The Speciation and Coordination of a Deep Earth Carbonate-Silicate-Metal Melt

Abstract

Ab initio molecular dynamics calculations on a carbonate-silicate-metal melt were performed to study speciation and coordination changes as a function of pressure and temperature. We examine in detail the bond abundances of specific element pairs and the distribution of coordination environments over conditions spanning Earth's present-day mantle. Average coordination numbers increase continuously from 4 to 8 for Fe and Mg, from 4 to 6 for Si, and from 2 to 4 for C from 1 to 148 GPa (4,000 K). Speciation across all pressure and temperature conditions is complex due to the unusual bonding of carbon. With the increasing pressure, C-C and C-Fe bonding increase significantly, resulting in the formation of carbon polymers, C-Fe clusters, and the loss of carbonate groups. The increased bonding of carbon with elements other than oxygen indicates that carbon begins to replace oxygen as an anion in the melt network. We evaluate our results in the context of diamond formation and of metal-silicate partitioning behavior of carbon. Our work has implications for properties of carbon and metal-bearing silicate melts, such as viscosity, electrical conductivity, and reactivity with surrounding phases.Plain Language Summary Carbonate melts play an important role in the Earth's upper mantle, but their role in the lower mantle is less well understood. Carbonate melts reacting with lower mantle silicates or metals could yield melts with unusual structural and physical properties. We simulate a carbonate-silicate-metal melt under a host of lower mantle conditions to understand the evolution of speciation and coordination in the melt with pressure and temperature. We find that coordination numbers increase continuously for all cations with pressure. We also find that with the increasing pressure, carbon begins to replace oxygen in the melt network, leading to the formation of unusual chemical species, such as carbon-carbon and carbon-iron clusters. Our work highlights that carbonate-silicate-metal melt compositions in the lower mantle could be parent melts for diamonds or for dense iron-carbon liquids that would carry carbon to the core.

The distribution of volatile elements during rocky planet formation

“…Recent first principles/ab initio molecular dynamics and density functional theory (DFT) simulations have been used to gain further insights on how volatile elements are distributed among cores, magmas, and atmospheres. For example (Solomatova and Caracas, 2019;Davis, et al, 2022), found C coordination in silicate melts to be largely pressure dependent, implying that terrestrial magma oceans could have contained oxidized carbon polymers. The latter study also found strong clustering between C and Fe, which increases as a function of pressure, suggesting C could be present in cores.…”