Oxidation State of Vanadium in Glass and Olivine from Terrestrial and Martian Basalts: Implications for Oxygen Fugacity

INTRODUCTION Several studies have demonstrated the usefulness of Synchrotron micro x-ray absorption near edge structure (XANES), or (SmX) spectroscopy, to determine the oxidation state of elements in planetary materials. Delaney et al. [1] used SmX to investigate the oxidation states Fe, Cr, and V in extraterrestrial samples, and later determined the oxidation state of V in experimental glasses as a function of oxygen fugacity [2]. More recently, Sutton et al. [3] studied the oxidation state of V in meteoritic fassaite and also synthetic pyroxene. This paper reports our first results using SmX spectroscopy to determine the oxidation state of vanadium in olivine and glass from a terrestrial ocean floor (OF) basalt and a martian basaltic shergottite meteorite, Dar Al Gani 476. The goal of this study, and of future studies, is to use V (and Cr, Fe) valence states to determine the oxygen fugacity of basalts from different planetary bodies. METHODS All SmX analyses were performed with the GeoSoilEnviroCARS (GSECARS) x-ray microprobe at the Advanced Photon Source (APS), Argonne National Laboratory, IL. The microprobe consisted of an APS undulator x-ray source, a silicon monochromator, Kirkpatrick-Baez microfocussing mirrors and a germanium solid state x-ray fluorescence detector. The beam size was approximately 5 um. XANES spectra were measured over the V K absorption edge (5465 eV) from 5450 eV to 5600 eV. Measurements made below and above the absorption edge provided for normalization of the peaks. The pre-edge peak (near 5470 eV) was distinguished by the energy and normalized intensity at the maximum. Previous studies have shown these parameters to be useful for determining the valence state of V [1-3]. Three sets of standards were used to calibrate the experiment in terms of pre-edge peak intensity versus oxidation state of V, and oxygen fugacity. The first standards are two basaltic glasses by H. Schreiber [4] that provide independent calibration of the valence state of V at high fO 2 conditions. The second standard is a basaltic glass produced at known fO 2 conditions (IW + 1) by Hanson and Jones [e.g., 5]. A third set, by Canil [6], are komatiite compositions produced at known oxygen fugacities. RESULTS A summary of the XANES data, as well as the standard information is given in Table 1. For