Oxidation during metasomatism in ultramafic xenoliths from the Wesselton kimberlite, South Africa: implications for the survival of diamond

Garnets in xenoliths from the Wesselton kimberlite show significant zoning in major and trace elements. The garnets were studied using room temperatur e Mossbauer spectroscopy with high spatial resolution, and show an increase in Fe3+/Sigma Fe from core to secondary rim. Temperatures and pressures we re determined using the garnet-olivine, garnet-orthopyroxene and Ni in garn et formulations, and indicate conditions close to 1,000 degreesC and 37 kba r for most of the garnets. Oxygen fugacities calculated using the garnet-ol ivine-orthopyroxene oxybarometer show an increase of approximately one log- bar unit from garnet core to secondary rim, relative to the quartz-fayalite -magnetite buffer curve. Combined with reanalysis of literature data from u naltered material from the same locality, there was an increase in relative oxygen fugacity of approximately two log-bar units during the course of me tasomatism. Existing data from other South African garnet peridotites were recalculated using the same thermobarometers and oxybarometers, and indicat e relative oxygen fugacities that lie at least two log-bar units below the diamond-carbonate equilibrium in peridotitic systems, which defines the max imum limit of diamond stability in peridotite. Diamond would hence be prese rved during the initial stages of metasomatism, but in later stages fluid w ould react with the diamonds, leading to their resorption and eventual dest ruction.