[S-IT17] 地球型惑星内部での液体の特性とその役割

[SIT17-P01] H₂O-induced melting at the uppermost lower mantle

*坂巻 竜也¹ (1.東北大学大学院理学研究科)

キーワード：mantle melting、lower mantle、hydrous magma、low-velocity anomaly、high pressure

Seismological low-velocity anomalies have been observed above and below the mantle transition zone, which is a water reservoir in the interior of the Earth. These anomalies can be caused by mantle melting. In the case of the downflow zone at the uppermost lower mantle, a dehydration decomposition of ringwoodite to bridgmanite and ferropericlase results in H₂O-induced mantle melting. For a better understanding of the melting phenomena at the top of the lower mantle, melting experiments of hydrous peridotite with 2.0 wt.% H₂O have been conducted at 26 GPa and at temperatures from 1673 K to 2023 K. Under the experimental temperature, all recovered samples showed the quenched melt texture. This indicates that the solidus temperature of the hydrous peridotite with 2.0 wt.% H₂O is below 1673 K and normal mantle geotherm. This result strongly supports the H₂O-induced melting at the top of the lower mantle. The composition of partial melt coexisted with bridgmanite is SiO₂-poor and MgO-rich, and is inconsistent with that of dry peridotite system. The water content of the partial melt is about 30-50 wt.%, and the melt is less dense than the surrounding lower mantle. The H₂O-rich melt goes upward and the mantle transition zone can be kept in hydrous condition.