TRACE-ELEMENT ANALYSES OF FLUID-BEARING DIAMONDS FROM JWANENG, BOTSWANA

Fibrous diamonds from Botswana contain abundant micro-inclusions, whic h represent syngenetic mantle fluids under high pressure. The major el ement composition of the fluids within individual diamonds was found t o be uniform, but a significant compositional variation exists between different diamond specimens. The composition of the fluids varies bet ween a carbonatitic and a hydrous endmember. To constrain the composit ion of fluids in the mantle, the trace element contents of thirteen mi cro-inclusion-bearing fibrous diamonds from Botswana was studied using neutron activation analysis. The concentrations of incompatible eleme nts (including K, Na, Br, Rb, Sr, Zr, Cs, Ba, Hf, Ta, Th, U, and the L REEs) in the fluids are higher than those of mantle-derived rocks and melt inclusions. The compatible elements (e.g., Cr, Co, Ni) have abund ances that are similar to those of the primitive mantle. The concentra tions of most trace elements decrease by a factor of two from the carb onate-rich fluids to the hydrous fluids. Several models may explain th e observed elemental variations. Minerals in equilibrium with the flui d were most likely enriched in incompatible elements, which does not a gree with derivation of the fluids by partial melting of common perido tites or eclogites. Fractional crystallization of a kimberlite-like ma gma at depth may yield carbonatitic fluids with low mg numbers (atomic ratio [Mg/(Mg+Fe)]) and high trace element contents. Fractionation of carbonates and additional phases (e.g., rutile, apatite, zircon) may, in general, explain the concentrations of incompatible elements in th e fluids, which preferably partition into these phases. Alternatively, mixing of fluids with compositions similar to those of the two endmem bers may explain the observed variation of the elemental contents. The fluids in fibrous diamonds might have equilibrated with mineral inclu sions in eclogitic diamonds, while peridotitic diamonds do not show ev idence of interaction with these fluids. The chemical composition of t he fluids in fibrous diamonds indicates that, atp, T conditions that a re characteristic for diamond formation, carbonatitic and hydrous flui ds are efficient carriers of incompatible elements.