Protracted U-Pb age spectra from complex zircon crystals resolved using high-precision geochronology and selective sample pre-treatment

Geochronology is fundamental for the understanding of rates and mechanisms of Earth processes, including tectonics, crust formation, ore formation and magmatism. Analytical techniques are mostly applied to the mineral zircon, particularly LA-ICPMS and ID-TIMS dating, which offer the required accuracy, precision and analytical throughput to solve outstanding scientific questions. However, zircon can record multiple geological events within discrete crystallographic domains, so it is crucial to ensure that measurements are completed using optimal precision and accuracy while specifically targeting crystal domains of interest to resolve potentially complex zircon systematics. We explore here a case where the combination of xenocrystic and autocrystic growth zones within same crystals, together with decay damage related lead loss, leads to apparently protracted age spectra, which can erroneously be interpreted in terms of magmatic evolution.

We present LA-ICP-MS and ID-TIMS U-Pb zircon data from a Variscan, 335 Ma old granodiorite from the Alpine basement in the Aar massif (Switzerland), which highlight the potential complexities present in zircon samples and address the need for careful zircon pre-treatment. CL imagery of zircon reveals minor but pervasive secondary alteration, leading to the observed excess scatter in LA-ICPMS dates. Chemical abrasion (CA) as a pre-treatment prior to LA-ICPMS analysis significantly reduces this scatter. CA-ID-TIMS analyses of zircon from this sample yield extremely high precision due to very high radiogenic/common Pb ratios (Pb*/Pb_c), with significant ²⁰⁶Pb/²³⁸U scatter. Due to the elevated precision of these analyses, it is possible to resolve a linear discordance for these data. This indicates that Pb-loss is not the only age component observed, and the volume of zircon analyzed via CA-ID-TIMS does not purely reflect Variscan igneous crystallization. Since CL images also show thin and poorly visible metamorphic rims, we carried out a physical abrasion (PA) pre-treatment prior to chemical abrasion to isolate the Variscan zircon zones from later Alpine overgrowth for CA-ID-TIMS analysis. We interpret a high-precision PA-CA-ID-TIMS ²⁰⁶Pb/²³⁸U age of 335.479 ± 0.041/0.096 Ma (internal non-systematic/external systematic error; MSWD=0.27) as best estimate for Variscan zircon crystallization for this sample. This age overlaps with the result of CA-LA-ICPMS analyses when properly accounting for the total analytical uncertainty, including matrix effects on concentration ratio standardization.

From these data we conclude: (1) mixing of two age components in zircon may lead to an apparent protracted range in ²⁰⁶Pb/²³⁸U age, which can be resolved if isotope analyses yield very high Pb*/Pb_c ratios and thus are very precise. At lower precision zircon age spectra can be erroneously interpreted as reflecting protracted growth, since they will overlap concordia due to elevated ²⁰⁷Pb/²³⁵U uncertainties, as well as in between individual ²⁰⁶Pb/²³⁸U ages. (2) By combining physical and chemical abrasion, we can resolve the observed complexities, by selectively analyzing zircon domains of interest while simultaneously mitigating diffusive Pb-loss. (3) This study shows how analytical precision may dramatically impact on scientific interpretation, as less precise data can easily be mistaken to reflect prolonged magmatic growth, rather than two-component mixing with xenocrystic material. This difference can significantly impact the interpreted lifespan of magmatic systems.