Title: Petrography, composition, and origin of large, chromian spinels from the Murchison meteorite
Abstract: Most spinel grains in Murchison acid residues are Mg-, Al-rich, 16O-rich (δ18O = −50%.), small (10–30 μm) and probably from refractory inclusions. They are quite unlike spinels we have recovered from Murchison by freeze-thaw disaggregation, density separation, and handpicking. As reported here, the latter spinels contain up to 37 wt% Cr2O3 and up to 17 wt% FeO, are not 16O-enriched (δ18O = 1.9 ± 2.4%.), are coarse (60–325 μm), and are not from refractory inclusions. From backscattered electron images of fifty-seven such grains, we recognize five zoning types defined by variations in Cr2O3 contents: patchy (56%); homogeneous (21%); chevron (10.5%); gradational (9%); and core-rim (3.5%). Many grains have silicate inclusions, the most common being small, anhedral grains of diopside with 12–24 wt% Al2O3 and up to 3.7 wt% TiO2. Eleven spinel samples occur with forsteritic (Fo95–99) olivine; in most cases, the spinel partially encloses the olivine. Cr-bearing spinel was found in situ in two Al-rich chondrules (one with homogeneous spinel, the other with homogeneous, gradational and core-rim spinel, and both with forsterite and aluminous diopside); in two irregularly shaped, olivine-bearing inclusions (one with homogeneous spinel, the other patchy); and attached to an isolated olivine grain (patchy). Observation of homogeneous, gradational, and core-rim type spinels in chondrules and basalts shows that grains with these zoning patterns can crystallize from liquids, although, in Murchison, chondrules with the appropriate compositions and sufficiently coarse textures to yield the separated spinels are exceedingly rare. Chevronzoned grains also could have formed in chondrules; alternatively, they may have acquired their oscillatory zoning patterns by cycling through different P-T-ƒO2 regimes in the solar nebula during their formation. The patchy spinel grains were probably never molten and they most likely formed by sintering of aggregates of smaller spinel grains which were enriched in Cr and Fe to varying degrees. In spite of their various crystallization and thermal histories, the spinels all have normal oxygen and chromium isotopic compositions, consistent with formation from a single, well-mixed nebular reservoir. Based on the known slow rates of diffusion of oxygen in MgO, Al2O3, and MgAl2O4, it is unlikely that the spinels of this study formed from an isotopically anomalous reservoir and later re-equilibrated with a normal one; it is more likely that they have retained their original isotopic compositions. We see no evidence for anomalous Cr, which had been reported by others.