The Role of Halogens in the Lithospheric Mantle

Abstract

The present chapter focuses on the still poorly known sources, distribution and fractionation of halogens in the oceanic and continental lithospheric mantle in intraplate and extensional tectonic settings, and in the deep cratonic sub-continental mantle that hosts diamonds and kimberlites. In the lithospheric mantle, halogens are considered to be mainly stored in volatile-bearing metasomatic minerals, such as apatite, amphibole, and phlogopite. However, recent studies demonstrate that the nominally anhydrous minerals, incorporating measurable amounts of water as hydroxyl, are important storage sites for F, and to a lesser extent Cl. The halogen contents of minerals influence their stability in the lithospheric mantle, and they might be responsible for halogen enrichment and trace element signatures in some primitive basaltic melts. Over the last 20 years a growing body of evidence, based mainly on fluid inclusion data, has confirmed that relatively high amounts of halogens can be dissolved in aqueous-carbonic fluids, which may be locally immiscible, and in hydrous carbonate melts which ascend through the lithospheric mantle in intraplate and extensional settings. Metasomatic halide-bearing fluids are important carriers of incompatible trace elements (e.g., LILE and LREE). The highest halogen concentrations in fluids are observed in inclusions in fibrous and cloudy diamondsfrom the deep cratonic lithosphere. It is now well established that diamond-forming fluids consist of complex silicate-carbonate-Cl-aqueous mixtures formed by mixing of a hydrous-silicic fluid (rich in SiO2, Al2O3, and K2O), carbonatitic fluid (containing high concentrations of carbonate, CaO, FeO, MgO, and variable Na2O and K2O), and saline aqueous fluid (rich in Cl, K, and Na) end-member types. Halogen enrichment in lithospheric mantle fluids, both in the vicinity of, and away from, subduction zones, suggests a dynamic balance in the halogen geochemical cycle, which is ruled by recycling of surface halogens and mantle fractionation processes.