Mantle‐wide sequestration of carbon in silicates and the structure of magnesite II

Abstract

The participation of the deep mantle in the global carbon cycle and its ability to sequester carbon over billion‐year time scales depends upon the mineralogical host for carbon. Density‐functional theory calculations for MgCO 3 ‐magnesite and structures with tetrahedrally coordinated carbon reveal the stability of magnesite up to ∼80 GPa, with a bulk modulus of 110 (±2) GPa. Magnesite undergoes a structural transition to a pyroxene‐like structure at ∼80–100 GPa, with a density increase of 4.5–7.1%. Combined with thermodynamic models for the MgSiO 3 —MgCO 3 system, the inter‐solubility of MgCO 3 with MgSiO 3 orthoenstatite and perovskite constrains the carbon content in the silicates to an upper bound of 4 and 20 ppm (wt), respectively. The carbon content in lower mantle silicates is estimated to be no more than 1% of the mantle's total carbon budget for degassed regions, such that in even the mantle's most depleted regions, most carbon must be stored in carbonates or diamond.