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In this study, we use micro-X-ray absorption near-edge structures (µ-XANES) spectroscopy at the S K-edge to investigate the oxidation state of S in natural magmatic-hydrothermal apatite (Durango, Mexico, and Mina Carmen, Chile) and experimental apatites crystallized from volatile-saturated lamproitic melts at 1000 °C and 300 MPa over a broad range of oxygen fugacities [log(fO2)=FMQ, FMQ+1.2, FMQ+3; FMQ = fayalite-magnetite-quartz solid buffer]. The data are used to test the hypothesis that S oxidation states other than S6+ may substitute into the apatite structure. Peak energies corresponding to sulfate S6+ (∼2482 eV), sulfite S4+ (∼2478 eV), and sulfide S2- (∼2470 eV) were observed in apatite, and the integrated areas of the different sulfur peaks correspond to changes in fO2 and bulk S content. Here, multiple tests confirmed that the S oxidation state in apatite remains constant when exposed to the synchrotron beam, at least for up to 1 h exposure (i.e., no irradiation damages). To our knowledge, this observation makes apatite the first mineral to incorporate reduced (S2-), intermediate (S4+), and oxidized (S6+) S in variable proportions as a function of the prevailing fO2 of the system. Apatites crystallized under oxidizing conditions (FMQ+1.2 and FMQ+3), where the S6+/STotal peak area ratio in the coexisting glass (i.e., quenched melt) is ∼1, are dominated by S6+ with a small contribution of S4+, whereas apatites crystallizing at reduced conditions (FMQ) contain predominantly S2-, lesser amounts of S6+, and possibly traces of S4+. A sulfur oxidation state vs. S concentration analytical line transect across hydrothermally altered apatite from the Mina Carmen iron oxide-apatite (IOA) deposit (Chile) demonstrates that apatite can become enriched in S4+ relative to S6+, indicating metasomatic overprinting via a SO2-bearing fluid or vapor phase. This XANES study demonstrates that as the fO2 increases from FQM to FMQ+1.2 to FMQ+3 the oxidation state of S in igneous apatite changes from S2- dominant to S6+ > S4+ to S6+ >> S4+ Furthermore, these results suggest that spectroscopic studies of igneous apatite have potential to trace the oxidation state of S in magmas. The presence of three S oxidations states in apatite may in part explain the non-Henrian partitioning of S between apatite and melt. Our study reveals the potential to use the S signature of apatite to elucidate both oxygen and sulfur fugacity in magmatic and hydrothermal systems.

Oxygen fugacity (fO2) exerts first-order control on the geochemical evolution of planetary interiors, and the Fe3+/ΣFe ratios of silicate glasses provide a useful proxy for fO2. Fe K-edge micro-X-ray absorption near-edge structure (XANES) spectroscopy allows researchers to micro-analytically determine the Fe3+/ΣFe ratios of silicate glasses with high precision. In this study we characterize hydrous and anhydrous basalt glass standards with Mossbauer and XANES spectroscopy and show that synchrotron radiation causes progressive changes to the XANES spectra of hydrous glasses as a function of radiation dose (here defined as total photons delivered per square micrometer), water concentration, and initial Fe3+/ΣFe ratio. We report experiments from eight different radiation dose conditions and show that Fe in hydrous silicate glasses can undergo rapid oxidation upon exposure to radiation. The rate and degree of oxidation correlates with radiation dose and the product of water concentration and ferrous/ferric iron oxide ratio on a molar basis (Φ = XHO0.5·XFeO/XFeO1.5). For example, a basalt glass with 4.9 wt% dissolved H2O and Fe3+/ΣFe = 0.19 from its Mossbauer spectrum may appear to have Fe3+/ΣFe ≥ 0.35 when analyzed over several minutes at a nominal flux density of ∼2 × 109 photons/s/µm2. This radiation-induced increase in Fe3+/ΣFe ratio would lead to overestimation of fO2 by about two orders of magnitude, with dramatic consequences for the interpretation of geological processes. The sample area exposed to radiation shows measureable hydrogen loss, consistent with radiation-induced breaking of O-H bonds, associated H migration and loss, and oxidation of Fe2+. This mechanism is consistent with the observation that anhydrous glasses show no damage under any beam conditions. Cryogenic cooling does not mitigate, but rather accelerates, iron oxidation. The effects of beam damage appear to persist indefinitely. We detect beam damage at the lowest photon flux densities tested (3 × 106 photons/s/µm2); however, at flux densities ≤6 × 107 photons/s/µm2, the hydrous glass calibration curve defined by the centroid (derived from XANES spectra) and Fe3+/ΣFe ratios (derived from Mossbauer spectra) is indistinguishable from the anhydrous calibration curve within the accuracy achievable with Mossbauer spectroscopy. Thus, published Fe3+/ΣFe ratios from hydrous glasses measured at low photon flux densities are likely to be accurate within measurement uncertainty with respect to what would have been measured by Mossbauer spectroscopy. These new results demonstrate that to obtain accurate Fe3+/ΣFe ratios from hydrous, mafic, silicate glasses, it is first necessary to carefully monitor changes in the XANES spectra as a function of incident dose (e.g., fixed-energy scan). Defocusing and attenuating the beam may prevent significant oxidation of Fe in mafic water-bearing glasses.

Fe-bearing carbonates have been proposed as possible candidate host minerals for carbon inside the Earth's interior and hence their spectroscopic properties can provide constraints on the deep carbon cycle. Here we investigate high-pressure spin crossover in synthetic FeCO3 (siderite) using a combination of Mossbauer, Raman, and X-ray absorption near edge structure spectroscopy in diamond-anvil cells. These techniques sensitive to the short-range atomic environment show that at room temperature and under quasi-hydrostatic conditions, spin crossover in siderite takes place over a broad pressure range, between 40 and 47 GPa, in contrast to previous X-ray diffraction data that described the transition as a sharp volume collapse at approximately 43 GPa. Based on these observations we consider electron spin pairing in siderite to be a dynamic process, where Fe atoms can be either high spin or low spin in the crossover region. Mode Gruneisen parameters extracted from Raman spectra collected at pressures below and above spin crossover show a drastic change in stiffness of the Fe-O octahedra after the transition, where they become more compact and hence less compressible. Mossbauer experiments performed on siderite single crystals as well as powder samples demonstrate the effect of differential stress on the local structure of siderite Fe atoms in a diamond-anvil cell. Differences in quadrupole splitting values between powder and single crystals show that local distortions of the Fe site in powder samples cause spin crossover to start at higher pressure and broaden the spin crossover pressure range.

Micro-X-ray absorption near-edge structure (µ-XANES) spectroscopy has been used by several recent studies to determine the oxidation state and coordination of iron in silicate glasses. Here, we present new results from Fe µ-XANES analyses on a set of 19 Fe-bearing felsic glasses and 9 basaltic glasses with known, independently determined, iron oxidation state. Some of these glasses were measured previously via Fe XANES (7 rhyolitic, 9 basaltic glasses; Cottrell et al. 2009), while most felsic reference glasses (12) were analyzed for the first time. The main purpose of this study was to understand how small changes in glass composition, especially at the evolved end of silicate melt compositions occurring in nature, may affect a calibration of the Fe µ-XANES method. We performed Fe µ-XANES analyses at different synchrotron radiation sources [Advanced Photon Source (APS), Argonne, U.S.A., and Angstromquelle Karlsruhe (ANKA), Germany] and compared our results to existing calibrations obtained at other synchrotron radiation sources worldwide. The compiled results revealed that changes in instrumentation have a negligible effect on the correlation between the centroid energy of the Fe pre-edge peak and the Fe oxidation state in the glasses. Oxidation of the glasses during extended exposure (up to 50 min) to the X-ray beam was not observed. Based on the new results and literature data we determined a set of equations for different glass compositions, which can be applied for the calculation of the iron valence ratio (Fe3+/ΣFe) in glasses by using XANES spectra collected at different synchrotron beamlines. For instance, the compiled felsic reference material data demonstrated that the correlation between the centroid energy of the Fe pre-edge peak CFe (eV) and the Fe3+/ΣFe ratio of felsic glasses containing 60.9 to 77.5 wt% SiO2 and 1.3 to 5.7 wt% FeOtot can be accurately described by a single linear trend, if the spectra were collected at 13-ID-E beamline at APS and for 0.3 ≤ Fe3+/ΣFe ≤ 0.85: CFe[eV] = 0.012395 (±0.00026217) × Fe3+/ΣFe + 7112.1 (±0.014525);R2 = 0.987. Based on this equation, the Fe oxidation state of felsic glasses can be estimated at an absolute uncertainty of ±2.4% Fe3+/ΣFe.In general, the differences between the calibrations for felsic and mafic glasses were small and the compiled data set (i.e., results collected at four different beamlines on 79 reference glass materials) is well described by a single second-order polynomial equation.

Many igneous rocks contain mineral assemblages that are not appropriate for application of common mineral equilibria or oxybarometers to estimate oxygen fugacity. Spinel-structured oxides, common minerals in many igneous rocks, typically contain sufficient V for XANES measurements, allowing use of the correlation between oxygen fugacity and V K pre-edge peak intensity. Here we report V pre-edge peak intensities for a wide range of spinels from source rocks ranging from terrestrial basalt to achondrites to oxidized chondrites. The XANES measurements are used to calculate oxygen fugacity from experimentally produced spinels of known fO2. We obtain values, in order of increasing fO2, from IW-3 for lodranites and acapulcoites, to diogenites, brachinites (near IW), ALH 84001, terrestrial basalt, hornblende-bearing R chondrite LAP 04840 (IW+1.6), and finally ranging up to IW+3.1 for CK chondrites (where the ΔIW notation = logfO2 of a sample relative to the logfO2 of the IW buffer at specific T). To place the significance of these new measurements into context we then review the range of oxygen fugacities recorded in major achondrite groups, chondritic and primitive materials, and planetary materials. This range extends from IW-8 to IW+2. Several chondrite groups associated with aqueous alteration exhibit values that are slightly higher than this range, suggesting that water and oxidation may be linked. The range in planetary materials is even wider than that defined by meteorite groups. Earth and Mars exhibit values higher than IW+2, due to a critical role played by pressure. Pressure allows dissolution of volatiles into magmas, which can later cause oxidation or reduction during fractionation, cooling, and degassing. Fluid mobility, either in the sub-arc mantle and crust, or in regions of metasomatism, can generate values >IW+2, again suggesting an important link between water and oxidation. At the very least, Earth exhibits a higher range of oxidation than other planets and astromaterials due to the presence of an O-rich atmosphere, liquid water, and hydrated interior. New analytical techniques and sample suites will revolutionize our understanding of oxygen fugacity variation in the inner solar system, and the origin of our solar system in general.

Chlorite is a ubiquitous product of metamorphism, alteration of magmatic rocks and hydrothermal processes owing to its large stability field and wide compositional range. Its composition is governed by several substitutions and has been used as a geothermometer, on the basis of empirical, semi-empirical, and thermodynamic models. As in some other phyllosilicates of petrological interest, the oxidation state of iron in chlorite may differ from the usually assumed divalent state. However, the crystal chemistry of trivalent iron in chlorite remains poorly known, and the thermodynamic properties of ferric chlorite are missing from databases used for petrological modeling. As part of an attempt to fill this gap, we present results from in situ, micrometer-scale measurements of the oxidation state of iron in various chlorite-bearing samples. X-ray absorption near-edge spectroscopy (XANES) was combined with electron probe microanalysis (EPMA) on the same crystals. Results show iron oxidation states varying from ferrous to ferric; iron is in octahedral coordination in all ferromagnesian chlorites but to ∼25% tetrahedral in the lithian chlorite cookeite (1.0 wt% Fe2O3(total)). Absolute amounts of ferric iron cover an unprecedented range (0 to ∼30 wt % Fe2O3). For highly magnesian, ferric chlorite, Fe concentrations are low and can be accounted for by Al = Fe3+ substitution. In Fe-rich samples, Fe3+ may exceed 2 atoms per formula unit (pfu, 18 oxygen basis). When structural formulas are normalized to 28 charges corresponding to the standard O10(OH)8 anionic basis, these measurements define the exchange vector of a di-trioctahedral-type substitution: 3 VI(Mg, Fe2+) = VI∎ + 2 VIFe3+, as described in earlier studies. However, structural formulas calculated on the basis of the oxygen contents actually measured by EPMA show that this trend is an artifact, due to the neglect of variations in the number of protons in the structure. Our measurements indicate increasing hydrogen deficiency with increasing Fe3+ content, up to ∼2 H+ pfu in the Fe3+-rich chlorite samples, corresponding to a net exchange vector of the type R2+ + H+ = Fe3+. These results do not support substitutions toward di-trioctahedral ferric end-members, and highlight the need for considering substitution toward an \"oxychlorite\" (i.e., H-deficient) ferric component, close to tri-trioctahedral, with an O12(OH)6 anionic basis, even in green, pristine-looking chlorite. The effects of iron oxidation and H deficiency on chlorite geothermometers were explored. They are deterring if H deficiency is ignored but, given the sensitivity of most thermometers to octahedral vacancy, the assumption FeTotal = Fe2+ is still safer than using high measured Fe3+ contents and the standard 28 charge basis, which artificially increases vacancies. In such ferric chlorites, EPMA measurement of oxygen allows a fair estimate of H content if Fe3+/Fe2+ is known; it should be more systematically implemented. For the same reasons, literature data reporting Fe3+-rich chlorite with vacancy content along the possibly artificial di-trioctahedral-type substitution should be verified. With the help of constraints from thermodynamic models, charge balance, crystal symmetry, and proton loss, a new cation site distribution is proposed for di-tri- to tri-trioctahedral chlorites in the Fe2+-Fe3+-Mg-Al-Si-O-H system, allowing a more realistic thermodynamic handling of their solid solutions.

Peridotites dredged from mid-ocean ridges and glassy mid-ocean ridge basalts (MORB) transmit information about the oxygen fugacity (fO2) of Earth's convecting upper mantle to the surface. Equilibrium assemblages of olivine+orthopyroxene+spinel in abyssal peridotites and Fe3+/ΣFe ratios in MORB glasses measured by X-ray absorption near-edge structure (XANES) provide independent estimates of MORB source region fO2, with the former recording fO2 approximately 0.8 log units lower than the latter relative to the quartz-fayalite-magnetite (QFM) buffer. To test cross-compatibility of these oxybarometers and examine the compositional effects of changing fO2 on a peridotite plus melt system over a range of Earth-relevant fO2, we performed a series of experiments at 0.1 MPa and fO2 controlled by CO-CO2 gas mixes between QFM-1.87 and QFM+2.23 in a system containing basaltic andesite melt saturated in olivine, orthopyroxene, and spinel Oxygen fugacities recorded by each method are in agreement with each other and with the fO2 measured in the furnace. Measurements of fO2 from the two oxybarometers agree to within 1σ in all experiments. These results demonstrate that the two methods are directly comparable and differences between fO2 measured in abyssal peridotites and MORB result from geographic sampling bias, petrological processes that change fO2 in these samples after separation of melts and residues, or abyssal peridotites may not be residues of MORB melting. As fO2 increases, spinel Fe3+ concentrations increase only at the expense of Cr from QFM-1.87 to QFM-0.11. Above QFM, Al is also diluted in spinel as the cation proportion of Fe3+ increases. None of the three spinel models tested, MELTS (Ghiorso and Sack 1995), SPINMELT (Ariskin and Nikolaev 1996), and MELT_CHROMITE (Poustovetov and Roeder 2001), describe these compositional effects, and we demonstrate that MELTS predicts residues that are too oxidized by >1 log unit to have equilibrated with the coexisting liquid phase. Spinels generated in this study can be used to improve future thermodynamic models needed to predict compositional changes in spinels caused by partial melting of peridotites in the mantle or by metamorphic reactions as peridotites cool in the lithosphere. In our experimental series, where the ratio of Fe2O3/FeO in the melt varies while other melt compositional parameters remain nearly constant, experimental melt fraction remains constant, and Fe3+ becomes increasingly compatible in spinel as fO2 increases. Instead of promoting melting, increasing the bulk Fe3+/ΣFe ratio in peridotite drives reactions analogous to the fayalite-ferrosilite-magnetite reaction. This may partly explain the absence of correlation between Na2O and Fe2O3 in fractionation-corrected MORB.

Minerals of the Fe-As-S system are the main components of Au ores in many hydrothermal deposits, including Carlin-type Au deposits, volcanogenic massive sulfide deposits, epithermal, mesothermal, sedimentary-hosted systems, and Archean Au lodes. The \"invisible\" (or refractory) form of Au is present in all types of hydrothermal ores and often predominates. Knowledge of the chemical state of \"invisible\" Au (local atomic environment/structural position, electronic structure, and oxidation state) is crucial for understanding the conditions of ore formation and necessary for the physical-chemical modeling of hydrothermal Au mineralization. In addition, it will help to improve the technologies of ore processing and Au extraction. Here we report an investigation of the chemical state of \"invisible\" Au in synthetic analogs of natural minerals (As-free pyrite FeS2, arsenopyrite FeAsS, and lollingite FeAs2). The compounds were synthesized by means of hydrothermal (pyrite) and salt flux techniques (in each case) and studied by X-ray absorption fine structure (XAFS) spectroscopy in a high-energy resolution fluorescence detection (HERFD) mode in combination with first-principles quantum chemical calculations. The content of \"invisible\" Au in the synthesized lollingite (800 ± 300 ppm) was much higher than that in arsenopyrite (23 ± 14 ppm). The lowest Au content was observed in zonal pyrite crystals synthesized in a salt flux. High \"invisible\" Au contents were observed in hydrothermal pyrite (40-90 ppm), which implies that this mineral can efficiently scavenge Au even in As-free systems. The Au content of the hydrothermal pyrite is independent of sulfur fugacity and probably corresponds to the maximum Au solubility at the experimental P-T parameters (450 °C, 1 kbar). It is shown that Au replaces Fe in the structures of lollingite, arsenopyrite, and hydrothermal pyrite. The Au-ligand distance increases by 0.14 A (pyrite), 0.16 A (lollingite), and 0.23 A (As), 0.13 A (S) (arsenopyrite) relative to the Fe-ligand distance in pure compounds. Distortions of the atomic structures are localized around Au atoms and disappear at R > ∼4 A. Chemically bound Au occurs only in hydrothermal pyrite, whereas pyrite synthesized without hydrothermal fluid contains only Au°. The heating (metamorphism) of hydrothermal pyrite results in the decomposition of chemically bound Au and formation of Au° nuggets, which coarsen with increasing temperature. Depending on the chemical composition of the host mineral, Au can play a role of either a cation or an anion: the Bader atomic partial charge of Au decreases in the order pyrite (+0.4 e) > arsenopyrite (0) > lollingite (-0.4 e). Our results suggest that other noble metals (platinum group elements, Ag) can form a chemically bound refractory admixture in base metal sulfides/chalcogenides. The content of chemically bound noble metals can vary depending on the composition of the host mineral and ore history.