Alteration assemblages in martian meteorites: Implications for near-surface processes

The SNC (Shergotty-Nakhla-Chassigny) meteorites have recorded interactions between martian crustal fluids and the parent igneous rocks. The resultant secondary minerals - which comprise up to similar to1 vol.% of the meteorit es - provide information about the timing and nature of hydrous activity an d atmospheric processes on Mars. We suggest that the most plausible models for secondary mineral formation involve the evaporation of low temperature (25 - 150 degreesC) brines. This is consistent with the simple mineralogy o f these assemblages - Fe-Mg-Ca carbonates, anhydrite, gypsum, halite, clays - and the chemical fractionation of Ca-to Mg-rich carbonate in ALH84001 "r osettes". Longer-lived, and higher temperature, hydrothermal systems would have caused more silicate alteration than is seen and probably more complex mineral assemblages. Experimental and phase equilibria data on carbonate c ompositions similar to those present in the SNCs imply low temperatures of formation with cooling taking place over a short period of time (e.g. days) . The ALH84001 carbonate also probably shows the effects of partial vapouri sation and dehydration related to an impact event post-dating the initial p recipitation. This shock event may have led to the formation of sulphide an d some magnetite in the Fe-rich outer parts of the rosettes. Radiometric dating (K-Ar, Rb-Sr) of the secondary mineral assemblages in on e of the nakhlites (Lafayette) suggests that they formed between 0 and 670 Myr, and certainly long after the crystallisation of the host igneous rocks . Crystallisation of ALH84001 carbonate took place 0.5 Gyr after the parent rock. These age ranges and the other research on these assemblages suggest that environmental conditions conducive to near-surface liquid water have been present on Mars periodically over the last similar to1 Gyr. This fluid activity cannot have been continuous over geological time because in that case much more silicate alteration would have taken place in the meteorite parent rocks and the soluble salts would probably not have been preserved. The secondary minerals could have been precipitated from brines with seawat er-like composition, high bicarbonate contents and a weakly acidic nature. The co-existence of siderite (Fe-carbonate) and clays in the nakhlites sugg ests that the pCO(2) level in equilibrium with the parent brine may have be en 50 mbar or more. The brines could have originated as flood waters which percolated through the top few hundred meters of the crust, releasing catio ns from the surrounding parent rocks. The high sulphur and chlorine concent rations of the martian soil have most likely resulted from aeolian redistri bution of such aqueously-deposited salts and from reaction of the martian s urface with volcanic acid volatiles. The volume of carbonates in meteorites provides a minimum crustal abundance and is equivalent to 50-250 mbar of CO2 being trapped in the uppermost 200 -1000 m of the martian crust. Large fractionations in delta O-18 between ig neous silicate in the meteorites and the secondary minerals (less than or e qual to 30 parts per thousand) require formation of the latter below temper atures at which silicate-carbonate equilibration could have taken place (si milar to 400 degreesC) and have been taken to suggest low temperatures (e.g . less than or equal to 150 degreesC) of precipitation from a hydrous fluid .