Diffusion-induced disturbances of the U–Pb isotope system in pre-magmatic zircon and their influence on SIMS dating. A numerical study

Pre-magmatic zircons provide a unique source of information about the nature and origin of magmas, but their original U–Pb ages can be disturbed by the thermal shock caused by entrainment in magma. Disturbances result from the migration of Pb from U-rich to U-poor domains rather than Pb-loss to the surrounding. Our 3D numerical models of Pb diffusion and zircon solution kinetics reveal that pre-magmatic zircons suspended in magma can only be perceptibly disturbed at 800–830 °C, but this requires more than 106 years. Higher T not only increases the diffusion rate but also exponentially increases the solubility of zircon in the melt so that at T > 830–850 °C all pre-magmatic zircons in contact with common magmas would be dissolved before diffusion to be effective. Zircons shielded from the melt, on the other hand, can resist much higher temperatures and are surrounded by solid minerals that, in contrast to a melt, do not act as sinks for Pb2 +. At T ≥ 1100 °C these zircons require very short times to develop large disturbances, as illustrated in xenolithic zircons from a lamproite from SE Spain. By contrast, at 1000 °C even moderate disturbances require 104 to 107 years, depending on the inter-domain concentration ratio. Accordingly, the diffusive redistribution of Pb in pre-magmatic zircons is most likely limited to two natural scenarios (i) crustal magmas derived from long-lived anatectic complexes, and (ii) crust-contaminated mafic magmas in which zircons survived shielded as inclusions in primocrysts or within xenoliths. Diffusion-induced disturbances tend to yield higher-than-original U–Pb ages that are extremely difficult to detect as illustrated in an autochthonous leucosome from NW Spain, in which the age of pre-magmatic zircon cores fails to reflect the age of the spatially-related source. The interpretation of SIMS U–Pb data for pre-magmatic zircons is not straightforward, so the resulting ages should be considered carefully.