Extremely rapid cooling of a carbonaceous-chondrite chondrule containing very O-rich olivine and a Mg-excess

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.