Diffusion of calcium in forsterite and ultra-high resolution of experimental diffusion profiles in minerals using local electrode atom probe tomography

Abstract

We report measurements of experimentally induced diffusion profiles in minerals using local electrode atom probe (LEAP) tomography, and demonstrate that this is a viable technique for use in experimental diffusion studies, with distinct advantages over current state-of-the-art techniques such as secondary ion mass spectrometry and Rutherford backscattering spectroscopy. For this purpose, we have investigated Ca diffusivity in synthetic forsterite, which we measured parallel to [1 0 0] from 750 to 1300 °C, at 1 atm. pressure in air, with silica activity buffered by forsterite-enstatite or forsterite-periclase assemblages. We observe no dependence of Ca diffusion on silica activity, and argue that this may be due the strong preference of Ca for the M2 site, while M-site vacancies in olivine are more favorably located in M1 sites. Taken together, our data yield the following Arrhenius relation for Ca diffusion parallel to [1 0 0] in synthetic forsterite at 1 atm. pressure: logDCa_[100]m2s-1=-8.42±0.36-250±9kJmol-12.303RT. Time series experiments at 750 °C reveal that the shortest profiles measured by secondary ion mass spectrometry depth profiling (using an oxygen primary ion beam) are compromised by ion-beam mixing, while LEAP tomography yields accurate diffusion coefficients from the same samples. Because LEAP tomography does not suffer from the type of analytical artefacts that complicate the measurement of extremely short profiles via secondary ion mass spectrometry depth profiling, and also typically produces an order of magnitude better sensitivity than Rutherford backscattering spectroscopy, this technique offers a valuable new tool for quantifying the diffusivities of slow-diffusing trace elements in crystalline materials.