Experimental hydrothermal alteration of a Martian analog basalt: Implications for Martian meteorites

— A number of martian meteorite samples contain secondary alteration minerals such as Ca-Mg-Fe carbonates, Fe oxides, and clay minerals. These mineral assemblages hint at hydrothermal processes occurring in the martian crust, but the alteration conditions are poorly constrained. This study presents the results of experiments that examined the alteration of a high-Fe basalt by CO2-saturated aqueous fluids at 23 and 75 °C and by mixed H2O-CO2 vapors at 200 and 400 °C and water-rock ratios of 1:1 and 1:10. Results indicate that observable alteration of the basalt takes place after runs of only seven days. This alteration includes mobilization of silica into phases such as opal-CT and quartz, as well as the formation of carbonates, oxides, and at some conditions, zeolites and hydrous silicates. The degree of alteration increases with run temperature and, in high-temperature vapor experiments, with increasing water content of the vapor. The degree of alteration and the mineralogy observed in the martian meteorites suggests that none of these samples were exposed to aqueous fluids for long periods of time. Nakhla and Lafayette probably interacted with water for relatively brief periods of time; if so, silica may have been leached from the parent rocks by the altering fluids. Allan Hills 84001 shows possible evidence for very limited interaction with an aqueous fluid, but the overall slight degree of alteration described for this meteorite strongly suggests that it never interacted extensively or at high temperature with any water-bearing fluid. Elephant Moraine A79001 may not have been altered by aqueous fluids at all. The results of this study best support models wherein the meteorite parent rocks were wetted intermittently or for brief periods of time rather than models that invoke long-term reaction with large volumes of water. Our experiments studied alteration of a high-Fe basalt by dilute, CO2-saturated, aqueous solutions at 23 and 75 °C and by mixed H2O-CO2 vapors at 200 and 400 °C. The results suggest that alteration of the parent rock takes place even after very short reaction times of seven days. All experiments produced carbonate minerals, including calcite, and in some cases, magnesite, siderite, and ankerite. A free silica phase, either opal, quartz, or hydrated silica, formed in most experiments. More altered experiments also contained minerals such as zeolites and hydrous phyllosilicates. Clay minerals were not observed to form in any experiments. In aqueous fluids, higher temperature corresponded with a higher degree of alteration, whereas changing fluid composition had no observable effect. In high-temperature vapors, the degree of alteration was controlled by temperature and the proportion of H2O to CO2, with water-rock ratio also playing a role in transport of silica. Application of these results to martian meteorites that contain secondary alteration minerals suggests that none of the martian rocks underwent extensive interaction with aqueous fluids. Nakhla and Lafayette contain clay minerals, which suggests that they interacted with water to some extent, possibly at elevated temperatures. Although ALH84001 shows possible evidence of very limited interaction with aqueous fluids, EETA79001 does not. These results support models for the alteration of these meteorites that do not invoke long-term interaction with water or reaction with large volumes of water. Except for some models for alteration of ALH84001, this conclusion agrees with most of the literature on alteration of martian meteorites.