查看更多>>摘要：The search for traces of life is one of the principal objectives of Mars exploration. The global distribution of minerals associated with serpentinization in the Noachian terrains on Mars likely suggest that serpentinization commonly occurred early in the planets history. A consensus is that these reactions involved in the serpentinization may have provided the initial material and energy for the origin and early evolution of life on Earth. Comparably, the serpentine regions of Mars are potential target sites for the detection of the putative life on Mars. Herein, an actinobacterial species Streptomyces spp. FXJ8.102 is used as a model microbe to investigate the microbe-mediated dissolution of antigorite in liquid culture. The experimental results show that the strain can couple biomechanical and biochemical mechanisms in the serpentine dissolution, leading to the development of "cell-sized" and "cell-shaped" eroded pits/channels on the mineral surfaces, and the low Mg/Si ratios on the eroded sites. In addition, the cells can also induce the formation of secondary glushinskite during the dissolution process. By contrast, no distinguishable surface change on antigorite and secondary mineral formation can be observed in the control experiments without inoculation. Therefore, a collection of the features including the distinctive eroded texture and composition on serpentine surfaces, and the special mineral assemblage including the formation of secondary oxalate minerals may serve as a compelling biosignature for the search of life on Mars.
查看更多>>摘要：Lipid compound-specific hydrogen isotopes (delta D) have been widely applied in studies of biogeochemistry and paleoclimatology. delta D values of lipid homologues with different chain lengths from a single analysis are often utilized together in order to make full use of the delta D information. However, this approach could be potentially impacted by the large analytical error due to the inappropriate amount injected for some homologues. Here, we systematically investigate compound-specific delta D deviations with varying contents injected on the isotope ratio mass spectrometry, expressed as amplitudes, using both lab working standards and natural samples. The n-alkane delta D values of lab working standards (mixed C-21, C-25, C-27, C-29, C-31, and C-33) could vary by 40 parts per thousand to 70 parts per thousand when the amplitudes of n-alkanes change from 0.5 V to 10 V. For natural samples (27 n-alkane and 77 fatty acid samples), we have made repeated analyses of the same samples with different homologues targeted for the optimum range. The measured delta D values are higher by 20 parts per thousand to 40 parts per thousand with the amplitudes lower than the optimum range, and lower by 10 parts per thousand to 20 parts per thousand with higher amplitudes. All the results consistently show higher delta D values with decreasing amplitudes, and larger deviations occurring in low amplitude range, implying that special caution should be taken with the delta D values measured at low amplitude range. We have attempted amplitude-based correction of lipid delta D values, however, this approach should be cautious owing to their large residual errors. Hence, delta D values of different homologues in the same samples have to be measured in separate analysis if they could not fall within the optimum range at the same time.
Apen, Francisco E.Wall, Corey J.Cottle, John M.Schmitz, Mark D....
16页
查看更多>>摘要：In situ apatite U-Pb petrochronology and Sr-Nd isotope geochemistry requires well-characterized and matrix-matched references materials (RMs), yet only a few suitable apatite RMs are currently available. To ameliorate this issue, we determined the U-Pb, Sm-Nd, and Sr isotopic and elemental compositions of a suite of prospective apatite RMs using isotope dilution (ID) TIMS and laser ablation (LA) ICP-MS. The two RMs, from Morocco (MRC-1) and Brazil (BRZ-1), are cm-sized and available in significant quantities. The U-Pb ID-TIMS data yield an isochron age of 153.3 +/- 0.2 Ma for MRC-1. This age is consistent with laser ablation split stream ICP-MS (LASS) analyses that produce an isochron age of 152.7 +/- 0.6 Ma. The weighted mean of ID-TIMS analyses for Nd-143/Nd-144 analyses is 0.512677 +/- 3, for Sm-147/Nd-144 is 0.10923 +/- 9, and for Sr-87/Sr-86 is 0.707691 +/- 2. The range and mean of TIMS Sm-Nd isotopic data are reproducible by LA-ICP-MS, but laser ablation Sr data are consistently offset towards more radiogenic values. For BRZ-1 apatite, ID-TIMS U-Pb analyses are dispersed, but a subset of the data yields a coherent age intercept of 2078 +/- 13 Ma. The vast majority of LASS spot transects across the apatite produce an isochron that define a younger age of 2038 +/- 14 Ma. We interpret this as incorporation of cryptic, younger altered domains within BRZ-1. Discordant U-Pb spot analyses are associated with chemically distinct cracks, likely a result of fluid infiltration. The weighted means of ID-TIMS analyses of BRZ-1 yield Nd-143/Nd-144 = 0.510989 +/- 5, Sm-147/Nd-144 = 0.10152 +/- 8, and Sr-87/Sr-86 = 0.709188 +/- 3. The distribution of Nd isotopic compositions of this RM measured by LA-MC-ICP-MS analyses are comparable to TIMS analyses. By contrast, Sr-87/Sr-86 measurements by LA-ICP-MS are inaccurate and exhibit large uncertainties, but this RM can be useful for empirically correcting in situ Sr-87/Sr-86 measurements. The data indicate that MRC-1 apatite may serve well as a U-Pb, Sm-Nd, and Sr RM, whereas BRZ-1 apatite has the most potential as a Sm-Nd RM. These potential RMs provide new benchmarks for in situ apatite chemical analyses and inter-laboratory calibrations.
Strong, J. W. D.Cawood, P. A.Cruden, A. R.Nebel, O....
12页
查看更多>>摘要：The Winnipeg River terrane is one of three plutonic-gneiss terranes that contains the oldest rocks in the Archean Superior Province and is integral to understanding the evolution of Earth's largest Archean craton. We evaluate the evolution of the Winnipeg River terrane using whole-rock Sr-Nd-Pb isotope data for a suite of 17 samples of the Cedar Lake gneiss, along with U-Pb-Lu-Hf isotopes and trace elements in zircon grains from one sample. Most whole-rock Sr-Nd-Pb isotope data trend along 3.25 Ga reference isochrons, which overlap with the dominant population of ca. 3.25 Ga zircon, and are interpreted to mark the igneous crystallization of the pmtolith to the Cedar Lake gneiss. An older population of zircon aged ca. 3.5-3.4 Ga indicate either Hadean or Eoarchean crust was reworked at ca. 3.25 Ga. At ca. 2.7 Ga, new zircon grew with limited dissolution of older grains resulting in near-chondritic time-integrated Hf isotope signatures and low Th/U ratios relative to the older grains. As a result of the overlap between whole-rock reference isochrons at 3.25 Ga and the dominant zircon population ca. 3.25 Ga, the radiogenic Hf isotope signature of the ca. 2.7 Ga zircon grains imply Hf was recycled from non-zircon constituents within the rock rather than from external input. Additional support is drawn from elevated Nb and Ta concentrations in ca. 2.7 Ga zircon, hornblende and biotite. Our data highlights a complexity in the Lu-Hf isotope systematics of zircon, where metamorphic zircon inherited radiogenic Hf isotope compositions as a result of internal redistribution of Hf from non-zircon phases in the rock rather than juvenile input.
查看更多>>摘要：Cosmogenic noble gas isotope Ne-21 in terrestrial rocks has been used primarily to determine timing and rates of the Earth's surface evolution. Here we explore the ability of detrital Ne-21 as a provenance tracer, considering that Ne isotopes produced in source rocks could be preserved in minerals over geological time and might be predominant in total Ne inventory of sediments sunk in basins. This ability is predicated on potential source terranes of a given stratigraphic section with distinct neon isotopic signatures. Here we analyze neon isotopes of a well-dated Miocene-Pleistocene sedimentary archives in Kuqa foreland basin of southern Tianshan. The data suggest that the neon isotopic signature, which is expressed as c m and defined in this work as the excess Ne-21/Ne-20-ratio relative to atmospheric ratio, is stratigraphically sensitive to changes in local source terranes. This result is compatible with U/Pb provenance analysis and also supported by evidences from sandstone petrography and heavy mineral analysis. Influence of other non-source related Ne-21 components in sedimentary archives on sensitivity of epsilon(Ne) has proven to be negligible. Furthermore, the integrated stratigraphic signatures of neon isotope and U/Pb age permit the detection of differential erosion in drainage basin, by which the tectonic or climatic effects on geomorphic evolution could be deciphered.
查看更多>>摘要：Calcium is a major element in terrestrial planets and has six stable isotopes, including Ca-40 which is a product of radioactive decay of K-40 (T-1/2 = 1.25 Gyrs). Calcium isotope geochemistry has therefore been used to trace biological and geological processes in various environments as well as radiogenic dating. However, it has been technically impossible to analyze stable and radiogenic isotopes simultaneously and high precision measurements usually require tens of micrograms of Ca. Here, we present high-precision Ca isotopic data, including the stable isotopes Ca-42, Ca-43 and Ca-44 as well as the radiogenic Ca-40 using a Nu Sapphire multi-collector inductively-coupled-plasma mass-spectrometer (MC-ICP-MS), that is equipped with a collision cell to minimize Ar based interferences. The stable isotopic variations are reported using the delta notation (delta Ca-40/44, delta(42/44)ca, delta Ca-43/44), which is the permit deviation of a given ratio from the NIST SRM 915b standard. The radiogenic excess is reported as epsilon Ca-40, the per ten thousand deviation of the Ca-40/Ca-44 ratio from the same standard, after internal normalization to the Ca-42/Ca-44 ratio. We tested the influence of various parameters (Ca concentration, Ca and HNO3 concentration mismatch between sample and standard, effect of matrix elements) on the precision and accuracy of the measurements. We show that, using similar to 100 ng is sufficient to obtain high precision (<100 ppm, 25D) delta Ca-40/44 and epsilon Ca-40 using a standard bracketing technique-this represents an improvement by over an order of magnitude in sensitivity compared to any previous methods. It should be noted that a limitation is that the intensities of the standard and sample must be matched very closely (better than 2%). Following this recommendation, the Ca isotopic compositions of 9 geological samples reported here is consistent with previous data. For example, the epsilon Ca-40 value of GSP-2 (3.92 +/- 0.48 parts per thousand) is within error with literature data (values between 3.69 and 4.32 parts per thousand). Using this method, it is therefore possible, for the first time, to obtain simultaneously high-precision radiogenic and stable Ca isotopes on small samples, which will certainly open up many novel applications of Ca stable isotopes.
查看更多>>摘要：It is not currently well understood how a changing climate controls methane releases in the Okinawa Trough. In this study, a piston core was collected from a seep-impacted area at a water depth of approximately 1000 m from the Okinawa Trough, East China Sea, and was studied using comprehensive analyses of carbon and oxygen stable isotopes, lipid biomarkers, and major and trace element geochemistry. Extremely low delta C-13 values of total inorganic carbon and organic carbon, increased Methane Index, and molybdenum (Mo) and uranium (U) enrichments at depths of 225-255 cm below the sea-floor (bsf) and 75-142.5 cm bsf were found. Combined with published pore water data, fossil and current sulfate-methane transitions (SMTs) were identified. Mass balance equations were applied to estimate the fraction and content of authigenic carbonates at the two SMTs and their corresponding delta O-18 values. The calculations revealed that the estimated delta O-18 values of authigenic carbonates for the fossil SMT (from 4.9%o to 5.8%o) and the current SMT (from -2.1%o to 4.4%o) were higher and lower than the theoretical equilibrium delta O-18 value (4.8%.), respectively. These results suggest that fossil seepage is possibly induced by the dissociation of gas hydrates. The fluids of the current seepage may be derived from the equilibrium between the ambient seawater and the fluids of gas hydrate dissociation. Based on the accumulation of authigenic carbonate in sediments, previously published pore-water calcium and magnesium fluxes, and foraminifera C-14 dating, the fossil methane seepage was confirmed to have occurred during the period 8.2 to 4.5 ka B. P., with its SMT depth less than 105 cm bsf, while the present-day methane seepage with a low fluid intensity was estimated to have started before 1.0 ka and formed its SMT at approximately 250 cm bsf. The timing of this fossil methane seepage suggests that the rising temperature of the North Pacific Intermediate Water during the early and middle Holocene mainly controlled the methane release in the Okinawa Trough, whereas the ongoing methane emission was likely induced by decreased pressure driven by back-arc extension. The geochemical data demonstrate past methane release as a response to environmental changes and suggest that the Okinawa Trough gas hydrates were sensitive to temperature fluctuations during the middle Holocene.
查看更多>>摘要：Zebra rock, found in the eastern Kimberley region of northern Western Australia, is a Late Proterozoic sedimentary rock with a rhythmic Liesegang-type Fe-oxide banding. The striped rhythmic pattern in sedimentary rocks is an important key to infer chemical conditions of water-rock reactions. Although past studies have discussed the zebra rock formation for decades, the process remains unclear. Here, we introduce a new formation model, suggesting that zebra rock formed in an acidic hydrothermal system and that pH buffering of Fe2+ bearing acidic fluid, in a neutralization reaction with primary carbonate minerals, induced rhythmic Fe-precipitation. The Fe profile clearly shows a reaction front, indicating unidirectional diffusive fluid migration along bedding planes. The two types of zebra rock distinguished by color and by clay mineral assemblages correspond with typical acidic hydrothermal alteration zonation. Although a specific volcanic event related to the hydrothermal activity cannot be identified presently, this study provides a new model for the zebra rock, recording both hydrothermal alteration and post-Late Proterozoic fluid migration.
NSTL
Elsevier