Advances in ID-TIMS and LA-MC-ICP-MS U-Pb mass spectrometry with applications to geochronology and environmental U analysis
Abstract
Recent advances highlighted are the analysis of tiny quantities of U and Pb isotopes, ID-TIMS U-Pb dating using EARTHTIME double spike tracers and improved detectors, and the performance of integrated LA-MC- ICP-MS systems for U-Pb dating. Detector improvements: The use of 1012 ohm resistors with faraday detectors has shown that their noise levels conform closely to prediction and overlap ion intensities previously in the realm of ion counting detectors, enabling multiple faraday ion detection for <20 pg of radiogenic Pb. A trade-off in slower response time, however, hinders rapid peak-switching data acquisition and favours use in static multicollection mode. Sub-ng high precision U analysis of complex matrices: The accurate measurement of U isotopes, including ^{236}U at the <1e^{-5} level, in sub-ng quantities of uranium, represents a very significant challenge to the detection of human internal uranium contamination many years after exposure. We have used MC-ICP-MS instruments with mixed SEM-faraday detectors along with improvements in chemical separation and purification to measure uranium isotopes in urine with detection limits of a few fg. In a large study >95% of samples intentionally blind-doped with very small amounts (~3 fg to 40 pg) of either depleted uranium or pure ^{236}U were accurately quantified from a sample cohort of more than 300 urine samples. ID-TIMS: Recent developments spearheaded by the EARTHTIME project (www.earth-time.org) has driven a community effort to improve inter-laboratory U-Pb comparability (and accuracy) via the development of mixed U-Pb tracer solutions for community use. In addition a ^{202}Pb-^{205}Pb-^{233}U-^{235}U double spike has been mixed as part of this project aimed at achieving increased precision via real time Pb mass bias determination. LA-MC-ICP-MS: The Pb/U and 207Pb/206Pb precision by LA-(MC)-ICP-MS can be comparable to SIMS U-Pb dating of zircon. A weakness of LA-ICP-MS U-Pb analysis has been the large consumption of sample, compromising both spatial resolution and the potential for subsequent Hf-O isotopes and/or trace element analysis. New protocols involving the New Wave 193nm solid state laser with the Nu Plasma HR configured with multiple SEMs and faraday cups (including Hg and 204Pb correction) demonstrate that for most zircons this problem has been overcome. 95% confidence Pb/U uncertainties of <3% are achievable with zircon consumption only a factor 6-10x larger than SIMS for most zircons, therefore consuming <1% of the mass of typical crystals. This has been achieved with limited laser-induced elemental fractionation, reduced need to discard time-resolved data, a shallow pit aspect ratio, and up to 30 analyses per hour. The incredibly varied applications of Pb and U isotopes to problems in earth and environmental science will continue to challenge detectors on TIMS and MC-ICP-MS instruments for years to come.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.V11E..03P
- Keywords:
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- 0454 Isotopic composition and chemistry (1041;
- 4870);
- 1040 Radiogenic isotope geochemistry;
- 1094 Instruments and techniques;
- 1115 Radioisotope geochronology;
- 1194 Instruments and techniques