Orthopyroxene stability along the peridotite solidus and the origin of cratonic lithosphere beneath southern Africa

Abstract

The stability of orthopyroxene within the melting interval of natural fertile peridotite between 4.5 and 6.5 GPa was investigated. Orthopyroxenes rich in CaO and Al 2O 3 were found stable at and above the fertile peridotite solidus in this pressure range. These results contradict a previous melting study of fertile peridotite, which found orthopyroxene absent in the melting interval above 4.5 GPa, but are consistent with phase equilibrium investigations which show orthopyroxene stable with olivine between 4.5 and 6.0 GPa along the liquidi of komatiites in both simple and complex systems, and near the solidus of depleted abyssal and cratonic peridotites. The new melting relationships for fertile peridotite are used in a simplified model for the origin of low-temperature, coarse-grained, garnet peridotite xenoliths from the Kaapvaal craton, southern Africa. The model depicts cratonic peridotites as former harzburgitic residues of Archean komatiitic liquid extraction which cooled to become lherzolites at ambient mantle temperatures within the mantle root of the craton. The large and varied abundance of orthopyroxene, and published petrographical and bulk chemical observations for cratonic peridotites from the Bultfontein and Matsoku pipes, southern Africa, are generally explainable by this ‘exsolution’ origin. Cratonic peridotites from other localities in the Kaapvaal craton, however, may contain excess modal pyroxene and/or garnet of uncertain origin, which cannot easily be accounted for by the ‘exsolution’ hypothesis.