Electrical Conductivity of F-H2O-bearing Rhyolitic Melts: Implications for High-Conductivity Anomalies in the Crust

Fluorine (F) and water (H₂O) are critical volatiles in magmatic systems. They play a vital role in magmatism, hydrothermal metallogenesis and so on. Previous studies have studied the effects of F on melt viscosity and element diffusion. However, the impact of fluorine on electrical conductivity, and the coupled effect of F-H₂O, remain poorly constrained. We performed in-situ electrical conductivity measurements on metaluminous rhyolitic melts in a piston cylinder apparatus combined with a Solartron 1260 impedance analyzer. The experimental conditions spanned 0.5–1.0 GPa and 700–1200 °C with different contents of F and H₂O. The results show that H₂O can significantly enhance the electrical conductivity of metaluminous rhyolitic melt, with an increase of 0.5-1.0 log units by adding ~4 wt% H₂O. In contrast, adding ~4 wt% F can only increase the electrical conductivity by 0.2–0.3 log units. Moreover, the fluorine-water coupling effect is less than the sum of their independent contributions. This result indicates that the non-linear coupling mechanisms between the two volatiles must be considered when evaluating their speciation and transport behavior. Additionally, we found that the influence of F on the electrical conductivity of rhyolitic melts varies with the aluminum saturation index (ASI). The effect of fluorine becomes more pronounced with increasing ASI. Based on the measurement data, we established a general electrical conductivity model for F-H₂O-bearing metaluminous rhyolitic melts, which can be applied to constrain high-conductivity anomalies in the Earth's crust. For example, the high-conductivity anomaly beneath the Gangdese belt in southern Tibet can be explained by the existence of 8–22 vol% of melt with >6 wt% H₂O; and the conductivity anomaly in the upper crust of the Yellowstone volcano corresponds to 11–24 vol% of melt. This study highlights the characteristics of electrical conductivity for F-H₂O-bearing melts, providing key physical constraints for understanding volatile migration in magmatic-hydrothermal systems.