Harzburgite to lherzolite and back again: metasomatic processes in ultramafic xenoliths from the Wesselton kimberlite, Kimberley, South Africa

Abstract

Garnets from phlogopite harzburgite xenoliths from the Wesselton kimberlite show zoning from low-Ca harzburgitic cores to rims with lherzolitic Ca-Cr relations. Garnet cores are depleted in Y and HREE, but have sinuous REE patterns enriched in the MREE. Rimwards increase in Ca and decrease in Cr and Mg is accompanied by increases in Zr, Y, Ti and HREE. Secondary replacement rims on some garnets consist of garnet with low Ca and Cr, but high Mg, Ti and HREE. The zoning, and the secondary replacement rims, are attributed to different stages of a metasomatic process that has converted harzburgite to lherzolite, at temperatures near 1000 °C. Modelling of zoning profiles suggests that the process can be divided into three parts: (a) Inwards diffusion of Ca, Zr and Y over periods of 10,000–30,000 years, from a fluid depleted in Ti, Ga and Y; (b) formation of overgrowths high in Ca, Zr, Y and Ti, followed by annealing over periods of several thousand years; (c) formation of secondary reaction rims of low-Ca garnet, on very short timescales prior to eruption. The sinuous REE patterns of the garnet cores are regarded as “primary” features reflecting an ancient metasomatic event superimposed on a depleted protolith. The high Zr/Y, Zr/Ti and Zr/Ca of the fluids corresponding to stage (a) are ascribed to the presence of phlogopite and garnet in the matrix near the fluid source (presumed to be a melt, possibly a kimberlite precursor), leading to the development of concentration fronts in the percolating fluid. The overgrowths of stage (b) appear to coincide with the precipitation of phlogopite in the rock. The low Ca of the fluid responsible for the secondary replacement rims of stage (c) may reflect the late precipitation of clinopyroxene or Ca-carbonate as part of the metasomatic assemblage. These processes have significantly modified the modal, major- and trace-element composition of the mantle volume sampled by the Wesselton kimberlite, within <1 Ma of eruption. Recognition of such effects and their distribution in time and space is essential to understanding of the evolution of the subcontinental lithospheric mantle.