Compaction profiles of ash-flow tuffs: Modeling versus reality
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This work constitutes the first geochemical, mineralogical and textural analysis of the early fumarolic deposits that appeared in the Tajogaite Volcano (La Palma island, Spain) one month after the official end of the eruptive process. Fourteen fumaroles grouped into 5 study areas have been characterized following a novel methodology that includes on-site microscopic observations and weak samples preservation for their laboratory characterization. Three main mineralogical zones were identified in the fumaroles: 1) sulphur-sulphate; 2) halides; and 3) salammoniac zone. Zones 1 and 2 are included in the S-gas domain and the predominance of sulphur-sulphate minerals or halides in the mineral deposits is dependent on the gas temperature (halides appear in the high temperature fumaroles). Zone 3 overlaps the (HCl-CO) and (HCl) -gas domains. The thermal analysis of the fumaroles provides the sequence of mineral suites according to the vent temperature. Sylvite-halite-tenorite are revealed as typical minerals of high-temperature fumaroles (>530 °C), whilst Ca, Mg and Na sulphates appear in low-temperature fumaroles (<200 °C). Salammoniac-Mascagnite forms in intermediate ranges (170–350 °C). Finally, three different fumarole types are recognized (punctual, diffuse and associated to fissures), establishing different environmental conditions for mineral precipitation. Differences are based on the gas flux mechanism, the gas-rock interaction intensity, temperature and gas concentrations. All these data will serve as a referenced starting point for later studies focused on fumarole evolution and the degassing process of volcanic systems in oceanic volcanic islands framework.
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We investigated the gas geochemistry of fumaroles close to the Voragine crater of Mt Etna that have a temperature of 90–95°C, are CO 2 ‐dominated, and have an air content as low as <1%. This is the first report of the monitoring of such air‐free fumaroles at the Etnean crater area—previous studies indicated an air contribution of 70% or more. The helium and carbon isotopes (Rc/Ra = 6.5 ± 0.4, δ 13 C CO2 = −1.7 ± 0.5‰) suggest that the released gas is directly related to the magmatic degassing. The fumaroles were sampled 12 times between June 2007 and June 2008, which revealed an increase in Rc/Ra from 6.1 to 6.9 that can be related to the increasing volcanic activity at the summit area of Mt Etna. These fumaroles offer a new tool for detecting magmatic processes (magma ascent, refilling, degassing, etc.), and will be useful for volcano surveillance.
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Abstract. Fumarolic fields, especially those with near-surface soil temperature <100 ∘C, are very common features of active or quiescent volcanoes, with both open or closed conduits. Their spatial extent, as well as the time variability of their temperature, are conditioned by three main factors: (1) Local hydro-meteorological conditions; (2) Vapor flow from the underlying volcanic-hydrothermal system; (3) Permeability variation induced by stress field changes and/or deposition dissolution cycles of hydrothermal alteration minerals. Once depurated from the exogenous noise, time variations of the thermal signal, in term of both short-lasting transients and medium/long term trends, reflect changes in the activity state of the related volcanic system, and/or of seismic activity, also of tectonic origin, affecting volcanoes. Theoretical models of heat transfer processes are discussed, highlighting how it is very difficult distinguish between conductive and convective mechanisms or calculating heat fluxes: as a consequence, thermal data from low temperature fumaroles should be used as qualitative proxies of volcano-tectonic phenomena acting on the monitored volcanoes. Following the description of the measuring systems and of the criteria for designing a performing network for thermal monitoring of fumaroles, some case histories from Italian volcanoes (Vulcano, Stromboli, Mt. Etna, Mt. Vesuvius) are presented, illustrating how in the last years the monitoring of low temperature fumaroles have given useful insights on the evolution of the activity state of these volcanoes.
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Fumarole temperatures are the ultimate results of many processes that are encountered by deep fluids during their passage to the surface. Here, the time variations of high-temperature fumaroles acquired by continuous monitoring are presented, to show the effects of the forces that act on the system. Data acquired by continuous monitoring of fumaroles and the time relationships with the different parameters related to the activity of the volcanic system are discussed. From 1998 to 2010, the temperature and compositional changes of fumarolic gases were monitored at the same time as variations in the number of volcano-seismic events, which indicate frequent variations of energy release (heat and mass flow, and seismic strain release). Geochemical modeling applied to the volcanic system of Vulcano Island suggests that the overall expansion of magmatic gas through the fractured system is an almost iso-enthalpic process at depth, which shifts to an adiabatic process at shallow depth, where the rock permeability increases. Thus, the time variations of the fumarole temperatures reflect various physical variations of the system that can either occur at depth or close to the surface. The temperature monitoring performed in the fumarolic area of La Fossa Cone showed short-term effects related to rain events, and negligible effects related to other external agents (ambient temperature and atmospheric pressure variations). At the same time, the long-term monitoring highlighted some mean-term and long-term variations. These last are the main characters observed in the time-series, and they both appear to be related to endogenous forces that perturb the equilibrium of this complex geochemical system.
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Monitoring the volcanic activity of Teide, the only active stratovolcano in Tenerife, the largest island of the Canary Islands, is extremely important for the prevention and reduction of the volcanic disasters of the island. As part of the geochemical monitoring of the Teide volcanic activity, during the last three decades the volcano has been the subject of a geochemical monitoring of the fumarole discharges, characterized by low flux emission of fluids with temperatures of ∼83°C, located at the Teide summit crater (Pérez et al., 1992 and 1996; Melián et al., 2012). Teide fumaroles show chemical compositions typical of hydrothermal fluids, i.e., meteoric steam dominates the gas composition, followed by CO2, N2, H2, H2S, HCl, Ar, CH4, He, and CO (Pérez et al., 1992). The temporal variations in fumarole gas chemistry at Teide volcano was useful to detect significant changes in the chemical composition of the Teide fumarole, including the appearance of SO2, and increases in the HCl and CO concentrations, one year before a seismic crisis that occurred in Tenerife Island between April and June 2004, what suggested that the associated temporal changes in seismic activity and magmatic degassing indicate that geophysical and fluid geochemistry signals in this system are unequivocally related (Melián et al., 2012). The average of the air-corrected 3He/4He ratio during the period 1991-2022 was 6.80 RA (being RA the atmospheric ratio), with the maximum value of the time series (7.57 RA) measured in August 2016, when an input of magmatic fluids triggered by an injection of fresh magma and convective mixing took place beneath Teide volcano (Padrón et al., 2021). After such input of magmatic fluids, increases in the CO2/H2O, C/S and He/CO2 ratios, a decrease in the CO/CO2 ratio were observed together with a significant increase in the seismic activity recorded in the island of Tenerife. This work highlights the important role of volcanic gases in the monitoring of volcanic activity, paying attention to different chemical and isotopic species in the fumarolic discharges. Melián G.V. et al., (2012), Bull. VolcanoL. 74, 1465–1483.Padrón E. et al., (2021), J. Geophys. Res. 126, e2020JB020318.Pérez N.M. et al., (1992), Actas de las sesiones científicas. III Congreso Geológico de España, 1, 463–467.Pérez N.M. et al., (1996), Geophys. Res. Lett. 23(24), 3531–3534.  
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