Other
Scientific paper
Nov 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997gecoa..61.4689h&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 61, p. 4689
Other
6
Chondrule, Metamorphism (Geology), Chondrites, Meteoritic Composition, High Temperature Environments, Petrography, Chemical Composition
Scientific paper
This paper represents the first attempt to describe and discuss the unique group A5 class of chondrules. Group A5 chondrules are one of the four primary chondrule groups occurring in the essentially unmetamorphosed Semarkona chondrite, the others being groups A1, A2, and B1. Group A5 chondrules have olivine compositions resembling those of group B1 chondrules, and unlike the FeO-poor compositions (less than 4 wt pct) of olivines from group A1 and A2 chondrules, they undergo compositional changes with metamorphism similar to group B1 olivines. Group A5 chondrules in six LL chondrites were studied by cathodoluminescence, optical microscopy, electron-microprobe, and, in the case of four chondrules removed from Semarkona (type 3.0) and Krymka (type 3.1), by instrumental neutron activation analysis. The major conclusions of the study are: (1) Group A5 chondrules are not simply xenolithic objects from equilibrated chondrites, because their olivine is compositionally more heterogeneous than olivine in equilibrated chondrules, and the mesostasis is not recrystallized. (2) Group A5 chondrules probably experienced similar conditions and peak temperatures during chondrule formation as group B1, because their redox state and bulk composition are similar. (3) Mesostasis compositions suggest important differences in crystallization and cooling history for group A5 and B1 chondrules.
Huang Shaoxiong
Sears Derek W. G.
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