1Takayuki Ushikubo,2Makoto Kimura
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.11.003]
1Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 200 Monobe-otsu, Nankoku, Kochi 783-8502 Japan
2National Institute for Polar Research, 10-3 Midoricho, Tachikawa, Tokyo 190-8518 Japan
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 (C2-anomalous) 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 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.