Contemporaneous formation of chondrules in distinct oxygen isotope reservoirs

Abstract

We investigated the 26Al–26Mg systematics of type I (FeO-poor) chondrules from one of the most primitive carbonaceous chondrites, Acfer 094. The inferred initial 26Al/27Al ratios of chondrules from Acfer 094 ranged from (4.2±2.0)×10−6 to (9.0±1.5)×10−6. These chondrules have distinct oxygen isotope ratios (Δ17O: −5‰, −2‰, and 0‰), though no correlation between inferred initial 26Al/27Al ratios and oxygen isotope ratios were observed. These results indicate the regional oxygen isotope heterogeneity in the solar nebula over the duration of chondrule formation.A few low-Ca pyroxene grains in chondrules have small but resolvable 26Mg excesses at the level of ∼0.1‰, which may be the result of a partial melting of chondrules by reheating events. If this is the case, the true inferred initial 26Al/27Al ratios of these particular chondrule could be slightly lower (<10%) than estimates using Mg isotope ratios of coexisting plagioclase and olivine.Stable isotope ratios of Mg (25Mg/24Mg) of many olivine grains in chondrules are positively fractionated by ∼1‰ relative to those in other phases within the same chondrules. This suggests that olivine preserved a positively fractionated Mg isotope ratios of the chondrule-forming melt prior to significant condensation during the cooling stage of chondrule formation.