Geochemical diversity and K-rich compositions found by the MSL APXS in Gale Crater, Mars

Along the Curiosity rover’s traverse toward Glenelg (through sol 102) the Alpha Particle X-ray Spectrometer (APXS) analysed four rocks and one soil. Microscopic images and compositions of unbrushed rock surfaces are consistent with 5-20% dust contamination. Nevertheless, the underlying characteristics of these rocks may still be discerned. As a group, they span nearly the entire range in FeO^* and MnO of the Martian dataset. In addition, they are particularly enriched in volatile metals (K, Zn, Ge), and these elements do not correlate with Cl or S. One rock, Jake_Matijevic is notably alkaline and evolved; its composition is that of a nepheline normative mugearite. The other three rocks plot in the basanite field of a TAS diagram, with high K_2O (up to 3.0%) and low SiO_2. These three rocks are otherwise SNC-like (high Fe and low Al). Three out of the four rocks (including Jake_Matijevic) plot along a line in variation diagrams, suggesting mixing of Fe-rich and Al-rich components, likely by sedimentary processes. With only four rocks analyzed so far and ambiguity as to their geologic context (e.g. outcrop vs. float; igneous vs. sedimentary) additional measurements are needed to fully understand the region. It is nevertheless clear that Curiosity landed in a lithologically diverse, K-rich region of Mars