LINKING MARTIAN ROCKS FROM GUSEV CRATER AND THE CHASSIGNITE METEORITES

The MER Rover Spirit found alkalic igneous rocks on Mars at Columbia Hills (Champagne, Wishstone, and Backstay) in keeping with the proposed alkalic parent magma for the Chassigny meteorite (Nekvasil et al., 2003). The Columbia Hills rocks are silica-saturated and share many characteristics with the silica-saturated alkalic rocks of terrestrial intra-plate magmatic suites. The rock Backstay lies very close to the sodic silica-saturated trend exemplified by suites of lavas from Ascension Island and from the Nandewar Volcano of NSW, Australia. Champagne and Wishstone have many of the characteristics of potassic silica-saturated alkalic suites of the Snake River Plain and the Fe-rich rocks associated with terrestrial anorthosite complexes. The hawaiite postulated to exemplify the liquid trapped as melt inclusions in cumulate olivine of the Chassigny meteorite lies along the sodic silica-saturated alkalic trend, as does the rhyolitic glass in the melt inclusions. If this hawaiite is similar to Backstay then it has a slightly lower K2O, titania, and alumina and slightly higher Fe contents than terrestrial hawaiite.

Experimental work indicates that both silica-saturated alkalic trends can be linked through a common tholeiitic parent crystallizing at pressures between 4.3 kbar and 12.3 kbar, with the differences in mineralogy and the resulting liquid lines-of-descent arising from different dissolved water contents. Parental tholeiite may be represented by the Gusev Crater basalts Humphrey, Adirondack, and Mazatzal, provided that at least some of the observed megacrystic olivine has accumulated and modified the bulk compositions. As is the case for terrestrial rocks from each trend, the mineral assemblages forming at elevated pressures will differ from those crystallizing from the ascending residual liquids at low pressures. Understanding the changes in phase equilibria should prove invaluable to the interpretation of TES data for the Martian lithologies.