Ancient Stratified Thermochemical Piles Due To High Intrinsic Viscosity

Abstract The two Large Low Velocity Provinces (LLVPs) in the lowermost Earth mantle are thought to affect large‐scale heat and material transport, governing mantle evolution. LLVPs have been interpreted as thermochemical piles of recycled oceanic crust (ROC) and/or other dense rock types. However, the role of ROC intrinsic viscosity in pile formation and related effects on mantle evolution remain poorly understood. Using mantle convection models, we show that, while ROC intrinsic density controls pile formation, intrinsic viscosity determines whether piles are internally convecting or stratified. Only high‐viscosity, stratified piles can preserve material over several billions of years. Pile stratification is therefore required to reconcile geochemical evidence for the survival of ancient reservoirs. Compositionally layered piles are also consistent with geophysical observations that point to vertical gradients in LLVP properties. As mineral physics constraints point to low‐viscosity ROC, our results suggest that LLVPs may be partly formed by early basal‐magma‐ocean cumulates.