The electrical conductivity of granite: The role of hydrous accessory minerals and the structure water in major minerals

Abstract

The electrical conductivity of granitic rocks (granitoids) is crucial to understand the rock composition, physiochemical state and structure of the crust. However, it is not clear how the conductivity of granitoids is affected by the incorporated water in major minerals and hydrous accessory minerals. Here, we measured the electrical conductivity of two natural granites and two synthetic aggregates compositionally similar to the granites at temperatures of 573–1273 K and pressures of either ambient pressure or 1 GPa. No mineral preferred orientation was observed for the natural samples. The hydrous accessory minerals in the samples, including muscovite, biotite and amphibole, start to dehydrate at about 700 K at ambient pressure, but barely affect the bulk conductivity. The increase in conductivity of granite by more than half orders of magnitude at 1273 K and 1GPa indicated the onset of melting due to the dehydration of biotite. The conductivities of synthetic aggregates are higher than the granite samples, suggesting that the structural water in feldspars could influence, to a moderate degree, the conductivity of granite. We combined the effect of modal composition and structural water on the conductivity of granite and derived a maximal electrical conductivity of a granitoid at different geological settings. The maximal conductivity can be 0.004 ± 0.0001 S/m at 60 km under orogens, over one order of magnitude higher than a cratonic crust. The maximal conductivity provides an upper limit for interpreting the electrical anomalies in the crust.