Titaniferous magnetite-ilmenite thermometry and titaniferous magnetite-ilmenite-orthopyroxene-quartz oxygen barometry in granulite facies gneisses, Bamble Sector, SE Norway: implications for the role of high-grade CO2-rich fluids during granulite genesis

Oxygen fugacities have been estimated for a wide distribution of samples fr om the granulite facies terrane (region C) of the Bamble Sector, SE Norway using both the titaniferous magnetite-ilmenite and orthopyroxene-titanifero us magnetite-quartz oxygen barometers. These oxygen fugacities are estimate d using temperatures calculated from the titaniferous magnetite-ilmenite th ermometer of Ghiorso and Sack (1991) and are both internally consistent wit h each other as well with the thermometer. In samples for which the estimat ed temperature is high, the two oxygen barometers show good agreement where as agreement is poor for low temperature samples. In these low temperature samples, oxygen fugacities estimated from titaniferous magnetite-ilmenite a re considerably less than those estimated from orthopyroxene-titaniferous m agnetite-quartz. An increase in this discrepancy with decrease in temperatu re appears to reflect preferential resetting of the hematite component in t he ilmenite grains without significant alteration of the more numerous tita niferous magnetite grains. This is due, in part, to greater re-equilibratio n of the ilmenite grains during retrograde interoxide resetting between the ilmenite grains and the titaniferous magnetite grains. The mean temperatur e for the non-reset samples. 791 +/- 17 degrees C (1 sigma), is in good agr eement with temperatures obtained from garnet-orthopyroxene KD exchange the rmometry in the same region, 785-795 degrees C (1 sigma) (Harlov 1992, 2000 a). Most non-reset oxygen fugacities range from log(10)fO(2) = - 14 to -11. 8 or approximately 0.5-1.5 log units above quartz-fayalite-magnetite at 7.5 kbar. Both these temperatures and the range of oxygen fugacities are in go od agreement with those estimated using the titaniferous magnetite-ilmenite thermometer/oxygen barometer of Andersen et al. (1991). The QUIIP equilibr ium (quartz-ulvospinel-ilmenite-pyroxene) is used to project equilibrium te mperatures and oxygen self-consistent Im temperatures and oxygen fugacities for reset due to hematite loss from the ilmenite grains. These projected t emperatures and oxygen fugacities agree reasonably well with the non-reset samples. The mean projected QUIIP temperature is 823 +/- 6 degrees C (1 sig ma). This result supports the conclusion that low titaniferous magnetite-il menite temperatures (down to 489 degrees C) and accompanying low oxygen fug acities are the result of hematite loss from the ilmenite grains. Non-reset oxygen fugacities lie approximately 1.5 log(10) units above the upper grap hite stability curve indicating that the stable C-O-H fluid phase interacti ng with these gneisses, whether regionally or locally, was CO2. This is bor ne out by the presence of numerous CO2-rich fluid inclusions in these rocks .