U Pb Age Dating Rule - cybertime.ru

U Pb Age Dating Rule

u pb age dating rule

The isotopes[ edit ] There are a number of isotopes of interest in U-Pb dating. U pb age dating rule has a half-life of 4. U pb age dating rule is also useful to know of the existence of Pb leadwhich is neither unstable nor radiogenic. Isochron dating and U-Pb[ edit ] We can always try U-Pb dating using the isochron methodbut this often doesn't work: There seem to be two reasons for this. First of all, the straight-line property of the isochron diagram is destroyed when the isotopes involved get shuffled between minerals. Now lead and uranium are particularly susceptible to such shuffling in the event of even mild metamorphism.

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The ingrowth equations for the three radiogenic Pb isotopes are given by: The corresponding age equations are: This assumption cannot be made for other minerals, young ages, and high precision geochronology. The corresponding age equations then become: This built-in redundancy provides a powerful internal quality check which makes the method arguably the most robust and reliable dating technique in the geological toolbox.

The initial Pb composition can either be determined by analysing the Pb composition of a U-poor mineral e. Note that isotopic closure is required for all intermediary isotopes as well. Initially, the U-Pb method was applied to U-ores, but nowadays it is predominantly applied to accessory minerals such zircon and, to a lesser extent, apatite, monazite and allanite. Note that these are only a function of time. Equations 5.

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.

From Equation 5. With time, the data shift further away from the origin. First of all, the straight-line property of the isochron diagram is destroyed when the isotopes involved get shuffled between minerals. Now lead and uranium are particularly susceptible to such shuffling in the event of even mild metamorphism. The other problem is that uranium is particularly susceptible to weathering. Now since all rocks are somewhat porous, and since we are pretty much obliged to date rocks from near the surface, it's hard to find instances in which uranium has not been lost.

Zircons[ edit ] Zircon. Zircon is the mineral Zr Si O 4; as you can see from its chemical formula, it is one of the silicate minerals. Although it is not abundant in igneous rocks , it is sufficiently common to be used for the purposes of radiometric dating. It has two properties which make it useful for this purpose. First of all, uranium will readily substitute for the zirconium Zr in the mineral , whereas lead is strongly rejected.

For this reason we expect zircons, when formed, to contain some uranium, but virtually no lead. Zircons, then, are relatively immune to the problems that make isochron U-Pb dating so difficult. But of course for isochron dating we need more than one mineral ; zircons alone would not be enough. However, these facts about zircons, combined with what we know about uranium, suggest an alternative method of dating.

The method[ edit ] If there is no lead in the zircon originally, and if no lead or uranium has been added or subtracted to the zircon since its formation, then the following formula will hold: It is possible to refine this date still further.

If we suspect that the zircon, despite its chemical properties, still managed to incorporate a little lead at or after its formation, then since all lead isotopes are chemically the same, we can measure the amount of Pb the zircon contains. Since we know the ratios in which the various lead isotopes are usually found, we can then apply the same sort of correction we used to account for atmospheric argon in the K-Ar method.

While zircon has been the most popular mineral for U-Pb dating, other minerals have been employed, including apatite, monazite, titanite, allanite and, most interesting of all, xenotime. Xenotime[ edit ] 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. Speleothems[ edit ] A cave with speleothems.

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.

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