The Archean Slave craton in northwestern Canada is an ideal natural laboratory for investigating lithosphere formation and evolution, and has become an international focus of broad geoscientific investigation following the discovery of economic diamondiferous kimberlite pipes. Three deep-probing magne-totelluric (MT) surveys have recently been carried out on the craton using novel acquisition procedures. The MT responses reveal an unexpected and remarkable anomaly in electrical conductivity, collocated with the kimberlite field, which is modeled as a spatially confined upper-mantle region of low resistivity (<30 W·m) at depths of 80-100+ km, and interpreted to be due to disolved hydrogen or carbon in graphite form. This geophysically anomalous upper-mantle region is also spatially coincident with a geochemically defined ultradepleted harzburgitic layer. The tectonic processes that emplaced this structure are possibly related to the lithospheric subduction and trapping of overlying oceanic mantle at 2630-2620 Ma.