The growth of fibrous, cloudy and polycrystalline diamonds

Abstract

We have performed an experimental study designed to reproduce the growth of fibrous/coated, cloudy and polycrystalline diamonds in hydrous silica–carbonate-rich fluids. Multi-anvil experiments have been conducted at pressures between 7 and 9GPa and temperatures between 1200 and 1675°C, with run durations ranging from a few hours to a few days. Significant diamond growth was observed in all experiments. The growth was either homogeneous, with new diamond crystals growing spontaneously in the fluid, or heterogeneous, by rim overgrowth on pre-existing diamond seeds. A wide range of the morphology observed in nature has been reproduced and identified in the run products (aggregates, fibrous, and monocrystalline areas). For the first time, multi-phase inclusions, closely mimicking those found in natural samples, have been experimentally produced during diamond growth. These arise either from a mixture of coexisting liquids (hydrous silicate melt and aqueous fluid) or from a unique supercritical fluid resulting from the total miscibility of the two fluids, which occurs at high pressure and temperature. These results strengthen the hypothesis that diamonds grow from carbon and water-rich fluids and support models proposing episodic growth from successive fluids. The results raise the possibility that natural inclusions with different compositions found in a single diamond may form through the trapping of immiscible liquids, which had locally unmixed prior to trapping. Finally, considering the composition of the diamond’s parent fluids, a subduction origin is likely, at least for fibrous/coated, cloudy diamonds, and to some extent, for polycrystalline diamonds also.