Lithospheric magnetotelluric imaging in Canada: significance to diamond exploration

Abstract

Deep magnetotelluric (MT) surveys provide information on thickness, electrical resistivity and tectonic history of Precambrian lithosphere that is relevant to diamond exploration. In regions where lithosphere is electrically resistive, its thickness can be inferred from the observed depth of a conductor at 100 km to 300 km taken as defining the top of the electrical asthenosphere. In some locations high conductivity is observed within the lithosphere and, at depths above the diamond stability field, may be attributed to the presence of graphite. The form of crustal and upper mantle conductors, interpreted with deep seismic information, provides information that can be used to constrain the tectonic history of a region, eg, defining the geometry of past subduction. Surveys in the Slave craton in northern Canada have identified an MT phase anomaly that is coincident with both a diamond-bearing Eocene kimberlite field and an upper mantle geochemical anomaly. The phase anomaly is explained by a conductor, with its upper surface at 80 to 120 km depth and the enhanced conductivity is interpreted as being due to graphite or an interconnected carbon grain-boundary film. In the western Superior Province in central Canada, MT surveys have delineated a mantle conductor at a depth of 130 km. The MT responses define the geoelectric fabric of both the crust and subcrustal lithosphere and indicate that the subcrustal lithosphere is of Archaean age.