# Uranium–lead dating

If a material that selectively rejects the daughter nuclide is heated, any daughter nuclides that have been accumulated over time will be lost through diffusion , setting the isotopic "clock" to zero. Fluorine absorption Nitrogen dating Obsidian hydration Seriation Stratigraphy. To try to set a reasonable bound on the age, we could presume that the Earth formed at the same time as the rest of the solar system.

## Multiple ages for a single rock; the thermal effect

It is not affected by external factors such as temperature , pressure , chemical environment, or presence of a magnetic or electric field. In a similar development, the use of highly sensitive thermal ionization mass spectrometers is replacing the counting techniques employed in some disequilibrium dating see below. So what do the observational scientists in the radiometric dating lab do? Uranium-lead dating uses four different isotopes to find the age of the rock. Isotopic systems, on the other hand, can yield either the primary age or the time of a later event, because crystalline materials are very specific in the types of atoms they incorporate, in terms of both the atomic size and charge. Note that the decay constant scale in the table below was kept the same as the table above for comparison.

To the intriguing question "How old is the Earth? While there are numerous natural processes that can serve as clocks, there are also many natural processes that can reset or scramble these time-dependent processes and introduce uncertainties. To try to set a reasonable bound on the age, we could presume that the Earth formed at the same time as the rest of the solar system. If the small masses that become meteorites are part of that system, then a measurement of the solidification time of those meteorites gives an estimate of the age of the Earth.

The following illustration points to a scenario for developing such an age estimate. Some of the progress in finding very old samples of rock on the Earth are summarized in the following comments. But later in It is a compound of zirconium, silicon and oxygen which in its colorless form is used to make brilliant gems. Samples more than 3. Older ages in the neighborhood of 4. The graph below follows the treatment of Krane of Rb-Sr studies of meteorite samples from Wetherill in order to show the nature of the calculation of age from isochrons.

Considering the relative scale of nuclei and atoms , nuclei are so remote from the outer edge of the atoms that no environmental factors affect them. However, there are two obvious problems with radioactive dating for geological purposes: The relative amounts of strontium and are determined with great precision and the fact that the data fits a straight line is a strong argument that none of the constituents was lost from the mix during the aging process.

Similar results are also obtained from the study of spontaneous fission events from uranium and plutonium One of the standard references for modeling the age of the Earth is G. From such data, and from estimates of how long it would take to produce the quantities of various lead isotopes now found on the earth, geochronologists feel that the 4. The ages of rocks returned to Earth from the Apollo missions range from 3. The older age determinations are derived from rocks collected on the lunar highland, which may represent the original lunar crust.

Our best clues to the age of the Moon are the radiometric dates of the oldest Moon rocks, those from the lunar highlands. Dalrymple reports that thirteen samples from the lunar highlands gave the oldest ages. These were collected by Apollo 15, 16, 17 and Luna The radiometric dates range from 3.

If we take the oldest ages to be the age of the Moon, then we place that age at about 4. Clocks in the Rocks The following radioactive decay processes have proven particularly useful in radioactive dating for geologic processes: Lead isochrons are also an important radioactive dating process.

Some of the decays which are useful for dating, with their half-lives and decay constants are: Uranium-Lead Dating Ages determined by radioactive decay are always subject to assumptions about original concentrations of the isotopes.

Clocks in the Rocks. Index Beta decay concepts. Potassium-Argon Method Potassium-Argon dating has the advantage that the argon is an inert gas that does not react chemically and would not be expected to be included in the solidification of a rock, so any found inside a rock is very likely the result of radioactive decay of potassium.

More detail on Potassium-Argon dating. Rubidium-Strontium The rubidium-strontium dating method is often used in geologic studies. Age of the Earth To the intriguing question "How old is the Earth? Index Reference Krane Sec 6. Meteorite Dating "Meteorites, which many consider to be remnants of a disrupted planet that oriaginally formed at about the same time as the earth, have provided uranium-lead and rubidium-strontium ages of about 4. Index Reference Dalrymple Ch 5.

Age of the Moon Our best clues to the age of the Moon are the radiometric dates of the oldest Moon rocks, those from the lunar highlands. A sample of the kind of data that leads to such a projected age is the rubidium-strontium isochron of lunar sample which yields a time to last melting of 4. This isochron was discussed in Dalrymple and credited to Papanastassiou and Wassenburg, 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]. This method started to be used in [2].

Uranium-lead dating is one of the first radiometric dating method that found the supposed age of the earth to be 4. 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 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]. 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]. Where is the time from starting point, the original amount of uranium, the amount of uranium at the measurement, the original amount of lead, the amount of lead at the measurement, the rate uranium changes to lead, the average rate of loss and gain in the amount of lead, the average rate of loss and gain in the amount of uranium.

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. This means that to begin with, any rock dated with this process will be in the 10's of millions [5]. 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 [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.

### Imsges: uranium-lead method of dating

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. The procedures used to isolate and analyze the parent and daughter nuclides must be precise and accurate. Another example of short-lived extinct radionuclide dating is the 26 Al — 26 Mg chronometer, which can be used to estimate the relative ages of chondrules.

The isotopic age then is called a cooling age.

To share with more than one person, separate addresses with a comma. The disintegration products of uranium: United States Geological Survey. This temperature is what is known as *uranium-lead method of dating* temperature and represents the temperature below which the mineral is a uranijm-lead system to uarnium-lead. Nuclear Methods of Dating. Clocks in the Rocks The following radioactive decay smwch speed dating have proven *uranium-lead method of dating* useful in radioactive dating for geologic processes: Many radioactive dating methods are based on minute additions of daughter products to a rock or mineral in which a considerable amount of daughter-type isotopes already exists.

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