Textural and modal analyses of picritic basalts with ChemCam Laser-Induced Breakdown Spectroscopy

The ChemCam instrument suite on the Mars Science Laboratory rover Curiosity provides standoff compositional information using the first Laser-Induced Breakdown Spectroscopy (LIBS) on a planetary mission and provides visual data on sample texture using a remote microimager. The LIBS technique is normally used to provide both qualitative (PCA, ICA) and quantitative (PLS) measurements. In this study we test its ability to evaluate variations in the proportion of different mineral phases, the distribution of mineral grains and the relative proportions of pyroxene and plagioclase in five basaltic rocks: (1) three picritic samples: Martian shergottite Dar al Gani 476 with olivine phenocrysts in a coarse-grained pyroxene-rich matrix; a terrestrial analog of Gusev basalt with a fine-grained matrix and equal amounts of pyroxene and plagioclase; a synthetic microlitic glass and (2) two basaltic terrestrial rocks. LIBS measurements were acquired at a 3-m standoff distance using a constant 400 mu m beam size. The number of analysis locations per sample was fixed to five regardless of the rock texture. Results show that olivine phenocrysts can be easily distinguished from groundmass. The standard deviation of Ca/Al emission line ratios correlates with the grain size of the analyzed rocks and corresponds to the amount of plagioclase and pyroxene in the sample, even when the grain size is smaller than the laser spot size.