Geochemical characterization of the Nirano mud volcano, Italy

Abstract

The Nirano mud volcano is located in the western sector of the Modena Apennine margin (Italy). It represents one of the most spectacular phenomena of sedimentary volcanism in the entire Italian territory and is among the largest in Europe. Here numerous aligned gryphon clusters and seeping pools constantly burst gas and mud inside a morphological depression. Besides the obvious surface expressions of these emission spots, until now the type and amount of gas released in the rest of the large Nirano caldera zone remained unknown. An extensive geochemical soil gas survey (O2, N2, CO2, CH4, 222Rn, He, H2, and light hydrocarbons) and exhalation fluxes (CO2 and CH4), was carried out inside the mud volcano field with the aim of identifying soil degassing distribution, and to estimate the micro- and macro-seepage budget for both CO2 and CH4. Soil gas data highlight the presence of two zones characterized by high concentrations and flux values. These enhanced seepage zones are located in the SW and NE sectors of the mud volcano suggesting that the enhanced gas emissions present in the peripheral zones, are controlled by caldera collapse structures. The most significant CO2 flux (up to 91 g m-2 d-1) and 222Rn anomalies are located in the central part of the crater in correspondence of a morphological escarpment. Here we infer the presence of a buried tectonic system of collapsed terraces that facilitate fluids degassing. In contrast, CH4 fluxes show a scattered distribution and low values (mean 221 mg m-2 d-1). Overall the CH4 degassing budget is low (27.09 t km-2 y-1) when compared with other Italian mud volcanoes. This could be related to a relative low emission activity during the period of the geochemical survey and to a more homogeneous dilution of surface distribution of the emission point-s. Chemical and isotopical composition of the gas discharged from the active gryphons is methane-dominated and the thermogenic signature (ranging from -41 to -47‰) suggests a deep reservoir source. This conclusion is supported by noble-gas measurements (He, Ne, Ar, Kr, Xe) conducted in the pore water phase of the emitted mud, indicating a secondary gas exchange occurring at a depth of a few kilometers. The geochemical anomalies found in this study, successfully predicted the occurrence of new degassing phenomena towards the NE sector of the caldera. Indeed recently (i.e. after the survey data acquisition) new manifestations of mud and gas emissions appeared in the north-eastern edge of the caldera.