Mineral disequilibrium in the Merensky Reef, western Bushveld Complex, South Africa: New Sm-Nd isotopic evidence

The Sm-Nd isotopic data from the Merensky Reef and its immediate footwall suggest that orthopyroxene and plagioclase are in isotopic disequilibrium with one another, such that plagioclase is enriched in radiogenic Nd relative to a Bushveld-aged reference isochron passing through the pyroxene and whole-rock data. The whole rocks and orthopyroxenes give epsilon(Nd) values at 2.06 Ga between -7.46 and -8.46, whereas the plagioclases have epsilon(Nd) values of -1.13 to -3.37. Orthopyroxene was derived from a liquid affected by pre-emplacement crustal contamination, and settled by density-driven accumulation into a liquid-crystal mush dominated by plagioclase, derived from a relatively uncontaminated previous liquid. Isotopic constraints on the potential contaminant favour pre-emplacement contamination by late Archaean granitoids, and also require that orthopyroxene preceded plagioclase on the liquidus of the incoming magma. The presence of isotopic disequilibrium suggests that isotopic compositions of whole-rock samples in cumulus rocks must be interpreted with caution.