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

Goethals, M.M., Hetzel, R., Niedermann, S., Wittmann, H., Fenton, C., Kubik, P.W., Christl, M. and von Blanckenburg, F. (2009) An improved experimental determination of cosmogenic ¹⁰Be/²¹Ne and ²⁶Al/²¹Ne production ratios in quartz. Earth and Planetary Science Letters, 284(1-2), pp. 187-198. (doi: 10.1016/j.epsl.2009.04.027)

Abstract

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.

• Item Type: Articles
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Fenton, Dr Cassandra
• Authors: Goethals, M.M., Hetzel, R., Niedermann, S., Wittmann, H., Fenton, C., Kubik, P.W., Christl, M., and von Blanckenburg, F.
• Subjects: G Geography. Anthropology. Recreation > GE Environmental Sciences
• College/School: College of Science and Engineering > Scottish Universities Environmental Research Centre
• Journal Name: Earth and Planetary Science Letters
• Journal Abbr.: Earth Planet Sc Lett
• Publisher: Elsevier
• ISSN: 0012-821X
• ISSN (Online): 1385-013X
• Published Online: 19 May 2009