Carbon-bearing silicate melt at deep mantle conditions

Abstract

Knowledge about the incorporation and role of carbon in silicate magmas is crucial for our understanding of the deep mantle processes. CO2 bearing silicate melting and its relevance in the upper mantle regime have been extensively explored. Here we report first-principles molecular dynamics simulations of MgSiO3 melt containing carbon in three distinct oxidation states - CO2, CO, and C at conditions relevant for the whole mantle. Our results show that at low pressures up to 15 GPa, the carbon dioxide speciation is dominated by molecular form and carbonate ions. At higher pressures, the dominant species are silicon-polyhedral bound carbonates, tetrahedral coordination, and polymerized di-carbonates. Our results also indicate that CO2 component remains soluble in the melt at high pressures and the solution is nearly ideal. However, the elemental carbon and CO components show clustering of carbon atoms in the melt at high pressures, hinting towards possible exsolution of carbon from silicate melt at reduced oxygen contents. Although carbon lowers the melt density, the effect is modest at high pressures. Hence, it is likely that silicate melt above and below the mantle transition zone, and atop the core-mantle boundary could efficiently sequester significant amounts of carbon without being gravitationally unstable.

Density‐Pressure Profiles of Fe‐Bearing MgSiO3 Liquid: Effects of Valence and Spin States, and Implications for the Chemical Evolution of the Lower Mantle

“…The reference isotherm ( T 0 = 3,000 K) is taken to be the fourth‐order Birch‐Murnaghan equation: V 0 = 1,180 Å 3 , K 0 = 21.1 GPa, , and GPa −1 for pure MgSiO 3 liquid. The thermal pressure increases systematically with compression, which is reflected in strongly volume‐dependent B TH (inset of Figure , left), as also found previously (Ghosh et al, ; Stixrude & Karki, ). The thermal contributions are the same within computational uncertainty for pure and iron‐bearing melts.…”

“…Our simulations covered a wide range of volume ( V / V X = 1.2 to 0.5, where V X = 1,033.6 Å 3 for the pure phase), mapping the whole mantle pressure regime. At each volume, initial melting and equilibration were performed at 8,000 K followed by quenching to lower temperatures 5,000, 4,000, and 3,000 K (Ghosh et al, ). A plane wave cutoff energy of 400 eV with Gamma‐point Brillouin‐zone sampling was used.…”

Polymerized 4-Fold Coordinated Carbonate Melts in the Deep Mantle

“…A striking characteristic of glassy FeCO 3 is its strong first sharp diffraction peak (FSDP) that persists in the structure factor up to the highest P investigated (Figure 3A), indicative of a strong medium-range order. This is in stark contrast to silicate glasses that lose their medium-range order with increased P (Sato and Funamori, 2008), but consistent with ab initio calculations on carbon-bearing silicate melts reporting Pinduced polymerization of carbonate species into dimers and with the silicate network (Ghosh et al, 2017;Solomatova and Asimow, 2019). A second noticeable feature is the decrease of the contribution at 4 Å −1 attributed in molten carbonates to the O-O bond (Wilding et al, 2016).…”

Fate of Hydrous Fe‐Rich Silicate Melt in Earth's Deep Mantle

“…Improved data will help constrain the Fe # and water content in partial melts, thus providing a better understanding of the stability of partial melts at the CMB. Effects of other volatiles, such as carbon, should be also included in the future to evaluate the stability of partial melt at the CMB (e.g., Ghosh et al, ; Solomatova et al, ). Understanding the fate of these volatile‐rich melts will be the key to unfolding Earth's early history and thermochemical evolution.…”