|Title:||Extremely rapid cooling of a carbonaceous‐chondrite chondrule containing very 16O−rich olivine and a 26Mg‐excess|
|Authors:||H. Yurimoto, and J.T. Wasson|
|Publication:||Geochim. Cosmochim. Acta, v. 66, p. 4355‐4363.|
|Publish Date:||December 2002|
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 5 to 6 K hr–1. This estimate is much higher than the cooling rates of 1 to 3 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.