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

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 pa tterns 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 replacemen t rims on some garnets consist of garnet with low Ca and Cr, but high Mg, T i 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 degrees C. Modelling of zoning pr ofiles suggests that the process can be divided into three parts: (a) inwar ds diffusion of Ca. Zr and Y over periods of 10,000-30,000 years, from a fl uid depleted in Ti, Ca 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 times cales prior to eruption. The sinuous REE patterns of the garnet cores are r egarded as "primary" features reflecting an ancient metasomatic event super imposed on a depleted protolith. The high Zr/Y, Zr/Ti and Zr/Ca of the flui ds corresponding to stage (a) are ascribed to the presence of phlogopite an d garnet in the matrix near the fluid source (presumed to be a melt, possib ly a kimberlite precursor), leading to the development of concentration fro nts in the percolating fluid. The overgrowths of stage (b) appear to coinci de with the precipitation of phlogopite in the rock. The low Ca of the flui d responsible for the secondary replacement rims of stage (c) may reflect t he late precipitation of clinopyroxene or Ca-carbonate as part of the metas omatic assemblage. These processes have significantly modified the modal, m ajor- and trace-element composition of the mantle volume sampled by the Wes selton kimberlite, within <1 Ma of eruption. Recognition of such effects an d their distribution in time and space is essential to understanding of the evolution of the subcontinental lithospheric mantle.