Valence of titanium and vanadium in pyroxene in refractory inclusion interiors and rims

Abstract

The clinopyroxene in coarse-grained refractory inclusions contains significant amounts of Ti and V, two elements that are multivalent over the range of temperatures and oxygen fugacities under which the inclusions formed. The Ti 3+/Ti 4+ ratios and the valence of V of these pyroxenes are valuable recorders of nebular conditions. The former can be calculated stoichiometrically from electron probe analyses, but only for relatively Ti-rich grains (i.e., >∼4 wt% TiO 2 tot ). For Ti-poor pyroxene, and for measurement of V valence, another technique is needed. We have, for the first time, applied K-edge X-ray absorption near edge structure (XANES) spectroscopy to the measurement of Ti and V valence in meteoritic clinopyroxene in refractory inclusions. Use of MicroXANES, a microbeam technique with high (μm-scale) spatial resolution, allowed measurement of Ti and V valence along traverses across (a) Ti-, V-rich “spikes” in pyroxene in Type B1 inclusions; (b) typical grains in a Type B2 inclusion; and (c) the pyroxene layer of the Wark–Lovering rim sequence on the outsides of two inclusions. Measurements of Ti 3+/(Ti 3+ + Ti 4+), or Ti 3+/Ti tot, by XANES agree with values calculated from electron probe analyses to within ∼0.1, or ∼2 σ. The results show that Ti 3+/Ti tot increases sharply at the spikes, from 0.46 ± 0.03 to 0.86 ± 0.06, but the V valence, or V 2+/(V 2+ + V 3+), does not change, with V 2+ ≈ V 3+. We found that pyroxene in both Types B1 and B2 inclusions has Ti 3+/Ti tot and V 2+/V tot ratios between 0.4 and 0.7, except for the spikes. These values indicate, to first order, formation at similar, highly reducing oxygen fugacities that are consistent with a solar gas. The pyroxene in the rim on an Allende fluffy Type A coarse-grained refractory inclusion, TS24, has an average Ti 3+/Ti tot of 0.51 ± 0.08 and an average V 2+/V tot of 0.61 ± 0.06, determined by XANES. These values are within the range of those of pyroxene in the interiors of inclusions, indicating that the rims also formed under highly reducing conditions. Measurements of Ti 3+/Ti tot of pyroxene in the rim of a Leoville compact Type A inclusion, 144A, by both XANES and electron probe give a wide range of results. Of our 72 XANES analyses of this rim, 66% have Ti 3+/Ti tot of 0.40–0.71, and the remaining analyses range from 0 to 0.38. In data from Simon et al. [Simon J. I., Young E. D., Russell S. S., Tonui E. K., Dyl K. A., and Manning C. E. (2005) A short timescale for changing oxygen fugacity in the solar nebula revealed by high-resolution 26Al– 26Mg dating of CAI rims. Earth Planet. Sci. Lett. 238, 272–283.] for this sample, 7 electron probe analyses yield calculated Ti 3+/Ti tot values that are positive and 15 do not. In the probe analyses that have no calculated Ti 3+, Ca contents are anticorrelated and Al contents directly correlated with the total cations per 6 oxygens, and the data fall along trends calculated for addition of 1–7% spinel to pyroxene. It appears likely that electron probe analyses of pure pyroxene spots have Ti 3+/Ti tot values that are typical of refractory inclusions, in agreement with the majority of the XANES results. The average of the XANES data for 144A, 0.41 ± 0.14, is within error of that for TS24. The rim of 144A probably formed under reducing conditions like those expected for a solar gas, and was later heterogeneously altered, resulting in an uneven distribution of secondary, FeO-, Ti-bearing alteration products in the rim, and accounting for the measurements with low Ti 3+/Ti tot values.