The potassium-argon K-Ar isotopic dating method is especially useful for determining the age of lavas. Developed in the s, it was important in developing the theory of plate tectonics and in calibrating the geologic time scale. Potassium occurs in two stable isotopes 41 K and 39 K and one radioactive isotope 40 K. Potassium decays with a half-life of million years, meaning that half of the 40 K atoms are gone after that span of time. Its decay yields argon and calcium in a ratio of 11 to
Because the spike is precisely known, the 40 Ar is determined by comparison to it. Variations in this data may point to errors anywhere in the process, which is why all the steps of preparation are recorded in detail.
A variant of the K-Ar method gives better data by making the overall measurement process simpler. The key is to put the mineral sample in a neutron beam, which converts potassium into argon Because 39 Ar has a very short half-life, it is guaranteed to be absent in the sample beforehand, so it's a clear indicator of the potassium content.
The advantage is that all the information needed for dating the sample comes from the same argon measurement.
Accuracy is greater and errors are lower. This method is commonly called "argon-argon dating.
A-Z of Archaeology: 'K - K-Ar Dating' (Potassium - Argon Dating)
The physical procedure for 40 Ar- 39 Ar dating is the same except for three differences:. These effects must be corrected, and the process is intricate enough to require computers. The Ar-Ar method is considered superior, but some of its problems are avoided in the older K-Ar method.
Apr 19, Dating very young materials, e.g., Ar-Ar method due to very short irradiation times and larger uncertainties resulting from variations in the neutron flux and proportionally larger corrections from blanks, neutron interferences, and mass discrimination in the mass spectrometer. This method is commonly called "argon-argon dating." The physical procedure for 40 Ar- 39 Ar dating is the same except for three differences: Before the mineral sample is put in the vacuum oven, it is irradiated along with samples of standard materials by a neutron source. Ar-ar dating method. From 40k is the k-ar dating, abbreviated k and deposits yielded k-ar dating, but. Branching diagram showing the potassium-argon k-ar ages obtained by comparing the layer. Best samples whose conventional k-ar isochron dating method is set. Jack dymond arrived at the kar dating, over a radiometric click this dating method.
Also, the cheaper K-Ar method can be used for screening or reconnaissance purposes, saving Ar-Ar for the most demanding or interesting problems. These dating methods have been under constant improvement for more than 50 years. The learning curve has been long and is far from over today.
With each increment in quality, more subtle sources of error have been found and taken into account. Good materials and skilled hands can yield ages that are certain to within 1 percent, even in rocks only 10, years old, in which quantities of 40 Ar are vanishingly small. Share Flipboard Email.
Andrew Alden. Geology Expert. Andrew Alden is a geologist based in Oakland, California. He works as a research guide for the U. Geological Survey. In general, Ar-Ar dating has been applied most often in the following areas of study which are described briefly below.
Ar-ar dating method
Ar-Ar dating and U-Pb dating are currently the most commonly used techniques in establishing the timing of geological events that are used to mark the various temporal boundaries in the geological time scale. The accuracy and precision of both of these methods remain unmatched by any other geochronological technique.
Ar-Ar dating has been used to refine the age of many boundaries such as the Cretaceous-Paleogene boundary by dating the age of the Chicxulub crater off the coast of Mexico e. The high precision of the Ar-Ar technique makes it an excellent method in which to date fossil evidence of the earliest origins of the human species. By precisely dating minerals such as sanidines from volcanic ash beds which lie above and below fossilized remains, Ar-Ar dating has proven to be extremely useful in understanding the evolution of hominids.
Examples of Ar-Ar studies include the dating of hominid evolution in western Africa e. Because Ar is a noble gas, it is particularly sensitive to temperature. If ambient temperatures are high enough, Ar will preferentially leave the mineral structure via diffusive transfer and escape to the surroundings.
Thus, rocks which have been reheated after crystallization e. The temperature dependence of Ar-Ar ages has opened up an entire field of geochronology known as thermochronology where the thermal evolution of rocks and regions can be studied using multiple geochronological techniques; of these, Ar-Ar technique is one of the most versatile. As mentioned in the section on Modes of Analysis, the interpretation of argon data is fundamentally based on solid-state diffusion theory.
There is now a considerable body of knowledge associated with the application of diffusion theory in geochronology, starting with the landmark paper by Dodsonwho developed the mathematics defining the concept of closure temperature T c.
This key concept, which is defined as the temperature of a cooling system at the time defined by the age of a mineral, has led to the development of field thermochronology, in which the thermal i.
Because argon diffuses at different rates in minerals due to difference in chemical composition and the structure of the crystal latticedifferent K-bearing minerals span a different range of Ar closure temperatures. Argon diffusion experiments have determined that, in general, for K-bearing minerals having the same length scale of diffusion i.
Complicated numerical models attempting to deconvolute thermal histories from the step-heating of single crystals of K-feldspar have been developed e.
The entry on Thermochronology provides more details on the fundamental principles of this field. One of the earliest Ar-Ar thermochronological studies was by Berger and Yorkwho analyzed hornblende, biotite, plagioclase, and K-feldspar to determine the cooling history of the Haliburton Highlands in the Grenville Province, Ontario.
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An overview of many of these studies and a more detailed analysis of thermochronological principles can be found in McDougall and Harrison Ar-Ar dating has found widespread application in solving structural geology problems because it can date K-bearing phases especially micas that form during deformational events.
Many recent studies have also highlighted the importance of considering defect structures associated with deformation in controlling Ar mobility and therefore in interpreting Ar-Ar ages; the study of white micas by Kramar et al. The ability to document a significant portion of the low-mid-temperature history of geological terranes has made Ar-Ar dating the method of choice in many tectonic studies.
In general, thermochronology is used to elucidate regional thermal T - t histories by dating a variety of K-bearing minerals that record different parts of the geological evolution of a terrane.
Many Ar-Ar applications in tectonics have been published.
A novel application of Ar-Ar thermochronology was presented by Camacho et al. Ar-Ar dating is well suited for the timing of mineralization in ore deposits because K-bearing minerals are commonly associated either with the ore or with ore-forming processes.
Minerals such as biotite, K-feldspar, muscovite, and alunite are commonly dated in such studies. Ar-Ar dating has been applied in the study of a variety of ore deposits including porphyry Cu-Mo e. Ar-Ar dating of single crystals of K-bearing minerals has provided useful insights into the provenance of sediments derived from adjacent source regions.
Analogous to the U-Pb dating of single crystals of zircon, the dating of grains of detrital biotite and muscovite can be equally useful if the thermal history of the rocks has not been substantially disturbed. A recent overview of the use of Ar-Ar dating in detrital provenance studies is given in Hodges et al. Ar-Ar dating has been particularly useful in dating paleomagnetic poles for plate reconstructions as well as in refining the geomagnetic polarity time scale.
An excellent summary is given in York with subsequent developments outlined in Spell and McDougall Sedimentary basins have been the subject of intense study because of the opportunity to study a wealth of crustal processes as well as a rich source of ore and hydrocarbon deposits. Ar-Ar dating is generally considered to record higher-temperature processes in such basins and is also used in the study of the provenance of sediments.
A recent overview of the use of Ar-Ar and other thermochronometers in understanding the thermal histories of sedimentary basins is given in Armstrong One of the earliest applications of 40 Ar- 39 Ar dating was in the study of meteorites as well as lunar samples brought back from the US Apollo missions.
In particular, there is a vast body of literature on the 40 Ar- 39 Ar dating of whole-rock basalts and related rocks which led not only to an increased understanding of the genesis of the moon and solar system but also to many refinements in the development of the technique. Many of these results are summarized in McDougall and Harrison With a 40 K half-life of 1, Ma, the K-Ar and Ar-Ar methods are well-suited for dating most geological materials from the Paleocene to the age of the Earth and solar system.
Dating very young materials, e. However, extremely young rocks dating from historical times i. In addition, the K-Ar and Ar-Ar dating methods occupy a unique position among all geochronological methods because Ar is a noble gas.
As K-Ar and Ar-Ar ages are therefore particularly dependent on the temperature of geological systems, this has both its advantages and disadvantages. In particular, one of the greatest strengths is the ability to record the geological history of a sample over a range of temperatures.
However, in the absence of additional geological and petrological information, it may not be possible to know if a K-Ar or Ar-Ar age represents a crystallization, metamorphic, or cooling age.
At present, there are two main advantages of the conventional K-Ar method. First, it is a method grounded on first principles, i.
Title: ar/ar dating method is a leica dm microscope to the noble gasbag. Published 40ar/39ar dating laboratory at the ar/ar dating in accuracy. Ar-Ar dating of caledonian and dense cave calcites are the direct dating of metamorphic minerals and gabbros from volcanic rock. Argon-Argon or the amount of metamorphic minerals were combined to. The aim of this chapter is to present the K-Ar and Ar-Ar dating techniques in the context of noble gas studies, since there are already several recent texts on K-Ar and Ar-Ar dating (Dickin ; McDougall and Harrison ). The focus of this section will be cts of argon transport and storage in the crust, which affect K-Ar and Ar-Ar dating. Ar/ Ar dating into the historical realm: calibration against Pliny. the Y ounger. «Science, , The K-Ar dating method was applied to the groundmass, using the unspiked Cassignol.
Second, it is well suited for dating potassium-bearing fine-grained materials such as clays or glauconites where 39 Ar recoil would be problematic. A major disadvantage of the K-Ar method is the sensitivity of a K-Ar age to the thermal history of the rocks. While a K-Ar age may reflect the formation age or crystallization age of rocks, this age can be readily disturbed or even reset due to a variety of geological processes such as slow cooling, thermal reheating, or regional metamorphism.
For example, in many early studies, it was noted that K-Ar dates did not agree with either a dates for other potassium-bearing minerals from the same rock or b ages using other geochronological methods e. Thus, one of the most serious challenges with the conventional K-Ar method is that, in isolation, it is impossible to tell whether a K-Ar date reflects the time of crystallization, time of a subsequent metamorphism, or some time in between.
There are some other disadvantages with conventional K-Ar dating. In the K-Ar method, absolute quantities of potassium and argon are measured on separate aliquots of the sample in order to calculate the 40 Ar- 40 K ratio and, hence, the age of the sample; consequently, K-Ar ages tend to have much larger uncertainties than Ar-Ar ages.
Moreover, if the sample is inhomogeneous, this practice can significantly reduce the accuracy of the final age.
In addition, because the sample is completely fused to measure K and Ar, only a single date can be obtained per analysis. Finally, relatively large quantities milligrams of pure sample are required for a K-Ar analysis which means that there must be significant quantities of sample available to be analyzed, also increasing sample preparation requirements. Fortunately, all of these disadvantages are effectively eliminated by using the 40 Ar- 39 Ar technique.
There are numerous advantages in using the 40 Ar- 39 Ar method. It is more convenient in terms of analysis because only isotopes of Ar need to be measured from a single aliquot of sample, unlike K-Ar dating in which different techniques are required to measure K and Ar, respectively.
Problems with sample inhomogeneity and differing machine calibrations are eliminated because all of the measurements are performed on the same sample with the same equipment. Because only ratios are needed i.
The method can yield significant information on the thermal history of the sample. Finally, other argon isotopes can be used to infer some information about the chemical composition K, Ca, Cl of the sample.
Ar-Ar dating does have some disadvantages. A nuclear reactor with a suitable fast-neutron fluence is required. This also means that irradiated samples are radioactive and must be appropriately stored; safe handling and sample preparation procedures must also be developed. Neutron irradiation results in the formation of artificial Ar isotopes from K, Ca, and Cl which must be corrected in calculating an Ar-Ar age, leading to more complicated data processing. Finally, all of these factors also mean that the method is generally more time consuming and costly in comparison to K-Ar dating.
The Ar-Ar method has largely superseded conventional K-Ar dating because of its numerous advantages. The presence of K-bearing minerals in a variety of rocks has led to the widespread application of Ar-Ar dating to solve a variety of geological problems involving the geological time scale, hominid evolution, structural geology and tectonics, economic geology, detrital provenance, paleomagnetism, the evolution of sedimentary basins, and the origin of extraterrestrial materials. Moreover, the strong relationship between argon mobility and temperature makes the Ar-Ar method an ideal thermochronometer.
To apply it successfully, however, knowledge of the microstructural, mineralogical, petrological, and geological context of the dated samples must be known in order to obtain meaningful interpretations of the age data.
With this information, the real power of the Ar-Ar method lies in its unique ability to solve diverse problems in the earth sciences and to elucidate a significant portion of the temperature-time histories of geological terranes.
Age of the Earth. Cretaceous-Tertiary Boundary. Geological Time Scale. Hominid Evolution Timescale Chixculub. Mass Spectrometry. Minerals Ar-Ar. Noble Gas Mass Spectrometer.
Editors: W. Contents Search. Ar-Ar and K-Ar Dating.
Living reference work entry First Online: 19 April Download entry. How to cite. This process is experimental and the keywords may be ated as the learning algorithm improves. Any potassium-bearing solid material has the potential to be dated by either the K-Ar or Ar-Ar methods.
Because many common rock-forming minerals contain K, the Ar-Ar method is one of the most commonly employed geochronological techniques in many geological studies.
The most common materials that are particularly well suited to dating using the K-Ar or Ar-Ar technique include: K-feldspar Biotite Phlogopite Glasses e. The K-Ar method is unique among geochronological schemes because the radioactive parent 40 K undergoes a complex branched radioactive decay to both radiogenic 40 Ca and 40 Ar Fig. Open image in new window.
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The Ar-Ar method is fundamentally based on the K-Ar method. Substitution of Eq. The determination of the neutron irradiation parameters in Eq. Therefore, the Ar-Ar age determination can be greatly simplified if an age standard sometimes called a monitor mineral is used.
If the age of the standard is known designated as t sthen Eq. However, for a sample of unknown age herein designated by the subscript uwe obtain from Eq. Thus, by substituting Eq. Potassium The analysis of potassium typically involves wet-chemical or physical methods.
Blank Correction Although argon is measured in an ultrahigh vacuum environment in both K-Ar and Ar-Ar dating, system blanks from both the extraction line and mass spectrometer must be subtracted from the total amount of each argon isotope measured. Neutron-Induced Interferences In Ar-Ar dating, neutron bombardment of most minerals generates numerous reactions with elements other than K that produce a variety of argon isotopes.
Radioactive Decay Correction In Ar-Ar dating, two of the artificial argon isotopes produced by neutron irradiation 37 Ar Ca and 39 Ar K are themselves radioactive with half-lives that are on a time scale of days to years. Conventional Total Fusion Age The analysis of a sample in conventional K-Ar dating produces a single age, since the sample must be completely dissolved and fused in order to extract the 40 K and 40 Ar, respectively.
This incremental-heating or step-heating technique thus was a major advantage over conventional K-Ar dating because it provided a way to examine the distribution of 40 Ar and hence, ages within minerals.
The fundamental principle is based on solid-state diffusion theory and is illustrated in Fig. By plotting the apparent ages of each of the steps t 1 - t 9 in a diagram of apparent age versus the cumulative fraction of 39 Ar K released, a so-called apparent age spectrum can be constructed.
For example, consider the same spherical grain in Fig. If 39 K is homogeneous throughout the grain i. The significance of a plateau age is discussed in the following section. Plateau Ages There is no convention that is widely accepted by the geochronology community as a standard definition of a plateau in an Ar-Ar age spectrum. Ar Loss There are three main scenarios in which an age spectrum showing apparent Ar loss can be generated. The first is due to reheating.
If a mineral has experienced a moderate-high T geological event, such as proximity to an igneous intrusion, contact metamorphism, hot fluid flow, etc.
If argon diffusion is not sustained for protracted periods of time, it is possible that the mineral may experience only partial argon loss, such that argon is lost from a limited region of the grain nearest the grain boundary and the original argon is retained in the grain core. In this case, an apparent age spectrum may look like that in Fig. A second scenario involves slow regional cooling.
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If a mineral is subject to slow cooling resulting from regional-scale metamorphism or exhumation, partial argon loss can occur over long periods of time if the rate of cooling is not rapid enough to retard Ar diffusion.
This will also yield an age spectrum similar to Fig. Finally, a third scenario involves impurities e. Therefore, knowledge of the sample and its local geological context is important in interpreting such age spectra.
Thermal activation can also result in the diffusion of 40 Ar from the external environment into mineral grains, if the chemical potential of argon often referred to as the argon partial pressure is much greater outside the crystal than inside. Because this 40 Ar is not associated with the in situ decay of 40 K i. There are many possible sources of excess Ar. Another common source is hot Ar-rich fluids which may carry 40 Ar E from the lower crust or distal regions.
Incorporation of 40 Ar E from the external environment means that age spectra tend to have anomalously old ages in the lower T steps Fig. Sometimes, anomalously old ages are reflected in the highest T steps. Although the reasons for this will vary from sample to sample, one potential source is 40 Ar E released from contaminant phases e.
Such phases may only release excess argon at high T or the phases may occur as internal inclusions which can only degas when the host mineral itself decomposes at high T.
An example of an age spectrum displaying excess argon in the low and high T steps is shown in Fig. An excellent review of the role of excess Ar in Ar-Ar dating may be found by Kelley This is illustrated in Fig. For example, Huneke and Smith and Lo and Onstott observed this phenomenon occurring between a K-rich phase and olivine and biotite and chlorite, respectively.
In such cases, the resultant age spectrum can display anomalously high ages in the low T steps reflecting the loss of 39 Ar recoiled near the grain boundary monotonically decreasing to anomalously low ages in the high T steps as the 39 Ar is degassed from the K-poor phases.
Such an age spectrum is shown in Fig. One useful ct about the 40 Ar- 39 Ar technique is that it has the ability to yield significant information about the chemical composition of the samples that are being dated. The foundation of the 40 Ar- 39 Ar technique through the 39 K n,p 39 Ar reaction already provides chemical information on the K content in the sample.
As noted in the section on Data Corrections, however, there are also other neutron-induced interferences, leading to the production of 37 Ar from 40 Ca and 38 Ar from 37 Cl. This is analogous to many other geochronological systems, e. By substituting the K-Ar age equation of Eq. The difficulty in using Eq. In Ar-Ar dating, we substitute Eq. This is analogous to the K-Ar isochron equation [Eq. An example of such an isochron diagram is shown in Fig.
Dividing Eq. An example of an inverse isochron diagram is shown in Fig.
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