Oxygen isotopes of an Early Archaean layered ultramafic body, southern West Greenland: implications for magma source and post-intrusion history

Abstract

A large number of metamorphosed ultramafic bodies are found in the region SW of the Isua Greenstone Belt in southern West Greenland, enclosed in Archaean banded tonalitic gneisses. These enclaves of dunite and harzburgite range in size from a few to several hundred meters. The Ujarassuit Nunat layered body occurs as a large enclave (800 m×100 m) entrained within tonalitic gneisses and preserves primary igneous layering and textures. This enclave has been subjected to amphibolite-grade metamorphism, but for the most part the magmatic mineralogy remains intact. Separated minerals have been analysed for δ18O by CO2 laser fluorination. δ18O values range from +4.49 to +4.89‰ for olivine and from +5.70 to +5.86‰ for orthopyroxene. All chromite values are <+2.5‰. Closure temperatures recorded for olivine–orthopyroxene pairs vary from 700 to 900 °C, much lower than expected for an ultramafic magma. Widespread late-magmatic re-equilibration has resulted in significant decreases in δ18O for chromites and small changes of <0.3‰ in olivine, but orthopyroxene values show little variation from typical magmatic signatures. The lowering of 18O in chromite is associated with significant Fe-enrichment. Thermometry suggests that the re-equilibration took place at 680–750 °C, with equilibration between plagioclase and amphibole in residual gabbro anorthosite melt around 700 °C. Hydration, possibly associated with this late melt stage, resulted in further changes to chromite chemistry, formation of secondary amphibole, phlogopite and chlorite and may have further enhanced 18O-depletion in disseminated chromites. All these events are related to late-magmatic processes. Only the later metamorphic mineral, ruby corundum, present in amphibole-harzburgite adjacent to an intrusive gneiss vein, clearly indicates an external oxygen input to this system. Olivines from an unlayered dunite body, 3.5 km away from Ujarassuit Nunat, have higher δ18O values around 5.25‰, typical of more recent high temperature mantle peridotite nodules, and suggesting that mantle oxygen composition at 3.8 Ga was already similar to the present composition.