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-1yr1, 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 3Hesub(c) below 170 g cm-2, which is attributed to the contribution from 6Li(n,α)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. (author)