High speed theory for the planing of a flat plate at high Froude number     

Y.K. Chung  H.H. Chun   《Ocean Engineering》2007,34(11-12):1552-1560

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Persistence of mantle lithospheric Re–Os signature during asthenospherization of the subcontinental lithospheric mantle: insights from in situ isotopic analysis of sulfides from the Ronda peridotite (Southern Spain)     

Claudio Marchesi  William L. Griffin  Carlos J. Garrido  Jean-Louis Bodinier  Suzanne Y. O’Reilly  Norman J. Pearson 《Contributions to Mineralogy and Petrology》2010,159(3):315-330

The western part of the Ronda peridotite massif (Southern Spain) consists mainly of highly foliated spinel-peridotite tectonites and undeformed granular peridotites that are separated by a recrystallization front. The spinel tectonites are interpreted as volumes of ancient subcontinental lithospheric mantle and the granular peridotites as a portion of subcontinental lithospheric mantle that underwent partial melting and pervasive percolation of basaltic melts induced by Cenozoic asthenospheric upwelling. The Re–Os isotopic signature of sulfides from the granular domain and the recrystallization front mostly coincides with that of grains in the spinel tectonites. This indicates that the Re–Os radiometric system in sulfides was highly resistant to partial melting and percolation of melts induced by Cenozoic lithospheric thermal erosion. The Re–Os isotopic systematics of sulfides in the Ronda peridotites thus mostly conserve the geochemical memory of ancient magmatic events in the subcontinental lithospheric mantle. Os model ages record two Proterozoic melting episodes at ~1.6 to 1.8 and 1.2–1.4 Ga, respectively. The emplacement of the massif into the subcontinental lithospheric mantle probably coincided with one of these depletion events. A later metasomatic episode caused the precipitation of a new generation of sulfides at ~0.7 to 0.9 Ga. These Proterozoic Os model ages are consistent with results obtained for several mantle suites in Central/Western Europe and Northern Africa as well as with the Nd model ages of the continental crust of these regions. This suggests that the events recorded in mantle sulfides of the Ronda peridotites reflect different stages of generation of the continental crust in the ancient Gondwana supercontinent.  相似文献   

Aerobic oxidation of mackinawite (FeS) and its environmental implication for arsenic mobilization   总被引：1，自引：0，他引：1  

Hoon Y. Jeong  Sung W. Park 《Geochimica et cosmochimica acta》2010,74(11):3182-5194

Oxidation of mackinawite (FeS) and concurrent mobilization of arsenic were investigated as a function of pH under oxidizing conditions. At acidic pH, FeS oxidation is mainly initiated by the proton-promoted dissolution, which results in the release of Fe(II) and sulfide in the solution. While most of dissolved sulfide is volatilized before being oxidized, dissolved Fe(II) is oxidized into green rust-like precipitates and goethite (α-FeOOH). At basic pH, the development of Fe(III) (oxyhydr)oxide coating on the FeS surface inhibits the solution-phase oxidation following FeS dissolution. Instead, FeS is mostly oxidized into lepidocrocite (γ-FeOOH) via the surface-mediated oxidation without dissolution. At neutral pH, FeS is oxidized via both the solution-phase oxidation following FeS dissolution and the surface-mediated oxidation mechanisms. The mobilization of arsenic during FeS oxidation is strongly affected by FeS oxidation mechanisms. At acidic pH (and to some extent at neutral pH), the rapid FeS dissolution and the slow precipitation of Fe (oxyhydr)oxides results in arsenic accumulation in water. In contrast, the surface-mediated oxidation of FeS at basic pH leads to the direct formation of Fe (oxyhydr)oxides, which provides effective adsorbents for As under oxic conditions. At acidic and neutral pH, the solution-phase oxidation of dissolved Fe(II) accelerates the oxidation of the less adsorbing As(III) to the more adsorbing As(V). This study reveals that the oxidative mobilization of As may be a significant pathway for arsenic enrichment of porewaters in sulfidic sediments.  相似文献   

Ferrous phosphate surface precipitates resulting from the reduction of intragrain 6-line ferrihydrite by Shewanella oneidensis MR-1     

T.S. Peretyazhko  J.M. Zachara  D.W. Kennedy  J.K. Fredrickson  B.W. Arey  J.P. McKinley  C.M. Wang  A.C. Dohnalkova  Y. Xia 《Geochimica et cosmochimica acta》2010,74(13):3751-60

The reductive biotransformation of 6-line ferrihydrite located within porous silica (intragrain ferrihydrite) by Shewanella oneidensis MR-1 was investigated and compared to the behavior of 6-line ferrihydrite in suspension (free ferrihydrite). The effect of buffer type (PIPES and NaHCO₃), phosphate (P), and an electron shuttle (AQDS) on the extent of reduction and formation of Fe(II) secondary phases was investigated under anoxic conditions. Electron microscopy and micro X-ray diffraction were applied to evaluate the morphology and mineralogy of the biogenic precipitates and to study the distribution of microorganisms on the surface of porous silica after bioreduction. Kinetic reduction experiments with free and intragrain ferrihydrite revealed contrasting behavior with respect to the buffer and presence of P. The overall amount of intragrain ferrihydrite reduction was less than that of free ferrihydrite [at 5 mmol L⁻¹ Fe(III)_T]. Reductive mineralization was not observed in the intragrain ferrihydrite incubations without P, and all biogenic Fe(II) concentrated in the aqueous phase. Irrespective of buffer and AQDS addition, rosettes of Fe(II) phosphate of approximate 20-30 μm size were observed on porous silica when P was present. The rosettes grew not only on the silica surface but also within it, forming a coherent spherical structure. These precipitates were well colonized by microorganisms and contained extracellular materials at the end of incubation. Microbial extracellular polymeric substances may have adsorbed Fe(II) promoting Fe(II) phosphate nucleation with subsequent crystal growth proceeding in different directions from a common center.  相似文献   

Unsteady diagenetic processes and sulfur biogeochemistry in tropical deltaic muds: Implications for oceanic isotope cycles and the sedimentary record   总被引：1，自引：0，他引：1  

Robert C. Aller  Vanessa Madrid  Josephine Y. Aller 《Geochimica et cosmochimica acta》2010,74(16):4671-4692

Sedimentary S cycling is usually conceptualized and interpreted within the context of steadily accreting (1-D) transport-reaction regimes. Unsteady processes, however, are common in many sedimentary systems and can result in dramatically different S reaction balances and diagenetic products than steady conditions. Globally important common examples include tropical deltaic topset and inner shelf muds such as those extending from the Amazon River ∼1600 km along the Guianas coast of South America. These deposits are characterized by episodic reworking of the surface seabed over vertical depths of ∼0.1-3 m. Reworked surface sediments act as unsteady, suboxic batch reactors, unconformably overlying relict anoxic, often methanic deposits, and have diagenetic properties largely decoupled from net accumulation of sediment. Despite well-oxygenated water and an abundant reactive organic matter supply, physical disturbance inhibits macrofauna, and benthic communities are dominated by microbial biomass across immense areas. In the surficial suboxic layer, molecular biological analyses, tracer experiments, sediment C/S/Fe compositions, and δ³⁴S, δ¹⁸O of pore water indicate close coupling of anaerobic C, S, and Fe cycles. δ¹⁸O- can increase by 2-3‰ during anaerobic recycling without net change in δ³⁴S-, demonstrating reduction coupled to complete anaerobic reoxidation to and a δ¹⁸O- reduction + reoxidation fractionation factor?12‰ (summed magnitudes). S reoxidation must be coupled to Fe-oxide reduction, contributing to high dissolved Fe²⁺ (∼1 mM) and Fe mobilization-export. The reworking of Amazon-Guianas shelf muds alone may isotopically alter δ¹⁸O- equivalent in mass to?25% of the annual riverine delivery of to the global ocean. Unsteady conditions result in preservation of unusually heavy δ³⁴S isotopic compositions of residual Cr reducible S, ranging from 0‰ to >30‰ in physically reworked deposits. In contrast, bioturbated facies adjacent to physically reworked regions accumulate isotopically light S (δ³⁴S to −20‰) in otherwise similar decomposition regimes. The isotopic patterns of both physically and biologically reworked regions can be simulated with simple diagenetic models. Heavy S isotopic signatures are largely a consequence of unsteady diffusion and progressive anaerobic burndown into underlying deposits, whereas isotopically depleted bioturbated deposits predominantly reflect biogenic diffusive scaling and isotopic distillation/diffusive pumping associated with reoxidation in burrow walls immediately adjacent to reduced zones. The S isotopic transition from unsteady physically controlled regions of the Amazon delta moving laterally into bioturbated facies mimics the transition of S isotopic patterns temporally in the geologic record during the rise of bioturbation. No special role for S disproportionation is required to explain these differences. The potential role of unsteady, suboxic diagenesis and dynamic reworking of sediments has been largely ignored in models of the evolution of surficial elemental cycling and interpretations of the geologic record.  相似文献   

Late Carboniferous-Early Permian Sequence Stratigraphy and Depositional Evolution in the Northeast Ordos Basin,North China     

YANG Minghui  LIU Chiyang  LAN Chaoli  LIU Le  LI Xin  ZHANG Kunshan 《《地质学报》英文版》2010,84(5)

<正>Sequence stratigraphical analysis was applied to the Upper Carboniferous-Lower Permian sedimentary succession of the northeastern Ordos Basin,north China based on data acquired from ten entire logging curves and eight outcrops.The facies framework of the lithostratigraphical unit,the Taiyuan Formation comprises seven facies in two facies associations,varying from fluvio-delta to shelf-barrier islands.The facies are presented within a chronostratigraphical framework,linked by systems tract,which in turn are limited by flooding surfaces and sequence boundaries.Six third-order depositional sequences are recognised,bounded by six type 2 unconformities.An upwards-shallowing epicontinental sea sedimentary model is created,which consists of a sandstone,coal seam and carbonate succession.  相似文献   

The growth of the continental crust: Constraints from zircon Hf-isotope data     

E.A. Belousova  Y.A. Kostitsyn  W.L. Griffin  G.C. Begg  S.Y. O'Reilly  N.J. Pearson 《Lithos》2010,119(3-4):457-466

A worldwide database of over 13,800 integrated U–Pb and Hf-isotope analyses of zircon, derived largely from detrital sources, has been used to examine processes of crustal evolution on a global scale, and to test existing models for the growth of continental crust through time. In this study we introduce a new approach to quantitatively estimating the proportion of juvenile material added to the crust at any given time during its evolution. This estimate is then used to model the crustal growth rate over the 4.56 Ga of Earth's history. The modelling suggests that there was little episodicity in the production of new crust, as opposed to peaks in magmatic ages. The distribution of age-Hf isotope data from zircons worldwide implies that at least 60% of the existing continental crust separated from the mantle before 2.5 Ga. However, taking into consideration new evidence coming from geophysical data, the formation of most continental crust early in Earth's history (at least 70% before 2.5 Ga) is even more probable. Thus, crustal reworking has dominated over net juvenile additions to the continental crust, at least since the end of the Archean. Moreover, the juvenile proportion of newly formed crust decreases stepwise through time: it is about 70% in the 4.0–2.2 Ga time interval, about 50% in the 1.8–0.6 Ga time interval, and possibly less than 50% after 0.6 Ga. These changes may be related to the formation of supercontinents.  相似文献   

Provenance of Lower Cretaceous Wölong Volcaniclastics in the Tibetan Tethyan Himalaya: Implications for the final breakup of Eastern Gondwana     

Xiumian Hu  L. Jansa  Lei Chen  W.L. Griffin  S.Y. O'Reilly  Jiangang Wang 《Sedimentary Geology》2010,223(3-4):193-205

Lower Cretaceous volcanic lithic arenites, widely distributed in the Tethyan Himalaya, provide insights into the continental breakup of Eastern Gondwana. In southern Tibet they are represented by the Wölong Volcaniclastics. The volcanic rocks that contributed clastic material to the lower parts of this unit were predominantly alkali basalts, whereas rhyolitic/dacitic volcanism becomes the predominant source of the upper strata. Geochemical analyses of basaltic grains and of detrital Cr-spinels from the Wölong Volcaniclastics demonstrate the alkaline character of the volcanism and suggest “within-plate” tectonic setting for the volcanism. Zircon U–Pb ages confirm that this volcanism continued from ～ 140 Ma to ～ 119 Ma. Hf-isotope data on these Early Cretaceous zircons indicate that their parental magmas were mantle-derived, but in the later stage of magmatic activity mantle-derived magmas were mixed with partial melts derived from the continental crust.The Lower Cretaceous volcaniclastics occur along a broad belt paralleling the northern margin of Greater India. The onset of volcaniclastic deposition in the Himalayas appears to become progressively younger toward the west, but it ended synchronously during the Late Albian (～ 102 Ma). The low volume of volcanic rocks and their intra-plate tectonic setting suggest that they are the result of decompressional melting along extensional deep-seated fractures cross-cutting the continental crust, and reflect changes in regional intra-plate tectonic stresses when Greater India began to separate from the Australia–Antarctica supercontinent.  相似文献   

Biological influences on modern sulfates: Textures and composition of gypsum deposits from Guerrero Negro,Baja California Sur,Mexico     

Marilyn B. Vogel  David J. Des Marais  Mary N. Parenteau  Linda L. Jahnke  Kendra A. Turk  Michael D.Y. Kubo 《Sedimentary Geology》2010,223(3-4):265-280

Gypsum (CaSO₄·2H₂O) deposits from a range of sedimentary environments at Guerrero Negro, Baja California Sur, Mexico were investigated for microscale texture and composition in order to differentiate features formed under substantial microbial influence from those for which microbial effects were relatively minor or absent. Gypsum deposits were classified according to their sedimentary environment, textures, crystal habit, brine composition and other geochemical factors. The environments studied included subaqueous sediments in anchialine pools and in solar salterns, as well as subsurface sediments of mudflats and saltpans. Gypsum that developed in the apparent absence of biofilms included crystals precipitated in the water column and subsedimentary discs that precipitated from phreatic brines. Subsedimentary gypsum developed in sabkha environments exhibited a sinuous microtexture and poikilitically enclosed detrital particles. Water column precipitates had euhedral prismatic habits and extensive penetrative twinning. Gypsum deposits influenced by biofilms included bottom nucleated crusts and gypsolites developing in anchialine pools and saltern ponds. Gypsum precipitating within benthic biofilms, and in biofilms within subaerial sediment surfaces provided compelling evidence of biological influences on crystal textures and habits. This evidence included irregular, high relief surface textures, accessory minerals (S°, Ca-carbonate, Sr/Ca-sulfate and Mg-hydroxide) and distinctive crystal habits such as equant forms and crystals having distorted prism faces.  相似文献   

SHRIMP zircon dating and Nd isotopic systematics of Palaeoproterozoic migmatitic orthogneisses in the Epupa Metamorphic Complex of northwestern Namibia     

A. Kröner  Y. Rojas-Agramonte  E. Hegner  K.-H. Hoffmann  M.T.D. Wingate 《Precambrian Research》2010

The Epupa Metamorphic Complex constitutes the southwestern margin of the Congo Craton and is exposed in a hilly to mountainous terrain of northwestern Namibia, bordering the Kunene River and extending into southern Angola. It consists predominantly of granitoid gneisses which are migmatized over large areas. This migmatization locally led to anatexis and produced crustal-melt granites such as the Otjitanda Granite. We have undertaken reconnaissance geochemical studies and single zircon U–Pb SHRIMP and Pb–Pb evaporation dating of rocks of the Epupa Complex. The granitoid gneisses, migmatites and anatectic melts are similar in composition and constitute a suite of metaluminous to peraluminous, calc-alkaline granitoids, predominantly with volcanic arc geochemical signatures. The zircon protolith ages for the orthogneisses range from 1861 ± 3 to 1758 ± 3 Ma. Anatexis in the migmatitic Epupa gneisses was dated from a melt patch at 1762 ± 4 Ma, and the anatectic Otjitanda Granite has a zircon age of 1757 ± 4 Ma. Migmatization and anatexis therefore occurred almost immediately after granitoid emplacement and date a widespread high-temperature Palaeoproterozoic event at ∼1760 Ma which has not been recorded elswhere in northern Namibia. The Nd isotopic systematics of all dated samples are surprisingly similar and suggest formation of the protolith from a source region that probably separated from the depleted mantle about 2.4–2.0 Ga ago. A major Archaean component in the source area is unlikely.  相似文献