the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Jun 2024
Interpreting cooling dates and histories from laser ablation in-situ (U-Th-Sm)/He thermochronometry
Abstract. Recent applications of the in-situ (U-Th-Sm)/He thermochronometry technique demonstrate its potential to address some of the analytical challenges associated with the whole-grain technique. In this study, we adapted state-of-the-art apatite and zircon production-ejection-diffusion models for application to in-situ dating methods, aiming to enhance the applicability of this technique to a broad range of geologic samples and applications. Our modifications to thermal history models include accommodation of the full range of stopping distances for alpha-particles and cylindrical grain geometries. This investigation focuses on several key aspects of in-situ data interpretation: (i) exploring the relationship between in-situ dates and the position of ablation spots across individual grains, (ii) assessing differences and similarities between whole-grain and in-situ dates, (iii) determining optimal strategies and performance for reconstructing cooling histories from in-situ (U-Th-Sm)/He data, and (iv) reporting the effects of radionuclide zoning on (U-Th-Sm)/He thermochronology. Results indicate that the measured in-situ helium distribution is a function of grain size, ablation spot position and size, and cooling history. Together, these analytical and natural factors result in systematic variations in in-situ dates with distance from the grain rim. Therefore, similar to whole-grain analyses, robust interpretation requires determining grain geometry and the distance of the laser spot to the nearest prismatic face. In most cases, resulting in-situ dates are approximately 30 % older than corresponding whole-grain dates, except for samples exhibiting negligible diffusional helium loss. Reconstruction of cooling histories using in-situ (U-Th-Sm)/He data can be achieved through single measurements in several grains with varying grain size and/or effective uranium content, or within a single grain with measurements taken at different distances from the grain rim. In addition, statistical analysis of a large compilation of measured radionuclide variations in apatite and zircon grains reveals that radionuclide zoning strongly impacts whole-grain analyses, but can be directly measured with the in-situ method. Overall, our results suggest that in-situ measurements for (U-Th-Sm)/He date determination offer a means to extract meaningful cooling signals from samples with poor reproducibility from traditional whole-grain techniques.
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Status: closed
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RC1: 'Comment on gchron-2024-12', István Dunkl, 16 Jul 2024
The manuscript is well written, the structure is logic. It has an important role for the proper interpretation of in-situ, laser ablation-based He-thermochronology.
I think the manuscript can give a kind of orientation rather than explicit solutions for the decay-ejection-diffusion processes.
The authors mention that approaching of the zonation by a two shell model is a simplification. That was for me the most disturbing issue while reading the manuscript. In the every-day practice we see irregular, chaotic patterns and less frequently concentric zonations. The CL patterns and the Raman or LA mapping confront us with the simplified two shell model.
Minor issues:
Line 33: “In most cases, resulting in-situ dates are approximately 30% older than corresponding whole-grain dates, except for samples exhibiting negligible diffusional helium loss.” This statement is in this form misleading. Such a general percentage can’t be determined, it needs explanation, in which conditions calculated the authors this value. Or when the length of the Abstract does not allow, then better to delete this sentence that can easily be the starting point of a misused, and endlessly recycled “urban legend”.
Line 444: alph --> alpha
Line 492: “other r-planes” Explain it, please.
Line 620: “radius of 100 μm radius”
Line 675: The alpha-damage density should also be mentioned.
Lines 687, 690: “C/Ma” --> C/Myr
Lines 730, 736: Tipathy --> Tripathy-Lang
Line 797: ablation bit --> ablation pit
Line 835: “Our observations revealed that radionuclide zoning is not an anomaly but a prevalent
occurrence in both apatite and zircon.” I am afraid it is already a common knowledge. Maybe it does not fit in this form in the conclusions.
Line 840: The issue of the recommended minimum number of grains is important, however, the details of their estimation method is not completely transparent.
Figs. 2, 5, 6: Explain, please, the “Distance” on the X axis. Is it from the center of the grain? Consider a more detailed explanation in the captions – without the text it is difficult the follow.
In Fig. 2 (at the comparison of cylindrical and spherical geometries) it should be also indicated that the “distance” is perpendicular to C-axis.
In Fig. 6 the “distance” in the left and right panels are different.
Fig. 4: (A): add a color scale. (B): To be consequent, and use blue line for the U concentration. Delete Th and Sm from the caption, when the vertical axis indicates only U.
SD1: It is not clear, how the laser ablation ICPMS measurements were done. Did the authors use (i) down-hole (drilling in a spot) analyses or (ii) the zonation is detected across polished grains by LA line analyses. In case of (i) how was considered/compensated the down-hole fractionation?
SD3: It is not clear how the radionuclide heterogeneity was considered at the calculation. Did the authors take the variation detected in the own laboratory or took synthetic, modelled zonation patterns?
Fig. S8: Correct “Fig. X”, please.
The References needs several corrections
Missing:
Brown et al., 2013
Tripahy-Lang et al., 2013
Boyce et al., 2006
Evans et al., 2015
Ketcham et al., 2011
Wolf et al., 1998
Horne et al., 2018
Vermeesch et al., 2012
Dunkl et al., 2024
Flowers and Farley, 2012
Does not cited in the text:
McDowell et al., 2005
Alphabetic order:
Horne / House
Citation: https://doi.org/10.5194/gchron-2024-12-RC1 -
AC1: 'Reply on RC1', Christoph Glotzbach, 19 Aug 2024
We would like to thank both reviewers for their comments and corrections on the original manuscript. Most of the more significant comments deal with updating figures and better explaining our modeling approach. We do not see any larger issue implementing those changes and are optimistic that the manuscript will be a significant contribution to better understand and interpret in-situ (U-Th-Sm)/He data.
RC1: The authors mention that approaching of the zonation by a two shell model is a simplification. That was for me the most disturbing issue while reading the manuscript. In the every-day practice we see irregular, chaotic patterns and less frequently concentric zonations. The CL patterns and the Raman or LA mapping confront us with the simplified two shell model.
AC: We do agree that this two- or multi-shell model is a simplification required to do thermal history modeling in a reasonable time. Full spatial-variable 3D modeling is not practical and would also require additional information than 2D mapping approaches, but 3D synchrotron information. We will add a more detailed discussion on the limitations in thermal history modeling that we are facing in the thermochronological community.
RC1: Line 33: “In most cases, resulting in-situ dates are approximately 30% older than corresponding whole-grain dates, except for samples exhibiting negligible diffusional helium loss.” This statement is in this form misleading. Such a general percentage can’t be determined, it needs explanation, in which conditions calculated the authors this value. Or when the length of the Abstract does not allow, then better to delete this sentence that can easily be the starting point of a misused, and endlessly recycled “urban legend”.
AC: We do think that this is a fundamental difference between whole grain and in-situ dating and should be stated in the abstract, but to prevent misuse we will add details on the conditions under which this will occur.
RC1: Line 444: alph --> alpha
Line 492: “other r-planes” Explain it, please.
Line 620: “radius of 100 μm radius”
Line 675: The alpha-damage density should also be mentioned.
Lines 687, 690: “C/Ma” --> C/Myr
Lines 730, 736: Tipathy --> Tripathy-Lang
Line 797: ablation bit --> ablation pit
AC: Thanks for the detailed corrections, we will change the manuscript accordingly.
RC1: Line 835: “Our observations revealed that radionuclide zoning is not an anomaly but a prevalent occurrence in both apatite and zircon.” I am afraid it is already a common knowledge. Maybe it does not fit in this form in the conclusions.
AC: We do agree that this is common knowledge, but we provided a detailed analyses of our LA-ICP-MS data on this and think that it is still important to report. Whole grain (U-Th-Sm)/He analyses is usually ignoring this and/or interpreting date/eU relations that can be an artefact of zonation.
RC1: Line 840: The issue of the recommended minimum number of grains is important, however, the details of their estimation method is not completely transparent.
AC: Thanks for the clarification, to fill this gap in transparency, we will move some of the details from the supplement data to the main text.
RC1: Figs. 2, 5, 6: Explain, please, the “Distance” on the X axis. Is it from the center of the grain? Consider a more detailed explanation in the captions – without the text it is difficult the follow.
AC: We will change the X axis label to Distance from center of grain in Figs. 2, 5 and 6.
RC1: In Fig. 2 (at the comparison of cylindrical and spherical geometries) it should be also indicated that the “distance” is perpendicular to C-axis.
AC: Good point, that will be added to the figure caption.
RC1: In Fig. 6 the “distance” in the left and right panels are different.
AC: To be consistent we will change the X axis in panels A and C to start with 0 from the center of the grain.
RC1: Fig. 4: (A): add a color scale. (B): To be consequent, and use blue line for the U concentration. Delete Th and Sm from the caption, when the vertical axis indicates only U.
AC: Thanks for the details, we will change the figure accordingly.
RC1: SD1: It is not clear, how the laser ablation ICPMS measurements were done. Did the authors use (i) down-hole (drilling in a spot) analyses or (ii) the zonation is detected across polished grains by LA line analyses. In case of (i) how was considered/compensated the down-hole fractionation?
AC: The data was generated by drilling in a spot and correction for down-hole fractionation was done based on NIST612/610 calibration standards using 43Ca (for apatite) and 29Si (for zircon). We will add the required details to the supplement (SD1).
RC1: SD3: It is not clear how the radionuclide heterogeneity was considered at the calculation. Did the authors take the variation detected in the own laboratory or took synthetic, modelled zonation patterns?
AC: We have used the radionuclide measurements of our own laboratory. We will add this information in the revised manuscript.
RC1: Fig. S8: Correct “Fig. X”, please.
AC: Will be corrected to S7.
RC1: The References needs several corrections
Missing:
Brown et al., 2013
Tripahy-Lang et al., 2013
Boyce et al., 2006
Evans et al., 2015
Ketcham et al., 2011
Wolf et al., 1998
Horne et al., 2018
Vermeesch et al., 2012
Dunkl et al., 2024
Flowers and Farley, 2012
Does not cited in the text:
McDowell et al., 2005
Alphabetic order:
Horne / House
AC: The references will be added or corrected according to the reviewers suggestions.
Citation: https://doi.org/10.5194/gchron-2024-12-AC1
-
AC1: 'Reply on RC1', Christoph Glotzbach, 19 Aug 2024
-
RC2: 'Comment on gchron-2024-12', Rebecca Flowers, 29 Jul 2024
This study modifies apatite and zircon He production and diffusion models so that they can be applied to predict in situ laser-ablation (U-Th)/He dates while accounting for alpha ejection and parent-isotope zonation. This model is then used to compare whole grain and in situ dates, evaluate the effects of parent isotope zonation on data interpretation, and consider the best strategies for acquiring laser-ablation data in order to facilitate thermal history interpretation. Given the number of labs that are now implementing laser-ablation (U-Th)/He methods, this is a timely contribution focused on developing a modeling approach for quantitatively interpreting laser-ablation (U-Th)/He datasets – such tools are currently lacking.
I enjoyed reading this paper and include some suggestions in the manuscript pdf for how it might be further clarified. A few notes here:
You might consider revising the title to better convey the manuscript’s content. Based on the title I had assumed that this paper would present laser-ablation (U-Th)/He data and interpret it, but no laser-ablation (U-Th)/He data are presented or interpreted and with the exception of the eU zonation data this is entirely a modeling study.
It may be helpful to include a first figure to be referred to in the introduction that is a schematic showing why a whole grain date with an alpha-ejected rim will be younger than an in situ date in the core of the grain. This would be effective in explaining the basic concepts to the reader before getting into the details in the rest of the manuscript.
On several of the figures it would be helpful to show the date profiles across the grains to show how the He concentrations relate to the laser-ablation (U-Th)/He dates. For example, on Figures 2, 10, and 11.
Infinite cylinder vs. sphere geometry: In Figure 2, it appears that the dates in each grain's core could be substantially different for the sphere and infinite cylinder models - is that right or not? It’d be helpful to plot the date profiles as suggested above for this reason. Also, what are the whole grain dates for each of these models and how much do they differ for the sphere vs. infinite cylinder models? This would be worth discussing and then better justifying why an infinite cylinder geometry should be used instead of a sphere (especially if the dates are different between the two geometries). The choice of geometry appears to matter a lot more than, for example, whether complete stopping distances are used or not as shown in Figure 5.
I appreciate the modeling exercises in this paper. Are the spot sizes used in the Figures 7, 10, 11 and 12 models analytically reasonable given the He, U, and Th concentrations of the modeled grains? Given real analytical and blank limits? I think this is likely true for the Figure 10 and 11 examples, but am unsure of the others. The He detection limit on the Tuebingen system and how this relates to the youngest grains that can be measured for given pit volumes are noted in Lines 62-67, but it’s unclear if all of the modeling exercises are done for analytically reasonable scenarios. It would be good to explicitly state this for each modeling exercise. Since an important conclusion of the paper is that that in situ (U-Th-Sm)/He dating can improve thermal history interpretations, to make this point I think it’s important to be clear that the theoretical data modeled are analytically feasible.
Given your detailed discussion of the importance of radionuclide zonation effects, including a thermal history modeling exercise (like those in Figures 10 and 11) that includes zonation would be a good addition to this paper. Radionuclide zonation is part of the challenge of laser-ablation (U-Th)/He dating because of alpha redistribution combined with the inability to measure the exact same volumes for daughter and parent. This would make for a slightly more complex, but arguably more realistic, thermal history modeling example.
Becky Flowers, University of Colorado Boulder
-
AC2: 'Reply on RC2', Christoph Glotzbach, 19 Aug 2024
We would like to thank both reviewers for their comments and corrections on the original manuscript. Most of the more significant comments deal with updating figures and better explaining our modeling approach. We do not see any larger issue implementing those changes and are optimistic that the manuscript will be a significant contribution to better understand and interpret in-situ (U-Th-Sm)/He data.
RC2: It may be helpful to include a first figure to be referred to in the introduction that is a schematic showing why a whole grain date with an alpha-ejected rim will be younger than an in situ date in the core of the grain. This would be effective in explaining the basic concepts to the reader before getting into the details in the rest of the manuscript.
AC: We will follow this suggestion and will prepare a schematic figure showing the basic difference between whole-grain and in-situ (U-Th-Sm)/He method and resulting date.
RC2: On several of the figures it would be helpful to show the date profiles across the grains to show how the He concentrations relate to the laser-ablation (U-Th)/He dates. For example, on Figures 2, 10, and 11.
AC: We will provide this information, either as additional sub-figures (e.g. Fig. 2C,D) or in the supplement data.
RC2: Infinite cylinder vs. sphere geometry: In Figure 2, it appears that the dates in each grain's core could be substantially different for the sphere and infinite cylinder models - is that right or not? It’d be helpful to plot the date profiles as suggested above for this reason. Also, what are the whole grain dates for each of these models and how much do they differ for the sphere vs. infinite cylinder models? This would be worth discussing and then better justifying why an infinite cylinder geometry should be used instead of a sphere (especially if the dates are different between the two geometries). The choice of geometry appears to matter a lot more than, for example, whether complete stopping distances are used or not as shown in Figure 5.
AC: The corresponding whole-grain and in-situ dates will be added to the figure after revision. The dates will be largely different, with older dates for the infinite cylinders compared to spheres for similar grain radii. We will also add a few sentences discussing the He profile/date difference between the different geometric models.
RC2: I appreciate the modeling exercises in this paper. Are the spot sizes used in the Figures 7, 10, 11 and 12 models analytically reasonable given the He, U, and Th concentrations of the modeled grains? Given real analytical and blank limits? I think this is likely true for the Figure 10 and 11 examples, but am unsure of the others. The He detection limit on the Tuebingen system and how this relates to the youngest grains that can be measured for given pit volumes are noted in Lines 62-67, but it’s unclear if all of the modeling exercises are done for analytically reasonable scenarios. It would be good to explicitly state this for each modeling exercise. Since an important conclusion of the paper is that that in situ (U-Th-Sm)/He dating can improve thermal history interpretations, to make this point I think it’s important to be clear that the theoretical data modeled are analytically feasible.
AC: This is an important point and we will address this in the revised manuscript. There is a trade-off between the spot size (or better volume) required to achieve a reasonable analytical uncertainty with the in-situ method. What we will do in the revised manuscript, is to report which spot size results in an analytical uncertainty above for instance 10%, likely by reporting those scenarios with a different colour. We can use our lab-specific values for reference, but this will be lab-specific and dependent on the reproducibility of the line-blank He content.
RC2: Given your detailed discussion of the importance of radionuclide zonation effects, including a thermal history modeling exercise (like those in Figures 10 and 11) that includes zonation would be a good addition to this paper. Radionuclide zonation is part of the challenge of laser-ablation (U-Th)/He dating because of alpha redistribution combined with the inability to measure the exact same volumes for daughter and parent. This would make for a slightly more complex, but arguably more realistic, thermal history modeling example.
AC: This is a good idea and we will provide two modeling scenarios where a set of zoned zircons are used for thermal history modeling with the in-situ and whole grain approach. In the first scenario we will have a set of grains with random zonation of radionuclides and in the second scenario we will use a set of grains with common zoning trend (e.g. higher radionuclide concentration of the rim compared to the core).
RC2: Details given in the annotated manuscript pdf.
AC: Thanks for the detailed correction of the manuscript. We went through all comments (see comments by the author above) and minor corrections. We do agree with all reviewers corrections and will revise the manuscript accordingly.
Citation: https://doi.org/10.5194/gchron-2024-12-AC2
-
AC2: 'Reply on RC2', Christoph Glotzbach, 19 Aug 2024
Status: closed
-
RC1: 'Comment on gchron-2024-12', István Dunkl, 16 Jul 2024
The manuscript is well written, the structure is logic. It has an important role for the proper interpretation of in-situ, laser ablation-based He-thermochronology.
I think the manuscript can give a kind of orientation rather than explicit solutions for the decay-ejection-diffusion processes.
The authors mention that approaching of the zonation by a two shell model is a simplification. That was for me the most disturbing issue while reading the manuscript. In the every-day practice we see irregular, chaotic patterns and less frequently concentric zonations. The CL patterns and the Raman or LA mapping confront us with the simplified two shell model.
Minor issues:
Line 33: “In most cases, resulting in-situ dates are approximately 30% older than corresponding whole-grain dates, except for samples exhibiting negligible diffusional helium loss.” This statement is in this form misleading. Such a general percentage can’t be determined, it needs explanation, in which conditions calculated the authors this value. Or when the length of the Abstract does not allow, then better to delete this sentence that can easily be the starting point of a misused, and endlessly recycled “urban legend”.
Line 444: alph --> alpha
Line 492: “other r-planes” Explain it, please.
Line 620: “radius of 100 μm radius”
Line 675: The alpha-damage density should also be mentioned.
Lines 687, 690: “C/Ma” --> C/Myr
Lines 730, 736: Tipathy --> Tripathy-Lang
Line 797: ablation bit --> ablation pit
Line 835: “Our observations revealed that radionuclide zoning is not an anomaly but a prevalent
occurrence in both apatite and zircon.” I am afraid it is already a common knowledge. Maybe it does not fit in this form in the conclusions.
Line 840: The issue of the recommended minimum number of grains is important, however, the details of their estimation method is not completely transparent.
Figs. 2, 5, 6: Explain, please, the “Distance” on the X axis. Is it from the center of the grain? Consider a more detailed explanation in the captions – without the text it is difficult the follow.
In Fig. 2 (at the comparison of cylindrical and spherical geometries) it should be also indicated that the “distance” is perpendicular to C-axis.
In Fig. 6 the “distance” in the left and right panels are different.
Fig. 4: (A): add a color scale. (B): To be consequent, and use blue line for the U concentration. Delete Th and Sm from the caption, when the vertical axis indicates only U.
SD1: It is not clear, how the laser ablation ICPMS measurements were done. Did the authors use (i) down-hole (drilling in a spot) analyses or (ii) the zonation is detected across polished grains by LA line analyses. In case of (i) how was considered/compensated the down-hole fractionation?
SD3: It is not clear how the radionuclide heterogeneity was considered at the calculation. Did the authors take the variation detected in the own laboratory or took synthetic, modelled zonation patterns?
Fig. S8: Correct “Fig. X”, please.
The References needs several corrections
Missing:
Brown et al., 2013
Tripahy-Lang et al., 2013
Boyce et al., 2006
Evans et al., 2015
Ketcham et al., 2011
Wolf et al., 1998
Horne et al., 2018
Vermeesch et al., 2012
Dunkl et al., 2024
Flowers and Farley, 2012
Does not cited in the text:
McDowell et al., 2005
Alphabetic order:
Horne / House
Citation: https://doi.org/10.5194/gchron-2024-12-RC1 -
AC1: 'Reply on RC1', Christoph Glotzbach, 19 Aug 2024
We would like to thank both reviewers for their comments and corrections on the original manuscript. Most of the more significant comments deal with updating figures and better explaining our modeling approach. We do not see any larger issue implementing those changes and are optimistic that the manuscript will be a significant contribution to better understand and interpret in-situ (U-Th-Sm)/He data.
RC1: The authors mention that approaching of the zonation by a two shell model is a simplification. That was for me the most disturbing issue while reading the manuscript. In the every-day practice we see irregular, chaotic patterns and less frequently concentric zonations. The CL patterns and the Raman or LA mapping confront us with the simplified two shell model.
AC: We do agree that this two- or multi-shell model is a simplification required to do thermal history modeling in a reasonable time. Full spatial-variable 3D modeling is not practical and would also require additional information than 2D mapping approaches, but 3D synchrotron information. We will add a more detailed discussion on the limitations in thermal history modeling that we are facing in the thermochronological community.
RC1: Line 33: “In most cases, resulting in-situ dates are approximately 30% older than corresponding whole-grain dates, except for samples exhibiting negligible diffusional helium loss.” This statement is in this form misleading. Such a general percentage can’t be determined, it needs explanation, in which conditions calculated the authors this value. Or when the length of the Abstract does not allow, then better to delete this sentence that can easily be the starting point of a misused, and endlessly recycled “urban legend”.
AC: We do think that this is a fundamental difference between whole grain and in-situ dating and should be stated in the abstract, but to prevent misuse we will add details on the conditions under which this will occur.
RC1: Line 444: alph --> alpha
Line 492: “other r-planes” Explain it, please.
Line 620: “radius of 100 μm radius”
Line 675: The alpha-damage density should also be mentioned.
Lines 687, 690: “C/Ma” --> C/Myr
Lines 730, 736: Tipathy --> Tripathy-Lang
Line 797: ablation bit --> ablation pit
AC: Thanks for the detailed corrections, we will change the manuscript accordingly.
RC1: Line 835: “Our observations revealed that radionuclide zoning is not an anomaly but a prevalent occurrence in both apatite and zircon.” I am afraid it is already a common knowledge. Maybe it does not fit in this form in the conclusions.
AC: We do agree that this is common knowledge, but we provided a detailed analyses of our LA-ICP-MS data on this and think that it is still important to report. Whole grain (U-Th-Sm)/He analyses is usually ignoring this and/or interpreting date/eU relations that can be an artefact of zonation.
RC1: Line 840: The issue of the recommended minimum number of grains is important, however, the details of their estimation method is not completely transparent.
AC: Thanks for the clarification, to fill this gap in transparency, we will move some of the details from the supplement data to the main text.
RC1: Figs. 2, 5, 6: Explain, please, the “Distance” on the X axis. Is it from the center of the grain? Consider a more detailed explanation in the captions – without the text it is difficult the follow.
AC: We will change the X axis label to Distance from center of grain in Figs. 2, 5 and 6.
RC1: In Fig. 2 (at the comparison of cylindrical and spherical geometries) it should be also indicated that the “distance” is perpendicular to C-axis.
AC: Good point, that will be added to the figure caption.
RC1: In Fig. 6 the “distance” in the left and right panels are different.
AC: To be consistent we will change the X axis in panels A and C to start with 0 from the center of the grain.
RC1: Fig. 4: (A): add a color scale. (B): To be consequent, and use blue line for the U concentration. Delete Th and Sm from the caption, when the vertical axis indicates only U.
AC: Thanks for the details, we will change the figure accordingly.
RC1: SD1: It is not clear, how the laser ablation ICPMS measurements were done. Did the authors use (i) down-hole (drilling in a spot) analyses or (ii) the zonation is detected across polished grains by LA line analyses. In case of (i) how was considered/compensated the down-hole fractionation?
AC: The data was generated by drilling in a spot and correction for down-hole fractionation was done based on NIST612/610 calibration standards using 43Ca (for apatite) and 29Si (for zircon). We will add the required details to the supplement (SD1).
RC1: SD3: It is not clear how the radionuclide heterogeneity was considered at the calculation. Did the authors take the variation detected in the own laboratory or took synthetic, modelled zonation patterns?
AC: We have used the radionuclide measurements of our own laboratory. We will add this information in the revised manuscript.
RC1: Fig. S8: Correct “Fig. X”, please.
AC: Will be corrected to S7.
RC1: The References needs several corrections
Missing:
Brown et al., 2013
Tripahy-Lang et al., 2013
Boyce et al., 2006
Evans et al., 2015
Ketcham et al., 2011
Wolf et al., 1998
Horne et al., 2018
Vermeesch et al., 2012
Dunkl et al., 2024
Flowers and Farley, 2012
Does not cited in the text:
McDowell et al., 2005
Alphabetic order:
Horne / House
AC: The references will be added or corrected according to the reviewers suggestions.
Citation: https://doi.org/10.5194/gchron-2024-12-AC1
-
AC1: 'Reply on RC1', Christoph Glotzbach, 19 Aug 2024
-
RC2: 'Comment on gchron-2024-12', Rebecca Flowers, 29 Jul 2024
This study modifies apatite and zircon He production and diffusion models so that they can be applied to predict in situ laser-ablation (U-Th)/He dates while accounting for alpha ejection and parent-isotope zonation. This model is then used to compare whole grain and in situ dates, evaluate the effects of parent isotope zonation on data interpretation, and consider the best strategies for acquiring laser-ablation data in order to facilitate thermal history interpretation. Given the number of labs that are now implementing laser-ablation (U-Th)/He methods, this is a timely contribution focused on developing a modeling approach for quantitatively interpreting laser-ablation (U-Th)/He datasets – such tools are currently lacking.
I enjoyed reading this paper and include some suggestions in the manuscript pdf for how it might be further clarified. A few notes here:
You might consider revising the title to better convey the manuscript’s content. Based on the title I had assumed that this paper would present laser-ablation (U-Th)/He data and interpret it, but no laser-ablation (U-Th)/He data are presented or interpreted and with the exception of the eU zonation data this is entirely a modeling study.
It may be helpful to include a first figure to be referred to in the introduction that is a schematic showing why a whole grain date with an alpha-ejected rim will be younger than an in situ date in the core of the grain. This would be effective in explaining the basic concepts to the reader before getting into the details in the rest of the manuscript.
On several of the figures it would be helpful to show the date profiles across the grains to show how the He concentrations relate to the laser-ablation (U-Th)/He dates. For example, on Figures 2, 10, and 11.
Infinite cylinder vs. sphere geometry: In Figure 2, it appears that the dates in each grain's core could be substantially different for the sphere and infinite cylinder models - is that right or not? It’d be helpful to plot the date profiles as suggested above for this reason. Also, what are the whole grain dates for each of these models and how much do they differ for the sphere vs. infinite cylinder models? This would be worth discussing and then better justifying why an infinite cylinder geometry should be used instead of a sphere (especially if the dates are different between the two geometries). The choice of geometry appears to matter a lot more than, for example, whether complete stopping distances are used or not as shown in Figure 5.
I appreciate the modeling exercises in this paper. Are the spot sizes used in the Figures 7, 10, 11 and 12 models analytically reasonable given the He, U, and Th concentrations of the modeled grains? Given real analytical and blank limits? I think this is likely true for the Figure 10 and 11 examples, but am unsure of the others. The He detection limit on the Tuebingen system and how this relates to the youngest grains that can be measured for given pit volumes are noted in Lines 62-67, but it’s unclear if all of the modeling exercises are done for analytically reasonable scenarios. It would be good to explicitly state this for each modeling exercise. Since an important conclusion of the paper is that that in situ (U-Th-Sm)/He dating can improve thermal history interpretations, to make this point I think it’s important to be clear that the theoretical data modeled are analytically feasible.
Given your detailed discussion of the importance of radionuclide zonation effects, including a thermal history modeling exercise (like those in Figures 10 and 11) that includes zonation would be a good addition to this paper. Radionuclide zonation is part of the challenge of laser-ablation (U-Th)/He dating because of alpha redistribution combined with the inability to measure the exact same volumes for daughter and parent. This would make for a slightly more complex, but arguably more realistic, thermal history modeling example.
Becky Flowers, University of Colorado Boulder
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AC2: 'Reply on RC2', Christoph Glotzbach, 19 Aug 2024
We would like to thank both reviewers for their comments and corrections on the original manuscript. Most of the more significant comments deal with updating figures and better explaining our modeling approach. We do not see any larger issue implementing those changes and are optimistic that the manuscript will be a significant contribution to better understand and interpret in-situ (U-Th-Sm)/He data.
RC2: It may be helpful to include a first figure to be referred to in the introduction that is a schematic showing why a whole grain date with an alpha-ejected rim will be younger than an in situ date in the core of the grain. This would be effective in explaining the basic concepts to the reader before getting into the details in the rest of the manuscript.
AC: We will follow this suggestion and will prepare a schematic figure showing the basic difference between whole-grain and in-situ (U-Th-Sm)/He method and resulting date.
RC2: On several of the figures it would be helpful to show the date profiles across the grains to show how the He concentrations relate to the laser-ablation (U-Th)/He dates. For example, on Figures 2, 10, and 11.
AC: We will provide this information, either as additional sub-figures (e.g. Fig. 2C,D) or in the supplement data.
RC2: Infinite cylinder vs. sphere geometry: In Figure 2, it appears that the dates in each grain's core could be substantially different for the sphere and infinite cylinder models - is that right or not? It’d be helpful to plot the date profiles as suggested above for this reason. Also, what are the whole grain dates for each of these models and how much do they differ for the sphere vs. infinite cylinder models? This would be worth discussing and then better justifying why an infinite cylinder geometry should be used instead of a sphere (especially if the dates are different between the two geometries). The choice of geometry appears to matter a lot more than, for example, whether complete stopping distances are used or not as shown in Figure 5.
AC: The corresponding whole-grain and in-situ dates will be added to the figure after revision. The dates will be largely different, with older dates for the infinite cylinders compared to spheres for similar grain radii. We will also add a few sentences discussing the He profile/date difference between the different geometric models.
RC2: I appreciate the modeling exercises in this paper. Are the spot sizes used in the Figures 7, 10, 11 and 12 models analytically reasonable given the He, U, and Th concentrations of the modeled grains? Given real analytical and blank limits? I think this is likely true for the Figure 10 and 11 examples, but am unsure of the others. The He detection limit on the Tuebingen system and how this relates to the youngest grains that can be measured for given pit volumes are noted in Lines 62-67, but it’s unclear if all of the modeling exercises are done for analytically reasonable scenarios. It would be good to explicitly state this for each modeling exercise. Since an important conclusion of the paper is that that in situ (U-Th-Sm)/He dating can improve thermal history interpretations, to make this point I think it’s important to be clear that the theoretical data modeled are analytically feasible.
AC: This is an important point and we will address this in the revised manuscript. There is a trade-off between the spot size (or better volume) required to achieve a reasonable analytical uncertainty with the in-situ method. What we will do in the revised manuscript, is to report which spot size results in an analytical uncertainty above for instance 10%, likely by reporting those scenarios with a different colour. We can use our lab-specific values for reference, but this will be lab-specific and dependent on the reproducibility of the line-blank He content.
RC2: Given your detailed discussion of the importance of radionuclide zonation effects, including a thermal history modeling exercise (like those in Figures 10 and 11) that includes zonation would be a good addition to this paper. Radionuclide zonation is part of the challenge of laser-ablation (U-Th)/He dating because of alpha redistribution combined with the inability to measure the exact same volumes for daughter and parent. This would make for a slightly more complex, but arguably more realistic, thermal history modeling example.
AC: This is a good idea and we will provide two modeling scenarios where a set of zoned zircons are used for thermal history modeling with the in-situ and whole grain approach. In the first scenario we will have a set of grains with random zonation of radionuclides and in the second scenario we will use a set of grains with common zoning trend (e.g. higher radionuclide concentration of the rim compared to the core).
RC2: Details given in the annotated manuscript pdf.
AC: Thanks for the detailed correction of the manuscript. We went through all comments (see comments by the author above) and minor corrections. We do agree with all reviewers corrections and will revise the manuscript accordingly.
Citation: https://doi.org/10.5194/gchron-2024-12-AC2
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AC2: 'Reply on RC2', Christoph Glotzbach, 19 Aug 2024
Model code and software
Interpreting cooling dates and histories from laser ablation in-situ (U-Th-Sm)/He thermochronometry Christoph Glotzbach and Todd A. Ehlers https://zenodo.org/records/10531763
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