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 andshallower processes, such as fluid-rock hydrothermal interaction and mixing with atmospheric-derivedfluids. 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 investigatethe deep magmatic fluid composition at the Nisyros caldera (Aegean Arc, Greece) by measuring nitrogenand noble gas abundances and isotopes in naturally degassing fumaroles. Gas samples were collectedfrom 32 fumarolic vents at water-boiling temperature between 2018 and 2021. These fumaroles areadmixtures of magmatic fluids typical of subduction zones, groundwater (or air saturated water, ASW),and air. The N2, He, and Ar composition of the magmatic endmember is calculated by reverse mixingmodeling and shows N2/He = 31.8 - 4.5, N2/Ar = 281.6, d15N = +7 - 3 %o, 3He/4He = 6.2 Ra (where Ra isair 3He/4He), and 40Ar/36Ar = 551.6 - 19.8. Although N2/He is significantly low with respect to typical val-ues for arc volcanoes (1,000-10,000), the contribution of subducted sediments to the Aegean Arc magmageneration is reflected by the positive d15N values of Nisyros fumaroles. The low N2/He ratio indicatesN2-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 N2, He, andAr values collected from the same fumaroles during a hydrothermal unrest following the seismic crisisin 1996-1997. Results show additions of both magmatic fluid and ASW during this unrest. In the sameperiod, fumarolic vents display an increase in magmatic species relative to hydrothermal gas, such asCO2/CH4 and He/CH4 ratios, an increase of circa50 °C in the equilibrium temperature of the hydrothermalsystem (up to 325 °C), and greater amounts of vapor separation. These variations reflect an episode ofmagmatic fluid expulsion during the seismic crisis. The excess of heat and mass supplied by themagmatic fluid injection is then dissipated through boiling of deeper and peripheral parts of thehydrothermal system. Reverse mixing modeling of fumarolic N2-He-Ar has therefore important ramifica-tions not only to disentangle the magmatic signature from gases emitted during periods of dormancy, butalso to trace episodes of magmatic outgassing and better understand the state of the upper crustalreservoir.