Helium Release During Fracture and Granular Fragmentation of Rocks

Geogenic Helium-4 (4He) in-situ increases locally in regions of large deformation generated naturally or anthropogenically. This gas release by deformation is a potential geochemical precursor signal for subsurface deformation. To evaluate the applicability of 4He degassing for correlating deformation in different lithologies, we conducted high force crush tests, up to 97,800 N axial load, to assess the total 4He released during fragmentation of the rocks. We observed that the highest 4He released occurred in the sedimentary rocks and that release correlated strongly with lithologic age and U/Th content. Microstructural changes of the pre- and post-test rocks indicate that the degree of grain size reduction relates directly to the total 4He released during crushing. The range of in-place 4He was calculated based XRF measurements of uranium and thorium in each lithology, with the results indicating that the majority of the trapped 4He was not released. However, the 4He released by deformation depended upon how the each rock deformed during deformation and the degree of grain size reduction. We postulate that 4He precursor signals can be used to understand subsurface deformation only if geomechanical and geochemical conditions for 4He enrichment in a lithology are met.