Noble Gas Release from Bedded Rock Salt during Deformation

Abstract

Geogenic noble gases are contained in crustal rocks at inter- and intracrystalline sites. In this study, bedded rock salt from southern New Mexico was deformed in a variety of triaxial compression states while measuring the release of naturally contained helium and argon utilizing mass spectrometry. Noble gas release is empirically correlated to volumetric strain and acoustic emissions. At low confining pressures, rock salt deforms primarily by microfracturing, rupturing crystal grains, and releasing helium and argon with a large amount of acoustic emissions, both measured real-time. At higher confining pressure, microfracturing is reduced and the rock salt is presumed to deform more by intracrystalline flow, releasing less amounts of noble gases with fewer acoustic emissions. Our work implies that geogenic gas release during deformation may provide an additional signal which contains information on the type and amount of deformation occurring in a variety of earth systems.

Investigating Fracture Network Deformation Using Noble Gas Release

“…He and 40 Ar release has been shown to be highly sensitive to triaxial deformation [4,5] and show a similar pattern to Rn, with (1) constant or slightly lower gas release during initial compression, (2) increasing gas release after reaching around 1/3 of the ultimate yield stress due to microfracture during compression, and (3) rapidly increasing gas release just before and during macrofracture, subsequently dropping after macrofracture [5,7]. The gas release signal size has been shown to change as a function of rock type and as a result of deformation style [4][5][6]. These noble gas release signals have been used to infer changes in rock flow and transport properties due to fracture creation at the lab scale, using dynamic fracture flow models and matching to 4 He release signals [7].…”

“…Thus, it is clear we sampled noble gas composition from several locations that saw significant deformation. In our experience in the lab, radiogenic noble gas release is extremely sensitive to rock deformation across a broad spectrum of rock types and deformation styles [4][5][6][7]. These experiments have shown that when new fractures are being created as a result of deformation, radiogenic noble gases are released in significant quantities.…”

“…As this methodology is still relatively new, this inference is subject to increased scrutiny. However, we have now tested a variety of lithologies and ages in the laboratory setting and have found radiogenic noble release to be a sensitive indicator of new fracture creation [4][5][6][7]. Radiogenic gas release will be dependent upon the lithology and geologic history and cannot be expected for every rock type in every setting.…”

“…These nobles gases are likely released as a result of episodic fracturing and brittle deformation in the subsurface [2,3]. Recent research has shown that radiogenic noble gases released from rocks as they are deformed can be observed [4][5][6][7][8]. Thus, monitoring noble gas release and isotopic composition could provide a new method of measuring and inferring subsurface deformation mechanics.…”

Helium in Stream Water as a Volcanic Monitoring Tool

“…Groundwater helium is a potential indicator of stress, strain, and seismicity in volcanic systems where subsurface fluid pressure and stress are tightly coupled. Dilation, microfracture, and macroscopic failure break crystal lattices and create new transport pathways to release accumulated radiogenic 4 He into adjacent pore networks and groundwater (Bauer et al, ; Bauer, Gardner, & Lee, ; Bauer, Gardner, & Heath, ). The release of helium due to mechanical deformation during seismic events has been shown to alter helium isotopic composition of groundwater (Brauer et al, ).…”