potassium argon dating problems

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Potassium argon dating problems

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. Because it is present within the atmosphere, every rock and mineral will have some quantity of Argon. Argon can mobilized into or out of a rock or mineral through alteration and thermal processes. Like Potassium, Argon cannot be significantly fractionated in nature. However, 40 Ar is the decay product of 40 K and therefore will increase in quantity over time.

The quantity of 40 Ar produced in a rock or mineral over time can be determined by substracting the amount known to be contained in the atmosphere. This ratio is The decay scheme is electron capture and positron decay. Certain assumptions must be satisfied before the age of a rock or mineral can be calculated with the Potassium-Argon dating technique.

These are:. Argon loss and excess argon are two common problems that may cause erroneous ages to be determined. Excess argon may be derived from the mantle, as bubbles trapped in a melt, in the case of a magma. Both techniques rely on the measurement of a daughter isotope 40 Ar and a parent isotope. Because the relative abundances of the potassium isotopes are known, the 39 Ar K produced from 39 K by a fast neutron reaction can be used as a proxy for potassium.

Instead, the ratios of the different argon isotopes are measured, yielding more precise and accurate results. The amount of 39 Ar K produced in any given irradiation will be dependant on the amount of 39 K present initially, the length of the irradiation, the neutron flux density and the neutron capture cross section for 39 K. However, because each of these parameters is difficult to determine independantly, a mineral standard, or monitor, of known age is irradiated with the samples of unknown age.

The monitor flux can then be extrapolated to the samples, thereby determining their flux. This flux is known as the 'J' and can be determined by the following equation:. In addition to 39 Ar production from 39 K, several other 'interference' reactions occur during irradiation of the samples. Other isotopes of argon are produced from potassium, calcium, argon and chlorine. As the table above illustrates, several "undesirable" reactions occur on isotopes present within every geologic sample.

These reactor produced isotopes of argon must be corrected for in order to determine an accurate age. The monitoring of the interfering reactions is performed through the use of laboratory salts and glasses. Petrology, 5, 82— Mcdougall, I. Naughton, J. Nature, , — Reynolds, J. Richter, D. Hawaiian Acad. Roedder, E. Download references. Funkhouser, I. You can also search for this author in PubMed Google Scholar.

Reprints and Permissions. Funkhouser, J. Problems in the dating of volcanic rocks by the potassium-argon method. Bull Volcanol 29, — Download citation. Issue Date : December Search SpringerLink Search. Abstract 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.

Immediate online access to all issues from Subscription will auto renew annually. References Dalrymple, G. Google Scholar Evernden, J. Article Google Scholar Fitch, F. Article Google Scholar Lovering, J. Article Google Scholar Macdonald, G. Google Scholar Macdonald, G. Google Scholar Mcdougall, I. Article Google Scholar Naughton, J.

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The quantity of 40 Ar produced in a rock or mineral over time can be determined by substracting the amount known to be contained in the atmosphere. This ratio is The decay scheme is electron capture and positron decay. Certain assumptions must be satisfied before the age of a rock or mineral can be calculated with the Potassium-Argon dating technique. These are:. Argon loss and excess argon are two common problems that may cause erroneous ages to be determined.

Excess argon may be derived from the mantle, as bubbles trapped in a melt, in the case of a magma. Both techniques rely on the measurement of a daughter isotope 40 Ar and a parent isotope. Because the relative abundances of the potassium isotopes are known, the 39 Ar K produced from 39 K by a fast neutron reaction can be used as a proxy for potassium. Instead, the ratios of the different argon isotopes are measured, yielding more precise and accurate results.

The amount of 39 Ar K produced in any given irradiation will be dependant on the amount of 39 K present initially, the length of the irradiation, the neutron flux density and the neutron capture cross section for 39 K. However, because each of these parameters is difficult to determine independantly, a mineral standard, or monitor, of known age is irradiated with the samples of unknown age.

The monitor flux can then be extrapolated to the samples, thereby determining their flux. This flux is known as the 'J' and can be determined by the following equation:. In addition to 39 Ar production from 39 K, several other 'interference' reactions occur during irradiation of the samples. Other isotopes of argon are produced from potassium, calcium, argon and chlorine. As the table above illustrates, several "undesirable" reactions occur on isotopes present within every geologic sample.

These reactor produced isotopes of argon must be corrected for in order to determine an accurate age. The monitoring of the interfering reactions is performed through the use of laboratory salts and glasses. For example, to determine the amount of reactor produced 40 Ar from 40 K, potassium-rich glass is irradiated with the samples. The desirable production of 38 Ar from 37Cl allows us to determine how much chlorine is present in our samples.

Multiple argon extractions can be performed on a sample in several ways. Step-heating is the most common way and involves either a furnace or a laser to uniformily heat the sample to evolve argon. The individual ages from each heating step are then graphically plotted on an age spectrum or an isochron.

Mechanical crushing is also a technique capable of releasing argon from a single sample in multiple steps. Laser probes also allow multiple ages to be determined on a single sample aliquot, but do so using accurate and precise spatial control. For example, laser spot sizes of microns or less allow a user to extract multiple argon samples from across a small mica or feldspar grain.

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. The determination of the age of such a material would give an old age, and thus account for the anomalies found.

For meaningful dating of volcanics by the potassium argon method it is concluded that phenocryst-containing materials should be examined for fluid inclusion content, and samples which contain these should be rejected. This is a preview of subscription content, access via your institution. Rent this article via DeepDyve. Dalrymple, G. Google Scholar. Evernden, J. Article Google Scholar. Fitch, F. London, , 55—69 and — Lovering, J. Macdonald, G. Hawaii Div. Petrology, 5, 82— Mcdougall, I.

Naughton, J. Nature, , — Reynolds, J. Richter, D. Hawaiian Acad. Roedder, E.

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Dalrymple, referring to metamorphism and melting of rocks in the crust, has commented: "If the rock is heated or melted at some later time, then some or all the 40 Ar may escape and the K-Ar clock is partially or totally reset. Indeed, a well-defined law has been calculated for 40 Ar diffusion from hornblende in a gabbro due to heating.

They are the lower mantle below km , upper mantle, continental mantle lithosphere, oceanic mantle lithosphere, continental crust and oceanic crust, the latter four constituting the earth's crust. Each is a distinct geochemical reservoir. A steady-state upper mantle model has been proposed for mass transfer of rare gases, including Ar.

Assuming a 4. Thus all K-Ar and Ar-Ar "dates" of crustal rocks are questionable, as well as fossil "dates" calibrated by them. Notes: "Ma" represents a million years Mega-annum ; "Ga" represents a billion years Giga-annum.

The remainder has no radiogenic source. The two are identical. Cite this article: Snelling, A. Snelling, Ph. Helens Volcano by Steven A. Skip to main content. References 1 A. Karpinskaya, I. Ostrovskiy and L. Geology Series , 8 : pp. Patterson, M. Honda and I. Burnard, D. Graham and G.

Moreira, J. Kunz and C. Funkhouser, I. Barnes and J. Lanphere and G. Reynolds, J. Richter, D. Hawaiian Acad. Roedder, E. Download references. Funkhouser, I. You can also search for this author in PubMed Google Scholar.

Reprints and Permissions. Funkhouser, J. Problems in the dating of volcanic rocks by the potassium-argon method. Bull Volcanol 29, — Download citation. Issue Date : December Search SpringerLink Search. Abstract 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. Immediate online access to all issues from Subscription will auto renew annually.

References Dalrymple, G. Google Scholar Evernden, J. Article Google Scholar Fitch, F. Article Google Scholar Lovering, J. Article Google Scholar Macdonald, G. Google Scholar Macdonald, G. Google Scholar Mcdougall, I. Article Google Scholar Naughton, J. Article Google Scholar Reynolds, J. Article Google Scholar Richter, D. Google Scholar Download references. Naughton Authors J.