Disentangling xenon components in Nakhala: Martian atmosphere spallation and Martian interior

A powdered sample of Nakhla was separated into 3 subsamples. One was left otherwise untreated, one was washed in water and one etched with HNO3 removing 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; some aliquots were neutron irradiated before isotopic analysis (to allow determination of I, Ba and U as daughter xenon isotopes) and some were not. There is evidence that water soluble phases contain both martian atmospheric xenon and a component with low 129Xe/132Xe, either martian interior xenon or terrestrial atmospheric. Higher temperature data from unirradiated aliquots of the water and acid treated samples reveal two-component mixing. One is a trapped xenon component with 129Xe/132Xe = 2.350± 0.026, isotopically identical to the martian atmosphere as measured in shock glass from shergottites. It is associated with leachable iodine, suggesting it is trapped 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 interior xenon and spallation xenon. The martian interior component is present at a concentration of ∼ 10 -12 cm3 STP g-1 132Xe, around 40 times lower than that observed in Chassingny. Its association with spallation xenon (produced from Ba and light rare earth elements) suggest it is in the feldspathic mesostasis. We propose that it was trapped during crystallisation and reflects the mantle source of the parental magma. Copyright © 2001 Elsevier Science Ltd.