Toward epsilon levels of measurement precision on 234U/ 238U by using MC-ICPMS

Abstract

Variations in 234U/ 238U have wide-ranging applications as tracers for ground- and river-water fluxes and is an essential component in U-series dating. Analytical developments for measuring 234U/ 238U have progressed from direct alpha-counting, with precisions at the percent level, to thermal ionization and multiple-collector inductively coupled plasma mass spectrometry (TIMS and MC-ICPMS, respectively) isotopic measurement techniques. However, 234U/ 238U is difficult to measure with better than permil precision because of the small atomic ratios for most geological samples (∼10 −4 range). Using a Nu Instruments Nu Plasma MC-ICPMS, we have developed two analytical techniques for the precise measurement of 234U/ 238U: (1) a conventional standard-bracketing protocol using multiple Faraday cups and electron multipliers with ion counting capabilities (FM) and (2) a standard-bracketing Faraday cup protocol (FF). Both are capable of measuring 234U/ 238U with precisions at the epsilon level (1 epsilon = 1 part in 10 4): (1) The conventional standard-bracketing FM measurements are conducted as static measurements with the minor 234U isotope measured in a conventional discrete dynode electron multiplier (SEM) equipped with ion counter and a retardation filter. The Faraday-multiplier gain is measured using bracketing measurements of the U metal standard CRM-145. The external reproducibility of 234U/ 238U (reformulated into δ-notation as δ 234U), interspersed with frequent measurements of the gain, is at the ±0.6‰ level (2 σ) for both uraninite and carbonate standards, takes ∼75 min and consumes ∼120 ng of U per measurement. (2) The static standard-bracketing FF protocol measures all three natural U isotopes in Faraday collectors. This is not usually possible using a standard multiple-Faraday array due to the large differences in the abundances of naturally occurring U isotopes. In our study, this is achieved by replacing the standard 10 11 Ω resistor for the 238U Faraday cup with a 10 9 Ω resistor. The 10 9 Ω resistor enables the measurement of ion beams that are ∼100 times larger than can be accommodated by the normal 10 11 Ω resistor, so 238U and 234U are measured simultaneously in Faraday cups with intensities of ∼9 × 10 −9 and ∼5 × 10 −13 A, respectively. All measurements are normalized to bracketing CRM-145 standard measurements (measured with similar 238U signal intensities) thereby correcting for significant tailing from the large 238U ion beam below the smaller 234U beam. Measurements are conducted over 2 min on-peak and 400–650 ng of 238U is required per analysis. External reproducibility for samples with low matrix/U ratios (e.g., uraninites) is better than ±0.3‰ (2 σ). Coral samples show a slightly poorer external reproducibility of ±0.4‰ (2 σ) due to a higher matrix/U ratios of these samples. Repeat measurements of CRM-145 give respective δ 234U values of −36.44 ± 0.10‰ (2 σ m, n = 9) and −36.50 ± 0.14‰ (2 σ m, n = 54) using the FF and FM analytical technique, assuming Harwell uraninite (HU-1) is in secular equilibrium with respect to 234U/ 238U. The improved analytical precision achieved in this study for 234U/ 238U measurement is superior to any other reported measurements and is of great importance for U-series dating errors, particularly for samples older than 300,000 years before present.