[S-CG45] 岩石・鉱物・資源

[SCG45-06] Electrical conductivity of siderite and the effect of the spin transition of iron

*増野 いづみ¹、芳野 極¹、三井 隆也²、藤原 孝将²、Garai Máté³、山下 茂¹ (1.岡山大学惑星物質研究所、2.量子科学技術研究開発機構、3.ユニバーシティ・オブ・ザ・サウス)

キーワード：シデライト、下部マントル、電気伝導度、スピン転移

Carbonates can be transported into the Earth’s deep interior through subducting slabs, and thus have been proposed as host minerals for carbon in the Earth’s mantle. Therefore, revealing the behavior of carbonates at extreme conditions is a key to understanding the deep carbon cycle. In this study, we have conducted electrical conductivity measurements of FeCO₃ siderite under high pressure up to 62 GPa using diamond anvil cells in order to understand the nature and effect of iron spin transition and its influence on the geophysical properties of siderite, which is an end-member of major carbonate minerals. The results from Raman and Mössbauer spectroscopic measurements show that the high- to low-spin transition of iron occurs at around 50 GPa. A sharp decrease of the electrical conductivity was also observed at around 50 GP, which is associated with the iron spin transition. Although the stability of FeCO₃ siderite may be limited under high-temperature conditions along with the mantle geotherm, solid solutions in the MgCO₃-FeCO₃ system, Mg_1-xFe_xCO₃, could be stable up to the pressure-temperature condition of the lowermost mantle. The pressure-temperature range of the iron spin transition of Mg_1-xFe_xCO₃ would be narrower than those of the major lower mantle minerals of ferropericlase and bridgmanite, and thus the drop of the electrical conductivity induced by the spin transition could be clearer under the lower mantle conditions. Therefore, the existence of Mg_1-xFe_xCO₃ may affect the observed heterogeneity of electrical conductivity in the mid-lower mantle.