We have measured Cl-36 in iron oxide minerals from slowly-eroding ferricrete surfaces in Brazil and western Australia. Based on exposure histories derived from Be-10 and Al-26 measurements on co-existing quartz, we calculate a terrestrial production rate of approximately 1.9 atom/gFe/yr at sea level and high latitude. Although this production rate is low, cosmogenic Cl-36 from Fe makes an important contribution to the overall production rate of this nuclide in mafic rocks and minerals, by virtue of their high Fe contents. Previous Cl-36 calibrations based on measurements of whole-rock basalt samples have not considered production by spallation of Fe. Because Fe concentrations tend to correlate with Ca concentrations in basalt, Cl-36 contributions from Fe are likely to have been attributed to Ca spallation. This can explain a significant part of the discrepancy between existing calibrations of the Ca production rate based on pure Ca minerals, which indicate a low Ca spallation rate [1], and those based on Fe-rich whole-rocks, which yield higher values [2,3]. Among neighbouring-Z elements, Ti is the only element likely to have a high enough production rate, concentration in common rock types, and abundance broadly correlated with Ca, to explain the remaining discrepancy between existing calibrations of the Ca reaction.
References: [1] Stone, J.O. et al. (1996) Geochim. Cosmochim. Acta 60, 679-692. [2] Phillips, F.M. et al. (1996) Geophys. Res. Lett. 23, 949-952. [3] Swanson, T. and Caffee, M.W. (2001) Quaternary Res. 56, 366–382.
Supported by NSF grant EAR9805132
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