Revealing Histories of Exposure Using In Situ Produced 26Al and 10Be in Libyan Desert Glass

Abstract

We present the results of measurements of 26Al and 10Be produced in situ in 12 samples of Libyan Desert Glass by cosmic rays during the last ten million years. Based on the variability of the concentrations of 10Be and of the 26Al/10Be ratios we measured, we conclude that individual fragments of glass have experienced different exposure histories, implying several major redistributions of the glass within the past 106 years. The 26Al and 10Be concentrations are inconsistent with the theoretical estimates of the rates of in situ production. We estimate minimum production rates of 70 atoms g-1 yr-1 and 10 atoms g-1 yr-1 for 26Al and10Be, respectively, produced in quartz at sea level between 60-90 degree latitude. Despite the present uncertainty in the rates of production, we feel that these results show clearly the effectiveness of in situ produced 26Al and10Be in studying earth-surface processes.