Is the illite group the cause of high electrical conductivityin certain lithospheric areas?

Abstract

This article discusses the problem of the mineralogical-petrographic interpretation of the "anomalous" conductive layer inside the lithosphere, which has resulted from electromagnetic deep soundings. Three situations can be found: 1) a resistivity of less than 1 Ω ·m in the Very High Conductivity Layer (VHCL); 2) a resistivity of 1÷100 Ω ·m in the High Conductivity Layer (HCL); 3) a resistivity of 102 ÷ 103 Ω ·m in the Continental Lower Crust (CLC). We have focused our attention on the HCL because of its widespread distribution. Most authors attribute HCL conductivity to the presence of salt water or graphitic materials, that are uniformly and continuously distributed inside rocks. Other hypotheses from the literature are the presence of oxides and/or sulphides, rock melting, and brine-bearing rocks. Each one of these elements can cause high electrical conductivity, depending on the petrophysical conditions, but much discussion involves the necessary physical continuity of the conductive elements. We forward one more hypothesis as follows. By means of experiments on rock samples from different geographical and deep areas, under simulated in situ physical conditions (e.g., pressure, temperature, saturation), we have found resistivity values similar to the in situ ones. Chemical, mineralogical, and petrographic analyses have shown the presence of large amounts of phyllosilicate minerals, such as illite. These minerals act in two ways: they produce the physical continuity of associated conductive minerals, and they decrease the resistivity of rocks. This second hypothesis is consistent with both the very widespread diffusion of illite inside the Earth's crust, and with the good conductivity found in depth.