WATER AND GRAPHITE IN THE EARTHS CRUST - AN APPROACH TO INTERPRETATION OF CONDUCTIVITY MODELS

In the present discussion on the origin of highly conducting layers in the middle and lower crust, water (brines) and graphite are widely accepted as the two most promising explanations. In this review, petrophysical and petrological aspects are summarized aiming at an understanding of how the indispensable interconnectedness of these highly conducting phases within a normally insulating rock matrix may be established and maintained under conditions of the upper and the lower crust. Accordingly, interpretation of conductivity models will be conclusive in cases of conductance values reaching several thousand S for layers at deeper crustal level since they can not be explained other than by the existence of metaanthracitic or graphitic rocks. In cases of low to moderate conductance values as often observed, e.g., at the transition zone from the upper to the lower crust, interpretation may remain ambiguous and may need further multidisciplinary investigations.