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

Abstract

We 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.

Diffusion Creep Characteristics of Anorthite Revealed by Uniaxial and Pure Shear Deformation Experiments

“…In previous work from our laboratory, we have conducted 1-atm high-temperature creep tests on fine-grained olivine aggregates at small stress, where dislocation processes would be suppressed (Maruyama & Hiraga, 2017a;Miyazaki et al, 2013;Okamoto & Hiraga, 2022;, and found different mechanical and microstructural characteristics from what we have believed for a long time. One is power-law creep identified at lower stresses and smaller grain sizes than the conditions at which grain-boundary diffusion creep dominates .…”

“…This situation complicates the determination of the actual Al 2 O 3 activity of anorthite during the deformation tests. Okamoto and Hiraga (2022) recently showed that grain-boundary diffusion processes for creep and grain growth in olivine are strongly sensitive to MgO activity. However, the present study could not clarify such activity effect on the creep properties of anorthite.…”

The Effect of Secondary‐Phase Fraction on the Deformation of Olivine + Ferropericlase Aggregates: 2. Mechanical Behavior

“…Specifically, Ol single crystals buffered by Per are stronger than those buffered by Opx in the dislocation creep regime (Bai et al., 1991; Ricoult & Kohlstedt, 1985). Additionally, in the diffusion creep regime, polycrystalline samples of Fo 100 + 10% Per 100 (Okamoto & Hiraga, 2022) are approximately an order of magnitude stronger than samples of Fo 100 + 20% En 100 (Nakakoji et al., 2018). To test the effect of silica activity on the strength of our samples deforming in the dislocation‐accommodated grain‐boundary sliding creep (disGBS) regime, an additional experiment was conducted on a sample of Ol + <10% Per 70 that yielded a strength within error of, if not slightly weaker than, that predicted by the disGBS flow law determined by Hansen et al.…”

A Common Diffusional Mechanism for Creep and Grain Growth in Polymineralic Rocks: Application to Lower Mantle Viscosity Estimates

“…In this section, we discuss feasible grain growth and diffusion creep processes in the lower mantle, drawing insights from our previous work, especially regarding grain growth and creep of forsterite + enstatite (Nakakoji & Hiraga, 2018; Nakakoji et al., 2018) and forsterite + periclase (Okamoto & Hiraga, 2022).…”

“…Interestingly, a similar deviation has been identified for grain growth. Using the classical grain growth model for a polyphase system (Ardell, 1972a), $${\delta D}_{\text{GB}\_\text{gr}}$$ has been found to be systematically larger by a factor of 9 compared to the values predicted based on $${\delta D}_{\text{Si}\_\text{GB}}$$ (Nakakoji & Hiraga, 2018; Okamoto & Hiraga, 2022). Our proposition that grain growth can be viewed as part of the primary phase creep process led us to infer that this deviation is also due to a contribution from GBS during grain growth, which is not considered in the classical grain growth model.…”