Impact of Phase Change Kinetics on the Mariana Slab Within the Framework of 2-D Mantle Convection

Abstract:

Recent studies of high pressure and high temperature experiments indicate that metastable olivine might persist in a cold core of a slab due to the low rate of reaction of olivine to wadsleyite phase transformation. These experimental results correlate with recent seismological observations that a metastable olivine wedge may survive up to a depth of 630 km in the Mariana slab.

To study the problem of a non-equilibrium phase transformation in detail, we developed a 2D Cartesian numerical code which incorporates self-consistently transition kinetic effects into a thermo-mechanical convection model. The kinetics of the 410-km olivine to wadsleyite and the 660-km ringwoodite to Pv+Mw phase transformations, including effects of water content at the 410-km phase boundary and latent heat release (respectively absorption), are taken into account.

The results show a positive correlation for some of the controlling parameter with respect to the size of the metastable olivine wedge: A thicker and deeper metastable olivine wedge is obtainable by a higher dip angle, a faster subduction velocity, an older slab age, and/or by less water content of the subducting mantle lithosphere. The effects of latent heat release is enhanced with increasing depth; heating of about 100 °C occurs when olivine transforms into wadsleyite at depths deeper than 550 km.

We also attempted to explain the recent seismological observations, by calculating the temperature and phase structures in the Mariana slab. Assuming an age of 150 Myr and a subducting velocity of 10 cm/yr for the Mariana slab, a grain-boundary nucleated reaction for the olivine to wadsleyite transformation and a water content of 700 wt. ppm, the metastable olivine wedge survives to a depth of 630 km, which is in good agreement with the seismological observation and available experimental data. This suggests that the Mariana slab is relatively dry.

Furthermore, assuming that depression of the 660-km discontinuity by ~ 20-30 km within the Mariana slab, which is indicated by seismological observation, is related to the combination of depression caused by negative Clapeyron slope in the cold slab and due to the kinetics of the 660-km phase transformation, we obtain a gentle Clapeyron slope of 0.7 MPa/K for the phase transformation from ringwoodite to Pv+Mw.