U–Pb TIMS geochronology using ATONA amplifiers

V11D-0124Dawid Szymanowski & Blair Schoene

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

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).