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Department of Earth and Planetary Sciences - USRG

U-Series Research Group

U-series isotope analysis




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Analytical issues and challenges


  • traditional α-counting yields a precision of ∼4-10%
  • Th and Pa have thermal ionisation efficiency <0.01% (PIMS)
  • Th and Pa are highly insoluble
  • Ra has a thermal ionisation efficiency ∼5-14% (TIMS)
  • Ra and Pa only present at fg/g levels
  • 234U/238U ratios ∼10-5, 230Th/232Th ratios ∼10-6
  • 238U and 232Th "tail" onto 234U and 230Th, respectively

Spikes

  • Mixed 236U-229Th spike for U-Th analysis (Oak Ridge)
  • 228Ra spike milked from equilibrium 232Th solution (NIST3159)
  • 233Pa spike milked from 237Np or derived by irradiation of 232Th
  • 209Po spike available from NIST
  • Calibration against SRM solution standards
  • Cross-check with analysis of secular equilibrium material (TML)
  • Th and Pa spikes need ∼0.01M HF to keep in solution
    Decay chains of the U-series isotopes.

    Decay chains of the U-series isotopes (activity = λN). Click to enlarge


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    Analytical facilities

    Laboratories


    • Newly constructed clean lab facilities with four self-contained laboratories, including two dedicated solely for U-series work.
      The clean lab area includes 7 laminar flow workstations and two separate balance rooms
      Laboratories

      U-series clean lab facilities

      • Nu Plasma high resolution multi-collector ICPMS, equipped with an RPQ filter (Installed Nov. 2003)
      • Sample in solution of 2% HNO3, sprayed into the plasma using a DSN 100
      Nu Plasma MC-ICP-MS

      Nu Plasma MC-ICP-MS

      • Finnigan Triton TIMS (Installed March 2005) Equipped with RPQ filter, SEM and oil-free scroll pump.
      • Ra samples loaded onto outgassed, zone-refined Re filamentsusing a Ta-HF-H3PO4 activator solution
      Triton

      Finnigan Triton TIMS


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      U isotope analysis by MC-ICP-MS


      • U analysis using CRM145 or CRM960 standard
      • Comparison of measured 238U/235U ratio to the true ratio (137.88) yields mass bias fractionation.
      • Comparison of mass fractionation-corrected 234U/238U to the true ratio (5.286x10-5) yields ion counter gain
      • Intersperse samples with CRM145 analyses

      • Cycle 1

      • 234U on Ion Counter 0 (IC0)
      • 235U on Faraday cup L2
      • 238U on Faraday cup H1

      • Cycle 2

      • 236U on IC0
      • 238U on Faraday cup L1
      Standard measurements of TML

      234U/238U standard measurements of TML. True value= 1. Click to enlarge.


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        Th isotope analysis by MC-ICP-MS


        • A solution of 20ppb Th yields a beam of and ∼1V of 232Th, 100s of cps of 230Th
        • 230Th is collected on an ion counter preceded by a RPQ deceleration lens. Typical abundance sensitivity is ∼450ppb at 1 a.m.u. for a transmission of 95-98% (190ppb at 2 a.m.u. but transmission not determined)

        • Th analysis using CRM145 or CRM960 and Th 'U' standards
        • Comparison of measured 238U/235U ratio to the true ratio (137.88) yields mass bias fractionation.
        • Comparison of mass fractionation-corrected 234U/238U to the true ratio (5.286x10-5) yields ion counter gain
        • Intersperse samples with Th 'U' analyses to get βTh

        Standard measurements of TML

        TML-230Th/232Th bracketed with Th'U'. True value= 5.80x10-6. Click to enlarge.

          Cycle 1

        • 230Th on IC0
        • 232Th on Faraday cup L1
        • Cycle 2

        • 229Th on IC0
        • 232Th on axial Faraday cup (Ax)
        Standard measurements of TML

        TML-230Th/238U bracketed with Th'U'. True value= 1. Click to enlarge


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        Ra isotope analysis by TIMS

        • Slow heating to 1200°C then use 138Ba to focus
        • Scan masses 224-230 to check for Ra and hydrocarbons
        • Build Ra beam (100-1000 cps @ 1250-1350°C)
        • Dynamic collection on SEM ∼80 ratios with 8 second integration times (precision <0.5%)
        TML for<sup> 226</sup>Ra/<sup>230</sup>Th

        TML standard measurements for 226Ra/230Th. True value= 1. Click to enlarge.


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        210Pb isotope analysis by α-counting

        • 210Pb concentrations are determined from 210Po on separate 1-2 g sample dissolutions which are spiked with a 209Po tracer calibrated against a value of 7.95 dpm/g for TML
        • Samples are loaded onto a 1 ml anionic column in 1N HCl containing H3BO3 where most major and trace elements were washed off in 1N and 0.5N HCl after which Po is eluted in warm 7.5N HNO3
        • Po is autoplated onto a Ag disc above a magnetic stirrer for 6-8 hours in 150 ml of warm 0.5N HCl which contained ascorbic acid. 210Po and 209Po were counted for ∼1 week to achieve a precision of <4%
        α-counter

        Ortec Octete Plus α-counter.




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