Potassium-argon (K-Ar) dating (video) | Khan Academy
But, for the purposes of the KAr dating system, the relative abundance of 40K is Once the 40Ar and potassium in a rock/mineral are accurately measured, the. potassium–argon dating* A dating technique  for certain rocks that depends on the decay of the radioisotope potassium–40 to argon–40, a process with a. Potassium occurs in two stable isotopes (41K and 39K) and one radioactive isotope (40K). Potassium decays with a half-life of million.
Because the relative abundances of the potassium isotopes are known, the 39ArK produced from 39K by a fast neutron reaction can be used as a proxy for potassium.
Instead, the ratios of the different argon isotopes are measured, yielding more precise and accurate results. The amount of 39ArK produced in any given irradiation will be dependant on the amount of 39K present initially, the length of the irradiation, the neutron flux density and the neutron capture cross section for 39K.
However, because each of these parameters is difficult to determine independantly, a mineral standard, or monitor, of known age is irradiated with the samples of unknown age. The monitor flux can then be extrapolated to the samples, thereby determining their flux. This flux is known as the 'J' and can be determined by the following equation: In addition to 39Ar production from 39K, several other 'interference' reactions occur during irradiation of the samples.
Other isotopes of argon are produced from potassium, calcium, argon and chlorine. As the table above illustrates, several "undesirable" reactions occur on isotopes present within every geologic sample.
These reactor produced isotopes of argon must be corrected for in order to determine an accurate age.
K-Ar dating calculation (video) | Khan Academy
The monitoring of the interfering reactions is performed through the use of laboratory salts and glasses. For example, to determine the amount of reactor produced 40Ar from 40K, potassium-rich glass is irradiated with the samples.
The desirable production of 38Ar from 37Cl allows us to determine how much chlorine is present in our samples. Multiple argon extractions can be performed on a sample in several ways.
Step-heating is the most common way and involves either a furnace or a laser to uniformily heat the sample to evolve argon. The individual ages from each heating step are then graphically plotted on an age spectrum or an isochron.
With 18 protons and 22 neutrons, the atom has become Argon Aran inert gas. For every K atoms that decay, 11 become Ar How is the Atomic Clock Set? When rocks are heated to the melting point, any Ar contained in them is released into the atmosphere. When the rock recrystallizes it becomes impermeable to gasses again.
As the K in the rock decays into Ar, the gas is trapped in the rock. The Decay Profile In this simulation, a unit of molten rock cools and crystallizes. The ratio of K to Ar is plotted.
Potassium-argon (K-Ar) dating
Note that time is expressed in millions of years on this graph, as opposed to thousands of years in the C graph. Click on the "Show Movie" button below to view this animation. How are Samples Processed? And what's really interesting to us is this part right over here. Because what's cool about argon, and we study this a little bit in the chemistry playlist, it is a noble gas, it is unreactive. And so when it is embedded in something that's in a liquid state it'll kind of just bubble out. It's not bonded to anything, and so it'll just bubble out and just go out into the atmosphere.
So what's interesting about this whole situation is you can imagine what happens during a volcanic eruption.
Let me draw a volcano here. So let's say that this is our volcano. And it erupts at some time in the past. So it erupts, and you have all of this lava flowing.
K-Ar dating calculation
That lava will contain some amount of potassium And actually, it'll already contain some amount of argon But what's neat about argon is that while it's lava, while it's in this liquid state-- so let's imagine this lava right over here. It's a bunch of stuff right over here.
I'll do the potassium And let me do it in a color that I haven't used yet. I'll do the potassium in magenta. It'll have some potassium in it. I'm maybe over doing it. It's a very scarce isotope. But it'll have some potassium in it. And it might already have some argon in it just like that. But argon is a noble gas. It's not going to bond anything. And while this lava is in a liquid state it's going to be able to bubble out.
It'll just float to the top. It has no bonds. And it'll just evaporate. I shouldn't say evaporate. It'll just bubble out essentially, because it's not bonded to anything, and it'll sort of just seep out while we are in a liquid state.
And what's really interesting about that is that when you have these volcanic eruptions, and because this argon is seeping out, by the time this lava has hardened into volcanic rock-- and I'll do that volcanic rock in a different color.
By the time it has hardened into volcanic rock all of the argon will be gone. It won't be there anymore. And so what's neat is, this volcanic event, the fact that this rock has become liquid, it kind of resets the amount of argon there.
- K–Ar dating
- Potassium-argon dating
So then you're only going to be left with potassium here. And that's why the argon is more interesting, because the calcium won't necessarily have seeped out. And there might have already been calcium here. So it won't necessarily seep out.