Increase of electrical conductivity with pressure as an indicator of conduction through a solid phase in midcrustal rocks

Abstract

Rocks freshly cored from depth at the German continental scientific drilling site (KTB) offer an opportunity to study transport properties in relatively unaltered samples resembling material in situ. Electrical conductivity σ was measured to 250 MPa pressure, and room temperature on 1 M NaCl‐saturated amphibolites from 4 to 5 km depth. An unexpected feature was an increase of σ with pressure P that appeared (anisotropically) in most samples. To characterize this behavior, we fitted the linear portion of log σ versus P to obtain two parameters: the slope dlogσ/dP (of order 10−3 MPa−1) and the zero‐pressure intercept σ0. Samples of positive and negative slopes behave differently. Those having negative slopes show strong correlation of σ0 with a fluid property (permeability). This behavior indicates that fluids exert the dominant control on σ0 at low pressure when σ0 is greatest, which is typical behavior observed in previous studies. In contrast, samples with positive slopes lack a correlation of σ0 with permeability, indicating that fluids are less important to positive pressure behavior. Another result is that samples of negative dlogσ/dP have uncorrelated slopes and initial conductivities. In significant contrast, samples of positive slopes have the greatest P dependence for lowest initial conductivity σ0, that is, the less fluid, the more positive dlogσ/dP. Hence positive dlogσ/dP is consistent with reconnection of solid phases into a conductive texture better resembling that of rock at depth. Detailed examination of one sample by electron probe and scanning electron microscope reveals the presence of carbon on internal cleavage surfaces in amphibole, the most abundant mineral present. Thus carbon probably dominates the reconnection, but total σ still involves fluids as well as Fe‐Ti oxides. For the KTB location it is inferred that the reason mid to deep crustal electrical conductivities modeled from geophysical measurements are so much higher than conductivities of silicates is the presence of interconnected good conductors involving films of carbon on surfaces and other solid phases.