U-Pb Concordia

The Lac Turgeon Granite was previously dated at >980 Ma, based on a U-Pb Concordia intercept (Rimsaite 1982), and 948 23 Ma from a whole-rock Rb-Sr isochron. It has an initial Sr/ Sr of 0.7090 0.0043 (Fowler Doig 1983) it is relatively young suggesting it has been partially reset or remained open. 

Ludwig, K.R., 2000. Decay constant errors in U-Pb concordia-intercept ages. Chem. Geol., 166 315-318. 

Figure 1 U-Pb Concordia diagram showing the results of an analysis of a detrital zircon crystal by ion microprobe (SHRIMP) followed by analysis of the same crystal using TIMS. Both error ellipses are plotted at 2a. The best estimate of the age of crystallization of the zircon is identical for both techniques however, the TIMS analysis is an order of magnitude more precise than that obtained using the ion microprobe (source Samson et aL, 2003).

BOX 1.1 FIGURE 3 U-Pb Concordia curve showing the compositions of individual analyses as error ellipses. The data, which are for a single grain, show that the grain has a core which is about 3,350 Ma old and a rim which is about 2,700 Ma old. The rim has experienced subsequent lead loss, giving rise to a series of discordant points on the Concordia diagram. This lead loss reflects a later thermal event at about 1,000 Ma. 

Fig. 3.17 The U-Pb concordia curve is plotted by solving Eqs. 3.16 and 3.17 for the same value of t in order to obtain the 206p /238 j 207p 3 /235u j- tios of points that represent concor-...

The isochron diagram was described above in the section about the Rb-Sr method. The U-Pb concordia method differs from simple isochron methods in that it utilizes the simultaneous decay and accumulation of two parent-daughter pairs 2 U- ° Pb and U- ° Pb. If we rewrite Eq. (5) in terms of U and its final daughter product... 

FIGURE 4 U-Pb concordia diagram. Concordia is iocus of aii points representing equai 207pb 35(j 206p[3/238y gges. Lo-... 

Figure 13. Tera-Wasserbnrg style three-dimensional concoidia plot for the WHCl stalactite (Richards et al. 1988). Shown is the projection onto the U/ ° Pb- ° Pb/ ° Pb plane (see Fig. 10 caption). The intercept of the three-dimensional fit with this plane is shown by the shaded error ellipse. A) Three-dimensional regression, disequilibrium concordia (5 " U(0) = 320 dashed line], and plane intercept. B) Detail of intercept with 2 U/ ° Pb- ° Pb/ Pb plane, disequilibrium concordia (2o error), regression line, and the secular equilibrium concordia.

Figure 5-6 The concordia and an example of discordia for U-Pb dating. The x-axis is The y-axis is ° Pb / U. The...

Another coupled system is the Sm-Nd system, with two Sm isotopes ( Sm and Sm) undergoing ot-decay to become two Nd isotopes ( Nd and Nd). The half-life of Sm is 106 billion years and that of is 103 million years. In principle, the concepts for the U-Pb system (such as concordia and discordia, Nd-Nd isochron) can also be applied to the Sm-Nd system. However, the Sm-Nd coupled system has not found many applications. One reason is that the half-life of " Sm is so short that it is an extinct nuclide. Secondly, the half-lives of Sm and " Sm are very different, by a factor of 1000 (in contrast, the half-lives of and 235 differ only by a factor of 6.3). Hence, the coupled system has found only limited applications to very old rocks, such as meteorites and very old terrestrial rocks. 

Wetherill s concordia diagram for interpreting discordant U-Pb dates caused by episodic loss of radiogenic lead. 

One drawback of the Pb-Pb system is that two-stage or three-stage evolution of the U-Pb isotopic system can produce linear arrays on a Pb-Pb isochron plot (Gale and Mussett, 1973). If these arrays are interpreted as single-stage isochrons, incorrect dates may be obtained. In some cases, measurements of the U/Pb ratio can help identify multistage samples. But if several events occurred sufficiently early in solar system history, even a concordia approach may not be able to identify a multistage system (Tera and Carlson,... 

SHRIMP 238U/206Pb ages as old as 7.2 Ga, much older than the Pb/ Pb Concordia age of 2.68 Ga. Isotope dilution thermal mass spectrometry analysis of the same zircons demonstrated that the ion microprobe U/Pb ratios were too low. This was attributed to the presence of a labile Pb component within amorphous microdomains that had an anomalously low U/Pb ion yield, which invalidated the SHRIMP U-Pb ion yield calibration. 

Figure 9. U-Pb ages of phosphates from ordinary chondrites. Numbers on the Concordia line are in Ga. The oldest solidification age (for H chondrites) is --4.563 Ga ago and thermal metamorphism occupied the next 60-70 Ma. (Reproduced with permission from reference 1. Copyright 2006 Elsevier.)...

Figure 1. Concordia diagram illustrating misbehavior in the U-Th-Pb system that is relevant to U-Pb dating. Arrows show how concordant data can be affected by inheritance, Pb loss, disequilibrium, and common Pb (Pb°) corrections.

Size-fractions of zircon extracted from a pluton of unfoliated quartz diorite which intrudes the intensely folded metasedimentary rocks of the Anthill and Cocks formations in the Skelton Glacier-Cocks Glacier area at 78°39 and 161°00 E, were dated by Rowell et al. (1993) by the U-Pb method. The data define a chord that intersects concordia at a point that corresponds to a well-constrained date of 551 4 Ma. Zircon from a second pluton, composed of granite and located only 1 km from the quartz-diorite pluton, yielded an identical date of 551 4 Ma (Encarnacion and Grunow 1996). This date not only establishes the age of the two plutons, but also sets a lower limit to the time of deposition and deformation of the metasedimentary rocks that were intruded by these plutons. 

These equations are used to calculate compatible sets of 2o pb p u and Pb / U ratios for selected values of t. The resulting values of these ratios are the coordinates of points that define the concordia curve which is the locus of all points whose coordinates yield concordant U-Pb dates. Any point that represents a mineral whose measured 206pjj /238u 207p j /235pj j-gbos do... 

Concordia plots allow the interpretation of the geological histories of uranium/lead systems and provide information on the past disturbances of these systems. Several concordia models have been developed in order to interpret the discordancy of the U/Pb dates of uranium-bearing minerals. 

Concordia diagram showing the effects of lead loss or uranium gain on U/Pb systems, fcryst is the time elapsed since original crystallization, fcios is the time elapsed since closure of the system after lead loss or uranium gain. A235 and A238 are the decay constants of the respective primary parents and... 

FIGURE 31.12 U-Pb age determination on zircon from Cinque Frati granite (Corsica, France). (A) Common-Pb uncorrected Tera and Wasserburg diagram for data obtained with LA-MC-ICP-MS (290.7 2.4 Ma [2o] MSWD = 2.7 [n = 20]). (B) Conventional Concordia diagram data corrected for common-Pb using Pb (ID-TIMS) (286.4 + 1.7 Ma [2oj MSWD = 1.2 [n = 7]). 

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