Crystallization and Disturbance Histories of Single Zircon Crystals From Hadean‐Eoarchean Acasta Gneisses Examined by LA‐ICP‐MS U‐Pb Traverses

Abstract

Zircon U‐Pb geochronology is a very robust dating method but the accuracy of determined ages can sometimes be compromised. This is because, as commonly observed, the U‐Pb isotope system can be reopened during the post‐crystallization evolution of a zircon. The present manuscript investigates the capability of traverses by LA‐ICP‐MS to identify zones of well‐preserved U‐Pb isotope systematics within zircon crystals. The data for the different zones can be used to construct internal Discordia lines that provide age information about crystallization and metamorphism of single zircon crystals. To test our approach, we analyzed zircons from three ca. 3.96 Ga Acasta gneisses. Results demonstrate that such a method allows retrieval of original crystallization age in most of the studied zircons, even in those that experienced ancient U‐Pb disturbances resulting in discordant U‐Pb data. It was also possible to estimate disturbance ages, though not as precisely as for the igneous crystallization, in many of the studied crystals.

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

“…The igneous activity of the AGC essentially consists in several granitic pulses that occurred between 4020 and 2900 Ma, with a major event at 3600 Ma that involved both juvenile addition and crustal anatexis (Table A.1). The metamorphic history of the AGC is less constrained and mostly derived from U-Pb dating of zircon, apatite and titanite (e.g., Hodges and Bowring, 1995;Bleeker and Stern, 1997;Sano et al, 1999;Iizuka et al, 2007;Mojzsis et al, 2014;Guitreau et al, 2018;Fisher et al, 2019;Kirkland et al, 2020). However, with the available dataset, it clearly appears that during the AGC geological history, most metamorphic events are closely linked to the magmatic activity (e.g., Bleeker and Stern, 1997).…”

“…Some more discrete events also likely occurred afterwards (e.g. Hodges and Bowring, 1995;Guitreau et al, 2018).…”

“…Sample AG09009 is more potassic (K2O= 3.64 wt %; K2O/ Na2O ~ 1) despite high La/Yb, and strong depletion in HREE, and therefore falls within the field of granite in a normative An-Ab-Or plot. Samples AG09009, AG09014, and AG09015 were dated by zircon U-Pb geochronology at 3600 Ma (Guitreau et al, 2012), whereas AG09016 was dated at 3947 Ma (Guitreau et al, 2012(Guitreau et al, , 2018.…”

“…However, it is presently unclear to which extent zircon-hosted apatite inclusions can resist metamorphic resetting, especially in ancient zircon crystals that experienced significant radiation damage such as those in the AGC (e.g. Guitreau et al, 2018). Consequently, the aim of this paper is to study the preservation potential of trace element and U-Th-Pb isotope signatures in zircon-hosted inclusions and, hence, better constrain the extent to which such approach can allow retrieval of information about the formation of Earth's first crusts.…”

“…Consequently, the aim of this paper is to study the preservation potential of trace element and U-Th-Pb isotope signatures in zircon-hosted inclusions and, hence, better constrain the extent to which such approach can allow retrieval of information about the formation of Earth's first crusts. For this purpose, we carefully and systematically measured trace element contents and U-Th-Pb isotope systematics in zirconhosted apatite as well as in matrix apatite crystals in well-characterized 3.6-3.96 Ga samples from the Acasta Gneiss Complex (Guitreau et al, 2012(Guitreau et al, , 2014(Guitreau et al, , 2018Mojzsis et al, 2014).…”