Strain-induced olivine-ringwoodite phase transformation at room temperature in rotational diamond anvil cell: Implication for deep-focus earthquake mechanism

Deep-focus earthquakes are assumed to be caused by olivine-spinel phase transformation (PT) during slab subduction. However how does metastable olivine, which does not transform to spinel for over a million years, suddenly transform in seconds to produce large seismic rates? A recent analytical model explains this phenomenon by treating the olivine-spinel PT like plastic strain-induced PT instead of pressure- or stress-induced PTs. The kinetics of strain-induced PT depends on plastic strain instead of time. That is why the transformation rate is proportional to the strain rate. To conceptually confirm this hypothesis, we deform San Carlos olivine at room temperature using a dynamic Rotational Diamond Anvil Cell (dRDAC). We implement rough polished diamond anvils to increase friction and plastic strain. After severe plastic straining, we identify and quantify the olivine-ringwoodite PT at 16-21 GPa, which was never observed at room temperature under compression in DAC. This PT occurs within tens of seconds and could be further accelerated by increasing the rotation rate. Besides, the nano crystallite size of ringwoodite and decreasing PT pressure with increasing strain are the features of strain-induced PT. These findings identify the existence of strain-induced olivine-ringwoodite PT and conceptually validate the proposed deep-focus earthquake mechanism. According to model, in nature, high strain rates are achieved within self-blown-up PT-PT induced plasticity-heating process in shear band.