As the magma chamber is depleted in daughter products, subsequent lava flows and ash beds would have younger dates. Conclusion Scientists do not assume that rocks have been closed systems, but they test for it. Teach the controversy: Radiometric geochronology reappraised. Thanks for your comment, Kevin. Anyway, suppose we throw out all isochrons for which mixing seems to be a possibility. You could talk about the tiny quartz crystal and the piezoelectric effect used to provide a stable time base for the electronic movement. Finally, the inferred age of the shocked quartz, as determined from the age of the melted feldspar in the Manson impact structure Such a high melting point would imply fractionation in the magma. Progressive geochemical differentiation of the upper mantle of the Earth has resulted in the concentration of U and Th into the rocks of the continental crust compared to those of the upper mantle.