The geological materials in Earth's lowermost mantle control the characteristics and interpretation of seismic ultra-low velocity zones at the base of the core-mantle boundary. Partial melting of the bulk lower mantle is often advocated as the cause, but this does not explain the nonubiquitous character of these regional seismic features. We explored the melting properties of mid-oceanic ridge basalt (MORB), which can reach the lowermost mantle after subduction of oceanic crust. At a pressure representative of the core-mantle boundary (135 gigapascals), the onset of melting occurs at ~3800 kelvin, which is ~350 kelvin below the mantle solidus. The SiO2-rich liquid generated either remains trapped in the MORB material or solidifies after reacting with the surrounding MgO-rich mantle, remixing subducted MORB with the lowermost mantle.

1The phase transition between a face-centered cubic (fcc) and hexagonal close-packed 2 (hcp) structures in Fe-4wt% Si alloy was examined in an internally resistive heated 3 diamond anvil cell (DAC) under high-pressure (P) and -temperature (T) conditions to 4 71 GPa and 2000 K by in-situ synchrotron X-ray diffraction. Complementary 5 laser-heated DAC experiments were performed in Fe-6.5wt% Si. The fcc-hcp phase 6transition boundaries in the Fe-Si alloys are located at higher temperatures than that in 7 pure Fe, indicating that the addition of Si expands the hcp stability field. The dP/dT 8 slope of the boundary of the entrant fcc phase in Fe-4wt% Si is similar to that of pure Fe, 9 but the two-phases region is observed over a temperature range increasing with pressure, 10

No abstract