[S-IT18] GEOPHYSICAL PROPERTIES AND TRANSPORT PROCESSES IN THE DEEP CRUST AND MANTLE

[SIT18-P05] Acoustic wave velocities of ferrous-bearing MgSiO₃ glass up to 158 GPa with implications for dense silicate melts at the base of the Earth’s mantle

*増野 いづみ^1,2、村上 元彦²、北尾 真司³、三井 隆也⁴、増田 亮⁵、瀬戸 誠³ (1.岡山大学惑星物質研究所、2.スイス連邦工科大学チューリッヒ校、3.京都大学、4.量子科学技術研究開発機構、5.弘前大学)

キーワード：ケイ酸塩ガラス、下部マントル、部分溶融

We conducted in-situ high-pressure acoustic wave velocity measurements of Fe²⁺-bearing MgSiO₃ glass up to 158 GPa by means of Brillouin scattering spectroscopy in a diamond anvil cell to clarify the effect of iron on the elasticity and structural evolution of silicate melts in the lower mantle. The change in trend of the acoustic wave velocity profile, likely induced by the structural transition of Si-O coordination number from 6 to 6+ proposed in previous studies of silicate glasses, was confirmed to be located at around 129 GPa. Given the iron contents of partial melts derived from a pyrolitic or chondritic mantle, the transition pressure where Si-O coordination number likely becomes higher than 6 would be at around 125-128 GPa, which is well within the lowermost mantle pressure regime. Our data show the substitution of 12 mol% Fe in MgSiO₃ glass decreases the V_S by ~5.5 %. This study implies that iron considerably affects the buoyancy relations between melts/crystals and the iron-bearing melts at the lowermost mantle will have the higher coordination number than 6.