Recent Compositional Trends within the Murray Formation, Gale Crater, Mars, as seen by APXS: Implications for Sedimentary, Diagenetic and Alteration History.

The >230 m thick Murray Formation is the lower-most unit of the Mount Sharp Group, and interpreted as primarily lacustrine. Representative mudstone, siltstone and fine sandstone targets, encountered above -4330 m elevation, trend to lower Si, Al, Ti, Cr and Ca, and higher Fe, Mn, Zn, P and Mg than the Murray below. Less common, distinctive, coarser grained sandstone lenses tend to exhibit slightly different compositions to the more typical Murray but, overall, show similar elemental trends with elevation, albeit exaggerated. This suggests that the variations observed with elevation in Al, Ti, Cr, K, Fe, Mn, Zn and P within both the coarser sandstones and finer grained Murray are the result of diagenetic and/or alteration processes rather than provenance or physical sedimentary processes such as sorting. This is supported by the chemistry of obvious diagenetic, dark grey nodules, and other potential diagenetic/alteration features within this section, which show variations in the same element concentrations (i.e., P, Mn, Fe, Zn, Mg, Ca and S), distinct from diagenetic features lower down in the stratigraphy, indicating mobility of these elements within this section and changing fluid chemistry. Trends in FeO/MnO generally mimic the presence of ferric absorption features observed in visible/near infrared passive spectra from the ChemCam instrument and from CRISM orbital data, which may be consistent with changes in redox conditions as we climb up section towards Vera Rubin Ridge (Hematite Ridge). Layer-parallel CaSO4 is also common, and not observed below -4330 m. This may represent syndepositional evaporite layers, or late bedding/laminae parallel veins emplaced after lithification, in conjunction with cross-cutting veins. The overall differences in composition between the sandstone targets and finer grained Murray are attributed to distinct provenances and/or sorting during transport. We will discuss the implications of the trends and composition of the Murray above -4330 m elevation and how this pertains to the history and evolution of the Murray Formation as a whole, climatic conditions during the formation of the Murray and the nature of Gale crater lake. Also, what do the trends imply about how circulating fluids have evolved within the Murray sediments and pH, redox, salinity conditions of these fluids?