[S-IT27] 地球内部での液体の特性とその役割

[SIT27-P02] Melting phenomenon at the top of the lower mantle

*中嶋 彩乃¹、坂巻 竜也¹、川添 貴章²、鈴木 昭夫¹ (1.東北大学、2.広島大学)

キーワード：脱水溶融、下部マントル、含水マグマ、第二鉄

There are various igneous activities in the Earth. On the surface of the Earth, for example, igneous activities occur in island arcs, mid-ocean ridges, hot spots and petit spots. However, not only on the surface, but also in the interior, melting phenomena are also suggested (e.g., the top of the asthenosphere [e.g., Barazangi & Isacks, 1971 JGR; Schmerr, 2012 Science]; the bottom of the upper mantle [e.g., Bercovici & Karato, 2003 Nature; Song et al., 2004 Nature]). In this study, we focused on the melting of hydrous peridotite at the top of the lower mantle because seismological observation indicates the low velocity anomaly [Schmandt et al., 2014 Science; Liu et al., 2016 GRL; Liu et al., 2018 EPSL]. The low-velocity region is expected to be caused by mantle melting due to dehydration decomposition of ringwoodite to bridgmanite and ferro-periclase with a downward flow. Here, we performed melting experiments of peridotite with 6.98 wt % H₂O at 21-26 GPa and 900-1600 °C. As a starting material, a mixture of several oxide powders was used. In this study, Fe₂O₃ was used instead of FeO because the lower mantle is expected to be Fe³⁺-rich [e.g., Frost et al., 2004 Nature; McCammon, 1997 Nature; Wang et al., 2015 Nature Geosci.]. Recovered samples from 1300-1600 °C showed partial melting texture, and clearly demonstrate that mantle melting can be occurred under the experimental conditions. The composition of melt was SiO₂- and Al₂O₃-poor and CaO-rich. Mg/(Mg+Fe) in atomic ratio of the melt (= 0.90) is similar to that of bridgmanite (= 0.89). This result is completely inconsistent with melting experiment under dry condition [e.g., Ito & Takahashi, 1987 Nature; Trønnes & Frost, 2002 EPSL] but consistent with hydrous melting (Fe²⁺) [Kawamoto, 2004 PEPI]. In addition to decrease in the melting temperature of mantle rock, hydrogen (water) can affect the melt composition such as the partitioning of FeO and MgO between crystal and melt. The density and compressibility of the magma were calculated based on the obtained melt composition. Comparing with seismological model, the melt is lighter than that of the lower mantle. This implies that the melt can be ponded at the top of lower mantle and form the seismological low velocity zone. This work was supported by the JSPS Japanese-German Graduate Externship.