Iron spin state in silicate glass at high pressure: Implications for melts in the Earthʼs lower mantle

We report a Mössbauer spectroscopic study of a Fe2+-rich aluminous silicate glass and a Fe3+-rich sodium silicate glass measured in a diamond anvil cell up to 84 GPa. The hyperfine parameters vary smoothly with pressure and are consistent with a gradual increase in coordination number with pressure. Fe2+ and Fe3+ remain in the high-spin state and show no evidence of spin crossover over the measured pressure range. A spin crossover may eventually occur at higher pressures; however the strong thermal broadening of the crossover region due to Boltzmann statistics would prevent any spin crossover from occurring sharply at P,T conditions down to the base of the lower mantle. Our results in combination with recent solid/melt partitioning data in a chondritic system imply that strong preferential partitioning of iron into the melt phase cannot give rise to negatively buoyant melts in the Earthʼs lower mantle.