Calculating the partitioning of the isotopes of Mo between oxidic and sulfidic species in aqueous solution   总被引：1，自引：0，他引：1  

J.A. Tossell 《Geochimica et cosmochimica acta》2005,69(12):2981-2993

The fractionation of the isotopes of Mo between different geological environments has recently been determined to high accuracy using mass spectrometry (Barling et al., 2001). Fractionation is observed between Mo in seawater, where it exists primarily in the form of the Mo(VI) anion molybdate, MoO₄⁻², and in oxic sediments, where the Mo is isotopically lighter than in sea water by ∼1.8‰ (in terms of the ⁹⁷Mo, ⁹⁵Mo isotope pair). EXAFS evidence exists for a five- or six-coordinate Mo environment in the Fe,Mn oxyhydroxides of ferromanganese nodules (Kuhn et al., 2003). In sediment regimes which are anoxic and sulfidic (sometimes referred to as euxinic), where the Mo(VI) is expected to exist as a sulfide, no fractionation is observed compared to seawater. This is presumably because of the stoichiometric conversion of the Mo from MoO₄⁻² to MoS₄⁻² (Erickson and Helz, 2000) and then to other sulfides. If the conversion is stoichiometrically complete, mass balance requires the same isotopic distribution in reactant and product. This is a result of the very high equilibrium constant for this reaction. Thus, to understand isotopic fractionation processes both the equilibrium constants for the isotopic fractionation reactions and the equilbrium constants for transformation of one chemical compound to another must be considered.We here present quantum mechanical calculations of the isotopic fractionation equilibrium constants for the isotopes ⁹²Mo and ¹⁰⁰Mo between MoO₄⁻², MoO₃(OH)⁻, MoO₂(OH)₂, MoO₃, MoO₃(OH₂)₃, MoS₄⁻² and a number of other oxidic and sulfidic complexes of Mo. The fractionation equilibrium constants are calculated directly from the computed vibrational, rotational and translational contributions to the free energy in the gas-phase using quantum methods. Calculated vibrational frequencies and ratios of frequencies for different isotopomers are first obtained using a number of different quantum methods and compared with available experimental data to establish the most reliable methodology.We have also calculated free energy changes in aqueous solution for a range of reactions of MoO₄⁻² and MoO₂(OH)₂ with H₂O and H₂S. We present evidence for the instability of the monomeric octahedral species Mo(OH)₆ commonly assumed to exist in acid solution and suggest highly distorted six-coordinate MoO₃(OH₂)₃ or three-coordinate MoO₃ as better representations of the species present. We have also calculated visible-UV absorption spectra to support our interpretation of the speciation. MoO₃ is calculated to be isotopically lighter than MoO₄⁻² by 1.6‰, consistent with the experimentally observed difference between sea water and oxic sediments. We explain the isotopic lightness of oxic sediments as arising from an intermediate step in which a three coordinate MoO₃ species is formed in aqueous solution, and subsequently attaches to the surface of a Fe,Mn oxyhydroxide mineral.  相似文献   

Capture of molybdenum in pyrite-forming sediments: role of ligand-induced reduction by polysulfides     

Trent P. Vorlicek  Mani D. Kahn  George R. Helz 《Geochimica et cosmochimica acta》2004,68(3):547-556

Capture of Mo by FeS₂ is an important sink for marine Mo. X-ray spectroscopy has shown that Mo forms Mo-Fe-S cuboidal clusters on pyrite. Reduction of Mo^VI must occur to stabilize these structures. Sulfide alone is a poor reductant for Mo, producing instead a series of Mo^VI thioanions (MoO_xS_4−x²⁻, x = 0-3). In solutions that contain both H₂S and S⁰-donors (i.e. polysulfides; dissolved S₈), Mo is transformed to Mo^IV or Mo^V₂ polysulfide/sulfide anions. This intramolecular reduction requires no external reducing agent. Remarkably, an oxidizing agent (S⁰ donor), rather than a reducing agent, stabilizes the reducible Mo^VI complex. Thiomolybdates and their reduction products do not precipitate spontaneously; solutions supersaturated by 10⁹ with respect to molybdenite, MoS₂, produce no precipitate in 40 days. In 10-minute exposures, pyrite can scavenge MoOS₃²⁻ and MoS₄²⁻ weakly at mildly alkaline pH but can scavenge an unidentified product of the S⁰-induced reduction of MoOS₃²⁻ very strongly. On the basis of these observations, a reaction pathway for Mo capture by pyrite is proposed. Conditions that favor Mo capture by this pathway also favor pyrite growth. Ascribing Mo capture simply to low redox potential is too simplistic and neglects the likely role of oxidizing S⁰-donors. The aqueous speciation of Mo in anoxic environments will be a function of the activity of zero-valent sulfur as well as the activity of H₂S(aq).  相似文献   

The partitioning of molybdenum(VI) between aqueous liquid and vapour at temperatures up to 370 °C     

Kirsten U. Rempel  Anthony E. Williams-Jones  Artas A. Migdisov 《Geochimica et cosmochimica acta》2009,73(11):3381-6609

We have conducted experiments to evaluate the vapour-liquid fractionation of Mo(VI) in the system MoO₃-NH₃-H₂O at 300-370 °C and saturated vapour pressure, using a two-chamber autoclave that allows separate trapping of the vapour and liquid. The measured total Mo concentrations in each phase were used to calculate a distribution coefficient, , which increases as the density of the vapour approaches that of the liquid, and is greater than one for pH ? 4. Molybdenum speciation in the vapour is described by a single complex, MoO₃H₂O. By contrast, thermodynamic modeling of the distribution of Mo species in the liquid indicates that bimolybdate (HMoO₄⁻) is the dominant aqueous species at the conditions of our experiments, and that molybdate (MoO₄²⁻) and molybdic acid (H₂MoO₄⁰) are present in smaller quantities. As vapour-liquid fractionation occurs between neutral species, it is governed by the reaction H₂MoO₄⁰(aq) = MoO₃ · H₂O(g). Fractionation is therefore controlled by the concentration of H₂MoO₄⁰ in the liquid, which increases with increasing temperature and decreasing pH. Owing to the pH dependence of , it cannot be used to describe Mo fractionation in aqueous vapour-liquid systems with compositions different than those of this study. We have therefore calculated a composition-independent (Henry’s Law) constant, , for each experimental point, using the measured total Mo concentration in the vapour and the modeled concentration of H₂MoO₄⁰ in the liquid. This constant may be applied to aqueous vapour-liquid systems of known liquid composition to estimate the concentration of Mo in a vapour for which little chemical information is available, and thereby supplement the available fractionation data for natural porphyry-forming systems. The results of this study demonstrate that at conditions typical of natural porphyry ore-forming systems, a significant amount of molybdenum fractionates into the vapour over the liquid, and the vapour may transport quantities of Mo in excess of that in the liquid at pH conditions below those of the muscovite-microcline reaction boundary.  相似文献   

Catalysis by mineral surfaces: Implications for Mo geochemistry in anoxic environments     

Trent P. VorlicekGeorge R. Helz 《Geochimica et cosmochimica acta》2002,66(21):3679-3692

Fixation of Mo in sulfidic environments is believed to be preceded by conversion of geochemically passive MoO₄²⁻ to particle-reactive thiomolybdates (MoO_xS_4−x²⁻). In aqueous solution, these transformations are general-acid catalyzed, implying that proton donors can accelerate both the forward and reverse reactions. Here, we explore whether mineral surfaces also catalyze thiomolybdate interconversions. The rate of MoS₄²⁻ hydrolysis is investigated in the presence and absence of natural kaolinite (KGa-1b) and synthetic Al₂O₃ and SiO₂ phases. Comparison of rates achieved with these phases suggests that the Al oxyhydroxide component in kaolinite furnishes the catalytic activity. An anhydrous Al₂O₃ phase is catalytically inactive until hydrated (and therefore protonated). Surface kinetics with kaolinite at mildly alkaline pH are consistent with rate limitation by formation or decomposition of monomeric surface complexes; oligomeric surface intermediates may become important at MoS₄²⁻ > 20 μmol/L, higher than is likely to be found in nature. The pH dependence of the kaolinite-catalyzed reaction suggests that weak-acid surface sites promote hydrolysis. Intermediate thiomolybdates or molybdate appears to compete for active sites, inhibiting MoS₄²⁻ hydrolysis. Catalysis of MoOS₃²⁻ hydrolysis is also observed but has not been studied systematically. Thiomolybdate hydrolysis is inhibited slightly by sulfate and more strongly by phosphate. Low NaCl concentrations (<10⁻² mol/L) promote hydrolysis, but higher NaCl concentrations retard the reaction to a small extent. A mechanism is postulated involving expansion of the coordination number around Mo from 4 to 6 under the influence of the surface. The effective concentration of surface sites available to Mo in sediment pore waters is likely to be large enough to greatly accelerate thiomolybdate hydrolysis and sulfidation. Possibly this explains why Mo capture in seasonally or intermittently anoxic environments often occurs through processes operating within sediments but not in overlying waters.  相似文献   

Re-assessing the surface cycling of molybdenum and rhenium     

Christian A. Miller  Bernhard Peucker-Ehrenbrink  Franco Marcantonio 《Geochimica et cosmochimica acta》2011,75(22):7146-7179

We re-evaluate the cycling of molybdenum (Mo) and rhenium (Re) in the near-surface environment. World river average Mo and Re concentrations, initially based on a handful of rivers, are calculated using 38 rivers representing five continents, and 11 of 19 large-scale drainage regions. Our new river concentration estimates are 8.0 nmol kg⁻¹ (Mo), and 16.5 pmol kg⁻¹ (Re, natural + anthropogenic). The linear relationship of dissolved Re and in global rivers (R² = 0.76) indicates labile continental Re is predominantly hosted within sulfide minerals and reduced sediments; it also provides a means of correcting for the anthropogenic contribution of Re to world rivers using independent estimates of anthropogenic sulfate. Approximately 30% of Re in global rivers is anthropogenic, yielding a pre-anthropogenic world river average of 11.2 pmol Re kg⁻¹. The potential for anthropogenic contribution is also seen in the non-negligible Re concentrations in precipitation (0.03-5.9 pmol kg⁻¹), and the nmol kg⁻¹ level Re concentrations of mine waters. The linear Mo- relationship (R² = 0.69) indicates that the predominant source of Mo to rivers is the weathering of pyrite. An anthropogenic Mo correction was not done as anthropogenically-influenced samples do not display the unambiguous metal enrichment observed for Re. Metal concentrations in high temperature hydrothermal fluids from the Manus Basin indicate that calculated end-member fluids (i.e. Mg-free) yield negative Mo and Re concentrations, showing that Mo and Re can be removed more quickly than Mg during recharge. High temperature hydrothermal fluids are unimportant sinks relative to their river sources 0.4% (Mo), and 0.1% (pre-anthropogenic Re). We calculate new seawater response times of 4.4 × 10⁵ yr (τ_Mo) and 1.3 × 10⁵ yr (τ_Re, pre-anthropogenic).  相似文献   

Estimates of the second dissociation constant of H₂S from the surface sulfidation of crystalline sulfur     

Art A. Migdisov  A.E. Williams-JonesL.Z. Lakshtanov  Yu V. Alekhin 《Geochimica et cosmochimica acta》2002,66(10):1713-1725

The adsorption of hydrogen sulfide (Γ_H2S) and protons (Γ_H⁺) on the surface of crystalline sulfur was investigated experimentally in H₂S-bearing solutions at temperatures of 25, 50, and 70°C, NaCl concentrations of 0.1 and 0.5 mol/dm⁻³ and log C_H⁺ values in the range −2.3 to −5. At all temperatures, the dominant process on the surface of the sulfur was deprotonation, and the average values of Γ_H2S were very close to the highest values determined for Γ_H⁺. This finding, combined with the lack of detectable proton adsorption in H₂S-free solutions, suggests that proton adsorption/desorption on the surface of sulfur occurs through formation of ≡ S − H₂S complexes in the presence of H₂S.We propose that this complexation represents sulfidation of the sulfur surface, a process analogous to hydroxylation of oxide surfaces, and that the sulfidation can be described by the reaction: ≡ S + H₂S = ≡SSH₂⁰ β° The deprotonation of the ≡ SH° complex occurs via the reaction: ≡ SSH₂⁰ = ≡SSH⁻ + H⁺ β⁻ Values of 2.9, 2.8, and 2.9 (± 0.23) were obtained for −log β⁻ at 25, 50, and 70°C, respectively. These data were employed to estimate the second dissociation constant for hydrogen sulfide in aqueous solutions using the extrapolation method proposed by Schoonen and Barnes (1988) and yielded corresponding values for the constant of 17.4 ± 0.3, 15.7, and 14.5, respectively. The value for 25°C is in very good agreement with the experimentally determined values of Giggenbach (1971) at 17 ± 0.1; Meyer et al. (1983) at 17 ± 1; Licht and Manassen (1987) at 17.6 ± 0.3; and Licht et al. (1990) at 17.1 ± 0.3.  相似文献   

Energetics of anhydrite, barite, celestine, and anglesite: a high-temperature and differential scanning calorimetry study     

J. Majzlan  A. NavrotskyJ.M. Neil 《Geochimica et cosmochimica acta》2002,66(10):1839-1850

The thermochemistry of anhydrous sulfates (anglesite, anhydrite, arcanite, barite, celestine) was investigated by high-temperature oxide melt calorimetry and differential scanning calorimetry. Complete retention and uniform speciation of sulfur in the solvent was documented by (a) chemical analyses of the solvent (3Na₂O · 4MoO₃) with dissolved sulfates, (b) Fourier transform infrared spectroscopy confirming the absence of sulfur species in the gases above the solvent, and (c) consistency of experimental determination of the enthalpy of drop solution of SO₃ in the solvent. Thus, the principal conclusion of this study is that high-temperature oxide melt calorimetry with 3Na₂O · 4MoO₃ solvent is a valid technique for measurement of enthalpies of formation of anhydrous sulfates. Enthalpies of formation (in kJ/mol) from the elements (ΔH_f^o) were determined for synthetic anhydrite (CaSO₄) (−1433.8 ± 3.2), celestine (SrSO₄) (−1452.1 ± 3.3), anglesite (PbSO₄) (−909.9 ± 3.4), and two natural barite (BaSO₄) samples (−1464.2 ± 3.7, −1464.9 ± 3.7). The heat capacity of anhydrite, barite, and celestine was measured between 245 and 1100 K, with low- and high-temperature Netzsch (DSC-404) differential scanning calorimeters. The results for each sample were fitted to a Haas-Fisher polynomial of the form C_p(245 K < T < 1100 K) = a + bT + cT⁻² + dT^−0.5 + eT². The coefficients of the equation are as follows: for anhydrite a = 409.7, b = −1.764 × 10⁻¹, c = 2.672 × 10⁶, d = −5.130 × 10³, e = 8.460 × 10⁻⁵; for barite, a = 230.5, b = −0.7395 × 10⁻¹, c = −1.170 × 10⁶, d = −1.587 × 10³, e = 4.784 × 10⁻⁵; and for celestine, a = 82.1, b = 0.8831 × 10⁻¹, c = −1.213 × 10⁶, d = 0.1890 × 10³, e = −1.449 × 10⁻⁵. The 95% confidence interval of the measured C_p varies from 1 to 2% of the measured value at low temperature up to 2 to 5% at high temperature. The measured thermochemical data improve or augment the thermodynamic database for anhydrous sulfates and highlight the remaining discrepancies.  相似文献   

The behavior of molybdenum and its isotopes across the chemocline and in the sediments of sulfidic Lake Cadagno, Switzerland   总被引：1，自引：0，他引：1  

Tais W. Dahl  Ariel D. Anbar  Minik T. Rosing  Donald E. Canfield 《Geochimica et cosmochimica acta》2010,74(1):144-274

Molybdenum (Mo) isotope studies in black shales can provide information about the redox evolution of the Earth’s oceans, provided the isotopic consequences of Mo burial into its major sinks are well understood. Previous applications of the Mo isotope paleo-ocean redox proxy assumed quantitative scavenging of Mo when buried into sulfidic sediments. This paper contains the first complete suite of Mo isotope fractionation observations in a sulfidic water column and sediment system, the meromictic Lake Cadagno, Switzerland, a small alpine lake with a pronounced oxygen-sulfide transition reaching up to H₂S ∼ 200 μM in the bottom waters (or about 300 μM total sulfide: ΣS²⁻ = H₂S + HS⁻ + S²⁻). We find that Mo behaves conservatively in the oxic zone and non-conservatively in the sulfidic zone, where dissolved Mo concentrations decrease from 14 nM to 2-8 nM across this transition. Dissolved Mo in the upper oxic waters has a δ⁹⁸Mo_oxic = 0.9 ± 0.1‰, which matches that of the riverine input, δ⁹⁸Mo_river = 0.9 ± 0.1‰. In the deeper sulfidic waters, a subaquatic source delivers Mo at 1.55 ± 0.1‰, but the dissolved Mo is even heavier at δ⁹⁸Mo_sulfidic = 1.8‰. Sediment traps in the sulfidic zone of the lake collect particles increasingly enriched in Mo with depth, with δ⁹⁸Mo values significantly fractionated at −0.8‰ to −1.2‰ both near the chemocline and in the deepest trap. Suspended particulates in the sulfidic waters carry lighter Mo than the ambient dissolved Mo pool by ∼0.3-1.5‰. Sedimentary Mo concentrations correlate with total organic carbon and yield Mo levels which are two orders of magnitude higher than typical crustal values found in rocks from the catchment area. Solid-phase Mo in the sediment shows a slightly positive δ⁹⁸Mo trend with depth, from δ⁹⁸Mo = 1.2‰ to 1.4‰ while the pore waters show dramatic enrichments of Mo (>2000 nM) with a relatively light isotope signature of δ⁹⁸Mo = 0.9-1.0‰.These data are explained if Mo is converted to particle-reactive oxythiomolybdates in the sulfidic waters and is fractionated during removal from solution onto particles. Isotope fractionation is expressed in the water column, despite the high sulfide concentrations, because the rate of Mo removal is fast compared to the slow reaction kinetics of thiomolybdate formation. However, elemental and isotopic mass balances show that Mo is indeed quantitatively removed to the lake sediments and thus the isotopic composition of the sediments reflects sources to the sulfidic water. This efficient Mo drawdown is expected to occur in settings where H₂S is very much in excess over Mo or in a restricted setting where the water renewal rate is slow compared to the Mo burial rate. We present a model for the Mo isotope fractionation in sulfidic systems associated with the slow reaction kinetics and conclude that quantitative removal will occur in highly sulfidic and restricted marine systems.  相似文献   

Calculation of the energetics for the oxidation of Sb(III) sulfides by elemental S and polysulfides in aqueous solution     

J.A. Tossell 《Geochimica et cosmochimica acta》2003,67(18):3347-3354

Recent experimental studies have reported the existence of two new Sb sulfide species, Sb₂S₅²⁻ and Sb₂S₆²⁻, in alkaline sulfidic solutions in equilibrium with stibnite, Sb₂S₃, and orthorhombic S. These species contain Sb(V), which has also recently been identified in similar solutions using EXAFS by other researchers. This represents a significant change from the consensus a decade ago that sulfidic solutions of Sb contained only Sb(III) species. I have calculated from first principles of quantum mechanics the energetics for the oxidation of the Sb(III) sulfide dimer Sb₂S₄²⁻ to the mixed Sb(III,V) dimer Sb₂S₅²⁻ and then to the all Sb(V) dimer, Sb₂S₆²⁻. Gas-phase reaction energies have been evaluated using polarized valence double zeta effective core potential basis sets and Moller-Plesset second order treatments of electron correlation. All translational, rotational and vibrational contributions to the gas-phase reaction free energy have been calculated. Hydration energies have been obtained using the COSMO version of the self-consistent reaction field polarizable continuum method. Negative free energy changes are calculated for the oxidation of the dianion of the III,III dimer to the III,V dimer by both small polysulfides, like S₄H⁻, and elemental S, modeled as S₈. For the further oxidation of the III,V dimer to the V,V dimer the reaction free energies are calculated to be close to zero. The partially protonated Sb III,III dimer monoanion HSb₂S₄⁻ can also be oxidized, but the reaction is not so favorable as for the dianion. Comparison of the calculated aqueous deprotonation energies of H₂Sb₂S₄, H₂Sb₂S₅ and H₂Sb₂S₆ and their dianions with values calculated for various oxyacids indicates that the III,V and V,V dimers will have pK_a2 values <5, so that their dianions will be the dominant species in alkaline solutions. These results are thus consistent with the recent identification of Sb₂S₅²⁻ and Sb₂S₆²⁻ species. I have also calculated the Raman spectra of Sb₂S₅²⁻ and Sb₂S₆²⁻ to assist in their identification. The calculated vibrational frequencies of the III,V and V,V dimers are characteristically higher than those of the III,III dimer I previously studied. The III,V dimer may contribute shoulders to the Raman spectrum.  相似文献   

Re-Os and Mo isotope systematics of black shales from the Middle Proterozoic Velkerri and Wollogorang Formations, McArthur Basin, northern Australia   总被引：3，自引：0，他引：3  

Brian Kendall  Robert A. Creaser  Ariel D. Anbar 《Geochimica et cosmochimica acta》2009,73(9):2534-2558

Joint application of the Mo isotope paleoredox proxy and Re-Os deposition-age geochronometer to euxinic black shales has potential for tracing the evolution of ocean redox chemistry over geological time. Here, we report new Re-Os and Mo isotope data for the Mesoproterozoic Velkerri Formation (Roper Group) and Paleoproterozoic Wollogorang Formation (Tawallah Group), McArthur Basin, northern Australia. New Re-Os ages of 1361 ± 21 Ma (2σ, n = 14, mean square of weighted deviates [MSWD] = 1.3, Model 1) and 1417 ± 29 Ma (2σ, n = 12, MSWD = 1.3, Model 1) constrain the depositional age of the Velkerri Formation and its contained biomarkers, as well as acritarchs and microfossils from the Roper Group. Black shales from the upper Velkerri Formation have high Mo abundances (105-119 ppm) and degree of pyritization [DOP] values (0.90-0.92) implying quantitative conversion of molybdate (MoO₄²⁻) to thiomolybdate (MoS₄²⁻) in overlying bottom waters. The average δ^97/95Mo (0.72 ± 0.10‰, 2σ, n = 6) of these shales is consistent with previous data, but represents a significantly more precise determination for global seawater δ^97/95Mo at 1.4 Ga. This value is lighter than present-day seawater by ∼0.85‰ and reflects expanded strongly euxinic deep ocean conditions ([H₂S]_aq > 11 μM) relative to oxic, suboxic, and weakly/intermittently euxinic ([H₂S]_aq < 11 μM) marine deposition in the 1.4 Ga oceans. Mass-balance modelling suggests Mo removal into strongly euxinic and oxic sediments may have comprised 30-70% and less than 15%, respectively, of the oceanic Mo sink at 1.4 Ga as opposed to 5% and 35% today, respectively.The Re-Os radioisotope system in organic-rich shales serves as a test for post-depositional alteration that could affect the usefulness of paleoredox tracers such as Mo stable isotopes. Re-Os isotope data for the Wollogorang Formation black shales are scattered and yield a highly imprecise date of 1359 ± 150 Ma (2σ, n = 21, MSWD = 85, Model 3). This age is younger than U-Pb zircon ages from interbedded tuffs that constrain the age of deposition at ca. 1730 Ma. In conjunction with previous petrological, geochemical, and paleomagnetic data, the Re-Os isotope data suggest hydrothermal fluid flow through the Wollogorang Formation, possibly associated with formation of the ca. 1640 Ma McArthur River Pb-Zn-Ag sedimentary exhalative deposit, resulted in post-depositional mobilization of Re and Os. Based on the degree of deviation of the Re-Os data from a 1730 Ma reference line, open-system behavior of Re and Os was greatest near the base of the black shale unit. Wollogorang Formation black shales are enriched in Mo (41-58 ppm), but are characterized by variable δ^97/95Mo (0.3-0.8‰) and DOP (0.57-0.92). The lightest δ^97/95Mo values occur near the base of the black shale unit. Based on the Re-Os systematics, hydrothermal fluids have probably overprinted the authigenic δ^97/95Mo signature in those shales. However, the heaviest δ^97/95Mo values in the Wollogorang Formation come from stratigraphically higher shales, and are similar to those observed for the Velkerri Formation, and thus may reflect seawater δ^97/95Mo at 1.73 Ga.  相似文献   

Chemical evolution of metal in refractory inclusions in CV3 chondrites     

Andrew J. Campbell  Munir Humayun 《Geochimica et cosmochimica acta》2003,67(17):3119-3134

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was used to measure distributions of the siderophile elements V, Fe, Co, Ni, Mo, Ru, Rh, Pd, W, Re, Os, Ir, Pt, and Au in Fremdlinge with a spatial resolution of 15 to 25 μm. A sulfide vein in a refractory inclusion in Allende (CV3-oxidized) is enriched in Rh, Ru, and Os with no detectable Pd, Re, Ir, or Pt, indicating that Rh, Ru, and Os were redistributed by sulfidation of the inclusion, causing fractionation of Re/Os and other siderophile element ratios in Allende CAIs. Fremdlinge in compact Type-A inclusions from Efremovka (CV3-reduced) exhibit subsolidus exsolution into kamacite and taenite and minimal secondary formation of V-magnetite and schreibersite. Siderophile element partitioning between taenite and kamacite is similar to that observed previously in iron meteorites, while preferential incorporation of the light PGEs (Ru, Rh, Pd) relative to Re, Os, Ir, and Pt by schreibersite was observed. Fremdling EM2 (CAI Ef2) has an outer rim of P-free metal that preserves the PGE signature of schreibersite, indicating that EM2 originally had a phosphide rim and lost P to the surrounding inclusion during secondary processing. Most Fremdlinge have chondrite-normalized refractory PGE patterns that are unfractionated, with PGE abundances derived from a small range of condensation temperatures, ∼1480 to 1468 K at P_tot = 10⁻³ bar. Some Fremdlinge from the same CAI exhibit sloping PGE abundance patterns and Re/Os ratios up to 2 × CI that likely represent mixing of grains that condensed at various temperatures.  相似文献   

The oxidation state of sulfur in synthetic and natural glasses determined by X-ray absorption spectroscopy   总被引：1，自引：0，他引：1  

Nicole Métrich  Andrew J. Berry  Jean Susini 《Geochimica et cosmochimica acta》2009,73(8):2382-7631

Sulfur K-edge X-ray absorption near edge structure (XANES) spectra were recorded for experimental glasses of various compositions prepared at different oxygen fugacities (fO₂) in one-atmosphere gas-mixing experiments at 1400 °C. This sample preparation method only results in measurable S concentrations under either relatively reduced (log fO₂ < −9) or oxidised (log fO₂ > −2) conditions. The XANES spectra of the reduced samples are characterised by an absorption edge crest at 2476.4 eV, typical of S²⁻. In addition, spectra of Fe-bearing compositions exhibit a pronounced absorption edge shoulder. Spectra for all the Fe-free samples are essentially identical, as are the spectra for the Fe-bearing compositions, despite significant compositional variability within each group. The presence of a sulfide phase, such as might exsolve on cooling, can be inferred from a pre-edge feature at 2470.5 eV.The XANES spectra of the oxidised samples are characterised by an intense transition at 2482.1 eV, typical of the sulfate anion SO₄²⁻. Sulfite (SO₃²⁻) has negligible solubility in silicate melts at low pressures. The previous identification of sulfite species in natural glass samples is attributed to an artefact of the analysis (photoreduction of S⁶⁺). S⁴⁺ does, however, occur unambiguously with S⁶⁺ in Fe-free and Fe-poor compositions prepared in equilibrium with CaSO₄ at 4-16 kbar, and when buffered with Re/ReO₂ at 10 kbar. Solubility of S⁴⁺ thus requires partial pressures of SO₂ considerably in excess of 1 bar. A number of experiments were undertaken in an attempt to access intermediate fO₂s more applicable to terrestrial volcanism. Although these were largely unsuccessful, S²⁻ and S⁶⁺ were found to coexist in some samples that were not in equilibrium with the imposed fO₂.The XANES spectra of natural olivine-hosted melt inclusions and submarine glasses representative of basalts at, or close to, sulfide saturation show mainly dissolved S²⁻, but with minor sulfate, and additionally a peak at 2469.5 eV, which, although presumably due to immiscible sulfide, is 1 eV lower than that typical of FeS. These sulfate and sulfide-related peaks disappear with homogenisation of the inclusions by heating to 1200 °C followed by rapid quenching, suggesting that both these features are a result of cooling under natural conditions. The presence of small amounts of sulfate in otherwise reduced basaltic magmas may be explained by the electron exchange reaction: S²⁻ + 8Fe³⁺ = S⁶⁺ + 8Fe²⁺, which is expected to proceed strongly to the right with decreasing temperature. This reaction would explain why S²⁻ and S⁶⁺ are frequently found together despite the very limited fO₂ range over which they are thermodynamically predicted to coexist. The S XANES spectra of water-rich, highly oxidised, basaltic inclusions hosted in olivine from Etna and Stromboli confirm that nearly all S is dissolved as sulfate, explaining their relatively high S contents.  相似文献   

Pyrite (FeS₂) oxidation: A sub-micron synchrotron investigation of the initial steps   总被引：1，自引：0，他引：1  

Anand P. Chandra 《Geochimica et cosmochimica acta》2011,75(20):6239-6254

Pyrite is an environmentally significant mineral being the major contributor to acid rock drainage. Synchrotron based SPEM (scanning photoelectron microscopy) and micro-XPS (X-ray photoelectron spectroscopy) have been used to characterise fresh and oxidised pyrite (FeS₂) with a view to understanding the initial oxidation steps that take place during natural weathering processes. Localised regions of the pyrite surface containing Fe species of reduced coordination have been found to play a critical role. Such sites not only initiate the oxidation process but also facilitate the formation of highly reactive hydroxyl radical species, which then lead the S oxidation process.Four different S species are found to be present on fresh fractured pyrite surfaces: S₂²⁻_(bulk) (4-fold coordination), S₂²⁻_(surface) (3-fold coordination), S²⁻ and S⁰/S_n²⁻ (metal deficient sulfide and polysulfide respectively). These species were found to be heterogeneously distributed on the fractured pyrite surface. Both O₂ and H₂O gases are needed for effective oxidation of the pyrite surface. The process is initiated when O₂ dissociatively and H₂O molecularly adsorb onto the surface Fe sites where high dangling bond densities exist. H₂O may then dissociate to produce OH radicals. The adsorption of these species leads to the formation of Fe-oxy species prior to the formation of sulfoxy species. Evidence suggests that Fe-O bonds form prior to Fe-OH bonds. S oxidation occurs through interactions of OH radicals formed at the Fe sites, with formation of SO₄²⁻ occurring via S₂O₃²⁻/SO₃²⁻ intermediates. The pyrite oxidation process is electrochemical in nature and was found to occur in patches, where site specific adsorption of O₂ and H₂O has occurred. Fe and S oxidation was found to occur within the same area of oxidation probably in atomic scale proximity. Furthermore, the O in SO₄²⁻ arises largely from H₂O; however, depending on the surface history, SO₄²⁻ formed early in the oxidation process may also contain O from O₂.  相似文献   

H₂S fluxes from Mt. Etna, Stromboli, and Vulcano (Italy) and implications for the sulfur budget at volcanoes     

A. Aiuppa  S. Inguaggiato  M. O’Dwyer  M.J. Padgett  M. Valenza 《Geochimica et cosmochimica acta》2005,69(7):1861-1871

We present here new measurements of sulfur dioxide and hydrogen sulfide emissions from Vulcano, Etna, and Stromboli (Italy), made by direct sampling at vents and by filter pack and ultraviolet spectroscopy in downwind plumes. Measurements at the F0 and FA fumaroles on Vulcano yielded SO₂/H₂S molar ratios of ≈0.38 and ≈1.4, respectively, from which we estimate an H₂S flux of 6 to 9 t · d⁻¹ for the summit crater. For Mt. Etna and Stromboli, we found SO₂/H₂S molar ratios of ≈20 and ≈15, respectively, which combined with SO₂ flux measurements, suggest H₂S emission rates of 50 to 113 t · d⁻¹ and 4 to 8 t · d⁻¹, respectively. We observe that “source” and plume SO₂/H₂S ratios at Vulcano are similar, suggesting that hydrogen sulfide is essentially inert on timescales of seconds to minutes. This finding has important implications for estimates of volcanic total sulfur budget at volcanoes since most existing measurements do not account for H₂S emission.  相似文献   

Anaerobic methane oxidation and a deep H₂S sink generate isotopically heavy sulfides in Black Sea sediments     

Bo Barker Jørgensen  Michael E. Böttcher  Lev N. Neretin 《Geochimica et cosmochimica acta》2004,68(9):2095-2118

The main terminal processes of organic matter mineralization in anoxic Black Sea sediments underlying the sulfidic water column are sulfate reduction in the upper 2-4 m and methanogenesis below the sulfate zone. The modern marine deposits comprise a ca. 1-m-deep layer of coccolith ooze and underlying sapropel, below which sea water ions penetrate deep down into the limnic Pleistocene deposits from >9000 years BP. Sulfate reduction rates have a subsurface maximum at the SO₄²⁻-CH₄ transition where H₂S reaches maximum concentration. Because of an excess of reactive iron in the deep limnic deposits, most of the methane-derived H₂S is drawn downward to a sulfidization front where it reacts with Fe(III) and with Fe²⁺ diffusing up from below. The H₂S-Fe²⁺ transition is marked by a black band of amorphous iron sulfide above which distinct horizons of greigite and pyrite formation occur. The pore water gradients respond dynamically to environmental changes in the Black Sea with relatively short time constants of ca. 500 yr for SO₄²⁻ and 10 yr for H₂S, whereas the FeS in the black band has taken ca. 3000 yr to accumulate. The dual diffusion interfaces of SO₄²⁻-CH₄ and H₂S-Fe²⁺ cause the trapping of isotopically heavy iron sulfide with δ³⁴S = +15 to +33‰ at the sulfidization front. A diffusion model for sulfur isotopes shows that the SO₄²⁻ diffusing downward into the SO₄²⁻-CH₄ transition has an isotopic composition of +19‰, close to the +23‰ of H₂S diffusing upward. These isotopic compositions are, however, very different from the porewater SO₄²⁻ (+43‰) and H₂S (−15‰) at the same depth. The model explains how methane-driven sulfate reduction combined with a deep H₂S sink leads to isotopically heavy pyrite in a sediment open to diffusion. These results have general implications for the marine sulfur cycle and for the interpretation of sulfur isotopic data in modern sediments and in sedimentary rocks throughout earth’s history.  相似文献   

The role of sulfur in chemical weathering and atmospheric CO₂ fluxes: Evidence from major ions, δC_DIC, and δS_SO4 in rivers of the Canadian Cordillera     

Jody Spence  Kevin Telmer 《Geochimica et cosmochimica acta》2005,69(23):5441-5458

Water samples from the Fraser, Skeena and Nass River basins of the Canadian Cordillera were analyzed for dissolved major element concentrations (HCO₃⁻, SO₄²⁻, Cl⁻, Ca²⁺, Mg²⁺, K⁺, Na⁺), δ¹³C of dissolved inorganic carbon (δ¹³C_DIC), and δ³⁴S of dissolved sulfate (δ³⁴S_SO4) to quantify chemical weathering rates and exchanges of CO₂ between the atmosphere, hydrosphere, and lithosphere. Weathering rates of silicates and carbonates were determined from major element mass balance. Combining the major element mass balance with δ³⁴S_SO4 (−8.9 to 14.1‰_CDT) indicates sulfide oxidation (sulfuric acid production) and subsequent weathering of carbonate and to a lesser degree silicate minerals are important processes in the study area. We determine that on average, 81% of the riverine sulfate can be attributed to sulfide oxidation in the Cordilleran rivers, and that 25% of the total weathering cation flux can be attributed to carbonate and silicate dissolution by sulfuric acid. This result is validated by δ¹³C_DIC values (−9.8 to −3.7‰ _VPDB) which represents a mixture of DIC produced by the following weathering pathways: (i) carbonate dissolution by carbonic acid (−8.25‰) > (ii) silicate dissolution by carbonic acid (−17‰) ≈ (iii) carbonate dissolution by sulfuric acid derived from the oxidation of sulfides (coupled sulfide-carbonate weathering) (+0.5‰).δ³⁴S_SO4 is negatively correlated with δ¹³C_DIC in the Cordilleran rivers, which further supports the hypothesis that sulfuric acid produced by sulfide oxidation is primarily neutralized by carbonates, and that sulfide-carbonate weathering impacts the δ¹³C_DIC of rivers. The negative correlation between δ³⁴S_SO4 and δ¹³C_DIC is not observed in the Ottawa and St. Lawrence River basins. This suggests other factors such as landscape age (governed by tectonic uplift) and bedrock geology are important controls on regional sulfide oxidation rates, and therefore also on the magnitude of sulfide-carbonate weathering—i.e., it is more significant in tectonically active areas.Calculated DIC fluxes due to Ca and Mg silicate weathering by carbonic acid (38.3 × 10³ mol C · km⁻² · yr⁻¹) are similar in magnitude to DIC fluxes due to sulfide-carbonate weathering (18.5 × 10³ mol C · km⁻² · yr⁻¹). While Ca and Mg silicate weathering facilitates a transfer of atmospheric CO₂ to carbonate rocks, sulfide-carbonate weathering can liberate CO₂ from carbonate rocks to the atmosphere when sulfide oxidation exceeds sulfide deposition. This implies that in the Canadian Cordillera, sulfide-carbonate weathering can offset up to 48% of the current CO₂ drawdown by silicate weathering in the region.  相似文献   

PGE, Re-Os, and Mo isotope systematics in Archean and early Proterozoic sedimentary systems as proxies for redox conditions of the early Earth     

C. Siebert  Th. Meisel  Th.F. Nägler 《Geochimica et cosmochimica acta》2005,69(7):1787-1801

Re-Os data and PGE concentrations as well as Mo concentrations and isotope data are reported for suites of fine clastic sediments and black shales from the Barberton Greenstone Belt, South Africa (Fig Tree and Moodies Groups, 3.25-3.15 Ga), the Belingwe Greenstone Belt, Zimbabwe (Manjeri Formation, ca. 2.7 Ga) and shales from the Witwatersrand, Ventersdorp and Transvaal Supergroups, South Africa ranging from 2.95 to 2.2 Ga. Moderately oxidizing conditions are required to mobilize Re and Mo in the environment, Mo fractionation only occurs in solution, and these parameters thus have potential use as paleoredox proxies for the early Earth.PGE + Re abundance patterns of Barberton Greenstone Belt sediments are uniform and very similar in shape to those of komatiites. This indicates (1) that the PGE came from a source of predominantly ultramafic composition and, (2) that PGE were transported and deposited essentially in particulate form. Sediments from the younger Belingwe Greenstone Belt show more fractionated PGE + Re patterns and have Re/Os ratios 10 to 100× higher than those of Barberton sediments. Their PGE abundance patterns and Re/Os ratios are intermediate between those of the mid-Archean shales and Neoproterozoic to Recent black shales. They reflect scavenging of Re from solution in the sedimentary environment.δ^98/95Mo values of black shales of all ages correlate with their concentrations. The Barberton Greenstone Belt samples have ∼1-3 ppm Mo, similar to a granitoid-basaltic source. This Mo has δ^98/95Mo between −1.9 and −2.4‰ relative to present day mean ocean water molybdenum, MOMO and is thus not isotopically fractionated relative to such a source. Similar to the PGE this indicates transport in solid form. Sediments from the Belingwe Greenstone Belt show in part enhanced Mo concentrations (up to 6 ppm) and Mo isotope fractionation (δ^98/95Mo up to −1.4‰ relative to MOMO). The combined PGE + Re and Mo data show mainly reducing conditions in the mid-Archean and suggest that by 2.7 Ga, the atmosphere and oceans had become more oxidizing.Substantially younger samples from the Transvaal Supergroup (to ca. 2.2 Ga) surprisingly have mainly low Mo concentrations (around 1 ppm) and show no significant Mo isotope fractionation relative to the continental source. Among possible explanations for this are a return to reducing atmospheric conditions after 2.7 Ga, reservoir effects, or Mo removal by sulfide precipitation following sulfate reduction in early Proterozoic oceans.  相似文献   

The solubility of molybdenum dioxide and trioxide in HCl-bearing water vapour at 350 °C and pressures up to 160 bars     

K.U. Rempel  A.E. Williams-Jones  A.A. Migdisov 《Geochimica et cosmochimica acta》2008,72(13):3074-3083

The solubility and speciation of the assemblage MoO₂-MoO₃ in water vapour were investigated in experiments conducted at 350 °C, P_total from 59 to 160 bar and fHCl from 0 to 3.4 bar (0-2.0 mol%). Measured solubility at these conditions ranges from 22 to 2500 ppm (∑fMo from 4.4 × 10⁻⁴ to 6.5 × 10⁻² bar). The concentration of Mo in the vapour at fHCl below 0.1 bar is similar to that in pure water vapour, but increases by two orders of magnitude at fHCl above 0.1 bar. The fugacity of gaseous Mo species is independent of chloride concentration at fHCl below 0.1 bar, but increases with increasing fHCl above this pressure. The dominant Mo species at fHCl below 0.1 bar is interpreted to be the same as it is in pure water vapour, and to form as a result of the reaction