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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 40Ar/39Ar Systems; Radiation-damage Methods of Geo- and Thermochronology; The (U-Th)/He System; Uranium-series Geochronology; Cosmogenic Nuclides; and Extinct Radionuclide Chronology.
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
Peter W. Reiners, University of Arizona, USA
Richard W. Carlson, Carnegie Institution for Science, USA
Paul R. Renne, Berkeley Geochronology Center and University of California, USA
Kari M. Cooper, University of California, USA
Darryl E. Granger, Purdue University, USA
Noah M. McLean, University of Kansas, USA
Blair Schoene, Princeton University, USA
Preface ix
1 Introduction 1
1.1 Geo and chronologies 1
1.2 The ages of the age of the earth 2
1.3 Radioactivity 7
1.4 The objectives and significance of geochronology 13
1.5 References 15
2 Foundations of radioisotopic dating 17
2.1 Introduction 17
2.2 The delineation of nuclear structure 17
2.3 Nuclear stability 19
2.3.1 Nuclear binding energy and the mass defect 19
2.3.2 The liquid drop model for the nucleus 20
2.3.3 The nuclear shell model 22
2.3.4 Chart of the nuclides 23
2.4 Radioactive decay 23
2.4.1 Fission 23
2.4.2 Alpha-decay 24
2.4.3 Beta-decay 25
2.4.4 Electron capture 25
2.4.5 Branching decay 25
2.4.6 The energy of decay 25
2.4.7 The equations of radioactive decay 27
2.5 Nucleosynthesis and element abundances in the solar system 30
2.5.1 Stellar nucleosynthesis 30
2.5.2 Making elements heavier than iron: s- r-, p-process nucleosynthesis 31
2.5.3 Element abundances in the solar system 32
2.6 Origin of radioactive isotopes 33
2.6.1 Stellar contributions of naturally occurring radioactive isotopes 33
2.6.2 Decay chains 33
2.6.3 Cosmogenic nuclides 33
2.6.4 Nucleogenic isotopes 35
2.6.5 Man-made radioactive isotopes 36
2.7 Conclusions 36
2.8 References 36
3 Analytical methods 39
3.1 Introduction 39
3.2 Sample preparation 39
3.3 Extraction of the element to be analyzed 40
3.4 Isotope dilution elemental quantification 42
3.5 Ion exchange chromatography 43
3.6 Mass spectrometry 44
3.6.1 Ionization 46
3.6.2 Extraction and focusing of ions 49
3.6.3 Mass fractionation 50
3.6.4 Mass analyzer 52
3.6.5 Detectors 57
3.6.6 Vacuum systems 60
3.7 Conclusions 62
3.8 References 63
4 Interpretational approaches: making sense of data 65
4.1 Introduction 65
4.2 Terminology and basics 65
4.2.1 Accuracy, precision, and trueness 65
4.2.2 Random versus systematic, uncertainties versus errors 66
4.2.3 Probability density functions 67
4.2.4 Univariate (one-variable) distributions 68
4.2.5 Multivariate normal distributions 68
4.3 Estimating a mean and its uncertainty 69
4.3.1 Average values: the sample mean, sample variance, and sample standard deviation 70
4.3.2 Average values: the standard error of the mean 70
4.3.3 Application: accurate standard errors for mass spectrometry 71
4.3.4 Correlation, covariance, and the covariance matrix 73
4.3.5 Degrees of freedom, part 1: the variance 73
4.3.6 Degrees of freedom, part 2: Student's t distribution 73
4.3.7 The weighted mean 75
4.4 Regressing a line 76
4.4.1 Ordinary least-squares linear regression 76
4.4.2 Weighted least-squares regression 77
4.4.3 Linear regression with uncertainties in two or more variables (York regression) 77
4.5 Interpreting measured data using the mean square weighted deviation 79
4.5.1 Testing a weighted mean's assumptions using its MSWD 79
4.5.2 Testing a linear regression's assumptions using its MSWD 80
4.5.3 My data set has a high MSWD-what now? 81
4.5.4 My data set has a really low MSWD-what now? 81
4.6 Conclusions 82
4.7 Bibliography and suggested readings 82
5 Diffusion and thermochronologic interpretations 83
5.1 Fundamentals of heat and chemical diffusion 83
5.1.1 Thermochronologic context 83
5.1.2 Heat and chemical diffusion equation 83
5.1.3 Temperature dependence of diffusion 85
5.1.4 Some analytical solutions 86
5.1.5 Anisotropic diffusion 86
5.1.6 Initial infinite concentration (spike) 86
5.1.7 Characteristic length and time scales 86
5.1.8 Semi-infinite media 87
5.1.9 Plane sheet, cylinder, and sphere 88
5.2 Fractional loss 88
5.3 Analytical methods for measuring diffusion 89
5.3.1 Step-heating fractional loss experiments 89
5.3.2 Multidomain diffusion 92
5.3.3 Profile characterization 93
5.4 Interpreting thermal histories from thermochronologic data 94
5.4.1 "End-members" of thermochronometric date interpretations 94
5.4.2 Equilibrium dates 95
5.4.3 Partial retention zone 95
5.4.4 Resetting dates 96
5.4.5 Closure 97
5.5 From thermal to geologic histories in low-temperature thermochronology: diffusion and advection of heat in the earth's crust 105
5.5.1 Simple solutions for one- and two-dimensional crustal thermal fields 107
5.5.2 Erosional exhumation 108
5.5.3 Interpreting spatial patterns of erosion rates 109
5.5.4 Interpreting temporal patterns of erosion rates 113
5.5.5 Interpreting paleotopography 113
5.6 Detrital thermochronology approaches for understanding landscape evolution and tectonics 116
5.7 Conclusions 121
5.8 References 123
6 Rb-Sr, Sm-Nd, and Lu-Hf 127
6.1 Introduction 127
6.2 History 127
6.3 Theory, fundamentals, and systematics 128
6.3.1 Decay modes and isotopic abundances 128
6.3.2 Decay constants 128
6.3.3 Data representation 129
6.3.4 Geochemistry 131
6.4 Isochron systematics 133
6.4.1 Distinguishing mixing lines from isochrons 136
6.5 Diverse chronological applications 137
6.5.1 Dating diagenetic minerals in clay-rich sediments 137
6.5.2 Direct dating of ore minerals 138
6.5.3 Dating of mineral growth in magma chambers 140
6.5.4 Garnet Sm-Nd and Lu-Hf dating 141
6.6 Model ages 143
6.6.1 Model ages for volatile depletion 144
6.6.2 Model ages for multistage source evolution 146
6.7 Conclusion and future directions 148
6.8 References 148
7 Re-Os and Pt-Os 151
7.1 Introduction 151
7.2 Radioactive systematics and basic equations 151
7.3 Geochemical properties and abundance in natural materials 154
7.4 Analytical challenges 154
7.5 Geochronologic applications 156
7.5.1 Meteorites 156
7.5.2 Molybdenite 158
7.5.3 Other sulphides, ores, and diamonds 159
7.5.4 Organic-rich sediments 161
7.5.5 Komatiites 161
7.5.6 Basalts 163
7.5.7 Dating melt extraction from the mantle-Re-Os model ages 164
7.6 Conclusions 167
7.7 References 167
8 U-Th-Pb geochronology and thermochronology 171
8.1 Introduction and background 171
8.1.1 Decay of U and Th to Pb 171
8.1.2 Dating equations 173
8.1.3 Decay constants 173
8.1.4 Isotopic composition of U 174
8.2 Chemistry of U, Th, and Pb 176
8.3 Data visualization, isochrones, and concordia plots 176
8.3.1 Isochron diagrams 176
8.3.2 Concordia diagrams 177
8.4 Causes of discordance in the U-Th-Pb system 178
8.4.1 Mixing of different age domains 180
8.4.2 Pb loss 180
8.4.3 Intermediate daughter product disequilibrium 182
8.4.4 Correction for initial Pb 183
8.5 Analytical approaches to U-Th-Pb geochronology 184
8.5.1 Thermal ionization mass spectrometry 185
8.5.2 Secondary ion mass spectrometry 187
8.5.3 Laser ablation inductively coupled plasma mass spectrometry 188
8.5.4 Elemental U-Th-Pb geochronology by EMP 188
8.6 Applications and approaches 188
8.6.1 The age of meteorites and of Earth 188
8.6.2 The Hadean 192
8.6.3 P-T-t paths of metamorphic belts 194
8.6.4 Rates of crustal magmatism from U-Pb geochronology 197
8.6.5 U-Pb geochronology and the stratigraphic record 200
8.6.6 Detrital zircon geochronology 202
8.6.7 U-Pb thermochronology 204
8.6.8 Carbonate geochronology by the U-Pb method 209
8.6.9 U-Pb geochronology of baddeleyite and paleogeographic reconstructions 211
8.7 Concluding remarks 212
8.8 References 212
9 The K-Ar and 40Ar/39Ar systems 231
9.1 Introduction and fundamentals 231
9.2 Historical perspective 232
9.3 K-Ar dating 233
9.3.1 Determining 40Ar* 233
9.3.2 Determining 40K 234
9.4 40Ar/39Ar dating 234
9.4.1 Neutron activation 234
9.4.2 Collateral effects of neutron irradiation 237
9.4.3 Appropriate materials 240
9.5 Experimental approaches and geochronologic applications 242
9.5.1 Single crystal fusion 242
9.5.2 Intragrain age gradients 243
9.5.3 Incremental heating 243
9.6 Calibration and accuracy 248
9.6.1 40K decay constants 248
9.6.2 Standards 249
9.6.3 So which is the best calibration? 250
9.6.4 Interlaboratory issues 252
9.7 Concluding remarks 252
9.7.1 Remaining challenges 252
9.8 References 253
10 Radiation-damage methods of geochronology and thermochronology 259
10.1 Introduction 259
10.2 Thermal and optically stimulated luminescence 259
10.2.1 Theory, fundamentals, and systematics 259
10.2.2 Analysis 260
10.2.3 Fundamental assumptions and considerations for interpretations 264
10.2.4 Applications 265
10.3 Electron spin resonance 266
10.3.1 Theory, fundamentals, and systematics 266
10.3.2 Analysis 267
10.3.3 Fundamental assumptions and considerations for interpretations 268
10.3.4 Applications 269
10.4 Alpha decay, alpha-particle haloes, and alpha-recoil tracks 270
10.4.1 Theory, fundamentals, and systematics 270
10.5 Fission tracks 273
10.5.1 History 273
10.5.2 Theory, fundamentals, and systematics 273
10.5.3 Analyses 274
10.5.4 Fission-track age equations 276
10.5.5 Fission-track annealing 278
10.5.6 Track-length analysis 280
10.5.7 Applications 281
10.6 Conclusions 284
10.7 References 285
11 The (U-Th)/He system 291
11.1 Introduction 291
11.2 History 291
11.3 Theory, fundamentals, and systematics 292
11.4 Analysis 294
11.4.1 "Conventional" analyses 294
11.4.2 Other analytical approaches 306
11.4.3 Uncertainty and reproducibility in (U-Th)/He dating 307
11.5 Helium diffusion 310
11.5.1 Introduction 310
11.5.2 Apatite 311
11.5.3 Zircon 322
11.5.4 Other minerals 332
11.5.5 A compilation of He diffusion kinetics 334
11.6 4He/3He thermochronometry 342
11.6.1 Method requirements and assumptions 346
11.7 Applications and case studies 348
11.7.1 Tectonic exhumation of normal fault footwalls 348
11.7.2 Paleotopography 349
11.7.3 Orogen-scale trends in thermochronologic dates 350
11.7.4 Detrital double-dating and sediment provenance 353
11.7.5 Volcanic double-dating, precise eruption dates, and magmatic residence times 353
11.7.6 Radiation-damage-and-annealing model applied to apatite 355
11.8 Conclusions 355
11.9 References 356
12 Uranium-series geochronology 365
12.1 Introduction 365
12.2 Theory and fundamentals 367
12.2.1 The mathematics of decay chains 367
12.2.2 Mechanisms of producing disequilibrium 369
12.3 Methods and analytical techniques 369
12.3.1 Analytical techniques 369
12.4 Applications 372
12.4.1 U-series dating of carbonates 372
12.4.2 U-series dating in silicate rocks 378
12.5 Summary 389
12.6 References 390
13 Cosmogenic nuclides 395
13.1 Introduction 395
13.2 History 395
13.3 Theory, fundamentals, and systematics 396
13.3.1 Cosmic rays 396
13.3.2 Distribution of cosmic rays on Earth 396
13.3.3 What makes a cosmogenic nuclide detectable and useful? 397
13.3.4 Types of cosmic-ray reactions 398
13.3.5 Cosmic-ray attenuation 399
13.3.6 Calibrating cosmogenic nuclide-production rates in rocks 400
13.4 Applications 401
13.4.1 Types of cosmogenic nuclide applications 401
13.4.2 Extraterrestrial cosmogenic nuclides 401
13.4.3 Meteoric cosmogenic nuclides 402
13.5 Conclusion 415
13.6 References 416
14 Extinct radionuclide chronology 421
14.1 Introduction 421
14.2 History 422
14.3 Systematics and applications 423
14.3.1 26Al-26Mg 423
14.3.2 53Mn-53Cr chronometry 425
14.3.3 107Pd-107Ag 428
14.3.4 182Hf-182W 430
14.3.5 I-Pu-Xe 433
14.3.6 146Sm-142Nd 436
14.4 Conclusions 441
14.5 References 441
Index 445
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.
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 problems, like rates of erosion, sedimentation, crystallization, or groundwater flow, the degree to which these processes are steady or episodic, and the scale at which these questions even make sense.
There is no denying geochronology's utility for addressing some of the most fundamental and, in many cases, simple questions in Earth and planetary science. This is true in both a historical sense, as geochronology provided key foundations for geoscience progress over the century, as well as in a continuing sense, as it continues to provide simple formation and cooling ages essential to many geologic studies. So it is reasonable to begin here with a review...
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