Oxygen three-isotope ratios of silicate particles returned from asteroid Itokawa by the Hayabusa spacecraft: A strong link with equilibrated LL chondrites

Abstract Oxygen three-isotope ratios of four mineral phases (olivine, low-Ca and high-Ca pyroxene, and plagioclase) in seven silicate particles from asteroid Itokawa were measured to investigate oxygen isotope systematics of asteroidal materials. In order to obtain highly precise and accurate oxygen isotope ratios ( ∼ ± 0.3 ‰ in δ 18 O), we used a new indium mounting method that minimizes potential instrumental mass fractionation due to surface topography. The oxygen isotope data of the seven Itokawa particles are distributed above the terrestrial fractionation line with indistinguishable Δ O 17 ( = δ O 17 − 0.52 × δ O 18 ) values of + 1.34 ± 0.36 ‰ (2SD; n = 22 ), which is similar to those of Itokawa particles analyzed by Yurimoto et al. (2011) and Nakamura et al. (2012) . However, δ 18 O values of the seven Itokawa particles are tightly clustered ( + 3.9 ‰ to + 5.9 ‰ VSMOW) compared to previous analyses of the Itokawa particles ( + 1.4 ‰ to + 8.8 ‰ ). The average Δ 17 O values and 2SD of the two equilibrated ordinary chondrites (EOCs), which were analyzed for comparison, are + 1.31 ± 0.49 ‰ for St. Severin (LL6) and + 0.83 ± 0.64 ‰ for Guarena (H6). The Δ 17 O values of the seven Itokawa particles are similar to those of St. Severin. The average bulk δ 18 O value of the seven Itokawa particles is estimated as + 4.8 ‰ , which is within the δ 18 O range of bulk LL5 and LL6 chondrites. It is suggested that Itokawa particles are similar to those of equilibrated LL chondrites, which strengthens a link between asteroid Itokawa and equilibrated LL chondrites that fell to Earth. The seven Itokawa particles and two EOCs have δ 18 O values of high-Ca pyroxene that are lower than those of olivine, indicating that oxygen isotopes between the two mineral phases are not in equilibrium. Apparent oxygen isotope equilibrium temperatures estimated from a pair of low-Ca pyroxene and olivine are excessively higher than those from a pair of plagioclase and high-Ca pyroxene (∼800 °C) and peak metamorphic temperatures. Oxygen isotopes in olivine and low-Ca pyroxene do not represent equilibrium. On the other hand, oxygen isotope ratios in high-Ca pyroxene and plagioclase were mass-dependently fractionated accompanied by recrystallization (and possibly by diffusion) during thermal metamorphism. Values of δ 18 O in high-Ca pyroxene were lowered by exchange, which may have resulted in the reverse δ 18 O fractionation between olivine and high-Ca pyroxene.