PETROGRAPHY, COMPOSITION, AND ORIGIN OF LARGE, CHROMIAN SPINELS FROM THE MURCHISON METEORITE

Most spinel grains in Murchison acid residues are Mg-, Al-rich. O-16-r ich (deltaO-18 = -50 parts per thousand), small (10-30 mum) and probab ly from refractory inclusions. They are quite unlike spinels we have r ecovered from Murchison by freeze-thaw disaggregation. density separat ion, and handpicking. As reported here, the latter spinels contain up to 37 wt% Cr2O3 and up to 17 wt% FeO, are not O-16-enriched (deltaO-18 = 1.9 +/- 2.4 parts per thousand), are coarse (60-325 mum), and are n ot from refractory inclusions. From backscattered electron images of f ifty-seven such grains, we recognize five zoning types defined by vari ations 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 occ ur with forsteritic (Fo95-99) olivine; in most cases, the spinel parti ally encloses the olivine. Cr-bearing spinel was found in situ in two Al-rich chondrules (one with homogeneous spinel, the other with homoge neous, gradational and core-rim spinel, and both with forsterite and a luminous diopside); in two irregularly shaped, olivine-bearing inclusi ons (one with homogeneous spinel, the other patchy); and attached to a n isolated olivine grain (patchy). Observation of homogeneous, gradati onal, and core-rim type spinels in chondrules and basalts shows that g rains with these zoning patterns can crystallize from liquids, althoug h, in Murchison, chondrules with the appropriate compositions and suff iciently coarse textures to yield the separated spinels are exceedingl y rare. Chevron-zoned grains also could have formed in chondrules; alt ernatively, they may have acquired their oscillatory zoning patterns b y cycling through different P-T-f(O2), regimes in the solar nebula dur ing their formation. The patchy spinel grains were probably never molt en and they most likely formed by sintering of aggregates of smaller s pinel grains which were enriched in Cr and Fe to varying degrees. In s pite of their various crystallization and thermal histories, the spine ls all have normal oxygen and chromium isotopic compositions, consiste nt with formation from a single, well-mixed nebular reservoir. Based o n the known slow rates of diffusion of oxygen in MgO, Al2O3, and MgAl2 O4, it is unlikely that the spinels of this study formed from an isoto pically anomalous reservoir and later re-equilibrated with a normal on e; it is more likely that they have retained their original isotopic c ompositions. We see no evidence for anomalous Cr, which had been repor ted by others.