Defining Earth's elusive thermal budget in the presence of a hidden reservoir

Abstract

A hidden geochemical reservoir has long been a popular concept in the solid Earth sciences, often invoked to explain geochemical paradoxes. Easily overlooked, however, is that such a hidden reservoir invalidates traditional estimates of bulk silicate Earth (BSE) composition, including BSE heat production. Unconstrained BSE heat production threatens our ability to model Earth's thermal evolution. We present a novel method for estimating BSE heat production in the presence of a basal magma ocean—a putative hidden reservoir that has been linked to deep mantle seismic anomalies known as ultralow velocity zones. Our model tracks the thermal evolution of a fully coupled mantle, basal magma ocean, and core. Monte Carlo sampling, together with constraints on upper mantle cooling history and a physical bound on the degree of fractional crystallization, quantifies the major characteristics of Earth's thermal evolution. Chief among these constraints is the previously undefined present-day BSE heat production, which we estimate to be 19±3 TW. Our approach resolves the fundamental difficulty of self-consistent thermal modeling in the presence of a hidden geochemical reservoir, while simultaneously exploring the relevant parameter space. The mantle budget of non-heat-producing elements may require substantial revisions.