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

Experiments demonstrate that partial evaporation of solid silica at 1600-17 00 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 fract ionation at the ablating grain surfaces, the rate of sublimation, and the e fficacy of oxygen self diffusion in the solid. Observed isotopic effects ar e consistent with numerical simulations, confirming that vaporization of so lid silicate and oxide minerals is a viable cause for non-Rayleigh fraction ation of O-16, O-17, and O-18. Experiment and theory suggest that partial m elting during evaporation is not required a priori to explain mass-dependen t variations in oxygen isotope ratios in primitive meteoritical materials. Experimental determinations of the rates of ablation of appropriate mineral s are required to evaluate the meteoritical data.