Bradley, in Paleoclimatology Third Edition , 3. However, potassium-argon and argon-argon dating have indirectly made major contributions to Quaternary studies Walker, The techniques have proved to be invaluable in dating seafloor basalts and enabling the geomagnetic polarity timescale to be accurately dated and correlated on a worldwide basis Harland et al. Potassium-argon dating has also been used to date lava flows and volcanic tuff, which in some areas of the world may be juxtaposed with glacial deposits or be stratigraphically related to early hominid fossils.
In this way, limiting dates on the age of the glacial event or fossil occurrence may be assigned e. Potassium-argon dating is based on the decay of the radioisotope 40K to a daughter isotope 40Ar. Potassium is a very common component of minerals and occurs in the form of three isotopes, 39K and 41K, both stable, and 40K, which is unstable.
Although the decay to 40Ca is more common, the relative abundance of 40Ca in rocks precludes the use of this isotope for dating purposes, as the incremental production of 40Ca from the decay of 40K would be miniscule. Argon is a gas that can be driven out of a sample by heating. With the passage of time, 40Ar is produced and retained within the mineral crystals, until driven off by heating in the laboratory during the dating process Dalrymple and Lanphere, As the abundance ratios of the isotopes of potassium are known, the 40K content can be derived from a measurement of total potassium content or by measurement of another isotope, 39K.
Because of the relatively long half-life of 40K, the production of argon is extremely slow. Dating is usually carried out on minerals such as sanidine, plagioclase, biotite, hornblende, and olivine in volcanic lavas and tuffs. It may also be useful in dating authigenic minerals i. The former assumption may be invalid in the case of some deep-sea basalts, which retain previously formed argon during formation under high hydrostatic pressure.
Such factors result in the sample age being overestimated Fitch, Similar errors result from modern argon being absorbed on to the surface and interior of the sample, thereby invalidating the second assumption.
Fortunately, atmospheric argon contamination can be assessed by measurement of the different isotopes of argon present. Atmospheric argon occurs as three isotopes, 36Ar, 38Ar, and 40Ar. This may result from a number of factors, including diffusion, recrystallization, solution, and chemical reactions as the rock weathers Fitch, Obviously, any argon loss will give a minimum age estimate only.
Fortunately, some assessment of these problems and their effect on dating may be possible. Instead of measuring 40K directly, it is measured indirectly by irradiating the sample with neutrons in a nuclear reactor. This causes the stable isotope 39K to transmute into 39Ar; by collecting both the 40Ar and 39Ar, and knowing the ratio of 40K to 39K which is a constant , the sample age can be calculated.
Furthermore, several dates can be obtained from one sample and the results treated statistically to yield a date of high precision Curtis, The advantages stem from the fact that the 40K, which yields the 40Ar by decay, occupies the same position in the crystal lattice of the mineral as the much more abundant 39K, which produces the 39Ar on irradiation.
Heating of the sample thus drives off the argon isotopes simultaneously. Any atmospheric argon contaminating the sample occurs close to the surface of the mineral grains, so it is liberated at low temperatures.
Similarly, loss of radiogenic argon by weathering would mainly be confined to the outer surface of a mineral. At higher temperatures, the deeper-seated argon from the unweathered, uncontaminated interiors of the crystals will be driven off and can be measured repeatedly as the temperature rises to fusion levels.
If such gas increments indicate a stable and consistent age, considerable confidence can be placed in the result.
Such interpretations are discussed further by Curtis and by Miller