Mantle fluid evolution—a tale of one diamond

Microinclusions analyzed in a coated diamond from the Diavik mine in Canada comprise peridotitic minerals and fluids. The fluids span a wide compositional range between a carbonatitic melt and brine. The diamond is concentrically zoned. The brine microinclusions reside in an inner growth zone and their endmember composition is K19Na25Ca5Mg8Fe3Ba2Si4Cl32 (mol%). The carbonatitic melt is found in an outer layer and its endmember composition is K11Na21Ca11Mg26Fe7Ba2Si10Al3P2Cl5. The transition in inclusion chemistry is accompanied by a change in the carbon isotopic composition of the diamond from −8.5‰ in the inner zone to −12.1‰ in the outer zone. We suggest that this transition reflects mixing between already evolved brine and a freshly introduced carbonatitic melt of different isotopic composition. The compositional range found in diamond ON-DVK-294 is the widest ever recorded in a single diamond. It closes the gap between brine found in cloudy octahedral diamonds from South Africa and carbonatitic melt analyzed in cubic diamonds from Zaire and Botswana. Thus, all microinclusions analyzed to date fall along two arrays connecting the carbonatitic melt composition to either a hydrous-silicic endmember or to a brine endmember. This connection suggests that many diamonds are formed from fluids derived form a mantle source not significantly influenced by local heterogeneities.