Experimental investigation of the electrical behavior of olivine during partial melting under pressure and application to the lunar mantle

Electrical conductivity measurements were performed during melting experiments of olivine compacts (dry and hydrous Fo₇₇ and Fo₉₀) at 4 and 6 GPa in order to investigate melt transport properties and quantify the effect of partial melting on electrical properties. Experiments were performed in the multi-anvil apparatus and electrical measurements were conducted using the impedance spectroscopy technique with the two-electrode method. Changes in impedance spectra were used to identify the transition from an electrical response controlled by the solid matrix to an electrical response controlled by the melt phase. This transition occurs slightly above the solidus temperature and lasts until T_solidus+75°C (±25). At higher temperature, a significant increase in conductivity (corresponding to an increase in conductivity values by a factor ranging from ~30 to 100) is observed, consistent with the transition from a tube-dominated network to a structure in which melt films and pools become prominent features. This increase in conductivity corresponds to an abrupt jump for all dry samples and to a smoother increase for the hydrous sample. It is followed by a plateau at higher temperature, suggesting that the electrical response of the investigated samples lacks sensitivity to temperature at an advanced stage of partial melting. Electron microprobe analyses on quenched products indicated an increase in Mg# (molar Mg/(Mg+Fe)) of olivine during experiments (~77-93 in the quenched samples with an initial Fo₇₇ composition and ~92-97 in the quenched samples with an initial Fo₉₀ composition) due to the partitioning of iron to the melt phase. Assuming a respective melt fraction of 0.10 and 0.20 before and after the phase of significant increase in conductivity, in agreement with previous electrical and permeability studies, our results can be reproduced satisfactorily by two-phase electrical models (the Hashin and Shtrikman bounds and the modified brick layer model), and provide a melt conductivity value of 78 (±8) S/m for all Fo₇₇ samples and 45 (±5) S/m for the Fo₉₀ sample. Comparison of our results with electromagnetic sounding data of the deep interior of the Moon supports the hypothesis of the presence of interconnected melt at the base of the lunar mantle. Our results underline that electrical conductivity can be used to investigate in situ melt nucleation and migration in the interior of terrestrial planets.