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A spiked (with REE, V, Sc) martian basalt Yamato 980459 (Y98) composition was used to synthesize olivine, spinel, and pyroxene at 1200 °C at five oxygen fugacities: IW-1, IW, IW+1, IW+2, and QFM. These run products were analyzed by electron microprobe, ion microprobe, and X-ray absorption near-edge spectroscopy to establish four oxybarometers based on vanadium partitioning behavior between the following pairs of phases: V spinel-melt, V/(Cr+Al) spinel-melt, olivine-melt, and spinel-olivine. The results for the spinel-melt, olivine-melt, and V/(Cr+Al) spinel-melt are applicable for the entire oxygen fugacity range while the spinel-olivine oxybarometer is only applicable between IW-1 and IW+1. The oxybarometer based on V partitioning between spinel-olivine is restricted to basalts that crystallized under low oxygen fugacities, some martian, all lunar, as well as samples from 4 Vesta. The true potential and power of the new spinel-olivine oxybarometer is that it does not require samples representative of a melt composition or samples with some remnant of quenched melt present. It just requires that the spinel-olivine pairs were in equilibrium when the partitioning of V occurred. We have applied the V spinel-olivine oxybarometer to the Y98 meteorite as a test of the method. [ABSTRACT FROM AUTHOR]

The relative proportion of divalent and trivalent Eu has proven to be a useful tool for estimating fO2 in various magmatic systems. However, in most cases, direct determination of the Eu valence state has not been made. In this study, direct determination of Eu valence by XANES and REE abundance in merrillite provide insights into the crystal chemistry of these phosphates and their ability to record conditions of magmatism. Merrillite strongly prefers Eu3+ to Eu2+, with the average valence state of Eu ranging between 2.9 and 3 over approximately six orders of magnitude in fO2. The dramatic shift in the REE patterns of merrillite in martian basaltic magmas, from highly LREE-depleted to LREE-enriched, parallels many other trace element and isotopic variations and reflects the sources for these magmas. The behavior of REE in the merrillite directly reflects the relationship between the eightfold-coordinated Ca1 site and adjacent sixfold Na and tetrahedral P sites that enables charge balancing through coupled substitutions. [ABSTRACT FROM AUTHOR]

CO2-rich solutions are common in geological environments. An XANES (X-ray absorption near-edge structure) study of Br in CO2-bearing synthetic fluid inclusions has revealed that Br exhibits a strong pre-edge feature at temperatures from 298 to 423 K. Br in CO2-free solutions does not show such a feature. The feature becomes smaller and disappears as temperature increases, but reappears when temperature is reduced. The size of the feature increases with increasing X(CO2) in the fluid inclusion, where X(CO2) is the mole fraction of CO2 in the solution [ nCO2/( nCO2 + nH2O + nRbBr); n indicates the number of moles]. The pre-edge feature is similar to that shown by covalently bonded Br, but observed and calculated concentrations of plausible Br-bearing covalent compounds (Br2, CH3Br and HBr) are vanishingly small. An alternative possibility is that CO2 affects the hydration of Br sufficiently that the charge density changes to favour the 1 s– p level transitions that are thought to cause the pre-edge peak. The distance between the first two post-edge maxima in the XANES also decreases with increasing X(CO2). This is attributed to a CO2-related decrease in the polarity of the solvent. The proposed causes of the observed features are not integrated into existing geochemical models; thus CO2-bearing solutions could be predicted poorly by such models, with significant consequences for models of geological processes such as ore-formation and metamorphism. [ABSTRACT FROM AUTHOR]

Highlights: [•] XANES and optical characterization of [5]Co2+ in phosphates. [•] 4-, 5- and 6-fold Co2+ superimpose but can be discriminated. [•] Optical and XANES pre-edge transition intensities are larger in (D3h) than (C4v). [•] [5]Co2+ geometrical distortions affect transition energy splitting. [ABSTRACT FROM AUTHOR]

Andesitic glasses equilibrated at 1350 °C over a range of oxygen fugacities (log f O 2 from − 8.63 to − 0.68) were examined with Fe K-edge X-ray absorption near-edge structure (XANES) and Mössbauer spectra. XANES spectral features were then calibrated as a function of Mössbauer-derived Fe 3+ /∑Fe ratios. Additionally, both methods help characterize the local structure of iron ions in andesitic glasses. Fe 3+ /∑Fe ratios were determined from Mössbauer spectra collected at room temperature but corrected with recoilless fractions obtained from previously reported Mössbauer data collected on one of the glasses from 47 to 293 K. An empirical model was derived for the correlation between the pre-edge centroid energy and Fe 3+ /∑Fe ratio for andesitic glasses. This trend is intermediate between those previously determined for rhyolitic and basaltic glasses, but the distinction from basaltic compositions may be owing chiefly to differences in calibrations for Fe 3+ /∑Fe ratio, rather than to intrinsic differences in the spectra as a function of Fe 3+ /∑Fe ratio for mafic glasses. The ratios of intensities of pre-edge sub-peaks and Fe 3+ /∑Fe ratios for andesitic, basaltic, and rhyolitic glasses plot along a common trend, indicating that these measures provide a XANES calibration for Fe 3+ /∑Fe ratio that is less dependent on silicate composition. The coordination numbers of Fe 3+ and Fe 2+ ions in andesitic glass can be calculated from observations of pre-edge centroid energies and total intensities, combined with independent constraints on Fe 3+ /∑Fe ratio from Mössbauer spectra. The mean coordination of Fe 2+ ions calculated this way is close to 5.5 for reduced and oxidized compositions, and this is consistent with inferences from hyperfine features of the Mössbauer spectra. The mean coordination number of Fe 3+ inferred from XANES increases from ~ 4.5 to ~ 5 as andesitic glasses vary from reduced to oxidized; Mössbauer hyperfine parameters also suggest network-forming behavior of Fe 3+ , but with higher coordination for more reduced glasses. [ABSTRACT FROM AUTHOR]