Analysis of micro-fabricated, controlled geometry samples for determination of material transport properties of core materials

Chemical diffusion rates in iron and nickel provide key constraints on the behavior of planetary core materials. Samples were created for use in the laser-heated or externally-heated diamond anvil cell by sputtering 300 nm of Ni onto the surface of a 10-micron thick Fe64Ni36 alloy foil under ultra-high vacuum, after ion-etching to remove surface oxidation. Samples heated under pressure up to 1200 K show that a depth profile through the sample using secondary ionization mass spectrometry (SIMS) can accurately constrain the diffusivity of iron and nickel in these samples. Diffusivities over 10-19 m2/s can be determined with 0.1 log unit accuracy. Initial experiments at 1063K give a diffusivity of 1.2x10-17 m2/s, consistent with Yunker and van Orman (2004). Issues of the pressure medium, initial surface roughness, and knock-on effects will also be discussed. The diffusivity of hcp iron and iron-nickel alloys as determined in the laser-heated diamond anvil cell can be used to infer the solid-state viscosity of the inner-core, which may govern the mechanism to develop inner-core anisotropy.