Anisotropic copper diffusion in olivine: Implications for Cu preservation in olivine and olivine-hosted melt inclusions

Abstract

The rate of copper diffusion in olivine governs the preservation of copper in olivine and olivine-hosted melt inclusions, but Cu diffusivity in olivine has not been well determined especially regarding its anisotropy. In this study, Cu diffusion in single crystal olivine (Fo91Fa09) cuboid, prepared to comply with the three principle crystallographic axes, was investigated at 1273–1673 K and 1–2.5 GPa in a piston cylinder apparatus, using a diffusion-in approach with Cu or Cu2O as the source. Copper concentration profiles were analyzed by LA-ICP-MS scanning on the recovered samples along multiple directions. We found Cu diffusion in olivine to be appreciably anisotropic, with Cu diffusivity along the c axis being greater than that along the a axis by nearly an order of magnitude (Dc ≈ 3Db ≈ 7.5 Da). The activation energy for Cu diffusion is ∼210 kJ/mol, with little dependence on crystallographic orientation. Cu diffusivity at 2.5 GPa is only slightly lower than that at 1 GPa, yielding a small activation volume of 2.3 cm3/mol. While not being as rapid as the migration of H and interstitial Li, Cu diffusion, especially along the c axis, is significantly faster than the major elements of olivine (Fe-Mg, Si and O). Diffusion modeling indicates that the timescale for re-equilibration of Cu between olivine-hosted melt inclusion and external melt is relatively short (<60 yr at 1000 °C and <3000 yr at 800 °C) compared to the typical lifetime of magma chambers. Cu concentrations in olivine-hosted melt inclusions may therefore have been significantly modified from the pristine values. For Cu profiles preserved in olivine phenocryst, our determined Cu diffusivity can be used to constrain its residence time in magma chamber, and the diffusion anisotropy can provide a cross-check.