Mass spectrometry and geochronology

The physical method of geochronology is based on the measurement of isotopi c ratios by mass spectrometry. Geochronology, as an essential field of ende avor in the earth sciences, would not have been possible without mass spect rometry, and these two endeavors have developed in a mutually cooperative m anner over the past 50 years. In fact, the mutual development of these two fields is an excellent example of the interplay between science and technol ogy. The demand for improved mass spectrometric instrumentation to satisfy the increasingly exacting requirements of age determinations has led to a s uccession of technological advances in mass spectrometry. These advances ha ve, in turn, enabled geochronology to reach a level of scientific achieveme nt of impressive dimensions. The deceptively simple design of the 60 degree s sector field mass spectrometer by A. O. Nier in the 1940s was the foundat ion on which geochronology was established. Over the past 50 years, technol ogical advances have enabled the sector field instrument to meet most of th e requirements of the various geochronological techniques that have been de veloped during this period of time. During the past 20 years or so, however , radically new mass spectrometers have been developed. Secondary ion mass spectrometry (SIMS) enables geochronologists to analyze U- or Th-bearing mi nerals in situ to obtain accurate U, Th-Pb ages. These double-focusing mass spectrometers have revolutionized mineral age determinations. Accelerator mass spectrometry (AMS) has applied techniques developed in nuclear physics to measure short-lived commogenic nuclides such as C-14, (26) Al, and I-12 9 to provide information on geologically short-term events in a more effici ent manner than by radioactive-counting methods. Inductively coupled plasma mass spectrometry (ICPMS) allows for the rapid determination of the abunda nces of almost all elements in solid and liquid samples, and is now being a pplied to geochronology systems. This review examines the interplay between the evolution of mass spectrometric instrumentation and the development of geochronology over the past 50 years. (C) 1999 John Wiley & Sons, Inc.