High Precision Pb Isotope Techniques
from the WHOI NEPTUNE PIMMS
S.R. Hart, R.K. Workman, L. Ball and J. Blusztajn
Woods Hole Oceanographic Institution
Woods Hole MA 02543, USA
WHOI Plasma Facility Open File Report 10
1. NEPTUNE NUTS AND BOLTS – NBS 981
NEBULIZER – 0.12 mL/minute.
LEAD SENSITIVITY – 30-50 volts/ppm.
TYPICAL RUN INTENSITY – 1.2 – 9 volts.
RUN LENGTH – 9 to 18 minutes.
204Hg CORRECTION – 25-50 ppm of mass 204.
ABUNDANCE SENSITIVITY - 1.9 ppm of 205Tl at mass 204; for Pb/Tl = 3,
correction on 204Pb ~ 35 ppm.
EFFECT OF Pb/Tl RATIO – less than 70 ppm from Pb/Tl = 2.5 to Pb/Tl = 12.
INTERNAL PRECISION (2s) – 30 – 80 ppm on 20X/204 ratios.
EXTERNAL PRECISION (2s, n = 38) – 80 ppm on 206/204 Pb and 58 ppm on 206/207 Pb.
SUMMARY - EXTERNAL PRECISION (2-Sigma)
RATIO
206/204
207/204
208/204
207/206
208/206
NBS 981
80 ppm
94
126
58
92
SAMOA BASALTS
101 ppm
89
117
17
42
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2. Pb Isotopes by NEPTUNE PIMMS
Thallium Tailing, NEPTUNE
8000
Counts per Second
7000
205
Tl = 22 volts
6000
204
Hg
5000
4000
3000
1.9 ppm of
205
Tl
2000
1000
0
203.0
203.5
204.0
204.5
205.0
MASS, amu
Figure 1. The abundance sensitivity of the NEPTUNE at mass 204 is ~ 1.7 ppm/amu
(down-mass) and ~0.8 ppm/amu (up-mass); this leads to ~1.9 ppm from combined 203Tl
and 205Tl. For Pb/Tl ratios greater than 3, the effect on 204Pb is less than 40 ppm. By
using baselines at –0.5 amu, tailing effects are insignificant (< 10 ppm for 20X/204 Pb
ratios).
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Thallium Mass Bias versus Time
7/30/02 - 8/05/02
3/22/02 - 4/01/02
2.430
8/29/02 - 8/30/02
2.429
2.428
205/203 Tl
2.427
2.426
2.425
2.424
2.423
2.422
2.421
2.420
0
10
20
30
Run Number
40
50
Figure 2. The variation of mass bias with time on the NEPTUNE is quite small, as shown
by the time variation of 205Tl/203Tl for Pb runs spiked with NBS 997. The average mass
bias for 37 runs over a period of 6 months is +0.79%/amu; the standard deviation of these
runs (1s) is 860 ppm.
207
0.91465
Pb/206 Pb in NBS981 versus Pb/Tl Ratio
D
207Pb/206Pb
0.91460
0.91455
A
B
0.91450
0.91445
0.91440
A - 0.5N HCl
B - 1% Seawater
C - 33 ppb Pb
D - ± 50 ppm
C
1
2
3
4
5
208Pb/205Tl
6
7
8
Figure 3. Because of the excellent abundance sensitivity and mass bias stability of the
NEPTUNE, there is virtually no effect of varying Pb/Tl ratios on the external precision. For
example, there is no statistical variation of 207Pb/206Pb over a Pb/Tl range from 2.5 to 12
(208Pb/205Tl = 1.6 - 7.4). Based on initial tests, there is also no obvious matrix effect
between HNO3 (typical run solution), HCl and weak seawater.
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SRM 981 NEPTUNE Runs
[ Diamonds - 3/22/02; Squares -7/30/02; Triangles - 8/29/02 ]
15.489
15.488
15.487
fractionation
line
207/204 Pb
15.486
15.485
15.484
15.483
±100ppm in
206/204
15.482
±100ppm in
207/204
15.481
±50ppm in
207/206
15.480
16.929
16.930
16.931
16.932
16.933
16.934
206/204 Pb
Figure 4. External precision of 37 runs of the NBS 981 standard (14 days, 5 sessions, 6 months) is
80 ppm for 206Pb/204Pb and 94 ppm for 207Pb/204Pb respectively (2s). The data are well aligned
along a fractionation line, showing that the external precision is principally limited by the Tl massbias correction. However, these runs also encompass the use of a variety of baseline protocols
during development of the technique; these contribute to variability along a 204Pb “error” line
(which is flatter but sub-parallel to the fractionation line). Use of a consistent baseline protocol is
expected to improve external precision for ratios involving 204Pb. For illustration, an error polygon
is drawn for ±100ppm errors in 206/204 and 207/204, and ±50ppm in 207/206.
15.505
SRM 981 - COMPARISON WITH LITERATURE VALUES
Diamonds - TIMS
Squares - PIMMS
+ 200 ppm/amu
207/204 Pb
15.500
15.495
grand mean
15.490
NBS Certified value
15.485
fractionation line
- 200 ppm/amu
WHOI NEPTUNE
15.480
16.926
16.930
16.934
16.938
16.942
16.946
206/204 Pb
Figure 5. A comparison of NEPTUNE data on NBS 981 with other literature values, both TIMS
and PIMMS, shows that all data is aligned closely along a fractionation line, but there is ~
480ppm/amu spread along the fractionation line. Some of this relates to the error uncertainties
assigned to the absolute values of the NBS Pb and Tl standards (180ppm/amu and 210ppm/amu
respectively) that are used to “calibrate” fractionation effects.
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3. SAMPLE ANALYSES
15.610
Savai'i Basalts - Replicate Pb Analyses
S15
15.608
± 100ppm in 207/204
207/204 Pb
15.606
S25
15.604
15.602
15.600
S16
15.598
15.596
15.594
18.78
18.79
18.80
18.81
18.82
18.83
18.84
18.85
18.86
18.87
206/204 Pb
Figure 6. External precision of repeat runs on three Samoan basalts; filled diamonds are single
runs on leached samples; diamonds connected by bars are runs on different days of a single
chemistry (unleached samples), repeat solutions, 2s = 90 ppm for 207Pb/204Pb. Runs on duplicate
chemistries are not as precise, as there is obvious sample heterogeneity (see especially S25).
Sample heterogeneity issues may now be the limiting factor in Pb isotope studies, given the large
improvement in mass spec precision afforded by PIMMS and double/triple spike TIMS techniques.
Western Samoan Lavas
15.64
207/204 Pb
15.62
Savai'i TIMS
15.60
Upolu
TIMS
Savai'i
Neptune
15.58
15.56
15.54
18.65
Upolu
Neptune
18.70
18.75
18.80
18.85
18.90
18.95
19.00
19.05
206/204 Pb
Figure 7. High-precision NEPTUNE Pb data for basalts from the Upolu and Savai’i shields,
compared to literature data for Upolu shield basalts and post-erosional basalts from Savai’i. The new
Upolu data form a tight array with a “normal” positive slope, as a lower bound to the existing field;
there is no evidence for a large dispersion in 207Pb/204Pb as seen in the existing data. There is no
existing data for Savai’i shield basalts, but our new data bound the field for post-erosional basalts,
with much less scatter. The negative slope is very unusual for OIB lavas, and reflects mixing with a
high, probably ancient, D7/4 Pb component.
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4. ACKNOWLEDGMENTS
The NSF/EAR Instrumentation and Facilities Program was instrumental in facilitating
the acquisition of a NEPTUNE multi-collector ICP/MS at WHOI. The robust health of
analytical geochemistry in the US is due largely to this NSF Program, carefully nurtured
over several decades by Dan Weill’s vision and leadership. Our enduring thanks to Dan for
this contribution.
We also appreciate the patient and detailed help given to us by Wafa Abouchami and
Steve Galer, during our conversion to the Mainz Pb chemistry.
NEPTUNE “steerage” at WHOI has been a joint collaboration involving also Ken Sims,
Bernhard Peucker-Ehrenbrink, Roger Francois and Simon Thorrold.
For reference, also see Hart et al., EOS 83, F20, 2002.
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