High Precision Pb Isotope Techniques from the WHOI NEPTUNE PIMMS

Transcription

1 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

2 1. NEPTUNE NUTS AND BOLTS NBS 981 NEBULIZER 0.12 ml/minute. LEAD SENSITIVITY volts/ppm. TYPICAL RUN INTENSITY volts. RUN LENGTH 9 to 18 minutes. 204Hg CORRECTION ppm of mass 204. ABUNDANCE SENSITIVITY 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) ppm on 20X/204 ratios. EXTERNAL PRECISION (2s, n = 38) 80 ppm on and 58 ppm on 206/207 Pb. SUMMARY - EXTERNAL PRECISION (2-Sigma) RATIO NBS 981 SAMOA BASALTS 206/ ppm 101 ppm 207/ / / / of 7

3 2. Pb Isotopes by NEPTUNE PIMMS 8000 Thallium Tailing, NEPTUNE Counts per Second Hg 1.9 ppm of 205 Tl 205 Tl = 22 volts 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). 3 of 7

4 205/203 Tl Thallium Mass Bias versus Time 7/30/02-8/05/02 3/22/02-4/01/02 8/29/02-8/30/ Run Number 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 Pb/ 206 Pb in NBS981 versus Pb/Tl Ratio D 207Pb/206Pb A C B A - 0.5N HCl B - 1% Seawater C - 33 ppb Pb D - ± 50 ppm Pb/205Tl 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 = ). Based on initial tests, there is also no obvious matrix effect between HNO3 (typical run solution), HCl and weak seawater. 4 of 7

5 SRM 981 NEPTUNE Runs [ Diamonds - 3/22/02; Squares -7/30/02; Triangles - 8/29/02 ] fractionation line ±100ppm in 206/ ±100ppm in 207/ ±50ppm in 207/ 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/ SRM COMPARISON WITH LITERATURE VALUES Diamonds - TIMS Squares - PIMMS grand mean NBS Certified value ppm/amu ppm/amu WHOI NEPTUNE fractionation line 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. 5 of 7

6 3. SAMPLE ANALYSES Savai'i Basalts - Replicate Pb Analyses S15 ± 100ppm in 207/204 S25 S 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 Savai'i TIMS Savai'i Neptune Upolu TIMS Upolu Neptune 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. 6 of 7

7 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, of 7