Controls on density stratification in the early mantle

Abstract

Previous numerical models of convection in the hot early mantle, incorporating simulated subducted oceanic crust and some viscosity stratification, developed thermal and compositional stratification, with cooler, strongly depleted mantle above hotter, slightly enriched mantle. The robustness of these results is explored through variation of some important parameters. The stratification is minor at present mantle temperatures but stronger at higher mantle temperatures because mantle viscosities are reduced. The results are not much affected by variations in plate velocity, nor by whether internal heat sources are directly associated with the mafic component, nor by 20% of heat entering from below. However, stratification was reduced somewhat at higher temperatures by increasing the number of tracers, indicating that the effect was moderately overestimated in a previous paper. The present results still indicate that oceanic crust would be no more than 6–8 km thick at high mantle temperatures, because of the strong depletion of the upper mantle. When the effect of a phase transformation in the transition zone is included, the stratification is moderately enhanced and crustal thicknesses of 3–5 km result at 1650°C. Oceanic crustal thickness is highly variable in space and time, with some plates having essentially zero crust for significant periods. The new results support the previous conclusions that the stratification may explain the strong depletion of incompatible elements documented for the earliest mantle and that early plate tectonics may have been more viable that hitherto thought.