Original Calibration of a Garnet Geobarometer in Metapelite
Round 1
Reviewer 1 Report
Review of: Original calibration of a garnet geobarometer in metapelite
This is an interesting paper that has potentially very high impact. As the author writes, Ca-poor pelites are common in metamorphic terranes and conventional barometry using net transfer reactions is difficult. Many studies then apply phase diagram modeling to constrain P-T conditions, but alternative techniques, as described herein, would be useful.
I have two general questions. Firstly, have the effects of Fe3+ been considered in calibration of the geobarometer? Not all iron in garnet is Fe2+, and of course ilmenite can have substantial substitution towards the hematite end-member. The bottom line is that metapelites are not entirely reduced and anybody who wants to use this new geobarometer will want to know the uncertainty associated with variable oxidation state. I see there is brief mention of this in lines 99-102, but this does not explain these issues entirely.
Secondly, titanite is not exceptionally common in pelites. At low-grade conditions, hematite and rutile commonly form, and transition to ilmenite and magnetite at higher grade. What proportion of the samples used for calibration contained titanite? How would the absence of titanite affect this geobarometer?
Author Response
I have two general questions. Firstly, have the effects of Fe3+ been considered in calibration of the geobarometer? [REPLY ‒ Yes, the effects of Fe3+ have been considered in calibration of the geobarometer, and this is concisely described in Lines 99—101. In fact, the Fe3+ quantity of garnet is quite small] Not all iron in garnet is Fe2+, and of course ilmenite can have substantial substitution towards the hematite end-member. [REPLY ‒ Ilmenite is nearly pure FeTiO3 phase for most of the natural metapelite, and therefore, the Fe3+ content of ilmenite was neglected] The bottom line is that metapelites are not entirely reduced and anybody who wants to use this new geobarometer will want to know the uncertainty associated with variable oxidation state. I see there is brief mention of this in lines 99-102, but this does not explain these issues entirely. [REPLY ‒ Yes, some new words have been added to the text, see Lines 102—104]
Secondly, titanite is not exceptionally common in pelites. At low-grade conditions, hematite and rutile commonly form, and transition to ilmenite and magnetite at higher grade. What proportion of the samples used for calibration contained titanite? How would the absence of titanite affect this geobarometer? [REPLY ‒ Yes, titanite is not very common in ordinary metapelite. In fact, all of the calibrant samples contain ilmenite, among which about 70% of the samples contain titanite. However, due to TiO2-saturation behavior of the samples, it is anticipated that the other 30% samples reported in the literature also contain titanite, this is largely because titanite was not paid attention in the past days and the amount of titanite in the rock is very small. As we know, titanite is chemically pure CaTiSiO5 phase in natural metapelite, and the estimated pressures will be overestimated when titanite is absent in the metapelite.]
Author Response File: 
Author Response.docx
Reviewer 2 Report
The manuscript presents what is certainly a good idea. However it needs extensive rewriting and rewording to make it readable and understandable by a broader audience. Some of the statements need to be supported by evidence. Also some of the choices (e.g. the garnet activity model) need to be better argument. Also some references in the elastic barometry sentence are completely missing. For all of these reasons I would recommend to accept the manuscript only if these major changes are fixed in the revised version. I would encourage the authors to consider the criticisms here below and to also consider some support from English native speakers when preparing their revised manuscript.
Line 25 using major elements of minerals does not mean anything. Perhaps the author meant the chemical partitioning or the exchange of major elements between mineral phases in chemical equilibrium?
Line 27 substitute concise for precise
27-31 rewrite. It does not have any significance and is poorly written.
Line 31. The definition of concise does not agree with what I think the author meant. I think that a geothermometer cannot be succinct brief or concise.
Line 34 – 35 most of the “recent literature” is actually missing. I suggest a considerable extension to this part with a considerable update of the literature adding for example some of the novel works by the group at the University of Pavia that provide seminal basis for the readers. Here below some example of references that can be added. These represent the latest advances on the topic.
Mara Murri, Mattia L. Mazzucchelli, Nicola Campomenosi, Andrey V. Korsakov, Mauro Prencipe, Boriana D. Mihailova, Marco Scambelluri, Ross J. Angel, Matteo Alvaro (2018) Raman elastic geobarometry for anisotropic mineral inclusions. American Mineralogist, 103 (11), 1869-1872. Angel R.J., Murri M., Mihailova B., Alvaro M. (2019) Stress, Strain and Raman Shifts. Zeitschrift für Kristallographie, 234(2), 129–140. Murri, M., Alvaro, M., Angel, R.J., Prencipe, M., and Mihailova, B.D. (2019) The effects of non-hydrostatic stress on the structure and properties of alpha-quartz. Physics and Chemistry of Minerals Anzolini C., Nestola F., Mazzucchelli M.L., Alvaro M., Nimis P., Gianese A., Morganti S., Marone F., Campione M., and Harris J. (2019) Depth of diamond formation obtained from single periclase inclusions. Geology Nimis, P., Angel, R.J., Alvaro, M., Nestola, F., Harris, J.W., Casati, N., and Marone, F. (2019) Crystallographic orientations of magnesiochromite inclusions in diamonds: what do they tell us? Contributions to Mineralogy and Petrology Stangarone, C., Angel, R.J., Prencipe, M., Campomenosi, N., Mihailova, B., and Alvaro, M. (2019) Measurement of strains in zircon inclusions by Raman spectroscopy. European Journal of Mineralogy Stangarone, C., Angel, R.J., Prencipe, M., Mihailova, B., and Alvaro, M. (2019) New insights into the zircon-reidite phase transition. American Mineralogist Zaffiro, R.J. Angel, M. Alvaro (2019) Constraints on the Equations of State of stiff anisotropic minerals: rutile, and the implications for rutile elastic barometry. Mineralogical Magazine Joseph P. Gonzalez, Jay B. Thomas, Suzanne L. Baldwin, Matteo Alvaro (2019) Quartz-in-garnet and Ti-in-Quartz (QuiG-TiQ) thermobarometry: Methodology and first application to a quartzofeldspathic gneiss from the (ultra)high-pressure terrane in eastern Papua New Guinea. Journal of Metamorphic Geology
In addition I would also remove reference 18 and 19 as they have been demonstrated to be wrong both in the approach and in the matlab code.
Line 38 – 39 I sueggest rewording as follows: “.., which makes unusable most of the commonly adopted plagioclase geobarometers..”
Line 60: if units are provided for P and T also for the other terms units should be provided.
Line 70: the reasons for that choice of activity model should be given in the main text. How do the results compare with different models? If we assume linear dependence does it make significant difference beyond acceptable estimated uncertainties?
Line 86-87: check the subscripts for formulas.
Line 91: replace “in concert” with “in agreement”
Line 101 – 102: How does an incorrect estimate of the Fe3+ content affects the results? Can teh authors make an estimate? If yes I would then add it in the text.
Line 110 I would expect nothing less. The authors should anticipate here the sentence at line 174-175 otherwise seems that they have tested their calibration on the samples they used to produce the calibration that must be in agreement by definition.
Line 115 again concert does not mean in agreement.
Line 149 remove the word random. Just call it error. Also in other parts of the manuscript. I think I understand the authors want to make clear that these are not systematic errors and it is already clear wth no need to call them random as it does not mean tha are not systematic.
Line 149 – 163 I would add also some considerations about correlation factors. What happens if we consider the dependence of P on T? How does the 2% change in Ca or Fe translate into the total P and T error? The descritpion of Fig 3 itself is not enough. If I understood that correctly that is sipmly a misfit that does not represent the uncertainty from measuremtns error but simply from calculations.
Author Response
The manuscript presents what is certainly a good idea. However it needs extensive rewriting and rewording to make it readable and understandable by a broader audience. Some of the statements need to be supported by evidence. Also some of the choices (e.g. the garnet activity model) need to be better argument. Also some references in the elastic barometry sentence are completely missing. For all of these reasons I would recommend to accept the manuscript only if these major changes are fixed in the revised version. I would encourage the authors to consider the criticisms here below and to also consider some support from English native speakers when preparing their revised manuscript.
[REPLY ‒ Thanks to your best suggestions, and all the questions have been considered and answered below, point-to-point.]
Line 25 using major elements of minerals does not mean anything. Perhaps the author meant the chemical partitioning or the exchange of major elements between mineral phases in chemical equilibrium? [REPLY ‒ Yes, corrected. Now Lines 25-26]
Line 27 substitute concise for precise [REPLY ‒ Yes, corrected. Now Line 28]
27-31 rewrite. It does not have any significance and is poorly written. [REPLY ‒ Yes, revised. Now Lines 28-32]
Line 31. The definition of concise does not agree with what I think the author meant. I think that a geothermometer cannot be succinct brief or concise. [REPLY ‒ Yes, the word “concise” has been corrected to “precise” throughout the text.]
Line 34 – 35 most of the “recent literature” is actually missing. I suggest a considerable extension to this part with a considerable update of the literature adding for example some of the novel works by the group at the University of Pavia that provide seminal basis for the readers. Here below some example of references that can be added. These represent the latest advances on the topic. [REPLY ‒ Yes, these references have been added and accordingly, the reference numbers have been revised. Now Lines 35-36]
Mara Murri, Mattia L. Mazzucchelli, Nicola Campomenosi, Andrey V. Korsakov, Mauro Prencipe, Boriana D. Mihailova, Marco Scambelluri, Ross J. Angel, Matteo Alvaro (2018) Raman elastic geobarometry for anisotropic mineral inclusions. American Mineralogist, 103 (11), 1869-1872.
Angel R.J., Murri M., Mihailova B., Alvaro M. (2019) Stress, Strain and Raman Shifts. Zeitschrift für Kristallographie, 234(2), 129–140.
Murri, M., Alvaro, M., Angel, R.J., Prencipe, M., and Mihailova, B.D. (2019) The effects of non-hydrostatic stress on the structure and properties of alpha-quartz. Physics and Chemistry of Minerals
Anzolini C., Nestola F., Mazzucchelli M.L., Alvaro M., Nimis P., Gianese A., Morganti S., Marone F., Campione M., and Harris J. (2019) Depth of diamond formation obtained from single periclase inclusions. Geology
Nimis, P., Angel, R.J., Alvaro, M., Nestola, F., Harris, J.W., Casati, N., and Marone, F. (2019) Crystallographic orientations of magnesiochromite inclusions in diamonds: what do they tell us? Contributions to Mineralogy and Petrology
Stangarone, C., Angel, R.J., Prencipe, M., Campomenosi, N., Mihailova, B., and Alvaro, M. (2019) Measurement of strains in zircon inclusions by Raman spectroscopy. European Journal of Mineralogy
Stangarone, C., Angel, R.J., Prencipe, M., Mihailova, B., and Alvaro, M. (2019) New insights into the zircon-reidite phase transition. American Mineralogist
Zaffiro, R.J. Angel, M. Alvaro (2019) Constraints on the Equations of State of stiff anisotropic minerals: rutile, and the implications for rutile elastic barometry. Mineralogical Magazine
Joseph P. Gonzalez, Jay B. Thomas, Suzanne L. Baldwin, Matteo Alvaro (2019) Quartz-in-garnet and Ti-in-Quartz (QuiG-TiQ) thermobarometry: Methodology and first application to a quartzofeldspathic gneiss from the (ultra)high-pressure terrane in eastern Papua New Guinea. Journal of Metamorphic Geology
In addition I would also remove reference 18 and 19 as they have been demonstrated to be wrong both in the approach and in the matlab code. [REPLY ‒ Yes, these two references have been omitted.]
Line 38 – 39 I sueggest rewording as follows: “.., which makes unusable most of the commonly adopted plagioclase geobarometers..” [REPLY ‒ Yes, corrected. Now Lines 39-40]
Line 60: if units are provided for P and T also for the other terms units should be provided. [REPLY ‒ Yes, the units have been added. Now Lines 61-62]
Line 70: the reasons for that choice of activity model should be given in the main text. How do the results compare with different models? If we assume linear dependence does it make significant difference beyond acceptable estimated uncertainties? [REPLY ‒ Now Line 71. Here I preferred the Holdaway’s (2001) garnet activity model, and the reason is given in Liens 270-282.]
Line 86-87: check the subscripts for formulas. [REPLY ‒ Yes, corrected. Now Lines 87-88]
Line 91: replace “in concert” with “in agreement” [REPLY ‒ Yes, corrected. Now Line 92]
Line 101 – 102: How does an incorrect estimate of the Fe3+ content affects the results? Can teh authors make an estimate? If yes I would then add it in the text. [REPLY ‒ Estimation errors of Fe3+ of garnet, when taken to be of ~±15%, would introduce pressure errors to be around ±0.05~0.10 kbar. This is added in Lines 157-159.]
Line 110 I would expect nothing less. The authors should anticipate here the sentence at line 174-175 otherwise seems that they have tested their calibration on the samples they used to produce the calibration that must be in agreement by definition. [REPLY ‒ Yes, you are right. But I think to test the reproducibility of the garnet geobarometer to the calibrant samples, is also necessary, even preliminary.]
Line 115 again concert does not mean in agreement. [REPLY ‒ Yes, corrected. Now Line 117]
Line 149 remove the word random. Just call it error. Also in other parts of the manuscript. I think I understand the authors want to make clear that these are not systematic errors and it is already clear wth no need to call them random as it does not mean tha are not systematic. [REPLY ‒ Yes, the word “random” was deleted throughout the text.]
Line 149 – 163 I would add also some considerations about correlation factors. [REPLY ‒ Yes, you are right. However, correlation factors between error sources can be hardly estimated.] What happens if we consider the dependence of P on T? [REPLY ‒ Standard error of the garnet-biotite geothermometer is ±25 ºC. If this temperature error is adopted, the propagated pressure errors of the garnet geobarometer will be well restricted to ±0.54 kbar for all the calibrant samples. ] How does the 2% change in Ca or Fe translate into the total P and T error? REPLY ‒ See Line 153-155] The descritpion of Fig 3 itself is not enough. If I understood that correctly that is sipmly a misfit that does not represent the uncertainty from measuremtns error but simply from calculations. [REPLY ‒ Yes, you are right.]
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
My initial comments were very minor and the author has responded to them very well. I see no reason that this manuscript cannot be published in this form. It is a good contribution!
Author Response
Thank you very much. All the corrections were done and are in red.
Reviewer 2 Report
The work has considerably improved. Can be published as is.
Author Response
Thank you very much. All the corrections were done and are in red.
