CARBON-ISOTOPE CONSTRAINTS ON VOLATILE MIXING AND MELT TRANSPORT IN GRANULITE-FACIES MIGMATITES

Migmatites at Brattstrand Bluffs, East Antarctica, comprise granulite- facies mineral assemblages produced by high-temperature metamorphism o f pelitic sediments. These rocks preserve an advanced stage of partial melting, with recrystallized melt accounting for similar to 25% of ou tcrop area. Many of the melt veins contain cordierite, which traps vol atile 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 CO2 b y step-heating mass spectrometry. Channel CO2 in cordierite from Bratt strand Bluffs has not been significantly affected by retrograde isotop e exchange as it preserves high-temperature carbon fractionations with coexisting graphite. Cordierite samples from the same locality have s imilar isotope compositions (+/- 0.6 parts per thousand), reflecting e fficient mixing of partial melt and dissolved volatiles on a metre sca le. However, samples from different localities have distinct delta(13) C values (-10 to -20 parts per thousand), and indicate that there was little mixing of melt or volatiles on a scale of similar to 100 m desp ite the high degree of partial melting. The 10 parts per thousand vari ation between localities reflects mixing of variable proportions of is otopically light biogenic carbon with carbon from a heavier reservoir. Although this mixing can be interpreted in terms of carbon influx fro m an external source, CO2 was not a driving force for granulite metamo rphism in this case as granulite assemblages occur at all localities i rrespective of isotope systematics. CO2 played a complex but passive r ole during metamorphism and melting at Brattstrand Bluffs, where the d issolution of H2O into partial melt bodies was the dominant dehydratio n mechanism.