Halogen contents of eclogite facies fluid inclusions and minerals: Caledonides, western Norway

Primary multiphase brine fluid inclusions in omphacite and garnet from low- to medium-temperature eclogites have been analysed for Cl, Br, I, F, Li an d SO4. Halogen contents and ratios provide information about trapped lower crustal fluids, even though the major element (Na, K, Ca) contents of inclu sion fluids have been modified by fluid-mineral interactions and (step-) da ughter-crystal formation after trapping. Halogens in the inclusion fluids w ere analysed with crush-leach techniques. Cl/Br and Cl/I mass ratios of ecl ogite fluids are in the range 31-395 and 5000-33 000, respectively. Most fl uids have a Cl/Br ratio lower than modern seawater and a Cl/I ratio one ord er of magnitude lower than modern seawater. Fluids with the lowest Cl/Br an d highest Cl/I ratios come from an eclogite that formed by hydration of gra nulite facies rocks, and may indicate that Br and I are fractionated into h ydrous minerals. Reconstructions indicate that the inclusion fluids origina lly contained 500-4000 ppm Pr, 1-14 ppm I and 33-438 ppm Li. Electron micro probe analyses of eclogite facies amphibole, biotite, phengite and apatite indicate that F and Cl fractionate most strongly between phengite (F/Cl mas s ratio of 1469 +/- 1048) and fluid (F/Cl mass ratio of 0.008), and the lea st between amphibole and fluid. The chemical evolution of Cl and Br in pore fluids during hydration reactions is in many ways analogous to Cl and Br i n seawater during evaporation: the Cl/ Br ratio remains constant until the a(H2O) value is sufficiently lowered for Cl to be removed from solution by incorporation into hydrous minerals.