Potassium-argon dating , method of determining the time of origin of rocks by measuring the ratio of radioactive argon to radioactive potassium in the rock. This dating method is based upon the decay of radioactive potassium to radioactive argon in minerals and rocks; potassium also decays to calcium Thus, the ratio of argon and potassium and radiogenic calcium to potassium in a mineral or rock is a measure of the age of the sample. The calcium-potassium age method is seldom used, however, because of the great abundance of nonradiogenic calcium in minerals or rocks, which masks the presence of radiogenic calcium. On the other hand, the abundance of argon in the Earth is relatively small because of its escape to the atmosphere during processes associated with volcanism.
The geomagnetic polarity time scale was calibrated largely using K—Ar dating. Potassium naturally occurs in 3 isotopes: Conversion to stable 40 Ca occurs via electron emission beta decay in Conversion to stable 40 Ar occurs via electron capture in the remaining Argon, being a noble gas , is a minor component of most rock samples of geochronological interest: When 40 K decays to 40 Ar argon , the atom typically remains trapped within the lattice because it is larger than the spaces between the other atoms in a mineral crystal.
Entrained argon—diffused argon that fails to escape from the magma—may again become trapped in crystals when magma cools to become solid rock again. After the recrystallization of magma, more 40 K will decay and 40 Ar will again accumulate, along with the entrained argon atoms, trapped in the mineral crystals.Absolute dating methods (ANT)
Measurement of the quantity of 40 Ar atoms is used to compute the amount of time that has passed since a rock sample has solidified. Despite 40 Ca being the favored daughter nuclide, it is rarely useful in dating because calcium is so common in the crust, with 40 Ca being the most abundant isotope.
Potassium argon dating in archaeology
Thus, the amount of calcium originally present is not known and can vary enough to confound measurements of the small increases produced by radioactive decay. The ratio of the amount of 40 Ar to that of 40 K is directly related to the time elapsed since the rock was cool enough to trap the Ar by the equation.
The scale factor 0. In practice, each of these values may be expressed as a proportion of the total potassium present, as only relative, not absolute, quantities are required.
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Please note that our editors may make some formatting changes or correct spelling or grammatical errors, and may also contact you if any clarifications are needed. Potassium-argon dating Written By: The Editors of Encyclopaedia Britannica. Learn More in these related Britannica articles: This is possible in potassium-argon K-Ar dating, for example, because most minerals do not take argon into their structures initially. In rubidium-strontium dating, micas exclude strontium when they form but accept much rubidium.
In uranium-lead U-Pb dating of zircon, the zircon is found to exclude initial lead…. The radioactive decay scheme involving the breakdown of potassium of mass 40 40 K to argon gas of mass 40 40 Ar formed the basis of the first widely used isotopic dating method.
Since radiogenic argon was first detected in by the American geophysicist…. Potassium—argon dating has made it possible to establish that the earliest remains of man and his artifacts in East Africa go back at least 2,, years, and probably further.
Potassium-argon dating in archaeology
Potassium-argon dating , for instance, can provide the age of a specimen by clocking the rate at which radioactive isotopes of these elements have decayed. These methods are briefly summarized in terms of their initial inception and key advances in technique development, and they are illustrated by a limited number of applications.
Archaeologically-relevant Dating Techniques for the Next Century. This paper presents a brief review of the development and application of dating techniques, in addition to radiocarbon, which are useful in an archaeological context. For hominid evolution these include potassium—argon, argon—argon and fission-track dating.
For dating the emergence of anatomically-modern man these include electron spin resonance, uranium series, amino acid racemization and luminescence dating. New approaches are discussed which provide the basis for exciting new applications in the coming decades.
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