Metallic lead nanospheres discovered in ancient zircons

Abstract

Zircon (ZrSiO 4 ) 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.metallic Pb | nanospheres | zircon | Antarctica | early Earth Z ircon is the mineral of choice for precisely determining the timing of both magmatism and metamorphism in a wide range of geological samples as well as providing constraints on the source and time of deposition of clastic sedimentary rocks. Accurate zircon geochronometry is facilitated by zircon having a lattice structure that is stable over a wide range of temperatures and pressures (1), together with the fact that whatever Pb is present derives almost entirely from radioactive decay of U and Th. During its growth, zircon incorporates small amounts of nonformula elements, including Hf, Ti, and Y, and rare earth elements, U and Th, but generally excludes Pb. This incompatibility of Pb raises questions about how radiogenic Pb is retained in zircon, especially through geological events where elevated temperature, fluid activity, and deformation can enhance element mobility. Furthermore it has been established that an irregular redistribution of lead in metamorphic zircon can degrade the precision and accuracy of U-Pb isotopic data, in extreme cases leading to spurious ages (2, 3).Experimental and natural studies have revealed that element mobility in zircon is strongly dependent on the accumulation of α-recoil damage (4, 5). Multiple α-decay events along the U and Th decay chains leading to stable daughter Pb isotopes destroy the crystal lattice, creating amorphous ...

Metallic Pb nanospheres in ultra-high temperature metamorphosed zircon from southern India

“…At the time of UHT metamorphism, the accumulated α doses of 1.5 × 10 18 and 2.5 × 10 18 α/g based on the spot U-Pb analyses of the two grains (Table S1 in Whitehouse et al 2014) are similar to two of the three grains investigated from Enderby Land. This observation strengthens the conclusion of Kusiak et al (2015) that metamictization of zircon is not a prerequisite for Pb nanosphere formation. These doses bracket the value of 2.4 × 10 18 α/g (Pidgeon 2014, revised after Murakami et al 1991 andSalje et al 1999), which defines the so-called Bfirst percolation point^where amorphous domains in zircon begin to form an interconnected network.…”

“…The detection of randomly distributed metallic Pb nanospheres in zircon from these two KKB samples shows that this phenomenon is not unique to Enderby Land, from where it was originally reported (Kusiak et al 2015), and that their presence may be a more common phenomenon in UHT terranes. In both cases, Si-rich glass inclusions are also found, containing trace levels of Al and Ti in the Enderby Land example and trace levels of Fe in the KKB.…”

“…The Pb nanospheres are generally smaller and of lower abundance in the KKB zircons and in all observed cases, occur independently of the Si-rich glass inclusions, while in the Enderby Land example, most of the Pb nanospheres occur themselves as inclusions in the Si-rich glass. Additionally, in the KKB example, the zircon surrounding the nanospheres is highly crystalline, whereas in the Enderby Land example the nanospheres occur only in regions of lower crystallinity (Kusiak et al 2015).…”

“…No correlation between Pb distribution and degree of crystallinity nor significant plastic deformation was observed in the analysed grains (Kusiak et al 2017). Kusiak et al (2015) proposed that during metamorphism of radiation damaged zircon, nanoscale islands of crystallinity act as nucleation sites for lattice recrystallisation, trapping radiogenic Pb in amorphous zircon domains. As the temperature increases, melting of these amorphous domains results in the localised concentration of Pb, with subsequent cooling accompanied by metal (Pb) -silicate immiscibility.…”

“…5-35 nm diameter metallic nanospheres (Kusiak et al 2015). In these zircon grains, the Pb nanospheres occur either as individual droplets or are associated with an amorphous silica-rich phase and an unidentified Ti-Al amorphous phase.…”