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

Citation

Material Information

Title:
U–Pb dating of calcite–aragonite layers in speleothems from hominin sites in South Africa by MC-ICP-MS
Series Title:
Quaternary Geochronology
Creator:
Pickering, Robyn
Kramers, Jan D.
Partridge, Tim
Kodolany, Janos
Pettke, Thomas
Publisher:
Springer Nature
Publication Date:
Language:
English

Subjects

Subjects / Keywords:
Uranium-Lead Dating ( local )
Speleothems ( local )
Calcite–Aragonite ( local )
Mc-Icp-Ms ( local )
Beta-Scanning Imagery ( local )
Lead Isotopes ( local )
234U/238U Ratios ( local )
Genre:
serial ( sobekcm )

Notes

Abstract:
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 μg/g) to provide useful ages. For this reason, we used a simple non-invasive beta-scanner imaging screening step to identify U-rich (≥1 μg/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 μg/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 ∼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.
Original Version:
Quaternary Geochronology, Vol. 5, no. 5 (2010-10).

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