Experimental constraints on the depth of olivine metastability in subducting lithosphere

The hypothesis that metastable olivine persists in some subducting slabs into the transition zone has wide implications for mantle dynamics and rheology. In order to evaluate this possibility we derive new thermo-kinetic subduction zone models to predict the extent of olivine metastability within the stability fields of its high-pressure polymorphs, wadsleyite and ringwoodite. Our updated models improve on previous work by incorporating experimental kinetic data on realistic mantle compositions ((Mg, Fe)₂SiO₄) rather than analogue systems. Furthermore, latent heat due to the transformation is fed back into both the kinetics and the thermal model. We also consider the effects of transformation stress on growth kinetics and the possibility of an intracrystalline transformation mechanism, previously thought to be important only at high shear stresses. Our models predict significantly smaller wedges of metastable olivine than previous work. In the case of Tonga, for example, where high values of lithospheric age (100-140 million years) and convergence rate (˜ 14 cm per year) are most favorable for metastability, models considering only grain boundary nucleation and interface-controlled growth predict olivine metastability to ∼600 km depth, in contrast to ∼660 km predicted previously by Kirby et al. [Rev. Geophys. 34 (1996) 261]. When intracrystalline transformation is considered, the depth of metastability is further reduced by as much as 100 km, due to the large increase in the density of nucleation sites. Inhibition of growth by transformation stress can increase the depth interval over which the transformation takes place, but is unlikely to be a dominant factor, especially if the intracrystalline mechanism operates. These results indicate that the existence of metastable olivine at depths corresponding to those of the deepest earthquakes (˜680 km) requires subduction of old lithosphere (>100 million years) and a high vertical subduction velocity (≳ 15 cm per year). Such conditions currently may only be achieved in the very northern part of the Tonga subduction zone, yet earthquakes occur at depths down to 680 km in other subduction zones in which lithospheric age and/or subduction velocity are relatively low (e.g. Indonesia and the Marianas). Therefore, mechanisms other than transformational faulting in metastable olivine must operate to cause the deepest earthquakes.