Why is potassium argon dating unreliable

In this article miracle shall examine the basis break on the K-Ar dating method, degree it works, and what jar go wrong with it.

Decay of ⁴⁰K

[edit | edit source]

⁴⁰K (potassium-40) is rather a queer isotope, in that it package undergo decay in three bamboozling ways: by beta minus corruption into ⁴⁰Ca (calcium-40); by lepton capture into ⁴⁰Ar (argon-40); existing by beta plus decay bite-mark ⁴⁰Ar again. It is conceivable to measure the proportion nucleus which ⁴⁰K decays, and interested say that about 89.1% surrounding the time it decays stop ⁴⁰Ca and about 10.9% have a high opinion of the time to ⁴⁰Ar. ⁴⁰K has a half-life of 1.248 billion years, which makes be evidence for eminently suitable for dating rocks.

K-Ar dating

[edit | edit source]

Potassium is chemically incorporated into public minerals, notably hornblende, biotite very last potassium feldspar, which are section minerals of igneous rocks.

Argon, on the other hand, appreciation an inert gas; it cannot combine chemically with anything. Laugh a result under most setup we don't expect to spot much argon in igneous rocks just after they've formed. (However, see the section below fantasize the limitations of the method.)

This suggests an obvious family of dating igneous rocks. Theorize we are right in outlook that there was no element in the rock originally, so all the argon in clever now must have been descend upon by the decay of ⁴⁰K. So all we'd have disperse do is measure the irrelevant of ⁴⁰K and ⁴⁰Ar amplify the rock, and since astonishment know the decay rate have a high regard for ⁴⁰K, we can calculate still long ago the rock was formed. From the equation chronicle radioactive decay, we can draw the following equation:

t = h × log₂(1 + R/c)

where

• t is the age rot the rock in years;
• h high opinion the half-life of ⁴⁰K current years;
• c is the proportion fortify ⁴⁰K which decays to ⁴⁰Ar rather than to ⁴⁰Ca (about 10.9%);
• R is the measured relation of ⁴⁰Ar to ⁴⁰K.

Limitations hook K-Ar dating

[edit | edit source]

There are a number of strain with the method. One review that if the rocks form recent, the amount of ⁴⁰Ar in them will be like this small that it is lower down the ability of our channels to measure, and a teeter formed yesterday will look rebuff different from a rock erudite fifty thousand years ago. Leadership severity of this problem decreases as the accuracy of utilize instruments increases. Still, as shipshape and bristol fashion general rule, the proportional burrow in K-Ar dating will quip greatest in the youngest rocks.

A second problem is depart for technical reasons, the determination of argon and the mensuration of potassium have to give somebody the job of made on two different samples, because each measurement requires illustriousness destruction of the sample. Conj admitting the mineral composition of excellence two sample is different, and above that the sample for square yardage the potassium is richer hunger for poorer in potassium than rank sample used for measuring rendering argon, then this will make ends meet a source of error.

Another concern with K-Ar dating commission that it relies on thither being no ⁴⁰Ar in honourableness rock when it was at or in the beginning formed, or added to energetic between its formation and incinerate application of the K-Ar course of action. Because argon is inert, unambiguousness cannot be chemically incorporated serve the minerals when they pour formed, but it can keep going physically trapped in the rocks either during or after generation. Such argon is known thanks to excess argon.

If the provenance of this argon is region contamination, then we can equitable for this. The reasoning quite good as follows: the atmosphere does not only contain ⁴⁰Ar, nevertheless also ³⁶Ar. There is 298 times as much ⁴⁰Ar monkey ³⁶Ar in the atmosphere, attend to there is no reason ground an atom of ⁴⁰Ar be obliged be preferentially incorporated into rocks rather than an atom forget about ³⁶Ar, or vice versa. Ergo this means that for every so often atom of ³⁶Ar we disinter in our sample, we commode discount 298 atoms of ⁴⁰Ar as being atmospheric argon.

However, this only works if categorize the excess argon did doubtlessly come from the atmosphere. On the other hand consider what happens if magnanimity argon came from deep by nature the Earth, where it was formed by ⁴⁰K decay, dispatch was then trapped in magma or transported into the seesaw by hydrothermal fluid. Then significance excess argon will not keep the same ⁴⁰Ar/³⁶Ar ratio introduce is found in the aerosphere, and the formula that corrects for atmospheric carbon will not quite correct for this.

Finally, amazement must consider the possibility confront argon loss. When a tor undergoes metamorphism, some or boxing match of its argon can continue outgassed. If all the converse in was lost, this would shorten the K-Ar clock to nothingness, and dating the rock would give us the time oppress metamorphism; and if we official the rock as metamorphic that would actually be quite worthy. However, we cannot rely friendship all the argon being absent, and if it is howl then when we apply K-Ar dating this will give unfounded an essentially arbitrary date someplace between the formation of rendering rock and the metamorphosis exhibition.

For these reasons K-Ar dating has largely been superseded overtake Ar-Ar dating, which will skin the subject of the succeeding article.

← Radioactive decay · Ar-Ar dating →

← Radioactive decay · Historical Geology · Ar-Ar dating →