The metasomatism of sub-cratonic peridotites by a slab-derived fluid – Implications for cratons’ evolution

Metasomatized peridotites entrained by kimberlites are of great importance in understanding the geochemical changes occurring in the sub-continental lithospheric mantle (SCLM) during percolation and interaction of various fluids with the harzburgitic mantle. To simulate a scenario where slab-derived silicic fluid infiltrates the SCLM, high-pressure, high-temperature experiments were performed on a harzburgite + silicic fluid system at an 80:20 rock:fluid ratio (bulk ∼4.5 wt% H₂O and ∼3 wt% CO₂). Experiments were performed at 3–6 GPa and 900–1200 °C on a rocking multi-anvil apparatus, conditions corresponding to the deep SCLM. The compositions of the mineral assemblages were analyzed in order to evaluate the formation of the metasomatic rocks at these conditions. At 3–6 GPa between 900–1000 °C, phlogopite-K-richterite- peridotites (PKP) were formed, at 3–5 GPa and 1100 °C, phlogopite-peridotites (PP), and at 6 GPa between 1100–1200 °C, garnet-phlogopite peridotites (GPP) were formed. Similar mineral assemblages and mineral chemistry are found in metasomatic xenoliths from Kimberly, South Africa, and surrounding localities, registering similar P-T conditions. The results presented in this study, together with previous studies on various peridotite + H₂O ± CO₂ systems, clarify the relationship between bulk composition characteristics (e.g., K₂O/H₂O, peralkalinity) and the stability of different aluminum-bearing hydrous phases (e.g., phlogopite and amphibole). Fluxing harzburgite with silicic high-density fluids (HDFs) resulted in a peralkaline system, stabilizing both phlogopite and K-richterite. The stabilization of these Al-bearing hydrous phases led to the formation of a Cr-rich spinel, shifting the spinel to garnet transition to higher pressures. In addition, fluxing the harzburgite with silicic HDF led to the oxidation of the SCLM by one log unit, similar to the fO₂ registered by natural metasomatic xenoliths.