[S-IT18] GEOPHYSICAL PROPERTIES AND TRANSPORT PROCESSES IN THE DEEP CRUST AND MANTLE

[SIT18-P04] Diffusion in stishovite and CaCl₂-type silica from first-principles calculations: implications for MORB viscosity in the lower mantle

*Bowen Chen¹, Wenzhong Wang^1,2,3, Zhongqing Wu¹, Xiaoping Wu¹ (1.University of Science and Technology of China, 2.University College London, 3.Carnegie Institution for Science)

Keywords:diffusion, viscosity, MORB, crustal recycling

The segregation of the subducted oceanic crust has been widely considered as the main mechanism to generate the numerous compositional heterogeneities in the lower mantle. Knowledge of the rheological properties of the crustal components is essential for understanding the origin of this separation. Since the lower mantle deforms by diffusion creep, ionic diffusion controls the mineral viscosity. Here, we reported first-principles results for Si and O diffusion of stishovite and CaCl₂-type silica and evaluated the relative viscosity of oceanic crust along different mantle geotherms. Our results show that the Si diffusion in CaCl₂-type silica has instinct negative activation volumes along <111> direction, leading to the Si diffusion enhancement with both pressure and temperature. The rheological weak CaCl₂-type silica substantially reduces the crustal viscosity and facilitates the segregation of subducted oceanic crust with increasing depths. The delaminated crustal fragments would be stirred and stretched sufficiently by the vigorous mantle convection and account for the ubiquitous heterogeneities in the lower mantle. Our results provide new constraints on the crustal rheology for the fine geodynamic simulations in the future to better understand the crustal recycling in the earth`s interior.