Oxygen fugacity of the metasomatizing melt in a polymict peridotite from Kimberley

Kimberlitic melts are not only the transport vehicles for diamonds and mantle material to the Earth's surface but probably also one of the most common media responsible for mantle metasomatism in the subcontinental lithospheric mantle. We determined the Fe2+/Fe3+ in matrix and overgrowth garnets of a polymict breccia by electron microprobe and, from coexisting grt, opx and ol, calculated the oxygen fugacity of the melt which percolated, interacted and solidified this peridotite breccia. The percolating melt is inferred to be a Si-poor, Mg-rich and fluid-charged melt with a low Ca-activity, presumably of kimberlitic composition. The calculated oxygen fugacity is about 0.5 log units below the FMQ and just above the EMOG/D buffer at the likely conditions of solidification of the melt at around 1150 °C and 3.5 GPa. Such high oxygen fugacities would explain the common resorbed nature of diamonds from southern African occurrences if the transporting kimberlites of Cretaceous age were of a similar nature.