09:30 〜 09:45
[SIT17-03] In-situ Raman spectroscopy and DFT insights into the behavior of early transition metals in hydrothermal fluids
キーワード：前期遷移金属元素、流体、その場ラマン分光法、密度汎関数理論
The hydrothermal mobility of the early transition metals, particularly V, Cr, Mo, and W, has been documented in epithermal or magmatic-hydrothermal systems within the Earth's crust. In recent years, non-traditional stable isotopes of these elements have also gained attention as potential geochemical tracers for deciphering fluid-related geological processes. However, many thermodynamic properties of the aqueous metal complexes are predicted based on low-temperature solubility experiments, and understanding their speciation in geological fluids under high-pressure and high-temperature conditions remains limited.
We examined the behavior of V(V), Cr(VI), Mo(VI), and W(VI) species in aqueous solutions up to 800 °C and 1 GPa by in-situ Raman spectroscopy with an externally-heated hydrothermal diamond anvil cell. Our results showed that inMo(VI) and W(VI) solutions with pH greater than 7, the symmetric stretching mode of tetrahedral anions [MoO4]2– and [WO4]2– remained up to 800 °C, along with a new higher-frequency band becoming dominant above 400–600 °C. In the W(VI) and V(V) oxides-saturated solutions, the bands in higher-frequency regions than those of tetrahedral monomeric species [WO4]2– and [VO4]3– were detected at elevated temperatures. In contrast, aqueous Cr(VI) solutions showed no new bands in the high-frequency regions, and no bands corresponding to dimeric species [Cr2O7]2- were observed. Previous studies suggest that polyoxometalates, such as paratungstate A [W7O24]6–, could produce higher-frequency bands compared to the symmetric stretching mode of tetrahedral monomeric anionic species. Our density-functional theory (DFT) calculations further demonstrated that monomeric tungstic acid of the several optimized structures consistent with the H2WO4 formula could reproduce the high-frequency features. These findings have implications for assessing the thermodynamic properties of aqueous metal complexes, which are critical for modeling hydrothermal metal transport in high-temperature geological environments, and insights into the mechanisms of stable isotope fractionation between fluids and minerals. Ongoing work aims to expand the experimental conditions to a broader range of fluid compositions and assess the behavior of the early transition metals in hydrothermal fluids.
We examined the behavior of V(V), Cr(VI), Mo(VI), and W(VI) species in aqueous solutions up to 800 °C and 1 GPa by in-situ Raman spectroscopy with an externally-heated hydrothermal diamond anvil cell. Our results showed that inMo(VI) and W(VI) solutions with pH greater than 7, the symmetric stretching mode of tetrahedral anions [MoO4]2– and [WO4]2– remained up to 800 °C, along with a new higher-frequency band becoming dominant above 400–600 °C. In the W(VI) and V(V) oxides-saturated solutions, the bands in higher-frequency regions than those of tetrahedral monomeric species [WO4]2– and [VO4]3– were detected at elevated temperatures. In contrast, aqueous Cr(VI) solutions showed no new bands in the high-frequency regions, and no bands corresponding to dimeric species [Cr2O7]2- were observed. Previous studies suggest that polyoxometalates, such as paratungstate A [W7O24]6–, could produce higher-frequency bands compared to the symmetric stretching mode of tetrahedral monomeric anionic species. Our density-functional theory (DFT) calculations further demonstrated that monomeric tungstic acid of the several optimized structures consistent with the H2WO4 formula could reproduce the high-frequency features. These findings have implications for assessing the thermodynamic properties of aqueous metal complexes, which are critical for modeling hydrothermal metal transport in high-temperature geological environments, and insights into the mechanisms of stable isotope fractionation between fluids and minerals. Ongoing work aims to expand the experimental conditions to a broader range of fluid compositions and assess the behavior of the early transition metals in hydrothermal fluids.