Extremely deuterium depleted methane revealed in high-temperature volcanic gases

Active volcanoes often discharge hot (T >> 100 °C) magmatic gases whose original composition has been modified through partial interaction with an externally fed hydrothermal system. The study of methane (CH4) in these volcanic discharges may provide useful information on the interplay between deep magmatic gases and shallow circulation of hydrothermal fluids. However, the origin of CH4 in high-temperature volcanic gases and the factors exerting control on its abundance and stable isotope composition are still largely unknown. Here, we present the abundances and stable isotopic composition of CH4 in hot (99-387 °C) volcanic gases from the La Fossa volcanic crater of Vulcano Island (Southern Italy). Our investigation revealed low (<1.5 micromol/mol) CH4 concentrations and an extraordinarily large variability in CH4 stable isotopic composition, with delta13C and delta2H values being positively correlated and varying from -35 to -9.2 %o and -670 to -102 %o, respectively. Notably, CH4 isotopes measured at Vulcano almost encompasses the global-scale variability observed in natural fluids, with delta2H values <= -500 %o being the first ever reported in nature. Gases showing extremely negative delta13C-CH4 and delta2H-CH4 values systematically display higher CH4 abundances. We propose two possible scenarios in order to explain the observed huge variation in delta13C and delta2H: (1) mixing of 13C- and 2H-depleted CH4 with 13C- and 2H-enriched CH4 of thermogenic origin formed under hydrothermal conditions; (2) post-genetic removal and isotopic alteration of 13C- and 2H-depleted CH4 occurring during the ascent of volcanic gases. Comparing our dataset with available isotopic data from naturally occurring and artificially produced CH4, a thermogenic origin for the isotopically light CH4 seems unlikely. We postulate that the 13C- and 2H-depleted CH4 may have formed via kinetically-controlled abiotic synthesis through CO (or CO2) hydrogenation reactions in the hot ascending gas phase, possibly at temperatures intermediate between those typical of magmatic and hydrothermal conditions.