Constraining mantle viscosity structure for a thermochemical mantle using the geoid observation

Long‐wavelength geoid anomalies provide important constraints on mantle dynamics and viscosity structure. Previous studies have successfully reproduced the observed geoid using seismically inferred buoyancy in whole‐mantle convection models. However, it has been suggested that large low shear velocity provinces (LLSVPs) underneath Pacific and Africa in the lower mantle are chemically distinct and are likely denser than the ambient mantle. We formulate instantaneous flow models based on seismic tomographic models to compute the geoid and constrain mantle viscosity by assuming both thermochemical and whole‐mantle convection. Geoid modeling for the thermochemical model is performed by considering the compensation effect of dense thermochemical piles and removing buoyancy structure of the compensation layer in the lower mantle. Thermochemical models well reproduce the observed geoid, thus reconciling the geoid with the interpretation of LLSVPs as dense thermochemical piles. The viscosity structure inverted for thermochemical models is nearly identical to that of whole‐mantle models. In the preferred model, the lower mantle viscosity is ∼10 times higher than the upper mantle viscosity that is ∼10 times higher than the transition zone viscosity. The weak transition zone is consistent with the proposed high water content there. The geoid in thermochemical mantle models is sensitive to seismic structure at midmantle depths, suggesting a need to improve seismic imaging resolution there. The geoid modeling constrains the vertical extent of dense and stable chemical piles to be within ∼500 km above CMB. Our results have implications for mineral physics, seismic tomographic studies, and mantle convection modeling.