Thus, the ratio of argon and potassium and radiogenic calcium to potassium in a mineral or rock is a measure of the age of the sample. 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.
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. We welcome suggested improvements to any of our articles. You can make it easier for us to review and, hopefully, publish your contribution by keeping a few points in mind.
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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.
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When radiometric methods cannot be applied, investigators may still ascribe a relative age to a fossil by relating it to the…. We look at the periodic table of elements. And I have a snapshot of it, of not the entire table but part of it here.
Potassium has 19 protons. And we could write it like this. And this is a little bit redundant. We know that if it's potassium that atom has 19 protons.
And we know if an atom has 19 protons it is going to be potassium. Now, we also know that not all of the atoms of a given element have the same number of neutrons. And when we talk about a given element, but we have different numbers of neutrons we call them isotopes of that element. So for example, potassium can come in a form that has exactly 20 neutrons. And we call that potassium And 39, this mass number, it's a count of the 19 protons plus 20 neutrons.
And this is actually the most common isotope of potassium. It accounts for, I'm just rounding off, Now, some of the other isotopes of potassium. You also have potassium-- and once again writing the K and the 19 are a little bit redundant-- you also have potassium So this would have 22 neutrons. This accounts for about 6. And then you have a very scarce isotope of potassium called potassium Potassium clearly has 21 neutrons.
And it's very, very, very, very scarce. It accounts for only 0. But this is also the isotope of potassium that's interesting to us from the point of view of dating old, old rock, and especially old volcanic rock. And as we'll see, when you can date old volcanic rock it allows you to date other types of rock or other types of fossils that might be sandwiched in between old volcanic rock.
And so what's really interesting about potassium here is that it has a half-life of 1. So the good thing about that, as opposed to something like carbon, it can be used to date really, really, really old things. So argon is right over here. It has 18 protons. So when you think about it decaying into argon, what you see is that it lost a proton, but it has the same mass number. So one of the protons must of somehow turned into a neutron. And it actually captures one of the inner electrons, and then it emits other things, and I won't go into all the quantum physics of it, but it turns into argon And you see calcium on the periodic table right over here has 20 protons.
So this is a situation where one of the neutrons turns into a proton. This is a situation where one of the protons turns into a neutron. 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.
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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. 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.
Potassium-argon (K-Ar) dating
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.
(K/Ar) Potassium Argon Dating Techniques I
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.