In his book Faith, Form, And Time, Dr. Kurt Wise gives various reasons why radiometric methods are not near as reliable as we have been told. Among them is this statement with regard to the isochron method of dating rocks.
"And fourth, radiometric isochron dating assumes that the only way a straight isochron line can be produced is when multiple samples are taken of the same age and the same rock. In fact, straight isochron lines can be produced from a group of different rocks with very different ages.7" P. 63
The reference is in the book -- along with numerous other reasons why Dr. Wise believes this way.