Quantifying diffusion-driven metal stable isotope fractionation in olivine through powder source experiments

Chemical and isotopic zoning in olivine has frequently been used to determine timescales of magmatic processes by diffusion modeling. Furthermore, combining the information from chemical- and Fe-Mg- or Li isotopic zoning has been shown to be a powerful tool to elucidate complex crystal growth and diffusion histories [1,2]. However, the extent of diffusion-driven Fe-Mg-Li isotope fractionation in olivine (typically termed as βFe, βMg, and βLi) and the parameters that control them, are not yet well constrained. The latter may bear a potential to better determine the boundary conditions of diffusion in order to (1) better unravel complex diffusion histories and (2) receive more precise diffusion timescales. Here, we have performed a series of diffusion experiments in which a powder source was used to produce Fe-Mg- and Li chemical and isotopic diffusion profiles in crystallographically oriented San Carlos olivine crystal cubes. Chemical and isotopic diffusion profiles were analyzed by electron microprobe (Fe-Mg concentrations), by femtosecond-laser ablation-ICP-MS (minor and trace elements), and by fs-LA-MC-ICP-MS (Fe-Mg and Li isotopic variations). We have been investigating the dependence of βFe and βMg on temperature, crystallographic orientation, and composition, and additionally the dependence of βLi on fO2. Our preliminary results indicate that the Fe-Mg isotope fractionation during diffusion parallel to the crystallographic a- and b-axes is larger than that for diffusion along the c-axis. Furthermore, Li diffusion and the associated Li isotope fractionation appear to be strongly influenced by fO2.

References:

[1] Sio & Dauphas, Geology 2017.

[2] Oeser et al., American Mineralogist 2018.