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1.
Oxygen and iron isotope analyses of low-Ti and high-Ti mare basalts are presented to constrain their petrogenesis and to assess stable isotope variations within lunar mantle sources. An internally-consistent dataset of oxygen isotope compositions of mare basalts encompasses five types of low-Ti basalts from the Apollo 12 and 15 missions and eight types of high-Ti basalts from the Apollo 11 and 17 missions. High-precision whole-rock δ18O values (referenced to VSMOW) of low-Ti and high-Ti basalts correlate with major-element compositions (Mg#, TiO2, Al2O3). The observed oxygen isotope variations within low-Ti and high-Ti basalts are consistent with crystal fractionation and match the results of mass-balance models assuming equilibrium crystallization. Whole-rock δ56Fe values (referenced to IRMM-014) of high-Ti and low-Ti basalts range from 0.134‰ to 0.217‰ and 0.038‰ to 0.104‰, respectively. Iron isotope compositions of both low-Ti and high-Ti basalts do not correlate with indices of crystal fractionation, possibly owing to small mineral-melt iron fractionation factors anticipated under lunar reducing conditions.The δ18O and δ56Fe values of low-Ti and the least differentiated high-Ti mare basalts are negatively correlated, which reflects their different mantle source characteristics (e.g., the presence or absence of ilmenite). The average δ56Fe values of low-Ti basalts (0.073 ± 0.018‰, n = 8) and high-Ti basalts (0.191 ± 0.020‰, n = 7) may directly record that of their parent mantle sources. Oxygen isotope compositions of mantle sources of low-Ti and high-Ti basalts are calculated using existing models of lunar magma ocean crystallization and mixing, the estimated equilibrium mantle olivine δ18O value, and equilibrium oxygen-fractionation between olivine and other mineral phases. The differences between the calculated whole-rock δ18O values for source regions, 5.57‰ for low-Ti and 5.30‰ for high-Ti mare basalt mantle source regions, are solely a function of the assumed source mineralogy. The oxygen and iron isotope compositions of lunar upper mantle can be approximated using these mantle source values. The δ18O and δ56Fe values of the lunar upper mantle are estimated to be 5.5 ± 0.2‰ (2σ) and 0.085 ± 0.040‰ (2σ), respectively. The oxygen isotope composition of lunar upper mantle is identical to the current estimate of Earth’s upper mantle (5.5 ± 0.2‰), and the iron isotope composition of the lunar upper mantle overlaps within uncertainty of estimates for the terrestrial upper mantle (0.044 ± 0.030‰).  相似文献   

2.
In addition to equilibrium isotopic fractionation factors experimentally derived, theoretical predictions are needed for interpreting isotopic compositions measured on natural samples because they allow exploring more easily a broader range of temperature and composition. For iron isotopes, only aqueous species were studied by first-principles methods and the combination of these data with those obtained by different methods for minerals leads to discrepancies between theoretical and experimental isotopic fractionation factors. In this paper, equilibrium iron isotope fractionation factors for the common minerals pyrite, hematite, and siderite were determined as a function of temperature, using first-principles methods based on the density functional theory (DFT). In these minerals belonging to the sulfide, oxide and carbonate class, iron is present under two different oxidation states and is involved in contrasted types of interatomic bonds. Equilibrium fractionation factors calculated between hematite and siderite compare well with the one estimated from experimental data (ln α57Fe/54Fe = 4.59 ± 0.30‰ and 5.46 ± 0.63‰ at 20 °C for theoretical and experimental data, respectively) while those for Fe(III)aq-hematite and Fe(II)aq-siderite are significantly higher that experimental values. This suggests that the absolute values of the reduced partition functions (β-factors) of aqueous species are not accurate enough to be combined with those calculated for minerals. When compared to previous predictions derived from Mössbauer or INRXS data [Polyakov V. B., Clayton R. N., Horita J. and Mineev S. D. (2007) Equilibrium iron isotope fractionation factors of minerals: reevaluation from the data of nuclear inelastic resonant X-ray scattering and Mössbauer spectroscopy. Geochim. Cosmochim. Acta71, 3833-3846], our iron β-factors are in good agreement for siderite and hematite while a discrepancy is observed for pyrite. However, the detailed investigation of the structural, electronic and vibrational properties of pyrite as well as the study of sulfur isotope fractionation between pyrite and two other sulfides (sphalerite and galena) indicate that DFT-derived β-factors of pyrite are as accurate as for hematite and siderite. We thus suggest that experimental vibrational density of states of pyrite should be re-examined.  相似文献   

3.
Although iron isotopes provide a new powerful tool for tracing a variety of geochemical processes, the unambiguous interpretation of iron isotope ratios in natural systems and the development of predictive theoretical models require accurate data on equilibrium isotope fractionation between fluids and minerals. We investigated Fe isotope fractionation between hematite (Fe2O3) and aqueous acidic NaCl fluids via hematite dissolution and precipitation experiments at temperatures from 200 to 450 °C and pressures from saturated vapor pressure (Psat) to 600 bar. Precipitation experiments at 200 °C and Psat from aqueous solution, in which Fe aqueous speciation is dominated by ferric iron (FeIII) chloride complexes, show no detectable Fe isotope fractionation between hematite and fluid, Δ57Fefluid-hematite = δ57Fefluid − δ57Fehematite = 0.01 ± 0.08‰ (2 × standard error, 2SE). In contrast, experiments at 300 °C and Psat, where ferrous iron chloride species (FeCl2 and FeCl+) dominate in the fluid, yield significant fluid enrichment in the light isotope, with identical values of Δ57Fefluid-hematite = −0.54 ± 0.15‰ (2SE) both for dissolution and precipitation runs. Hematite dissolution experiments at 450 °C and 600 bar, in which Fe speciation is also dominated by ferrous chloride species, yield Δ57Fefluid-hematite values close to zero within errors, 0.15 ± 0.17‰ (2SE). In most experiments, chemical, redox, and isotopic equilibrium was attained, as shown by constancy over time of total dissolved Fe concentrations, aqueous FeII and FeIII fractions, and Fe isotope ratios in solution, and identical Δ57Fe values from dissolution and precipitation runs. Our measured equilibrium Δ57Fefluid-hematite values at different temperatures, fluid compositions and iron redox state are within the range of fractionations in the system fluid-hematite estimated using reported theoretical β-factors for hematite and aqueous Fe species and the distribution of Fe aqueous complexes in solution. These theoretical predictions are however affected by large discrepancies among different studies, typically ±1‰ for the Δ57Fe Fe(aq)-hematite value at 200 °C. Our data may thus help to refine theoretical models for β-factors of aqueous iron species. This study provides the first experimental calibration of Fe isotope fractionation in the system hematite-saline aqueous fluid at elevated temperatures; it demonstrates the importance of redox control on Fe isotope fractionation at hydrothermal conditions.  相似文献   

4.
The δ18O of ground water (−13.54 ± 0.05 ‰) and inorganically precipitated Holocene vein calcite (+14.56 ± 0.03 ‰) from Devils Hole cave #2 in southcentral Nevada yield an oxygen isotopic fractionation factor between calcite and water at 33.7 °C of 1.02849 ± 0.00013 (1000 ln αcalcite-water = 28.09 ± 0.13). Using the commonly accepted value of ∂(αcalcite-water)/∂T of −0.00020 K−1, this corresponds to a 1000 ln αcalcite-water value at 25 °C of 29.80, which differs substantially from the current accepted value of 28.3. Use of previously published oxygen isotopic fractionation factors would yield a calcite precipitation temperature in Devils Hole that is 8 °C lower than the measured ground water temperature. Alternatively, previously published fractionation factors would yield a δ18O of water, from which the calcite precipitated, that is too negative by 1.5 ‰ using a temperature of 33.7 °C. Several lines of evidence indicate that the geochemical environment of Devils Hole has been remarkably constant for at least 10 ka. Accordingly, a re-evaluation of calcite-water oxygen isotopic fractionation factor may be in order.Assuming the Devils Hole oxygen isotopic value of αcalcite-water represents thermodynamic equilibrium, many marine carbonates are precipitated with a δ18O value that is too low, apparently due to a kinetic isotopic fractionation that preferentially enriches 16O in the solid carbonate over 18O, feigning oxygen isotopic equilibrium.  相似文献   

5.
Iron isotopes fractionate during hydrothermal processes. Therefore, the Fe isotope composition of ore-forming minerals characterizes either iron sources or fluid histories. The former potentially serves to distinguish between sedimentary, magmatic or metamorphic iron sources, and the latter allows the reconstruction of precipitation and redox processes. These processes take place during ore formation or alteration. The aim of this contribution is to investigate the suitability of this new isotope method as a probe of ore-related processes. For this purpose 51 samples of iron ores and iron mineral separates from the Schwarzwald region, southwest Germany, were analyzed for their iron isotope composition using multicollector ICP-MS. Further, the ore-forming and ore-altering processes were quantitatively modeled using reaction path calculations. The Schwarzwald mining district hosts mineralizations that formed discontinuously over almost 300 Ma of hydrothermal activity. Primary hematite, siderite and sulfides formed from mixing of meteoric fluids with deeper crustal brines. Later, these minerals were partly dissolved and oxidized, and secondary hematite, goethite and iron arsenates were precipitated. Two types of alteration products formed: (1) primary and high-temperature secondary Fe minerals formed between 120 and 300 °C, and (2) low-temperature secondary Fe minerals formed under supergene conditions (<100 °C). Measured iron isotope compositions are variable and cover a range in δ56Fe between −2.3‰ and +1.3‰. Primary hematite (δ56Fe: −0.5‰ to +0.5‰) precipitated by mixing oxidizing surface waters with a hydrothermal fluid that contained moderately light Fe (δ56Fe: −0.5‰) leached from the crystalline basement. Occasional input of CO2-rich waters resulted in precipitation of isotopically light siderite (δ56Fe: −1.4 to −0.7‰). The difference between hematite and siderite is compatible with published Fe isotope fractionation factors. The observed range in isotopic compositions can be accounted for by variable fractions of Fe precipitating from the fluid. Therefore, both fluid processes and mass balance can be inferred from Fe isotopes. Supergene weathering of siderite by oxidizing surface waters led to replacement of isotopically light primary siderite by similarly light secondary hematite and goethite, respectively. Because this replacement entails quantitative transfer of iron from precursor mineral to product, no significant isotope fractionation is produced. Hence, Fe isotopes potentially serve to identify precursors in ore alteration products. Goethites from oolitic sedimentary iron ores were also analyzed. Their compositional range appears to indicate oxidative precipitation from relatively uniform Fe dissolved in coastal water. This comprehensive iron isotope study illustrates the potential of the new technique in deciphering ore formation and alteration processes. Isotope ratios are strongly dependent on and highly characteristic of fluid and precipitation histories. Therefore, they are less suitable to provide information on Fe sources. However, it will be possible to unravel the physico-chemical processes leading to the formation, dissolution and redeposition of ores in great detail.  相似文献   

6.
Bacterial sulfate reduction is one of the most important respiration processes in anoxic habitats and is often assessed by analyzing the results of stable isotope fractionation. However, stable isotope fractionation is supposed to be influenced by the reduction rate and other parameters, such as temperature. We studied here the mechanistic basics of observed differences in stable isotope fractionation during bacterial sulfate reduction. Batch experiments with four sulfate-reducing strains (Desulfovibrio desulfuricans, Desulfobacca acetoxidans, Desulfonatronovibrio hydrogenovorans, and strain TRM1) were performed. These microorganisms metabolize different carbon sources (lactate, acetate, formate, and toluene) and showed broad variations in their sulfur isotope enrichment factors. We performed a series of experiments on isotope exchange of 18O between residual sulfate and ambient water. Batch experiments were conducted with 18O-enriched (δ18Owater = +700‰) and depleted water (δ18Owater = −40‰), respectively, and the stable 18O isotope shift in the residual sulfate was followed. For Desulfovibrio desulfuricans and Desulfonatronovibrio hydrogenovorans, which are both characterized by low sulfur isotope fractionation (εS > −13.2‰), δ18O values in the remaining sulfate increased by only 50‰ during growth when 18O-enriched water was used for the growth medium. In contrast, with Desulfobacca acetoxidans and strain TRM1 (εS < −22.7‰) the residual sulfate showed an increase of the sulfate δ18O close to the values of the enriched water of +700‰. In the experiments with δ18O-depleted water, the oxygen isotope values in the residual sulfate stayed fairly constant for strains Desulfovibrio desulfuricans, Desulfobacca acetoxidans and Desulfonatronovibrio hydrogenovorans. However, strain TRM1, which exhibits the lowest sulfur isotope fractionation factor (εS < −38.7‰) showed slightly decreasing δ18O values.Our results give strong evidence that the oxygen atoms of sulfate exchange with water during sulfate reduction. However, this neither takes place in the sulfate itself nor during formation of APS (adenosine-5′-phosphosulfate), but rather in intermediates of the sulfate reduction pathway. These may in turn be partially reoxidized to form sulfate. This reoxidation leads to an incorporation of oxygen from water into the “recycled” sulfate changing the overall 18O isotopic composition of the remaining sulfate fraction. Our study shows that such incorporation of 18O is correlated with the stable isotope enrichment factor for sulfur measured during sulfate reduction. The reoxidation of intermediates of the sulfate reduction pathway does also strongly influence the sulfur stable isotope enrichment factor. This aforesaid reoxidation is probably dependent on the metabolic conversion of the substrate and therefore also influences the stable isotope fractionation factor indirectly in a rate dependent manner. However, this effect is only indirect. The sulfur isotope enrichment factors for the kinetic reactions themselves are probably not rate dependent.  相似文献   

7.
There is considerable debate about the mode and age of formation of large (up to ∼200 m long) hematite and goethite ironstone bodies within the 3.2 to 3.5 Ga Barberton greenstone belt. We examined oxygen and hydrogen isotopes and Rare Earth Element (REE) concentrations of goethite and hematite components of the ironstones to determine whether these deposits reflect formation from sea-floor vents in the Archean ocean or from recent surface and shallow subsurface spring systems. Goethite δ18O values range from −0.7 to +1.0‰ and δD from −125 to −146‰, which is consistent with formation from modern meteoric waters at 20 to 25 °C. Hematite δ18O values range from −0.7 to −2.0‰, which is consistent with formation at low to moderate temperatures (40-55 °C) from modern meteoric water. REE in the goethite and hematite are derived from the weathering of local sideritic ironstones, silicified ultramafic rocks, sideritic black cherts, and local felsic volcanic rocks, falling along a mixing line between the Eu/Eu* and shale-normalized HREEAvg/LREEAvg values for the associated silicified ultramafic rocks and felsic volcanic rocks. Contrasting positive Ce/Ce* of 1.3 to 3.5 in hematite and negative Ce/Ce* of 0.2 to 0.9 in goethite provides evidence of oxidative scavenging of Ce on hematite surfaces during mineral precipitation. These isotopic and REE data, taken together, suggest that hematite and goethite ironstone pods formed from relatively recent meteoric waters in shallow springs and/or subsurface warm springs.  相似文献   

8.
Computer modelling techniques were used to elucidate the hydration behaviour of three iron (hydr)oxide minerals at the atomic level: white rust, goethite and hematite. A potential model was first adapted and tested against the bulk structures and properties of eight different iron oxides, oxyhydroxides and hydroxides, followed by surface simulations of Fe(OH)2, α-FeO(OH) and α-Fe2O3. The major interaction between the adsorbing water molecules and the surface is through interaction of their oxygen ions with surface iron ions, followed by hydrogen-bonding to surface oxygen ions. The energies released upon the associative adsorption of water range from 1 to 17 kJ mol−1 for Fe(OH)2, 26 to 80 kJ mol−1 for goethite and 40 to 85 kJ mol−1 for hematite, reflecting the increasing oxidation of the iron mineral. Dissociative adsorption at goethite and hematite surfaces releases larger hydration energies, ranging from 120 to 208 kJ mol−1 for goethite and 76 to 190 kJ mol−1 for hematite.The thermodynamic morphologies of the minerals, based on the calculated surface energies, agree well with experimental morphologies, where these are available. When the partial pressures required for adsorption of water from the gas phase are plotted against temperature for the goethite and hematite surfaces, taking into account experimental entropies for water, it appears that these minerals may well be instrumental in the retention of water during the cyclic variations in the atmosphere of Mars.  相似文献   

9.
Evaporite outcrops are rare in the Basque Cantabrian basin due to a rainy climate, but saline springs with total dissolved solids ranging from 0.8 to 260 g/L are common and have long been used to supply spas and salterns. New and existing hydrochemistry of saline springs are used to provide additional insight on the origin and underground extent of their poorly known source evaporites. Saline water hydrochemistry is related to dissolution of halite and gypsum from two evaporitic successions (Triassic “Keuper” and Lower Cretaceous “Wealden”), as supported by rock samples from outcrops and oil exploration drill cuttings. The δ34S value of gypsum in the Keuper evaporites and sulfate in the springs is δ34SSO4 = 14.06 ± 1.07‰ and δ18OSO4 = 13.41 ± 1.44‰, and the relationship between Cl/Br ratio of halite and water shows that waters have dissolved halite with Br content between 124 and 288 ppm. The δ34S value of gypsum in the Wealden evaporites and sulfate in the springs is δ34SSO4 = 19.66 ± 1.76‰, δ18OSO4 = 14.93 ± 2.35‰, and the relationship between Cl/Br ratio of halite and water shows that waters have dissolved halite with Br content between 15 and 160 ppm. Wealden evaporites formed in a continental setting after the dissolution of Keuper salt. Gypsum δ34SSO4 and δ18OSO4 modification from Keuper to Wealden evaporites was due mainly to bacterial SO4 reduction in an anoxic, organic matter-rich environment. Saline springs with Wealden δ34SSO4 values are present in a 70 × 20 km wide area. Saline water temperatures, their δ2HH2O and δ18OH2O values, and the geological structure defines a hydrogeological model, where meteoric water recharges at heights up to 620 m above spring levels and circulates down to 720 m below them, thereby constraining the height range of evaporite dissolution. Groundwater flow towards saline springs is driven by gravity and buoyancy forces constrained by a thrust and fault network.  相似文献   

10.
Oxidation of pyrite by hydrogen peroxide (H2O2) at millimolar levels has been studied from 4 to 150 °C in order to evaluate isotopic effects potentially associated with radiolytic oxidation of pyrite. Gaseous, aqueous, and solid phases were collected and measured following sealed-tube experiments that lasted from 1 to 14 days. The dominant gaseous product was molecular oxygen. No volatile sulfur species were recovered from any experiment. Sulfate was the only aqueous sulfur species detected in solution, with sulfite and thiosulfate below the detection limits. X-ray diffraction patterns and images from scanning electron microscopy reveal solid residues composed primarily of hydrated ferric iron sulfates and sporadic ferric-ferrous iron sulfates. Hematite was detected only in solid residue produced during high temperature experiments. Elemental sulfur and/or polysulfides are inferred to be form on reacting pyrite surface based on extraction with organic solvents. Pyrite oxidation by H2O2 increases in rate with increasing H2O2concentration, pyrite surface area, and temperature. Rates measured in sealed-tube experiments at 25°C, for H2O2 concentration of 2 × 10−3 M are 8.8 × 10−9 M/m2/sec, which are higher than previous estimates. A combination of reactive oxygen species from H2O2 decomposition products and reactive iron species from pyrite dissolution is inferred to aggressively oxidize the receding pyrite surface. Competing oxidants with temperature-dependent oxidation efficiencies results in multiple reaction mechanisms for different temperatures and surface conditions. Sulfur isotope values of remaining pyrite were unchanged during the experiments, but showed distinct enrichment of 34S in produced sulfate and depletion in elemental sulfur. The Δsulfate-pyrite and Δelemental sulfur-pyrite was +0.5 to +1.5‰ and was −0.2 to −1‰, respectively. Isotope data from high-temperature experiments indicate an additional 34S-depleted sulfur fraction, with up to 4‰ depletion of 34S, in the hematite. Sulfur isotope trends were not influenced by H2O2 concentration, temperature, or reaction time. Results of this study indicate that radiolytically produced oxidants, such as hydrogen peroxide and hydroxyl radicals, could efficiently oxidize pyrite in an otherwise oxygen-limited environment. Although H2O2 is generally regarded as being of minor geochemical significance on Earth, the H2O2 molecule plays a pivotal role in Martian atmospheric and soil chemistry. Additional experimental and field studies are needed to characterize sulfur and oxygen isotope systematics during radiolytical oxidation of metallic sulfides and elemental sulfur.  相似文献   

11.
Variations in the oxygen isotope composition (δ18O) of five cherts from the 1.9 Ga Gunflint iron formation (Canada) were studied at the micrometer scale by ion microprobe to try to better understand the processes that control δ18O values in cherts and to improve seawater paleotemperature reconstructions. Gunflint cherts show clearly different δ18O values for different types of silica with for instance a difference of ≈15‰ between detrital quartz and microquartz. Microquartz in the five samples is characterized by large intra sample variations in δ18O values, (δ18O of quartz varies from 4.6‰ to 6.6‰ at the 20 μm scale and from ≈12‰ to 14‰ at 2 μm scale). Isotopic profiles in microquartz adjacent to hydrothermal quartz veins demonstrate that microquartz more than ≈200 μm away from the veins has preserved its original δ18O value.At the micrometer spatial resolution of the ion probe, data reveal that microquartz has preserved a considerable δ18O heterogeneity that must be regarded as a signature inherited from its diagenetic history. Modelling of the δ18O variations produced during the diagenetic transformation of sedimentary amorphous silica precursors into microquartz allows us to calculate seawater temperature (Tsw at which the amorphous silica precipitated) and diagenesis temperature (Tdiagenesis at which microquartz formed) that reproduce the δ18O distributions (mean, range and shape) measured at micrometer scale in microquartz. The two critical parameters in this modelling are the δ18O value and the mass fraction of the diagenetic fluid. Under these assumptions, the most likely ranges for Tsw and Tdiagenesis are from 37 to 52 °C and from 130 to 170 °C, respectively.  相似文献   

12.
Atmospheric carbon dioxide is widely studied using records of CO2 mixing ratio, δ13C and δ18O. However, the number and variability of sources and sinks prevents these alone from uniquely defining the budget. Carbon dioxide having a mass of 47 u (principally 13C18O16O) provides an additional constraint. In particular, the mass 47 anomaly (Δ47) can distinguish between CO2 produced by high temperature combustion processes vs. low temperature respiratory processes. Δ47 is defined as the abundance of mass 47 isotopologues in excess of that expected for a random distribution of isotopes, where random distribution means that the abundance of an isotopologue is the product of abundances of the isotopes it is composed of and is calculated based on the measured 13C and 18O values. In this study, we estimate the δ13C (vs. VPDB), δ18O (vs. VSMOW), δ47, and Δ47 values of CO2 from car exhaust and from human breath, by constructing ‘Keeling plots’ using samples that are mixtures of ambient air and CO2 from these sources. δ47 is defined as , where is the R47 value for a hypothetical CO2 whose δ13CVPDB = 0, δ18OVSMOW = 0, and Δ47 = 0. Ambient air in Pasadena, CA, where this study was conducted, varied in [CO2] from 383 to 404 μmol mol−1, in δ13C and δ18O from −9.2 to −10.2‰ and from 40.6 to 41.9‰, respectively, in δ47 from 32.5 to 33.9‰, and in Δ47 from 0.73 to 0.96‰. Air sampled at varying distances from a car exhaust pipe was enriched in a combustion source having a composition, as determined by a ‘Keeling plot’ intercept, of −24.4 ± 0.2‰ for δ13C (similar to the δ13C of local gasoline), δ18O of 29.9 ± 0.4‰, δ47 of 6.6 ± 0.6‰, and Δ47 of 0.41 ± 0.03‰. Both δ18O and Δ47 values of the car exhaust end-member are consistent with that expected for thermodynamic equilibrium at∼200 °C between CO2 and water generated by combustion of gasoline-air mixtures. Samples of CO2 from human breath were found to have δ13C and δ18O values broadly similar to those of car exhaust-air mixtures, −22.3 ± 0.2 and 34.3 ± 0.3‰, respectively, and δ47 of 13.4 ± 0.4‰. Δ47 in human breath was 0.76  ± 0.03‰, similar to that of ambient Pasadena air and higher than that of the car exhaust signature.  相似文献   

13.
The acidophilic iron-oxidizing bacterium, Acidithiobacillus ferrooxidans, plays a part in the pyrite oxidation process and has been widely studied in order to determine the kinetics of the reactions and the isotopic composition of dissolved product sulphates, but the details of the oxidation processes at the surface of pyrite are still poorly known. In this study, oxygen and sulphur isotopic compositions (δ18O and δ34S) were analyzed for dissolved sulphates and water from experimental aerobic acidic (pH < 2) pyrite oxidation by A. ferrooxidans. The oxidation products attached to the pyrite surfaces were studied for their morphology (SEM), their chemistry (Raman spectroscopy) and for their δ18O (ion microprobe). They were compared to abiotically (Fe3+, H2O2, O2) oxidized pyrite surface compounds in order to constrain the oxidation pathways and to look for the existence of potential biosignatures for this system.The pyrite dissolution evolved from non-stoichiometric (during the first days) to stoichiometric (with increasing time) resulting in dissolved sulphates having distinct δ18O (e.g. +11.0‰ and −2.0‰, respectively) and δ34S (+4.5‰ and +2.8‰, respectively) values. The “oxidation layer” at the surface of pyrite is complex and made of iron oxides, sulphate, polysulphide, elemental sulphur and polythionates. Bio- and Fe3+-oxidation favour the development of monophased micrometric bumps made of hematite or sulphate while other abiotic oxidation processes result in more variable oxidation products. The δ18O of these oxidation products at the surface of oxidized pyrites are strongly variable (from ≈−40‰ to ≈+30‰) for all experiments.Isotopic fractionation between sulphates and pyrite, Δ34SSO4-pyrite, is equal to −1.3‰ and +0.4‰ for sulphates formed by stoichiometric and non-stoichiometric processes, respectively. These two values likely reflect either a S-S or a Fe-S bond breaking process. The Δ18OSO4-H2O and Δ18OSO4-O2 are estimated to be ≈+16‰ and ≈−25‰, respectively. These values are higher than previously published data and may reflect biological effects. The large δ18O heterogeneity measured at the surfaces of oxidized pyrites, whatever the oxidant, may be related (i) to the existence of local surface environments isolated from the solution in which the oxidation processes are different and (ii) to the stabilization at the pyrite surface of reaction intermediates that are not in isotopic equilibrium with the solution. Though the oxygen isotopic composition of surface oxidation products cannot be taken as a direct biosignature, the combined morphological, chemical and isotopic characterization of the surfaces of oxidized pyrites may furnish clues about a biological activity on a mineral surface.  相似文献   

14.
Samples produced in piston cylinder experiments were used to document the thermal isotopic fractionation of all the major elements of basalt except for aluminum and the fractionation of iron isotopes by chemical diffusion between a natural basalt and rhyolite. The thermal isotopic fractionations are summarized in terms of a parameter Ωi defined as the fractionation in per mil per 100 °C per atomic mass units difference between the isotopes. For molten basalt we report ΩCa = 1.6, ΩFe = 1.1, ΩSi = 0.6, ΩO = 1.5. In an earlier paper we reported ΩMg = 3.6. These fractionations represent a steady state balance between thermal diffusion and chemical diffusion with the mass dependence of the thermal diffusion coefficient being significantly larger than the mass dependence of the chemical diffusion coefficients for isotopes of the same element. The iron isotopic measurements of the basalt-rhyolite diffusion couple showed significant fractionation that are parameterized in terms of a parameter βFe = 0.03 when the ratio of the diffusion coefficients D54 and D56 of 54Fe and 56Fe is expressed in terms of the atomic mass as D54/D56 = (56/54)βFe. This value of βFe is smaller than what we had measured earlier for lithium, magnesium and calcium (i.e., βLi = 0.215, βCa = 0.05, βMg = 0.05) but still significant when one takes into account the high precision with which iron isotopic compositions can be measured (i.e., ±0.03‰) and that iron isotope fractionations at magmatic temperatures from other causes are extremely small. In a closing section we discuss technological and geological applications of isotopic fractionations driven by either or both chemical and thermal gradients.  相似文献   

15.
We have investigated the transfer of oxygen isotope signals of diatomaceous silica (δ18Odiatom) from the epilimnion (0-7 m) through the hypolimnion to the lake bottom (∼20 m) in freshwater Lake Holzmaar, Germany. Sediment-traps were deployed in 2001 at depths of 7 and 16 m to harvest fresh diatoms every 28 days. The 7 m trap collected diatoms from the epilimnion being the main zone of primary production, while the 16 m trap collected material already settled through the hypolimnion. Also a bottom sediment sample was taken containing diatom frustules from approximately the last 25 years. The δ18Odiatom values of the 7 m trap varied from 29.4‰ in spring/autumn to 26.2‰ in summer according to the temperature dependence of oxygen isotope fractionation and represent the initial isotope signal in this study. Remarkably, despite the short settling distance δ18Odiatom values of the 7 and the 16 m trap were identical only during spring and autumn seasons while from April to September δ18Odiatom values of the 16 m trap were roughly ∼1.5‰ enriched in 18O compared to those of the 7 m trap. Isotopic exchange with the isotopically lighter water of the hypolimnion would shift the δ18Odiatom value to lower values during settling from 7 to 16 m excluding this process as a cause for the deviation. Dissolution of opal during settling with intact organic coatings of the diatom cells and near neutral pH of the water should only cause a minor enrichment of the 16 m values. Nevertheless, opal from the bottom sediment was found to be 2.5‰ enriched in 18O compared to the weighted average of the opal from the 7 m trap. Thus, resuspension of bottom material must have contributed to the intermediate δ18Odiatom signal of the 16 m trap during summer. Dissolution experiments allowed further investigation of the cause for the remarkably enriched δ18Odiatom value of the bottom sediment. Experiments with different fresh diatomaceous materials show an increase of opaline 18O at high pH values which is remarkably reduced when organic coatings of the cells still exist or at near neutral pH. In contrast, high pH conditions do not affect the δ18Odiatom values of sub-fossil and even fossil opal. IR analyses show that the 18O enrichment of the sedimentary silica is associated with a decrease in Si-OH groups and the formation of Si-O-Si linkages. This indicates a silica dehydroxylation process as cause for the isotopic enrichment of the bottom sediment. Silica dissolution and dehydroxylation clearly induce a maturation process of the diatom oxygen isotope signal presumably following an exponential behaviour with a rapid initial phase of signal alteration. The dynamics of this process is of particular importance for the quantitative interpretation of sedimentary δ18Odiatom values in terms of palaeothermometry.  相似文献   

16.
We present high-precision measurements of Mg and Fe isotopic compositions of olivine, orthopyroxene (opx), and clinopyroxene (cpx) for 18 lherzolite xenoliths from east central China and provide the first combined Fe and Mg isotopic study of the upper mantle. δ56Fe in olivines varies from 0.18‰ to −0.22‰ with an average of −0.01 ± 0.18‰ (2SD, n = 18), opx from 0.24‰ to −0.22‰ with an average of 0.04 ± 0.20‰, and cpx from 0.24‰ to −0.16‰ with an average of 0.10 ± 0.19‰. δ26Mg of olivines varies from −0.25‰ to −0.42‰ with an average of −0.34 ± 0.10‰ (2SD, n = 18), opx from −0.19‰ to −0.34‰ with an average of −0.25 ± 0.10‰, and cpx from −0.09‰ to −0.43‰ with an average of −0.24 ± 0.18‰. Although current precision (∼±0.06‰ for δ56Fe; ±0.10‰ for δ26Mg, 2SD) limits the ability to analytically distinguish inter-mineral isotopic fractionations, systematic behavior of inter-mineral fractionation for both Fe and Mg is statistically observed: Δ56Feol-cpx = −0.10 ± 0.12‰ (2SD, n = 18); Δ56Feol-opx = −0.05 ± 0.11‰; Δ26Mgol-opx = −0.09 ± 0.12‰; Δ26Mgol-cpx = −0.10 ± 0.15‰. Fe and Mg isotopic composition of bulk rocks were calculated based on the modes of olivine, opx, and cpx. The average δ56Fe of peridotites in this study is 0.01 ± 0.17‰ (2SD, n = 18), similar to the values of chondrites but slightly lower than mid-ocean ridge basalts (MORB) and oceanic island basalts (OIB). The average δ26Mg is −0.30 ± 0.09‰, indistinguishable from chondrites, MORB, and OIB. Our data support the conclusion that the bulk silicate Earth (BSE) has chondritic δ56Fe and δ26Mg.The origin of inter-mineral fractionations of Fe and Mg isotopic ratios remains debated. δ56Fe between the main peridotite minerals shows positive linear correlations with slopes within error of unity, strongly suggesting intra-sample mineral-mineral Fe and Mg isotopic equilibrium. Because inter-mineral isotopic equilibrium should be reached earlier than major element equilibrium via chemical diffusion at mantle temperatures, Fe and Mg isotope ratios of coexisting minerals could be useful tools for justifying mineral thermometry and barometry on the basis of chemical equilibrium between minerals. Although most peridotites in this study exhibit a narrow range in δ56Fe, the larger deviations from average δ56Fe for three samples likely indicate changes due to metasomatic processes. Two samples show heavy δ56Fe relative to the average and they also have high La/Yb and total Fe content, consistent with metasomatic reaction between peridotite and Fe-rich and isotopically heavy melt. The other sample has light δ56Fe and slightly heavy δ26Mg, which may reflect Fe-Mg inter-diffusion between peridotite and percolating melt.  相似文献   

17.
Hydrogen fractionation laws between selected hydrous minerals (brucite, kaolinite, lizardite, and gibbsite) and perfect water gas have been computed from first-principles quantum-mechanical calculations. The β-factor of each phase was calculated using the harmonic phonon dispersion curves obtained within density functional theory. All the fractionation laws show the same shape, with a minimum between 200 °C (brucite) and 500 °C (gibbsite). At low temperatures, the mineral/liquid water fractionation laws have been obtained using the experimental gas/liquid water fractionation laws. The resulting fractionation laws systematically overestimate measurements by 15‰ at low temperatures to 8‰ at ≈400 °C. Based on this general agreement, all calculated laws were empirically corrected with reference to brucite/water data. These considerations suggest that the experimental or natural calibrations by Xu and Zheng (1999) and Horita et al. (2002) (brucite/water), Gilg and Sheppard (1996) (kaolinite/water), Wenner and Taylor (1973) (lizardite/water), and in some extents Vitali et al. (2001) (gibbsite/water) are representative of equilibrium fractionations. Besides, internal isotopic fractionation of hydrogen between inner-surface and inner hydroxyl groups has been computed for kaolinite and lizardite. The obtained fractionation is large, of opposite sign for the two systems (respectively, −23‰ and +63‰ at 25 °C) and is linear in T-2. Internal fractionation of hydrogen in TO phyllosilicates might thus be used in geothermometry.  相似文献   

18.
Fossils of megaherbivores from eight late Pleistocene 14C- and OSL-dated doline infillings of Ajoie (NW Switzerland) were discovered along the Transjurane highway in the Swiss Jura. Carbon and oxygen analyses of enamel were performed on forty-six teeth of large mammals (Equus germanicus, Mammuthus primigenius, Coelodonta antiquitatis, and Bison priscus), coming from one doline in Boncourt (~ 80 ka, marine oxygen isotope stage MIS5a) and seven in Courtedoux (51–27 ka, late MIS3), in order to reconstruct the paleoclimatic and paleoenvironmental conditions of the region. Similar enamel δ13C values for both periods, ranging from − 14.5 to − 9.2‰, indicate that the megaherbivores lived in a C3 plant-dominated environment. Enamel δ18OPO4 values range from 10.9 to 16.3‰ with a mean of 13.5 ± 1.0‰ (n = 46). Mean air temperatures (MATs) were inferred using species-specific δ18OPO4–δ18OH2O-calibrations for modern mammals and a present-day precipitation δ18OH2O-MAT relation for Switzerland. Similar average MATs of 6.6 ± 3.6°C for the deposits dated to ~ 80 ka and 6.5 ± 3.3°C for those dated to the interval 51–27 ka were estimated. This suggests that these mammals in the Ajoie area lived in mild periods of the late Pleistocene with MATs only about 2.5°C lower than modern-day temperatures.  相似文献   

19.
The oxygen-isotope compositions (obtained by laser fluorination) of hand-picked separates of isolated forsterite, isolated olivine and chondrules from the Tagish Lake carbonaceous chondrite describe a line (δ17O = 0.95 * δ18O − 3.24; R2 = 0.99) similar to the trend known for chondrules from other carbonaceous chondrites. The isolated forsterite grains (Fo99.6-99.8; δ18O = −7.2‰ to −5.5‰; δ17O = −9.6‰ to −8.2‰) are more 16O-rich than the isolated olivine grains (Fo39.6-86.8; δ18O = 3.1‰ to 5.1‰; δ17O = −0.3‰ to 2.2‰), and have chemical and isotopic characteristics typical of refractory forsterite. Chondrules contain olivine (Fo97.2-99.8) with oxygen-isotope compositions (δ18O = −5.2‰ to 5.9‰; δ17O = −8.1‰ to 1.2‰) that overlap those of isolated forsterite and isolated olivine. An inverse relationship exists between the Δ17O values and Fo contents of Tagish Lake isolated forsterite and chondrules; the chondrules likely underwent greater exchange with 16O-poor nebular gases than the forsterite. The oxygen-isotope compositions of the isolated olivine grains describe a trend with a steeper slope (1.1 ± 0.1, R2 = 0.94) than the carbonaceous chondrite anhydrous mineral line (CCAMslope = 0.95). The isolated olivine may have crystallized from an evolving melt that exchanged with 16O-poor gases of somewhat different composition than those which affected the chondrules and isolated forsterite. The primordial components of the Tagish Lake meteorite formed under conditions similar to other carbonaceous chondrite meteorite groups, especially CMs. Its alteration history has its closest affinities to CI carbonaceous chondrites.  相似文献   

20.
To understand oxygen and carbon stable isotopic characteristics of aragonite stalagmites and evaluate their applicability to paleoclimate, the isotopic compositions of active and fossil aragonite speleothems and water samples from an in situ multi-year (October 2005-July 2010) monitoring program in Furong Cave located in Chongqing of China have been examined. The observations during October 2005-June 2007 show that the meteoric water is well mixed in the overlying 300-500-m bedrock aquifer, reflected by relatively constant δ18O, ±0.11-0.14‰ (1σ), of drip waters in the cave, which represents the annual status of rainfall water. Active cave aragonite speleothems are at oxygen isotopic equilibrium with drip water and their δ18O values capture the surface-water oxygen isotopic signal. Aragonite-to-calcite transformation since the last glaciation is not noticeable in Furong stalagmites. Our multi-year field experiment approves that aragonite stalagmite δ18O records in this cave are suitable for paleoclimate reconstruction. With high U, 0.5-7.2 ppm, and low Th, 20-1270 ppt, the Furong aragonite stalagmites provide very precise chronology (as good as ±20s yrs (2σ)) of the climatic variations since the last deglaciation. The synchroneity of Chinese stalagmite δ18O records at the transition into the Bølling-Allerød (t-BA) and the Younger Dryas from Furong, Hulu and Dongge Caves supports the fidelity of the reconstructed East Asian monsoon evolution. However, the Furong record shows that the cold Older Dryas (OD) occurred at 14.0 thousand years ago, agreeing with Greenland ice core δ18O records but ∼200 yrs younger than that in the Hulu record. The OD age discrepancy between Chinese caves can be attributable to different regionally climatic/environmental conditions or chronological uncertainty of stalagmite proxy records, which is limited by changes in growth rate and subsampling intervals in absolute dating. Seasonal dissolved inorganic carbon δ13C variations of 2-3‰ in the drip water and 5-7‰ in the pool and spring waters are likely attributed to variable degrees of CO2 degassing in winter and summer. The variable δ13C values of active deposits from −11‰ to 0‰ could be caused by kinetically mediated CO2 degassing processes. The complicated nature of pre-deposition kinetic isotopic fractionation processes for carbon isotopes in speleothems at Furong Cave require further study before they can be interpreted in a paleoclimatic or paleoenvironmental context.  相似文献   

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