  Oxygen isotope compositions of chondrules reflect the environment of
chondrule formation and its spatial and temporal variations. Here, we present a
theoretical model of oxygen isotope exchange reaction between molten silicate
spherules and ambient water vapor with finite relative velocity. We found a new
phenomenon, that is, mass-dependent fractionation caused by isotope exchange
with ambient vapor moving with nonzero relative velocity. We also discussed the
plausible condition for chondrule formation from the point of view of oxygen
isotope compositions. Our findings indicate that the relative velocity between
chondrules and ambient vapor would be lower than several 100 m/s when
chondrules crystallized.
