Hydrogen-isotopic compositions in Allan Hills 84001 and the evolution of the martian atmosphere

Abstract

Hydrogen-isotopic compositions of carbonate and maskelynite in Allan Hills (ALH) 84001 were measured by secondary ion mass spectrometry (SIMS). The δD values of both minerals show considerable deviation. The deviation seems to be caused by addition of varying amounts of terrestrial water in the case of carbonate. In the case of maskelynite, H is heterogeneously distributed and the deviation in δD values seems to be due to mixing of this indigenous heavy H with isotopically normal H present in the SIMS chamber. The indigenous δD value in ALH 84001 seems to be ~2000‰. Carbonate rather than maskelynite seems to be the main carrier of H in ALH 84001. Because ALH 84001 is ~4 Ga old, the H-isotopic composition suggests that a large fraction of the initial martian atmosphere had already escaped by 4 Ga.