COMMENTARY — The origin of diamonds

Diamonds are being found in geological environments outside cratons and in rocks other than kimberlites. As a result, different sources and origins of diamonds must be considered.

Experiments at several laboratories have shown that high restraining pressures of 35 kbar and temperatures of 700!C are required for the formation and preservation of diamonds. In nature, the introduction of diamonds into near-surface terrain depends on the maintenance of temperature and pressure to surface, as the loss of these properties causes the carbon to revert to other forms, such as graphite or carbon dioxide. This is what causes rounding and frosting in some diamonds.

The geological environment of adequate temperatures, pressure and chemical composition that are required for the formation and retention of diamonds occurs at the interface of mantle and continental rock.

N.L. Bowen and other academics believe the differentiation of basaltic melts at this interface produces kimberlites. Recent exploration has shown that rocks of diverse composition contain diamonds and that the normal indicator minerals of kimberlite are absent.

Carbon, water, carbon dioxide and other light elements need not be primary, but may well have been marine sediments subducted during the migration of continents, or they have been derived from the melting of the bases of cratons or the roots of folded masses of continental rocks during mountain-building. These conditions would place carbon in an environment containing adequate physical and chemical conditions that could produce diamonds.

The melting and mixing of continental and mantle rocks would produce melts of diverse composition. Crystallization of such melts would produce different minerals. Diamonds, graphite and other elements, such as gold, could be components of some of these minerals.

As the rocks in these situations differ greatly in composition, so will their indicator minerals. In these instances, a statistical study of the mineral composition must be completed to determine the new suite of indicator minerals.

There is little to be gained by following the typical South African pundits, who may be correct in their hypothesis concerning kimberlites and their indicator minerals but who generally have a closed mind regarding the possible existence of indicator minerals from other sources.

The author is a consulting geologist based in Corbyville, Ont.