Isotopic links between atmospheric chemistry and the deep sulphur cycle on Mars

Isotopic analyses of 40 Martian meteorites indicate that assimilation of sulphur into Martian magmas was a common occurrence throughout much of the planet's history and that the atmospheric imprint of photochemical processing preserved in Martian meteoritic sulphide and sulphate is distinct from that observed in terrestrial analogues. Martian sulphur chemistry Many questions remain regarding the surface processes operating on Mars and the magmatic assimilation of crustal material, and sulphur chemistry is crucial to these and other phenomena on the planet. A new sulphur isotope analysis of 40 Martian meteorites now suggests an atmospheric sulphur chemistry, notably the photochemical processing preserved in meteoritic sulphide and sulphate, very different from that seen on Earth. The data provide evidence to suggest that the assimilation of sulphur into Martian magmas was a common occurrence throughout much of the planet's history. The authors conclude that sulphur isotope systematics, used in conjunction with data for other isotopic systems and trace element characteristics, is a powerful tool for reconstructing the geological history of Mars. The geochemistry of Martian meteorites provides a wealth of information about the solid planet and the surface and atmospheric processes that occurred on Mars. The degree to which Martian magmas may have assimilated crustal material, thus altering the geochemical signatures acquired from their mantle sources, is unclear.sup.1. This issue features prominently in efforts to understand whether the source of light rare-earth elements in enriched shergottites lies in crustal material incorporated into melts.sup.1,2 or in mixing between enriched and depleted mantle reservoirs.sup.3. Sulphur isotope systematics offer insight into some aspects of crustal assimilation. The presence of igneous sulphides in Martian meteorites with sulphur isotope signatures indicative of mass-independent fractionation suggests the assimilation of sulphur both during passage of magmas through the crust of Mars and at sites of emplacement. Here we report isotopic analyses of 40 Martian meteorites that represent more than half of the distinct known Martian meteorites, including 30 shergottites (28 plus 2 pairs, where pairs are separate fragments of a single meteorite), 8 nakhlites (5 plus 3 pairs), Allan Hills 84001 and Chassigny. Our data provide strong evidence that assimilation of sulphur into Martian magmas was a common occurrence throughout much of the planet's history. The signature of mass-independent fractionation observed also indicates that the atmospheric imprint of photochemical processing preserved in Martian meteoritic sulphide and sulphate is distinct from that observed in terrestrial analogues, suggesting fundamental differences between the dominant sulphur chemistry in the atmosphere of Mars and that in the atmosphere of Earth.sup.4.
Author(s): Heather B. Franz [sup.1] [sup.2] , Sang-Tae Kim [sup.3] , James Farquhar [sup.2] , James M. D. Day [sup.4] , Rita C. Economos [sup.5] , Kevin D. McKeegan [sup.5] [...]