So there may not be anything to explain. Such a high melting point would imply fractionation in the magma. Price after trial Starting Price starting today. Initially, we would expect most of it to come from subducted oceanic plates, which are uranium and thorium poor and maybe lead rich. Since fractionation and mixing are so common, we should expect to find isochrons often.
Weather and Climate Science:
17.6: Radiocarbon Dating: Using Radioactivity to Measure the Age of Fossils and Other Artifacts
Radiometric Dating The aging process in human beings is easy to see. If this happens, the isochron can be measuring an age older than the date of the eruption. For example, heavier substances will tend to sink to the bottom of a magma chamber. At successively lower temperature, other minerals begin to crystallize as shown in Figure 3. One could say that some of the radiogenic lead has diffused into neighboring rocks, too. Tarbuck and Lutgens carefully explain the process of fractional crystallization in The Earth:
Later we will see that mixing of two kinds of magma, with different proportions of lead isotopes, could also lead to differences in concentrations. From the measurement performed in the Dead Sea Scrolls were determined to be years old giving them a date of 53 BC, and confirming their authenticity. The thing that makes this decay process so valuable for determining the age of an object is that each radioactive isotope decays at its own fixed rate, which is expressed in terms of its half-life. He was also not happy with the published dates on the flows in the Nevada Atomic Bomb Test site where one of the volcanic flows showed a reversal of isotope ratios and gave a value of 20, years in the future! The trapped charge accumulates over time at a rate determined by the amount of background radiation at the location where the sample was buried.