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.