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

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