[S-IT17] 地球型惑星内部での液体の特性とその役割

[SIT17-P02] 高圧下のマグマ中に存在するFe₂O₃の部分モル体積

坂 凛太郎¹、*鈴木 昭夫¹、坂巻 竜也¹ (1.東北大学大学院理学研究科地学専攻)

キーワード：マグマ、Fe2O3、密度、部分モル体積、マントル

Density of magma is one of the most important properties to understand the magma behavior in the interior of the Earth. Considering the composition of Earth’s magma, iron is the heaviest component and has a strong influence on the melt density. Therefore, we need to investigate the behavior of iron and iron oxides in magmas under high pressure in order to discuss the magma density in the Earth’s mantle. Since the high concentration of Fe₂O₃ in the lower mantle has been proposed (e.g., McCammon, 2005), the generation of Fe₂O₃-bearing magma at the top of lower mantle has been implied (Nakajima et al., 2019). Although the partial molar volume of Fe₂O₃ in magma is essential to calculate the density of magma in the deep mantle, there is no data of the partial molar volume of Fe₂O₃ at high pressure and temperature. Here, we obtained the partial molar volume of Fe₂O₃ in silicate melts under high pressure based on high-pressure density measurement of the melts. Several Fe³⁺-rich starting material were prepared. For high-pressure generation, we used a Kawai-type multi-anvil apparatus installed at Tohoku University. We succeeded in measuring the density of Fe₂O₃-bearing magma based on sink-float method (Agee and Walker, 1988; Suzuki et al., 1995). Combining the obtained melt densities and partial molar volumes of other oxides calculated from Wakabayashi and Funamori (2013), the partial molar volumes of Fe₂O₃ at 8, 10 and 12 GPa were determined. Assuming that the thermal expansibility of Fe₂O₃ is constant regardless of pressure, the compression curve of Fe₂O₃ in magmas was derived. Although the compressibility of Fe₂O₃ is larger than FeO, it was not observed that the volume crossover between Fe₂O₃ and “2FeO+1/2O₂”. It was indicated that the Fe₂O₃-bearing magma is about 0.1g/cm³ lighter than the FeO-bearing magma at the top of the lower mantle.