Understandable Earth Science

Posts tagged ‘Potassium’

How old is that rock? part 2: K-Ar dating

In the previous blog I talked about radioactivity and how we can use radioactive decay as a kind of clock. In this blog I am going to talk about a specific dating technique called potassium-argon (K-Ar) dating. This is a geological dating technique that has been in use since the 1950s and is the basis of a more versatile technique called argon-argon (⁴⁰Ar/³⁹Ar) dating, that I will talk about later.

Let’s start with some details about K and Ar:

  • Potassium (K) is an alkali metal that has 3 naturally occurring isotopes –³⁹K, ⁴⁰K and ⁴¹K. It is relatively common in the earth’s crust, especially in continental crust.
  • Argon (Ar) is a noble gas that has 3 naturally occurring isotopes – ⁴⁰Ar, ³⁸Ar and ³⁶Ar.
  • ⁴⁰K is radioactive and this radioactively decays to form ⁴⁰Ca (calcium) and ⁴⁰Ar. As discussed in the last blog, we call ⁴⁰K the parent isotope and radiogenic (made by radioactive decay) ⁴⁰Ca* and ⁴⁰Ar* daughter isotopes, because ⁴⁰Ca and ⁴⁰Ar are produced by ⁴⁰K. The star on daughter isotopes is a common notation that means “this isotope was formed by radioactive decay”.

Now let’s think about the geological processes we want to date. In terms of dating, one of the simplest geological events is a volcanic eruption, because these happen instantaneously on geological timescales.

Dating volcanic eruptions

Volcanoes exist because of pockets of magma (molten rock) stored in the crust. As magma cools (or changes pressure) it starts to grow crystals. Some of these crystals contain K. If the volcano erupts explosively (e.g. Mount Pinatubo, The Philippines) we call the deposits “tephra”, which is a catch-all term for “broken bits of rock, pumice and ash that come out of the volcano” (in volcanology, “ash” is a kind of tephra that specifically has a grain size of < 2 mm).  When the volcano erupts, the molten rock forces its way out of the ground, becomes solid and no more crystals form. This is when our geological clock starts. Over time, the ⁴⁰K in these new crystals starts turning into ⁴⁰Ar (and ⁴⁰Ca). Eventually, a geologist might come along and take a sample of the tephra to analyse.

For K-Ar dating, we take a rock sample and measure the amount of ⁴⁰K and the amount of ⁴⁰Ar in the rock and from this we calculate the age.  Measuring K is quite straightforward, but things are a little more complicated for Ar.

Ar makes up nearly 1% of the earth’s atmosphere, and ⁴⁰Ar is the most abundant Ar-isotope. 1% doesn’t sound like much, but it is enough to guarantee that the vast majority of samples will contain a little bit of Ar from the atmosphere. If we were to measure the amount of ⁴⁰Ar in a sample and use that to calculate a radiometric age, our age would be too old, because some of that ⁴⁰Ar would be from the atmosphere (let’s call that “⁴⁰Arₐ”), instead of being the ⁴⁰Ar*produced by decay of ⁴⁰K.

The first way to deal with this is to measure Ar using a special noble gas mass spectrometer. These  use ultra-high-vacuum (UHV) extraction lines. This means that the Ar is released from the sample and passed to the mass spectrometer along a tube that is under vacuum – it is completely empty, meaning the gas we measure is only the gas released from the sample and we don’t have to worry about Ar from the atmosphere in the lab contaminating the sample.

But, as I previously said, most *samples* contain at least some atmospheric contamination, and that needs to be removed for our age calculation. Fortunately, the Ar-isotopic composition of air is consistent and well known. ³⁶Ar, is the second most abundant isotope of Ar in the atmosphere and it ONLY occurs in the atmosphere. We know what the ratio of ⁴⁰Ar/³⁶Ar is in air, so if we measure the amount of ³⁶Ar in a sample we can calculate how much ⁴⁰Arₐ is in our sample and simply subtract it to get the ⁴⁰Ar*.

So, in summary, we measure the amount of ⁴⁰K in a sample to get a value for the parent isotope. Then we measure the amount of ⁴⁰Ar and ³⁶Ar in a sample and use those to calculate the amount of the daughter isotope ( ⁴⁰Ar*). We then use these values of ⁴⁰K and ⁴⁰Ar* to calculate how old the sample is.

Hang on a minute!! What if the crystals grow a long time before the volcano erupts? When does the clock start? – This is a question at the cutting edge of dating research at the moment. Fortunately, Ar is an inert gas. This means that it doesn’t form bonds with any parts of the crystal lattice and it is free to diffuse through the crystal. How fast something diffuses is controlled by temperature.  At magmatic temperatures (often between 800 °C and 1300 °C) the Ar is able to diffuse so quickly that it doesn’t build up in the crystal. The radiometric clock starts when the temperature drops (immediately after eruption) and the Ar is no longer able to diffuse out of the crystal. This means that the K-Ar technique dates geological *cooling* events, rather than *crystallisation* events, which makes it very useful for dating volcanic eruptions. There are other isotope systems (e.g. Rb-Sr, U-Th-Pb) that can be used to date crystallisation, instead of cooling.

In the next blog, I will talk about some of the problems with K-Ar dating, and how these can be solved using the ⁴⁰Ar/³⁹Ar technique.

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