- © 2013 by the Mineralogical Society of America
High-precision geochronology is integral to testing hypotheses regarding the correlation, causes, and rates of events and processes in Earth history. Recent studies have sought to reconcile very precise, but apparently conflicting, ages for the same geological samples and events using different chronometers. Both systematic (decay constants, ages of standard materials) and geological (daughter-nuclide loss, inheritance) complexities contribute to the challenges of rock-clock calibration. Community-wide efforts to improve radioisotope geochronology have successfully mitigated many of these factors, and have brought high-precision geochronology to a threshold of unprecedented integration with stratigraphic and geochemical proxies of Earth systems dynamics.
Geochronology places our observations of the Earth into a robust temporal framework. Studies of the Earth's processes, phenomena, and origins are unique in the physical sciences for their need to observe and measure past events over spatial and temporal scales that range from the micron and second to the astronomical unit and gigaannum. The radioactive decay of unstable atomic nuclei is the basis of the geochronometers used by Holmes (1913) to first calibrate the geologic timescale using measurements of U/Pb ratios in minerals. Radioisotopic dating has matured over the past century into a vibrant field of enquiry, exploring and testing hypotheses regarding the global correlation of events, causal mechanisms for dramatic changes in the Earth system, and the rates of geological, paleobiological, geophysical, and geochemical processes.
What Drives High-Precision Geochronology?
Some Earth processes can be studied in the recent past or via methods that have intrinsic high temporal resolution—for example, using short-lived nuclides to develop chronologies for the Quaternary (Richards and Andersen 2013 this issue) and the early Solar System (Amelin and Ireland 2013 this issue). Other aspects of the Earth system, like plate motion and orogenesis, are integrated over longer timescales, thus skirting the need for precise ages (Nemchin et al. 2013 this issue). But many phenomena occur over timescales of only thousands of years, and yet took place tens or even hundreds of millions of years ago in Earth's past—in the “deep time” record. Because the duration is short relative to the long span of time since the phenomenon occurred, its resolution demands a much greater relative precision and accuracy for geochronological measurements. As an example, Earth's orbital short eccentricity cycle (variation in shape of the Earth's orbit around the sun) has an average period of about 100,000 years, and resolving this cycle as a driver of Pleistocene glaciations requires a precision of just a few percent of the absolute age. By contrast, resolving the same cyclicity in the 300-million-year-old rocks of the Late Paleozoic ice age requires a relative precision 100 times greater.
Over the past two decades, paleoclimate and paleobiological studies have spurred the science of high-precision geochronology forward because of their demands for multimillennial-scale resolution of orbital cycles and biological extinction or evolutionary mechanisms in deep time. Similarly, igneous petrology and volcanology have also driven advances in high-precision geochronology, for example, in the study of the role of melting and differentiation processes operating during the millennial-scale lifetimes of magma batches in the mantle and crust. In these fields of enquiry, the targets for geochronology are often the eruptive products of explosive volcanism—volcanic “ash” or tuff beds that create geologically instantaneous, datable marker horizons in a variety of volcanic and sedimentary successions. Precise and accurate radioisotopic ages of these volcanic ash beds in combination with the stratigraphic, paleobiological and geochemical information from the rock record provide geoscientists with tools to unravel the history of the Earth in critical intervals of geologic time. This article focuses on the state-of-the-art methods in high-precision geochronology that have evolved to meet the challenges of studying the deep-time rock record.
Rock Clocks in Crisis?
The premier high-precision geochronometers are the U–Pb method (particularly 238U decay to 206Pb) in zircon (see Box 1 and Fig. 1) and the 40Ar/39Ar method (based on 40K decay to 40Ar) in sanidine feldspar. The strengths of these two methods include low initial daughter nuclide contents, a propensity for closed-system behavior and the availability of means for quantitatively assessing it, and our ability to precisely measure the parent/daughter ratio in these minerals. State-of-the-art measurements of both chronometers can yield precisions on the order of ≤0.1% in the age of these minerals.
Box 1 WHAT IS “HIGH-PRECISION” GEOCHRONOLOGY?
A variety of analytical methodologies can now analyze U–Pb isotope ratios in accessory minerals like zircon. In situ techniques, like laser ablation inductively coupled mass spectrometry (LA–ICP–MS) and secondary ion mass spectrometry (SIMS), with inherently high spatial resolution and rapid analysis times, can produce large data sets with individual spot and group weighted mean isotope ratios (and interpreted ages) of ±3–5% and ±1% relative precision, respectively. High-precision geochronology relies on isotope dilution thermal ionization mass spectrometry (ID-TIMS), a technique that can provide individual crystal-fragment and weighted mean isotope ratios (and interpreted ages) of ±0.1% and ±0.03% relative precision, respectively. In situ and ID-TIMS techniques are complementary approaches best applied, whether alone or in tandem, to crystals characterized for their internal complexity using imaging techniques like cathodoluminescence.
Notwithstanding the potential of these chronometers, the past decade has seen intense scrutiny of radioisotopic methods and ages as the scientific demands on precision and accuracy have increased. Particular attention has been focused on the accuracy of radioisotope decay constants (Min et al. 2000; Begemann et al. 2001; Schoene et al. 2006; Mattinson 2010) and the accuracy of isotopic measurements and their age interpretation (Mundil et al. 2001; Kuiper et al. 2008). Schoene et al. (2013 this issue) describe the metrologic framework that must underpin the measurement and decay systematics of highly precise radioisotope methods.
The emergence of an apparent “crisis” in high-precision geochronology stemmed from published differences between results from different radioisotope methods—or from the same methods measured in different laboratories—for the same critical samples and events in Earth history. Three examples of this discordance are particularly noteworthy: the Permo-Triassic extinction recorded in the boundary sections of South China; the astronomical tuning of the cyclostratigraphy of Miocene sediments in the Mediterranean Basin; and the magmatic history and eruption of the Bishop Tuff, the archetypical eruption product of a zoned, large-volume, silicic magma chamber.
Each of these examples is a case study in the evolution of high-precision geochronology, and together they illustrate the progress that has been made over the past decade in reconciling rock clocks.
THE PERMO-TRIASSIC EXTINCTION EVENT: A WATERSHED FOR U–PB GEOCHRONOLOGY
Geochronology is uniquely suited to testing hypotheses regarding cause and effect. The historically strong focus on stratigraphic boundaries like the Permo-Triassic or the Cretaceous–Paleogene stems in large part from the fact that those boundaries represent extraordinary reorganizations of the Earth system. Testing the competing hypotheses for the major biological extinction events accompanying these reorganizations has been a major goal of high-precision geochronology.
As host to the greatest biological extinction event in Earth history, the Permo-Triassic (P-T) boundary transition has been scrutinized by geologists for decades. The boundary sections in marine strata of South China provide some of the most complete records of the lead-up to, and recovery from, this extinction event. These strata are unique because they host both a rich pre- and postextinction biota and abundant volcanic ash beds, and these can be used to constrain the timing and rates of change across the boundary.
Claoué-Long et al. (1991) used ion probe (SIMS) analyses of the “boundary clay” bentonite bed to establish the first U–Pb zircon age of 251.2 ± 3.4 Ma. Subsequent 40Ar/39Ar feldspar step-heating analyses of the same boundary beds were reported by Renne et al. (1995), who commented on the synchrony of their apparent age—249.98 ± 0.20 Ma—with the beginning of Siberian Traps volcanism at 250.0 ± 1.6 Ma, also dated via 40Ar/39Ar analysis. Bowring et al. (1998) were the first to bring high-precision U–Pb ID-TIMS dates on zircon to bear on the problem, and they interpreted the age of the boundary as 251.4 ± 0.3 Ma on the basis of a large data set of physically abraded, single- and multigrain zircon analyses. These results provided the first glimpse of a potential systematic discrepancy between the U–Pb and 40Ar/39Ar rock clocks. Subsequent ID-TIMS U–Pb zircon studies (Mundil et al. 2001) questioned the ability to precisely date the boundary because of the apparent ubiquity of open-system behavior in bentonite-hosted zircon. Perhaps just as important for U–Pb geochronology, these studies also highlighted the averaging biases inherent to multigrain analyses containing crystals suffering from both Pb loss and Pb inheritance—the result being an overestimation of age precision and accuracy.
An unexpected analytical breakthrough would overcome this impasse for U–Pb geochronology of the P-T boundary. The chemical abrasion (CA-TIMS) method of Mattinson (2005) overcomes leaching artefacts to provide a more selective and effective chemical method for removing Pb-loss domains from zircon crystals compared with traditional physical abrasion. Mundil et al. (2004) demonstrated the effectiveness of CA-TIMS for zircon from the Meishan and Shangsi boundary sections in South China, and they arrived at a more robust age for the P-T extinction event of 252.6 ± 0.2 Ma. Most recently, Shen et al. (2011) revisited the same ash beds using the chemical abrasion technique and applying improvements in analytical blank and ionization efficiency. Their results demonstrate the reproducibility of the CA-TIMS method among laboratories (Fig. 2), and they have refined the age estimate for the extinction event to just before 252.28 ± 0.08 Ma.
The 40Ar/39Ar method was also in flux during this period of rapid innovation in U–Pb geochronology. Researchers carrying out experiments on the intercalibration of the 40Ar/39Ar chronometer with both U–Pb ages (Min et al. 2000) and astrochronology (Kuiper et al. 2008) were arriving at similar conclusions—that the consensus values of the 40K decay constants used since the late 1970s are likely in error by ~1% (see Box 2). Renne et al. (2010) used pairs of high-precision U–Pb and 40Ar/39Ar ages for selected rocks to arrive at a joint determination of the 40K decay constants and the radiogenic argon-to-potassium ratio (40Ar*/40K) of the most commonly used irradiation monitor, the Fish Canyon Tuff sanidine (Fig. 3). Using their intercalibration produces a 40Ar/39Ar age for the P-T extinction of 252.27 ± 0.18 Ma, in remarkable agreement with the U–Pb zircon ages for Bed 25 in the Meishan section.
The Permo-Triassic boundary is also an excellent example of the integration of quantitative biostratigraphy and high-precision geochronology. Several computationally intensive approaches, including constrained optimization and matrix permutation (Sadler 2004), provide a means of quantitatively correlating disparate stratigraphic sections or fossil occurrences into an ordinal framework including robust uncertainty assessment. Constrained optimization (using the program CONOP9) is a computer-aided method for finding the best-fit ordering of biological events and marker beds in multiple stratigraphic sections. The method incorporates constraints from stratigraphic thickness and radioisotopically dated event beds. Shen et al. (2011) applied this method to the South China boundary sections, sequencing over 1450 species occurrences from 18 stratigraphic sections with high-precision U–Pb zircon ages. This quantitative combination of bio- and geochronology constrains an absolute maximum of 200 ± 100 ky for the duration of the major extinction event, and demonstrates synchrony between an abrupt (<20 ky duration) negative carbon isotope excursion and the beginning of the extinction event.
This intensive study of the Permo-Triassic boundary has irrevocably changed the practice of high-precision U–Pb geochronology by promoting (1) the exclusive analysis of single crystals, (2) a switch from physical to chemical abrasion, and (3) the scrutiny and recalibration of tracer solutions and decay constants. These studies also motivated developments in 40Ar/39Ar geochronology, driven by intercomparison
Box 2 SYSTEMATIC AGE BIAS AND DECAY CONSTANTS
Several 40Ar/39Ar and U–Pb geochronological studies have highlighted discrepancies at the percent level between the eruption ages for silicic volcanics estimated by the two methods—with U–Pb zircon ages yielding consistently older ages. Two explanations for this discrepancy have been proposed: either historic calibrations of widely used 40Ar/39Ar standards and/or the consensus values for the 40K decay constants are significantly in error (Min et al. 2000), or zircon crystallization in silicic magmas occurs well before eruption (Simon et al. 2008). Recent compilations of data for quickly cooled volcanic and shallow plutonic rocks now quantify a ~1.0% younger bias in 40Ar/39Ar dates due to inaccuracy in the 40K decay constants (Schoene et al. 2006; Renne et al. 2010). This bias has been demonstrated in rocks whose relative ages render magmatic residence irrelevant, and several recent studies have proposed new decay constants and/or monitor standard ages on the basis of intercalibration with either U–Pb ages (Renne et al. 2010) or astronomical dating (Kuiper et al. 2008; Rivera et al. 2011).
ASTROCHRONOLOGY AND RADIOISOTOPES: BRINGING 40AR/39AR INTO HARMONY
The geologic timescale is a fundamental tool for correlating and establishing the rate of Earth events. In reality, the geologic timescale is a composite of both relative and numerical dating techniques, stitched together with varying degrees of intercalibration. For much of the Cenozoic and Mesozoic, a combination of cyclostratigraphy or “astrochronology” and radioisotopically calibrated magnetostratigraphy provides the backbone of the geologic timescale.
Astrochronology is based on the correlation or tuning of cyclic sedimentary successions (cyclostratigraphy) to astronomical target curves (e.g. precession, obliquity, eccentricity, insolation) that are computed based on astronomical solutions for the Solar System (Laskar et al. 2011). Oxygen isotope compositions of deep-sea sediment cores correlated to astronomically derived target curves led to the first commonly employed high-resolution timescale for the last 800,000 years (the SPECMAP project) (Imbrie et al. 1984). Neogene sediments of the Mediterranean Basin also preserve a robust signal of orbital cyclicity in marl–sapropel alternations and have provided an astrochronological calibration for the entire Neogene timescale (Hilgen et al. 2012).
The 40Ar/39Ar method relies on the reproducible analysis of a standard material with a known age and a homogeneous ratio of radiogenic daughter 40Ar* to parent 40K, in order to calibrate the neutron fluence controlling the 39Ar production reaction in a sample. Primary standards are those materials that have absolute ages determined by the K–Ar or other methods, while secondary standards are those materials whose ages are known based on 40Ar/39Ar intercalibration with primary standards. A surprisingly limited number of primary standards—mainly biotite and hornblende—have their ages linked to first principles 40K/40Ar measurements, and unfortunately, even the best primary standard K–Ar analyses cannot achieve the potential precision of 40Ar/39Ar ratio analyses. Instead, select volcanic sanidine crystal populations—notably including the Fish Canyon Tuff (FCT)—are the most reproducible and commonly used secondary standard materials for 40Ar/39Ar analysis. However, determining the absolute age of FCT sanidine has proven a difficult task, in part because sanidine is difficult to quantitatively degas and thus cannot be confidently dated by the K–Ar method. Other studies have illustrated the difficulties in calibrating the age of FCT sanidine directly to the 238U decay constant using ages for U-bearing accessory minerals (Bachmann et al. 2007), due to the petrologic complexity of the storage and eruption dynamics of the Fish Canyon magma body.
An alternative approach is to intercalibrate the 40Ar/39Ar chronometer with astronomical time as recorded by cyclostratigraphy. Kuiper et al. (2008) measured the 40Ar*/40K of populations of sanidine grains from several tuffs intercalated within an astronomically tuned marine succession of the Messinian Melilla Basin in Morocco. Using the astronomical ages for seven tuffs, they arrived at an absolute age for the FCT sanidine standard of 28.201 ± 0.046 Ma, incorporating all sources of analytical and systematic uncertainty. This result has been recently confirmed by Rivera et al. (2011) using a tuff layer in a Messinian astronomically tuned section in Crete. This new age for the FCT sanidine succeeds in bringing a majority of U–Pb and 40Ar/39Ar age pairs for the same volcanic and quickly cooled plutonic rocks into agreement, thus harmonizing astrochronology and the two premier radioisotope chronometers.
High-precision geochronology depends on standard materials to ensure the accurate translation of isotope ratios into isotopic “dates.” However, just as important to geochronological interpretation are the basic assumptions regarding the geologic significance of the “ages” generated from these dates. This latter challenge weaves itself into our next example.
THE BISHOP TUFF: WHAT ARE WE DATING?
Few volcanic deposits have inspired more discussion of the timescales of silicic-magma residence and differentiation than the Bishop Tuff of Long Valley caldera, California. The eruptive products of this volcanic center have been used to test models in which the longevity of large silicic magma chambers is explained by repeated injection of mafic magmas and/or the rejuvenation of crystal mushes; the models include predictions that the crystal load of silicic magmas could have formed tens or hundreds of thousands of years before eruption. Early estimates of up to 0.7 My for the lifetime of active silicic magma chambers in the Long Valley caldera system based on Sr isotope model ages and 40Ar/39Ar measurements on quartz-hosted melt inclusions have subsequently been refined to shorter residence times of <100 ky by in situ analyses of zircon (Simon and Reid 2005; Reid 2008).
The Bishop Tuff illustrates how different mineral–isotope chronometers may be dating different events or processes. For example, 40Ar/39Ar ages for sanidine feldspar are usually interpreted to date eruption, as this mineral can accumulate radiogenic Ar only after posteruptive cooling. By contrast, zircon begins to crystallize in Zr-saturated silicic magmas tens of degrees above commonly observed eruption temperatures. Combined with its propensity to retain accumulating radiogenic Pb even at the high temperatures of silicic-magma genesis, U–Pb ages for domains in zircon crystals could span a range encompassing progressive differentiation of the host magma. These potential differences between chronometers are both a boon and a liability. On the one hand, the intercomparison of U–Pb and 40Ar/39Ar ages has brokered a rich field of research into silicic-magma dynamics—one seeking to disentangle the operative petrologic and volcanic processes via the geochemical, isotopic, and age archives in single zircon crystals. On the other hand, a potential ≤100 ky overestimation of zircon-based eruption and deposition ages for volcanic tephras has been proposed from a recent compilation of ion microprobe U–Pb zircon ages for silicic volcanic rocks compared to their 40Ar/39Ar-based eruption ages (Simon et al. 2008).
Do preeruptive ages for zircon record the protracted hypersolidus evolution of single or coeval magma batches? Or is zircon, by its refractory nature, prone to recycling from earlier-crystallized magmas? Some ion probe U–Th and U–Pb zircon investigations of Pleistocene ash-flow tuffs and lavas have concluded that silicic magma bodies may remain as crystal mushes for up to hundreds of thousands of years prior to eruption (Schmitt 2011). However, other studies of Pleistocene to Miocene eruptive rocks have emphasized crystal recycling by rapid melting and assimilation of earlier-formed plutons or volcanic caldera fill (Bindeman et al. 2001). These contrasting models for the crystal cargo of silicic volcanics have profound implications for high-precision geochronology, as zircon grown from rapidly formed and segregated magmas would more accurately date eruption, if methods could be derived to isolate the magmatic from antecedent crystals.
In the products of the Bishop Tuff eruption, Reid and Coath (2000) and Simon and Reid (2005) interpreted the dispersion of spot ion probe U–Pb ages in zircon crystals as indicating at least 90 ky of magma differentiation and zircon crystallization prior to the climactic eruption dated by sanidine 40Ar/39Ar ages at 768 ka (Fig. 4a, b). Subsequently, Crowley et al. (2007) overcame challenges of analytical blank and ionization to present the first high-precision ID-TIMS results for zircon crystals from the Bishop Tuff. In contrast to the dispersion in ion probe ages, these ID-TIMS results are much more precisely clustered, with the majority of single crystals and crystal halves (polished and imaged by cathodoluminescence—Fig. 4c) recording an apparently short crystallization interval at 767.1 ± 0.9 ka, within a few thousand years of the 40Ar/39Ar-constrained eruption age (Sarna-Wojcicki et al. 2000). This short timescale of differentiation is in agreement with independent estimates based on quartz zoning, melt inclusions, crystal size distributions, and numerical models (Gualda et al. 2012). Rivera et al. (2011) recently reported a refined 40Ar/39Ar sanidine age of 767.4 ± 0.2 ka for the Bishop Tuff eruption, which further supports rapid zircon crystallization just prior to the climactic eruption.
The jury is still out on how to interpret the differences between the ID-TIMS and ion probe zircon measurements. Setting aside possible systematic analytical difficulties with either U–Pb methodology, it could be argued that greater analytical precision of individual measurements is important for recognizing analytical versus geological dispersion within the probability density functions of different experiments (including comparisons to 40Ar/39Ar results; Fig. 4b). Alternatively, reconciliation may lie in the different volume (ID-TIMS) versus surface (ion probe) averaging of the two techniques. By their sampling nature, ID-TIMS analyses will tend to emphasize volumetrically dominant late zircon growth, while essentially two-dimensional ion probe spots on polished-grain interiors are more sensitive to intracrystalline age variation in crystal cores (Fig. 4d). This hypothesis can be tested in the future with intracrystal ID-TIMS sampling and innovative crystal surface and serial depth-profiling ion probe work (Reid et al. 2010). In conclusion, the example of the Bishop Tuff suggests a rich potential for the next generation of complementary 40Ar/39Ar, ion probe, and ID-TIMS studies of Quaternary volcanic systems. These studies will likely explore more sophisticated modeling of the averaging effects of different sampling techniques, as well as the integration of geochemical, thermometric, and geochronological data via ion probe, LA–ICP–MS, and ID-TIMS.
WORKING TOGETHER: THE EARTHTIME COMMUNITY
Viewed in isolation, each of these case studies reflects the challenges of making and interpreting geochronologic measurements, and each was important in driving innovations in both scientific practice and thought. Yet each of these studies can also be viewed as a motif within the overarching response of the scientific community to the challenges of high-precision geochronology.
The response to these challenges has been arguably one of the most remarkable outpourings of synergetic activity in the past quarter century of isotope geochronology. Beginning in 2003, a series of community workshops under the moniker “EARTHTIME Initiative” have fostered (1) unprecedented cooperation among geochronologists to resolve interlaboratory and interchronometer calibration, sample handling, and data-analysis issues; and (2) the community-wide involvement of stratigraphers, geochronologists, geochemists, magnetostratigraphers, and paleontologists, with the goal of producing a highly calibrated geologic timescale as a new temporal stratigraphic framework for geoscience research (www.earth-time.org).
In bringing together a critical mass of scientific expertise across methodologies and disciplines, these workshops set the stage for intensive efforts to identify and mitigate interlaboratory biases through group intercalibration experiments. An international quorum of high-precision U–Pb and 40Ar/39Ar laboratories participated in these experiments, analyzing a variety of reference materials. In the U–Pb community, an initial experiment in 2005 using natural zircon gave one of the first clear snapshots of the significant variations among laboratories. This experiment was a key motivator for subsequent innovations sponsored by EARTHTIME, including the production of synthetic Pb–U isotope solutions for assessing mass spectrometer performance, and one of the most ambitious efforts in the history of isotope ratio measurements to procure, mix, and distribute a common set of carefully calibrated, enriched-isotope tracer solutions for isotope dilution analysis. Armed with these tracer and synthetic standard solutions, the community embarked upon a second intercalibration experiment in 2008, which demonstrated that 0.05% reproducibility was achievable among and between different laboratories.
In the 40Ar/39Ar community, two intercalibration experiments using three common sanidine standards have revealed a surprising amount of dispersion (>1%) among results from over a dozen established 40Ar/39Ar laboratories. A new experiment with a traveling pipette system promises to isolate the analytical sources of this dispersion in the near future. Analytical innovations with the next generation of multicollector mass spectrometers have the potential to eliminate sources of variance (Coble et al. 2011).
The EARTHTIME Initiative has also fostered a new style of collaborative study among geochronologists, stratigraphers, and paleobiologists. Multidisciplinary teams of scientists are now tackling key intervals in Earth history, combining geochronological methods and innovative stratigraphic and geochemical analyses.
What is the future of high-precision geochronology? The geochronological community has emerged from the aforementioned “crisis” with a new set of tools and methodologies and a broadened base of scientists. Can we contemplate how these resources will be used to study the Earth system over the next 100 years of geochronology?
In 40Ar/39Ar geochronology, a new generation of multicollector mass spectrometers will soon be in widespread use. These instruments, with higher mass resolution and ion sensitivity, will likely help to elucidate and promote interlaboratory reproducibility. Much like in U–Pb zircon geochronology, the greater analytical precision of these instruments will help to reveal and interpret geological complexities in crystal populations. 40Ar/39Ar geochronology will also continue to grow into its role as an important method for dating Pleistocene materials and processes. The lessons learned from high-precision 40Ar/39Ar analysis will also benefit its application to thermochronology.
Frontier problems critical to the petrologic and chronostratigraphic application of U–Pb geochronology revolve around the origins of complex mineral age spectra in silicic volcanic and plutonic rocks, and the precise link between the ages recorded by zircon crystals and the thermal evolution of their magmas. Answers to some of these questions will likely come from the integration of in situ and isotope dilution techniques and various age, thermobarometric, and geochemical proxies on an intragrain scale.
Within the context of these continued innovations in high-precision geochronology, the original goal of EARTHTIME—the high-resolution calibration of the past 600 million years of Earth history—appears to be within reach. As the geological and methodological barriers to precise and accurate ages are overcome, a broad community of interdisciplinary scientists will pursue new opportunities to combine high-precision geochronology with other quantitative disciplines in order to develop high-resolution timescales. The next 100 years of isotope geochronology look to be exciting ones!
The authors thank the guest editors for their invitation to contribute this article and for their deft handling of the manuscript, which benefited from thoughtful reviews by Troy Rasbury, Mary Reid, and Urs Schaltegger.