[M-TT42] 地球化学の最前線

[MTT42-05] Interaction between Cerium and H₂O in Hydrous Rhyolitic Melts

★Invited Papers

*近藤 望¹、河野 義生¹、尾原 幸治²、中田 亮一³、伊奈 稔哲²、Skrzypek Etienne^4,7、山田 明寛⁵、齊藤 哲⁶ (1.愛媛大学地球深部ダイナミクス研究センター、2.高輝度光科学研究センター、3.海洋研究開発機構高知コア研究所、4.京都大学大学院理学研究科、5.滋賀県立大学工学部ガラス工学研究センター、6.愛媛大学大学院理工学研究科、7.グラーツ大学地球科学科)

キーワード：セリウム、流紋岩質メルト、水

Cerium (Ce) in silicate melt is considered to provide important information about the nature of silicate magmas in the Earth’s and planetary interiors, especially about their oxidation state. However, the behavior of Ce in a silicate melt may not be simple, particularly in hydrous silicate melt, where a strong depression of the Ce⁴⁺/Ce³⁺ ratio occurs. Here we investigate the interaction between cerium and H₂O in the structure of hydrous rhyolitic melts. Hydrous rhyolitic glasses quenched from melts at 1 GPa and 1300 °C show a significant decrease of H₂O solubility by the incorporation of Ce. Pair distribution function analyses show no distinct change in the local structures of hydrous rhyolitic glasses with the incorporation of Ce, while we found a distinct change in the intensity of the T-OH (where T is Si or Al) Raman peak. The incorporation of Ce decreases the proportion of T-OH species and increases those of Q⁴ and Q³ species. These results suggest the occurrence of a direct charge transfer reaction between OH^- in the melt and incorporated Ce as Ce⁴⁺_melt + 2(OH)^-_melt → Ce³⁺_melt + H₂O + O^-_melt. This reaction causes a charge transfer from Ce⁴⁺ to Ce³⁺ in the hydrous rhyolitic melt, and it inevitably produces Ce³⁺. X-ray absorption near-edge structure (XANES) measurements show almost completely trivalent Ce (Ce³⁺) in the Ce-incorporated hydrous rhyolitic glasses, which supports the production of Ce³⁺ through the charge transfer reaction between Ce and OH^-. The decrease of Ce⁴⁺/Ce³⁺ ratio reported in hydrous silicate melts would be due to the charge transfer reaction between Ce and OH^-, which implies that the Ce⁴⁺/Ce³⁺ ratio in hydrous silicate melts may bear a complex relationship to the oxidation state of magmas.