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Lecture 9 Spring 2003
GEOCHRONOLOGY V
THE U-TH-PB SYSTEM: ZIRCON DATING
Zircon (ZrSiO 4 ) is a mineral with a number of properties that make it extremely useful for geochro- nologists. First of all, it is very hard (hardness 71 / 2 ), which means it extremely resistant to me- chanical weathering. Second, it is extremely resistant to chemical weathering and metamorphism. For geochronological purposes, these properties mean it is likely to remain a closed system. Third, i t concentrates U (and Th to a lesser extent) and excludes Pb, resulting in typically very high 238 U/^204 Pb ratios. It is quite possibly na- ture's best clock. Finally, it is reasonably common as an ac- cessory phase in a variety of igneous and metamorphic rocks. The very high 238 U/^204 Pb ra- tios in zircon (and similar high μ minerals such as sphere and apatite) provide some special geochronological opportunities and a special diagram, the concordia d i a - gram , has been developed to take advantage of them. The discussion that follows can be applied to any other system w i t h e x t r e m e l y h i g h (^238) U/ (^204) Pb ratios, but in prac- tice, zircons constitute t h e principle target for Pb geo- chronologists. A concordia diagram is sim- ply a plot of 206 Pb/^238 U vs. (^207) Pb/ (^235) U. You should satisfy yourself that both of these ra- tios are proportional to time. In essence, the concordia dia- gram is a plot of the 238 U–^206 Pb age against the 235 U–^207 Pb age. The ‘concordia’ curve on such a diagram that is the locus of points where the 238 U–^206 Pb age equals the 235 U–^207 Pb age. Such ages are said to be concordant. Figure 9.2 is an example of a concordia diagram. The best way to think about evolution of Pb/U ratios is to imagine that the diagram i t - self evolves with time, along with its axes, while the ac-
Figure 1. Upper. Separated Zircon crystals. Notice the zoning.
Lower. Strongly zoned zircon showing differing ages of spots ana- lyzed by ion probe.
Lecture 9 Spring 2003 tual data point stays fixed. Let’s take a 4.0 Ga old zircon as an example. When it first formed, or “closed”, it would have plotted at the origin, because had anyone been around to analyze it, they would have found the 207 Pb/^235 U and 206 Pb/^238 U ratios to be 0. Initially, 207 Pb/^235 U would have in- creased rapidly, while the 206 Pb/^238 U would have been increasing only slowly. This is because 4.0 Ga ago there was a lot of 235 U around (recall that 235 U has a short half-life). As time passed, the in- crease in 207 Pb*/^235 U would have slowed as the 235 U was ‘used up’. So imagine that the diagram ini- tially 'grows' or 'expands' to the left, expanding downward only slowly. Had someone been around 3.0 Ga ago to determine 'zircon' ages, he would have drawn it as it appears in Figure 9.3 (of course, h e would have labeled the 3.0 Ga point as 0, the 4.0 Ga point as 1.0, etc.). Any zircon that has remained as a completely closed system since its crystallization must plot on the concordia line. What happens when a zircon gains or looses U or Pb? Let’s take the case Pb loss, since that is the most common type of open-system behavior in zircons. The zircon must lose 207 Pb and (^206) Pb in exactly the proportions they exist in the zircon because the two are chemically identical. In other words, a zircon will not lose 206 Pb in preference to 207 Pb or visa versa. Let’s take the specific case of a 4.0 Ga zircon t h a t experienced some Pb loss during a metamorphic event at 3.0 Ga. If the loss was complete, the zircon would have been reset and would have plotted at t h e origin in Figure 9.3. W e cannot, of course, distin- guish a zircon completely reset at 3.0 Ga from one that crystallized at 3. Ga, but suppose it lost only half its Pb at that time. During the Pb loss, t h e (^206) Pb/ (^238) U and 207 Pb/ (^235) U
4.5 Ga
4.0 Ga
3.5 Ga
3.0 Ga
2.5 Ga
2.0 Ga
1.5 Ga
1.0 Ga
0.5 Ga
207 Pb/ 235 U
238 U
206 Pb
Figure 9.2. The concordia diagram. 0 10 20 30 40 50 60 70 3.0 Ga 4.0 Ga 4.55 Ga (^207) Pb/ (^235) U 0
(^238) U (^206) Pb Figure 9.3. A concordia diagram as it would have been drawn at 3.0 Ga.
Lecture 9 Spring 2003 the concordia. As is the case for Pb loss, the upper inter- cept of the cord gives the ini- tial age and the lower inter- cept gives the age of U loss. U loss in less common than Pb loss. This is true for two rea- sons. First, U is happy in t h e zircon, Pb is not. Second, Pb will occupy a site damaged by the alpha decay, making diffusion out of this site eas- ier. Radiation damage is a significant problem in zircon geochronology, and one of the main reasons ages can be imprecise. U-rich zircons are particularly subject to radia- tion damage. Heavily dam- aged crystals are easily rec- ognized under the microscope and are termed metamict. Pb gain in zircons is not predictable because the iso- topic composition of the Pb gained need not be the same as the composition of the Pb in the zircon. Thus Pb gain would destroy any age rela- tionships. However, Pb gain is much less likely than other open system behaviors. Zircons that have suffered multiple episodes of open system behavior will have U-Pb systematics that are difficult to interpret and could be incorrectly inter- preted. For example, zircons lying on a cord between 4. and 3.0 Ga that subsequently lose Pb and move on a second cord toward the 2.0 Ga could be interpreted as having a metamorphic age of 2.0 Ga and a crystallization age of between 4.0 and 3.0 Ga. Continuous Pb loss from zircons can also complicate the task of interpretation. The reason is that in continuous Pb loss, zircons do not define a straight line cord, but rather a slightly curved one. Again imagining that the concordia diagram grows with time, a zircon loosing Pb will always move toward the origin. However, the position of the origin relative to the position of the zircon moves with time in a non-linear fashion. The result is a non-linear evolution of the isotopic composition of the zircon. Given the mechanical and chemical stability of zircon, it should not be surprising that the oldest terrestrial material yet identified is zircon. Until a decade ago, the oldest dated terrestrial rocks were the Isua gneisses in Greenland. These are roughly 3850 Ma old. Work published in 1989, re- vealed that the Acasta gneisses of the Slave Province (Northwest Territories, Canada) are 3.96 Ga .
20.0 (^) 30.0 40.0 (^) 50.
20.0 (^) 30.0 40.0 (^) 50. 4000 3800 3600 3400 3200 3800 4000 3600 3400 3200 (^207) Pb/ (^235) U
Pb/
U
Pb/
U Cores Altered Cores Altered Massive Massive Zoned Cores Clear Prismatic Equant Turbid Prismatic Banded Gneiss BGXM Porphyritic Gneiss SP- H H HH H H H H H Figure 9.6. Concordia diagrams showing ion probe Pb-U analyses of Acasta gneiss zircons. Size of point is proportional to 1 s analytical uncertainty. Triangles are zircon analyses done by conventional mass spectrometry. From Bowring, et al, 1989.
Lecture 9 Spring 2003 old. These ages were determined using an ion probe to date the cores of zircon crystals extracted from these gneisses. Concordia diagrams for these gneisses are shown in Figure 9.6. Zircons having ages in the range of 4100-4260 Ma have been identified in quartzites at Mt. Narryer and the Jack Hills in western Australia (e.g., Compston and Pidgeon, 1986). The quartzites them- selves are metamorphosed sandstones that were probably deposited about 3100-3300 Ma. They con- tain zircons derived from a number of sources. A small fraction of these zircons has cores that are in the range of 4100-4200 Ma. The zircons were analyzed by a specially built high resolution ion probe at the Australian National University nicknamed ‘SHRIMP’. The great advantage of this instru- ment over conventional analysis of zircons is not only that individual zircons can be analyzed, but in- dividual parts of the zircons can be analyzed. The Mt. Narryer zircons have had complex histories suffering multiple metamorphic events between 4260 and 2600 Ma. The principle effect was t h e growth of rims of new material on the older cores around 3500 Ma. Conventional analysis of these zir- cons would not have recognized the older ages. The cores of these zircons, however, proved to be nearly concordant at the older ages. These ages determined by ion probe were initially highly con- troversial. By and large, however, the community has come to accept them as reliable, when per- formed carefully. Subsequently even older zircons (would be more correct to say parts of zircons), were discovered in the Jack Hills of Australia. An ion probe date on one part of one of these zircons (Figure 9.6) is shown 4.404 Ga ±8 Ma. Thus the oldest known terrestrial materials are approaching the oldest ages from other planetary bodies, including the Moon, Mars, and asteroids (as represented by meteorites). They remain, however, significantly younger than the age of the Solar System, which is 4.556 Ga. Never- theless, these very old ages seem to demonstrate that it is zircons, not diamonds, that “are forever”. REFERENCES AND SUGGESTIONS FOR FURTHER READING Bowring, S. A., I. S. Williams, and W. Compston, 3.96 Ga gneisses from the Slave province, North- west Territories, Canada, Geology , 17: 971-975, 1989. Compston, W. and R. T. Pidgeon, Jack Hills, evidence of more very old detrital zircons in Western Australia, Nature , 321:766-769, 1986. Wilde, S. A., J. W. Valley, W. H. Peck and C. M. Graham, Evidence from detrital zircons for the exis- tence of continental crust and oceans on the Earth 4.4 Gyr ago, Nature , 409:175-178, 2001.
