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Dating Rocks and Fossils Using Geologic Methods . Learn Science at Scitable

Radioactive Dating

Most absolute age determinations in geology rely on radiometric methods. The earth is billions of years old. The main condition for the method is that the production rate of isotopes stays the same through ages, i. An isotope is a particular type of atom of a chemical element, which differs from other isotopes of that element in the number of neutrons it has in its nucleus. By definition, all atoms of a given element have the same number of protons. However, they do not all have the same number of neutrons.

The discovery of radioactivity and the radiogenic decay of isotopes in the early part of the 20th century opened the way for dating rocks by an absolute, rather than relative, method. Up to this time estimates of the age of the Earth had been based on assumptions about rates of evolution, rates of deposition, the thermal behaviour of the Earth and the Sun or interpretation of religious scriptures. Radiometric dating uses the decay of isotopes of elements present in minerals as a measure of the age of the rock: to do this, the rate of decay must be known, the proportion of different isotopes present when the mineral formed has to be assumed, and the proportions of different isotopes present today must be measured.

This dating method is principally used for determining the age of formation of igneous rocks, including volcanic units that occur within sedimentary strata. It is also possible to use it on authigenic minerals, such as glauconite, in some sedimentary rocks. Radiometric dating of minerals in metamorphic rocks usually indicates the age of the metamorphism.

Different radioactive isotopes have different half lives and so they are useful for dating different types and ages of rocks. Who would want to?. Principles of Radiometric Dating. Radioactive decay is Igneous & sometimes metamorphic rocks and minerals. U. Pb. m.y. Th. Using relative and radiometric dating methods, geologists are able to answer The sedimentary rock layers exposed in the cliffs at Zumaia, Spain, are now tilted.

A number of elements have isotopes forms of the element that have different atomic masses that are unstable and change by radioactive decay to the isotope of a different element. Each radioactive decay series takes a characteristic length of time known as the radioactive half-life, which is the time taken for half of the original parent isotope to decay to the new daughter isotope.

The decay series of most interest to geologists are those with half-lives of tens, hundreds or thousands of millions of years. If the proportions of parent and daughter isotopes of these decay series can be measured, periods of geological time in millions to thousands of millions of years can be calculated. To calculate the age of a rock it is necessary to know the half-life of the radioactive decay series, the amount of the parent and daughter isotopes present in the rock when it formed, and the present proportions of these isotopes.

It must also be assumed that all the daughter isotope measured in the rock today formed as a result of decay of the parent. This may not always be the case because addition or loss of isotopes can occur during weathering, diagenesis and metamorphism and this will lead to errors in the calculation of the age. It is therefore important to try to ensure that decay has taken place in a 'closed system', with no loss or addition of isotopes, by using only unweathered and unaltered material in analyses.

The radiometric decay series commonly used in radiometric dating of rocks are detailed in the following sections. The choice of method of determination of the age of the rock is governed by its age and the abundance of the appropriate elements in minerals. The samples of rock collected for radiometric dating are generally quite large several kilograms to eliminate inhomogeneities in the rock.

The samples are crushed to sand and granule size, thoroughly mixed to homogenise the material and a smaller subsample selected. In cases where particular minerals are to be dated, these are separated from the other minerals by using heavy liquids liquids with densities similar to that of the minerals in which some minerals will float and others sink, or magnetic separation using the different magnetic properties of minerals.

Isotopic dating of rocks, or the minerals in them, is based on the fact that we know the Radiocarbon dating can be used on sediments or sedimentary rocks that. It is also possible to use it on authigenic minerals, such as glauconite, in some sedimentary rocks. Radiometric dating of minerals in metamorphic rocks usually . Geologists use radiometric dating to estimate how long ago rocks formed, and to When molten rock cools, forming what are called igneous rocks, radioactive.

The mineral concentrate may then be dissolved for isotopic or elemental analysis, except for argon isotope analysis, in which case the mineral grains are heated in a vacuum and the composition of the argon gas driven off is measured directly. Measurement of the concentrations of different isotopes is carried out with a mass spectrometer.

In these instruments a small amount micrograms of the sample is heated in a vacuum to ionise the isotopes and these charged particles are then accelerated along a tube in a vacuum by a potential difference. Part-way along the tube a magnetic field induced by an electromagnet deflects the charged particles. The amount of deflection will depend upon the atomic mass of the particles so different isotopes are separated by their different masses.

Detectors at the end of the tube record the number of charged particles of a particular atomic mass and provide a ratio of the isotopes present in a sample. This is the most widely used system for radiometric dating of sedimentary strata, because it can be used to date the potassium-rich authigenic mineral glauconite and volcanic rocks lavas and tuffs that contain potassium in minerals such as some feldspars and micas.

One of the isotopes of potassium, 40 K, decays partly by electron capture a proton becomes a neutron to an isotope of the gaseous element argon, 40 Ar, the other product being an isotope of calcium, 40 Ca. The half-life of this decay is However, the proportion of potassium present as 40 K is very small at only 0. Argon is an inert rare gas and the isotopes of very small quantities of argon can be measured by a mass spectrometer by driving the gas out of the minerals.

K—Ar dating has therefore been widely used in dating rocks but there is a significant problem with the method, which is that the daughter isotope can escape from the rock by diffusion because it is a gas.

Dating Rocks and Fossils Using Geologic Methods

The amount of argon measured is therefore commonly less than the total amount produced by the radioactive decay of potassium. This results in an underestimate of the age of the rock.

The problems of argon loss can be overcome by using the argon—argon method. The first step in this technique is the irradiation of the sample by neutron bombardment to form 39 Ar from 39 K occurring in the rock.

How Old is that Rock?

The ratio of 39 K to 40 K is a known constant so if the amount of 39 Ar produced from 39 K can be measured, this provides an indirect method of calculating the 40 K present in the rock.

Measurement of the 39 Ar produced by bombardment is made by mass spectrometer at the same time as measuring the amount of 40 Ar present. Before an age can be calculated from the proportions of 39 Ar and 40 Ar present it is necessary to find out the proportion of 39 K that has been converted to 39 Ar by the neutron bombardment. This can be achieved by bombarding a sample of known age a 'standard' along with the samples to be measured and comparing the results of the isotope analysis.

The principle of the Ar—Ar method is therefore the use of 39 Ar as a proxy for 40 K. Although a more difficult and expensive method, Ar—Ar is now preferred to K—Ar. The effects of alteration can be eliminated by step-heating the sample during determination of the amounts of 39 Ar and 40 Ar present by mass spectrometer.

Alteration and hence 40 Ar loss occurs at lower temperatures than the original crystallisation so the isotope ratios measured at different temperatures will be different.

The sample is heated until there is no change in ratio with increase in temperature a 'plateau' is reached : this ratio is then used to calculate the age.

If no 'plateau' is achieved and the ratio changes with each temperature step the sample is known to be too altered to provide a reliable date. This is a widely used method for dating igneous rocks because the parent element, rubidium, is common as a trace element in many silicate minerals.

Radiometric dating types of rocks

The isotope 87 Rb decays by shedding an electron beta decay to 87 Sr with a half-life of 48 billion years. The proportions of two of the isotopes of strontium, 86 Sr and 87 Sr, are measured and the ratio of 86 Sr to 87 Sr will depend on two factors. First, this ratio will depend on the proportions in the original magma: this will be constant for a particular magma body but will vary between different bodies. Second, the amount of 87 Sr present will vary according to the amount produced by the decay of 87 Rb: this depends on the amount of rubidium present in the rock and the age.

The rubidium and strontium concentrations in the rock can be measured by geochemical analytical techniques such as XRF X-ray fluorescence. The principle of solving simultaneous equations can be used to resolve these two unknowns.

An alternative method is whole-rock dating, in which samples from different parts of an igneous body are taken, which, if they have crystallised at different times, will contain different amounts of rubidium and strontium present. This is more straightforward than dating individual minerals as it does not require the separation of these minerals.

Isotopes of uranium are all unstable and decay to daughter elements that include thorium, radon and lead. Two decays are important in radiometric dating: U to Pb with a half-life of 4.

One good example is granite, which normally has some potassium feldspar Figure 8. Feldspar does not have any argon in it when it forms. Over time, the 40 K in the feldspar decays to 40 Ar.

Argon is a gas and the atoms of 40 Ar remain embedded within the crystal, unless the rock is subjected to high temperatures after it forms.

The sample must be analyzed using a very sensitive mass-spectrometer, which can detect the differences between the masses of atoms, and can therefore distinguish between 40 K and the much more abundant 39 K. Biotite and hornblende are also commonly used for K-Ar dating. An important assumption that we have to be able to make when using isotopic dating is that when the rock formed none of the daughter isotope was present e.

A clastic sedimentary rock is made up of older rock and mineral fragments, and when the rock forms it is almost certain that all of the fragments already have daughter isotopes in them.

The age of new minerals crystallizing in metamorphic rocks can also be determined by radiometric dating. The problem is that metamorphism. Common Types of Radiometric Dating. Carbon 14 Dating. As shown in the diagram above, the radioactive isotope carbon originates in the Earth's. Most absolute age determinations in geology rely on radiometric methods. An isotope is a particular type of atom of a chemical element, which differs from.

Furthermore, in almost all cases, the fragments have come from a range of source rocks that all formed at different times. If we dated a number of individual grains in the sedimentary rock, we would likely get a range of different dates, all older than the age of the rock.

It might be possible to date some chemical sedimentary rocks isotopically, but there are no useful isotopes that can be used on old chemical sedimentary rocks. Radiocarbon dating can be used on sediments or sedimentary rocks that contain carbon, but it cannot be used on materials older than about 60 ka. Assume that a feldspar crystal from the granite shown in Figure 8. The proportion of 40 K remaining is 0. Using the decay curve shown on this graph, estimate the age of the rock.

An example is provided in blue for a 40 K proportion of 0. This is determined by drawing a horizontal line from 0.

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