Secondary graphitization in mantle-derived rocks

The assemblage magnesite + "graphite" ± CH₄ has been identified by laser Raman microprobe spectroscopy in fluid inclusions in olivine grains in the Kao kimberlite and the Duluth Complex troctolite. In both cases, the assemblage (essentially Eggler's EMOG oxygen-fugacity buffer) is believed to arise from secondary reactions rather than from primary igneous processes. The known stability field in P-T-f_O2 space of the EMOG buffer, coupled with some other petrologic constraints imposed by the host rocks, indicates in both cases that the carbon-bearing assemblage was precipitated at low pressures and temperatures (≤2 kbar,≤550 °C) at oxygen fugacities within about 1 log (f_O2) unit below the fayalite-magnetite-quartz (FMQ) buffer. The laser Raman microprobe provides compositional and structural data on the coexisting phases (including carbon) that further constrain the stability field of the natural assemblages.