Oxygen isotope reservoirs in the outer asteroid belt inferred from oxygen isotope systematics of chondrule olivines and isolated forsterite and olivine grains in Tagish Lake-type carbonaceous chondrites, WIS 91600 and MET 00432

To understand oxygen isotope ratios and redox conditions of the chondrule formation environments of the outer rigions of the asteroid belt, we analyzed major element concentrations and oxygen isotope ratios of olivine grains in chondrules, isolated forsterite, and isolated olivine from the WIS 91600 and MET 00432 carbonaceous chondrites, which are thought to have originated from D-type asteroids located in the outer asteroid belt. The oxygen isotope ratios of individual chondrules and isolated grains show a wide variation in δ¹⁸O from −9.9‰ to +9.1‰ along the carbonaceous chondrite anhydrous mineral (CCAM) and primitive chondrule mineral (PCM) lines. The Δ¹⁷O (= δ¹⁷O – 0.52 × δ¹⁸O) values of the measured objects increase with decreasing Mg#; i.e., FeO-poor objects (Mg# > 90; type I chondrules and isolated forsterites) mainly have Δ¹⁷O values of ca. −6‰, and FeO-rich objects (Mg# < 90; type II chondrules and isolated olivines) have Δ¹⁷O values ranging from −3‰ to +2‰. Similar trends are observed for ferromagnesian silicate particles from comet Wild2 and CR chondrite chondrules, particularly in terms of FeO-rich objects with Δ¹⁷O values ranging from −3‰ to +2‰. It is suggested that FeO-rich objects formed in the outer regions of the asteroid belt and were transported to the outer solar nebular regions where comet Wild2 formed.