Elsevier

Chemical Geology

Volume 181, Issues 1–4, 15 October 2001, Pages 1-12
Chemical Geology

Precise analysis of 234U/238U ratio using UO2+ ion with thermal ionization mass spectrometry for natural samples

https://doi.org/10.1016/S0009-2541(01)00270-4Get rights and content

Abstract

We have developed a new, highly precise method for U isotopic analysis using a thermal ionization mass spectrometry (TIMS) by measuring UO2+ ions emitted from a very small amount (100–10 ng) of U. A new ionization activator, a silicic acid–diluted phosphoric acid solution, was found to be effective not only for producing a strong and stable UO2+ ion beam, but also for keeping the mass fractionation during measurement small and constant. U isotope analysis was carried out by a “dynamic multi collection” method using three Faraday cup collectors and a secondary electron multiplier in ion counting mode with RPQ (RPQ-SEM). For precise isotope analysis, baselines of the Faraday cups and a conversion factor between Faraday cups and the RPQ-SEM were precisely determined simultaneously with U isotope collection. Analytical precision and reproducibility of 234U/238U ratio ranged from 0.10% to 0.38% (2σmean) and from 0.13% to 0.31% (2σ), respectively, for 100–10 ng of U extracted from a natural rhyolitic obsidian. The precision and reproducibility were improved compared with previous studies, and thus, our method is especially useful for samples with very low abundance of U.

Introduction

Uranium and thorium series disequilibrium is a powerful geochemical tracer especially for determining various geological processes that occur on the scales ranging from ∼106 years to a few days, by measuring the activity of short lived radioactive nuclides (e.g., 234U, 230Th, 226Ra, 228Th and 234Th). The 234U/238U dating method has been applied to samples formed in marine environments, such as corals, ferromanganese nodules, and marine sediments (e.g., Ku and Broecker, 1967, Edwards et al., 1986, Moore et al., 1990, Henderson et al., 1993, Stirling et al., 1995, Henderson and Burton, 1999) because sea water is in 234U/238U disequilibrium (234U/238U activity ratio=1.144±0.002; Chen et al., 1986). If the system remains closed after the achievement of 234U/238U disequilibrium, 234U/238U age can be determined by the following equation234U238Upresent−1=234U238Uinitial−1exp(−λ234t)where λ234 is the decay constant of 234U, t is the time from the closure of the system to present, and (234U/238U) represents an activity ratio. It is clear from this equation that precise determination of 234U/238U ratio in the sample is required in order to carry out precise dating. By the classical method of using α counting, typical analytical uncertainties in the 234U/238U ratio are 2–3%, resulting in a few percentage uncertainty in age dating, thereby limiting the application for processes ranging from 0.1–100 ka.

Chen et al. (1986) developed precise 234U/238U measurement with TIMS and improved the analytical precision to 0.5% (2σmean) by using 30 ng of U. Edwards et al. (1986) applied this technique to various corals and obtained useful age data ranging from a few years old to ∼500 ka, which cannot be obtained using the α counting method. However, Stirling et al. (1995) pointed out that this precision in 234U/238U measurement is still insufficient for the estimation of the precise timing and duration of the Last Interglacial period around 125 ka recorded in fossil reefs. To overcome this problem, they used large amounts of sample, such as 1000 ng of U per analysis, and achieved an analytical precision of 0.1–0.2% (2σmean) in 234U/238U ratio. Luo et al. (1997) also developed high precision (0.15%, 2σmean) 234U/238U measurement employing a static-multi-collector ICP-MS, although 450 ng of U was required per analysis. However, there still exists no method capable of measuring 234U/238U ratios using small amounts (<100 ng) of U with 0.1–0.2% analytical precision.

In the study by Chen et al. (1986), the U isotope ratio was measured by collecting U+ ions with a colloidal graphite loading technique onto a “V” shaped rhenium filament, which was originally developed by Arden and Gale (1974). However, this technique has a difficulty in keeping the amount of loaded colloidal graphite constant. In our preliminary experiments, we found that even small differences in the amount of loaded colloidal graphite directly affects the intensity of the U+ ion beam. Furthermore, the colloidal graphite sometimes easily overflowed over the top of the “V” shaped filament during the loading procedure when the amount of colloidal graphite becomes large.

Recently, Gerstenberger and Haase (1997) reported that a mixture of colloidal silicic acid and dilute phosphoric acid is an effective activator for Pb+ ionization when compared to the conventional silica gel activator. Because silica gel is known to produce stable Pb+ as well as UO2+ ion beam (Gancarz and Wasserburg, 1977), we examined this new activator and found that it also promotes better UO2+ ionization, producing a stronger and more stable ion beam compared to the graphite method. In this study, we present a new and highly precise method of 234U/238U ratio analysis collecting UO2+ ions using a new activator of silicic acid–diluted phosphoric acid mixture. Throughout this paper, isotope ratios represent atomic ratio unless stated otherwise.

Section snippets

Experimental section

All the experiments in this study were carried out under clean conditions better than class 100 at the Pheasant Memorial Laboratory (PML), Institute for Study of the Earth's Interior in Misasa.

Baseline

In this study, the baseline values of the Faraday cups is one of the important factors that control the accuracy of the measurement, because the intensity of 235U16O2+ (m/z=267) during data collection ranged from 0.7 to 7×10−14 A, which is close to the detection limit of the Faraday cup collector. Faraday cup baselines were continuously monitored over 30 h in order to examine time drift. Two types of baseline data were acquired as follows: Faraday cup baselines with the deflection plate voltage

Acknowledgements

We would like to thank N. Imai (Geological Survey of Japan) for offering JR-2. We are grateful to M.J. Walter for improving this paper. We also thank all the member of PML for their analytical support and useful discussion. This research was supported by Grants-in-Aid for Scientific Research from the Ministry of Education, Science and Culture, Japan (Monbusho) and the Japanese Society for the Promotion of Science (JSPS) to E.N.

References (24)

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