Oxygen-isotope systematics of chondrules and olivine fragments from Tagish Lake C2 chondrite: Implications of chondrule-forming regions in protoplanetary disk

Abstract

Oxygen-isotope ratios of olivine in type I (MgO-rich) and type II (FeO-rich) chondrules and olivine fragments in the matrix from the Tagish Lake meteorite (ungrouped C2) were measured to understand the characteristics of the formation environment of the Tagish Lake chondrules. Of the 43 samples analyzed, 31 are MgO-rich and 16O-rich (Δ17O ∼ −5‰ [=δ17O – 0.52 × δ18O]), which is typical of chondrules in CM, CO, and CV chondrites. Six samples are FeO-rich and 16O-poor (Δ17O ∼ −2‰), while three samples are FeO-rich chondrules with Δ17O ≥ 0‰, the latter being a major component of chondrules and similar to the majority of crystalline silicates recovered from comet Wild 2.The correlation between Mg# [=MgO/(MgO + FeO) mol%] and Δ17O values of the samples defines an intermediate trend between those of CO-CM chondrite chondrules and comet Wild 2 samples. Assuming that the CM chondrites, Tagish Lake meteorite, and comet Wild 2 represent C-type asteroids, D-type asteroids, and Kuiper belt objects, respectively, the results of this study indicate that type II chondrules with Δ17O ≥ 0‰ formed at a location much farther out than that where the Tagish Lake meteorite parent body accreted, more than 3.1 million years after the CAI formation assuming homogeneous distribution of 26Al in the early Solar System (Tenner et al., 2019). These two aspects, namely the broad range of heliocentric distance and the prolonged period of chondrule formation, are important constraints when considering appropriate mechanisms of chondrule formation in the protoplanetary disk.