KINETICS OF THE OLIVINE-SPINEL TRANSFORMATION IN SUBDUCTING LITHOSPHERE - EXPERIMENTAL CONSTRAINTS AND IMPLICATIONS FOR DEEP SLAB PROCESSES

The persistence of metastable olivine to depths greater than 400 km in subducting slabs has implications for the generation of deep-focus earthquakes, the magnitude of buoyancy forces driving plate motion, and the state of stress in the slab. The depth to which metastable olivine (alpha) can survive in a subduction zone and the depth interval over which transformation to beta- or gamma-(Mg,Fe)(2)SiO4 occurs have been evaluated from experimental kinetic data for the Mg2GeO4 and Ni2SiO4 alpha-gamma transformations and the Mg2SiO4 alpha-beta transformation. The data were extrapolated using a kinetic model for grain-boundary nucleation and interface-controlled growth under non-isothermal and non-isobaric conditions. The results predict that metastable Mg1.8Fe0.2SiO4 olivine survives to depths greater than 500 km in the cold interior of rapidly subducting slabs of old lithosphere. The onset of transformation to gamma-(Mg,Fe)(2)SiO4 (spinel) depends only on growth kinetics and coincides with the 550(+/- 50)degrees C isotherm. Including the effects of latent heat production causes the transformation to occur by a runaway process over a very narrow depth interval. At the onset of transformation, high nucleation rates and low growth rates are consistent with the formation of very fine-grained reaction products which are required for the transformational faulting mechanism of deep-focus earthquakes. When olivine crosses the equilibrium boundaries at higher temperatures (e.g. higher than 700 degrees C at 400 km depth), transformation to beta or gamma occurs much closer to equilibrium, at a depth controlled by nucleation kinetics. In this case, the effect of latent heat production on the transformation kinetics is small and microstructural evolution is unlikely to result in transformational faulting. Below the depth of olivine breakdown, cold slabs are likely to have a complex theological structure owing to temperature-dependent microstructural evolution during the phase transformation.