INTEGRATED GEOCHEMICAL HISTORY OF THE SHEEPBED MUDSTONE, GALE CRATER BY ALPHA PARTICLE X-RAY SPECTROMETER

The Sheepbed mudstone is fine-grained and the stratigraphically lowest unit examined thus far by the Curiosity rover. Compositions of bedrock surfaces determined by the Mars Science Lab (MSL) alpha particle X-ray spectrometer (APXS) reflect its integrated provenance, depositional, diagenetic, surface weathering, and dust coating histories. To constrain relative contributions of primary and secondary processes to Sheepbed bulk chemistry, we consider APXS analyses, including a short traverse across strike along with small-scale (2.5 to 5 cm) offset raster measurements (Sols 129-287). APXS analyses provide context for the John Klein drilling campaign and is complementary to ChemCam LIBS analyses, which are at a smaller scale (350-550 µm vs. ~1.7 cm diameter spots). Dust variably coats rock surfaces (4 to 24% coverage by particles >5 µm) and most enriches S and Cl concentrations. S and Cl are also associated with the rock itself; brushed targets contain up to 1.78 wt% Cl, indicating a primary or secondary Cl phase, and a strong correlation between Ca and S in rocks laced by white, late diagenetic veins indicates a Ca-sulfate (CheMin identified anhydrite and bassanite). Surface weathering and atmospheric processes most affect highly soluble elements such as Br, which can be enriched in these rocks (up to 570 ppm) and it is most abundant in unbrushed targets. Correcting rock compositions for dust and for late Ca-sulfate reveals that the Sheepbed mudstone is basaltic with high Fe (up to 23.5 wt% FeO*). Lower K₂O and higher Ni in the Sheepbed than in overlying units (e.g., 0.3-0.8 wt% vs. 1.2-2.8 wt% K₂O) suggest a different, less alkaline sediment source. Relatively uniform compositions and overall similarity to average martian crust (Taylor and McLennan, 2009) suggest that transport and depositional processes moderate bulk compositions. Secondary processes (apart from cross-cutting veins) are largely isochemical, yet redox reactions (e.g., olivine breakdown to saponite + magnetite as suggested by CheMin) are inferred by variable FeO*/MnO (53-112). This ratio correlates with Fe³⁺/Fe_T determined by Mössbauer at the MER sites (e.g., Rieder et al., 2004). For Sheepbed, the highest FeO*/MnO are found among analyses of drill tailings, indicating that it represents subsurface alteration likely associated with diagenesis.