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1896 marked the beginning of a decade that spawned both modern physics and the science of geochronology based on radioactive decay. The decade started with the discovery of radioactivity by Henri Becquerel in 1896, and ended with the formal publication of ages for natural mineral samples by Ernest Rutherford in 1906. The next fifty years witnessed the discovery of isotopes and nuclear fission; the development of the mass spectrograph and the mass spectrometer; application of the isotope dilution method to dating trace, accessory, and major minerals in typical crustal rocks; and publication of the ca. 4.55 Ga age for the Earth. Yet, after all this, geochronology was still viewed with suspicion by some geologists. In the past fifty years, with additional major advances in instrumentation, technique, and interpretation, geochronology is fully integrated into almost all fields of geology. The three major dating methods from the 1950s and 1960s, U-Pb, K-Ar, and Rb-Sr, have been refined repeatedly. In particular, U-Pb and Ar-Ar, a modern variant of K-Ar, are now capable of <0.1% precisions, with spectacular results in recent studies of crucial problems such as the exact timing and duration of mass extinctions. Many new methods are now available to attack problems ranging from rates of metamorphic mineral growth to rates of uplift and erosion, to the time of surface exposure of geomorphic surfaces. It is a good time to be a geochronologist, or to collaborate with one or more. The future looks very bright.

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