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Halogens in Eclogite Facies Minerals from the Western Gneiss Region, NorwayHughes; orcid: 0000-0002-4363-8675; email: email@example.com; Cuthbert; orcid: 0000-0002-1029-6357; email: firstname.lastname@example.org; Quas-Cohen; email: email@example.com; Ruzié-Hamilton; orcid: 0000-0002-2802-8123; email: firstname.lastname@example.org; Pawley; orcid: 0000-0002-3022-3235; email: email@example.com; Droop; email: firstname.lastname@example.org; Lyon; email: Ian.Lyon@manchester.ac.uk; Tartèse; email: email@example.com; Burgess; orcid: 0000-0001-7674-8718; email: firstname.lastname@example.org (MDPI, 2021-07-14)Ultra-high-pressure (UHP) eclogites and ultramafites and associated fluid inclusions from the Western Gneiss Region, Norwegian Caledonides, have been analysed for F, Cl, Br and I using electron-probe micro-analysis, time-of-flight secondary ion mass spectrometry and neutron-irradiated noble gas mass spectrometry. Textures of multi-phase and fluid inclusions in the cores of silicate grains indicate formation during growth of the host crystal at UHP. Halogens are predominantly hosted by fluid inclusions with a minor component from mineral inclusions such as biotite, phengite, amphibole and apatite. The reconstructed fluid composition contains between 11.3 and 12.1 wt% Cl, 870 and 8900 ppm Br and 6 and 169 ppm I. F/Cl ratios indicate efficient fractionation of F from Cl by hydrous mineral crystallisation. Heavy halogen ratios are higher than modern seawater by up to two orders of magnitude for Br/Cl and up to three orders of magnitude for I/Cl. No correlation exists between Cl and Br or I, while Br and I show good correlation, suggesting that Cl behaved differently to Br and I during subduction. Evolution to higher Br/Cl ratios is similar to trends defined by eclogitic hydration reactions and seawater evaporation, indicating preferential removal of Cl from the fluid during UHP metamorphism. This study, by analogy, offers a field model for an alternative source (continental crust) and mechanism (metasomatism by partial melts or supercritical fluids) by which halogens may be transferred to and stored in the sub-continental lithospheric mantle during transient subduction of a continental margin.