INTRACRYSTALLINE TRANSFORMATION OF OLIVINE TO WADSLEYITE AND RINGWOODITE UNDER SUBDUCTION ZONE CONDITIONS

Experiments on Mg1.8Fe0.2SiO4 olivine at 18 to 20 gigapascals and 1000 degrees to 1400 degrees C show that transformation to the high-pressu re polymorphs wadsleyite and ringwoodite involves two competing nuclea tion mechanisms when the grain size of the olivine is large: (i) incoh erent nucleation on olivine grain boundaries and (ii) coherent intracr ystalline nucleation of ringwoodite lamellae on shear-induced stacking faults in olivine un the (100) planes, followed by the nucleation of wadsleyite on the ringwoodite. The large amount of Intracrystalline tr ansformation, which occurs under low differential stress under the pre ssure and temperature conditions of the mantle transition zone, sugges ts that shear-induced intracrystalline transformation of olivine to wa dsleyite and ringwoodite is important in subducting slabs.