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

Abstract

Oxygen fugacities have been estimated for a wide distribution of samples from the granulite facies terrane (region C) of the Bamble Sector, SE Norway using both the titaniferous magnetite–ilmenite and orthopyroxene–titaniferous magnetite–quartz oxygen barometers. These oxygen fugacities are estimated using temperatures calculated from the titaniferous magnetite–ilmenite thermometer of Ghiorso and Sack (1991) and are both internally consistent with each other as well with the thermometer. In samples for which the estimated temperature is high, the two oxygen barometers show good agreement whereas agreement is poor for low temperature samples. In these low temperature samples, oxygen fugacities estimated from titaniferous magnetite–ilmenite are considerably less than those estimated from orthopyroxene–titaniferous magnetite–quartz. An increase in this discrepancy with decrease in temperature appears to reflect preferential resetting of the hematite component in the ilmenite grains without significant alteration of the more numerous titaniferous magnetite grains. This is due, in part, to greater re-equilibration of the ilmenite grains during retrograde interoxide resetting between the ilmenite grains and the titaniferous magnetite grains. The mean temperature for the non-reset samples, 791 ± 17 °C (1σ), is in good agreement with temperatures obtained from garnet–orthopyroxene KD exchange thermometry in the same region, 785–795 °C (1σ) (Harlov 1992, 2000a). Most non-reset oxygen fugacities range from log10 f O2=−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 good agreement with those estimated using the titaniferous magnetite–ilmenite thermometer/oxygen barometer of Andersen et al. (1991). The QUIlP equilibrium (quartz–ulvospinel–ilmenite–pyroxene) is used to project self-consistent equilibrium temperatures and oxygen fugacities for samples reset due to hematite loss from the ilmenite grains. These projected temperatures and oxygen fugacities agree reasonably well with the non-reset samples. The mean projected QUIlP temperature is 823 ± 6 °C (1σ). This result supports the conclusion that low titaniferous magnetite–ilmenite temperatures (down to 489 °C) and accompanying low oxygen fugacities are the result of hematite loss from the ilmenite grains. Non-reset oxygen fugacities lie approximately 1.5 log10 units above the upper graphite stability curve indicating that the stable C–O–H fluid phase interacting with these gneisses, whether regionally or locally, was CO2. This is borne out by the presence of numerous CO2-rich fluid inclusions in these rocks.