of grains between top and bottom is always proportional to the time elapsed.In principle, the potassium-argon (K-Ar) decay system is no different.Using a nuclear reactor, the mineral sample is bombarded with neutrons, which interact with a particular isotope of potassium (39K), essentially by knocking a single proton out of the nucleus and replacing it with a neutron.
Since this value is within uncertainty of the atmospheric ratio (295.5), we know that the mineral did not contain excess argon when it formed.
As mentioned, argon is a noble gas, which does not form chemical bonds with the mineral itself.
In this case, it wouldn’t matter if the scale were slightly inaccurate (and most scales are), because both measurements will be off by the same amount, and you only need the of weights.
Potassium-argon dating, however, is akin to weighing yourself on a bathroom scale at home, while weighing the baby on the vegetable scale at the local grocery store—each instrument has its own calibration and uncertainty.
Could you find the position of the starting line if it were not marked?
Well, if you knew each runner’s speed and location, you could simply extrapolate back in time to the point when runners were at the same position.
While this assumption holds true in the vast majority of cases, excess argon can occasionally be trapped in the mineral when it crystallizes, causing the K-Ar age to be a few hundred thousand to a few million years older than the actual cooling age.
Secondly, K-Ar dating assumes that very little or no argon or potassium was lost from the mineral since it formed. it does not bond to any other elements), it can readily escape from minerals if they are exposed to significant amounts of heat for a prolonged period of time.
In an Ar-Ar isochron, geochronologists essentially measure all three isotopes in different parts of the same mineral and then plot the points (Fig. The resulting best-fit line indicates the amount of .
If that amount is significantly higher than the atmospheric ratio of 295.5, then we know that the mineral contained excess argon when it crystallized and likely will not yield a reliable date.
Although this detail may sound minor, it implies that we cannot measure 40K and 40Ar simultaneously on the same instrument. Imagine that you were tasked with measuring the ratio of weight versus that of a newborn baby.