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

Abstract.

Garnets in xenoliths from the Wesselton kimberlite show significant zoning in major and trace elements. The garnets were studied using room temperature Mössbauer spectroscopy with high spatial resolution, and show an increase in Fe³⁺/ΣFe from core to secondary rim. Temperatures and pressures were determined using the garnet–olivine, garnet–orthopyroxene and Ni in garnet formulations, and indicate conditions close to 1,000 °C and 37 kbar for most of the garnets. Oxygen fugacities calculated using the garnet–olivine–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 unaltered material from the same locality, there was an increase in relative oxygen fugacity of approximately two log-bar units during the course of metasomatism. Existing data from other South African garnet peridotites were recalculated using the same thermobarometers and oxybarometers, and indicate relative oxygen fugacities that lie at least two log-bar units below the diamond–carbonate equilibrium in peridotitic systems, which defines the maximum limit of diamond stability in peridotite. Diamond would hence be preserved during the initial stages of metasomatism, but in later stages fluid would react with the diamonds, leading to their resorption and eventual destruction.