Low Melting Temperature of Anhydrous Mantle Materials at the Core‐Mantle Boundary

One of the central challenges in accurately estimating the mantle melting temperature is the sensitivity of the probe for detecting a small amount of melt at the solidus. To address this, we used a multichannel collimator to enhance the diffuse X‐ray scattering from a small amount of melt and probed an eutectic pyrolitic composition to increase the amount of melt at the solidus. Our in situ detection of diffuse scattering from the pyrolitic melt determined an anhydrous melting temperature of 3,302 ± 100 K at 119 ± 6 GPa and 3,430 ± 130 K at the core‐mantle boundary (CMB) conditions, as the upper bound temperature. Our CMB temperature is approximately 700 K lower than the previous estimates, implying much faster secular cooling and higher concentrations of S, C, O, and/or H in the region, and nonlinear, advocating the basal magma ocean hypothesis. Plain Language Summary The heat stored in the Earth's deep interior has been the primary fuel for a range of global processes from mantle convection to surface tectonics, but quantitative estimation of the heat remains uncertain. The melting temperature of mantle materials is one of the key parameters to understanding the thermal evolution and present‐day state of the Earth's interior, but it has been poorly constrained, with recent measurement discrepancies as large as 600 K. Here, we report melting temperatures of mantle compositions measured over a wide range of pressures expected for the lower mantle. We investigated a eutectic mantle composition using multichannel collimator filtered X‐ray diffraction in combination with the laser‐heated diamond‐anvil cell. Fitting our melting data over the range of 46–145 GPa led to a solidus temperature of 3,430 ± 130 K at the core‐mantle boundary. This temperature is approximately 700 K lower than the previous estimates, implying much faster secular cooling at the lower mantle than previously believed. Furthermore, our solidus curve constrained for a wide pressure range is strongly nonlinear and thus supports the basal magma ocean hypothesis. Key Points Combined usage of multichannel collimator and laser‐heated diamond‐anvil cell improved detection of silicate melt This led to the lowest solidus temperature of anhydrous pyrolite, 3,430 ± 130 K, at the core‐mantle boundary conditions We advocate Fe‐enriched provinces at the core‐mantle boundary to be originated from the magma ocean