A Common Diffusional Mechanism for Creep and Grain Growth in Polymineralic Rocks: Experiments

Abstract

AbstractWe conducted uniaxial compression and grain growth experiments on fine‐grained forsterite (Mg2SiO4) + 10 vol% periclase (MgO) aggregates. This aggregate is a unique polymineralic system in which all of the constituent elements in the secondary phase constitute the primary mineral phase, except for Si, which is the slowest diffusing species in the primary phase. Grain growth in polymineralic aggregates, where the presence of a secondary phase results in grain boundary pinning of the primary phase, proceeds via Ostwald ripening of the secondary phase. Grain growth and diffusion creep rates were analyzed, and both rates were found to be limited by the same grain‐boundary diffusivity. For periclase ripening, forsterite grains surrounding the periclase grains must be deformed, such that diffusion of Si as well as Mg and O is necessary for periclase ripening, similarly to diffusion creep of the aggregates. Most rocks are polymineralic and usually contain a silicate mineral as their main phase, in which Si is the slowest diffusing species. This leads to the prediction that a single diffusivity, which is Si diffusivity in most cases, determines the rates of grain growth and diffusion creep in much of the Earth's interior. The grain size, which is the result of grain growth, and the time it takes to achieve that size, indicate the diffusivity. Viscosity during diffusion creep is determined by these grain size and diffusivity values. We view these findings as a unique opportunity to estimate the viscosity from the Earth's crust to the lower mantle.