MINERALOGY AT BRADBURY LANDING AND YELLOWKNIFE BAY, GALE CRATER, MARS AS MEASURED BY THE CHEMCAM LIBS

More than 50,000 laser shots of rocks, soils, and dust have been acquired by the ChemCam laser-induced breakdown spectrometer (LIBS) on the Mars Science Laboratory Curiosity rover over its first 269 sols. As the rover has traversed from Bradbury landing site to Yellowknife Bay, LIBS data have been acquired in sets with 30-150 shots at individual locations, often with 9-25 different locations on the same rock being sampled. In ~10% of those data sets, the material lased with the ~400 mm diameter ChemCam beam contains three or fewer phases, allowing compositions of individual phases with specific compositions to be directly extracted. This capability makes ChemCam a powerful tool for measuring Mars mineralogy.

In this study, compositions of individual laser shots were determined using partial least-squares (PLS-1) multivariate analysis and then converted to moles to identify minerals by stoichiometry or through use of elemental ratios. In rare cases, individual minerals were sampled by the laser and the mineral stoichiometries of feldspar and Ca-pyroxene can be identified. In others, multiple LIBS shots on the same material produce constant molar ratios that pass through the origin for elements that occur in only a single phase or two phases. For example, a rock in which the laser samples only varying amounts of feromagnesian silicate and plagioclase (plag) with constant compositions will produce linear arrays of points on a graph of Fe vs. Fe+Mg that pass through the origin and have constant slopes equal to the Fe/(Fe+Mg) content of the ferromag. Olivine (ol) vs. pyroxene (px) may then be distinguished on the basis of partitioning by other elements such as Ni. Results suggest that Fe>Mg compositions predominate for both ol and px in sols through 269. Martian plagioclases were studied using the ratio of Na/(Na+Ca), and have an average composition for these sols of Na_0.45Ca_0.55Al_1.5Si_2.5O₈ with individual spots spanning the entire range from Ca to Na end-members. A small K-feldspar component, generally <10 mole%, is present either in plagioclase or as a distinct phase. Finally, it is also possible to discern compositions of mineral pairs from linear trends with non-zero intercepts in the plots described above, enabling compositions of, for example, albite + nepheline, plagioclase+pigeonite, and plag + olivine to be determined.