An improved experimental determination of cosmogenic 10Be/21Ne and 26Al/21Ne production ratios in quartz

The confidence in surface exposure dating and related research, such as erosion rate studies or burial dating, strongly depends on the accuracy and precision of the currently used production rates of in situ-produced cosmogenic nuclides. Reducing the uncertainties of nuclide production rates by more accurate calibrations with independently dated natural rock surfaces is crucial for further improving the quantification of earth surface processes. Here we use surface samples from the 760 ± 2 ka old Bishop Tuff in eastern California to quantify the 10Be/21Ne and 26Al/21Ne production rate ratios in quartz. Our determination is based on (1) measured nuclide concentrations of cosmogenic 10Be, 21Ne, and 26Al, (2) a conservative estimate for the erosion of the tuff in the Volcanic Tableland area, which we base on our previously published 21Ne concentrations [Goethals, M.M., Niedermann, S., Hetzel, R., Fenton, C.R., 2009. Determining the impact of faulting on the rate of erosion in a low-relief landscape: A case study using in situ produced 21Ne on active normal faults in the Bishop Tuff, California. Geomorphology 103, 401-413] and a conservative estimate for the uncertainty of the 21Ne production rate, and (3) the assumption of steady-state erosion. Other assumptions, such as the applied scaling procedure, the muon contribution to nuclide production, or the attenuation lengths of neutrons and muons in rock, do not substantially affect the results. Based on 13 samples, the following average production rate ratios and conservative uncertainty estimates are obtained for sea level, high latitude, open sky, and rock surface: 0.249 ± 0.009 or 0.232 ± 0.009 for 10Be/21Ne using 10Be half-lives of 1.51 and 1.39 Ma, respectively, and 1.80 ± 0.09 for 26Al/21Ne (for an 26Al half-life of 0.72 Ma). The 10Be/21Ne and the 26Al/21Ne production ratios are consistent with currently used production rates but the ratios are much more precise than previous determinations. The resulting 26Al/10Be production ratio of 7.23 ± 0.45 (for a 10Be half-life of 1.51 Ma) or 7.76 ± 0.49 (for a 10Be half-life of 1.39 Ma) is high when compared to previously published values. We discuss reasons for this difference, amongst them the possibility that 26Al analyses in general might be compromised by artefacts affecting the stable Al concentration measurements. When combined with a 10Be production rate of 5.01 at g− 1 a− 1 for the 1.51 Ma half-life (or 4.61 at g− 1 a− 1 for the 1.39 Ma half-life), our production ratios convert to 21Ne and 26Al production rates of 20.1 and 36.2 at g− 1 a− 1 (or 19.9 and 35.7 at g− 1 a− 1), respectively.