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

Abstract

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 low2,3, only 10–100 atoms g−1 yr−1, with the consequence that earlier attempts4,5 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.