Preprints
https://doi.org/10.5194/gchron-2022-7
https://doi.org/10.5194/gchron-2022-7
 
18 Mar 2022
18 Mar 2022
Status: this preprint is currently under review for the journal GChron.

Technical note: Rapid phase identification of apatite and zircon grains for geochronology using X-ray micro-computed tomography

Emily H. G. Cooperdock1,, Florian Hofmann1,2,, Ryley M. Collins1, Anahi Carrera1, Aya Takase3, and Aaron J. Celestian4 Emily H. G. Cooperdock et al.
  • 1University of Southern California, Department of Earth Sciences, 3651 Trousdale Parkway, Los Angeles, CA 90089, USA
  • 2University of Alaska Fairbanks, Geophysical Institute, 900 Yukon Dr, Fairbanks, AK 99775, USA
  • 3Rigaku Americas Corporation, 9009 New Trails Drive, The Woodlands, TX 77381, USA
  • 4Mineral Sciences Department, Natural History Museum of Los Angeles County, 900 Exposition Boulevard, Los Angeles, California 90007, USA
  • Authors contributed equally to this work.

Abstract. Apatite and zircon are among the best-studied and most widely used accessory minerals for geochronology and thermochronology. Given that apatite and zircon are often present in the same lithologies, distinguishing the two phases in crushed mineral separates is a common challenge that many laboratories face. Here we present a method for efficient and accurate apatite and zircon mineral phase identification using X-ray micro-computed tomography (microCT) of grain mounts that provides additional 3-dimensional grain size, shape, and inclusion suite information. In this study, we analyzed apatite and zircon grains from Fish Canyon Tuff samples that underwent methylene iodide (MEI) and lithium heteropolytungstate (LST) heavy liquids density separations. We validate the microCT results using known standards and phase identification with Raman spectroscopy demonstrating that apatite and zircon are distinguishable from each other and other common phases, e.g., titanite, based on microCT X-ray density. We present recommended microCT scanning protocols after systematically testing the effects of different scanning parameters and sample positions. This methodology can help to reduce time spent performing density separations with highly toxic chemicals and visually inspecting grains under a light microscope, and the improved mineral identification and characterization can make geochronologic data more robust.

Emily H. G. Cooperdock et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on gchron-2022-7', Anonymous Referee #1, 11 Apr 2022
  • RC2: 'Comment on gchron-2022-7', Anonymous Referee #2, 12 May 2022

Emily H. G. Cooperdock et al.

Emily H. G. Cooperdock et al.

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Short summary
Apatite and zircon are the most widely used minerals for dating rocks, but they can be difficult to identify in crushed rock samples. Incorrect mineral identification results in wasted analytical resources and inaccurate data. We show how X-ray computed tomography can be used to rapidly and accurately distinguish apatite from zircon based on density variations, and provide non-destructive 3D grain-specific size, shape, and inclusion information for improved data quality.