Radiometric dating of rocks and minerals devon aoki is dating
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.
The samples are crushed to sand and granule size, thoroughly mixed to homogenise the material and a smaller subsample selected.
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 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.
Part-way along the tube a magnetic field induced by an electromagnet deflects the charged particles.