MANTLE METASOMATISM IN MARS: POTASSIC BASALTIC SANDSTONE IN GALE CRATER DERIVED FROM PARTIAL MELT OF PHLOGOPITE-PERIDOTITE

Among the surprises found by the Curiosity MSL rover in Gale Crater, Mars, is a range of alkali-rich basaltic sandstones. These sandstones show little evidence of alteration, and their bulk compositions are consistent with those of fresh basalts, affected little by fractionation during sediment transport. Thus, it is reasonable (though tentative) to consider the chemical components of these sandstones as magma compositions.

At the Windjana area, Curiosity obtained APXS and ChemCam LIBS chemical analyses of many basaltic sandstones; their range of compositions implies at least three sources: tholeiitic, sodic, and potassic (Treiman et al. 2016). The last is of particular interest (LeDeit et al. 2016)– the average of the four most K-rich samples (LIBS) is: SiO2=48.4%, Al2O3=11.5%, FeO=18.2%, MgO=9.0%, CaO=6.0%, Na2O=1.7%, K2O=5.4%. This composition is consistent with that of a partial melt from a phlogopite-bearing martian mantle peridotite, in Fe and Mg contents and Mg* (Collinet et al. 2015), and in K2O (Condamine et al. 2014). These samples are also rich in Rb and F (LeDeit et al. 2016), as expected from a phlogopite-peridotite. The relatively low Si and Al contents could suggest melting at pressure >~0.2GPa (Grove et al. 2013).

Mars is richer in Na and K than is the Earth, with a similar Na/K ratio, and this overall enrichment does not explain the existence of ultrapotassic basalt as inferred here (K2O/Na2O=3.2). By analogy with ultrapotassic rocks on Earth (Gupta, 2015), it seems likely that the sub-Gale mantle was metasomatically enriched in K, producing significant volumes of phlogopite peridotite. On Earth, ultrapotassic basalts similar to these Gale crater compositions occur in many tectonic settings, but are most common after continent collisions (Wilson, 1989). This analogy is likely not relevant to Mars.