Discussion

The retention of radiogenic Pb in zircon is controlled by three main factors: the diffusivity of Pb in crystalline zircon, enhanced diffusivity in areas where domains of α-recoil damage overlap, and the recovery of radiation-damaged zircon through recrystallization or thermal annealing. Previously, Pb particles consisting of Pb–oxide and Pb–phosphate have been observed in synthetic zircon (26, 27). However, the reduced nature of native Pb documented here, along with the absence of nonradiogenic ²⁰⁴Pb (2), argues against the action of external agents such as percolating fluids or melts in the formation of the nanospheres. Consequently, we suggest that thermal annealing is the sole agent of damage recovery and element mobilization in zircon with the Pb nanospheres. Because the mobility and compatibility of Pb is greatly enhanced in amorphous damaged zircon, the annealing process will concentrate Pb into these domains. It is likely that recovery will progress from the margins of predominantly crystalline domains, which can survive in parts of the zircon crystal with lower U and Th contents, into the more amorphous domains. In such cases, it is expected that crystal recovery along reaction fronts would force Pb into the unrecovered domains, leaving the healed domains Pb-free. This likely explains why the more crystalline domains are free of Pb nanospheres (Fig. 3). Importantly, whereas some of the differences in crystallinity observed in Fig. 3 may have been preserved through the annealing process, some, if not all, of the differences were likely produced by the accumulation of further radiation damage, owing to the geologically ancient timing of the metamorphism that induced annealing.

To account for the observations, we propose a thermal annealing model for crystal recovery and phase separation. Annealing involves the recovery of crystallinity in metamict zircon (Fig. 4A). At the first and second percolation thresholds, amorphous zircon makes up 30% and 70% of the zircon volume, respectively (28). According to calculation of the alpha-dose at the time of metamorphism (Table 2), two of the three zircons analyzed in this study were metamict. However, the zircon with the lowest ²³⁸U content (165 ppm) may not have been, indicating that damage accumulation above the first percolation threshold is not a prerequisite for the formation of Pb nanospheres. Even in highly metamict zircon, nanoscale “islands” of crystallinity have been observed (29), and they would act as nucleation sites for lattice recovery. In such cases, recovery would occur until the crystalline domains merge and percolation is lost, trapping Pb in isolated amorphous domains. At a critical concentration, Pb, Si, and other elements that cannot be accommodated in the zircon structure formed inclusions of new mineral phases (Fig. 4B). It is unknown what solid or liquid phases may have been present at the high temperatures that were prevalent when the inclusions formed; however, the polyphase nature of the inclusions, the amorphous nature of the silica-rich phase, and the low melting point of metallic lead (327.5 °C at 1 atmosphere) suggest that coexisting silicate and lead melts were present at some stage. The presence of nanospheres of Pb indicates the separation of a metallic liquid phase from a silicate before the crystallization of native Pb. Once phase separation occurred, the Pb droplets embedded in silica melt inclusions grew in size and decreased in number through Ostwald ripening, thus minimizing surface energy. The presence of Al- and Ti-bearing phases may also be attributed to crystallization from a silicate melt (Fig. 4C). These and other elements that may have been originally compatible in the zircon structure when it first formed would have migrated along with incompatible Pb into nanoinclusions during annealing, possibly due to changes in compatibility, as the conditions of zircon annealing may not have been the same as those under which zircon initially grew (e.g., where lowering the temperature would reduce Ti solubility in zircon). Indeed, silicate melt inclusions in zircon, both glassy and crystalline, are well documented in the literature (30).

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Fig. 4.

Model of Pb nanosphere formation. (A) Zircon structure damaged by radioactive decay leaving “crystalline islands.” Radiogenic Pb (red) is distributed in the noncrystalline areas. (B) During heating, individual incompatible elements, including lead, are concentrated in the noncrystalline areas forming melt inclusions (blue). Larger lead spheres form at the expense of smaller ones, thus minimizing surface energy (Ostwald ripening). (C) During cooling, the metal melt (Pb) separates from the Si-rich phase, forming nanospheres; Pb clusters are formed and are commonly associated with silica melt (yellow) locally enriched in Al and Ti (green). The spherical shape of the silicate–Pb inclusions suggests a melt at peak metamorphic conditions. Finally, the zircon crystalline islands grow and join together, forming a mosaic crystal.

The native Pb nanoparticles observed in our study represent much higher concentrations of Pb than in the clusters observed in a recent atom probe study of a metamorphosed Hadean zircon (13) and are more heterogeneously distributed. In that study, a typical “Pb cluster,” with a maximum concentration of about 0.2% Pb, contains about 4,000 atoms, and such clusters are distributed tens to hundreds of nanometers apart. Such small degrees of Pb heterogeneity would not affect SIMS analyses to any significant degree given the micrometer scale of the analytical spots. However, a sphere of crystalline lead 20 nm across (typical for the nanoinclusions observed in this study) composed of ²⁰⁶Pb only (for ease of calculation) contains about 4.75 × 10⁻¹⁷ g or 139,000 atoms and is derived from a much larger volume of the host zircon. To understand how large a volume of zircon is required to produce such a nanosphere, we have calculated the values for examples of zircon with observed inhomogeneous Pb. For one example, the lowest concentration of ²³⁸U in zircon as estimated by spot analysis (3), in which Pb inclusions were observed by SIMS mapping, is 165 µgg⁻¹ (grain n3847-30). The volume of zircon associated with each ²⁰⁶Pb sphere can be derived from the concentration of ²³⁸U remaining in zircon at the present day, the age of formation of the Pb nanosphere (i.e., about 2.5 Ga), and the age at which the zircon first formed. Assuming this zircon had grown at the upper intercept age of 3.3 Ga (2), the concentration of radiogenic ²⁰⁶Pb accumulated by 2.5 Ga would be 32 µgg⁻¹. Each 20-nm-wide nanosphere of ²⁰⁶Pb would be derived from a spherical volume of zircon 792 nm in diameter, with a corresponding density of 3.8 Pb nanospheres per cubic micrometer of zircon. In this case, the maximum accumulated α-decay dose (which assumes that for the entire period between zircon formation at 3.3 Ga and metamorphism at 2.5 Ga, the zircon resided at temperatures low enough for damage to accumulate) is about 1.0 × 10¹⁸ events per gram, lower than the 2.2 × 10¹⁸ events per gram required for the zircon to become metamict (at the first percolation point). This demonstrates that the condition of complete metamictisation is not a prerequisite for the development of Pb melt inclusions (Table 2). In an example of zircon with higher U and observed Pb heterogeneity, (n3844-47), with 4,614 μgg^{–1 238}U and an upper intercept age of 3.7 Ga, the concentration of radiogenic ²⁰⁶Pb accumulated by 2.5 Ga would be 1,366 µgg⁻¹, with each ²⁰⁶Pb sphere derived from a spherical volume of zircon 226 nm in diameter, corresponding to a density of 163 Pb spheres per cubic micrometer of zircon. In both cases, Pb inhomogeneity on the scale of a few tens of nanometer would be readily observable during analysis under a SIMS ion beam, which typically has a width of 10–30 microns but much finer depth resolution (12), with the sequential measurement of Pb isotopes varying over time as the zircon surface is sputtered at a typical rate of ca. 100 nm per magnet cycle, leading to significant deviations in the estimation of both U–Pb and Pb–Pb ages.

Uranium–Pb zircon ages obtained from SIMS analyses are calculated by calibration against a matrix-matched standard reference zircon. This assumes a similar ablation behavior between the standard and the analyzed material. However, the separation of Pb in zircon into a metallic phase may produce changes in ablation and ionization behavior, especially considering the weakly bonded nature of native lead. Whereas Pb heterogeneity on a submicron scale may only result in increased imprecision in U–Pb and Pb–Pb age estimates, sputtering behavior may induce systematic discordance. Systematic discordance may also be induced by micrometer-scale redistribution of Pb inclusions, as crystallization fronts sweep Pb out of recovered domains and concentrate them in more amorphous domains, as indicated in Figs. 3 and 4. Furthermore, as noted in previous studies (2, 3, 12), if the redistribution of Pb occurred in geologically ancient times, it would have a higher ²⁰⁷Pb/²⁰⁶Pb ratio than any radiogenic lead that accumulated from the time of annealing to the present day. This would inevitably produce reverse discordance of U–Pb and Pb–Pb ages, with spuriously old ²⁰⁷Pb/²⁰⁶Pb age estimates obtained from domains where mobilized radiogenic lead was preferentially concentrated.

The occurrence of metallic Pb nanospheres in zircon affected by UHT metamorphism can explain the unusual U–Pb behavior of such grains during SIMS U–Pb analysis (2, 3, 12). Their formation is related to an annealing process under high-grade conditions, and once formed, the transformation of the enclosing metamict domains into newly crystallized zircon effectively arrests Pb loss, explaining why zircon that has experienced such extreme conditions is not completely reset to its metamorphic age. However, inhomogeneity in the distribution of radiogenic lead, and possible matrix effects, may result in inaccurate age estimates of such zircon when analyzed by microbeam techniques. In samples that contain zircon grains with a uniform age, such inaccuracies can be monitored; however, removed from the context of their host rock, for example in a detrital population, there is great potential for erroneous age interpretation.