Methods of absolute (direct and chronometric) age determination : Part 2 - Potassium-argon dating and other Radioactive dating Methods 

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Potassium -argon dating

This method is very similar to radiocarbon dating. It is based on the decay of radioactive material into a non-radioactive substance at a fixed rate. The rate of radioactive potassium (40K)into argon (40Ar) can be measured since it is known that 40K has a half-life of 1.3 billion years. 

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Potassium/argon dating (often abbreviated as K/Ar) is an absolute dating technique based on the known rate at which potassium changes to argon in volcanic sediments.More specifically it is based on the rate at which potassium and radioactive argon change into stable argon gas.

Volcanic materials are especially suitable for dating purposes. Potassium-argon dating has been applied to determine the age of early fossil hominins found in association with volcanic layers in East Africa. For example, remains of Australopithecus boisei from Olduvai Gorge in Tanzania have been dated to 1.75 million years ago on the basis of this method ranges from the origins of the earth to 100 000 years ago.

The scientific principles on which the method is based

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~ Potassium-argon (K/Ar) dating and dendrochronology are, like radiocarbon, examples of absolute (chronometric) dating methods.

~ This method of dating is based on the disintegration of radioactive potassium (40K) into non-raddioactive argon (40Ar) at a fixed and measurable rate.

~ Potassium (K) is present in most minerals and in its natural form contains a small proportion of radioactive 40K atoms.

~ 40K has a half-life of 1.3 billion years and for every 100 40K atoms that decay, 11 atoms become 40Ar, which is an inactive gas.

~ This inactive gas (40Ar) escapes by diffusion when lava and other igneous rocks are formed and the amount of 40AAr consequently drops to almost nothing. The 40K continues to decay at this fixed and measurable rate and new concentrations of 40Ar start to accumulate in the rock. In the case of volcanic rock the 40Ar remains trapped in the rock.

~ Spectrometer is used to measure the 40Ar which has accumulated in the rock as a result of decay of 40K and this information is used to determine the age of the rock.

~ It is important to note that the dates obtained refer to the age of the volcanic material. Archaeological remains associated with periods of volcanic activity can be dated through association with the volcanic material. The collection of datable archaeological samples, the laboratory procedures and the limitations of the method

~ Potassium -argon (K/Ar) dating and dendrochronology are like radiocarbon, examples of absolute (chronometric) dating methods.

~ This method of dating is based on the disintegration of radioactive potassium (40k) into non- radioactive potassium (40K) into non-radioactive argon (40Ar) at a fixed and measurable rate.

~ Potassium (K) is present in most minerals and in its natural form contains a small proportion of radioactive 40K atoms.

~ 40K has a half-life of 1.3 billion years and for every 100 40K atoms that decay, 11 atoms become 40Ar, which is an inactive gas.

~ This inactive gas (40Ar) escapes by diffusion when lava and other igneous rocks are formed and the amount of 40Ar consequently drops to almost nothing. The 40K continues to decay at this fixed and measurable rate and new concentrations of 40Ar start to accumulate in the rock. In the case of volcanic rock the 40Ar remains trapped in the rock.

~ A spectrometer is used to measure the 40Ar which has accumulated in the rock as a result of the decay of 40 K and this information is used to determine the age of the rock.

~ It is important to note that the dates obtained refer to the age of the volcanic material. Archaeological remains associated with periods of volcanic activity can be dated through association with the volcanic material.The collection of datable archaeological samples, the laboratory procedures and the limitations of the method

~ It can only be used on volcanic and other igneous rocks.

~ A unique set of conditions is necessary for K/Ar dating. The techniques is specialized and certain procedures have to be followed if the method is to be reliable.The relationship between lava deposition and archaeological activity must be accurately determined. It is vital that the context of the context of the artefact in relation to the artefact in relation to the volcanic layer is well known. This method is very complex, so an experienced geochronologists (an expert in dating rocks) should take samples at the site.

~ The samples are investigated under microscope to check that there are contamination and then processed and treated through the application of a member of tests. After the argon gas has been isolated, a standard formula is used to determine the age of the sample. 

~ Potassium-argon dating is an absolute method of dating archaeological sites. This means that it determines age of a specific scale or in definite units of time, as in years Before Present or BP. It does not, however, date an archaeological artefact and is, therefore, an indirect form of absolute dating. It can only date a layer of rock associated with an archaeological site or archaeological remains. K/Ar dating can therefore only be used when a clear relationship exists between a site and one or two layers of volcanic rock. The archaeologist therefore needs to be very careful when working out the relationship between the site and the layer of rock, which can be dated by means of potassium-argon dating.

~ K/Ar dating can be used to date archaeological remains between approximately 5 million years (my) and 100 000 years ago. 

~ The deviation for a date, however, is about a quarter of a million years each way. Because of this large standard deviation high accuracy in dating the acrhaeological material is not possible. 

Although statistically it may not be meaningful deviation, it nevertheless limits the accuracy of the dates. According to Renfrew and Bahn (1991:131) it only represents a statistical deviation of approximately 2% of the total age.

Examples of applications in archaeology

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This is the best method available at present for dating very early archaeological material and it is therefore very relevant to archaeology. Volcanic materials are particularly suited to potassium-argon dating. In East Africa periods of volcanic activity both preceded and covered Early Stone Age deposits and this dating method has been used to determine the age of early fossil hominins found in association with the volcanic material. This dating method is not suitable for the travertines and cave breccias where the early southern African hominins occur and the most reliable estimates used in dating southern African fossils are based on faunal analyses and comparison with dated assemblages from East Africa.

Potassium-argon dating can be used to accurately determine dates from the origins of the earth up to 100 000 years ago. This method has therefore been important in establishing dates that range from more than 4 million years ago for hominin evolution to almost 2 million years ago for artefacts manufactured by humans. Early fossil hominins or sites in East Africa have been dated through association with volcanic material. The most famous example of K/Ar dating was when it was used to date the skull of Australopithecus boisei  (also referred to as Zinjanthropus) from Olduvai Gorge in Tanzania at approximately 1.75 +0.25my (million years). The deviation of a quarter of a million means that the skull could, in fact, be anything from 2my to 1.5my old. K/Ar dating was also used to date Lucy, the well-known Australopithecus afarensis fossil from Hadar in Europe, at over 3my. The stone tools recovered from Koobi Fora in northern Kenya were dated to approximately 2.5my, which represents one of the earliest dates for the manufacture of stone tools.

Other Radioactive clocks

There are number of radioactive clocks that cover the time span between radiocarbon dating and potassium-argon dating. Three of the most widely used in archaeology are:

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Uranium-series dating (1 million to 50 000 years ago)

This method, which is based on the radioactive decay of isotopes of uranium, covers the period 1 000 000 - 50 000 years ago. It is used to date rocks rich in calcium carbonate, for examples stalagmites found in lime stone caves where early humans lived, as well as teeth that have absorbed water - soluble uranium. Two radioactive isotopes of the element uranium, namely U238 and U235 into (Th230) and protactinium (Pa231). These daughter elements are also subject to radioactive decay and their half-lives are known. By measuring the ratio of the parent uranium isotopes and their daughter elements, usually the Th230/U238 ratio, the age of the calcareous deposits, and thus the associated archaeological finds, can be determined.

Fission-track dating (3 million to 100 000 years ago)

This absolute dating method is based on the spontaneous fission of an isotope of uranium (U238) in a wide range of rocks and minerals such as obsidian, pumice, zircon, apatite, and mica, as well as obsidian and  other volcanic glasses. When U238 divides into half during fission, the halves move apart at high speed and cause much damage to structure in their path. These pathways of damage, which are referred to as fission tracks, can be counted under an optical microscope and provide a key to determining the age of the mineral or glass. The older the sample, the more tracks it will display. The method has been used to date geological samples containing some of the earliest hominin fossils found at Olduvai Gorge in Tanzania. Though time-consuming, it has also been applied to date artificial glass and pottery glazes less than 2000 years old.

Thermoluminescence dating

Through this method, generally known as TL dating, inorganic material such as pottery and burnt flint can be dated. Materials with a crystalline structure contain radioactive material such as uranium, thorium and potassium. As they decay, electrons are displaced which are then trapped in the crystalline matrix. When the clay is fired to 5008 degrees celsius or more,the electrons are released as thermoluminescent light and the radioactive clock is set to zero. The amount of time that has elapsed since the last heating of the material, that is since the pot was fired, can be counted by reheating the clay in the laboratory. TL dating is regarded as less accurate than radiocarbon dating, but can potentially be used to date burnt stone artefacts made more than 40 000 years ago, as well as ceramics produced within the last few thousand years .

Thermoluminescence dating is based on the premise that energy becomes trapped in objects when they are heated to very high temperatures. Reheating the objects releases stored energy in the form of light in a way that can determine the length of time since the object was originally heated. TL is most commonly used for dating ceramics, but stone tools have also been dated with this technique.

 Thank you for reading

References: Renfrew & Bahn 

                          Fagan 

Part 3 to follow.....

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