TY - JOUR
T1 - Oxygen isotopes of an early Archaean layered ultramafic body, southern West Greenland: implications for magma source and post-intrusion history
AU - Lowry, D.
AU - Appel, P.W.U.
AU - Rollinson, H.R.
N1 - Funding Information:
This study of Ujarassuit Nûnat forms part of the Isua Multidisciplinary Research Project (IMRP) that is funded by the Danish Natural Science Research Council, to the Commission for Scientific Research in Greenland and to the Bureau of Minerals and Petroleum, Nuuk, Greenland. The Greenland oxygen isotope work is a follow-up to studies by D. Lowry on chromitites from ophiolites and layered intrusions under NERC grant GR9/2193. Dave Mattey is thanked for setting up and maintaining the laser fluorination facility at Royal Holloway and Nathalie Grassineau for analysis of some of the chromite samples. The authors would also like to thank Gilles Chazot and Hilary Downes for their constructive reviews. The article is published with the permission of the Geological Survey of Denmark and Greenland.
PY - 2003/10/10
Y1 - 2003/10/10
N2 - 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 Nûnat 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 δ
18O by CO
2 laser fluorination. δ
18O 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 δ
18O 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 Nûnat, have higher δ
18O 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.
AB - 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 Nûnat 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 δ
18O by CO
2 laser fluorination. δ
18O 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 δ
18O 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 Nûnat, have higher δ
18O 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.
KW - Chromitite
KW - Early Archaean
KW - Greenland
KW - Layered ultramafic
KW - Oxygen isotopes
UR - http://www.scopus.com/inward/record.url?scp=0347481421&partnerID=8YFLogxK
U2 - 10.1016/S0301-9268(03)00099-8
DO - 10.1016/S0301-9268(03)00099-8
M3 - Article
VL - 126
SP - 273
EP - 288
JO - Precambrian Research
JF - Precambrian Research
SN - 0301-9268
IS - 3-4
ER -