Duvalo “Volcano” (North Macedonia): A Purely Tectonic‐related CO2 Degassing System

Abstract

Duvalo “volcano” is a site of anomalous geogenic degassing close to Ohrid (North Macedonia) not related to volcanic activity, despite its name. CO2 flux measurements made with the accumulation chamber (321 sites over ∼50,000 m2) showed fluxes up to nearly 60,000 g m-2 d-1, sustaining a total output of ∼67 t d-1. Soil gas samples were taken at 50 cm depth from sites with high CO2 fluxes and analyzed for their chemical and isotope composition. The gas is mainly composed by CO2 (> 90%) with significant concentrations of H2S (up to 0.55 %) and CH4 (up to 0.32 %). The isotope compositions of He (R/RA 0.10) and of CO2 (δ13C ∼0‰) exclude significant mantle contribution, while δ13C-CH4 (∼ -35‰) and δ2H-CH4 (∼ -170‰) suggest a thermogenic origin for CH4. The area is characterized by intense seismic activity and Duvalo corresponds to an active tectonic structure bordering the Ohrid graben. The production of H2S within the stratigraphic sequence may be explained by thermochemical reduction of sulfate. The uprising H2S is partially oxidized to sulfuric acid that, reacting with carbonate rocks, releases CO2. The tectonic structure of the area favors fluid circulation, sustaining H2S production and oxidation, CO2 production and allowing the escape of the gases to the atmosphere. In the end, Duvalo represents a tectonic-related CO2 degassing area whose gases originate mostly, if not exclusively, in the shallowest part of the crust (<10 km). This finding highlights that even systems with trivial mantle contribution may sustain intense CO2 degassing (> 1000 t km-2 d-1).