Production of 3He in crustal rocks by cosmogenic thermal neutrons

Accurate determination of cosmogenic 3He concentrations in minerals has to account for nucleogenic 3He produced from the reaction of thermalized neutrons with 6Li. Thermal neutrons from the cosmic ray cascade are typically not considered. However, in exposed basalts and granites the flux of cosmogenic thermal neutrons (CTN) is generally 10–200 times higher than the thermal neutron flux produced by (α,n) reactions in the rock, with the absolute values dependent on composition and geographic location. The reaction of CTN with Li can account for 5–50% of the cosmogenic 3He in minerals in typical granitiod rocks, depending on the Li content and distribution, and petrography of the rock, and may be considerably higher in special cases. In basalts this contribution is usually lower (1.5–6%) but may still warrant consideration. The strong depth-dependence of CTN-produced 3He in rock may be used to assess erosion of landforms, similar to CTN-produced 36Cl; however, as 3He is stable it has the potential to extend the useful time range to older surfaces.