Hydrogen isotope geochemistry of SNC meteorites

Abstract

We report the yields and hydrogen isotopic compositions of water extracted by stepwise heating of eight whole-rock SNC meteorites. D/H values are consistent with terrestrial water at low temperatures and increase with temperature to values that far exceed terrestrial. The data are consistent with the water in the samples originating from two sources: a terrestrial component/contaminant, released largely at low temperature, and an extraterrestrial component, released at high temperature. The variation in δD values of the high-temperature hydrogen (∼+250 to +900 for the nakhlites, ∼+1200 to +2100 for the shergottites, and ∼+800 for ALH84001) could represent true variation of the δD of the extra-terrestrial water in the samples, or may reflect varying contributions of the terrestrial endmember, even at high temperature. The high δD values are consistent with a martian origin for the meteorites since the current martian atmosphere contains water with a δD of ∼+4000. The presence of alteration products in at least some of the samples suggests the D-enriched water was probably incorporated into the rocks through interaction at low temperature with aqueous crustal fluids that had exchanged with the martian atmosphere. Chassigny contains water that has terrestrial δD values at all temperatures (with the possible exception of the highest temperature step, with δD up to ∼+50), suggesting contamination of this sample by terrestrial water. Carbon and oxygen isotopic results for CO_2 extracted by stepwise heating are also reported. These data are consistent with formation of carbonates in the SNC meteorites by secondary processes on Mars (from fluids that had exchanged C and O with the atmosphere), perhaps in the same alteration events that formed the D-enriched minerals.