Zircon U-Th-Pb Geochronology by Isotope Dilution -- Thermal Ionization Mass Spectrometry (ID-TIMS)

INTRODUCTION ID-TIMS is the acronym for Isotope Dilution Thermal Ionization Mass Spectrometry. This refers to the addition of an isotope tracer to a dissolved sample to make a homogeneous isotopic mixture, and the measurement of isotopic composition of the mixture using a thermal ionization mass spectrometer. The method is one of the most accurate and precise methods of isotopic techniques because it is relatively insensitive to chemical yields or mass spectrometric sensitivity. It is a method very widely applied both in earth and many other areas of science involving the measurement of element or isotope concentrations and isotopic ratios. The ID-TIMS technique was first applied to the U-Th-Pb dating of zircon in the 1950s (Tilton et al. 1955, Wetherill 1956, Tilton et al. 1957), exploiting the general availability to academia of enriched uranium isotopes developed in the 1940s and 1950s related to nuclear energy re-search. ID-TIMS has remained the main foundation to zircon geochronology ever since, in spite of the proliferation of other analytical methodologies. In the past 50 years, many improvements have been made and they have contributed to the maturity and reliability of the method. As a method, it was effectively unchallenged until the 1980s when secondary ionization mass spec-trometry (Anderson and Hinthorne 1972) was further developed and applied to zircon geochro-nology by W. Compston and colleagues at the Australian National University (Compston et al. 1984). The instrument developed by the ANU group (SHRIMP, or Sensitive High Resolution Ion Microprobe) and the associated measurement protocols facilitated measurement of Pb/U isotopic ratios within a small region of a single zircon grain, and it proved to be a powerful tool to address complex age structure of multi-component zircons. In the 1990s, laser ablation quadrupole ICP-MS methods came on stream and offered an alternate way to make intra-grain U-Th-Pb isotopic measurements. The advent of double-focusing ICP-MS instruments has offered much better mea-surement precision compared to quadrupole machines, and while still developing, they have yet to make quite the same impact, though many current laboratories are working to close the gap with SIMS methods. There has been an unfortunate tendency for advocates of TIMS on the one hand and SIMS on the other to unfairly criticize each other's methodology, or at least the ways those methodolo-gies have been applied to zircon geochronology. While this trend has decreased in recent years it is reflected in some of the literature and can give a biased impression of the complementary capabilities of both methods, often ignoring the strengths and weaknesses of the alternative. It is imperative that students of zircon geochronology become more aware of the capabilities of all useful methods, so that when confronted with a problem to solve, they know how to design their measurement experiment. The lack of a comprehensive textbook means we must rely on indi-vidual research and review papers and our own abilities at synthesis to gain a reasonable under-standing of the field. As an alternative, short courses can be very effective at bringing the views of experts together, as this and other volumes (Heaman and Parrish 1991) attempt to do. Parrish and Noble 184 A great range of geochronological problems can be addressed effectively with either ID-TIMS or intra-grain microbeam techniques (SIMS or LA-ICP-MS). While this is the case for many appli-cations, some problems are best-solved using ID-TIMS methods and vice-versa. This chapter de-scribes TIMS methods and it comments on the strengths and weaknesses of method variations, using examples of zircon dating mainly from the literature. Naturally it will pay special attention to those applications where ID-TIMS analysis is preferred, if not essential, and it will portray the field of U-Th-Pb geochronology using ID-TIMS as having a very bright future in modern earth science.