Xenotime (YPO) is an isotopically robust chronometer, which is increasingly being recognized as a trace constituent in siliciclastic sedimentary rocks.
For example, a geologist may examine a cutting where the rocks appear as shown in Figure 1.
Here he can see that some curved sedimentary rocks have been cut vertically by a sheet of volcanic rock called a dyke.
From the mapped field relationships, it is a simple matter to work out a geological cross-section and the relative timing of the geologic events.
His geological cross-section may look something like Figure 2.
Let us imagine that the date reported by the lab was 150.7 ± 2.8 million years.
Our geologist would be very happy with this result.
Would he have concluded that the fossil date for the sediments was wrong? Would he have thought that the radiometric dating method was flawed? Instead of questioning the method, he would say that the radiometric date was not recording the time that the rock solidified.
He may suggest that the rock contained crystals (called xenocrysts) that formed long before the rock solidified and that these crystals gave an older date.
He may suggest that some other very old material had contaminated the lava as it passed through the earth.