The high pressure reaction orthopyroxene + clinopyroxene + spinel to garnet + olivine is applicable to the mantle of Mars. Experimental reveals of this reaction in the system CaO-MgO-FeO-Al2O3-SiO2 have been made in the range 1000¿--1200 ¿C using the bulk composition proposed by Morgan and Anders <1980>. At 1000 ¿C and 1200 ¿C the garnet-out equilibrium is bracketed at 10.8¿0.5, 12.6¿0.5, and 15.2¿0.15 kbars, respectively. A dP/dT slope of 14.0 bar/K at 1000 ¿C increasing to 26.0 bar/K at 1200 ¿C has been inferred. Depending on bulk composition, the mantle of Mars is inferred to have a phase assemblage of either olivine + clinopyroxene + orthopyroxene + garnet or olivine + clinopyroxene + garnet + spinel. In the second, the partial melts will not be buffered by olivine and orthopyroxene and will have markedly lower silica activities than those of the terrestrial mantle.