Argon-argon dating gets around many of the issues by measuring only multiple isotopes of argon.
The trick is to irradiate the sample with neutrons along with samples of known age.
Potassium-40 is radioactive but has such a long half-life that it is primordial - it has been around since the earth was being formed. About 10.7% decays to Argon-40 by emitting a positron.
Because of this, we can assume that the potassium-40:potassium-39 ratio in minerals at any time is a constant. If the argon-40 stays trapped in the crystal and you can measure the ratio of potassium-40 to argon-40, then you know how long it has been since the mineral formed.
decay or the rate of other cumulative changes in atoms resulting from radioactivity. The various isotopes of the same element differ in terms of atomic mass but have the same atomic number..
A problem is that it takes two separate measurements to get the potassium concentration and the argon isotope ratios, increasing the uncertainty.
As a result, all of the argon-40 in a volcanic rock sample is assumed to date from that time.
When a fossil is sandwiched between two such volcanic deposits, their potassium-argon dates provide a minimum and maximum age.
In the example below, the bone must date to sometime between 1.75 and 1.5 million years ago.
For instance, a date of 100,000 5,000 years ago means that there is a high probability the date is in the range of 95,000 and 105,000 years ago and most likely is around 100,000.