V XANES in spinels as an oxy-barometer in meteorites with implications for redox variations in the inner solar system

Introduction: The variation of oxygen fugacity within inner solar system materials spans a range of nearly 15 orders of magnitiude [1]. Igneous and metamorphic rocks commonly contain a mineral assemblage that allows oxygen fugacity to be calculated or constrained such as FeTi oxides, olivine-opx-spinel, or some other oxybarometer [2]. Some rocks, however, contain a limited mineral assemblage and do not provide constraints on fO2 using mineral equilibria. Good examples of the latter are orthopyroxenites or dunites, such as diogenites, ALH 84001, chassignites, or brachinites. In fact it is no surprise that the fO2 of many of these achondrites is not well known, other than being "reduced" and below the metal saturation value. In order to bridge this gap in our understanding, we have initiated a study of V in chromites in achondrite. Because the V pre-edge peak intensity and energy in chromites varies with fO2 (Fig. 1) [3], and this has been calibrated over a large fO2 range, we can apply this relation to rocks for which we otherwise have no fO2 constraints. Samples: We have selected a suite of samples for which there are no phases for use of standard oxybarometers, and for which there are large and accessible chromites: ALH84001 (martian orthopyroxenite), EET 99402 and ALH 84025 (brachinites), GRA 06128/129 (ungrouped achondrites), ALH 77256 and GRA 98108 (diogenites), ALH 77081, EET 84302, MAC 88177, MET 01198, RBT 04228 (acapulcoites/lodranites), and QUE 99679 (CK chondrite). XANES and EMPA measurements: All major elements in spinels were analyzed with a CAMECA SX100 electron microprobe, using an accelerating voltage of 20 kV, sample current of 20 nA, and standardization and corrections as described in [3]. Measurements of the valence of V were made using synchrotron microXANES (X-ray Absorption Near-Edge Structure) spectroscopy (SmX), at the Advanced Photon Source (APS), Argonne National Laboratory (beamline 13-ID, the Consortium for Advanced Radiation Sources or CARS). SmX measurements are made by focusing a monochromatic (cryogenic, Si (111) double crystal monochromator) X-ray beam (3x3 m) from the synchrotron onto a spot on the sample and measuring the fluorescent X-ray yield from that spot as a function of incident X-ray energy. Changes of fluorescent X-ray intensity and energy of features in the XANES spectrum (notably the preedge peak) depend on oxidation state and coordination (e.g., [4,5]). In the present work, the intensity of the XANES pre-edge