Oxygen fugacities of olivine-orthopyroxene-ilmenite-bearing, high-pressure megacrysts, mineral intergrowths, and peridotites from kimberlites and from a minette have been calculated from the equilbrium 2Fe2O3O2+4FeSiO3=8FeSi0.5+O2. Resultant fO2's. which are interpreted to reflect the subcontinental source regions of alkalic magmas, are clustered between the quartz-fayalite-magnetite and magnetite-wustite buffers. These relatively oxidized conditions are compatible with at least some estimates of fO2 of typical basalts. They are also compatible with existence of carbonate minerals and diamond or graphite and, if fluids are present, with water-rich H2O-CO2 fluids rather than CH4-H2O fluids.