Mechanism(s)and Conditions(s) of Growth of Metamorphic Diamonds from Ultra-High Pressure Felsic Gneisses in Kazakhstan and Germany

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DobrizhinetskayaEAR-0107118Microdiamonds of the Kokchetav massif in Kazakhstan grew under conditions of "ultra-high-pressure"(UHP) metamorphism of quartzofeldspathic sediments in a subduction zone. The recognition of this phenomenon a decade ago initiated a revolution in the understanding of continental collision terranes. Similar diamond-bearing rocks have now been discovered in the Saxony region of Germany, in the Erzgebirge of Variscan age. Erzgebirge and Kokchetav diamonds are strikingly similar. However, Erzgebirge diamonds appear to have suffered greater back-reaction to graphite and/or dissolution at lower pressures. The study of diamonds in situ in garnet and zircon shows that in both terranes, diamond inclusions are very frequently accompanied by a hydrous phase and cavities indicative of a free fluid. This pilot project will pursue two studies relating to the genesis of microdiamonds. The first will be a study of inclusions in recently discovered diamonds from Saxonian Erzgebirge, Germany by TEM utilizing techniques similar to those we recently successfully developed for Kokchetav diamonds. Determinations of the composition and structure of inclusions in Erzgebirge diamonds coupled with our previous research on similar diamond formation from Kasakhstan will provide constraints for the evaluation of pressure conditions of diamond crystallization and an understanding of the depth to which crustal quartzofeldspathic rocks have been taken by tectonic processes. Such techniques could later be used in many other studies in both Earth and material sciences dealing with TEM research on the nanometric sized natural and synthetic minerals. The second study will be an experimental test of the diamond growth hypotheses suggested by De Corte et al. (1999) and Dobrzhinetskaya et al. (2001) to put constraints on the process(es) which may be responsible for preferential growth of hopper/hollow, skeletal, cuboidal and cube-octahedral diamonds in fluid-rich subduction-zone environments. Traditionally, experimental attempts to replicate natural diamond formation within the diamond stability field have been based on syntheses in metal-carbon systems in a highly reduced environment. Only recently has experimental modeling of natural diamonds at high P and T conditions been extended to the carbonate-carbon system in presence of H2O. The latter ideas have been corroborated by results of fluid-inclusion studies in natural diamonds. It is now believed that C-O-H fluids can be very effective in diamond formation. Our focus will be on experimental syntheses of microdiamonds in the diamond stability field to test current models of growth. We will conduct several pilot experiments using a starting material composed of natural mineral mix of composition similar to the natural diamond-bearing felsic gneiss from Kokchetav upon which we reported in Dobrzhenitskaya et al. (2001). We will explore the effects of presence and absence of a C-O-H fluid phase, although we expect little results under fluid-absent conditions. Oxygen fugacity will be controlled by buffering with metal-oxide pairs. This pilot study will allow us to develop initial understanding of some aspects of graphite-diamond kinetics in presence of H2O as well as technical aspects of such experimentation to enable subsequent pursuit. Our experimental program will be the first study of diamond growth under such conditions. Specifically, we will synthesize diamonds under relatively oxidizing conditions (conditions under which diamonds and carbonates coexist stably) using C-O-H fluid in starting material whose bulk composition is close to Kokchetav and Erzgebirge diamond-bearing quatzofeldspathic rocks.