Carbon sequestration during core formation implied by complex carbon polymerization

Abstract

Current estimates of the carbon flux between the surface and mantle are highly variable, and the total amount of carbon stored in closed hidden reservoirs is unknown. Understanding the forms in which carbon existed in the molten early Earth is a critical step towards quantifying the carbon budget of Earth's deep interior. Here we employ first-principles molecular dynamics to study the evolution of carbon species as a function of pressure in a pyrolite melt. We find that with increasing pressure, the abundance of CO2 and CO3 species decreases at the expense of CO4 and complex oxo-carbon polymers (CxOy) displaying multiple C-C bonds. We anticipate that polymerized oxo-carbon species were a significant reservoir for carbon in the terrestrial magma ocean. The presence of Fe-C clusters suggests that upon segregation, Fe-rich metal may partition a significant fraction of carbon from the silicate liquid, leading to carbon transport into the Earth's core.

Carbon Speciation and Solubility in Silicate Melts

“…) ratio is 0.15 for the basaltic melt and 0.03 ), as a function of pressure and temperature. Guillot and Sator (2011) predict significantly more CO 2 relative to CO 3 2in MORB melt than , Solomatova et al (2019), and this study in enstatite melt (en), pyrolite melt (py), and forsterite melt (fo), if an extrapolation to lower temperatures is made. In fact, the high CO 2 /(CO 2 +CO 3 2-) ratios predicted by Guillot and Sator (2011) at 2273 K are only achieved at 5000 K in and Solomatova et al (2019).…”

“…used ab initio molecular dynamics to examine the speciation of carbon in MgSiO 3 melt with 5-16 wt.% CO 2 at 2200-6000 K and 140 GPa, while examined the transport prop erties of MgSiO 3 melt with 16.1 wt.% CO 2 at 2200-5000 K up to 140 GPa. Most recently, Solomatova et al (2019) used ab initio molecular dynamics to examine the specia tion and polymerization of carbon in pyrolite melt with 5-10 wt.% CO 2 at 3000-5000 K up to 140 GPa. The com positions and carbon concentrations of the ab initio molecular dynamics simulations are summarized in Table 16.1.…”

“…) ratio is plotted for the computational studies on carbon-bearing MgSiO 3 and carbon-bearing pyrolite (Solomatova et al, 2019), the concentrations of CO 2 are substantially lower than in Guillot and Sator (2011) at comparable temperatures (Figure 16.2). For example, at 2-3 GPa, ) at 2273 K in MORB.…”

“…The discrepancy bet ween the computational studies may be best explained by the difference in methodology. Guillot and Sator (2011) used classical molecular dynamics on a CO 2 saturated composition (~25 wt.% CO 2 at 10 GPa), but and Solomatova et al (2019) used ab initio molec ular dynamics on melts with 5-16 wt.% CO 2 . Vuilleumier et al (2015) used a combination of classical and ab initio molecular dynamics to study the speciation of carbon in basaltic and kimberlitic melts with ~20 wt.% CO 2 at 2073 K and 12 GPa, finding that most of the carbon exists as CO 3 2-.…”

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.…”