Thermoelastic Properties of Aluminous Phases in MORB From First‐Principle Calculation: Implications for Earth's Lower Mantle

Abstract

AbstractThe mantle dynamics and mineralogy of the deep Earth are fundamentally influenced by the intrusion of subducted mid‐ocean ridge basalt (MORB). Calcium ferrite (CF) structure and calcium titanate (CT) structure phases are probable candidates for the end members of the complex aluminous solid solution in MORB under Earth's lower mantle (LM) conditions. In this paper, we investigate the thermoelastic properties of CF‐type MgAl2O4 (Mg‐CF), CT‐type MgAl2O4 (Mg‐CT), and CF‐type NaMg2Al5SiO12 (NaMg‐CF) under 32 to 142 GPa at 0, 2,000, and 3,000 K by ab initio molecular dynamics calculations. Using the thermal equations of state of the aluminous phases and considering the effects of composition and phase transition, we confirm that MORB is 0.7–1.8% denser than the surrounding mantle and support that oceanic plate could subduct at least to the bottom 200–300 km of the LM. At LM pressure‐temperature conditions, MORB has 0.3% faster VP, 2.2% slower Vs, and 1.9% faster VΦ than does the preliminary reference Earth model. Temperature dependencies of elasticity of Mg‐CF, Mg‐CT, and NaMg‐CF are calculated under LM conditions. We find that the temperature effect on different elastic constant components is complicated and the trend in the changes of derivatives of wave velocities with respect to temperature is contrary to other system (e.g., bridgmanite), so it is worth obtaining the elastic properties of mantle minerals under their relevant conditions. In addition, the significant elastic anisotropy due to the phase transition from CF structure to CT structure may lead to detectable seismic anisotropy at the bottom of the LM.