Non-Rayleigh oxygen isotope fractionation by mineral evaporation

Geochim. Cosmochim. Acta, 62, 3109-3116, 1998

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

Edward D. Young 1,
Hiroko Nagahara 2,
Bjorn O. Mysen 3, and
Denis Marc Audet 1

1 Department of Earth Sciences, University of Oxford, Parks Road, Oxford, OXI 3PR, UK

2 Geological Institute, University of Tokyo, 7-3-1 Hongo, Tokyo 113, Japan

3 Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, NW, Washington D.C., 20015-1305, USA

Abstract

Experiments demonstrate that partial evaporation of solid silica at 1600° to 1700°C and low pressure (10-9 bar) results in enrichment of 180/160 and 170/160 in solid products. Evaporative residues formed in H2 or N2 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 160 , 170 , and 180 . 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.

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[1], [2], [3], [4],
[5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18],
[19], [20], [21], [22], [23], [24], [25]

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Email: mysen@gl.ciw.edu