70 research outputs found

    Thermoelasticity of Fe2+-bearing bridgmanite

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    We present LDA+U calculations of high temperature elastic properties of bridgmanite with composition (Mg(1x)_{(1-x)}Fex2+_{x}^{2+})SiO3_3 for 0x0.1250\le{x}\le0.125. Results of elastic moduli and acoustic velocities for the Mg-end member (x=0) agree very well with the latest high pressure and high temperature experimental measurements. In the iron-bearing system, we focus particularly on the change in thermoelastic parameters across the state change that occurs in ferrous iron above \sim30 GPa, often attributed to a high-spin (HS) to intermediate spin (IS) crossover but explained by first principles calculations as a lateral displacement of substitutional iron in the perovskite cage. We show that the measured effect of this change on the equation of state of this system can be explained by the lateral displacement of substitutional iron, not by the HS to IS crossover. The calculated elastic properties of (Mg0.875_{0.875}Fe0.1252+_{0.125}^{2+})SiO3_3 along an adiabatic mantle geotherm, somewhat overestimate longitudinal velocities but produce densities and shear velocities quite consistent with Preliminary Reference Earth Model data throughout most of the lower mantle.Comment: Accepted for Geophysical Research Letters (DOI: 10.1002/2014GL062888

    Velocity and density characteristics of subducted oceanic crust and the origin of lower-mantle heterogeneities

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    Seismic heterogeneities detected in the lower mantle were proposed to be related to subducted oceanic crust. However, the velocity and density of subducted oceanic crust at lower-mantle conditions remain unknown. Here, we report ab initio results for the elastic properties of calcium ferrite‐type phases and determine the velocities and density of oceanic crust along different mantle geotherms. We find that the subducted oceanic crust shows a large negative shear velocity anomaly at the phase boundary between stishovite and CaCl2-type silica, which is highly consistent with the feature of mid-mantle scatterers. After this phase transition in silica, subducted oceanic crust will be visible as high-velocity heterogeneities as imaged by seismic tomography. This study suggests that the presence of subducted oceanic crust could provide good explanations for some lower-mantle seismic heterogeneities with different length scales except large low shear velocity provinces (LLSVPs)