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

摘要: SURFACE electromagnetic measurements commonly reveal that the intermediate and lower continental crust has an appreciable electrical conductivity, much higher than found for laboratory analogues of deep-crustal rocks or upper-crustal either formed at shallow depths uplifted from deeper levels. Conducting films grain boundaries are thought to play important part in enhancing with brines graphite being main candidates1. At present, however, there is a lack direct evidence such films. Frost et al.2 have reported enhancement conductivity igneous lower-crustal La ramie complex apparently owing presence films, but possible role was not clear this work. Here we present study surface composition metamorphic (and one igneous) exhumed 20 km depth by Kapuskasing uplift Canadian shield3. We expect these samples be representative deep Archaean crust. Auger spectroscopy provides boundaries, which can account signature crust4–6. also detect traces chlorine, sulphur iron, suggest solid conductors other may helped enhance their abundance suggests best minor relative graphite.