# 3 radioactive elements commonly used in the absolute dating

### Radiometric dating - Wikipedia

Radiometric dating is used to estimate the age of rocks and other objects based on the fixed decay rate of radioactive isotopes. Learn about. Is the release of subatomic particles from the nuclei of radioactive elements until a stable Parent material used in radiometric dating of organic remains. Geological Time Chapter 4 Section 3 / 4 Radioactive Dating / Geologic Time Scale. Third, magnetism in rocks can be used to estimate the age of a fossil site. . Geologists commonly use radiometric dating methods, based on the natural radioactive decay of Figure 5: Radioactive isotopes and how they decay through time.

Dendrochronology The growth rings of a tree at Bristol ZooEngland. Each ring represents one year; the outside rings, near the bark, are the youngest. Dendrochronology or tree-ring dating is the scientific method of dating based on the analysis of patterns of tree rings, also known as growth rings. Dendrochronology can date the time at which tree rings were formed, in many types of wood, to the exact calendar year. Dendrochronology has three main areas of application: In some areas of the world, it is possible to date wood back a few thousand years, or even many thousands.

Currently, the maximum for fully anchored chronologies is a little over 11, years from present.

### Radiometric Dating

Amino acid dating Amino acid dating is a dating technique [5] [6] [7] [8] [9] used to estimate the age of a specimen in paleobiologyarchaeologyforensic sciencetaphonomysedimentary geology and other fields. This technique relates changes in amino acid molecules to the time elapsed since they were formed. All biological tissues contain amino acids. All amino acids except glycine the simplest one are optically activehaving an asymmetric carbon atom.

After one half-life has elapsed, one half of the atoms of the nuclide in question will have decayed into a "daughter" nuclide or decay product. In many cases, the daughter nuclide itself is radioactive, resulting in a decay chaineventually ending with the formation of a stable nonradioactive daughter nuclide; each step in such a chain is characterized by a distinct half-life. In these cases, usually the half-life of interest in radiometric dating is the longest one in the chain, which is the rate-limiting factor in the ultimate transformation of the radioactive nuclide into its stable daughter.

Isotopic systems that have been exploited for radiometric dating have half-lives ranging from only about 10 years e. It is not affected by external factors such as temperaturepressurechemical environment, or presence of a magnetic or electric field. For all other nuclides, the proportion of the original nuclide to its decay products changes in a predictable way as the original nuclide decays over time.

This predictability allows the relative abundances of related nuclides to be used as a clock to measure the time from the incorporation of the original nuclides into a material to the present. Accuracy of radiometric dating[ edit ] Thermal ionization mass spectrometer used in radiometric dating. The basic equation of radiometric dating requires that neither the parent nuclide nor the daughter product can enter or leave the material after its formation.

The possible confounding effects of contamination of parent and daughter isotopes have to be considered, as do the effects of any loss or gain of such isotopes since the sample was created. It is therefore essential to have as much information as possible about the material being dated and to check for possible signs of alteration.

## Absolute dating

Alternatively, if several different minerals can be dated from the same sample and are assumed to be formed by the same event and were in equilibrium with the reservoir when they formed, they should form an isochron. This can reduce the problem of contamination. In uranium—lead datingthe concordia diagram is used which also decreases the problem of nuclide loss. Finally, correlation between different isotopic dating methods may be required to confirm the age of a sample. For example, the age of the Amitsoq gneisses from western Greenland was determined to be 3.

The procedures used to isolate and analyze the parent and daughter nuclides must be precise and accurate. This normally involves isotope-ratio mass spectrometry. For instance, carbon has a half-life of 5, years.

**How to solve radiometric dating problems**

After an organism has been dead for 60, years, so little carbon is left that accurate dating cannot be established. On the other hand, the concentration of carbon falls off so steeply that the age of relatively young remains can be determined precisely to within a few decades.

Closure temperature If a material that selectively rejects the daughter nuclide is heated, any daughter nuclides that have been accumulated over time will be lost through diffusionsetting the isotopic "clock" to zero. The temperature at which this happens is known as the closure temperature or blocking temperature and is specific to a particular material and isotopic system. These temperatures are experimentally determined in the lab by artificially resetting sample minerals using a high-temperature furnace.

As the mineral cools, the crystal structure begins to form and diffusion of isotopes is less easy. At a certain temperature, the crystal structure has formed sufficiently to prevent diffusion of isotopes. This temperature is what is known as closure temperature and represents the temperature below which the mineral is a closed system to isotopes. Thus an igneous or metamorphic rock or melt, which is slowly cooling, does not begin to exhibit measurable radioactive decay until it cools below the closure temperature.

The age that can be calculated by radiometric dating is thus the time at which the rock or mineral cooled to closure temperature.

This field is known as thermochronology or thermochronometry. The age is calculated from the slope of the isochron line and the original composition from the intercept of the isochron with the y-axis. The equation is most conveniently expressed in terms of the measured quantity N t rather than the constant initial value No.

## Radiometric dating

The above equation makes use of information on the composition of parent and daughter isotopes at the time the material being tested cooled below its closure temperature. This is well-established for most isotopic systems. Plotting an isochron is used to solve the age equation graphically and calculate the age of the sample and the original composition. Modern dating methods[ edit ] Radiometric dating has been carried out since when it was invented by Ernest Rutherford as a method by which one might determine the age of the Earth.

The radioactive minerals in sedimentary rocks are derived from the weathering of igneous rocks. If the sedimentary rock were dated, the age date would be the time of cooling of the magma that formed the igneous rock. The date would not tell anything about when the sedimentary rock formed.

To date a sedimentary rock, it is necessary to isolate a few unusual minerals if present which formed on the seafloor as the rock was cemented. Glauconite is a good example. Glauconite contains potassium, so it can be dated using the potassium-argon technique. How does Carbon dating work? Cosmic rays from the sun strike Nitrogen 14 atoms in the atmosphere and cause them to turn into radioactive Carbon 14, which combines with oxygen to form radioactive carbon dioxide.