Trace Element Behavior in Martian Evaporite Minerals: Experimental Constraints

Introduction: The Mars Exploration Rovers have detected distinctive suites of evaporite minerals at the Martian surface precipitated from fluids derived from basaltic weathering under low pH conditions. Orbital spectroscopic observations have confirmed the global distributions of these materials. The formation and stability of evaporite assemblages have been well modeled at Meridiani Planum for major element chemistry. In addition to providing major element abundances of Martian surface materials, the APXS (Alpha-Particle 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 [1, 2]. These measurements have shown considerable trace element abundance variability that is not well-understood (see Table 1, below). Accordingly, it is of interest to constrain the trace element partitioning behavior for the distinctive evaporite minerals that likely exist at Meridiani Planum, including gypsum, Mgand Fe-sulfates. Background: Although relatively uncommon in terrestrial environments, acidic weathering of basaltic rock is likely the primary chemical weathering regime for much of the Martian surface [3, 4]. Acidic, sulfurrich brines evaporate along a particular pathway producing the specific set of evaporite mineral assemblages (Fig. 1, right) observed or implied throughout Meridiani Planum. These evaporite minerals have then been modified, reworked, and mixed with altered siliciclastic debris to form sulfur-rich, layered sandstone outcrops (most notably observed at the Burns Formation and surrounding terrains). Periods of diagenic modification have further altered these deposits [6]. Trace element behavior during the formation and evolution of these specific mineralogies has not previously been studied experimentally in any detail. Method & Results: To fully characterize the partitioning of a trace element throughout an evolving evaporating system, one must first characterize the trace element behavior into each precipitating mineral phase, starting with the earliest precipitated (Fig. 1, above). Determining trace element partitioning during the formation of evaporites 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 [7]. We use a Chemo-Stat and/or pH-Stat titration experimental setup to reliably characterize the trace element partitioning coefficients into a precipitating evaporite mineral. These experiments are ongoing and