Low-δ18O (−8 ‰, VSMOW) Paleoproterozoic and discordant zircon: Lessons learned from using a combination of traditional bulk and in situ approaches

Abstract

Zircon has become an important archive of early Earth's history relating the surface water cycle and magmatism via its O isotope composition. However, metamictization of zircon presents a challenge when dealing with ancient detrital grains that are removed from their host rock. Here we use an extremely low-δ¹⁸O zircon (−8 ‰) that is collected from an intact low-δ¹⁸O host rock in order to refine our analytical approaches to O isotope and UPb determinations. The δ¹⁸O of the hosting quartz is −4.2 ± 0.4 ‰, attesting to the primary nature of low δ¹⁸O values from an ancient water-rock interaction that originated in the Neoarchean. Highly variable ¹⁸O/¹⁶O of the zircon correlate positively with ¹⁶O¹H/¹⁶O measured by SIMS, representing secondary hydration accompanied by O isotope exchange, from the least altered δ¹⁸O = −8.0 ± 0.1 ‰ to the highly hydrated domains with δ¹⁸O around +2 ‰. Previously measured near-concordant in situ UPb ages accompanied by high U (up to 0.5 wt%) present a puzzling observation of a preserved UPb system and highly disturbed O isotope compositions. Here we test the accuracy of these interpretations by combining several “gold standard” techniques including elemental X-ray mapping, bulk laser fluorination (δ¹⁸O), and chemical abrasion isotope dilution thermal ionization mass spectrometry (CA-ID-TIMS) for UPb ages. The zircon bulk δ¹⁸O values (−5.7 ‰ to −3.6 ‰) agree with the average values measured by SIMS, corroborating the effect of chemical removal of damaged domains by bulk that approach values in domains with low ¹⁶O¹H/¹⁶O. Further, the triple O isotope compositions of zircon and quartz measured by laser fluorination have values similar to the isotope composition of the low-δ¹⁸O protoliths found within the same area. The δD value of −175 ‰ measured in a bulk zircon with 2.4 wt% H₂O points to a near-surface source of hydrating waters. The CA-ID-TIMS UPb dating of such highly metamict zircon is challenged by almost complete loss of the analyte during the chemical abrasion step and by UPb discordant ages. Leaching at 130–210 °C for 3–12 h produces variably discordant ages, with one zircon grain yielding concordance and the ²⁰⁷Pb²⁰⁶Pb age of 1773 ± 2 Ma after leaching at 170 °C for 12 h, whereas leaching at higher temperature for less time produced inferior concordance. Both normal and reverse discordance are observed, indicating high and near-contemporaneous mobility of U/Pb. Here we document both major and trace elemental mobility that are mechanistically difficult to explain, however both are related to a fluid-zircon interaction and are only observable by a combination of in situ and bulk methods.