Geochronology and Thermochronology

Peter W. Reiners, Richard W. Carlson, Paul R. Renne, Kari M. Cooper, Darryl E. Granger, Noah M. McLean, Blair Schoene

• Éditeur: American Geophysical Union
• Année: 2017
• ISBN:
  9781118455906
• Édition: 1
• Sujet: Physical Sciences
• Sous-sujet: Mineralogy

This book is a welcome introduction and reference for users and innovators in geochronology. It provides modern perspectives on the current state-of-the art in most of the principal areas of geochronology and thermochronology, while recognizing that they are changing at a fast pace. It emphasizes fundamentals and systematics, historical perspective, analytical methods, data interpretation, and some applications chosen from the literature. This book complements existing coverage by expanding on those parts of isotope geochemistry that are concerned with dates and rates and insights into Earth and planetary science that come from temporal perspectives.

Geochronology and Thermochronology offers chapters covering: Foundations of Radioisotopic Dating; Analytical Methods; Interpretational Approaches: Making Sense of Data; Diffusion and Thermochronologic Interpretations; Rb-Sr, Sm-Nd, Lu-Hf; Re-Os and Pt-Os; U-Th-Pb Geochronology and Thermochronology; The K-Ar and 40 Ar/ 39 Ar Systems; Radiation-damage Methods of Geo- and Thermochronology; The (U-Th)/He System; Uranium-series Geochronology; Cosmogenic Nuclides; and Extinct Radionuclide Chronology.

• Offers a foundation for understanding each of the methods and for illuminating directions that will be important in the near future
• Presents the fundamentals, perspectives, and opportunities in modern geochronology in a way that inspires further innovation, creative technique development, and applications
• Provides references to rapidly evolving topics that will enable readers to pursue future developments

Geochronology and Thermochronology is designed for graduate and upper-level undergraduate students with a solid background in mathematics, geochemistry, and geology. "Geochronology and Thermochronology is an excellent textbook that delivers on the difficult balance between having an appropriate level of detail to be useful for an upper undergraduate to graduate-level class or research reference text without being too esoteric for a more general audience, with content and descriptions that are understandable and enlightening to the non-specialist. I would recommend this textbook for anyone interested in the history, principles, and mechanics of geochronology and thermochronology." --American Mineralogist, 2021 Read an interview with the editors to find out more:
https://eos.org/editors-vox/the-science-of-dates-and-rates

CHAPTER 1
Introduction

Occasionally debates arise and hands are wrung about what parts of a scientific discipline really distinguish it from others. Geoscientists often find themselves trying to define the unique perspectives or essential skills at the heart of their field as if failure to properly indoctrinate students in them might put the entire profession at risk. Without commenting on the wisdom of such disciplinary exceptionalism, a reasonable person asked to engage in it could, after some thought, suggest that if there is something distinctive about Earth science, it might have something to do with time. Naturalistic thinking about the evolution and workings of the Earth have been around for centuries if not millennia, and considerations of time at scales far surpassing human experience are a central and obligatory part of any serious endeavor in this area. The facility to deal easily with enormous timescales is such an ingrained part of Earth and planetary science that occasional meditative realizations of even the most hardened scientists are sometimes required to remind them that our ability to envision geologic time accurately and precisely has been in some ways hard won. Before quantitative measurements were available of the durations of time separating events of the past from the present, and of the rates of geologic processes, practically all attempts to understand Earth were, to paraphrase a key historical figure in geochronology (Lord Kelvin), meagre and of a most unsatisfactory kind. Quantitative geochronology as a concept, and especially radioisotopic geochronology as a field in and of itself, revolutionized our understanding of the Earth and planets. More importantly, geochronology continues to be one of, if not the most, important foundation and means of exploration in modern geoscience.

The tools and applications of geochronology find use in a variety of fields besides Earth and planetary science, including archeology, evolutionary ecology, and environmental studies. But the impact of geochronology on Earth science was fundamentally transformative. For one thing, it laid out the boundary conditions for reconstructing the history of the planet and quantitative understanding of the significance of ongoing physical processes like erosion, sedimentation, magmatism, and deformation. It also established, for the first time, a realistic temporal context of existence—not just of life as we know it, but for the recognizable planetary environment that hosts life. This is because the timescales of Earth history and Earth processes (including biotic evolution at that scale) require a fundamentally different temporal perspective than human experience (much less historical records) can offer. While some important geologic and evolutionary processes happen over very short timescales and require chronometers with commensurate sensitivity, many of the most challenging and important observations we make about the Earth reflect processes that occur either very slowly or very rarely, relative to the perspective of humans as individuals, civilizations, or even species. Modern radioisotopic techniques span vast timescales from seconds to billions of years, finding application in problems ranging from the age and pace of individual volcanic eruptions to condensation of the solar nebula and ongoing planetary accretion. The transformative power of geochronology comes from its capacity to expand our understanding beyond the reach of the pathetically short timescales of intuitive human or social perspectives.

1.1 GEO AND CHRONOLOGIES

Extending the timescale of our understanding does not mean just establishing a chronology of events that occurred earlier than historical records or generational folklore allow. It goes without saying that establishing pre‐historical records of changes on and in Earth and other planets is practically useful: knowing when a volcano erupted or a nearby fault last ruptured or the age of an extinction or diversification event may be important. Establishing historical chronologies of tectonic events is clearly necessary for practical purposes. But a list of dates or sequence of regional events is of limited value in and of itself, and does little to represent geochronology as way of exploring how the planet works using time as an organizing principle or mode of inquiry.

For one thing, there is the question of how to define an event. At one level the question of the age of the Earth is simple, and has been the focus of countless studies since human curiosity began. Modern perspectives on the problem however, shifted years ago from simplistic numerical answers of around 4.56 Ga, to more sophisticated ones that raise issues of how to assign a single age to a protracted evolutionary process complicated by questions of the initial uniformity of and chemical fractionation in the solar nebula, and timescales of accretion, mass loss, and differentiation. Many other questions in Earth and planetary science have evolved similarly as understanding deepened. Continuing efforts to understand the geologic record are no longer satisfied with just knowing “the age” of a particular event such as the Permo‐Triassic boundary, the Paleocene–Eocene Thermal Maximum (PETM), or meltwater pulse 1A, but now we need to know the duration, pace, and number of perturbations composing an event, and the detailed sequence and timing of resulting effects. Geochronology has been central to all of these as not only the intended accuracy and precision, but also the essence of the question, changed. Geochronology shows that “events” are not only finite and messy, but manifestations of more interesting phenomena in themselves.

Also, while some scientists see geochronology as a useful tool for addressing pre‐defined geologic problems, using geochronology is not the same thing as doing it. The power of geochronology arises from innovative approaches. There is no single template for this, but one could make an argument for at least two types of creative geochronology. The first is adapting new geochemical, physical, or analytical insight or technology to addressing suitable geologic problems. Fission‐track dating was developed after methods for observing cosmic ray tracks in insulators were extended to tracks produced by natural radiation sources in situ [Fleischer and Price, 1964]. Inductively coupled plasma mass spectrometry and its pairing with laser ablation sample introduction both changed isotope geochemistry and geochronology in key ways [e.g., Halliday et al., 1998; Lee and Halliday, 1995; Kosler et al., 2008]. K–Ar dating was adapted into one of the most precise and powerful geochronological techniques ever developed (⁴⁰Ar/³⁹Ar dating) using fast neutron irradiation to create proxies of parent nuclides of the same element and chemical behavior as the daughter nuclides [Merrihue and Turner, 1966]. And of course the first radioisotopic date itself was calculated as a marginalia to a nuclear physics study much more concerned with “radioactive transmutations” than with determining the age of anything [Rutherford, 1906].

Second, and as is true in many other fields, some impactful advances in geochronology have come not from deliberate engineering but more as refusals to ignore complications. Solutions to such problems often hold potential for illuminating unknown unknowns, which may then be trained to address previously unsolvable problems. When a particular technique appears to “not work” for answering the question originally posed, it may be time to ask why the answer is unexpected and what can be learned from it by reframing the question. Thermochronology, for example, owes a great deal of its modern utility to this sort of lemons‐to‐lemonade evolution, as the diffusive loss of daughter products was initially considered a debilitating limitation of noble‐gas‐based techniques [e.g., Strutt, 1906] but is now recognized as its defining strength, as increasingly complex as it appears to be [e.g., Shuster et al., 2006; Guenthner et al., 2013].

This is all to say that geochronology is not just a “tool” serving other fields, but is a field unto itself, and one that originates the new ideas and approaches that allow for advances in the areas to which it is applied. Geochronology generates the innovative ways to use nuclear physics and geochemistry to understand natural processes, often by using initially problematic aspects of these systems, and adapting them to questions that initially may not have been asked. It was not until long after we started wondering about the age of the Earth that we started to appreciate questions about the duration of events, stratigraphic boundaries, and diachroneity. And it was not until we developed quantitative tools (serendipitously, in many cases) for measuring dates and rates in new ways that we began to realize the value of understanding many more nuanced time‐related proble...