## 2 Results and Discussion
### Distribution and Origin of the Heteroaromatic Compounds  
Concentrations (in ppbv, referred to the dry gas phase) of C\({}_{4}\)H\({}_{4}\)O, 3-C\({}_{5}\)H\({}_{6}\)O, C\({}_{4}\)H\({}_{8}\)O, 2-C\({}_{5}\)H\({}_{10}\)O, C\({}_{4}\)H\({}_{4}\)S, 3-C\({}_{5}\)H\({}_{6}\)S and 2,5-C\({}_{6}\)H\({}_{8}\)S, and those of the simplest aromatics (C\({}_{6}\)H\({}_{6}\) and C\({}_{7}\)H\({}_{8}\)), are reported in Table 3. In the M gases, the concentrations of furans tend to be higher than those ofiophenes (their sum ranging from 1.9 to 35 and from 0.2 to 26 ppbv, respectively); C\({}_{4}\)H\({}_{4}\)S is largely the most abundant S-bearing compound (up to 1.9 ppbv), whereas C\({}_{4}\)H\({}_{4}\)O (up to 31 ppbv) dominates the furan composition. Conversely, the H gases have relatively high concentrations of C\({}_{4}\)H\({}_{4}\)S and 3-C\({}_{5}\)H\({}_{6}\)S (up to \begin{table}
\begin{tabular}{l c c c c c c c c} \hline \hline  & & & & & & **Cl-** & **O-** & & \\ \(\mathbf{n}^{\circ}\) & **sample** & **Alkanes** & **Aromatics** & **Cyclies** & **Alkenes** & **Bearing** & **bearing** & **heteroaromatics** & **sum** \\ \hline
125 & Tatun 7 & 29548 & 2581 & 697 & 362 & _5.5_ & 783 & 45 & 34022 \\
126 & Yellowstone 1 & 339119 & 3659 & 4020 & 2694 & 26 & 2274 & 22 & 351814 \\
127 & Yellowstone 2 & 121602 & 8281 & 3237 & 4040 & 21 & 1599 & 24 & 138804 \\
128 & Yellowstone 3 & 174226 & 16987 & 1739 & 4662 & 49 & 1387 & 15 & 199066 \\
129 & Yellowstone 4 & 190197 & 18516 & 3805 & 2091 & 65 & 1561 & 69 & 216304 \\
130 & Yellowstone 5 & 150632 & 8311 & 1634 & 1295 & 28 & 1275 & 43 & 163218 \\
131 & Yellowstone 6 & 258551 & 10288 & 3019 & 2757 & 29 & 1767 & 43 & 276453 \\
132 & Yellowstone 7 & 489610 & 9410 & 10816 & 4226 & 75 & 3605 & 40 & 517782 \\
133 & Yellowstone 8 & 79168 & 2164 & 1327 & 2274 & 35 & 639 & 35 & 85641 \\
134 & Yellowstone 9 & 320241 & 3399 & 1987 & 3556 & 28 & 1250 & 35 & 330496 \\
135 & Yellowstone 10 & 40923 & 1187 & 871 & 1296 & 49 & 356 & 43 & 44725 \\
136 & Yellowstone 11 & 1012181 & 8996 & 9685 & 9178 & 78 & 6698 & 27 & 1046843 \\
137 & Yellowstone 12 & 47727 & 671 & 560 & 1313 & 20 & 210 & 21 & 50522 \\
138 & Yellowstone 13 & 413725 & 5214 & 9650 & 4224 & 66 & 1818 & 41 & 434738 \\
139 & Yellowstone 14 & 1156785 & 27956 & 34552 & 9520 & 76 & 6999 & 54 & 1235942 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: _Cont._  
Figure 1: Relative concentrations, expressed as % of the total VOC abundances, of alkane, aromatic, cyclic, alkene, _Cl_-bearing, _O_-bearing and heteroaromatic compounds in **(a)**_M_ and **(b)**_H_ gases. 191 and 121 ppbv, respectively), minor 2,4-C\({}_{6}\)H\({}_{8}\)S (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 H\({}_{2}\)S, (in hydrothermal environment SO\({}_{2}\) 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 H\({}_{2}\)S is also produced. This hypothesis is consistent with the composition of fluids from carbonate reservoirs affected by thermochemical sulphate reduction: the higher the H\({}_{2}\)S fugacity, the higher content of organic sulphur compounds in the coexisting hydrocarbon phase [48, 49]. Moreover, sulphidation of organic matter giving rise toiophenes 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 \({}^{\circ}\)C [5, 6].