Definitions
from Wiktionary, Creative Commons Attribution/ShareAlike License.
 noun physics The major
stable isotope ofargon , 4018Ar, having eighteenprotons and twentytwoneutrons ; it amounts to about 99.6% of theelement innature
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Examples

After a few hundreds of millions of years have elapsed, a scientist comes upon the same enclosed space and measures the relative proportions of potassium40 and argon40.
THE GREATEST SHOW ON EARTH RICHARD DAWKINS 2009

Notice that we must know the ratio of parent (potassium40) to daughter (argon40) isotopes.
THE GREATEST SHOW ON EARTH RICHARD DAWKINS 2009

So, imagine that you start with some quantity of potassium40 in an enclosed space with no argon40.
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But, for the reason I have just explained, this quantity is meaningful only if expressed as the ratio of potassium40 to argon40.
THE GREATEST SHOW ON EARTH RICHARD DAWKINS 2009

So geologists, by measuring the ratio between potassium40 and argon40 in a piece of igneous rock that they pick up today, can tell how long ago the rock first crystallized out of its molten state.
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If you start with some quantity of potassium40, after 1.26 billion years half of the potassium40 will have decayed to argon40.
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The accumulating quantity of argon40 is a measure of the time that has elapsed since the rock was formed.
THE GREATEST SHOW ON EARTH RICHARD DAWKINS 2009

In 2005, the Huygens probe also discovered the presence of argon40 on Titan, the largest moon of Saturn.
Argon 2009

After another 1.26 billion years, half of the remaining potassium40 will have been converted to argon40, and so on.
THE GREATEST SHOW ON EARTH RICHARD DAWKINS 2009

It is often called the potassium argon clock, because argon40 (one lower in the periodic table) is one of the elements to which potassium40 decays (the other, resulting from a different kind of radioactive decay, is calcium40, one higher in the periodic table).
THE GREATEST SHOW ON EARTH RICHARD DAWKINS 2009
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