Radon and thoron emissions from high to low porosity rocks under increasing deformation

""Anomalous radon and thoron emissions are regarded as valuable precursors of earthquakes and volcanic eruptions and are related to increasing damage of the medium. However both positive and negative anomalies are observed in the field prior ruptures, thus indicating competing deformation mechanisms. Initial porosity (P) plays a key role for natural lithologies in controlling damage accumulation and strain localisation processes. Thus investigating how elastic properties of low to high porosity rocks are affected from increasing loading is required to quantitatively interpret the positive and negative anomalies observed. . Here we present an experimental dataset where measurements of radon (222Rn) and thoron (220Rn) emissions are carried out on low to high porosity lithologies incrementally loaded up to failure. The following samples are investigated: a high porosity lithophysae-rich tuff (P = 47.0 %) and a low porosity phonolitic lava flow (P = 3.6%). Cylindrical samples of 120 (length) x 60 (diameter) mm, have been uniaxially loaded at constant strain rate in servo-controlled uniaxial machines. Samples have been either 1) mechanically damaged to the end of the elastic phase, before the onset of dilatancy or 2) loaded up to the failure. Radon and thoron exhalation rates are carried out by using a solid-state alpha detector, connected to a small accumulation chamber kept at the constant temperature of 60°C. Measurements are performed on two samples at once to achieve stronger signals and repeated several times in order to verify their reproducibility.. Results show that a decrease of radon and thoron emissions are evidenced at the end of the ‘damage phase’ for the high porosity tuff, while a slight increase is observed for the low porosity lava flow. At the end of the elastic phase the tuff reduces its radon and thoron exhalation rates, because of pervasive pore collapse of the macropores, with a P reduction up to 40.1 %. In contrast, for the lava flows the loading slightly increases the crack damage and, consequently, P increases up to 5.1 %. This change does not significantly affect the radon exhalation. At the failure, new rupture surfaces are generated. The total exhaling surface of the samples increases and an increase of radon emission is observed for both lithologies. ""