Density and viscosity changes between depleted and primordial mantle at ∼1000 km depth influence plume upwelling behavior

Earth's mantle is more than 2800 km deep, compositionally heterogenous, and potentially stratified. However, understanding of its heterogeneities and stratification is limited. Recently, plume-like anomalies were detected to exhibit different types of anomalous behavior at ∼1000 km depth, where they laterally pond, neck or broadly penetrate to the upper mantle, suggesting that their behavior may be influenced by possible viscosity or density stratification near this depth. However, the specific key reasons are unclear. Here, we use 2D thermal-mechanical-chemical modeling to constrain the key factors causing these large-scale plume-related anomalies. Upward mantle plume penetration at ∼1000 km depth is mainly caused by large source volume and excess temperature, whereas its necking and lateral extension at this depth can only be caused by viscosity and composition-related density stratification, respectively. Considering the various plume behaviors at this proposed boundary, we show that Earth's mantle is likely heterogeneously stratified at ∼1000 km depth, with regional viscosity and/or density changes of different lateral scales and vertical gradients. We speculate that this boundary separates the upper depleted mantle from the primordial mantle domain below. This fundamental boundary has been progressively evolving during stratification of Earth's mantle through melt extraction and mantle stirring throughout Earth history.