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In situ U-Th-Pb geochronology was born some two decades ago with the introduction and development of high-resolution secondary ion mass spectrometry (SIMS or SHRIMP [Sensitive High Mass Resolution Ion MicroProbe]; Compston et al. 1984, Williams 1998, Compston 1999, Davis et al.; this volume, Ireland and Williams, this volume). This technique clearly demonstrated the existence of age heterogeneities within the single crystals of zircon and other accessory phases and therefore the need for high-spatial resolution (tens to hundreds of cubic micrometers) geochronological data. In situ dating by ion probe is capable of achieving an analytical precision that is only an order of magnitude worse than the conventional isotope dilution-thermal ionization mass spectrometry (ID-TIMS) dating technique. It has the advantage, however, of more readily identifying concordant portions of grains, does not require chemical treatment of samples prior to the analysis, is essentially nondestructive, and can achieve greater sample throughputs. A major obstacle to the wider use of ion probe dating has always been the high cost of instrumentation and hence relative scarcity of suitably equipped geologic laboratories.
Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) emerged in 1985 and rapidly became an important analytical tool for trace element determinations in geological samples (Jackson et al. 1992). It was soon realized that the large variations in radiogenic Pb and Pb/ U isotopic ratios found in nature could be resolved by ICPMS techniques and, when coupled to a laser, ICPMS could be used as a dating tool similar to the ion probe. The pioneering work of Feng et al. (1993), Fryer et al. (1993), Hirata and Nesbitt (1995) and Jackson et al. (1996) illustrated the potential usefulness of laser sampling for in situ dating by ICPMS particularly well. However, these studies and others that followed also revealed the major …