Noble gas contents of shergottites and implications for the Martian origin of SNC meteorites

Isotopic concentrations of the noble gases have been measured in several different phases of Elephant Moraine A 79001 and in whole rock samples of Zagami and Allan Hills A 77005, three meteorites which belong to the rare group of SNC achondrites that may have originated from the planet Mars. Shocked phases of EETA 79001 contain a trapped Ar, Kr, and Xe component characterized by 84Kr/132Xe approx. 15, 40Ar/36Ar > 2000, 129Xe/ 132Xe >= 2, and 4He/40Ar <= 0.1. These elemental and isotopic ratios are unlike those for any other noble gas component except analyses of the Martian atmosphere made by Viking spacecraft. The isotopic composition of the trapped Kr shows an approximate 1% per mass unit enrichment of lighter isotopes compared to terrestrial Kr, and the trapped Xe may show either a fission component or a fractionated enrichment of heavier isotopes compared to terrestrial Xe. It is hypothesized that these gases represent a portion of the Martian atmosphere which was shock-implanted into EETA 79001, and that they constitute direct evidence of a Martian origin for the shergottite meteorites. Cosmic ray-produced gases in the eight known SNC meteorites form three distinct groups with exposure ages of approx. 11 MY (Chassigny and the nakhlites), approx. 2.6 MY (Shergotty, Zagami, and ALHA 77005), and approx. 0.5 MY (EETA 79001). (author)