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Project Progress

Sulfate-bearing evaporite deposits are regarded as key exploration targets for Mars sample return. Sulfates form in water, over a broad range of environmental conditions (e.g., pH, salinity, temperature and hydration) and often provide a sensitive indicators of habitability, including how conditions evolved after deposition. Key research questions for sulfate studies include the long-term preservation potential of sulfates and in particular, how biosignature capture and preservation varies under different depositional conditions and post-burial (diagenetic) histories. To shed light on these questions, we have been studying Miocene sulfate evaporites of the Camp Verde Formation, central AZ. Results show that capture and preservation of organic matter (as fossil kerogen) is higher for matrix-dominated, clay-rich subfacies, but significantly lower for environments where sulfates grew displacively within in sediments during early diagenesis. We also show that during later diagenesis (i.e., the post-depositional history of these sediments), primary gypsum is often dissolved early, producing crystal molds that are later infilled with finer-grained mosaics of secondary gypsum having low preservaiton potential. Comparing another diagenetic pathway, primary gypsum and halite are often replaced by secondary alteration rinds dominated by thernardite with an intermediate preservation potential (see Shkolyar and Farmer 2015; AGU).

The second part of this study examines the potential for detecting kerogen preserved in rocks, using in situ laser Raman methods. Our study confirms previous reports that sulfate evaporites (and also associated carbonates) often show very high background fluorescence under conventional Continuous Wave (CW) Raman (532 nm). This can preclude in situ mineral and/or kerogen identifications in target samples. We have undertaken comparative studies of a large sample suite of sulfates, using both conventional Continuous Wave (CW) and Time Resolved Raman (TRR) systems (at longer wavelengths) to see if detection can be improved and if so, how to use this information to improve outcomes for Mars 2020.