Thus the fraction of bioalteration of the total alteration of the glass ranges from 20-90% in the upper 300 m down to a maximum of 10% at about 500 m depth. This might be due to a natural variability in the abundance of bioaltered glass or to biased sampling from low drilling recovery of relatively young crust. The proportion of bioaltered to abiotically altered glass does not show any systematic variations with age of the crust. Thus bioalteration lasts as long as abiotic alteration, i.e., for as long as water is available to the hydration of the oceanic crust. Evidence from heat flow measurements suggests that hydrothermal circulation lasts until at least ˜70 Ma, and thus the deep biosphere is likely to expand at least into crust of this age.

  3. Consortium study of lunar meteorites Yamato-793169 and Asuka-881757: Geochemical evidence of mutual similarity, and dissimilarity versus other mare basalts

    NASA Technical Reports Server (NTRS)

    Warren, Paul H.; Lindstrom, Marilyn M.

    1993-01-01

    Compositions of bulk powders and separated minerals from two meteorites derived from the mare lava plains of the Earth's Moon, Yamato-793169 and Asuka-881757, indicate a remarkable degree of similarity to one another, and clearly favor lunar origin. However, these meteorites are unlike any previously studied lunar rock. In both cases, the bulk-rock TiO2 content is slightly greater than the level separating VLT from low-Ti mare basalt, yet the Sc content is much higher than previously observed except among high-Ti mare basalts. Conceivably, the Sc enrichment in A881757 reflects origin of this rock as a cumulate from a mare magma of 'normal' Sc content, but this seems unlikely. Mineral-separate data suggest that most of the Sc is in pyroxene, and a variety of evidence weighs against the cumulus hypothesis as a major cause for the high Sc. The remarkable similarity between Y793169 and A881757 suggests the possibility that they were derived from a single source crater on the Moon.

  4. 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.

  5. Collescipoli - An unusual fusion crust glass. [chondrite

    NASA Technical Reports Server (NTRS)

    Nozette, S.

    1979-01-01

    An electron microprobe study was conducted on glass fragments taken from the fusion crust and an internal glass-lined vein in the H-5 chondrite Collescipoli. Microprobe analyses of the glasses revealed an unusual fusion crust composition, and analyses of glass from inside the meteorite showed compositions expected for a melt of an H-group chondrite. Studies of fusion crusts by previous workers, e.g., Krinov and Ramdohr, showed that fusion crusts contain large amounts of magnetite and other oxidized minerals. The Collescipoli fusion crusts do contain these minerals, but they also contain relatively large amounts of reduced metal, sulphide, and a sodium-rich glass. This study seems to indicate that Collescipoli preserved an early type of fusion crust. Oxidation was incomplete in the fusion crust melt that drained into a crack. From this study it is concluded that fusion crust formation does not invariably result in complete oxidation of metal and sulphide phases.

  6. 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.

  7. Sedimentological and geochemical evidence for multistage failure of volcanic island landslides: A case study from Icod landslide on north Tenerife, Canary Islands

    NASA Astrophysics Data System (ADS)

    Hunt, James E.; Wynn, Russell B.; Masson, Douglas G.; Talling, Peter J.; Teagle, Damon A. H.

    2011-12-01

    Volcanic island landslides can pose a significant geohazard through landslide-generated tsunamis. However, a lack of direct observations means that factors influencing tsunamigenic potential of landslides remain poorly constrained. The study of distal turbidites generated from past landslides can provide useful insights into key aspects of the landslide dynamics and emplacement process, such as total event volume and whether landslides occurred as single or multiple events. The northern flank of Tenerife has undergone multiple landslide events, the most recent being the Icod landslide dated at ˜165 ka. The Icod landslide generated a turbidite with a deposit volume of ˜210 km3, covering 355,000 km2of seafloor off northwest Africa. The Icod turbidite architecture displays a stacked sequence of seven normally graded sand and mud intervals (named subunits SBU1-7). Evidence from subunit bulk geochemistry, volume, basal grain size, volcanic glass composition and sand mineralogy, combined with petrophysical and geophysical data, suggests that the subunit facies represents multistage retrogressive failure of the Icod landslide. The basal subunits (SBU1-3) indicate that the first three stages of the landslide had a submarine component, whereas the upper subunits (SBU4-7) originated above sea level. The presence of thin, non-bioturbated, mud intervals between subunit sands suggests a likely time interval of at least several days between each stage of failure. These results have important implications for tsunamigenesis from such landslides, as multistage retrogressive failures, separated by several days and with both a submarine and subaerial component, will have markedly lower tsunamigenic potential than a single-block failure.

  8. Extending the deep biosphere through ocean drilling: Bioalteration of volcanic glass in the oceanic crust

    NASA Astrophysics Data System (ADS)

    Banerjee, N. R.; Furnes, H.; Muehlenbachs, K.; Staudigel, H.; French, J.

    2003-12-01

    Scientific ocean drilling through the Deep Sea Drilling Program (DSDP) and Ocean Drilling Program (ODP) has provided a window into the deep biosphere. Microbial communities have been identified within hydrothermal vent systems at ocean floor spreading centers, deep within oceanic sediments, and within glassy portions of the basaltic oceanic crust of varying age. Questions still remain regarding the utilized metabolic pathways, how long such microbial activity persists within the oceanic crust, and how well it may be preserved on Earth or other planets. Microbial alteration of basaltic glass from ODP/DSDP cores and ophiolites can be documented by petrographic and biogeochemical techniques. Microbial alteration is seen as either tubular or granular textures. Tubular textures are characterized by micron-scale channel-like features extending from palagonite alteration rims into fresh glass. Granular textures appear as irregular patches of spherical bodies protruding into fresh glass. Detailed SEM imaging of these features commonly reveals delicate filament-like structures and material resembling desiccated biofilm. X-ray element maps invariably show elevated levels of C, N, P, and K associated with suspected microbial alteration features. Comparison of glasses from different ODP/DSDP holes indicates that carbon isotope ratios of carbonates in samples of microbially altered volcanic glass are commonly depleted by as much as -20 per mil, except in samples from slow-spreading ridges where both elevated and depleted C-isotope ratios are observed. In general, the microbes appear to be living off of dissolved organic matter with some evidence of lithoautotrophy at slow-spreading ridges. Maximum microbial activity seems to occur at ˜70° C but is moderated by pore water flow. We have applied nucleic acid staining techniques to many samples and imaged them with epifluorescence and laser scanning confocal microscopy. This has produced exceptional images that provide high

  9. Geochemical transition shown by Cretaceous granitoids in southeastern China: Implications for continental crustal reworking and growth

    NASA Astrophysics Data System (ADS)

    Chen, Jing-Yuan; Yang, Jin-Hui; Zhang, Ji-Heng; Sun, Jin-Feng

    2014-05-01

    -derived materials. The monzogranites have distinctly different zircon Hf and O isotope compositions from the enclaves, indicating that the parental magmas were mainly derived from ancient crust that interacted with underplated depleted mantle-derived magmas. The monzogranites have relatively high HREE contents, suggesting a garnet-free source (< 32 km), distinct from the Early Cretaceous adakite-like granites that were generated from a garnet-bearing source. Combined with previously published data, it is evident that Early Cretaceous adakite-like magmatic rocks (107-99 Ma) and associated mafic rocks (100-107 Ma) were widespread in SE China, indicating a crustal thickening event that was possibly induced by underplating of mantle-derived magma. Subsequently, between 99 and 87 Ma, crustal extension and lithospheric thinning induced the later widespread I-type granites with similar geochemical features to the Zaoshan pluton. A-type granites and syenites of this age are also present in SE China. The transition in geochemical and isotopic data, from enriched to depleted Hf isotope compositions as seen in the monzogranites, their enclaves and intrusive dolerites, suggest that depleted mantle-derived materials were involved in the generation of the monzogranites, further indicating continental crustal reworking and later crustal growth in SE China during the early Late Cretaceous.

  10. Evidence for tropical South Pacific climate change during the Younger Dryas and the Bølling-Allerød from geochemical records of fossil Tahiti corals

    NASA Astrophysics Data System (ADS)

    Asami, Ryuji; Felis, Thomas; Deschamps, Pierre; Hanawa, Kimio; Iryu, Yasufumi; Bard, Edouard; Durand, Nicolas; Murayama, Masafumi

    2009-10-01

    We present monthly resolved records of strontium/calcium (Sr/Ca) and oxygen isotope ( δ18O) ratios from well-preserved fossil corals drilled during the Integrated Ocean Drilling Program (IODP) Expedition 310 "Tahiti Sea Level" and reconstruct sea surface conditions in the central tropical South Pacific Ocean during two time windows of the last deglaciation. The two Tahiti corals examined here are uranium/thorium (U/Th)-dated at 12.4 and 14.2 ka, which correspond to the Younger Dryas (YD) cold reversal and the Bølling-Allerød (B-A) warming of the Northern Hemisphere, respectively. The coral Sr/Ca records indicate that annual average sea surface temperature (SST) was 2.6-3.1 °C lower at 12.4 ka and 1.0-1.6 °C lower at 14.2 ka relative to the present, with no significant changes in the amplitude of the seasonal SST cycle. These cooler conditions were accompanied by seawater δ18O ( δ18O sw) values higher by ~ 0.8‰ and ~ 0.6‰ relative to the present at 12.4 and 14.2 ka, respectively, implying more saline conditions in the surface waters. Along with previously published coral Sr/Ca records from the island [Cohen and Hart (2004), Deglacial sea surface temperatures of the western tropical Pacific: A new look at old coral. Paleoceanography 19, PA4031, doi:10.1029/2004PA001084], our new Tahiti coral records suggest that a shift toward lower SST by ~ 1.5 °C occurred from 13.1 to 12.4 ka, which was probably associated with a shift toward higher δ18O sw by ~ 0.2‰. Along with a previously published coral Sr/Ca record from Vanuatu [Corrège et al. (2004), Interdecadal variation in the extent of South Pacific tropical waters during the Younger Dyras event. Nature 428, 927-929], the Tahiti coral records provide new evidence for a pronounced cooling of the western to central tropical South Pacific during the Northern Hemisphere YD event.

  11. 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

  12. Relamination and the Differentiation of Continental Crust

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  13. Magnetic structure of the crust

    NASA Technical Reports Server (NTRS)

    Wasilewski, P.

    1985-01-01

    The bibuniqueness aspect of geophysical interpretation must be constrained by geological insight to limit the range of theoretically possible models. An additional step in depth understanding of the relationship between rock magnetization and geological circumstances on a grand scale is required. Views about crustal structure and the distribution of lithologies suggests a complex situation with lateral and vertical variability at all levels in the crust. Volcanic, plutonic, and metamorphic processes together with each of the observed anomalies. Important questions are addressed: (1) the location of the magnetic bottom; (2) whether the source is a discrete one or are certain parts of the crust cumulatively contributing to the overall magnetization; (3) if the anomaly to some recognizable surface expression is localized, how to arrive at a geologically realistic model incorporating magnetization contrasts which are realistic; (3) in the way the primary mineralogies are altered by metamorphism and the resulting magnetic contracts; (4) the effects of temperature and pressure on magnetization.

  14. Chronology of early lunar crust

    NASA Technical Reports Server (NTRS)

    Dasch, E. J.; Nyquist, L. E.; Ryder, G.

    1988-01-01

    The chronology of lunar rocks is summarized. The oldest pristine (i.e., lacking meteoritic contamination of admixed components) lunar rock, recently dated with Sm-Nd by Lugmair, is a ferroan anorthosite, with an age of 4.44 + 0.02 Ga. Ages of Mg-suite rocks (4.1 to 4.5 Ga) have large uncertainties, so that age differences between lunar plutonic rock suites cannot yet be resolved. Most mare basalts crystallized between 3.1 and 3.9 Ga. The vast bulk of the lunar crust, therefore, formed before the oldest preserved terrestrial rocks. If the Moon accreted at 4.56 Ga, then 120 Ma may have elapsed before lunar crust was formed.

  15. 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

  16. 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.

  17. 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

  18. 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.

  19. 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

  20. 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

  1. Petrological and geochemical constraints on the origin of adakites in the Garibaldi Volcanic Complex, southwestern British Columbia, Canada

    NASA Astrophysics Data System (ADS)

    Fillmore, Julie; Coulson, Ian M.

    2013-07-01

    The Garibaldi Volcanic Complex (GVC) is located in southwestern British Columbia and comprises two related but distinct volcanic fields: the Garibaldi Lake and the Mount Garibaldi volcanic fields. The rocks of the GVC range from basalt to rhyolite, and analyses of samples from both fields distinguish these as adakites. The GVC magmas have high Sr/Y, Mg#, and Al2O3; low K2O/Na2O; and fractionated rare earth element compositions. Models of adakite genesis fall into two main groups: slab melting and non-slab melting. Adakites generated by slab melting commonly occur from young subducting crust (≤25 Ma) and are felsic partial melts of the subducting slab that interact with the mantle wedge during ascent. Non-slab melting models vary widely and include basalt fractionation, assimilation, fractional crystallization processes and partial melting of mafic lower crust. Data from the GVC are too limited to fully elucidate the mechanisms of adakite genesis; however, the petrographical and geochemical characteristics of the GVC rocks in this study do not refute an origin by slab partial melts. Variations in trace elements that reflect non-adakitic values (e.g., low La, low Cr) are likely the result of magma mixing at shallow depths within the magma reservoirs of each center, for which there is mineralogical and textural evidence. The adakite rocks of the GVC share geochemical traits akin to both low-SiO2 adakite (LSA) and high-SiO2 adakite (HSA) groups, though additional data are needed to investigate whether LSA- or HSA-type dominates within the GVC, and by extension, which should be the preferred model of adakite genesis.

  2. 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.

  3. 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.

  4. Crust Formation and Stabilization of the Western Archean Kaapvaal Craton: Evidence from U-Pb Geochronology of Basement Blocks and Deep Crustal Xenoliths from the Kimberley Region, South Africa

    NASA Astrophysics Data System (ADS)

    Schmitz, M. D.; Bowring, S. A.

    2001-05-01

    The kimberlites of the Kimberley region of South Africa have yielded one of the most abundantly sampled and studied suites of lithospheric mantle xenoliths in the world, providing a detailed picture of the composition and thermal evolution of the continental mantle beneath the western Kaapvaal craton. Surprisingly however, little published data exist regarding the nature of the basement and deeper crustal rocks in the western craton, with which to contrast the evolution of the crustal and mantle portions of this Archean cratonic region. Crustal xenoliths collected in the various mine dumps around Kimberley are predominantly large blocks of near-surface basement lithologies, including deformed granitic to tonalitic gneisses and amphibolites, weakly deformed pegmatoids, and non-deformed biotite granite. U-Pb zircon geochronological data for a number of xenoliths have been used to develop a preliminary framework for the age and evolution of the Archean crust of the Kimberley region. The youngest component of the Kimberley basement is a non-deformed sample of biotite granite with an age of 2724+/-2 Ma. A major episode of metamorphism and crustal anatexis is recorded by 2928+/-2 Ga metamorphic zircon growth in amphibolitic and tonalitic components of banded gneisses, and igneous zircons of identical age in weakly deformed cross-cutting pegmatoids. Zircons from these same pegmatoids also have inherited cores which yield 207Pb/206Pb dates as old as 3265 Ma. These inherited zircons, as well as cores of zircons from a foliated granodioritic xenolith with 207Pb/206Pb dates as old as 3184 Ma, indicate the antiquity of the oldest crustal components of the Kimberley basement. These data are consistent with cursory SHRIMP U-Pb zircon geochronological information reported for lithologies collected in situ in the diamond mine walls of Kimberley. Two important implications of this data are considered: first, we interpret the major metamorphism and crustal anatexis at 2.93 Ga as

  5. 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.

  6. 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.

  7. 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.

  8. Mars Crust: Made of Basalt

    NASA Astrophysics Data System (ADS)

    Taylor, G. J.

    2009-05-01

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

  9. 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

  10. 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.

  11. A Geochemical Comparison of the Northern Peninsular Ranges Batholith in Southern California and the Coastal Batholith in Southern Peru

    NASA Astrophysics Data System (ADS)

    Clausen, B. L.; Martínez Ardila, A. M.; Morton, D. M.

    2010-12-01

    An extensive geochemical data set from the northern Peninsular Ranges Batholith (PRB) in southern California is compared and contrasted with the Arequipa segment of the Peruvian Coastal Batholith, including new granitoid samples recently collected near Ica (14°S, 76°W). The data include major and trace elements and Sr isotope ratios. This is part of an on-going study of subduction-related magmatism to refine a petrogenetic model of crust formation at plate boundaries, with a particular interest in the role of magma mixing. Research in the northern PRB suggests that continental crust is formed in several cycles: (1) mantle melting to give mafic volcanics and gabbroic intrusives, (2) basalt/gabbro melting to give felsic granitoids uncontaminated by continental crust and having low initial 87Sr/86Sr (Sri) values less than 0.704, and (3) crustal melting to give high Sri values greater than 0.704. Geochemical evidence was used to determine the extent of mixing between mafic and felsic magma that produced rocks of intermediate SiO2 composition. These differentiation cycles formed a west to east chronologic sequence and yielded granitoids of gabbro, tonalite, and granodiorite composition. Using principal component analysis on the northern PRB granitoids, the four factors affecting geochemical composition were categorized as differentiation, crustal contamination, depth of magma source, and conditions that yield a range from calcic to more alkaline granitoids. A similar major and trace element analysis is being done for a classic result of subduction in the Peruvian Coastal Batholith. The Peruvian samples recently collected include granitoids of the upper Cretaceous Coastal Batholith, as well as the associated volcanics of Cretaceous and Jurassic age. The Coastal Batholith samples include a range of granitoids from the early gabbros and from the four batholithic super-units (from west to east: Linga, Pampahuasi, Tiabaya, and Incahuasi) containing a combination of diorite

  12. 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

  13. 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.

  14. 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

  15. Apulian crust: Top to bottom

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  16. 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.

  17. 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.

  18. 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.

  19. Eclogite xenoliths in west African kimberlites as residues from Archaean granitoid crust formation

    NASA Astrophysics Data System (ADS)

    Rollinson, Hugh

    1997-09-01

    Eclogites are a comparatively rare but petrologically important member of kimberlite xenolith suites. Their broadly basaltic chemistry has led many authors to propose that they represent ancient, subducted ocean crust. Recent studies, however, have suggested an alternative origin and propose that kimberlitic eclogites are residues from the process of Archaean granitoid crust formation. Geochemical arguments in support of this new model were previously based on the trace-element chemistry of eclogitic minerals. Here I report that the major-element chemistry of eclogite xenoliths also supports a crustal residue model. I examine eclogite xenoliths from kimberlite pipes at Koidu, Sierra Leone, which sample the lithospheric mantle underlying the Archaean (2.8Gyr) granitoid crust of the West African craton. Geochemical plots of major elements measured in unaltered, whole-rock samples of low-silica eclogite demonstrate that they are complementary to the granitoids of the West African craton and have compositions which indicate that both were derived from a common basaltic parent rock.

  20. 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

  1. The Search for Water in the Lunar Crust

    NASA Astrophysics Data System (ADS)

    Smith, D. E.; Zuber, M. T.

    2015-10-01

    There is evidence from several sources that water ice is present at a number of locations on the surface at the lunar poles. But there seems little agreement on the source of the ice with most suggestions being of external nature, such as impacts by ice-rich comets. Here we discuss the lunar crust as a possible source of the water ice and investigate whether there is any evidence from the recent GRAIL [1,2] and Lunar Reconnaissance Orbiter (LRO) [3] missions to support this possibility.

  2. 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.

  3. Seismicity in the outer rise offshore southern Chile: Indication of fluid effects in crust and mantle

    NASA Astrophysics Data System (ADS)

    Tilmann, Frederik J.; Grevemeyer, Ingo; Flueh, Ernst R.; Dahm, Torsten; Goßler, Jürgen

    2008-05-01

    We examine the micro-earthquake seismicity recorded by two temporary arrays of ocean bottom seismometers on the outer rise offshore southern Chile on young oceanic plate of ages 14 Ma and 6 Ma, respectively. The arrays were in operation from December 2004-January 2005 and consisted of 17 instruments and 12 instruments, respectively. Approximately 10 locatable events per day were recorded by each of the arrays. The catalogue, which is complete for magnitudes above 1.2-1.5, is characterized by a high b value, i.e., a high ratio of small to large events, and the data set is remarkable in that a large proportion of the events form clusters whose members show a high degree of waveform similarity. The largest cluster thus identified consisted of 27 similar events (average inter-event correlation coefficient > 0.8 for a 9.5 s window), and waveform similarity persists far into the coda. Inter-event spacing is irregular, but very short waiting times of a few minutes are far more common than expected from a Poisson distribution. Seismicity with these features (high b value, large number of similar events with short waiting times) is typical of swarm activity, which, based on empirical evidence and theoretical considerations, is generally thought to be driven by fluid pressure variations. Because no pronounced outer rise bulge exists on the very young plate in the study region, it is unlikely that melt is accessible from decompression melting or opening of cracks. A fluid source related to processes at the nearby ridge is conceivable for the younger segment but less likely for the older one. We infer that the fluid source could be seawater, which enters through fractures in the crust. Most of the similar-earthquake clusters are within the crust, but some of them locate significantly below the Moho. If our interpretation is correct, this implies that water is present within the mantle. Hydration of the mantle is also indicated by a decrease of Pn velocities below the outer

  4. 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.

  5. 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.

  6. 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.

  7. 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

  8. Geochemical Speciation Mass Transfer

    SciTech Connect

    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 mineral 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.

  9. Geochemical Speciation Mass Transfer

    Energy Science and Technology Software Center (ESTSC)

    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

  10. Assessing the role of upwelling hydrothermal fluids in altering the upper ocean crust

    NASA Astrophysics Data System (ADS)

    Gillis, K. M.; Coogan, L. A.

    2008-12-01

    spreading mid-ocean ridges, Geochem., Geophys., Geosyst., v. 8, doi:10.1029/2007GC001787. [2] Heft et al. [2008], Role of upwelling hydrothermal fluids in the development of alteration patterns at fast spreading ridges: Evidence from the sheeted dike complex at Pito Deep, Geochem., Geophys., Geosyst., v. 9, doi:10.1029/2007GC001926.

  11. Modification of an oceanic plateau, Aruba, Dutch Caribbean: Implications for the generation of continental crust

    NASA Astrophysics Data System (ADS)

    White, R. V.; Tarney, J.; Kerr, A. C.; Saunders, A. D.; Kempton, P. D.; Pringle, M. S.; Klaver, G. T.

    1999-01-01

    The generation of the continental crust may be connected to mantle plume activity. However, the nature of this link, and the processes involved, are not well constrained. An obstacle to understanding relationships between plume-related mafic material and associated silicic rocks is that later tectonic movements are liable to obscure the original relationships, particularly in ancient greenstone belts. Studies of younger analogous regions may help to clarify these relationships. On the island of Aruba in the southern Caribbean, a sequence of partly deformed mafic volcanic rocks intruded by a predominantly tonalitic batholith is exposed. The mafic lavas show geochemical and isotopic affinities with other basaltic, picritic and komatiitic rocks that crop out elsewhere in the Caribbean—these are well documented as belonging to an 88-91 Ma plume-related oceanic plateau, which is allochthonous with respect to the Americas, and is thought to have been formed in the Pacific region. The ˜85 to ˜82 Ma tonalitic rocks share some geochemical characteristics (high Sr and Ba, low Nb and Y) with Archaean tonalite-trondhjemite-granodiorite (TTG) suites. Field relationships suggest that deformation of the plateau sequence, possibly related to collision with a subduction zone, was synchronous with intrusion of the Aruba batholith. New incremental heating 40Ar/ 39Ar dates, combined with existing palaeontological evidence, show that cooling of the batholith occurred shortly after eruption of the plateau basalt sequence. Sr-Nd isotopic data for both rock suites are uniform ( 87Sr/ 86Sr i≈0.7035 , ɛNd i≈+7), whereas Pb isotopes are more variable (Plateau sequence: 206Pb/ 204Pb =18.6-19.1 , 207Pb/ 204Pb =15.54-15.60 , 208Pb/ 204Pb =38.3-38.75 ; Aruba batholith: 206Pb/ 204Pb =18.4-18.9 , 207Pb/ 204Pb =15.51-15.56 , 208Pb/ 204Pb =38.0-38.5 ). This suggests that there has been a minor sedimentary input into the source region of the batholith. However, the limited time interval

  12. Geochemical consequences of the metasomatic conversion of an Early Archean komatiite sequence into chert

    SciTech Connect

    Hanor, J.S.; Duchac, K.C.

    1985-01-01

    Duchac and Hanor (1985) have demonstrated from field and petrographic evidence that the stratiform, muscovite-bearing cherts of Skokhola Ridge, Barberton Mountain Land, South Africa, represent a pervasively silicified sequence of komatiites and komatiitic basalts of Early Archean age. Most alteration took place early, prior to any significant tectonic deformation. The isovolumetric nature of the alteration, as confirmed by excellent preservation of igneous textures, and the immobility of Al make it possible to quantify elemental gains and losses during metasomatism. The sequence was strongly depleted in Na, Mg, Ca, Sr, Fe, and Mn, and enriched in K, Rb, and Ba, Al, Y, Zr, Ti, and P were immobile. It is most likely that the sequence was altered by large volumes of ascending hydrothermal solutions, with the decrease in T upward favoring precipitation of silica and muscovite. The net effect was to convert an ultramafic igneous rock into a rock having a composition approaching that of the average sandstone. Such alteration, if regionally extensive, undoubtedly affected the geochemical evolution of both the Early Archean crust and hydrosphere in the Barberton greenstone belt.

  13. 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

  14. 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