MESA Journal 42 August 2006 27 LA-ICPMS U–Pb dating A new geochronological capability for South Australia: U–Pb zircon dating via LA-ICPMS Anthony J Reid 1 , Justin L Payne 2 and Ben P Wade 2 1 Geological Survey Branch, PIRSA 2 School of Earth and Environmental Sciences, University of Adelaide Introduction High-precision geochronology is an essential component of any geological investigation into crustal evolution, sedimentary provenance, or metallogenic process. Geochronology is based on the natural decay of unstable isotopes to daughter products, and our ability to measure these isotopic abundances in a wide variety of minerals. The U–Pb isotopic decay scheme is one of the most widely used by geochronologists, largely because of the chemical and physical properties of accessory minerals such as zircon, monazite and rutile. These minerals are able to partition trace amounts of U (and Th) into their crystal structure during formation and the presence of these minerals in a variety of geological environments makes them suitable for a wide range of geochronological applications. The Geological Survey Branch of PIRSA is able to access U–Pb geochronology on a routine basis through two methodologies — sensitive high- resolution ion microprobe (SHRIMP) and laser ablation - inductively coupled plasma mass spectrometry (LA-ICPMS). The SHRIMP is accessed through an agreement with Geoscience Australia and is overseen by PIRSA’s senior geochronologist, Dr Liz Jagodzinski, in conjunction with PIRSA geologists. The LA-ICPMS instrumentation has recently been installed at Adelaide Microscopy within the University of Adelaide and can now be accessed by PIRSA staff through an agreement with the University of Adelaide and the University of South Australia. PIRSA has made a commitment to the technique through joint funding installation of the instrument. The SHRIMP methodology is the industry standard for zircon geochronology due to its high-isotopic precision and ability to analyse very small portions of individual accessory mineral grains. The LA-ICPMS technique was developed during the mid 1990s and has often been overlooked as a geochronological tool by geologists, who tend to perceive SHRIMP data as being more reliable. However, a number of studies have shown that the LA-ICPMS technique is able to generate accurate and precise geochronological results (e.g. Jackson et al., 2004; Chang et al., 2006). The LA-ICPMS also has the advantage of being relatively low cost and because the analysis of the isotopic composition of a single zircon grain can be completed in ~5 minutes, large data sets can be accumulated rapidly. The latter feature makes the LA-ICPMS highly useful for studies of detrital zircons, which is perhaps the best application for This article presents the results of U–Pb dating of South Australian igneous zircons of known age using the instrumentation installed at Adelaide Microscopy. The LA-ICPMS is able to reproduce previous U–Pb ages obtained by both isotope dilution - thermal ionisation mass spectrometry (ID-TIMS) and SHRIMP techniques, which should with this instrumentation. LA-ICPMS instrumentation and method The LA-ICPMS system consists of a laser, a transport medium, an ion source and a mass spectrometer (Fig. 1). The laser heats the sample surface causing it to evaporate (‘ablate’). This ablation process creates a pit within the sample, which in the case of zircon dating is typically ~40 µm in diameter and up to ~200 µm deep (Fig. 2). The ablated zircon is then transported in a combined He–Ar carrier gas into a plasma, which operates at a temperature of ~6725 °C. The high temperatures encountered in the plasma ionise the zircon particles enabling them to be transported through an electrostatic spectrometer, which registers the ions as counts per second at a given mass. The mass spectrometer is able to rapidly switch between different masses, in order to provide a quasi-simultaneous measurement of the isotopic composition of the zircon. A time resolved output of the isotopic composition is thus generated by the mass spectrometer, from which a representative signal is selected and integrated to enable calculation of the isotopic ratios 207 Pb/ 206 Pb, 206 Pb/ 238 U, and 208 Pb/ 232 Th (Fig. 3). Because of the low abundance of 235 U, 207 Pb/ 235 U is calculated assuming the naturally occurring abundance ratio of these isotopes: 235 U = 238 U/137.88. Data reduction is conducted using the online software GLITTER (van Achterbergh et al., 1999), which then calculates the 207 Pb/ 206 Pb and 206 Pb/ 238 U age and error for each analysis. GLITTER calculates isotopic ratios from background-subtracted signals for the relevant isotopes. Uncertainties from counting statistics for the signal and background for the standard and unknown analyses are added in quadrature. One of the limitations of the laser ablation method is the differential fractionation of U and Pb (and other elemental species) during an individual analysis. Fractionation occurs at a number of stages during the analytical procedure, including at the site of ablation, during transport and during plasma ionisation. The ICPMS methodology aims to ensure that fractionation is consistent between analyses, as it uses a zircon standard of known fractionation characteristics and known age in order to apply a correction to the unknown analyses for instrument-induced mass fractionation. Since the standard has a precisely known age, GLITTER is able to determine the fractionation trends between U and Pb during an individual analysis of the standard. The fractionation characteristics of the standard zircon are then assumed to mimic those of the unknown zircons and a correction algorithm is then applied to each unknown analysis. abundances of U and Pb that are fed into the age equation in order to generate the age of the unknown zircon. In practice, each analytical ‘run’ of ten unknowns is bracketed by three or four analyses of a standard, the gem quality zircon, GJ, which has a 207 Pb/ 206 Pb age of 608.5 ±