Compositional controls on subducted slab dynamics in the early Earth

Abstract:

Most modeling studies of subduction under early Earth conditions have focused on the effect of higher mantle potential temperatures reducing mantle viscosity and affecting slab behavior. Very few, however, have adequately considered the fact that higher mantle temperatures should also result in thicker, more mafic oceanic crust entering subduction systems. Here we investigate this "compositional effect" by means of petrologic and geodynamic modeling.

First, we construct the density profiles of sinking slabs under modern and early Earth conditions. To do so, we input a large set (>1200) of real mafic crust and mantle compositions into the thermodynamic modeling program Perple_X to estimate the density of predicted mineral assemblages stable in a slab within the upper 1000 km of the mantle. Upon combining these calculations with an analytical model for the thermal structure of subducted slabs, we find that unlike modern MORB compositions that are less dense than ambient mantle within the transition zone, predicted early earth MORB compositions are denser than ambient mantle at all depths.

Second, we investigate the geodynamical consequences of this effect with a finite-difference/particle-in-cell model of a subducted slab system. We find that, in addition to the previously identified influence of mantle temperature on viscosity, the key control on subducted slab behavior is the viscosity increase imposed at the base of the transition zone. However, the negative buoyancy resulting from an early Earth-like thick mafic crust may also play an important role under certain conditions. This compositional effect, previously underappreciated, results in slabs that more readily penetrate the transition zone, thereby promoting single-layered convection and effective mantle mixing.