High abundances of noble gas and chlorine delivered to the mantle by serpentinite subduction

The Earth's mantle contains non-radiogenic noble gas isotopes that imply transfer of noble gases from the atmosphere into the mantle through subduction. Hydrated serpentinite rocks within subducting oceanic lithosphere are recognized as key carriers of water and chlorine, but the pathways for noble gas subduction have been poorly constrained. Here we analyse the concentration of noble gas isotopes and halogens in rocks from the Ligurian Alps, Italy and the Betic Cordillera, Spain. These rocks and the fluid inclusions trapped within them preserve a record of serpentinite dehydration during progressively deeper stages of subduction. We find that the noble gas and halogen signature of serpentinites reflects that of sea water and sediment pore fluids. The fluids released become progressively depleted in noble gases and chlorine is subducted to greater depths than bromine or iodine. After complete breakdown of the serpentinites at mantle depths of around 70 km, the dehydrated rock residues still retain a seawater-derived noble gas signature and significant chlorine. We suggest that these samples are proxies for serpentinite dehydration in cold slabs at mantle depths of 200 km or greater. We conclude that atmospheric noble gases are readily incorporated into hydrous minerals formed close to the sea floor, and incompletely removed by subduction zone metamorphism. This implies that noble gases can be subducted to great depths in the Earth's mantle.