Evolutionary trends in volatiles of the nakhlite source region of Mars

摘要

We report new data on isotopic systematics in nitrogen, Ar, and Xe of nakhlite NWA998 and discuss the implications of nakhlite signatures on the sources, evolution, and closed system properties of the nakhlite magma in the Martian mantle. The isotopic signatures of xenon in nakhlites are consistent and reveal a source region with the Chass-S xenon signature, but which had assimilated fission gas starting with the initial early differentiation of Mars to the time of magma eruption 1.3 Gyr ago. We note a trend of correlated enrichments of the fission gas and of trace elements (REE) in the source. Nakhlite NWA817, which contains abundant mesostasis and a hydrous phase, shows a distinct nitrogen component as well as a fission gas excess in low-temperature steps. Radiogenic 129Xe in nakhlites is associated with neither the fission gas nor the indigenous components and indicates a later incorporation, possibly during magma ascent. Indigenous nitrogen isotopic signatures of δ15N = 13‰ as observed in nakhlites (except NWA817) attest to very limited exchanges of modern atmospheric gases with the solid planet. This nakhlite nitrogen signature corresponds to the heavier of two indigenous signatures in Chassigny and supports other evidence for an origin in similar source regions.