Rubidium 87 radiometric dating
The “correct” answer is chosen on the basis of stratigraphic sequences, that is, what kinds of fossils are buried nearby.
Of course, the fossil dates depend on the assumption of evolution.
It is well known that argon, which is a gas, diffuses easily through rock, and there is no way of knowing whether that may have happened in any given case.
Errors are particularly bad with the K-Ar (potassium-argon) method. Joan Engels wrote: It is now well known that K-Ar ages obtained from different minerals in a single rock may be strikingly discordant.3 Skull 1470 In 1972 Richard Leakey found a skull, near Lake Rudolf in Kenya, that he said was “virtually indistinguishable” from that of a modern human.
Each team criticized the others’ techniques of rock sample selection.Reasons given usually involved detrital intrusion, leakage or leaching of some of the isotopes in the sample, and sometimes the initial isotopic content of the sample.For K-Ar dates, it’s easy to blame argon loss if the reported age is too short, or argon absorption if it’s too long.Marvin Lubenow gives a good description of the ten years of controversy surrounding the dating of this skull.4 In the first attempt at dating the KBS Tuff, Fitch and Miller analyzed the raw rocks, and got dates ranging from 212 to 230 MY-the Triassic period, vastly older than expected.Because mammal bones had been found below this stratum, they said these dates were obviously in error because of “the possible presence of extraneous argon derived from inclusions of pre-existing rocks.” Even though the rock looked good, anything older than 5 MY was obviously wrong in view of their knowledge of the “sequence of evolutionary development.” Meanwhile a team from the University of California at Berkeley, led by G. Curtis, analyzed several KBS pumice rocks and found some that were around 1.6 MY and some that were about 1.8 MY.