ARTHUR L. DAY MEDAL LECTURE

Unlike other physical scientists, almost every geologist’s story of how they came to the field is different. Mine came when, as a community college flunkout, I was cajoled to leave Canada for Australia where a series of unlikely circumstances led me to apprentice as a thin sectionist at Melbourne Uni. I enrolled in Geology 1 and hung with the students – including Andy Gleadow who envisioned that one day mineral dating would tell us how mountain belts cool. Eventually returning to B.C., Dick Armstrong supervised my UBC honors thesis on the cooling history of the Coast Range. There I discovered the duality of the diffusion equation with Garry Clarke and a copy of Dodson 1973. Dissatisfied with the limitations of bulk closure theory, I headed to ANU to learn the ⁴⁰Ar/³⁹Ar method with Ian McDougall thinking that imaging ⁴⁰Ar within crystals would open up vast new possibilities. Back then, K-spar was the least favored member of the K-Ar family which was itself a poor relative of such prized methods as – I kid you not – Rb-Sr isochron dating. Working with Frank Richter and Oscar Lovera, the long-standing problem of the surface singularity in a slowly cooled solid was transcended ushering in the multi-domain diffusion model for K-spars (and now muscovite). An Yin, Rick Ryerson, Paul Tapponnier and I then applied the now widely emulated approach of mating tectonic studies with thermal history reconstructions to the evolution of Tibet and the Himalaya. At this time, Kevin McKeegan and I specified a new generation ion microprobe that could undertake both radiogenic and stable isotope analysis. Our inaugural studies led to the development of prograde thermochronomety – the culmination of investigations on accessory minerals thermometry begun years before with Bruce Watson – and the discovery of a possible biologic signature on earliest Earth that ultimately reunited us in applying new accessory mineral thermometers to the oldest known terrestrial minerals. Results of these studies support Dick Armstrong’s once lonely challenge of the view that early Earth was a desiccated, inhospitable, continent-free, wasteland. Looking back, I’d say the only lessons that someone just starting out in science could draw from my career would be: work with smart, visionary people and assume that much of what we think we know is in some way wrong.