K–Ar dating

<h2 id="decay-series">Decay series</h2>
Further information: <a href="/facts/Isotopes_of_potassium/xxMtp7JP">Isotopes of potassium</a>
Potassium naturally occurs in 3 isotopes: 39K (93.2581%), 40K (0.0117%), 41K (6.7302%). 39K and 41K are stable. The 40K isotope is radioactive; it decays with a <a href="/facts/Half-life/kGVH8BAB">half-life</a> of 1.248×109 years to <a href="/facts/Calcium-40/YhuBa6LI">40Ca</a> and <a href="/facts/Argon-40/BhxtyJGu">40Ar</a>. Conversion to stable 40Ca occurs via electron emission (<a href="/facts/Beta_decay/vxTBlckp">beta decay</a>) in 89.3% of decay events. Conversion to stable 40Ar occurs via <a href="/facts/Electron_capture/EIccoV1u">electron capture</a> in the remaining 10.7% of decay events.<a class="footnote-ref" id="fnref:3" href="#fn:3">3</a>
Argon, being a <a href="/facts/Noble_gas/j0yHMEAE">noble gas</a>, is a minor component of most rock samples of <a href="/facts/Geochronology/U8hRTGwL">geochronological</a> interest: It does not bind with other atoms in a crystal lattice. When 40K decays to 40Ar; the atom typically remains trapped within the lattice because it is larger than the spaces between the other atoms in a mineral crystal. However, it can escape into the surrounding region when the right conditions are met, such as changes in pressure or temperature. 40Ar atoms can 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 are 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 in dating because calcium is so common in the crust, with 40Ca being the most abundant isotope. Thus, the amount of calcium originally present is unknown and can vary enough to confound measurements of the small increases produced by radioactive decay.

<h2 id="formula">Formula</h2>
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 equation:

t
 =
 
 t
 
 
 1
 2
 
 
 
 
 log
 
 2
 
 
 ⁡
 
 (
 
 
 
 
 
 K
 
 
 f
 
 
 +
 
 
 
 
 Ar
 
 
 f
 
 
 0.109
 
 
 
 
 
 K
 
 
 f
 
 
 
 
 )
 
 
 
 {\displaystyle t=t_{\frac {1}{2}}\log _{2}\left({\frac {{\ce {K}}_{f}+{\frac {{\ce {Ar}}_{f}}{0.109}}}{{\ce {K}}_{f}}}\right)}
 
,
Where:

<ul><li>t is time elapsed</li>
<li>t1/2 is the <a href="/facts/Half-life/kGVH8BAB">half-life</a> of 40K</li>
<li>Kf is the amount of 40K remaining in the sample</li>
<li>Arf is the amount of 40Ar found in the sample.</li></ul>
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 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.

<h2 id="obtaining-the-data">Obtaining the data</h2>
To obtain the content ratio of isotopes 40Ar to 40K in a rock or mineral, the amount of Ar is measured by <a href="/facts/Mass_spectrometry/RH5WBHJ2">mass spectrometry</a> of the gases released when a rock sample is volatilized in a vacuum. The potassium is quantified by <a href="/facts/Emission_spectrometry/fxumR717">flame photometry</a> or <a href="/facts/Atomic_absorption_spectroscopy/NkJQHxSB">atomic absorption spectroscopy</a>.
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 40Ar is also measured to assess how much of the total argon is atmospheric in origin.

<h2 id="assumptions">Assumptions</h2>
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:<a class="footnote-ref" id="fnref:4" href="#fn:4">4</a>

<ul><li>The parent nuclide, 40K, decays at a rate independent of its physical state and is not affected by differences in pressure or temperature. This is a well-founded major assumption, common to all dating methods based on radioactive decay. Although changes in the electron capture partial decay constant for 40K possibly may occur at high pressures, theoretical calculations indicate that for pressures experienced within a body the size of the Earth, the effects are negligibly small.<a class="footnote-ref" id="fnref:5" href="#fn:5">5</a></li>
<li>The 40K/39K ratio in nature is constant so the 40K is rarely measured directly, but is assumed to be 0.0117% of the total potassium. Unless some other process is active at the time of cooling, this is a very good assumption for terrestrial samples.<a class="footnote-ref" id="fnref:6" href="#fn:6">6</a></li>
<li>The radiogenic argon measured in a sample was produced by in situ decay of 40K in the interval since the rock crystallized or was recrystallized. Violations of this assumption are not uncommon. Well-known examples of incorporation of extraneous 40Ar include chilled glassy deep-sea basalts that have not completely outgassed preexisting 40Ar*,<a class="footnote-ref" id="fnref:7" href="#fn:7">7</a> and the physical contamination of magma by the inclusion of older xenolitic material. The <a href="/facts/Argon%25E2%2580%2593argon_dating/kALXQu7H">Ar–Ar dating</a> method was developed to measure the presence of extraneous argon.</li>
<li>Great care is needed to avoid contamination of samples by absorption of non-radiogenic 40Ar from the atmosphere. The equation may be corrected by subtracting from the 40Armeasured value of the amount present in the air where 40Ar is 295.5 times more plentiful than 36Ar. 40Ardecayed = 40Armeasured − 295.5 × 36Armeasured.</li>
<li>The sample must have remained a closed system since the event was dated. Thus, there should have been no loss or gain of 40K or 40Ar*, other than by radioactive decay of 40K. Departures from this assumption are quite common, particularly in areas of complex geological history, but such departures can provide useful information that is of value in elucidating thermal histories. A deficiency of 40Ar in a sample of a known age can indicate a full or partial melt in the thermal history of the area. Reliability in the dating of a geological feature is increased by sampling disparate areas which have been subjected to slightly different thermal histories.<a class="footnote-ref" id="fnref:8" href="#fn:8">8</a></li></ul>
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. <a href="/facts/Argon%25E2%2580%2593argon_dating/kALXQu7H">Ar–Ar dating</a> is a similar technique that compares isotopic ratios from the same portion of the sample to avoid this problem.

<h2 id="applications">Applications</h2>
Due to the long <a href="/facts/Half-life/kGVH8BAB">half-life</a> of 40K, the technique is most applicable for dating minerals and rocks over 100,000 years old. For shorter timescales, it is unlikely that enough 40Ar will have had time to accumulate to be accurately measurable. K–Ar dating was instrumental in developing the <a href="/facts/Geomagnetic_polarity_time_scale/dsINRz5g">geomagnetic polarity time scale</a>.<a class="footnote-ref" id="fnref:9" href="#fn:9">9</a> Although it finds the most utility in <a href="/facts/Geology/WGvVRIYF">geological</a> applications, it plays an important role in <a href="/facts/Archaeology/K3Q5P38R">archaeology</a>. One archeological application has been bracketing the age of archeological deposits at <a href="/facts/Olduvai_Gorge/0bnoQHm5">Olduvai Gorge</a> by dating <a href="/facts/Lava/q2an5SVN">lava</a> flows above and below the deposits.<a class="footnote-ref" id="fnref:10" href="#fn:10">10</a> It has also been indispensable in other early East <a href="/facts/Africa/XkqQUP9s">African</a> sites with a history of <a href="/facts/Volcanic/c2M5upml">volcanic</a> activity such as <a href="/facts/Hadar%2c_Ethiopia/3QW3h7BB">Hadar, Ethiopia</a>.<a class="footnote-ref" id="fnref:11" href="#fn:11">11</a> The K–Ar method continues to have utility in dating clay mineral <a href="/facts/Diagenesis/F1nATwdp">diagenesis</a>.<a class="footnote-ref" id="fnref:12" href="#fn:12">12</a> In 2017, the successful dating of <a href="/facts/Illite/m4u7Ee4X">illite</a> formed by <a href="/facts/Weathering/uSYGlv0H">weathering</a> was reported.<a class="footnote-ref" id="fnref:13" href="#fn:13">13</a> This finding indirectly led to the dating of the <a href="/facts/Strandflat/l66jqrmC">strandflat</a> of <a href="/facts/Western_Norway/oBn6qBBV">Western Norway</a> from where the illite was sampled.<a class="footnote-ref" id="fnref:14" href="#fn:14">14</a> Clay minerals are less than 2 μm thick and cannot easily be irradiated for <a href="/facts/Argon%25E2%2580%2593argon_dating/kALXQu7H">Ar–Ar</a> analysis because Ar recoils from the crystal lattice.
In 2013, the K–Ar method was used by the <a href="/facts/Mars_Curiosity/mVLawAfA">Mars Curiosity</a> 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.<a class="footnote-ref" id="fnref:15" href="#fn:15">15</a><a class="footnote-ref" id="fnref:16" href="#fn:16">16</a>

<h2 id="notes">Notes</h2>

<ul><li>Aronson, J. L.; Lee, M. (1986). <a href="https://doi.org/10.1346%2FCCMN.1986.0340415">"K/Ar systematics of bentonite and shale in a contact metamorphic zone"</a>. <a href="/facts/Clays_and_Clay_Minerals/k5KgQldl">Clays and Clay Minerals</a>. 34 (4): 483–487. <a href="/facts/Bibcode_(identifier)/9HtdQSGB">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/1986CCM....34..483A">1986CCM....34..483A</a>. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1346%2FCCMN.1986.0340415">10.1346/CCMN.1986.0340415</a>.</li>
<li><a href="/facts/Ian_McDougall_(geologist)/7KJDTXTr">McDougall, I.</a>; Harrison, T. M. (1999). Geochronology and thermochronology by the 40Ar/39Ar method. <a href="/facts/Oxford_University_Press/CYyTzzWG">Oxford University Press</a>. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 978-0-19-510920-7.</li>
<li>Tattersall, I. (1995). <a href="https://archive.org/details/fossiltrailhowwe00tatt">The Fossil Trail: How We Know What We Think We Know About Human Evolution</a>. <a href="/facts/Oxford_University_Press/CYyTzzWG">Oxford University Press</a>. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 978-0-19-506101-7.</li></ul>
<h2 id="further-reading">Further reading</h2>

The Wikibook Historical Geology has a page on the topic of: K-Ar dating

<ul><li><a href="https://web.archive.org/web/20060417234339/http://www.ees.nmt.edu/Geol/labs/Argon_Lab/Methods/Methods.html">"K/Ar and 40K/39K methodology"</a>. New Mexico Geochronology Research Laboratory. Archived from <a href="http://www.ees.nmt.edu/Geol/labs/Argon_Lab/Methods/Methods.html">the original</a> on 17 April 2006.</li>
<li>Michaels, G. H.; Fagan, B. M. (15 December 2005). <a href="https://web.archive.org/web/20100810084507/http://id-archserve.ucsb.edu/anth3/courseware/Chronology/09_Potassium_Argon_Dating.html">"Chronological Methods 9: Potassium-Argon Dating"</a>. <a href="/facts/University_of_California/M9AWowUd">University of California</a>. Archived from <a href="http://id-archserve.ucsb.edu/anth3/courseware/Chronology/09_Potassium_Argon_Dating.html">the original</a> on 10 August 2010.</li>
<li>Moran, T. J. (2009). <a href="http://mast.unco.edu/JGE-PDFs/MAR/p101-105MAR2009Moran.pdf">"Teaching Radioisotope Dating Using the Geology of the Hawaiian Islands"</a> (PDF). <a href="/facts/Journal_of_Geoscience_Education/jS868HxG">Journal of Geoscience Education</a>. 57 (2): 101–105. <a href="/facts/Bibcode_(identifier)/9HtdQSGB">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/2009JGeEd..57..101M">2009JGeEd..57..101M</a>. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.5408%2F1.3544237">10.5408/1.3544237</a>.</li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1">McDougall & Harrison 1999, p. 10 - McDougall, I.; Harrison, T. M. (1999). Geochronology and thermochronology by the 40Ar/39Ar method. Oxford University Press. ISBN 978-0-19-510920-7. <a href="#fnref:1" class="footnote-back-ref">↩</a></li>
<li id="fn:2">McDougall & Harrison 1999, p. 9 - McDougall, I.; Harrison, T. M. (1999). Geochronology and thermochronology by the 40Ar/39Ar method. Oxford University Press. ISBN 978-0-19-510920-7. <a href="#fnref:2" class="footnote-back-ref">↩</a></li>
<li id="fn:3">ENSDF decay data in the MIRD format for 40Ar (Report). National Nuclear Data Center. December 2019. Retrieved 29 December 2019. <a href="https://www.nndc.bnl.gov/useroutput/40k_mird.html" target="_blank">https://www.nndc.bnl.gov/useroutput/40k_mird.html</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></li>
<li id="fn: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." - McDougall, I.; Harrison, T. M. (1999). Geochronology and thermochronology by the 40Ar/39Ar method. Oxford University Press. ISBN 978-0-19-510920-7. <a href="#fnref:4" class="footnote-back-ref">↩</a></li>
<li id="fn:5">McDougall & Harrison 1999, p. 10 - McDougall, I.; Harrison, T. M. (1999). Geochronology and thermochronology by the 40Ar/39Ar method. Oxford University Press. ISBN 978-0-19-510920-7. <a href="#fnref:5" class="footnote-back-ref">↩</a></li>
<li id="fn:6">McDougall & Harrison 1999, p. 14 - McDougall, I.; Harrison, T. M. (1999). Geochronology and thermochronology by the 40Ar/39Ar method. Oxford University Press. ISBN 978-0-19-510920-7. <a href="#fnref:6" class="footnote-back-ref">↩</a></li>
<li id="fn:7">40Ar* means radiogenic argon <a href="#fnref:7" class="footnote-back-ref">↩</a></li>
<li id="fn:8">McDougall & Harrison 1999, pp. 9–12 - McDougall, I.; Harrison, T. M. (1999). Geochronology and thermochronology by the 40Ar/39Ar method. Oxford University Press. ISBN 978-0-19-510920-7. <a href="#fnref:8" class="footnote-back-ref">↩</a></li>
<li id="fn:9">McDougall & Harrison 1999, p. 9 - McDougall, I.; Harrison, T. M. (1999). Geochronology and thermochronology by the 40Ar/39Ar method. Oxford University Press. ISBN 978-0-19-510920-7. <a href="#fnref:9" class="footnote-back-ref">↩</a></li>
<li id="fn:10">Tattersall 1995 - Tattersall, I. (1995). The Fossil Trail: How We Know What We Think We Know About Human Evolution. Oxford University Press. ISBN 978-0-19-506101-7. <a href="https://archive.org/details/fossiltrailhowwe00tatt" target="_blank">https://archive.org/details/fossiltrailhowwe00tatt</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></li>
<li id="fn:11">Tattersall 1995 - Tattersall, I. (1995). The Fossil Trail: How We Know What We Think We Know About Human Evolution. Oxford University Press. ISBN 978-0-19-506101-7. <a href="https://archive.org/details/fossiltrailhowwe00tatt" target="_blank">https://archive.org/details/fossiltrailhowwe00tatt</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></li>
<li id="fn:12">Aronson & Lee 1986 - Aronson, J. L.; Lee, M. (1986). "K/Ar systematics of bentonite and shale in a contact metamorphic zone". Clays and Clay Minerals. 34 (4): 483–487. Bibcode:1986CCM....34..483A. doi:10.1346/CCMN.1986.0340415. <a href="https://doi.org/10.1346%2FCCMN.1986.0340415" target="_blank">https://doi.org/10.1346%2FCCMN.1986.0340415</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></li>
<li id="fn:13">Fredin, Ola; Viola, Giulio; Zwingmann, Horst; Sørlie, Ronald; Brönner, Marco; Lie, Jan-Erik; Margrethe Grandal, Else; Müller, Axel; Margeth, Annina; Vogt, Christoph; Knies, Jochen (2017). "The inheritance of a Mesozoic landscape in western Scandinavia". Nature. 8: 14879. Bibcode:2017NatCo...814879F. doi:10.1038/ncomms14879. PMC 5477494. PMID 28452366. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5477494" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5477494</a> <a href="#fnref:13" class="footnote-back-ref">↩</a></li>
<li id="fn:14">Fredin, Ola; Viola, Giulio; Zwingmann, Horst; Sørlie, Ronald; Brönner, Marco; Lie, Jan-Erik; Margrethe Grandal, Else; Müller, Axel; Margeth, Annina; Vogt, Christoph; Knies, Jochen (2017). "The inheritance of a Mesozoic landscape in western Scandinavia". Nature. 8: 14879. Bibcode:2017NatCo...814879F. doi:10.1038/ncomms14879. PMC 5477494. PMID 28452366. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5477494" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5477494</a> <a href="#fnref:14" class="footnote-back-ref">↩</a></li>
<li id="fn:15">NASA Curiosity: First Mars Age Measurement and Human Exploration Help, Jet Propulsion Laboratory, 9 December 2013 <a href="https://www.jpl.nasa.gov/news/news.php?release=2013-356" target="_blank">https://www.jpl.nasa.gov/news/news.php?release=2013-356</a> <a href="#fnref:15" class="footnote-back-ref">↩</a></li>
<li id="fn:16">Martian rock-dating technique could point to signs of life in space, University of Queensland, 13 December 2013 <a href="https://www.uq.edu.au/news/article/2013/12/martian-rock-dating-technique-could-point-signs-of-life-space" target="_blank">https://www.uq.edu.au/news/article/2013/12/martian-rock-dating-technique-could-point-signs-of-life-space</a> <a href="#fnref:16" class="footnote-back-ref">↩</a></li>
</ol>

K–Ar dating open-in-new

K–Ar dating