Hydrogen isotope analyses of alteration phases in the nakhlite martian meteorites

Abstract

Secondary alteration phases, such as carbonates, smectite clays and Fe-oxides, are found within the martian meteorites. If these meteorites were seen to fall, the assumption can be reasonably made that the secondary phases have a martian origin. However, for martian meteorite finds, this is not the case. Deuterium/hydrogen (D/H) ratio analysis can be used to distinguish between terrestrial and martian secondary alteration phases – the martian atmosphere is currently five times enriched in deuterium compared to terrestrial seawater, producing a deuterium enrichment in the martian alteration phases large enough to be detected with modern ion microprobe techniques. We aimed to measure the D/H ratio of the iddingsite-like alteration veins in the nakhlite martian meteorites Nakhla, MIL 03346, MIL 090030 and Y 000593, to help confirm the martian origin of this material, and determine the relative amounts of terrestrial contamination in each meteorite. As an observed fall Nakhla is nominally uncontaminated by terrestrial alteration, and shows elevated δD values in its alteration veins. However, even the highest of these values is much below that of the martian atmosphere, indicating that terrestrial contamination, probably combined with fractionation effects between the martian atmosphere and the alteration material, have reduced the δD values of these veins. The speed of hydrogen isotope exchange in the nakhlite iddingsite-like alteration veins is demonstrated by the purely terrestrial δD values in the veins of Nakhla 110, a thin-section exposed to the terrestrial atmosphere since its preparation in 1998. Mineralogical heterogeneity also affects the hydrogen isotope exchange rate in these veins – the wide variation in δD within the veins of Nakhla and Y 000593 appears to be due to the heterogeneous nature of the vein material, where some phases exchange hydrogen with the atmosphere at a faster rate than others.