The role of fluid in the growth of fibrous diamond: a study of African and Canadian coated diamonds

This multi-disciplinary study determines the composition and structure of mantle fluid inclusions in natural fibrous diamonds, and their role in diamond growth. Coated diamonds from Mbuji Mayi (Democratic Republic of Congo) and Panda (Canada) were analysed by electron microprobe, laser ablation inductively coupled mass spectrometry and infrared spectroscopy. Carbonate and KC1 are important components of the complex fluid. Therefore, KCI-K2CO3 was used as a solvent catalyst for graphite in high-pressure high-temperature diamond growth experiments. Grown diamonds have skeletal morphologies growth was rapid and occurred at temperatures as low as 1050 °C. Fluid-inclusions in Panda and Mbuji Mayi diamond coats are dominated by liquid water and carbonate. The absence of silicate melt indicates fibrous diamond growth occurred below or close to the host rock solidus. Three end-member fluid compositions are recognised: Fe-Mg-Ca carbonate, KC1 and alumnosilicate. The trace element composition of the Mbuji Mayi coat fluids is consistent with K-rich carbonatite. The carbonate fluid component was derived externally before infiltration into peridotitic and eclogitic growth environments. The silicate fluid is restricted to eclogitic samples and is thought to have been derived locally by reactions between the oxidising fluid and more reduced mantle minerals. Carbonate reduction and crystallisation concentrated KC1. Panda diamond coats contain silicate inclusions whose chemistries indicate formation in eclogitic and peridotitic (lherzolitic) hosts. There fore, diamond coats grow in the same paragenetic environments as octahedral diamonds. Inclusion geothermobarometry indicates that peridotitic inclusions equilibrated at 930-1010 °C and 4.2-4.6 GPa. These conditions are below the lherzolite solidus and are 200 °C lower than inclusions in Panda Octahedral diamonds. A model is presented in which fibrous diamonds grow in a solid host rock containing older octahedral diamonds, following the influx of H2O-rich carbonate melt. This melt metasomatised and fertilised the host rock. Diamond was precipitated by the reduction of carbonate and nucleated on diamond cores