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Thus, the chemical Pb-U, Th approach depends upon all lead in an uranium mineral being radiogenic in origin, its amount increasing as a function of time.Th are longer by orders of magnitude than those of their respective daughters.For this reason, this mineral is widely utilised in dating by the U, Th-Pb isotope method, for instance by C. It is very difficult, however, to link a discordant date with a particular geological event.The loss of radiogenic Pb is less serious and the ‘207- 206' date derived may well approach the actual age of the zircon fairly closely.Although such discordancy in dating is common, there are numerous instances of concordant dates (Bowen, 1988).Decay of the naturally occurring isotopes of uranium to form stable, radiogenic lead provides two independent geochronometers.

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When in pegmatites, zircons contain more of these elements than they do in normal igneous rocks.

Uranium and thorium are found in zircon through the isomorphous replacement of Zr is excluded altogether because its ionic radius is 1.32 and it bears a lower charge. Sometimes model dates for these minerals are discordant as consequence of non-fulfilment of some of the dating assumptions.

The result is that zircon does not contain much lead at the time of its formation and has very high ratios for U/Pb and Th/Pb, making it a valuable geochronometer (Bowen, 1988). Loss of radiogenic Pb in zircons may be due to continuous diffusion or to the influence of metamorphic episodes.

Zircon is ubiquitous in granites, and is more resistant to isotopic disturbance by secondary geological processes such as metamorphism, alteration and weathering than most other minerals. These have resulted in its being possible to make increasingly precise analyses of ever smaller zircon samples (Williams, 1992).

Also, the U-Pb system itself has the unique feature that, because it consists of two U isotopes that decay at different rates to two Pb isotopes, any isotopic disturbance that does occur can be detected by a simple test for internal consistency between the radiogenic Pb isotopic composition and the Pb/U ratios (Williams, 1992). Much slower to develop, however, has been an understanding of the properties of the zircon U-Pb system as such.




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