SYNTHESIS OF DIAMONDS FROM C-O-H RICH SUPERCRITICAL FLUID AND ITS IMPLICATION FOR THE ORIGIN OF MICRODIAMONDS FROM METAMORPHIC ROCKS RELATED TO CONTINENTAL COLLISIONS

The microdiamonds associated with ultra-high pressure metamorphic rocks are now known in Kazakhstan (Sobolev and Shatsky, 1990), China (Xu et al., 1992), Norway (Dobrzhinetskaya et al., 1995) and Germany (Massonne, 1998). Various researches have demonstrated that they are crystallized from a multicomponent COH-rich supercritical fluid, whose compositional diversity reflects an intermixture of crustal and mantle materials (Dobrzhinetskaya et al., 2000) although some have postulated the diamonds grew from a carbonate melt (Shatsky et al.,2001). Molecular water has also been detected in diamonds from Kazakhstan ( De Corte et al., 1998).

Because of the reaction Graphite - Diamond is sluggish due to a high kinetic barrier, synthetic diamonds are usually produced at very high temperature and pressure in the presence of a metal catalyst. Only several years ago perfect octahedral and cube-octahedral diamonds were synthesized from graphite (e.g. Wang et al., 1999; Yamaoka et al., 2000) and carbonate material (Pal'anov et al., 1999) in the presence of water in the diamond stability field. According to the classical theory of crystal growth, at thermodynamic equilibrium, only shapes with a minimum surface energy are stable (e.g. Chernov,1974). It is also known that a skeletal-hopper crystal is one that develops under conditions of rapid growth, a high degree of supersaturation, and in the presence of impurities (Gornitz and Schreiberg, 1981). Because most metamorphic diamonds are characterized by imperfect crystallographic forms, they have been interpreted as metastable. However, the discovery of the other high-pressure phases associated with this diamond indicates that the rocks have been subjected to UHP metamorphism within the diamond stability field.

We have recently synthesized imperfect diamond crystals from graphite + 2% brucite and from graphite + 5%talc in calcite and dolomite systems as well as in quatrz and albite systems. These systems roughly reproduce a natural rock environment, where the brucite and talc release water to generate a supercritical fluid at high T and P. Conditions of experiments are: P=8-8.5 GPa, T=1673-1773K, t=14 to 136 hours. These conditions have the effect of increasing the rate of breakage of graphite bonds, a prerequisite to the formation of diamond from liberated carbon atoms.