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Rant 19 - Potassium Argon Dating Proves Old Earth

The calcium-potassium age method is seldom used, however, because of the great abundance of nonradiogenic calcium in minerals or rocks, which masks the presence of radiogenic calcium. On the other hand, the abundance of argon in the Earth is relatively small because of its escape to the atmosphere during processes associated with volcanism.


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The potassium-argon dating method has been used to measure a wide variety of ages. The potassium-argon age of some meteorites is as old as 4,,, years, and volcanic rocks as young as 20, years old have been measured by this method.

Radiometric dating

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Our editors will review what you've submitted, and if it meets our criteria, we'll add it to the article. Please note that our editors may make some formatting changes or correct spelling or grammatical errors, and may also contact you if any clarifications are needed. Potassium-argon dating Written By: The Editors of Encyclopaedia Britannica.

Learn More in these related Britannica articles: This is possible in potassium-argon K-Ar dating, for example, because most minerals do not take argon into their structures initially. In rubidium-strontium dating, micas exclude strontium when they form but accept much rubidium. In uranium-lead U-Pb dating of zircon, the zircon is found to exclude initial lead…. The radioactive decay scheme involving the breakdown of potassium of mass 40 40 K to argon gas of mass 40 40 Ar formed the basis of the first widely used isotopic dating method.

Since radiogenic argon was first detected in by the American geophysicist…. Potassium—argon dating has made it possible to establish that the earliest remains of man and his artifacts in East Africa go back at least 2,, years, and probably further. Potassium-argon dating , for instance, can provide the age of a specimen by clocking the rate at which radioactive isotopes of these elements have decayed. When radiometric methods cannot be applied, investigators may still ascribe a relative age to a fossil by relating it to the….

More About Potassium-argon dating 5 references found in Britannica articles Assorted References major reference In dating: The severity of this problem decreases as the accuracy of our instruments increases.

Still, as a general rule, the proportional error in K-Ar dating will be greatest in the youngest rocks. A second problem is that for technical reasons, the measurement of argon and the measurement of potassium have to be made on two different samples, because each measurement requires the destruction of the sample. If the mineral composition of the two sample is different, so that the sample for measuring the potassium is richer or poorer in potassium than the sample used for measuring the argon, then this will be a source of error.

Another concern with K-Ar dating is that it relies on there being no 40 Ar in the rock when it was originally formed, or added to it between its formation and our application of the K-Ar method. Because argon is inert, it cannot be chemically incorporated in the minerals when they are formed, but it can be physically trapped in the rocks either during or after formation.


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Such argon is known as excess argon. If the source of this argon is atmospheric contamination, then we can correct for this.

Historical Geology/K-Ar dating

The reasoning is as follows: There is times as much 40 Ar as 36 Ar in the atmosphere, and there is no reason why an atom of 40 Ar should be preferentially incorporated into rocks rather than an atom of 36 Ar, or vice versa. So this means that for every atom of 36 Ar we find in our sample, we can discount atoms of 40 Ar as being atmospheric argon.

However, this only works if all the excess argon did indeed come from the atmosphere. But consider what happens if the argon came from deep within the Earth, where it was formed by 40 K decay, and was then trapped in magma or transported into the rock by hydrothermal fluid. Finally, we must consider the possibility of argon loss. When a rock undergoes metamorphism , some or all of its argon can be outgassed.

Potassium-Argon Dating

If all the argon was lost, this would reset the K-Ar clock to zero, and dating the rock would give us the time of metamorphism ; and if we recognized the rock as metamorphic this would actually be quite useful. However, we cannot rely on all the argon being lost, and if it is not then when we apply K-Ar dating this will give us an essentially arbitrary date somewhere between the formation of the rock and the metamorphosis event. For these reasons K-Ar dating has largely been superseded by Ar-Ar dating, which will be the subject of the next article.

From Wikibooks, open books for an open world. Decay of 40 K [ edit ] 40 K potassium is rather a peculiar isotope, in that it can undergo decay in three different ways: