Xenon and other noble gases in shergottites

Abstract

We have determined the isotopic composition of the xenon component trapped in EETA 79001 lithologies B and C, which we refer to as SPB-xenon. SPB-xenon is isotopically distinct from known xenon reservoirs, but differs in regular fashion. Normalized to 132Xe, the light isotope ratios are indistinguishable from air, the 129Xe/132Xe ratio is about 2.4, and 134Xe and 136Xe are enhanced relative to the terrestrial atmosphere or AVCC. The apparent heavy-isotope enrichments are not generated by in situ fission and there is no spectral evidence for the presence of 244Pu fission xenon. However, the xenon composition does match that of fractionated AVCC except at 129Xe, and consequently may be derived from or related to that component. ALHA 77005, Shergotty and EETA 79001 lithology A also have enhanced 129Xe/132Xe ratios in most temperature steps, and are seemingly consistent with varying mixtures of SPB-xenon and terrestrial xenon.Our results for neon and argon in EETA 79001 confirm earlier results on the exposure ages. We have also verified that the trapped 38Ar/36Ar ratio in lithology C is apparently substantially different from the terrestrial or meteoritic value. Krypton in EETA 79001,C is more fractionated with respect to AVCC than is terrestrial krypton and in the opposite direction as xenon. EETA 79001,C contains excess 80Kr (and perhaps 82Kr and 128Xe), presumably from neutron capture on bromine and iodine, but these neutron captures do not appear to have occurred by in situ processes.