Roberts, N.M.W., Pawlig, S., Horstwood, M.S.A., The Nu Attom High Resolution ICP-MS: Laser Ablation U-Pb Geochronology. Nu Instruments Application Note AN11. more

NU ATTOM HR-ICP-MS APPLICATION NOTE AN11 THE NU ATTOM HIGH RESOLUTION ICP-MS: LASER ABLATION U-Pb GEOCHRONOLOGY Nick M W Roberts1, Sabine Pawlig and Matthew Horstwood1, 1: NERC Isotope Geosciences Laboratory, Keyworth, Nottingham , NG12 5GG INTRODUCTION Modern high resolution ICP-MS (HR-ICP-MS) instruments offer a number of performance advantages compared to more widely used quadrupole ICP-MS instruments, including increased sensitivity, superior detection limits and faster scan speeds. For laser ablation acquisition, rapid peak scanning is a distinct advantage, as it allows for increased temporal resolution of time-resolved data. The advantage of single-collector ICP-MS over multi-collector ICP-MS, is that a wider mass range can be scanned in a single analysis. This means that a range of elemental concentrations can be determined, as well as precise isotope ratios. Laser ablation sampling coupled to measurement via ICP-MS is an increasingly used tool within earth science, and can be used for determining quantitative trace element concentrations of materials, as well as for isotopic dating of minerals in particular within U-Pb geochronology. Here, we report the use of the Nu Attom for determining U-Th-Pb isotopes in zircon and monazite crystals, and demonstrate the ability to combine these isotope ratio measurements with other trace element concentrations using the wide mass range available in rapid peak-scanning mode. Experiment Laser ablation analysis used a spot size of 20-35µm (figure 1), with a fluence of 1.8 to 2.2 j/cm2, for 30 seconds of integration. An on-peak zero was measured every 5 to 10 analyses. The Pb-Pb, U-Pb and Th-Pb ratios were normalised to a bracketing primary standard, based on the average measured value of the standard compared to the ‘true’ value determined by ID-TIMS. The measured masses and dwell time for each of the four experiments are shown in Table 1; also shown are average count rates for the isotopes of interest for the standards shown in the figure. Trace element concentrations are semi-quantitative, and use repeat analyses of NIST 612 glass for normalisation. The Attom can measure large signals by means of an attenuation mechanism; this was used for 232Th which is particularly concentrated in monazite. To measure the degree of attenuation, 236U is introduced via a spike solution and is measured with both a normal and an attenuated signal; the average value of the attenuation/normal signal is then applied to the 232Th offline. Data were collected using the time-resolved-analysis function in the Nu Attolab software; with ratio calculations performed using the Nu calculations editor. Discussion Using typical ablation parameters (20 to 35 µm spot @ 1.5-2.5 j/cm-2), the Nu Attom is capable of measuring 207Pb/206Pb, 208Pb/232Th 206Pb/238U ratios with an external reproducibility of <3% (2SD) after normalisation to a standard, these ratios are accurate to <2% (2SD) (figure 2). This makes the Attom ideal for UTh-Pb geochronology of U-bearing accessory minerals such as zircon and monazite; although not shown, dating of other minerals such as titanite, allanite and apatite is feasible. To gain the most from U-Th-Pb geochronology it is commonly useful to determine trace element concentrations of the dated minerals. For example, REE patterns in zircon can aid the determination of the co-precipitating mineralogy, and thus whether the dated growth-zone within the zircon represents a magmatic or metamorphic event. Ideally, trace element concentrations will relate to the individual growth zone that has been dated. This can be done using one ablation for a U-Th-Pb measurement, and a separate ablation for a trace element measurement; however, this assumes that the same zone has been analysed each time. For consumption of less material and allowing a greater spatial resolution, a preferred approach is to analyse U-Th-Pb isotopes and trace elements in one ablation. The large mass range of the Nu Attom allows for certain trace elements to be simultaneously determined along with precise U-Th-Pb isotopic ratios. Experiment 3 shows that 207Pb/206Pb and 206Pb/238U ratios can be precisely and accurately measured along with determination of the heavy REE content; whilst experiment 4 shows that a complete REE pattern can be determined along with precise and accurate 207Pb/206Pb age determinations (figure 2). Figure 1: 20µm diameter ablation pits in Moacyr monazite. Instrumentation The Nu Attom is a double-focusing, high-resolution magnetic sector mass spectrometer. The instrument is entirely purpose designed and built to provide the best performance and reliability coupled with flexibility and ease-of-use for precise and accurate elemental and isotope ratio analysis. A unique detector system gives the Nu AttoM a large dynamic range, and its electrostatic scanning capability has the widest range in its class (40%). Furthermore, the continuously variable high resolution means that sufficient resolution for isobaric separation can be achieved with minimum compromise in sensitivity. For the laser ablation work presented here, a Nu Attom was coupled to a New Wave Research UP193FX excimer laser ablation system. Helium was used as a carrier gas, and mixed with argon before entering the ICP-MS. For some experiments a solution was simultaneously aspirated using the Nu Instruments DSN-100 that contained 203,205Tl, 230Th and 236U; this allows for on-line correction of mass-bias and drift in the inter-element fractionation. Conclusions The Nu Attom ICP-MS allows for rapid peak-scanning across a wide mass range. The ability to determine precise and accurate U-Th-Pb isotope ratios, whilst at the same time determining concentrations of other trace elements makes it an ideal tool for geochronological dating of a range of natural materials. INSTRUMENTS THAT WORK www.nu-ins.com NU ATTOM HR-ICP-MS APPLICATION NOTE AN11 A B C D E F Figure 2: (A) U-Pb concordia diagram for GJ-1 zircon normalised to 91500 (ellipses are 2σ). (B) Th/Pb vs. U/Pb isochron for Moacyr monazite normalised to Stern monazite (error bars are 2σ). (C) U-Pb Concordia for GJ-1 zircon normalised to 91500, and (D) weighted mean Pb-Pb age of 91500 zircon normalised to Plesovice. (E) Chondrite normalised HREE pattern, and (F) chondrite normalised REE pattern. (1) 35µm, ~2.2 j.cm-2, 5Hz peak dwell (µs) cps 202Hg 70 203Tl 100 204Hg,Pb 70 205Tl 100 206Pb 200 620000 207Pb 400 38000 208Pb 70 230Th 70 232Th(att) 70 235U 400 47000 236U 70 236U(att) 70 (2) 20µm, ~1.8 j.cm-2, 7Hz peak dwell (µs) cps 202Hg 100 203Tl 100 204Hg,Pb 100 205Tl 100 206Pb 200 650000 207Pb 200 41000 208Pb 200 29000 230Th 100 232Th(att) 200 89000 235U 200 65000 236U 100 236U(att) 1000 (3) 25µm, ~1.8 j.cm-2, 7Hz peak dwell (µs) cps 159Tb 100 16600 165Ho 100 56000 169Tm 100 63000 172Yb 100 164000 175Lu 100 85000 206Pb 200 308000 207Pb 400 19000 235U 400 26000 (4) 25µm, ~1.8 j.cm-2, 7Hz peak dwell (µs) cps 139La 100 8 140Ce 100 157000 141Pr 100 350 146Nd 70 1300 147Sm 70 2600 153Eu 70 7000 157Gd 70 10500 159Tb 70 20000 165Ho 70 65000 169Tm 70 72000 172Yb 70 185000 175Lu 70 95000 206Pb 250 320000 207Pb 400 19500 Table 1: Measured masses and dwell times for each of the four experiments INSTRUMENTS THAT WORK www.nu-ins.com