Grain-boundary graphite in Kapuskasing gneisses and implications for lower-crustal conductivity

Abstract

SURFACE electromagnetic measurements commonly reveal that the intermediate and lower continental crust has an appreciable electrical conductivity, much higher than that found for laboratory analogues of deep-crustal rocks or for upper-crustal rocks either formed at shallow depths or uplifted from deeper levels. Conducting films at grain boundaries are thought to play an important part in enhancing deep-crustal conductivity, with brines and graphite being the main candidates¹. At present, however, there is a lack of direct evidence for such films. Frost et al.² have reported an enhancement in conductivity of igneous lower-crustal rocks from the La ramie complex apparently owing to the presence of graphite films, but the possible role of brines was not clear from this work. Here we present a study of the grain surface composition of metamorphic (and one igneous) rocks exhumed from 20 km depth by the Kapuskasing uplift of the Canadian shield³. We expect these samples to be representative of metamorphic and igneous rocks of the deep Archaean crust. Auger spectroscopy provides evidence for graphite films at grain boundaries, which can account for the present electrical signature of the Kapuskasing crust^4–6. We also detect traces of chlorine, sulphur and iron, which suggest that brines and solid conductors other than graphite may have helped to enhance the conductivity, but their abundance suggests at best a minor role relative to graphite.