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The method of dating rocks and minerals is known as geochronology. Although in principle this term could be applied to estimation of relative ages according to traditional geological observation, it is nowadays usually restricted to the quantitative measurement of geological time using the constant-rate natural process of radioactive decay. The halflife of this decay is only years. Even using pre-concentration techniques and highly sensitive detectors, the practical range of the dating method does not extend back beyond about years-a period utterly insignificant in terms of the geological evolution of the Earth, which extends over the past million years.
For geological dating one requires naturally occurring elements with much longer halflives. Most of the handful of appropriate decay schemes are listed. Most of the parent elements are rare metal constituents in the bulk chemical composition of the Earth.
In principle the uptake rate of 14 C by animals is in equilibrium with the atmosphere. As soon as a plant or animal dies, they stop the metabolic function of carbon uptake and with no replenishment of radioactive carbon, the amount of 14 C in their tissues starts to reduce as the 14 C atoms decay. Libby and his colleagues first discovered that this decay occurs at a constant rate.
They found that after years, half the 14 C in the original sample will have decayed and after another years, half of that remaining material will have decayed, and so on. This became known as the Libby half-life.
Radiometric dating in geology
After 10 half-lives, there is a very small amount of radioactive carbon present in a sample. At about 50 to 60 years, the limit of the technique is reached beyond this time, other radiometric techniques must be used for dating. By measuring the 14 C concentration or residual radioactivity of a sample whose age is not known, it is possible to obtain the number of decay events per gram of Carbon.
By comparing this with modern levels of activity wood corrected for decay to AD and using the measured half-life it becomes possible to calculate a date for the death of the sample. As a result of atomic bomb usage, 14 C was added to the atmosphere artificially.
This affects the 14 C ages of objects younger than Any material which is composed of carbon may be dated. Herein lies the true advantage of the radiocarbon method.
Potassium-Argon K-Ar dating is the most widely applied technique of radiometric dating. Potassium is a component in many common minerals and can be used to determine the ages of igneous and metamorphic rocks. The Potassium-Argon dating method is the measurement of the accumulation of Argon in a mineral. It is based on the occurrence of a small fixed amount of the radioisotope 40 K in natural potassium that decays to the stable Argon isotope 40 Ar with a half-life of about 1, million years. In contrast to a method such as Radiocarbon dating, which measures the disappearance of a substance, K-Ar dating measures the accumulation of Argon in a substance from the decomposition of potassium.
Argon, being an inert gas, usually does not leech out of a mineral and is easy to measure in small samples. This method dates the formation or time of crystallisation of the mineral that is being dated; it does not tell when the elements themselves were formed. It is best used with rocks that contain minerals that crystallised over a very short period, possibly at the same time the rock was formed.
This method should also be applied only to minerals that remained in a closed system with no loss or gain of the parent or daughter isotope.
Radiometric Dating: Methods, Uses & the Significance of Half-Life
Uranium-Lead U-Pb dating is the most reliable method for dating Quaternary sedimentary carbonate and silica, and fossils particulary outside the range of radiocarbon. Quaternary geology provides a record of climate change and geologically recent changes in environment.
U-Pb geochronology of zircon , baddelyite , and monazite is used for determining the age of emplacement of igneous rocks of all compositions, ranging in age from Tertiary to Early Archean. U-Pb ages of metamorphic minerals, such as zircon or monazite are used to date thermal events, including terrestrial meteoritic impacts. U-Pb ages of zircon in sediments are used to determine the provenance of the sediments. The Fission track analysis is based on radiation damage tracks due to the spontaneous fission of U. Fission-tracks are preserved in minerals that contain small amounts of uranium, such as apatite and zircon.
Fission-track analysis is useful in determining the thermal history of a sample or region. By determining the number of tracks present on a polished surface of a grain and the amount of uranium present in the grain, it is possible to calculate how long it took to produce the number of tracks preserved.
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- Radiometric Dating.
- Radiometric dating in geology - IOPscience.
As long as the mineral has remained cool, near the earth surface, the tracks will accumulate.