A process for low-temperature olivine-spinel transition under quasi-hydrostatic stress

[1] The behavior of fayalite-the Fe2SiO4 olivine end-member (Fa(100))-has been monitored during high-pressure and low-temperature experiments using in-situ x-ray synchrotron radiation, in order to investigate the effect of stress on the olivine (alpha-phase)-spinel (gamma-phase, or ringwoodite) transition. The run products were investigated by optical and transmission electron microscopy (TEM). A low-temperature mechanism for the alpha-gamma transformation is documented. This shear induced mechanism is operative in hydrostatically compressed samples at temperature as low as 450degreesC, and is driven by the shear stress generated by the volume change of the transformation itself. Projection to mantle compositions suggests that metastable subduction of olivine will not occur in the deep transition zone.