the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 Dec 2024
Technical Note: Improved volume and derived value calculations for polished zircon
Abstract. Polishing mounted mineral crystals prior to bulk grain (U-Th)/He thermochronology analysis offers many advantages for characterizing and subsampling each grain via in situ methods to obtain the maximum geologically relevant information. However, polishing introduces complications for calculating grain volume, on which many derived (U-Th)/He data partially depend. Impacted data include isotope concentrations, effective uranium (a proxy for radiation damage), and alpha-ejection correction factors (FT) which are used to correct (U-Th)/He dates. These derived data are integral to interpreting (U-Th)/He dates; without a way to accurately calculate these values for polished grains, the benefits of polishing and in situ measurements can be greatly reduced or negated. This reality has resulted in many studies forgoing polishing and thus missing potentially important data. To address this issue, this paper presents a set of equations encoded in an R script to calculate volume and derived values for polished zircon that can be easily integrated into existing workflows for bulk grain (U-Th)/He analysis.
- Preprint
(1077 KB) - Metadata XML
-
Supplement
(48 KB) - BibTeX
- EndNote
Status: open (until 22 Jan 2025)
-
RC1: 'Comment on gchron-2024-33', Christoph Glotzbach, 17 Jan 2025
reply
Dear Author,
I have read your technical note on improving grain geometry estimates required for (U-Th)/He measurements in case grains are mounted and polished prior to He measurements. The reasoning why a more accurate method is needed is clear to me, but I would ask you to be more specific in describing the method. From what I understand from your manuscript, your proposed approach will likely under- or overestime the Ft value since you are using volume and surface area calculation of the polished grains for your calculation of Ft. The correct Ft value is dependent on both the original grain geometry and the resulting after polishing and only if half a grain is polished both will be similar. The resulting Ft value calculated with your approach will be smaller than the corresponding Ft value of the whole grain, in case polishing removes less than half of the grain. The opposite is in case more than half the grain is removed during polishing. Either show the difference to the correct value and state the limitation or implement the correct calculation.
Please see my scientific comments and technical corrections for more details:
Technical corrections:
Line 1,16: Specify what you mean with ‘derived value calculation’. Also, later you often say something like ‘other values’. Please make sure that you always specify the measurements you are referring to.
Line 8-17: You state that the proposed protocol is beneficial for in situ measurements (line 14) and later for bulk grain (e.g. line 16). Please state clearly which method (in-situ and/or bulk grain) would benefit. Â Â
Line 49-50: You may want to reference to my approach using a set of orthogonal microscopic pictures to derive whole-grain geometries (Glotzbach et al. 2019 – Chemical Geology).
Line 48-58: Please clearly state what your approach is. Measuring only after mounting/polishing or, what I guess, two measurements are required before and after mounting/polishing.
Line 72: The word ‘irrelevant’ is somewhat misleading here, since the depth to which grains are polished is impacted V, SA and other related parameters and is not irrelevant. I guess you mean that it is easy to account/correct for.
Line 74-77: See above, in case you meausure individual grains before and after mounting/polishing this would not be required. Therefore I guess you are measuring only after mounting/polishing and derive the depth of polishing from the glass beads.
Line 85-90: I do not fully understand how you can estimate the correct values of a and b for an ellipsoid (r for cylinder) in case more than half of the grain is polished away. The equations that you state will be minimum values for a and b (e.g. b=W1/2).
Line 86: Specify what the ellipsoid coefficient is.
Line 114-115: Same as for the ellipsoid does apply for a cylinder, it is not possible to correctly determine r when more than half the grain is gone. The equation that you are using r= min(W1,W2) will underestimate r. Why not using equation 1 to estimate the correct radius?
Line 158-186: It is unclear to me if you calculate the Ft for the whole grain or the mounted/polished grain. The Ft value of mounted/polished grains will in most cases be higher/lower than the theoretical value of the whole grain (similar only if exactly half the grain is removed).
Line 187: Please add more details on how you did the comparison, are this read data or synthetic data and give details how the methods of Ketcham and Reiners differ from your approach.
Line 189: Please clarify what you mean with ‘uncorrected method’?
Citation: https://doi.org/10.5194/gchron-2024-33-RC1
Interactive computing environment
polished_ZHe_derived_values_v1.0.R Barra A. Peak https://github.com/Barra-Peak/polished-ZHe-derived-values
Viewed
| HTML | XML | Total | Supplement | BibTeX | EndNote | |
|---|---|---|---|---|---|---|
| 78 | 9 | 3 | 90 | 9 | 2 | 4 |
- HTML: 78
- PDF: 9
- XML: 3
- Total: 90
- Supplement: 9
- BibTeX: 2
- EndNote: 4
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1