Fluid-inclusions in fibrous diamonds from Jwaneng (Botswana) contain water, carbonates, silicates, apatite, and CO2. Average compositions of fluids trapped in individual diamonds span a wide range, and vary linearly and continuously between two endmember compositions, a carbonatitic fluid rich in carbonate, CaO, FeO, MgO, and P2O2, and a hydrous fluid rich in water, SiO2, and Al2O2. K2O contents are high in both endmembers. The mg numbers (Mg/(Mg + Fe)) of the trapped fluids are low (0.55-0.44) and decrease towards the hydrous endmember. Fluid compositions are broadly similar to those reported for Zairean diamonds, but cover a wider range. Intra-diamond compositional variation is limited. We examine three simple models for the formation and evolution of the fluid in the earth's mantle: 1. (1) Mixing of hydrous and carbonatitic fluids, 2. (2) partial melting of a carbonate-bearing source rock, and 3. (3) fractional crystallization of a carbonatitic melt at depth. The low mg numbers of both endmembers suggest that the source rocks for the melting scenario must be more Fe-rich than common mantle peridotites. Fractional crystallization of ferroan dolomite and magnesite with small amounts of rutile and apatite can account for the observed variation of most elements. Crystallization of an additional K-rich phase is needed to explain the potassium trend. Recent experimental studies have demonstrated that carbonatitic melts and hydrous fluids may exist in equilibrium 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.