Trace-element geochemistry of diamondite: Crystallisation of diamond from kimberlite-carbonatite melts

Quantitative trace-element analyses of 38 elements have been carried out in 11 samples of polycrystalline diamond (diamondite) from southern Africa by LAM-ICPMS. The samples are divided into “peridotitic” and “eclogitic” types based on the compositions of garnet and clinopyroxene intergrown with the diamond. The trace-element patterns of the diamondite samples reflect mixtures of diamond and submicroscopic inclusions of fluid ± solid phases, and are interpreted as representing the fluids from which the diamondite crystallised. They are similar to parent-melt compositions calculated from macroscopic garnet and clinopyroxene inclusions in the diamondites, and suggest that the diamondites, like many fibrous diamonds, crystallised from melts in the kimberlite-carbonatite spectrum. Multi-element spikes seen in the time-resolved LAM-ICPMS analyses indicate the presence of several, probably submicroscopic, solid phases: garnet, clinopyroxene, an Y-Yb rich (fluoride?) phase, a sulfide rich in Cu-Pb-Zn-Co-Ni, a LIMA-type phase, a carbonate, ilmenite, chromite and mica. The similarity in trace-element patterns of the “peridotitic” and “eclogitic” diamondites suggests that both types have crystallised from the same type of metasomatic fluid. This fluid may have evolved from “peridotitic” to “eclogitic” by the removal of chromite ± sulfide ± ilmenite.