Lower Mantle Melting: Experiments and Thermodynamic Modeling in the System MgO‐SiO2

• Published in: Journal of geophysical research. Solid earth Vol. 126; no. 12
• Main Authors: Yao, Jie, Frost, Daniel J., Steinle‐Neumann, Gerd
• Format: Journal Article
• Language: English
• Published: 01.12.2021
• Subjects: eutectic compositions; lower mantle; melting relations; thermodynamic models
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1029/2021JB022568

Characterizing and modeling melting relations in the system MgO‐SiO2 at lower mantle pressures rely on the location of the eutectic points for MgO‐MgSiO3 and MgSiO3‐SiO2. While at an uppermost lower mantle pressure there is general consensus on the eutectic composition in the former, large discrepancies exist for MgSiO3‐SiO2 from experiments in the diamond anvil cell, ab‐initio simulations and models built on them. In order to address this discrepancy, we have performed multi‐anvil press experiments at 24 GPa for Mg4Si6O16 and Mg3Si7O17 at temperatures of 2650±100 K and 2750±100 K. In the experiments at 2750±100 K, we observe the presence of partial melt. The recovered Mg4Si6O16 sample shows SiO2 stishovite as the liquidus phase, and electron microprobe analysis of the quenched melt determines XSiO2=0.53±0.03 as the eutectic composition. We fit a thermodynamic model to describe the melting relations in the MgO‐SiO2 system, and extrapolate to core‐mantle boundary pressure. At 136 GPa, we predict that the eutectic points have moved further away from enstatite composition, and solidus temperatures remain similar for MgO‐MgSiO3 and MgSiO3‐SiO2. Plain Language Summary Melting of rock in the Earth's deep interior is an important process that has shaped its geological history, and continues to do so to date. However, melting processes at high pressure have not been fully characterized to date, and here we perform experiments that help to better describe models for it. We do so for compositions in the MgSiO3‐SiO2 system that complement prior work for MgO‐MgSiO3; combined, these experiments put important constraints on melting for the most important chemical components in the Earth's mantle, MgO‐SiO2. We find that when silicate melts crystallize, they do form the dominant lowermost mantle mineral, MgSiO3 bridgmanite, only for a limited composition range in terms of the SiO2/MgO ratio; otherwise, MgO or SiO2 form first, and generally at much higher temperatures. Melts in both the MgO‐MgSiO3 and MgSiO3‐SiO2 systems become fully crystallized at very similar temperatures which suggests that a prior idea that the occurrence of partial melts in the Earth's deepest mantle is associated with different chemical composition does not hold if only the MgO‐SiO2 system is considered. Key Points Multi‐anvil press experiments in the system MgSiO3‐SiO2 at 24 GPa located the eutectic at 53 mol% SiO2 and 2750 K We build a thermodynamic model for MgO‐SiO2 based on the data and other constraints that allows extrapolation to the core‐mantle boundary Based on melting curves of MgO and SiO2 and their liquid interaction parameter, the model predicts eutectic temperatures and compositions