Using Vanadium in Spinel as a Sensor of Oxygen Fugacity in Meteorites: Applications to Mars, Vesta, and Other Asteroids

Introduction: 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 [1]. 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 samples 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 natural meteorite samples. Because the V pre-edge peak intensity and energy in chromites varies with fO2 (Fig. 1) [2], 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 available phases for standard oxybarometers, and for which there are large and accessible chromites. ALH84001 is a martian orthopyroxenite, for which there has been some debate regarding its oxygen fugacity; some have argued an oxidized fO2 near FMQ [3], while others have argued a more reduced value, well below FMQ [4,5]. Brachinites are olivine-rich achondrites that also contain chromite, plagioclase and pyroxene. They have recently been linked to the ungrouped achondrite GRA 06128/129, via O isotopes, age, and composition [6-8]. Chromites in EET 99402, ALH 84025, and GRA 06129 were measured and compared to assess whether they record comparably oxidized conditions. Diogenites contain orthopyroxene and chromite, and although they have been linked to eucrites and the HED parent body which is thought to be ~ IW-1 [9], there have not been redox estimates for diogenites. We have measured chromites in ALH 77256 and GRA98108, and can compare the results to previous estimates related to the HED parent body. Finally, we have analyzed a suite of lodranites and acapulcoites again because the redox conditions in this parent body have not been discussed in detail, yet seem to be of fundamental importance on understanding and constraining the processes that led to their formation. All samples were provided by MWG except for the acapulcoites/lodranites (J. Herrin), GRA 06128 (A. Treiman), and ALH 84001 (M. Righter). 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 [2]. Measurements of the valence of V were made using synchrotron micro-XANES (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 μm) 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 pre-edge peak) depend on oxidation state and coordination (e.g., [10,11]). In the present work, the intensity of the