A Post-Perovskite Lens and D'' Heat Flux Beneath the Central Pacific

Abstract

Temperature gradients in a low-shear-velocity province in the lowermost mantle ( D″ region) beneath the central Pacific Ocean were inferred from the observation of a rapid S -wave velocity increase overlying a rapid decrease. These paired seismic discontinuities are attributed to a phase change from perovskite to post-perovskite and then back to perovskite as the temperature increases with depth. Iron enrichment could explain the occurrence of post-perovskite several hundred kilometers above the core-mantle boundary in this warm, chemically distinct province. The double phase-boundary crossing directly constrains the lowermost mantle temperature gradients. Assuming a standard but unconstrained choice of thermal conductivity, the regional core-mantle boundary heat flux (∼85 ± 25 milliwatts per square meter), comparable to the average at Earth's surface, was estimated, along with a lower bound on global core-mantle boundary heat flow in the range of 13 ± 4 terawatts. Mapped velocity-contrast variations indicate that the lens of post-perovskite minerals thins and vanishes over 1000 kilometers laterally toward the margin of the chemical distinct region as a result of a ∼500-kelvin temperature increase.