Chemical, optical, and isotopic investigations of fibrous diamonds from Brazil

One coated and two cubic fibrous diamonds from Brazil carry microinclusions that contain fluids with a wide range of composition. Fluid chemistry is similar to that found in diamonds from Botswana and varies between a carbonatitic end-member rich in carbonate, CaO, FeO, MgO, and K₂O and a silicic end-member rich in water, SiO₂, Al₂O ₃, and K₂O. The main difference from the Botswanan set is the wider range of compositions sampled by individual diamonds. One diamond, BR-5, is unique and records growth from two contrasting compositions. The inner part grew from silicic fluid, and the outer part, from a carbonate-rich one. Carbon isotopic compositions vary between diamonds and radially within individual diamonds. Silicic fluids are associated with heavier isotopic compositions (most analyses >-5‰); carbonate-rich fluids, with lighter values (most analyses <-5‰). Radial evolution in different diamonds is contrasting but is mostly toward the median value of -5‰. Nitrogen isotopes show more scatter but correlate positively with carbon isotopic composition. It is suggested that fluid chemistry and diamond isotopic composition are affected mainly by fractionation of carbonates and diamonds (and, possibly, silicates). Separation of CO₂ and interaction of the fluid with host-rock carbon may also be important in controlling the isotopic composition.