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Uranium-Lead dating

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Uranium-Lead dating is a radiometric dating method that uses the decay chain of uranium and lead to find the age of a rock. As uranium decays radioactively, it becomes different chemical elements until it stops at lead. The reason for stopping at lead is because lead is not radioactive and will not change into a different element. It may sound straight-forward, but there are many variables that have to be considered. The three main parameters that have to be set are the original amount of uranium and lead in the sample, the rate at which uranium and lead enter and leave the sample, and how much the rate of decay changes.


Basic Principle

Concordia diagram that is used to check if the finding from U-Pb dating match other accepted dates for this radiometric dating method

Uranium-lead dating uses four different isotopes to find the age of the rock. The four isotopes are uranium-235, uranium-238, lead-207, and lead-206. The process of dating finds the two ratios between uranium-235 and lead-207; and uranium-238 and lead-206. The radiometric dater then uses the half-life of all four isotopes to find an age range the rock should be in. The half-lives of the cascade from uranium-235 to lead-207 has been been extrapolated to about 704 million years and the cascade form uranium-238 to lead-206 has been calculated to about 4.47 billion years[1].

This data is compared to a curve called the Concordia diagram. This diagram has been made by using the ratio of uranium to lead of all the rocks dated with this method and their assumed age. Scientists know that there are geological events that can disturb the zircon and release the lead created from the uranium. This would reset the time recorded by this method. To try to account for this, a radiometric dater will use many different samples and use the ones that fit the Concordia curve. If they do not fit, it is assumed that it signifies a large geological event[1].


History

This method started to be used in 1907[2]. Uranium-lead dating is one of the first radiometric dating method that found the supposed age of the earth to be 4.4 billion years old[1].

Detail of Process

A zircon crystal in a rock

The part of the rock a dater will use to date the rock is normally the zircon in the rock. It is assumed that when the rock cools to the point that it makes the zircon, all of the lead is excluded from the zircon. If this is true, it makes the dating simple because if the half-lifes are correct, the dater only has to find the ratio of the amount of lead and uranium in the sample[1].

The benefits of using zircon is that the trapping temperature is 900^{\circ } C. This temperature makes the zircon hard to pull out substances out of it. From what has been observed, even small amounts of rock metamorphosis should not disturb the elements in the zircon. Another benefit is that zircon has been found in most igneous rocks. The last of the benefits is that the zircon, itself, is very hard. This fact helps with extracting the zircon out of the rock it was in[1].

Most radiometric daters prefer using zircon for these reasons, but it is not the only compound used for uranium-lead dating. Some other compounds used that have zirconium are zirconolite, and badeleyite. Other compounds that do not contain zirconium but are commonly used for this method are titanite, and monazite. Since most radiometric daters prefer using zircon for this process, geologists often call uranium-lead dating zircon dating[1].

Problems

With all radiometric dating processes, the accuracy of uranium-lead dating is called into question. Some of the classic problems with this kind of dating process include what the process can really date, how far the radiometric process can date accurately, and the assumptions taken so the dating process works. One assumption is to use a worldview that uniformitarianism is accepted[3].

They use this equation to find the age of a rock:

T={\frac  {(B_{T}-B_{0})+(A_{0}-A_{T})\pm (Da\pm Db)}{2R_{T}}}

Where T is the time from starting point, B_{0} the original amount of uranium, B_{T} the amount of uranium at the measurement, A_{0} the original amount of lead, A_{T} the amount of lead at the measurement, R_{T} the rate uranium changes to lead, Da the average rate of loss and gain in the amount of lead, Db the average rate of loss and gain in the amount of uranium.[4].

Limitations

Uranium-Lead dating only works on igneous and metamorphic rocks because sedimentary layers contain small pieces of a other rock layers[3].

Like all radiometric dating methods, uranium-lead dating has a range that it works best. For uranium-lead has a range of 10 million to 4.6 billion years. This means that to begin with, any rock dated with this process will be in the 10's of millions [5].

Assumptions

For Uranium-Lead dating to work, scientists have to make three assumptions. These assumptions are that the system being dated is a closed system; at the beginning of the time period, there are no daughter isotopes present; and the rate of radioactive decay stays the same through the whole time period. Once all these assumptions are taken, the equation above simplifies to T={\frac  {B_{T}}{R_{T}}}[4].

Without a closed system, uranium-lead dating, like all other radiometric dating methods, falls apart. Assuming a closed system means that nothing on the outside of the rock affected the sample. This means that none of the parent or daughter isotope leaked in or out. It also implies that none of the factors that might affect the rate of the radioactive decay could not. This is an ideal concept that cannot happen. If the ages this dating process generates are true, it gets harder to assume that nothing on the outside of the sample has any effect on the system. After a few million or billion years of a near-closed system, it will have a large error[6].

To find the age of a rock, a person trying to find it has to know the original amount of the parent isotope, and the original amount of the daughter isotope. The common assumption evolutionary scientists use is that the original amount was zero. This is not scientific because at the beginning of that rock, there were no scientific observers to measure original amount of daughter isotope, in this case that would be lead-206 and lead-207[4].

All radiometric dating systems depend on the idea that radioactive decay happens at a constant rate. It has been found that the rates fluctuate for an unknown reason. One of the explanations has been found that the rates of decay of some radioactive isotopes change depending on the its proximity to the sun. Two examples of an isotope that exhibits this behavior is silicon-32 and radon-222. Not all radioactive isotopes follow this kind of behavior; others have irregular rate changes that still have no explanation. The scientists that have studied these changes in rates are not sure if the sun really has anything to do with the change in the decay rate[7].

References

  1. 1.0 1.1 1.2 1.3 1.4 1.5 Alden, Andrew. Uranium-Lead Dating About.com Geology. Web. May 3, 2012.
  2. Mutley. Radioactive Dating: Uranium-Lead. Science and Evolution. Web. May 23, 2012.
  3. 3.0 3.1 Unknown Author. Radiometric Dating Methods Institute of Origins Education. Web. May 4, 2012.
  4. 4.0 4.1 4.2 Morris, Henry. The Young Earth Institute for Creation Research. Web. May 10, 2012.
  5. Noeller. P.Exercise in Radiometric Dating.Community College of Baltimore County. Web. May 23,2012.
  6. Drake, Jonathan. Radiometric Dating Methods: A Thoughtful Analysis. True Authority. Web. May 23, 2012.
  7. Thomas, Brian. Fluctuations Show Radioisotope Decay Is Unreliable Institute for Creation Research. Web. April 30, 2012.