A Century of U-Pb Geochronology

Downloaded from gsabulletin.gsapubs.org on May 1, 2013 Geological Society of America Bulletin A century of U-Pb geochronology: The long quest towards concordance F. Corfu Geological Society of America Bulletin 2013;125, no. 1-2;33-47 doi: 10.1130/B30698.1 Email alerting services Subscribe Permission request click www.gsapubs.org/cgi/alerts to receive free e-mail alerts when new articles cite this article click www.gsapubs.org/subscriptions/ to subscribe to Geological Society of America Bulletin click http://www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. Individual scientists are hereby granted permission, without fees or further requests to GSA, to use a single figure, a single table, and/or a brief paragraph of text in subsequent works and to make unlimited copies of items in GSA's journals for noncommercial use in classrooms to further education and science. This file may not be posted to any Web site, but authors may post the abstracts only of their articles on their own or their organization's Web site providing the posting includes a reference to the article's full citation. GSA provides this and other forums for the presentation of diverse opinions and positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflect official positions of the Society. Notes © 2013 Geological Society of America org/pubs/ft2013 . allow an immediate verification of the validity of their ages. and only that one is good enough. In a review of the methods current by ca. 33–47. a critical advance was the development of mass spectrometry. A different type of concordia diagram was later introduced by Tera and Wasserburg (1972) (see Appendix in the GSA Data Repository [see footnote 1]). although under favorable circumstances. the problems of U-Pb discordance have not yet been completely resolved and will be one of the main hurdles to overcome in the future. p. Ahrens (1955a) used data for monazite from Rhodesia (Zimbabwe) reported by Holmes (1954) to show that the divergence between 206Pb/ 238U and 207Pb/ 235U and 207Pb/ 206Pb changed in systematic ways. as well as a history of a long struggle toward concordance.Downloaded from gsabulletin.corfu@geo. (1954) pointed out that analyses tended to give discrepant ages for 206Pb/ 238U. toward an understanding of the causes of discordance. Only one answer is the right one. Wetherill. and implications of U-Pb discordance in modern geochronology. and 207Pb/ 206Pb. The progress in developments of the U-Pb method is both a history of technical discoveries and advances. Bowring and Schmitz (2003). January/February 2013. and of the data obtained by then. and Condon and Bowring (2012).uio. 1 Kulp et al. he discussed the causes of discordance. contact editing@geosociety. focusing especially on the style.org. (LA-)ICP-MS—(laser ablation) inductively coupled plasma–mass spectrometry. E-mail: fernando. Data Repository item 2013061. His formalism became the basis for the subsequent treatment of discordant data (e.gsapubs. University of Oslo. and discovered the reality of U-Pb discordance. but also a second one that bypasses the problem and solves the Gordian knot by essentially eliminating the concept. and he interpreted the pattern as being due principally to Pb loss. This paper follows these historical developments. For permission to copy. (1955) of isotope dilution techniques2 to the determination of U.org on May 1. Ahrens (1955b) presented a semilogarithmic plot of t (time) 207Pb/ 235U versus log t 206Pb/ 238U. and. and will not be repeated here. also establishing that 207Pb/ 206Pb ages were those closest to the real age. was then provided by Wetherill (1956a. The modern version of the concordia diagram. respectively. In a subsequent paper. Postbox 1047. declaring U-Pb systems concordant by definition (see section “Administrative Concordancy”). 207 Pb/ 235U. and concluded that 207 Pb/ 235U was the most reliable. Corfu† Department of Geosciences. and Th in minerals was the 2 Abbreviations used in this paper: ID-TIMS—isotope dilution–thermal ionization mass spectrometry. 60 yr later. Norway ABSTRACT The U-Pb system is a prime geochronometer. The second and subsequent waves searched for ways to accommodate U-Pb discordance and explored various strategies to eliminate it. no. † 18 8 8 2 013 CELEBRATING ADVANCES IN GEOSCIENCE Invited Review such as that by Davis et al. the degree of discordance can vary greatly. These twin decay systems.E. 1957. (2003). In detail. Pb.. using preliminary versions of concordia diagrams. mainly due to its occurrence as a pair of isotopically distinct but chemically identical decay systems with 235U decaying to 207Pb and 238 U to 206Pb.htm or by request to [email protected] © 2013 Geological Society of America 33 . 1963). and after several decades dedicated to research into fundamental aspects of radioactive systems. doi: 10.geosociety. 1950. 1956b). Russell and Ahrens.1.1 More detailed descriptions of various aspects of the analytical technique and error analysis can be found in Parrish and Noble (2003). the opposite of the conclusion of Kulp et al. and discordance can have many different causes. showing that the Rhodesian data fitted a straight line. discordant data can be extrapolated to the correct age. They discussed the various factors affecting the analyses. The first wave of applications in the 1950s developed basic techniques. U has suitable half-lives and is hosted in some very convenient minerals. Brief description of the U-Pb method and specific technical aspects. and demonstrated graphically the convergence along curved trajectories toward a common age. and toward ways to eliminate it. Research on this subject during the first 50 yr of the twentieth century prepared the ground for the emergence of U-Pb geochronology as a robust dating technique. which up to then had been based on bulk chemical ratios. Discordant data from Manitoba and Madagascar could in part be explained by the same scheme. —T. we see two contrasting trends. 125.1130/B30698. causes. 1/2. A brief description of the U-Pb method and specific technical aspects is given in an Appendix in the GSA Data Repository. two philosophies: one that continues the battle to eliminate discordance. running at different speeds. SIMS—secondary ionization mass spectrometry. such as the widespread and robust mineral zircon. In addition.g. together with a detailed mathematical treatment of the effects of loss and gain of Pb and U.no GSA Bulletin. undertook the first geological case studies. This paper reviews some of the main stages in the evolution of the method. Now. Krogh INTRODUCTION The development of U-Pb geochronology is rooted in the early days of discovery of radioactivity and the radioactive decay of U with emission of alpha and beta particles and a gradual accumulation of radiogenic Pb. EVOLUTION OF THE TECHNIQUE The Pioneers—Realization of U-Pb Discordance Following the discovery of radioactivity. A description of the most fundamental discoveries and of the progressive development of mass spectrometry is given in some detail in earlier reviews. The first measurements of Pb isotopic compositions by Nier (1939) were an important analytical step in geochronology. N-0316 Oslo. and 207Pb/ 206Pb was the least reliable age. Despite the enormous progress achieved in this field. (1954). v. 6 figures. is available at http://www. 2013 A century of U-Pb geochronology: The long quest towards concordance F. considering especially loss of Rn. Blindern. Isotope Dilution–Thermal Ionization Mass Spectrometry (ID-TIMS) The application by Tilton et al. which must be concordant to be considered valid. GSA Data Repository item 2013061. 1973. who proposed that radiogenic Pb forms in the tetravalent state. Goldich and Mudrey (1972) had discussed a model of Pb loss by dilatancy with loss of water from microcapillaries in zircon during uplift. 1960. although it was more easily reset (or newly formed) during high-temperature events. and (3) the use of an artificial 205Pb isotope as a tracer (Krogh. remained the immense analytical effort needed to produce individual analyses from zircon fractions of 100–500 mg (Silver and Deutsch. expanding its application to address the evolution of orogens and decipher the timing of granitic magmatism and metamorphism. He understood that zircon populations in rocks can be extremely variable in terms of composition.Downloaded from gsabulletin. There was a gradually growing consensus that the dominant cause of discordance was loss of Pb. In addition. A major hurdle. The U-Pb results defined collinear arrays for which intercepts could be interpreted in terms of primary crystallization and secondary Pb loss (Silver. Cameron et al. 2011) represents the next major step in the evolution of the method. several models were elaborated that linked discordance to various diffusion mechanisms. 1969). but it represented a great step forward. (1981) showed that it was possible to find zircon and monazite and date rocks of mafic and ultramafic composition. Silver and Deutsch. 1974). the former may be lost preferentially. (1976) on early Archean Amitsoq gneisses of West Greenland. (1966) conducted a series of experiments using portions of a large metamict zircon immersed in hot NaCl solutions and observed considerable leaching of Pb and a reduction of the Pb-U ratio. 1982).. 1963a. 1963). and his measurements of the U-Pb ratios of the different fractions revealed extreme variation in the degree of discordance. but also focusing on specific mechanisms causing U-Pb discordance. (1969) had introduced the Si-gel loading technique. diffusion linked to gradual metamictization of the host. (1964) showed that some of the discordance reflected mixing with old Pb components. Krogh Revolution 1: Simplifying the Method The change of focus by Tom Krogh. He also showed that volume diffusion alone could not explain the pattern. Good examples of the former are the studies by Pidgeon and Aftalion (1978) on the granites and metamorphic basement of Scotland. Grünenfelder et al. Steiger and Wasserburg. Wetherill. In a study of discordance. these models and discussions could not provide unique and fully reliable solutions. While intellectually stimulating. in part combining the age of different minerals. when much larger quantities had previously been necessary. 1966. 2013 Corfu fundamental technical innovation that opened the doors to work with zircon. The new technique was rapidly embraced by the growing U-Pb community. The main advantages of these combined modifications were the drastic reduction (by three orders of magnitude) of the background contamination. The procedure adopted in that study was extremely complex and time consuming. Subsequently. much smaller sample sizes. a more sophisticated multistage Pb loss model was added to the toolbox (Allègre et al. 1975a). He experimented in separating fractions according to size and magnetic variability. at Caltech in Pasadena. Gulson and Krogh (1973) on the Bergell granite. 1963a. and the problems of discordant U-Pb data remained a challenge (e. and the one that really enabled the more widespread application and testing of the principles and analytical approaches defined by Leon Silver’s work. to U-Pb geochronology from his previous involvement with Rb-Sr dating (Kamo et al. and different attempts were made to rationalize the effect and extract reliable ages. p.gsapubs. observing a much lower susceptibility to partial Pb loss. perhaps causing the commonly observed nonzero lower-intercept ages of discordant arrays of Precambrian zircon populations. He realized that the weak point in the analytical procedure was “the absence of a suitable approach in selecting samples from which data could be successfully systematized and interpreted” (Silver. They further point out that the last α-decay in the chain to 207Pb involves more energy and causes more damage than the corresponding last decay to 206Pb. Cliff (1980) and Schärer (1980). 1957. the strategy became that of maximizing the spread in order to establish more robust discordia lines and increase the reliability of their intercept ages. crystallinity. Wasserburg. this mineral became the dominant geochronometer. 1963b. Given that discordance was next to unavoidable in most samples. (1974) found that a major mechanism affecting detrital zircon ages was gain of U during metamorphism. January/February 2013 . and Van Schmus et al. Other studies tested techniques to expand the range of discordance in order to obtain 34 Geological Society of America Bulletin. which correlated with U content and degree of radioactivity. and hence remains immobile until it is exposed to fluids that change its redox state and make it mobile. Tilton. 1963). and Mattinson (1972) had devised an efficient way to distill the reagents used in the analysis. much simpler analytical protocols. attention was focused on the general problems of discordance of U-Pb systems in nature.. seen from our presentday perspective. established the evolution of nappe complexes. 1963b. (1987) on Paleoproterozoic Trans-Hudson orogen.g. 1976. then at the Carnegie Institution of Washington. or diffusion linked to episodic activation (Nicolaysen. hence. Schärer and Allègre. 1963). Silver’s developments set up analytical approaches that were to become standard procedures in the following two decades.. and other parameters. Besides zircon.org on May 1. Krogh and Davis. 281). New Strategies to Understand and Work around Discordance The rebound from these rather discouraging early phases in the development of U-Pb can be credited largely to the work and imagination of Leon Silver. Zircon gave U-Pb and 207Pb/ 206Pb ages that were considered equal within the limits of uncertainty. Gebauer et al. Tilton and coworkers tested the method using a sample from a granitic rock from the Grenville Province. either simple volume diffusion. In the following half century. however. The main components of the new technique were: (1) the change from flux-based decomposition of zircon to hydrothermal dissolution in Teflon bombs. Baadsgaard et al. All chemical steps in this procedure were carried out in simple laminar flow hoods. This mechanism is touched on also by a concept more recently discussed by Kramers et al.. and the loss of Pb required additional controls from the internal parts of the zircon. which greatly facilitated mass spectrometry due to enhanced ionization of Pb. (2) the chemical separation of U and Pb using ion-exchange resin. 1966.g. The analysis of coarse titanite also yielded the same 207 Pb/ 206Pb age as the zircon. (2009). Lancelot et al. permitting analysis of gram quantities of minerals. Kouvo and Tilton. Tilton and Grünenfelder (1968) also examined the behavior of titanite (sphene) in different settings. His tests established that the earlier-discussed Rn loss effect had no great influence on the discordance of zircon.. and better reproducibility. Modeling Phase With improvements in technique and increased production of data. The reduction in sample size permitted by the new technical advances led to the first single-grain zircon studies (e. Grauert et al. Along a different track. The presence of uranothorite inclusions was also shown to have a strong influence on the degree of discordance. Subsequently. Pidgeon et al. 1963. Wetherill (1956b) had provided the basic graphic and mathematical tools for dealing with discordant data. which severely limited the utility of the method. 2009). surrounded by more complex regular and sector-zoned domain. whereas the fractures formed during later mylonitization without causing significant Pb loss (Austrheim and Corfu. itself rimmed by high-luminescence outer layer. Alteration was caused by fluids attacking the rims of zircon or penetrating along fractures and attacking metamict domains. (F) A similar case. he opted to remove the outside of the grains. several workers experimented with partial dissolution techniques. January/February 2013 35 .. Turek et al. 2007).. 1E and 1F). or of fragments of grains.org on May 1. The texture indicates overgrowth and new reworking of a magmatic zircon during two separate high-grade metamorphic events. and refine these techniques and eventually discovered how to master them a decade later (see later herein). Cathodoluminescence (A–E) and backscattered electron (F) images illustrating typical features of zircon affected by secondary modifications causing partial resetting and discordance of U-Pb systems. To avoid the side effects of the chemical attacks. 2010).. isolating. Although this flurry of activities and new applications helped to solve many geological questions. but locally fractured and externally resorbed and overgrown by new highly luminescent (and low U) rims during a metamorphic overprint (modified from Corfu. resulting in partially reset and discordant data (modified from Corfu. and analyzing concordant domains of zircon. (A) Zircon crystal with regular growth zoning. in many cases. with alteration attacking metamict zones in the zircons and penetrating along cracks perpendicular to the zoning (modified from Nasdala et al. Scale bar indicates 100 μm. Krogh (1982a) chose instead a different track. 1981. His tests showed that. By means of leaching and etching experiments. and since extrapolating the ages from discordant arrays could be problematic in all those cases that had undergone a Pb evolution involving more than just two stages. 2007). however. (B) Small core with regular growth zoning. botryoidal trains of alteration advancing along cracks and interfaces and forcing their way into metamict domains (from Medenbach. the thin rims represent a metamorphic overprint that caused strong Pb loss. The groundwork laid by Silver and coworkers. 2012). 1982).Downloaded from gsabulletin. The fractures formed as a result of tensions caused by volume expansion during metamictization. Krogh Revolution 2: Reducing Discordance Since model solutions to discordance did not work very well. 1983. 2013 A century of U-Pb geochronology: The long quest towards concordance better-constrained intercept ages (Aleinikoff. Because altered domains are easily soluble and could be separated from the unaltered zircon by partial dissolution. The penetrating fluid removed radiogenic Pb while it introduced elements such as Ca and Fe. which in complex cases had detrimental effects on the precision and accuracy of the ages. Alteration causes chemical changes and produces severe Pb loss. Mattinson (1994) reviewed these experiments and concluded that the fractionation of U from Pb was likely related to the precipitation of fluorides during dissolution. The results. 1975. (D) large zircon crystal strained by mylonitization. (C) Zircon crystal with regular and sector zoning. Geological Society of America Bulletin. Mattinson continued to experiment with. 1975b) discovered that some zircon crystals contain easily dissolvable altered domains (Figs. he also experimented with a high-gradient magnetic device that separated A B C D E F Figure 1. (E) Zircon affected by alteration expressed by the black. Todt and Büsch. The strain in this case caused severe resetting of the U-Pb system (from Roffeis et al. Pidgeon and Hopgood. many of the studies involving zircon had to rely on extrapolating the age from discordant arrays. Steiger et al. and also by others. the technique could indeed strip most of the discordant zircon matter. but is especially prominent in specific parts of the grains.gsapubs. which caused internal fracturing and local distortion of the lattice and fragmentation of the rims in addition to full recrystallization and/or new growth of external neoblastic zircon grains.g.. 1993). Tom Krogh concluded that the only reliable strategy had to be that of finding. In a parallel development. Krogh and Davis (1974. and locally thin rims of low luminescence cut by brittle fractures. by spinning them around in a steel chamber with a jet of compressed air (air abrasion method). in the 1960s had shown that discordance is generally not a uniform property of zircon. 1976). were not encouraging because the technique introduced secondary effects.. fractionating U from Pb in the remaining parts of the zircons (e. As exemplary applications in that period. High-Resolution ID-TIMS Geochronology The array of technical improvements.org on May 1.g. Kröner et al. or the common but less suitable apatite.g. Barth et al. allowing verification of whether the composition remained constant. Hansmann and Oberli. LA-ICP-MS Technique: Problems and Solutions The latest addition to the arsenal of U-Pb dating methods is the laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) technique. 1991.. 1984. but also the less common baddeleyite.. from where it could subsequently be ionized and measured (Kober. the genesis of granites. Corfu and Davis. by providing accurate ages that permitted correlation of geological elements across belts. In the ideal case. Nutman et al. An additional benefit of the technique was the fact that it only required a thermal ionization mass spectrometer without facilities for low-blank chemical dissolution and separation.. 1979) encountered difficulties because of problems correcting for molecular interferences on the desired Pb isotope masses. 1991. 2013 Corfu zircon based on faint paramagnetic properties. Because the sputtering of U and Pb is controlled to some degree by the composition and/or structure of the sample (McLaren et al. and between continents and.. Another critical step was the determination of the U-Pb ratios. 1986. allanite. or changed progressively.. 1994). the main contribution for U-Pb dating was the development of the sensitive high-resolution ion microprobe (SHRIMP) at the Australian National University (ANU) in Canberra by Bill Compston and his group (Williams.g. 2001). The technique was used successfully in a number of studies (e. 1987). Krogh et al. These disadvantages have limited the number of applications of this technique. thereby eliminating the zircons that had undergone even very small amounts of alteration. The efficiency of these treatments was further greatly improved by the growing ability to measure smaller and smaller samples due to progressive refinements of the Krogh (1973) technique by miniaturizing dissolution bombs and columns. Davidson and van Breemen. The measurement procedure is also relatively time consuming. 2008. January/February 2013 . Kober Technique: Zircon Evaporation An interesting alternative for dating zircon was developed by Berndt Kober in Heidelberg.-old zircon grains in conglomerates at Mount Narryer and Jack Hills in Western Australia (Froude et al. Ion probe dating has been applied to the entire range of terrestrial geological problems. One of the most celebrated studies was the discovery of ≥4-b. The SHRIMP main solution was to build a large ion microprobe that was capable of achieving high mass resolution without having to compromise the sensitivity. 1986. a modified version has been proposed that promises to provide a better control of the isotopic fractionation and thus contribute to high-age-resolution geochronology (Davis. Dunning and Pedersen.. however.. one can mention the systematic determination of the chronostratigraphy of Archean and Proterozoic greenstone belts.. Jaeckel et al. and perovskite. This procedure allowed. 1996a. which released Pb. (2000). contributing to many improve- 36 Geological Society of America Bulletin. especially titanite and monazite. is the fact that it provides only Pb-Pb but no U-Pb information. More specific dating applications are those dealing with the formation of very young rocks. In less than a decade. and modern analytical protocols are generally concerned with proper matching of the reference zircons. which by the end of the 1980s in some laboratories had started to creep below 1 picogram Pb. The first experiments with U-Pb dating by LA-ICP-MS were done in the early 1990s (Fryer et al.gsapubs. suggesting a closed system. 1997). discrepancies can arise between sample and standard when they have quite distinct basic properties. the dating of ophiolite complexes. and taking advantage of improved mass spectrometers. and the sources of xenocrystic and detrital zircons (e. 2008). the dating of mafic dike swarms. and hence no direct way to verify the degree of discordance. these methods had to be combined with careful inspection and selection of the grains under a binocular microscope. and depositing it on a facing cold filament. 1988. rutile. 1987.. Corfu et al. It involved mounting a zircon directly on a Refilament and then.g. and hence loss of temporal resolution. by providing the detailed age resolution necessary to address the speed and kinematics of rock-forming and rock-deforming processes. Davis et al. 1983). which in SIMS must be calibrated against the corresponding ratios of an external standard.y. the extraction procedure would provide a number of sequential steps. 1996). Williams. 1996b). Kamo et al. to achieve the best results. the extraction and purging of disturbed Pb from discordant domains before reaching the more firmly held undisturbed Pb. 1992.. Compston and Pidgeon.. 1992. and the development of abrasion. 1988. The anomalous behavior is evident in some zircons very rich in U.. where the ratios are obtained by mixing a tracer of known composition with the sample. with the further advantages of a high analytical speed and the preservation of most of the analyzed objects.. The SHRIMP instrument attracted a large interest from the community.. suggesting that Pb was disturbed because of loss or mixing with older components.g. Wilde et al. 1982b). which spawned many other studies on these rare and very critical earliest witnesses of Earth evolution (e. 1998. in principle. made it possible to address a variety of geological problems at a level of detail that would not have been possible before.. in contrast to ID-TIMS. Nemchin et al. which permitted the analysis of discrete domains of polished sections through grains. The Ion Probe: The Early Challenges and Progress Around 1980. across continents. Besides the ubiquitous zircon. gaining Fe and losing Pb (Krogh. with its much improved control on discordance. 1998). Kinny and Dawson. but especially to the study of multistage crustal growth and metamorphism. and constraining the age of ore deposits and of major meteorite impacts (e. Following early steps in Cambridge and Chicago. 1995. mainly because of the high spatial resolution. Recently. and in specific cases with imaging (cf. Davis and Smith. 1989.. such as the progressive deposition of opal in veins at Yucca Mountain studied by Neymark et al.. When compared to ID-TIMS. the main disadvantage of the ion probe was a considerable decrease in precision. xenotime.Downloaded from gsabulletin. 2003). the resolution of the emplacement sequence of multistage granitoid complexes and polymetamorphic evolution of lower-crustal terrains. such as the cases reported by Harrison et al. Its main drawback.. 1993). after inserting it into a mass spectrometer.. This stemmed mainly from the much smaller amount of material available for measurement. Williams et al. Hinton and Long. ID-TIMS was joined by secondary ion mass spectrometry (SIMS). The original attempts to use regular ion probes for U-Pb dating (e. 2001. many laboratories worldwide had adopted it. 1991. among them. a variety of other minerals could be routinely analyzed. on the other hand. subjecting it to rounds of progressive heating. and from there the technique rapidly improved. Special applications of the SIMS technique were also those focused on the rare zircons in meteorites and in rocks brought back from the moon by the Apollo missions (Ireland and Wlotzka. 1989. In general. Parrish. The new analytical approaches contributed at two distinct levels: on the one hand. reducing the blank. (1987) and Wiedenbeck (1995). 1992. This potential problem is now largely recognized in the SIMS community. 2009). Svensen et al. thus limiting the ability to evaluate the state of concordance of the analyses. 2002. 1995. but the latter has the advantage of consuming much less of the target (spots of 25 μm × 3 μm for SIMS vs. 2002. in a period of time over which it is insufficient to repair the damage of much more strongly metamict domains (Mattinson. 2013 A century of U-Pb geochronology: The long quest towards concordance ments and refinements. after the discovery of radioactivity a century ago. the opposite effect has been suggested by Kramers et al. but after the advent of efficient isotopic measurement techniques around 1940–1950. but it has been used mostly to pursue a different type of problem. thus making it possible to isolate the latter. though rich in U. 2000). In my experience. by extension.. In addition. 1975.g. without assistance from U-Pb. Occasionally. with Geological Society of America Bulletin. and the consequent increase in sensitivity. January/February 2013 37 .. 1987.Downloaded from gsabulletin. 2010b). which may have been frozen-in during earlier geological annealing events (Davis and Krogh. 1996. combined with U-Pb and other isotopic systems. Pb-Pb Dating The Pb-Pb method is a special branch of U-Pb geochronology.. i. not only because it protects the crystalline parts from easy dissolution. The method is attractive mainly because of the high throughput capability. 2011). due to the presence of mercury in the argon gas used to produce the plasma that causes an isobaric interference by 204Hg on 204Pb. Gerdes and Zeh.. Gehrels et al. This method had represented one of the initial steps into U-Pb dating in the early decades of the twentieth century. 1994. One of the most prominent applications of Pb isotopic analysis to dating was that of Patterson (1956). but especially because it immobilizes the Pb and U in the crystalline parts. Bau et al. although the limited precision makes it most attractive for the study of detrital zircon populations (e. the two tracks overlap and complement each other. 1999. Mattinson: Development of Chemical Abrasion As mentioned earlier herein. The main area of application of chemical dating has been monazite. An interesting line of experiments exploring geochronological applications involved differential leaching and dissolution of silicates. 2012). Terrestrial applications of Pb-Pb techniques have been less notable from the perspective of precise dating. 2000). 2009).. 1997). largely thanks to its very high levels of U and especially Th. Jim Mattinson in Santa Barbara understood this (Mattinson. and (2) absence of initial Pb. potentially making 206Pb more leachable than 207Pb (but. The two basic requirements of the method are (1) closed-system behavior. Jercinovic and Williams. chemical abrasion has taken a firm place in the toolbox of most ID-TIMS laboratories and has undoubtedly contributed to improve the quality of many geochronological studies (e. THE ROAD TOWARD CONCORDANCE: A UTOPIAN GOAL? As reviewed earlier herein.. Cottle et al..e. or Pb since its formation. Amelin et al. as mentioned already. 2005. developing a new reliable technique. 1995. Subsequent work. 1990). even in cases where the grains. Frei et al. focusing mainly on genetic questions. it is generally difficult to apply the method to U-rich and metamict grains because they dissolve too easily. the planet Earth. 2005). [2009] in consideration of the more energetic last decay in the 235U compared to the 238U chain). 2008. 2010a. 2009). Subsequent exposure to hydrofluoric acid dissolves the metamict domains much more easily than the annealed crystalline zircon components. A more subtle effect of the original partial dissolution experiments was the preferential leaching from crystalline zircon of “young” Pb (Corfu. Tera and Wasserburg.. mainly because the process introduced some artificial fractionation of U from Pb. 2002. Jahn and Cuvellier... who determined the first (essentially) correct age of 4.. These experiments yielded important insights into the distribution of U and Th inside specific minerals and into their control on Pb evolution and closed-system behavior of potential chronometers (e.gsapubs. and with progressive improvements of the techniques. which makes it ideal for the study of large populations. the method only works if the zircon grains include some domains that have remained closed systems throughout their history. Th. the chemical method cannot take advantage of the information from the two separate decay chains of 235U to 207Pb and 238 U to 206Pb. presumably due to the precipitation of fluorides during dissolution. progress in U-Pb geochronology has been enormous. As pointed out by Mattinson (2011). separating it from the earlier-produced Pb. Crowley and Ghent.g.. which facilitate the measurement. has greatly refined the scale of details of the chronology of the early solar system (e. Košler et al. The LA-ICP method is now widely used to address many different geological problems. Chemical U + Th–Total Pb Dating Chemical dating is based on the determination of the ratio between the total amount of radiogenic Pb and U + Th.. 1991.g. Frei and Kamber. Annealing is important. The main handicap of the method has been the necessity to rely solely on the Pb composition. 1973. Tilton. but these volumes can vary considerably depending on the task envisaged and the operator choices). 1994) but continued nonetheless to experiment with partial dissolution until he eventually succeeded. preventing the smallscale leaching that was an important cause of U-Pb fractionation in the original experiments. Schoene et al. Horn et al. employing the Pb composition of U-poor or U-free minerals and rocks to study the genesis of rocks rather than to date them. 1973. and correcting for common Pb.. The precision tends to be comparable to that achieved by SIMS.. 2000. Another interesting application of the Pb-Pb method is the dating of ancient limestones (Moorbath et al.. which act as independent chronometers and enable us to evaluate whether or not the system has remained closed. 30 μm × 30 μm for LA-ICP-MS. Goncalves et al. In the decade since its development. Jackson et al. the early attempts to remove discordant domains from zircon by means of leaching or partial dissolution were not very successful. which also benefit from improved emitters (Gerstenberger and Haase. namely... The main advantage of chemical dating over other methods is the ability to analyze very small areas of minerals (5 μm or less). and hence Pb. The main initial challenges for LA-ICP-MS were achieving control of the analytical fractionation between U and Pb. Different groups have adopted different strategies for coping with these problems (Košler et al. the method has now undergone a considerable revival (Suzuki and Adachi.. Tatsumoto et al. Because it is not based on the measurement of isotopes.org on May 1.g. recovering the original zircon lattice.. and hence more likely to be concordant or nearly so (Parrish. Montel et al. 2005.. 2004.g. The effect could also be caused by the fact that the 238U chain has one more alpha emission than the 235U chain. are free of alteration and yield concordant data when treated with air abrasion (e.55 Ga for meteorites and. 2008. helping to connect the obtained age to the structural and petrologic context. The critical element in chemical abrasion is the fact that crystalline zircon with a low dose of radioactivity-induced defects can be thermally annealed (at temperatures of 800– 1000 °C). Allègre et al. such as in detrital zircon studies. however. 1997). it cannot restore a lost age.. chemical dating had all but fallen into disuse. with chemically identical pairs of parent and daughter isotopes but different decay speeds. 1999). Thanks to the technical progress of modern electron microprobes.. Schaltegger et al. monazite is much less prone to Pb loss than zircon. 2006). the system has not lost or gained U. but it has been discussed as a possible cause in many other cases where the discordance toward too high 207Pb/235U ratios for uniform 206Pb/238U is distinct. 2001) where the effect is very strong. 665. metamorphism. 2013 Corfu the development of different methods that have been tested and refined. This behavior is very evident in young monazites (Schärer. 2006) plotting in a cluster to the right of the concordia curve (calculated with the decay constants of Jaffey et al. If the Th/U ratio of the magma is known or can be approximated. which produces an excess of 207Pb (Fig. is related to initial disequilibrium in the decay chains from U to Pb. For zircon.88 recommended by Steiger and Jäger (1977). Amelin and Zaitsev.08 m. Another factor that can affect the degree of discordance is the U isotopic composition. do we really know how to navigate around it? Concordant U-Pb ages are “qualitatively. and one concordant analyses. At present. Schmitz and Bowring. (2006). The preoccupation with discordance. (B) Typical example of high-precision isotope dilution–thermal ionization mass spectrometry (ID-TIMS) data (from Schoene et al.g. 1971)... 1998. (A-III) Interpretation of nearly concordant data assuming ancient partial resetting (age 1 by projection from the time of the early disturbance) or modern Pb loss (age 2 by projection from the origin). Early studies had shown that 238U/235U in zircon was homogeneous within 0. which laid a cloud of skepticism over the initial enthusiasm. Thousands of studies have examined the behavior of the decay system. However. the behavior is generally the reverse. the effect can be corrected. it is common to observe that high-quality. An important reason for this behavior is the apparently slightly incorrect U decay constant (Jaffey et al. where the intercepts reflect two distinct events that affected the population. A second set of causes that has been widely discussed. 38 Geological Society of America Bulletin. 2A-I). A related phenomenon is the excess of 231Pa. because the mineral forms mostly with a deficit of 230 Th. However. Parrish. Concordia diagram with data points (ellipses representing analytical uncertainty) illustrating some terms and concepts discussed in the text. the correction causes a shift of ~0. as soon as the analytical error is considered....gsapubs. highly reproducible sets of U-Pb analyses do not plot exactly on the concordia curve but variously to the right of it (Fig. or.y. more detailed modern investigations show that the ratio is likely too high. 2010. … analyses in which the 206Pb/238U. Anczkiewicz et al. such as high Th/U zircon in carbonatites. Yet. 2A-I). the term becomes somewhat fuzzy. Kamo et al. 1965). especially because some of the earliest solar system materials show larger deviations of up to 0. and 207 Pb/206Pb ages are equal within analytical error. do we really understand discordance. The effect of such a change on the calculated ages is relatively minor 206Pb/238U A-III age 1 age 2 concordant A-II discordia line two-stage evolution projected from the origin of the concordia curve (= 0 Ma) projected from an ancient lower intercept age discordant A-I reversely discordant B discordant concordant 207Pb/235U Figure 2. 2003.y. 2006). Today. 2001.4% in the 238U/235U ratio.).org on May 1. which is not affected by 230Th excess. Fig. and the response in different geological settings. The phenomenon has been demonstrated convincingly in two cases (Parrish and Noble. even the use of the better matching decay constant does not always succeed in bringing data sets into concordance. 2012). 2010). and in the following half a century all laboratories used the value of 238U/235U = 137. Hiess et al. and convincingly proven in some cases. Correcting such data with a revised decay constant for 235 U. 2B. occasionally progressing into extreme fogginess. U-Pb dating is the method of choice in many studies of magmatic systems. 2003. can often shift the concordia curve to match the data much more closely. (A-II) Discordia line defined by a suite of discordant. January/February 2013 . U/Pb and Pb/Pb ages whose corresponding isotope ratios have a 95% confidence error ellipse that (to a greater or lesser degree) overlaps the concordia curve” (Ludwig.08% lower (Condon et al. (Brennecka et al.. Near-Concordance In modern high-precision ID-TIMS studies. and measurements of various reference materials give values that are up to 0. and provenance. and resulting in changes of the calculated 207Pb/206Pb ratios of up to 5 m. e.5% (Doe and Newell. which then translates into a deficit of 206Pb (but there are exceptions. 207Pb/235U. 1990) because of the strong geochemical affinity for Th in monazite (and allanite). has been largely dispersed by the subsequent developments. visually. we have reasonably good control of the behavior of U-Pb systems. p.. Parrish and Noble. 1971) as discussed by Schoene et al.. some dropped and picked up again and further expanded. 2002). 2A). Incorporation of excess 230Th at the time of formation of a mineral results in excess 206Pb with data that plot reversely discordant (Figs... but not extreme. in the 28 Ma Fish Canyon tuff zircons investigated by Schmitz and Bowring (2001). as proposed by Mattinson (2010). the behavior of U-Pb in different minerals.Downloaded from gsabulletin. (A-I) Concordant versus discordant analyses.y. The alternative is to use the 207Pb/235U age alone. for example. 1996a. Although the basic definition of concordance is clear-cut. 1984. attributed to the decay of 247Cm. Schoene et al. but it becomes a more important element in meteorite studies. for relatively young minerals (causing an effect of ≤1 m. The effect from 230Th deficit is rather minor. Connelly. 1997.. Direct measurements of 231 Pa in Holocene zircon crystals suggest. Krogh. 1996). 1974.. 2009). These cases are typical of high-grade metamorphic terrains. Example of reverse discordance most likely caused by U loss or Pb gain during a metamorphic overprint. (2005). causing shock metamorphism where zircons or other minerals have been very strongly reworked so that only discordant data can be extracted (Fig. such minerals incorporate high amounts of Th. including 230Th. 1994. 2006). Natural reverse discordance has. January/February 2013 39 .1 Figure 3. i.gsapubs. 2001). 2001. Crowley et al. 2001). or simply analytical biases from improper blank corrections. McFarlane et al. 1E and 1F. Reddy et al. for the 0. fractionation.. Geological Society of America Bulletin. as suggested by the high degree of collinearity of the data and the lower-intercept age of 424 ± 20 Ma. which corresponds to the Caledonian orogeny affecting this region. 1995). or laboratory leaching processes. in terms of loss of 222Rn rather than 231Pa excess (Heaman and LeCheminant.. Corfu et al. 1993. personal observation). and instrumental nonlinearities. or sites affected by meteorite impact. 2011). incorrect common Pb corrections. that the effect on the ages should be very small (Schmitt. northern Norway (Corfu. The results of such thorough partial resetting by recrystallization. 1989). commonly paired with local new growth. 2003). subtle disequilibrium effects can be difficult to distinguish from effects caused by other factors. 1998. and Dunning and Hodych (1990). Pidgeon et al. or in titanite (Tucker et al. (2007) documented an effect corresponding to ~50 k..7 Ma MSWD = 0. data with too high 206Pb/238U and/or too low 207Pb/235U ages plotting above the concordia curve in Wetherill diagrams (Fig. heterogeneities which eventually can result in reversely discordant data due to paleo-annealing events.. hence..31 207Pb/235U 4.. Amelin and Zaitsev (2002). In addition. Moser et al. such as in an Early Archean gneiss from Antarctica where reversely discordant analyses plot along a paleodiscordia array. making it possible to isolate the concordant parts by some of the techniques mentioned previously herein. Pb loss by recrystallization and/or redistribution often means that there may not be any parts of a mineral left with preserved close-system relationships and thus that are capable of providing concordant ages. 1A–1D. In its oxidized form. which are prone to alteration and chemical mobility (Poitrasson et al. such as small amounts of inheritance of xenocrystic material. In practice. By contrast.g.g. Reverse discordance can be produced artificially by SIMS in U-rich zircon in which the sputtering behavior is different from that of the reference zircon (e.... 2013 A century of U-Pb geochronology: The long quest towards concordance Among the most interesting observations are those of Kamo et al. 1987. Cornell et al. e.30 0. focusing in particular on the role of alpha recoil in creating small-scale heterogeneities in zircon or other minerals (see also Mattinson et al. however. 2A-II. or differential sputtering during SIMS analyses. Krogh et al. Reverse Discordance Reversely discordant data.. are linear arrays of data points between the time of crystallization and that of secondary reworking and/or local new growth (e. Krogh and Davis.. Wiedenbeck.g. Solid state diffusion of Pb from zircon is extremely slow and unlikely to be effective at normal crustal temperatures (Mezger and Krogstad. 1975b) or by expulsion and/or intracrystalline redistribution of Pb from the lattice during deformation and recrystallization (Figs.. resulting in reversely discordant data.g. can be the results of several processes that remove U or introduce unsupported radiogenic Pb. Moser et al. The former mechanism generally implies the parallel existence of concordant and discordant domains. In zircon.org on May 1. loss of Pb must be accomplished either by extraction of Pb from altered domains by fluids (Figs. Connelly. 1984). however. Cherniak and Watson.. 1994.33 1750 0. 2011). e.. and another is shown in Figure 3. 2001. 2000). 1994) and in rutile (Blackburn et al. 1996). reverse discordance is more rarely observed. A distinct deviation of 207Pb/235U in baddeleyite has also been interpreted.e. A similar case has also been described by McFarlane et al. The zircon population is from a Paleoproterozoic gabbro (Hamn gabbro) on the island of Senja. 2007). Zircons from a gabbro from the island of Senja in northern Norway plot both normally as well as reversely discordant but define a well-constrained discordia line for which the lower intercept corresponds to the time of the Caledonian orogeny.. been observed in some particular cases. and obtain concordant ages. and hence will normally build up excess 206Pb. 1984). MSWD—mean square of weighted deviates. Hard-Wired Discordance More severely discordant data will generally have been produced by either Pb loss or mixing. Pa+5 is predicted to be more enriched with respect to U (Barth et al. showing that the discordance is not an analytical artifact but is due to ancient redistribution of U and Pb (Williams et al.. 2005.3 ± 0. most likely because U is anchored much more strongly in the crystal lattice than Pb. The same phenomenon can in part also be seen in xenocrystic zircon populations where targeted analyses only provide partially reset discordant ages (e.Downloaded from gsabulletin. The phenomenon is not uncommon in minerals such as monazite or allanite.7 5. 2005.g. hence demonstrating that a gain of radiogenic Pb in parts of some zircon grains during metamorphism was the likely cause. Schmitz and Bowring (2001). 2A-I). as discussed before (Schärer. which affected the region. 206Pb/238U 1850 1830 1810 1790 1770 Intercepts at 424±20 & 1802. Romer (2003) discussed potential mechanisms capable of creating reversely discordant data. Pb loss. McLaren et al.. however.. 1990.y. (1989.8 Ma Bishop Tuff zircon.. 2007.g. A similar uncertainty. the time of residence in crustal magma chambers. it is not possible to construct an accurate projection to the real age. but in the opposite direction.. and calibrating accurately the time scale (e.. and the speed of cooling. Some magmatic systems develop very fast. The alternative is to analyze parts of the minerals with LA-ICP-MS or using combinations of ion probe and ICP-MS (e. or more. (2010).. The technique has the advantage of linking the Hf composition to the age of the same specific parts of multistage zircon and thus makes it possible to evaluate mixing relationships and the composition of end members (Kemp et al. Amelin et al. Antecrysts. January/February 2013 . 2009). especially those concerning the earliest evolution of Earth. Because of the dependence of εHf on the age. at 767. and Hf before the measurement (e. Subsequent analyses were carried out with LA-ICP-MS by measuring Hf and Pb concurrently on the same zircon spots. In felsic magmas that evolve over a period of time. ancient Pb loss from zircon will reduce the 207Pb/206Pb ratio without affecting the Hf composition (Corfu and Noble... yielded a similar spread in 207Pb/206Pb age from 3941 to 4067 Ma. of the Hf isotopic composition concurrently with the 207Pb/206Pb ratio in the same targeted zircon domain.. but their 207Pb/206Pb ages show a distinct dispersion between 3901 Ma and 4029 Ma.Downloaded from gsabulletin. In favorable cases.1 ± 0. Mundil et al. 2007.9% discordant data with a 207Pb/206Pb age of 3465... can be problematic if the data are discordant. the not always easily controllable role of disequilibrium in 230Th. 2001.. 2010b). Harrison et al. The Hf isotopic data are commonly discussed in frames of reference defined by the chondritic evolution. which yields a reproducible cluster of data with just subordinate outliers. Schmitt.. where air-abraded zircon yielded a main group of 0. mixing.. 2005). alternatively.g. it is generally the youngest data points that are considered to represent the most likely age of emplacement.. Modern versus Ancient Pb Loss A somewhat related dilemma. and Pb Loss in High-Resolution Geochronology High-precision U-Pb geochronology has now advanced to a stage where it is possible to study in detail the chronology of magmatism and emplacement processes of complex multistage plutons. In some cases. whereby 176Hf/177Hf ratios are transformed into deviations from the chondritic value at their specific age. These studies employ zircon. whereas in other cases. One critical question affecting modern studies done with chemical abrasion ID-TIMS is whether Pb loss effects can be completely eliminated by the chemical abrasion procedure. including a few as much as 3% discordant. Shen et al. 231Pa.. Recent advances in the analysis of Hf isotopic compositions using multicollector ICP-MS techniques have greatly improved the opportunities to study crustal growth processes.g. yielding precious information into the U-Pb behavior of each grain. and crystallization (e. Pb.. 2003. 2007).g. The determination of the trace-element composition of the dated zircon grains yields additional information that can help to discriminate the genesis of grains with different ages. it is very much open to debate and often decided on the basis of the interpreter’s bias. 2008. yielding the three U-Pb data points shown in Figure 4A. For the sake of discussion.g. by LA-ICP-MS. and 222 Rn. concerns the interpretation of data that are nearly. the choice is strongly supported by the combined available evidence. The subsequent LA-ICP-MS analyses. or. Grain jh17–7 was analyzed in three spots with the ion probe.4 ± 0. 1999. and the discordance is related to ancient disturbances rather than modern Pb loss. studies of plutonic or volcanic rocks reveal more complicated patterns with individual zircon grains defining ages spread over longer periods of up to 1 m. The study utilized zircon initially dated in multiple spots with the ion probe. An interesting example of this dilemma was reported by Stern et al. generally higher. This relationship can be illustrated using an example from Kemp et al. One approach is to dissolve the zircon and separate U. 2013 Corfu SOME MODERN DILEMMAS Role of Xenocrysts.y.1 ± 0.g. Disequilibrium dating by SIMS of zircon in recent magmatic systems also provides fundamental new insights into the evolution of magma chambers (e. mixing processes (Schoene et al. and thought to reflect growth during early as well as late stages of the magma evolution. The main difficulty in interpreting such U-Pb data is in identifying correctly the reasons for the specific distribution of discrete data patterns. however. defining rates and speed of magma emplacement.. and hence the age can be reasonably based on the coherent group.org on May 1. 2A-III). discordia lines are projected through the three analyses from arbitrary lower-intercept ages of 2000 Ma and 3000 40 Geological Society of America Bulletin. 2011). but not fully. 2A-III). therefore facilitating the determination of initial Hf isotopic compositions.. Reid et al.y. Lima et al. discussed previously. zircons are shown to have crystallized within 5 k. Kemp et al. The question as to whether discordant data should be interpreted in terms of modern or ancient disturbances can also have a large impact on the use of Hf isotopic compositions in zircon. calculations with ages that are too low or too high will result in erroneous εHf values. 2011). Bowring and Schmitz. 1992. the consistency with coexisting minerals. their consistency with the various properties of the analyzed zircons. Finally.gsapubs. the ages can be extrapolated by projecting a line through the data from lower-intercept ages corresponding to the time of the supposed event (age 1 in Fig. Some examples show that this assumption is not always valid (Schoene et al. 2001. Schaltegger et al. 2012). Schmitz and Bowring.. indicative of an ancient disturbance. The alternative possibility is that the slight discordance is due to ancient Pb loss: In such cases. Without further information. 2007).6 Ma.. which can be dated by U-Pb and contains much Hf (1%) and little Lu. however. evaluating whether they may have been linked by fractionation.. which is equivalent to using the 207Pb/206Pb age of the near-concordant cluster and assuming a modern disturbance (age 2 in Fig.. The three analyses are concordant within error. 176 Hf/177Hf. 2006).9 ka (Crowley et al. 2010a). done in the same spots. 2006). and the consistency within the geological framework (e. but two chemically abraded zircon grains yielded data points closer to concordia and with a slightly higher 207Pb/206Pb age of 3467. The use of 207Pb/206Pb ages. Miller et al.6 Ma. Whereas new growth of zircon involves the buildup of layers with lower 207Pb/206Pb but distinct. Matzel et al.. Ultimately. which overlapped the upper-intercept age of a line through all the analyses of abraded zircons. can further complicate the interpretation of high-resolution data. The second choice is generally made based on the evidence from independent geological factors. (2009). is introduced by the difficulty of evaluating whether some slightly too old grains might indicate the presence of antecrysts or traces of xenocrystic material.. in the Bishop Tuff. concordant (Fig. More commonly. the overriding criterion for the validity of an interpretation is the mutual coherence of the data. One common solution is to extrapolate to the concordia curve by projecting the data from a lower-intercept age of 0 Ma. a sample contains a uniform zircon population. One of the most recent techniques developed is based on the determination. who reported ages and Hf compositions of Archean detrital zircons from the Jack Hills conglomerates in Western Australia. whereas the older ones are taken to represent antecryst or xenocrysts formed before final consolidation (Miller et al. Ramezani et al. paired with the trends of the data and the behavior dictated by comparable data patterns in related units. which becomes more acute in interpreting Paleozoic and older U-Pb data sets. 2010). 2A-III). for example. Two examples are discussed here. 1997. 6 to –5. Corfu et al. grain jh17–26. and once integrated with the textural information of the zircon grains. These cases are representative of the approach followed in a number of studies focused on early crustal evolution. In this case. is largely a function of the analytical uncertainty. the εHf values calculated with an inferred primary (= concordant age) are dramatically different (giving slightly positive values) from those calculated using the discordant 207Pb/206Pb ages.3 jh17-26. there is again evidence for nonzero Pb loss.7 –4 projected from 2000 Ma –8 4280 ± 77 Ma MSWD =1. as the present examples show.7 to –3 (Fig. Geological Society of America Bulletin.5. In the chosen examples. as well as studies that use Hf isotopes in zircon to deal with the younger history of Earth. making an extrapolation to concordia is much more uncertain. each representing the concordant age of a particular grain. when the difference between the two intercept ages is small enough. (2009). Corfu. age for the data obtained by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) from the same zircon grains as in A. calculated and plotted using both the concurrent 207Pb/ 206Pb ages (the procedure followed by Kemp et al. however. yielding distinct εHf values of –7. whereas the extrapolated age of 4280 Ma produces εHf values that are identical within error (Fig. the two sets of data complement each other very well. Use of a lower-intercept age of 3000 Ma leads to an impossibly old age. they back up a coherent interpretation of two specific events. 4B). The field delimited by the stippled line encompasses all the εHf values calculated using the concurrent 207Pb/ 206Pb ages in Kemp et al. dependent on the magnitude of the uncertainty. is more extreme. and the causes and the effects of U-Pb discordance on the results and their interpretations. 2010) and the inferred upper-intercept age of the discordant arrays. the Hf data were calculated using the concurrent 207Pb/206Pb ages.. The second example. indistinguishable from it (e. whereas a lower-intercept age of 2000 Ma yields an upper-intercept age of 4280 Ma.. MSWD—mean square of weighted deviates. (B) Plot of εHf vs. 4A). 4B). January/February 2013 41 . become subparallel to the concordia curve and. discordance matters.org on May 1. generations. because. or whether it represents a random distribution of dates. Use of the 207Pb/ 206Pb ages of discordant analyses introduces an artificial spread in εHf and a significant bias toward more negative values. respectively.17 Hf 4100 jh17-7.9 A error ellipses are 68. The merits of this projection are also supported by the fact that the upper-intercept age best matches the oldest reported 207Pb/206Pb.. Ma.8 MSWD = 2.gsapubs. whereas the less precise SIMS data all overlap the concordia curve within error (Fig. whereas the 207Pb/206Pb ages obtained concurrently with the Hf composition by LAICP-MS gave 4015 Ma and 3841 Ma. which overlap each other within error. It is not the purpose of this section to advocate specific ages and argue in favor or against specific εHf values for the ancient Jack Hills zircons. which. The more precise ID-TIMS data are discordant but plot on a discordia line (Fig. (2010). Resolving whether a set of data defines a subconcordant linear array representing a single disturbed. Evidently simply ignoring discordance distorts the reality. 2007).9. it is clear that the dispersion of the calculated εHf data is both artificially enlarged and biased toward too negative values. By contrast.1+2 calculated with concurrent 207Pb/206Pb age 3300 30 40 207 Pb/ 235 U projected from 0 Ma 4050 ± 9 Ma MSWD = 3. Here. Subconcordant Data and the Handicap of Low Precision The “hard-wired” discordance discussed in the previous section can produce linear arrays of data points. In Kemp et al. Such a case is demonstrated by the example in Figure 5 using data reported by Moser et al. The lower-intercept age of 2000 Ma is used here only to demonstrate the general concept. 5B).1+2+3 projected from 3000 Ma 4055 ± 130 Ma MSWD = 0. CHUR—chondritic uniform reservoir. (2010). where the probability of fit improves for the older lower-intercept age. The εHf calculated using the concurrent 207Pb/206Pb ages shows a large variation between –6. but the examples stress the importance of a proper consideration of the reality.Downloaded from gsabulletin. 1994. In this particular case.1+2+3 calculated with concurrent 207Pb/206Pb age B CHUR approximate 2 uncertainty 0 + calculated for 4055 Ma all εHf data calculated with concurrent 207Pb/206Pb ages DM 3900 projected from 2000 Ma 3994 ± 45 Ma 0. (A) Concordia diagram showing U-Pb spot analyses by secondary ionization mass spectrometry (SIMS) on two zircon grains jh17–7 and jh17–26 from a Jack Hills detrital zircon population (Kemp et al.3% conf 4 U 206 Pb/ 238 jh17-7.2 and –9. Abbreviations: DM—depleted mantle.g. MC—mafic crust.0 - MC C UC + + 3700 jh17-26. However.1 50 60 –12 calculated for 4280 Ma 3700 3900 4100 age [Ma] 4300 4500 Figure 4. UCC—upper continental crust. the ion probe data yielded two discordant data points with slightly divergent 207Pb/206Pb ages of 4054 Ma and 4031 Ma (Fig. calculation of εHf using the extrapolated upper-intercept age of 4055 Ma yields values of –4.1+2 3500 0. The fact that the resulting εHf values are identical is simply due to the fact that the measured 176Hf/177Hf values are also essentially identical. yielding upper-intercept ages of 3994 Ma and 4055 Ma. 2013 A century of U-Pb geochronology: The long quest towards concordance 0. or two mixed. In absolute terms.. and the geological context. 5A). Hence. 2010). This may not be a very important factor in sedimentary rocks produced from relatively simple sources. 2009).. In good ID-TIMS laboratories.50 2600 A ID-TIMS data 0. (C) Relative probability plot of the SIMS 207Pb/ 206Pb ages extracting a number of peaks and subpeaks. it is likely that some contamination will affect every analysis. such a pattern would normally be interpreted as indicating real ages. (2009). 2009). However.2 pg. Practitioners of all three methods are concerned with keeping the common Pb levels as low as possible. In SIMS and LA-ICP-MS measurements. 2670 Ma 0.. constructed using the data from Moser et al. and the loading on a filament in preparation for the mass spectrometry. For Precambrian zircons.46 2400 206Pb/238U 0. Zircon U-Pb data from a mafic xenolith from the Lace kimberlite in South Africa.gsapubs. and have devised specific techniques to achieve this goal. The zircon formed magmatically at 2670 Ma and was strongly disturbed by the processes caused by deep crustal deformation and metamorphism after the Vredefort meteorite impact (Moser et al. where the data are considered “concordant” as long as they are not more than several percent discordant (often a 5% or 10% cutoff limit is used). a level still easily detected in a thermal ionization mass spectrometer.. The statistical treatment extracts the two main age peaks corresponding to the initial crystallization at 2670 Ma and secondary overprint in response to the Vredefort impact at 2023 Ma.e. but it is likely to have serious repercussions on the apparent age distribution produced in zircon derived from more complex high-grade terranes. Spot analyses show that zircon lost Pb due to crystal-plastic deformation. if ever. the Pb blank reaches levels as low as 0. The initial and the final peaks correspond to the two real events. such as an island arc. 2013 Corfu taken alone without the supplementary information.42 2200 2600 2023 Ma 0. January/February 2013 . considered. there are also several other peaks of variable magnitude. although full resetting was achieved only in a few domains (Moser et al. or those that are altered or contain inclusions of Pb-bearing minerals. (B) Secondary ionization mass spectrometry (SIMS) data showing the same trend as the ID-TIMS data but at lower resolution.ages 2000 2200 207Pb/206Pb 2400 2600 . and to some degree also monazite. the 207Pb/206Pb ages will generally be used and interpreted by means of probability density diagrams. Hence. 42 Geological Society of America Bulletin. in a modern detrital zircon study.34 5 11 13 Relative probability C SIMS 207Pb/206Pb . In fact.age [Ma] Figure 5. The ID-TIMS method introduces Pb and U contamination during the dissolution. and the possibility that individual subconcordant data may be recording partial resetting or mixing is rarely. contaminant Pb (and U) can be introduced mainly during the mounting and polishing process. Some common geochronometers such as titanite and apatite.38 2000 2400 0. (A) Isotope dilution–thermal ionization mass spectrometry (ID-TIMS) data plot along a discordia line. However. but the intermediate ones are spurious and geologically meaningless. a few femtograms 204Pb. or multiple sources if this were a detrital zircon population. except crystals with rare compositions.38 2000 9 11 13 B SIMS data 7 9 207Pb/235U 0. Administrative Concordancy Some contamination by common Pb (and U) is essentially unavoidable in all dating techniques. Zircon is generally virtually free of initial common Pb. the SIMS data could also be explained in terms of multiple growth periods. The example shows that an awareness of discordance and precision are important for accurate interpretations. which in this case are known to be geologically meaningless.34 Vredefort 7 crater 2200 rebound 0. Nevertheless. is shown in Figure 5C. Such an artificial example.org on May 1.Downloaded from gsabulletin. or of detrital zircon populations done by SIMS or LA-ICP-MS. the chemical separation. i. the pattern mimics that seen in many modern studies of complex high-grade terrains. incorporate some initial Pb. Conversely. out of convenience.. 2013 A century of U-Pb geochronology: The long quest towards concordance ID-TIMS analyses always include the measurement of 204Pb.. U. measuring the 204Pb peak can be difficult because of the very small signal produced during the analysis and because of overlaps with potential isobaric interferences. FINAL REMARKS Progress in the development of U-Pb dating techniques has been driven forward by the spirit of technical innovation. and hence zircon in highly evolved rocks will simply inherit the low Th/U from the magma. with both a general increase in U and Th levels during evolution of the systems and a gradual decline in their Th/U ratio (from Rybach. however. An important branch also deals with stable isotopes in zircon (e. and the mineral’s chemical composition. 2002). based on the assumption that zircon has no common Pb and that the blank is negligible. many exceptions.. but at the same time. their compositions and uncertainties.. for samples that are actually discordant. 2005. which is then used to correct for the nonradiogenic Pb. 1994). however.g. the main problem is caused by the interference on 204Pb by 204Hg introduced with the Ar gas.. Moser and Heaman. 2007) and should only be considered in combination with other lines of evidence. Discordance. One of the most commonly discussed geochemical discriminants of U-bearing minerals is the Th/U ratio. One factor affecting the ratio is the scavenging of Th by other minerals. high Th/U ratios can be very prominent in metamorphic zircon and titanite of high-grade rocks (e. In LA-ICP-MS analyses. The latter assumes instead that Th. For example. 1997.2. the so-called 207Pb and 208 Pb corrections are also used. 2008).Downloaded from gsabulletin. the historical consciousness of discordance simply disappears from the collective awareness.0. Fu et al. 1998). January/February 2013 1: 1 1 6: U [ppm] Th /U U [ppm] 43 . The recognition of the causes of U-Pb discordance and establishment of techniques to deal with this problem have contributed to the emergence of U-Pb as a prime geochronological tool.0–0. Questions affecting the precision are usually related to the proportion of blank versus initial Pb. It is. or it can be estimated from 208Pb/206Pb. The disadvantage of the 207Pb method is that. the dictated administrative concordance will result in an overcorrection and a biased age. whereas metamorphic zircon in amphibolite.g. Hoskin and Schaltegger.. Belousova et al. and to how well one knows. remains a general problem. the type and spatial arrangement of inclusions. 2009).and eclogite-facies rocks tends to have ratios of 0. but another important reason is the fact that Th/U tends to decrease progressively during the evolution of granitic systems (Fig. 2003). it has been moderated by some inherent limitations. Whitehouse and Platt. Harley et al. it is important to stress the fact that Th/U alone has a very limited diagnostic value (Whitehouse and Kamber. An increasing number of studies have been devoted to the systematic assessment of the role of trace elements in zircon (Heaman et al. pegmatites. and also implies a loss of information on the mutual isotopic relationships between grains. in principle. aplite stocks 10 aplites 1 10 100 Th 3: 1 [ppm] schist gneiss granite 10 1 + pegmatite + + + 1 10 0. Valley et al. Whitehouse and Nemchin. especially of the rare earth elements (REEs) (Rubatto. besides measuring 204Pb directly and correcting as in ID-TIMS. such as monazite or epidote (Bingen et al. 6). and Pb were not disturbed in the mineral and calculates the amount of common Pb from the difference between the measured and the expected 208Pb/206Pb.2–0. and choose instead to either make no correction at all. 2011). possible to minimize the amount of Hg present and correct for it by measuring the abundance of 202Hg (Gerdes and Zeh. the minimization of which still requires work and creative solutions. 1973. the latter for geothermometry.. aplites. 2002. Thus. Many imaginative approaches had to be developed to make the system work.org on May 1. 2008). Whole-rock Th-U ratios in granitic systems from the central Alps show a very systematic behavior.1 Geological Society of America Bulletin. Kramers and Mouri. Corfu et al. and for very small samples they become its dominant element.gsapubs. Rubatto. has been shown to introduce significant uncertainties in samples with normal to high Th/U (Williams. 2002. 2005. or they correct the common Pb with a model calculation that assumes a coherent behavior of Th/Pb and U/Pb and estimates the time of the isotopic disturbance (Andersen. information which can be important for the interpretation. Discriminating Origins of Zircon (and Other Minerals’) Based on Th/U Factors that are important for a proper interpretation of U-Pb data include the geological and mineralogical context of the studied minerals. These factors propagate into the total uncertainty. 2003) and Ti (Watson and Harrison. It is indeed correct that magmatic zircon and titanite quite commonly yield Th/U of 1. the internal textures of the minerals. or can estimate. unless. very low Th/U ratios are very common in magmatic zircons grown in highly evolved granites. Both approaches can have problematic implications for the accuracy of the data and the geological deductions. 2001. The 208Pb correction. In SIMS analyses. and leucosomes in migmatites. Hence. 1973).. 2005. but most users consider that this correction introduces too much uncertainty. The ratio is especially low in pegmatites and aplites. The ratio of Th/U is popular mainly because of the widespread perception that it can be used to distinguish magmatic from metamorphic zircon (or titanite) (Bingen et al.. the age relationships between different components of a mineral and between different coexisting minerals. however. 2001). 2002. 1990. Rybach and Labhart.. whole-rock values Th [ppm] 1 4: 100 main body Central Aaregranite Bergell Giuv 2: Th /U 1: 1 marginal facies Mont Blanc Vallorcine Grimsel Figure 6. 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