A new model for the evolution of diamond-forming fluids: Evidence from microinclusion-bearing diamonds from Kankan, Guinea

Abstract We report major, trace and volatile element compositions for seven coated diamonds from Kankan, Guinea. Four diamonds trapped microinclusions with high-Mg carbonatitic high-density fluids (HDFs) and three carry silicic to low-Mg carbonatitic HDFs. Mineral inclusions and nitrogen aggregation in the cores indicate growth at ∼ 5 GPa, 1100–1200 °C. Significant differences in the nitrogen aggregation state (A + B, A and A + C centres in cores, inner coats and rims of three diamonds) indicate growth during multiple events, well separated in time. All diamonds show radial chemical evolution. The HDFs in high-Mg carbonatitic diamonds grow more carbonatitic towards the rim, the silicic one grows more silicic, and the zoned diamonds show contrasting trends in the different parts of the coats. The HDFs are highly enriched in Ba, Th, U and the light REE; enrichment in K, Rb and Cs is somewhat lower. Normalized Sr, Ti and Y concentrations are low relative to the middle and heavy REE. Trace elements in the low-Mg carbonatitic to silicic suite vary systematically with increasing silicic character of the HDFs. The high-Mg HDFs commonly deviate from such arrays. A new model is proposed for the evolution of diamond-forming fluids, suggesting that high-Mg carbonatitic HDFs are formed through interaction of saline HDF with peridotitic rock, while low-Mg silicic to carbonatitic HDFs are the result of penetration of K-rich hydrous fluid into eclogitic rock. Another possible source for the high potassium content of the HDFs is K-bearing phases in the mineral assemblage of the rock. Sinusoidal REE patterns that are common in garnet inclusions in monocrystalline diamonds may form when HDFs with steep REE patterns are introduced into a LREE-depleted harzburgite. Unless garnet and diamond were formed in two separate events, this similarity suggests growth of monocrystalline diamonds from fluids similar to the HDFs.