THE EARLY GENIUS AND LEGACY OF ARTHUR HOLMES TO U-PB GEOCHRONOLOGY

Arthur Holmes was a very remarkable earth scientist who on several fronts, made seminal scientific contributions that were decades ahead of his time. When Holmes was an undergraduate student, the concept of isotopes was just being conceived by Frederick Soddy, radioactivity had only been recently discovered, and most believed the earth began somewhere between 4004 BC and 200,000,000 years ago. Nevertheless 100 years ago, he published what can only be regarded as a remarkable paper that presented for the first time chemical measurements of the geological age of minerals that were placed within a stratigraphical context. Then in 1913 at the age of 23, he wrote a ‘little book’ entitled “The Age of The Earth” wherein he discussed all manner of implications of its antiquity and radioactive decay. In doing so, he laid the groundwork for a temporally-graduated geological time scale, for modern views about mantle convection, and for the science of geochronology using radioactive decay. In geochronology he was so far ahead of his time that significant progress was not made until after 1940 when instruments resembling modern mass spectrometers became available to exploit isotope geochronology using multiple decay schemes relevant to earth science.

Expansion of U-Pb geochronology was gradual during the prior to ~1970 but in the last 30 years it has exponentially increased in terms of instrumentation, diversity of analysis methods and applications, integration with petrology and chemistry of igneous and metamorphic minerals in terrestrial and lunar materials and meteorites, and in precision and spatial resolution. Advances in methods and creativity will be illustrated by recent work; we now know that the age of the solar system is 4567 Ma, that the oldest surviving crustal material on earth is about 4360 Ma, and that thousands of rocks have been precisely dated using the U-Pb system to fill in the complex intervening history of the planet. Currently the frontier of the U-Pb system is in the late Pleistocene, in an area of time clearly overlapping the U-series dating method. The system has thus provided a remarkable set of diverse insights for a radioactive decay system recognised by Holmes very early on to have such enormous scientific significance and potential.