In igneous/metamorphic zircon geochronology, the Th/U ratio is a petrogenetic indicator: magmatic (igneous) zircon typically has higher Th/U (~0.5–1+), while metamorphic zircon or zircon growing in the presence of a Th-sequestering mineral like monazite typically shows much lower Th/U, since monazite preferentially incorporates Th (being a Th- and LREE-rich phosphate), depleting the coexisting melt/fluid in Th and leaving less available for subsequently crystallizing zircon.
The zoned zircon grains show high-Th/U cores and low-Th/U rims. Since monazite strongly partitions and sequesters thorium, its crystallization removes Th from the local melt/fluid...
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