Radiometric age dating geology

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.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.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.

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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.

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.

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.

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.

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.

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.

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