Grain-boundary graphite in rocks and implications for high electrical conductivity in the lower crust

Abstract

THE origin of zones of high electrical conductivity in the lower continental crust is a long-standing mystery; possible explanations include the presence of brines1,2, partial melt3, serpentine4 and graphite5. When discussing the occurrence of graphite in the crust many petrologists have considered phase relations as they would have existed at the peak of metamorphism or during igneous emplacement6-10. Here we show that a fine film of graphite is present on the grain boundaries in three rocks from the Laramie Anorthosite Complex. In two of these rocks graphite was stable during igneous crystallization but in the other it was not. We maintain that in all of the rocks the grain-boundary graphite precipitated from a CO2-rich fluid during cooling. The chemical processes that produced the grain-boundary graphite in these rocks are likely to operate in many lower-crustal rocks. We therefore contend that, because the films we observe are capable of producing the high conductivity that is seen in the lower crust, grain-boundary graphite should be considered as a possible cause for some conductivity anomalies.