The interaction of slab-derived silicic fluid and harzburgite – Metasomatism in the sub cratonic lithospheric mantle

Understanding the interaction between fluids and the depleted peridotite is essential in deciphering the geochemical and mineralogical variability of the sub-continental lithospheric mantle (SCLM). High-density fluid (HDF) inclusions trapped in diamonds during their formation in the SCLM are potential metasomatic agents. In this study, experiments were performed on a harzburgite + silicic fluid system at an 80:20 rock:fluid ratio (bulk 5 wt% H₂O and 3 wt% CO₂), simulating the infiltration of a slab-derived silicic fluid into the SCLM. Experiments were performed at 3–6 GPa and 900–1200 °C on a rocking multi-anvil apparatus, conditions corresponding to the deep SCLM. The fluid and melt compositions were directly determined using the cryogenic LA-ICP-MS technique. The metasomatized peridotite consisted of olivine, clinopyroxene, phlogopite, orthopyroxene, amphibole, and magnesite. The H₂O-CO₂-saturated solidus in this system is located between 900–1000 °C at 3 GPa and between 1000–1100 °C at 4–6 GPa. The fluids at all pressures are alkali-rich. With increasing temperature and crossing the solidus, the melts evolve towards the initial silicic fluid composition. Together with previous studies on various peridotite + H₂O ± CO₂ systems, the results presented here suggest a positive correlation between the bulk alkali/volatile ratio and the solidus temperatures. An increase in this parameter and in the mantle fertility (i.e., Ca#) increases the near-solidus isobaric melt production rate. The experimental fluid compositions found in this study are similar to saline HDFs. We propose a scenario where the percolation of a slab-derived silicic fluid into the depleted cratonic peridotite leads to the formation of near-solidus saline fluids. Such sequence of metasomatic events was reported from fluid inclusions in diamonds from several localities in the Kaapvaal craton in South Africa (e.g., De Beers, Finsch, Voorspoed).