Radon and thoron emission from lithophysae‐rich tuff under increasing deformation: An experimental study

Abstract

Radon and thoron emissions from lithophysae‐rich tuff under increasing deformation are measured to determine how mechanical damage affects gas emission levels in tuffs. Mechanical properties of rocks under stresses should be carefully considered to properly interpret data from geochemical field monitoring. Two samples are uniaxially loaded up to failure, while two others are unloaded at the end of the elastic phase, in order to achieve the highest compaction of existing pores. Changes in the porosity of deformed samples are evidenced by helium pycnometer and microscopy analyses. Radon and thoron exhalation rates are measured on groups of two samples by alpha spectrometer technique. Results show that tuff samples are characterised by a dual porosity consisting of a macroporosity, given by isolated large pores with sizes from mm‐ up to cm‐scale and a microporosity ranging between microns to hundreds of microns. At the end of the elastic phase pervasive pore collapse is observed, due to the closure of the cm‐scale macropores. This is mirrored by a significant decrease of radon and thoron release. After failure, a further reduction of porosity in the rock adjacent to the fault planes is observed due to extensive closure of both macropores and micropores. At this stage radon and thoron emissions increase. The formation of new exhaling surfaces is the main carrier of the bulk increase of radon and thoron exhalations, strongly prevailing over the densification carried out from the compaction mechanisms. In terms of volcanic hazard, negative anomalies in radon emissions should be considered as indicators of forthcoming ruptures. Key words: radon and thoron exhalation, tuff deformation, seismic precursor.