Cl isotope fractionation in magmatic and hydrothermal eudialyte, sodalite and tugtupite (Ilímaussaq intrusion, South Greenland)

Abstract

We determined the Cl stable isotope composition of Cl-rich minerals (eudialyte, sodalite and tugtupite) from evolved peralkaline rocks of the Ilímaussaq complex (South Greenland) to determine the effects of orthomagmatic and hydrothermal processes on Cl isotope variations and to estimate the Cl isotopic composition of the lithospheric mantle source for the complex.The total variation of δ37Cl values is between +0.1 and + 0.4‰ for sodalites (N = 16), 0 and + 0.3‰ for tugtupites (N = 6) and + 0.2 and + 1.7‰ for eudialytes (N = 31). This data set allowed for estimating Cl isotope fractionation between minerals, melts and fluids in a well-defined natural system. The Cl isotope fractionation between magmatic sodalite and melt is slightly negative (about −0.05‰), decreasing to about −0.1‰ during hydrothermal conditions; Cl isotope fractionation between tugtupite and fluid (about −0.2‰) is slightly larger. Magmatic eudialyte has δ37Cl values that are approximately 0.35‰ higher than magmatic sodalite, indicating Cl isotope fractionation between eudialyte and melt of about +0.3‰.The Cl isotopic composition of the melt was estimated to a δ37Cl of about +0.3‰, which may be representative for the lithospheric mantle below South Greenland. This value is within the range of δ37Cl values of various mantle reservoirs (varying between −3‰ and + 3‰) including a recently suggested value of +0.9‰ derived from Mexican mantle-derived deep geothermal waters, and the generally accepted average mantle value of −0.2‰. The data indicate that large Cl isotope heterogeneities exist within the mantle that question the validity of using a general average Cl isotope ratio when Cl isotope mantle values are discussed, and that it is important to better understand the global variability of Cl isotopes in the mantle and Cl isotope fractionation between the fluid and crystalline phases.Cl isotopes in sodalite and tugtupite samples are compared to previously published Br isotope values. The range in Cl isotope values is less than the range of Br isotope values, and different sample groups are better constraint with Br isotopes. Br isotope fractionation is larger due to competition of Br with Cl which better fits at mineral site locations in the crystals. This results in better isotope separation between different sample groups using Br isotopes. Due to low Br concentrations in most samples, it is only rarely possible to measure Br isotope compositions so that generally it is not possible to apply this comparison.