Geochemistry and microstructure of diamondites

Polished plates of diamondites (polycrystalline diamond aggregates) from Africa and in eclogites from Lesotho, were prepared for study of internal structure and in-situ geochemical analysis, to understand diamond-forming processes. The diamondites in eclogites are enclosed in large garnet grains, while monocrystalline diamonds occur in cpx-bearing metasomatic veins that crosscut the garnets. CL imaging and Electron BackScattered Diffraction (EBSD) show that diamondites underwent plastic deformation, recrystallization and grain-size reduction under mantle conditions. Therefore they should not be interpreted as products of primary crystallisation. Oxygen-isotope analysis of interstitial garnets and zoning in carbon isotope composition indicate that the diamondites have been deformed and recrystallised during the penetration of mantle fluids/melts, which provided carbon for the formation of diamond in metasomatic veins. Interstitial silicates reacted with metasomatic fluids percolating along diamond grain boundaries; interstitial garnets have higher mg0.78) than original inclusions (0.70 to 0.53). The REE patterns of eclogitic garnets become flatter (higher LREE/HREE) with increasing metasomatism. The compositions of large garnet grains from Lesotho (mg.78-0.82) are similarto those of interstitial garnets from diamondites. Coexisting cpx and gnt inclusions from the least deformed diamondites give T=1200 C for diamond formation.