[S-IT15] Mass and energy transport properties and processes in the crust and the mantle

[SIT15-02] Experimental investigation of tungsten speciation in hydrothermal fluids under subduction-zone conditions

*高橋 菜緒子¹、中村 美千彦²、鍵 裕之¹ (1.東京大学大学院理学系研究科、2.東北大学大学院理学研究科)

キーワード：タングステン、流体、ダイヤモンドアンビルセル、ラマン分光法

Tungsten(W) is recognized as a critical raw material essential for increasing economic demand and applications in green technology. W is an incompatible and fluid mobile element in the Earth’s crust and mantle, and its stable isotopes serve as tracers to decipher geological processes. Despite its importance, the mechanisms controlling W transport in the Earth’s crust and upper mantle remain uncertain, requiring a fundamental understanding of W speciation in hydrothermal solutions. Early solubility experiments have suggested that W(VI) primarily dissolves as simple monomers [WO₄]^2- and [HWO₄]^- and their alkali ion pairs[1]. This view of W speciation contrasts with the prevailing view that W(VI) forms large polynuclear metal-oxygen anions under ambient conditions. Recent in-situ Raman spectroscopic analyses using a fused-silica capillary tube have suggested the persistence of polyanions in acidic to neutral solutions up to 400 °C and 60 MPa [2]. However, the speciation of W(VI) at deep crustal pressures and temperatures has not yet been explored. In this study, we performed in situ Raman spectroscopic measurements of W-dissolved hydrothermal solutions up to 750 °C and 1.2 GPa using an externally-heated hydrothermal diamond anvil cell. In the WO₃–H₂O system, the Raman spectra of tungsten oxide-saturated fluids showed no detectable W=O stretching band from monomeric tungstate species [WO₄]^2- appearing at ~930 cm^-1. Instead, the Raman bands centered at ~950 cm^-1 and ~975 cm^-1, which are typically assigned to tungsten polyanions, were respectively detected above 500 °C and 600 °C. These findings indicate that polymeric tungsten species are dominant in high-pressure and high-temperature fluids, challenging existing thermodynamic modeling of electrolyte fluids. In the Na₂WO₄–H₂O±HCl systems at constant concentration, the initial pH of solutions primarily controls tungsten speciation at elevated pressures and temperatures, which is in broad agreement with the previous studies. In addition, we observed that polymeric tungsten species that appeared at ~950 cm^-1 became more pronounced in acidic to alkaline solutions with increasing pressures and temperatures. Consequently, our results suggest that tungsten polyclusters govern tungsten hydrothermal transport in the Earth’s deep crust and upper mantle and may be potential precursors in mineralizing fluids.

References: [1] Wood (1992) Geochim. Cosmochim. Acta, 56, 1827–1836. [2] Carocci et al. (2022) Geochim. Cosmochim. Acta, 317, 306–324.