Non-Rayleigh oxygen isotope fractionation by mineral evaporation: Theory and experiments in the system SiO2

Experiments demonstrate that partial evaporation of solid silica at 1600-1700 degrees C and low pressure (10(-9) bar) results in enrichment of O-18/O-16,d O-17/O-16 in solid products. Evaporative residues formed in H-2 or N-2 gas at higher pressures (>10(-5) bar) exhibit limited or negligible heavy isotope enrichment. The degree of enrichment is controlled by kinetic fractionation at the ablating grain surfaces, the rate of sublimation, and the efficacy of oxygen self diffusion in the solid. Observed isotopic effects are consistent with numerical simulations, confirming that vaporization of solid silicate and oxide minerals is a viable cause for non-Rayleigh fractionation of O-16, O-17, and O-18. Experiment and theory suggest that partial melting during evaporation is not required a priori to explain mass-dependent variations in oxygen isotope ratios in primitive meteoritical materials. Experimental determinations of the rates of ablation of appropriate minerals are required to evaluate the meteoritical data.