the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 23 Jan 2024
Cosmogenic 3He chronology of postglacial lava flows at Mt. Ruapehu, New Zealand
Abstract. Accurate volcanic hazard assessments rely on a detailed understanding of the timing of past eruptions. While radiometric methods like 40Ar/39Ar or K/Ar are by far the most conventional lava flow dating tools, their low resolution for young (<20 ka) deposits interferes with the development of precise recent effusive chronologies on most volcanoes. Mt. Ruapehu (New Zealand) has produced many lava flows throughout its history, but the precise timing of many recent eruptions remains largely unknown. In this study, we use cosmogenic 3He exposure dating to provide 23 eruption ages of young lava flows at Ruapehu. We then compare our results with existing 40Ar/39Ar and palaeomagnetic constraints, highlighting the value of cosmogenic nuclides exposure dating in refining recent eruptive chronologies. Of the 23 sampled flows, 16 provided robust eruption ages (5–20 % internal 2σ; n≥3) between ca 20 and 8 ka, except for one lava erupted at around 43 ka, and their age distribution indicates that, during the last 20 thousand years, effusive activity at Ruapehu peaked at 17–12 ka and at 9–7.5 ka. Nearly identical eruption ages of lavas located in different flanks of the volcanic edifice suggest concurrent activity from multiple vents during relatively short time intervals (0–2 kyr) at around 13, 10 and 8 ka. We analysed four lavas previously dated by 40Ar/39Ar, two of which yielded eruption ages older than the older limit of the 2σ interval of the radiometric dates, but the good clustering of individual samples from our sites suggests that our results better represent these lava flows’ real eruption ages. Our 3He-based dates show excellent agreement with palaeomagnetic constraints, suggesting that production rate uncertainties are unlikely to impact the accuracy of our eruption ages. This study demonstrates how cosmogenic nuclides dating can provide greater detail on the recent effusive chronology of statovolcanoes, helping to resolve the low resolution and difficulty in applying radiometric dating methods to young lava flows.
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RC1: 'Comment on egusphere-2024-163', Eric Portenga, 28 Feb 2024
Review of Doll et al., submitted to Geochronology.
In their submission, “Cosmogenic 3He chronology of postglacial lava flows at Mt. Ruapehu, New Zealand,” Doll et al. present a vast new dataset of cosmogenic 3He ages of olivine and pyroxene minerals isolated from 23 lava flows circling the summit of New Zealand’s Ruapehu volcano, which belong to 8 previously described members of at least 2 formations. The authors use 3He exposure-age dating because it has greater precision on lava flow ages than 40Ar/39Ar methods, which were previously used to broadly constrain Ruapehu’s lava eruption history since 20 ka. Six of the eight members erupted in the post-glacial late-Pleistocene and Holocene, so constraining Ruapehu’s eruption history through these time periods is important to assess volcanic activity and hazards associated with eruptions.
The study is well organized and detailed and it was an enjoyable read. The authors come to several conclusions and many refinements to Ruapehu’s eruption history and the lithological correlations of disparate lava flows as they are currently known. Overall, I find the Scientific Significance and Scientific Quality to be Excellent and the authors are commended for putting together such a comprehensive and detailed report! However, I believe the Presentation Quality needs some minor reorganization, particularly with regards to repetition of content and narrative presented for each lava flow or Member, which currently feels very disconnected across sections.
I find it really wonderful (maybe it was required, regardless) that the researchers consulted with local Iwi and that this is clearly established at the beginning of the Sample Collection Section. These details may not be necessary to the science presented, but they are necessary to demonstrate proper means of doing geological research in landscapes with Indigenous significance. It is an example we should all strive to follow.
The organization of the remainder of my review follows the list of aspects Referees are asked to consider:
1. Does the paper address relevant scientific questions within the scope of GChron?
Yes. The paper focuses on a suite of 3He ages that facilitate mean ages of 23 lava flows to be determined. The ages of these flows are inspected both through the lens of existing 40Ar/39Ar and paleomagnetic ages of the lava flows. Interpreting new geochronological constraints of lava flows in the context of existing ages determined by other means, for an active and prominent volcano in New Zealand makes this paper relevant for Geochronology. Given the deadly 2019 eruption of Whakaari, reassessments of stratovolcano eruption histories for New Zealand also seems timely.
2. Does the paper present novel concepts, ideas, tools, or data?
Yes. 3He is used because of the availability of olivine and pyroxene in the basaltic andesite to andesitic lava flows found at Ruapehu. 3He ages also provide tighter constraints on lava flow ages than those presently available in the literature; existing ages identify many young flows (<20 kyr), but the authors clearly state and support the need for reanalysis with new dating methods because of the poor accuracy and precision of 40Ar/39Ar ages.
One question on this topic kept coming to mind: How novel is 3He dating on olivines and pyroxenes for stratovolcanoes? The wording in paragraph 2 of the Introduction implies that 3He dating of olivines and pyroxenes for stratovolcanoes is novel, or at least not frequently used, but support from the literature about the scarcity of 3He data would be helpful for readers to understand better the novelty of this approach. In my own quick search of existing literature, it seems like 3He dating has been used on lava flows before, but mainly basaltic lava flows from hot spot or extensional settings, in which case it seems this paper’s application of 3He on andesitic lava flows for a stratovolcano is novel.
3. Are substantial conclusions reached?
Yes. Through their analysis, the Authors identify two periods of heightened volcanic activity at Ruapehu 17–12 ka and 9–7.5 ka. The accuracy of the 3He dating methods for young lava flows also allowed the Authors to propose several reorganizations and associations of lava flows around Ruapehu between the Whakapapa and Mangawhero Formations. Overall, the Authors demonstrate, conclusively, that using 3He exposure-age dating can greatly improve our knowledge of effusive eruption histories of stratovolcanoes.
Building on the novelty of 3He dating for andesitic lava flows, I think the Authors could be stronger with their language about how effective this method can be for dating volcanoes elsewhere or at least constraining better the timing of young eruptions.
4. Are the scientific methods and assumptions valid and clearly outlined?
Yes. The Authors do a great job in the Methods section describing the 3He dating method and all aspects, assumptions, and data required to fully interpret their samples as lava flow exposure ages. The Authors also do a great job in outlining the different analyses that are done on samples from each site.
However, the paper presents new ages for 23 lava flows belonging to eight different Members of two (possibly three) different Formations. It is a lot for the reader to keep track of. With regards to Methods and Assumptions, there is a significant amount of field site/lava flow and sample selection details that ends up in the Results section:
- Tawhainui Flows: Lines 286-287, 290-291
- Mangatoetoenui Flows: Lines 300-301
- Taranaki Falls Flows: Line 316
- Saddle Cone Member: Lines 324-325, 327-328
- Rangataua Member: Lines 340-342
- Paretetaitonga Member: Line 350
- Turoa Member: Lines 356-357, 362-364
- Makotuku Member: Lines 375-377, 384-385
- Mangahuehu Member: Lines 387-391
5. Are the results sufficient to support the interpretations and conclusions?
Yes. The Authors make it very clear what data are used to produce each sample’s exposure age and how individual sample exposure ages are averaged to produce new exposure ages for each dated lava flow. The Authors have clearly considered the strength of each lava flow’s 3He exposure age based on the number of ages comprising the mean age, and the Authors present and support each lava flow’s new exposure age as a mean age or a minimum age, which guides the Discussion of lava flows, consistency of new 3He ages with existing 40Ar/39Ar and paleomagnetic ages, and proposed revisions to the eruption chronology of Ruapehu.
6. Is the description of experiments and calculations sufficiently complete and precise to allow their reproduction by fellow scientists (traceability of results)?
Yes. The Methods are very clearly outlined and detailed with all relevant equations and data provided for recalculation and reproducibility of new exposure ages presented in this study.
7. Do the authors give proper credit to related work and clearly indicate their own new/original contribution?
Yes. Again, because of the number of individual lava flows dated in this study, though, it was difficult to follow along with the narrative, at times, and remember which lava flow was which. To this end, some of the Background information about each lava flow sampled in this study did not appear until later in Results section, and it would have been helpful to have full descriptions of field sites, including previous work, earlier in the paper:
- Tawhainui Flows: Lines 284-286
- Mangatoetoenui Flows: Lines 296, 299-300, 304-305
- Taranaki Falls Flows: Lines 314-316
- Saddle Cone Member: Lines 319-322
- Pinnacle Ridge Member: Lines 332-333
- Rangataua Member: Lines 336-340
- Paretetaitonga Member: Lines 349-350
- Turoa Member: Lines 354-359
- Mangaehuehu Member: Lines 388-389
8. Does the title clearly reflect the contents of the paper?
Yes.
9. Does the abstract provide a concise and complete summary?
Yes. Line 11 suggests 4 lava flows dated with 40Ar/39Ar methods were revisited, but according to Table 1, it seems there are 5 lava flows previously dated with 40Ar/39Ar:
- Mangatoetoenui Flows
- Paretetaitonga Member
- Turoa Member
- Makotuku Member
- Mangaehuehu Member
10. Is the overall presentation well structured and clear?
The paper as written seems to follow the standard Introduction > Methods > Results > Discussion > Conclusion format, but I am not convinced that this is the best way of structuring analysis, reanalysis, and chronological refinement of 23 lava flows. I personally struggled as I read through the paper to keep each flow organized in my head because background, field methods, results, and interpretation of new data from each flow were scattered throughout the paper. Upon finishing the paper and re-reading, I really wonder if a better format for the paper would be to lump the pertinent background, field methods, results, and interpretation of new data for each lava flow into its own mini-narrative set within a broader narrative of the need for higher-resolution age constraints on effusive eruptions at stratovolcanoes? The current Results section is basically organized in this way. In my personal opinion, I think the Authors might consider the following organization:
- Introduction — Leave as is; it is great. Use this to set up the challenge of 40Ar/39Ar dating young lava flows and why (presumably for eruption hazards reasons) it is important to contrain the ages of young flows.
- Geological Background — Keep it focused on Ruapehu and broad scale geological setting. Introduce the readers to the various lava flows and names for initial orientation and introduce previous geochronological work that dates Ruapehu lava flows, but keep it broad for now. Reiterate that other methods are available to date young lava flows.
- Methods — Explain to the reader that specific sample collection for each flow involved its own considerations and focus broadly on what new methods will be applied to all samples collected (i.e. mineral separation, geochemical analyses, measurement of He isotopes, calculation of cosmogenic 3He, determination of exposure ages).
- Specific Lava Flow Details — Here, I would go lava flow by lava flow and present all specific and relevant Background information (i.e. exiting chronology, assumptions of correlation between flows), specific field sampling site considerations, results, and interpretation/discussion of results. Constructing individual narratives for each lava flow would really aid the reader in following along with the importance and significance of each new exposure age. Much of what is presented in Sections 5.1 and 5.2 “Consistency with previous age constraints,” and “Inconsistencies with previous classification of units,” could be brought into individual narratives.
- Discussion — Return to a broader Discussion about how 3He exposure ages really complement existing 40Ar/39Ar ages and highlight the refinements proposed to unit age and classification. Sections 5.3 and 5.4 were great sections that highlight the importance of the datasets overall in better understanding the recent effusive eruption history at Ruapehu and broader applicability of this seemingly novel 3He dating approach.
- Conclusions
The Authors should feel free to agree or disagree with this suggestion, but I think tightening the narrative in this, or another way, would take this paper to the next level.
The only other comment about the structure of the paper is this: The first sentence of the Abstract highlights the importance of having a detailed knowledge of the timing of past eruptions for making accurate volcanic hazards assessments, but the Authors never return to this topic beyond the first paragraph of the Introduction. It leaves me wondering what new guidance can be made about effusive volcanic hazards, at least at Ruapehu, now that we have this new and wonderful 3He dataset? Does the clustering of effusive eruptions between 17-12 ka and 9-7.5 ka mean that Ruapehu is in a quiescent state? Are these age clusters related to specific activity within Ruapehu or the broader tectonic setting of the Taupō Volcanic Zone or Tonga-Kermadec subduction zone? The Authors did such a good job of setting up the need for better chronological constraints on effusive eruptions to supplement existing argon-based chronologies, but the broader significance of new findings seen through the lens of volcanic hazards is missing.
11. Is the language fluent and precise?
Generally, yes. Previous comments highlight the need for some reorganization to reduce redundancies in content, however.
12. Are mathematical formulae, symbols, abbreviations, and units correctly defined and used?
Yes. Methods section is great.
13. Should any parts of the paper (text, formulae, figures, tables) be clarified, reduced, combined, or eliminated?
Possibly. See previous comments.
14. Are the number and quality of references appropriate?
Yes. Perhaps missing some literature on other applications of 3He dating or importance of new data in context of hazards associated with past effusive activity of Ruapehu.
Lines 33-45: This section is a good introduction to TCNs, but I think it is lacking in its support from the large existing body of literature. For example, the assumptions of TCN dating should each be supported by references, as should the statement that 3He diffuses through quartz and feldspars and volcanic groundmass.
15. Is the amount and quality of supplementary material appropriate?
Yes.
Minor Comments:
Line 48: First use of 3Hecos, but at this point the reader does not know there is any other kind, but they will in Section 3.6.1, so it should be defined. Authors could do this in Line 39, “Cosmogenic 3He (3Hecos) is a stable isotope…”
Section 2.1: The authors might find Gabrielson et al. (2018) to be useful: Reflections from an Indigenous Community on Volcanic Event Management, Communications and Resilience (Advances in Volcanology, https://doi.org/10.1007/11157_2016_44)
Figure 1. I do not see any vents on Panel C despite a symbol being listed in the key.
Line 126: A reference to Figure 3 seems to be missing/out of order.
Section 3.6.1: I am not an expert in 3He calculations. The authors have done an excellent job explaining each measurement, its significance, where it comes from, and whether it needs to be addressed or is negligible. Very straightforward and well done.
Lines 256-257: Put “ppm U” after measurement values and do the same for “ppm Th” and “ppm Sm.” It will just make the sentence easier to process.
Lines 266-267: The sentence is oddly worded following “…in Table A3,” and the Authors should revise for clarification.
Section 4.3: Already stated above, but some of these sections include a significant amount of field area background information, rather than new Results. Mini-narratives on each flow (as suggested above) can help with this.
Table 2: The layout and placement of individual Lava Flow Member mean ages and uncertainties is confusing because the summary age appears to be in the 3Hecos column instead of under Exposure Age. It's obvious that the n = value comes from the subsequent rows, so I don't think that is necessary. The table is already long, so perhaps putting the (INT 2 sig) uncertainties in a new row beneath the summary age could help with the alignment of the summary age?
Line 388: Define which ages “These ages…” refers to
Line 402: Might just say that the Taranaki Falls and the Lava Cascade Flows’ 3He ages are not just outside 2σ of Conway et al. (2016) but are older than 40Ar/39Ar ages. Also, it would help to remind readers that the Lava Cascade Flows are part of the Mangatoetoenui Flows of the Iwikau Member of the Whakapapa Formation since “Lava Cascade” is not shown in Table 1.
Lines 401-402: 3Hecos for some reason has a “/” through it here, and only here.
Line 404: Please define the term “rootless nature” of a lava flow. Volcano readers may be familiar but others may not. It’s use a few time throughout the paper suggests it is important for age interpretation or unit correlation, so a definition would be helpful for all readers.
Lines 404-405: I am uncertain of the significance of the second half of this sentence (i.e. “as it precedes…”), both in terms of the order of events, rootless nature of the flow, and the geomorphology. Can the Authors please clarify?
Line 408: Parentheses needed around “Figure 5”
Lines 408-409, 415, 426-427: These are all statements about inconsistencies and seemingly should be discussed in the next section on Inconsistencies?
Lines 453-457: Seems like these two bullet-points could be merged?
Lines 459-460: Consider rewording, “Our 3He based eruption ages allow two periods of enhanced effusive activity since the LGM to be indentified on Ruapehu, during which lava emplacement surrounding the summit of Ruapehu occurred nearly simultaneously.”
Section 5.4: This whole section seems to justify the use of 3He on stratovolcanoes. 3He has been used on lava flows before, so is the significance here that it's never been used on stratovolcanoes? Whether 3He has or has not been used on stratovolcanoes elsewhere should be expressed clearly in the Introduction so that the significance of this new dataset and the novelty of this approach at Ruapehu is clear to the reader. Overall, it seems like this section does not really interpret or discuss any of the new data or geospatial relationships between flows of similar age, so I'm not certain that this is necessary here. The first paragraph seems entirely like Background and the second paragraph seems like a Lit Review on Methods; I would consider what can be moved to Intro or Methods. The third paragraph seems relevant for the Discussion, but statements made are disconnected from the new data presented here. I think this could be strengthened by using new data-driven interpretations about the usefulness of 3Hecos on stratovolcanoes. If 3Hecos hasn't been used on stratovolcanoes before, this certainly is a great study to make the case for its broader use, but it doesn't seem the authors take full advantage of this opportunity.
Line 498: The two age clusters of 17–12 ka and 9-7.5 ka are mentioned here, in the Abstract, and in the first sentence of Section 5.3, but I still do not have a clear sense of the geological significance of these clusters beyond the fact that new ages identify two clusters of activity. If there is anything more the Authors want readers to take away from these two clusters, I think that needs to be more evident in the Discussion.
Citation: https://doi.org/10.5194/egusphere-2024-163-RC1 -
AC1: 'Reply on RC1', Pedro Doll, 09 Mar 2024
We are very grateful for the reviews, comments and suggestions of Eric Portenga (Referee 1), and we believe that addressing them will greatly improve the quality of this manuscript. We use R1 to quote the comments of Referee 1, and AC to indicate the author comments.
First general comment:
R1:“ (…)However, I believe the Presentation Quality needs some minor reorganization, particularly with regards to repetition of content and narrative presented for each lava flow or Member, which currently feels very disconnected across sections.”
AC: We appreciate the general manuscript reorganization suggestion (in particular, results and discussion section). This is an aspect where I (the main author) struggled with at the time of writing, and we agree with the suggestion that a more “lava flow led” presentation (sampled lava flows, previous work, results and short discussion for each site) can help with the narrative, as suggested by the referee in Item 10).
We would appreciate the opinion of the editor on this particular matter before we apply these changes on our manuscript.
Item 2) Does the paper present novel concepts, ideas, tools, or data?
R1: “One question on this topic kept coming to mind: How novel is 3He dating on olivines and pyroxenes for stratovolcanoes? The wording in paragraph 2 of the Introduction implies that 3He dating of olivines and pyroxenes for stratovolcanoes is novel, or at least not frequently used, but support from the literature about the scarcity of 3He data would be helpful for readers to understand better the novelty of this approach. In my own quick search of existing literature, it seems like 3He dating has been used on lava flows before, but mainly basaltic lava flows from hot spot or extensional settings, in which case it seems this paper’s application of 3He on andesitic lava flows for a stratovolcano is novel.”
AC: As the referee pointed, 3He dating has not been frequently used on andesitic stratovolcanoes. We will emphasize this and add supporting literature in the introduction section of our next version of the manuscript, particularly extending the length of the paragraph now ending in line 57. See also "Comment on section 5.4".
Item 3) Are substantial conclusions reached?
R1: “(…) Building on the novelty of 3He dating for andesitic lava flows, I think the Authors could be stronger with their language about how effective this method can be for dating volcanoes elsewhere or at least constraining better the timing of young eruptions.”
AC: We acknowledge that we might have been timid in our wording when writing about the method’s potential to improve eruptive chronologies of andesitic stratovolcanoes globally, so we will extend and modify the discussion section 5.4 (see comment on Section 5.4).
Item 4) Are the scientific methods and assumptions valid and clearly outlined?
R1: “(…)However, the paper presents new ages for 23 lava flows belonging to eight different Members of two (possibly three) different Formations. It is a lot for the reader to keep track of. With regards to Methods and Assumptions, there is a significant amount of field site/lava flow and sample selection details that ends up in the Results section:”
AC: We believe that the “field site/lava flow and sample selection details” now in the results section will seem more natural and in place when we change the results and discussion section to a “lava flow led” section, as suggested by the referee in Item 10).
Item 7) Do the authors give proper credit to related work and clearly indicate their own new/original contribution?
R1: “Yes. Again, because of the number of individual lava flows dated in this study, though, it was difficult to follow along with the narrative, at times, and remember which lava flow was which. To this end, some of the Background information about each lava flow sampled in this study did not appear until later in Results section, and it would have been helpful to have full descriptions of field sites, including previous work, earlier in the paper(…)”
AC: See items 4) and 10)
Item 9) Does the abstract provide a concise and complete summary?
R1: “Yes. Line 11 suggests 4 lava flows dated with 40Ar/39Ar methods were revisited, but according to Table 1, it seems there are 5 lava flows previously dated with 40Ar/39Ar: 1) Mangatoetoenui Flows; 2) Paretetaitonga Member; 3) Turoa Member; 4) Makotuku Member; 5) Mangaehuehu Member”
AC: We understand that there might be a confusion between individual lava flows previously dated with 40Ar/39Ar (i.e. Delta Corner, Taranaki Falls, Lava Cascade and Tukino Slopes-b) and units (groups of flows, i.e. Formations, Members or submembers) that have one or more lavas dated with 40Ar/39Ar as shown in Table 1 (all units of Ruapehu except for the Saddle Cone, Rangataua and Pinnacle Ridge members). Direct comparison between 3He and 40Ar/39Ar dates are only done for the individual flows with both types of ages available. This confusion may be caused by a lack of clarity in our manuscript, and we will change the abstract wording in our next version to address this.
Item 10) Is the overall presentation well structured and clear?
R1:
“In my personal opinion, I think the Authors might consider the following organization:
- Introduction (…)
- Geological Background (…)
- Methods (…)
- Specific Lava Flow Details — Here, I would go lava flow by lava flow and present all specific and relevant Background information (i.e. exiting chronology, assumptions of correlation between flows), specific field sampling site considerations, results, and interpretation/discussion of results. Constructing individual narratives for each lava flow would really aid the reader in following along with the importance and significance of each new exposure age. Much of what is presented in Sections 5.1 and 5.2 “Consistency with previous age constraints,” and “Inconsistencies with previous classification of units,” could be brought into individual narratives.
- Discussion — Return to a broader Discussion about how 3He exposure ages really complement existing 40Ar/39Ar ages and highlight the refinements proposed to unit age and classification. Sections 5.3 and 5.4 were great sections that highlight the importance of the datasets overall in better understanding the recent effusive eruption history at Ruapehu and broader applicability of this seemingly novel 3He dating approach.
- Conclusions (…)”
AC: We appreciate the suggestion in this matter. We believe that this will greatly improve the quality of the manuscript and we will be happy to merge specific site and sample collection details, as well as individual result interpretation and discussion with the results section towards a “lava flow led” type presentation, carefully separating between background, results and interpretations for each flow.
As mentioned, we would appreciate the editor’s opinion in this matter.
R1: “ The only other comment about the structure of the paper is this: The first sentence of the Abstract highlights the importance of having a detailed knowledge of the timing of past eruptions for making accurate volcanic hazards assessments, but the Authors never return to this topic beyond the first paragraph of the Introduction. It leaves me wondering what new guidance can be made about effusive volcanic hazards, at least at Ruapehu, now that we have this new and wonderful 3He dataset? Does the clustering of effusive eruptions between 17-12 ka and 9-7.5 ka mean that Ruapehu is in a quiescent state? Are these age clusters related to specific activity within Ruapehu or the broader tectonic setting of the Taupō Volcanic Zone or Tonga-Kermadec subduction zone? “
We acknowledge that we do not emphasize on the hazard implications of our results, we definitely need to delve into that point with more depth. We will address this as an extension of section 5.3, mentioning (between other points) that these temporal clusters suggest that Ruapehu has the capacity to produce lava flows from multiple vents in short time periods, even after thousands of years from the last large effusive activity occurred.
Item 14) Are the number and quality of references appropriate?
R1: “Yes. Perhaps missing some literature on other applications of 3He dating or importance of new data in context of hazards associated with past effusive activity of Ruapehu.
Lines 33-45: This section is a good introduction to TCNs, but I think it is lacking in its support from the large existing body of literature. For example, the assumptions of TCN dating should each be supported by references, as should the statement that 3He diffuses through quartz and feldspars and volcanic groundmass.”
AC: We appreciate the indication on where more references can help set up the context of 3He applications and volcanic hazards at Ruapehu. We would appreciate, however, a clarification on which assumptions of TCN dating should be supported by more references, as we tried to provide references for each point made. Some references, though, support multiple statements, which might bring some confusion.
Minor comments:
Comment on Section 2.1:
R1: “The authors might find Gabrielson et al. (2018) to be useful: Reflections from an Indigenous Community on Volcanic Event Management, Communications and Resilience”
AC: We had considered it but decided not to add it as it does not directly tackle any statement done. However, we do realise that it might help bring attention to important research linking native communities to the landscape, so we will gladly add it. Note that Hollei Gabrielsen helped us with the permitting process, as stated in the acknowledgements.
Comment on Section 5.4
R1: ” This whole section seems to justify the use of 3He on stratovolcanoes. 3He has been used on lava flows before, so is the significance here that it's never been used on stratovolcanoes? Whether 3He has or has not been used on stratovolcanoes elsewhere should be expressed clearly in the Introduction so that the significance of this new dataset and the novelty of this approach at Ruapehu is clear to the reader. Overall, it seems like this section does not really interpret or discuss any of the new data or geospatial relationships between flows of similar age, so I'm not certain that this is necessary here. The first paragraph seems entirely like Background and the second paragraph seems like a Lit Review on Methods; I would consider what can be moved to Intro or Methods. The third paragraph seems relevant for the Discussion, but statements made are disconnected from the new data presented here. I think this could be strengthened by using new data-driven interpretations about the usefulness of 3Hecos on stratovolcanoes. If 3Hecos hasn't been used on stratovolcanoes before, this certainly is a great study to make the case for its broader use, but it doesn't seem the authors take full advantage of this opportunity.”
AC: We agree that the first and second paragraph will fit better in the introduction. That said, we would be happy to expand on why this study is significant as the first large-scale use of 3He on andesitic stratovolcanoes, showing how our results validate the use of this technique in these environments. However, we are aware that a case-by-case approach needs to be taken into account when applying 3He dating on other stratovolcanoes, which will be addressed too.
AC: We agree with the rest of the minor comments made by Referee 1, and we will address them in the next version of the manuscript.
Citation: https://doi.org/10.5194/egusphere-2024-163-AC1 -
AC2: 'Reply on RC1', Pedro Doll, 09 Mar 2024
Publisher’s note: this comment is a copy of AC1 and its content was therefore removed.
Citation: https://doi.org/10.5194/egusphere-2024-163-AC2
-
RC2: 'Comment on egusphere-2024-163', David Marchetti, 03 Mar 2024
I reviewed the pre-print manuscript “Cosmogenic 3He chronology of postglacial lava flows at Mt Ruapehu, New Zealand” by Pedro Doll and others. I generally find it to be in good shape, and would consider this needing only minor revisions. I applaud the authors for a putting so many ages into one paper! I’ll answer the GChron specific reviewer rubric questions below and then offer broad comments first, shorter thoughts about particular points second, and then line by line comments and edits below that.
1. Does the paper address relevant scientific questions within the scope of GChron?
Yes, certainly does.
2. Does the paper present novel concepts, ideas, tools, or data?
I’ll say Yes, but I would just say that the use of ‘novel’ in the question is a bit anachronistic. The manuscript has a lot of data (exposure ages). I applaud the authors for using all of it in a single paper.
3. Are substantial conclusions reached?
Yes. And they are well supported.
4. Are the scientific methods and assumptions valid and clearly outlined?
For the most part, I’ll mention one possible methodological problem below.
5. Are the results sufficient to support the interpretations and conclusions?
Yes. I think the conclusions are well supported and uncertainties are realistic.
6. Is the description of experiments and calculations sufficiently complete and precise to allow their reproduction by fellow scientists (traceability of results)?
Yes. For the most part. See main comment.
7. Do the authors give proper credit to related work and clearly indicate their own new/original contribution?
In most cases, yes, but given the wide background of work on the 3He method and myriad application papers, some readers may feel that a few instances are lacking more citation. For example, on the use of the crush or magmatic correction and how to best perform a crush.
8. Does the title clearly reflect the contents of the paper?
Yes, dead on.
9. Does the abstract provide a concise and complete summary?
Yes, fine abstract, however I have some edits given below to perhaps make it read better.
10. Is the overall presentation well structured and clear?
I think so, but it’s a lot of data (which is a good thing) and keeping track of the various flows is difficult, not sure how to fix.
11. Is the language fluent and precise?
In most places, line by line edits below to help clarify and smooth the reading.
12, Are mathematical formulae, symbols, abbreviations, and units correctly defined and used?
Yes.
13. Should any parts of the paper (text, formulae, figures, tables) be clarified, reduced, combined, or eliminated?
No, not really. Added too in a few places perhaps.
14. Are the number and quality of references appropriate?
Yes, for the most part.
15. Is the amount and quality of supplementary material appropriate?
Yes. But I would like a little more info on the magmatic correction that I describe below.
Overarching Issue
My main overarching scientific/methodological concern is with the magmatic correction used to generate the cosmogenic 3He values which are then used to determine the exposure ages. The authors did not analyze (crush) any of their samples for magmatic He ratios but rather used previously published 3He/4He ratio data from Patterson et al 1994 (GCA v 58). It should be noted that those data (Patterson et al., 1994) were from fused (heated) mineral separates and not from crushes either. I think the assumption is that since those 1994 samples were not collected for cosmogenic exposure dating (that paper says they are from flow interiors from road cuts, but the source for all samples isn’t clear) that those samples will approximate the non-cosmogenic 3He/4He abundances, and for young flows with minimal nucleogenic 3He and radiogenic 4He, approximate the magmatic correction. Probably a good assumption.
The tight exposure age clusters for most flows as well as the low % of non-cosmogenic 3He, extremely low 4He total values (some have zero 4He, which is interesting), indicate that determining the 3Hecosmo amounts this way is fine. However, this all needs more explanation. For example, where did the 7.5 ± 1.5 x 10-6 3He/4He magmatic ratio come from? Looking at Patterson et al 1994, I can get close to that number (7.58 or 7.6) from averaging the three Waimarino data (they are replicates of the same sample) measured in olivine, and then averaging the Waimarino and Ohakune data (Ohakune from pyroxene – just 1 sample). Ultimately the magmatic ratio used in this (2024) manuscript, which is applying 3He exposure age dating to almost exclusively pyroxenes (save for one olivine sample), is based on the 3He/4He ratio from 1 pyroxene sample averaged with olivines that are really gassy, measured from furnace heating 30+ years ago from rocks likely to not have experienced any cosmogenic exposure? Yes, more explanation please! Perhaps as much as was given to the nucleogenic 3He component which was found to be insignificant.
As I said at the beginning of the last paragraph – this is likely ok, and even really cranking on the magmatic values only changes the resulting 3Hec very slightly and well within the 1sigma uncertainty of the 3Hec values; but when 3Hec and exposure ages of one sample are different from others from the same flow it might be worth mentioning that the largely unconstrained 3He/4He magmatic ratio correction could be a factor. Sometimes there is a ‘hot’, high ratio inclusion that gets released during heating. A simple sensitivity test of the possible effect of having a gassy inclusion with a really high 3He/4He ratio would help. Would only take a few sentences to explain this all. Could just look up the range of 3He/4He magmatic ratios from subduction zone volcanism and extend beyond the stdev used in the 7.5 ± 1.5 E-6 ratio.
A He isotope data presentation suggestion; this could be me being pedantic, but I much prefer 3He/4He ratios to be given relative to air (R/Rair) as well as the true ratio.
Circling back around to the really low 4He total values. That is really interesting, they’re all really low. Those are lower than similar age basalts in many cases? Might be worth mentioning this and suggesting that there is essentially no magmatic gas in almost every sample (except for MA samples – with older crystallization ages; but why not NR samples of the same exposure duration?). In almost every case those pyroxenes (perhaps the key, often less gassy than olivine, certainly so in the Patterson et al data and many other papers that measure both) didn’t pick up but a tiny wiff of magmatic gas. Sorry to go on but the 3He/4He fusion ratios relative to air (Rair) are in the hundreds to thousands for samples with closure ages of like 10 ka. Just really cool.
Secondary Points
- Figure 2 could use at least two more photos. I would actually like to see a more than 4 in the text if allowed by editors? If not allowed, then a bunch more in the appendices maybe? 3He dating is a great tool for determining the timing of young volcanism, showing readers what to sample from andesitic flows is important, and more examples are better
- You mention Figure 4 before first mention of Figure 3 – I think.
- I think Figure 3 is really valuable. It’s great to see a complete understanding of the minerals used in the analyses. In that light I think Figure A1 should be moved out of the appendices and into the text. It won’t take up too much room and helps show a deep understanding of the minerals and rocks.
- At the first mention of the R correction factor I think the authors should briefly explain WHY the R correction factor is essential when determining 3He c values using the magmatic correction.
- For Figure 3. It might be interesting to determine the partition coefficients for these trace elements in the rock/mineral pairs. Could compare to https://kdd.earthref.org/KdD/
- Use of ca This is just me, but in geochronology papers I read ca to mean circa annum or ‘about in time’. I know it can also just mean circa or “about” (and this partially stems from the abbreviation being either c or ca), but it’s off putting for me to see it used relative to distances, like with ‘ca 10 km’, in the same way as dates. This is because when I see the latin 'a’ in a geochronology paper, I read it as annum or year. As a reader I would prefer to see ca just reserved for dates, as a reviewer I understand that usage varies and it’s probably ok as is. If you want to take my route here then the tilde ~ , or “approximately” works great for approximate distances, as used in the manuscript in a few places.
- In Table 2, perhaps another digit in the latitude and longitude data. The elevations are down to the decimeter while the lat/long--which are usually much more precise that z--are coarser?
- Also in Table 2, I don’t like the averaged eruption age being above the data it comes from. I know you may not want to use another line for the average age below the data, as you have a line above the data for the name of the flow, but I think it would be better.
- Figure 5 could be improved a bit. It just looks a little odd. Perhaps just shorten it vertically? So many long lines require moving your head up and down to try and figure it out. Or perhaps better...move part b to a different spot and then extend horizontally and shorten vertically?
- Not sure what this journal wants but some journals want ka to only be used for a determined age, and kyr to only be used for a time duration between two events or ages.
- Table A2, would prefer ’minerals’ to ‘crystals’
- Table A3, the significant figures for the thicknesses seem unrealistic at a tenth of a millimeter? I know you were very precise and used calipers etc. but still, the surface of an aa flow top just can’t realistically be measured to that precise of a thickness across the whole thing.
- Plural possessive apostrophes in many places, like “minerals’ surfaces”. This may be grammatically and stylistically correct but its off putting to me a bit. Could just rewrite most of those sentences so that it’s not needed.
- Table A3 could have clearer explanations for the headings. And I’m not sure I follow the P3 to He nuc argument? The first part of TA3 has a 10^2 in the Pnuc column, the second part of the same table does not? This needs to be fixed and clarified.
Line by line edits
3 - ‘effusive’ is awkward here, could rewrite
14 – maybe change ‘dates’ to chronology?
20 – maybe change to: “and social fabric and livelihoods of local communities”
43 – this line is a bit awkward, could rewrite for clarity.
67 – ‘is formed by’ confuses the relationship, the TVZ has andesitic stratovolcanoes in it, but did those stratovolcanoes form the TVZ? – The TVZ is an idea or grouping of stuff?
69 – replace ‘which has experienced’ with “with”
70 – I would list Tongariro before Ruapehu as you are describing N to S and Rup. before Tong. messes that up.
73 – delete ‘have been’
149 – not a big deal but I prefer concentration before the acid or chemical name, so 5% HF
153 – were the final impurity removals done visually?
156 – would be good to mention how the samples were introduced into the ICP-OES and ICP MS. They were probably dissolved and then introduced?
168 - ‘condensed’ ….I’m not sure He would condense at that T but maybe under low P? Maybe “focused” would be better. And ‘cryogenic’ and ‘cold’ seem redundant next to each other.
173 – was the hot blank done prior to each analysis, or the days analyses? Could clarify.
181 – delete ‘emitted’
186 – ‘was’ should become “were”
222 – could LSD be represented as LSDn, is it the neutron flux monitor based routine? Because LSDn sure seems like a better ‘framework’ than a LSD one….
232 – ‘fully described’…I seem to remember something about that age calculator having self-described terrible documentation….
383 - need to fix the superscripts for Ar
329 - the 'rubbly nature of the flow' argument isn't very convincing
402 - errant / after He
461 - i'd change 'affected' to "occurred in"
481 - i'd change 'comprise' to "include"
496 - i'd add "significantly" before affect
499 - perhaps delete 'We have demonstrated how' and just capitalize "Cosmogenic" to start the sentence.
594 - in the references you have J twice (don't think its JJ) and then Phillips twice?
Citation: https://doi.org/10.5194/egusphere-2024-163-RC2 -
AC3: 'Reply on RC2', Pedro Doll, 09 Mar 2024
We appreciate the highly constructive comments and points raised by David Marchetti as a referee, and we believe that the main issue raised is very fair, so we are more than happy to provide an adequate answer to it. We use R2 to quote the referee’s comments and AC to indicate author’s comments.
Item 7) Do the authors give proper credit to related work and clearly indicate their own new/original contribution?
R2: “In most cases, yes, but given the wide background of work on the 3He method and myriad application papers, some readers may feel that a few instances are lacking more citation. For example, on the use of the crush or magmatic correction and how to best perform a crush.”
AC: We appreciate that the Referee pointed this out. We agree on this, including more detail here will help the reader understand the assumptions and uncertainties that may arise from this. This is related to the main comment raised by Referee 2, see below.
Item 10) Is the overall presentation well structured and clear?
R2: “I think so, but it’s a lot of data (which is a good thing) and keeping track of the various flows is difficult, not sure how to fix.”
AC: Referee 1 suggests a solution to this, see Item 10) from Referee 1.
Overarching issue:
R2: “My main overarching scientific/methodological concern is with the magmatic correction used to generate the cosmogenic 3He values which are then used to determine the exposure ages. The authors did not analyze (crush) any of their samples for magmatic He ratios but rather used previously published 3He/4He ratio data from Patterson et al 1994 (GCA v 58). It should be noted that those data (Patterson et al., 1994) were from fused (heated) mineral separates and not from crushes either. I think the assumption is that since those 1994 samples were not collected for cosmogenic exposure dating (that paper says they are from flow interiors from road cuts, but the source for all samples isn’t clear) that those samples will approximate the non-cosmogenic 3He/4He abundances, and for young flows with minimal nucleogenic 3He and radiogenic 4He, approximate the magmatic correction. Probably a good assumption.
The tight exposure age clusters for most flows as well as the low % of non-cosmogenic 3He, extremely low 4He total values (some have zero 4He, which is interesting), indicate that determining the 3Hecosmo amounts this way is fine. However, this all needs more explanation. For example, where did the 7.5 ± 1.5 x 10-6 3He/4He magmatic ratio come from? Looking at Patterson et al 1994, I can get close to that number (7.58 or 7.6) from averaging the three Waimarino data (they are replicates of the same sample) measured in olivine, and then averaging the Waimarino and Ohakune data (Ohakune from pyroxene – just 1 sample). Ultimately the magmatic ratio used in this (2024) manuscript, which is applying 3He exposure age dating to almost exclusively pyroxenes (save for one olivine sample), is based on the 3He/4He ratio from 1 pyroxene sample averaged with olivines that are really gassy, measured from furnace heating 30+ years ago from rocks likely to not have experienced any cosmogenic exposure? Yes, more explanation please! Perhaps as much as was given to the nucleogenic 3He component which was found to be insignificant.
As I said at the beginning of the last paragraph – this is likely ok, and even really cranking on the magmatic values only changes the resulting 3Hec very slightly and well within the 1sigma uncertainty of the 3Hec values; but when 3Hec and exposure ages of one sample are different from others from the same flow it might be worth mentioning that the largely unconstrained 3He/4He magmatic ratio correction could be a factor. Sometimes there is a ‘hot’, high ratio inclusion that gets released during heating. A simple sensitivity test of the possible effect of having a gassy inclusion with a really high 3He/4He ratio would help. Would only take a few sentences to explain this all. Could just look up the range of 3He/4He magmatic ratios from subduction zone volcanism and extend beyond the stdev used in the 7.5 ± 1.5 E-6 ratio. ”
AC: We acknowledge that we have not explained enough:
-How we obtained the magmatic ratio used for our corrections, the fact that it is largely unconstrained and that carries a significant uncertainty.
-Why this uncertainty has a small effect in our final results.
That said, we appreciate this being raised by the referee. Similarly, Referee 3 mentions that: “(…) this is reasonable, in this case, but would not necessarily work at lower elevations, or for younger lava flows (…)”
Our coauthors from CRPG performed last week three crushed analyses of samples MA058; WG326 and DC329 . They obtained very low He concentrations (3He 3-9x104 at/g; 4He 0.3-2.6x1010 at/g) and hence large associated uncertainties, which supports the relatively small impact on the magmatic correction to the calculated 3Hecos from our samples .
These analyses resulted in 3He/4He ratios of 13±5; 7±4 and 3±1 Ra, the latter being from a MA sample (erupted at, at least, 55 ka) which may be affected by radiogenic 4He contamination, so its real magmatic 3He/4He ratio is probably larger than 3 Ra.
These results are consistent with the magmatic correction we previously applied, but indicate that we need, as the Referee 2 suggested, to extend the uncertainty of the used magmatic ratio.
Considering the new crushed data and the data from Patterson et al (1994), we obtained an uncertainty-weighted magmatic ratio of 4.8 Ra (6.6x10-6), within error of the ratio used in the first version of the manuscript. Hence, we would prefer to modify the ratio used from 5,4±1.2 to 4.8±1.9 Ra (1sigma extended from 20 to 40%), This change will have a minor impact in the calculated 3Hecos, and the derived ages, while encompassing a more realistic estimate of the uncertainty arising from the magmatic correction.
We will add the new crushed data and all calculations made for the magmatic ratio as a new supplementary file (see file attached).
Additionally, we will add the description of the helium crushing analyses in the methods section and their results in table 2.
We will also show why the large magmatic ratio uncertainty has a small impact in our results. We will add the sensitivity test suggested by Referee 2, showing what would be the effect of a sample carrying a higher (or lower) 3He/4He magmatic ratio (within reasonable limits based on our measurements and on literature).
Secondary points
R2: “Figure 2 could use at least two more photos. I would actually like to see a more than 4 in the text if allowed by editors? If not allowed, then a bunch more in the appendices maybe? 3He dating is a great tool for determining the timing of young volcanism, showing readers what to sample from andesitic flows is important, and more examples are better”
AC: We agree with this point (also raised by Referee 3). One of the main takeaway points of this (2024) manuscript is that 3He dating can be effectively applied in andesitic 'a'ā flows, and showing what kind of surfaces are suitable for this method is key. We would appreciate a comment of the editor on this matter, but we will add more photos from more sites as a supplementary file regardless.
R2: “I think Figure 3 is really valuable. It’s great to see a complete understanding of the minerals used in the analyses. In that light I think Figure A1 should be moved out of the appendices and into the text. It won’t take up too much room and helps show a deep understanding of the minerals and rocks.”
AC: Thank you. We had Figure A1 together with the rest of Figure 3 in a previous version of the manuscript, and we decided to move it to the appendices for some reason. If the editor agrees, we’d happily add it as Figure 3a
R2: “Use of ca This is just me, but in geochronology papers I read ca to mean circa annum or ‘about in time’. I know it can also just mean circa or “about” (and this partially stems from the abbreviation being either c or ca), but it’s off putting for me to see it used relative to distances, like with ‘ca 10 km’, in the same way as dates. This is because when I see the latin 'a’ in a geochronology paper, I read it as annum or year. As a reader I would prefer to see ca just reserved for dates, as a reviewer I understand that usage varies and it’s probably ok as is. If you want to take my route here then the tilde ~ , or “approximately” works great for approximate distances, as used in the manuscript in a few places.”
AC: We appreciate this detail, we think this can help with the clarity and the precision of the vocabulary/terminology used.
R2: “In Table 2, perhaps another digit in the latitude and longitude data. The elevations are down to the decimeter while the lat/long--which are usually much more precise that z--are coarser?”
AC: Yes, lat/long resolution is better than the elevation data, but the latter has a much larger influence in production rates, hence our choosing of decimal places. However, having another digit for lat/long values may help identify the exact sampling place easier, if future researchers so wish. We’ll consider adding another digit in the final version, assessing the width of the table (as we will add R/Ra values as per requested by Referee 3).
R2: “Figure 5 could be improved a bit. It just looks a little odd. Perhaps just shorten it vertically? So many long lines require moving your head up and down to try and figure it out. Or perhaps better...move part b to a different spot and then extend horizontally and shorten vertically?”
AC: We do not find a better place to locate subfigure b, but we made the figure vertically shorter and so it will be included in the next version of the manuscript.
R2: “Not sure what this journal wants but some journals want ka to only be used for a determined age, and kyr to only be used for a time duration between two events or ages.”
AC: That is what we wanted to do, we will double check in all uses of the abbreviations to be sure that is what we use them for.
R2: “Table A3 could have clearer explanations for the headings. And I’m not sure I follow the P3 to He nuc argument? The first part of TA3 has a 10^2 in the Pnuc column, the second part of the same table does not? This needs to be fixed and clarified.”
We apologise and appreciate this finding. Both headings should have a 10^2 factor in the Pnuc column, and we will fix this. I consider that moving the “closure age” column before the “Pnuc” column will help make the reading easier, as the closure ages*Pnuc=Henuc.. P3 and P4 are used for the R factor.
R2: “168 - ‘condensed’ ….I’m not sure He would condense at that T but maybe under low P? Maybe “focused” would be better. And ‘cryogenic’ and ‘cold’ seem redundant next to each other.”
We do not prefer “focused” over “condensed”. The trap works indeed under very low pressures, which we can mention in that line.
R2: “Line 222 – could LSD be represented as LSDn, is it the neutron flux monitor based routine? Because LSDn sure seems like a better ‘framework’ than a LSD one…”
AC: We acknowledge that the CREp calculator uses the Lifton et a. (2014) approach of “take into account specific cross sections for each particle, nuclide and target element” (Martin et al, 2017). However, LSDn is not formerly described in Martin et al (2017) nor mentioned in the CREp website, and the mention of a geomagnetic database with a different name than that of the CREp might raise confusion.
AC: We agree with the rest of the comments and suggestions provided by Referee 2, and we will address them in the final version of the manuscript.
-
RC3: 'Comment on egusphere-2024-163', Mark Kurz, 04 Mar 2024
This is an interesting paper that presents important new cosmogenic 3He measurements from lava flows at Mt. Ruapehu, New Zealand. It will be of interest to isotope geochemists, volcanologists, and also the wider community interested in volcanic hazards, particularly because dating of young lava flows is so difficult. The number of measurements presented here is impressive, as is the number of samples per lava flow, so this is a prodigious effort. The new 3He age determinations are compared to previous age determinations, using other methods, which is rare in the literature and therefore quite useful.
Overall, this paper significantly improves the eruption chronology of this important volcano, nicely demonstrates the utility of cosmogenic 3He in dating lava flows, and should definitely be published. I do have some suggestions that I think would improve the manuscript if addressed:
- It is implied, throughout the paper, that the main geochronological tools for determining lava flow ages are Ar-Ar and K-Ar, as the “most conventional” methods. This ignores the important work that has been done by radiocarbon dating of charcoal beneath lava flows. This is not necessarily a common method, because it takes a lot of effort to find charcoal underneath lava flows, which is scarce in some environments. However, it is probably one of the best methods for lava flows in the age range discussed here. This method has been widely used in Hawaiian volcanoes, and was used in several of the papers cited (Trusdell, 1995; Sherrod et al., 2006). As it is written, one would get the impression that radiocarbon is not a viable method, which I think is misleading. I recommend that the authors at least mention radiocarbon dating, even briefly, in the text. Are there any lava flow radiocarbon dates from this region, aside from the sediments that are mentioned, and did the authors consider it? Is there some reason that charcoal is not plausible here, perhaps the high altitudes?
-In the section on calculation of cosmogenic 3He (pages 9-10), the discussion could be clearer on one important point. The samples analyzed here all come from fairly high elevations (> 1500 meters), so cosmogenic 3He is dominant compared to magmatic helium, for this suite of samples (i.e. this age range at this elevation). Therefore, magmatic 3He/4He is a small contribution to uncertainties. For this reason, the authors did not crush the samples in vacuum as a means of estimating magmatic helium contributions, they simply used previous measurements of the magmatic 3He/4He. I think this is reasonable, in this case, but would not necessarily work at lower elevations, or for younger lava flows. I recommend that the authors state this more clearly, so that future researchers are not led astray. One way to illustrate this would be to just point out that the magmatic 3He/4He assumed is ~ 5.4 Ra, and the measured (total fusion) values obtained here are > 200 Ra. It would also help if the measured 3He/4He values were given in table 2, and perhaps give the range of 3He/4He mentioned in the text.
-In the methods section, the authors should state clearly what was used as running standard, and how reproducible they were, as well as values and reproducibility for secondary standards (HESJ and CRONUS-P).
Figure 1. Is the geological map and age assignments based on the new data presented here, or the previous work?
Table 1. This table should make clearer what ages have been revised with the new 3He ages. It is difficult to compare the new ages with the previous constraints, so the table is confusing as it stands. How about a separate column for 3He ages?
Figure 2 and text on Page 7. This figure shows two examples of sampled sites. I suggest providing the sample numbers in the caption. Are these typical? It would be helpful if there was more discussion of the features that were sampled. This is mentioned in the text but only in a general sense. I suggest providing photos of the “representative” range of features that were sampled, in the supplemental information. In the cases where there is good agreement for one flow, were all the features sampled similar? Do sampling site characteristics contribute to uncertainties? What can be learned from the large number of samples collected?
Page 8. How big are the topographic corrections?
Line 170: replace “R/Ra = 20.63” with “3He/4He = 20.63 R/Ra”. R/Ra is the unit, not the parameter measured.
Page 11 and Figure 3. The trace element measurements (Th, U, Sm) in the minerals were determined by ICP methods, which means that some amount of mineral separate was dissolved. This should be stated and more details provided (how much mineral and what was the procedure?). A tiny amount of host rock contamination within the mineral separates, either on the surfaces or as melt inclusions, would dramatically increase the measured concentrations. I therefore suspect that the measured mineral concentrations are actually maximum values. This should be discussed in the text.
Figure 3. How do the pyroxene compositions presented here, and in the table, compare to compositions of previous samples used for 3He determinations and production rate determinations, in the literature? Is there any impact of composition on 3He production rates?
Figure 4. I found this figure to be quite hard to read, and recommend enlarging the fonts or the individual figures to make them easier to read.
Citation: https://doi.org/10.5194/egusphere-2024-163-RC3 -
AC4: 'Reply on RC3', Pedro Doll, 09 Mar 2024
We are thrilled to receive such positive reviews from one of the pioneers of cosmogenic 3He, and we are believe that addressing the points raised will contribute to have a better final manuscript.
R3: “- It is implied, throughout the paper, that the main geochronological tools for determining lava flow ages are Ar-Ar and K-Ar, as the “most conventional” methods. This ignores the important work that has been done by radiocarbon dating of charcoal beneath lava flows. This is not necessarily a common method, because it takes a lot of effort to find charcoal underneath lava flows, which is scarce in some environments. However, it is probably one of the best methods for lava flows in the age range discussed here. This method has been widely used in Hawaiian volcanoes, and was used in several of the papers cited (Trusdell, 1995; Sherrod et al., 2006). As it is written, one would get the impression that radiocarbon is not a viable method, which I think is misleading. I recommend that the authors at least mention radiocarbon dating, even briefly, in the text. Are there any lava flow radiocarbon dates from this region, aside from the sediments that are mentioned, and did the authors consider it? Is there some reason that charcoal is not plausible here, perhaps the high altitudes?”
AC: This is an excellent point that I (the main author) missed. It will definitely be adequate to mention the method and recognise the vast amount of research done with it.
Regarding its use in the area: Besides the mass deposits mentioned in the manuscript, only pyroclastic deposits have been dated in this area with radiocarbon. As the referee suggests, the elevation (and lack of vegetation in the area close to the vents) prevents the use of this method for dating lavas at Ruapehu. It is important to note, however, that the treeline now at approximately 1200 masl on the western half of the edifice (see Figure 1) would have likely been lower at the time of most of these eruptions, when climate conditions were colder. This considered, this (2024) manuscript is an example on how 3He can be valuable in areas where 14C is not applicable.
R3: “-In the section on calculation of cosmogenic 3He (pages 9-10), the discussion could be clearer on one important point. The samples analyzed here all come from fairly high elevations (> 1500 meters), so cosmogenic 3He is dominant compared to magmatic helium, for this suite of samples (i.e. this age range at this elevation). Therefore, magmatic 3He/4He is a small contribution to uncertainties. For this reason, the authors did not crush the samples in vacuum as a means of estimating magmatic helium contributions, they simply used previous measurements of the magmatic 3He/4He. I think this is reasonable, in this case, but would not necessarily work at lower elevations, or for younger lava flows. I recommend that the authors state this more clearly, so that future researchers are not led astray. One way to illustrate this would be to just point out that the magmatic 3He/4He assumed is ~ 5.4 Ra, and the measured (total fusion) values obtained here are > 200 Ra. It would also help if the measured 3He/4He values were given in table 2, and perhaps give the range of 3He/4He mentioned in the text.”
AC: We agree with the referee in this matter. As indicated in the answer to the “overarching issue” point of Referee 2, we will address this in detail, providing new crushed 3He/4He data (which have large uncertainties associated due to the low total 3He and 4He values from crushed minerals), emphasizing that such big uncertainties will not work in all cases. We will, as suggested, add measured 3He/4He values in table 2 and mention the obtained ranges in the text. See supplementary file on comment "Repply on RC2".
R3: “Figure 1. Is the geological map and age assignments based on the new data presented here, or the previous work?“
AC: This figure only shows previous data (summarised by Townsend et al. 2017), we will clarify this in the figure’s legend
R3: “Table 1. This table should make clearer what ages have been revised with the new 3He ages. It is difficult to compare the new ages with the previous constraints, so the table is confusing as it stands. How about a separate column for 3He ages?”
AC: We realise that this table might raise confusion. A separate column for 3He ages is a good idea (or maybe merging this new column with the last column of the old table), and we will attempt to do this while keeping this table as a one-page table.
R3: “Figure 2 and text on Page 7. This figure shows two examples of sampled sites. I suggest providing the sample numbers in the caption. Are these typical? It would be helpful if there was more discussion of the features that were sampled. This is mentioned in the text but only in a general sense. I suggest providing photos of the “representative” range of features that were sampled, in the supplemental information. In the cases where there is good agreement for one flow, were all the features sampled similar? Do sampling site characteristics contribute to uncertainties? What can be learned from the large number of samples collected?”
AC: Similarly the point raised by Referee 2, we acknowledge that further detail on the sampled surfaces will be very beneficial. Showing the range of features we sampled can help both understand possible problems/contribution to uncertainties and lead future researchers to better results. We will include this in the supplementary material, and highlight those samples which did not yield consistent results with other of the same sites.
R3: “Page 8. How big are the topographic corrections?”
AC: All topographic corrections can be found in table 2 under the column (“shielding factor”). We will indicate this in the text. If the Referee 3 strongly prefers to include this range in page 8 (methods section), we will add it.
R3: “Page 11 and Figure 3. The trace element measurements (Th, U, Sm) in the minerals were determined by ICP methods, which means that some amount of mineral separate was dissolved. This should be stated and more details provided (how much mineral and what was the procedure?). A tiny amount of host rock contamination within the mineral separates, either on the surfaces or as melt inclusions, would dramatically increase the measured concentrations. I therefore suspect that the measured mineral concentrations are actually maximum values. This should be discussed in the text.”
AC: This is a very good point, we will add more detail on the methodology of the ICP prep. In section 4.1, we will indicate that trace elements values measured in the minerals represent maximum due to the possibility of groundmass and/or melt inclusion contamination that may be not accounted for at the time of measurement.
R3: “Figure 3. How do the pyroxene compositions presented here, and in the table, compare to compositions of previous samples used for 3He determinations and production rate determinations, in the literature? Is there any impact of composition on 3He production rates?”
AC: Apologies in advance for the lengthy response. This is an interesting comment, and very relevant for future 3He research.
Based on the literature presented below, we do not want to delve into this discussion in this (2024) manuscript, as we believe this discussion can (and should) be the centre of future research.
Thad said, we could mention that our concentrates have higher contents of orthopyroxene that those of Blard (2006) and higher clinopyroxene contents than those of Eaves et al (2015).
To the best of our knowledge, there is still no conclusive evidence that pyroxene compositions impact the 3He production rates (see review in Blard, 2021). Not all calibration data sets include mineral chemistry, and global estimates incorporate pyroxene, olivine and amphibole data.
We acknowledge that Ackert et al (2003) and Fenton et al (2009) suggested slightly higher production rates for olivines compared to clinopyroxenes, but this effect is almost statistically insignificant.
In a more recent study, Delunel et al (2016) found no significant difference between the production rates calculated for clinopyroxenes and orthopyroxenes. Additionally, Fenton and Niedermann (2014), as previous data from Blard et al (2006), provided results implying that olivine and pyroxenes have similar amounts of 3Hecos.
At some point, we thought about performing several analyses of the same samples after separating clinopyroxenes and orthopyroxenes, to test this, as we managed to obtain a relatively large (3-5 gr in most samples) amount of pure pyroxenes for each sample. This was, although interesting, not the main focus of this research, and due to the large number of samples I (main author) preferred to leave that test for future work. Instead, we measured our mixture of clino- and orthopyroxene (see paragraph in lines 251-254 of the submitted pdf).
When comparing to calibration sites using pyroxenes that inform their compositions, we found that our samples have:
-Higher Al than minerals analysed by Eaves et al (2015). Enstatite/Ferrosilite ratios are similar, ours being closer to the Diopside-Hedenbergite endmember.
-Higher Mg, Fe and lower Al and Ca content than pyroxenes analysed by Blard et al (2006). That means that pyroxenes studied by Blard et al (2006) are closer to the Diopside-Hedenbergite endmember.
R3: “Figure 4. I found this figure to be quite hard to read, and recommend enlarging the fonts or the individual figures to make them easier to read.”
AC: This is an issue I (main author) faced before and hoped to have solved with enlarging the fonts once before. I tried to find a balance between large enough for easy reading and small enough that the text does not cover a significant amount of map. I will re-assess this balance and increase the font size for the next version.
AC: We agree with the rest of the minor comments of Referee 3 and will address them.
Citation: https://doi.org/10.5194/egusphere-2024-163-AC4
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AC4: 'Reply on RC3', Pedro Doll, 09 Mar 2024
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