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 ~ 1 vol.% of the meteorites – provide information about the timing and nature of hydrous activity and atmospheric processes on Mars. We suggest that the most plausible models for secondary mineral formation involve the evaporation of low temperature (25-150C) brines. This is consistent with the simple mineralogy of these assemblages – Fe-Mg-Ca carbonates, anhydrite, gypsum, halite, clays – and the chemical fractionation of Ca-to Mg-rich carbonate in ALH84001 ‘rosettes’. 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 compositions 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 vapourisation and dehydration related to an impact event postdating the initial precipitation. This shock event may have led to the formation of sulphide and some magnetite in the Fe-rich outer parts of the rosettes. Radiometric dating (K-Ar, Rb-Sr) of the secondary mineral assemblages in one of the nakhlites (Lafayette) suggests that they formed between 0 and 670 Ma, and certainly long after the crystallisation of the host igneous rocks. Crystallisation of ALH84001 carbonate took place 0.5 Ga 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 ~1 Ga. 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 seawater-like composition, high bicarbonate contents and a weakly acidic nature. The co-existence of siderite (Fe-carbonate) and clays in the nakhlites suggests that the pCO2 level in equilibrium with the parent brine may have been 50 mbar or more. The brines could have originated as flood waters which percolated through the top few hundred metres of the crust, releasing cations from the surrounding parent rocks. There is no spectroscopic or photographic evidence for extensive evaporite deposits on the current martian surface but some salt precipitation through solute concentration must have occurred as floodwaters associated with many of the martian channels ponded in low-lying areas. The high sulphur and chlorine concentrations of the martian soil have most likely resulted from aeolian redistribution of such aqueously-deposited salts and from reaction of the martian surface with volcanic acid volatiles. However, the SNC salt assemblages were precipitated under more reducing conditions – goethite is a minor phase compared to the other SNC secondary minerals than the ferric iron-rich martian soil has experienced. The SNC secondary assemblages record the effects of atmospheric loss, mass-independent isotope fractionation within the martian atmosphere and trapping of CO2 within the crust. The D values of up to +4400 ‰ reported in SNC minerals reflect the effects of atmospheric loss but the sulphur S and oxygen isotopic ( O) values determined in the secondary minerals are affected by mass-independent fractionation processes such as photolysis in the atmosphere. 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. The atmosphere of early Mars was thinned through a combination of CO2 sequestration in carbonate and impact erosion. Large fractionations in O between igneous silicate in the meteorites and the secondary minerals ( 30 ‰) require formation of the latter below temperatures at which silicate-carbonate equilibration could have taken place (~400C) and have been taken to suggest low temperatures (e.g. 150C) of precipitation from a hydrous fluid. These estimates are similar to the experimental data on the carbonate formation. However, the presence of mass-independent fractionation means that the precise modelling of temperatures using isotopic compositions is problematic. Space Science Reviews 96 365–392, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands. MS-Bridges.tex; 13/03/2001; 11:58; p.1