Metallic lead (Pb) nanospheres discovered in Hadean and Eoarchean zircon crystals at Jack Hills.

Here, we report small randomly-distributed crystalline lead (Pb) nanospheres occurring in detrital zircon grains obtained from a weakly metamorphosed Archean conglomerate at Jack Hills, Western Australia, making this the third known global example of this phenomenon. They form in zircon crystals ranging from Hadean (> 4 billion years-Ga) to Eoarchean (> 3.6 Ga) in age, but are absent from Paleoarchean (~ 3.4 Ga) crystals. Unlike previous discoveries of nanospheres in zircon from Precambrian gneisses in Antarctica and India, detrital zircon from Jack Hills shows no evidence of ever undergoing ultra-high temperature (UHT) metamorphism, either before or after deposition, therefore implying that nanospheres can form at temperatures lower than ca. 900 °C. The nanospheres are composed of radiogenic Pb released by the breakdown of uranium (U) and thorium (Th) and are present in zircon irrespective of its U, Th and water contents, its oxygen isotopic composition, and the degree of discordance due to Pb loss or gain. The nanospheres pre-date annealed cracks in the crystals, showing that, once formed, they effectively 'freeze' radiogenic Pb in the zircon structure, precluding any further interaction during subsequent geological processes. Both Pb nanoclusters and nanospheres are now reported from Jack Hills, and it appears likely the former is a precursor stage in the formation of the latter. Although the precise mechanism for this transition remains unresolved, a later thermal event is required, but this likely did not reach UHT conditions at Jack Hills.

Pb nanospheres in ancient zircon yield model ages for zircon formation and Pb mobilization.

Nanospheres of lead (Pb) have recently been identified in zircon (ZrSiO4) with the potential to compromise the veracity of U-Pb age determinations. The key assumption that the determined age is robust against the effects of Pb mobility, as long as Pb is not lost from the zircon during subsequent geological events, is now in question. To determine the effect of nanosphere formation on age determination, and whether analysis of nanospheres can yield additional information about the timing of both zircon growth and nanosphere formation, zircons from the Napier Complex in Enderby Land, East Antarctica, were investigated by high-spatial resolution NanoSIMS (Secondary Ion Mass Spectrometry) mapping. Conventional SIMS analyses with >µm resolution potentially mixes Pb from multiple nanospheres with the zircon host, yielding variable average values and therefore unreliable ages. NanoSIMS analyses were obtained of 207Pb/206Pb in nanospheres a few nanometres in diameter that were resolved from 207Pb/206Pb measurements in the zircon host. We demonstrate that analysis for 207Pb/206Pb in multiple individual Pb nanospheres, along with separate analysis of 207Pb/206Pb in the zircon host, can not only accurately yield the age of zircon crystallization, but also the time of nanosphere formation resulting from Pb mobilization during metamorphism. Model ages for both events can be derived that are correlated due to the limited range of possible solutions that can be satisfied by the measured 207Pb/206Pb ratios of nanospheres and zircon host. For the Napier Complex zircons, this yields a model age of ca 3110 Ma for zircon formation and a late Archean model age of 2610 Ma for the metamorphism that produced the nanospheres. The Nanosphere Model Age (NMA) method constrains both the crystallization age and age of the metamorphism to ~±135 Ma, a significant improvement on errors derived from counting statistics.

Metallic lead nanospheres discovered in ancient zircons.

Zircon (ZrSiO4) is the most commonly used geochronometer, preserving age and geochemical information through a wide range of geological processes. However, zircon U-Pb geochronology can be affected by redistribution of radiogenic Pb, which is incompatible in the crystal structure. This phenomenon is particularly common in zircon that has experienced ultra-high temperature metamorphism, where ion imaging has revealed submicrometer domains that are sufficiently heterogeneously distributed to severely perturb ages, in some cases yielding apparent Hadean (>4 Ga) ages from younger zircons. Documenting the composition and mineralogy of these Pb-enriched domains is essential for understanding the processes of Pb redistribution in zircon and its effects on geochronology. Using high-resolution scanning transmission electron microscopy, we show that Pb-rich domains previously identified in zircons from East Antarctic granulites are 5-30 nm nanospheres of metallic Pb. They are randomly distributed with respect to zircon crystallinity, and their association with a Ti- and Al-rich silica melt suggests that they represent melt inclusions generated during ultra-high temperature metamorphism. Metallic Pb is exceedingly rare in nature and previously has not been reported in association with high-grade metamorphism. Formation of these metallic nanospheres within annealed zircon effectively halts the loss of radiogenic Pb from zircon. Both the redistribution and phase separation of radiogenic Pb in this manner can compromise the precision and accuracy of U-Pb ages obtained by high spatial resolution methods.