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

Abstract

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 research. 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 spectrometry (Anderson and Hinthorne 1972) was further developed and applied to zircon geochronology 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 …