5 The U-Pb system
Th, U and U each form the start of long decay series comprising and Pb. The ingrowth equations for the three radiogenic Pb isotopes are given by: This assumption cannot be made for other minerals, young ages, and high. 22 a. Decay of U and Th to Pb . Minerals used in U-Th-Pb dating. Mineral. Formula. U content. (ppm). Th/U Detection of age domains in complex zircon. GEOL U-Pb dating handout p. 2. Note that this is linear on a plot of 87Sr/ 86Sr vs87Rb/86Sr, with the slope related to the age and the intercept to the initial .
Such trapped Ar is not problematical when the age of the rock is in hundreds of millions of years. The dating equation used for K-Ar is: Some of the problems associated with K-Ar dating are Excess argon. This is only a problem when dating very young rocks or in dating whole rocks instead of mineral separates.
Minerals should not contain any excess Ar because Ar should not enter the crystal structure of a mineral when it crystallizes. Thus, it always better to date minerals that have high K contents, such as sanidine or biotite. If these are not present, Plagioclase or hornblende. If none of these are present, then the only alternative is to date whole rocks. Some 40Ar could be absorbed onto the sample surface.Radiometric or Absolute Rock Dating
This can be corrected for. Most minerals will lose Ar on heating above oC - thus metamorphism can cause a loss of Ar or a partial loss of Ar which will reset the atomic clock. If only partial loss of Ar occurs then the age determined will be in between the age of crystallization and the age of metamorphism.
If complete loss of Ar occurs during metamorphism, then the date is that of the metamorphic event. The problem is that there is no way of knowing whether or not partial or complete loss of Ar has occurred. Thus the ratio of 14C to 14N in the Earth's atmosphere is constant. Living organisms continually exchange Carbon and Nitrogen with the atmosphere by breathing, feeding, and photosynthesis.
When an organism dies, the 14C decays back to 14N, with a half-life of 5, years.
Measuring the amount of 14C in this dead material thus enables the determination of the time elapsed since the organism died. Radiocarbon dates are obtained from such things as bones, teeth, charcoal, fossilized wood, and shells. Because of the short half-life of 14C, it is only used to date materials younger than about 70, years.
Other Uses of Isotopes Radioactivity is an important heat source in the Earth. The Pb-Pb method has the following advantages over conventional U-Pb dating: There is no need to measure uranium.
The method is insensitive to recent loss of U and even Pb, because this would not affect the isotopic ratio of the Pb. In practice, the Pb-Pb method is rarely applied by itself but is generally combined with the U-Pb technique. With time, the data shift further away from the origin.
The upper intercept of the linear array aka discordia line can be used to estimate the crystallisation age, whereas the lower intercept yields the age of metamorphism. The greater the distance from the expected composition at t, the greater the degree of Pb loss and the greater the linear extrapolation error on the crystallisation age Figure 5.
Zircon is a very durable mineral that undergoes minimal chemical alteration or mechanical abrasion. Therefore, zircon crystals can be considered time capsules carrying the igneous and metamorphic history of their proto-sources. The probability distribution of a representative sample of zircon U-Pb ages from a detrital population can serve as a characteristic fingerprint that may be used to trace the flow of sand through sediment routing systems.
As a provenance tracer, zircon U-Pb data are less susceptible to winnowing effects than conventional petrographic techniques. Due to the robustness of zircons as a tracer of sedimentary provenance, and the relative ease of dating them, the use of detrital zircon U-Pb geochronology has truly exploded in recent years. At present, nearly a thousand detrital zircon publications appear each year.
An extensive survey of late Archaean sandstones from the Jack Hills in Australia have revealed a subpopulation of detrital zircons with Hadean 4. These are the oldest terrestrial minerals known to science, predating the oldest igneous rocks by million years.