Effects of heat-producing elements on the long-term evolution of thermochemical piles at the base of the mantle

Geochemical observations of ocean island and mid-ocean ridge basalts suggest heat producing element (HPE: U, Th, K) abundances vary within the mantle. Based on bulk silicate Earth (BSE) models, mass balance may require a more enriched, possibly deeper source for ocean island basalts compared to mid-ocean ridge basalts. The enriched reservoir may occupy a significant portion of the mantle, suggesting the presence of Stable HPE-Enriched Provinces (SHEEP). It is conceivable that the enriched reservoir is related to some observed or hypothetical deep mantle structures (e.g. LLSVPs, ULVZs, or viscous blobs). We test the specific hypothesis that SHEEP correspond to the present-day seismically imaged LLSVPs. Heat production within the LLSVPs poses a dynamical challenge when considering the thermal evolution of both the LLSVPs and the mantle, including decay of heat production over geologic time.

As part of the 2018 CIDER initiative we explore the stability of HPE enriched thermochemical piles. One of the contributing factors to the stability of the thermochemical piles is the ratio of compositional to thermal buoyancy. The distribution of HPE affects the cooling history of both the ambient mantle and thermochemical piles, affecting their buoyancy over time. We perform a comprehensive suite of 2D simulations of mantle convection to quantify the effects of HPE-enrichment. We address the stability and morphology of the thermochemical piles over time. Model results are evaluated against geochemical constraints and constraints on the present-day morphology, seismic wavespeed, and density of the LLSVPs.