Temperature dependence of indentation size effect in olivine and its implications to low-temperature plasticity

The strength of the lithosphere strongly influences the plate tectonics and mantle convection. The flow behavior of the lithospheric mantle is largely controlled by low-temperature plasticity of olivine, the dominant mineral in the upper mantle. Many experimental studies have explored the low-temperature rheological behaviors of olivine but result in strengths that are highly variable when extrapolated to geological conditions. Kumamoto et al. (2017) performed nanoindentation experiments using Berkovich and spherical indenters on olivine at room temperature and proposed that the strength of olivine depends on the length scale of deformation, with experiments on smaller volumes of material exhibiting larger yield stress, that is, the indentation size effect (ISE). However, their nanoindentation tests were done at room temperature, while traditional creep tests were often done at elevated temperatures of ⩾400°C, the temperature dependence in the ISE must be considered in synthesizing experimental results from different studies. Here, we conducted nanoindentation experiments on a single crystal of Fe-free olivine, eliminating the influence from grain size, using a diamond Berkovich indenter at temperatures of 28, 100, 200, 400 and 600°C. In all tests, the hardness decreases with increasing contact depth that is characteristic of the ISE. Taking our data into the classic hardness-depth relationship of H = H₀(1+h^*/h_c)^1/2, where H is hardness, H₀ is the so-called “infinite hardness”, corresponding to the hardness at the infinite indentation depth, h_c is contact depth, and h^∗ is the material length scale parameter. We found h^∗decreases with increasing temperature, which can be attributed to an increase of the storage volume of geometrically necessary dislocations during nanoindentation test. The decrease of h^∗means that the ISE weakens with increasing temperature, suggesting that at lithospheric temperatures the size effect is not strong enough to explain the disagreements between different experiments and between experiments and geophysical observations. Other aspects, such as grain size effect (Hall-Petch effect) and strain-weakening mechanisms may contribute significantly and need to be revisited.