EXTREMELY RAPID COOLING OF A CARBONACEOUS-CHONDRITE CHONDRULE CONTAINING VERY 16O-RICH OLIVINE AND A 26MG-EXCESS

Abstract

We describe a phenocryst in a CO-chondrite type-II chondrule that we infer to have formed by melting an amoeboid olivine aggregate (AOA). This magnesian olivine phenocryst has an extremely 16O-rich composition Δ17O (=δ17O – 0.52 · δ18O) = −23‰. It is present in one of the most pristine carbonaceous chondrites, the CO3.0 chondrite Yamato 81020. The bulk of the chondrule has a very different Δ17O of −1‰, thus the Δ17O range within this single chondrule is 22‰, the largest range encountered in a chondrule. We interpret the O isotopic and Fe-Mg distributions to indicate that a fine-grained AOA assemblage was incompletely melted during the flash melting that formed the chondrule. Some Fe-Mg exchange but negligible O-isotopic exchange occurred between its core and the remainder of the chondrule. A diffusional model to account for the observed Fe-Mg and O-isotopic exchange yields a cooling rate of 105 to 106 K hr−1. This estimate is much higher than the cooling rates of 101 to 103 K hr−1 inferred from furnace simulations of type-II chondrule textures (e.g. Lofgren, 1996); however, our cooling-rate applies to higher temperatures (near 1900 K) than are modeled by the crystal-growth based cooling rates. We observed a low 26Al/27Al initial ratio ((4.6 ± 3.0) · 10−6) in the chondrule mesostasis, a value similar to those in ordinary chondrites (Kita et al., 2000). If the 26Al/27Al system is a good chronometer, then chondrule I formed about 2 Ma after the formation of refractory inclusions.