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Chemical Stratigraphy of the Yellowknife Bay Formation and use of LIBS and APXS in Characterizing Martian Strata


ChemCam is a remote sensing instrument on the Mars Science Laboratory (MSL) Curiosity rover. ChemCam relies on Laser-Induced Breakdown Spectroscopy (LIBS) to provide elemental composition from 1.3 to 7 m, and a micro-camera to return context images. The ChemCam LIBS instrument yields semi-quantitative analyses for major and some minor and trace elements (e.g. Ba, Rb, Sr, Li). Spinoff of ChemCam-LIBS technology for use in the oil and gas industry is in advanced development. ChemCam data (several ten thousands of shots) constrain the compositions of siliciclastic sedimentary rocks exposed through the ∼5 m thick section of the Yellowknife Bay formation (YB) in Gale Crater. The YB formation likely was deposited in a prograding alluvial fan fluvio-lacustrine depositional system. The YB formation is subdivided into four members which are, in ascending order: Sheepbed member (bedded mudstones); Gillespie Lake member (medium- to very coarse-grained sandstones); Point Lake member (enigmatic vuggy sandstones); Shaler member (cross-bedded sandstones and recessive finer-grained sediments). Compositions of Sheepbed/Gillespie siliciclastics, including saponitic mudstones, correspond to iron-rich basalts, with no more than 1-2% SO3 on average, but with elevated Cl (=1%) consistent with the presence of perchlorate/akaganeite salts. In contrast, Point Lake/ Shaler have bulk compositions that show lower relative amounts of FeOT+MgO and higher K2O in some samples. Both major and trace elements exhibit stratigraphic trends, for example the Point Lake and Shaler members show higher K2O/Al2O3 compared to the Sheepbed and Gillespie Lake members, signaling a change in provenance to include more alkali-rich basalts. Stratigraphic variations in Cr/Ni versus Zn/Ni indicate that Point Lake and Shaler have higher and more variable ratios than the Sheepbed, related to combinations of elevated Zn and Cr and lower Ni. In Shaler sandstones, Ni is similarly low (∼200 ppm) as in the Gillespie Lake sandstones but Cr reaches very high levels (>5,000 ppm) consistent with the heavy mineral chromite. Lithium data reveal that Sheepbed mudstones are low and uniform in abundance whereas overlying units are higher and much more variable.