Preprints
https://doi.org/10.5194/gchron-2024-17
https://doi.org/10.5194/gchron-2024-17
02 Jul 2024
 | 02 Jul 2024
Status: this preprint is currently under review for the journal GChron.

µID-TIMS: Spatially-resolved high-precision U-Pb zircon geochronology

Sava Markovic, Jörn-Frederik Wotzlaw, Dawid Szymanowski, Joakim Reuteler, Peng Zeng, and Cyril Chelle-Michou

Abstract. We present a novel methodology for spatially-resolved high-precision U-Pb geochronology of individual growth domains in complex zircon. Our approach utilizes a combined plasma (Xe+/Ar+) focused ion beam (PFIB)–femtosecond (fs) laser system equipped with a scanning electron microscope (SEM). This system enables micrometer resolution sampling of zircon growth domains with real-time monitoring by cathodoluminescence (CL) SEM imaging. Microsamples are then extracted, chemically abraded, dissolved and analyzed by isotope dilution thermal ionization mass spectrometry (ID-TIMS) to obtain high-precision U-Pb dates. Because of its superior beam precision (~8–20 µm diameter), cleaner cuts, and negligible, nanometer-scale damage imparted on the zircon structure, PFIB machining (30 kV) is preferred for microsamples of sizes expected in most future studies focusing on texturally complex natural zircon (20–120 µm length scales). Femtosecond laser machining is significantly faster and therefore more appropriate for larger microsamples (>120 µm length scales) but it is also coarser (≥20 µm probe size), produces rougher cuts, and creates a minimum of two orders of magnitude wider (micrometer-scale) structurally damaged zone along the laser cuts. Our experiments show that PFIB machining can be conducted on zircon coated with carbon (minor drift of ion beam during machining) and protective metal coatings (no CL signal) as neither offset the U-Pb systematics nor do they introduce trace amounts of common Pb. We used Xe+ PFIB and femtosecond laser to obtain U–Pb dates for Mud Tank and GZ7 zircon microsamples covering a range of sizes (40 × 18 × 40 µm – 100 × 80 × 70 µm) and found that microsampling does not bias the accuracy of the resulting µID-TIMS U-Pb dates. The accuracy and precision of µID-TIMS dates for zircon of any given age and U concentration depend, as for non-microsampled zircon, on Utotal/Ublank and Pb*/Pbc – both a function of sample size. Our accompanying open-source code can aid researchers in estimating the necessary microsample size needed to obtain accurate dates at precision sufficient to resolve the processes under study. µID-TIMS bridges the gap between conventional bulk-grain high-precision dating and high-spatial resolution in situ techniques, enabling the study of the timescales of a variety of processes recorded on the scale of individual growth zones in zircon. This method can be applied to zircon of any age and composition, from terrestrial systems to precious samples from other planetary bodies.

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Sava Markovic, Jörn-Frederik Wotzlaw, Dawid Szymanowski, Joakim Reuteler, Peng Zeng, and Cyril Chelle-Michou

Status: open (until 13 Aug 2024)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on gchron-2024-17', Donald Davis, 16 Jul 2024 reply
Sava Markovic, Jörn-Frederik Wotzlaw, Dawid Szymanowski, Joakim Reuteler, Peng Zeng, and Cyril Chelle-Michou
Sava Markovic, Jörn-Frederik Wotzlaw, Dawid Szymanowski, Joakim Reuteler, Peng Zeng, and Cyril Chelle-Michou

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Short summary
We present a pioneering method for high-precision absolute dating of individual growth zones in mineral zircon. The micrometer-wide growth zones record key processes in Earth and Planetary sciences, such as conditions in magma reservoirs prior to supereruptions or planetary formation during the early stages of the Solar system. In our approach, we directly sample the growth zones with a focused ion beam and high pulse laser, allowing to tackle a number of long-standing research questions.