2 Radiometric Constraints on the Timing, Tempo, and Effects of Large Igneous Province Emplacement Jennifer Kasbohm1, Blair Schoene1, and Seth Burgess2 ABSTRACT There is an apparent temporal correlation between large igneous province (LIP) emplacement and global envi- ronmental crises, including mass extinctions. Advances in the precision and accuracy of geochronology in the past decade have significantly improved estimates of the timing and duration of LIP emplacement, mass extinc- tion events, and global climate perturbations, and in general have supported a temporal link between them. In this chapter, we review available geochronology of LIPs and of global extinction or climate events. We begin with an overview of the methodological advances permitting improved precision and accuracy in LIP geochro- nology. We then review the characteristics and geochronology of 12 LIP/event couplets from the past 700 Ma of Earth history, comparing the relative timing of magmatism and global change, and assessing the chronologic support for LIPs playing a causal role in Earth’s climatic and biotic crises. We find that (1) improved geochronol- ogy in the last decade has shown that nearly all well-dated LIPs erupted in < 1 Ma, irrespective of tectonic set- ting; (2) for well-dated LIPs with correspondingly well-dated mass extinctions, the LIPs began several hundred ka prior to a relatively short duration extinction event; and (3) for LIPs with a convincing temporal connection to mass extinctions, there seems to be no single characteristic that makes a LIP deadly. Despite much progress, higher precision geochronology of both eruptive and intrusive LIP events and better chronologies from extinc- tion and climate proxy records will be required to further understand how these catastrophic volcanic events have changed the course of our planet’s surface evolution. 2.1. INTRODUCTION heating of organic-, carbonate-, or evaporite-rich sedi- ments (Bond & Sun, Chapter 3 this volume; Mather & The observation that large igneous province (LIP) Schmidt, Chapter 4 this volume; Self et al., 2006; Svensen magmatism has in some cases occurred concurrently et al., 2004; Thordarson & Self, 1996). Determining with mass extinctions (Courtillot & Renne, 2003; Ernst, whether any of these mechanisms are important contrib- 2014; Ernst & Youbi, 2017; Ernst et al., Chapter 1 this utors to climate change requires understanding the rela- volume) leads to the question of whether there is a cause- tive rate of both extrusive and intrusive magmatism, and and-effect relationship between them. While various correlating these with paleoenvironmental records. The hypotheses exist as to how LIPs lead to catastrophic cli- only way to directly date the rates of LIP emplacement is mate change that can lead to ecosystem collapse and through radioisotope geochronology. Furthermore, as mass extinctions, they generally focus on volatile release LIP lava flows and biological evidence for mass extinc- from either magma-derived gases or indirectly through tions rarely occur in the same stratigraphic sections, making direct stratigraphic temporal correlation impos- 1Department of Geosciences, Princeton University, Princeton, sible, geochronology is also essential to test the plausibil- New Jersey, USA ity of causality. 2United States Geological Survey, Volcano Science Center, Although a wide range of geochronologic techniques Menlo Park, California, USA have been applied to dating LIPs, uranium-lead (U-Pb) Large Igneous Provinces: A Driver of Global Environmental and Biotic Changes, Geophysical Monograph 255, First Edition. Edited by Richard E. Ernst, Alexander J. Dickson, and Andrey Bekker. © 2021 The Authors. Co-published 2021 by the American Geophysical Union and John Wiley and Sons, Inc. DOI: 10.1002/9781119507444.ch2 This is an open access publication under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. 28 LARGE IGNEOUS PROVINCES and argon-argon (40Ar/39Ar) geochronometers are the and/or the environmental event had been the subject of most widely used geochronometers for this application recent high-precision geochronological study. To com- because their parent isotopes are characterized by long plement our narrative review below, we also present half-lives and relatively high concentrations in a variety tables detailing the samples included in our analysis of mineral phases (Reiners et al., 2017; Renne et al., (see Tables 2.1 and 2.2). This new compilation is able 1998a). Temporal correlation between some LIPs and to highlight some similarities and some differences mass extinctions was suggested over 30 years ago between LIPs that permit discussion of both the geody- (McLean, 1985), and this was rapidly tested using namics of LIPs and their timing relative to environmen- 40Ar/39Ar geochronology on basalts (Courtillot et al., tal catastrophes. 1986; Duncan & Pyle, 1988). However, not until the 1990s were 40Ar/39Ar and U-Pb geochronology used to demon- 2.1.1. Dating Methods for Large Igneous Provinces strate a temporal coincidence between the Siberian Traps LIP and the end-Permian mass extinction to within ~1 Most dates for LIPs and their corresponding environ- Ma (Campbell et al., 1992; Dalrymple et al., 1995; Renne mental perturbations are derived from the 40Ar/39Ar or & Basu, 1991; Kamo et al., 2003; Reichow et al., 2002). U-Pb techniques. Historically, 40Ar/39Ar studies have Attempts to further correlate LIPs and mass extinctions yielded a temporal framework of LIP emplacement at have both capitalized on and pushed the development of ~1% resolution (e.g., Barry et al., 2013; Kerr et al., 2003; higher precision geochronology. Because the duration of Marzoli et al., 2018; Renne et al., 1995), sufficient to the hypothesized individual environmental effects of establish a broad correlation, but in excess of that LIPs can vary in timescale from years (cooling associated required to resolve the relative timing of LIP emplace- with sulfate aerosols, acid rain, ozone depletion) to 100 ment relative to environmental change. This relatively ka (warming associated with CO2) (Black & Manga, coarse resolution is due to small concentrations of the 2017; Wignall, 2001), greater and greater precision is parent isotope, 40K, found in whole-rock basalt samples required to test hypotheses relating LIPs to paleoclimate or plagioclase separates. U-Pb geochronology can yield and paleontological records. Geochronological uncer- higher-precision dates (to better than ± 0.1%), but ame- tainties >1 Ma, which were commonplace in data sets nable minerals (e.g., zircon, baddeleyite) are not ubiqui- from the early efforts to date these events, obscure the tous in LIPs. With their particular strengths and relative timing of LIP eruptions versus global change on shortcomings, both techniques have been crucial to relevant climatic timescales, and limit assessment of the improving understanding of the connection between plausibility of a causal connection. However, the last dec- LIPs and environmental change, and working to better ade has seen dramatic improvement in the precision and understand that connection has produced methodologi- accuracy of dates generated by radiogenic geochronol- cal breakthroughs in both geochronometers. In this sec- ogy, such that recently published data sets can begin to tion, we will briefly describe the methodological advances compare and resolve relative timing differences between leading to improved accuracy and precision for both the onset and duration of LIP magmatism and potential techniques, with a stronger focus on U-Pb methods as downstream environmental effects, such as biotic decline, these have provided the majority of high-precision ages ecosystem deterioration, global warming and carbon discussed in this review. We follow the definitions of cycling, and sea level change. With methodological accuracy and precision summarized in Schoene et al. advances in U-Pb geochronology, it is now possible to (2013): An accurate date agrees with the actual age of an discern a coincidence of events with precision of 10–100 event within uncertainty, while precision is defined as the ka, allowing testing of a potential causal connections at reported uncertainties, which may include both random the sub-100 kyr level. However, further increases in ana- and systematic uncertainties (Renne et al., 1998b; lytical precision of dates for LIPs and of age models for Schoene, 2014). stratigraphic sections that contain paleoenvironmental The accuracy of 40Ar/39Ar dates has increased as meth- records will be required in order to resolve the timing of odology evolved from analyzing primarily LIP whole- environmental effects operating on shorter timescales, rock/groundmass samples, to more recent efforts focused such as cooling due to SO2. on analysis of aliquots of plagioclase or biotite separated In this chapter, after describing the methodological from basalts, or sanidines separated from intercalated advances that have led to improved accuracy and preci- silicic ashes. These recent studies have also used more sion in geochronology, we will review the geochronology careful means to handpick plagioclase grains to avoid the of 12 LIP–global environmental change couplets effects of weathering, and choosing grains of sufficient (Fig. 2.1). We chose to focus on LIPs that coincided in size to mitigate effects from 39Ar recoil (Renne et al., time with a dramatic global environmental