Constraints on the Crystal Chemistry of Fe/Mg-Rich Smectitic Clays on Mars and Links to Global Alteration Trends
Abstract: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. We show how these clays, detected from orbit, fall into four groups. We compare the Fe/Mg ratios of each of these groups with the Fe/Mg ratios of various candidate source materials, such as olivines and pyroxenes in Martian meteorites. One of these groups of clays has similar chemistry to candidate protoliths, consistent with little segregation of Fe with respect to Mg compared to potential protoliths. The other three groups suggest significant segregation, indicating Mg-mobility in what was likely open-system alteration. We propose that these differences are related to oxidative, open system weathering in groups 1-3, and reduced, closed system hydrothermal alteration in the case of group 4.
Figure caption: The sample suite of analog materials studied here includes pure nontronite as well as mixed layer (or interstratified) glauconite-nontronite, talc-saponite, and talc-nontronite. This diagram illustrates the range of chemical and structural space represented by our sample suite, which contains 35 well characterized, nearly pure clay samples.