The Role of Compositionally-Dependent Phase Transitions in Slab Deformation and Trench Motion

It has long been understood that mineral phase transitions have a profound effect on the deformation of subducting lithosphere in the transition zone. This effect is due to the first-order force balance between the buoyancy forces from the phase transition induced density anomalies in the slab and the gradients or jumps in the viscous forces, which are also related to some of these phase transitions. Although olivine comprises only ~55% of mantle material (pyrolite), most models of subduction include only the olivine to wadsleyite (410 km) and ringwoodite to bridgmanite and ferropericlase (660 km) transitions, over-estimating the magnitude of these density anomalies, and ignoring the contributions from the pyroxene/garnet component. In addition, the lithosphere includes crustal and harzburgitic layers, and these compositions undergo different phase transitions due to the different mineralogy. Using 2D simulations of subduction we show that the deformation of the slab is different for models which account for compositionally-dependent phase transitions. These slabs experience less resistance to sinking across the 660 and therefore tend to sink slowly into the lower model. However, the series of density anomalies in the slab also lead to episodic folding and buckling. This oscillatory slab motion translates into an oscillation in subducting plate speed and in trench motion. While these models demonstrate that more complete incorporation of phase transitions is important to accurately model subduction dynamics, this implementation is still incomplete. Future progress will require extending this approach to use tables that self-consistently describe the density for multiple compositions at appropriate P-T conditions (e.g., Sitxrude and Lithgow-Bertelloni, GJI 2011), as well as considering non-equilibrium effects, more directly coupling phase transitions and rheology, and using a compressible formulation of the governing equations.