Helium release during deformation w

Helium release during deformation was observed to follow a systematic pattern indicative of the mechanical processes effective during different stages of deformation (Figure 2). During the initial portions of the deformation, axial loading and elastic compaction are the dominant deformation modes, causing mechanically induced reduction of the pore volume and intrinsic permeability of the rock. During this portion of the experiment a subsequent continuous reduction in the 4He release rate from the rock was observed. At roughly one third to one half of the total prefailure yield strength (an elapsed time of ∼4.5 h, Figure 2), the 4He release rate reaches a minimum. After this point the helium release rate then begins to increase indicating formation of intragranular microfractures.