Open AccessIncrease of electrical conductivity with pressure as an indicator of conduction through a solid phase in midcrustal rocks
Author(s) -
Shankland T. J.,
Duba A. G.,
Mathez E. A.,
Peach C. L.
Publication year - 1997
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/96jb03389
Subject(s) - electrical resistivity and conductivity , scanning electron microscope , geology , conductivity , thermal conduction , mineralogy , permeability (electromagnetism) , phase (matter) , analytical chemistry (journal) , materials science , chemistry , composite material , membrane , physics , biochemistry , organic chemistry , chromatography , quantum mechanics
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 σ 0 . Samples of positive and negative slopes behave differently. Those having negative slopes show strong correlation of σ 0 with a fluid property (permeability). This behavior indicates that fluids exert the dominant control on σ 0 at low pressure when σ 0 is greatest, which is typical behavior observed in previous studies. In contrast, samples with positive slopes lack a correlation of σ 0 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 σ 0 , 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.