THE FORMATION OF DIAMOND

This article summarises data on the formation of commercial-sized diamonds (macrodiamonds) from kimberlitic rocks. A brief review of previous work and ideas is presented, together with new studies on the growth of diamonds from Yakutian kimberlites. A new approach to deciphering the origin of diamond is presented, involving the collation of all primary genetic information collected from large single crystals. The main primary features of diamond that have been studied are inclusion paragenesis, internal morphology, mechanism of growth and the evolution of the growth environment. The differences in the growth history of the peridotitic and eclogitic suites of diamonds from kimberlites is in agreement with data about the differences between diamonds from mantle eclogites and peridotites. Evidence for the primary nature of zoning within diamond as revealed by IR-microscopy and CL-data, the reasons for it, and the evolution of the morphology of diamond crystals during their growth are discussed. From the internal structure it is argued that single crystalline diamonds were formed in a free (fluid) environment by tangential, layer by layer, growth mechanisms. They crystallised within the mantle from a slightly supersaturated solution of carbon in a sulphide-silicate melt (fluid). Most diamonds nucleated heterogeneously on mineral seeds that could lower the energy barrier to nucleation of monocrystals. All of the mineral species that have been identified in the genetic centres of diamonds (sulphide + native iron + wustite + monocrystalline graphite) may have acted as seeds and/or catalysts for the process of diamond nucleation. This assemblage indicates that redox conditions of diamond formation corresponded to the Fe-FeO buffer. It is concluded that the formation of diamond monocrystals of both eclogitic and peridotitic parageneses took place during early igneous events within the mantle. Temperature generally decreased slightly during the process of diamond growth. However, some peridotitic diamonds reveal a very complicated history of growth and dissolution, indicative of large scale changes in the growth environment within the mantle. It is concluded that diamonds in kimberlitic rocks originated from a variety of mantle rock source environments.