Geochemical diversity in first rocks examined by the curiosity rover in gale crater: Evidence for and significance of an alkali and volatile-rich igneous source

The first four rocks examined by the Mars Science Laboratory Alpha Particle X-ray Spectrometer indicate that Curiosity landed in a lithologically diverse region of Mars. These rocks, collectively dubbed the Bradbury assemblage, were studied along an eastward traverse (sols 46-102). Compositions range from Na- and Al-rich mugearite Jake-Matijevic to Fe-, Mg-, and Zn-rich alkali-rich basalt/hawaiite Bathurst-Inlet and span nearly the entire range in FeO* and MnO of the data sets from previous Martian missions and Martian meteorites. The Bradbury assemblage is also enriched in K and moderately volatile metals (Zn and Ge). These elements do not correlate with Cl or S, suggesting that they are associated with the rocks themselves and not with salt-rich coatings. Three out of the four Bradbury rocks plot along a line in elemental variation diagrams, suggesting mixing between Al-rich and Fe-rich components. ChemCam analyses give insight to their degree of chemical heterogeneity and grain size. Variations in trace elements detected by ChemCam suggest chemical weathering (Li) and concentration in mineral phases (e.g., Rb and Sr in feldspars). We interpret the Bradbury assemblage to be broadly volcanic and/or volcaniclastic, derived either from near the Gale crater rim and transported by the Peace Vallis fan network, or from a local volcanic source within Gale Crater. High Fe and Fe/Mn in Et-Then likely reflect secondary precipitation of Fe³⁺ oxides as a cement or rind. The K-rich signature of the Bradbury assemblage, if igneous in origin, may have formed by small degrees of partial melting of metasomatized mantle. Key Points Rocks at the MSL landing site are diverse and alkali- and volatile metal-rich. Three of the first four rocks studied by APXS form a mixing line in element plots. The alkali-rich nature reflects an igneous source affected by metasomatism.