Hydrous Silicate Melts: A new Growth Medium for Diamond?
Creators: Fagan, Andrew J.
Publisher: name: University of Alberta Library
Year: 2009

<p>Natural peridotitic diamonds form in the &#39;diamond window&#39; (DFW), an area of the upper mantle bounded by temperatures of 900-1400°C and pressures between 4 and 7 GPa. This study has experimentally tested and shown the viability of a hydrous, halide bearing silicate melt as a possible diamond growth medium.</p>

<p>Many mechanisms have been suggested for diamond growth in the Earth&#39;s mantle, and despite much work in the last 40 years, this issue is still unresolved. Experimental petrology can help to determine the effect of different chemical constituents on diamond growth. Many systems have been investigated, including carbonate, sulphide and silicate. We have conducted experiments in the system MgO-Si02-C-H20 to test the efficacy of a hydrous silicate melt (HSM) in nucleating and growing diamond, in a C-saturated system. Natural diamonds have been found to contain alkali halides within their fluid inclusions; these trapped &#39;brine-like&#39; mantle fluids have been cited as &#39;diamond forming fluids&#39;. For this reason, we have also evaluated the effect of alkali halides in catalyzing diamond-forming reactions by investigating the addition of KC1 and NaCl. Previous studies in carbonate bearing systems have concluded that alkali halides have a catalytic effect on diamond formation reactions; this study tests the extension of this proposal to hydrous silicate melts. </p>

<p>Experiments were conducted in a multi-anvil apparatus housed within the CM. Scarfe Laboratory for Experimental Petrology at the University of Alberta. In these experiments, diamond was successfully grown on seed crystals at temperatures of 1400-1600°C and pressures of 7 GPa, over 4 hour durations. </p>

<p>The HSM precipitated diamonds coexisting with enstatite and forsterite crystals in a simplified upper mantle assemblage. H20 plays an important role in the diamond forming reaction. The presence of H20 lowered the silicate solidus and allowed melting to occur at lower temperatures. This is especially relevant for experiments bounded by the physical conditions within the diamond window. Spontaneous nucleation of diamond was observed during the course of these experiments along with the formation of diamond coats, &lt;10um thick, created by the amalgamation of numerous single octahedral crystals. The addition of KC1, to the HSM system enabled diamond to form 200°C cooler than the previously published minimum experimental temperature. We observed that temperature is critical to the formation of diamond. The presence of the KC1 likely affects the diffusion rate of carbon, allowing diamond growth to proceed at a lower temperature. KC1 shows a clear effect on diamond growth, producing larger crystals at a lower temperature compared to an un-fluxed system. The NaCl system is different; in these experiments the diamond seed crystals show triangular and hexagonal etch pits, a common feature of resorption on natural stones and facets with small dissolution holes spread homogeneously over the surface of the seeds. Thus we suggest that NaCl is a possible diamond growth inhibitor within a silicate growth system. </p>

<p>This experimental study demonstrates that hydrous silicate melts, especially containing Kbearing alkali halides, are a viable medium for diamond growth in the Earth&#39;s upper mantle. We suggest that the necessary components used in this study could be present and can easily interact with one another in the sub-continental lithospheric mantle (SCLM), inside an imbricated oceanic slab. The gradual heating of the old slab will cause dehydration and eventually the generation of a HSM at P-T conditions inside the DFW. </p>