The average ¦¤17O values and 2SD of the two equilibrated ordinary chondrites (EOCs), which were analyzed for comparison, are for St. S¨¦verin (LL6) and for Guare?a (H6). The ¦¤17O values of the seven Itokawa particles are similar to those of St. S¨¦verin. The average bulk ¦Ä18O value of the seven Itokawa particles is estimated as , which is within the ¦Ä18O 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 ¦Ä18O 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?¡ãC) 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 ¦Ä18O in high-Ca pyroxene were lowered by exchange, which may have resulted in the reverse ¦Ä18O fractionation between olivine and high-Ca pyroxene.