Those dates are done using whole rock specimens separated by meters to kilometers. Even there, one cannot simply pick up rock specimens, send them to the laboratory, and get the "right" dates. A substantial number of the dates are too old, too young, or even in some cases of negative age!. Evaporite minerals (which might be expected to have full mixing at the beginning), consistently give "unacceptably" young ages, and are therefore not used. When "acceptable" dates are found, however, they are usually published.
2010 Arthur V. Chadwick, Ph.D.