A double focusing mass spectrometer for geochronology John R. De Laeter*, Allen K. Kennedy Department of Applied Physics, Curtin University of Technology, Perth 6001, Western Australia Received 6 August 1997; accepted 10 March 1998 Abstract The performance characteristics of a double focusing mass spectrometer designed specifically for geochronology are described. The sensitive high resolution ion micro probe (SHRIMP II) mass spectrometer is ideally suited for in-situ analysis of uranium and/or thorium-bearing minerals to provide geochronological information on micron-sized domains of the minerals. SHRIMP II simultaneously meets the requirements of high mass resolution and high ion transmission efficiency together with good abundance sensitivity. SHRIMP II’s excellent spatial and mass resolution, coupled with its high sensitivity, has enabled zircon geochronology to be revolutionized. (Int J Mass Spectrom 178 (1998) 43–50) © 1998 Elsevier Science B.V. 1. Introduction Geochronology—the dating of geological events by radioactive isotopes—was initiated when Nier [1] demonstrated that the isotopic composition of lead extracted from uranium and thorium-rich minerals varied significantly, depending on the chemical com- position and age of the minerals. This allowed mea- surement of the age of the minerals provided the half-life of the parent nuclides ( 235 U, 238 U, and 232 Th) were known. Aldrich and Nier [2] demonstrated that 40 K decays to both 40 Ca and 40 Ar thus providing the basis for two further methods for measuring geolog- ical age. Nier [3,4] departed from accepted mass spectro- metric practice by designing a single stage machine based on a 60° sector field magnet. This mass spec- trometer not only reduced the weight and power consumption of the electromagnet, but enabled the ion source and detector to be removed from the influence of the magnetic field. The relative simplicity of the Nier mass spectrometer compared to the large 180° spectrometers and double focusing mass spectrome- ters hitherto in use, enabled mass spectrometry to be accessible to a wide group of scientists rather than remain as a specialised instrument in physics. The juxtaposition of the availability of this new type of mass spectrometer and the establishment of age determination techniques by the U and Th–Pb and K–Ar radioactive decay systems, precipitated the development of geochronology in the early 1950s. At that time, gas source mass spectrometry pos- sessed sufficient sensitivity to analyze lead as lead hexafluoride or as lead tetraethyl for U and Th–U dating, and the noble gas argon for K–Ar geochronol- ogy. An electron impact ion source produced ions with a limited spread in energy which could be accommodated by the single stage magnetic sector field mass spectrometer. Another chronological tech- nique, based on the decay of 87 Rb to 87 Sr, was later developed. Solid source mass spectrometers that use a * Corresponding author. Dedicated to the memory of Al Nier. 1387-3806/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved PII S1387-3806(98)14092-7 International Journal of Mass Spectrometry 178 (1998) 43–50