## 2 Results and Discussion
### Distribution and Origin of the Heteroaromatic Compounds
**Figure 3.**(**a**) 3-C\({}_{5}\)H\({}_{6}\)S _vs._ C\({}_{4}\)H\({}_{4}\)S and (**b**) C\({}_{6}\)H\({}_{6}\)_vs._ C\({}_{4}\)H\({}_{4}\)S binary diagrams. Symbols as in Figure 2.  
It is worth noting that the \(H\) gases have higher 3-C\({}_{5}\)H\({}_{6}\)S/C\({}_{4}\)H\({}_{4}\)S and C\({}_{7}\)H\({}_{8}\)/C\({}_{6}\)H\({}_{6}\) ratios than the \(M\) ones (Figure 4). This may be caused by the large availability of CH\({}_{4}\) (Table 1) and light hydrocarbons (Table 2) that at hydrothermal conditions can produce free and halogenated radicals that favour the production of 3-C\({}_{5}\)H\({}_{6}\)S from C\({}_{4}\)H\({}_{4}\)S, as well as that of C\({}_{7}\)H\({}_{8}\) from C\({}_{6}\)H\({}_{6}\). Attach of XCH\({}_{3}\)\({}^{+}\) (X = F or Cl), whose formation likely occurs in both geothermal and volcanic fluid reservoirs where halogenated species are abundant [6, 7, 8, 9], on thiophene may give rise to the corresponding methylated derivatives [59]. It is worthy of noting that 3-C\({}_{4}\)H\({}_{4}\)S 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-C\({}_{4}\)H\({}_{4}\)S that results the thermodynamically most stable isomer in natural environments. Alternatively, 3-C\({}_{4}\)H\({}_{4}\)S may be produced through H\({}_{2}\)S 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). **Figure 4.** 3-C\({}_{5}\)H\({}_{6}\)S/C\({}_{4}\)H\({}_{4}\)S _vs._ C\({}_{7}\)H\({}_{8}\)/C\({}_{6}\)H\({}_{6}\) binary diagram. Symbols as in Figure 2.  
**Scheme 2. ** 3-Methylthiophene production from 2-methylbutadiene. In the \(M\) gases, C\({}_{4}\)H\({}_{4}\)O is inversely correlated to C\({}_{4}\)H\({}_{4}\)S (Figure 5). This suggests that the production of C\({}_{4}\)H\({}_{4}\)O is particularly efficient in a magmatic-related environment, where thermodynamic conditions promote the destruction of thiophenes and aromatics. The main mechanism of formation of C\({}_{4}\)H\({}_{4}\)O 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 SO\({}_{2}\)) 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: C\({}_{4}\)H\({}_{8}\)O is dominant in the \(H\) gases, whereas C\({}_{4}\)H\({}_{4}\)O is the most abundant O-bearing heteroaromatic species in the \(M\) gases. This suggests that reducing conditions and relatively low temperature (\(<\)350 \({}^{\circ}\)C), typical of hydrothermal environments, tend to favour the consumption of C\({}_{4}\)H\({}_{4}\)O to produce C\({}_{4}\)H\({}_{8}\)O through catalytic hydrogenation (Scheme 4) [60, 61]. Figure 6: C\({}_{4}\)H\({}_{4}\)O-C\({}_{5}\)H\({}_{6}\)O-C\({}_{4}\)H\({}_{8}\)O triangular diagram. Symbols as in Figure 2.  
**Scheme 4. ** Furan hydrogenation to form C\({}_{4}\)H\({}_{8}\)O.