Growth of diamond in hydrous silicate melts

Abstract

This study demonstrates that a hydrous, halide bearing silicate melt is a viable medium for diamond growth. Experiments were conducted in the MgO–SiO2–H2O–C ± KCl ± NaCl system, which was used as a model for harzburgitic mantle. In no case did we observe crystals that could be interpreted as spontaneously nucleated, but growth of diamond on seed crystals at 1,400–1,600°C and 7 GPa in experiments of 4 h duration was observed. The addition of KCl to the system produced crystallization of diamond at temperatures as low as 1,400°C. At higher temperatures, larger growth features were produced than those that seen in the KCl-free system at the same conditions. The NaCl-bearing system is different; in these experiments, the diamond seed crystals show evidence of possible dissolution and layer growth, albeit more subdued growth than in the KCl system. Therefore, NaCl may be an inhibitor of diamond growth in a hydrous silicate melt. Based on these results, hydrous silicate melts could play a role in formation of diamond in either deep subduction zones, or above slabs imbricated against a lithospheric ‘root’ in the sub-continental lithospheric mantle. The water and halide necessary for their formation could be transported into the mantle in hydrous phases such as serpentine in subducting lithospheric slabs. Dehydration of serpentine at >200 km depth would release hydrous, halide-bearing fluids into the overlying mantle wedge or lithospheric root, triggering melting at conditions similar to those of the formation of natural diamond.