In this article we shall examine the basis of the K-Ar dating method, how it works, and what can go wrong with it. It is possible to measure the proportion in which 40 K decays, and to say that about Potassium is chemically incorporated into common minerals, notably hornblende , biotite and potassium feldspar , which are component minerals of igneous rocks. Argon, on the other hand, is an inert gas; it cannot combine chemically with anything. As a result under most circumstances we don't expect to find much argon in igneous rocks just after they've formed. However, see the section below on the limitations of the method. This suggests an obvious method of dating igneous rocks.
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The Editors of Encyclopaedia Britannica Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree See Article History. Read More on This Topic. This is possible in potassium-argon K-Ar dating, for example, because most minerals do not take argon into their structures initially 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. Anomalously high ages are seen in the first few steps which gradually decline to anomalously low ages in the final few steps.
For this reason, Ar-Ar studies involving fine-grained samples must interpret the results with care. In such specific situations, it may also be worthwhile to consider using conventional K-Ar dating, which avoids such difficulties.
These spectra correspond to the same sample analyzed in Fig. Such spectra can be useful in interpreting degassing patterns in Ar-Ar step-heating experiments. These argon isotope ratios can provide useful information on the degassing of phases during a step-heating run.
For example, consider Figs. These spectra were all derived from the same sample, and a comparison of the first three steps in all of the spectra shows that the apparent argon-loss profile in Fig.
This strongly suggests that the first three steps showing apparent argon-loss profile do not necessarily reflect a geologically meaningful event, but rather likely reflect argon released from a small amount of a contaminating phase, such as plagioclase or epidote.
Note that this also does not necessarily affect the validity of the plateau age, but rather helps to discern the geological significance of various portions of an apparent age spectrum. In K-Ar dating, several minerals which have varying amounts of K e. If the data form a straight line, this is termed a mineral isochron and the slope can therefore be used to calculate the age of the sample. An analogous procedure can be used to analyze whole-rock samples across a region, in which case a resultant line would be termed a whole - rock isochron.
The goodness-of-fit of such lines is usually indicated by the mean square of the weighted deviates MSWD McIntyre et al.
Ar-Ar isochron diagram using the data obtained from the sample shown in Fig. Note that the first three temperature steps do not lie on a line corresponding to the steps defining the plateau.
A best-fit line through these steps yields a slope corresponding to an Ar-Ar isochron age of One feature of such isochron diagrams in Ar-Ar dating is that there is a strong correlation of uncertainties between the abscissa 39 Ar- 36 Ar and ordinate 40 Ar- 36 Arshown by the rotated error ellipses which lie subparallel to the line in Fig.
This is a result of the common denominator 36 Ar in each; because 36 Ar is such a small peak to measure in relation to 40 Ar, the relative uncertainty associated with measuring 36 Ar is much larger in relative terms and so this error correlation is significant. To reduce this large correlation of errors, the inverse isochron also known as isotope correlation diagram places the largest measured peak 40 Ar in the denominator.
Ar-Ar inverse isochron diagram using the data obtained from the sample shown in Fig. A best-fit line through these steps yields an intercept age of The data which form a line on an isotope correlation diagram can be interpreted to reflect mixing between two Ar reservoirs or components in the sample.
More complex mixing patterns between different argon reservoirs within a crystal may also be discerned using such diagrams e. Sometimes, Ar-Ar isochron diagrams may be somewhat misleading because nonlinear data may appear to have a good linear correlation because of the strong correlation of errors; however, Dalrymple et al.
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Probability distribution diagram of the Ar-Ar ages from 58 single crystals of detrital muscovite derived from a sandstone. The higher the peak, the greater the number of grains of that age. Artificially induced Ar - argon typically 36 Ar, 37 Ar, 38 Ar, 39 Ar, or 40 Ar produced in a mineral during fast-neutron irradiation in a nuclear reactor, due to neutron interactions with Cl, Ca, and K.
Atmospheric Ar - argon from the atmosphere, may be either 36 Ar a38 Ar aor 40 Ar a. Cosmogenic Ar - any Ar isotope typically 36 Ar, 37 Ar, 38 Ar, 39 Ar, or 40 Ar produced from cosmic ray interactions with various elements including Ca, Ti, Fe in a mineral, mainly through spallation reactions, but also through neutron capture, normally only a concern when dealing with extraterrestrial samples.
Excess Ar - any Ar typically 40 Ar which is incorporated into minerals by processes other than the in situ radioactive decay of 40 K in the crystal, includes inherited Ar but may also include Ar derived from physical contamination by other minerals or Ar introduced into a crystal from other sources after its formation, e. This is one form of excess Ar. At present, there are two main uses of the conventional K-Ar method. First, it can be used to calibrate fundamental physical parameters such as the K decay constant, since it is a method grounded on first principles, i.
Second, it is well-suited for dating potassium-bearing fine-grained materials such as clays or glauconites where 39 Ar recoil would be problematic. In general, however, the method of choice for dating most K-bearing materials in a variety of geological studies is the Ar-Ar method. The presence and abundance of K-bearing minerals in a variety of rocks, coupled with the strong temperature dependence of argon diffusion in minerals, have led to the widespread application of Ar-Ar dating in a variety of geological applications.
There are a plethora of studies in each of the general areas below which are far too numerous to list; some illustrative references are included, and more can also be found in more specific entries cross-referenced in this work.
In general, Ar-Ar dating has been applied most often in the following areas of study which are described briefly below. 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.
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.
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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.
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.
Apr 19, The potassium-argon (K-Ar) geochronological method is one of the oldest absolute dating methods and is based upon the occurrence of a radioactive isotope of potassium (40 K), which naturally decays to a stable daughter isotope of argon (radiogenic 40 Ar, also known as 40 Ar*).For this reason, the K-Ar method is one of the few radiometric dating techniques in which the parent (40 K, a. Jan 31, 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. 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 .
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. 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. 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.
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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.
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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. Probability distribution diagrams Fig. Analogous to histograms, the diagrams plot the age on the x-axis versus the relative probability i. They are particularly useful in detrital provenance studies where source regions of different ages can be distinguished.
There are several possible sources of argon in rocks. The following glossary describes these various sources: Artificially induced Ar - argon typically 36 Ar, 37 Ar, 38 Ar, 39 Ar, or 40 Ar produced in a mineral during fast-neutron irradiation in a nuclear reactor, due to neutron interactions with Cl, Ca, and K.
K-Ar At present, there are two main uses of the conventional K-Ar method. Ar-Ar The presence and abundance of K-bearing minerals in a variety of rocks, coupled with the strong temperature dependence of argon diffusion in minerals, have led to the widespread application of Ar-Ar dating in a variety of geological applications. Refinement of the Geological Time Scale and Time-Scale Boundaries 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.
Hominid Evolution 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. Thermochronology Because Ar is a noble gas, it is particularly sensitive to temperature.
Structural Geology 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. Tectonics 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.
Economic Geology 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.
Detrital Provenance Ar-Ar dating of single crystals of K-bearing minerals has provided useful insights into the provenance of sediments derived from adjacent source regions. Paleomagnetism and the Geomagnetic Polarity Time Scale Ar-Ar dating has been particularly useful in dating paleomagnetic poles for plate reconstructions as well as in refining the geomagnetic polarity time scale. Sedimentary Basin Evolution 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.
Cosmochronology 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.
K-Ar At present, there are two main advantages of the conventional K-Ar method.
Ar-Ar There are numerous advantages in using the 40 Ar- 39 Ar method. Early dispersal of homo from Africa. Annual Reviews in Anthropology33- CrossRef Google Scholar. Armstrong, P. Thermochronometers in sedimentary basins. Reviews in Mineralogy and Geochemistry58- Begemann, F. Call for an improved set of decay constants for geochronological use. Geochimica et Cosmochimica Acta65- Berger, G. Geochimica et Cosmochimica Acta45- Bissig, T. Miocene landscape evolution in the Chilean flat-slab transect: uplift history and geomorphologic influences on epithermal processes in the El Indio-Pascua Au -Ag, Cu belt.
Economic Geology97- Camacho, A. Short-lived orogenic cycles and the eclogitization of cold crust by spasmodic hot fluids. Nature, - Dalrymple, G. Potassium-Argon dating: principles, techniques, and applications to geochronology.
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