THE FATE OF OLIVINE IN SUBDUCTING SLABS - A RECONNAISSANCE STUDY

To explore the transformation behavior of metastable olivine at large overpressures equivalent to 500-670 km depth, a reconnaissance study u sing Mg2GeO4 olivine at pressures up to 16 GPa was conducted. Optical microscopy and transmission electron microscopy of the specimens were used to observe the behavior of both the reconstructive and martensiti c-like transformation mechanisms. The experiments revealed that the ki netics of the thermally activated reconstructive transformation are in sensitive to pressure; comparison with previous growth rate measuremen ts made at 1-2 GPa lead to an estimate of the activation volume for gr owth of V = 0 +/- 2 cm(3)/mol. In contrast, the martensitic-like mech anism becomes more important at high pressure, producing optically vis ible features in Mg2GeO4 olivine by 9 GPa. Similar conditions would be experienced by natural olivine that has metastably persisted in cold subducting slabs to the bottom of the transition zone, these observati ons suggest that the conversion of natural olivine to its spinel struc ture by the martensitic-like mechanism could become significant in tha t environment. This conversion would provide a uniform final cut-off d epth for earthquakes caused by transformational faulting independent o f slab thermal structure.