Nitrogen, helium, and argon reveal the magmatic signature of fumarole gases and episodes of outgassing from upper-crustal magma reservoirs: The case of the Nisyros caldera (Aegean Arc, Greece)

The chemical composition of gases emitted by active volcanoes reflects both magma degassing and shallower processes, such as fluid-rock hydrothermal interaction and mixing with atmospheric-derived fluids. Untangling the magmatic fluid endmember within surface gas emission is therefore challenging, even with the use of well-known magma degassing tracers such as noble gases. Here, we investigate the deep magmatic fluid composition at the Nisyros caldera (Aegean Arc, Greece) by measuring nitrogen and noble gas abundances and isotopes in naturally degassing fumaroles. Gas samples were collected from 32 fumarolic vents at water-boiling temperature between 2018 and 2021. These fumaroles are admixtures of magmatic fluids typical of subduction zones, groundwater (or air saturated water, ASW), and air. The N-2, He, and Ar composition of the magmatic endmember is calculated by reverse mixing modeling and shows N-2/He = 31.8 +/- 4.5, N-2/Ar = 281.6, delta N-15 = +7 +/- 3 parts per thousand, He-3/He-4 = 6.2 Ra (where Ra is air He-3/He-4), and Ar-40/Ar-36 = 551.6 +/- 19.8. Although N-2/He is significantly low with respect to typical values for arc volcanoes (1,000-10,000), the contribution of subducted sediments to the Aegean Arc magma generation is reflected by the positive delta N-15 values of Nisyros fumaroles. The low N-2/He ratio indicates N-2-depletion due to solubility-controlled differential degassing of an upper-crustal silicic (dacitic/rhyodacitic) melt in a high-crystallinity reservoir. We compare our 2018-2021 data with N-2, He, and Ar values collected from the same fumaroles during a hydrothermal unrest following the seismic crisis in 1996-1997. Results show additions of both magmatic fluid and ASW during this unrest. In the same period, fumarolic vents display an increase in magmatic species relative to hydrothermal gas, such as CO2/CH4 and He/CH4 ratios, an increase of similar to 50 degrees C in the equilibrium temperature of the hydrothermal system (up to 325 degrees C), and greater amounts of vapor separation. These variations reflect an episode of magmatic fluid expulsion during the seismic crisis. The excess of heat and mass supplied by the magmatic fluid injection is then dissipated through boiling of deeper and peripheral parts of the hydrothermal system. Reverse mixing modeling of fumarolic N-2-He-Ar has therefore important ramifications not only to disentangle the magmatic signature from gases emitted during periods of dormancy, but also to trace episodes of magmatic outgassing and better understand the state of the upper crustal reservoir. (C) 2022 The Author(s). Published by Elsevier Ltd.