Carbon and oxygen isotopes in eclogites, amphibolites, veins and marbles from the Western Gneiss region, Norway

The ¹³C ¹²C and ¹⁸O ¹⁶O ratios have been measured in carbonates and silicates from pods of eclogite, garnet-amphibolite and marbles and also some veins which outcrop in the Western Gneiss Region, Norway, in an attempt to clarify the origin of these rocks. The eclogite carbonates (magnesite or dolomite) display carbon and oxygen isotope values (-9‰ < δ ¹³C < -2.4‰ PDB; +6.8‰ < δ ¹⁸O < +10.4‰ SMOW) which lie within the field of mantle-effluxed carbonate as represented by primary igneous carbonatites. Two garnet-amphibolites have heavier oxygen in the same δ ¹³C range. Two marbles have heavier carbon and differing δ ¹⁸O. Three types of carbonate-bearing vein show different isotopic relationships to the carbonate of their host rocks. Oxygen isotope fractionations amongst minerals indicate preservation of a high-temperature equilibration (650-850°C) comparable to the temperature previously deduced from FeMg partitioning amongst silicates (700-850°C). The eclogite carbonate carbon and oxygen isotope compositions may have been produced either by metamorphism of crustal carbonates or by carbonation of carbonate-free rocks by mantle-effluxed CO₂ before or during the eclogitisation processes. The whole-rock δ ¹⁸O-values (4-8‰ SMOW) are within the range of crustal basalts and gabbros thus supporting a crustal origin for these eclogites. Several other lines of petrological and geochemical evidence also support the model of crustal protoliths of these rocks. These data may contribute to the geodynamic modelling of the carbon cycle in indicating that subducted carbon reinjected into the mantle could have essentially the same isotope composition as that of carbon effluxed from the mantle and that a crustal protolith origin for some erupted xenoliths gains in credibility.