GEODYNAMIC SETTING OF DIAMOND DEPOSITS

Many factors influence the surface distribution of diamond deposits. These include the stability of the thermal structure of the continental lithosphere, the frequency of kimberlitic/lamproitic eruptions, and the survival of the surface expression of such a feature. Seismic tomography and xenolith thermobarometry all suggest that Archean cratons generally constitute the thickest, coolest portions if the continental lithosphere. This has even led to an empirical relationship between on-craton diamondiferous kimberlites, and off-craton barren pipes. Furthermore, the dating of inclusions in South African diamonds suggest that they are Archean, which implies that the cratonic roots have survived since that time. Australia's two economic diamond deposits do not conform to this simple exploration model. Argyle, Australia's largest diamond mine, is located in the Proterozoic Hall's Creek mobile belt, adjacent to the Kimberley block. Similarly, the Merlin deposit is located in the central Australian Proterozoic mobile belts. Additionally, tomographic results suggest that the thickest portions of the Australian lithosphere are not the western Australian Archean cratons, but the central Australian Proterozoic terrains. We use a particle-in-cell finite element code to simulate the nature and stability of the geotherms in different continental regions in a convecting mantle. Cratonic regions generally show less surface deformation than their non-cratonic counterparts. Geotherms are generally low and quite stable in cratonic regions with thermochemically stable root zones. For non-cratonic regions, the geotherms are elevated and less stable. However, the continental structure can exert a large influence on the mantle dynamics around it, and consequently the thermal structure of the continent as a whole. We show that cratonic roots can help stabilise the thermal structure of adjacent terrains, and explain the distribution of economic diamond deposits in Australia.