Impact of phase change kinetics on the Mariana slab within the framework of 2-D mantle convection

Abstract

Recent high-pressure and high-temperature experiments indicate that metastable olivine might persist in the cold core of a slab due to the low reaction rate of the olivine–wadsleyite phase transformation. Recent seismological observations detected a metastable olivine wedge that survives to a depth of 630km in the Mariana slab. To consider the problem of non-equilibrium phase transformation, we developed a two-dimensional (2-D) Cartesian numerical code that incorporates the effects of kinetics into a thermal convection model. We consider the kinetics of the 410-km olivine–wadsleyite and the 660-km ringwoodite–Pv+Mw phase transformations, including the effects of water content at the 410-km phase boundary. The latent heat release of the 410-km non-equilibrium phase transformations inside the slab is also considered. The results show positive correlations between some of the controlling parameters and the length of the metastable olivine wedge: the faster the subducting velocity, and the lower the water content, the deeper is the metastable olivine wedge. With increasing depth of phase transformation, the effect of latent heat release is enhanced: heating of, at most, 100°C occurs if olivine transforms into wadsleyite at a depth of approximately 570km in our model setting. Temperature increase due to the latent heat released stimulates further phase transformation, resulting in further temperature increase, acting as a positive feedback effect. We also attempt to explain the seismological observations by calculating the temperature and phase structures in the Mariana slab. If we assume that the age of the Mariana slab is 150Myr, the subduction velocity is 9.5cm/yr, phase transformation occurs from the grain boundary of the parental phase, and the water content is 250wt.ppm for a grain size of 1mm, 300wt.ppm for one of 5mm, and 100wt.ppm for intracrystalline transformation, then the metastable olivine wedge survives to a depth of 630km, which is in good agreement with the seismological observations. This suggests that the deeper portion of the Mariana slab is relatively dry. Assuming that the depression of the 660-km discontinuity by ∼20–30km within the Mariana slab, as is indicated by seismological observation, is explained by a combination of the depression caused by a negative Clapeyron slope in the cold slab and that due to the kinetics of the 660-km phase transformation, we obtain a gentle Clapeyron slope of −0.9MPa/K for the phase transformation from ringwoodite to Pv+Mw.