MODELING THE GROWTH OF NATURAL DIAMONDS

In the following article we have reviewed the mineralogical, chemical, physical and isotopic evidence concerning the growth of coated diamon ds, and have combined it to produce a comprehensive model for their fo rmation. We suggest that the coatings formed during a period of metaso matic activity which preceded the eruption of their host kimberlites. Ultrapotassic CO2-H2O-rich fluids resulted from the crystallisation of diamond-free kimberlitic magmas intruded into the deep lithosphere. T he kimberlite magmas were derived from an asthenospheric source which was globally quite uniform in terms of the isotopic composition of car bon and nitrogen: deltaC-13 and deltaN-15 both being close to -5%. The fluids produced during crystallisation invaded overlying diamondifero us lithosphere in which different regions could contain diamond popula tions with different mineralogical, isotopic and physical characterist ics depending on the history of their source region. Either intrinsic to their formation or due to reaction with reduced lithosphere, the fl uids were super-saturated with carbon although not to such an extent a s to allow for the nucleation of new diamonds. When these super-satura ted fluids encountered older diamonds, new and fibrous diamond growth commenced immediately. This period of metasomatic activity was followe d rapidly by one or more successful kimberlite eruptions which transpo rted the now coated diamonds to the surface. In the crust, carbonates within hypabyssal facies kimberlites will have deltaC-13-values which reflect the deep-seated source (approximately - 5%) whereas those with in diatreme facies may be more variable due to outgassing of CO2 durin g eruption. Finally, we consider isotope fractionation related to grow th. We suggest that approaches based on equilibrium are not appropriat e to diamonds but rather that it is better to consider the nature and kinetics of the surface reactions leading to growth. Where diamond gro wth is very rapid, as may be the case for fibrous diamond, we propose that there will be no difference in isotopic composition between the f luid and diamond and not the 3-4%0 C-13 depletion in the latter predic ted by theoretical treatments. Octahedral diamonds are more problemati c since far less is known about their growth conditions. However, we s how in principle how the content and perhaps isotopic composition of n itrogen within a diamond may be controlled by its growth rate. We are uncertain whether deltaC-13 would be affected.