Ja. Tyburczy et al., Shock-induced devolatilization and isotopic fractionation of H and C from Murchison meteorite: some implications for planetary accretion, EARTH PLAN, 192(1), 2001, pp. 23-30
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 (< 1.5 km/s) the solid shoc
ked products were combusted and isotopic analysis of the resulting H2O and
CO2 was performed. H and C-13 are partitioned preferentially over D and C-1
2, respectively, into the released gas suggesting that the inorganic (miner
al) portion of Murchison is devolatilized preferentially over the organic (
kerogen) fraction (which is relatively enriched in D and C-12) at the shock
pressures studied. These results are combined with previous results on ser
pentine devolatilization to derive an empirical H fractionation versus devo
latilization 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 primitiv
e 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 (re
lative to D) compared to the residual solid, with a fractionation factor of
-18 to -23 parts per thousand. Assuming that current planetary atmospheres
have resulted from degassing of planetary interiors after loss of the earl
iest H-enriched atmosphere, the above degree of isotopic fractionation is n
ot sufficient by itself to explain the large positive deltaD values of the
present Martian and Venusian atmospheres. However, this mechanism in conjun
ction with tectonic recycling over geologic time could contribute to prefer
ential H loss for Earth and Mars. (C) 2001 Elsevier Science B.V. All rights
ae reserved.