An oxygen barometer for rutile-ilmenite assemblages: oxidation state of metasomatic agents in the mantle

摘要

Oxygen fugacity has been calculated for rutile-ilmenite assemblages from the reaction 2Fe_2O_3 (in ilmenite) + 4TiO_2 (rutile) = 4FeTiO_3 (in ilmenite) + O_2. The equation log fO_2 = 22.59 - 25925/T - 3.09 log T + 0.0016535P + 48.836P/T - 4 log a_(FeTiO_3)~(Ilm) + 2 log a_(Fe_2O_3)~(Ilm) + 4 log a_(TiO_2)~(Rut), where T is in kelvin and P is in kbar, was derived from available thermodynamic data. The hypothetical end-member rutile-ilmenite reaction is located between the magnetite-hematite and Ni-NiO (NNO) buffers. The rutile-ilmenite oxygen barometer has been applied to ilmenite-*bearing assemblages in mantle xenoliths from kimberlites, including the metasomatic MARID (mica-amphibole-rutile-ilmenite-diopside) suite and a MORID (mica-orthopyroxene-rutile-ilmenite-diopside) vein, along with rutile-ilmenite assemblages in eclogites and in Granny Smith diopside megacrysts. The oxygen fugacities of MARID and MORID lie around the NNO buffer and are comparable to those in metasomatized spinel lherzolites. Most MARID and MORID assemblages yield a more oxidizing fO_2 than the EMOD (enstatite-magnesite-olivine-diamond) buffer, such that MARID and MORID fluid or melt would tend to destroy diamond or graphite by oxidation.