Metamorphic diamonds: Mechanism of growth and inclusion of oxides

We report a detailed series of electron microscope observations of metamorp hic microdiamonds included in and separated from garnets and zircons from a single specimen of garnet-biotite-feldspar gneiss from the Kokchetav massi f, Kazakhstan. The morphology of the diamonds ranges from skeletal forms co mposed of thin {111} plates through cuboid and octahedral forms. Included w ithin the diamonds is a diverse suite of nanometric oxides, suggesting that the C-O-H fluid from which the diamonds grew may have carried chemical com ponents derived from both the sediments and the mantle. The spectrum of mor phologies and their abundant tiny inclusions can all be explained by a simp le model based on the ratio of the rate at which {111} plates grow and the rate of random nucleation of new plates at their edges.