Problems with dating metamorphic rocks

20-Dec-2017 14:45

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.

Finally, we must consider the possibility of argon loss.

(However, see the section below on the limitations of the method.) This suggests an obvious method of dating igneous rocks.

If we are right in thinking that there was no argon in the rock originally, then all the argon in it now must have been produced by the decay of Ar in them will be so small that it is below the ability of our instruments to measure, and a rock formed yesterday will look no different from a rock formed fifty thousand years ago.

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.

However, not as well known is the fact that such methods have serious flaws which are often glossed over, or ignored when writing on, or discussing this subject in public. that (the) half-lives (of uranium-thorium-lead) are not constant but vary with time. comes from the study of pleochroic haloes which form in a rock in the following way.

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They consist of measuring the amount of radioactive (mother) element and comparing it to the amount of stable (daughter) element. Uranium is radioactive, which means it is in the process of changing from an unstable element into a stable one. And after 9 billion years there would be 75% lead and 25% uranium, and so on. (an) episode of drastically accelerated decay has ... When the crystal is looked at under a microscope, these discolourations appear as dark ringshence the name "pleochroic halo".