In situ production of terrestrial cosmogenic helium and some applications to geochronology

The concentrations of cosmogenic ³He have been measured in a series of basaltic drill core samples from Hawaiian volcanoes Haleakala and Mauna Loa. The ³He concentration in the surface of a radiocarbon dated Mauna Loa flow (20,000 years) gives reasonable agreement with a theoretical production rate of 140 atoms g−1yr−1 and suggests that the uncertainty in this rate is roughly 10%. The results illustrate the feasibility of using ³He to measure exposure ages of young basaltic lava flows and for measuring erosion rates. Erosion rates calculated from the three Haleakala cores range from 7 to 11 meters/million years. The drill core data demonstrate that accurate depth control is crucial to the use and evaluation of cosmogenic helium. Depth profiles from several of the older cores display a non-exponential depth dependence of ³He_c below 170 g cm⁻², which is attributed to the contribution from ⁶Li(n, )T, where the neutrons are from stopped muons. This has important implications for depth dependence of cosmogenic ³He because muons are weakly attenuated compared to the nucleonic component that produces spallation.