Tungsten(VI) speciation in aqueous fluids at high pressures and high temperatures: in-situ Raman spectroscopy supported by DFT calculations

Tungsten (W) is mobile in magmatic–hydrothermal and deep subduction zone fluids, and its stable isotopes offer potential as tracers of fluid-rock interactions. We investigated W behavior in aqueous fluids using in-situ Raman spectroscopy on slightly acidic to alkaline Na2WO4 solutions (10−3–10−1 M W) and WO3–H2O/D2O ± NaCl systems up to 1.4 GPa and ∼800 °C, with the band assignments supported by density functional theory (DFT) calculations. Raman spectra showed that the ν1 mode of WO42− persisted to the highest P–T conditions in alkaline fluids, while additional high-frequency bands near 950 or 973 cm−1 appeared at elevated T in the studied fluids. Their relative intensities depended on T, pH, and NaCl content; the 973 cm−1 band was only observed above 650 °C in Na-free fluids. Substitution of H2O with D2O did not significantly affect the band frequencies. Based on analogous molybdenum experiments, previous in-situ studies, and thermodynamic models, these high-T bands are assigned to mononuclear species rather than polynuclear species, consistent with the stepwise formation of hydrogentungstate anion, HWO4−, and then neutral tungstic acid, H2WO4, as T increases. DFT vibrational frequency calculations for HWO4−, H2WO4, and their D-substituted species support this interpretation, suggesting tetrahedral HWO4− and higher-coordinated H2WO4 as plausible species. The present study clarifies Raman band assignments for HWO4− and paratungstate A (W7O246−) to address previous ambiguities in the literature, and confirms that polynuclear species play a negligible role in high-T ore-forming fluids. The tetrahedral HWO4− appears to be a predominant transport species in NaCl-bearing metamorphic fluids and may govern stable W isotope fractionation during deep fluid-rock interactions.