Elsevier

Quaternary Geochronology

Volume 31, February 2016, Pages 28-39
Quaternary Geochronology

Research paper
Rapid uranium-series age screening of carbonates by laser ablation mass spectrometry

https://doi.org/10.1016/j.quageo.2015.10.004Get rights and content
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Highlights

  • We develop a rapid laser ablation method for U-series carbonate age determination.

  • Ages can be determined for fifty samples in one 12 h measurement session.

  • The method can be used in any lab with a laser and an MC-ICPMS with an ion counter.

  • Uncertainties range from <1ka in Holocene samples to 15 ka for samples of 125 ka.

Abstract

Uranium-series dating is a critical tool in quaternary geochronology, including paleoclimate work, archaeology and geomorphology. Laser ablation (LA) methods are not as precise as most isotope dilution methods, but can be used to generate calendar ages rapidly, expanding the range of dating tools that can be applied to late Pleistocene carbonates. Here, existing LA methods are revisited for corals (cold- and warm-water) and speleothems spanning the last 343 thousand years (ka). Measurement of the required isotopes (238U, 234U, 230Th and 232Th) is achieved by coupling a laser system to a multi-collector inductively-coupled-plasma mass spectrometer (MC-ICPMS) using a combination of a single central ion counter and an array of Faraday cups. Each sample analysis lasts for ∼4.3 min, and fifty samples can be measured in 12 h with an automated set up, after a day of sample preparation. The use of different standard materials and laser systems had no significant effect on method accuracy. Uncertainty on the measured (230Th/238U) activity ratios ranges from 5.4% to 7.6% for (230Th/238U) ratios equal to 0.7 and 0.1 respectively. Much of this uncertainty can be attributed to the heterogeneity of the standard material (230Th/238U) at the length scale of LA. A homogeneous standard material may therefore improve measurement uncertainty but is not a requirement for age-screening studies. The initial (234U/238U) of coral samples can be determined within ∼20‰, making it useful as a first indicator of open-system behaviour. For cold-water corals, success in determination of (232Th/238U) – which can affect final age accuracy – by LA depended strongly on sample heterogeneity. Age uncertainties (2 sigma) ranged from <0.8 ka at 0–10 ka, ∼1.5 ka at 20 ka to ∼15 ka at 125 ka. Thus, we have demonstrated that U-series dating by LA-MC-ICPMS can be usefully applied to a range of carbonate materials as a straightforward age-screening technique.

Keywords

Laser ablation
Coral
Speleothem
Uranium
Thorium

Abbreviations

PM
Photon Machines
NW
New Wave
SEM
Secondary Electron Multiplier

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