Systematics of water isotopic composition and chlorine content in arc-volcanic gases

Abstract

Abstract In this review paper a set of chemical (H 2 O, CO 2 , S, HCl, HF) and water isotopic (δD, δ 18 O) data is compiled for 26 subduction zone volcanoes and 3 volcanoes of other tectonic settings. To the arc-volcano dataset we added new data on high-temperature gas vents from three Kamchatkan volcanoes (Gorely, Bezymianny and Tolbachik) and also published data on the chemical composition of volcanic plumes obtained by remote instrumental techniques (Open-Path Fourier Transform Infrared Spectroscopy (OP-FTIR), Differential Optical Absorption Spectroscopy (DOAS) MultiGas) for volcanoes of different tectonic settings. Three typical cases of the persistent volcanic degassing in terms of δD, δ 18 O of the volcanic vapour and its HCl content are discussed in detail: (1) Kudryavy type with a clear two-end-member mixing behaviour; (2) degassing of active or cooling lava domes with fractionation of both water isotopes and Cl content; and (3) degassing of small volcanic islands with a complex admixing of meteorics and seawater. We show that independently of the complexity of the shallow processes which affect volcano degassing, the highest temperature volcanic gases are characterized by δD values and HCl concentrations close to −25‰ V-SMOW and 0.5–1.0 mol%, respectively. These values are the same for any volcanic arc (except maybe Mediterranean volcanoes) and are the result of seawater recycling at subduction zones on the global scale. In contrast to volcanic fluids of non-arc settings, the arc-magmatic fluid has uniformity not only in water isotopic composition and Cl content but also in the total composition (H–C–O–S–Cl–F).