230Th/234U/238U and 231Pa/235U ages from a single fossil coral fragment by multi-collector magnetic-sector inductively coupled plasma mass spectrometry

Details 230Th/234U/238U and 231Pa/235U ages from a single fossil coral fragment by multi-collector magnetic-sector inductively coupled plasma mass spectrometry 230Th/234U/238U and 231Pa/235U ages from a single fossil coral fragment by multi-collector magnetic-sector inductively coupled plasma mass spectrometry

Feb 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005gecoa..69..649m&link_type=abstract

Geochimica et Cosmochimica Acta, Volume 69, Issue 3, p. 649-657.

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The 230Th/234U/238U 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 230Th/234U/238U ages is elevated 234U/238Uinitial values, very common in samples older than 100,000 yr. For samples younger than 100,000 yr that have 234U/238Uinitial values close to seawater, there is a need for age validation. Redundant 230Th/234U/238U and 231Pa/235U ages in a single fossil coral fragment are possible by Multi-Collector Magnetic Sector Inductively Coupled Plasma Mass Spectrometry (MC-MS-ICPMS) 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 230Th/234U/238U and 231Pa/235U ages in fragments as small as 500 mg. We have obtained excellent agreement between 230Th/234U/238U and 231Pa/235U ages in Barbados corals (30 ka) and suggest that the methods described in this paper can be used to test the 230Th/234U/238U age accuracy. Separate fractions of U, Th, and Pa are measured by employing a multi-dynamic procedure, whereby 238U 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 230Th/234U/238U and 231Pa/235U 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 231Pa and from undetermined sources currently limit the 231Pa/235U age uncertainty to about ±2.5%. U isotope data and 230Th/234U/238U ages agree with National Institute of Standards and Technology (NIST) reference materials and with measurements made by Thermal Ionization Mass Spectrometry (TIMS) in our laboratory.

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