First-principles investigation on diffusion in stishovite and CaCl2-type silica: Implication for MORB viscosity in the lower mantle

Abstract

Subducted oceanic crust is thought to play a key role in generating the compositional heterogeneities in the lower mantle. Its rheological properties are essential for understanding the segregation of oceanic crust from the slab and the further interaction with the surrounding mantle. Here, we report first-principles results for Si and O diffusions in stishovite and CaCl2-type silica and evaluate the viscosity of the oceanic crust under lower-mantle conditions. Our results show that the Si diffusion in CaCl2-type silica is enhanced by pressure, with a negative activation volume of ∼−0.2 cm3/mol along the 〈111〉 direction. We find that the Si diffusivity of CaCl2-type silica is two orders of magnitude faster than that of bridgmanite in the deep lower mantle, and therefore, it becomes a rheological weaker phase compared to bridgmanite. In addition, CaCl2-type silica could substantially reduce the crustal viscosity and facilitate the segregation of upper oceanic crust from the lower lithosphere with increasing depths. The delaminated crustal fragments may be sufficiently stirred and stretched by the vigorous mantle convection and account for the ubiquitous small-scale heterogeneities in the lower mantle. Our results provide new constraints on the crustal rheology data for future geodynamic simulations to better understand the crustal recycling in the lower mantle.