Along-arc, inter-arc and arc-to-arc variations in volcanic gas CO2/ST ratios reveal dual source of carbon in arc volcanism

Some 300–600 Tg of volatiles are globally vented each year by arc volcanism. Such arc gas emissions have contributed to past and present-day evolution of the Earth atmosphere and climate by recycling mineral-bound volatiles subducted along active slabs. Carbon dioxide (CO2) and total sulphur (ST) are, after water, the major components of volcanic arc gases. Understanding their relative abundances (e.g., the CO2/ST ratio) in arc volcanic gases is important to constrain origin and recycling efficiency of these volatiles along the subduction factory, and to better constrain the global arc volcanic CO2 flux. Here, we review currently available information on global variations of volcanic arc CO2/ST gas ratios. We analyse a dataset of > 2000 published volcanic arc gas measurements that comprise (i) low-temperature hydrothermal gas emissions, in which ST is dominated by hydrothermal hydrogen sulphide (H2S), and (ii) high temperature “magmatic” gases rich in sulphur dioxide (SO2). We show that the global CO2/ST population of hydrothermal gases is mainly controlled by S loss to hydrothermal fluids/rocks. We then select a subset of high-temperature (≥ 450 °C) arc gases which, being less affected by S hydrothermal loss, can be used to infer the “deep” source of volatiles. Using a subset of time-averaged high-T gas compositions for 56 arc volcanoes, we identify sizeable along-arc and inter-arc variations in the “magmatic” arc gas CO2/ST ratio, which we ascribe to distinct volatile origins in the magma generation/storage zone. In the attempt to resolve the slab vs. crustal contributions to arc gas budgets, we explore the global association between volcanic gas CO2/ST ratios and non-volatile (trace elements) tracers in arc magmas. For the first time in a global study, we find evidence for higher carbon output (CO2/ST) in arcs where carbonate sediment subducts on the seafloor. Indeed, most arc volcanoes exhibit gas vs. trace element relationships that are explained by addition of slab-sediment melts ± fluids to the mantle wedge. We also identify a subset of CO2-rich arc volcanoes with unusually high CO2/ST ratios (Etna, Stromboli, Vulcano Island, Popocatépetl, Soufriere of St Vincent, Bromo and Merapi), which we interpret as the product of magma-limestone interactions in the upper crust. Evidence for this process comes from carbonate xenoliths and/or carbonate basement that characterise these volcanic systems. Although the mean global CO2/ST ratio of arc gas (~ 2.5) reflects a predominant source from subducted sediment, limestone-assimilation-derived C may account for a substantial (~ 19–32%) fraction of the present-day global arc budget, and may have contributed to elevated atmospheric CO2 levels and warmer climate in Earth's past. Our global CO2/ST vs. trace element association paves the way to identifying the gas signature of volcanoes (or arc segments) for which gas information is currently missing, and so improve our current global volcanic arc CO2 flux inventory.