Diamonds and Kimberlites

Diamonds are formed at depth (150-200km, up to 700km) under very specific physico-chemical conditions (T=900-1200 C; P=4.5-5 GPa) in the earth mantle under geolo-gically stable cratons of the continents.

Diamonds crystallize within 2 main types of mantle-magmas:

  • Peridotitic (rock mainly composed of olivine, orthopyroxene, clinopyroxene, chromite and pyrope garnet);
  • Eclogitic (rock mainly composed of pyrope-almandine garnet, omphacitic clino-pyroxene).

These deep diamond-bearing rocks are later picked up and assimilated as fragments (xenoliths) within deeper fast-ascending  (150-200 km/hour) kimberlite volcanic magmas on its way to surface. The kimberlite is a volatile-rich (H2O, CO2), potassic ultrabasic  igneous rock that is mainly composed of olivine with some phlogopite, carbonate, serpentine, clinopyroxene, orthopyroxene, spinels, perovskite, ilmenite, magnetite and garnets.

Specific conditions (Temperature and Pressure) of formation within the mantle give the minerals of the diamond-bearing xenoliths as well as the ones of the associated kimberlites some specific chemical compositions that characterized and differentiated them from other rocks. These characteristics of these indicator minerals are widely used in diamond exploration to identify anomaly in glacial sediments or soils, and to trace them back to the sources.

Typical kimberlite bodies generally occur as volcanic pipes that may expand (cone-shape) in its upper part near the surface. They are often associated with dykes and sills that are also associated with deep structures that controlled the distribution. The kimberlites are generally grouped in clusters that expand themselves over some 20-30 square km. The surface diameters of the rounded pipes generally vary from 100 meters to 1000 meters. The abundance of iron minerals within the kimberlite also allows the identification of its bodies by detailed magnetic survey.