DIAMOND SYNTHESIS FROM GRAPHITE IN THE PRESENCE OF WATER AND SiO2: IMPLICATIONS FOR DIAMOND FORMATION IN QUARTZITES FROM KAZAKHSTAN

Abstract

Recent studies of microdiamonds from orogenic belts related to continental collisions induced experimentalists to explore new crystallization media possible for diamond synthesis. This has led to considerable progress in diamond-synthesis experiments under high pressures and high tempera-tures. Diamond was found to grow in a wide variety of systems, which drastically differ from the metal-solvent-catalyst media that have been known since the 1950s in industrial diamond syntheses. The newer systems include a variety of melts consisting of silicate, carbonate, sulfur-carbon, as well as carbon-oxygen-hydrogen (C-O-H) supercritical fluid systems in the presence of different oxides. Our experimental program has focused on systems compositionally approximating natural sites of ultrahigh-pressure metamorphic rocks, where abundant microdiamonds occur. Yet to be explored are SiO 2 -rich systems that constitute an important diamond-bearing lithology in the world-famous Kokchetav ultrahigh-pressure metamorphic (UHPM) terrane. The purpose of this study is to deter-mine what effect the presence of SiO 2 has on the synthesis of diamond from C-O-H supercritical fluid. Experiments performed in a Walker-style multianvil apparatus at T = 1450–1500°C and P = 8–8.5 GPa on the Si-C-O-H system at different levels of oxygen fugacity (f O 2) have led us to the conclu-sion that diamond crystallization requires more reduced media than its counterpart synthesized in Mg-C-O-H and Ca-Mg-C-O-H systems at similar pressures, temperatures, and times.