# Is potassium argon dating accuracy

Potassium—argon dating , abbreviated K—Ar dating , is a radiometric dating method used in geochronology and archaeology. It is based on measurement of the product of the radioactive decay of an isotope of potassium K into argon Ar. Potassium is a common element found in many materials, such as micas , clay minerals , tephra , and evaporites. In these materials, the decay product 40 Ar is able to escape the liquid molten rock, but starts to accumulate when the rock solidifies recrystallizes. The amount of argon sublimation that occurs is a function of the purity of the sample, the composition of the mother material, and a number of other factors. Time since recrystallization is calculated by measuring the ratio of the amount of 40 Ar accumulated to the amount of 40 K remaining.

## Problems in the dating of volcanic rocks by the potassium-argon method

Potassium—argon dating , abbreviated K—Ar dating , is a radiometric dating method used in geochronology and archaeology. It is based on measurement of the product of the radioactive decay of an isotope of potassium K into argon Ar. Potassium is a common element found in many materials, such as micas , clay minerals , tephra , and evaporites. In these materials, the decay product 40 Ar is able to escape the liquid molten rock, but starts to accumulate when the rock solidifies recrystallizes.

The amount of argon sublimation that occurs is a function of the purity of the sample, the composition of the mother material, and a number of other factors. Time since recrystallization is calculated by measuring the ratio of the amount of 40 Ar accumulated to the amount of 40 K remaining. The long half-life of 40 K allows the method to be used to calculate the absolute age of samples older than a few thousand years. The quickly cooled lavas that make nearly ideal samples for K—Ar dating also preserve a record of the direction and intensity of the local magnetic field as the sample cooled past the Curie temperature of iron.

The geomagnetic polarity time scale was calibrated largely using K—Ar dating. Potassium naturally occurs in 3 isotopes: Two are stable, while the radioactive isotope 40 K decays with a half-life of 1. 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. 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.

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.

To obtain the content ratio of isotopes 40 Ar to 40 K in a rock or mineral, the amount of Ar is measured by mass spectrometry of the gases released when a rock sample is volatilized in vacuum. The potassium is quantified by flame photometry or atomic absorption spectroscopy. The amount of 40 K is rarely measured directly. The amount of 40 Ar is also measured to assess how much of the total argon is atmospheric in origin.

Both flame photometry and mass spectrometry are destructive tests, so particular care is needed to ensure that the aliquots used are truly representative of the sample. Ar—Ar dating is a similar technique which compares isotopic ratios from the same portion of the sample to avoid this problem. Due to the long half-life of 40 K , the technique is most applicable for dating minerals and rocks more than , years old. For shorter timescales, it is unlikely that enough 40 Ar will have had time to accumulate in order to be accurately measurable.

K—Ar dating was instrumental in the development of the geomagnetic polarity time scale. One archeological application has been in bracketing the age of archeological deposits at Olduvai Gorge by dating lava flows above and below the deposits. In , the K—Ar method was used by the Mars Curiosity rover to date a rock on the Martian surface, the first time a rock has been dated from its mineral ingredients while situated on another planet. From Wikipedia, the free encyclopedia. Further information: Isotopes of potassium.

National Nuclear Data Center. June Retrieved 20 September Retrieved 22 February Retrieved from " https: Radiometric dating. Hidden categories: Use dmy dates from May Namespaces Article Talk. Views Read Edit View history. This page was last edited on 15 March , at By using this site, you agree to the Terms of Use and Privacy Policy. The Wikibook Historical Geology has a page on the topic of: K-Ar dating.

## Chronological Methods 9 - Potassium-Argon Dating Potassium-argon dating is accurate from billion years (the age of the Earth) to about. The accuracy of the K-Ar dating is dependent upon the following: Hawaii in the years were dated by the potassium-argon method.

Some updates to this article are now available. The sections on the branching ratio and dating meteorites need updating. Radiometric dating methods estimate the age of rocks using calculations based on the decay rates of radioactive elements such as uranium, strontium, and potassium. On the surface, radiometric dating methods appear to give powerful support to the statement that life has existed on the earth for hundreds of millions, even billions, of years. We are told that these methods are accurate to a few percent, and that there are many different methods.

Most people envision radiometric dating by analogy to sand grains in an hourglass:

The good dates are confirmed using at least two different methods, ideally involving multiple independent labs for each method to cross-check results. Sometimes only one method is possible, reducing the confidence researchers have in the results. Kidding aside, dating a find is crucial for understanding its significance and relation to other fossils or artifacts.

## Everything Worth Knowing About ... Scientific Dating Methods

The belief that radiodating methods give absolute measurements of time is widespread as a result of scientific popularization in journals, conferences, and the media. In fact, due to the difficulties in applying the experimental method to events in the past, all chronometers based on natural or artificial nuclear disintegration need a calibration. This calibration supposes a great number of basic assumptions concerning initial conditions. The choice of these initial conditions affects the results. These assumptions are not demonstrated, only supposed to be true.

## Potassium-Argon Dating Methods

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 The K-Ar method works by counting these radiogenic 40 Ar atoms trapped inside minerals. What simplifies things is that potassium is a reactive metal and argon is an inert gas: Potassium is always tightly locked up in minerals whereas argon is not part of any minerals. Argon makes up 1 percent of the atmosphere. So assuming that no air gets into a mineral grain when it first forms, it has zero argon content.

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This service is more advanced with JavaScript available, learn more at http: Bulletin Volcanologique. The potassium-argon method is attractive for dating volcanics since it can be applied to rocks of Pleistocene age and older, thus encompassing important periods of general volcanic activity. However it has been found that dates obtained on whole rocks and on included minerals frequently show gross discordances. In order to establish this dating method in this application an attempt has been made to trace the sources of the anomalies. To illustrate these efforts, dating results from a rhyodacite of Mauna Kuwale, Oahu, Hawaii, are reported. Determinations on several minerals and the whole rock of this ridge give a concordant age of 2. It has been noted that xenoliths in certain Hawaiian volcanics contain fluid inclusions which show evidence of formation at depth. We have found that gas released from such inclusions by crushing contains radiogenic argon, and that the constituent minerals give very old potassium-argon ages circa million years. Similar gaseous inclusions have been noted in a variety of other lava phenocrysts, and their presence in a dated sample may produce an anomalous old age. In the minerals from Mauna Kuwale sporadic occurrences of inclusions have been noted in biotites and hornblendes, and crushing of the mineral releases the excess radiogenic argon.

## Potassium-argon (K-Ar) dating

Because radiometric dating utterly refutes their biblical interpretations, young-Earth creationists YECs are desperate to undermine the reliability of these dating methods. As part of their efforts, YECs clearly believe that they can discredit K-Ar dating if they can show that excess argon routinely enters rocks and minerals as they form. That is, they believe that excess argon will cause rocks and minerals that are supposedly less than 10, years old to have 'deceptively' old K-Ar dates of millions or billions of years. In particular, YECs attempt to demonstrate that excess argon is a 'problem' for K-Ar dating by locating examples of historically erupted volcanics, which yield K-Ar dates that are hundreds of thousands or millions of years older than their eruption dates. YECs Dr.

## Potassium-argon dating

Potassium-Argon Dating Potassium-Argon dating is the only viable technique for dating very old archaeological materials. Geologists have used this method to date rocks as much as 4 billion years old. It is based on the fact that some of the radioactive isotope of Potassium, Potassium K ,decays to the gas Argon as Argon Ar By comparing the proportion of K to Ar in a sample of volcanic rock, and knowing the decay rate of K, the date that the rock formed can be determined. How Does the Reaction Work? Potassium K is one of the most abundant elements in the Earth's crust 2. One out of every 10, Potassium atoms is radioactive Potassium K

Potassium, an alkali metal, the Earth's eighth most abundant element is common in many rocks and rock-forming minerals. The quantity of potassium in a rock or mineral is variable proportional to the amount of silica present. Therefore, mafic rocks and minerals often contain less potassium than an equal amount of silicic rock or mineral. Potassium can be mobilized into or out of a rock or mineral through alteration processes. Due to the relatively heavy atomic weight of potassium, insignificant fractionation of the different potassium isotopes occurs. However, the 40 K isotope is radioactive and therefore will be reduced in quantity over time. But, for the purposes of the KAr dating system, the relative abundance of 40 K is so small and its half-life is so long that its ratios with the other Potassium isotopes are considered constant. Argon, a noble gas, constitutes approximately 0.

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.

How Accurate is K-Ar Dating? Messel, "A Modern Introduction to Physics" vol. Sydney, p: The radiogenic argon that builds up in potassium-rich minerals after they have crystallized, therefore, furnishes a good measure of the age of the sample. Del Mar California , p: The rubidium-strontium and uranium-lead techniques are very difficult to use with such samples, because the slow decay rates of the parent isotopes have not allowed a significant increase in the daughter isotopes.

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