Chondrite thermal histories: Clues from electron microscopy of orthopyroxene |
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| Authors: | JR Ashworth |
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| Institution: | Department of Geological Sciences, University of Aston in Birmingham, Gosta Green, Birmingham B4 7ET England |
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| Abstract: | Optically “striated” orthopyroxenes in two ordinary chondrites, Allegan (H5) and Quenggouk (H4), are compared with shock-affected orthopyroxenes in Saint-Sévérin (LL6) and Ambapur Nagla (H5) by high-voltage transmission electron microscopy. The striated orthopyroxenes have very many, thin, evenly distributed lamellae of clinopyroxene. They are undeformed and also lack evidence of partial inversion from clinopyroxene to orthopyroxene. Striated orthopyroxene does not seem to be a reliable indicator of prograde metamorphism. Instead, it is interpreted as inverted protopyroxene, produced during the cooling of chondrules at slower rates than the rapid quenching of Type 3 chondrules. The conclusions are consistent with retrograde models for the evolution of H-group chondrites, in which the higher Petrologic Types are attributed to retarded cooling due to accretionary processes leading to the growth of the parent body. The thermal histories of ordinary chondrites could be greatly clarified by further experimental work on inversions in bronzitic pyroxenes. |
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