Dating zircon grains
Zircon forms in silicate melts with large proportions of high field strength incompatible elements.For example, hafnium is almost always present in quantities ranging from 1 to 4%.Colorless specimens that show gem quality are a popular substitute for diamond and are also known as "Matura diamond".The name derives from the Persian zargun meaning gold-hued. It occurs as a common accessory mineral in igneous rocks (as primary crystallization products), in metamorphic rocks and as detrital grains in sedimentary rocks.Late Indosinian zircons (200–230 Ma) were eroded and deposited in the Lower Jurassic Beipiao Formation, and this implies that intensive tectonic activation and uplift of the Yan-Liao Orogenic Belt in the Mesozoic commenced in the Late Indosinian. Why is it that different locations within a zircon have differing ratios of Pb/U?
Because of their uranium and thorium content, some zircons undergo metamictization.
So to answer the question posed in the title, we should not assume that there had been different amounts of common Pb incorporated during crystallization.
Th activity was measured to be varying from 2.0 to 2.8.
Zircon is rare within mafic rocks and very rare within ultramafic rocks aside from a group of ultrapotassic intrusive rocks such as kimberlites, carbonatites, and lamprophyre, where zircon can occasionally be found as a trace mineral owing to the unusual magma genesis of these rocks.
Zircon forms economic concentrations within heavy mineral sands ore deposits, within certain pegmatites, and within some rare alkaline volcanic rocks, for example the Toongi Trachyte, Dubbo, New South Wales Australia in association with the zirconium-hafnium minerals eudialyte and armstrongite.
Most of the U–Pb ages from zircons collected from a sandstone in the Lower Jurassic Beipiao Formation range from 194.3±2.9 to 233.8±4.2 Ma, reflecting the major sediment source during the Early Jurassic.