High pressure, near-liquidus phase equilibria of the Home Plate basalt Fastball and melting in the Martian mantle

Near‐liquidus phase equilibria experiments have been conducted on a synthetic Fastball basalt composition, as analyzed at Home Plate plateau of Mars (Gusev Crater), to test if it represents a primitive mantle derived melt and place constraints on the temperature of the ancient mantle and on the lithosphere‐asthenosphere boundary of Mars. The Fastball basalt is multiply saturated with olivine and orthopyroxene at ∼1.2 GPa and 1430°C. Based on melting models, we predict that the Fastball composition could be produced by 13–23% equilibrium melting of the Martian mantle, with extraction of melt from the base of ∼105 km thick lithosphere. The multiple saturation for Fastball also constrains the potential temperature of the Martian mantle to be approximately 1480–1530°C with an initial melting pressure of 4.0–4.7 GPa. This potential temperature is much lower than that of the terrestrial mantle derived from similarly ancient magmas, i.e., komatiites.