Diamonds, dunites, and metasomatic rocks formed by melt/rock reaction in craton roots

Abstract

The thick mantle lithosphere beneath cratons consists of strongly reduced rocks that have reacted with oxidized melts. These low-silica, incipient melts are rich in CO2 and H2O and react with surrounding rocks forming an enriched zone at the base of the lithosphere, which is the source region for many diamonds. Here, we reproduce these reactions in novel experiments in which oxidised, hydrous carbonate-rich melts are reacted with reduced, depleted peridotite at 5 GPa pressure. Results explain several key features of the mantle sample in kimberlites as products of a single process, namely the formation of diamonds, phlogopite and alkali-amphibole bearing rocks, iron-rich dunites, and garnets and clinopyroxenes with pyroxenitic compositions. Initially, redox freezing occurs where melts meet the reduced peridotite, depositing diamonds and associated garnet and clinopyroxenes. The spreading reaction front leaves behind Fe-rich dunite, and crystallizing phlogopite and amphibole when the melt solidifies at the reaction front.