Amorphization and Plasticity of Olivine During Low‐Temperature Micropillar Deformation Experiments

Abstract

Experimentally quantifying the viscoplastic rheology of olivine at the high stresses and low temperatures of the shallow lithosphere is challenging due to olivine's propensity to deform by brittle mechanisms at these conditions. In this study, we use microscale uniaxial compression tests to investigate the rheology of an olivine single crystal at room pressure and temperature. Pillars with nominal diameters of 1.25 μm were prepared using a focused ion beam milling technique and were subjected to sustained axial stresses of several gigapascal. The majority of the pillars failed after dwell times ranging from several seconds to a few hours. However, several pillars exhibited clear evidence of plastic deformation without failure after 4–8 hr under load. The corresponding creep strain rates are estimated to be on the order of 10−6 to 10−7 s−1. The uniaxial stresses required to achieve this deformation (4.1–4.4 GPa) are in excellent agreement with complementary data obtained using nanoindentation techniques. Scanning transmission electron microscopy observations indicate that deformation occurred along amorphous shear bands within the deformed pillars. Electron energy loss spectroscopy measurements revealed that the bands are enriched in Fe and depleted in Mg. We propose that inhomogeneities in the cation distribution in olivine concentrate stress and promote the amorphization of the Fe‐rich regions. The time dependence of catastrophic failure events suggests that the amorphous bands must grow to some critical length scale to generate an unstable defect, such as a shear crack.

On the formation mechanisms of intragranular shear bands in olivine by stress-induced amorphization

“…Here, the interest is on unravelling the mechanisms at the origin of the intragranular amorphous lamellae. Some evidence of shear lamellae resulting from stress-induced amorphization were recently reported in olivine in single crystals micropillars compressed at low temperature [32] or in shock recovered samples [9] but no information was provided regarding the deformation mechanism. The detailed observation in our specimen of the tip of a lamella interrupted inside the core of a grain provide novel information to elucidate the local amorphization mechanism.…”

“…The first report of amorphization of olivine has been made by Jeanloz et al [31] in shocked samples recovered from peak pressure of about 56 GPa (see also recent report of amorphization shear bands in shocked olivine from Zhao et al [9] ). The contribution of stress-induced amorphization to plastic deformation of olivine has first been reported in single crystal micropillars deformed in compression [32] at room pressure and temperature. Then Samae et al [30] reported pervasive occurrence of amorphization along grain boundaries in specimens deformed at low temperature, i.e.…”