Brine inclusions in diamonds: a new upper mantle fluid

Micro-inclusions in cloudy diamonds from Koffiefontein consist of three main types: silicates, carbonates and brine inclusions. The silicates belong to either the eclogitic or the peridotitic paragenesis and both are associated with carbonates and brine. The brine composition is roughly (K,Na)8(Ca,Fe,Mg)4SiO(CO3)4Cl10(H2O)28–44. Average mass proportions are about 30–42% water, 19–22% chlorine, 14–17% sodium and potassium, 22–25% Fe–Ca–Mg–carbonates, and 3–4% silica. The brine composition is distinct from that of fluids trapped in fibrous diamonds mainly by its high chlorine and low silica content. The close association of carbon-bearing brine, silicate minerals and diamonds suggests that such brines are important for diamond growth in both eclogitic and peridotitic environments. The similarity of brine composition in both environments may indicate that diamonds of both suites grew in a single event.