Chlorine isotope composition of volcanic gases and rocks at Mount Etna (Italy) and inferences on the local mantle source

• We investigated the chlorine isotope composition in gases and lavas from Mount Etna (Italy).
• The Etnean δ37Cl value for magmatic chlorine is ∼0±0.7‰.
• Magma degassing does not modify the isotopic composition of chlorine.
• High-temperature gases collected in 2009 were affected by secondary process (i.e., HCl removal).
• Mantle source below Mount Etna is characterized by altered oceanic crust added to depleted mantle.

We present the first chlorine isotope compositional data for gases and lavas from Mount Etna (Italy), and to our knowledge, of active Mediterranean volcanism. We investigated lavas erupted and gases discharged during 2008–2011 from a high-temperature fumarole (HT; >300 °C) and plume gases from both North East and Central Craters. Most of the samples vary in a narrow range of chlorine isotope composition (δ37Cl values≈0±0.7‰) with gases partially overlapping with rocks. Only HT gases sampled in 2009 have been clearly affected by secondary processes (δ37Cl values>15.9‰), resulting in partial removal of chlorine and isotopic fractionation producing a 37Cl enrichment in the residual gaseous HCl. These secondary processes also affect, although to a lesser extent, plume gases from North East Crater (NEC). Although post-magmatic processes are able to modify the chlorine isotope composition, δ37Cl values are not affected by magma degassing for residual fractions ≥0.3 in the melt, or any effect is within our data variability. Finally, δ37Cl values and Cl/K ratios of magmatic chlorine constrain the Etnean source to be compatible with depleted mantle (DMM) contaminated by altered oceanic crust (AOC), in agreement with indications from more common isotopic tracers of mantle processes.