Amp-TB2 Protocol and Its Application to Amphiboles from Recent, Historical and Pre-Historical Eruptions of the Bezymianny Volcano, Kamchatka

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

General Comments:

This manuscript represents an application of the single-mineral thermobarometry program Amp-TB2 developed by the first author for application specifically to calc-alkaline and alkaline volcanic rocks of low thermodynamic variance.  This approach, though empirical, appears to provide meaningful results.  In this study the authors look specifically at a suite of samples from the Bezymianny volcano of Kamchatka to derive information about the conditions of amphibole formation as a function of the age of eruption.  I believe this study provides an interesting example of how the Amp-TB2 program can be used. 

One of my main concerns about this study is the concept of chemical homogeneity.  The authors provide Equation (2) to help distinguish chemical variation arising from normal statistical variations in the analysis of presumably homogeneous standard materials.  Unfortunately, the authors do not discuss one of the main issues regarding homogeneity, namely that homogeneity is a scale or distant-dependent term.  Compositional consistency in a chemical species (Al, Si, Ca, etc.) cannot be discussed without considering the length-scale of the analysis.  Perhaps it is the smallest excitation volume of an electron microprobe analysis, which can probably be defined relatively well for a typical amphibole, or perhaps it is the analytical distance that the analyst chooses.  Whatever it is, there should be some discussion of the distance over which a sample is considered homogeneous.  This leads to a second concern.

The authors comment in lines 72-83 that heterogeneous analyses should be avoided as they may represent non-steady state conditions and therefore give unreliable results.  But how is this established and, again, on what scale?  One can simply look at the EMPA profiles provided in Figures 2 to 5 and easily identify regions of relatively constant composition over distances of 100-200 micron compared to rapidly changing chemical compositions without having to resort to Equation (2).  The authors have focused on these homogenous domains for extracting physical-chemical values; however, some may find the regions of rapid change to be the most interesting as these regions are recording changes to the amphibole growth on (presumably) a much shorter time scale than the steady-state regions and are recording changes as they occur.  Before dismissing these regions as being useless, it would be useful to include some discussion about the issue of heterogeneity in the context of length scale and growth rate of a phenocryst from a typical magma.  Some suggestion of what a minimum distance might be that would represent sufficient time to avoid disequilibrium caused by, for example, diffusion-controlled growth for the migration of cations in the magma to the site of crystal growth, would be interesting.  This topic is probably beyond the scope of the paper, but any discussion about what a minimum length scale might be used for extracting physical-chemical values using Amp-TB2 would be helpful. 

Concerning Equations (1) and (2), presumably the 1-σ uncertainties are based only on counting statistics for the EMPA line traverses and are not the standard deviation from the average of multiple analyses at a given spot (which would be considerably longer to acquire).  This should be stated explicitly.

Regarding Figure 1, it would be useful to include a second figure (Fig 1b ?) which takes the numerical values listed (in detail) in the text in lines 116-118 and translates these into a figure showing the 2σ limits vs Oxide wt%.  Most readers will probably find this sort of diagram more useful than the current Figure 1, especially if the authors provide an equation (3^rd-order polynomial, power law, or some other form) that closely models the curve.

The English is a little rough in places.  Attached is a PDF file with revisions on the manuscript that includes many small changes to improve the English and grammar.

Comments for author File: Comments.pdf

Comments on the Quality of English Language

The English is a little rough in places.  Attached is a PDF file with revisions on the manuscript that includes many small changes to improve the English and grammar.

Author Response

General Comments of Reviewer 1 (R1) with Authors (A) replies:

(1)

R1: This manuscript represents an application of the single-mineral thermobarometry program Amp-TB2 developed by the first author for application specifically to calc-alkaline and alkaline volcanic rocks of low thermodynamic variance.  This approach, though empirical, appears to provide meaningful results.  In this study the authors look specifically at a suite of samples from the Bezymianny volcano of Kamchatka to derive information about the conditions of amphibole formation as a function of the age of eruption.  I believe this study provides an interesting example of how the Amp-TB2 program can be used. 

A: We are glad that R1 appreciated our work, and thank him/her for this.

(2)

R1: One of my main concerns about this study is the concept of chemical homogeneity.  The authors provide Equation (2) to help distinguish chemical variation arising from normal statistical variations in the analysis of presumably homogeneous standard materials.  Unfortunately, the authors do not discuss one of the main issues regarding homogeneity, namely that homogeneity is a scale or distant-dependent term.  Compositional consistency in a chemical species (Al, Si, Ca, etc.) cannot be discussed without considering the length-scale of the analysis.  Perhaps it is the smallest excitation volume of an electron microprobe analysis, which can probably be defined relatively well for a typical amphibole, or perhaps it is the analytical distance that the analyst chooses.  Whatever it is, there should be some discussion of the distance over which a sample is considered homogeneous.  This leads to a second concern.

Plus:

Concerning Equations (1) and (2), presumably the 1-σ uncertainties are based only on counting statistics for the EMPA line traverses and are not the standard deviation from the average of multiple analyses at a given spot (which would be considerably longer to acquire).  This should be stated explicitly.

Plus:

Regarding Figure 1, it would be useful to include a second figure (Fig 1b ?) which takes the numerical values listed (in detail) in the text in lines 116-118 and translates these into a figure showing the 2σ limits vs Oxide wt%.  Most readers will probably find this sort of diagram more useful than the current Figure 1, especially if the authors provide an equation (3^rd-order polynomial, power law, or some other form) that closely models the curve.

 

A: Thanks a lot for this comment too. Accordingly changes and additional discussions have been reported in section 2.1, including the recommended Fig. 1b. They are as follows (highlighted):

In order to find a criterion to exclude problematic analyses of amphibole and establish a threshold between homogeneous and heterogeneous compositional domains, we have performed multiple EPMA measurements (from 10 to 30 analyses profiles or different spots) using a set of mineral and glass standards (Smithsonian Institution) and synthetic glasses with different composition (which so far are considered to be homogeneous; i.e. shows intra-crystal chemical variations within the uncertainty of EPMA measurements; e.g. [26]), and calculated the average amount of element oxides (in wt%), the relative standard deviation (σ, in wt%) and its percentage value (σ% = σ x 100/ element oxide wt%; Figure 1a)….

In other words, homogeneous compositions should show σ% of all its element oxides lower than or equal to a threshold calculated by equation (2) whereas heterogeneous compositions are invariably characterized by higher uncertainties. It is worth noting that “homogeneity” is a scale-dependent concept depending by the length-scale of the analytical method and the sample/s used to test it. In the case reported in this work, homogeneity is defined by the interaction of an EPMA (i.e. Cameca SX 100, operating in profile analysis) with minerals and glasses assumed to have intra-crystal compositional consistency of their major chemical species (Fig. 1). In addition, this condition is also respected by our new JEOL JXA-iHP 200F (at Leibniz University Hannover) as all mineral and glass standards measured so far, show σ% values ≤ to those estimated by equation (2).

 

The σ values calculated with (2) are 0.47 wt% at a level of measured element oxide of 70 wt%, 0.42 at 50 wt%, 0.39 at 40 wt%, %, 0.31 wt% at 20 wt%, 0.24 wt% at 10 wt%, 0.19 wt% at 5 wt%, 0.14 wt% at 2 wt%, 0.11 wt% at 1 wt% and 0.05 wt% at 0.1 wt% levels. This pattern is also reported in Figure 1b, also showing that most of the measured element oxides have σ values lower than those predicted by equation (2), red-dashed curve.

 

(3)

R1: The authors comment in lines 72-83 that heterogeneous analyses should be avoided as they may represent non-steady state conditions and therefore give unreliable results.  But how is this established and, again, on what scale?  One can simply look at the EMPA profiles provided in Figures 2 to 5 and easily identify regions of relatively constant composition over distances of 100-200 micron compared to rapidly changing chemical compositions without having to resort to Equation (2).  The authors have focused on these homogenous domains for extracting physical-chemical values; however, some may find the regions of rapid change to be the most interesting as these regions are recording changes to the amphibole growth on (presumably) a much shorter time scale than the steady-state regions and are recording changes as they occur.  Before dismissing these regions as being useless, it would be useful to include some discussion about the issue of heterogeneity in the context of length scale and growth rate of a phenocryst from a typical magma.  Some suggestion of what a minimum distance might be that would represent sufficient time to avoid disequilibrium caused by, for example, diffusion-controlled growth for the migration of cations in the magma to the site of crystal growth, would be interesting.  This topic is probably beyond the scope of the paper, but any discussion about what a minimum length scale might be used for extracting physical-chemical values using Amp-TB2 would be helpful. 

A: Also this comment helped to clarify on the application of our protocol. Related changes and discussions are as follow.

Section 2.2, at the end of the discussion of Fig. 5:

Al₂O₃ decreases at the heterogeneous domain to increase again towards rim 1. This pattern suggests a process of interdiffusion of aluminium and should be not considered for thermobarometric quantification.

Section 3.2:

At this stage, it is convenient to input the compositional profiles into Amp-TB2.1 and perform a qualitative intra-crystal analysis by compositional, P, T, ∆NNO, H₂O_melt vs. profile distance diagrams (Figs. 2–5).

Section 3.3:

Finally, average and standard deviation values must be calculated also for the physico-chemical parameter values given by the model in the zones constrained with eq. (2). σ and σ% values for the heterogeneous zones are generally higher than the uncertainties predicted by Amp-TB2.1. The physico-chemical parameters of these heterogeneous domains can not be quantitatively constrained. If an amphibole did not grow at “steady-state” (equilibrium) magmatic conditions or underwent high degrees of post-magmatic alteration it may not show any homogeneous domains. In this case, the calculated parameters do not have any qualitative and quantitative meaning.

Secion 5. Final remarks:

This article shows that the application of this protocol is essential to quantify the “steady-state” conditions of amphibole crystallization (through the identification of homogeneous crystals or intra-crystal domains) and retrieve reliable estimations on the depths of the magma storage levels and their vertical size. Heterogeneous domains can be eventually considered for a qualitative analysis on how much the physic-chemical parameters “apparently” change during kinetic (disequilibrium) processes such as magma ascent, mixing and convection. However, the readers should be aware that the Amp-TB2 model has been calibrated at equilibrium conditions where the physico-chemical and compositional parameters are assumed to be constant for a long time (ideally infinite) and can give unpredictable results when the sub-volcanic processes are too rapid for allowing the amphibole-melt chemical equilibrium to be sufficiently approached. This phenomenon mostly affects the estimated P (and depth) values. For instance, Kiss et al. [46] reported Amp phenocryst domains showing oscillatory zonings with large decreases and increases in major elements over intra-crystal distances of a few microns, associated with large P variations (⁓100-300 MPa). In addition, the application of Amp thermobarometry to four cooling experiments performed at 150 MPa by Shea and Hammer [47] shows slight temperature overestimations (6-27 °C; approximately consistent with the uncertainty of Amp-TB2) and pronounced pressure overestimations of ~100-200%, [6].

(4)

R1: The English is a little rough in places.  Attached is a PDF file with revisions on the manuscript that includes many small changes to improve the English and grammar.

A: All English changes has been accepted.

Relevant comments in the pdf file:

(5)

R1: Line 102: What is the σ based on, EPMA counting statistics or the standard deviation of multiple analyses?

 

R1: Lines 117-118: It would be more useful to show this information in a graph, perhaps as Figure 1b, to help the reader visualize the relationship between oxide wt% and the 2-sigma values that you derived for your microprobe analytical conditions. See General Comments.

A: See point 2.

(6)

 

R1: Lines 174-175: But over what length scale? Is this really a bad thing? See General Commetns.

A: See point 3.

(7)

 

R1: Lines 130-131: Lower than what? I have provided some suggested wording, but please see that this does not change your meaning.

 

A: The sentence has been reformulated as an additional diagram (Fig. 1b) has been added:

The diagrams show that homogeneous mineral and glass samples, containing major oxides in a wide range, are mostly characterized by lower σ% and σ values than those of equation (2), i.e. red-dashed curve.

 

(7)

 

R1: Lines 312-314: This illustrates the main question this reviewer has about applying equation (2) to microprobe data. Rather than excluding data that vary strongly across a given compositional profile, application of your Amp-TB2.1 analysis may be useful to point to changes in conditions that are occurring while a particular phenocryst is forming.

A: See point 3.

(8)

R1: Fig. 7: Please check the abbreviation Pre-Benz in the figure. Supposedly it should be Pre-Bez.

A: corrected.

(9)

References: Many of the references have words merged together without any space. These have been corrected where they were found.

A: corrected, hopefully all of them.

 

 

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

 

It is a paper for application study to estimate the volcanic activities related to evolution of magma with time. A proposed methodology of Amp-TB2 Amp-TB2 (single-amphibole thermobarometry) seems to be an interesting approach. The submitted paper includes both the development of methodology of Amp-TB2 and application study to Bezymianny volcano, however, it is quite difficult to identify the reliability of proposed method and availability of application due to ambiguous descriptions of the manuscript. The reviewer recommends separating the contents with another paper and each paper should be described in detail with scientific points of view. Additionally, the paper is summited as the review article, but it is not the review paper. The authors should reconsider the category again. It is therefore that the paper should be revised as major revision. Several minor comments identified in review are listed as notification for the revision of manuscript.

Line 48: Description of citation should be changed as Ridolfi et al. [2], not [2].

Line 54: Ditto

Line 68: Not EMP but EPMA

Line 116 to 118: Should be checked carefully the descriptions for sigma value and element oxide. Former unit is not wt%.

Line 129: Not EMP but EPMA

Line 144: what does mean “mantle” in the crystal?

Figures: For BSE image, it is difficult to identify the line profile of EPMA, should be clear. Scale is also not visible. Labels of axis are too small and not easy to read, should be enlarged.

If the authors revise the paper for the former part (i.e., methodology of Amp-TB2), it should be reconsidered the structure of paper as the contents of style for category article.

 

Author Response

Comments of Reviewer 2 (R2) with Authors (A) replies:

It is a paper for application study to estimate the volcanic activities related to evolution of magma with time. A proposed methodology of Amp-TB2 Amp-TB2 (single-amphibole thermobarometry) seems to be an interesting approach. The submitted paper includes both the development of methodology of Amp-TB2 and application study to Bezymianny volcano, however, it is quite difficult to identify the reliability of proposed method and availability of application due to ambiguous descriptions of the manuscript. The reviewer recommends separating the contents with another paper and each paper should be described in detail with scientific points of view. Additionally, the paper is summited as the review article, but it is not the review paper. The authors should reconsider the category again. It is therefore that the paper should be revised as major revision.

A: We appreciate the suggestion of R2, but dividing the paper in two articles is not possible cause we believe that the article is self-consistent the way it is. R2 two stated that the manuscript has an ambiguous description but didn´t really said why.

From our point of view, the protocol has been developed thanks to the detailed profile analyses performed on Bezymianny amphiboles that are reported as examples of application. In turn, we show how the application of the protocol can lead to great details on the magma feeding systems of sub-volcanic systems such as the one of Bezymianny.

About the article category, it is not important for us and we leave the decision to the editors. 

 

R2: Several minor comments identified in review are listed as notification for the revision of manuscript.

A: Corrected

R2: Line 48: Description of citation should be changed as Ridolfi et al. [2], not [2].

A: Corrected

R2: Line 54: Ditto

A: Corrected

R2: Line 68: Not EMP but EPMA

A: Corrected

R2: Line 116 to 118: Should be checked carefully the descriptions for sigma value and element oxide. Former unit is not wt%.

A: We checked it and looks fine to us. Sure, the former parameter described was σ% that like any percentage of two parameters with the same unit has no dimension. Whereas here we describe the values of σ, standard deviation of the element oxide content (that is measured in wt%) obtained by equation (2) at several level of element oxide wt%.

 

 

R2: Line 129: Not EMP but EPMA

A: Corrected

Line 144: what does mean “mantle” in the crystal?

A: Mantle is a commonly used term in petrology and mineralogy to describe intermediate zones within the crystals, between core and rim

 

Figures: For BSE image, it is difficult to identify the line profile of EPMA, should be clear. Scale is also not visible. Labels of axis are too small and not easy to read, should be enlarged.

A: They have been enlarged of about twice

 

If the authors revise the paper for the former part (i.e., methodology of Amp-TB2), it should be reconsidered the structure of paper as the contents of style for category article.

A: See reply to first comment

Reviewer 3 Report

Comments and Suggestions for Authors

Dear author,

I read your paper. It's unusual and interesting for me. I think that it would be interesting to some specialist in the field  of volcanic activity.  I hope that in the next researches you'll pay more attention on the determination of FO2 in the volcano processes.

Author Response

Comments of Reviewer 3 (R3) with Authors (A) replies:

Dear author,

I read your paper. It's unusual and interesting for me. I think that it would be interesting to some specialist in the field  of volcanic activity.  I hope that in the next researches you'll pay more attention on the determination of FO2 in the volcano processes.

A: We thanks R3 for his/her interest and suggestion for our future research.