the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 09 Jan 2025
40Ar/39Ar age constraints on the formation of fluid-rich quartz veins from the NW Rhenohercynian zone (Rursee area, Germany)
Abstract. The late Palaeozoic Variscan orogeny (~350 Ma) dictates a significant part of the subsurface geology in north-western and central Europe. Our focus is particularly on veining that occurred in metamorphosed sedimentary rocks that are affected by this orogeny. Vein minerals serve as repositories for documenting the origin of subsurface fluid flows and dynamics, and dating them provides crucial insight into the timing of orogenic and possible reactivation events. The Rursee area (Rhenish Massif, Germany) that is part of the Variscan foreland zone on the Avalonia micro-continent represents a key locality for studying Variscan quartz vein formation. Based on structural grounds, the two different groups/types of Rursee quartz veins have been linked with the early stages of Variscan, but their absolute ages are still unknown.
The aim of this study is to date these quartz veins using the 40Ar/39Ar stepwise crushing method based on the radioactive decay of 40K dissolved in high salinity fluid inclusions (FIs). We obtained Jurassic to Cretaceous ages, and the isotopic analysis of argon gases revealed that the fluid-rich quartz fractions release 39Ar in two distinct phases. Regardless of quartz veins FIs salinity, stepwise crushing provides apparent K/Cl >1. Electron Probe Micro Analyser data confirm the presence of the K (39Ar) in the K-bearing mineral inclusions (e.g., sericite, mica, and chlorite) and in microcracks and possibly in the crystal lattice of quartz.
K-bearing mineral inclusions and/or crystal lattice of quartz, which form in the Variscan-origin vein fractures, provide a plausible explanation for the young apparent isotopic ages. The presence of the quartz sub-grains may suggest that obtained ages are likely to reflect post-Variscan reactivation-recrystallisation due to tectonic activity or its cooling moment during the Jurassic-Cretaceous period rather than the original Variscan vein formation.
This study emphasizes the complexities of isotopic dating of FIs, as well as the importance of careful interpretation of such data, especially in cases where different K-bearing mineral inclusions and/or radiogenic argon from crystal lattice obscure the initial FIs signal.
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RC1: 'Comment on gchron-2024-35', Alexei Ivanov, 17 Jan 2025
I would like to start my comments by saying that I have never dated fluid and mineral inclusions in quartz by 40Ar/39Ar or any other method myself. However, I have experience in 40Ar/39Ar dating, including experience in dating mineral inclusions in pyrite, and I am also quite familiar with the U,Th-He method of dating mineral inclusions in sulfides. Of course, different approaches are used in all of the above cases, but I think I am qualified to evaluate the paper. Speaking directly about the paper, I read it with great interest. The article is impeccably written and the research itself is technically sound with ample material. On the whole, I have no complaints about this part of the paper, except perhaps that the colours of the regression lines in Figures 5, 11 and B1 are not contrasted enough. I would recommend showing one of the lines in black and the other in pink, as the authors have done. As for the results of the studies, I have a clear opinion that 40Ar/39Ar dating of fluid and mineral inclusions in quartz is absolutely unpromising from a geochronological point of view. In general, the authors write this in the discussion section, but in the final fourth conclusion they offer a geological interpretation that I do not think is supported by the data presented.
The authors also write (lines 410-411):
"Unlike studies that obtained consistent ages from FIs (Qiu & Wijbrans, 2006; Qiu et al., 2011; Bai et al., 2013, 2019), we were unable to date FIs in Rursee quartz samples, likely due to high 40ArE concentrations and/or low salinity."
I, and probably other readers, would like to understand why other studies have succeeded in dating fluid inclusions and the authors have not. Is it a coincidence that such unsuccessful samples were found, or is the dating of fluid inclusions in quartz in principle impossible? A short sentence here is clearly not enough. I would like to see more generalisation. In other words, it would be useful to have a more detailed analysis of the work of predecessors where such dating was possible. Is it really possible, or did the authors of the earlier works draw too optimistic conclusions about such dating?
Citation: https://doi.org/10.5194/gchron-2024-35-RC1 -
AC1: 'Reply on RC1', Akbar Aydin Oglu Huseynov, 30 Jan 2025
Dear Referee,
Thank you for your contribution to our submitted manuscript. We deeply appreciate your comments and consider your valuable thoughts on our revised version of the manuscript.
" I would like to start my comments by saying that I have never dated fluid and mineral inclusions in quartz by 40Ar/39Ar or any other method myself. However, I have experience in 40Ar/39Ar dating, including experience in dating mineral inclusions in pyrite, and I am also quite familiar with the U,Th-He method of dating mineral inclusions in sulfides. Of course, different approaches are used in all of the above cases, but I think I am qualified to evaluate the paper. Speaking directly about the paper, I read it with great interest. The article is impeccably written and the research itself is technically sound with ample material. On the whole, I have no complaints about this part of the paper, except perhaps that the colours of the regression lines in Figures 5, 11 and B1 are not contrasted enough. I would recommend showing one of the lines in black and the other in pink, as the authors have done."
Dear Referee,
We agree on Figures 5, 11 and B1, and we modified the lines to be more contrasted in the revised version.
"As for the results of the studies, I have a clear opinion that 40Ar/39Ar dating of fluid and mineral inclusions in quartz is absolutely unpromising from a geochronological point of view. In general, the authors write this in the discussion section, but in the final fourth conclusion they offer a geological interpretation that I do not think is supported by the data presented. "
Dear Referee,
Thank you for your comments regarding the implications section of our discussion. We acknowledge that the interpretation of our data, derived from 40Ar/39Ar dating, is limited. However, we have endeavoured to provide the most plausible interpretations based on the available data.
We recognize the possibility of low-salinity Variscan remnant fluid circulation, as indicated by Kirnbauer et al. (2012). Furthermore, the high-temperature, low-salinity meteoric water circulation described by Schroyen & Muchez (2008) supports an interpretation involving convective cells of meteoric water circulation in study zone. In addition, we know that Virgo et al. (2013) showed that geomechanically veins can reactivate easily if their hosts are stronger than the veins themselves, which is the case in our study.
Nevertheless, despite the poor quality of the age results, they still suggest an overprinting event during the Mesozoic. This interpretation aligns with similarly ambiguous age signals reported from other areas of the North German Foreland Basin.
Taking your feedback into account, we have revised this section to present a more cautious interpretation of the data in the revised manuscript.
Thank you for your thoughtful review, which has helped us refine our discussion.
"The authors also write (lines 410-411):
"Unlike studies that obtained consistent ages from FIs (Qiu & Wijbrans, 2006; Qiu et al., 2011; Bai et al., 2013, 2019), we were unable to date FIs in Rursee quartz samples, likely due to high 40ArE concentrations and/or low salinity."
I, and probably other readers, would like to understand why other studies have succeeded in dating fluid inclusions and the authors have not. Is it a coincidence that such unsuccessful samples were found, or is the dating of fluid inclusions in quartz in principle impossible? A short sentence here is clearly not enough. I would like to see more generalisation. In other words, it would be useful to have a more detailed analysis of the work of predecessors where such dating was possible. Is it really possible, or did the authors of the earlier works draw too optimistic conclusions about such dating?"
Dear Referee,
Studies by Qiu & Wijbrans (2006), Qiu et al. (2011), and Bai et al. (2013, 2019) on fluid inclusions in wolframite and eclogite minerals have shown that these host minerals preserve primary fluid inclusions in growth zones as well as secondary fluid inclusions. Notably, these fluid inclusions exhibit high salinity (>20 eq. wt.% NaCl) and elevated potassium concentrations from KCl, which enable differentiation of fluid inclusion 39Ar spectra during crushing.
The key distinction between previous studies and our research, as discussed, lies in the recrystallization of quartz veins. This process results in the complete loss of the original high-salinity primary fluid inclusions in studied quartz samples. Consequently, we only observe low-salinity pseudosecondary and secondary fluid inclusions (3–8 eq. wt.% NaCl), which inherently contain lower K concentrations. Without sufficient K in the system, the 40Ar/39Ar dating method cannot be effectively applied. Furthermore, given that recrystallization occurred at low temperatures (as inferred from the homogenization temperatures of pseudosecondary fluid inclusions), it is likely that this process reset the argon source, affecting any potential age determinations.
In conclusion, while previous studies on fluid inclusions were well-conducted and provide realistic insights, the 40Ar/39Ar dating method is not viable for low-salinity, low-potassium fluid inclusions, particularly when argon resetting occurs due to recrystallization.
Citation: https://doi.org/10.5194/gchron-2024-35-AC1
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AC1: 'Reply on RC1', Akbar Aydin Oglu Huseynov, 30 Jan 2025
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CC1: 'Comment on gchron-2024-35', Hua-Ning Qiu, 28 Jan 2025
Publisher’s note: this comment is a copy of RC2 and its content was therefore removed on 29 January 2025.
Citation: https://doi.org/10.5194/gchron-2024-35-CC1 -
RC2: 'Comment on gchron-2024-35', Hua-Ning Qiu, 29 Jan 2025
The crushing technique is very important to date fluid inclusions, and to exclude the trapped excess argon and obtain high-precision age messages for K minerals. The authors make a great effort to complete the research with more than sixty crushing steps (excluding the blanks) for each sample and relevant supportive analyses. The manuscript is well written and should be recommended for publication after modification. The main points include:
- “To date, three main hypotheses are being debated as to the origin of the released argon in a stepwise crushing experiment. The first group (Qiu & Wijbrans, 2006, 2008; Bai et al., 2019) suggests that progressive crushing releases gases mainly from FIs and therefore represents FIs ages” (Lines 272 – 274). This group also suggested the radiogenic argon within the K-mineral lattices might be released by prolonged crushing when the grain sizes were reduced to tens of nanometers: Bai et al. (2018) proposed that “the K/Ca ratios of 09PT37Q in the final crushing steps are gradually rising up (Fig. 7c, purple lines), indicating gas release from a K-rich phase. Therefore, the gas mixing line is reasonable”; “The gas release sequence with sufficient crushing can be summarized as from microcracks → SFIs → PFIs → micrometer- to nanometer-sized minerals” (Bai et al., 2022); “The radiogenic argon (40ArR) within the microcline lattices might be obviously liberated by prolonged crushing when grain sizes were reduced to tens of nanometers” (Bai et al., 2024).
- “In the latter stages of the experiment (from the 20th analysing steps), the substantial release of 39ArK isotopes may support the hypothesis proposed by Kendrick and Philips (2007) and Kendrick et al., (2011), suggesting the presence of K-bearing mineral inclusions in the samples and/or 40Ar* from the crystal lattice and also non-crushed small-sized FIs (<5 μm)” (Lines 372 – 374). I do not agree with this point. The K-bearing mineral inclusions in quartz should release the lattice argon in the final “pseudo-plateau” steps and a few earlier steps. The gas released in the first 20 steps should be from the secondary fluid inclusions. The reasons include: 1) Fig.3 shows that the quartz samples contain many fine fluid inclusions; 2) The sizes of K-bearing mineral inclusions should be tens of microns because the larger impurities had been excluded under binocular microscope; 3) A positive correlation between K and Cl is a characteristic of geofluids, not the solid minerals; 4) Your crusher with a pestle of 69.5 g hits sample more gently than that at CUG Wuhan (218 g) and much weaker than that of the air-actuated (90 psi) crushing device used by Kendrick and Phillips (2009).
- The K–Cl–Ar* correlation diagrams of (40Ar*/K vs Cl/K), (Cl/40Ar* vs K/40Ar*) and (40Ar*/Cl vs K/Cl) are helpful to obtain the ages of secondary and primary fluid inclusions respectively, and to distinguish the gas sources.
- The total 39ArK of R1 Rursee (Rursee 1b BNV?) in Supplementary 1.xlsx is only 89 fA, indicating too low K in the sample. This is the main reason that the samples do not yield good isochron ages.
Minor points:
- The unit of argon isotopes should be fA, not A in “Supplementary 1.xlsx” and “Supplementary 3.xlsx”.
- I do not understand the grain sizes in “Supplementary 4.xlsx”.
References:
Bai X.J., Li Y.L., Hu R.G., Liu X., Tang B., Gu X.P. & Qiu H.N., 2024, High-precision microcline 40Ar/39Ar dating by combined techniques: CHEMICAL GEOLOGY, 655. https://doi.org/10.1016/j.chemgeo.2024.122086.
Bai X.J., Liu M., Hu R.G., Fang Y., Liu X., Tang B. & Qiu H.N., 2022, Well-Constrained Mineralization Ages by Integrated 40Ar/39Ar and U-Pb Dating Techniques for the Xitian W-Sn Polymetallic Deposit, South China: Economic Geology, 117: 833–852. https://doi.org/10.5382/econgeo.4889.
Bai X.J., Jiang Y.D., Hu R.G., Gu X.P. & Qiu H.N., 2018, Revealing mineralization and subsequent hydrothermal events: Insights from 40Ar/39Ar isochron and novel gas mixing lines of hydrothermal quartzs by progressive crushing: Chemical Geology, 483: 332‒341. https://doi.org/10.1016/j.chemgeo.2018.02.039.
Kendrick M.A. & Phillips D., 2009, New constraints on the release of noble gases during in vacuo crushing and application to scapolite Br-Cl-I and 40Ar/39Ar age determinations: Geochimica et Cosmochimica Acta, 73: 5673‒5692. https://doi.org/10.1016/j.gca.2009.06.032.
Citation: https://doi.org/10.5194/gchron-2024-35-RC2 -
AC2: 'Reply on RC2', Akbar Aydin Oglu Huseynov, 30 Jan 2025
Dear Referee,
Thank you for your contribution to our submitted manuscript. We deeply appreciate your comments and consider your valuable thoughts on our revised version of the manuscript.
“The crushing technique is very important to date fluid inclusions, and to exclude the trapped excess argon and obtain high-precision age messages for K minerals. The authors make a great effort to complete the research with more than sixty crushing steps (excluding the blanks) for each sample and relevant supportive analyses. The manuscript is well written and should be recommended for publication after modification. The main points include:
- “To date, three main hypotheses are being debated as to the origin of the released argon in a stepwise crushing experiment. The first group (Qiu & Wijbrans, 2006, 2008; Bai et al., 2019) suggests that progressive crushing releases gases mainly from FIs and therefore represents FIs ages” (Lines 272 – 274). This group also suggested the radiogenic argon within the K-mineral lattices might be released by prolonged crushingwhen the grain sizes were reduced to tens of nanometers: Bai et al. (2018) proposed that “the K/Ca ratios of 09PT37Q in the final crushing steps are gradually rising up (Fig. 7c, purple lines), indicating gas release from a K-rich phase. Therefore, the gas mixing line is reasonable”; “The gas release sequence with sufficient crushing can be summarized as from microcracks → SFIs → PFIs → micrometer- to nanometer-sized minerals” (Bai et al., 2022); “The radiogenic argon (40ArR) within the microcline lattices might be obviously liberated by prolonged crushing when grain sizes were reduced to tens of nanometers” (Bai et al., 2024).”
Dear Referee,
We deeply appreciate your comments and consider your valuable thoughts on our revised version of the manuscript. Therefore, we add, “The gas release sequence with sufficient crushing can be summarised as from microcracks → SFIs → PFIs → micrometer- to nanometer-sized minerals (Bai et al., 2022);” in our revised version.
- ““In the latter stages of the experiment (from the 20th analysing steps), the substantial release of 39ArKisotopes may support the hypothesis proposed by Kendrick and Philips (2007) and Kendrick et al., (2011), suggesting the presence of K-bearing mineral inclusions in the samples and/or 40Ar* from the crystal lattice and also non-crushed small-sized FIs (<5 μm)” (Lines 372 – 374). I do not agree with this point. The K-bearing mineral inclusions in quartz should release the lattice argon in the final “pseudo-plateau” steps and a few earlier steps. The gas released in the first 20 steps should be from the secondary fluid inclusions. The reasons include: 1) Fig.3 shows that the quartz samples contain many fine fluid inclusions; 2) The sizes of K-bearing mineral inclusions should be tens of microns because the larger impurities had been excluded under binocular microscope; 3) A positive correlation between K and Cl is a characteristic of geofluids, not the solid minerals; 4) Your crusher with a pestle of 69.5 g hits sample more gently than that at CUG Wuhan (218 g) and much weaker than that of the air-actuated (90 psi) crushing device used by Kendrick and Phillips (2009).”
Dear Referee,
We respectfully disagree with your points regarding the degassing of K-bearing minerals only occurring during the final stage of the stepwise crushing process. Unfortunately, quartz grains that we analyzed are free from the primary fluid inclusion and they contain mostly pseudo- and secondary fluid inclusions. Knowing the low salinity of both FIs generations (3-8 eq. wt.% NaCl), the K/Cl ratio should be mostly varied around ~1, knowing KCl from the salinity. However, we do not observe that after the 20th analysing step. It is important to note that by the 20th pestle drop, there have already been over ~1000 impacts.
This differs from the findings of Bai et al. (2019), which resulted in different outcomes than expected from their previous work. Given this, we believe it is highly plausible that any occurring K-bearing minerals may degas together with the fluid phase.
We understand that about queries that our pestle weight is less heavy than the Wuhan CUG pestle and much weaker than Kendrick and Philips (2009) crusher device to release radiogenic argon within the K-bearing minerals directly in the early stage of crushing, however, considering that we have at the end of the stepwise crushing more than ~40,000 cumulative pestle drops, which should not be dramatically different from the above crusher system.
- “The K–Cl–Ar*correlation diagrams of (40Ar*/K vs Cl/K), (Cl/40Ar* vs K/40Ar*) and (40Ar*/Cl vs K/Cl) are helpful to obtain the ages of secondary and primary fluid inclusions respectively, and to distinguish the gas sources.”
Dear Referee,
We appreciate your thoughtful comments. As per Bai et al. (2019), we generated all the relevant plots during our study; unfortunately, none of the samples provided clear differentiation between the gas sources. We think that due to recrystallization, our vein quartz lost any primary high salinity brine. Given the resultant low concentration of K in the fluid, the chances of finding any correlation in the isotope correlation plots are low.
We believe that, as mentioned in your second comment, there may be a mixing of fluid and solid sources after the 20th crushing step. A PDF file with all plots is attached to this file.
- “The total 39ArKof R1 Rursee (Rursee 1b BNV?) in Supplementary 1.xlsx is only 89 fA, indicating too low K in the sample. This is the main reason that the samples do not yield good isochron ages.”
Dear Referee,
We agree that in our example we are limited, most probably due to the loss of the primary high salinity brine and consequent low K-contents of our fluid inclusions.
Minor points:
- “The unit of argon isotopes should be fA, not A in “Supplementary 1.xlsx” and “Supplementary 3.xlsx”.”
- Dear Referee,
We will correct that on our revised manuscript.
- “I do not understand the grain sizes in “Supplementary 4.xlsx”.”
Dear Referee,
This is a grain size distribution analysis of the crushed quartz materials, aimed at determining whether all the fluid inclusions are decrypted during the crushing process or not. Unfortunately, after ~1000 (20th analyzing step) of pestle drop, most of the grain sizes were still higher than fluid inclusions sizes, which revealed that the fluid phase and solid phase after 20th analyzing steps released K simultaneously rather than fluid phase -> solid phase.
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RC3: 'Reply on AC2', Hua-Ning Qiu, 07 Feb 2025
Thank you very much for your rapid response to my comments. Obviously you did not discuss with your senior supervisors in such a short time.
The results of K-rich microcline by 40Ar/39Ar crushing indicated that a lot of gas was released from the microcracks and secondary fluid inclusions in the first several steps (Bai et al., 2024), implying that the gas trapped in fluid inclusions is much more than that released from the microcline lattices. The small mineral inclusions with your quartz samples contain much less potassium than microcline. You also admitted that “after ~1000 (20th analyzing step) of pestle drop, most of the grain sizes were still higher than fluid inclusions sizes”, the gas from fluid inclusions would still dominate for more crushing steps until the very fine fluid inclusions (<1 μm) were exhausted.
Citation: https://doi.org/10.5194/gchron-2024-35-RC3
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