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Multiple generations of hibonite in spinel‐hibonite inclusions from Murchison
Author(s) -
SIMON Steven B.,
GROSSMAN Lawrence,
DAVIS Andrew M.
Publication year - 1997
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/j.1945-5100.1997.tb01264.x
Subject(s) - murchison meteorite , carbonaceous chondrite , chondrite , mineralogy , melilite , spinel , chemistry , meteorite , materials science , astrobiology , physics , metallurgy
— 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 TiO 2 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 TiO 2 ‐rich hibonite, some that is virtually TiO 2 ‐free but contains ∼1 wt% MgO and ∼1.5 wt% SiO 2 . 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 <2.5 times 10 5 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).