Electrical conductivity measurement criteria for silicate and carbonate melts at high pressure: Two-electrode versus pseudo-four-electrode methods

Magnetotelluric surveys often detect high electrical conductivity anomalies in Earth's interior. Interpreting these anomalies requires quantitative laboratory measurements of electrical conductivity on various melts at high temperatures and pressures. Although the two-electrode and pseudo-four-electrode configurations have been used for such measurement, their comparability at high pressures has not been systematically evaluated. In this study, we performed electrical conductivity measurements on silicate melts and carbonate melt at 0.5–1.0 GPa and 700–1300 °C using a piston-cylinder apparatus with sweeping-frequency impedance analysis. In the experiments, two-electrode and pseudo-four-electrode methods were conducted simultaneously for comparison. The results demonstrate that after proper resistance correction, the two-electrode method can provide reliable electrical conductivity values for most silicate melts. The electrode-induced measurement error can be effectively constrained within 30 %, showing good agreement with pseudo-four-electrode measurement. However, for highly conductive carbonate melt, significant discrepancies persist even after correction. The values from two-electrode method are systematically lower than those from pseudo-four-electrode method. Based on the comparative measurements, we recommend that the two-electrode method remains valid for melts with electrical conductivity below 45 S/m. In contrast, the pseudo-four-electrode technique is essential for obtaining accurate results at higher electrical conductivity ranges, particularly for carbonate-rich systems where electrical conductivity typically exceeds this threshold.