INCREASE OF ELECTRICAL-CONDUCTIVITY WITH PRESSURE AS AN INDICATOR OF CONDUCTION THROUGH A SOLID-PHASE IN MIDCRUSTAL ROCKS

Rocks freshly cored from depth at the German continental scientific dr illing site (KTB) offer an opportunity to study transport properties i n relatively unaltered samples resembling material in situ. Electrical conductivity sigma was measured to 250 MPa pressure, and room tempera ture on 1 M NaCl-saturated amphibolites from 4 to 5 km depth. An unexp ected feature was an increase of sigma with pressure P that appeared ( anisotropically) in most samples. To characterize this behavior, we fi tted the linear portion of log sigma versus P to obtain two parameters : the slope dlog sigma/dP (of order 10(-3) MPa-1) and the zero-pressur e intercept sigma(0). Samples of positive and negative slopes behave d ifferently. Those having negative slopes show strong correlation of si gma(0) with a fluid property (permeability). This behavior indicates t hat fluids exert the dominant control on sigma(0) at low pressure when sigma(0) is greatest, which is typical behavior observed in previous studies. In contrast, samples with positive slopes lack a correlation of sigma(0) with permeability, indicating that fluids are less importa nt to positive pressure behavior. Another result is that samples of ne gative dlog sigma/dP have uncorrelated slopes and initial conductiviti es. In significant contrast, samples of positive slopes have the great est P dependence for lowest initial conductivity sigma(0), that is, th e less fluid, the more positive dlog sigma/dP. Hence positive dlog sig ma/dP is consistent with reconnection of solid phases into a conductiv e texture better resembling that of rock at depth, Detailed examinatio n of one sample by electron probe and scanning electron microscope rev eals the presence of carbon on internal cleavage surfaces in amphibole , the most abundant mineral present. Thus carbon probably dominates th e reconnection, but total sigma still involves fluids as well as Fe-Ti oxides. For the KTB location it is inferred that the reason mid to de ep crustal electrical conductivities modeled from geophysical measurem ents are so much higher than conductivities of silicates is the presen ce of interconnected good conductors involving films of carbon on surf aces and other solid phases.