Why is potassium 40 useful in dating igneous rocks matte babel 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.
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For these reasons K-Ar dating has largely been superseded by Ar-Ar dating, which will be the subject of the next article.
When a rock undergoes metamorphism, some or all of its argon can be outgassed.
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.
K has a half-life of 1.248 billion years, which makes it eminently suitable for dating rocks.
Potassium is chemically incorporated into common minerals, notably hornblende, biotite and potassium feldspar, which are component minerals of igneous rocks.
However, this only works if all the excess argon did indeed come from the atmosphere.
The attraction of the method lies in the fact that one of the daughter elements is argon which is an inert gas.
This means that the geologist can plausibly assume that all argon gas escapes from the molten magma while it is still liquid.
Another concern with K-Ar dating is that it relies on there being no 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. If the source of this argon is atmospheric contamination, then we can correct for this.
But consider what happens if the argon came from deep within the Earth, where it was formed by Ar ratio as is found in the atmosphere, and the formula that corrects for atmospheric carbon will not correct for this.