Carbon isotope constraints on volatile mixing and melt transport in granulite-facies migmatites

Abstract

Migmatites at Brattstrand Bluffs, East Antarctica, comprise granulite-facies mineral assemblages produced by high-temperature metamorphism of pelitic sediments. These rocks preserve an advanced stage of partial melting, with recrystallized melt accounting for ∼ 25% of outcrop area. Many of the melt veins contain cordierite, which traps volatile molecules within open channels in its mineral framework and can be used to monitor the syn-metamorphic volatile contents of the melts. We have analysed the isotopic composition of cordierite channel CO 2 by step-heating mass spectrometry. Channel CO 2 in cordierite from Brattstrand Bluffs has not been significantly affected by retrograde isotope exchange as it preserves high-temperature carbon fractionations with coexisting graphite. Cordierite samples from the same locality have similar isotope compositions (±0.6‰), reflecting efficient mixing of partial melt and dissolved volatiles on a metre scale. However, samples from different localities have distinct δ 13C values (−10 to −20‰), and indicate that there was little mixing of melt or volatiles on a scale of ∼ 100 m despite the high degree of partial melting. The 10‰ variation between localities reflects mixing of variable proportions of isotopically light biogenic carbon with carbon from a heavier reservoir. Although this mixing can be interpreted in terms of carbon influx from an external source, CO 2 was not a driving force for granulite metamorphism in this case as granulite assemblages occur at all localities irrespective of isotope systematics. CO 2 played a complex but passive role during metamorphism and melting at Brattstrand Bluffs, where the dissolution of H 2O into partial melt bodies was the dominant dehydration mechanism.