查看更多>>摘要：Dolomitization is controlled by numerous interacting factors, causing the mechanism of dolomite reservoir formation to remain unclear. In particular, the influences of lithological minerals and crystal lattice characteristics on dolomitization may be the main driver of large-scale dolomite development. In this study, physical laboratory experiments involving the dissolution, precipitation, and transformation of carbonate minerals were conducted, along with real-time testing and analysis of rock samples and water samples, to determine the mechanism underlying metasomatic dolomitization. Additionally, multiple two-dimensional and threedimensional homogeneous and heterogeneous numerical models were established. Changes in mineral content, ion concentration, and porosity were calculated using multiphase flow numerical simulation technology, and dolomitization was comparatively analyzed under various lithological conditions. The combination of physical experiments and numerical simulation effectively demonstrated the influence of dolomite crystal nucleus and lattice defects on dolomite formation. The presence of a dolomite crystal nucleus can promote the dissolution of calcite and allow more Mg to enter the calcite crystal lattice, promoting further dolomitization. Dolomitization occurs preferentially at lattice defects, and calcite grain boundaries break easily. The results of this study verify that surface conditions with low temperature and pressure are unsuitable for the development of dolomite, but microbes may produce small dolomite nuclei, leading to the large-scale development of dolomite. In the deep layer, because of defects in the crystal lattice, numerous pores form through dissolution. Because the lattice of deep dolomite is less stable than the lattice of limestone, dolomite has better physical properties than limestone after dissolution.
查看更多>>摘要：The geochemistry of oceanic intraplate (primarily oceanic island and seamount) lavas can provide essential information on the composition and evolution of their mantle source and geodynamics. Due to very limited rock sampling of the many mid-Cretaceous Wake seamounts in NW Pacific, the mantle source lithology and petrogenesis of their lavas, and the geodynamic mechanism responsible for generating the lavas have not been fully delineated. In order to help resolve these issues, here we present whole-rock major-trace element and Sr-Nd-Pb- Hf isotopic data for twenty-one lava samples collected from Lamont, Dacheng, Xufu, Penglai, Niulang, Zhinyu, and Zhanlu seamounts in the Southern Wake seamount trail (WST). These lava samples are silica-undersaturated alkali basalt and basanites/nephelinite. They have high CaO, FeOT and TiO_2 contents and CaO/Al_2O_3 ratios, consistent with their derivation from partial melting of a carbonated peridotite or reaction between carbonated MORB-eclogite-derived silicate melts and fertile peridotite. High Zr/Hf and negative Zr-Hf-Ti anomalies in the most mafic lavas further suggest a contribution from carbonated components in their mantle source. These lavas show FOZO (focal zone)-HIMU (high μ = ~(238)U/~(204)Pb)-like Sr-Nd-Pb-Hf isotopic compositions (e.g., (~(206)Pb/~(204)Pb)i = 19.36-20.72), falling within the Arago (also known as "Young Rurutu" or "Atiu") hotspot field. Combined with the sparse previous age and geochemical data, we propose that WST lavas were most likely derived from partial melting of secondary plume clusters emanating from the top of Arago mantle plume trapped at the mantle transition zone. Alternatively, WST lavas could have come from a number of secondary plumelets emanating from the top of the Pacific Large Low Shear Velocity Province (LLSVP). The simultaneously upwelling secondary plumes or plumelets generated chronologically overlapping, compositionally similar and closelyspaced Wake seamounts atop the moving Pa
查看更多>>摘要：Jarosite and other metal sulphates are common on Mars and are interpreted to have formed in an extreme acidic saline aqueous environment, similar to modern Earth analogues where jarosite precipitates. We examined an alternative Earth analogue, at the Smoking Hills (known as Ingniryuat by Inuvialuit), Northwest Territories, Arctic Canada. The Smoking Hills are characterised by auto-combusting pyritic mudstones of the Smoking Hills Formation in a polar desert. The Smoking Hills Formation was deposited in an outer shelf to slope marine environment, during a Late Cretaceous Ocean Anoxic Event. Oxidative weathering of this unit creates extensive jarosite-rich deposits, and banded jarosite- and phyllosilicate-rich mudstones, similar to those observed on Mars. Slumping of these mudstones exposes large masses to atmospheric oxygen leading to generation of high temperatures (sufficient to produce paralavas) through pyrite oxidation, and the subsequent formation of a diverse suite of hydrated metal-sulphate minerals. Weathered combustion sites are characterised by a simpler jarositerich mineralogy along with thermally-altered mudstone. Away from sites of auto-combustion (bocannes), pyrite-rich layers within the mudstone oxidise to jarosite, creating a yellow-banded appearance in outcrops. These jarosite-rich layers, similar to those observed interbedded in mudstones on Mars, reflect post depositional oxidation processes - not acidic conditions during the time of deposition. As such, this could reflect a more habitable environment than is commonly interpreted for Mars.
查看更多>>摘要：The relationship between Δ~(36)S and Δ~(33)S in Archean sedimentary pyrites has been used to evaluate early geologic processes, including photochemical reactions in the anoxic atmosphere, biological activity and thermochemical alteration during sediment deposition. We have applied statistical methods to quadruple S isotope analyses of Archean sedimentary pyrites, using data compiled from the literature. Most of the best-fit lines, on plots of Δ~(36)S against Δ~(33)S, have Archean reference array-like Δ~(36)S/Δ~(33)S slopes that vary between - 1.5 and - 0.9. Rigorous statistical tests were conducted to calculate the probability of the best-fit lines passing through the origin. Seventeen of 23 Δ~(36)S-Δ~(33)S regression lines, which pass our reliability filter of R~2 ≥ 75% and Δ~(33)S range ≥ 2‰, have positive intercepts on the Δ~(36)S axis, and 13 of these have a probability of < 5% of a zero intercept on the Δ~(36)S axis. The observed Δ~(36)S/Δ~(33)S slopes and the non-negative intercepts, which requires at least two massindependent fractionation source reactions to operate simultaneously, can be produced by UV radiation in the atmosphere at low SO_2 partial pressures by combining collision-induced intersystem crossing in the SO_2 photoexcitation band (240-340 nm), with the self-shielding effect in the SO_2 photolysis band (190-220 nm). The two SO_2 photochemical processes must occur simultaneously in a single atmospheric reservoir in order that the fraction contributed by the end-member process remains constant across the full range of Δ~(33)S values. We call this process simultaneous fractionation. We applied a two-end-member model to calculate the fraction of S contributed by the SO_2 photoexcitation end-member (f) needed to produce the observed Δ~(36)S/Δ~(33)S gradients and variable intercepts on the Δ~(36)S axis in the Archean sedimentary pyrites, when the other end-member is SO_2 photolysis with the self-shielding. The simplest explanation
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