Mapping the mantle lithosphere for diamond potential using teleseismic methods

Abstract

Recent developments in seismic, magnetotelluric and geochemical analytical techniques have significantly increased our capacity to explore the mantle lithosphere to depths of several hundred kilometres, to map its structures, and through geological interpretations, to assess its potential as a diamond reservoir. Several independent teleseismic techniques provide a synergistic approach in which one technique compensates for inadequacies in another. Shear wave anisotropy and discontinuity studies using single seismic stations define vertical mantle stratigraphic columns. For example, beneath the central Slave craton seismic discontinuities at depths of 38, 110, 140 and 190 km appear to bound two distinct anisotropic layers. Tomographic (3-D) inversions of seismic wave travel-times and 2-D inversions of surface or scattered waves use arrays of stations and provide lateral coverage. In combination, and by correlation with electrical conductivity and xenolith petrology studies, these techniques provide maps of key physical properties within parts of the cratons known to host diamonds. Beneath the Slave craton, the discontinuity at 38 km is the base of the crust; the boundaries at 110 and 140 km appear to bound a layer of depleted harzburgite that is interpreted to contain graphite. To date, only some of these techniques have been applied to the Slave and Kaapvaal cratons so that the origin and geological history of the currently mapped mantle structures are not, as yet, generally agreed.