THE ELECTRIC LITHOSPHERE OF THE SLAVE CRATON

Alan G. Jones(1), Ian J. Ferguson(2), Alan D. Chave(3), Rob Evans(3), 
Gary W. McNeice(4)
(1) Geological Survey of Canada
    615 Booth St., Ottawa, Ontario, K1A 0E9, Canada
(2) Department of Geological Sciences
    University of Manitoba
    Winnipeg Manitoba, R3T 2N2, Canada
(3) Woods Hole Oceanographic Institution
    360 Woods Hole Road, MS 8
    Woods Hole, MA 02543-1539, U.S.A.
(4) Geosystem-Canada Inc., Pickering, Ontario, Canada
ajones@cg.nrcan.gc.ca

The Archean Slave craton, in the northwestern part of the Canadian 
Shield, has become an international focus of broad geoscientific 
investigation following the discovery of diamondiferous kimberlite 
pipes in the early 1990s and the opening of North America's first 
commercial diamond mine in October 1998. As part of LITHOPROBE's 
efforts to understand the history and tectonic development along the 
SNORCLE transect, three electromagnetic surveys, using the 

magnetotelluric, MT, technique, have been carried out on the craton 
since 1996, of which two involved novel acquisition on the frozen lake 
ice and on the lake bottoms. MT responses sampling 50-150 km depths 
exhibit a maximum spatially located with the Eocene kimberlite field 
in the Lac de Gras region. Resistivity models show a 
spatially-confined anomalous upper mantle conducting region, of 
resistivity less than 30 ohm.m, at depths of 80-120 km beneath the Lac 
de Gras region. The depth and spatial position of this anomalous 
region coincides with a geochemically-defined harzburgitic 
ultradepleted layer interpreted as oceanic or arc-related lithosphere 
emplaced during early tectonism. Speculative interpretation of the 
conducting region is in terms of either ionic conduction from hydrogen 
diffusion or electronic conduction due to the presence of carbon in 
graphite form, neither of which can be excluded based on the existing 
evidence. The crust and uppermost mantle above the conductor are more 
heterogeneous than other regions of the Slave, suggestive of 
disruption during kimberlite emplacement.