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

Abstract

Primary multiphase brine fluid inclusions in omphacite and garnet from low‐ to medium‐temperature eclogites have been analysed for Cl, Br, I, F, Li and SO4. Halogen contents and ratios provide information about trapped lower crustal fluids, even though the major element (Na, K, Ca) contents of inclusion fluids have been modified by fluid–mineral interactions and (step‐) daughter‐crystal formation after trapping. Halogens in the inclusion fluids were analysed with crush–leach techniques. Cl/Br and Cl/I mass ratios of eclogite fluids are in the range 31–395 and 5000–33 000, respectively. Most fluids have a Cl/Br ratio lower than modern seawater and a Cl/I ratio one order of magnitude lower than modern seawater. Fluids with the lowest Cl/Br and highest Cl/I ratios come from an eclogite that formed by hydration of granulite facies rocks, and may indicate that Br and I are fractionated into hydrous minerals. Reconstructions indicate that the inclusion fluids originally contained 500–4000 ppm Br, 1–14 ppm I and 33–438 ppm Li. Electron microprobe analyses of eclogite facies amphibole, biotite, phengite and apatite indicate that F and Cl fractionate most strongly between phengite (F/Cl mass ratio of 1469 ± 1048) and fluid (F/Cl mass ratio of 0.008), and the least 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 in seawater during evaporation: the Cl/Br ratio remains constant until the aH2O value is sufficiently lowered for Cl to be removed from solution by incorporation into hydrous minerals.

Diffusion versus recrystallization processes in Rb–Sr geochronology: Isotopic relics in eclogite facies rocks, Western Gneiss Region, Norway

“…The fact that large proportions of the original granulite were metastably preserved through eclogite-facies metamorphism indicates that the rock volume now forming Bårdsholmen was overall fluid-deficient during eclogite facies conditions. This is corroborated by the finding that eclogitization was largely due to the infiltration of water-rich fluids from external sources, and that H 2 O was internally consumed by the eclogitization reactions, leaving behind residual brines rich in highly incompatible elements, like Br and Pb (Svensen et al, 1999;Svensen et al, 2001). For the fairly well preserved granulites this means that the eclogitization reactions stranded at low reaction progress, due to full consumption of free fluids shortly after the rocks' exposure to eclogite facies P,T conditions.…”

Fluids in deeply subducted continental crust: Petrology, mineral chemistry and fluid inclusion of UHP metamorphic veins from the Sulu orogen, eastern China

“…Veins containing HP and UHP mineral assemblages are common in eclogites from a number of metamorphic terranes (e.g., Austrheim, 1987;Philippot and Selverstone, 1991;Philippot, 1993;Nadeau et al, 1993;Castelli et al, 1998;Becker et al, 1999;Bruusmann et al, 2000;Scambelluri and Philippot, 2001;Franz et al, 2001;Gao and Klemd, 2001;Svensen et al, 2001;Widmer and Thompson, 2001;Rubatto and Hermann, 2003;Molina et al, 2004). The complex allanite-bearing quartz vein associations, however, have not been reported previously in the DabieSulu orogen.…”

New constraints on the release of noble gases during in vacuo crushing and application to scapolite Br–Cl–I and 40Ar/39Ar age determinations

“…Svensen et al, 2001;Mark et al, 2005) and therefore high Br/Cl values could be generated in ultra-saline desiccation fluids (Markl and Bucher, 1998;Kullerud and Erambert, 1999), or by incongruent breakdown of hydrous minerals. If this sample's Br/Cl value is representative of such fluids, similar processes would have acted on the I/Cl value.…”

“…Further measurement of scapolite Br/Cl values, complemented by analyses of I/Cl, are desirable to test the range of halogen compositions attainable in scapolite. This could further constrain the partitioning behaviour of halogens in scapolite (Pan and Dong, 2003), and provide an indication as to the extent of halogen fractionation during metamorphism (Markl and Bucher, 1998;Kullerud and Erambert, 1999;Svensen et al, 2001). Constraining halogen fractionation during metamorphic fluid-rock interactions is important for utilising Br/Cl and I/Cl as fluid tracers in a variety of crustal and/or mantle environments (e.g.…”