Elastic Wave Velocities of Lower Mantle Perovskite With Intermediate-spin Iron and Consequences for Mantle Properties and Dynamics

The recent discovery that the bulk of the lower mantle contains Fe2+ in the intermediate-spin (IS) state raises important issues with regard to modelling of lower mantle properties using mineral physics and seismological data. So far there are no elastic wave velocity measurements of lower mantle perovskite containing intermediate-spin iron, and the currently used experimental constraints from the MgSiO3 endmember are likely not realistic due to the inferred coupling between IS state stability and structure distortion combined with the large observed difference in anisotropic compression between MgSiO3 and (Mg,Fe)SiO3 perovskite. We therefore undertook a nuclear inelastic scattering (NIS) study of (Mg,Fe)SiO3 perovskite at beamline ID18 at the European Synchrotron Radiation Facility to address this problem. We collected room temperature NIS data for Mg0.88Fe0.12SiO3 perovskite at pressures from 0 to 115 GPa using a specially designed panoramic diamond anvil cell, and we made nuclear forward scattering measurements to confirm the spin state, and X-ray diffraction measurements for pressure calibration and to verify the structural state of the sample. We performed annealing using an infrared laser at low power after nearly all increases or decreases of pressure above 30 GPa to relieve stresses in the sample. Vp and Vs were calculated from the NIS data using the Debye model, and both show a marked deviation from the trend for the MgSiO3 endmember, which is likely related to iron spin crossover in the perovskite structure. We will present the implications for modelling of lower mantle compositions and temperatures, and discuss the consequences of the heterogeneous spin region for perovskite at the top of the lower mantle for dynamic mantle processes.