Radiometric age dating rocks financialdating com
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.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.
The half-life of this decay is 11.93 billion years.
Potassium is a very common element in the Earth’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.
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.