AN OVERVIEW OF ALTERATION IN THE MURRAY FORMATION, GALE CRATER, MARS
Characterizing the geochemical and mineralogical trends of the Murray formation, the lowermost unit of the Mt. Sharp group in Gale crater, is essential to interpreting ancient aqueous environments and post-depositional diagenetic alteration of this generally lacustrine sedimentary unit. To date, the Mars Science Laboratory Curiosity rover has drilled eleven samples from the Murray formation. Drilled sample fines were delivered to the CheMin instrument, a combination X-ray diffractometer and X-ray fluorescence spectrometer, and quantitative abundances of minerals and amorphous phases were determined.
Mineralogical Trends in the Murray Formation:
Mineralogical variations observed in the Murray formation include changes in abundance and speciation of Fe-oxides, sulfates, and phyllosilicates, reflecting changes in lake water chemistry and/or diagenetic conditions. At the base of the Murray fm. in the Pahrump Hills member, hematite, smectite, plagioclase, and pyroxene are the dominant mineral phases, indicating a basaltic igneous source for the sediments and relatively oxidizing aqueous conditions. Up-section ~10 m in the Pahrump Hills member, phyllosilicate is absent and plagioclase, cristobalite, tridymite, opaline silica, and magnetite are observed, suggesting a silicic igneous and/or hydrothermal source with more reducing aqueous conditions. Further up-section a few 10s of m, in the Hartmann’s Valley member, smectite re-emerges and is inferred to persist for at least a few 100 m of vertical section. The structure and composition of the smectite changes within the stratigraphy, where smectite has more dioctahedral character and is more enriched in Fe3+ and Al going up section into the Karasburg and Sutton Island members. Calcium sulfate and hematite abundances also increase significantly up-section. This change in mineralogy within the Murray formation indicates a greater intensity of near-surface chemical weathering and/or diagenesis over time.