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Presentation on theme: "U-Th-Pb Decay Systems 9/9/10"— Presentation transcript:
Therefore igneous rocks of granitic composition are more enriched in U and Th than basaltic or ultramafic rocks Consequently, the continental crust has more U and Th than the upper mantle. Similar to previous systems, the decay equations can be solved if the concentrations of U, Th and Pb are determined along with the isotopic composition of Pb.
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Therefore in theory, you end up with three independent dates based on three separate decay systems. The mineral has remained closed to U, Th and Pb and all intermediate daughters throughout its history Correct values are used for the initial Pb isotope ratios The decay constants of U, U and Th are known accurately The isotopic composition of U is normal and has not been modified by isotope fractionation or by the occurrence of a natural chain reaction based on fission of U.
All analytical results are accurate and free of systematic errors. This ratio is insensitive to recent lead loss, ie when the Pb that was lost had the same isotope composition as the Pb that remained in the mineral being analysed. What this means is that the calculation of dates does not require the concentration of Pb or U to be known, just the isotope ratios of Pb.
It can indicate the time of Pb-loss if Pb loss is episodic, ie occurs as a single event If however Pb-loss occurs via continuous diffusion then the lower intercept has less meaning Monazite sometimes displays U-loss which causes it to plot above the concordia curve on an extrapolation of the discordia line. Those indicated by the light blue the points between roughly — Ma are discordant and plot on a discordia line.
5 The U-Pb system
There is a difficulty in using radiometric dating to put an age on sedimentary rock. The problem is that sediment is made up of clasts of some parent rock , and when we date these clasts , we are in effect dating the parent rock rather than the sediment as such. If, for example, we apply U-Pb dating to a grain of zircon found in sandstone , we aren't dating the formation of the sandstone, we're dating the formation of the granite that the zircon came from; all we could say about the sandstone is that it must be younger than that. However, it is possible to put a date on some sedimentary rocks using the mineral xenotime Y P O 4.
Uranium can and often does substitute for the element yttrium, whereas lead cannot, making xenotime suitable for radiometric dating. The key fact about xenotime is that since it has the same crystal structure as zircon, it can grow on zircon crystals , forming a crust; and this process, of course, cannot begin to take place while the zircon crystal is still locked inside its parent rock. The zircon will only start acquiring its xenotime crust after weathering and erosion have freed it from its parent rock and it becomes sediment.
So by dating the xenotime crust, we can find out when the zircon it's growing on became sediment ; dating the zircon itself would tell you the age of the parent rock. A speleothem , more colloquially known as a cave formation , is formed when minerals dissolved in water precipitate out of the water as it drips, seeps, or flows into a cave. The reader will probably be familiar with stalagmites and stalagtites ; more speleothems are shown in the photograph to the right.
Now, compounds of uranium are often highly soluble in water this, indeed, is one of the major problems with U-Pb isochron dating whereas compounds of lead are stubbornly insoluble. As a result, we expect speleothems when they are first formed to contain some uranium but little or no lead — just like zircons.
So we can apply the same technique to speleothems as we do to zircons. We can exploit our double system of U- Pb and U- Pb in another way.
Presentation on theme: "GEOCHRONOLOGY 2006 Lecture 04 U-Th-Pb Dating"— Presentation transcript:
It can be shown mathematically that if the rock has been undisturbed, so that the isotope ratios reflect nothing but the passage of time, then just as with the isochron diagrams we've already discussed though for a different reason the minerals so plotted will lie on a straight line on the graph; and the age of the rock can be calculated from the slope of the line. Unlike the ordinary isochron methods such as Rb-Sr , the Pb-Pb method does not allow us to deduce the original proportions of the various lead isotopes from the data acquired from the sample.
Instead, we need to find this out some other way. We can do this by finding minerals that contain lead but never contained any uranium, or only ever contained it in negligible quantities.
Troilite Fe S from iron-rich meteorites fits the bill: You might perhaps doubt that meteorites would have the same initial lead isotope ratios as the Earth. Planetary scientists maintain that they should, for reasons which are somewhat beyond the scope of this textbook. These fission tracks inevitably act as conduits deep within the crystal, thereby providing a method of transport to facilitate the leaching of lead isotopes from the zircon crystal.
Under conditions where no lead loss or gain from the outside environment has occurred, the age of the zircon can be calculated by assuming exponential decay of Uranium. These are said to yield concordant ages.
Historical Geology/U-Pb, Pb-Pb, and fission track dating
Loss leakage of lead from the sample will result in a discrepancy in the ages determined by each decay scheme. This effect is referred to as discordance and is demonstrated in Figure 1. If a series of zircon samples has lost different amounts of lead, the samples generate a discordant line. The upper intercept of the concordia and the discordia line will reflect the original age of formation, while the lower intercept will reflect the age of the event that led to open system behavior and therefore the lead loss; although there has been some disagreement regarding the meaning of the lower intercept ages.
Zircon is very chemically inert and resistant to mechanical weathering—a mixed blessing for geochronologists, as zones or even whole crystals can survive melting of their parent rock with their original uranium-lead age intact. Zircon crystals with prolonged and complex histories can thus contain zones of dramatically different ages usually, with the oldest and youngest zones forming the core and rim, respectively, of the crystal , and thus are said to demonstrate inherited characteristics.
Unraveling such complications which, depending on their maximum lead-retention temperature, can also exist within other minerals generally requires in situ micro-beam analysis via, say, ion microprobe SIMS or laser ICP-MS. From Wikipedia, the free encyclopedia. The disintegration products of uranium: American Journal of Science Retrieved 7 January Alpha-recoil in U-Pb geochronology: Effective sample size matters.
Contributions to Mineralogy and Petrology , 4: Combined annealing and multi-step dissolution analysis for Improved precision and accuracy of zircon ages.