Production of 10Be and 26Al by cosmic rays in terrestrial quartz in situ and implications for erosion rates

We present results of determinations of ¹⁰Be (half life = 1.6 Myr) and ²⁶Al (half-life = 0.705 Myr) produced by cosmic rays in situ in several terrestrial rock samples exposed at altitudes of 1-4 km. These experiments were designed to determine the feasibility of using quartz extracted from natural surfaces for studying continental weathering/erosion processes¹. Quartz is attractive as a target material for several reasons. Its low aluminium concentration allows measurement of ²⁶Al after exposures as short as 10³ yr; its integrity minimizes the possibility of contamination by rain with ¹⁰Be produced in the atmosphere; and its ubiquity ensures that it is found in a wide variety of geological settings. However, the cosmic-ray production rates of ²⁶Al and ¹⁰Be even at mountain altitudes are low^2,3, only 10-100 atoms g^-1 yr^-1, with the consequence that earlier attempts^4,5 to determine in situ produced ³⁶Cl and ²⁶Al in rocks were not continued. Recent development of accelerator mass spectrometry (AMS) however, has made the measurement of 10-20g samples feasible. The results presented here demonstrate the feasibility of quantitatively measuring ¹⁰Be and ²⁶Al produced in situ by cosmic rays in quartz and the possible applications of ¹⁰Be and ²⁶Al as a pair for studying continental weathering/erosion processes.