MULTIPLE GENERATIONS OF HIBONITE IN SPINEL-HIBONITE INCLUSIONS FROM MURCHISON

Through freeze-thaw disaggregation of the Murchison meteorite, we have recovered, in addition to many spinel-hibonite spherules, several hib onite-rich inclusion fragments in which the hibonite has wider ranges in TiO2 contents (e.g., 0.07-8.6 wt% in one inclusion and 2-10 wt% in another) than previously observed within single inclusions. In these i nclusions, there are sharp contacts between texturally early, Ti-poor hibonite and relatively late, Ti-rich hibonite, and the two types occu r in complex intergrowth textures that are not consistent with crystal lization from a melt in a single-stage cooling event. One inclusion ha s, in addition to relatively TiO2-rich hibonite, some that is virtuall y TiO2-free but contains similar to 1 wt% MgO and similar to 1.5 wt% S iO2. Instead of the common substitution of Mg + Ti for 2Al, Mg coupled with Si in this case, probably reflecting crystallization from an unu sual, Ti-free silicate liquid. Ion microprobe analyses of Ti-rich and Ti-poor hibonite from this inclusion yield quite similar trace-element patterns and Mg-isotopic compositions. The results are most consisten t with formation of Ti-rich hibonite from Ti-poor hibonite by addition of Mg and Ti to the latter by exchange with a hot, Ti-rich liquid. Th at this occurred without a resolvable change in the Mg-isotopic compos ition requires that the Ti-rich, second generation of hibonite formed <2.5 x 10(5) yr later than the Ti-poor hibonite. Observations of spine l-hibonite spherules and spherule fragments in our sample suite provid e additional evidence that, despite the claims by Greenwood et al. (19 94), spinel-hibonite spherules crystallized from individual, molten dr oplets, as earlier suggested by Macdougall (1981) and MacPherson et al . (1983).