Stable isotopes of volatile elements as a window into the crust and mantle beneath Icelandic volcanoes

Abstract

Volatile species (e.g., H2O, He, B, CO2, S, Cl) and their isotopes in volcanic materials provide a unique, but underexplored archive of information about magmatic processes and planetary evolution. This study aims to improve our understanding regarding the origins of volatiles in ocean island basalts and the roles played by volatiles in evolving magmas in the crust. To this end, data are presented on the abundances and isotopic compositions of volatiles in lavas and hydrothermal fluids from active volcanoes in Iceland. Special focus is given to the Kverkfjöll volcanic system, for which a comprehensive geochemical dataset is provided. The main results are: (1) Chlorine isotope ratios of silicic volcanic rocks are introduced as a novel tracer of fluid-melt interaction in silicic magma domains, showing that magmatic brine assimilation may be a fundamental, but previously unrecognized process during rhyolite genesis. (2) Sulfur isotopic compositions of Icelandic basalts are shown to reflect both crustal magma evolution and mantle heterogeneity. Distinct Δ33S- δ34S signatures are assigned for recycled and primordial mantle components. (3) Deep intrusive degassing is identified as the source of CO2 and S in Icelandic volcanichydrothermal systems, which are proposed to be the main conduits of mantle-toatmosphere degassing for these volatiles. (4) The geochemistry of the Kverkfjöll magma suite highlights the lithospheric lid control on mantle melting, and an asymmetric distribution of enriched components in the Iceland mantle (5) A subduction fluid-enriched mantle component beneath Iceland is suggested to explain the chlorine-enrichment and back-arc basin-like isotopic signatures of hydrogen, helium, boron, sulfur and chlorine of the Kverkfjöll basalts.