Dating carbonate rocks with in situ produced cosmogenic Be-10 : why it often fails

Surface exposure dating of carbonate rocks using cosmogenic 10Be is problematic. We have performed step-wise leaching of calcite-rich samples in order to investigate the reasons for this. Results on different grain size fractions clearly indicate the source of atmospheric 10Be is clay. We demonstrate that partial-leaching procedures, which result in moderate pH levels will not release 10Be (in-situ produced or atmospheric) due to the instant re-absorption on grain surfaces. By contrast, under strongly acidic conditions, all absorbed 10Be is leached from aluminosilicates giving abnormally high 10Be concentrations and consequently exposure ages that are too old. Dating is only possible if samples do not contain any clay minerals or if they can be removed prior to carbonate dissolution.