Linking Surface and Cumulus Lithologies on Mars: Evidence From Experimental Phase Relations

A major crustal differentiation process in regions of thick crust on Earth is deep-seated fractionation. In this process, the crystalline material formed at depth through crystallization of mantle-derived magma ponded at or near the base of the crust remains in the lower crust while residual liquids ascend to the upper crust. An example of this process on Mars is afforded by the volcanic rock Backstay analyzed by the MER Spirit and the Chassigny dunite of the SNC meteorites. Experimental investigation of phase equilibria of Backstay composition liquids indicates that at the base of the Martian crust Backstay composition liquid with 1 wt% water is saturated in the cumulus minerals of the meteorite and, upon further crystallization, produces the mineral assemblages of the polyphase melt inclusions. The residual liquids become increasingly silica-rich, alumina-rich and Na-rich, and the Fe contents become increasingly similar to those of alkalic suites on Earth. Depending upon the temperature of separation of the residual liquid from the accumulating minerals, the residual material varies from dunitic to gabbroic. The compositional characteristics of the evolved alkalic liquids are not reflected in bulk surface chemistry of the planet which suggests that as on Earth alkalic rocks may be rare on Mars. Such experimental investigations are being applied to other Martian surface lithologies (i.e., Humphrey, Irvine) in order to identify further links between the cumulus meteorite lithologies and surface igneous lithologies, and in order to constrain possible igneous processes responsible for inducing compositional variability of the martian crust.