the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Technical note: Rapid phase identification of apatite and zircon grains for geochronology using X-ray micro-computed tomography
- 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.
- 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.
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Emily H. G. Cooperdock et al.
Status: final response (author comments only)
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RC1: 'Comment on gchron-2022-7', Anonymous Referee #1, 11 Apr 2022
I appreciate the detailed and careful work that the authors have done developing and optimizing a protocol for microCT imaging of zircon and apatite. I believe that there is great, untapped potential for geochronological applications of microCT imaging of loose grains, grain mounts, and whole rock samples. Others have shown that microCT imaging can be used to quantify grain size, characterize crystal morphology, estimate surface-to-volume ratios for alpha-ejection corrections, and to identify inclusions and fractures in minerals. As such, microCT imaging could well become an important, routine tool for zircon and apatite chronology like it has become for (U-Th)/He dating of magnetite and other opaque oxides.
I also appreciate that the authors highlight the health risks of toxic heavy liquids like methylene iodide and bromoform. I agree that whenever possible, use of these chemicals should be avoided. Non-toxic heavy liquids such as LST, panning, and Wilfley tables are effective methods for separating out less dense minerals. However, as stated by the authors these methods are not capable of – or are not always terribly effective at – separating apatite from zircon.
While I believe that the work presented here represents an important contribution to the field of geochronology, I question the practicality of using microCT as a routine tool for mineral identification. Mineral identification is indeed a challenge in geochronology; geochronological opportunities are missed if less routinely dated minerals like baddeleyite or xenotime are not accurately identified in mineral separates. Apatite and zircon, however, are routinely encountered and have distinct crystal habits, optical properties, and differing solubilities. In the case of mineral identification challenges, other tools like Raman spectroscopy and EDS analysis have databases at hand that can identify mineral phases more directly as compared to relying on microCT density contrast. Further, not many geochronology labs are equipped with in-house microCT scanners. That said, having another analytical tool available to help with mineral identification – in the event that Raman and EDS are not available, and optical identification has truly failed – is useful.
In my personal experience, the challenge separating zircon from apatite in LST dense fractions that have not undergone MEI separation is often a problem of relative abundance. Some rocks have significantly more apatite than zircon. Identifying a small number of zircon crystals in an ocean of apatite can mean significant time spent at the picking scope. While spending more time at the microscope is safer than using MEI, I doubt that using microCT imaging to aid in mineral identification helps to save time.
16-18: Is it really that challenging to distinguish between apatite and zircon? The two minerals have very different crystal morphologies, optical properties, and acid solubilities.
36: Characterization of crystal shape, size, and inclusion content seems a stronger motivation.
57: I appreciate the discussion about health risks. I feel that most users don’t know much beyond the fact that these chemicals are “toxic” or carcinogenic.
104: I would think that mounting and scanning grains likely adds additional time and cost to the process as well – not many labs have their own CT scanner.
160: It would be useful/interesting to demonstrate how often trained graduate students actually misidentify zircon and apatite. It would help to justify the study’s stated motivation.
291-294: If a worker has already invested the time in preparing a grain mount for microCT imaging, why not simply go for the longer scan that will yield additional, more useful information about grain morphology, inclusions, ect?
Figure 3: Great figure. Very useful for predicting which minerals CT scans may be useful for. Maybe include other minerals of geochronological interest? How do zircon and apatite compare to magnetite and other oxide minerals that are now the focus of many (U-Th)/He studies?
Figure 7: As someone new to microCT imaging, this is also a great figure illustrating how different scan conditions affect image quality.
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RC2: 'Comment on gchron-2022-7', Anonymous Referee #2, 12 May 2022
This manuscript presents a detailed description of a microCT method for distinguishing apatite from zircon. The authors clearly lay out an optimized methodology, along with excellent figures that illustrate various aspects of the data and measurements. Besides some suggestions that I provide below for clarification, I have no issue with this manuscript being published. I appreciate the authors’ efforts to develop another way to use microCT together with geochronology methods. However, I’m skeptical this method will be widely adopted for distinguishing apatite from zircon. The authors might consider shifting the manuscript emphasis in places, as suggested below, to potentially make this contribution more impactful.
- In my view the problem being addressed (distinguishing apatite from zircon in mineral separates) is greatly overstated. In the vast majority of circumstances, it isn’t challenging to distinguish apatite from zircon under the microscope after separation with LST. These minerals are distinct in morphology, relief, and other properties. Even for newbies, after a few hours of getting one’s eyes calibrated at the microscope, it is not particularly difficult to distinguish these mineral phases. It arguably becomes more important for detrital mineral suites – it may be effective for the authors to specifically emphasize this challenge, rather than trying to argue that this is a routine problem when it really isn’t and most reading this paper will know this.
- Agreed that performing mineral separation with toxic chemicals is undesirable, but again this strikes me as overstated given that one can alternatively use LST and then i.d. the minerals typically without too much trouble.
- Although not emphasized in the abstract or introduction, elsewhere in the paper the authors highlight attempting to distinguish apatite from titanite with microCT. These phases are even easier to differentiate than apatite and zircon, with titanite typically coming off at a more magnetic level on the frantz than apatite. It may be better to eliminate this comparison in this paper entirely.
- It would be helpful to provide some estimate of the total time required per grain (including mineral selection, mount making, analysis, data reduction) to 1) use the proposed microCT method to distinguish different mineral phases and 2) additionally identify inclusions and acquire grain geometry information.
- Is the microCT method for mineral i.d. faster than alternative analytical methods that could be used to identify these phases? For example, in my experience, mineral identification using an EDS system on an electron microprobe or SEM requires only a few minutes to place individual crystals on carbon tape and then seconds per grain for EDS identification. This seems faster than the microCT method described here, and EDS systems are more common and therefore more accessible than microCT systems. If this is incorrect, then it would be helpful to clarify this in the paper.
- If one is going to the trouble of making the mount, then why not use the longer scan times to acquire the additional information about inclusions and grain geometry? This strikes me as a more compelling reason to use this method, and could be emphasized more strongly as a motivation in the paper. Or perhaps this could become the paper’s primary motivation.
- Lines 33-43: Suggest revising the second sentence. The characterization of the crystal shape does not matter for U-Pb and fission-track, unlike what is implied by the structure of these two opening sentences.
Emily H. G. Cooperdock et al.
Emily H. G. Cooperdock et al.
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