The role of exchange reactions in oxygen isotope fractionation during CAI and chondrule formation


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Abstract— The role of oxygen isotope exchange during evaporation and condensation of silicate melt is quantitatively evaluated. Silicate dusts instantaneously heated above liquidus temperature are assumed to cool in gas and experience partial evaporation and subsequent recondensation. The results show that isotopic exchange effectively suppresses mass-dependent O-isotope fractionation even if the degree of evaporation is large, which is the fundamental difference from the case without isotopic exchange. The final composition of silicate melt strongly depends on the initial abundance of oxygen in the ambient gas relative to that in silicate dust, but not on the cooling rate of the system. The model was applied to O-isotope evolution of silicate melts in isotopically distinct gas of the protoplanetary disk. It was found that deviation from a straight mixing line toward the δ18O-rich side on the three-oxygen isotope diagram is inevitable when mass-dependent fractionation and isotopic exchange take place simultaneously; the degree of deviation depends on the abundance of oxygen in an ambient gas and isotopic exchange efficiency. The model is applied to explain O-isotopic compositions of igneous CAIs and chondrules.