Kinetics of intracrystalline olivine-ringwoodite transformation

We studied the microstructural evolution and kinetics of intracrystalline olivine-ringwoodite transformation in order to evaluate the importance of this mechanism in subducting lithosphere. Intracrystalline transformation occurs within single crystals of San Carlos olivine (Fo(90)) at T greater than or equal to 900 degrees C and P greater than or equal to 18 GPa and involves four stages: (1) Formation of (100)(alpha) stacking faults in olivine, (2) coherent nucleation of thin ringwoodite platelets on these stacking faults, (3) semi-coherent growth of the platelets and (4) incoherent nucleation of ringwoodite and/or wadsleyite grains at the platelet interfaces. Because rates of incoherent growth are about two orders of magnitude faster than rates of semi-coherent growth, the final stage is most important for bulk transformation. However, the ringwoodite platelets act as essential nucleation sites for intracrystalline incoherent transformation. The growth of ringwoodite platelets proceeds by the nucleation and migration of interface ledges (1.0-1.5 nm high) and is thermally activated with an activation energy of similar to- 270 kJ mol(-1). This is similar to the activation energy for incoherent growth, suggesting that both processes are controlled by Si-O bond breaking. The nucleation rate of platelets, as determined from the densities of platelets observed in transmission electron microscopy (TEM) sections, is also strongly temperature-dependent and has a similar activation energy. These results suggest that intracrystalline transformation is likely to be important in subducting lithosphere in addition to the grain boundary nucleation mechanism. The intracrystalline mechanism will reduce the depth interval over which the transformation occurs and may enhance rheological weakening by reducing the grain size of the reaction products. (C) 2000 Elsevier Science B.V. All rights reserved.