It can help fix the maximum age of sedimentary rocks when they contain enough accessory zircon grains usually need about grains. In an undeformed sequence, the oldest rocks are at the bottom and the youngest rocks are at the top. In addition, published data concerning the length of fission tracks and the annealing of minerals imply that the basic assumptions used in an alternative procedure, the length reduction-correction method, are also invalid for many crystal types and must be approached with caution unless individually justified for a particular mineral. In short, isochron dating is not the independent dating method that it was once thought. That is why it does not matter how long the magma was in the volcano before it erupted.
However some isotopes, like 14 C, have an unstable nucleus and are radioactive.
Radiometric Dating: Methods, Uses & the Significance of Half-Life
Potassium—argon dating. It seems reasonable to me that the large radiometric ages are simply a consequence of mixing, and not related to ages at all, at least not necessarily the ages of the rocks themselves. Funkhouser and Naughton found that the excess 40 Ar in the Hualalai flow, Hawaii, resided in fluid and gaseous inclusions in olivine, plagioclase, and pyroxene in ultramafic xenoliths in the basalt, and was sufficient to yield "ages" of 2.