EXTRACTING GEOLOGIC AGES FROM ISOTOPICALLY-DISTINCT NANOSCALE RESERVOIRS OF PB IN DISCORDANT ZIRCON BY ATOM PROBE MICROSCOPY

Isotopic discordance in zircon is commonly associated with mobilization and loss of radiogenic Pb during metamorphism, deformation, and/or metasomatism. Within single grains, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and/or secondary ion mass spectrometry analyses indicate that discordance can differ at length scales <10 μm. Heterogeneous distribution of Pb may explain these differences, but the controls on small-scale Pb retention in zircon—and its effects—remain largely unknown.

We used LA-ICP-MS and atom probe microscopy to analyze a single zircon from a garnet-kyanite schist from the Rhodope Metamorphic Complex (Greece). LA-ICP-MS data (collected at 15 µm spot size) indicate a strongly discordant core rimmed by concordant c. 150 Ma metamorphic zircon, consistent with the timing of granulite-facies metamorphism in the region. The upper intercept indicates crystallization at 2144 ± 33 Ma, consistent with published detrital core ages from this locality.

Atom probe microscopy analysis indicates homogeneous distributions of Zr, Si, O, and Hf in all specimens from the zircon core. In contrast, Pb is heterogeneously distributed in clusters measuring <20 nm in diameter. Although the cluster sizes and their relative spacing differ among specimens, all clusters have a toroidal morphology that is consistent with the size and shape of dislocation loops. Furthermore, all clusters indicate local Pb enrichment with minor enrichment in Y and Al (~ 1 at.%) and no U discernable above background. ²⁰⁷Pb/²⁰⁶Pb ratios measured from eight clusters are equivalent (within error) and yield a mean ²⁰⁷Pb/²⁰⁶Pb age of 2258 ± 90 Ma.

The toroidal morphology and trace element enrichment within the clusters are consistent with dislocation loops that formed and trapped diffusing radiogenic Pb during metamorphism. Recalculating the ²⁰⁷Pb/²⁰⁶Pb ratio from the clusters with respect to the granulite-facies event yields an age of 2186 ± 93 Ma, which agrees well with the crystallization age. These data indicate that discrete nanoscale reservoirs of Pb with different isotopic compositions can preserve geologically meaningful ages. Variations in discordance measured by microbeam techniques may be attributed to differences in size, number, density, and distribution of Pb-rich nanoclusters.