Applications of accurate, high-precision Pb isotope ratio measurement by multi-collector ICP-MS

Collerson, KD, Kamber, BS and Schoenberg, R (2002) Applications of accurate, high-precision Pb isotope ratio measurement by multi-collector ICP-MS. Chemical Geology, 188 1-2: 65-83. doi:10.1016/S0009-2541(02)00059-1


Author Collerson, KD
Kamber, BS
Schoenberg, R
Title Applications of accurate, high-precision Pb isotope ratio measurement by multi-collector ICP-MS
Journal name Chemical Geology   Check publisher's open access policy
ISSN 0009-2541
Publication date 2002-01-01
Sub-type Article (original research)
DOI 10.1016/S0009-2541(02)00059-1
Volume 188
Issue 1-2
Start page 65
End page 83
Total pages 19
Editor C.J. Allegre
P. Deines
S.L. Goldstein
E. H. Oelkers
R.L. Rudnick
L..M. Walter
Place of publication Netherlands
Publisher Elsevier Science BV
Language eng
Subject C1
260301 Geochronology and Isotope Geochemistry
780104 Earth sciences
Abstract The isotope composition of Ph is difficult to determine accurately due to the lack of a stable normalisation ratio. Double and triple-spike addition techniques provide one solution and presently yield the most accurate measurements. A number of recent studies have claimed that improved accuracy and precision could also be achieved by multi-collector ICP-MS (MC-ICP-MS) Pb-isotope analysis using the addition of Tl of known isotope composition to Pb samples. In this paper, we verify whether the known isotope composition of Tl can be used for correction of mass discrimination of Pb with an extensive dataset for the NIST standard SRM 981, comparison of MC-ICP-MS with TIMS data, and comparison with three isochrons from different geological environments. When all our NIST SRM 981 data are normalised with one constant Tl-205/Tl-203 of 2.38869, the following averages and reproducibilities were obtained: Pb-207/Pb-206=0.91461+/-18; Pb-208/Ph-206 = 2.1674+/-7; and (PbPh)-Pb-206-Ph-204 = 16.941+/-6. These two sigma standard deviations of the mean correspond to 149, 330, and 374 ppm, respectively. Accuracies relative to triple-spike values are 149, 157, and 52 ppm, respectively, and thus well within uncertainties. The largest component of the uncertainties stems from the Ph data alone and is not caused by differential mass discrimination behaviour of Ph and Tl. In routine operation, variation of sample introduction memory and production of isobaric molecular interferences in the spectrometer's collision cell currently appear to be the ultimate limitation to better reproducibility. Comparative study of five different datasets from actual samples (bullets, international rock standards, carbonates, metamorphic minerals, and sulphide minerals) demonstrates that in most cases geological scatter of the sample exceeds the achieved analytical reproducibility. We observe good agreement between TIMS and MC-ICP-MS data for international rock standards but find that such comparison does not constitute the ultimate. test for the validity of the MC-ICP-MS technique. Two attempted isochrons resulted in geological scatter (in one case small) in excess of analytical reproducibility. However, in one case (leached Great Dyke sulphides) we obtained a true isochron (MSWD = 0.63) age of 2578.3 +/- 0.9 Ma, which is identical to and more precise than a recently published U-Pb zircon age (2579 3 Ma) for a Great Dyke websterite [Earth Planet. Sci. Lett. 180 (2000) 1-12]. Reproducibility of this age by means of an isochron we regard as a robust test of accuracy over a wide dynamic range. We show that reliable and accurate Pb-isotope data can be obtained by careful operation of second-generation MC-ICP magnetic sector mass spectrometers. (C) 2002 Elsevier Science B.V. All rights reserved.
Keyword Pb-isotope
Analysis
Reproducibility
Isochron
Multi-collector Inductively Coupled Plasma Spectrometer
Geochemistry & Geophysics
Multi-collector Inductively Coupled Plasma Mass Spectrometer
Great Dyke
Mass-spectrometry
Double Spike
U-pb
Zimbabwe
Discrimination
History
Crust
Ages
Q-Index Code C1

Document type: Journal Article
Sub-type: Article (original research)
Collections: Excellence in Research Australia (ERA) - Collection
School of Physical Sciences Publications
 
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