Helium (He), the lightest of the noble gases, can make an important contribution to understanding the processes that control the transfer of gases through the earth's crust and can be useful as a tool to investigate and monitor the deformations of the earth's rocks , in some cases, they can also generate seismic events.
These are the results of a multidisciplinary study entitled “ Continental degassing of helium in an active tectonic setting (northern Italy): the role of seismicity ” conducted by the National Institute of Geophysics and Volcanology (INGV) and the University of Palermo, just published in the international journal "Scientific Reports" of Nature. “ The production of radiogenic helium in the earth's crust (i.e. that produced by the natural decay of uranium and thorium) and its diffusion over time scales of even millions of years may not be the main process regulating the transfer of this gas through the earth's crust towards the atmosphere”, state the authors of the study, coordinated by Antonio Caracausi, a researcher at INGV. "The transport of gaseous substances through the earth's crust, on the other hand, can also have an episodic component, determined by the deformation of rock volumes, which can be associated with seismicity ".
The researchers studied the dynamics that regulate this phenomenon in continental areas characterized by the presence of earthquakes, to understand if the outgassing through the earth's crust was dominated only by diffusion processes over long time scales or if it was also linked to the state of deformation and fracturing of the rocks.
"We analyzed the gases released into the atmosphere by the mud volcanoes located in the towns of Regnano and Nirano, in the province of Modena”, continues Antonio Caracausi. “These are geological structures whose formation is linked to the emission of mud mixed with salt water and gases, including methane, carbon dioxide, nitrogen and helium. In addition to the chemical composition, in the geochemical laboratories of the Palermo Section of INGV we also analyzed the isotopic composition of the carbon species and noble gases present. Subsequently, the University of Palermo elaborated the 3D models of the gaseous reservoirs present in the crust below the mud volcanoes and which fuel their activity, together with the geological-structural structure of the area ”. In this way, the researchers were able to estimate the volumes of gas contained in natural reservoirs.
“We focused on helium (He) which, being a noble gas, is characterized by the so-called chemical inertness and is, therefore, an excellent tracer of the source from which it derives (mantle, crust or atmosphere). The different sources, in fact, are characterized by a clearly different isotopic signal and this helps to identify the origin of the natural fluids ".
The helium isotopes (i.e. gas atoms with different masses) contained in the fluids examined have unequivocally shown that this noble gas contained in the natural reservoirs below the mud volcanoes of Nirano and Regnano is of crustal origin and that the contribution of helium from the mantle or from the atmosphere can be considered negligible. Once the amount of helium contained in natural reservoirs has been calculated, it has emerged that this cannot be explained by the accumulation of helium produced in the earth's crust and then transferred to the reservoirs through the rocks (since the age of formation of the reservoirs themselves from 1,8 to 4,5 million years).
The authors therefore state that “experimental studies have shown that rocks subjected to stress release helium more easily, thanks to the presence of micro-fractures produced during the deformation process. Since the geographical area studied is seismically active, we first investigated the effects of ground deformations connected to local seismicity, on the basis of the information contained in the INGV instrumental and historical catalogues.".
“Our Study”, says the researcher, “showed that the production of radiogenic helium in the crust and its diffusion over long time scales may not be the main process regulating degassing in continental areas: the results demonstrate that the transport of volatile substances across the crust may also have an episodic character, therefore impulsive, as a function of the deformation of the rock volumes associated with seismicity. The important consequence of this research is that the helium emission can contribute to investigate and monitor the changes over time of crustal deformations as a function of tectonics. This gaseous species can be used as a geochemical tracer for the understanding of the earthquake preparation phase where there is a good knowledge of the origin of the fluids in the earth's crust, of the dynamics of their circulation and of the processes that can modify their quantity and composition during the their migration to the surface”, concludes the researcher.
Extended
Continental degassing of helium in an active tectonic setting (northern Italy): the role of seismicity
Dario Buttitta, Antonio Caracausi, Lauro Chiaraluce, Rocco Favara, Maurizio Gasparo Morticelli, Attilio Sulli
In order to investigate the variability of helium degassing in continental regions, its release from rocks and emission into the atmosphere, here we studied the degassing of volatiles in a seismicly active region of northern Italy (MwMAX = 6) at the Nirano-Regnano mud volcanic system. The emitted gases in the study area are CH4–dominated and it is the carrier for helium (He) transfer through the crust. Carbon and He isotopes unequivocally indicate that crustal-derived fluids dominate these systems.
A high-resolution 3-dimensional reconstruction of the gas reservoirs feeding the observed gas emissions at the surface allows to estimate the amount of He stored in the natural reservoirs. Our study demonstrated that the in-situ production of 4He in the crust and a long-lasting diffusion through the crust are not the main processes that regulate the He degassing in the region. Furthermore, we demonstrated that micro-fracturation due to the field of stress that generates the local seismicity increases the release of He from the rocks and can sustain the excess of He in the natural reservoirs respect to the steady-state diffusive degassing. These results prove that (1) the transport of volatiles through the crust can be episodic as a function of rock deformation and seismicity and (2) He can be used to highlight changes in the stress field and related earthquakes.
Rome, January 21, 2020
Some photos of the mud volcanoes of Nirano and Regnano during the sampling campaign
