CONSTRAINTS ON THE CRYSTAL CHEMISTRY OF MARTIAN FE/MG-RICH SMECTITIC CLAYS: LINKS TO GLOBAL ALTERATION TRENDS AND PROTOLITHS

Near infrared remote sensing data of Mars have revealed thousands of ancient, astrobiologically interesting deposits of Fe/Mg-rich smectitic clay minerals within the crust. Diagnostic (Fe,Mg)-OH infrared spectroscopic absorptions used to interpret the mineralogy of these clays occur at (λ=) 2.27-2.32 μm, indicating variable Fe/Mg ratios in the clay structures. Using a suite of Fe/Mg-rich seafloor clays as a mineralogical and spectroscopic analog for Martian clays, we show how crystal chemical substitution and mixed-layering affect the position of the diagnostic metal-OH feature in smectitic clays. We show how substitution of divalent cations such as Fe²⁺ and Mg²⁺ into dioctahedral clays predictably affects the placement of the metal-OH absorptions used to interpret the composition of smectitic clays from remote sensing data. The effect of substitution of trivalent cations (chiefly Fe³⁺) into trioctahedral clays is less pronounced, but discernable. Taken together, we are able to constrain the actual composition, especially the Fe/Mg ratio of Martian smectitic clays from near infrared data.

One of the goals of this work is to compare the derived chemistry of Martian smectitic clays with the known chemistry of possible protoliths, such as olivines, pyroxenes, and glasses measured in Martian meteorites, in order to determine if the Martian clays likely formed in closed systems with low water/rock ratios from known source rocks, or alternatively, if the smectitic clays formed in open systems with significant chemical mobility. Using our samples as a framework, we categorized Martian smectitic clays into four groups based on remote near infrared measurements. Of these, three of the groups have high Fe/Mg ratios that likely represent open-system alteration in high water/rock ratio, oxidizing conditions. The fourth group represents Fe²⁺ and Mg²⁺-rich clays that likely formed in reducing conditions at depth, and possibly in closed systems.