Variations of sulfur isotopes in metamorphic rocks from Bamble Sector, southern Norway: a laser probe study

Abstract

Variations of sulfur isotopes in metamorphic rocks from Bamble Sector, southern Norway, are investigated using a Q-switched, pulsed mode, laser combustion technique. The results indicate that laser-related fractionations are reproducible (1 σ=0.2‰), provided uniform conditions are maintained during analysis. Bamble is a Precambrian high-grade metamorphic terrain. A transition from amphibolite- to granulite-facies occurs across the 25-km width of the Sector. Supracrustal rocks of mid-Proterozoic ages were intruded by felsic and mafic magmas and metamorphosed during the Sveconorwegian orogenic episode. Earlier studies indicated that Bamble rocks equilibrated under a relatively high f O 2 , 2–3 log units above QFM buffer, during peak metamorphism [J. Geochemical Exploration 31 (1989) 149; J. Geology 100 (1992) 447; Contrib. Mineral Petrol. 1139 (2000) 180], which led to phase changes in sulfides. Pyrrhotite, or Po mss, has variably been replaced by pyrite±magnetite. Mantling of pyrite by magnetite is common. Metasupracrustal rocks display a large variation in δ 34S CDT (−1‰ to +12‰), consistent with data from sedimentary rocks of similar age. No distinction can be made between amphibolite, transition, and granulite zones. The isotopic signature of sulfides was determined by bacteriogenic reduction of seawater sulfate with an initial δ 34S value of 20–30‰. Tonalitic–trondhjemitic gneisses from granulite zone show a large scatter in δ 34S CDT (0‰ to +8‰), implying that the parent magmas were contaminated by sedimentary materials. This is supported by S/Se ratios. Mafic rocks from various metamorphic zones display a clustering of δ 34S CDT values around 0‰, and S/Se ratios in the range 3×10 3 to 6×10 3. This is consistent with a mantle source for S. Ni–Cu sulfide ores locally associated with the mafic rocks, however, are enriched in 34S, indicating the involvement of sedimentary S in the formation of the ores. Mass and isotopic balance calculations indicate that oxidation of pyrrhotite, or Po mss, proceeded without significant addition or removal of S. Sulfur liberated during mantling of Fe–sulfides by magnetite was in a reduced form and reprecipitated in short distances as secondary overgrowth and fracture filling pyrite. The large variation of δ 34S values for the supracrustal rocks, and the preservation of the original isotopic features for various rock units suggest that no isotopic homogenization occurred in spite of the high-grade metamorphism.