Anomalously high amounts of orthopyroxene are an unexpected feature of some depleted peridotite xenoliths from the cratonic lithosphere. This investigation presents new petrographic, geochemical and thermodynamic modelling data for silica-rich spinel harzburgites and one garnet–spinel harzburgite from the Kaapvaal Craton, South Africa, to constrain the pressure–temperature (P–T) conditions of orthopyroxene formation. At the time of sampling by the host kimberlite, the garnet–spinel harzburgite resided on the geotherm at 25 kbar, 765°C, wheras the compositionally similar garnet-free samples probably equilibrated at broadly comparable P–T conditions. The samples are subdivided into ‘bronzite type’ peridotites characterised by orthopyroxene with micrometre scale exsolutions of spinel ± clinopyroxene, and ‘normal type’ peridotites in which orthopyroxene does not contain visible exsolved phases. The presence of these exsolved phases indicates unmixing from a more Al-Cr-(Ca)-rich precursor orthopyroxene i.e. an original orthopyroxene that precipitated from, and was, in equilibrium with the Si-enriching agent at high temperature, before cooling and undergoing sub-solidus re-equilibration to form the exsolved orthopyroxene we observe now. Major and trace element compositions, geochemical interpretation and thermodynamic modelling of the samples show that the precursor orthopyroxene coexisted with melt at high pressure and temperature (~40–50 kbar, 1600–1700°C) within the garnet stability field. We consider these to be the conditions of orthopyroxene formation, and thus silica enrichment. These conditions, together with the refractory composition of the host peridotite (olivine forsterite contents > 92) suggest that silica enrichment is a secondary process potentially resulting from interaction with komatiitic melts. Our observations demonstrate that orthopyroxene enrichment in spinel-facies peridotites is not limited to low-pressure processes but can also occur via high-pressure melt–rock interaction, offering new insights into the thermal and chemical evolution of the sub-cratonic lithospheric mantle.
