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.Potassium is a very common element in the Earthâ€™s crust and its concentration in **rocks** is easily measured.However, the proportion of potassium present as 40 K is very small at only 0.012%, and most of this decays to 40 Ca, with only 11% forming 40 Ar.

Radiometric *dating* of minerals in metamorphic *rocks* usually indicates the age of the metamorphism.

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.

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* of minerals in metamorphic *rocks* usually indicates the age of the metamorphism.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.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 decay series of most interest to geologists are those with half-lives of tens, hundreds or thousands of millions of years.