From sea level to summit: Evidence of altitude dependence for the production rate of cosmogenic nuclides 3He and 36Cl in lava flows on Mount Erebus volcano, Antarctica

The need for greater accuracy and precision to solve geologic questions using terrestrial cosmogenic nuclide dating demands our continued pursuit to understand cosmogenic nuclide production rates. Scaling methods developed prior to the LSDn model of Lifton et al. (2014) assumed all cosmogenic nuclide production rates had the same altitude dependence. Theoretically, production rates should vary with elevation due to known differences in excitation functions for production reactions and the decreasing intensity and energy spectrum of the secondary cosmic ray flux with atmospheric depth. Studies conducted at low and mid-latitudes have investigated nuclide specific scaling empirically, however, conflicting results and analytical uncertainties have hindered establishment of unambiguous conclusions. In this study we measured concentrations of cosmogenic 3He in clinopyroxene and 36Cl in anorthoclase feldspar from phonolitic lava flows sampled between sea level and 3609 m elevation at Erebus volcano, Antarctica (77°S). Twenty-one clinopyroxene-anorthoclase pairs were examined. The 36Cl/3He concentration ratio was used to examine altitude dependence of production rates and assess the magnitude and direction of change in production with altitude. The LSDn model predicts, at most, a 10% reduction in the 36Cl/3He ratio over 3600 m elevation. Our data show the 36Cl /3He ratio decreases ~25% from sea level to 3600 meters elevation. Assessment of exposure ages shows the LSDn model produces 3He ages consistently younger (0.5-11.5 %) than the LSD model and the difference is directly proportional to elevation. Corresponding 36Cl ages show the opposite relationship between LSDn and LSD and a smaller difference between the ages (0.2-1.1%) which also appears directly proportional to elevation. This dataset confirms 3He production rates increase significantly with altitude compared to 36Cl and can help direct future research for further improvements of scaling methods and cosmogenic nuclide production rates. Our results also have important implications for 3He cosmogenic exposure age dating at mid- and high altitudes when samples are collected or compared over a wide range of elevations or when multiple cosmogenic nuclide systems are used. Lifton et al., 2014. Earth Planet. Sci. Lett. 386, 149-160.