Production of He-3 in crustal rocks by cosmogenic thermal neutrons

摘要

Accurate determination of cosmogenic He-3 concentrations in minerals has to account for nucleogenic He-3 produced from the reaction of thermalized neutrons with Li-6. 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 (alpha,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 3 He in rock may be used to assess erosion of landforms, similar to CTN-produced Cl-36; however, as He-3 is stable it has the potential to extend the useful time range to older surfaces. (C) 2007 Elsevier B.V. All rights reserved.