[S-IT14] 地球深部科学

[SIT14-10] ウォズリアイトの熔融温度に及ぼす高酸素分圧と水の影響の
解明とEELSによるFe³⁺/ΣFeの測定

*山口 和貴¹、川添 貴章¹、井上 徹¹、境 毅²、富岡 尚敬³ (1.広島大学、2.愛媛大学・地球深部、3.高知コア研究所)

キーワード：ウォズリアイト、急冷結晶、酸素フィガシティー、熔融温度、マントル遷移層

The melting of the mantle minerals is very important for the material cycle in the Earth's interior. Seismological observations indicate the existence of a low-velocity layer of seismic waves atop 410 km depth (Tauzin et al., 2010). The low-velocity layer is believe to be caused by dehydration melting of the mantle. However, hydrous melt at the topmost lower mantle is less than the transition zone (Fei, 2021). We think that the low-velocity layer is not generated only by the dehydration melting of the mantle. Recent years have seen many attempts to measure the Fe³⁺/ΣFe ratio of melt inclusions from hot-spot volcanoes. These attempts show that oxygen fugacity of mantle source can be higher than previously thought (e.g., Moussallam et al. 2019). If melting temperature of mantle minerals decrease at high oxygen fugacity conditions, the low-velocity layer can be caused by the melt generated at high oxygen fugacity conditions. There has been little research on effect of high oxygen fugacity on melting temperature of mantle minerals. Therefore, we investigated the effects of high oxygen fugacity and water on the melting temperature of wadsleyite. A Kawai-type multi-anvil apparatus was used for the high-temperature and high-pressure experiments at Hiroshima University. The starting materials were a powder and single crystals of olivine from San Carlos, Arizona. The use of the single crystals can decrease the water content in recovered samples by minimizing surface adsorbed water. Temperature and pressure conditions were 1300 to 1900°C and 13.7 to 16.6 GPa, respectively. Re-ReO₂ and Mo-MoO₂ buffers were used to control oxygen fugacity. Au and AuPd were used as a capsule material to enclose the starting material and the buffer materials. After keeping the target temperature for desired duration, the samples were quenched by turning off the heating power supply. Microstructural observations were made with a scanning electron microscope. Phase identification was performed by micro-Raman spectroscopy and micro-focus X-ray diffraction. Chemical compositions were measured by EPMA. The water content of the samples was measured by FTIR spectroscopy. At 14.6 GPa and 1400°C using a powder as the starting material, no quenched melt was observed in both samples buffered with the Re-ReO₂ and Mo-MoO₂ buffers. At 14.6 GPa and 1500°C using a powder as the starting material, quenched melt was observed in the sample with oxygen fugacity controlled by the Re-ReO₂ buffer. In contrast, no quenched melt was observed in the sample with oxygen fugacity controlled by the Mo-MoO₂ buffer. The water content of the samples was 0.4 wt%. Additional experiments were performed between 14.6 to 16.3 GPa and between 1600 to 1900℃ using the single crystals of olivine as the starting material to minimize water in the recovered sample. In this case, the water content of the recovered samples was 0.02 wt%, and no quenched melt was observed in both samples buffered by the Re-ReO₂ and Mo-MoO₂ buffers of all runs. The solidus temperature of wadsleyite under low oxygen fugacity conditions was determined to be about 2300°C (Ohtani et al., 1998). In this study, wadsleyite melted under high oxygen fugacity and 0.4 wt% water content conditions at 1500°C. The results of this study indicate that the melting temperature of wadsleyite was decreased by ~800°C with high oxygen fugacity and 0.4 wt% water. In addition, when the water content is 0.02 wt%, a decrease in the melting temperature of wadsleyite due to oxygen fugacity is less than 400°C. Temperature of the mantle transition zone is estimated to be ~1600℃ (Katsura, 2022). We can conclude that If water content of mantle transition zone is more than 0.4 wt%, partial melting layer is present at upper mantle transition zone of oxidized conditions. We are also developing quantitative measures of the Fe³⁺/ΣFe ratios of the recovered samples by Electron energy loss spectroscopy (EELS) and will report the results in the presentation.