Abstract
Several igneous activities occur on the surface of the Earth, including island arcs, mid-ocean ridges and hot spots. Based on geophysical observations, melting phenomena in the interior also occur at the asthenosphere’s top and the upper mantle’s bottom. Additionally, a seismological low-velocity anomaly was observed at the top of the lower mantle that may result from mantle melting due to dehydration decomposition of ringwoodite to bridgmanite and ferropericlase with a downward flow. However, the corresponding high-pressure experimental data are too poor to understand the melting phenomena under the lower mantle condition. Herein, we conducted hydrous peridotite melting experiments at pressures from 23.5 to 26 GPa and at temperatures from 1300 to 1600 °C for demonstrating the melt composition and the gravitational stability of magma at the top of the lower mantle. The melt had a SiO2-poor and MgO-rich composition, which is completely different than that of dry peridotite melting experiments. Compared with the seismological lower mantle, the experimental melt is gravitationally lighter; thus, a similar melt could be observed as seismological low-velocity zone at the lower mantle’s top. The generated magma plays as a filter of down-welling mantle and can contribute to a formation of a silicate perovskitic lower mantle.
Highlights
Hydrous ringwoodite with about 1.5 wt.% H2O1 and phase egg[2,3,4] has been discovered in the diamond inclusions of mantle xenoliths
Geophysical observations support the existence of melt at the base of the upper mantle based on low seismic wave velocity anomalies[9,10] and high electrical conductivity[10]
Hydrous peridotite melting experiments were conducted at 1300 °C–1600 °C at 23.5 and 26 GPa
Summary
Hydrous ringwoodite with about 1.5 wt.% H2O1 and phase egg[2,3,4] has been discovered in the diamond inclusions of mantle xenoliths. We conducted melting experiments with a hydrous peridotite composition with Fe3+ to more accurately reproduce the melting phenomenon that occurs at the boundary between the mantle transition zone and the lower mantle. Hydrous peridotite melting experiments were conducted at 1300 °C–1600 °C at 23.5 and 26 GPa. All recovered samples contained bridgmanite, CaSiO3-perovskite and ferropericlase with quench microcrystals from the melt.
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