Experimental Constraints on Trace Element Behavior in Martian Evaporite Minerals

The Mars Exploration Rovers, Spirit and Opportunity, have detected a distinctive suite of evaporite minerals at the Martian surface precipitated from fluids derived from basaltic weathering under low pH conditions. Orbital spectroscopic methods have confirmed global distributions of these materials. The formation and stability of these mineral assemblages have been well modeled at Meridiani Planum for major element chemistry. However, in addition to providing major element chemistry of Martian surface materials, the APXS (Alpha-Proton X-Ray Spectrometer) instruments onboard the Mars Exploration Rovers have returned a selection of trace element abundances in soils and rocks including Ni, Zn, and Cr. These measurements have shown considerable trace element abundance variability that is not well-understood. Accordingly, it is of some interest to constrain the trace element partitioning behavior for the distinctive evaporite minerals that likely exist at Meridiani Planum, including gypsum, Mg- and Fe-sulfates. Determining trace element partitioning during the formation of these materials is difficult through experimental means due to various kinetic effects and complicating factors. Also, analysis of trace element abundances in the final precipitated mineral can be further complicated by the presence of ubiquitous fluid inclusions. Here we describe an experimental approach and preliminary results of trace element incorporation into gypsum (CaSO4\dot2H2O) - which precipitates early in an evolving evaporation system and for which the precipitation process is well-understood. We record the behavior of Ni, Zn and Cr individually during the carefully controlled precipitation of gypsum at constant pH and temperature (25°C). Future experiments will examine the behavior of these elements for increasingly more complex evaporite formation systems more fully analogous to Meridiani Planum mineralogy.