An exact knowledge of rock formation ages is perhaps the single most important tool needed for assembling the geologic record into a coherent history. Moreover, the age of Earth and the time scale of pre-human events are central to a civilization’s sense of origin and purpose. Therefore, the quest for precise and reliable geochronometers has had a scientific and a cultural importance that few other enterprises can match.

Since the beginning of the last century it has been recognized that long-lived radioactive decay systems provide the only valid means of quantifying geologic time. The uranium-lead decay system has always played...

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