A halite-siderite-anhydrite-chlarapatite assemblage in Nakhla: Mineralogical evidence for evaporites on Mars

We report the results of a study of a halite-siderite-anhydrite-chlorapatite assemblage in the Shergotty-Nakhla-Chassigny (SNC) Martian meteorite Nakhla. These minerals are found associated with each other in interstitial areas, with halite often being adjacent to or enclosing siderite. We suggest the halite and other minerals are Martian in origin because (1) the conditions of fall preclude significant amounts of terrestrial contamination or weathering having taken place; (2) textures indicate that the minerals within this assemblage crystallized at the same stage as some silicate and oxide minerals within the Nakhla parent; and (3) the association with siderite, which previous studies have shown has C and O-isotopic compositions incompatible with an origin on the Earth. Siderite has the range of compositions: CaCO3, 0.1-5.7; MgCO3, 2.0-40.9; FeCO3, 23.2-87.0; MnCO3,1.0-39.9 mol%. There are two compositional groupings: high-MnCO3 (greater than or equal to 30 mol%) and low-MnCO3/high-FeCO3, with a gap identified between the two. This may be a miscibility gap or, alternatively, the two compositional groupings may mark separate generations of carbonate. We have not found any textural evidence for the latter explanation, but acceptance of the presence of a miscibility gap would require independent work on Fe-Mn carbonates to verify its existence. Trace element abundances have been determined by ion microprobe analysis on three siderite and one anhydrite grains. Siderite has light rare earth elements (2.2-7.3 x C1) greater than heavy rare earth elements (0.32-0.79 x C1) without Ce or Eu anomalies, and the anhydrite has a similar pattern. These abundances reflect the source composition rather than partitioning or complexing controls. They are not typical of hydrothermal signatures which generally do not have such smooth REE abundance patterns. The nature of the mineral assemblage suggests that its source rocks on Mars were evaporites. These may be common in the craters and flood plains of the Martian southern highlands. Two models are suggested in this paper to explain the incorporation of evaporitic material into the Nakhla igneous parent. It may have happened as a low-temperature process (<200 degrees C) by crystallisation from an aqueous fluid. An origin at low temperature is compatible with the available experimental data on siderite stability. Alternatively, we suggest evaporitic material may have been incorporated into the Nakhla parent while melt was still present and crystallized similar to 800 degrees C. The latter model can more readily explain the trace element abundances and also the siderite textures that imply intergrowth wit residual interstitial melt. Both high- and low-temperature models are consistent with the presence of evaporite sediments on Mars.