Variations in electrical conductivity of rocks above metamorphic conditions

This study of the electrical conductivity of metamorphic rocks as a function of temperature highlights the importance of conductivity variations associated with dehydration. We observed conductivity changes for anhydrous rocks and hydrous amphibolites up to 1100 K at 0.5 GPa. Our experimental results revealed that dehydration takes place and that there is a non-linear variation above metamorphic temperatures. To evaluate the observed changes in conductivity after dehydration, numerical calculations were conducted. The calculation results showed that the bulk conductivity of rock plus pure water cannot account for the conductivity values of reacted rocks if the crust dehydrates. However, the data for 0.1 m NaCl solutions are consistent with those for amphibolites between 530 and 1100 K. In this temperature range, the estimated volume fractions of dewatered saline solutions vary between 0.04 and 0.36 vol.%. These results show that although dehydration is insignificant, and only small amounts of high-salinity fluids are formed in the crust, the total conductivity of amphibolites is high after dehydration.

Research highlights
► Dehydration takes place above metamorphic temperatures.
► The estimated volume fractions of dewatered saline solutions varied between 0.04 and 0.36 vol.% in the amphibolite.
► Only small amounts of high salinity fluids are formed in the crust.