Primordial oxygen isotope reservoirs of the solar nebula recorded in chondrules in Acfer 094 carbonaceous chondrite

Abstract

Highly precise and accurate ion microprobe analyses of oxygen three-isotope ratios in chondrules from the Acfer 094, one of the most primitive carbonaceous chondrites, show that chondrules preserve evidence for oxygen isotope heterogeneity in chondrule-forming regions of the solar nebula. Identical Δ17O values in most minerals and glass within each chondrule indicate that the oxygen isotope ratio in chondrule melt did not change during or after crystallization. Nearly half of the chondrules studied contain small amounts of olivine grains that have an oxygen isotope anomaly relative to other minerals and glass in the same chondrule. Most chondrules in Acfer 094 can be classified into two oxygen isotope groups (Δ17O∼−2‰ and Δ17O∼−5‰) indicating that the final melting of chondrules occurred within two distinct oxygen isotope reservoirs, probably representing the local protoplanetary disk immediately before planetesimal formation. One of these reservoirs (Δ17O∼−2‰) is observed from chondrules in other carbonaceous chondrites and from crystalline silicates in comet Wild 2, suggesting that crystalline silicates formed in an oxygen isotope reservoir of Δ17O∼−2‰ were widely distributed in the outer asteroid belt and throughout the outer solar nebula. Oxygen three-isotope ratios of minerals in chondrules from Acfer 094 are distributed along a newly defined Primitive Chondrule Minerals (PCM) line, which has slope ∼1 [δ17O=(0.987±0.013)×δ18O−(2.70±0.11)] and intersects the terrestrial fractionation line at δ18O=5.8±0.4‰. These data are distinct from, and plot between, the CCAM, and Young and Russell lines. The PCM line is interpreted to represent the mixing trend of extreme oxygen isotope reservoirs in the early solar system that were the primary oxygen isotope reservoir of solids that accreted to form planets including the Earth.