Growth of diamond in hydrous silicate melts |
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| Authors: | Andrew J Fagan Robert W Luth |
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| Institution: | (1) C.M. Scarfe Laboratory of Experimental Petrology, Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, T6G 2E3, Canada |
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| 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. |
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