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James M. Mattinson The geochronology revolution Geological Society of America Special Papers

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303

The Geological Society of America

Special Paper 500

2013

The geochronology revolution

James M. Mattinson*

Department of Earth Science, University of California, Santa Barbara, California 93106-9630, USA

ABSTRACT

1896 marked the beginning of a decade that spawned both modern physics and

the science of geochronology based on radioactive decay. The decade started with the

discovery of radioactivity by Henri Becquerel in 1896, and ended with the formal pub-

lication of ages for natural mineral samples by Ernest Rutherford in 1906. The next

fifty years witnessed the discovery of isotopes and nuclear fission; the development of

the mass spectrograph and the mass spectrometer; application of the isotope dilution

method to dating trace, accessory, and major minerals in typical crustal rocks; and

publication of the ca. 4.55 Ga age for the Earth. Yet, after all this, geochronology was

still viewed with suspicion by some geologists. In the past fifty years, with additional

major advances in instrumentation, technique, and interpretation, geochronology is

fully integrated into almost all fields of geology. The three major dating methods from

the 1950s and 1960s, U-Pb, K-Ar, and Rb-Sr, have been refined repeatedly. In particu-

lar, U-Pb and Ar-Ar, a modern variant of K-Ar, are now capable of <0.1% precisions,

with spectacular results in recent studies of crucial problems such as the exact timing

and duration of mass extinctions. Many new methods are now available to attack

problems ranging from rates of metamorphic mineral growth to rates of uplift and

erosion, to the time of surface exposure of geomorphic surfaces. It is a good time to bea geochronologist, or to collaborate with one or more. The future looks very bright.

*[email protected]

Mattinson, J.M., 2013, The geochronology revolution, in Bickford, M.E., ed., The Web of Geological Sciences: Advances, Impacts, and Interactions: GeologicalSociety of America Special Paper 500, p. 303–320, doi:10.1130/2013.2500(09). For permission to copy, contact [email protected]. © 2013 The GeologicalSociety of America. All rights reserved.

INTRODUCTION

Fifty years ago, at the time of the previous GSA commemo-

rative book volume celebrating the 75th anniversary of The Geo-

logical Society of America, geochronology and other quantita-

tive methods were not always welcomed with open arms by the

geological community. Writing in that 75th anniversary volume,

J. Hoover Mackin began his contribution as follows: “Most of usare concerned, and some of us have strong feelings, pro or con,

about what has been happening to geology in the past 25 years:

greatly increased use of nongeologic techniques in the solution of

geologic problems, such as dating by radioisotope methods…. At

meetings of our societies, when older brethren gather together in

hotel rooms after technical sessions, the discussion usually comes

around to these changes. There are apt to be sad postmortems

for certain departments, once powerful, which are now, owing

to the retirement or flight of their older stalwarts, largely staffed

by dial twisters and number jugglers” (Mackin,1963, p. 135).

Mackin (1963) wisely went on to caution the old-timers againstblindly resisting all quantitative methods, and the young “dial

twisters and number jugglers” against ignoring sound classical

geological reasoning in interpreting their numerical data. Now, in

CELEBRATING ADVANCES IN GEOSCIENCE

1 8 8 8 2 0 1 38 2



304 Mattinson

the modern era of geology, it is hard to imagine research in petrol-

ogy, tectonics, stratigraphy, paleobiology, and indeed, the geologic

time scale itself, that is not robustly based on geochronology.

In this chapter, after a brief review of the early roots of geo-

chronology, I will discuss some of the spectacular advances in

techniques and instrumentation of the past 50 years that have led

to the present state-of-the-art of geochronology, and its integra-tion into almost all fields of geology.

BRIEF HISTORY OF DEVELOPMENT OF

GEOCHRONOLOGY: 1896–1963

This account of the early years of discovery and research

into radioactivity and its application to geochronology is drawn

from original papers where specifically referenced, and more

broadly from Harper (1973—a compilation of “benchmark

papers” in geochronology from 1906 to 1971), Faure (1977—

this early edition contains a much richer discussion of the history

of isotope geology than does the latest edition), Quinn (1995),

Lewis (2000), Davis et al. (2003), Parrish and Noble (2003), and

Mattinson (2013).

Discovery

Modern geochronology traces its roots to Henri Becquerel,

a French physicist. In some ways, Becquerel was an unlikely

and perhaps accidental hero (Badash, 1996). Following in his

father’s footsteps, Becquerel had conducted research in phos-

phorescence, the property of some materials, including some

uranium compounds, to emit light after exposure to sunlight or

ultraviolet light. However, by 1895, the 43-year-old Becquerel

evidently had been inactive in research for about five years. The

discovery in late 1895 by Wilhelm Roentgen of “X rays” emitted

from a cathode-ray apparatus spurred Becquerel back into action.

Becquerel began experiments with some of his phosphorescent

uranium samples to determine if they might emit not just visible

light after exposure to sunlight, but also invisible penetrating rays

similar to Roentgen’s X-rays. By early 1896 Becquerel found

that phosphorescent compounds of uranium indeed emitted pen-

etrating radiation, but he also found, perhaps by accident, that

exposure to sunlight was not required. The radiation, later called

radioactivity by Marie and Pierre Curie, was a property of the

material itself. Becquerel had discovered radioactivity.

Within a few years of Becquerel’s discovery, Marie Curie

had discovered two new elements, polonium and radium, bothradioactive daughter products of uranium, and Ernest Rutherford

and Frederick Soddy established the nature and mathematical

principles of radioactive decay. Rutherford and Soddy further

suggested that helium might be one of the stable daughter prod-

ucts of U decay. Rutherford realized that the ratio He/U could be

used to date U-bearing minerals and, based on a suggestion by

Bertrand Boltwood, predicted that Pb/U might prove even more

useful. Rutherford presented the first dates based on the radioac-

tive decay of uranium to helium in a 1904 lecture, and formally

published the results in 1906. In 1907, with the encouragement

of Rutherford, Boltwood published the first dates based on the

decay of uranium to lead. Thus, only about a decade after the dis-

covery of radioactivity, the results of Rutherford and Boltwood

demonstrated that Paleozoic and Precambrian geological materi-

als ranged from hundreds of millions to billions of years old.

However, at this stage there was little interest in the geologiccontext of the samples dated. Enter Arthur Holmes. Holmes was

a student in physics in 1907 at the Royal College of Science,

London, but switched his emphasis to geology in his third year.

He was able to combine his interests in radioactivity and geology

working with professor Robert Strutt, who had narrowly lost out

to Rutherford in the race to publish the first He/U dates. Strutt’s

detailed studies had shown that helium leaked out of rocks and

minerals quite readily, and he suggested that Holmes focus on

the potentially more reliable Pb/U system. By 1911 Holmes had

developed improved methods for Pb/U dating that he applied to

a range of minerals from samples with well-defined stratigraphic

ages. Within two years of this initial success, Holmes (1913) pub-

lished a 196-page monograph, The Age of the Earth, in which he

laid out the principles of radiometric dating, and presented a geo-

logic time scale for the Pleistocene to the Precambrian, based on

all He/U and Pb/U dates available at the time. We can view this

as the beginning of modern geochronology in the sense of rigor-

ous integration of dating and geology. Holmes devoted the rest of

his long career to geochronology in general, and the problem of

determining the age of the Earth in particular.

At about the same time that Holmes was beginning his career

in geochronology, Rutherford discovered the atomic nucleus with

his famous experiments in which he bombarded thin metal foils

with collimated beams of alpha particles. About two years later,

J.J. Thomson, Rutherford’s professor, used a primitive ancestor

of today’s sophisticated mass spectrometers to make the first

direct observation of isotopes: He discovered that neon had two

forms with different masses, about mass 20 and mass 22, respec-

tively. Almost simultaneously, the American chemist T.W. Rich-

ards made highly precise measurements of the atomic weights of

Pb, analyzing Pb from Pb-ore minerals as well as Pb from U-ore

minerals. He proved that Pb produced by the decay of U in U-ore

minerals had a significantly lower atomic weight than the “ordi-

nary” Pb from Pb-ore minerals, a clear indication that Pb also

consisted of different isotopes, depending on the origin of the

Pb. These insights into the nature of matter would be central to

further developments in geochronology, but only after significant

improvements in instrumentation.

Early Isotopic Research

Francis Aston (1919) improved on Thomson’s primitive

instrument. Aston’s “mass spectrograph” recorded the mass

spectra of various elements on photographic plates, and allowed

semiquantitative determinations of the ratios between different

isotopes (Fig. 1). Aston initially confirmed Thomson’s results

on neon, then began working his way through the periodic



The geochronology revolution 305

table. In 1927 he analyzed “ordinary” Pb from Pb ore, and in

1929 he analyzed “uranogenic” Pb produced by the radioac-

tive decay of uranium in uranium ore. The latter produced a

stunning result. As expected, most of the Pb in the U ore was

the 206Pb isotope, produced by the decay of 238U. However, the

uranium ore also contained a significant amount of 207Pb, much

more than could be explained by contamination by ordinary Pb.It was clear that the excess 207Pb must have been generated by

the decay of a previously unknown isotope of U, i.e., 235U. Sev-

eral years later, after the different half-lives for 238U and 235U

were approximately known, the 207Pb/ 206Pb ratio of uranogenic

Pb could be used to determine an age based only on the Pb

isotopic composition of a sample, independent of the concen-

trations of U and Pb. These discoveries greatly enhanced the

power of the U-Pb dating system.

Still further instrumental improvements replaced the photo-

graphic plates of Aston’s mass spectrograph with collection and

electronic amplification of the small electrical currents produced

by the separate ion beams of each isotope. The resulting “mass

spectrometer” allowed much more precise and accurate isotope

ratio measurements than the more primitive mass spectrograph,

and analysis of samples a thousand times smaller. The brilliant

Alfred Nier further improved mass spectrometer design (Fig. 2),

and made several crucial analytical and conceptual contributions

to the applications of radiogenic Pb and “ordinary” or “common”

Pb to geochronology in general, and to determination of the age

of the Earth in particular.

The Development of the Isotope Dilution Method

At about the same time that Nier was reporting his new iso-

topic results for Pb from Pb ores, another revolution was about

Figure 1. One of F.W. Aston’s mass spectrographs. (A) is the cathoderay tube that produced high-energy electrons. The electrons were col-limated in the (B) section of the instrument, and ionized the sample,which was introduced in gaseous form in the (C) section of the instru-ment. (D) marks the curved section of the flight tube that was posi-tioned during operation between the poles of a large electromagnet.Just to the left of (E) the separated ion beams impinged on a photo-graphic plate for detection and semiquantitative isotope ratio measure-ment. Courtesy of Cambridge Physics Outreach.

Figure 2. A.O. Nier, ca. 1940, with one of his early mass spectro-meters, a 180-degree instrument; most of the curved tube would bebetween the poles of a large magnet. Ions were produced by electronbombardment in the mass spectrometer “source,” held in Nier’s righthand, and were measured in the “collector” section of the instrument,held in Nier’s left hand. This is apparently the instrument used by Nierfor his superb measurements of the isotopic composition of uranium,radiogenic lead, and common lead in the late 1930s (University ofMinnesota, courtesy of AIP Emilio Segre Visual Archives).

to begin in nuclear physics. Again, the Curie laboratory played

a leading role. Marie Curie’s daughter, Irene Joliot-Curie, and

her husband, Frederic Joliot-Curie, bombarded a stable element

with alpha particles, transforming it into a new radioactive ele-

ment. This marked the first laboratory transformation of one

element into another. Over the next several years numerous

labs conducted similar experiments on a variety of elements. Inevery case, the newly created radioactive isotopes were always

within a few atomic mass units (amu) of the target element. In

1938, Irene Joliot-Curie and a visitor to her lab, Pavel Savitch,

published results of experiments in which they bombarded U

with neutrons. Their chemical analyses of the irradiation prod-

ucts of the experiment strongly suggested the presence of the

rare earth element lanthanum. But because lanthanum is ~100

amu lighter than uranium, Joliot-Curie and Savitch were doubt-

ful about the identification. In fact, their identification was cor-

rect. Joliot-Curie and Savitch had actually induced the nuclear

fission of 235U but failed to recognize it. The German team of

Otto Hahn and Fritz Strassmann, who also had doubted the

original result, duplicated the Joliot-Curie and Savitch neutron

irradiation of U. They conducted exquisitely detailed chemical



306 Mattinson

analyses of the irradiated products and provided convincing

evidence that barium, also ~100 amu lighter than uranium, was,

along with several other elements, present after the neutron irra-

diation. By providing definitive proof that the atom had been

“split,” Hahn and Strassmann received credit for the discovery

of atomic fission.

Further research on fission, the onset of World War II, andthe Manhattan Project, led to the atomic bomb. But the path-

way to the bomb required developing techniques to separate

the fissionable 235U isotope from the much more abundant non-

fissionable 238U isotope. After the war, some scientists realized

that the isotope separation techniques could be used to produce

purified isotopes of many elements that could then be used as

tracers for accurate measurements of very small amounts of

material. This became known as the isotope dilution method.

One such scientist was Harrison Brown at the University of

Chicago. Brown was interested in applying new methods, such

as low-level radiation counting, neutron activation, and isotope

dilution–mass spectrometry, to measurement of trace elements

in meteorites. Brown’s group included graduate students Clair

(“Pat”) Patterson and George Tilton, along with mass-spec-

trometer and isotope-dilution expert M.G. Inghram. Isotope

dilution analysis of U using highly purified 235U as a tracer or

“spike” was an immediate success. However, the Pb isotopic

work on meteorites went more slowly, owing to low Pb levels

in most meteorite samples and high levels of Pb contamination

in the environment, so the team turned its attention to minerals

in terrestrial crustal rocks. The mass spectrometry methods they

developed reduced the amount of Pb needed for analysis by a

factor of ~1,000 compared with Nier’s earlier work. This, along

with reductions in Pb contamination, allowed ages to be deter-

mined on reasonable amounts of common U and/or Th-bearing

trace minerals, such as zircon, in ordinary crustal rocks—the

foundation of “modern” U-Pb geochronology. After some

delay owing to “top secret” classification of the very existence

of purified 235U, the final results (Tilton et al., 1955) presented

Pb, U, and Th data for all the minerals in a ca. 1 Ga granite.

The work, published in the GSA Bulletin, reported the highly

radiogenic nature of Pb in zircon, and published the first isotope

dilution ages for zircon. Ironically, the title of the paper: “Isoto-

pic composition and distribution of lead, uranium, and thorium

in a Precambrian granite,” does not include the word zircon.

Thus, some discussions of the history of U-Pb zircon dating

have overlooked this seminal contribution. Tilton et al. (1955)

also reported the moderately radiogenic nature of Pb in sphene(titanite), apatite, and magnetite, and the presence of non-radio-

genic (“common”) Pb in feldspar. Patterson continued with the

meteorite work, improved “clean lab” techniques still further,

and determined a ca. 4.55 Ga age for meteorites and the Earth

(Patterson et al., 1955; Patterson, 1956), quite compatible with

modern measurements.

At about the same time that Brown’s group developed the

next generation of U-Th-Pb geochronology, similar techniques

using advanced chemical separation, mass spectrometry, and iso-

tope dilution were applied to the Rb-Sr and K-Ar parent-daughter

systems. Rb and K were discovered to be “weakly radioactive”

by 1906, but could only be exploited for geochronology by about

the 1950s when instrumentation and methods had sufficiently

improved. Also developed in the early 1950s was the carbon-14

dating method.

State-of-the-Art by 1963

By 1963, the “big three” of geochronology, U-Th-Pb, Rb-Sr,

and K-Ar, were well established. Complexities in the U-Pb sys-

tem caused by loss of radiogenic Pb by either episodic or con-

tinuous diffusion mechanisms (Wetherill, 1956, and Tilton, 1960,

respectively) were reasonably well understood, and could be

“seen through” by using the Wetherill (1956) concordia diagram

(Fig. 3). Silver and Deutsch (1963) convincingly correlated the

degree of Pb loss in zircon to the amount of radiation damage

related to sample age and U + Th concentrations, an insight that

would provide the basis for future generations of sample selec-

tion and sample “pre-treatment” strategies. Nicolaysen (1961)

devised the “isochron” method for interpreting Rb-Sr age data

(Fig. 4). The basic isochron method would later be applied to a

wide variety of other isotopic “parent-daughter” systems. Evern-

den et al. (1960) presented experimental data on the diffusion of

Ar in several minerals, and Hart (1964) provided further insights

into “closure temperature” for both the K-Ar and Rb-Sr dating

systems. Other systems, such as fission track dating and U-series

disequilibrium dating, were in their infancy.

Figure 3. Example of a concordia diagram plotted with syntheticdata, showing different fractions of a 2 Ga zircon sample that havelost varying amounts of Pb during a 200 Ma disturbance. Even in theabsence of the concordant 2 Ga point, the remaining data would stillproject to the 2 Ga “upper intercept” on concordia, giving the origi-nal age. MSWD—mean square of weighted deviates.




THE NEXT FIFTY YEARS: THE EXPLOSION IN

GEOCHRONOLOGY

The fifty years between 1963 and the present have seen,

quite literally, an explosion in the amount of geochronological

research, the diversity of methods, the quality of the analyses,

and the sophistication of data interpretation. Major driving forceshave included development of new instrumentation, analytical

techniques, data analysis techniques, and, especially in the last

decade, laboratory collaboration at national and international

levels. This section provides a few highlights of this tremendous

“geochronology revolution.”

Instrumentation

“Conventional” Mass Spectrometers

In the early 1960s, most mass spectrometers were mod-

estly updated versions of the Nier-type design (e.g., Inghram

and Hayden, 1954; Shields, 1962), and many instruments in

academic labs were custom-built, using off-the-shelf electronics

and the skills of local machine shops and glassblowers. In these

mass spectrometers, highly purified samples of the elements of

interest were loaded into the instrument, either dried onto a fila-

ment and ionized by “thermal ionization” or introduced as a gas

and ionized by electron bombardment. Signals were recorded

on, and laboriously read from, paper charts. Calculations were

made on large mechanical calculating machines. By the mid- to

late-1960s, development of mini-computers allowed some level

of computer control of mass spectrometer operation, plus digital

collection and reduction of data, with major increases in preci-

sion and accuracy (e.g., Wasserburg, et al., 1969). The grow-

ing demands of the nuclear industry for large numbers of mass

spectrometers to monitor enrichment levels of 235U throughout

the nuclear fuel production cycle led to increased commercial

production of high-quality mass spectrometers, thus providing

greater access to high-quality instruments for academic labs.Highly linear ion-counting systems such as the Daly detector

also became available in the 1960s, allowing accurate measure-

ments on still smaller samples. Solid-state electronics gradually

replaced vacuum tubes, and by the early 1980s a new generation

of multi-collector mass spectrometers with modern electronics,

high levels of computer control, computer designed ion optics,

and modern vacuum systems emerged. These remarkable instru-

ments continue to be improved, and to provide astonishing levels

of precision and accuracy, beyond the wildest imagination of the

likes of Aston and Nier, the pioneers to whom we owe such a

great debt.

Secondary Ion Mass SpectrometersIn the early 1980s, a revolutionary new approach to zir-

con U-Pb dating emerged: secondary ion mass spectrometry

or SIMS (Fig. 5). This approach is exemplified by the sensitive

high-resolution ion microprobe or SHRIMP; e.g., Compston

et al. (1982, 1984); and see Ireland and Williams (2003) for an

excellent summary. The use of an ion microprobe for U-Pb dat-

ing had been demonstrated earlier by Andersen and Hinthorne

(1972), but the standard ion microprobe used in that study lacked

the resolution for effective zircon dating. Compston successfully

overcame these limitations, and the instrument has been widely

adopted around the world. The SHRIMP and competing CAM-

ECA instruments typically use a beam of oxygen ions to sputter

spots typically ~5–50 µm diameter and a few micrometers deep

on the polished surfaces of thin sections or grain mounts. Dur-

ing the sputtering process, a fraction of the sputtered atoms are

ionized and introduced into the mass spectrometer section of the

instrument, permitting measurement of the relative intensities of

isotopes of Pb, U, Th, and other elements. The intensities are nor-

malized to results for zircon standards that have been accurately

dated by isotope dilution–thermal ionization mass spectrometry

or ID-TIMS methods.

Owing to the very small amount of sample actually ana-

lyzed, the need for standardization, small amounts of instru-

mental drift with time, and slight variations in results related to

matrix effects, SIMS zircon206

Pb/ 238U

dates have uncertaintiesin the 1%–2% 2-sigma range (e.g., Nemchin et al., 2013). This

is significantly greater than the ≤0.1% for recent high-quality

ID-TIMS ages (e.g., Schoene et al., 2010). However, the lower

accuracy and precision of SIMS dating is offset by the ability

to image, then date different domains within individual zircon

grains. This high spatial resolution can resolve age complexi-

ties related to older inherited cores, multi-generational younger

overgrowths-replacements-alterations, etc. (Fig. 6). Many U-Pb

geochronology labs now complement the superb precision and

Figure 4. Example of a Rb-Sr isochron diagram re-plotted using datafrom a lunar meteorite from Rankenburg et al. (2007). The error el-lipses shown larger than actual size for visibility. The age is calcu-lated from the slope of the isochron. The Y-intercept gives the initialratio of 87Rr/ 86Sr at the time the lunar source rock was formed.



308 Mattinson

accuracy of the ID-TIMS method with the high-resolution capa-

bilities of SIMS analysis for the best possible results.

With SIMS dating, the ability to analyze grains directly

eliminates the arduous, time-consuming low-blank dissolution ±

chemical separation procedures required prior to ID-TIMS anal-

ysis. In addition, the actual data measurements using SIMS are

faster than for ID-TIMS. This rapid analysis capability opens up

new possibilities for projects that would be impractical with ID-

TIMS. A spectacular example is the “mass spectrometric mining

Figure 5. A recent example of the SHRIMP SIMS instrument. Samples are bombarded with ions in the right-hand section of the instrumentand after a lengthy passage, first through an electrostatic sector, then a magnetic sector, arrive at the left-hand side of the instrument to becounted. Source: Geoscience Australia.

Figure 6. 24 µm diameter laser ablation spots on a zircon grain datedby LA-ICPMS. Cathodoluminescence (CL) imaging prior to analysisreveals protolith zircon with classical igneous oscillatory zoning thathas been partially overgrown and replaced by younger zircon associat-ed with migmatization. For this figure, the grain mount was re-imagedby CL after LA-ICPMS analysis. Zircon analyzed by SIMS would ap-pear about identical, except that the analysis pits would be shallower(photo by the author).

of Hadean zircons” from the Narryer Gneiss Complex in Western

Australia (e.g., Holden et al., 2009). The host rock for the ancient

zircon, a conglomerate deposited at ca. 3 Ga, contains zircons

of a wide range of ages, and only a very small percentage of the

zircon grains are greater than 4 Ga. These oldest zircon grains are

a precious archive of geochemical information about the Earth’s

earliest igneous processes, and the evolution of the Earth’s early

mantle. Holden et al. (2009) used SHRIMP instruments set up

for automatic operation to eventually screen some 100,000 zircon

grains. As individual zircon grains with greater than 4 Ga ages

were identified, they were used for a wide array of petrologic

and isotopic studies (e.g., Hopkins et al., 2010, and references

therein). This work also highlights another advantage of SIMS

analysis—only a minute amount of material is consumed in a

typical SIMS analysis—the technique is essentially nondestruc-

tive. The identified ancient zircon grains were available for a

wide array of additional analyses.

Inductively Coupled Plasma–Mass Spectrometers

At about the same time as the emergence of the SHRIMP

in the early 1980s, another new instrument, the inductively cou-

pled plasma mass spectrometer, or ICP-MS, was developed. The




ICP-MS typically uses a plasma of Ar ions that can efficiently

ionize most elements in the periodic table. The ions are then

introduced into a high-resolution mass spectrometer for isoto-

pic analysis. Work by Gray and Date (e.g., Date and Gray, 1981;

Gray and Date, 1983) was seminal in the development of the

plasma source and its combination with a high-resolution mass

spectrometer. Initially, all samples were fed into the plasma insolution with use of spikes or standard solutions to measure trace

element concentrations and correct for instrumental drift, mass

bias, and fractionation. Within a few years, however, lasers were

being used to ablate material from solid samples and feed the

material into the plasma using a carrier gas, again, thanks to the

pioneering efforts of Gray (e.g., Gray, 1985). This became known

as laser ablation–inductively coupled plasma mass spectrometry,

or LA-ICPMS. By the early to mid-1990s, the laser ablation

technique was applied to U-Pb dating of zircon (e.g., Feng et al.,

1993; Fryer et al., 1993). Kosler and Sylvester (2003) provide

an excellent summary of the development of LA-ICPMS zircon

dating. Another major step forward was the coupling of the ICP

source with a high-resolution multi-collector mass spectrometer,

allowing high-accuracy isotopic measurements of elements of

interest in geochronology and radiogenic isotope tracer studies

(Walder et al., 1993).

As in the case of the SIMS, unknown samples are inter-

spersed with zircon standards of known age. In recent years,

LA-ICPMS U-Pb dating has made rapid advances spurred by

improvements in lasers and ICPMS instruments, including

simultaneous multi-collection with several ion counters, signifi-

cant decreases in spot size, depth, and amount of sample con-

sumed. These improvements have allowed development of more

powerful techniques such as depth profiling with single pulse

laser ablation analysis (e.g., Cottle et al., 2009). In terms of pre-

cision and accuracy, the LA-ICPMS U-Pb zircon dating tech-

nique is now comparable to the SIMS zircon dating technique.

A significant advantage of LA-ICPMS is that individual spot

analyses require even less time than individual spot analyses for

SIMS. High throughput is an important factor in studies requir-

ing analysis of large numbers of zircon grains, e.g., studies of

detrital zircon grains; see Gehrels (2000) and Fedo et al. (2003).

As in the case of SIMS, U-Pb dating by LA-ICPMS has been

extended to several other minerals.

At this stage, it is necessary to include an important caveat,

at the suggestion of a perceptive reviewer. The enormous volume

of SIMS and especially LA-ICPMS U-Pb dating made possible

by rapid analyses has a down side. To quote the reviewer, “Moreand more papers of poor quality data and even poorer interpreta-

tions that have considerable disregard for the known complexity

of zircon are being published with an emphasis on volume of

data. … The result is a ‘dumbing down’ of the discipline and

more and more difficult distinction to be made by geologists

between highly reliable work and that of sometimes unreliable

interpretations made using uncritical inspection of data and ana-

lytical quality.” This issue will be increasingly important in the

field of micro-beam geochronology in future years. The good

news is that many labs are paying close attention to data quality

and interpretation, and to developing improved techniques.

Tandem-Accelerator Mass Spectrometer

Also in the early 1980s, the tandem-accelerator mass spec-

trometer was designed, with major advantages for C-14 and cos-

mogenic nuclide studies (e.g., Purser et al., 1982). Earlier anal-yses relied on counting the radioactive decay of 14C and other

short-lived isotopes of interest. To use 14C with a half-life of 5730

a as an example, only ca. 4 × 10–7 of the total 14C atoms in a sam-

ple will decay and be counted in an entire day, assuming 100%

counting efficiency. The tandem-accelerator mass spectrometer

measures the 14C atoms present in the sample directly, rather than

having to wait for them to decay. The new instrument has allowed

sample size reduction from thousands of milligrams to <1 mg,

and dating of older samples in which the 14C is almost totally

decayed away. The instrument has also been applied to other dat-

ing systems with great success, e.g., cosmic ray exposure ages

used to determine geomorphic denudation rates.

Improved Techniques

K-Ar to 40 Ar- 39 Ar Method

The K-Ar dating system was the first of the “big three” to

experience a major innovation in technique. The K-Ar method,

based on the decay of 40K, the minor radioactive isotope of K,

requires physically splitting a rock or mineral sample into two

aliquots prior to analysis. One aliquot is used for analysis of the

daughter isotope, 40Ar, an inert gas, typically by isotope dilution

mass spectrometry after extracting the Ar by fusing the sample

in a vacuum. The other aliquot is used for the analysis of K,

using one of several possible techniques, most often by flame

photometry. Overall, conventional K-Ar dating tends to be some-

what limited in precision and accuracy by possible heterogeneity

between the two aliquots, by the imprecision of the methods used

for K determination, by difficulty in assessing whether or not

some Ar has escaped from the dated sample during its lifetime,

and whether or not any “excess” 40Ar is present (McDougall and

Harrison, 1988).

Development of the 40Ar-39Ar technique was a huge step

forward (Merrihue and Turner, 1966). An excellent detailed

reference on the theory of the method and its application to

a wide range of terrestrial and lunar samples is McDougall

and Harrison (1988). In the 40Ar-39Ar, or “Ar-Ar” method, the

unknown samples to be dated, along with interspersed well-dated “standards,” are irradiated with neutrons in a nuclear

reactor. During the irradiation, a small fraction of the major

non-radioactive isotope of potassium (39K) is converted to 39Ar

via a neutron in, proton out reaction. 39Ar is extremely rare

in nature, owing to its short half-life of 269 years, and thus

it serves as an ideal proxy for the amount of K in the sample.

With both the daughter and the proxy of the parent now in the

form of Ar gas, the parent/daughter ratio can be precisely mea-

sured by mass spectrometry after extraction from the sample



310 Mattinson

by heating. usually with a laser or a furnace based on electrical

resistance or radio frequency induction.

With the Ar-Ar method, the need to split the sample and the

imprecision of the old, separate K measurements are eliminated,

and precision of the calculated age is dependent primarily on

the mass spectrometry plus the quality of the ages for the stan-

dards. Also thanks to the elimination of sample splitting, verysmall samples, including single grains, can be analyzed. There

is still the issue of 40Ar loss, as discussed above for K-Ar, e.g.,

during slow cooling, reheating, or alteration. However, since the

daughter isotope and the proxy for the parent isotope are both in

the form of Ar gas, a spectrum of ages can be obtained by incre-

mentally heating the sample (Fig. 7). Thus, the Ar is released in

a series of separately analyzed steps at progressively higher tem-

peratures, rather than in one bulk step. The series of steps might

yield a complex progression of ages, or a well-defined “plateau”

of equivalent ages, permitting more rigorous age interpretations.

Laser heating of samples has been used successfully for analysis

of small single grains, in situ dating in polished rock slabs, and

studies of diffusion of Ar in sections of large mineral grains.

As with other geochronology methods, improvements in

instrumentation and techniques opened the way for more and

more precise measurements on smaller and smaller samples. Pre-

cisions of the best Ar-Ar dates improved to ±0.1% or better, com-

parable to precisions for the best 206Pb/ 238U zircon dates. However,

in cases where both high-quality Ar-Ar and 206Pb/ 238U zircon ages

were available from identical samples with simple rapid cool-

ing histories, the Ar-Ar ages were commonly ~1% younger. This

discrepancy has been resolved only in the past decade or so by a

major cooperative effort by geochronologists.

U-Pb Zircon Method

U-Pb dating by ID-TIMS. The U-Pb zircon dating method

experienced major innovations just a few years after the develop-

ment of the Ar-Ar technique. In the case of U-Pb, several factors,

some independent, and some inter-related, combined to drive a

major advance. For most of the 1960s, zircon analysis techniques

still followed the pioneering methods of Tilton et al. (1955): Zir-

con samples were fused in a borate-based flux in platinum cru-

cibles; Pb and U were separated by liquid-liquid extraction using

large amounts of reagents; and Pb was typically loaded as a sul-

fide for mass spectrometry. As a result, even with extreme care

in handling and purification of reagents, the Pb “blank”—the

amount of Pb contamination introduced during a typical zircon

analysis—was quite large. This, along with the Pb sulfide load-ing method, limited precise age determinations to zircon samples

that contained at least 10–20 µg of radiogenic Pb.

The first advance was the availability of clean, chemically

resistant Teflon for laboratory use. A second advance was a new

method of loading Pb for mass spectrometry. Cameron et al.

(1969) used a mixture of silica gel and phosphoric acid to load

Pb on rhenium filaments, producing yet another factor of 1,000

decrease in the amount of Pb needed for mass spectrometric anal-

ysis, while improving precision and accuracy. A third advance

was the measurement of new decay constants for 238U and 235U by

Jaffey et al. (1971). These constants are about an order of mag-

nitude more precise than those previously used, greatly reduc-

ing a major source of uncertainty in calculated U-Pb ages. These

developments opened the way for a generational improvement in

the zircon method by Tom Krogh of the Carnegie Institution Geo-

physical Lab. Krogh (1973) developed a new, low-contaminationmethod for digesting zircon using hydrofluoric acid in a Teflon

container inside a stainless steel pressure vessel. The chemical

resistance of the Teflon and strength of the steel jacket allowed

Figure 7. Examples of Ar-Ar step-heating release diagrams re-plotted using data from Lunar samples from Turner (1971). The up-per diagram, representing data for lunar rock 12013, yields a broadage plateau, suggesting minimal post-crystallization disturbance ofthe isotopic system. The lower diagram, representing data for lunarrock 10068, shows the effect of major Ar loss in the two lowest tem-perature release steps, shown as open rectangles, and a plateau for thefour higher temperature release steps, shown as shaded rectangles.




digestion of zircon at high temperatures and pressures in small

volumes of hydrofluoric acid with very low Pb blanks. Krogh

also developed small-scale ion exchange techniques to separate

Pb and U from the other components of zircon. At the same time,

Mattinson (1972), then a post-doc in Krogh’s lab, designed a

simple, all-Teflon sub-boiling still that produced very high-purity

hydrofluoric and other acids, further reducing Pb blanks. All ofthese developments opened the path for precise and accurate

U-Pb dating of very small zircon samples, including single grains

and fragments of grains.

Krogh was responsible for, or an inspiration for, many other

significant advances, such as synthesis of 205Pb for isotope dilu-

tion analysis of Pb (Krogh and Davis, 1975; Parrish and Krogh,

1987), which allowed full analysis of Pb from zircon in a single

mass spectrometer run. Previously, zircon was divided into two

aliquots—one for measuring concentration, and one for measur-

ing Pb isotopic composition. As in the case of Ar-Ar dating, the205Pb tracer eliminated possible errors associated with sample

splitting and opened the way for analyzing much smaller samples.

The distribution to the U-Pb geochronology community of 205Pb

made by Parrish and Krogh (1987) was immediately followed by

a marked improvement in the quality of ID-TIMS work world-

wide. Krogh (1982) also developed an “air abrasion” technique

for the selective removal of the outer layers of zircon grains. In

many igneous rocks in particular, the outer, late crystallized zones

of zircon grains are higher in U and Th than are the inner zones.

The higher U and Th concentrations generate higher degrees of

radiation damage, rendering the outer zones more susceptible

to Pb loss, as shown originally by Silver and Deutsch (1963).

Krogh’s air abrasion method was the first simple and reliable

method for selectively removing the outer zones prior to analysis

(Fig. 8). Air abrasion remained the “pre-treatment” method of

choice for over 20 years, until a new technique, CA-TIMS or

“chemical abrasion” (Mattinson, 2000a, 2003, 2005), began to

be widely adopted. The CA-TIMS technique uses a high-temper-ature annealing step prior to partial dissolution in hydrofluoric

acid. The annealing step eliminates undesirable leaching effects

that hampered earlier attempts to use partial dissolution as a pre-

treatment for zircons. CA-TIMS not only effectively removes

outer high U + Th radiation-damaged layers, but is also effective

at penetrating zircon grains along fractures and other defects to

“mine out” interior high U + Th zones. This selective removal of

highly-radiation-damaged zircon zones in many cases results in

a residue of minimally damaged, perfectly concordant “closed

system” zircon (Fig. 9).

In recent years, the silica gel method for running Pb has been

further improved (Gerstenberger and Haase, 1997), allowing pre-

cise measurements of still smaller Pb samples. Also in recent

Figure 8. Concordia diagram re-plotted from data for low uranium zir-con fractions from Krogh (1982). The open ellipses are from slightlydiscordant non-treated zircon fractions. The shaded ellipse is from astrongly air-abraded zircon fraction. Removal of the outer, higher-Uzones of the zircon grains by the air abrasion removes almost all of theparts of the zircon grains that have lost Pb, yielding an almost com-pletely concordant result for the abraded zircon fraction.

Figure 9. A CA-TIMS multi-step “release diagram” modified fromMattinson (2005, fig. 7). Diagram A is analogous to an Ar-Ar step-heating release diagram, showing major Pb loss for the first two rela-tively low-temperature steps, very minor Pb loss for the third step, anda plateau for the remaining ten steps. These latter steps are all perfectlyconcordant, based on the full isotopic data (not shown here). DiagramB reveals that the early steps are the highest in U, whereas the concor-dant plateau steps have U concentrations less than a tenth that of thefirst, highly discordant step. The progressive decrease in U concentra-tions from the early steps to the later steps demonstrates the highlyselective nature of the CA-TIMS dissolution process.



312 Mattinson

years, the constants used in calculation of U-Pb ages have been

“fine-tuned.” The decay constant of 235U, relative to the decay

constant of 238U, has been re-determined (Mattinson, 2000b,

2010; Schoene et al., 2006). The result is about an order of mag-

nitude more precise than the relative value of Jaffey et al. (1971).

The 235U/ 238U ratio of natural uranium in zircon and other miner-

als used for U-Pb geochronology of common igneous and meta-morphic rocks has also been re-determined with much greater

accuracy and precision (Hiess et al., 2012). The two improved

constants combine to greatly increase the accuracy of 207Pb/ 206Pb

ages, relative to 206Pb/ 238U ages, permitting better evaluation of

apparent concordancy versus apparent slight discordancy for

U-Pb ages, thus allowing more accurate age interpretations.

U-Pb dating by SIMS and LA-ICPMS. As mentioned in the

section on instrumentation earlier, micro-beam methods for dat-

ing zircons, then other minerals, emerged in the early 1980s and

early 1990s using SIMS and LA-ICPMS, respectively. These are

discussed in more detail in a later section.

Rb-SrThe third “big three” technique of the 1950s and 1960s,

Rb-Sr, also benefited from improved instrumentation and tech-

niques, but it has been slow to reach the level of the Ar-Ar and

U-Pb systems. Difficulty in accurately determining the 87Rb

decay constant has long limited the overall accuracy of Rb-Sr

geochronology. For example, starting in the mid- to late 1950s,

two quite disparate decay constants for 87Rb were used in dif-

ferent labs. One decay constant, 1.39 × 10–11 a–1, was based on

using concordant U-Pb dates on uraninite and monazite in peg-

matites to calculate the 87Rb decay constant from 87Sr/ 87Rb ratios

in coexisting micas in the pegmatites (Aldrich et al.,1956). Note:

Adjustment of this constant using the newer Jaffey et al. (1971) U

decay constants would give a decay constant of ~1.41 × 10–11 a–1.

The other decay constant in use, based on liquid scintillation

counting of the beta emissions from natural Rb (Flynn and Glen-

denin, 1959), was 1.47 × 10–11 a–1, more than 4% higher than the

1.39 value, casting considerable doubt on the accuracy of R-Sr

dates using either decay constant. Also damaging for Rb-Sr geo-

chronology: “The Rb-Sr whole-rock method was widely used

as a dating tool for igneous crystallization during the 1960s and

1970s, but lost credibility in the 1980s as evidence of whole-rock

open-system behavior mounted” (Dickin, 2005, p. 51).

Improvement, at least in terms of decay constant accuracy,

was gradual. By 1977, the publication date of recommenda-

tions on adoption of a standard set of decay constants by theInternational Subcommission on Geochronology (Steiger and

Jäger, 1977), several additional studies had been completed on

the 87Rb decay constant, but there was still considerable scatter.

Steiger and Jäger (1977, p. 360–361) noted that “the new value

for the 87Rb decay constant should lie between 1.41 and 1.43 ×

10−11 /yr,” recommended that a value of 1.42 × 10–11 /yr be accepted

“for provisional use,” but warned that “the problem of the 87Rb

decay constant was not definitely solved.” In the years following

the 1977 Steiger and Jäger report, several more measurements

of the 87Rb decay constant have been published. Most recently,

Rotenberg et al. (2012) presented results of a ~30-year laboratory

accumulation experiment, measuring the amount of radiogenic87Sr formed from a large batch of highly purified RbClO

4 over

that time span. The result of 1.3968 + 0.0027/–0.0018 × 10 –11 a−1

is the most precise measurement to date, with precisions compa-

rable to those for the U decay constants. The new result also isquite consistent, within errors, with six of the seven other studies

completed between 1982 and 2011 (Rotenberg et al., 2012, fig.

4). It will be interesting to see if, over the next several years,

Rb-Sr geochronology regains its place as one of the premier geo-

chronology systems, thanks to better understanding of the behav-

ior of Rb and Sr in rocks and minerals, and to the latest decay

constant results.

New Dating Systems

Since 1963, many new decay systems have been devel-

oped for geochronology. In many cases these new systems have

become viable, thanks to new generations of instrumentation,

such as multi-collector TIMS instruments, and multi-collector

ICP instruments. One way to track the growth is by surveying the

contents of widely used textbooks and monographs that focus on

isotope geology in general, and geochronology in particular. For

example, papers in the classic Geochronology of Rock Systems

(Kulp, 1961) are entirely devoted to Rb-Sr, K-Ar, and U-Pb, with

brief mention of C-14 and “development of other chronometers”

(Kulp, 1961, p. 165). The small book Ages of Rocks, Planets,

and Stars (Faul, 1966) also concentrates on C-14, Rb-Sr, K-Ar,

and U-Pb, but includes brief discussions of U-series disequilib-

rium, fission track, and the “now little used” U-Th-He dating

method. The first edition of Faure’s widely used text (Faure,

1977) adds coverage of the then-well-established Ar-Ar method,

and includes full chapters on fission track dating and other forms

of radiation damage, U-series disequilibrium dating, and C-14

dating. A single chapter (Faure, 1977, p. 183–196) describes the

Re-Os, Lu-Hf, and K-Ca systems as “most promising” but notes

that these decay systems “have found only limited applications

for age determinations of rocks and minerals.” Further, Faure

(1977) suggests that the Sm-Nd and La-Ce + Ba systems “may

be of some interest,” with recent work on Sm-Nd having “yielded

promising results.” Jäger and Hunziker (1979) cover Rb-Sr, K-Ar,

Ar-Ar, U-Pb, and fission track dating, with brief mention of C-14,

U-series disequilibrium, and Sm-Nd. In Faure’s second edition

(Faure, 1986), Sm-Nd, Lu-Hf, Re-Os, and K-Ca systems fill sixchapters, totaling 80 pages, and are joined by another full chapter

on dating with cosmogenic nuclides. More recent volumes, such

as Dickin (2005), Faure and Mensing (2005), and Allègre (2008),

include full coverage of still more systems, including La-Ce,

La-Ba, the renaissance of U-Th-He dating, and the use of extinct

nuclides for understanding very early solar-system history. All

told, depending on how one counts, close to 40 different dating

methods are now available, spanning age ranges from months to

billions of years.




It is beyond the scope of this broad survey to provide

detailed discussions of all of these newer methods, so a few brief

examples must suffice. Systems such as Sm-Nd and Lu-Hf have

proven invaluable for dating high-grade garnet-bearing meta-

morphic rocks, and even the time span of metamorphic garnet

growth. Re-Os has been used for direct dating of ore minerals,

basalts, and organic-rich shales. The renaissance of U-Th-He dat-ing is of particular interest inasmuch as the very first radiometric

dates by Rutherford were based on the decay of U and its numer-

ous intermediate daughter isotopes to helium. As discussed ear-

lier, Strutt soon demonstrated that helium was readily lost from

highly radioactive minerals, and dating based on helium accu-

mulation in U ± Th-rich minerals “fell into disrepute for nearly

twenty years” (Harper, 1973, p. 14). U-Th-He dating enjoyed a

short-lived revival from ca. 1929 to the mid-1930s. Helium dates

for a suite of stratigraphically well-constrained “mostly basaltic

rocks” appeared to agree closely with existing U-Th-Pb dates.

Holmes actually published an updated version of his geologic

time scale based on a combination of the new helium dates plus

existing U-Th-Pb dates. Shortly thereafter, the calibration of an

instrument used in the helium dating was found to be in error by

more than a factor of two (Harper, 1973, p. 69–70). After this

debacle, the U-Th-He dating method attracted only minor further

interest until its potential for low-temperature thermochronology

was realized (e.g., Zeitler et al., 1987; Wolf et al., 1996; Reiners

and Farley, 1999). Ironically, the method’s greatest “weakness,”

the relatively low temperatures at which helium diffuses out of

minerals, is the primary reason for its reincarnation as an impor-

tant low-temperature thermochronometer.

K-Ar, then Ar-Ar dating of perthitic alkali feldspars, has a

history similar in some respects to that of the U-Th-He dating

method (see McDougall and Harrison, 1988, for more detailed

discussion). Dating of perthitic alkali felspar from metamor-

phic rocks and slowly cooled igneous rocks usually yielded ages

that were “too low” compared with ages for other minerals. The

mineral was deemed unsuitable, and largely abandoned for dat-

ing work. Later, investigations using detailed Ar-Ar age spectra

revealed that perthitic alkali feldspar commonly contained a

record of cooling between ~250 and 150 °C, and could be an

important thermochronometer for this temperature range. In

summary, the modern geochronologist has available a wide array

of dating methods with which to approach geologic problems

ranging from the earliest history of the solar system to rates of

topsoil loss from erosion.

Data Analysis

In the 1950s and 1960s, “data analysis” was primitive at

best. Uncertainties for individual analyses were commonly based

on some general idea about overall reproducibility of mass spec-

trometer runs plus, in some cases, a provision for the typically

large decay constant uncertainties. Data were plotted by hand on

large sheets of graph paper. Isochrons and discordia lines through

arrays of data points were fitted “by eye,” using a long ruler, with

an overall age uncertainty deduced from how much the position

of the ruler could be varied while still overlapping most of the

data points. These primitive methods would soon be supplanted

by more rigorous data analysis. York (1968) developed the equa-

tions for fitting a straight line to data points with “correlated

errors,” typical of most geochronological data. Brooks et al.

(1972) expanded on York’s work, specifically for evaluation ofRb-Sr data. Ludwig (1980) and Davis (1982) focused on uncer-

tainties in U-Pb data. Subsequently, Ludwig (e.g., 1988, 1998a,

1998b, 2000, etc.) published a remarkable series of contributions

to calculation, plotting, and interpretation of U-Pb data, plus data

from many other decay systems used in geochronology.

Sample Size Reduction−Spatial Resolution

One consistent trend in geochronology has been the constant

push to obtain precise and accurate ages on ever-smaller sam-

ples. A major driving force behind this trend, especially in the

case of U-Pb zircon dating, has been increasing recognition of

the complexity of geochronological samples. Zircon populations

in many igneous rocks consist entirely of grains formed during

magmatic crystallization. However, in some igneous rocks, zir-

con grain populations might comprise primary igneous zircon

crystallized from the magma, ± older inherited zircon grains

or cores from older rocks in the magma’s source area, ± older

zircon grains entrained during the magma’s passage through

the crust, ± younger zircon overgrowths or replacements added

during subsequent metamorphism. Data from a large multigrain

sample of such a complex population can be challenging to inter-

pret. Zircon from a volcanic ash fall−flow would be subject to the

same possible complexities, with the added problem of possible

entrainment of xenocrystic zircon during eruption and flow over

the Earth’s surface. Ar-Ar analyses of volcanic minerals such as

sanidine in ash falls−flows are also susceptible to entrainment

of xenocrystic sanidine. For these reasons, analysis of individual

grains is a powerful tool, allowing “outliers” to be excluded from

the main igneous population of grains. In addition, for the zircon

method, grains can be studied in detail using various imaging

techniques plus micro-beam dating techniques either to directly

date the appropriate parts of individual grains or to assist in grain

selection and/or interpretation of single grain analyses. Recently,

additional ways of evaluating spot or single grain analyses have

been added to the geochronologist’s arsenal of tools. One par-

ticularly powerful tool is analysis of trace elements, particularly

the rare earth elements (REE) on the same volume of zircon usedfor U-Pb geochronology: single grains, Schoene et al. (2010);

and SIMS, e.g., Mazdab and Wooden (2006), Mattinson et al.

(2006, 2009), Grimes et al. (2007), and McClelland et al. (2009).

Incorporation of REE in zircon during crystallization reflects not

only the composition of the bulk rock system from which the zir-

con is forming, and the partition coefficients for the various REE

in zircon, but also the nature of other minerals that form before

or during zircon crystallization and compete with zircon for the

available REE. Thus, REE patterns can help place the measured



314 Mattinson

age of zircon in petrologic context. For example, garnet and zir-

con both strongly favor incorporation of the heavy rare earth

elements (HREE). In the absence of garnet, say, in typical igne-

ous rocks, zircon shows strong HREE enrichments. However,

in garnet-bearing high-pressure and ultra-high-pressure (UHP)

metamorphic rocks, the garnet competes strongly for the HREE,

and zircon growing under these conditions is characterized byrelatively flat HREE patterns (e.g., Rubatto, 2002; and see Fig.

10). Micro-inclusions in zircon can also provide important petro-

logic context. For example, Katayama et al. (2001) demonstrated

that cores of zircon that crystallized during UHP metamorphism

of the Kokchetav massif in northern Kazakhstan contained micro-

inclusions of diamond, coesite, and jadite. In contrast, the outer

rims of the zircons lacked these UHP inclusions, and evidently

crystallized during later, retrograde metamorphism.

Other dating problems, such as direct dating of garnet to

determining the time span of metamorphic garnet growth, are

less amenable to dating of tiny individual crystals or micro-beam

techniques. However, these problems can be approached through

physical sampling techniques such as “micro-milling” core and

rim samples of sectioned garnet grains, or by taking advantage

of the natural zoning of appropriate elements within individual

garnet grains. Some examples will be discussed below.

Single Grain Analyses: Zircon U-Pb, and Ar-Ar

One of the most fruitful areas of geochronological research

in the last 15–20 years has been the application of “high-

precision” methods such as U-Pb zircon analysis and Ar-Ar sani-

dine analysis to fundamental problems in stratigraphy and bio-

geology. To quote Bowring and Schmitz (2003, p. 305): “What

are the durations of mass extinctions? How long does ecologi-

cal recovery take following a major extinction? Do evolutionary

radiations correlate with changes in chemistry and temperature

of the ocean-atmosphere system and global climate?” With mod-

ern instrumentation and techniques, both the zircon U-Pb and theAr-Ar methods seemed ideally suited to answer these questions.

A number of groups attacked a particularly attractive target—

the timing and duration of the mass extinction at the Permian-

Triassic (P-T) boundary, the greatest extinction event in the strati-

graphic record. Several marine sections in south China expose

the Permo-Triassic boundary, including the type section at Meis-

han. The sedimentary beds are intercalated with numerous thin

volcanic ash beds that contain zircon and sanidine, seemingly

ideal for dating by the latest high-precision U-Pb and Ar-Ar tech-

niques, respectively.

However, the high-precision data sets yielded slightly differ-

ent ages for the Permo-Triassic boundary, outside of the stated

uncertainties, not only for U-Pb versus Ar-Ar results but also for

U-Pb results from different laboratories. At this point, it would

have been unsurprising if a “bunker mentality” prevailed. Instead

an international group formed the “EARTHTIME Initiative,” a

community effort primarily involving U-Pb geochronologists

and Ar-Ar geochronologists, to resolve the issue of disparity of

ages between different labs and different techniques. This will be

discussed in more detail in the section below on “Collaboration

and Inter-calibration.” For now, the important point is that the

controversy drove home the necessity of: (1) analyzing zircon

grains individually, rather than analyzing multi-grain samples;

(2) inter-calibrating isotope dilution tracers used in U-Pb labs;

(3) dealing with Pb loss in zircon grains by replacing air abra-

sion with CA-TIMS; and (4) reevaluating the accuracy of the

decay constants, age standards, and analytical protocols used in

Ar-Ar labs. See Schmitz and Kuiper (2013) for an excellent sum-

mary of these general issues, as well as the saga of dating the

P-T boundary specifically. At the present time, these issues have

been mostly resolved, and the high precision of modern U-Pb

and Ar-Ar methods has been matched by high accuracy (Fig. 11).

Micro-beam Analysis

Micro-beam analysis, predominantly using SIMS and LA-

ICPMS techniques, has become an increasingly vital part of mod-

ern geochronology. Zircon, with a combination of relatively high

concentrations of U and Th, and virtually no initial Pb, is particu-larly well suited for this approach, although several other minerals

are also attractive targets. As discussed earlier, the major advan-

tage of micro-beam techniques is high spatial resolution—the

ability to date small spots in the range of several microns to a few

tens of microns in diameter on polished sections of individual min-

eral grains. Typically, placement of spots is guided by fine-scale

imaging techniques such as cathodoluminescence, or CL, and

backscattered electrons, or BSE (e.g., Hanchar and Miller, 1993;

Corfu et al., 2003; Ireland and Williams, 2003; and see Fig. 6).

Figure 10. Simultaneous U-Pb age (ages not shown) and REE con-centration measurements on zircon from a UHP eclogite using theSHRIMP reveals older, pre-metamorphic protolith zircon cores withtypical igneous REE patterns, with strongly enriched HREE and neg-ative Eu anomalies. The cores are rimmed by younger UHP zircon,whose flat HREE patterns and lack of negative Eu anomalies indicatecrystallization in competition for HREE from metamorphic garnet,and the absence of plagioclase in the eclogitic host rock, respectively,both indicative of high-pressure metamorphic crystallization. Modi-fied from Mattinson et al. (2007).




These imaging techniques are sensitive to variations in trace ele-

ment chemistry and degree of crystallinity, revealing internal

zoning, inherited cores, and different types of overgrowths and

alterations. This, along with in situ trace-element studies as dis-

cussed above, provides petrologic context for ages determined

from individual spot ages (e.g., Fig. 10).

As discussed earlier, the precision of ages determined bySIMS and LA-ICPMS techniques is limited by a number of fac-

tors, typically to 1%–2% 2-sigma (e.g., Nemchin et al., 2013).

This renders micro-beam techniques, by themselves, less viable

for geochronologic problems that require <0.1% age uncertain-

ties, such as high-precision geologic time-scale work. In such

cases, micro-beam methods can be used to screen samples that

will be analyzed by single-grain ID-TIMS techniques. It is even

possible for individual grains, or more correctly, half-grains, that

have been characterized by micro-beam techniques to be plucked

out of their epoxy mounts and re-dated by the higher precision,

higher accuracy ID-TIMS method.

Before leaving the topic of micro-beam analysis, it is impor-

tant to note that the electron microprobe has been applied withconsiderable success to monazite U-Th-Pb geochronology.

Unlike SIMS and LA-ICPMS methods, electron microprobe

geochronology is a non-isotopic technique, based on measuring

the elemental amounts of U, Th, and Pb in samples. However, the

ability to make these measurements on the micron scale permits

in situ investigation of the chronology of complex metamorphism

and deformation fabrics (e.g., see Williams and Jercinovic, 2002,

for an excellent summary).

Micro-Milling

For some mineral dating techniques, such as Sm-Nd dating

of garnet, the concentrations of the parent and daughter isotopes

are too low, and the radiogenic enrichments of the daughter iso-

tope are too low to permit dating by micro-beam techniques.

Interesting problems, such as the growth rate of metamorphic

garnet, must be approached in other ways. One successful

approach has been to apply micro-milling techniques. Micro-

milling allows selective sampling of different parts of single

grains, typically in thin sections, thick sections, or polished

slabs. Material milled from the core region of the grain, and

from the rim of the grain, is collected and analyzed separately,

revealing the time span of metamorphic growth (e.g., Ducea et

al., 2003; Dragovic et al., 2012).

Mineral Zoning

An alternative method for dating the time span of garnet

crystallization combines the Lu-Hf and Sm-Nd dating techniques

in garnets (Fig. 12; and see Lapen et al., 2003; Kylander-Clark et

al., 2007; Kohn, 2009; and Smit et al., 2010). This method takes

advantage of the tendency of garnet to preferentially concen-

trate the HREE. Lu, the heaviest REE, is so strongly partitioned

into garnet during the early stages of garnet growth, i.e., into the

early-formed cores of garnet grains, that much less Lu is avail-

able for incorporation into the rims of the garnet grains. Thus,

dating whole garnet grains by the Lu-Hf method yields ages that

are strongly biased toward early garnet growth. In contrast, Sm

and Nd, both lighter REE, and very close together in the REE

spectrum, are more evenly distributed throughout garnet coresand rims, yielding a ca. mean age of garnet growth. With some

modeling of the actual distributions of Lu, Hf, Sm, and Nd within

a particular garnet, ages for both the early garnet formation (core)

and the late garnet formation (rim) can be deduced.

Collaboration and Inter-Calibration

Until about a decade ago, most U-Pb geochronology labs

operated quite independently, other than a few efforts to distribute

Figure 11. Dating the Permo-Triassic (P-T) boundary over the past~22 years. Dates from seven studies are plotted with 2-sigma errorbars (the original Ar-Ar results were quoted with 1-sigma errors):Solid circles are dates based on U-Pb zircon analyses; open circlesare dates based on Ar-Ar sanidine analyses. A plots results fromSHRIMP analyses of 34 zircon grains by Claoué-Long et al. (1991).B plots an Ar-Ar sanidine analysis by Renne et al. (1995). C plotsID-TIMS U-Pb air-abraded zircon analyses by Bowring et al. (1998).Zircon and sanidine for these first three studies were separated fromthe “boundary clay” layer just below the P-T boundary. D plots ID-TIMS U-Pb air abraded and “leached” data from Mundil et al. (2001)for a layer just 8 cm above the P-T boundary. Zircon data from the

boundary clay itself that are complex and, as indicated by the shadedbar above the error bar, suggest a possible older age (see Schmitzand Kuiper, 2013) for a more detailed discussion. E is Mundil et al.(2004), applying the CA-TIMS method, modified for single grainzircon analysis. Meanwhile, Renne and others had been working onrefining the decay constants for 40K. Applying the new constants tothe results of Renne et al. (1995) increased the earlier date by ~1%(Renne et al., 2010). This new result is shown as F. Finally, G plotsan exceptionally detailed CA-TIMS U-Pb zircon study by Shen et al.(2011), demonstrating that “the extinction peak occurred just before252.28 ± 0.08 million years ago….” (Shen et al., 2011, p. 1367). Thisfigure is adapted from Schmitz and Kuiper (2013).



316 Mattinson

samples for use as “standards,” e.g., Wiedenbeck et al. (1995), andthe usual diffusion of knowledge that took place when a Ph.D.

from one lab became a post-doc at another. That each labora-

tory had its own “in-house” U-Pb spike and calibration solutions

was the rule rather than the exception. Ar-Ar labs were perhaps

more closely linked, at least in terms of complete reliance on the

availability of well-characterized and widely accepted age stan-

dards for every single Ar-Ar analysis, but not necessarily in other

important ways. In retrospect, it should have come as no surprise

that different labs, even those using the same method, obtained

slightly different ages, outside of experimental errors, for crucial

geologic problems such as the exact timing of the Permo-Triassic

boundary−mass extinction, as discussed briefly in the section

above on “single grain analyses: zircon U-Pb, and Ar-Ar.” After

some preliminary discussions, leading geochronologists—Sam

Bowring, Randy Parrish, and Paul Renne—organized the “Earth-

time Initiative” for the purpose of bringing the internationalgeochronology community together to resolve these problems.

At an early stage, common samples for U-Pb zircon and Ar-Ar

dating were distributed to numerous international labs. A signifi-

cant number of the labs completed analyses and reported their

results “blind”—without being aware of the results from any of

the other labs. The results showed significant scatter outside esti-

mated analytical uncertainties for both the U-Pb zircon and the

Ar-Ar experiment.

For the U-Pb community, attention focused on possible

errors in isotope tracer calibrations and possible varying degrees

of minor Pb loss from the standard zircons. The first of these

issues was approached initially by distributing recently prepared

calibration solutions from a few labs, and eventually by prepar-

ing and distributing two new U-Pb isotope tracer solutions to

the international community. A major effort was put into highly

accurate calibration of the U-Pb tracer solutions, with reevalua-

tions of all possible sources of error, ranging from the exact level

of purity of the widely used NBS/NIST Pb isotopic standards to

the exact isotopic ratio of 238U/ 235U in a large collection of zircon

and other minerals used for U-Pb geochronology. The second

issue was approached initially by using the then-new CA-TIMS

method to pre-treat one particular zircon standard prior to distri-

bution, and later by preparing a set of standard synthetic U-Pb

age solutions, eliminating the possibility of heterogeneities in

zircon samples distributed to different labs. At this stage almost

all U-Pb labs were able to obtain agreement at the <0.1% 2-sigma

level. Moreover, re-analysis of single zircon grains from the

Permo-Triassic boundary section using the CA-TIMS technique

brought results for different U-Pb labs into excellent agreement,

within very small analytical uncertainties (Fig. 11; cf. Mundil et

al., 2004, and Shen et al., 2011).

Ar-Ar tests proceeded from standard sample distribution, to

standard samples irradiated together in the same reactor, and, in

progress, use of a “40Ar/ 39Ar dating intercalibration pipette sys-

tem” (Turrin et al., 2010), which will travel from lab to lab to

directly calibrate mass spectrometer performance. Meanwhile,

the major source of discordance between U-Pb zircon ages and

Ar-Ar ages has been identified as error in the40

K decay constants.Revised 40K decay constants, based on comparisons between

U-Pb zircon versus Ar-Ar pairs from the same samples (e.g., Min

et al., 2000; Renne et al., 2010) and independent inter-calibrations

between Ar-Ar dates and astronomical dating (e.g., Kuiper et al.,

2008; Rivera et al., 2011), now bring Ar-Ar ages for the Permo-

Triassic boundary into excellent agreement with the latest U-Pb

zircon results (e.g., Fig. 11, and Schmitz and Kuiper, 2013).

During the decade-long history of Earthtime, formal

and informal contacts among international labs have greatly

Figure 12. Lu-Hf and Sm-Nd isochrons for garnet and other mineralsfrom high-pressure to ultra-high-pressure (HP-UHP) rocks. Data re-plotted from sample MS09001 of Smit et al. (2010). Error ellipses havebeen greatly enlarged so that the data points are visible. The Lu-Hfisochron age is strongly biased toward the early stage of garnet growth,represented by the Lu-rich garnet cores. The Sm-Nd is biased towardthe later stages of garnet growth.




increased, sharing tips on “best practices,” lessons learned “the

hard way,” etc. Open collaboration and cooperation are now the

rule rather than the exception. Over the past decade this level

of communication, collaboration, and cooperation have been

essential to the numerous advances associated with Earthtime.

The entire international geochronological community will ben-

efit from continuing to use this model.

SUMMARY AND CONCLUSIONS

Fifty years ago, after a sometimes-difficult fifty-year-long

infancy and adolescence, geochronology reached young adult-

hood. Senior members of the geologic community did not always

appreciate the brash newcomer. In some cases, the lab-oriented

scientists were lacking in basic geological background, and made

incorrect interpretations. In other cases the newcomers commit-

ted the cardinal sin of clearly demonstrating that a long-held

conclusion was simply wrong—how could rocks that “looked

Precambrian” turn out to be Mesozoic? Over the last fifty years,

progress in geochronology has been spectacular as instrumen-

tation, analytical techniques, and integration of geochronology

with geologic-petrologic processes have made enormous leaps.

For example, in the early part of the past fifty years, geochro-

nologists were quite satisfied with age uncertainties of a few

percent. In the past decade, much smaller discrepancies between

ages determined by different labs and/or by different techniques

spurred a major effort to identify and correct the sources of errors

that might be responsible, and to drive age uncertainties below

the 0.1% level. As geochronology matured, it became more

and more deeply integrated into nearly every field of geology.

Evidently, derogatory terms such as “dial twisters and number

jugglers” fell out of favor as more geologists eagerly sought

collaborations with geochronologists, and as most young geo-

chronologists were educated in geology departments rather than

in chemistry and physics departments. Of course, some of the

increased acceptance of geochronology compared to the “early

days” might be explained by the old saying that “science makes

progress one funeral at a time.” Now, few would deny the phe-

nomenal contributions of geochronology to our knowledge of the

timing of important geological events and rates of geological pro-

cesses, ranging from early solar system processes to an increas-

ingly accurate and detailed understanding of Earth, Lunar, and

even Martian evolution.

As it happens, the advances in geochronology in the past

several years, especially in the U-Pb and Ar-Ar dating methods,make this a particularly fortuitous time for a review of prog-

ress in geochronology. The ability to date geologic events with

<0.1% age uncertainties opens the way for deciphering geologic

history at a remarkable level of detail. Two recent examples of

state-of-the-art zircon U-Pb geochronology can be found in

Schoene et al. (2010) and Shen et al. (2011). In Schoene et al.

(2010), high-precision U-Pb ages of zircon demonstrate that the

Triassic-Jurassic boundary, the end-Triassic mass extinction,

and the onset of Central Atlantic Magmatic Province terrestrial

flood volcanism are all coeval within less than ca. 150 ka. Shen

et al. (2011), using the same techniques, demonstrate that the

Permo-Triassic mass extinction event had a maximum duration

of 200 ± 100 ka. Comparable work is under way in many labs

to build up a detailed, very high quality chronology for the rest

of the Phanerozoic time scale.

Arthur Holmes died in 1965, just after the publication of theGSA 75th anniversary volume, with the “dial twisters and num-

ber jugglers” comment. Holmes undoubtedly would be pleased

with the astonishing progress in geochronology, the acceptance

and integration of geochronology into so many fields of geology,

the dogged pursuit of a more and more accurate geological time

scale, and especially with the outstanding young scientists who

continue to advance the field. The future looks particularly bright.

ACKNOWLEDGMENTS

I would like to dedicate this chapter to the late George R. Tilton,

my Ph.D. co-advisor and later fellow professor at UCSB, and

to the late Thomas E. Krogh, my post-doctoral advisor at the

Geophysical Laboratory, Carnegie Institution of Washington.

I regard them as the “grandfather” and “father,” respectively,

of modern U-Pb zircon geochronology. Just as they stood on

the shoulders of their predecessors, so we all stand on theirs. I

am especially grateful for helpful reviews from Drs. Chris Mat-

tinson, Randy Parrish, Paul Renne, and Scott Samson. Their

thoughtful and constructive suggestions have significantly

improved this paper.

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