GRAIN-BOUNDARY DIFFUSION WITH GLOBAL SIGNIFICANCE: METAL AND CARBON FLUXES FROM THE CORE?

Historically, diffusion has been considered to be of little or no importance to solid-Earth geochemical transport at scales exceeding a few meters. This view follows from measured diffusivities in crystals and melts and N.L. Bowen's observation that hot geologic systems will cool conductively before diffusion can be effective. Recent measurements of grain-boundary diffusion in mantle analog materials are changing perceptions about the potential importance of diffusion in the deep Earth, which has remained hot since Earth formation. Diffusivities as high as 10^-8-10^-7 m²/s have been determined, which implies diffusive transport distances as great as ~100 km in the age of the Earth. Our studies focus on elements that are concentrated in the core and thus potentially capable of 'contaminating' the mantle by outward diffusion. We have examined highly siderophile elements (HSE) Os, Ir, Re, Pt, Au, Ru, W, Rh, Cu, Co and Mo, as well as elemental C. These elements are incompatible in mantle oxides and silicates, so studies of their mobility in the grain boundaries of mantle phases require a new experimental technique. Our approach is to embed metal particles in the rock of interest that serve as 'sources' and 'sinks' for individual elements. The 'long-distance' alloying of initially pure particles can be used to compute grain-boundary diffusive fluxes. Experiments conducted with polycrystalline MgO at 1873K and 2.5 GPa reveal grain-boundary diffusivities ranging from 10^-11 to 10^-7 m²/s, with Au and W the fastest diffusers. The ranking of grain-boundary diffusivities in Fe-bearing harzburgite is similar to that in MgO, but the values are 10-100X lower. Diffusion of C in MgO is fastest of all elements examined to date, indicating that an outward flux of C from the outer core, as well as an overall upward flux in the mantle, is a realistic possibility. The measured diffusivities provide a plausible mechanism for core-to-mantle chemical communication: the implied diffusive length scales in 4 billion years reach into lower mantle that could be entrained in convection or plume flow of the mantle in general. A carbon flux toward Earth's surface could alter the electrical properties of upper-mantle rocks. It could also produce methane or other volatiles through interaction with H₂O recycled from the hydrosphere.