Diffusion modeling in olivine: influence of 3D crystal shape and zoning style on extracted timescales

Abstract:

Applying diffusion models to chemical transects collected across crystal sections is progressively becoming an essential utensil of the earth scientist’s toolbox to extract timescales of magmatic or metamorphic processes. The extent to which the crystal morphology in 3D space, the style of zoning (i.e. normal, reverse. Core-rim), the anisotropy of diffusion, and fluxes from the different dimensions affect timescales retrieved from 1D diffusion modeling remains largely unstudied. Here, we examine the influence of crystal shape and zoning style on calculated diffusion times via series of numerical models. Three-dimensional numerical olivines with various habits (spherical, rectangular parallelepiped, and polyhedral) were built and left to diffuse at magmatic temperatures for various durations. To cover a range of potentially magma mixing and crystallization scenarios, the simulations tested six forsterite zoning configurations. We show that even when diffusion anisotropy is corrected for in 1D models, timescales can still vary between 0.2‒10 times the true 3D diffusion time due to crystal shape and sectioning effects. Zoning style is found to only have a significant influence in the case of core-rim zoning configurations. Most of the problems associated with crystal shape and sectioning cannot be corrected empirically, but we highlight how they can be averted by using careful grain selection procedures.