Tracking episodes of outgassing from upper-crustal magma reservoirs through fumarole gas chemistry: the case of the Nisyros caldera (Aegean Arc, Greece)

The Nisyros caldera is located in the easternmost part of the South Aegean Volcanic Arc (SAVA), which formed in the seismically active region of the Aegean and has been site of violent explosive eruptions in the last 160 ky. Two 2–3 km³ dense-rock equivalent silicic explosions caused the collapse of the Nisyros volcanic edifice 60 ka, and were followed by effusive eruptions until 20 ka. More recently, the interplay between an upper-crustal magma reservoir and an active hydrothermal fluid circulation has led to hydrothermal explosions (the most recent in 1887), concurrently with periods of enhanced seismicity. In 1996–1997, a cluster of earthquakes at shallow depth (<10 km) affected the Nisyros area, growing concern for new hydrothermal/volcanic activity. Here, we compare new fumarole chemical and isotopic compositions (2018–2021) with previous data to gain new insights into the state of the magmatic-hydrothermal system and reconstruct its dynamics during the 1996–1997 seismic crisis. New N₂, He, and Ar contents and isotopes show that Nisyros gases are mixtures of magmatic fluids typical of subduction zones, groundwater (or air saturated water, ASW), and air. The composition of the magmatic endmember is calculated through reverse mixing modeling, and shows N₂/He = 31.8 ± 4.5, N₂/Ar = 281.6, δ¹⁵N = +7 ± 3‰, ³He/⁴He = 6.2 Ra (where Ra is air ³He/⁴He), and ⁴⁰Ar/³⁶Ar = 552 ± 20. Although N₂/He is significantly low with respect to typical values for arc volcanoes (1,000–10,000), the contribution of subducted sediments to the SAVA magma generation is reflected by the positive δ¹⁵N values of Nisyros fumaroles. Hence, we suggest that the low N₂/He ratio is not ascribed to the absence of subducted sediments, as previously thought, but to a N₂-depletion due to solubility-controlled differential degassing of an upper-crustal silicic (dacitic/rhyodacitic) reservoir at high-crystallinity. N₂, He, and Ar data reveal clear additions of both magmatic fluid and ASW during the unrest. In the same period, fumarolic vents display a significant increase in magmatic species relative to hydrothermal gas, such as CO₂/CH₄ and He/CH₄ ratios, an increase of ~50 °C in the equilibrium temperature of the hydrothermal system (up to 325 °C), and greater amounts of vapor separation, estimated through the H₂O-H₂-CO-CO₂-CH₄ gas system. 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, without culminating in hydrothermal eruptions. Gas chemistry and reverse mixing modeling of N₂, He, and Ar have therefore important implications for monitoring magmatic-hydrothermal systems, as enable us not only to better understand the state of upper-crustal magma reservoirs, but also to track episodes of magmatic outgassing.