Ian McDougall was a pioneer in the development of potassium-argon geochronology. Credit: Peter Zeitler
Ian McDougall, who played a key role in developments leading to the plate tectonic revolution, died in Canberra, Australia, on 10 November 2018, surrounded by family.
Born in Tasmania, Australia, McDougall earned a B.Sc. in geology at the University of Tasmania in Hobart and then moved to the Australian National University (ANU) in Canberra, where he then spent the rest of his career. At ANU Ian completed a dissertation in petrology under the supervision of Germaine Joplin, studying the nature and origin of the Tasmanian dolerites.
Potassium-Argon Dating and the Birth of the Magnetic Polarity Reversal TimescaleWhile McDougall was completing his Ph.D., the director of the then Department of Geophysics was John Jaeger (of heat conduction fame). Jaeger was a visionary leader who wished to put ANU on the international map by focusing on emerging fields, and one step was to get involved in the new method of potassium-argon (K-Ar) dating that was being pioneered by John Reynolds at the University of California, Berkeley.
Jaeger swayed McDougall to shift from petrology to the new field of geochronology and in 1960 arranged a postdoctoral year for McDougall at Reynolds’s lab. As is chronicled in William Glen’s classic book The Road to Jaramillo: Critical Years in the Revolution in Earth Science, McDougall returned from Berkeley to take up a position at ANU overseeing the K-Ar lab.
McDougall made a key detour in Hawaii before returning to Australia, however. The dating of the basalts he sampled across the Hawaiian chain demonstrated for the first time the age progression of the Hawaiian Islands and thus supported the hot spot model. At the time, basalts (rather than separated minerals) were seen as giving erratic results and thus unsuitable for K-Ar dating. However, as an experienced petrologist, Ian was able to cull altered materials and date only those samples that met his exacting criteria.
In collaboration with paleomagnetist Don Tarling, Ian began work on dating magnetic reversals, and a vigorous competition emerged between the American team of Brent Dalrymple, Allan Cox, and Richard Doell and the Australian team. Within a few years, this research quickly led to establishment of ever-more-refined versions of the geomagnetic polarity timescale, which in his book Glen argues was a key supporting link in the development of the Vine-Matthews-Morley hypothesis and acceptance of plate tectonics.
Applications Throughout Earth ScienceEarly in the 1970s, as a natural outgrowth of McDougall’s interest in precise and accurate K-Ar age determinations, he brought the 40Ar/39Ar method to his laboratory in ANU. At the time, the method was in some disrepute for dating terrestrial samples, and his systematic and exacting work helped bring the method into routine use.
This work influenced several other aspects of McDougall’s research and led to an authoritative book on the method with former student Mark Harrison.
In the late 1970s, McDougall was invited to address the long-standing controversy regarding the age of the Kay Behrensmeyer Site (KBS) Tuff, a key unit in the Turkana basin providing stratigraphic control on studies of hominid evolution.
McDougall’s work found a different, but robust, age for the KBS Tuff, ending the debate. These precise geochronological studies, conducted in collaboration with Richard Leakey, Meave Leakey, Frank Brown, and many others, continued for many years, well into McDougall’s retirement.
Although Ian’s primary interests were in geochronological applications of the K-Ar system, he was aware of the potential for the 40Ar/39Ar approach to reveal information about the diffusion characteristics and thermal histories of minerals.
Through the 1980s and 1990s, McDougall supported a series of Ph.D. students and postdocs working in this area, and as a result ANU can claim a seminal role in development of thermochronology. Key work in this period included a fundamental understanding of how diffusion theory can explain 40Ar/39Ar age spectra and be used to extract thermal histories, how K-feldspar analyses can reveal a wealth of information, including a continuous segment of a thermal history, and how (uranium-thorium)/helium dating of apatite can act as a very sensitive low-temperature thermochronometer.McDougall was an exacting mentor, whose care and rigor in getting things right and in “doing the sums” rubbed off to some degree on all his students.
Lasting LegacyMcDougall was an exacting mentor. His care and rigor in getting things right and in “doing the sums” rubbed off to some degree on all of us, and for that and for all the many chances he took in giving us career-changing opportunities, we will always be grateful.
The scope of his work over the years continues to be inspiring. It also did not escape our notice that his many sampling trips to the Pacific and other warm settings often happened during the cold Canberra winter.
McDougall published more than 150 papers and an influential and best-selling book on the 40Ar/39Ar method, Geochronology and Thermochronology by the 40Ar/39Ar Method.
In 2000, McDougall entered an active retirement from ANU, staying engaged with writing and research. He was a fellow of AGU, the Geological Society of America, and the Australian Academy of Science. He received the F. L. Stillwell Award from the Geological Society of Australia in 1974, the Australian government’s Centenary Medal in 2001, the Australian Academy’s Jaeger Medal in 2007, and an honorary degree from the University of Glasgow in 2009.
Ian is survived by his wife of 58 years, Pam McDougall; his children, Scott, Geoffrey, and Sandra; grandchildren Islay, Faye, Angus, and Elliane; and past students and postdocs at institutions around the world.
—Peter Zeitler (peter.zeitler@lehigh.edu), Lehigh University, Bethlehem, Pa.; Mark Harrison, University of California, Los Angeles; Suzanne Baldwin, Syracuse University, N.Y.; Robert Duncan, Oregon State University, Corvallis; Terry Spell, University of Nevada, Las Vegas; and Jan Wijbrans, Vrije Universiteit Amsterdam, Netherlands
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