Quantifying the P-T conditions of silica-enrichment in Archean cratonic spinel-facies peridotites.

Anomalously high orthopyroxene is an unexpected feature of some depleted peridotite xenoliths from the cratonic lithosphere.This investigation presents new petrographic, geochemical, and thermodynamic modelling data on silica-rich spinel harzburgites and one garnet-spinel harzburgite from the Kaapvaal Craton, South Africa, to constrain the pressuretemperature (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, 765C, while 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