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.

Kim, T.Ko, B.Greenberg, E.Prakapenka, V.Shim, S.H.Lee, Y. (2020). Low melting temperature of anhydrous mantle materials at the coremantle boundaryGeophysical Research Letters47, e2020GL089345. abstract

Xray diffraction setup and melting of estimated eutectic composition of the pyrolitic mantle. (a) A schematic
diagram of the experimental setup with MCC. Diffracted Xray beams from the sample (red arrows) at the rotation
center of MCC pass the inner and outer slits of MCC while most of the scattered Xray beams from anvils (blue fan
with gradient) are suppressed by MCC. (b) XRD patterns with (red) and without (black) the MCC setup.
(c) Two representative diffraction patterns at 119 GPa showing the growth of diffuse scattering (gray shaded area in the
above pattern) upon laser heating from 3,241 K. Peak identications: hkl for bridgmanite, pPv: postperovskite, Fp:
ferropericlase, and NaCl: NaCl pressure medium. (d) Twodimensional diffraction images of the patterns presented in (c).
The red colored lines are the gaps between the modules in the detector.