Stepwise chemical abrasion–isotope dilution–thermal ionization mass spectrometry with trace element analysis of microfractured Hadean zircon

This is a Preprint and has not been peer reviewed. The published version of this Preprint is available: https://doi.org/10.5194/gchron-1-85-2019.

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Authors

C. Brenhin Keller , Patrick Boehnke, Blair Schoene, T. Mark Harrison

Abstract

The Hadean Jack Hills zircons represent the oldest known terrestrial material, providing a unique and truly direct record of Hadean Earth history. This zircon population has been extensively studied via high spatial resolution, high throughput in situ isotopic and elemental analysis techniques such as secondary ionization mass spectrometry (SIMS), but not by comparatively destructive, high-temporal-precision (< 0.05 % two-sigma) thermal ionization mass spectrometry (TIMS). In order to better understand the lead loss and alteration history of terrestrial Hadean zircons, we conduct stepwise chemical abrasion isotope dilution thermal ionization mass spectrometry with trace element analysis (CA-ID-TIMS-TEA) on manually microfractured Hadean Jack Hills zircon fragments previously dated by SIMS. We conducted three successive HF leaching steps on each individual zircon fragment, followed by column chromatography to isolate U-Pb and trace element fractions. Following isotopic and elemental analysis, the result is an independent age and trace element composition for each leachate of each zircon fragment. We observe ~50 Myr of age heterogeneity in concordant residues from a single zircon grain, along with a protracted history of post-Hadean Pb-loss with at least two modes circa ~0 and 2–4 Ga. Meanwhile, step-wise leachate trace element chemistry reveals enrichments of light rare earth elements, uranium, thorium, and radiogenic lead in early leached domains relative to the zircon residue. In addition to confirming the efficacy of the LREE-I alteration index and providing new insight into the mechanism of chemical abrasion, the interpretation and reconciliation of these results suggests that Pb-loss is largely driven by low-temperature aqueous recrystallization, and that regional thermal events may act to halt – not initiate – Pb-loss from metamict domains in the Hadean Jack Hills zircons.

DOI

https://doi.org/10.31223/osf.io/gzr43

Subjects

Earth Sciences, Geochemistry, Geology, Physical Sciences and Mathematics

Keywords

Geochronology, Hadean, Zircon, Jack Hills

Dates

Published: 2019-06-16 13:19

Last Updated: 2019-12-13 03:09

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License

CC BY Attribution 4.0 International

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