Halogen behaviour in subduction zones: eclogite facies rocks from the Western and Central Alps

Abstract We examined F, Cl, Br and I concentrations and distributions in eclogite facies rocks and minerals from the Western and Central Alpine ophiolitic zone to determine halogen behaviour in subduction zones, and to identify potential host phases that may be able to transport halogens to the deeper mantle. Analysis was carried out on a range of ophiolitic lithologies — peridotites, serpentinites, metagabbros, metabasalts and metasediments — to assess the distribution of halogens within deeply subducted oceanic crust. Halogen abundances in individual mineral phases range from below detection (∼100 ppm) to ∼1900 ppm for F, ∼1 to ∼3000 ppm for Cl, ∼1 to ∼11,000 ppb for Br and from In situ analyses and chemical mapping using electron probe micro-analysis and time of flight secondary ion mass spectrometry indicate that measured halogen abundances are primarily hosted within the mineral structure. Overall, our dataset provides new constraints on the available inventory of halogens that can be transferred to the deeper mantle via the subduction of oceanic crust.