EXPERIMENTAL-STUDY OF HIGH-TEMPERATURE OXYGEN-ISOTOPE EXCHANGE DURING CHONDRULE FORMATION

Laboratory experiments have been conducted to determine rates and other details of the oxygen isotopic exchange process between meteoritic material and water vapor. The meteoritic samples were totally or partially melted, or completely unmelted. The furnace atmosphere consisted of water vapor and hydrogen, and was diluted with helium for some of the experiments. Both isothermal and flash heating experiments were performed. Extensive oxygen isotope exchange has been observed between chondrule analogue melt and surrounding dilute water vapor, such that 50% equilibration occurs in charges near their liquidus in five minutes or less. The principal rate-limiting mechanism of the exchange is diffusion within the melt. The initial exchange rate also depends on the partial pressure of water, which points to the effect of the surface exchange process. The kinetics of the surface reaction have not been determined due to the restrictions of our experimental configuration. The extent of exchange observed in short time isothermal runs and in flash heating runs suggests that much of the isotopic variability of natural chondrules can be produced during chondrule-forming events by the gas/melt isotopic exchange processes. However, due to the very low gas densities in the nebula, multiple melting events might be needed to produce the chondrules with highly exchanged isotopic compositions. For a partially melted sample, as is often the case in natural chondrules, the rate of isotopic change of the bulk sample is determined by the exchange rates of both the melt and unmelted relict minerals. If the melt and relict minerals have a large difference in isotopic compositions, the exchange trajectory becomes markedly nonlinear. These observations are consistent with the observed dependence of extent of exchange in natural chondrules on the degree of melting, and imply that for each of the linear trends on the three-isotope diagram observed for natural chondrules, only two initial isotopic reservoirs might have existed.