Tracking episodes of outgassing from upper-crustal magma reservoirs through fumarole gas chemistry: the case of the Nisyros caldera (Aegean Arc, Greece)
Authors: Bini Giulio , Chiodini Giovanni , Caliro Stefano , Tassi Franco , Vaselli Orlando , Rizzo Andrea , Mollo Silvio , Vougioukalakis Georgios , Bachmann Olivier 
Published: February 2023

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&#8211;3 km3&#160;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&#8211;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&#8211;2021) with previous data to gain new insights into the state of the magmatic-hydrothermal system and reconstruct its dynamics during the 1996&#8211;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 &#177; 4.5, N2/Ar = 281.6, &#948;15N = +7 &#177; 3&#8240;, 3He/4He = 6.2 Ra (where Ra is air 3He/4He), and 40Ar/36Ar = 552 &#177; 20. Although N2/He is significantly low with respect to typical values for arc volcanoes (1,000&#8211;10,000), the contribution of subducted sediments to the SAVA magma generation is reflected by the positive &#948;15N 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 &#176;C in the equilibrium temperature of the hydrothermal system (up to 325 &#176;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.