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The composition of non-methane organic volatile compounds (VOCs) determined in 139 thermal gas discharges from 18 different geothermal and volcanic systems in Italy and Latin America, consists of C2–C20 species pertaining to the alkanes, alkenes, aromatics and O-, S- and N-bearing classes of compounds. Thiophenes and mono-aromatics, especially the methylated species, are strongly enriched in fluids emissions related to hydrothermal systems. Addition of hydrogen sulphide to dienes and electrophilic methylation involving halogenated radicals may be invoked for the formation of these species. On the contrary, the formation of furans, with the only exception of C4H8O, seems to be favoured at oxidizing conditions and relatively high temperatures, although mechanisms similar to those hypothesized for the production of thiophenes can be suggested. Such thermodynamic features are typical of fluid reservoirs feeding high-temperature thermal discharges of volcanoes characterised by strong degassing activity, which are likely affected by conspicuous contribution from a magmatic source. The composition of heteroaromatics in fluids naturally discharged from active volcanoes and geothermal areas can then be considered largely dependent on the interplay between hydrothermal vs. magmatic contributions. This implies that they can be used as useful geochemical tools to be successfully applied in both volcanic monitoring and geothermal prospection.
Chemical constituents released from gas discharges in active and quiescent volcanic complexes and geothermal systems can be related to: 1) primary (magma degassing) and 2) secondary (gas-water-rock interactions occurring at relatively shallow depth) sources. The ratio between magmatic vs. hydrothermal contributions is generally indicative of the state of activity of a volcanic system and is a basic parameter in terms of volcanic surveillance [1–4]. Magma degassing produces highly acidic and corrosive gas compounds that may affect the geothermal potential of a hydrothermal reservoir. Hydrothermal fluid composition is constituted by water vapor and CO2 and show significant concentrations of reduced gas species, such as H2S, H2 CO and CH4 [5,6]. Magmatic-related fluid contributions, although mainly consisting of the same gases dominating hydrothermal fluids, i.e., water vapor and CO2, can unequivocally be recognized in thermal discharges by the presence of highly acidic compounds, especially SO2 [7–9]. Secondary interactions, such as gas scrubbing processes within shallow aquifers [10,11], are able to strongly affect this highly soluble and reactive gas compounds, frequently masking any clue of magmatic-related fluid contribution at surface. The behaviour of hydrocarbons in natural fluid discharges has recently been considered as a potential tool to investigate the thermodynamic conditions controlling fluid reservoirs feeding fumarolic exhalations in volcanic and geothermal systems [12–15]. These investigations have demonstrated that light hydrocarbons, especially the C2–C4 alkenes-alkanes pairs, play an important role in both geochemical surveillance of volcanic systems and geothermal prospection. On the contrary, little attention was devoted to heavier organic compounds for similar purposes.
In the present work, 139 gas emissions from active volcanoes and geothermal systems set in different geodynamical environments were analysed for the determination of non-methane VOC (Volatile Organic Compound) composition, especially that of heteroaromatic and aromatic compounds. On the basis of this dataset, the main goals were to: 1) assess the origin of thiophenes and furans in naturally discharged fluids and 2) evaluate the possible use of these compounds as geochemical tracers to discriminate different fluid source regions in volcanic-hydrothermal environment.
A representative composition of the main gas components in fluid discharges from the volcanic and geothermal systems investigated in the present study is listed in Table 1. Gas concentrations are expressed in μmol/mol and referred to the dry gas phase. Gas samples from Teide (Spain), Turrialba (Costa Rica), Vulcano (Italy), Lascar and Tacora (Chile) volcanoes have outlet temperatures varying in a wide range (from 72 to 405 °C) and show a chemical composition dominated by CO2 and characterised by variable amounts of SO2 (from 1.2 to 569394 μmol/mol).
Such features, coupled with relatively low concentrations of CH4 (<690 μmol/mol) and high concentrations of HCl (from 351 to 74540 μmol/mol) and H2 (up to 32591 μmol/mol), indicate that the gas chemistry of these systems is strongly controlled by magma degassing . This hypothesis is in agreement with previous studies [17–20] that investigated the source of fluids produced by the intense fumarolic activity recently observed at these volcanic systems. A different chemistry characterises gases from (1) El Tatio (Chile), Larderello (Italy) and Tendaho (Ethiopia) geothermal systems [21–23] (samples #48–64), and (2) volcanoes whose degassing activity is considered to be mainly related to boiling of extended hydrothermal reservoirs (Copahue, Argentina; Deception, Antarctica; El Chichon, Mexico; Ischia, Pantelleria, Phlegrean Fields and Vesuvio, Italy; Nisyros, Greece; Tatun, Taiwan; Yellowstone, USA) [24–31] (samples #65–139). This group shows SO2 below the instrumental detection limit (≈0.01μmol/mol: samples #48–138), relatively low outlet temperatures <118 °C, high CH4 concentrations (up to 64103 μmol/mol), and HCl not exceeding 500 μmol/mol.
Up to 129 different non-methane VOCs, pertaining to the alkane (27 compounds), aromatic (21 compounds), cyclic (17 compounds), alkene (15 compounds), Cl-bearing (13 compounds), O-bearing (ketones, aldehydes, organic acids and alcohols; 36 compounds) and heteroaromatic (7 compounds) groups, were determined (Table 2; gas concentrations are expressed in ppb by volume and referred to the dry gas phase). The total VOC concentrations in gases with a dominating magmatic contribution (samples #1–47, hereafter M gases) are relatively low (from 61 to 1664 ppbv), whereas they range from 180 to 1235942 ppbv (Table 2) in those gases (#48–141) characterised by prevalent hydrothermal contribution (hereafter H gases). Hydrothermal reservoirs, even when associated with volcanic systems, are commonly recharged by fluids circulating within organic-bearing sedimentary rocks. This organic source is then transformed into VOCs through biogenic and thermogenic processes . Therefore, relatively high VOC concentrations in the H gases are expected. On the contrary, the organic-rich component constitutes a minor fraction of the M gases, since it is generally destroyed by high-temperature, oxidizing fluids released from the magmatic melts . The relative percentages (mean values) of the different groups of VOCs can provide preliminary indications to distinguish the M and H gases: in the M gases, alkane, Cl-bearing, aromatic, heteroaromatic and alkene compounds are present in almost comparable amounts (31, 28, 16, 16 and 9% of total VOCs, respectively), whereas cyclic and O-bearing species represent a small VOC fraction (<0.04%) (Figure 1a). The organic fraction of the H gases is largely dominated by alkanes and aromatics (54 and 24%, respectively) with minor cyclic, alkene, heteroaromatic and O-bearing compounds (from 0.7 to 1.4%), and traces (<0.04%) of Cl-bearing compounds (Figure 1b). These evidences are consistent with recent investigations that have highlighted a recurrent relation between VOC speciation and thermodynamic conditions at the fluid source in gas discharges from volcanic and geothermal systems [33–36]. Predominance of alkanes and aromatics in both the M and H gases was considered to reflect the proceeding of “reforming” processes, which in geothermal areas, as well as in hydrothermal systems commonly surroundings active volcanoes, are favoured by the large availability of catalytic agents, such as free acids, allumosilicates and sulphur gas species [37,38]. Pyrolysis of organic material was found to produce mostly alkanes and, secondarily, aromatics [39,40], whereas Fischer-Tropsch reactions were invoked for the production of light alkanes and, at a minor extent, alkenes . The presence of halocarbons in volcanic gas emissions was attributed to either the product of pyrolysis of adjacent vegetation [42,43] or, alternatively, air contamination [44,45]. Conversely, organic geochemical evidence supported a pristine abiogenic origin by high-temperature gas-phase radical reactions [46,47].
Concentrations (in ppbv, referred to the dry gas phase) of C4H4O, 3-C5H6O, C4H8O, 2-C5H10O, C4H4S, 3-C5H6S and 2,5-C6H8S, and those of the simplest aromatics (C6H6 and C7H8), are reported in Table 3. In the M gases, the concentrations of furans tend to be higher than those of thiophenes (their sum ranging from 1.9 to 35 and from 0.2 to 26 ppbv, respectively); C4H4S is largely the most abundant S-bearing compound (up to 1.9 ppbv), whereas C4H4O (up to 31 ppbv) dominates the furan composition. Conversely, the H gases have relatively high concentrations of C4H4S and 3-C5H6S (up to 191 and 121 ppbv, respectively), minor 2,4-C6H8S (up to 13 ppbv), and no furans, with the only exception of those from Deception, Nisyros, Vesuvio, Copahue and El Chichon volcanoes. As shown in Figure 2, in the H gases thiophenes are strongly related to H2S, (in hydrothermal environment SO2 concentrations are the below detection limit; Table 1). This correlation would imply that the formation of the S-bearing heteroaromatics intimately depends on sulphur fugacity (fS) at the fluid source, and likely occurs within deep fluid reservoirs where H2S is also produced. This hypothesis is consistent with the composition of fluids from carbonate reservoirs affected by thermochemical sulphate reduction: the higher the H2S fugacity, the higher content of organic sulphur compounds in the coexisting hydrocarbon phase [48,49]. Moreover, sulphidation of organic matter giving rise to thiophenes was found to be associated with gold mineralization deriving from hydrothermal fluids [50,51]. According to these considerations, it is reasonable to suppose that thiophenes can efficiently be produced in a hydrothermal reservoir, this environment being commonly characterised by reducing conditions, relatively high fS and temperature <350 °C [5,6]. Production of C4H4S by reaction of light alkenes, such as C2H4, with FeS2 and H2S was invoked to explain their presence in the volcanic gases emitted from Mt. Etna .
On an industrial scale thiophene is synthesized through the following catalytic processes: 1) reaction of C4+ alcohols or carbonyls with CS2 over alkali-promoted alumina; 2) reaction of unsaturated aldehydes with H2S over an alkali-promoted alumina; 3) reaction of C4+ alkyl hydrocarbons or olefins with CS2, S, and H2S over alkali-promoted alumina; 4) catalytic dehydrogenation of tetrahydrothiophene; 5) synthesis from furan and H2S over alumina [53–56]. The thiophene Paal-Knorr synthesis involves the reaction of 1,4-diketones with H2S as sulphurising agent . Generally speaking, in gases from natural fluid discharges, the most reliable genetic mechanism for the formation of thiophene is through the addition of H2S to dienes in the presence of H+ and metal catalysts (Scheme 1).
In the M gases heteroaromatics and inorganic sulphur-bearing gases, the latter being constituted by SO2 and H2S at comparable concentrations (Table 1), are apparently showing an inverse correlation (Figure 2). This suggests that thiophenes, which are less reactive than other five-membered heteroaromatics, including furans, serving as dienes during Diels-Alder reactions , tend to be destroyed when fluid reservoirs are affected by conspicuous contribution from magmatic degassing.
The C4H4S concentrations and those of 3-C5H6S (Figure 3a) and C6H6 (Figure 3b) show a positive correlation in both H and M gases. This supports the following hypotheses: 1) at hydrothermal conditions mono-aromatics and thiophenes are efficiently produced by similar genetic processes; 2) all these compounds have a similar behaviour in response to thermodynamic conditions caused by presence of oxidizing and high temperature (>400 °C) magmatic fluids.
It is worth noting that the H gases have higher 3-C5H6S/C4H4S and C7H8/C6H6 ratios than the M ones (Figure 4). This may be caused by the large availability of CH4 (Table 1) and light hydrocarbons (Table 2) that at hydrothermal conditions can produce free and halogenated radicals that favour the production of 3-C5H6S from C4H4S, as well as that of C7H8 from C6H6. Attach of XCH3+ (X = F or Cl), whose formation likely occurs in both geothermal and volcanic fluid reservoirs where halogenated species are abundant [6–9], on thiophene may give rise to the corresponding methylated derivatives . It is worthy of noting that 3-C4H4S is the only methyl-thiophene recognized in both geothermal and volcanic gases (Table 3), although electrophilic methylation of thiophene is able to produce different isomers. This may be explained by the occurrence of secondary isomerization of methylated thiophenes favouring 3-C4H4S that results the thermodynamically most stable isomer in natural environments. Alternatively, 3-C4H4S may be produced through H2S adding to dienes, such as 2-methylbutadiene originated by isomerization of 1.3-pentadiene (Scheme 2). Double methylation seems to be favoured when methyl substitutions are stabilized at positions 2 and 4 (Table 3).
In the M gases, C4H4O is inversely correlated to C4H4S (Figure 5). This suggests that the production of C4H4O is particularly efficient in a magmatic-related environment, where thermodynamic conditions promote the destruction of thiophenes and aromatics.
The main mechanism of formation of C4H4O may be related to the Paal-Knorr synthesis (Scheme 3), which is efficient under acidic conditions, such as those determined by the huge amounts of highly acidic gas species (HF, HCl and SO2) occurring in the M gases (Table 1).
As shown in Figure 6, the H and M gases can also be clearly distinguished on the basis of the relative concentrations of furans: C4H8O is dominant in the H gases, whereas C4H4O is the most abundant O-bearing heteroaromatic species in the M gases. This suggests that reducing conditions and relatively low temperature (<350 °C), typical of hydrothermal environments, tend to favour the consumption of C4H4O to produce C4H8O through catalytic hydrogenation (Scheme 4) [60,61].
Gas samples for the determination of the main gas species were collected into pre-evacuated 60 mL glass flasks filled with 20 mL of a 4N NaOH and 0.15 M Cd(OH)2 suspension. Quartz-glass dewar tubes and a plastic funnel were used to convoy the gas into the sampling flasks from 1) fumarolic vents and 2) boiling pools, respectively. During sampling, CO2, SO2 and HCl dissolved into the alkaline solution, water vapour condensed, and H2S reacted with Cd2+ to form insoluble CdS, allowing the residual gases (N2, CH4, Ar, O2, H2, and light hydrocarbons) to be concentrated in the head-space [62–64]. Gas samples for the determination of VOC composition were collected with the same devices used for the conventional gas sampling, and stored into pre-evacuated 12 mL glass vials equipped with pierceable rubber septum (Labco Exetainer®).
Nitrogen, Ar, O2 and H2 were analysed with a Shimadzu 15A gas-chromatograph equipped with Thermal Conductivity Detector (TCD) and a 9 m, 5A molecular sieve column. Methane and C1–C4 alkanes and alkenes were analysed with a Shimadzu 14a gas-chromatograph equipped with Flame Ionization Detector (FID) and a 10 m long stainless steel column ( = 2 mm) packed with Chromosorb PAW 80/100 mesh coated with 23% SP 1700. The alkaline solution, separated from the solid precipitate by centrifugation at 4,000 rpm for 30 min, was used for the determination of: 1) CO2 as CO32− by titration with 0.5 N HCl solution; 2) SO2 as SO42−, after oxidation with H2O2, by ion-chromatography (Metrohm Compact 761); 3) HCl, as Cl− by ion-chromatography. The solid precipitate was oxidized by H2O2 to determine H2S as SO42− by ion-chromatography [63,64]. The analytical error is <5%.
The VOCs were pre-concentrated and transferred from the sampling vials into the column headspace of a Thermo Trace GC Ultra gas chromatograph by using a manual SPME (solid-phase micro-extraction) device introduced through the silicon membrane of the glass vial to expose the gas mixtures to a divinylbenzene (DVB)-Carboxen-polydimethylsiloxane (PDMS), 50/30 μm, 2 cm long fibre assembly (Supelco; Bellefonte, PA, USA) for 15 min . The usefulness of the SPME method  for the VOC analysis has widely been demonstrated [67–69]. The DVB-Carboxen-PDMS fibre was selected by its high retentive properties, a feature that is particularly appropriate for analysis aimed to the determination of the organic compounds of interest for the present paper. A Thermo Trace GC Ultra gas chromatograph coupled with a Thermo DSQ Quadrupole Mass Spectrometer was used for analytical separation and detection. The mass spectrometer operated in full scan mode, in the mass range 40–400 m/z. The transfer-line temperature was set at 230 °C. The mass detector was equipped with EI set at 70 eV. The source temperature was 250 °C. The gas chromatograph was equipped with a split/splitless injection port operating in the splitless mode with a dedicated SPME liner (0.75 mm i.d.). Analytes were desorbed from the SPME fiber through direct exposure for 2 min in the GC injection port, heated at 230 °C. The chromatographic column was a 30 m × 0.25 mm i.d. 0.25 μm film thickness TV1-MS fused silica capillary column (Thermo). The carrier gas was helium set to a flow-rate of 1.3 mL/min in constant pressure mode. The column oven temperature program was the following: 35 °C (hold 10 min), rate 5.5 °C/min to 180 °C (hold 3 min), rate 20 °C/min to 230 °C (hold 6 min) . Compounds were identified by comparison of the mass spectra with those of the NIST05 library (NIST, 2005).
The VOCs identified by mass spectrometry were quantified using an external standard calibration procedure performed on the basis of calibration curves created by analyzing gaseous standard mixtures of the main VOC groups, i.e., alkanes, alkenes, aromatics, cyclics, chlorofluorocarbons, ketones, aldehydes and heteroaromatics. The values of the Relative Standard Deviation (RDS), calculated from seven replicate analyses of a gaseous mixture in which the compounds of interest were present at a concentration of 2 ppmv, are <7%. Eventually, the detection limits were determined by linear extrapolation from the lowest standard in the calibration curve using the area of a peak having a signal/noise ratio of 5 .
The distribution of thiophenes and furans in gases from hydrothermal and magmatic-hydrothermal systems have been revealed to be strongly dependent on the physical-chemical conditions acting on fluid reservoirs, where VOCs are produced via a complex series of catalytic processes, involving organic matter buried in sedimentary formations. Thiophene seems to be efficiently produced at hydrothermal conditions and tend to be destroyed in presence of hot, highly oxidizing fluids from a magma source. On the contrary, the formation of C4H4O seems to be favoured at highly acidic and oxidizing conditions that are determined by the presence of fluids from magmatic degassing. Methylated and hydrogenated heteroaromatics are also preferentially associated with hydrothermal conditions. According to these considerations, the composition of O- and S-bearing heteroaromatics can be utilized in both volcanic and geothermal systems to evaluate contributions of fluids produced in different “natural dominions”, i.e., hydrothermal and magmatic. These results may imply useful applications in volcanic monitoring and geothermal prospection, although the existing dataset should be expanded to better constrain the behaviour of these new geochemical tracers. Experimental runs, able to test the mechanisms of formation and stability of heteroaromatics at temperature, redox and catalytic conditions resembling those of a volcano-hydrothermal environment, would probably useful to better constrain their behaviour.
Many thanks are due two anonymous reviewers who improved an early version of the manuscript. This work partly benefited from the financial support of the Laboratories of CNR-IGG and Department of Earth Sciences of Florence (Italy).
Sample Availability: Available from the authors.