Diamond exploration from the bottom up: regional geophysical signatures of lithosphere conditions favorable for diamond exploration

Abstract

On a local scale, regional geophysical trends may be used to locate the sites of volcanism which transports diamonds to the surface. By studying the conditions under which carbon is stable as diamond, a new regional geophysical exploration philosophy is espoused based upon the search for lithospheric conditions favorable to diamond genesis and preservation. Surface heat flow values of less than 40–45 mW m −2 and lithospheric thicknesses of greater than 150 km meet these target conditions. An analysis of two-dimensional thermal heterogeneities in the lithosphere indicates that target areas should have low surface heat flow over an areas of at least 300 to 400 km diameter. In addition to having low surface heat flow, such target regions are likely to be characterized by a large depth to the Curie Point isotherm, relatively great depths to lithospheric electrical conductors, slow seismic velocities, and great depths to the seismic low velocity zone. The region is likely to be characterized by a paucity of seismicity with high maximum hypocentral depths. The primary problem with these techniques in regional diamond exploration is that they provide information about modern lithospheric structure. Changes in lithospheric structure through time may be detected as elevation changes. These conditions are most commonly found in regions of Archean and Early Proterozoic lithosphere, and are consistent with Clifford's Rule, an empirical diamond exploration strategy. The restriction of these conditions to these ancient terrains may be associated with special conditions required to form these terrains which only existed early in Earth history, or with an increase in the thermal heterogeneity of the lithosphere through time which results in, on average, thinner lithosphere in younger terrains.