[S-MP38] 鉱物の物理化学

[SMP38-P04] Melting experiments on the MgSiO₃-SiO₂ system to deep lower mantle pressures

*安西 みゆき¹、小澤 佳祐¹、廣瀬 敬^1,2、新名 良介¹、舘野 繁彦² (1.東京大学地球惑星科学専攻、2.東京工業大学地球生命研究所)

キーワード：高圧、融解、下部マントル

MgSiO₃–SiO₂ system is important to understand SiO₂-rich chondritic mantle materials, but its melting phase relations have been determined experimentally only up to 1 GPa. Here we conducted melting experiments in a pressure range from 41 to 139 GPa using a laser-heated diamond-anvil cell (DAC), in order to determine the change in a eutectic melt composition. A cross section at the hot spot of a heated sample was prepared with a focused ion beam (FIB), and its textural and compositional characterizations were made with a SEM/EDS. Quenched molten samples always exhibited a concentric texture, with quenched melt at the center surrounded by liquidus phase(s) of MgSiO₃ and/or SiO₂. Our data show that eutectic composition changes with increasing pressure from SiO₂/(MgO+SiO₂) = 0.55 (molar ratio) at 1 GPa (Hudon et al., 2005 J. Petrol.) to ~0.60 at 41 GPa and further to ~0.65 at 135 GPa. Combining with the results on the MgO–MgSiO₃ system (Ozawa et al., this meeting), we discuss a large-scale differentiation in chondritic mantles starting from a wide range of MgO/SiO₂ ratios.