Diffusion in Stishovite and CaCl2-type Silica from First-Principles Calculations: Implications for MORB Viscosity in the Lower Mantle

Abstract:

Si and O diffusion in both stishovite and CaCl₂-type silica are simulated using first-principles calculations. Diffusion in both phases show weak anisotropies. Si diffuses along a,b-axis directly while migrating through 2 equivalent <111> jumps for c-axis. The migration enthalpy of Si in sitshovite for <111> directions is slightly lower than that of bridgmanite. After the phase transition, Si diffusion in CaCl₂-type silica is enhanced with both temperature and pressure because of the negative activation volumes. Our results show O migrate along the SiO₆ octahedra edges and diffusion of O is 2~3 orders of magnitute faster than that of Si. The effective diffusion of CaCl₂-type silica is assessed to be at least one order of magnitude faster than that of bridgmanite. The rheological weaker CaCl₂-type silica significantly reduce the MORB viscosity and facilitate the segregation of subducted oceanic crust with increasing depth during the slab heating.The crustal fragments will sink into the core-mantle boundry or be carried by mantle wind. And this can explain the various-scale of heterogeneities in the lower mantle.