Grain‐Boundary Diffusion Creep of Olivine: 2. Solidus Effects and Consequences for the Viscosity of the Oceanic Upper Mantle

Abstract

In Part 1 of this study (Yabe et al., 2020, https://doi.org/10.1029/2020JB019415), we established an olivine grain‐boundary diffusion creep law that incorporates the chemically induced enhancement of creep rates, which becomes significant with increasing temperature. The effect was attributed to grain‐boundary disordering that starts at ~0.92 × solidus (bulk eutectic temperature). In this study, we estimated solidus temperatures of the samples used in the previous diffusion creep experiments. These temperatures were used to compare previously reported diffusion creep rates for olivine with our established diffusion creep law. We found that the law explains a difference of up to 2 orders of magnitude in olivine creep rates at the same temperatures, stresses, grain sizes, and water contents in the previous studies. The geotherm normalized by the mantle solidus was calculated for the upper mantle with water contents ranging from 0 to 300 μg/g, which predicts depths where grain‐boundary diffusion creep is enhanced due to grain‐boundary disordering. Constructed viscosity‐depth profiles reveal a very thin mantle lithosphere beneath mid‐ocean ridges, with development of the lithosphere away from the ridge, leaving a low‐viscosity region below. Given a grain size of 1 mm and depending on the water content, a viscosity of 2–5 × 1019 Pa·s is predicted for the low‐viscosity mantle beneath 50‐million‐year‐old seafloor.

Grain‐Boundary Diffusion Creep of Olivine: 1. Experiments at 1 atm

“…In this study (Part 1), we established a combined flow law that best explains the creep rates of the various olivine aggregates synthesized in our laboratory. In Part 2 (Yabe & Hiraga, 2020), we compare our law with olivine creep data reported by different research groups that used experimental samples prepared by different sources and techniques than in this study. We will show that the data are all well summarized by the extended flow law that incorporates enhancement of grain-boundary processes due to grain-boundary segregation.…”

“…where N is the total number of grains, S is the total area of grains, and S i is the area of the ith grain with d i ), which will be discussed in Part 2 of this paper (Yabe & Hiraga, 2020), are included in the figure. The d area−weighted are systematically larger than d and can be described by d area − weighted ≈ 1.6 × d.…”

Diffusion Creep of Diopside

“… Arrhenius plot of strain rate as a function of 10000/ T of dry diopside (this study), Fe‐olivine (Yabe & Hiraga, 2020; Yabe et al., 2020) (Y), Fe‐free olivine (Nakakoji et al., 2018) (N), enstatite (Bruijn & Skemer, 2014) (B&S), and anorthite (Rybacki & Dresen, 2000) (R&D) in the diffusion creep regime are shown. All the plots are fixed to d = 10 μm and = 30 MPa.…”

“…Although the p value was not extracted from the data of Di (An) aggregate, but essentially the same Q values under Newtonian creep for both Di (Fo) and Di (An) aggregates support the operation of the same deformation mechanism (Figure 5). Since Di (Fo) and Di (An) aggregates were fine‐grained, we initially predicted to identify grain boundary diffusion creep, based on our earlier olivine diffusion creep results (Nakakoji et al., 2018; Yabe & Hiraga, 2020; Yabe et al., 2020). However, none of our data support p = 3 (Figure 4a).…”

“…This method differs from using flow law parameters to calibrate mechanical data obtained from various grain sizes, temperatures, stress, and so on under certain conditions. Previously, we have demonstrated its utility in comparing mechanical data of olivine from various research groups (Yabe & Hiraga, 2020; Yabe et al., 2020).…”