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Melting of Peridotite to 140 Gigapascals

Under Pressure In order to understand the behavior of materials in the solid deep Earth, it is important to be able to estimate how a material melts at high pressure. To this end, Fiquet et al. (p. 1516) performed experiments using a laser-heated diamond anvil cell coupled to in situ synchrotron measurements of peridotite rock—a mixture of minerals thought to represent Earth's upper mantle—across a wide pressure range. The results suggest that liquid phases may exist at very high pressure values, such that seismically anomalous zones near the boundary between the core and the mantle may result from isolated pockets of melt. Along similar lines, the base of primitive Earth's mantle may have acquired its trace element signature from partial melting of certain mineral phases higher up in the mantle. High-temperature and -pressure experiments reveal details about how and where the mantle melts. Interrogating physical processes that occur within the lowermost mantle is a key to understanding Earth’s evolution and present-day inner composition. Among such processes, partial melting has been proposed to explain mantle regions with ultralow seismic velocities near the core-mantle boundary, but experimental validation at the appropriate temperature and pressure regimes remains challenging. Using laser-heated diamond anvil cells, we constructed the solidus curve of a natural fertile peridotite between 36 and 140 gigapascals. Melting at core-mantle boundary pressures occurs at 4180 ± 150 kelvin, which is a value that matches estimated mantle geotherms. Molten regions may therefore exist at the base of the present-day mantle. Melting phase relations and element partitioning data also show that these liquids could host many incompatible elements at the base of the mantle.