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.After converting all 39K to 39Ar, geochronologists can effectively measure the isotopic the third assumption.What about the other two assumptions behind the K-Ar method?
This fact allows them to be measured and compared simultaneously on the same instrument.
Of the naturally occurring isotopes of potassium, 40K is radioactive and decays into 40Ar at a precisely known rate, so that the ratio of 40K to 40Ar in minerals is always proportional to the time elapsed since the mineral formed [ 40K is a potassium atom with an atomic mass of 40 units; 40Ar is an argon atom with an atomic mass of 40 units].
This relationship is useful to geochronologists, because quite a few minerals in the Earth’s crust contain measurable quantities of potassium (e.g. In theory, therefore, we can estimate the age of the mineral simply by measuring the relative abundances of each isotope.
Fortunately, the Ar-Ar method can address both by 1) building an isochron and 2) using a step-heating method during analysis.
Isochron methods work by measuring a third, stable isotope in addition to the pair that gauges radioactive decay (in this case, 36Ar alongside 39Ar and 40Ar).
Having this third isotope allows us to measure directly (and not simply assume) how much argon was in the mineral at the moment that it crystallized.