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-chlorapatit e assemblage in the Shergotty-Nakhla-Chassigny (SNC) Martian meteorite Nakh la. These minerals are found associated with each other in interstitial are as, with halite often being adjacent to or enclosing siderite. We suggest t he halite and other minerals are Martian in origin because (1) the conditio ns of fall preclude significant amounts of terrestrial contamination or wea thering having taken place; (2) textures indicate that the minerals within this assemblage crystallized at the same stage as some silicate and oxide m inerals within the Nakhla parent; and (3) the association with siderite, wh ich previous studies have shown has C and O-isotopic compositions incompati ble with an origin on the Earth. Siderite has the range of compositions: Ca CO3, 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 m ol%) 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 m iscibility gap would require independent work on Fe-Mn carbonates to verify its existence. Trace element abundances have been determined by ion microp robe analysis on three siderite and one anhydrite grains. Siderite has ligh t 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 simil ar pattern. These abundances reflect the source composition rather than par titioning or complexing controls. They are not typical of hydrothermal sign atures which generally do not have such smooth REE abundance patterns. The nature of the mineral assemblage suggests that its source rocks on Mars wer e evaporites. These may be common in the craters and flood plains of the Ma rtian southern highlands. Two models are suggested in this paper to explain the incorporation of evaporitic material into the Nakhla igneous parent. I t may have happened as a low-temperature process (<200 degrees C) by crysta llisation 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 intersti tial melt. Both high- and low-temperature models are consistent with the pr esence of evaporite sediments on Mars.