U-Pb TIMS Geochronology using ATONA® Amplifiers

Dawid Szymanowski & Blair Schoene 

Department of Geosciences, Princeton University; dszymanowski@princeton.edu; bschoene@princeton.edu 

V11D-0124 

Key Points 

  • ATONA [aA (10-18 A) to nA (10-9 A)] is a new Faraday cup signal amplifying technology for Isotopx Phoenix thermal ionisation mass spectrometers (TIMS)  
  • Main advantages for TIMS U–Pb geochronology (compared to conventional ion counting with peak-hopping): 
  • Better precision and accuracy for all but the smallest/youngest samples  
  • Shorter analysis time  
  • We present the results of our tests of the new ATONA system at Princeton University, the conditions in which it is advantageous to use it, and optimised Pb and U analysis methods. 
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Detector Parameters 

Figure 1. Low amplifier noise is key to measuring small beams precisely. The noise of the ATONA system improves with longer integration times, performing close to the theoretical (Johnson–Nyquist) noise of a 1012 Ω resistor at integration periods <10 s and approaching the theoretical limit of a 1013 Ω resistor for integrations of >100 s. This implies that for e.g. a 1000 s integration period, one should be able to quantify a beam of 50–100 cps with a signal-to-noise ratio of ~5.
Figure 2. Evolution of baseline parameters over time since installation (measured for 1h at 10 s integration period).
Figure 3. Gain calibration results since installation. The gain values of all channels are highly reproducible over hours to days (default calibration time is 4 h). We observed a slight drift of gain values since installation on the order of 1 ppm across all channels.

U–Pb methods and first results 

Measurement setup  

Cup configuration

Pb: 2-cycle FaraDaly routine with 204Pb (30s) alternated with 205Pb (10s) in the axial Daly photomultiplier (PM) to correct for Faraday–Daly gain. 

UO2: static Faraday routine allowing for simultaneous measurement of 18O/16O and UO2 interference corrections using mass 269  

Baseline: single at start, 300s at each half-mass 

Figure 4. U–Pb FaraDaly dating results for shards of megacrystic zircon GZ7 (Nasdala et al. 2018, GGR), the Earthtime 2 Ga and Early Time 4.5 Ga synthetic solutions. Synthetic solution data on loads >10 pg Pb* show good reproducibility and accuracy. GZ7 aliquots were prepared in a range of sizes down to 1.7 pg Pb*. The accuracy of 206Pb/238U date appears to scale with average intensity of the limiting beam (205Pb or 206Pb depending on zircon size and spike weight), with a drop-off below ~1 mV. 207Pb/235U dates of the smallest GZ7 aliquots remain accurate despite large uncertainties for 207Pb beams of a few 10s µV. Pb was analysed for 100–200 cycles (1.5–2.5h) following a single, long (3×300 s) baseline.
Figure 5. 18O/16O measured during our UO2 runs of zircon is consistent with the IUPAC recommended value of 0.002055.

ATONA vs (Daly) Ion Counting 

Figure 6. Results of automated 1 h-long runs of NBS 982 with the Daly/photomultiplier system and ATONA amplifiers at three different integration times and a range of intensities. Faraday runs reach higher precisions at average intensities >2 mV (208Pb/207Pb data); Daly performs better at lowest intensities <400 µV (~20,000 cps; 206Pb/204Pb data). Extended integration times of 60–100 s have little effect on either precision or accuracy; the marginal gains are in the intensity range where Daly outperforms Faradays.

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