Evolution of fluid redox in a fault zone of the Pic de Port-Vieux thrust in the Pyrenees Axial Zone (Spain)

Charpentier, Delphine; Milesi, Gaétan; Labaume, Pierre; Abd Elmola, Ahmed; Buatier, Martine; Lanari, Pierre; Muñoz, Manuel

doi:https://doi.org/10.5194/egusphere-2024-386

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https://doi.org/10.5194/egusphere-2024-386

Preprints

15 Mar 2024

| 15 Mar 2024

Evolution of fluid redox in a fault zone of the Pic de Port-Vieux thrust in the Pyrenees Axial Zone (Spain)

Delphine Charpentier, Gaétan Milesi, Pierre Labaume, Ahmed Abd Elmola, Martine Buatier, Pierre Lanari, and Manuel Muñoz

Abstract. In mountain ranges, crustal-scale faults localize multiple episodes of deformation. It is therefore common to observe current or past geothermal systems along these structures. Understanding the fluid circulation channelized in fault zones is essential to characterize the thermo-chemical evolution of associated hydrothermal systems. We present a study of a paleo-system of the Pic de Port-Vieux thrust fault. This fault is a second-order thrust associated with the Gavarnie thrust in the Axial Zone of the Pyrenees. The study focused on phyllosilicates, which permit to constrain the evolution of temperature and redox of fluids at the scale of the fault system. Combined X-ray absorption near-edge structure (XANES) spectroscopy and electron probe microanalysis (EPMA) on synkinematic chlorite, closely linked to microstructural observations were performed in both the core and damage zones of the fault zone. Regardless of their microstructural position, chlorite from the damage zone contains iron and magnesium (Fe_total/(Fe_total+Mg) about 0.4), with Fe³⁺ accounting for about 30 % of the total iron. Chlorite in the core zone is enriched in total iron, but individual Fe³⁺/Fe_total ratios range from 15 % to 40 % depending on the microstructural position of the grain. Homogeneous temperature conditions about 300 °C have been obtained by chlorite thermometry. A scenario is proposed for the evolution of fluid-rock interaction conditions at the scale of the fault zone. It involves the circulation of a single hydrothermal fluid with homogeneous temperature but several redox properties. A highly reducing fluid evolves due to redox reactions involving progressive dissolution of hematite, accompanied by crystallization of Fe²⁺-rich and Fe³⁺-rich chlorite in the core zone.

Received: 09 Feb 2024 – Discussion started: 15 Mar 2024

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Journal article(s) based on this preprint

28 Aug 2024

Evolution of fluid redox in a fault zone of the Pic de Port Vieux thrust in the Pyrenees Axial Zone (Spain)

Delphine Charpentier, Gaétan Milesi, Pierre Labaume, Ahmed Abd Elmola, Martine Buatier, Pierre Lanari, and Manuel Muñoz

Solid Earth, 15, 1065–1086, https://doi.org/10.5194/se-15-1065-2024,https://doi.org/10.5194/se-15-1065-2024, 2024

Short summary

Delphine Charpentier, Gaétan Milesi, Pierre Labaume, Ahmed Abd Elmola, Martine Buatier, Pierre Lanari, and Manuel Muñoz

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2024-386', Anonymous Referee #1, 08 Apr 2024

This article aims to better understand fluid circulation in fractured zones in the Pyrenees axial zone, based on chlorite geochemistry and thermometry.
The article is well-written and well-organized, and the conclusions seem well-founded. However, this article leaves an impression of incompleteness, a sense of lacking of data. In fact, the article heavily relies on the previous work by Trincal et al 2015, and Abd Elmola et al 2017, and provides only (as new data) about 30 microprobe analyses and 15 XANES data, obtained on 2 samples. That's all. Were there more analyses? Have some been discarded? On what basis? It is said that chlorite and quartz in veins are co-genetic, chlorite being intimately interwoven with quartz. Why not combine chlorite thermometry with microthermometry on fluid inclusions in quartz? Even at the end, it's hard to identify truly new conclusions from those already formulated by Trincal et al 2015 and Abd Elmola et al 2017.
And even in terms of form: Why no EMP mapping? Why not present XANES spectra representative of each "area"? Line 182: "Two representative samples were selected". In what way are they representative and, on their own (2), allow solid conclusions to be drawn? They have already been studied by Abd Elmolah et al 2017, so why not other samples?
Line 234: « Estimated XFe3+ can be compared with weasured XFe3+ ». Yes, but this is not done. The only mention is line 370 "the XFe3+ values are always underestimated compared to those calculated by XANES analyses, which can explain the underestimation of these calculated temperatures (i.e. calculated by Abd Elmola et al 2017)". Yet underestimating Fe3+ means underestimating octahedral vacancies, and therefore overestimating calculated temperatures. No?
In short, despite its editorial qualities, I am left with an appetite for more. The paucity of new analyses prevents me from describing this paper as innovative or very usefull.

Citation: https://doi.org/10.5194/egusphere-2024-386-RC1
- AC1: 'Reply on RC1', Delphine CHARPENTIER, 25 May 2024
  
  Dear Reviewer,
  Please find below the answers to your comments and questions (italic).
  
  The article is well-written and well-organized, and the conclusions seem well-founded. However, this article leaves an impression of incompleteness, a sense of lacking of data. In fact, the article heavily relies on the previous work by Trincal et al 2015, and Abd Elmola et al 2017, and provides only (as new data) about 30 microprobe analyses and 15 XANES data, obtained on 2 samples. That's all. Were there more analyses? Have some been discarded? On what basis? It is said that chlorite and quartz in veins are co-genetic, chlorite being intimately interwoven with quartz. Why not combine chlorite thermometry with microthermometry on fluid inclusions in quartz? Even at the end, it's hard to identify truly new conclusions from those already formulated by Trincal et al 2015 and Abd Elmola et al 2017.
  Authors response:
  The aim of this article is to investigate the main processes related to the paleo-fluid circulation within the fault zone. We therefore coupled microstructural analyses with geochemical and mineralogical characterisation. The novelty of the study is to focus on chlorite and specially on its iron oxidation state to show that: 1) chlorite is a good tracker of redox properties and their variation with time due to the mineral reactions of fluids with the pelitic rocks at the scale of a fault zone, 2) the Fe³⁺/Fe ratio is one of the weakest points of chlorite geothermometry (only accessible using µ-XANES spectroscopy) ; it is an information useful to calculate temperature of chlorite formation but usually neglected as it is difficult to quantify it. As suggested, we modified the manuscript to identify truly new conclusions from those already formulated by Trincal et al 2015 and Abd Elmola et al 2017. (Line 25, Line 79, Line 557-560).
  Line 25 in abstract: “This study shows the importance to determine redox state of iron in chlorite to calculate their temperature of formations and to consider the fluid evolution at the scale of a fault.”
  Line 79 in introduction: “First, chlorite chemistry, obtained by X-ray absorption near-edge spectroscopy (XANES) and electron probe microanalysis (EPMA) on the same synkinematic minerals in clearly identified microstructures of damage and core zones, was used to trace different fluid circulation events. Then, this dataset is used to refine the local mechanisms, and temperature and redox conditions of fluid-rock interactions during mineral growth. Finally, a new model of fluid circulation coupled with the tectonic evolution of the PPVT is also proposed.”
  Line 557 in conclusion: “Thanks to this integrated study, we characterize the main processes related to the paleo-fluid circulation within a fault zone. Micro-XANES spectroscopy provides unique insights, regarding redox properties and their variation with time due to the mineral-fluids interactions even at the scale of a fault zone. Moreover, iron state quantification is one of the weakest points of chlorite geothermometry that can be addressed by the methodology applied.”
  
  The µ-XANES spectroscopy is complex, expensive, and difficult-to-access. The twenty analyses we performed correspond to the 24h of the BM23 beamline of the European Synchrotron Radiation Facility we obtained through the program ESRF ES548 financed by INSU. All the µ-XANES data obtained are presented in this article (except one that failed).
  As shown in the results (Figure 9, modified following RC2 comments), fluid temperatures associated with chlorite formation are quite similar (270 to 285°C), only redox of fluid varies (µ-XANES data). Microthermometry on fluid inclusions (Grant, 1989) does not allow to be more precise on the temperature determination. Furthermore, the origin of the fluid is identical, as specified in Cathelineau et al; (2021).
  
  And even in terms of form: Why no EMP mapping? Why not present XANES spectra representative of each "area"? Line 182: "Two representative samples were selected". In what way are they representative and, on their own (2), allow solid conclusions to be drawn? They have already been studied by Abd Elmolah et al 2017, so why not other samples?
  Authors response:
  We selected one sample that present all the microstructural patterns observed in the core zone and that present a typical mineralogical composition of the core zone. The second sample is representative of the damage zone. Abd Elmola et al., 2017 provides the global composition of chlorite particles from the core zone and from the damage zone. We completed this study by performing microprobe analyses on the particles analysed by µ-XANES to be able to provide structural formulae with Fe²⁺ and Fe³⁺ contents, and clearly correlated the composition, the iron oxidation state and the microstructural position of chlorite. We performed punctual analyses because 1) chlorite particles are thin and small, unlike the rosette-shaped aggregates mapped in Trincal et al 2015 and 2) punctual analyses show homogeneous composition (see appendix tables). As µ-XANES data are scarce and precious, we added µ-XANES spectra in appendix.
  
  Line 234: « Estimated XFe3+ can be compared with weasured XFe3+ ». Yes, but this is not done. The only mention is line 370 "the XFe3+ values are always underestimated compared to those calculated by XANES analyses, which can explain the underestimation of these calculated temperatures (i.e. calculated by Abd Elmola et al 2017)". Yet underestimating Fe3+ means underestimating octahedral vacancies, and therefore overestimating calculated temperatures. No?
  Authors response:
  There was a discrepancy between the methodology, the description of the geothermometers used and the results (text and table). We have modified the text in the methodological part (Lines 237-240), the description of the results (Lines 357-377) and the results in table S4 in order to remedy this. The relation between XFe3+ underestimation and temperature modification are now clearly mentioned (Lines 371-375 and 400-405).
  Line 237 in methodology: “The estimated XFe³⁺ values were compared to the XFe³⁺ values measured by µ-XANES.
  Additionally, temperatures were calculated using the ChlMicaEqui program of Lanari (2012) and using the method of Vidal et al. (2005) with fixed XFe³⁺ ratio corresponding to the µ-XANES results. The semi-empirical thermometer developed by Inoue et al. (2009) was also applied because it was developed for low-temperature chlorite with known XFe³⁺ contents.”
  Line 357 in results: “The temperature conditions of chlorite formation for the four microstructural domains described above were estimated using the XFe³⁺ values determined by µ-XANES synchrotron analyses coupled with microprobe analyses.
  The results obtained with the ChlMicaEqui program of Lanari (2012) are presented in column 1 of Table 1. In the damage zone sample (PPV12-07), chlorites in the releasing overstep of area 1 and in the high angle vein of area 2 exhibit formation temperatures of 270±26 °C and 282±39 °C respectively. In PPV12-05 core zone sample, the temperature of formation of the chlorite in the interboudin of area 1 and at the edge of a mylonitized older V1 quartz vein in area 2 are 276±44 °C and 274±14 °C respectively.
  Regarding Inoue et al. (2009) calculation (Table S4, column 3), in the damage zone sample (PPV12-07), chlorite of area 1 and of area 2 present formation temperature of 282±25 °C and 292±35 °C respectively. In PPV12-05 core zone sample, the temperature of formation of the chlorite of area 1 is 278±30 °C; at the edge of a mylonitized older V1 quartz vein in area 2, the mean calculated temperature is 294±19 °C. These values are much lower than the value obtained without considering the XFe³⁺ ratio (Table 1, column 2).
  Temperatures estimated using Vidal et al. (2005) with fixed values of XFe³⁺ determined by µ-XANES are reported in column 6. In the damage zone sample (PPV12-07), chlorites of area 1 and of area 2 have a formation temperature of 283±20 °C and 292±36 °C respectively. In PPV12-05 core zone sample, the temperature of formation of the chlorites of area 1 and of area 2 are 293±41 °C and 274±11 °C respectively. Those temperature are most of the time slightly higher than the temperature estimated when we let the model estimates the XFe³⁺ ratio. Indeed, for PPV12-07 Area 1, PPV12-07 Area 2, and PPV12-05 Area 1 temperatures are underestimated by about 10°C whereas the modelled underestimate the XFe³⁺ ratio is 0.25 instead of 0.31, 0.23 instead of 0.32 and 0.22 instead of 0.39. Temperature of chlorite formation for PPV12-05 Area 2 are equivalent to the XFe³⁺ ratio.
  For each type of chlorite, the temperatures estimated by the three models considering the XFe³⁺ ratio are very similar. We therefore decided to plot the average values in Figure 9A of the discussion part: about 279°C for PPV12-07 Area 1, 289°C for PPV12-07 Area 2, 282°C for PPV12-05 Area 1, 281°C for PPV12-05 Area 2.”
  
  Line 400 in discussion: “This explanation is confirmed by the equivalent difference we observed between Vidal et al. (2006) temperature calculations with optimised calculated XFe³⁺ values and with XFe³⁺ values determined by µ-XANES. Indeed, considering the XFe³⁺ ratio, can reduce the R²⁺ occupancy and increase the number of octahedral vacancies (e.g. Vidal et al., 2005). As the octahedral vacancy is correlated with temperature (e.g. Lanari et al., 2014), modifying the amount of Fe³⁺can result in different estimated temperature. The temperature variation caused by the introduction of Fe³⁺ content is different for each thermometer (e.g. Inoue et al., 2009 and references therein; Bourdelle et al., 2013; Vidal et al., 2016), as shown table 1. Considering the XFe³⁺ ratio allows for accurate temperature calculation.”
  
  Citation: https://doi.org/10.5194/egusphere-2024-386-AC1
CC1:
'Comment on egusphere-2024-386', Giacomo Medici, 12 Apr 2024

General comments
Good research on circulation of fluids in fault zones. The manuscript needs further detail before publication and can be improved following the comments.

Specific comments
Lines 31-33. “These fault zones typically have important associated fractures...along these fractures”. Please, add recent review papers on channelized fluid circulation in geothermal systems:
- Review of Discrete Fracture Network Characterization for Geothermal Energy Extraction. Frontiers in Earth Science, 11, 1328397.
- Fault zone hydrogeology. Earth-Science Reviews, 127, 171-192.
Lines 60-61. “Phyllosilicates are highly sensitive to pressure, temperature and chemical (P–T–X) conditions”. Please, explain the physico-mineralogical reasons for this sensitivity in your introduction.
Lines 60-61. Explain in more detail the mechanism for the sensitivity of chloride.
Line 81. You should disclose the specific objectives by using numbers (e.g., i, ii, and iii) that looking at your conclusions should be three. Please, revise the final part of your introduction.
Lines 101-102. “Permo-Triassic and Upper Cretaceous strata”. Please, provide more detail on the stratigraphy and the sedimentology of these deposits.
Line 101. ““Permo-Triassic strata”. Are you talking about the fluvio-aolian deposits of Permo-Triassic age that widespread in Europe during that time? Please, specify this point if my observation is correct.
Line 525. Insert a “take home message” after your three conclusive points.
Lines 534-854. Integrate and expand the literature that you have proposed.

Figures and tables
Figure 3. No scale on some outcrop images. You can insert it graphically.
Figures 3 to 6. They can be larger if I compare with Figure 2.
Figure 10. Insert approximate spatial scales to your conceptual schemes.

Citation: https://doi.org/10.5194/egusphere-2024-386-CC1
- AC3: 'Reply on CC1', Delphine CHARPENTIER, 25 May 2024
  
  Dear Dr Medici,
  
  We thank you for the valuable comments you made on our manuscript.
  
  To answer your specific comments (in italic):
  
  1/ Lines 31-33. “These fault zones typically have important associated fractures...along these
  
  fractures”. Please, add recent review papers on channelized fluid circulation in geothermal
  
  systems:
  
  - Review of Discrete Fracture Network Characterization for Geothermal Energy Extraction.
  
  Frontiers in Earth Science, 11, 1328397.
  
  - Fault zone hydrogeology. Earth-Science Reviews, 127, 171-192.
  
  Lines 534 -854. Integrate and expand the literature that you have proposed.
  Authors response: We have added the references you suggested as they explain how the
  
  circulation of channelized fluids occurs in geothermal systems (Line 32).
  
  2/ Lines 60-61. “Phyllosilicates are highly sensitive to pressure, temperature and chemical (P–
  
  T–X) conditions”. Please, explain the physico-mineralogical reasons for this sensitivity in your
  
  introduction.
  
  Lines 60-61. Explain in more detail the mechanism for the sensitivity of chloride.
  
  Authors response: We include a short explanation regarding phyllosilicate sensitivity
  
  (Line 60). “because of their layered structure and variable chemical composition”
  
  3/Line 81. You should disclose the specific objectives by using numbers (e.g., i, ii, and iii) that
  
  looking at your conclusions should be three. Please, revise the final part of your introduction.
  Authors response: As suggested, the different objectives have been disclosed and correlated
  
  with the three points of the conclusion (Lines 78-83).
  
  “First, chlorite chemistry, obtained by X-ray absorption near-edge spectroscopy (XANES) and electron probe
  
  microanalysis (EPMA) on the same synkinematic minerals in clearly identified microstructures of damage and
  
  core zones, was used to trace different fluid circulation events. Then, this dataset is used to refine the local
  
  mechanisms, and temperature and redox conditions of fluid-rock interactions during mineral growth. Finally, a
  
  new model of fluid circulation coupled with the tectonic evolution of the PPVT is also proposed.”
  
  Lines 101-102. “Permo-Triassic and Upper Cretaceous strata”. Please, provide more detail
  
  on the stratigraphy and the sedimentology of these deposits.
  
  Line 101. ““Permo-Triassic strata”. Are you talking about the fluvio-aolian deposits of PermoTriassic age that widespread in Europe during that time? Please, specify this point if my
  
  observation is correct.
  Authors response: We also include more details regarding stratigraphy (Line 109-111).
  
  “The Triassic strata comprises a predominantly mudstone sequence with thin interbedded fining-upwards
  
  sandstone units; the Cretaceous strata is constituted by limestone (Grant, 1990).”
  
  Line 525. Insert a “take home message” after your three conclusive points.
  Authors response: A take home message has been included at the end of the conclusion (Lines
  
  556-559).
  
  “Thanks to this integrated study, we characterize the main processes related to the paleo-fluid circulation within
  
  a fault zone. Micro-XANES spectroscopy provides unique insights, regarding redox properties and their variation
  
  with time due to the mineral-fluids interactions even at the scale of a fault zone. Moreover, iron state quantification
  
  is one of the weakest points of chlorite geothermometry that can be addressed by the methodology applied.”
  
  Figures and tables
  
  Figure 3. No scale on some outcrop images. You can insert it graphically.
  
  Figures 3 to 6. They can be larger if I compare with Figure 2.
  
  Figure 10. Insert approximate spatial scales to your conceptual schemes.
  
  Authors response: The figures have been modified according to your remarks.
  
  Citation: https://doi.org/10.5194/egusphere-2024-386-AC3
RC2:
'Comment on egusphere-2024-386', Fernando Nieto, 23 Apr 2024

This is a careful study on the physical-chemical mechanisms in which fluids interact with the protolith in a fault system, using the case of a secondary fault, which has been very well studied in the previous literature. Even if many of the processes had been previously defined, the novelty of the study is the direct determination of the redox properties and their variation with time due to the mineral reactions of fluids with the pelitic rocks. The Fe³⁺/Fe ratio is one of the weakest points of chlorite geothermometry, as justified by previous literature, due to the lack of adequate methods of in situ determination at the mineral grain level and severely affects the result of the determined temperature. The authors have used complex, expensive, and difficult-to-access XANES spectroscopy, which is one of the very few possibilities for solving this problem. In this way, they checked how the real value of the oxidation state of Fe in chlorite could have affected the apparent temperature differences and inferred real differences along the time and space of fluid redox. Even if the temperatures of formation were roughly known, knowledge of the real variation in the Fe oxidation state would have been impossible without the applied methodology.

Even if the textural analysis of the studied areas is excellent and the methods have been carefully applied, the presentation of the geothermometry results is confusing and lacks significant explanations. As the geothermometry of chlorite using a real value of the Fe oxidation state in chlorite is a significant novelty of the paper, this point should be fully solved before the manuscript is accepted.

- There is a discrepancy between the description of the geothermometers used, as described in lines 350-352, and the presentation of the corresponding results in table S4. Its first column (T1) gives the Inoue´s temperatures, without the application of the Fe³⁺ data, without any explanation about the reason to be presented in this study, whose main interest is the determination of Fe³⁺. In fact, this point is contradictory with lines 351-352: “The latter two require knowledge of Fe³⁺/Fe^total”. Therefore, this sentence applies only to column T2 (not to T1). Column T3 (according to the caption of the table) includes both the Vidal´s and Lanari´s geothermometers, but they are different geothermometers. How do they produce a unique number? Which is more, Lanari´s geothermometer, according to the previous sentence in 351-352, requires the Fe³⁺/Fe data; from this sentence, we can deduce that the authors refers to Chl(1) geothermometer of Lanari, not to Chl(2) (never said in the text!), which does not need Fe³⁺. However, Vidal´s geothermometer does not require Fe³⁺knowledge, as correctly stated in the sentence. In fact, the last column (Modelled XFe³⁺) could have been calculated only using the Vidal´s geothermometer, if not, what is the origin of this column?

- After this confusing presentation, the authors represent in figure 9a, and use during all the discussion, the data coming from column T1, that is, the Inoue´s geothermometer without considering the Fe³⁺ data, just the main novelty of the paper. These temperatures are consequently different from those concluded in the corresponding chapter 4.4 of the results, which uses Fe³⁺ data. Moreover, this use of the Inoue´s geothermometer is not correct, according to the original paper.

- In lines 371-373, the authors claim “It can be observed that the XFe³⁺ values are always underestimated compared to those calculated by μ-XANES analyses, which can explain the underestimation of these calculated temperatures”. Right, this is a very important sentence in the paper and the reason why XANES determination justifies the study. Apparently, they refer to the previously cited column “Modelled XFe³⁺” in table S4, calculated using the Vidal´s geothermometer. This is because those Fe³⁺ values are operative data, necessary for the determination of the temperature, but probably not real values. This is a very important conclusion of the paper, but it is never explained or justified. In fact, for not expert readers, the sentence must be completely obscure, presented like an axiom.

- The opportunity to evaluate the effect of the lack of knowledge of Fe³⁺ on chlorite geothermometers is one of the strengths of this paper, but it has not been sufficiently developed. It would have been very interesting to compare the results with those of semi-empirical geothermometers that use an average Fe³⁺ of natural chlorites (implicit in the used databases of natural cases). Both Bourdelle´s and Inoue(2018)´s thermometers are valid in this range of temperatures, but they have not been calculated in the study.

- I have included an annotated PDF with minor corrections.

Citation: https://doi.org/10.5194/egusphere-2024-386-RC2
- AC2: 'Reply on RC2', Delphine CHARPENTIER, 25 May 2024
  
  Dear Reviewer,
  Thank you for the remarks included in the annotated PDF, they all have been considered. Please
  
  find below the answers to your comments and questions (In italic).
  
  - There is a discrepancy between the description of the geothermometers used, as described in
  
  lines 350-352, and the presentation of the corresponding results in table S4. Its first column (T1) gives
  
  the Inoue´s temperatures, without the application of the Fe3+ data, without any explanation about the
  
  reason to be presented in this study, whose main interest is the determination of Fe3+. In fact, this point
  
  is contradictory with lines 351-352: “The latter two require knowledge of Fe3+/Fetotal”. Therefore, this
  
  sentence applies only to column T2 (not to T1). Column T3 (according to the caption of the table)
  
  includes both the Vidal´s and Lanari´s geothermometers, but they are different geothermometers. How
  
  do they produce a unique number? Which is more, Lanari´s geothermometer, according to the previous
  
  sentence in 351-352, requires the Fe3+/Fe data; from this sentence, we can deduce that the authors refers
  
  to Chl(1) geothermometer of Lanari, not to Chl(2) (never said in the text!), which does not need Fe3+.
  
  However, Vidal´s geothermometer does not require Fe3+ knowledge, as correctly stated in the sentence.
  
  In fact, the last column (Modelled XFe3+) could have been calculated only using the Vidal´s
  
  geothermometer, if not, what is the origin of this column?
  
  Authors response:
  
  There was indeed a discrepancy between the methodology, the description of the geothermometers
  
  used and the results (text and table). We have modified the text in the methodological part (Lines 237-
  
  240) and the description of the results (Lines 357-377) in order to correct this.
  
  Line 237 in methodology: “The estimated XFe3+ values were compared to the XFe3+ values measured by µ-XANES.
  
  Additionally, temperatures were calculated using the ChlMicaEqui program of Lanari (2012) and using the method of
  
  Vidal et al. (2005) with fixed XFe3+ ratio corresponding to the µ-XANES results. The semi-empirical thermometer developed
  
  by Inoue et al. (2009) was also applied because it was developed for low-temperature chlorite with known XFe3+ contents.”
  
  Line 357 in results: “The temperature conditions of chlorite formation for the four microstructural domains described
  
  above were estimated using the XFe3+ values determined by µ-XANES synchrotron analyses coupled with microprobe
  
  analyses.
  
  The results obtained with the ChlMicaEqui program of Lanari (2012) are presented in column 1 of Table 1. In the damage
  
  zone sample (PPV12-07), chlorites in the releasing overstep of area 1 and in the high angle vein of area 2 exhibit formation
  
  temperatures of 270±26 °C and 282±39 °C respectively. In PPV12-05 core zone sample, the temperature of formation of the
  
  chlorite in the interboudin of area 1 and at the edge of a mylonitized older V1 quartz vein in area 2 are 276±44 °C and 274±14
  
  °C respectively.
  
  Regarding Inoue et al. (2009) calculation (Table S4, column 3), in the damage zone sample (PPV12-07), chlorite of area 1
  
  and of area 2 present formation temperature of 282±25 °C and 292±35 °C respectively. In PPV12-05 core zone sample, the
  
  temperature of formation of the chlorite of area 1 is 278±30 °C; at the edge of a mylonitized older V1 quartz vein in area 2,
  
  the mean calculated temperature is 294±19 °C. These values are much lower than the value obtained without considering the
  
  XFe3+ ratio (Table 1, column 2).
  
  Temperatures estimated using Vidal et al. (2005) with fixed values of XFe3+ determined by µ-XANES are reported in column
  
  6. In the damage zone sample (PPV12-07), chlorites of area 1 and of area 2 have a formation temperature of 283±20 °C and
  
  292±36 °C respectively. In PPV12-05 core zone sample, the temperature of formation of the chlorites of area 1 and of area 2
  
  are 293±41 °C and 274±11 °C respectively. Those temperature are most of the time slightly higher than the temperature
  
  estimated when we let the model estimates the XFe3+ ratio. Indeed, for PPV12-07 Area 1, PPV12-07 Area 2, and PPV12-05
  
  Area 1 temperatures are underestimated by about 10°C whereas the modelled underestimate the XFe3+ ratio is 0.25 instead
  
  of 0.31, 0.23 instead of 0.32 and 0.22 instead of 0.39. Temperature of chlorite formation for PPV12-05 Area 2 are equivalent
  
  to the XFe3+ ratio.
  
  For each type of chlorite, the temperatures estimated by the three models considering the XFe3+ ratio are very similar. We
  
  therefore decided to plot the average values in Figure 9A of the discussion part: about 279°C for PPV12-07 Area 1, 289°C
  
  for PPV12-07 Area 2, 282°C for PPV12-05 Area 1, 281°C for PPV12-05 Area 2.”
  
  Moreover, Table 4 presentation was confusing. It was completely re-organized. Now, we clearly
  
  indicate results obtained with XFe3+ determined by µ-XANES but we also present results obtained with
  
  Fetotal = Fe2+ (Inoue et al., 2009) and with modelled XFe (Vidal et al., 2005). In the results part, we focus
  
  the description of the results obtained with XFe3+ determined by µ-XANES, but we use results obtained
  
  with Fetotal = Fe2+ and obtained with modelled XFe as comparison (Lines 370). Now this table is widely
  
  used in the results section, we added it in the manuscript and it is no longer in the appendix (Line 378).
  
  - After this confusing presentation, the authors represent in figure 9a, and use during all the
  
  discussion, the data coming from column T1, that is, the Inoue´s geothermometer without considering
  
  the Fe3+ data, just the main novelty of the paper. These temperatures are consequently different from
  
  those concluded in the corresponding chapter 4.4 of the results, which uses Fe3+ data. Moreover, this
  
  use of the Inoue´s geothermometer is not correct, according to the original paper.
  
  Authors response:
  
  We fully agree that the values used in figure 9a are not the correct ones. We decide to plot on
  
  Figure 9 the average values obtained with the three modeling performed using XFe3+ determined by µ-
  
  XANES as the results are very closed (Lines 375-377).
  
  “For each type of chlorite, the temperatures estimated by the three models considering the XFe3+ ratio are very similar. We
  
  therefore decided to plot the average values in Figure 9A of the discussion part: about 279°C for PPV12-07 Area 1, 289°C
  
  for PPV12-07 Area 2, 282°C for PPV12-05 Area 1, 281°C for PPV12-05 Area 2.”
  
  - In lines 371-373, the authors claim “It can be observed that the XFe3+ values are always
  
  underestimated compared to those calculated by μ-XANES analyses, which can explain the
  
  underestimation of these calculated temperatures”. Right, this is a very important sentence in the paper
  
  and the reason why XANES determination justifies the study. Apparently, they refer to the previously
  
  cited column “Modelled XFe3+” in table S4, calculated using the Vidal´s geothermometer. This is
  
  because those Fe3+ values are operative data, necessary for the determination of the temperature, but
  
  probably not real values. This is a very important conclusion of the paper, but it is never explained or
  
  justified. In fact, for not expert readers, the sentence must be completely obscure, presented like an
  
  axiom.
  
  Authors response:
  
  We highlight that the XFe3+ values are always underestimated compared to those calculated using
  
  μ-XANES analyses, which can explain the underestimation of these calculated temperatures. It is now
  
  mentioned in the result part (Lines 364-373) and discussed in the first part of the discussion (Lines 399-
  
  405). This is also added in the conclusion (Lines 557-5609).
  
  Line 364 in results: “Regarding Inoue et al. (2009) calculation (Table S4, column 3), in the damage zone sample (PPV12-
  
  07), chlorite of area 1 and of area 2 present formation temperature of 282±25 °C and 292±35 °C respectively. In PPV12-05
  
  core zone sample, the temperature of formation of the chlorite of area 1 is 278±30 °C; at the edge of a mylonitized older V1
  
  quartz vein in area 2, the mean calculated temperature is 294±19 °C. These values are much lower than the value obtained
  
  without considering the XFe3+ ratio (Table 1, column 2).”
  
  Line 399 in discussion: “This explanation is confirmed by the equivalent difference we observed between Vidal et al.
  
  (2006) temperature calculations with optimised calculated XFe3+ values and with XFe3+ values determined by µ-XANES.
  
  Indeed, considering the XFe3+ ratio, can reduce the R2+ occupancy and increase the number of octahedral vacancies (e.g.
  
  Vidal et al., 2005). As the octahedral vacancy is correlated with temperature (e.g. Lanari et al., 2014), modifying the amount
  
  of Fe3+ can result in different estimated temperature. The temperature variation caused by the introduction of Fe3+ content is
  
  different for each thermometer (e.g. Inoue et al., 2009 and references therein; Bourdelle et al., 2013; Vidal et al., 2016), as
  
  shown table 1. Considering the XFe3+ ratio allows for accurate temperature calculation.”
  
  Line 557 in conclusion: “Thanks to this integrated study, we characterize the main processes related to the paleo-fluid
  
  circulation within a fault zone. Micro-XANES spectroscopy provides unique insights, regarding redox properties and their
  
  variation with time due to the mineral-fluids interactions even at the scale of a fault zone. Moreover, iron state quantification
  
  is one of the weakest points of chlorite geothermometry that can be addressed by the methodology applied.”
  
  - The opportunity to evaluate the effect of the lack of knowledge of Fe3+ on chlorite
  
  geothermometers is one of the strengths of this paper, but it has not been sufficiently developed. It would
  
  have been very interesting to compare the results with those of semi-empirical geothermometers that use
  
  an average Fe3+ of natural chlorites (implicit in the used databases of natural cases). Both Bourdelle´s
  
  and Inoue (2018)´s thermometers are valid in this range of temperatures, but they have not been
  
  calculated in the study.
  Authors response:
  
  We added more precision about this subject in the text (see previous comment). This paper
  
  constitutes a first point of discussion, but to strengthen our conclusions, it will be necessary to perform
  
  a large study based on chlorites formed at different temperature and from various context. We hope this
  
  study opens new perspectives and questions on the use of chlorite thermometers at scale of a fault zone.
  
  This paper also highlights the importance that in absence of XFe3+ determined by µ-XANES, chlorite
  
  temperatures must be considered carefully.
  
  Citation: https://doi.org/10.5194/egusphere-2024-386-AC2

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2024-386', Anonymous Referee #1, 08 Apr 2024

This article aims to better understand fluid circulation in fractured zones in the Pyrenees axial zone, based on chlorite geochemistry and thermometry.
The article is well-written and well-organized, and the conclusions seem well-founded. However, this article leaves an impression of incompleteness, a sense of lacking of data. In fact, the article heavily relies on the previous work by Trincal et al 2015, and Abd Elmola et al 2017, and provides only (as new data) about 30 microprobe analyses and 15 XANES data, obtained on 2 samples. That's all. Were there more analyses? Have some been discarded? On what basis? It is said that chlorite and quartz in veins are co-genetic, chlorite being intimately interwoven with quartz. Why not combine chlorite thermometry with microthermometry on fluid inclusions in quartz? Even at the end, it's hard to identify truly new conclusions from those already formulated by Trincal et al 2015 and Abd Elmola et al 2017.
And even in terms of form: Why no EMP mapping? Why not present XANES spectra representative of each "area"? Line 182: "Two representative samples were selected". In what way are they representative and, on their own (2), allow solid conclusions to be drawn? They have already been studied by Abd Elmolah et al 2017, so why not other samples?
Line 234: « Estimated XFe3+ can be compared with weasured XFe3+ ». Yes, but this is not done. The only mention is line 370 "the XFe3+ values are always underestimated compared to those calculated by XANES analyses, which can explain the underestimation of these calculated temperatures (i.e. calculated by Abd Elmola et al 2017)". Yet underestimating Fe3+ means underestimating octahedral vacancies, and therefore overestimating calculated temperatures. No?
In short, despite its editorial qualities, I am left with an appetite for more. The paucity of new analyses prevents me from describing this paper as innovative or very usefull.

Citation: https://doi.org/10.5194/egusphere-2024-386-RC1
- AC1: 'Reply on RC1', Delphine CHARPENTIER, 25 May 2024
  
  Dear Reviewer,
  Please find below the answers to your comments and questions (italic).
  
  The article is well-written and well-organized, and the conclusions seem well-founded. However, this article leaves an impression of incompleteness, a sense of lacking of data. In fact, the article heavily relies on the previous work by Trincal et al 2015, and Abd Elmola et al 2017, and provides only (as new data) about 30 microprobe analyses and 15 XANES data, obtained on 2 samples. That's all. Were there more analyses? Have some been discarded? On what basis? It is said that chlorite and quartz in veins are co-genetic, chlorite being intimately interwoven with quartz. Why not combine chlorite thermometry with microthermometry on fluid inclusions in quartz? Even at the end, it's hard to identify truly new conclusions from those already formulated by Trincal et al 2015 and Abd Elmola et al 2017.
  Authors response:
  The aim of this article is to investigate the main processes related to the paleo-fluid circulation within the fault zone. We therefore coupled microstructural analyses with geochemical and mineralogical characterisation. The novelty of the study is to focus on chlorite and specially on its iron oxidation state to show that: 1) chlorite is a good tracker of redox properties and their variation with time due to the mineral reactions of fluids with the pelitic rocks at the scale of a fault zone, 2) the Fe³⁺/Fe ratio is one of the weakest points of chlorite geothermometry (only accessible using µ-XANES spectroscopy) ; it is an information useful to calculate temperature of chlorite formation but usually neglected as it is difficult to quantify it. As suggested, we modified the manuscript to identify truly new conclusions from those already formulated by Trincal et al 2015 and Abd Elmola et al 2017. (Line 25, Line 79, Line 557-560).
  Line 25 in abstract: “This study shows the importance to determine redox state of iron in chlorite to calculate their temperature of formations and to consider the fluid evolution at the scale of a fault.”
  Line 79 in introduction: “First, chlorite chemistry, obtained by X-ray absorption near-edge spectroscopy (XANES) and electron probe microanalysis (EPMA) on the same synkinematic minerals in clearly identified microstructures of damage and core zones, was used to trace different fluid circulation events. Then, this dataset is used to refine the local mechanisms, and temperature and redox conditions of fluid-rock interactions during mineral growth. Finally, a new model of fluid circulation coupled with the tectonic evolution of the PPVT is also proposed.”
  Line 557 in conclusion: “Thanks to this integrated study, we characterize the main processes related to the paleo-fluid circulation within a fault zone. Micro-XANES spectroscopy provides unique insights, regarding redox properties and their variation with time due to the mineral-fluids interactions even at the scale of a fault zone. Moreover, iron state quantification is one of the weakest points of chlorite geothermometry that can be addressed by the methodology applied.”
  
  The µ-XANES spectroscopy is complex, expensive, and difficult-to-access. The twenty analyses we performed correspond to the 24h of the BM23 beamline of the European Synchrotron Radiation Facility we obtained through the program ESRF ES548 financed by INSU. All the µ-XANES data obtained are presented in this article (except one that failed).
  As shown in the results (Figure 9, modified following RC2 comments), fluid temperatures associated with chlorite formation are quite similar (270 to 285°C), only redox of fluid varies (µ-XANES data). Microthermometry on fluid inclusions (Grant, 1989) does not allow to be more precise on the temperature determination. Furthermore, the origin of the fluid is identical, as specified in Cathelineau et al; (2021).
  
  And even in terms of form: Why no EMP mapping? Why not present XANES spectra representative of each "area"? Line 182: "Two representative samples were selected". In what way are they representative and, on their own (2), allow solid conclusions to be drawn? They have already been studied by Abd Elmolah et al 2017, so why not other samples?
  Authors response:
  We selected one sample that present all the microstructural patterns observed in the core zone and that present a typical mineralogical composition of the core zone. The second sample is representative of the damage zone. Abd Elmola et al., 2017 provides the global composition of chlorite particles from the core zone and from the damage zone. We completed this study by performing microprobe analyses on the particles analysed by µ-XANES to be able to provide structural formulae with Fe²⁺ and Fe³⁺ contents, and clearly correlated the composition, the iron oxidation state and the microstructural position of chlorite. We performed punctual analyses because 1) chlorite particles are thin and small, unlike the rosette-shaped aggregates mapped in Trincal et al 2015 and 2) punctual analyses show homogeneous composition (see appendix tables). As µ-XANES data are scarce and precious, we added µ-XANES spectra in appendix.
  
  Line 234: « Estimated XFe3+ can be compared with weasured XFe3+ ». Yes, but this is not done. The only mention is line 370 "the XFe3+ values are always underestimated compared to those calculated by XANES analyses, which can explain the underestimation of these calculated temperatures (i.e. calculated by Abd Elmola et al 2017)". Yet underestimating Fe3+ means underestimating octahedral vacancies, and therefore overestimating calculated temperatures. No?
  Authors response:
  There was a discrepancy between the methodology, the description of the geothermometers used and the results (text and table). We have modified the text in the methodological part (Lines 237-240), the description of the results (Lines 357-377) and the results in table S4 in order to remedy this. The relation between XFe3+ underestimation and temperature modification are now clearly mentioned (Lines 371-375 and 400-405).
  Line 237 in methodology: “The estimated XFe³⁺ values were compared to the XFe³⁺ values measured by µ-XANES.
  Additionally, temperatures were calculated using the ChlMicaEqui program of Lanari (2012) and using the method of Vidal et al. (2005) with fixed XFe³⁺ ratio corresponding to the µ-XANES results. The semi-empirical thermometer developed by Inoue et al. (2009) was also applied because it was developed for low-temperature chlorite with known XFe³⁺ contents.”
  Line 357 in results: “The temperature conditions of chlorite formation for the four microstructural domains described above were estimated using the XFe³⁺ values determined by µ-XANES synchrotron analyses coupled with microprobe analyses.
  The results obtained with the ChlMicaEqui program of Lanari (2012) are presented in column 1 of Table 1. In the damage zone sample (PPV12-07), chlorites in the releasing overstep of area 1 and in the high angle vein of area 2 exhibit formation temperatures of 270±26 °C and 282±39 °C respectively. In PPV12-05 core zone sample, the temperature of formation of the chlorite in the interboudin of area 1 and at the edge of a mylonitized older V1 quartz vein in area 2 are 276±44 °C and 274±14 °C respectively.
  Regarding Inoue et al. (2009) calculation (Table S4, column 3), in the damage zone sample (PPV12-07), chlorite of area 1 and of area 2 present formation temperature of 282±25 °C and 292±35 °C respectively. In PPV12-05 core zone sample, the temperature of formation of the chlorite of area 1 is 278±30 °C; at the edge of a mylonitized older V1 quartz vein in area 2, the mean calculated temperature is 294±19 °C. These values are much lower than the value obtained without considering the XFe³⁺ ratio (Table 1, column 2).
  Temperatures estimated using Vidal et al. (2005) with fixed values of XFe³⁺ determined by µ-XANES are reported in column 6. In the damage zone sample (PPV12-07), chlorites of area 1 and of area 2 have a formation temperature of 283±20 °C and 292±36 °C respectively. In PPV12-05 core zone sample, the temperature of formation of the chlorites of area 1 and of area 2 are 293±41 °C and 274±11 °C respectively. Those temperature are most of the time slightly higher than the temperature estimated when we let the model estimates the XFe³⁺ ratio. Indeed, for PPV12-07 Area 1, PPV12-07 Area 2, and PPV12-05 Area 1 temperatures are underestimated by about 10°C whereas the modelled underestimate the XFe³⁺ ratio is 0.25 instead of 0.31, 0.23 instead of 0.32 and 0.22 instead of 0.39. Temperature of chlorite formation for PPV12-05 Area 2 are equivalent to the XFe³⁺ ratio.
  For each type of chlorite, the temperatures estimated by the three models considering the XFe³⁺ ratio are very similar. We therefore decided to plot the average values in Figure 9A of the discussion part: about 279°C for PPV12-07 Area 1, 289°C for PPV12-07 Area 2, 282°C for PPV12-05 Area 1, 281°C for PPV12-05 Area 2.”
  
  Line 400 in discussion: “This explanation is confirmed by the equivalent difference we observed between Vidal et al. (2006) temperature calculations with optimised calculated XFe³⁺ values and with XFe³⁺ values determined by µ-XANES. Indeed, considering the XFe³⁺ ratio, can reduce the R²⁺ occupancy and increase the number of octahedral vacancies (e.g. Vidal et al., 2005). As the octahedral vacancy is correlated with temperature (e.g. Lanari et al., 2014), modifying the amount of Fe³⁺can result in different estimated temperature. The temperature variation caused by the introduction of Fe³⁺ content is different for each thermometer (e.g. Inoue et al., 2009 and references therein; Bourdelle et al., 2013; Vidal et al., 2016), as shown table 1. Considering the XFe³⁺ ratio allows for accurate temperature calculation.”
  
  Citation: https://doi.org/10.5194/egusphere-2024-386-AC1
CC1:
'Comment on egusphere-2024-386', Giacomo Medici, 12 Apr 2024

General comments
Good research on circulation of fluids in fault zones. The manuscript needs further detail before publication and can be improved following the comments.

Specific comments
Lines 31-33. “These fault zones typically have important associated fractures...along these fractures”. Please, add recent review papers on channelized fluid circulation in geothermal systems:
- Review of Discrete Fracture Network Characterization for Geothermal Energy Extraction. Frontiers in Earth Science, 11, 1328397.
- Fault zone hydrogeology. Earth-Science Reviews, 127, 171-192.
Lines 60-61. “Phyllosilicates are highly sensitive to pressure, temperature and chemical (P–T–X) conditions”. Please, explain the physico-mineralogical reasons for this sensitivity in your introduction.
Lines 60-61. Explain in more detail the mechanism for the sensitivity of chloride.
Line 81. You should disclose the specific objectives by using numbers (e.g., i, ii, and iii) that looking at your conclusions should be three. Please, revise the final part of your introduction.
Lines 101-102. “Permo-Triassic and Upper Cretaceous strata”. Please, provide more detail on the stratigraphy and the sedimentology of these deposits.
Line 101. ““Permo-Triassic strata”. Are you talking about the fluvio-aolian deposits of Permo-Triassic age that widespread in Europe during that time? Please, specify this point if my observation is correct.
Line 525. Insert a “take home message” after your three conclusive points.
Lines 534-854. Integrate and expand the literature that you have proposed.

Figures and tables
Figure 3. No scale on some outcrop images. You can insert it graphically.
Figures 3 to 6. They can be larger if I compare with Figure 2.
Figure 10. Insert approximate spatial scales to your conceptual schemes.

Citation: https://doi.org/10.5194/egusphere-2024-386-CC1
- AC3: 'Reply on CC1', Delphine CHARPENTIER, 25 May 2024
  
  Dear Dr Medici,
  
  We thank you for the valuable comments you made on our manuscript.
  
  To answer your specific comments (in italic):
  
  1/ Lines 31-33. “These fault zones typically have important associated fractures...along these
  
  fractures”. Please, add recent review papers on channelized fluid circulation in geothermal
  
  systems:
  
  - Review of Discrete Fracture Network Characterization for Geothermal Energy Extraction.
  
  Frontiers in Earth Science, 11, 1328397.
  
  - Fault zone hydrogeology. Earth-Science Reviews, 127, 171-192.
  
  Lines 534 -854. Integrate and expand the literature that you have proposed.
  Authors response: We have added the references you suggested as they explain how the
  
  circulation of channelized fluids occurs in geothermal systems (Line 32).
  
  2/ Lines 60-61. “Phyllosilicates are highly sensitive to pressure, temperature and chemical (P–
  
  T–X) conditions”. Please, explain the physico-mineralogical reasons for this sensitivity in your
  
  introduction.
  
  Lines 60-61. Explain in more detail the mechanism for the sensitivity of chloride.
  
  Authors response: We include a short explanation regarding phyllosilicate sensitivity
  
  (Line 60). “because of their layered structure and variable chemical composition”
  
  3/Line 81. You should disclose the specific objectives by using numbers (e.g., i, ii, and iii) that
  
  looking at your conclusions should be three. Please, revise the final part of your introduction.
  Authors response: As suggested, the different objectives have been disclosed and correlated
  
  with the three points of the conclusion (Lines 78-83).
  
  “First, chlorite chemistry, obtained by X-ray absorption near-edge spectroscopy (XANES) and electron probe
  
  microanalysis (EPMA) on the same synkinematic minerals in clearly identified microstructures of damage and
  
  core zones, was used to trace different fluid circulation events. Then, this dataset is used to refine the local
  
  mechanisms, and temperature and redox conditions of fluid-rock interactions during mineral growth. Finally, a
  
  new model of fluid circulation coupled with the tectonic evolution of the PPVT is also proposed.”
  
  Lines 101-102. “Permo-Triassic and Upper Cretaceous strata”. Please, provide more detail
  
  on the stratigraphy and the sedimentology of these deposits.
  
  Line 101. ““Permo-Triassic strata”. Are you talking about the fluvio-aolian deposits of PermoTriassic age that widespread in Europe during that time? Please, specify this point if my
  
  observation is correct.
  Authors response: We also include more details regarding stratigraphy (Line 109-111).
  
  “The Triassic strata comprises a predominantly mudstone sequence with thin interbedded fining-upwards
  
  sandstone units; the Cretaceous strata is constituted by limestone (Grant, 1990).”
  
  Line 525. Insert a “take home message” after your three conclusive points.
  Authors response: A take home message has been included at the end of the conclusion (Lines
  
  556-559).
  
  “Thanks to this integrated study, we characterize the main processes related to the paleo-fluid circulation within
  
  a fault zone. Micro-XANES spectroscopy provides unique insights, regarding redox properties and their variation
  
  with time due to the mineral-fluids interactions even at the scale of a fault zone. Moreover, iron state quantification
  
  is one of the weakest points of chlorite geothermometry that can be addressed by the methodology applied.”
  
  Figures and tables
  
  Figure 3. No scale on some outcrop images. You can insert it graphically.
  
  Figures 3 to 6. They can be larger if I compare with Figure 2.
  
  Figure 10. Insert approximate spatial scales to your conceptual schemes.
  
  Authors response: The figures have been modified according to your remarks.
  
  Citation: https://doi.org/10.5194/egusphere-2024-386-AC3
RC2:
'Comment on egusphere-2024-386', Fernando Nieto, 23 Apr 2024

This is a careful study on the physical-chemical mechanisms in which fluids interact with the protolith in a fault system, using the case of a secondary fault, which has been very well studied in the previous literature. Even if many of the processes had been previously defined, the novelty of the study is the direct determination of the redox properties and their variation with time due to the mineral reactions of fluids with the pelitic rocks. The Fe³⁺/Fe ratio is one of the weakest points of chlorite geothermometry, as justified by previous literature, due to the lack of adequate methods of in situ determination at the mineral grain level and severely affects the result of the determined temperature. The authors have used complex, expensive, and difficult-to-access XANES spectroscopy, which is one of the very few possibilities for solving this problem. In this way, they checked how the real value of the oxidation state of Fe in chlorite could have affected the apparent temperature differences and inferred real differences along the time and space of fluid redox. Even if the temperatures of formation were roughly known, knowledge of the real variation in the Fe oxidation state would have been impossible without the applied methodology.

Even if the textural analysis of the studied areas is excellent and the methods have been carefully applied, the presentation of the geothermometry results is confusing and lacks significant explanations. As the geothermometry of chlorite using a real value of the Fe oxidation state in chlorite is a significant novelty of the paper, this point should be fully solved before the manuscript is accepted.

- There is a discrepancy between the description of the geothermometers used, as described in lines 350-352, and the presentation of the corresponding results in table S4. Its first column (T1) gives the Inoue´s temperatures, without the application of the Fe³⁺ data, without any explanation about the reason to be presented in this study, whose main interest is the determination of Fe³⁺. In fact, this point is contradictory with lines 351-352: “The latter two require knowledge of Fe³⁺/Fe^total”. Therefore, this sentence applies only to column T2 (not to T1). Column T3 (according to the caption of the table) includes both the Vidal´s and Lanari´s geothermometers, but they are different geothermometers. How do they produce a unique number? Which is more, Lanari´s geothermometer, according to the previous sentence in 351-352, requires the Fe³⁺/Fe data; from this sentence, we can deduce that the authors refers to Chl(1) geothermometer of Lanari, not to Chl(2) (never said in the text!), which does not need Fe³⁺. However, Vidal´s geothermometer does not require Fe³⁺knowledge, as correctly stated in the sentence. In fact, the last column (Modelled XFe³⁺) could have been calculated only using the Vidal´s geothermometer, if not, what is the origin of this column?

- After this confusing presentation, the authors represent in figure 9a, and use during all the discussion, the data coming from column T1, that is, the Inoue´s geothermometer without considering the Fe³⁺ data, just the main novelty of the paper. These temperatures are consequently different from those concluded in the corresponding chapter 4.4 of the results, which uses Fe³⁺ data. Moreover, this use of the Inoue´s geothermometer is not correct, according to the original paper.

- In lines 371-373, the authors claim “It can be observed that the XFe³⁺ values are always underestimated compared to those calculated by μ-XANES analyses, which can explain the underestimation of these calculated temperatures”. Right, this is a very important sentence in the paper and the reason why XANES determination justifies the study. Apparently, they refer to the previously cited column “Modelled XFe³⁺” in table S4, calculated using the Vidal´s geothermometer. This is because those Fe³⁺ values are operative data, necessary for the determination of the temperature, but probably not real values. This is a very important conclusion of the paper, but it is never explained or justified. In fact, for not expert readers, the sentence must be completely obscure, presented like an axiom.

- The opportunity to evaluate the effect of the lack of knowledge of Fe³⁺ on chlorite geothermometers is one of the strengths of this paper, but it has not been sufficiently developed. It would have been very interesting to compare the results with those of semi-empirical geothermometers that use an average Fe³⁺ of natural chlorites (implicit in the used databases of natural cases). Both Bourdelle´s and Inoue(2018)´s thermometers are valid in this range of temperatures, but they have not been calculated in the study.

- I have included an annotated PDF with minor corrections.

Citation: https://doi.org/10.5194/egusphere-2024-386-RC2
- AC2: 'Reply on RC2', Delphine CHARPENTIER, 25 May 2024
  
  Dear Reviewer,
  Thank you for the remarks included in the annotated PDF, they all have been considered. Please
  
  find below the answers to your comments and questions (In italic).
  
  - There is a discrepancy between the description of the geothermometers used, as described in
  
  lines 350-352, and the presentation of the corresponding results in table S4. Its first column (T1) gives
  
  the Inoue´s temperatures, without the application of the Fe3+ data, without any explanation about the
  
  reason to be presented in this study, whose main interest is the determination of Fe3+. In fact, this point
  
  is contradictory with lines 351-352: “The latter two require knowledge of Fe3+/Fetotal”. Therefore, this
  
  sentence applies only to column T2 (not to T1). Column T3 (according to the caption of the table)
  
  includes both the Vidal´s and Lanari´s geothermometers, but they are different geothermometers. How
  
  do they produce a unique number? Which is more, Lanari´s geothermometer, according to the previous
  
  sentence in 351-352, requires the Fe3+/Fe data; from this sentence, we can deduce that the authors refers
  
  to Chl(1) geothermometer of Lanari, not to Chl(2) (never said in the text!), which does not need Fe3+.
  
  However, Vidal´s geothermometer does not require Fe3+ knowledge, as correctly stated in the sentence.
  
  In fact, the last column (Modelled XFe3+) could have been calculated only using the Vidal´s
  
  geothermometer, if not, what is the origin of this column?
  
  Authors response:
  
  There was indeed a discrepancy between the methodology, the description of the geothermometers
  
  used and the results (text and table). We have modified the text in the methodological part (Lines 237-
  
  240) and the description of the results (Lines 357-377) in order to correct this.
  
  Line 237 in methodology: “The estimated XFe3+ values were compared to the XFe3+ values measured by µ-XANES.
  
  Additionally, temperatures were calculated using the ChlMicaEqui program of Lanari (2012) and using the method of
  
  Vidal et al. (2005) with fixed XFe3+ ratio corresponding to the µ-XANES results. The semi-empirical thermometer developed
  
  by Inoue et al. (2009) was also applied because it was developed for low-temperature chlorite with known XFe3+ contents.”
  
  Line 357 in results: “The temperature conditions of chlorite formation for the four microstructural domains described
  
  above were estimated using the XFe3+ values determined by µ-XANES synchrotron analyses coupled with microprobe
  
  analyses.
  
  The results obtained with the ChlMicaEqui program of Lanari (2012) are presented in column 1 of Table 1. In the damage
  
  zone sample (PPV12-07), chlorites in the releasing overstep of area 1 and in the high angle vein of area 2 exhibit formation
  
  temperatures of 270±26 °C and 282±39 °C respectively. In PPV12-05 core zone sample, the temperature of formation of the
  
  chlorite in the interboudin of area 1 and at the edge of a mylonitized older V1 quartz vein in area 2 are 276±44 °C and 274±14
  
  °C respectively.
  
  Regarding Inoue et al. (2009) calculation (Table S4, column 3), in the damage zone sample (PPV12-07), chlorite of area 1
  
  and of area 2 present formation temperature of 282±25 °C and 292±35 °C respectively. In PPV12-05 core zone sample, the
  
  temperature of formation of the chlorite of area 1 is 278±30 °C; at the edge of a mylonitized older V1 quartz vein in area 2,
  
  the mean calculated temperature is 294±19 °C. These values are much lower than the value obtained without considering the
  
  XFe3+ ratio (Table 1, column 2).
  
  Temperatures estimated using Vidal et al. (2005) with fixed values of XFe3+ determined by µ-XANES are reported in column
  
  6. In the damage zone sample (PPV12-07), chlorites of area 1 and of area 2 have a formation temperature of 283±20 °C and
  
  292±36 °C respectively. In PPV12-05 core zone sample, the temperature of formation of the chlorites of area 1 and of area 2
  
  are 293±41 °C and 274±11 °C respectively. Those temperature are most of the time slightly higher than the temperature
  
  estimated when we let the model estimates the XFe3+ ratio. Indeed, for PPV12-07 Area 1, PPV12-07 Area 2, and PPV12-05
  
  Area 1 temperatures are underestimated by about 10°C whereas the modelled underestimate the XFe3+ ratio is 0.25 instead
  
  of 0.31, 0.23 instead of 0.32 and 0.22 instead of 0.39. Temperature of chlorite formation for PPV12-05 Area 2 are equivalent
  
  to the XFe3+ ratio.
  
  For each type of chlorite, the temperatures estimated by the three models considering the XFe3+ ratio are very similar. We
  
  therefore decided to plot the average values in Figure 9A of the discussion part: about 279°C for PPV12-07 Area 1, 289°C
  
  for PPV12-07 Area 2, 282°C for PPV12-05 Area 1, 281°C for PPV12-05 Area 2.”
  
  Moreover, Table 4 presentation was confusing. It was completely re-organized. Now, we clearly
  
  indicate results obtained with XFe3+ determined by µ-XANES but we also present results obtained with
  
  Fetotal = Fe2+ (Inoue et al., 2009) and with modelled XFe (Vidal et al., 2005). In the results part, we focus
  
  the description of the results obtained with XFe3+ determined by µ-XANES, but we use results obtained
  
  with Fetotal = Fe2+ and obtained with modelled XFe as comparison (Lines 370). Now this table is widely
  
  used in the results section, we added it in the manuscript and it is no longer in the appendix (Line 378).
  
  - After this confusing presentation, the authors represent in figure 9a, and use during all the
  
  discussion, the data coming from column T1, that is, the Inoue´s geothermometer without considering
  
  the Fe3+ data, just the main novelty of the paper. These temperatures are consequently different from
  
  those concluded in the corresponding chapter 4.4 of the results, which uses Fe3+ data. Moreover, this
  
  use of the Inoue´s geothermometer is not correct, according to the original paper.
  
  Authors response:
  
  We fully agree that the values used in figure 9a are not the correct ones. We decide to plot on
  
  Figure 9 the average values obtained with the three modeling performed using XFe3+ determined by µ-
  
  XANES as the results are very closed (Lines 375-377).
  
  “For each type of chlorite, the temperatures estimated by the three models considering the XFe3+ ratio are very similar. We
  
  therefore decided to plot the average values in Figure 9A of the discussion part: about 279°C for PPV12-07 Area 1, 289°C
  
  for PPV12-07 Area 2, 282°C for PPV12-05 Area 1, 281°C for PPV12-05 Area 2.”
  
  - In lines 371-373, the authors claim “It can be observed that the XFe3+ values are always
  
  underestimated compared to those calculated by μ-XANES analyses, which can explain the
  
  underestimation of these calculated temperatures”. Right, this is a very important sentence in the paper
  
  and the reason why XANES determination justifies the study. Apparently, they refer to the previously
  
  cited column “Modelled XFe3+” in table S4, calculated using the Vidal´s geothermometer. This is
  
  because those Fe3+ values are operative data, necessary for the determination of the temperature, but
  
  probably not real values. This is a very important conclusion of the paper, but it is never explained or
  
  justified. In fact, for not expert readers, the sentence must be completely obscure, presented like an
  
  axiom.
  
  Authors response:
  
  We highlight that the XFe3+ values are always underestimated compared to those calculated using
  
  μ-XANES analyses, which can explain the underestimation of these calculated temperatures. It is now
  
  mentioned in the result part (Lines 364-373) and discussed in the first part of the discussion (Lines 399-
  
  405). This is also added in the conclusion (Lines 557-5609).
  
  Line 364 in results: “Regarding Inoue et al. (2009) calculation (Table S4, column 3), in the damage zone sample (PPV12-
  
  07), chlorite of area 1 and of area 2 present formation temperature of 282±25 °C and 292±35 °C respectively. In PPV12-05
  
  core zone sample, the temperature of formation of the chlorite of area 1 is 278±30 °C; at the edge of a mylonitized older V1
  
  quartz vein in area 2, the mean calculated temperature is 294±19 °C. These values are much lower than the value obtained
  
  without considering the XFe3+ ratio (Table 1, column 2).”
  
  Line 399 in discussion: “This explanation is confirmed by the equivalent difference we observed between Vidal et al.
  
  (2006) temperature calculations with optimised calculated XFe3+ values and with XFe3+ values determined by µ-XANES.
  
  Indeed, considering the XFe3+ ratio, can reduce the R2+ occupancy and increase the number of octahedral vacancies (e.g.
  
  Vidal et al., 2005). As the octahedral vacancy is correlated with temperature (e.g. Lanari et al., 2014), modifying the amount
  
  of Fe3+ can result in different estimated temperature. The temperature variation caused by the introduction of Fe3+ content is
  
  different for each thermometer (e.g. Inoue et al., 2009 and references therein; Bourdelle et al., 2013; Vidal et al., 2016), as
  
  shown table 1. Considering the XFe3+ ratio allows for accurate temperature calculation.”
  
  Line 557 in conclusion: “Thanks to this integrated study, we characterize the main processes related to the paleo-fluid
  
  circulation within a fault zone. Micro-XANES spectroscopy provides unique insights, regarding redox properties and their
  
  variation with time due to the mineral-fluids interactions even at the scale of a fault zone. Moreover, iron state quantification
  
  is one of the weakest points of chlorite geothermometry that can be addressed by the methodology applied.”
  
  - The opportunity to evaluate the effect of the lack of knowledge of Fe3+ on chlorite
  
  geothermometers is one of the strengths of this paper, but it has not been sufficiently developed. It would
  
  have been very interesting to compare the results with those of semi-empirical geothermometers that use
  
  an average Fe3+ of natural chlorites (implicit in the used databases of natural cases). Both Bourdelle´s
  
  and Inoue (2018)´s thermometers are valid in this range of temperatures, but they have not been
  
  calculated in the study.
  Authors response:
  
  We added more precision about this subject in the text (see previous comment). This paper
  
  constitutes a first point of discussion, but to strengthen our conclusions, it will be necessary to perform
  
  a large study based on chlorites formed at different temperature and from various context. We hope this
  
  study opens new perspectives and questions on the use of chlorite thermometers at scale of a fault zone.
  
  This paper also highlights the importance that in absence of XFe3+ determined by µ-XANES, chlorite
  
  temperatures must be considered carefully.
  
  Citation: https://doi.org/10.5194/egusphere-2024-386-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by Delphine CHARPENTIER on behalf of the Authors (25 May 2024) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (28 May 2024) by Federico Rossetti

RR by Fernando Nieto (10 Jun 2024)

Suggestions for revision or reasons for rejection

The main problems of the first version, related to the disorder in the correspondence among methodological description, presentation of results, tables, and figures, have been solved with the new organisation, in which the different data are coherent and clearly presented. Nevertheless, the changes have produced a new disorder, which probably superposed to some inherited one from previous successive versions of the manuscript, as, for example, there appear references to figures, whose numbers did not exist in this or in the first version. I am going to give only some limited examples of this disorder, as I have tried to carefully follow the correspondence between text, figures, and figure caption (or tables), but I have finally become lost and I have “thrown the towel”. So, please, cheque carefully all the mentioned examples and similar situations, as the collection of data (particularly images) is very complex and rich, and this kind of mistakes is too frequent:

- Table 2 (mentioned in line 326) does not exist. Neither does it table 3 (L 331). They are probably intended to be in the supplementary material in this new version, but I have checked the supplementary material and it is also there the material (S4) presented in Table 1 (or a part of it?)

- In the text, the reference is to Lanari (2012), for example, in line 238, but in table 1 it is to Lanari et al (2014), possibly it is the same geothermometer, but this is confusing for the reader.

- Most of the confusion is in the numbering of the figures. In figure 4f there is a window refereeing to figure 7d, which does not exist. In figure 4d, area 2 refers to fig 6b, but fig. 6b is a magnification of fig 6a. I have tried to cheque the validity of the assignation in figure 10 a1 and a2, respectively, to the most reducing or oxidant states, as it is fundamental for the conclusion and something sounds strange to me, but going to the original figures, I have become lost. Therefore, I am not sure if the correspondence of the two textural positions with the Fe2+- and Fe3+ -rich chlorites is correct or if it has been changed; maybe it is correct, but I am not sure.

It is very important to philtre carefully all these questions (not only those mentioned in my comments) because the excellent collection of data and conclusions apparently lose credibility if the reader cannot easily follow the presentation and reasoning.

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ED: Publish subject to minor revisions (review by editor) (15 Jun 2024) by Federico Rossetti

AR by Delphine CHARPENTIER on behalf of the Authors (19 Jun 2024) Author's response

EF by Sarah Buchmann (25 Jun 2024) Manuscript

EF by Sarah Buchmann (25 Jun 2024) Author's tracked changes

EF by Sarah Buchmann (25 Jun 2024) Supplement

ED: Publish as is (28 Jun 2024) by Federico Rossetti

ED: Publish as is (28 Jun 2024) by Federico Rossetti (Executive editor)

AR by Delphine CHARPENTIER on behalf of the Authors (04 Jul 2024) Author's response Manuscript

Journal article(s) based on this preprint

28 Aug 2024

Evolution of fluid redox in a fault zone of the Pic de Port Vieux thrust in the Pyrenees Axial Zone (Spain)

Delphine Charpentier, Gaétan Milesi, Pierre Labaume, Ahmed Abd Elmola, Martine Buatier, Pierre Lanari, and Manuel Muñoz

Solid Earth, 15, 1065–1086, https://doi.org/10.5194/se-15-1065-2024,https://doi.org/10.5194/se-15-1065-2024, 2024

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Delphine Charpentier, Gaétan Milesi, Pierre Labaume, Ahmed Abd Elmola, Martine Buatier, Pierre Lanari, and Manuel Muñoz

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https://doi.org/10.5194/egusphere-2024-386-supplement

Delphine Charpentier, Gaétan Milesi, Pierre Labaume, Ahmed Abd Elmola, Martine Buatier, Pierre Lanari, and Manuel Muñoz

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Understanding the fluid circulation in fault zones is essential to characterize the thermo-chemical evolution of hydrothermal systems in mountain ranges. The study focused on a paleo-system of the Pyrenees. Phyllosilicates permit to constrain the evolution of temperature and redox of fluids at the scale of the fault system. A scenario is proposed and involves the circulation of a single highly reducing hydrothermal fluid (~300 °C) that evolves due to redox reactions.


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