U-Pb dating of calcite-aragonite layers in speleothems from hominin sites in South Africa by MC-ICP-MS

Speleothems are found in association with hominin fossil-bearing cave deposits in South Africa and can be used to provide valuable chronological constraints. Such material is generally too old for U-Th dating and, although U-Pb geochronology presents a suitable alternative, bulk U concentrations are typically too low (<0.05 mu g/g) to provide useful ages. For this reason, we used a simple non-invasive beta-scanner imaging screening step to identify U-rich (>= 1 mu g/g) domains that could be analyzed with MC-ICP-MS techniques to provide U-Pb ages. We demonstrate the technique using samples from Sterkfontein cave that exhibit infrequent <1 cm-thick layers with U concentrations >= 1 mu g/g. Relict aragonite needles are found exclusively in these U-rich layers. We analyzed material from the same flowstone suite as Walker et al. (2006) and obtained a U-Pb age of similar to 2.3 Ma that agrees well with their estimate of 2.24 +/- 0.09 Ma. We also obtained similar U-Pb (0.164 +/- 0.026 to 0.200 +/- 0.052 Ma) and U-Th (0.148 +/- 0.003 Ma) ages for another sample exhibiting U-rich layers. We recognize that open-system behaviour during the partial transformation of aragonite to calcite is a potential problem and argue, on the basis of geochemistry and age consistencies, that recrystallization took place rapidly after speleothem formation and did not significantly affect the U-Pb ages. (C) 2010 Elsevier B.V. All rights reserved.