How lowermost mantle viscosity controls the chemical structure of Earth’s deep interior

Abstract Determining the fate of subducted oceanic crust is critical for understanding Earth’s chemical evolution, material cycling through Earth's deep interior, and sources of mantle heterogeneity. However, what happens to subducted slabs over long timescales and how their material is distributed within the mantle remains debated and poorly understood. A key control on this distribution is the bridgmanite to post-perovskite (ppv) phase transition in the lowermost mantle, thought to cause rheological weakening. Using high-resolution computational models, we show that weak ppv can facilitate or prevent the accumulation of basaltic oceanic crust at the base of the mantle, depending on the amount of weakening and the crustal thickness. Moderately weak ppv (~10–100 times weaker) increases basalt accumulation in dense piles, facilitating the segregation of crust from subducted slabs. Conversely, very weak ppv (more than 100 times weaker) promotes more vigorous plumes that entrain more crustal material, decreasing basalt accumulation. This process causes small-scale thermal instabilities leading to pulsating plumes with periodicities of a few million years, comparable to the timescale inferred from V-shaped ridges in Iceland. Our results reconcile the contradicting conclusions of previous studies and provide insights into the accumulation of subducted crust in the lowermost mantle throughout Earth’s history.