Ab Initio Atomistic Simulations of Ca‐Perovskite Melting

Abstract

The lower mantle is dominated by the two silicate perovskites, bridgmanite and Ca-perovskite, constituting about 77 and 5 mol%, respectively, of peridotites and about 34 and 24 mol%, respectively, of recycled oceanic crust of basaltic composition (e.g., Stixrude & Lithgow-Bertelloni, 2012). The melting curves of the end member components MgSiO 3 , CaSiO 3 and MgO in the system CaO-MgO-SiO 2 system (CMS) may inform about the crystallization history of the magma ocean and the nature of the ultra-low velocity zones (e.g., Trønnes et al., 2021). Although the melting curves for MgO-periclase and MgSiO 3 -bridgmanite are reasonably well established through the lower mantle pressure range, the Ca-perovskite melting curve remains poorly constrained, in particular at the lowermost mantle pressures (Figure 1). The melting of Ca-perovskite at 14-15 and 16-60 GPa, respectively, has been investigated by multi-anvil (Gasparik et al., 1994) and laser-heated diamond anvil cell (LH-DAC) experiments (Shen & Lazor, 1995;Zerr et al., 1997). Multi-anvil experiments with resistive heating and W-Re thermocouples might provide more accurate and reproducible melting temperatures than the LH-DAC experiments, which are in poor agreement with each other with a difference in melting temperature of about 880 K at 45 GPa. Nomura et al. (2017) performed multi-anvil experiments along the MgSiO 3 -CaSiO 3 join at 24 GPa and developed a thermodynamic model, anchored to the Zerr et al. (1997) melting curve. Although the experimentally based melting curves converge in the 2500-2800 K range at 14-16 GPa, the large variation in their dT/dp slopes yields increasing discrepancies with increasing pressure. The melting curve of Gasparik et al. (1994) is steeper (212 K/ GPa in the 13-15 GPa range) than those determined by the LH-DAC experiments of Shen and Lazor (1995) and Zerr et al. (1997). The latter slopes, measured in the 17-20 GPa range are 32 and 61 K/GPa, respectively. The Ca-perovskite melting curve has also been investigated by classical molecular dynamics (MD) based on empirical atomic potentials (Liu et al., 2010) and density functional theory (DFT) MD calculations (BraithwaiteAbstract Melting curves of Ca-perovskite (pure CaSiO 3 ) were determined by ab initio density functional theory, using two solid-liquid coexistence methods and two free energy approaches, in the form of thermodynamic integration and two-phase thermodynamics. The melting curves based on the solid-liquid coexistence methods and thermodynamic integration rise steeply from 2000 K at 14 GPa to 7000 K at 136 GPa. The melting temperature at 136 GPa is 1400 K higher than previous ab initio predictions. The high thermal stability of Ca-perovskite is linked to its high-symmetry isometric structure and consistent with experiments, demonstrating that Ca-perovskite is the most refractory phase in basaltic compositions in the lower mantle pressure range. The steep dT/dp slope of the melting curve also shows that the Ca-perovskite liquidus field expands relative to those of bridgmanite...

Davemaoite as the mantle mineral with the highest melting temperature

“…Although Hernandez et al ( 13 ) predicted much higher values of temperature than previous calculations ( 11 , 12 ), their melting data are still ~500 K lower than that of our predictions on average. To reconcile this discrepancy, we carefully investigate the procedure of their calculations.…”

“…They found the inconsistency of melting curves between Wilson and Stixrude ( 12 ) and theirs mainly results from the overestimation of liquid entropies obtained from the 2PT method. We have repeated the thermodynamic integration method calculations by ourselves and found good agreement with the results of Hernandez et al ( 13 ) (table S3). According to former experience of using 2PT method, entropy of liquid phase calculated by this method is strongly affected by the way of how to estimate the partial molar volume of each atomic species in a polyatomic system ( 40 ).…”

“…Unlike the MgSiO 3 bridgmanite, melting curves of davemaoite obtained based on DFT calculations display large discrepancies ( 11 – 13 ). Figure 1B indicates that our predicted melting curve of davemaoite is the highest one among all computational studies.…”

“…( B ) Melting of davemaoite. Multi-anvil experimental data: G94, Gasparik et al (1994) ( 9 ); LHDAC experimental data: SL95, Shen and Lazor (1995) ( 7 ) and Z97, Zerr et al (1997) ( 10 ); and previous computational data based on ab initio calculations: BS19, Braithwaite and Stixrude (2019) ( 11 ); WS21, Wilson and Stixrude (2021) ( 12 ); and H22, Hernandez et al (2022) ( 13 ). Black lines display the phase transition boundaries of solid phases below 13 GPa ( 9 , 16 , 17 ).…”

“…They derived the free energies by calculating absolute entropies using the vibrational density of states, which is obtained from Fourier transformation of the velocity autocorrelation function ( 38 ). Very recently, Hernandez et al ( 13 ) repeated the 2PT free-energy calculations. In addition to 2PT method, they also calculated the free energies using thermodynamic integration method ( 39 ).…”