Probing the status of felsic magma reservoirs: Constraints from the P–T–H2O dependences of electrical conductivity of rhyolitic melt

Abstract

Electrical conductivity of rhyolitic melts, combined with magnetotelluric data, can provide important constraints for the physicochemical conditions of active felsic magma reservoirs. Previous experimental investigations are limited to low H2O concentration (<3 wt% H2O) and low pressure (<0.2 GPa for hydrous melts). We here report new results from electrical conductivity measurements of peralkaline rhyolitic melts with 4.7 wt% Na2O and 0.1–7.9 wt% H2O at 868–1665 K and 0.5–1.0 GPa in piston cylinder apparatuses using sweeping-frequency impedance analyses. Logarithmic electrical conductivity (σ) is found to correlate linearly with H2O concentration and reciprocal temperature, and the influence of H2O has been considerably underestimated by previous work. The negative pressure effect on electrical conductivity appears to attenuate with increasing H2O concentration. Based on the new data, we develop the following electrical conductivity model for peralkaline rhyolitic melts under conditions up to 1665 K, 1.0 GPa, and 8 wt% H2O:log⁡σ=2.983−0.0732w−3528−233.8w+(763−7.5w2)PT where σ is electrical conductivity in S/m, T is temperature in K, w is H2O concentration in wt%, and P is pressure in GPa. According to the Nernst–Einstein relation, this conductivity model can also be applied to metaluminous and peraluminous rhyolitic melts by scaling down according to the diffusivity and concentration of Na+ (the primary charge carrier). With the new model, we suggest that the highest electrical anomaly beneath the Tianchi volcano corresponds to a melt fraction of ∼9% if the melt contains 5 wt% H2O, or to a melt fraction of ∼32% for 2 wt% H2O. Similarly, the magma reservoir beneath the Taupo volcanic zone, New Zealand is inferred to contain a fully molten rhyolite with 7.5 wt% H2O or 36% water-saturated melt with 10 wt% H2O.