Effects of Stress on the Olivine–Spinel Phase Transformation in the Mantle: Griggs-Type Deformation Experiments Using Mg2GeO4 as an Analogue.

The effects of pressure and temperature on the phase transformation of olivine to wadsleyite and then ringwoodite within the mantle is well understood. However, the extent to which stress affects this phase transformation is not clear. Understanding how stress influences the kinetics of the olivine to spinel phase transformation and the mechanism in which it does so at grain scale, will have broader implications for mantle dynamics. Deformation experiments using Mg₂GeO₄ have been used as an approximate analogue for fayalite as it transforms from olivine to ringwoodite at lower pressures and temperatures rather than the conditions found at d410 (Vaughan, 1981). This enables the use of larger samples than possible for the silicate system, and allows for extensive microstructural investigations. This session aims to discuss high pressure deformation experiments on Mg₂GeO₄ (olivine) during the transformation to ringwoodite using a Griggs-type, solid medium, deformation apparatus. These experiments expand on (Vaughan 1984) which linked kinetics of the reaction in a model that matches other stressed reactions in the mantle (Wheeler, 2020). Experiments were conducted at a range of confining pressures 0.8 - 1.2 GPa at a fixed temperature of 900 °C and a strain rate of 10^-6 /s. The four samples were deformed to finite strains ranging from 10 to 45 %. The aim of the conditions chosen was to apply varying amounts of differential stress and therefore differing the σ₁ stress on the sample as a whole. Samples were characterised down to the level of individual interfaces using Electron Backscatter Diffraction (EBSD) to understand the physical mechanism of the reaction and the kinetics that govern it.