Nominally volatile-free mantle minerals: a reservoir for water and fluorine in the Kaapvaal Craton

The extent to which Earth’s sub-cratonic lithospheric mantle acts as a major repository for volatile elements is poorly constrained. This study presents the first systematic investigation into water, fluorine and chlorine storage in nominally volatile-free mantle minerals from sub-cratonic mantle. Full petrologic and chemical characterisation (using EPMA, LA-ICP-MS, SIMS, FTIR, SEM, EBSD) of nineteen mantle xenoliths from the interior (Bultfontein, South Africa) and margin (Mothae, Lesotho) of the Kaapvaal craton has placed important constraints on the capacity of olivine and pyroxenes to store volatiles. Cryptically metasomatised olivine is found to be a major host of fluorine (up to 196 ppm). A new incorporation mechanism involving triple coupling of F, OH and Ti in olivine is recognised: Ti4+ in an octahedral site is charge balanced by F− in an oxygen site and OH− in a neighbouring tetrahedral vacancy. This triple coupling limits the independent diffusion of each element and hence preserves metasomatic enrichment on timescales of millions to billions of years. Four different types of metasomatism have been recognised in the Kaapvaal xenolith suites, each with different controls on the storage of volatile elements in olivine and pyroxenes. Proto-kimberlite metasomatism is found to flux the largest amount of volatile elements into the sub-cratonic lithospheric mantle. This has important implications for the pre-conditioning of the mantle prior to the host kimberlite eruption, because assimilation of volatile-rich mantle wall-rock contributes volatile species to kimberlite magmas and drives their ascent through hundreds of kilometres of lithosphere to the surface. The Kaapvaal craton has previously been considered as dry compared to the Slave and Siberian cratons but this new dataset includes water concentrations of clinopyroxene and orthopyroxene that are as high as those in the published literature. Olivine F concentrations are some of the highest recorded in global lithospheric mantle and provide evidence that F input at subduction zones is recycled through the convecting mantle, transported into the cratonic mantle by small-fraction melts and stored in both NVFMMs and new metasomatic phases. A metasomatic front at 120 km depth coincides with high concentrations of volatiles and corresponds to seismic mid-lithospheric discontinuities and has important implications for craton stability.NERC DTP Studentshi