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

Abstract

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.

Understanding Preservation of Primary Signatures in Apatite by Comparing Matrix and Zircon‐Hosted Crystals From the Eoarchean Acasta Gneiss Complex (Canada)

“…Instead, we propose that reversely discordant apatite inclusions were contaminated by radiogenic Pb coming from the host zircon during a thermal event such as the Wopmay orogen. Radiogenic Pb migration has already been described in Archean zircons, notably from the Napier Complex, Antarctica (Kusiak et al, 2013; Lyon et al, 2019; Williams et al, 1984). The evolution of the Pb isotope composition of a zircon with a Th/U of 0.5 is presented in Figure 9, and it is clearly visible that contamination by such radiogenic Pb would result in horizontal spread of data in a 208 Pb/ 206 Pb versus 207 Pb/ 206 Pb plot.…”

“…Instead, we propose that reversely discordant apatite inclusions were contaminated by radiogenic Pb coming from the host zircon during a thermal event such as the Wopmay orogen. Radiogenic Pb migration has already been described in Archean zircons, notably from the Napier Complex, Antarctica (Kusiak et al, 2013;Lyon et al, 2019;Williams et al, 1984). The evolution of the .…”

Understanding preservation of primary signatures in apatite by comparing matrix and zircon-hosted crystals from the Eoarchean Acasta Gneiss Complex (Canada)

“…Instead, we propose that reversely discordant apatite inclusions were contaminated by radiogenic Pb coming from the host zircon during a thermal event such as the Wopmay orogen. Radiogenic Pb migration has already been described in Archean zircons, notably from the Napier Complex, Antarctica (Williams et al, 1984;Kusiak et al, 2013;Lyon et al, 2019). The evolution of the Pb isotope composition of a zircon with a Th/U of 0.5 is presented in Figure 9 and it is clearly visible that contamination by such radiogenic Pb would result in horizontal spread of data in a 208 Pb/ 206 Pb versus 207 Pb/ 206 Pb plot.…”

Metallic-Pb nanospheres in zircon from the Challenger Au deposit, South Australia: probing metamorphic and ore formation histories

“…As temperatures increased, melting of amorphous domains generated locally anomalous concentrations of Pb, resulting in metal–silicate immiscibility during cooling. The ages of metallic-Pb nanospheres from the same Antarctic samples were obtained using nanoscale secondary ion mass spectrometry (Lyon et al , 2019), yielding model 207 Pb/ 206 Pb ages for host zircon crystallisation (3.11 Ga) and metallic-Pb nanospheres formation (2.61 Ga), substantiating that U–Pb ratios in metallic-Pb nanospheres record the timing of UHT metamorphism.…”