ARTHUR L. DAY MEDAL: THE MICROANALYSIS REVOLUTION IN ISOTOPE GEOCHEMISTRY

Sample sizes for stable isotope analysis have been reduced by factors of 10⁶ to 10¹² over the past 30 years. At the same time, analysis is faster, more accurate, and in situ. Thus, geochemistry can be correlated to textures and other features at nm- to μm-scale, opening new fields of study in areas of traditional interest to MSA: mineralogy, petrology and geochemistry. For instance, microanalysis of ²⁰⁷Pb/²⁰⁶Pb in zircons by SIMS lead to recognition of the earliest samples from Earth, and SIMS analysis of δ¹⁸O provides evidence for Hadean oceans and conditions habitable to life 800 Myr before the oldest accepted microfossils. The zircon age of 4.4 Ga was challenged as caused by Pb mobility, but recent studies by atom probe tomography show that post-crystallization migration of Pb in the oldest zircon was restricted to distances less than 50 nm, forming 10-nm nanoclusters of Pb, and had no effect on ages measured from SIMS pits 10,000 nm across (Valley et al. 2015, Am. Min.). Modelling of Pb-clusters suggests that this zircon experienced at least two high-temperature periods of nanoscale Pb diffusion to form clusters, but no micron- or grain-scale Pb mobility in its core (Blum 2019, PhD UW-Madison). Likewise, mildly elevated values of δ¹⁸O(zircon) were suggested to result from hydrous alteration during Cenozoic weathering rather than high-δ¹⁸O Hadean magmas, but recent studies by SIMS show that the critical high-δ¹⁸O zircons are not hydrous and are unaltered. The geochemical fidelity of zircons critically depends on high degrees of crystallinity, which is confirmed by Raman analysis showing low degrees of unannealed radiation damage (Cameron 2017 MS, UW-Madison). These cases document both the complexity and utility of fine-scale measurements, and illustrate their fundamental, sometimes perplexing, but sometimes transformative, application in areas as diverse as igneous petrology, the Early Earth, paleoclimate, and emergence of life.