Deformation modulates helium diffusion behaviour in apatite

Utilising the apatite (U-Th)/He thermochronometer to infer the thermal histories of geologic materials requires understanding the factors that influence helium diffusion kinetics. Here, we demonstrate that deformation in apatite modulates helium diffusion behaviour. We deformed single crystal Durango apatite under compression and torsion, used electron microscopy to characterise the deformation, and measured the evolution of 3He and 4He released during stepwise heating experiments on proton-irradiated fragments of the deformed crystals. Fragments deformed under compression contained distributed dislocations and resulted in mostly unimodal helium release comparable to undeformed, unannealed Durango apatite, while fragments deformed to higher stress under torsion contain a higher dislocation density, developed subgrain boundaries defined by dislocation arrays, and exhibited multimodal helium release, comparable to results from continuous ramped heating analysis of some natural apatite samples. We conclude that deformation-induced dislocations can modulate helium diffusion behaviour by impeding helium diffusion and functioning as diffusion sinks, and are likely an important source of (U-Th)/He date overdispersion in natural samples.