Methods and Applications of Geochronology

Methods and Applications of Geochronology provides a comprehensive, practical guide to the rapidly developing field of geochronology. Chapters are written by leading experts in their specific field of geochronology and discuss practical information and ‘rules of thumb’ for establishing laboratories and using analytical equipment. Methods and Applications of Geochronology is an authoritative guide not only for the foundational principles of geochronological research, but also descriptions of analytical methods, guidance for sample selection, all the way to data reduction and presentation.

• Features the latest techniques and recommended tools for each of the most common geochronological methods
• Includes perspectives from a variety of well-respected researchers in the field, each representing different specialties of geochronology
• Bridges the gap between theory and application, offering best practices and relevant case studies throughout

• Language: English
• Publisher: Elsevier Science
• Release date: Mar 12, 2024
• ISBN: 9780443188022

Book preview

Preface

A question as simple as how old is this rock? can provoke discussions on a number of topics that have wide-ranging implications for the geosciences. Crystal nucleation rates, mass bias in analytical instrumentation, elemental and isotopic diffusion under metamorphic conditions, the statistics of uncertainty, field relationships, crystal residence time in magma chambers, and values of radioactive decay constants are only a few among many, many relevant considerations. Such a question even carries the weight of discussion through the millennia on the antiquity of the Earth and the application of the scientific method (see Chapter 1).

Tom Krogh, a pioneer in the modern methods of U–Pb geochronology and the founder of the Jack Satterly Geochronology Laboratory in Toronto, is remembered by colleague Greg Dunning as suggesting geochronologists consider the physicians' Hippocratic Oath: First, do no harm. Meaning that geochronological data, still relatively rare and precious, underpin geological interpretations and models that range from mineral-scale to continental- or even planetary-scale, and it is therefore crucial to not only take care to produce data of the best possible quality, but also to present that data in a responsible way that accurately represents its limitations. Geochronologists, as specialists that generate such important data, owe it to the geological community to do their best to ensure that their data does no harm once it is out in the world.

Modern geochronology has advanced significantly in recent decades, and various methods have been developed to explore the ages of the Earth and extraterrestrial materials, the rates of paleobiological processes, thermal histories of uplifted and/or metamorphosed rocks, and other geological processes to a high resolution that enables us to answer long-standing fundamental questions in the Earth Sciences. This book presents examples of applications and best practices for a range of these methods, albeit not a complete list of all available chronometers and methods (at least, not this edition!).

In every chapter, leading researchers have contributed their knowledge of specific geochronological techniques, including sample selection and processing, instrumental analysis, and data handling. Examples are provided for specific applications as well.

While the applicability and theoretical understanding of geochronology, particularly radio isotopic geochronology, is increasingly taught as part of an undergraduate geology curriculum, exposure to actual practices and hands-on experience is not usually provided until postgraduate studies. This book is intended to be a reference for new researchers in the field, but as with most pursuits, there is no substitute for attentive learning alongside an experienced practitioner, in this case a geochronologist in a laboratory setting. Fortunately, in our experience, geochronologists are enthusiastic folks and happy to share their knowledge. We are grateful to those who have shared it within these pages.

J. G. Shellnutt

S. W. Denyszyn

K. Suga

2023

Chapter 1: Introduction to methods and applications of geochronology

A perspective on geological time

J. Gregory Shellnutt¹, Steven W. Denyszyn², and Kenshi Suga¹     ¹Department of Earth Sciences, National Taiwan Normal University, Taipei, Taiwan     ²Department of Earth Sciences, Memorial University of Newfoundland, St. John's, NL, Canada

Abstract

Since the 17th century, the geological sciences have transitioned from a mostly qualitative order of operations discipline to a fully quantitative scientific discipline. In order to properly contextualize the origin and evolution of the solid Earth, a firm understanding of physical time and how it can be measured is required. Geological philosophy primarily advanced through rock and mineral observations, experimentation, and the study of the fossil record, but it was the development of analytical geochronology that quantified the age of geological and extraterrestrial materials and constrained the rates of past geological processes. The ability to measure individual isotopes and isotopic ratios by fission track methods and mass spectrometry revealed the vastness of geological time and permitted the robust correlations between rock formations across the globe, the reconstruction of supercontinents, and provided temporal constraints on biological evolution and the longevity of ancient ecosystems. The development of whole rock, single crystal, and in situ methodologies enhanced the ability of geoscientists to push scientific boundaries in the investigation of deep time. The application of geochronological methods can now offer accurate and precise results at various scales that range from crystal growth rates to crustal growth rates. In this chapter, we provide a perspective on the concept of geological time, the development of analytical geochronology, and the application of instruments. It is intended that this chapter acts as the historic foundation to the subsequent chapters that provide greater insight and the scientific basis for methods and applications of analytical geochronology.

Keywords

Deep time; Geological timescale; Instrumentation; Radioisotopic geochronology; Relative geological time

Time is the school in which we learn,

Time is the fire in which we burn

Delmore Schwatz, 1959, Calmly We Walk Through This April's Day.

1. Introduction

The geological sciences are principally concerned with the physicochemical processes involved in the formation and evolution of the solid Earth (e.g., magmatism, metamorphism, sedimentation). Moreover, they provide insight for understanding the development of life, ancient ecosystems, and climate systematics. Until the late 19th century, the geological sciences were mostly qualitative and, although rudimentary experiments were conducted during this time, relied on keen observations and creative interpretations of exposed surface rocks to advance new concepts and ideas. By the mid to late 20th century, the application of mathematics, physics, and chemistry to geology expanded the scope of the geological sciences into the quantitative realm and permitted the collection of data from within the Earth's crust, beneath the oceans, and from other planetary bodies of the solar system. One of the most significant advancements in the geological sciences was the quantification of time and establishment of analytical geochronology (Rutherford and Soddy, 1903; Holmes, 1911).

Prior to measuring radioisotopes and the determination of absolute ages, relative geological relationships were relied upon to provide an order of operations view of rock formations, be they depositional, structural, magmatic, or metamorphic in nature. One should always keep in mind the views of Arthur Holmes (1890–1965) when using the term absolute age, that it is … not only redundant and both philosophically and scientifically without meaning: it is also misleading in its psychological suggestion of a higher degree of accuracy than can be justified (Holmes, 1962). The fossil record demonstrated that the Earth's surface and environment changed over time and that some plants and metazoans became extinct whereas others evolved. However, the rates of these changes and processes was poorly constrained. Nevertheless, it was evident before the development of radioisotopic geochronology that the age of the Earth was 10s–1000s of millions of years old and that most geological processes and speciation occur over similarly long time spans (Thomson, 1862; Holmes, 1913; Knopf et al., 1931; Dalrymple, 1991). The impact of analytical geochronology on the geological sciences is immeasurable as proper temporal context became achievable, and the depth of time could be fully understood with the first reliable and reproducible radioisotopic age of the Earth and meteorites (Patterson, 1955, 1956). The subsequent rapid development of analytical methods and their application to geological problems ushered in a new era of scientific accessibility to a once, and often still, laborious activity. In some respects, analytical geochronology has become the foundation of modern geological research as, to paraphrase the ancient Greek philosopher Aristotle (c. 384–c. 322 BCE):

Geochronology is the first of the geological qualities because it is the quality which guarantees the others.

The wide use of whole rock, single crystal, and in situ mineral geochronology by the end of the 20th century opened the door to a greater understanding of geological processes at sufficient temporal resolution and precision (Reiners et al., 2017; Dickin, 2018). It is now possible to date very specific aspects of geological processes such as the mineralization of ore deposits, individual mineral crystallization in igneous systems, deposition of sedimentary rocks, and distinct periods of deformation within metamorphic rocks (Schaltegger and Davies, 2017; Bosse and Villa, 2019; Hnatyshin et al., 2020; Vermeesch, 2021). Moreover, from these results, accurate process rates (e.g., crystal growth) can be established as well as the identification of secular variability in crustal growth, and as another example, extremely precise temporal correlation between flood basalt eruptions and mass extinction events can be achieved (Belousova et al., 2010; Burgess et al., 2014; Schoene et al., 2014; Phelps et al., 2020; Sorokin et al., 2022).

For many geoscientists, the introduction to the methods and applications of geochronology occurs at the postgraduate level of instruction with very few becoming specialists. However, a fundamental understanding of the methods of data acquisition and handling in geochronology is increasingly essential, as it is important to apply the proper method for the proper goal. What may be an applicable method for igneous petrology may not be applicable to metamorphic or sedimentary petrology. Thus, there is a need to provide useful information on the available methods and applications of geochronology for practitioners and nonspecialists alike. This book contains a series of discussion chapters that focus on specific methods and applications of geochronology within the solid Earth geosciences. We selected topics based on their relevance to addressing scientific problems and their accessibility to geoscientists. Although all chapters provide insight on methods and applications of geochronology, there are some chapters that are methodology-leaning and others that are application-leaning. The methodology-leaning chapters discuss best practices in whole rock, single crystal, and in situ geochronology techniques, whereas the application-leaning chapters discuss how to obtain specific information from geological materials and best practices for data interpretation. Each chapter is self-contained, but there is overlap between some subjects, and we have arranged the chapters such that the related topics flow from subject to subject.

This chapter summarizes the key observations in geology that led to the basic understanding of geological time and the seminal moments where the rates of geological processes were realized, as well as an outline for the importance of geochronology in the geological sciences. Moreover, we highlight the development of analytical instruments and their application to geochronology. Much of the following subject matter is covered within entry-level courses of an undergraduate program and books and journal articles that chronicle the history of geological thought (e.g., Gould, 1997; Lyle, 2016). We encourage the reader to explore these references as they may assist in developing a greater awareness or appreciation for the individuals that helped to create the discipline of analytical geochronology.

2. Awareness and observation of geological and biological time: pre-20th century

2.1. Quod tempus est?

On a day-to-day basis, one does not think too much about the concept of time or its precise definition. Humanity has long had a first order understanding of time based on relative markers such as the orbit of the Moon around the Earth or the orbit of the Earth around the Sun, but also by seasonal weather patterns (e.g., monsoon, typhoon, hurricane, snowfall, flooding) or the life cycle. It is important to understand that time was not created, but rather, it was discovered. The discovery of time did not occur exactly in the same manner as the discovery of the electron by J. J. Thomson and colleagues, for example. Instead, awareness of time likely occurred shortly after the moment our ancient ancestors or an older evolutionary branch of our ancestors became self-aware. Since that moment, there was an understanding of yesterday, today, and tomorrow.

The awareness of time is an evolutionary advantage, not exclusive to Homo sapiens, as it enables an individual or a group to prepare for a future that has not yet arrived. Furthermore, acquired knowledge can be accumulated and passed on generationally for a given society through oral histories or recordings (e.g., cave drawings, papyrus, library, internet). Ancient and modern civilizations function nicely within these relative parameters, and it is generally unnecessary for an individual to contemplate the philosophical or scientific implications of time. Perhaps as a consequence, societies have attached different behavioral meaning to time (i.e., psychological time vs. cosmological time; Meyer, 2016). Nevertheless, how time is perceived, is measured, functions, and is utilized on a day-to-day basis—whether it is psychological time (a product of the mind) or cosmological time (objective)—is mostly a consequence of relative meaning rather than of what time actually is or its purpose (Meyer, 2016).

From the scientific perspective in the present day, one can loosely categorize time into: (1) absolute time, (2) physical or cosmological time, and (3) biological time. Absolute time (not in the Newtonian sense; Newton, 1726) can simply be considered to be the beginning of existence to the present whether that was 13.787 ± 0.020 billion years ago or whether there have been multiple expansion/contraction cycles of the universe is incidental as there must be an initial beginning to existence (Planck Collaboration, 2020). In this sense, understanding absolute time can be considered principally as an academic endeavor and aligns with the arrow of time or asymmetry of time concept (Eddington, 1928; Gould, 1987; Hemmo and Shenker, 2016). The concept of time's arrow refers to the one-way direction of time, in that as time passes, nothing can be undone or repeated. In contrast, physical time and biological time exist within the framework of absolute time but are relative to the systems in which their matter exists. In this sense, physical time and biological time may, in part, be cyclic in nature. Time's cycle refers to the persistent repetition of cycles without direction. In other words, once time/existence begins, an asymmetric process by definition, a series of asymmetric but highly structured and/or cyclical events will occur. For example, the precise formation and development of the solar system is random and unidirectional, but the structure of the Sun and planets follows a comparatively ordered process and will internally develop eddy-like cycles (e.g., solar cycle, supercontinent cycle, life cycle) that are repeated but distinguishable. In some sense, the intertwining of time's arrow and time's cycle is similar to the Heisenberg uncertainty principle in that there is a limit to the accuracy in which the positions of different types of time in physical and biological realms can be predicted from initial conditions.

The age of the planetary portion of the solar system is known because of radioisotopic decay (physical time) which is a noncyclical process (i.e., follows time's arrow) and a physical reality that can be applied universally. If and when terrestrial extrasolar planets are visited, they will be composed of elements that have radioisotopes that can be measured and an age can be determined. In a significant way this has already been proven as meteorites and lunar rocks have been dated using radioisotopic methods, though they are not extrasolar materials. In contrast, biological time is not directly observed or measured on the basis of radioisotopic decay but rather on the finite process of successful cell replication that maintains stable metabolic processes of the host organism, including consciousness. The longevity of a given species is based on reproduction and ability to adapt to the natural world. This is known intuitively because radioactive decay or the number of passages one takes around the Sun has little to do with biological functionality. One's physical and mental acuity is relatively correlated to the absolute number of revolutions around the Sun but the onset of decline is directly related to an individual's genetic coding and the prolonged success in cell replication. A simple thought exercise within our solar system illustrates the point nicely. If a human is 40 years old on Earth, then that would be equivalent to ∼65 revolutions around the Sun on Venus or ∼21 revolutions around the Sun on Mars and yet all would be of similar biological maturity. Likewise, if biological evolution on extrasolar terrestrial (or nonterrestrial) planets occurs then its development, not functionality (c.f., Cockell, 2016), will be related to a number of parameters such as: distance from the Star, type of Star, composition of the atmosphere, planetary rotation rate, planetary revolution rate, gravitational force, presence or absence of plate tectonics, and random catastrophic occurrences (Brack et al., 2010; Scharf and Cronin, 2016; Haqq-Misra, 2019; Irwin and Schulze-Makuch, 2020). However, the life expectancy of most individual organisms is objectively short, but its species may continue to exist for 100s of millions or even billions of years, provided the environment in which they can exist is maintained. For example, the importance or significance of time to a cyanobacterium is vastly different than to that of a trilobite. One of the best approaches to understand the relationships between physical time and biological time in the natural world is through the geological sciences as these subjects are the focus of stratigraphy, paleontology, and isotopic geochronology.

2.2. The journey into deep time

The geological sciences, from a broad perspective, are among the oldest divisions of science as stone tools, possibly created by Australopithecus afarensis, date back to more than three million years ago (McPerron et al., 2010; Harmand et al., 2015). Australopithecus afarensis was able to determine, undoubtedly through trial and error, which stones available were most effective for a given purpose (e.g., carving animal carcasses or shelter). The subsequent development of civilizations from the Bronze Age (3300–1200 BCE) to the present day was dependent on geological materials and the work that they enable. Over the past 500 years or so, a deeper philosophical understanding of the geological record developed, and it became evident that geological processes occur over a variety of timescales, of which the duration of some were beyond comprehension. The evolution of thought on physical/biological time concomitant with the application of physics, chemistry, and biology to geological problems greatly expanded the understanding of the Earth and solar system and led to the concept of Deep Time (MacPhee, 1981).

The problem with chronicling the evolution of thought on the subjects of physical time and biological time on Earth is that the chronicler is introduced at some point after the story began (i.e., 4.54 billion years ago) and is tasked with determining the beginning of something that they did not witness. The obvious question is: Where does one begin in order to understand when the Beginning began? The simplest answer appears to be through the observation of the natural world. In the 5th century BCE, Herodotus (c. 484–c. 425 BCE) suggested that the amount of time needed to accumulate the sediments of the Nile River delta must have been tens of thousands of years (Louderbeck, 1936). Thus, it was the observed sedimentation rate in the natural world that led to the conclusion. Observations of marine fossils by Xenophanes (c. 570–c. 480 BCE), Anaximander (c. 610–c. 546 BCE), Herodotus, and Aristotle (c. 384–c. 322 BCE) developed ideas that water must have been present in regions that are now dry land and thus older life was preserved and that a significant amount of time must have passed. This concept was independently explored by Leonardo da Vinci (1452–1519 CE) during the European Renaissance, but ideas pertaining to fossils were also expressed by Shen Kuo (1031–1095 CE) during the Song Dynasty in China and by Ibn Sina (980–1037 CE) in Persia. In contrast, earnest attempts using theological documents considered to be insightful on the history of the Earth came later. James Ussher (1650), Archbishop of Armagh, in Annales Veteris Testamanti, estimated the age of the Earth to be 4004 years 296 days (October 23rd, 4004 BCE) after examining scripture. This age is similar to the estimates put forth by John Lightfoot (estimate of 3928 BCE), Johannes Kepler (3993 BCE), and Isaac Newton (3998 BCE), but contrasts significantly with estimates (i.e., 4.32 billion years) using Hindu cosmology (Klostermaier, 2007). Moreover, Thomas Burnet (1635–1715) in Telluris Theoria Sacra offered a view reconciling Earth's origin with biblical events (e.g., Noah's Flood) and presented geological processes within the concepts of time's arrow and time's cycle (Gould, 1987; Lyle, 2016). The point is not to claim the superiority of one method over the other, but merely to demonstrate that all journeys begin with the first step and that science thrives within a panoply of ideas.

The 2000 or so years between the views of Herodotus and those of James Ussher indirectly set the stage for the scientific debate on the age of Earth and charted the path to the discovery of Deep Time. The fundamental question of the age of the Earth leads to the testing of two end-member hypotheses: Young Earth (thousands of years) and Old Earth (tens of thousands of years). However, in practice, the test was between the Christian scriptural views on the age of the Earth (6000 years) versus a nontheological view of the natural world. Georges-Louis Leclerc (1707–1788), Comte de Buffon, attempted to constrain the age of the Earth through experimentation and observation. By extrapolating the cooling rate of near molten metallic and nonmetallic balls to the size of the Earth, Leclerc estimated the age of the Earth to be 50,000–97,000 years with a revised estimate of 75,000 years (Poirier, 2017). It is interesting to note that even the early estimates of Herodotus and Leclerc are both significantly older than that accounted for in scripture. It is no coincidence that both age estimates are the result of observing the rates of natural processes (i.e., sedimentation, cooling) and can be replicated by experimentation. A consequence of the work by Leclerc was the development of the Neptunist view of the Earth. Neptunism is the idea that rocks and minerals of the Earth formed by precipitation from a universal ocean that has since receded. It was developed, in part, by the distinguished mineralogist and geologist Abraham Gottlob Werner (1749–1817) and had specific relevance to the formation of basalt.

Around the same time as the emerging scripture-based ages of the Earth, Nicolas Steno (Niels Steenson, 1638–1686) and Robert Hooke (1635–1703) ruminated on the origins of fossils and sedimentation. Hooke (1665, 1705), using a microscope to examine fossilized wood, concluded that fossils, including shells, represent the remains of living things that were preserved and that they could help to understand the history of life on Earth. After dissecting the head of a shark caught from the Ligurian Sea, Steno (1667) suggested that the shark teeth resembled the tongue stones (glossopetrae) observed on Malta. He posited that the tongue stones were in fact deposits of shark teeth within sand or mud of the sea floor that now appear on land, which is true. The views of Steno and Hooke on the origin of fossils followed the thinking of previous philosophers but more importantly mark an important step in the integration of biological time with physical time. Steno (1669) would later develop the four principles of stratigraphy: (1) the law of superposition; (2) the principle of original horizontality; (3) the principle of lateral continuity; and (4) the principle of cross-cutting relationships. In some respect, the first three principles of stratigraphy verify the thoughts of Herodotus regarding the deposition of sediment in the Nile Delta; however, the insight into rate of sedimentation and the geological order of operations of stratigraphy laid the groundwork for the uniformitarian views of James Hutton (1726–1797). This view was later advocated by John Playfair (1748–1819) and Charles Lyell (1797–1875) and led to the development of the stratigraphic column and the geological time scale.

One of the central tenets from Theory of the Earth, with Proofs and Illustrations by James Hutton is that the Earth operates as a self-renewing (uplift and erosion) system within physical (pressure, temperature) and chemical (weathering) parameters. One could consider this as a whole-system approach by integrating various disciplines of the physical sciences (Ranalli, 2001). Nevertheless, the recognition by Hutton at Dail-an-eas Bridge in the Scottish Highlands that granite intruded the country rock (schist) and therefore must have been molten during its emplacement was a seminal moment that was not only related to geological processes but also for the concept of Deep Time (Fig. 1.1). Although the debate on the nature of granite continued well into the middle 20th century (Marmo, 1967), conceiving of the intrusive nature of granite was instrumental in the support of the Plutonist view of the Earth.

The principles of Plutonism are that pressure and temperature play important roles in the formation of igneous and metamorphic rocks as demonstrated by experiments (Hall, 1805). The Plutonist versus Neptunist debate on the origin of rocks would end in favor of the Plutonists; however, the concept of a Magma Ocean during planetary formation and accretion and the formation of chemically precipitated sedimentary rocks (e.g., of banded iron formations) may be viewed as a continuation of Neptunism (Klein, 2005; Elkins-Tanton, 2012). The notion that granite was molten, emplaced in the crust, and eventually exposed at the surface through erosion has direct implications for the rate of geological processes and thus the ability to chronicle time, but it was the observation of sedimentary rocks that was vital to understand Deep Time (Fig. 1.1). The description of tilted gray sandstones overlain by horizontal red sandstone at Jedburgh, SE Scotland and the vertical greywacke beds overlain by near horizontal beds of red sandstone at Siccar Point near Edinburgh led to the conclusion that a significant amount of time must had passed in order for deposition, tilting, and erosion of the underlying sedimentary rocks relative to the deposition of the overlying sedimentary rocks (Fig. 1.2). The angular unconformities at Jedburgh and Siccar Point and the magmatic origin of granite were the catalysts that led Hutton (1788) to write:

Figure 1.1  A photo of the outcrop at Dail-an-eas Bridge (Glen Tilt, Scotland) showing the intrusive nature of granite (light pink) with the metamorphic (green) country rock (Britt Bousman, CC BY-SA 4.0).

But if the ſucceſſion of worlds eſtabliſhed in the ſyſtem of nature, it is vain to look for anything higher in the origin of the earth. The reſult, therefore of our preſent enquiry is, that we find no veſtige for a beginning, —no proſpect of an end.

That Hutton could not find vestige of a beginning nor a prospect of an end with respect to the Earth was as philosophical as it is profound, but it carries with it a message that geologists must look at rocks far and wide for answers and develop the necessary tools to uncover their secrets. In some ways, the words of psychiatrist Carl Gustav Jung (1875–1961) are mirroring Hutton when referring to the absence of spirituality in modern life as being due to the fact that Modern man can't see God because he doesn't look low enough. Nevertheless, one could interpret melancholy in the words of Hutton since all that was truly understood was that there were more questions to be answered, but one can also sense the call to adventure and that the secrets of the undiscovered country may be revealed.

Figure 1.2  A photo of sloping sandstone above conglomerate and vertical bedding of greywacke at Siccar point, Berwickshire, Scotland (Dave Souza, CC BY-SA 4.0).

2.3. Uniformitarianism and the geological timescale

By the 19th century, the Huttonian view of the Earth was gaining the upper hand on the Neptunist view in large part due to the writings of John Playfair (1802) in Illustrations of the Huttonian Theory of the Earth and Charles Lyell (1833) in the three volumes of Principles of Geology (1830–1833). The uniformitarianism concept, a term coined by William Whewell (1794–1866), advocated that the same natural laws operating in the present have operated in the past; in other words, there is cyclicity in the natural world. Under the umbrella of uniformitarianism were included gradualism and the ideas that: (1) the laws of nature are constant across time and space; (2) that geological phenomenon of the past have a modern corollary; (3) that the cause of a particular geological event had the same kind of energy and produced the same effects, then as now; and (4) that geological circumstances have remained constant over time (Hooykaas, 1963). Adjacent to or in opposition to uniformitarianism was catastrophism, another term coined by Whewell, in which the Earth was shaped by sudden, short-duration violent events that may have been global in extent.

Catastrophism, in the geological sense, is attributed to observations of the fossil record by Jean Léopold Nicolas Frédéric Cuvier (1769–1832) also known as Georges Cuvier or the Baron Cuvier. Examining fossils and using comparative anatomy, Cuvier advocated that some species became extinct suddenly and/or were unknown to man. The catastrophism view was strongly opposed by Lyell as gradualism was the preferred rate of geological processes and, at the time, certain aspects of catastrophism were perceived as validating Biblical events (e.g., flooding). It is interesting to note that around the same time as the uniformitarianism versus catastrophism debate was being resolved that the ideas of natural selection and descent with modification arose from Alfred Russel Wallace (1823–1913) and Charles Robert Darwin (1809–1882). In addition to opposing catastrophism, a sensible position from a gradualist point of view that emphasizes the cyclical nature of geological processes, Lyell was initially reluctant to support biological evolution or biological nonprogression until the final two editions of Principles of Geology (1866, 1872). The absence of mammal fossils in Paleozoic sedimentary rocks and human remains in the fossil record probably helped to convince Lyell that uniformitarianism may not be operating as precisely as was thought (Gould, 1987). The conflicting nature between uniformitarianism and catastrophism relates, in part, to differences between the life cycle and the rock cycle but also between physical time and biological time, and by extension time's arrow and time's cycle. Physical time and biological time are intertwined by existence, but they are separate.

The attention to the fossil record in the 19th century and the concepts of Steno and Hooke discussed in the 17th century were applied to the debate on evolution, and already by the middle of the 18th century, Johann Gottlob Lehmann (1719–1767) and Giovanni Arduino (1714–1795) were organizing the rock record by applying the principles of stratigraphy. This initial step was followed by the recognition by Alexander von Humboldt (1769–1859) that there was a widespread occurrence of the Jura-Kalkstein (Jura Limestone) in the Jura Mountains along the France–Switzerland border that is distinct from the clam-fossil-bearing limestone of the German Muschelkalk (von Humboldt, 1799). Furthermore, William Smith (1769–1839) developed the principle of faunal succession by observing predictable fossil-bearing strata across Britain and subsequently published their distribution in map form (Smith, 1815). The rock types and fossils that they contain act as a geological finger printing system whereby correlations can be made across great distances. Around the same time as Smith, Alexandre Brongniart (1770–1847) would correlate the Jura-Kalkstein observed by von Humboldt (1799) with oolitic limestone in Britain and make reference to the terrain Jurassiques (Brongniart, 1829). Brongniart applied the principles of Steno (superpositioning, original horizontality, and lateral continuity) as well as the vertical dimension based on the fossil record (faunal succession) within the rocks to define the Jurassic Period.

The formal creation of the stratigraphic column accelerated shortly after the work of Smith with the adoption of the informal term Carboniferous for coal-rich strata throughout Europe in 1822. In the same year, Jean d’Omalius d’Halloy (1783–1875) defined the extensive chalk beds of the Paris Basin and Western Europe as the terrain Crétacé (d’Halloy, 1822) which were distinct from the terrain Jurassiques of Brongniart. Friedrich August von Alberti (1795–1878) defined the Triassic, from the Trias sequence of Germany that referred to a triad of lower colorful sandstone, middle shell-bearing limestone, and upper colored clay (von Alberti, 1834). The names Cambrian (1835), Ordovician (1879), Silurian (1839), Devonian (1839), and Permian (1841) would be used to define different sedimentary rock successions that would later include the Tertiary and its subdivisions, the Quaternary in Europe over the next few decades, and round out most of what is now known as the Phanerozoic Eon (Holmes, 1913; Gradstein et al., 2020). Later, Periods would be grouped into Eras that would become the Paleozoic (old life forms), Mesozoic (middle life forms), and Cenozoic (new life forms).

An important consequence of the stratigraphic column was the chronological order of fossil-rich sedimentary rocks that showed the evolution of life (biological time) to be asymmetrical and not cyclical and thus inconsistent with the uniformitarian concept of the present is the key to the past. There is nothing in the present that could have predicted the direction of biological evolution during the Mesozoic, for example, and it certainly was not cyclical as many species went extinct and did not re-evolve, though cycles of evolution and extinction continue apace. However, the present can help to understand the geological processes (i.e., sedimentation, metamorphism, magmatism) at work during the past. Although catastrophism was unfavorably viewed by Lyell, it could not be easily dismissed as it was clear that many species disappeared in a geological instant, never to return. Catastrophism would be broadly accepted within the scope of geological thought again in the 20th century, as evidence for massive short-duration flooding in the Pacific Northwest of the United States was compelling (Bretz, 1923), and mass extinctions were identified in the fossil record (Raup and Sepkoski, 1982). Moreover, during this time, the Oxford evolution debate of 1860, a reaction to On the Origin of Species by Charles Darwin, was an important moment as scripture-based thinking waned and logic and reason waxed. In some sense, one can view the Oxford evolution debate as completing the thought cycle that began with the ideas of Herodotus and James Ussher.

The geological timescale is still a work in progress as the Phanerozoic is constantly being refined (Gradstein et al., 2020) and the division of the Precambrian is ongoing, but it is hampered by the limited fossil record and its dependence on the precise development and timing of specific geological processes which may not have developed uniformly across the Earth (c.f., Windley, 1984). The evolution of life on Earth through faunal succession and the rate of geological processes demonstrated by the stratigraphic column shows that biological and geological temporal cycles were synchronizing after the Precambrian, as fossils became relatively abundant. This abundance of fossils in Phanerozoic sedimentary rocks is directly related to terrestrial ecosystem development and speciation of life with physical attributes that can be preserved by fossilization. By the middle 19th century, with additional observations of Alpine glaciers and their remnant features by Louis Agassiz (1807–1873), it was beyond doubt that the Earth was far more than 4000 years old as the ideas of Hutton, Cuvier, and others were coalescing around the Old Earth model. The question remained, however: exactly how old is the Earth?

3. In search of an absolute time

By the end of the 19th century, the stratigraphic column through Earth history was nearly complete, and the Periods and Eras of the modern geological timescale were established (Fig. 1.3). However, assigning the precise ages of the Periods was not possible as all stratigraphic relationships are relative whether they contain fossils or not. The need to quantify physical time was becoming more evident as geological observations could not provide better constraints. William Thomson (1824–1907), first Baron Kelvin of Largs, attempted to calculate the age of the Earth by applying the laws of thermodynamics. Calculating the cooling rate of the Earth, assuming no additional heat was generated by the Earth and that the Sun and the Earth were at one point of similar temperature, Thomson initially found that the age of the Earth should be between 20 and 400 million years, later refined to 20–40 million years (Thomson, 1862). The concepts assumed for the calculation were not incorrect, but rather the assumptions in the calculation were not well constrained, specifically the surficial thermal gradient (Perry, 1895). John Perry (1850–1920) debated with Thomson in a series of letters to Nature that the age of the Earth could be 1000 million years or possibly older (England et al., 2007). Given the understanding of the Earth at the time, the debate between Perry and Thomson is moot, but some of the ideas that sprouted from the discussions were somewhat prescient with respect to the internal processes of the Earth.

It was the discovery of radiation by Wilhelm Conrad Röntgen (1845–1923) in 1895 that began the journey of quantifying physical time through radioisotopic geochronology. Röntgen (1896), while working with Crookes tubes to study cathode rays (electron beam), noticed that barium platinocyanide Ba[Pt(CN)4] fluoresced when it came in contact with the cathode rays. The cause of the fluorescence was interpreted to be a new type of radiation called X-rays. The X-rays were shown to not only penetrate paper but also biological soft tissue. A short time later, Antoine Henri Becquerel (1852–1908) discovered that the element uranium emitted radiation without external stimulation (i.e., sunlight) and demonstrated that the radiation could be deflected by magnetic or electric fields and thus must be different from X-rays. Marie Salomean Skłodowska-Curie (1867–1934) with her husband Pierre Curie (1859–1906) were able to measure the intensity of the radiation (radioactivity) and discovered the elements thorium, polonium, and radium before the end of the century. The discovery of radioactivity, uranium, and thorium was the beginning of the final steps toward quantifying the age of the Earth through radioisotopic geochronology and for a true understanding of how deep Deep Time actually is.

Figure 1.3  The geological time scale as presented by Figuier (1865). At the time of publication, the Ordovician (1879) had not been defined nor had all of the epochs of the Cenozoic.

After the philosophical revolution that led to the acceptance of deep time, the early 20th century saw a technological/scientific revolution that cemented the time's arrow concept as it applies to Earth history. The early work on radioactivity immediately carried with it the knowledge that radioactivity was naturally occurring and rocks and minerals were common sources. Marie Curie observed that the mineral pitchblende is significantly more radioactive than refined uranium, and natural uranium ore was the source of the radium she studied. Rutherford and Soddy, over a series of publications (c.f., Rutherford and Soddy, 1903), not only determined that radioactivity resulted from the breakdown of unstable elements into stable ones but derived the mathematical rules for radioactive decay and established that a daughter product of uranium is helium. The determination that this disintegration occurred at a constant rate was a phenomenon that shaped the maturing field of geology irreversibly. Papers by Rutherford (1906) and Boltwood (1907) used Boltwood's discovery of Pb as another decay product of U to use U/Pb chemical ratios to calculate dates for uranium-bearing geological samples; the ages obtained went as far back as 2200 million years. Arthur Holmes (1890–1965), a British geologist and the first geochronologist in that unlike the earlier physicists his interest lay in the rock record, studied under Robert Strutt (1875–1947), who was the first to date zircon—using the U–He method (Strutt, 1910). Holmes's early work built upon Strutt's, calculating new ages for geological materials (Holmes, 1911) and culminating in his Age of the Earth monograph (Holmes, 1913) that summarized not only his own work but recalculations of earlier dates by Strutt and Boltwood.

Joseph Thomson (1856–1940) invented a positive ray detector, an early version of a mass spectrometer (Thomson, 1914), and used it to determine that neon had two varieties, chemically indistinguishable but with different masses; combined with Rutherford's postulation of neutrons in 1920 (confirmed by Chadwick in 1932), this set the stage for future work based on isotopes (a term first proposed by Soddy). Francis Aston (1877–1945) used his mass spectrograph (Aston, 1919) to identify 212 isotopes of various elements, including ²⁰⁶Pb and ²³⁸U, and later, the much less abundant ²⁰⁷Pb and ²³⁵U. Arthur Dempster (1886–1950) used a 180-degree magnetic field to guide a path of ions to a detector (Dempster, 1918) and discovered the isotope ²³⁵U in 1935. His mass spectrometer, the first of modern design and orders of magnitude more accurate than previous instruments, became the foundation for work by Alfred Nier (1911–1994), whose refinement of the magnetic sector mass spectrometer design is fundamentally the one used to the present day (e.g., Nier, 1940). Nier not only discovered ⁴⁰K, the basis for K–Ar and Ar–Ar geochronology (Nier, 1935), but characterized common and radiogenic Pb isotopes such that a reliable age of the Earth could be calculated (Nier et al., 1941).

The decade following World War II saw a spectacular development of geochronology using isotopic methods. The wartime discovery of nuclear fission and the use of isotope separation technology enabled the isolation of isotopes such as ²³⁵U, critical for isotope dilution mass spectrometry and the precise and accurate measurement of a variety of isotopes. George Tilton (1923–2010) and Clair Patterson (1922–1995) applied this method to the U–Th–Pb system as recorded in both meteorites and terrestrial materials (including the first U–Pb ID-TIMS date on zircon). The Rb–Sr method had been used to date geological materials by Otto Hahn and Ernst Walling (Hahn and Walling, 1938), and the K–Ar method was already widely in use (c.f., Houtermans, 1966). Perhaps the most notable outcome of this scientific explosion was the determination of the age of the Earth to ca. 4.55 billion years, using Pb isotopes of meteoritic and terrestrial samples (Patterson et al., 1955).

3.1. The modern approach

Today, in the context of geochronology, it might be said that time's arrow points backward. It is now natural to consider time as extending back into the past from the Present. The Present is marked by modern humans looking back through time and trying to see the Beginning, with geological time markers in between commonly represented in Ma: mega-anni that define a duration, in millions of years, from a geological event to the time of measurement in the Present. It is perhaps solipsistic, though probably not unrealistic, to consider the Present as a more or less instantaneous moment where we now sit, examining the vast history that has led us to this point, not particularly worried about any future need to recalculate the ages we measure to account for a citation published far enough in the future to push these ages beyond their uncertainty envelopes.

The past few decades have seen a constant push toward refinement of methods (e.g., Krogh, 1973) and breadth of samples available for geochronological analysis. In situ methodologies such as laser ablation inductively coupled mass spectrometry, and secondary ion mass spectrometry have enabled spatial resolution that reveals complexity on the scale of a few microns, with atom-probe tomography exploring the possibility of resolving time at the atomic scale. Advances in instrumentation have permitted analytical precision to be improved to the point where we are now resolving geological events such as magma chamber processes within single mineral crystals; where we once had to accept potential isotopic ratio heterogeneity of multi-crystal analyses in order to have a large enough ion beam to reliably measure, we are now able to interrogate isotopic ratio heterogeneity within single crystals. Sensitivity of detectors, mass resolution, controls over instrumental bias and other analytical parameters have seen significant improvements in recent years. And as always, geochronological methods have advanced through trial and error, playing in the lab, and sharing results both positive and negative with the international community of scientists in formal and informal communication. Tips and techniques are often shared between laboratories, though rarely formalized or codified.

We, therefore, invite the reader to take their place in time; consider the following chapters to be a starting point in the Present, necessarily representing the Past and forming the foundation for the Future of isotopic geochronology. Forward-looking statements in these chapters provide only some directions for where the field will be in the years to come, with innovations that change our perspective on Earth history arriving around every corner.

References

1. Aston F.W. Mass spectrograph. Philosophical Magazine. 1919;38:209–220.

2. Belousova E.A, Kostitsyn Y.A, Griffin W.L, Begg G.C, O'Reilly S.Y, Pearson N.J.The growth of the continental crust: constraints from zircon. Lithos. 2010;119:457–466.

3. Boltwood B.B. On the ultimate disintegration products of the radio-active elements. Part II. The disintegration products of uranium. American Journal of Science. 1907;23(4):77–88.

4. Bosse V, Villa I.M. Petrochronology and hygrochronology of tectono-metamorphic events. Gondwana Research. 2019;71:76–90.

5. Brack A, et al. Origin and evolution of life on terrestrial planets. Astrobiology. 2010;10:69–76.

6. Bretz J.H. The channeled scablands of the Columbia Plateau. The Journal of Geology. 1923;31:617–649.

7. Brongniart A. Tableau des Terrains qui Composent L'écorce du Globe ou Essai sur la Structure de la Partie Connue de la Terre. Paris: F.G. Levrault; 1829:435.

8. Burgess S.D, Bowring S, Shen S.-Z. High-precision timeline for Earth's most severe extinction. Proceedings of the National Academy of Science United States of America. 2014;111:3316–3321.

9. Cockell C.S. The similarity of life across the universe. Molecular Biology of the Cell. 2016;24:1553–1555.

10. Dalrymple G.B. In: The Age of the Earth. Stanford: Stanford University; 1991.

11. d'Halloy d’O.J.-J. Observations sur in essai de carte géologique de la France des Pays-Bas et des contrées voisines. Annales des Mines. 1822;7:353–376.

12. Dempster A.J. A new method of positive ray analysis. Physical Review. 1918;11(4):316.

13. Dickin A.P. Radiogenic Isotope Geology. third ed. Cambridge University Press; 2018:482.

14. Eddington A.S. The Nature of the Physical World. New York: The MacMillian Company; 1928:365.

15. Elkins-Tanton L.T. Magma oceans in the inner solar system. Annual Reviews of Earth and Planetary Sciences. 2012;40:113–139.

16. England P.C, Molnar P, Richter F.M. Kelvin, Perry and the age of the Earth. American Scientist. 2007;95:342–349.

17. Figuier L. The World Before the Deluge. London: Chapman and Hall; 1865:439.

18. Gould S.J. Time's Arrow, Time's Cycle: Myth and Metaphor in the Discovery of Geological Time. Cambridge, MA: Harvard University Press; 1987:222.

19. Gradstein F.M, Ogg J.G, Schmitz M.D, Ogg G.M, eds. Geological Time Scale 2020. Amsterdam: Elsevier; 2020:1390.

20. Hahn O, Walling E. Über die Möglichkeit geologischer Altersbestimmungen rubidiumhaltiger Mineralien und Gesteine. Zeitschrift für Anorganische und Allgemeine Chemie. 1938;236:78–82.

21. Hall J. Experiments on Whinstone and Lava. Transactions of the Royal Society of Edinburgh. 1805;5:43–75.

22. Harmand S, et al. 3.3-million-year-old stone tools from Lomekwi 3, West Turkana, Kenya. Nature. 2015;521:310–315.

23. Haqq-Misra J. Does the evolution of complex life depend on the stellar spectral energy distribution?Astrobiology. 2019;19:1292–1299.

24. Hemmo M, Shenker O. The arrow of time. In: Doleb Y, Roubach R, eds. Cosmological and Psychological Time. Boston Studies in the Philosophy and History of Science. vol 285. 2016:155–164.

25. Hnatyshin D, Creaser R.A, Meffre S, Stern R.A, Wilkinson J.J, Turner E.C. Understanding the microscale spatial distribution and mineralogical residency of Re in pyrite: examples from carbonate-hosted Zn-Pb ores and implications for pyrite Re-Os geochronology. Chemical Geology. 2020;533:119427. doi: 10.1016/j.chemgeo.2019.119427.

26. Holmes A. The association of lead with uranium in rock-minerals, and its application in the measurement of geological time. Proceedings of the Royal Society of London. 1911;A85:248–256.

27. Holmes A. The Age of the Earth. London: Harper & Brothers; 1913:196.

28. Holmes A. ‘Absolute’ age: a meaningless term. Nature. 1962;196:665.

29. Hooke R. Micrographia: or Some Physiological Descriptions of Minute Bodies Made by Magnifying Glasses. first ed. London: J. Martyn and J. Allestry; 1665:259.

30. Hooke R. The Posthumous Works of Robert Hooke, Containing His Cutlerian Lectures, and Other Discourses. London: S. Smith and B. Walford; 1705:590.

31. Hooykaas R. The Principle of Uniformity in Geology, Biology, and Theology: Natural Law and Divine Miracle. Leiden: E.J. Brill; 1963:237.

32. Houtermans F.G. History of the K/Ar-Method of Geochronology. Potassium Argon Dating. Berlin: Springer-Verlag; 1966:1–6.

33. Hutton J. On the theory of the Earth; or an investigation of the laws observable in the composition, dissolution and restoration of the globe. Transactions of Royal Society of Edinburgh. 1788;1:209–304.

34. Irwin L.N, Schulze-Makuch D. The astrobiology of alien worlds: known and unknown forms of life. Universe. 2020;6:130. doi: 10.3390/universe6090130.

35. Klein C. Some Precambrian banded iron-formations (BIFs) from around the world: their age, geological setting, mineralogy, metamorphism, geochemistry, and origin. American Mineralogist. 2005;90:1473–1499.

36. Klostermaier K.K. A Survey of Hinduism. third ed. Albany: State University of New York Press; 2007:718.

37. Knopf A, Schuchert C, Kovarik A.F, Holmes A, Brown E.W. Physics of the Earth – IV: The Age of the Earth. Washington: Bulletin of the National Research Council; 1931:80.

38. Krogh T.E. A low-contamination method for hydrothermal decomposition of zircon and extraction of U and Pb for isotopic age determinations. Geochimica et Cosmochimica Acta. 1973;37:485–494.

39. Louderback G.D. The age of the Earth from sedimentation. The Scientific Monthly. 1936;42:240–246.

40. Lyell C. The Principles of Geology. London: J. Murray; 1833:420.

41. Lyle P. The Abyss of Time: A Study of Geological Time and Earth History. Edinburgh: Dunedin Academic Press; 2016:204.

42. MacPhee J. Book 1: basin and range. In: Annals of the Former World. New York: Farrar, Straus, and Giroux; 1981:79.

43. Marmo V. On the granite problem. Earth-Science Reviews. 1967;3:7–29.

44. McPherron S.P, Alemseged Z, Marean C.W, Wynn J.G, Reed D, Geraads D, Bobe R, Béarat H.A.Evidence for stone-tool-assisted consumption of animal tissues before 3.39 million years ago at Dikika, Ethiopia. Nature. 2010;466:857–860.

45. Meyer U. Consciousness and the present. In: Doleb Y, Roubach R, eds. Cosmological and Psychological Time. Boston Studies in the Philosophy and History of Science. vol 285. 2016:143–154.

46. Newton I. Philosophiæ Naturalis Principia Mathematica. Londini: Apud G. & J. Innys; 1726.

47. Nier A.O. Evidence for the existence of an isotope of potassium of mass 40. Physical Review. 1935;48(3):283.

48. Nier A.O. A mass spectrometer for routine isotope abundance measurements. Review of Scientific