HYDROUS AND CARBONATITIC MANTLE FLUIDS IN FIBROUS DIAMONDS FROM JWANENG, BOTSWANA

Fluid-inclusions in fibrous diamonds from Jwaneng(Botswana) contain wa ter, carbonates, silicates, apatite, and CO2. Average compositions of fluids trapped in individual diamonds span a wide range, and vary line arly and continuously between two endmember compositions, a carbonatit ic fluid rich in carbonate, CaO, FeO, MgO, and P2O5, and a hydrous flu id rich in water, SiO2, and Al2O3. K2O contents are high in both endme mbers. The mg numbers (Mg/(Mg + Fe)) of the trapped fluids are low (0. 55-0.44) and decrease towards the hydrous endmember. Fluid composition s are broadly similar to those reported for Zairean diamonds, but cove r a wider range. Intra-diamond compositional variation is limited. We examine three simple models for the formation and evolution of the flu id in the earth's mantle: (1) Mixing of hydrous and carbonatitic fluid s, (2) partial melting of a carbonate-bearing source rock, and (3) fra ctional crystallization of a carbonatitic melt at depth. The low mg nu mbers of both endmembers suggest that the source rocks for the melting scenario must be more Fe-rich than common mantle peridotites. Fractio nal crystallization of ferroan dolomite and magnesite with small amoun ts of rutile and apatite can account for the observed variation of mos t elements. Crystallization of an additional K-rich phase is needed to explain the potassium trend. Recent experimental studies have demonst rated that carbonatitic melts and hydrous fluids may exist in equilibr ium with metasomatized peridotite. The data presented here provide the first direct evidence for the existence of both fluids in the diamond stability field, deep in the upper mantle.