Continental degassing of helium in an active tectonic setting (northern Italy): the role of seismicity

Dario Buttitta,Lauro Chiaraluce,Rocco Favara,Attilio Sulli,Antonio Caracausi,Maurizio Gasparo Morticelli

doi:10.1038/s41598-019-55678-7

Dario Buttitta, Lauro Chiaraluce + Show 4 more

Open Access

https://doi.org/10.1038/s41598-019-55678-7

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Abstract

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 seismically 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. An high-resolution 3-dimensional reconstruction of the gas reservoirs feeding the observed gas emissions at the surface permits 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 rule 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 function of rock deformation and seismicity and (2) He can be used to highlight changes in the stress field and related earthquakes.

Highlights

Large-scale vertical transport of fluids through the continental crust is not always dominated by steady-state diffusion processes but it can be advective and episodic[1,2,3,4]
Our results show that the field of stress associated to the seismicity generated a release of 4He from rock supporting the amount of 4He that accumulated in the natural reservoirs since their formation (1.8–4.5 Ma)
These results demonstrate that in tectonically active regions, the crustal 4He degassing can episodically occur and powered as an advective process by seismo-genetic processes

Summary

Introduction

Large-scale vertical transport of fluids through the continental crust is not always dominated by steady-state diffusion processes but it can be advective and episodic[1,2,3,4]. We investigated the origin and processes controlling the transfer of He through the crust in a seismically active region of the Northern Apennines (Fig. 1), to establish the possible contribution of the tectonic activity in enhancing the release of volatiles accumulated in the rock over time and how earthquakes occurrence may contribute to the episodic volatiles degassing. To this end we collect fluids from mud volcanoes (Fig. 1a,b) and we analyse their chemical and isotopic composition (He, δ13C-CO2, δ13C-CH4, δ2H-CH4) to figure out the origin of the outgassing volatiles and the process controlling the crustal degassing. The balance between input and output of He in the reservoirs allow us to unravel the processes that control the crustal degassing from He production until its transfer towards the surface, giving insights to the role and modality of tectonic and seismicity in the vertical transferring of fluids (diffusive vs. advective and episodic)

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