Buoyancy and Structure of Volatile‐Rich Silicate Melts

Abstract

The early Earth was marked by at least one global magma ocean. Melt buoyancy played a major role for its evolution. Here we model the composition of the magma ocean using a six-component pyrolite melt, to which we add volatiles in the form of carbon as molecular CO or CO 2 and hydrogen as molecular H 2 O or through substitution for magnesium. We compute the density relations from firstprinciples molecular dynamics simulations. We find that the addition of volatiles renders all the melts more buoyant compared to the reference volatile-free pyrolite. The effect is pressure dependent, largest at the surface, decreasing to about 20 GPa, and remaining roughly constant to 135 GPa. The increased buoyancy would have enhanced convection and turbulence, and thus promoted the chemical exchanges of the magma ocean with the early atmosphere. We determine the partial molar volume of both H 2 O and CO 2 throughout the magma ocean conditions. We find a pronounced dependence with temperature at low pressures, whereas at megabar pressures the partial molar volumes are independent of temperature. At all pressures, the polymerization of the silicate melt is strongly affected by the amount of oxygen added to the system while being only weakly affected by the specific type of volatile added.

Hydrous silicate melts and the deep mantle H2O cycle

“…A full description of the melt structure computed from the simulation trajectories is provided in the Supplementary Material. Consistent with previous studies (Mookherjee et al, 2008;Karki and Stixrude, 2010a;Karki et al, 2010;Bajgain et al, 2019;Yuan et al, 2020;Karki et al, 2020;Solomatova and Caracas, 2021;Karki et al, 2021), H 2 O dissolution proceeds via depolymerisation of the melt structure with little influence on the Si and Mg local coordination environment. Dissolved H 2 O leads to a substantial reduction in both melt density and viscosity, with the hydrous melts highly inviscid over all P -T space.…”

“…Hydrous melts produced at 24 GPa will also be less dense than surrounding mantle as defined by PREM (Fig. 5b) and there is little change in the relative densities compared to 13 GPa (Solomatova and Caracas, 2021). Only melts that have exceptionally high iron contents (e.g.…”

“…H 2 O dissolution in silicate melts proceeds by depolymerisation of the melt structure via reaction with bridging oxygen atoms to form hydroxyl (OH − ) units resulting in a profound reduction in viscosity (Kohn, 2000). Various ab initio MD simulations have been reported for hydrous silicate melts under a wide range of P -T conditions (Mookherjee et al, 2008;Karki and Stixrude, 2010a;Karki et al, 2010;Bajgain et al, 2019;Yuan et al, 2020;Karki et al, 2020;Solomatova and Caracas, 2021;Karki et al, 2021). Despite the lower density of hydrous melts relative to anhydrous compositions, Mookherjee et al (2008) argued that a buoyantly stable melt at the upper boundary of the transition zone would contain approximately 3 wt% H 2 O.…”

Thermoelasticity of Water in Silicate Melts: Implications for Melt Buoyancy in Earth's Mantle

“…The (Mg,Fe)-silicate and pyrolite melt simulations at 2,000 K from Karki et al (2021) and Solomatova and Caracas (2021) include the lowest temperature simulations of the FPMD data set that we consider. There is thus overlap in temperature between the hottest experimental measurements (2,473 K from Sakamaki et al, 2006) and the FPMD studies of Bajgain et al (2015), Karki et al (2021), and Solomatova and Caracas (2021). For the FPMD simulations, the quoted uncertainties in the volume of the H 2 O component range from 1% to 21%; uncertainties in pressure, associated with the equation of state, typically vary from less than 1%-20%.…”

“…The melts in the FPMD simulations are typically annealed at very high temperatures (1,000's of K above T melt ) to ensure thermodynamic equilibrium, and then are typically cooled to lower temperatures. The (Mg,Fe)-silicate and pyrolite melt simulations at 2,000 K from Karki et al (2021) and Solomatova and Caracas (2021) include the lowest temperature simulations of the FPMD data set that we consider. There is thus overlap in temperature between the hottest experimental measurements (2,473 K from Sakamaki et al, 2006) and the FPMD studies of Bajgain et al (2015), Karki et al (2021), and Solomatova and Caracas (2021).…”