FIRST Ti-XANES ANALYSES OF REFRACTORY INCLUSIONS FROM MURCHISON

Introduction: A system of solar composition is so reducing that Ti and Ti can coexist, making the valence of Ti a valuable indicator of fO2 conditions during formation of nebular materials. The Ti/Ti ratios observed in the Ti-rich phases fassaite and rhönite in coarse-grained refractory inclusions from CV3 chondrites have been shown [1,2] to be quantitatively consistent with formation in a gas of solar composition (log fO2=IW-6.8), but these are the only objects in chondrites for which this is the case. Here, we report the valence of Ti in various phases in refractory inclusions from the Murchison CM2 chondrite. The second-highest temperature, major-elementbearing phase predicted to condense from a gas of solar composition, hibonite (ideally CaAl12O19), can contain significant amounts of Ti, but the hibonite structure can have oxygen vacancies, so calculation of Ti valence from stoichiometry of electron probe analyses is not recommended for hibonite [3]. To date, the only reported measurement of Ti valence in meteoritic hibonite was done by electron spin resonance [3], on coarse crystals from a Murchison hibonite-perovskitemelilite inclusion [4]. Spinel and most of the pyroxene in CM inclusions contain too little Ti for derivation of Ti/Ti ratios from electron probe analyses. X-ray absorption near edge spectroscopy (XANES), however, allows determination of Ti valence in relatively Ti-poor phases. In the present work, we apply synchrotron microXANES to a large hibonite grain from Murchison and to spinel-hibonite (sp-hib) and spinel-pyroxene (sp-pyx) inclusions from Murchison, refractory materials whose Ti/Ti ratios have not been previously measured. Analysis of these samples allows comparison of Ti valence of 1) pyroxene in sp-pyx inclusions with that of fassaite; 2) spinel in hibonite-bearing with that of hibonite-free inclusions; and 3) hibonite in sp-hib inclusions with that of large, single grains and the previously analyzed sample [3]. Analytical methods: Samples were documented with a scanning electron microscope and analyzed by electron probe. Ti K XANES spectra were collected using the GSECARS X-ray microprobe (fluorescence mode, 3 μm X-ray beam). Ti K-edge XANES spectra of pure Ti-bearing minerals fall into three, distinct valence-coordination clusters on a plot of pre-edge peak intensity vs. energy [5].