From Hell to the Himalayas: Thermochronology across Deep Time

Abstract

Thwarted in pursuit of a career in aviation, academic underachiever Mark Harrison then kicked around the world for a couple of years. Doing so he stumbled into a series of geological technician jobs in the southern hemisphere that motivated a return to school in Canada to prepare for a research career. A series of inspirational mentors at Australian, Canadian, and American universities imbued him with an outsider’s perspective that encouraged intellectual grazing across several fields, including geochronology, tectonics, and early Earth evolution. The connective tissue between these disciplines was the development of thermochronology – the release of temperature history information stored in minerals to infer geophysical mechanisms acting in the distant past. Since most geodynamic processes involve heat flow discontinuities, this new science could image ancient events that might otherwise go undetected. A second unifying theme, reflective of both Harrison’s contrarian nature and his visionary mentors, was a pronounced mistrust of received wisdom. His early kinetic calibrations established ⁴⁰Ar/³⁹Ar thermochronology as a key tool in documenting epeirogenic histories. Of particular note, development of the multi-diffusion domain model required a natural test bed with the highest possible dynamic range of geologic rates, taking him first to Tibet and then the Himalaya. Together with UCLA colleague An Yin, they developed a widely emulated, hybridised application of geochemistry and tectonics to address longstanding questions in the evolution of that mountain system. On hand as a graduate student in Australia to witness development of the high sensitivity ion microprobe, Harrison and colleagues commissioned the first high sensitivity ion microscope, which could determine both in situ ages and stable isotope compositions. This unique capability permitted them to find microscale evidence that life likely emerged prior to 3.8 billion years (Ga) ago – 300 million years (Ma) earlier than previously thought. Harrison’s research groups, both at UCLA and the Australian National University, set out to create an unprecedented archive of >4 Ga zircons from which they documented evidence most simply interpreted as reflecting Hadean eon oceans, continental crust, plate boundary interactions, and the emergence of terrestrial life as early as 4.1 Ga. These interpretations challenged the longstanding paradigm that Earth had not evolved stable continental crust or life over the first 500 to 1000 million years of its history. That dialectic led Harrison to ponder the intellectual and philosophical underpinnings of historical geology, which he sees as underdeveloped. Ultimately, the field needs to adopt a multiple working hypothesis credo along with the scientific humility to acknowledge the limitations imposed by the fragmental and biased rock record. Doing so shouldn’t dampen our enthusiasm for understanding Earth history but instead only make its pursuit more intriguing, challenging, and communally rewarding.