DIAMOND-ANVIL CELL STUDY OF THE TRANSFORMATION MECHANISM FROM THE OLIVINE TO SPINEL PHASE IN CO2SIO4, NI2SIO4, AND MG2GEO4

Transformation textures and kinetics of the olivine to spinel phase tr ansformation were observed in situ in ungasketed samples using a diamo nd anvil cell (DAC). The low-temperature kinetic limit for reconstruct ive transformation from olivine (alpha) to spinel phase (gamma) observ ed in DAC experiments is approximately 150 degrees C lower than when o bserved under more hydrostatic conditions. The spinel phase, which in many of the samples is distributed in an annular pattern, exhibits rec onstructive textures including grain boundary nucleation, and lack of topotaxy; in some cases it forms lenses similar to those associated wi th transformation-induced mechanical failure. Although spinel phase la mellae, formed by a martensiticlike mechanism, are observed in the spe cimens, the lamellae remain extremely thin (similar to 10 nm) and do n ot produce enough spinel to be optically visible. The observation of r econstructive textures within the annular transformed regions leads us to conclude that high shear stress and plastic strain enables reconst ructive transformation at temperatures where transformation rates woul d otherwise be virtually zero. High transient differential stresses an d rapid deformation accompany deep earthquakes. Therefore knowledge of the kinetics of this transformation under these conditions is importa nt for understanding the connection between phase transformation and d eep earthquakes in subducting lithospheric slabs.