Shock-induced devolatilization and isotopic fractionation of H and C from Murchison meteorite: some implications for planetary accretion

Incipient shock-induced devolatilization of Murchison meteorite occurs upon subjecting samples to a minimum shock stress, or pressure, of about 5 GPa. This pressure is similar to that required to initiate devolatilization of 20% porous serpentine. Upon low velocity impact (61.5 km/s) the solid shocked products were combusted and isotopic analysis of the resulting H_2O and CO_2 was performed. H and ^(13)C are partitioned preferentially over D and ^(12)C, respectively, into the released gas suggesting that the inorganic (mineral) portion of Murchison is devolatilized preferentially over the organic (kerogen) fraction (which is relatively enriched in D and ^(12)C) at the shock pressures studied. These results are combined with previous results on serpentine devolatilization to derive an empirical H fractionation versus devolatilization relation that is used to evaluate the extent of impact-induced isotopic fractionation during planetary accretion. During accretion of the Earth, impact-induced devolatilization and formation of the early primitive atmosphere would have begun at a point where the `growing' Earth achieved a radius in the 480-800 km range. The present experimental results suggest that the Earth's early atmosphere would have been enriched in hydrogen (relative to D) compared to the residual solid, with a fractionation factor of -18 to -23‰. Assuming that current planetary atmospheres have resulted from degassing of planetary interiors after loss of the earliest H-enriched atmosphere, the above degree of isotopic fractionation is not sufficient by itself to explain the large positive δD values of the present Martian and Venusian atmospheres. However, this mechanism in conjunction with tectonic recycling over geologic time could contribute to preferential H loss for Earth and Mars