Potassium argon dating problems
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.
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 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.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.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 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.
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.
The samples of rock collected for radiometric dating are generally quite large (several kilograms) to eliminate inhomogeneities in the rock.