the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 15 Mar 2024
Evolution of fluid redox in a fault zone of the Pic de Port-Vieux thrust in the Pyrenees Axial Zone (Spain)
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 (Fetotal/(Fetotal+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+/Fetotal 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.
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RC1: 'Comment on egusphere-2024-386', Anonymous Referee #1, 08 Apr 2024
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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 -
CC1: 'Comment on egusphere-2024-386', Giacomo Medici, 12 Apr 2024
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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 -
RC2: 'Comment on egusphere-2024-386', Fernando Nieto, 23 Apr 2024
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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+/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?
- 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.
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