Hydrothermal Alteration Mineralogy of Home Plate: Thermochemical Constraints for Their Formation

Home Plate is a plateau in the Columbia Hills of Gusev Crater [1]. It is a layered sequence of clastic rocks with alkali basaltic composition. Based on stratigraphy, structure, sedimentology, mineralogy, and bulk chemistry it is interpreted as pyroclastic deposit and thought to be extensively hydrothermally altered [1-5]. The hydrothermal alteration is presumed to be of high and low temperature nature. In order to better understand the conditions (P,T,X) of hydrothermal alteration for the Home Plate region we have calculated the hydrothermal alteration mineralogy of Fastball, a primitive basalt from the Home Plate sequence, and compared these results to the mineralogy of the area. Home Plate alteration mineralogy: The MER Spirit has investigated the igneous and alteration mineralogy and chemistry of Home Plate. Home Plate contains significant amounts of igneous olivine, pyroxene, and magnetite with small amonts of hematite and nanophase oxides. Surrounding Home Plate are deposits rich in goethite, hematite and Fe-sulfates with one location possibly containing pyrite/marcasite. These have been interpreted as acid sulfate fumarolic and/or hydrothermal system deposits related to the Home Plate pyroclastic deposit [5]. However, if indeed Columbia Hills were part of the central uplift of Gusev [6], it could be related to impact-generated hydrothermal activtiy. Bulk chemical data support the assumption of high and low temperature aqueous alteration. Elements more susceptible to alteration show correlations with mineralogy and stratigraphic position [4]. Modeling backgound and input parameters. Thermochemical modeling to understand formation conditions of minerals has been applied to various Martian processes (e.g., weathering [7], jarosite [8] and methane [9,10] formation, and Martian volcanic [11,12] and impact-generated [13] hydrothermal systems). For this study, we use CHILLER [14] to evaluate mineral assemblages that are likely to form from the Martian rock Fastball in contact with a diluted fluid [see 13]. To explore different stages of the hydrothermal system we calculated mineral assemblages at three temperatures (13, 150 and 300°C), P=110 bar, and water to rock (W/R) ratios from 100,000 to 1. We chose Fastball because 1) it has recently been shown to represent a primitive basaltic composition [15] and 2) it has the least amount of Cl and SO3 of all of the Home Plate basalts suggesting it is the least altered composition [1]. We set the oxygen fugacity to 2 log units below the QFM oxygen fugacity buffer equivalent to ~10% of FeOT as Fe2O3 and added 0.3 % of F and Cl as CaF2 and CaCl2, respectively. These values are consistent with average values for the SNC meteorites and unaltered Gusev Basalts [16-18]. CO2 was set to 0.2x10 mole HCO3 in the brine. Results: Alteration mineralogy: Alteration at 300 °C can be grouped in three main alteration assemblages: high W/R cause magnetite to form. At the highest W/R, when ion activities of all species are low, only magnetite and few % of diaspore precipitate. If the activity of Si-containing species is high enough (W/R < ~20,000), chlorite and serpentine form. At intermediate W/R chlorite dominates the assemblage; magnetite is not stable, but quartz forms. The dominating minerals are chlorite and serpentine. At low W/R, chlorite and serpentine dominate the assemblage accompanied by a range of silicates (feldspar, pyroxene, epidote). If the system cools to 150 °C (Fig. 1), a different alteration assemblage forms. At high W/R, a very diluted fluid precipitates hematite, diaspore and eventually kaolinite. At intermediate W/R nontronite and kaolinite or chlorite dominate the precipitated minerals. Serpentine occurs in a small, intermediate W/R range only; and at very low W/R the silicate assemblage is amphibole, chlorite, feldspar quartz and pyroxene. If the system continues to cool to < 20°C the stable alteration assemblage would be nontronite–chlorite– goethite in the high and intermediate W/R rage. At low W/R chlorite, amphibole, zeolithe and quartz form. Models with higher CO2 contents are ongoing and based on previous results [11,19] should produce carbonates on the expense of oxides and silicates.