The isochron calculations can theoretically be done on different minerals from one rock sample. In fact, theoretically it should be much easier to homogenize strontium ratios in the minerals in a single rock than to do so for a suite of rocks (separated by meters or even kilometers). If the measured Sr-87/Sr-86 ratios versus Rb-87/Sr-86 ratios from several minerals in an igneous rock plot along a straight line, they are called "concordant". If they agree at a "geologically acceptable" date, they are assumed to be accurate. But most mineral suites do not plot on a straight line. That is, they are "discordant". They also consistently date much younger than the "correct" age.
2010 Arthur V. Chadwick, Ph.D.