10:45 AM - 11:00 AM
[SIT06-07] Grain boundary diffusion of siderophile elements in lower mantle phases
Keywords:siderophile element, lower mantle, grain boundary diffusion
The starting material for experiments to determine the grain boundary diffusion was mixture of San Carlos olivine or orthopyroxene with Mg#90 and 3 wt.% Al2O3 and Pt powder. Synthesis experiments of aggregates of postspinel or bridgmanite with tiny amount of Pt particle were conducted in a Kawai-type multianvil press at 25 GPa and 1873K. The synthesized aggregates were sliced into several disks with a few hundreds of micrometers. The disk was sandwiched by diffusion source (meta foil). This mixture was placed in an MgO cylinder. Hot-press and diffusion experiments were also performed using a Kawai-type multianvil press at 25 GPa and 1873 or 2073K. Experimental durations were more than 20 hours. The concentration of W and Re in the Pt sink was quantified using electron microprobe analyzer. The diffused siderophile element was recorded by the Pt particles behaving here as sink, which are implanted in the aggregate. A semi-infinite model for diffusion was used to calculate D from the concentration profile. The effective diffusivities of W and Re, which includes grain boundary effect were around 10-16 or 10-14 m2 s-1 at 1873K for bridgmanite and postspinel. These effective diffusivities are several orders of magnitude slower than the published diffusion data for MgO aggregates. On geological scales and for coarse-grain rocks, siderofile element diffusivity in grain boundaries is not fast enough to induce chemical interaction at the core-mantle boundary. The relative large grain size of mantle rocks will ensure a very limited siderophile element transport by effective diffusion, and a good conservation of siderophile element in the Earth's core. However, to estimate more reliable grain boundary diffusivities at the core-mantle boundary, more knowledge on effect of temperature and pressure on the siderophile element diffusivities is needed.