HR: 13:55h
AN: MR33A-02    [Abstracts]
TI: Grain Boundary Diffusion of Carbon in Mantle Minerals
AU: * Hayden, L A
EM: haydel@rpi.edu
AF: Rensselaer Polytechnic Institute, 110 8th St JRSC 1W19, Troy, NY 12180, United States
AU: Watson, E B
EM: watsoe@rpi.edu
AF: Rensselaer Polytechnic Institute, 110 8th St JRSC 1W19, Troy, NY 12180, United States
AB: The total amount of carbon in the Earth's near surface reservoirs is considered negligible compared to the amount believed to be present in the mantle. However, carbon is incompatible in most major mantle minerals and the mode of carbon storage is largely unknown. Carbon is also a candidate for being present as one of the light elements in the outer core. In light of recent findings that siderophile elements are mobile in grain boundaries of MgO, we have investigated whether grain boundaries of periclase (MgO) and olivine (Fo₉₀) represent potential fast pathways for diffusion (and storage) of carbon.
The experimental strategy utilized a carbon 'source' (high-purity graphite wafers) and a metal 'sink' (either Ni or Fe wire or foil) separated by a wafer of polycrystalline periclase or olivine. Experiments were annealed at 1573 and 1773K and 1.5 GPa for 5-10 hr in a modified 19-mm piston-cylinder assembly.
Results indicate that carbon is highly mobile in grain boundaries of both periclase and olivine. Estimated diffusivities range between 10^-11 and 10^-8 m²/s. There appears to be a correlation between fO₂ and carbon diffusivity, as fluxes in experiments featuring a Fe sink were significantly lower than those featuring a Ni sink, where fO₂ was approximately 5 log units higher. Preliminary data also suggests carbon is mobile in grain boundaries of β-quartz.
These results suggest that an outward flux of carbon from the outer core and an upward flux in the mantle is a possibility, and that grain boundary diffusivities of carbon are fast enough to result in transport distances as great as ~100s of km over geologically relevant time scales.
DE: 3630 Experimental mineralogy and petrology
DE: 3904 Defects
DE: 3947 Surfaces and interfaces
DE: 3999 General or miscellaneous
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting