Other
Scientific paper
Mar 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997m%26ps...32..259s&link_type=abstract
Meteoritics, vol. 32, pages 259-269
Other
5
Scientific paper
Through freeze-thaw disaggregation of the Murchison meteorite, we have recovered, in addition to many spinel-hibonite spherules, several hibonite-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 inclusions there are sharp contacts between texturally early, Ti-poor hibonite and relatively late, Ti-rich hibonite, and the two types occur in complex intergrowth textures that are not consistent with crystallization from a melt in a single-stage cooling event. One inclusion has, in addition to relatively TiO2-rich hibonite, some that is virtually TiO2-free but contains ~ 1 wt% MgO and ~ 1.5 wt% SiO2. Instead of the common substitution of Mg + Ti for 2Al, Mg coupled with Si in this case, probably reflecting crystallization from an unusual, 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 consistent 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. That this occurred without a resolvable change in the Mg isotopic composition requires that the Ti-rich, second generation of hibonite formed less than 2.5x105 yr later than the Ti-poor hibonite. Observations of spinel-hibonite spherules and spherule fragments in our sample suite provide additional evidence that, despite the claims by Greenwood et al. (1994), spinel-hibonite spherules crystallized from individual, molten droplets, as earlier suggested by Macdougall (1981) and MacPherson et al. (1983).
Davis Andrew M.
Grossman Lawrence
Simon Steven B. Simon
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