Experimental Constraints on Trace Element Mobility in Martian Basalt

It is widely believed that aqueous alteration on Mars has been strongly influenced by low pH conditions. Experimental work has shown that these low pH environments result in the mobility of elements, such as Fe (III) and Al, that are relatively insoluble under most near surface conditions on Earth. Although these studies have increased our understanding of major element behavior, little is known about the mobility of trace elements under Martian conditions. The Mars Exploration Rovers have provided and continue to provide analyses of Ni, Zn, and Cr at both the Spirit and Opportunity landing sites. In-situ rock and soil analyses show that these trace elements are highly variable, which generates many questions about the processes that control their distribution. Among the issues that need to be considered are the roles of meteoritic contributions, aqueous alteration, residual enrichments, evaporative concentrations, and so forth. A central question to addressing these issues is the nature of the mobility of Ni, Zn, and Cr during aqueous alteration under Martian conditions. Accordingly, we have begun a series of batch aqueous alteration experiments on synthesized Martian basalt in an attempt to better understand the mobility of Ni, Zn, and Cr on the Martian surface. Basalt analog compositions are the same as those used in previous studies from our lab and are based on average S- and Cl-free Pathfinder soil, with Ni, Zn, and Cr added in the form of oxides. Mixtures of sulfuric and hydrochloric acids, with varying concentrations (ranging from 1M H2SO4/0.25M HCl to 100 μM H2SO4/25 μM HCl and a S:Cl mole ratio of 4, comparable to typical soils), are added to synthesized basalt in order to achieve a water-to-rock ratio of 10. Fluid-rock mixtures are allowed to react in Teflon beakers at 25 ° C for a period of 14 days, during which time small amounts of fluid are extracted and analyzed for major and trace elements to constrain the nature of alteration mechanisms. At the end of the experiments, residual material is fully characterized for mineralogy, chemistry, and texture and final fluids are evaporated and evaporite minerals similarly characterized. In this poster, we will fully describe our experimental approaches and present initial results.