MINERALS/Zircons 605 Figure Cross section of the Eneabba (top) and Yoganup (bottom) beach placer deposits H.M., Heavy mineral Horizontal scale, 1:120; vertical scale, 1:24 Reproduced with permission from Evans AM (1993) Ore Geology and Industrial Minerals, 3rd edn Oxford: Blackwells Science Publications speculation, or crude physical measures (e.g., estimates based on ocean salinity) By 1903, Marie and Pierre Curie had been awarded the Nobel Prize in physics for showing that a radioactive parent element decays to various daughter elements at a constant rate, and this laid the foundations for precise physical measurement of mineral and rock ages Despite Wilhelm Rutherford realizing in 1905 that the decay of U to He and Pb provided a powerful technique for this purpose, precise dating methods were slow to develop, awaiting the discovery of isotopes by JJ Thomson and the development of mass spectrometric techniques in the 1920s by Francis Aston and Alfred Nier It was not until the 1940s that that a wide variety of decay schemes were tested in succession (U–Pb, K–Ar, K–Ca, Rb–Sr, Lu–Hf, Re–Os, and Sm–Nd) The basic requirements are (1) that the radioactive parent element is reasonably abundant geologically, (2) that, in the sample, the daughter element remains bound with the mineral in which it formed over time, and (3) that the latter can be distinguished reliably from any concentration of the same element that may have been originally present in the mineral or rock Zircon Samples Zircon forms an ideal sample material with respect to the U–Pb decay scheme, because it is an abundant