Abstract
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).
Highlights
One of the unique features of the Earth is its active plate tectonics driven by vigorous mantle convection
Previous experiments[32] demonstrated that the natural Mid-Ocean Ridge Basalt (MORB) assemblage at the P–T conditions of lower mantle consists of SiO2 silica, calcium perovskite (CaPv), bridgmanite (Bdg), and two types of aluminum-rich phases: the new Al-rich phase (NAL) and calcium ferrite‐type (CF-type)
Combining chemical compositions and mineral volume proportions present in natural MORB32, we find that MORB consists of approximately 39% Feand Al-bearing bridgmanite (Mg0.58Fe0.16Al0.26Si0.74Al0.26O3), 30% Ca-perovskite (CaSiO3), 16% SiO2, and 15% Fe-bearing CFtype phase (Na0.4Mg0.48Fe0.12Al1.6Si0.4O4)
Summary
One of the unique features of the Earth is its active plate tectonics driven by vigorous mantle convection. Previous studies[16,19,20] have attributed the detected strong small-scale heterogeneities in the mid mantle to the phase transition from stishovite to the CaCl2-type silica[25,26,27], which can cause a low velocity anomaly for the oceanic crust at the phase boundary[28]. Previous experiments[32] demonstrated that the natural MORB assemblage at the P–T conditions of lower mantle consists of SiO2 silica (stishovite and CaCl2-type silica), calcium perovskite (CaPv), bridgmanite (Bdg), and two types of aluminum-rich phases: the new Al-rich phase (NAL) and calcium ferrite‐type (CF-type). Previous ab initio calculations within the local density approximation (LDA) have obtained the reliable and accurate elasticity of Fe-free and Fe-bearing bridgmanite (MgSiO3 and Mg0.875Fe0.125SiO3)[34], corundum (Al2O3)[35], Ca-perovskite[36], and stishovite and CaCl2-type silica[37] at high P–T conditions. Our results show that the velocity anomalies produced by subducted oceanic crust strongly depend on depth and its presence can explain some seismic heterogeneities in the lower mantle
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