Disentangling xenon components in Nakhla: Martian atmosphere, spallation and Martian interior

A powdered sample of Nakhla was separated into 3 subsamples. One was left o therwise untreated, one was washed in water and one etched with HNO3 removi ng 6% of the original mass. We report results of isotopic analysis of xenon released by laser step heating on aliquots of each of these subsamples; so me aliquots were neutron irradiated before isotopic analysis (to allow dete rmination of I, Ba and U as daughter xenon isotopes) and some were not. There is evidence that water soluble phases contain both martian atmospheri c xenon and a component with low Xe-129/Xe-132, either martian interior xen on or terrestrial atmosphere. Higher temperature data from unirradiated ali quots of the water and acid treated samples reveal two-component mixing. On e is a trapped xenon component with Xe-129/Xe-132 = 2.350 +/- 0.026, isotop ically identical to the martian atmosphere as measured in shock glass from shergottites. It is associated with leachable iodine, suggesting it is trap ped close to grain boundaries. It may be a result of shock incorporation of adsorbed atmospheric gas. The second component is best explained as an intimate mixture of martian in terior xenon and spallation xenon. The martian interior component is presen t at a concentration of similar to 10(-12) cm(3) STP g(-1) Xe-132, around 4 0 times lower than that observed in Chassigny. Its association with spallat ion xenon (produced from Ba and light rare earth elements) suggests it is i n the feldspathic mesostasis. We propose that it was trapped during crystal lisation and reflects the mantle source of the parental magma. Copyright (C ) 2001 Elsevier Science Ltd.