The application of laser ablation-inductively coupled plasma-mass spectrometry to in situ U–Pb zircon geochronology
Introduction
The first demonstrations of the potential of LA-ICP-MS to perform in situ 207Pb/206Pb determinations on zircon with sufficient precision to be a useful tool for dating Proterozoic and older zircons took place in the early 1990s (Fryer et al., 1993, Feng et al., 1993). However, it is only since the development of UV laser ablation and high-sensitivity ICP-MS instrumentation in the mid-1990s that the technique has been applied widely to in situ zircon dating using the Pb/U decay schemes (e.g., Hirata and Nesbitt, 1995, Jackson et al., 1996, Fernandez-Suarez et al., 1998, Horn et al., 2000, Ketchum et al., 2001, Li et al., 2001, Košler et al., 2002, Tiepolo et al., 2003, Tiepolo, 2003). The measurement of 207Pb/235U and 206Pb/238U ratios allows assessment of concordance and extraction of a true age from zircon populations that have suffered variable Pb loss from grains or parts thereof.
The increasing use of LA-ICP-MS U–Pb dating derives from the fact that it is the cheapest, most widely available and fastest technique for in situ U–Pb dating. While it is not as well suited as SIMS for applications requiring high-resolution sampling (e.g., dating complex zircons with small cores and/or overgrowths), it is extremely well suited to projects needing large numbers of analyses (e.g., detrital zircon studies). Despite increasing usage, however, the LA-ICP-MS technique is not universally accepted as a robust technique for zircon dating. The most frequently cited drawbacks of U–Pb dating using LA-ICP-MS are as follows: (1) fractionation of Pb relative to U during the ablation/transport and ionisation processes (e.g., Hirata and Nesbitt, 1995, Fryer et al., 1995), and (2) difficulty in performing a useful common Pb correction based on 204Pb, due to the overwhelming isobaric interference from Hg.
It is clear that elemental fractionation during ablation is related to differential volatilisation and condensation processes, but the there has been considerable debate about the possible mechanisms involved (see Jackson, 2001, for a review). Possible mechanisms that give rise to ablation time-dependent elemental fractionation include the following: (1) dynamic differential volatilisation/condensation processes within or close to an ablation pit related to progressive defocusing of the laser as it penetrates into the sample (Hirata and Nesbitt, 1995) or to the evolving aspect ratio of the pit (Eggins et al., 1998, Mank and Mason, 1999); (2) partitioning of elements preferentially into a particulate (refractory elements) or vapour phase (volatile elements), which are differentially transported (Outridge et al., 1997); and (3) differential volatilisation of elements during incomplete vapourisation of particulates due to insufficient residence time in the ICP (Guillong and Günther, 2002).
A wide variety of procedures has been invoked in previous LA-ICP-MS U–Pb studies to minimise and/or correct for ablation-related elemental fractionation, in addition to the inherent mass bias of the ICP-MS instrument. “Active focussing” (raising the sample stage during ablation to maintain constant laser focus on the ablation surface) was used, together with standardisation on a NIST glass reference material, by Hirata and Nesbitt (1995). This method has not been widely adopted because of the design limitations of LA hardware and because it requires time-consuming optimisation of the rate of sample height adjustment for different ablation conditions.
A more practical approach to preventing formation of deep craters, and maintaining constant focus conditions, is to use raster ablation, in which the sample is constantly moved laterally during ablation (e.g., Li et al., 2001, Horstwood et al., 2001, Košler et al., 2002). The main drawback of this method is that it compromises the spatial resolution attainable (or requires use of a very small beam, which reduces the ablation rate and thus the signal/noise ratio). Use of an external standard or an alternate correction procedure is still required to correct instrumental mass bias.
Several studies have made use of a “jet cell”, which introduces the carrier gas as a high-velocity jet directly onto the ablation site (Jackson et al., 1996, Horn et al., 2000, Jackson, 2001) to reduce fractionation, together with calibration against a zircon standard (e.g., Fernandez-Suarez et al., 1998, Ketchum et al., 2001). While the jet cell can produce a significant reduction in fractionation (ca. 50%), maintaining precise alignment of the carrier gas jet onto the ablation site is difficult and represents a serious limitation to this cell design.
Horn et al. (2000) used an experimentally derived mathematical procedure that relates fractionation to spot geometry (at constant energy density) to correct for ablation-related elemental fractionation, together with use of a Tl/235U spike to correct for instrumental mass bias. This method requires elemental fractionation to be extremely reproducible from day to day. It is better suited to the flat energy profiles of excimer laser systems (e.g., Horn et al., 2000) than Nd:YAG systems (Tiepolo et al., 2003) for which the energy distribution within the laser beam can vary significantly depending upon laser tuning and operating conditions.
In this study, we used a high-sensitivity quadrupole ICP-MS coupled to a custom-built UV laser ablation microprobe based on a frequency quadrupled Nd:YAG laser (λ=266 nm). Also evaluated was a commercial frequency-quintupled (λ=213 nm) laser ablation system. We describe and evaluate a simple technique that employs spot ablations with no specific strategies to minimise ablation-related elemental fractionation. Fractionation and instrumental mass bias are corrected by direct calibration against a new zircon standard analysed under carefully matched conditions, using He as the ablation gas to increase the reproducibility of the Pb/U fractionation. Time-resolved data acquisition is employed to evaluate zircon homogeneity and to allow selective integration of signals to minimise common Pb contributions and Pb loss, and thus to maximise concordance. Tera–Wasserburg diagrams (Tera and Wasserburg, 1972) are also employed to assess, and correct for, residual common Pb contributions. These procedures are evaluated using data for five zircons ranging in age from 1065 to 7 Ma. Two of these zircons have been analysed more than 400 times over more than a year by two instrument operators, allowing an exhaustive evaluation of the long-term precision and accuracy of the technique.
Section snippets
Sample preparation
All zircons were mounted in epoxy in 2.5-cm-diameter circular grain mounts and polished until the zircons were just revealed. Images of the zircons were obtained using the back-scatter electron (BSE) detector on a Cameca SX50 electron microprobe. The BSE detector is a light-sensitive diode, so the image obtained of the internal structure of the zircon is a combination of the variation in mean atomic number (composition) as well as the cathodoluminescence (CL). BSE/CL images were taken of all
GLITTER software
The raw ICP-MS data were exported in ASCII format and processed using GLITTER (Van Achterbergh et al., 2001), an in-house data reduction program. GLITTER calculates the relevant isotopic ratios (207Pb/206Pb, 208Pb/206Pb, 208Pb/232Th, 206Pb/238U and 207Pb/235U where 235U=238U/137.88) for each mass sweep and displays them as a coloured pixel map and as time-resolved intensity traces. Ratios were examined carefully for anomalous portions of signal related to zones of Pb loss and/or common Pb gain,
Short-term precision (266 and 213 nm)
The short-term (2 h) precision of the method has been evaluated by multiple analyses of an isotopically homogeneous grain of our zircon standard, GJ-1. The 266 nm laser ablation has been compared with 213 nm ablation, using both Ar and He as ablation gases. The GJ-1 zircon was analysed 10 times using nominally the same conditions (spot size=50 μm, pulse energy 0.25 mJ measured at the sample, 10 Hz repetition rate, 60 s of data integrated) for each laser system. External precisions (1σ) for the
Discussion and conclusions
A new zircon standard, GJ-1, has been developed. Multiple LA-ICP-MS analyses of single grains have produced the best precision of any zircon that we have studied, including the Temora zircon standard. This, together with its large grain size (ca. 1 cm), combination of relatively high U content (230 ppm) and moderate age (ca. 609 Ma), extremely high 206Pb/204Pb (in excess of 146,000), make it a potentially highly suitable standard for in situ zircon analysis. The major drawback of this standard
Acknowledgements
We are most grateful to Jerry Yakoumelos of G and J Gems, Sydney, for the donation of the GJ-1 standard zircon, and to Fernando Corfu for performing TIMS analyses of the GJ-1 zircon. We thank Ayesha Saeed for providing her LA-ICP-MS data sets for the 91500 and Mud Tank zircons. Brent McInnes kindly provided the Gunung Celeng zircon and supporting data, and Lance Black provided the Temora zircon. Stirling Shaw kindly allowed us to use his data set for the Walcha Road zircon and provided
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