Flotation of Olivine in the Peridotite Melt at High Pressure

We have conducted experiments to measure the density of the FeO rich peridotite melt up to 8 GPa. The San Carlos olivine with a composition FO90 floats in the peridotite melt at 8 GPa. Pure forsterite floats and the San Carlos olivine sinks, whereas the Fo95 olivine is neutrally buoyant in the peridotite melt at 6 GPa. This result implies that the density crossover between the FeO rich peridotite melt and the equilibrating olivine exists at around 7 GPa. Since the FeO rich peridotite is similar to the Martian mantle composition proposed by Morgan and Anders (1979), present experiments clearly implies that the density crossover does exist in the Martian interior at the depths around 600 km. We also conducted a buoyancy test for the pyrollite melt. Olivine with the Fo₉₅ composition sinks in the melt at 8 GPa. This result together with that of the FeO rich peridotite melt implies that the density crossover will exist at the pressure around 12 GPa in the magma with the terrestrial mantle composition.