TephraNZ: a major- and trace-element reference dataset for glass-shard analyses from prominent Quaternary rhyolitic tephras in New Zealand and implications for correlation

Hopkins, Jenni L.; Bidmead, Janine E.; Lowe, David J.; Wysoczanski, Richard J.; Pillans, Bradley J.; Ashworth, Luisa; Rees, Andrew B. H.; Tuckett, Fiona

doi:https://doi.org/10.5194/gchron-3-465-2021

Articles | Volume 3, issue 2

https://doi.org/10.5194/gchron-3-465-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/gchron-3-465-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 3, issue 2

Research article

|

23 Sep 2021

Research article |

| 23 Sep 2021

TephraNZ: a major- and trace-element reference dataset for glass-shard analyses from prominent Quaternary rhyolitic tephras in New Zealand and implications for correlation

Jenni L. Hopkins, Janine E. Bidmead, David J. Lowe, Richard J. Wysoczanski, Bradley J. Pillans, Luisa Ashworth, Andrew B. H. Rees, and Fiona Tuckett

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Final revised paper (published on 23 Sep 2021)
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Preprint (discussion started on 09 Nov 2020)
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Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Formal review with emphasis on analytical data (Maxim Portnyagin)', Maxim Portnyagin, 02 Dec 2020
- AC1: 'Reply to RC1', Jenni Hopkins, 03 Mar 2021
RC2: 'Review of gchron-2020-34', Anonymous Referee #2, 23 Dec 2020
- AC2: 'Reply to RC2', Jenni Hopkins, 03 Mar 2021
RC3: 'comment on TephraNZ', Stephen Kuehn, 08 Jan 2021
- AC3: 'Reply to RC3', Jenni Hopkins, 03 Mar 2021

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Publish subject to revisions (further review by editor and referees) (23 Mar 2021) by Britta Jensen

AR by Jenni Hopkins on behalf of the Authors (01 Jun 2021) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (07 Jun 2021) by Britta Jensen

RR by Maxim Portnyagin (09 Jun 2021)

Suggestions for revision or reasons for rejection

Unfortunately, I have not received a detailed response to my previous comments.
On this iteration, I have checked only analytical techniques and tables. I see that the authors made some efforts towards publication. However, more detailed description of the analytical techniques revealed new problems.
EMPA
The authors provide more detailed description of their EMP analyses now. This description shows some fundamental problems, which were not obvious before. The major problem is that they used rhyolite glass VG568 for calibration of ALL elements, including Ti, Mn, Mg and Cl, which all occur in VG568 in concentrations below 0.1 wt%! Therefore, the authors obtained large variability of data for secondary standards, where these elements are more abundant. The best example is MgO in basaltic glass VGA99 (Figure in SM Table 3). The measured concentrations range from 4 to 6 wt%! Extremely poor accuracy. In essence, this figure illustrates that calibration of MgO on VG568 was not precise and varied between 7 sessions, as expected for such low concentration in standard and therefore imprecise peak positioning. Similar problems are evident for Mn, Ca, Ti, all minor elements poorly characterized in rhyolite.
The authors decided to use data for VG568 from Streck and Wacaster (2006) as reference values (“Streck” – not “Stracke“ in tables). Why? Simply because Streck and Wacaster reported some data for minor elements. These are not certified values. Moreover, these authors used 10nA and 5 um beam to analyze rhyolite glass, giving high electron beam density, and not surprisingly their Na2O is even lower than reported by Jarosevich et al. (1980). Na2O is also lower for secondary standard ATHO-G, and it must be higher as it has been discussed in literature already and mentioned in our previous reviews and by the editor. I see that counting times for elements are not reported in tables. This was requested.
Thus, the data suffer from inadequate calibration and imprecise standard values. Normally, I would recommend rejection of manuscript which reported such inadequate EMPA technique. However, I understand that most glasses studied in this work have very low Ti, Fe, Mn, Mg and Cl, and the accuracy of the analyses reduced to 20 rel.% for some of these elements is comparable with single point precision. These elements seem not very important for the regional systematics. The analyses are not perfect but, I guess, still usable for correlation of tephras in this region.
I suggest the authors to do next iteration of their corrections. Si, Al, Na, K should be re-calculated using reference data from Jarosevich et al. (1980) or even better ATHO-G, which is dry glass less affected by beam damage. Ti, Fe, Mg, Mn must be corrected using basaltic glass VGA99, where these elements are much more abundant. Cl can be kept as it is or slightly corrected using data from Portnyagin et al (2020) who reported average of c.2700 analyses obtained using Cl-rich scapolite as standard. What are these strange numbers written for “Cl” in Table SM6?
Na2O is definitely too low in the entire dataset because of too low accepted Na2O in primary standard after Streck and Wacaster. Correction using values from Jarosevich et al. (1980) for VG568 or GEOREM for ATHO-G data will bring the results to the lower range of accepted values. Lowe et al. (2017, page 8) – some co-authors of this manuscript- mentioned that GEOREM Na2O=3.75% for ATHO is likely too low. Portnyagin et al. (2020) suggested Na2O =4.1-4.2% in ATHO based on large set of EMP and LA-ICP-MS data. The authors may consider these results for further correction.
LA-ICP-MS.
There is still poor agreement between Ti in EMP and La-ICP-MS data. The authors probably misunderstood that the data falling out from the error envelope must be excluded from tables. This was not done. The data for Ti in standards are missing (SM Table 4), but I assume that 20 rel % is acceptable deviation. Larger deviation is symptomatic of contamination of analysis by mineral phases. The data should be excluded from tables.
Sc data for secondary standards is strongly affected by interference with SiO, as already said before and unevoidable at such high ThO/Th ratios. This is why they have up to 80 rel% (5 ppm) deviation from reference data for StHs6/80. This data should not be reported. The authors should admit that they likely had strong addition from SiO but because their samples have similar SiO2 content with ATHO, the data can still be usable. In other words, the relative contribution from SiO is similar for ATHO and unknown rhyolite samples. The data is however not usable for samples with lower SiO2 such as BHVO-2G or StHs60/8-G. Perhaps, other authors also reported ThO/Th>1%. It does not mean that this is good practice and that Sc data is not affected by oxides. It means that the authors run instrument with not optimal conditions and could generate poor results for some elements.

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RR by Stephen Kuehn (22 Jun 2021)

ED: Publish subject to minor revisions (further review by editor) (25 Jun 2021) by Britta Jensen

AR by Jenni Hopkins on behalf of the Authors (19 Jul 2021) Author's response Author's tracked changes Manuscript

ED: Publish as is (21 Jul 2021) by Britta Jensen

ED: Publish as is (22 Jul 2021) by Philippa Ascough (Editor)

AR by Jenni Hopkins on behalf of the Authors (23 Jul 2021) Author's response Manuscript

Short summary

Here we present the foundation dataset for TephraNZ, a formal, comprehensive, open-access reference dataset of glass-shard compositions for New Zealand tephras. We geochemically characterise 45 eruptive episodes ranging from Kaharoa (ca. 636 cal yr BP) to the Hikuroa Pumice member (ca. 2.0 Ma) from six or more caldera sources, most from the central Taupō Volcanic Zone.