U–Pb TIMS geochronology using ATONA amplifiers
V11D-0124Dawid Szymanowski & Blair Schoene
Department of Geosciences, Princeton University; [email protected]; [email protected]
Pb: 2-cycle FaraDaly routine with 204Pb (30 s) alternated with 205Pb (10 s) in the axial Daly photomultiplier (PM) to correct for Faraday–Daly gain
Figure 5. 18O/16O measured during our UO2 runs of zircon is consistent with the IUPAC recommended value of 0.002055.
Detector parameters
U–Pb methods and first results ATONA vs (Daly) ion counting
Measurement setup
Cup configuration
L5 L4 L3 L2 (Ax) PM H1 H2 H3 H4
202Pb 204Pb 205Pb 206Pb 207Pb 208Pb
205Pb 206Pb 207Pb 208Pb
265(UO2) 267(UO2) 269(UO2) 270(UO2) (272.7)
0.9944200
0.9944195
0.9944190
0.9944185
0.9944180
0.9944175
0.9944170JulyJuneMayApril
Jul Aug Sep Oct NovJunMayApr
H4 Gain1SD = 0.55 ppmL4–H3: 0.52 – 0.60 ppm
L2 Gain1SD = 0.60 ppmApr–Nov: +1.07 ppm
4 h means
4 h means
1.0064565
1.0064560
1.0064555
1.0064550
1.0064545
1.0064540
1.0064535
1.0064530
10-18
10-17
10-16
10-15 0.1 mV (100 μV) 5000
Intensity vs 1011 Ω Approx. cps
0.01 mV (10 μV) 500
0.001 mV (1 μV) 50
(0.1 μV) 5
10 11 Ω10 13 Ω
10 12 Ω
1 10
meas. time 3600–6000 s
100 1000
Integration time [s]
Cur
rent
noi
se [A
]
L5
Ax
L4 L3 L2
H1 H2 H3 H4
7
6
5
4
3
2
0
8 × 10-17
Baseline (10 s int.) Noise (1SD of baseline)
A A
Apr May Jun Jul Aug Sep Oct Nov Apr May Jun Jul Aug Sep Oct Nov
L5
Ax
L4 L3 L2
H1 H2 H3 H4
6 × 10-17
4
2
-2
18O/16O
30 s
30 s
10 s
2.08 × 10-3
18O/16O
mean 18O/16O (2.051 ± 0.010) × 10-3
2.072.062.052.042.032.022.01
n = 93
UO2: static Faraday routine allowing for simultaneous measurement of 18O/16O and UO2 interference corrections using mass 269
Baseline: single at start, 300 s at each half-mass
• 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.
Key points
532
531
530
529
528
206Pb/ 238U date [M
a]
206 P
b/23
8 U20
6 Pb/
238 U
207Pb/235U 207Pb/235U
207Pb/235U
206Pb/ 238U
6543210.66 0.67 0.68 0.69 0.70 0.71 0
Limiting beam size [mV]
527.5
528.0
528.5
529.0
529.5
530.0
530.5
531.0
531.5
532.0
1.0281.026
1.0241.030
0.08
540.
0856
0.08
580.
0860
45544555
4556
45604561
4562
45574558
4559
87.76.15 6.16 6.17 6.18 87.9 88.1 88.3
1998.0
1998.5
1999.0
1999.5
2000.0
2000.5
2001.0
2001.5
2002.0
0.36
300.
3635
0.36
400.
3645 Early Time solution
52–112 pg Pb*n = 8
Earthtime 2 Ga solution10–178 pg Pb*n = 15
207Pb/206Pb age4559.80 ± 0.27 Ma
(MSWD = 0.34)
207Pb/235U: 1999.53 ± 0.18 Ma (MSWD = 0.9)206Pb/238U: 1999.19 ± 0.12 Ma (MSWD = 1.6)
207Pb/206Pb: 1999.78 ± 0.31 Ma (MSWD = 1.0)
PU reference value530.24 ± 0.10 Ma
GZ7 zircon1.7–1368 pg Pb*
GZ7 zircon1.7–1368 pg Pb*n = 24
0.5
0.4
0.3
0.2
0.1
0.01000800600400
crossover~ 20,000 cps
2000
0.030
0.025
0.020
0.015
0.010
0.005
020151050
Limiting beam size [μV]
Limiting beam size [μV]
Limiting beam size [mV]
206 P
b/20
4 Pb
prec
isio
n [1
se%
]20
6 Pb/
204 P
b20
8 Pb/
207 P
b pr
ecis
ion
[1se
%]
37.4
37.2
37.00.5%
0.5%
1.0%
1.0%
36.8
36.6
36.4
36.2
36.010008006004002000
30 s60 s100 s
Daly
Faraday (ATONA)Integration times:
30 s60 s100 s
Daly
Faraday (ATONA)Integration times:
NBS 982 precision (1 h)
NBS 982 accuracy
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 (1.5–2.5 h) following a single, long (3 × 300 s) baseline.
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.
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 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.
Figure 2. Evolution of baseline parameters over time since installation (measured for 1 h at 10 s integration period).