Pa/U ages from a Single Fossil Coral Fragment by Multi-collector Magnetic-sector Inductively Coupled Plasma Mass Spectrometry

The Th/U/U age dating of corals via alpha counting or mass spectrometry has significantly contributed to our understanding of sea level, radiocarbon calibration, rates of ocean and climate change, and timing of El Nino, among many applications. Age dating of corals by mass spectrometry is remarkably precise, but many samples exposed to freshwater yield inaccurate ages. The first indication of open-system Th/U/U ages is elevated U/Uinitial values, very common in samples older than 100,000 yr. For samples younger than 100,000 yr that have U/Uinitial values close to seawater, there is a need for age validation. Redundant Th/U/U and Pa/U ages in a single fossil coral fragment are possible by Multi-Collector Magnetic Sector Inductively Coupled Plasma Mass Spectrometry (MC-MSICPMS) and standard anion exchange column chemistry, modified to permit the separation of uranium, thorium, and protactinium isotopes from a single solution. A high-efficiency nebulizer employed for sample introduction permits the determination of both Th/U/U and Pa/U ages in fragments as small as 500 mg. We have obtained excellent agreement between Th/U/U and Pa/U ages in Barbados corals (30 ka) and suggest that the methods described in this paper can be used to test the Th/U/U age accuracy. Separate fractions of U, Th, and Pa are measured by employing a multi-dynamic procedure, whereby U is measured on a Faraday cup simultaneously with all minor isotopes measured with a Daly ion counting detector. The multi-dynamic procedure also permits correcting for both the Daly to Faraday gain and for mass discrimination during sample analyses. The analytical precision of Th/U/U and Pa/U dates is generally better than 0.3% and 1.5%, respectively (2 Relative Standard deviation [RSD]). Additional errors resulting from uncertainties in the decay constant for Pa and from undetermined sources currently limit the Pa/U age uncertainty to about 2.5%. U isotope data and Th/U/U ages agree with National Institute of Standards and Technology (NIST) reference materials and with measurements made by 0016-7037/05 $30.00 .00 Thermal Ionization Mass Spectrometry (TIMS) in our laboratory. Copyright © 2005 Elsevier Ltd