"Dehydrated" chondrules from the Murchison (CM) chondrite

概要

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Two "dehydrated" chondrules (MC-5 and MC-27) were obtained from close to the fusion crust of the Murchison (CM) meteorite. They have a porphyritic texture with angular or rounded (relict) olivines containing abundant voids, recrystallized olivines, and numerous interstitial Fe oxide grains embedded in groundmass glass. This peculiar texture is similar to that of the fusion crust from the Orgueil (CI) chondrite, indicating that the texture formed during the reheating at the atmospheric entry of the meteorite. Isotope dilution analyses indicate that these chondrules have light-REE depleted pattern with a large negative Eu anomaly (CI normalized Sm/La ratio=1.3-2.4,Eu/Eu^*=0.40-0.57). They also show relatively low Ba and Sr (0.4-1.0×CI) concentrations and depletion of alkalies (0.2-0.4×CI). The lithophile element abundances of the two chondrules are in a similar range of those for other altered CM chondrules in the Murchison and the Yamato-793321 chondrites. It is suggested that the chemical compositions of chondrules, specifically REE and alkali abundances, remained unchanged but significant changes of petrographic texture occurred during atmospheric heating.
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