Phase equilibrium investigations of the Adirondack Class basalts from the Gusev plains, Gusev crater, Mars

Abstract

Phase equilibrium experiments have been performed on a synthetic analog of the Gusev plains basalt composition from the Spirit landing site on Mars, determined by the Alpha Particle X-ray Spectrometer on the Spirit Rover (Gellert et al., 2006; McSween et al., 2006). The near-liquidus phase relations were determined over the pressure range of 0.1 MPa to 1.5 GPa and at temperatures from 1105 °C to 1390 °C in a piston cylinder apparatus and l-atm gas mixing furnace. The composition is multiply saturated with olivine, orthopyroxene, and spinel on the liquidus at 1310 °C and 1.0 GPa, or 85 km depth on Mars, placing an upper limit constraint on the thickness of the Martian lithosphere at the time of eruption. Experiments carried out at 0.1 MPa show Fo77 olivine as the first liquidus silicate mineral, appearing near 1250 °C. Olivine and spinel co-crystallize until pigeonite and plagioclase (An73) join the crystallizing assemblage at 1125 °C. Low pressure crystallization of the Gusev composition does not produce liquids that could be parental magmas to surface types 1 and 2 of the Martian surface, nor do they appear to be related to olivine-phyric shergottites or other SNC meteorites. The iron-rich Gusev basalts appear to have been crystallized from a less depleted mantle source than the magnesian olivine-phyric shergottite Yamoto 980459.