Chlorine stable isotope composition of granulites from Lofoten, Norway: Implications for the Cl isotopic composition and for the source of Cl enrichment in the lower crust

The Cl isotopic composition of lower crustal rocks, as revealed from granulites from the Lofoten Islands, Northern Norway, centres around the value of Standard Mean Ocean Chloride (SMOC). The investigated rocks and mineral separates of Cl-amphibole and Cl-biotite have δ37Cl values (37Cl/35Cl normalized to SMOC) of −1.12‰ to +0.79‰ with most values concentrated in the range of −0.3‰ to +0.11‰ and an average of −0.15‰ Samples from gabbroic and from dry granitic rocks (magmatic mangerites and charnockites) with highly Cl-enriched hydrosilicates and from a later amphibolite with low-Cl amphiboles show no distinct differences in their δ37Cl values. sults indicate that there was only minor involvement of mantle-derived Cl in fluid-involving processes in the middle and lower crust. Assuming a δ37Cl of 4.7‰ for the undegassed mantle [1], Cl isotopic values of the granulites do not support the influx of significant amounts of mantle derived Cl. The maximum possible mantle contribution to the sample with the highest δ37Cl is about 33%. Thus the common occurrence of Cl-enriched minerals in granulite facies terrains is regarded as a product of remobilized crustal Cl, either during magmatism or during later fluid activity. The results support the hypothesis that the crust can be regarded as a reservoir with Cl isotopic values very similar to SMOC.