Mass spectrometry and geochronology

Abstract

The physical method of geochronology is based on the measurement of isotopic ratios by mass spectrometry. Geochronology, as an essential field of endeavor in the earth sciences, would not have been possible without mass spectrometry, and these two endeavors have developed in a mutually cooperative manner over the past 50 years. In fact, the mutual development of these two fields is an excellent example of the interplay between science and technology. The demand for improved mass spectrometric instrumentation to satisfy the increasingly exacting requirements of age determinations has led to a succession of technological advances in mass spectrometry. These advances have, in turn, enabled geochronology to reach a level of scientific achievement of impressive dimensions. The deceptively simple design of the 60° sector field mass spectrometer by A. O. Nier in the 1940s was the foundation on which geochronology was established. Over the past 50 years, technological advances have enabled the sector field instrument to meet most of the requirements of the various geochronological techniques that have been developed 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 minerals 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 cosmogenic nuclides such as 14C, 26Al, and 129I to provide information on geologically short-term events in a more efficient manner than by radioactive-counting methods. Inductively coupled plasma mass spectrometry (ICPMS) allows for the rapid determination of the abundances of almost all elements in solid and liquid samples, and is now being applied 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. © 1999 John Wiley & Sons, Inc., Mass Spec Rev 17, 97–125, 1998