Electrical conductivity of carbonbearing granulite at raised temperatures and pressures

IT has long been recognized that the electrical conductivity of the lower continental crust is anomalously high. Both pore-saturating brines 1–5 and conducting films of carbon at grain boundaries 6–10 have been proposed to explain this, but the evidence remains inconclusive. Here we report measurements of electrical conductivity at high temperatures and pressures 11–13 on samples of carbon-bearing and carbon-free granulites with a range of electrolyte saturations. The application of pressure to nominally dry carbon-free samples reduces the electrical conductivity as a result of a progressive reduction in pore connectivity, whereas the carbon-bearing samples show an increase in conductivity under the same conditions—an effect that we ascribe to reconnection of carbon conduction pathways during compaction. Moreover, we find a greater increase in conductivity with temperature for the carbon-bearing samples. In the light of work indicating that the abundance of carbon in high-grade rocks has been underestimated in the past 7,8 , our results provide strong evidence for the role of carbon in lower-crustal conductivity.