Fluid and mineral inclusions in cloudy diamonds from Koffiefontein, South Africa

Silicates, carbonates and brine were detected in microinclusions in cloudy diamonds from Koffiefontein. The silicates belong to either the eclogitic or the peridotitic paragenesis. They are similar in composition to the larger mineral inclusions found in other Koffiefontein diamonds and record similar pressures and temperatures of formation (1000-1200°C, 4-6 GPa). The clouds also carry additional mineral phases that were not detected before in Koffiefontein: phlogopite, high-Si mica, an Al-Mg-rich phase and carbonates. In many cases silicate minerals coexist together with brine in individual microinclusions. Brines associated with either eclogitic or peridotitic minerals have similar composition and both are rich in water, Cl, K and carbonates (Izraeli et al., 2001). The above findings, together with the high proportion of cloudy diamonds in Koffiefontein, suggest that brine penetration into neighboring peridotitic and eclogitic rocks was pervasive. The high carbon content of the brine (as dissolved carbonates) initiated diamond growth in both rock types, most, on preexisting diamonds that now form inclusion-free cores. The low aggregation state of nitrogen in the cloudy diamonds, relative to other diamonds from Koffiefontein indicates either a somewhat lower temperature of storage in the mantle (which may be the reason for the presence of brine rather than melt) or a young event of cloudy diamond formation.