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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 40Ar 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. These factors introduce error limits on the upper and lower bounds of dating, so that final determination of age is reliant on the environmental factors during formation, melting, and exposure to decreased pressure and/or open-air. Time since recrystallization is calculated by measuring the ratio of the amount of 40Ar accumulated to the amount of 40K remaining. The long "half-life of 40K allows the method to be used to calculate the "absolute age of samples older than a few thousand years.[1]

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.[2]

Contents

Decay series[edit]

Potassium naturally occurs in 3 isotopes: 39K (93.2581%), 40K (0.0117%), 41K (6.7302%). Two are stable, while the radioactive isotope 40K decays with a "half-life of 1.248×109 years to "40Ca and "40Ar. Conversion to stable 40Ca occurs via electron emission ("beta decay) in 89.1% of decay events. Conversion to stable 40Ar occurs via "electron capture in the remaining 10.9% of decay events.[3]

Argon, being a "noble gas, is a minor component of most rock samples of "geochronological interest: it does not bind with other atoms in a crystal lattice. When 40K decays to 40Ar (argon), the atom typically remains trapped within the lattice because it is larger than the spaces between the other atoms in a mineral crystal. But it can escape into the surrounding region when the right conditions are met, such as change in pressure and/or temperature. 40Ar atoms are able to diffuse through and escape from molten magma because most crystals have melted and the atoms are no longer trapped. 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 40K will decay and 40Ar will again accumulate, along with the entrained argon atoms, trapped in the mineral crystals. Measurement of the quantity of 40Ar atoms is used to compute the amount of time that has passed since a rock sample has solidified.

Despite 40Ca being the favored daughter nuclide, it is rarely useful dating as calcium is common in the crust, with 40Ca being the most abundant isotope. Thus, the amount of calcium originally present is not known with enough accuracy to be able to measure the small increase produced by radioactive decay.

Formula[edit]

The ratio of the amount of 40Ar to that of 40K is directly related to the time elapsed since the rock was cool enough to trap the Ar by the following equation:

The scale factor 0.109 corrects for the unmeasured fraction of 40K which decayed into 40Ca; the sum of the measured 40K and the scaled amount of 40Ar gives the amount of 40K which was present at the beginning of the elapsed time period. 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.

Obtaining the data[edit]

To obtain the content ratio of isotopes 40Ar to 40K in a rock or mineral, the amount of Ar is measured by "mass spectrometry of the gases released when a rock sample is melted in vacuum. The potassium is quantified by "flame photometry or "atomic absorption spectroscopy.

The amount of 40K is rarely measured directly. Rather, the more common 39K is measured and that quantity is then multiplied by the accepted ratio of 40K/39K (i.e., 0.0117%/93.2581%, see above).

The amount of 36Ar is also measured to assess how much of the total argon is atmospheric in origin.

Assumptions[edit]

According to McDougall & Harrison (1999, p. 11) the following assumptions must be true for computed dates to be accepted as representing the true age of the rock:[4]

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.

Applications[edit]

Due to the long "half-life, the technique is most applicable for dating minerals and rocks more than 100,000 years old. For shorter timescales, it is unlikely that enough argon-40 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.[2] Although it finds the most utility in "geological applications, it plays an important role in "archaeology. One archeological application has been in bracketing the age of archeological deposits at "Olduvai Gorge by dating "lava flows above and below the deposits.[8] It has also been indispensable in other early east "African sites with a history of "volcanic activity such as "Hadar, Ethiopia.[8] The K–Ar method continues to have utility in dating clay mineral "diagenesis.[9] Clay minerals are less than 2 micrometres thick and cannot easily be irradiated for "Ar–Ar analysis because Ar recoils from the crystal lattice.

In 2013 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.[10][11]

Notes[edit]

  1. ^ a b McDougall & Harrison 1999, p. 10
  2. ^ a b McDougall & Harrison 1999, p. 9
  3. ^ "ENSDF Decay Data in the MIRD Format for 40K". "National Nuclear Data Center. June 1993. Retrieved 20 September 2013. 
  4. ^ McDougall & Harrison 1999, p. 11: "As with all isotopic dating methods, there are a number of assumptions that must be fulfilled for a K–Ar age to relate to events in the geological history of the region being studied."
  5. ^ McDougall & Harrison 1999, p. 14
  6. ^ 40
    Ar
    * means radiogenic argon
  7. ^ McDougall & Harrison 1999, pp. 9–12
  8. ^ a b Tattersall 1995
  9. ^ Aronson & Lee 1986
  10. ^ NASA Curiosity: First Mars Age Measurement and Human Exploration Help, "Jet Propulsion Laboratory, 2013-12-09 
  11. ^ Martian rock-dating technique could point to signs of life in space, University of Queensland, 2013-12-13 

References[edit]

Further reading[edit]

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