Traces of Slab-derived Fluids Revealed by Halogens in Mantle-derived Rocks

Abstract

The distribution and behavior of halogens in the mantle are still poorly understood. Chlorine, bromine, and iodine are highly incompatible and strongly partitioned into fluids, whereas fluorine is partitioned into melts rather than fluids. As a result, during differentiation of the Earth, the former halogens became more abundant in surface reservoirs and scarcer in the mantle. Because their geochemical properties differ, each reservoir has a distinct composition. Therefore, the halogen compositions of mantle-derived materials are expected to enable material exchanges to be traced between the surface of the Earth and its interior, especially water transportation into the mantle accompanying a subducting slab. Exhumed peridotites in metamorphic belts and mantle-derived xenoliths in volcanic rocks can provide intrinsic halogen information on the mantle, whereas halogen compositions in volcanic rocks are changed from those of their mantle source during melting and degassing processes, in which halogen behavior is not well constrained. Very low halogen concentration in mantle-derived materials hampers analyses using conventional methods. Recent progress of analytical techniques involving the noble gas method overcomes such difficulties, making it possible to intensively investigate halogens in mantlederived materials during the last decade. Presented here is a review of studies of halogens in volcanic and mantle-derived rocks and minerals. Recent findings of distinctive Br/Cl and I/Cl ratios in exhumed peridotites in metamorphic belts, mantle xenoliths from arc volcanoes and kimberlites suggest subduction of halogen acquired from sedimentary pore water or crustal brine into the mantle.