K–Ar dating

The potassium - argon dating is a radiometric age determination with which one geological age of rocks is determined ( geochronology ), wherein the radioactive decay is used by 40Kalium to 40Argon. 40K is a beta emitter, and decays with a half-life of 1.25 x 109 years, argon and calcium. Here, 11% of the decays lead to stable argon, the remainder to calcium. Potassium is found in common rock-forming minerals such as micas, feldspars and hornblende, which is why this dating technique is often used successfully in terrestrial rocks. In addition, the potassium - argon dating is often applied for extraterrestrial rocks, such as the Apollo lunar samples and meteorites, which previously were age up to about 4.6 billion years, the estimated age of our solar system is determined.


40K decays according to the following formulas to 40Ar and 40Ca:

Is a rock mineral that is the potassium isotope 40K available ( the natural proportion of the isotope among all potassium isotopes is 0.0116 %), so does its frequency decreases with time, while the incidence of decay product increases 40Ar. The temporal evolution of the frequencies is determined by the radioactive decay law. From this the following formula can be derived for the disintegration time:

Thus, if the ratio 40 [Ar ] / 40 [ K] of daughter isotope 40Ar known to the parent isotope 40K by measurement, the age can be calculated.

40K - 40Ar method

The simplest application is the ease of measurement of the concentrations of potassium (for example, by means of atomic emission spectroscopy ) and 40Ar (using inert gas mass spectrometry) in a sample. From the concentration of potassium can be calculated because of the known isotopic ratios of the potassium isotopes, the concentration of the isotope 40K. The potassium -argon age can be calculated from the ratio of 40K to 40Ar then again to the formula shown in the "Basics " section.

This requires that the event be dated such as crystallization from a melt of a rock, the " potassium argon clock reset " has. That is, by the datable event everything before any existing radiogenic 40Ar ( 40Ar =, which is caused by radioactive decay, as opposed to primordial argon that comes from other sources, such as trapped atmospheric argon) from the rock has escaped, so that immediately after the end of the event no more 40Ar was present. Since argon as inert gas escapes very easy when complete melting of a rock, which is mostly given in this case, and this simple method yields in dating the crystallization of a rock from a melt usually reliable results.

If shock events, such as the impact of a large asteroid, be dated, is a complete outgassing no longer necessarily the case, as was already observed even at extremely high pressures, which create the shock waves in the rock that 40Ar not completely escape.

Furthermore, it must also be ensured that subsequent events such as diffusion of argon from the rock does not distort the age. In such cases, the outdiffusion of argon from the rock are systematically measured at young age.

If several different minerals with sufficient potassium content are present in a rock, you can exclude by mineral separation and independent determination of the potassium -argon ages in different minerals from the same rock falsification of age due to insufficient outgassing or diffusion from the rock, when the measured age the various minerals match. This is because different minerals have different diffusion behavior for argon, which would manifest itself in different argon ages, if any of the above conditions ( full reset of the potassium -argon clock, no diffusion of argon from the rock ) should not be given.

Furthermore, routinely, the other stable argon isotope 36Ar and 38aR also determined in the noble gas mass spectrometry. These two argon isotopes consist only of primordial argon and because of the known isotopic ratios of primordial argon (ie argon, which is not caused by radioactive decay, but comes from other sources) you can check the possible presence of primordial 40Ar and correct if necessary. This fix works well for aging over 100,000 years. In younger rocks, the radiogenic argon is usually too strong " hidden " by the primordial argon to still be able to make a correction, so that the conventional potassium - argon dating is not applied here.

Today, preferred the more sophisticated 39Ar - 40Ar method is applied, which is able to detect disorders of potassium -argon isotopic system without performing a costly mineral separation. Above all, can be measured with this method, even when partially ausdiffundiertem argon or argon reliable age. This can be ensured in many cases that a particular argon age is reliable and it can include much younger rocks are measured.

39Ar - 40Ar method

In 39Ar 40Ar method the sample to be measured is in a research reactor is irradiated with fast neutrons ( neutron activation ), wherein a part of the present in the sample 39K is converted into 39Ar. For calibration, this may also mitbestrahlt always a mineral standard ( eg hornblende ) of known age as a monitor sample. The samples are then gradually heated in certain temperature level and measured by mass spectrometry, the ratio of inert gas to 39Ar 40Ar of the outgassed in the individual temperature stages argon. The measured 39Ar/40Ar-Verhältnisse are then entered on the temperature in a chart. Displays in the diagram of the sample, a plateau in the high temperature range, ie, an extended temperature range in which the 39Ar/40Ar-Verhältnis is practically constant, so a disturbance of the potassium -argon system are excluded in this area. It is then on the 39Ar/40Ar-Verhältnis of the plateau, an argon argon age are calculated, wherein the monitor also certain 39Ar/40Ar-Verhältnis sample used for calibration. If, in the diagram of the sample, no plateau must be assumed that the potassium -argon isotopic system of the rock from which the sample was taken, is disturbed - often by argon loss due to diffusion. It can then be assigned a reliable argon age.

This 39Ar - 40Ar method is capable of much more recent events to date than the conventional potassium - argon dating. It has since been refined to the extent that it PR Renne et al. In 1997 has dated pumice from the eruption of Vesuvius, which destroyed Pompeii, an age of 1925 ± 94 years. This corresponds to the year 72 AD and agrees with it in error agreed with the historic date, which Pliny the Younger - converted to the Gregorian calendar - shows off 79 AD. At the same time it is also possible for example with the aid of this method, millions of years old hominin fossils - such as the remains of Ardipithecus ramidus - to date, in which the radiocarbon method is no longer applicable.