EXPERIMENTAL SIMULATION OF DIAMOND CRYSTALLIZATION IN QUARTZITE AT HIGH PRESSURES AND TEMPERATURES

Microdiamonds occur within ultra-high pressure metamorphic rocks exposed within collisional orogenic belts in Kazakhstan (Sobolev and Shatsky, 1990), China (Xu et al., 1992, Yang et al., 2003), Norway (Dobrzhinetskaya et al., 1995, Van Roermund et al., 2002), Germany (Massonne, 1998) and other locations. Various researchers have demonstrated that such diamonds are crystallized from a multicomponent COH-rich supercritical fluid, and that their compositional diversity reflects an intermixture of crustal and mantle components (Stokhert et al., 2000; Dobrzhinetskaya et al., 2000), although some have postulated the diamonds grew from a carbonate melt (Shatsky et al.,2003). Diamond is an important indicator of pressure, which determines the depth to which sedimentary rocks could be subducted during continental collision, and therefore, has a great significance for further modeling of plate tectonic events. Because the graphite-to-diamond transformation is sluggish due to a high kinetic barrier, synthetic diamonds were usually produced at high P & T in the presence of a metal catalysts. Only recently diamonds were synthesized from graphite (e.g. Yamaoka et al., 2000; Dobrzhinetskaya et al., 2002) and carbonate material (Pal'anov et al., 1999) in the presence of water in the diamond stability field. Diamond was easily synthesized from graphite in the presence of H2O in Ca or Mg rich systems (Renfro and Dobrzhinetskaya 2002) at P=8-8.5 GPa, T=1673-1773K without any partial melt. 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. However, we have also faced a problem with diamond synthesis at similar conditions in silica-rich media, leading to our assumption that silica plays a role as an anti-catalyst by decreasing the speed of diamond nucleation. We report here our new results on diamond synthesis from graphite in the presence of H2SiO3 as a source of H2O in silica-rich media. Such a bulk composition is an equivalent of the natural quartzite where many Kokchetav diamonds occur. Experiments were conducted at P=8GPa, T=1773K in a Walker style multianvil apparatus. Diamond crystals of micron scale were crystallized on a graphite disc inserted into silica-H2SiO3 powder. Speed of growth is calculated.