Preprints
https://doi.org/10.5194/gchron-2022-19
https://doi.org/10.5194/gchron-2022-19
 
26 Jul 2022
26 Jul 2022
Status: this preprint is currently under review for the journal GChron.

Chemical Abrasion: The Mechanics of Zircon Dissolution

Alyssa J. McKanna1, Isabel Koran1, Blair Schoene1, and Richard A. Ketcham2 Alyssa J. McKanna et al.
  • 1Department of Geosciences, Guyot Hall, Princeton University, Princeton, NJ 08544, USA
  • 2Jackson School of Geosciences, The University of Texas Austin, Austin, TX 78712, USA

Abstract. Chemical abrasion is a technique that combines laboratory annealing and partial dissolution in hydrofluoric acid (HF) to selectively remove radiation-damaged portions of zircon crystals prior to U-Pb isotopic analysis, and it is applied ubiquitously to zircon prior to U-Pb isotope dilution thermal ionization mass spectrometry (ID-TIMS). The mechanics of zircon dissolution in HF and the impact of different leaching conditions on the zircon structure, however, are poorly resolved. We present a microstructural investigation that integrates microscale X-ray computed tomography (µCT), scanning electron microscopy, and Raman spectroscopy to evaluate zircon dissolution in HF. We show that µCT is an effective tool for imaging metamictization and complex dissolution networks in three dimensions. We find that most grains do not dissolve predominantly from rim-to-core. Acid frequently reaches crystal interiors via fracture networks spatially associated with radiation damage zoning and inclusions to dissolve higher U zones, material in the vicinity of fractures, and some inclusions. Other acid paths to crystal cores include the dissolution of surface-reaching inclusions and the percolation of acid across zones with high defect densities. In highly crystalline samples dissolution is crystallographically-controlled with dissolution proceeding almost exclusively along the c-axis. Increasing the leaching temperature from 180 °C to 210 °C results in deeper etching textures, wider acid paths, more complex internal dissolution networks, and greater volume losses. We discuss the implications of our findings for zircon ID-TIMS U-Pb geochronology and paired trace element analyses, radiation damage annealing models, and for using µCT for imaging radiation damage zoning for (U-Th)/He thermochronology.

Alyssa J. McKanna et al.

Status: open (until 06 Sep 2022)

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Alyssa J. McKanna et al.

Alyssa J. McKanna et al.

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Short summary
Acid leaching is commonly used to remove damaged portions of zircon crystals prior to U-Pb dating. However, a basic understanding of the microstructural processes that occur during leaching is lacking. We present the first 3D view of zircon dissolution based on X-ray computed tomography data acquired before and after acid leaching. These data are paired with images of etched grain surfaces and Raman spectral data. We also reveal exciting opportunities for imaging radiation damage zoning in 3D.