Hydrothermal Alteration of the Martian Crust: an Experimental Approach

Introduction: On Mars, the geomorphology of numerous surface features has been attributed to the interaction of magmatic and impact heat sources with an H 2 O-rich Martian crust/megaregolith [1-3]. In addition , geochemical analyses of soil at the Pathfinder and Viking landing sites indicate high concentrations of SO 3 and Cl [4-6], consistent with interaction between a basaltic protolith and fluids rich in acidic sul-fate and chloride species. Such fluids are often found in association with subsurface hydrothermal systems and crater lake environments in terrestrial settings [7, 8]. These factors have led to the suggestion that hydrothermal alteration of the Martian crust may have been partially or wholly responsible for the production of the chemically unique sedimentary deposits observed on the Martian surface [9-11]. However, the secondary mineral phases formed via alteration of the Martian regolith under hydrothermal conditions relevant to Mars remain largely unconstrained [12-14]. The few relevant experimental hydrothermal studies published to date have emphasized the conditions of formation of alteration minerals observed in the Shergottite-Nahklite-Chassigny class of meteorites [15-17]. Given the exceedingly small volume of the observed alteration mineralogy, and the shock-related thermal history of these meteorites, there remains considerable doubt as to how much can be inferred about bulk Martian soil properties from these unique samples [18]. Thermodynamic modeling of hydrothermal alteration under conditions relevant to Mars has been performed utilizing only deionized water equilibrated with variable pCO 2 as the simulated solvent [19, 20]. To date, these types of studies have generally ignored other fluid compositions that may be of considerable importance in Martian hydrothermal systems, such as acid-sulfate and sulfate-chloride type fluids [9, 21]. The results presented herein represent the first in a series of experiments aimed at assessing the role that hydrothermal systems might play as a source of secondary minerals at the Martian surface. Utilizing the flow through reactor described in Tosca et al. [22], two successful experiments have recently been completed. In addition, four batch-type experiments will be initiated shortly. These experiments simulate the low-temperature (epithermal) alteration of a synthetic Martian basalt by an acidic sulfate-chloride type fluid; an analog to the near surface environment peripheral to a hydrothermal system.