During the mid–twentieth century, geochemistry—one of the core Earth sciences—underwent a spectacular transformation as a result of the introduction of electronic instruments based on physical principles. In this process, mass spectrometry became the workhorse analytical technique in isotope geochemistry. This essay concerns the dynamic relationship between discoveries of isotope systems and the variations in their relative abundances, on the one hand—discoveries that became the foundation of isotope geology—and the development of mass spectrometry, on the other. This relationship is illustrated by the career of physicist and instrument-builder Alfred O.C. Nier, who was based at the University of Minnesota in Minneapolis. Nier’s 60o-sector mass spectrometer design of 1940 endowed the instrument with powerful new capabilities, as well as facilitated its adoption outside the nuclear physics community. In the course of developing and applying the instrument, Nier also made important discoveries, about the relative abundances of isotopes, that paved the way for geochemical research on the deep past. My thesis is that Nier’s early career, spanning the 1930s and 1940s, illustrates a dynamic relationship in which science and technology co-evolved synergistically. This pattern of research spread beyond Nier—who largely moved on from this research after the 1950s—to develop into a research tradition, initially based at the University of Chicago’s Institute for Nuclear Studies and then spreading to other institutions, notably Caltech, the Carnegie Institution of Washington, and the University of California at Berkeley and San Diego. This tradition made crucial contributions to historical geology, including paleoclimatology, solar system history, and the tectonics revolution.

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