Modeling Alteration Minerals on Mars – Investigating the High Temperature Component

Introduction: Alteration minerals are important indicators of thermochemical environments at the time of their formation – and thus indicators for the potential habitability of a site [1,2]. Therefore, the detection of alteration minerals, especially nontronite [3], on Mars was an important step to understand Mars’ hydrous history. This initial finding was followed by the detection of a wide variety of alteration minerals, including but not limited to diverse phyllosilicates, carbonates, sulfates and hydrous silica in a variety of geologic settings in – mostly – Noachian terrains [e.g.,38], see [9] for review. The importance of thermochemical modeling lies in the fact that the observation of minerals on the ground delivers an end result, but information of alteration conditions, such as T, P, and composition of the fluid, are lost. For Mars, diverse geologic settings with their respective temperature and host rock requirements have been modeled, ranging from very low-T surface evaporation scenarios [10,11] and acid weathering [12,13] to hydrothermal silica deposition [14]. We have focused on impact-generated hydrothermal systems [14,15,16], i.e., systems at warm to hot water conditions. Those subsurface systems contain water, CO2, and host rock components, but no species are added from magma degassing or acid weathering. We used CHILLER [17] and limited the upper temperature range to ~250 °C. CHILLER is now replaced by CHIM-XPT [18], which allows for a much higher temperature range up to 600 °C. CHIM-XPT has been applied to terrestrial basaltic settings at temperatures up to 500°C [19]. High tempertures are important in the central peak settings of large impact-craters, where initial temperatures readily exceed 250 °C [20] and form high-temperature alteration phases – as is documented for terrestrial craters [e.g., 21]. With new exploration techniques on Mars, such as the CheMin instrument on the Mars Science Laboratory (MSL) rover [22], those rare but important phases might be found in the near future. Their detection will allow for a more complete understanding of the Martian alteration history. Here we compare our previous CHILLER results obtained on the Martian meteorite composition LEW 88516 [15] to CHIM-XPT and carry out models at 500 °C. CHILLER–CHIM-XPT results and discussion. We modeled LEW88516 earlier [15], and details on input data can be found there. Host rock and starting fluid chemistry are identical for all runs. W/R=100000