Experiment constraints on orthopyroxene enrichment in the Kaapvaal craton lithospheric mantle

Abstract

Peridotite xenoliths from the Kaapvaal cratonic lithosphere are characterized by high modal orthopyroxene (Opx), which is inconsistent with the mineralogy of residues of partial melting of primitive mantle. The high orthopyroxene content has been explained by the reaction between Opx-poor peridotite and silica-rich melts. To assess and further constrain this model, we conducted a series of experiments on the interaction of Si-rich melt with Opx-poor harzburgite at 1.5 GPa and 1100 to 1250 °C. Orthopyroxene is enriched in all experiments. In the low temperature run at 1100 °C, melt is consumed and phlogopite is precipitated with the generation of orthopyroxene. Mg# (atomic 100*Mg/(Mg+Fe)) values in olivines also exhibit a decrease compared with the starting olivines. At temperatures of ≥1200 °C, phlogopite is absent as the temperatures are higher than its upper stability field. The residues are composed of high modal orthopyroxene and high Mg# olivine. Only the residues in the high temperature runs are consistent with Kaapvaal cratonic mantle xenoliths. Combined our experiments with the distribution, geochronological and geochemical data of the xenoliths, we propose a two-stage ‘metasomatism-depletion’ model to explain the generation of Opx-rich xenoliths in Kaapvaal craton. In the early Archean, high-degree melting of primitive mantle produced a highly depleted lithospheric mantle, which was Opx-poor. Subsequently, Si-rich melts/fluids were released from the subducting oceanic slabs to interact with the Opx-poor mantle, generating an Opx-rich, but hydrous and fertile mantle. The metasomatic mantle is similar to the result of the run at 1100 °C. In the late Archean, the metasomatic mantle was re-melted in post-collisional extensional environments, which produced voluminous lavas with residues of highly depleted, Opx-rich peridotites. The Opx-rich residual peridotites are consistent with the runs at ≥1200 °C. Thus, Opx enrichment in the Kaapvaal craton lithospheric mantle can be well explained by the two-stage ‘metasomatism-depletion’ model.