the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 Jan 2026
Dating circulations of hydrothermal fluids in the crystalline basements of unconformity-related metallic deposits using in situ Rb/Sr geochronology: proof of concept
Abstract. The use of in situ Rb-Sr geochronology has boomed in recent years following its implementation using LA-ICP-QQQ-MS technology, which enables fast, in situ analyses at the micron scale on selected minerals. The Rb-Sr geochronometer applied to micas is now commonly used to date the crystallization or cooling of metamorphic and magmatic rocks, based on the assumptions of a closed isotopic system after passing the closure temperature and of a homogeneous Sr isotopic composition at the time of crystallization. In situ Rb-Sr geochronology applied to micas and related alteration products in geological contexts involving hydrothermal fluid circulation affecting micas after crystallization could provide a new way to decipher the timing and duration of fluid circulation in various settings such as mountain belts or sedimentary basins. The behavior and applicability of the Rb-Sr system in such contexts are, however, poorly understood, as the system may be partially reopened with differential redistribution of Rb and Sr at the grain scale. To test this hypothesis, we selected a case study related to unconformity-related U deposits from the Athabasca Basin (Canada), which formed through intense hydrothermal fluid circulation at the interface between crystalline basement and siliciclastic sedimentary rocks and represent archetypes of unconformity-related metallic deposits. Muscovite grains from metamorphic and magmatic rocks were targeted across a range of alteration states, from hydrothermally unaltered to strongly altered domains. We focused on a specific hydrothermal alteration linked to the formation of hydrothermal illite and sudoite at the expense of metamorphic or magmatic minerals. In unaltered zones, muscovite displayed variable but high Rb/Sr ratios, whereas the 87Sr/86Sr intercepts derived from Rb-Sr regressions were scattered and were not interpreted as meaningful initial isotopic compositions. The resulting ages ranged from ca. 1870 to ca. 1720 Ma and were consistent with the geological context. In distal-to-proximal alteration halos of U deposits, muscovite and related alteration products yielded lower 87Rb/86Sr ratios and highly variable regression intercepts. The mean age calculated across the different samples and investigated sites clustered around ~1640 Ma, a value previously obtained by Ar-Ar geochronology on illite and U-Pb geochronology on other hydrothermal phases and proposed to correspond to a major hydrothermal event linked to a geodynamic reorganization affecting the Canadian Shield at the circum-Laurentian scale. The ~1640 Ma age is geologically meaningful in the studied context and is interpreted as reflecting partial, micrometric-scale resetting of the Rb-Sr system in muscovite during this hydrothermal event. The wide range of regression intercept values commonly observed in disturbed Rb–Sr systems is interpreted as an apparent result of open-system behavior, reflecting partial system reopening and non-conservative redistribution of Rb and Sr at the grain scale, rather than as a physically meaningful initial isotopic composition. These results demonstrate that detailed analysis of Rb-Sr system perturbations in altered muscovite and related alteration products can constrain the timing of ancient hydrothermal activity and the spatial dynamics of fluid-rock interaction. This approach provides a valuable complement to conventional fluid-tracing methods and opens new perspectives for reconstructing paleo-hydrothermal systems in ancient basement terrains.
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Status: open (until 24 Apr 2026)
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RC1: 'Comment on egusphere-2025-6469', Anonymous Referee #1, 25 Feb 2026
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AC1: 'Reply on RC1', Quentin Boulogne, 10 Apr 2026
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Dear Referee,
We thank you for your careful evaluation of our manuscript and for your constructive comments. We have addressed all the technical points you raised. Please find below a point-by-point reply to your comments. Our responses are in bold.
Referee#1: Anonymous Referee
There is a lot to unpack in this contribution and I commend the authors on a comprehensive synthesis of this timely topic. The writing is excellent if somewhat dense in spots. I appreciate the attention to previous literature. The design of the project is thoughtful, and the data is collected using highly suitable methods. Incorporating textures and compositional data to support in situ Rb-Sr is essential and the authors do an overall excellent job of contextualizing the isotopic data. This is absolutely required in a setting like the Athabasca Basin where prolonged high-T to low-T hydrothermal activity has created a wide range of alteration assemblages during basin formation and readjustment. Integrating the in situ Rb-Sr data with models for U deposition is very novel indeed. Also particularly noteworthy is the suggestion, supported by the data, that partly altered white micas underwent preferential liberation of 87Sr* relative to non-radiogenic Sr. This could be a excellent case to follow-up using atom probe measurements to examine the distribution of Rb vs. 87Sr* and 86Sr in the mica lattice.
The only suggestion I can make is that the order of presentation of some important text could be improved. For example we read through all of the results and note that the initial Sr intercepts are impossibly low. I was questioning to myself if this was an analytical bias. It is only in the discussion that the robustness of La Posta secondary material is described which allays some fears of analytical bias. So perhaps it would be good to state more clearly the reproducibility of La Posta age AND initial Sr intercept at the end of the methods section.
We thank the reviewer for this constructive suggestion regarding the presentation of the La Posta secondary standard. In response, we have added a dedicated paragraph to the Methods section detailing the long-term reproducibility of both the isochron age and the initial 87Sr/86Sr intercept of La Posta. This section now includes a compilation of analytical sessions acquired over a three-year period. For each session, isochron ages and corresponding initial Sr isotopic ratios were calculated, and the resulting values were subsequently combined using weighted mean statistics, along with their associated uncertainties and dispersions. These results demonstrate robust long-term reproducibility of both age and initial Sr isotopic composition, and support the absence of instrumental drift, calibration bias, or matrix-dependent effects. We believe that this addition clarifies the reproducibility of La Posta prior to the Results section and addresses the reviewer’s concern.
Additionally, we provide the full results of this compilation as supplement attached to this response, in order to further document and illustrate the absence of analytical bias.
There are a few other minor things I’ve highlighted below. Notwithstanding these the manuscript is in very good shape and illuminates some very important processes affecting a world-class U camp.
Line 154: switching here from xxxx Ma to x.xx Ga.
We have revised Line 154 to report the ages in Ma rather than Ga, in order to maintain consistency with the other age values expressed in Ma throughout the manuscript.
Line 167: please consider italicizing P and T here and elsewhere
We have italicized P and T in Line 167 and throughout the manuscript for consistency with standard scientific notation.
Line 336-340: La Posta is listed as a secondary standard. But it is not obvious from the text how well it was reproduced based on NIST610/MicaMg-NP external calibration? I also note that no additional error was added to the final isochron ages and they are quoted as 1σ starting on line 456. This is a bit unconventional, with most studies using in situ Rb-Sr citing 2SE and including additional error reflecting the long-term reproducibility of secondary standards. Is the 1σ required to use the Gaussian Mixing Model? (this looks a lot like Isoplot ‘unmix age’ routine). What happens to this unmixing exercise if you use 2SE rather than 1σ? This gets back to La Posta results. What was the measured error on La Posta in this study? We are only given the Zack & Hogmalm 2016 reported value.
First, regarding the 1σ uncertainties reported for the isochron ages: in IsoplotR, the Gaussian Mixture Model (GMM) routine requires that each data point be associated with a 1σ uncertainty. These internal uncertainties are therefore used in the GMM analysis to weight contributions of individual analyses and to model the age distributions statistically. While some studies report 2SE to provide a conservative estimate of uncertainty, using 1σ is required for the GMM routine, and it accurately reflects the internal precision of the regression.
Second, to demonstrate long-term reproducibility, we compiled results from repeated analytical sessions of the La Posta secondary standard acquired over a three-year period. For each session, isochron ages and corresponding initial 87Sr/86Sr intercepts were calculated, and the resulting values were combined using weighted mean statistics, along with their dispersion. These results confirm stable and reproducible measurements. The total uncertainty on the reported isochrone ages is obtained by combining the internal regression error (1σ) with the external reproducibility derived from La Posta (~1.3%).
These additions are now described in a dedicated paragraph in the Methods section, making clear that the measured 87Sr/86Sr ratios and derived ages are robust, and that the reported 1σ uncertainties are both appropriate for the GMM analysis and consistent with long-term analytical reproducibility.
Line 499: Here and elsewhere in Results section I’m surprised to see some methodology details related to concentration calculations. Methods for calculating concentrations should be in an earlier section. If NIST610 was analyzed it would be a simple matter using the Trace Elements DRS in Iolite4.
We thank the reviewer for raising the point regarding the calculation of concentrations. We acknowledge that standard practice often employs the Trace Elements DRS routine in Iolite to derive concentrations automatically. However, in this study we calculated ⁸⁷Rb, ⁸⁶Sr, and ⁸⁷Sr concentrations using a rigorous, reproducible workflow based directly on measured isotopic ratios, regression outputs from Rb–Sr diagrams, and matrix corrections derived from the internal standard MicaMg.
To ensure transparency and clarity, we will add a dedicated paragraph in the Methods section detailing this calculation procedure, including the derivation of 87Rb from 85Rb, the calculation of 86Sr and 87Sr from measured ratios, and the application of the matrix correction factor. We believe that this addition will fully address the reviewer’s concern while demonstrating that our concentration calculations are scientifically robust and conform to standard practices in LA-ICP-MS geochronology.
Line 523: Is 1634.7 ± 1.7 Ma - an error of 0.1% - realistic? Again, how well as the secondary standard measured?
We thank the reviewer for raising this important point regarding the apparently small uncertainty associated with the isochron age.
The reported uncertainty of ±1.7 Ma corresponds to the 1σ internal regression error calculated by IsoplotR from 224 individual analyses defining the isochron. Although individual single-spot ages carry uncertainties on the order of ~20 Ma (1σ), the uncertainty on the isochron age reflects the statistical precision of the regression slope, which improves with the number of analyses. For a dataset of this size, the expected reduction in uncertainty follows approximately 1/√n. Given ~20 Ma single-spot uncertainties and n = 224 analyses, the theoretical reduction in uncertainty yields: 20 / √224 ≈ 1.3 Ma, which is consistent with the reported ±1.7 Ma. Therefore, the small reported internal uncertainty arises from the strong statistical constraint on the regression slope provided by the large number of analyses, rather than from unrealistically low analytical uncertainty on individual measurements.
We also note that the internal regression uncertainty does not account for long-term analytical variability. To provide a realistic estimate of the total uncertainty, the external reproducibility derived from the La Posta secondary standard (~1.3%, 1σ) is propagated. Combining the internal regression error with this external uncertainty yields a more representative total uncertainty on the isochron age, which in this case corresponds to approximately ±21 Ma (1σ). This ensures that reported ages reflect both the statistical precision of the regression and the long-term reproducibility of the analytical protocol.
Suggested statement for the manuscript:
“The Rb–Sr regression yields an age of 1634.7 ± 1.7 Ma (1σ internal regression error; the total uncertainty including external reproducibility is ~±21 Ma, 1σ).”
We have clarified in the revised manuscript that the quoted uncertainty represents the internal regression error (1σ) derived from the isochron fit.
Line 645: “…the mobility of radiogenic 87Sr* is significantly greater than that of Rb and non-radiogenic Sr…” The argument is that 87Sr* is preferentially located in interlayer site which is where alteration processes might start. Preferential removal of 87Sr* would, therefore, lower the 87Sr/86sr to values unsupported on earth. But the least-altered muscovite in basement gneiss also gives initial of 0.6709 ± 0.0115. This doesn’t make sense. The authors claim to have ruled out instrumental effects but is there some cryptic matrix-mismatch adversely affecting corrected 87Sr/86Sr? Some statement about the anomalously low initial in least-altered muscovite would be warranted (unless I missed it somewhere in the text)
Thank you for this comment. We believe there may be a misunderstanding, as the explanation for the anomalously low intercepts in the least-altered muscovite is already provided in the manuscript (Lines 686–705).
In that section, we explicitly state that the regression intercepts (e.g., 0.6704 ± 0.0075) fall below the minimum solar system 87Sr/86Sr value (~0.698) and therefore cannot represent physically meaningful initial isotopic compositions. We clearly indicate that these values are not interpreted as true initial 87Sr/86Sr ratios, but rather as apparent intercepts produced by disturbed Rb–Sr systematics.
We further explain that the well-correlated regressions, combined with low MSWD values (<1) and high p-values (p >> 0.05), indicate strong internal coherence despite the anomalous intercepts. We interpret this behavior as characteristic of rotated or disturbed isochrons formed under open-system conditions, where non-conservative redistribution of Rb and Sr (e.g., proportional Rb loss, selective Sr mobility, or partial isotopic resetting) modifies the intercept while preserving a geologically meaningful slope.
Line 697: I see here that the authors explain that age and initial intercept for La Posta were within accepted range and that anomalously low 87Sr/86Sr are not analytical artifacts. It would be nice to know this before launching into the results section where these anomalously low 87Sr/86Sr raise flags about this possibility.
Thank you for this constructive suggestion.
We agree that informing the reader earlier about the reproducibility of the La Posta secondary standard would improve clarity and help avoid potential concerns when encountering the anomalously low ⁸⁷Sr/⁸⁶Sr intercepts in the Results section.
Accordingly, we have added a dedicated paragraph to the Methods section explicitly detailing the analytical performance of the La Posta biotite secondary standard. This addition specifies both the reproducibility of the obtained age and the consistency of the initial 87Sr/86Sr intercept relative to accepted values. By presenting this information upfront, prior to the Results section, we aim to clearly demonstrate that the anomalously low intercepts observed in some samples are not attributable to analytical bias, calibration issues, or standardization problems.
We believe this modification strengthens the manuscript structure and improves the logical flow between the Methods and Results sections. Thank you again for highlighting this important point.
Line 723: typo in isochrone
The typo in “isochrone” (Line 723) has been corrected in the revised manuscript.
Line 728: back to x.xx Ga rather than xxxx Ma ages…
We have revised Line 728 to report the ages in Ma rather than Ga, in order to maintain consistency with the other age values expressed in Ma throughout the manuscript.
We believe that these revisions have significantly strengthened the manuscript, both scientifically and linguistically, and that it now meets the standards required for consideration in Geochronology. We sincerely thank you again for your insightful comments and for giving us the opportunity to resubmit an improved version of our work.
Yours sincerely,
Quentin Boulogne and co-authors
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AC1: 'Reply on RC1', Quentin Boulogne, 10 Apr 2026
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There is a lot to unpack in this contribution and I commend the authors on a comprehensive synthesis of this timely topic. The writing is excellent if somewhat dense in spots. I appreciate the attention to previous literature. The design of the project is thoughtful, and the data is collected using highly suitable methods. Incorporating textures and compositional data to support in situ Rb-Sr is essential and the authors do an overall excellent job of contextualizing the isotopic data. This is absolutely required in a setting like the Athabasca Basin where prolonged high-T to low-T hydrothermal activity has created a wide range of alteration assemblages during basin formation and readjustment. Integrating the in situ Rb-Sr data with models for U deposition is very novel indeed. Also particularly noteworthy is the suggestion, supported by the data, that partly altered white micas underwent preferential liberation of 87Sr* relative to non-radiogenic Sr. This could be a excellent case to follow-up using atom probe measurements to examine the distribution of Rb vs. 87Sr* and 86Sr in the mica lattice.
The only suggestion I can make is that the order of presentation of some important text could be improved. For example we read through all of the results and note that the initial Sr intercepts are impossibly low. I was questioning to myself if this was an analytical bias. It is only in the discussion that the robustness of La Posta secondary material is described which allays some fears of analytical bias. So perhaps it would be good to state more clearly the reproducibility of La Posta age AND initial Sr intercept at the end of the methods section.
There are a few other minor things I’ve highlighted below. Notwithstanding these the manuscript is in very good shape and illuminates some very important processes affecting a world-class U camp.
Line 154: switching here from xxxx Ma to x.xx Ga.
Line 167: please consider italicizing P and T here and elsewhere
Line 336-340: La Posta is listed as a secondary standard. But it is not obvious from the text how well it was reproduced based on NIST610/MicaMg-NP external calibration? I also note that no additional error was added to the final isochron ages and they are quoted as 1σ starting on line 456. This is a bit unconventional, with most studies using in situ Rb-Sr citing 2SE and including additional error reflecting the long-term reproducibility of secondary standards. Is the 1σ required to use the Gaussian Mixing Model? (this looks a lot like Isoplot ‘unmix age’ routine). What happens to this unmixing exercise if you use 2SE rather than 1σ? This gets back to La Posta results. What was the measured error on La Posta in this study? We are only given the Zack & Hogmalm 2016 reported value.
Line 499: Here and elsewhere in Results section I’m surprised to see some methodology details related to concentration calculations. Methods for calculating concentrations should be in an earlier section. If NIST610 was analyzed it would be a simple matter using the Trace Elements DRS in Iolite4.
Line 523: Is 1634.7 ± 1.7 Ma - an error of 0.1% - realistic? Again, how well as the secondary standard measured?
Line 645: “…the mobility of radiogenic 87Sr* is significantly greater than that of Rb and non-radiogenic Sr…” The argument is that 87Sr* is preferentially located in interlayer site which is where alteration processes might start. Preferential removal of 87Sr* would, therefore, lower the 87Sr/86sr to values unsupported on earth. But the least-altered muscovite in basement gneiss also gives initial of 0.6709 ± 0.0115. This doesn’t make sense. The authors claim to have ruled out instrumental effects but is there some cryptic matrix-mismatch adversely affecting corrected 87Sr/86Sr? Some statement about the anomalously low initial in least-altered muscovite would be warranted (unless I missed it somewhere in the text)
Line 697: I see here that the authors explain that age and initial intercept for La Posta were within accepted range and that anomalously low 87Sr/86Sr are not analytical artifacts. It would be nice to know this before launching into the results section where these anomalously low 87Sr/86Sr raise flags about this possibility.
Line 723: typo in isochrone
Line 728: back to x.xx Ga rather than xxxx Ma ages…