Grain boundary mobility of carbon in Earth's mantle: a possible carbon flux from the core.

The importance of carbon in Earth's mantle greatly exceeds its modest abundance of approximately 1,000-4,000 ppm. Carbon is a constituent of key terrestrial volatiles (CO, CO(2), CH(4)), it forms diamonds, and it may also contribute to the bulk electrical properties of the silicate Earth. In contrast to that of the mantle, the carbon content of Earth's metallic core may be quite high ( approximately 5 wt %), raising the possibility that the core has supplied carbon to the mantle over geologic time. The plausibility of this process depends in part upon the mobility of carbon atoms in the solid mantle. Grain boundaries of mantle minerals could represent fast pathways for transport as well as localized sites for enrichment and storage of carbon. Here, we report the results of an experimental study of grain-boundary diffusion of carbon through polycrystalline periclase (MgO) and olivine ([Mg,Fe](2)SiO(4)) that were obtained by determining the extent of solid solution formation between a graphite source and a metal sink (Ni or Fe) separated by the polycrystalline materials. Experimental materials were annealed at 1,373-1,773 K and 1.5-2.5 GPa pressure. Calculated diffusivities, which range up to 10(-11) m(2).s(-1), are fast enough to allow transport over geologically significant length scales ( approximately 10 km) over the age of the Earth. Mobility and enrichment of carbon on grain boundaries may also explain the high electrical conductivity of upper mantle rocks, and could result in the formation of C-H-O volatiles through interactions of core-derived C with recycled H(2)O in subduction zones.