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

Oxygen fugacity has been calculated for rutile-ilmenite assemblages from th e reaction 2Fe(2)O(3) (in ilmenite) + 4TiO(2) (rutile) = 4FeTiO(3) (in ilme nite) + O-2. The equation log fO(2) = 22.59 - 25925/T - 3.09 log T + 0.0016 535P + 48.836P/T - 4 log a(FeTiO3)(Ilm) + 2 log a(Fe2O3)(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 rutil e-ilmenite oxygen barometer has been applied to ilmenite-bearing assemblage s in mantle xenoliths from kimberlites, including the metasomatic MARID (mi ca-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 m etasomatized spinel lherzolites. Most MARID and MORID assemblages yield a m ore oxidizing fO(2) than the EMOD (enstatite-magnesite-olivine-diamond) buf fer, such that MARID and MORID fluid or melt would tend to destroy diamond or graphite by oxidation, (C) 1999 Elsevier Science B.V. All rights reserve d.