TRANSFORMATION MECHANISMS OF SAN-CARLOS OLIVINE TO (MGFE)(2)SIO4 BETA-PHASE UNDER SUBDUCTION ZONE CONDITIONS

To understand the mechanisms of the high-pressure transformations of (Mg0.9Fe0.1)(2)SiO4 under transition zone conditions, we have experimentally reacted San Carlos olivine (Fo(90)) at 1000 degrees C and 13 GPa in a multianvil split sphere apparatus. The reaction products and microstructures have been studied in detail by transmission electron microscopy. Under these disequilibrium conditions, San Carlos olivine transforms to beta-phase and spinel, with Mg/Fe ratios which are not consistent with equilibrium compositions. The transformation to both phases occurs by a nucleation and growth mechanism, involving nucleation at olivine grain boundaries. Both phases contain characteristic stacking faults on (010) (beta-phase) and {110} (spinel), and some spinel grains show extreme degrees of disorder. Although there is uncertainty in the exact stability field of the experiment, the results are consistent with formation of metastable spinel, probably within either the alpha + beta or beta stability fields. Metastable spinel progressively transforms back to beta-phase by a shear-like mechanism involving a highly disordered spinelloid phase, which may be a transitional phase in the transformation. Based on these results and available kinetic data metastable formation of spinel in a subducting oceanic slab, within the (alpha + beta or beta stability fields, is possible at temperatures below 1000 degrees C.