Mantle vs. crustal fluid sources in the gas discharges from Lesser Caucasus and Talysh Mountains (Azerbaijan) in relation to the regional geotectonic setting

Azerbaijan is famous for the impressive phenomenon of mud volcanism that dominates the landscape of the central sector of the country. The gases released are here strongly CH enriched. Conversely, the Lesser Caucasus to the NW host numerous thermo-mineral springs into which CO-rich gas are bubbling. In the Talysh area, located to SE at the border with Iran, CH-, N- and CO-rich gases are discharged. The chemical and isotopic composition of the free-gases from the Lesser Caucasus and Talysh was investigated to define their origin in a particularly complex geodynamical and geological setting. The CO in the Lesser Caucasus was apparently associated with thermometamorphic processes affecting the Jurassic and Cretaceous limestones, at which a minor contribution by a mantle component (helium) was also recognized, likely related to oceanic subduction and/or fluid interactions with the accreted ophiolite complex. The fold zone of Talysh, which represents the northern extension of the Iranian Alborz chain, produced the Early and Middle Eocene to Pleistocene volcanism that was invoked to explain the distinct chemical and isotopic composition of the discharged gases. The CO-rich gases at Buludul showed helium and carbon isotopic ratios that suggested mixing processes between (i) a crustal component produced by hydrolysis of limestones and interaction with volcano-sedimentary complexes rich in organic material, and (ii) tectonically driven mantle-derived fluids. The relatively high R/R values (~6) are indeed comparable with those recorded in the active volcanoes of the region, indicating the presence of degassing magmas at depth. Finally, typical hydrothermal gas compounds, e.g. CO and HS, were not detected in both the study areas. Unless secondary processes at shallow depth are occurring, the presence of developed hydrothermal systems at regional scale is unlikely.