Potassium argon dating

The K-Ar dating is a geochronologisches method for radiometric age determination of rocks in which the radioactive decay of potassium -40 ( 40 K) to argon -40 ( 40 utilizes Ar). The beta emitter 40 K decays with a half-life of 1.28 billion years in 11% of the cases to argon-40, in 89% to calcium-40. Potassium is found in common rock-forming minerals such as mica , feldspars and hornblende , which is why this dating technique is often used successfully with earthly rocks. In addition, potassium-argon dating is also used for extraterrestrial rocks, such as Apollo moon samples and meteorites ; so far, ages up to about 4.6 billion years, the estimated age of the solar system , have been determined.

A more precise variant is the 39 Ar- 40 Ar method. For them, the sample does not have to be divided into two, possibly unrepresentative halves, in order to determine argon and potassium separately. In addition, disturbances in the potassium-argon isotope system can be detected without the need for laborious mineral separation. Even with argon that has partially diffused out, this method can still be used to measure reliable “argon ages”, even on relatively young rocks.

Potassium-argon dating enables u. a. a reliable determination of the age of fossils that were embedded, for example, in the horizons between two volcanic tuff or lava layers: Because the intense heating of the volcanic rock expelled any argon that might previously have been present from the rock, the "radioactive clock" became as it were to zero and the argon measured afterwards consequently comes reliably from the decay of potassium.

Basics

40 K breaks down to 40 Ar and 40 Ca according to the following formulas :

• ${\ displaystyle {} ^ {40} \ mathrm {K} + \ mathrm {e} ^ {-} \ rightarrow {} ^ {40} \ mathrm {Ar} + \ gamma \ quad (1 {,} 505 \, \ mathrm {MeV})}$
with a rate of decay ${\ displaystyle \ lambda _ {\ epsilon} = 0 {,} 581 \ cdot 10 ^ {- 10} {\ frac {1} {a}}}$
• ${\ displaystyle {} ^ {40} \ mathrm {K} \ rightarrow {} ^ {40} \ mathrm {Ca} + \ beta ^ {-} \ quad (1 {,} 311 \, \ mathrm {MeV}) }$
with a rate of decay ${\ displaystyle \ lambda _ {\ beta} = 4 {,} 962 \ cdot 10 ^ {- 10} {\ frac {1} {a}}}$

If the potassium isotope 40 K is present in a rock mineral (the natural proportion of the 40 K isotope among all potassium isotopes is 0.0116%), its frequency decreases with time, while the frequency of the decay product 40 Ar increases. The development of frequencies over time is determined by the radioactive decay law. The following formula for the decay time can be derived from this:

• ${\ displaystyle t = {\ frac {1} {\ lambda}} \ cdot \ ln \ left (1 + {\ frac {\ lambda} {\ lambda _ {\ epsilon}}} \ cdot {\ frac {{} ^ {40} \ mathrm {[Ar]} _ {t}} {{} ^ {40} \ mathrm {[K]} _ {t}}} \ right)}$
${\ displaystyle \ lambda = \ lambda _ {\ epsilon} + \ lambda _ {\ beta}}$

Thus, the ratio 40 Ar at 40 K of daughter isotope 40 Ar to the parent isotope 40 K by measuring known, the age can be calculated.

With a frequency of 0.001%, a positron is formed when it decays to 40 Ar .

40 K- 40 Ar method

The simplest application is the direct measurement of the concentrations of potassium (e.g. using atomic emission spectroscopy ) and 40 Ar (using noble gas mass spectrometry ) in two parts of a sample. Because of the known isotope ratios of the potassium isotopes, the concentration of the isotope 40 K can be calculated from the concentration of potassium . The potassium-argon age can then be calculated from the ratio of 40 K to 40 Ar using the formula given in the “Basics” section.

This assumes that the event to be dated, e.g. B. the crystallization of a rock from a melt that has "reset the potassium-argon clock". This means that due to the event to be dated all previously possibly existing radiogenic 40 Ar (= 40 Ar, which was created by radioactive decay, in contrast to primordial argon, which comes from other sources, e.g. captured atmospheric argon) escaped from the rock, so that immediately after the end of the event there was no more 40 ares. Since argon, as a noble gas, escapes very easily when a rock is completely melted, this is usually the case in this case and this simple method usually provides reliable results when dating the crystallization of a rock from a melt.

Should shock events, e.g. B. the impact of a large asteroid, a complete outgassing is no longer necessarily given, since even at extremely high pressures that the shock waves generate in the rock, it has already been observed that 40 Ar does not escape completely.

Furthermore, it must be ensured that subsequent events such as diffusion of argon from the rock do not falsify the age. In such cases of argon diffusing out of the rock, too young ages are systematically measured.

If several different minerals with sufficient potassium content are present in a rock, one can rule out a falsification of the age through insufficient outgassing or diffusion from the rock by mineral separation and independent determination of the potassium-argon age in different minerals from the same rock, if the measured ages of the different minerals match. This is because different minerals show different diffusion behavior for argon, which would be noticeable in different argon ages if one of the above requirements (complete reset of the potassium-argon clock, no diffusion of argon from the rock) should not be met.

In addition, the other stable argon isotopes 36 Ar and 38 Ar are also routinely determined in noble gas mass spectrometry . These two argon isotopes consist only of primordial argon and because of the known isotope ratios of primordial argon ( i.e. argon, which was not created by radioactive decay, but from other sources), the possible presence of primordial 40 Ar can be checked and corrected if necessary. This correction works well for ages over 100,000 years. In younger rocks, the radiogenic argon is usually too “covered” by the primordial argon to be able to make a correction, so that conventional potassium-argon dating is not used here.

39 Ar- 40 Ar method

With the 39 Ar- 40 Ar method, the sample to be measured is irradiated with fast neutrons in a research reactor ( neutron activation ), with part of the 39 K in the sample being converted into 39 Ar. For calibration purposes, a mineral standard (e.g. hornblende ) of known age is always irradiated as a monitor sample. The samples are then gradually heated in certain temperature levels and the ratio of 39 Ar to 40 Ar of the argon outgassed in the individual temperature levels is measured using noble gas mass spectrometry . The measured 39 Ar / 40 Ar ratios are then plotted against the temperature in a diagram. If the diagram of the sample shows a plateau in the high temperature range, i.e. an extended temperature range in which the 39 Ar / 40 Ar ratio is practically constant, then a disruption of the potassium-argon system in this area can be excluded. An argon-argon age can then be calculated using the 39 Ar / 40 Ar ratio of the plateau, the 39 Ar / 40 Ar ratio of the monitor sample also determined being used for calibration. If there is no plateau in the diagram of the sample, it must be assumed that the potassium-argon isotope system of the rock from which the sample originates is disturbed - often due to argon loss due to diffusion. A reliable argon age cannot then be assigned.

This 39 Ar- 40 Ar method is able to date much more recent events than traditional potassium-argon dating. It has since been refined to such an extent that PR Renne et al. In 1997 it was possible to date pumice stone from the Vesuvius eruption , which destroyed Pompeii , to an age of 1925 ± 94 years. This corresponds to the year 72 AD and therefore coincides in error with the historical date, which Pliny the Younger gives - converted into the Gregorian calendar - as 79 AD. At the same time, however, with the help of this method it is also possible, for example , to date hominine fossils that are millions of years old - such as the finds of Ardipithecus ramidus - for which the radiocarbon method is no longer applicable.

Historical

The potassium-argon method was first scientifically described in 1950 by Friedolf ​​M. Smits and Wolfgang Gentner ( University of Freiburg ) in connection with the dating of tertiary salt deposits . Ten years later, hominine fossils were first dated with their help, or more precisely: the horizons of the areas explored by Louis Leakey and Mary Leakey in the Olduvai Gorge in northern Tanzania .

literature

• Alan P. Dickin: Radiogenic Isotopes Geology. Cambridge University Press, Cambridge 1995, ISBN 0-521-43151-4 .
• Gunter Faure, Teresa M. Mensing Isotopes. Principles and Applications. 3rd edition, completely updated and expanded. Wiley, Hoboken NJ 2005, ISBN 0-471-38437-2
• Ian McDougall , T. Mark Harrison, Geochronology and Thermochronology by the 40 Ar / 39 Ar Method. 2nd edition. Oxford University Press, New York NY 1999, ISBN 0-19-510920-1 .
• Etienne Roth, Bernard Poty (Eds.): Nuclear Methods of Dating. Kluwer, Dordrecht et al. 1989, ISBN 0-7923-0188-9 ( Solid earth science library 5).
• Fred Jourdan: Advances in 40 Ar / 39 Ar dating - from archeology to planetary sciences. The Geological Society, London 2014, ISBN 978-1-86239-360-8 .

Individual evidence

1. ^ PR Renne, WD Sharp, AL Deino, G. Orsi and L. Civetta: 40 Ar / 39 Ar Dating into the Historical Realm: Calibration Against Pliny the Younger. In: Science . Volume 277, No. 5330, 1997, pp. 1279-1280, doi: 10.1126 / science.277.5330.1279
2. Friedolf M. Smits and Wolfgang Gentner : Argon provisions in potassium minerals. I. Provisions on tertiary potash salts. In: Geochimica et Cosmochimica Acta. Volume 1, No. 1, 1950, pp. 22-27, doi: 10.1016 / 0016-7037 (50) 90005-6
3. Louis Leakey , Jack F. Evernden and Garniss Curtis : Age of Bed I, Olduvai Gorge, Tanganyika. In: Nature . Volume 191, 1961, pp. 478-479, doi: 10.1038 / 191478a0