Radon emanation from brittle fracturing in granites under upper crustal conditions

Abstract

Radon-222, a radioactive gas naturally produced in the Earth's crust, informs us about the migration of fluids and is sometimes considered as a potential earthquake precursor. Here we investigate the effects of mechanical and thermal damage on the radon emanation from various granites representative of the upper crust. Radon concentration measurements performed under triaxial stress and pore fluid pressure show that mechanical damage resulting from cycles of differential stress intensifies radon release up to 170±22% when the sample ruptures. This radon peak is transient and results from the connection of isolated micropores to the permeable network rather than new crack surface creation per se. Heating to 850°C shows that thermal fracturing irreversibly decreases emanation by 59-97% due to the amorphization of biotites hosting radon sources. This study, and the developed protocols, shed light on the relation between radon emanation of crustal rocks, deformation, and pressure-temperature conditions. Key Points Mechanical and thermal damage effects on radon release are studied in granites Measurements of radon concentration in pores are coupled with a triaxial cell Mechanical damage connects isolated pores leading to transient radon signals © 2014. American Geophysical Union. All Rights Reserved.