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The year 2011 marked the one hundredth anniversary of what may be the first real geochronology paper, published by Arthur Holmes, entitled “The Association of Lead with Uranium in Rock-Minerals and Its Application to the Measurement of Geological Time”(Holmes, 1911). Holmes’ early work was surprisingly accurate, even though it was carried out prior to the discovery of isotopes (Soddy, 1913) and restricted to whole-rock geochemical analyses. This and complementary efforts examining U decay and utilizing U–Pb chemical geochronology (eg, Barrell, 1917; Bateman, 1910; Boltwood, 1907; Holmes and Lawson, 1927) laid the foundation for what was to become one of the most important isotopic dating methods, capable of measuring the timescales of events from the early solar system $4.57 Ga into the Pleistocene. We now know that the element lead has four naturally occurring stable isotopes, 204Pb, 206Pb, 207Pb, and 208Pb, of which the latter three have a radiogenic component produced through the independent decay of 238U, 235U, and 232Th, respectively. The abundance of high-U minerals in most rock types, as well as the resistance of many of these minerals to chemical and physical weathering, contributes to the popularity and prolificacy of the U–Pb system. Though zircon is by far the most commonly utilized mineral for U–Pb dating (Hanchar and Hoskin, 2003), monazite, apatite, xenotime, titanite, rutile, baddeleyite, allanite, and perovskite are also commonly dated and provide a spectrum of geochronologic and thermochronologic applications in igneous, metamorphic, hydrothermal, and epithermal systems (Corfu, 1988 …