Variations in trapping temperatures and trace elements in peridotite-suite inclusions from African diamonds: evidence for two inclusion suites, and implications for lithosphere stratigraphy

The proton microprobe has been used to mea- sure the trace element contents of Cr-pyrope, olivine and orthopyroxene inclusions (DI) in >60 diamonds from southern Africa, and in concentrate garnets from the host kimberlites. Olivine inclusions show a negative correlation of Ca with Fo content, but olivines coexisting with very subcalcic garnets are anomously depleted in Ca relative to Fo. The maximum and median values of Ti, Zn, Ga, Zr and Y in DI garnets decrease as Ca decreases relative to Cr, consistent with increasing deple- tion by removal of silicate melts. However, the most subcalcic garnets are anomously enriched in Sr (and LREE). The contrasting depletion in Zr, Y, Ti etc. and enrichment in Sr is not consistent with a single-stage depletion event. The extreme Ca depletion of the most subcalcic garnets and their coexisting olivines, and the Sr enrichment, are interpreted as the result ofmetasoma- tism by a carbonatitic fluid following depletion. Trap- ping temperatures of garnet D1, estimated by nickel ther- mometry range from 950 to > 1500 ~ C. Most DI with T>1200~ are lherzolitic, rather than subcalcic. In most pipes studied, the trapping T of DI garnets is high- er than the T range of equivalent garnets from concen- trates. The high Ts are interpreted as reflecting the for- mation of diamonds during short-lived thermal pulses, followed by cooling toward a conductive geotherm. The T distribution of calcic and subcalcic garnets in concen- trates from kimberlites suggests that lherzolite and harz- burgite are intimately intermixed over the depth range 150-180km beneath the Kalahari craton. The abun- dance of calcic garnets with T>1200~ C suggests the presence of significant amounts of lherzolite at greater depths. This deeper lherzolite may be associated with eclogite, and may be the source region for some high-T lherzolitic DI garnets.