Determination of thorium and uranium isotope ratios ...

Determination of thorium and uranium isotope ratios in lowconcentration geological materials using a fixed multi-collector-ICPMS Simon Turner,a{ Peter van Calsteren,*a Nathalie Vigierb and Louise Thomasa a

Department of Earth Sciences, The Open University, Milton Keynes, UK MK7 6AA. E-mail: [email protected] b Laboratoire de Ge´ochimie et Cosmochimie, IPGP-CNRS UMR7579, 4, Place Jussieu, 75252 Paris Cedex 05, France

Received 17th October 2000, Accepted 22nd March 2001 First published as an Advance Article on the web 30th April 2001 Techniques used for the measurement of thorium and uranium isotope ratios in low-concentration (v0.1 ppm) geological materials on a Nu Instruments1 MC-ICP-MS are described. Using an Aridus1 micro-concentric, desolvating nebuliser sample introduction system, the beam current in a Faraday collector is typically 1.0 nA ppm21 232Th (total system efficiency~0.3%). The abundance sensitivity is less than 100 counts s21 at 1 mass unit below a large peak with an intensity of 16109 counts s21, or v0.1 ppm. The Faraday–ion counting gain is stable over a day and the method can provide 230Th/232Th ratio measurements on ng quantities of total dissolved Th to a precision that is better than 1% (95% confidence level). This is comparable to results obtained by thermal ionisation mass spectrometry on samples that are typically 10 to 100 times larger. Results for our internal laboratory standards are presented together with our results for standards used in other laboratories to facilitate inter-laboratory comparison.

Introduction The precise determination of 230Th/232Th and 234U/238U isotope ratios has applications in several diverse fields ranging from the chronology of young (v350 ka) silicate and carbonate rocks to investigations of mantle melting and elemental residence times in watershed and sea waters.1 The 230 Th/232Th ratio in natural silicate materials is typically of the order 1025–1026, and in thermal ionisation mass spectrometry (TIMS) the intensity of the 230Th peak is similar to the contribution of the tail of the 232Th peak to the 230Th peak. Multi-collector TIMS techniques, where the high abundance (232Th, 238U) isotopes are collected in Faraday collectors and the low intensity isotopes (230Th, 234U) are collected in ionmultiplier channels equipped with pulse counting electronics, allow static data collection without the loss of data collection time when jumping between masses. For multi-collector TIMS, various methods have been developed for removing the contribution of the 232Th tail on the 230Th peak, either by measurement and mathematical subtraction,2 energy focusing,3 or through elimination via a deceleration lens,4 and these techniques have improved precision to ¡1% (2s), sufficient for many Earth and environmental science applications. However, the extremely low thermal ionisation efficiency of Th (about 0.01%) means that typically hundreds of ng of thorium are required for analysis. This restricted the technique to samples with relatively high concentrations (w1 ppm) or the requirement of the processing of large amounts of material. The great potential of ICP-MS lies in the almost 100% ionisation of thorium and uranium at temperatures around 6000 K achieved in the plasma. Early investigations of thorium isotope ratio analysis by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) were presented by Luo et al.5 Here we describe the techniques we have used for the {Present address: Department of Earth Sciences, University of Bristol, Bristol, UK BS8 1RJ.

612

measurement of thorium isotope ratios in low concentration geological materials on a Nu Instruments1 MC-ICP-MS.6 The main distinguishing feature of this instrument is a fixed multicollector array with three ion-counting channels, one behind a deceleration lens. Individual isotope beams are aimed at their target collectors using two quadrupole lenses which can achieve a ‘zoom’ effect that is continuously variable between 0.5 and 2 mass units, depending on the mass range, without significant deterioration of the flat-top beam shape. The deceleration lens rejects ions that, because of collisions with residual gas molecules, have reduced energy or deviating direction and this improves the abundance sensitivity by up to two orders of magnitude.

Instrumentation and measurement protocol Measurements were performed on a Nu Instruments1, variable ‘zoom’ double-focusing multi-collector ICP-MS at The Open University. Details of this instrument have been described elsewhere.6 The arrangement of the collector array is shown in Fig. 1 and the working conditions for the analyses reported here are detailed in Table 1. Standards and purified samples were diluted in 2% HNO3 to concentrations of about 5– 100 ng ml21 and aspirated into the instrument via an Aridus1 micro-concentric nebuliser and desolvating system. The Aridus

Fig. 1 Schematic diagram of the collector array for the Nu Instruments1 MC-ICP-MS.

J. Anal. At. Spectrom., 2001, 16, 612–615 This journal is # The Royal Society of Chemistry 2001

DOI: 10.1039/b008351h

Table 1 Working conditions for U and Th analysis on the Nu Instrument1 MC-ICP-MS

Table 2 Collection and data reduction routines for determining IC1 gain and abundance sensitivity

Plasma conditions

Collection routine Collector FAR6 FAR7 FAR8 IC0 238 U 237U 236U Cycle 1 238 Cycle 2 U 237U 238 U 237U 236U 235U Cycle 3 Cycle 4

RF power Reflected power Ni sampler cone Ni skimmer cone

1300 W v4 W 1.15 mm diameter 0.6 mm diameter

FAR9 235 U 236 U 234 U 238 U

IC1 234 U 235 U 237

U

Gas flow rates Ar cooling flow Ar auxiliary flow Sample introduction Aspiration rate Aridus1 MCN Ar sweep flow

13 l min21 0.9 l min21 50 ml min21 2.9 l min21

Focussing optics Lens 1 ESA Resolution Zoom optics U : 1 mass unit spacing Quad 1 Quad 2

4000 V 231.7 V 400 per amu

Data reduction routinea 235 U/238U IC1/FAR8 235 U/238U IC0/FAR6 IC1 gain IC0 gain 236 U/238U Gain corrected 237 U/238U IC1 abundance sensitivity 237 U/238U IC0 abundance sensitivity 234 U/238U Corrected

R1~I(C2,235U)/I(C2,238U) R2~I(C3,235U)/I(C3,238U) R3~R1/0.00725 R4~R2/0.00725 R5~I(C1,236U)/I(C1,238U)/R4 R6~I(C4,237U)/I(C4,238U)/R3 R7~I(C2,237U)/I(C2,238U)/R4 R8~I(C1,234U)/I(C1,238U)/R3

a

R refers to a ratio calculation, I to a peak intensity and C refers to the cycle used. 34 V 286 V

Uranium isotope ratios

instrument with a mass resolution of 400. The discrimination level of the pulse pre-amplifiers was adjusted with the tube valve closed such that the dark noise was vv1 cps. Multiplier dead-time and linearity corrections are insignificant at count rates of hundreds to thousands counts s21 and have not been applied. The potential of the deceleration lens on IC1 was adjusted until the abundance sensitivity was satisfactory (v100 ppb) and the correct 234U/238U ratio was obtained. Once these conditions were met, it was found from repeated analyses that the Faraday–IC1 gain and IC1 abundance sensitivity were stable to better than 0.5% over one day. Fig. 2 shows traces of Faraday collectors 7 and 9 and ion counter IC1 when the magnet is scanned from mass 238.5 to 241.7 in Faraday 6. The trace for IC1 shows the 235U peak on the left, coincident with the 238U peak on Faraday 8. On the 236 position, the 238U peak would be on IC0, which is switched off to avoid overloading. The 237 position indicates the abundance sensitivity, and is coincident with the 238U peak on Faraday 9. The sharp increase on the right shows where the 238 U peak is approaching IC1. The IC1 trace gives a pessimistic impression of the abundance sensitivity, because there is a finite probability that 238U ions, deflected off the collectors and support structure, enter the ion multiplier. The abundance sensitivity at two mass units below the 238U peak, i.e., the 236 position, is indistinguishable from zero and this also indicates that any contribution from 235U hydride is insignificant.

The primary intention of this work was to investigate the sensitivity and stability of this instrument for Th isotope ratio measurements, but the instrument was set up by using the known4 isotope ratios of a uranium solution (NIST SRM U112a) to assess the gain and abundance sensitivity on the IC1 ion counting channel, which is equipped with a deceleration lens (see Fig. 1). The background of the ion counting channels is clear of oxide interferences. Natural uranium isotopes were collected in four magnet setting cycles using a combination of Faraday- and ion-counting collectors as indicated in Table 2. The gain factors for the two ion counting collectors can then be calculated by dividing the 235U/238U ratios from the first two cycles by the accepted ratio (0.00725) as indicated in Table 2. This calculation of gain factors implicitly includes a fractionation correction that, although less precise than an exponential correction, is appropriate for our purpose. Abundance sensitivity is determined by measuring the intensity at the position of the 237U isotope, which is assumed to be absent because of its short half life (6.75 d). The abundance sensitivity for IC0, which is not equipped with a deceleration lens, is around 2 ppm as expected for a magnetic sector

Fig. 2 Traces of Faraday collectors 7 and 9 and ion counter IC1 when the magnet is scanned from mass 238.5 to 241.7 in Faraday 6. The trace for IC1 shows 235U on the left, coincident with 238U on Faraday 8. On the 236 position, the 238U peak would be on IC0 which is switched off. The 237 position indicates the abundance sensitivity, and is coincident with the 238U peak on Faraday 9. The sharp increase on the right shows where the 238U peak is approaching IC1.

Th : 0.667 mass unit spacing Quad 1 Quad 2 Vacuum conditions Expansion chamber Analyser Data acquisition parameters Integration time Magnet settle time

74 V 2351 V 26102 Pa v161027 Pa 8s 2s

Typical sensitivity/efficiency 1.0 nA ppm21 or 0.3%

ensures better sample utilisation and reduced hydride formation than can be achieved with a pneumatic nebuliser and cooled spray chamber. Neither uranium nor thorium showed significant memory effects and between-sample wash-out was achieved by aspirating 2% HNO3 for 5 min. River water sample preparation involved U and Th pre-concentration using Fe(OH)3 co-precipitation.7 Both silicate samples and Fe(OH)3 precipitates were dissolved and further purified using the methodology described by Turner et al.8

J. Anal. At. Spectrom., 2001, 16, 612–615

613

Routine analysis of uranium isotopes can therefore be performed using a simplified, 3 cycle version of the routine detailed in Table 2. Thorium isotope ratios The instrument stability determined from the uranium analyses indicated that thorium isotope ratios could be analysed in static mode once the IC1 gain and abundance sensitivity had been established. The advantage of static collection is a rapid analysis that allows a minimum of material to be analysed. However, the collector configuration of the Nu Instrument1 is such that thorium isotopes cannot be analysed in static mode with the ‘zoom’ optics configured for a single mass spacing. Therefore, the ‘zoom’ optics were set (Table 1) to achieve a 0.667 mass spacing with 232Th in Faraday 8 and 230Th in IC1. There is no indication that the abundance sensitivity is affected by the setting of the ‘zoom’ optics. The disadvantage of this method of static thorium isotope ratio analysis is that the IC1 gain and abundance sensitivity cannot be assessed during the course of the analysis (see below). Therefore the following analysis protocol was assumed: (1) initial instrument set-up was achieved using a natural uranium solution; (2) fine-scale ‘‘tuning’’ of the abundance sensitivity and gain was made until the accepted 230Th/232Th ratio (6.13161026) was obtained for the in-house thorium standard solution Th‘U’;4 (3) one or more rock standard solutions with accepted 230 Th/232Th isotope ratios were then analysed as secondary standards in order to check the instrument calibration resulting from steps 1 and 2; and (4) unknowns were analysed using a standard–sample–standard sequence. Fine tuning (step 2) implicitly accounts for the small deviations caused by possible differences in fractionation behaviour between U and Th, possible small deviations caused by the different mass spacing of U and Th isotopes, and differences resulting from accumulated dead-time and non-linearity of the ion multiplier. In effect, step (2) ensures that data collected for unknowns are normalised to the value for our in-house Th‘U’ standard as determined by TIMS.4

Results and discussion The total sample consumption for a typical thorium isotope ratio analysis was less then 1 ml for 100 ratio determinations and sample concentrations varied from 100 to 5 ng ml21. On the basis of a beam current of 1.0 nA ppm21 of 232Th at an uptake rate of approximately 50 ml min21, this corresponds to an efficiency of 0.3%, which is significantly better than is achieved by TIMS or that reported by Luo et al.5 Thus, it is possible to analyse very low concentration materials using the Nu Instrument1 MC-ICP-MS in conjunction with an Aridus1 micro-concentric nebuliser. Table 3 shows triplicate analyses of a river water sample from the Mackenzie Basin in Canada7 and silicate minerals from a cumulate nodule from St. Vincent, Lesser Antilles,8 which would have either been impossible Table 3 230Th/232Th isotope ratios for river watera and silicate mineralsb Sample

Th concentrationc

Little Smoky river Duplicate Triplicate

57.5 ng ml21 57.5 ng ml21 57.5 ng ml21

230

Th/232Th (6106)

7.928 7.946 7.884

1 SE 0.029 0.037 0.048

WI-18 olivine 3.9 ppm 11.161 0.159 WI-18 amphibole 24.5 ppm 7.299 0.034 WI-18 plagioclase 13.5 ppm 7.931 0.088 a Mackenzie Basin, Canada (Vigier et al.7). bCumulate nodule, St. Vincent, Lesser Antilles (Turner et al.8). cTotal sample usage about 1.0 ml.

614


Fig. 3 Results of 230Th/232Th measurements (61026) for several standards (concentrations indicated in brackets) showing the mean and 2 standard deviations (illustrated by solid and dashed horizontal lines, respectively). Error bars on individual analyses are 1 standard error of the mean. (a) Variation over an 8 hour period of alternating analyses of the Open University Th‘U’ solution standard and the AThO (Icelandic rhyolite obsidian) rock standard. (b) TML#3 (Table Mountain Latite). (c) Clermont-Ferrand ThS1 standard. (d) University of California Santa Cruz ThA standard. Arrows indicate averages (value in italics) of published analyses by TIMS data sources: Th‘U’ (van Calsteren and Schwieters4); AThO (Williams et al.3); TML#2 (Williams et al.3) (no data have yet been published for TML#3); ThS1 (Claude-Ivanaj et al.9); ThA (Lundstrom et al.10).

previously or else required separation of thorium from unreasonably large amounts of material. The results of static thorium isotope ratio measurements for a series of other laboratory standards are shown in Fig. 3 and compared with the accepted values determined by TIMS. Fig. 3(a) shows the results from a day of alternately analyzing the in-house Th‘U’ standard solution and a 100 ng ml21 solution of the AThO (Icelandic rhyolite obsidian) rock standard. The data show no evidence of any memory effects and the mean value for the six determinations of AThO is indistinguishable from published TIMS values.3 Results for the Table Mountain Latite rock standard (TML Jar 3) are within error of the published TIMS value for TML.3 Finally, in Fig. 3(c) and (d) we show the results obtained for the Clermont-

Ferrand ThS1 and University of California Santa Cruz ThA solution standards, both of which are within error of their respective published values.9,10 Results for TML were obtained over a period of several weeks and the biggest discrepancies between the published TIMS values and our MC-ICP-MS measurements are for the TML and ThA standards, although all agree within 2s error. Several other aspects of the results in Fig. 3 are worthy of brief mention. Firstly, there is a minor decline in precision and reproducibility as the sample concentration is decreased from 100 to 5 ng ml21. Secondly, the internal precision (¡0.15%) is often much better than the reproducibility (about 0.5%). Thirdly, in Fig. 3(b), (c) and (d) there is evidence for a small progressive shift in the determined value over time. The latter two observations are almost certainly linked and probably reflect minor drift in the Faraday–IC1 gain or gradual changes in fractionation resulting from build-up and/or erosion of the sample and skimmer cones. It can be estimated that the combined effects are of the order of 0.5% over an 8 hour period. Although our purpose was to investigate the stability of the instrument, it should be recalled that the approach taken here is not intrinsically rigorous, in the sense that Faraday–IC1 gain, abundance sensitivity and mass fractionation are not measured directly. A straightforward and more rigorous approach would involve spiking unknowns with natural uranium and using a dynamic measurement routine based on that in Table 2 to explicitly measure the Faraday–IC1 gain, abundance sensitivity and mass fractionation during thorium isotope analysis. We anticipate that such an approach would result in reproducibility that is much closer to the internal precision and that reported by Luo et al.5 The disadvantage is that the longer analysis time required by this approach will result in higher sample consumption. Nevertheless, our method allows for a simple, fast and straightforward application to

thorium isotope determination in low-concentration geological materials where a precision of 1% at the 95% confidence level is acceptable.

Acknowledgements We thank Mabs Gilmour and Rhiannon George for assistance in the laboratories, Phil Freedman and Andy Burrows for help and advice with the instrument and three anonymous reviewers for their constructive comments. The NERC and Royal Society are acknowledged for financial support.

References 1 Uranium-series Disequilibrium: Applications to Earth, Marine and Environmental Sciences, ed. M. Ivanovich and R. S. Harmon, Clarendon Press, Oxford, UK, 2nd edn., 1992. 2 F. McDermott, T. R. Elliott, P. van Calsteren and C. J. Hawkesworth, Chem. Geol., 1993, 103, 283. 3 R. W. Williams, K. D. Collerson, J. B. Gill and C. Deniel, Earth Planet. Sci. Lett., 1992, 111, 257. 4 P. van Calsteren and J. B. Schwieters, Int. J. Mass Spectrom. Ion Processes, 1995, 146/147, 119. 5 X. Luo, M. Rehka¨mper, D.-C. Lee and A. N. Halliday, Int. J. Mass Spectrom. Ion Processes, 1997, 171, 105. 6 N. S. Belshaw, P. A. Freedman, R. K. O’Nions, M. Frank and Y. Guo, Int. J. Mass Spectrom. Ion Processes, 1998, 181, 51. 7 N. Vigier, B. Bourdon, S. Turner and C. J. Alle`gre, Earth Planet. Sci. Lett., 2000, submitted for publication. 8 S. P. Turner, C. J. Hawkesworth, P. van Calsteren, E. Heath, R. Macdonald and S. Black, Earth Planet. Sci. Lett., 1996, 142, 191. 9 C. Claude-Ivanaj, B. Bourdon and C. J. Alle`gre, Earth Planet. Sci. Lett., 1998, 164, 99. 10 C. C. Lundstrom, D. E. Sampson, M. R. Perfit, J. Gill and Q. Williams, J. Geophys. Res., 1999, 104, 13035.


615