Diverse, Alkali-Rich Igneous and Volcaniclastic Rocks Reflect a Metasomatised Mantle Beneath Gale Crater

Wednesday, 17 December 2014: 2:55 PM
Mariek E Schmidt, Brock University, St Catharines, ON, Canada, Michael B Baker, CALTECH, Pasadena, CA, United States, Jeffrey A Berger, University of Western Ontario, Earth Science, London, ON, Canada, Martin R Fisk, Oregon State Univ, Corvallis, OR, United States, Ralf Gellert, University of Guelph, Guelph, ON, Canada, Scott M McLennan, Stony Brook University, Stony Brook, NY, United States, Megan Eve Newcombe, Caltech, Pasadena, CA, United States, Edward M Stolper, California Institute of Technology, Pasadena, CA, United States and Lucy Thompson, University of New Brunswick, Earth Sciences, Fredericton, NB, Canada
Although Curiosity landed in a sedimentary setting, geochemical compositions determined by Alpha Particle X-ray Spectrometer (APXS) and ChemCam suggest that major element concentrations of some rocks were little modified by chemical weathering, and in these cases, the bulk (>70%) of the crystalline components determined by ChemMin are igneous. Gale rocks can therefore largely preserve the composition of their igneous protoliths and provide insight into the crystalline basement exposed in the north crater rim. Four end-member compositions are recognized on the basis of APXS analyses. (1) The diverse, evolved Jake M class (n=12) of inferred igneous origin includes float blocks and cobbles. Jake M rocks are phonotephritic/mugearitic to trachyandesitic and characterized by low MgO contents (3.0-5.7 wt%) and high Al and alkalis, particularly Na2O (up to 7.35 wt%). (2) The Bathurst class of siltstones to coarse sandstones (n=13) occurs as dark-toned float and bedded outcrop and is basaltic to trachybasaltic, ranging to high K2O (up to 3.8 wt%). Alteration of the protolith(s) or during diagenesis may have affected this class. (3) The Darwin class of conglomerates to coarse sandstones (n=10) has high Na and Al, likely reflecting a sodic plagioclase-rich mineralogy, but with higher Fe than Jake M class (13.0-17.1 vs. 6.0-12.5 wt%). (4) The low alkali “normal” Mars basaltic composition is typified by the Portage soils (n=6) and the John Klein class (n=13; includes the Sheepbed mudstone). Some degree of mixing and/or contamination with this low alkali basaltic compositon has affected all APXS analyses. Overall, Gale rocks are strongly enriched in total alkalis (at the same MgO) relative to basaltic shergottites and many have higher K2O than igneous rocks analyzed by Spirit and Opportunity, suggesting that the mantle beneath Gale is alkali-rich (likely as a result of a metasomatic event) and that alkalis are heterogeneously distributed in the planet’s interior.