Helium, carbon and nitrogen isotope evidence for slab influence on volcanic gas emissions at Rabaul caldera, Papua New Guinea

The chemical and isotopic composition of the gases emitted by subduction zone volcanoes can provide insights into the origin of magmatic volatiles. In volcanic arcs, magmatic volatiles and therefore emitted gases can be supplied from the mantle, the subducting slab, or the rocks of the arc crust. Determining the relative contributions of these distinct sources is important for understanding the transfer of volatiles between Earth’s interior and exterior reservoirs, which has implications for the physical and chemical evolution of both the mantle and the atmosphere. Each subduction zone is a different experiment in recycling efficiency according to the composition of the slab and the pressure-temperature path it experiences on subduction, and accordingly all volcanic arc emissions can be characterised by their particular chemical and isotopic compositions. In this study, we analyse the composition of volcanic gases from Rabaul caldera in the New Britain subduction zone, Papua New Guinea, and show that the emissions are substantially influenced by slab recycling of carbon and nitrogen. We find helium emissions are dominated by a mantle contribution, with little influence from the arc crust. Carbon isotopes point to a mixture of mantle, carbonate and organic sediment-derived contributions, with carbonate dominant. This may be of sedimentary origin, seafloor calcareous muds, or altered basalts of the subducting oceanic crust. Nitrogen isotopes also indicate a significant influence of organic sediments. Our study is the first comprehensive investigation of volatile sources in this subduction and our results and interpretation are consistent with previous studies of element recycling based on New Britain arc lavas.