Response of Rb-Sr system in biotite during contact metamorphism in the aureole of the Makhavinekh Lake Pluton, Labrador

McFarlane, Christopher R.M.

doi:10.5194/egusphere-2025-6320

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https://doi.org/10.5194/egusphere-2025-6320

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31 Dec 2025

| 31 Dec 2025

Status: this preprint is open for discussion and under review for Geochronology (GChron).

Response of Rb-Sr system in biotite during contact metamorphism in the aureole of the Makhavinekh Lake Pluton, Labrador

Christopher R.M. McFarlane

Abstract. High-temperature contact metamorphism in the aureole of the 1322 Ma Makhavinekh Lake Pluton, Labrador, led to progressive consumption of 1850 Ma garnet formed during upper-amphibolite facies regional metamorphism that produced migmatitic paragneiss (Tasiyuak Gneiss). Biotite Rb-Sr isotope measurements were carried out in situ by laser ablation MS/MS ICP-MS allowing biotite in a variety of textural settings to be characterized. This natural laboratory provides important constraints on the nature of Rb-Sr closure temperature (Tc) as a function of textural setting in high-grade metamorphic rocks. Intact biotite inclusions armoured in garnet preserved in the outer aureole (>4km from the contact) display a range of Rb-Sr isochron ages between ~1850 Ma and ~1322 Ma consistent with a zone of partial retention of Sr in biotite. Isotopic resetting in the outer aureole was controlled by microfractures in garnet that provided short-circuit diffusion pathways for redistribution of radiogenic Sr into plagioclase-bearing contact metamorphic assemblages; biotite inclusions isolated from microfractures retained 1850 Ma Rb-Sr isochron ages. Biotite grains falling along a ~1322 Ma isochron attest to efficient intra- and intercrystalline Sr diffusion at T ≥ 500 °C on timescales of ≥ 5 Myr. Samples in the central part of the contact aureole (3.7 to 1.1 km from the contact) contain partly resorbed biotite surrounded by contact metamorphic Opx + Crd coronal assemblages in addition to armoured inclusions in relict garnet. These display similar Rb-Sr behaviour to outer aureole samples with the exception that ~1322 Ma biotite domains display higher Rb/Sr due to more extreme loss of Sr. In the inner aureole, where garnet has been completely consumed by contact metamorphic assemblages, a new generation of biotite neoblasts occurs in textural equilibrium with Opx + Crd. This biotite preserves Rb-Sr ages ≤1322 Ma with initial ⁸⁷Sr/⁸⁶Sr best interpreted as a mixture of radiogenic Sr accumulated in regional biotite and whole-rock Sr liberated from low-Rb/Sr regional metamorphic garnet, apatite, and plagioclase. This study reveals how the exact textural setting of biotite in high-grade metamorphic rocks influences the preservation of Rb-Sr ages and demonstrates that there is no universal closure temperature for biotite Rb-Sr. It also reveals that in situ Rb-Sr dating of granulite or UHT rocks might provide robust chronometric data if grains isolated from intergranular diffusion are systematically evaluated to reveal zones of partial retention.

Received: 18 Dec 2025 – Discussion started: 31 Dec 2025

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Christopher R.M. McFarlane

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RC1:
'Comment on egusphere-2025-6320', Kyle Larson, 05 Feb 2026 reply
The work submitted is exactly the kind of study that needs to be carried out as the broader community continues to apply in situ beta-decay geochronometers to address geological problems. I find the work to be of high quality, from the experimental setup through to discussion.
As with any study, there are a few changes that could be made that would further improve the work and ensure consistency in interpretation.
The authors describe mineral assemblages and textures in detail, yet they are supported by a single mXRF map and 4 ppl images. It would be appreciated if the descriptions could be better illustrated by additional images either in the main text or as supplemental files so the reader can see what is being described. Directly parallel to this is better referencing of all supplied figures within the text.

Some of the main points of the paper are based on the interpretation of model Rb-Sr dates; however, detailed information about how the dates were calculated is not provided. The existing information in the Methods section is vague and does not articulate specifically how uncertainties were propagated. Given the central role these dates play in the overall narrative, their calculation needs to be well documented.

Finally, in the finalization of the Rb/Sr data, the data from the mica in the study are normalized against a biotite reference material of known age to compensate for a systematic offset between the primary reference material (pressed nanopowder) and actual mica grains. It is specifically noted that no additional uncertainty was added to the unknowns during the final normalization step. It is later noted in the discussion that the biotite reference material returned a date of 452 ± 7 Ma. That uncertainty is significant and, if converted to uncertainty on the slope of the 452 Ma isochron, it would equal ~ 0.777%. Given that all unknown mica are normalized relative to that 452 ± 7 Ma result, it is important to propagate the uncertainty associated with that normalization through to the final dates. Doing so has direct implications for interpretation. For example, an isochron date of 1338.4 ± 6.5 is noted as being older than the expected 1322 Ma date of pluton intrusion (source of head for the contact metamorphism), indicating something within the Rb-Sr systematics. If the uncertainty on the matrix normalization is propagated to the slope on the isochron in quadrature, however, the uncertainty increases to 21.60 Ma, which overlaps the expected 1322 Ma date. A similar interpretation is made about the 1332.2 ± 8.4 Ma date, which, once the uncertainty on the matrix normalization is propagated through, has an uncertainty of 22.16 Ma, which also overlaps with the 1322 Ma expected date. Perhaps there are other, more appropriate ways of propagating the uncertainty from the isochron used to matrix normalize the mica data, but regardless of how it is done, it should be incorporated into the study so that nuanced interpretations can be made.

I want to again emphasize the need for this kind of systematic study for these developing methodologies and my appreciation to the author for taking the time to do it.

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Citation: https://doi.org/10.5194/egusphere-2025-6320-RC1
- AC1: 'Reply on RC1', Christopher McFarlane, 07 Feb 2026 reply
  
  Many thanks for the constructive comments.
  1. On the question of additional figures and better referencing to figures in the text: I was reluctant to add too many figures in order to maintain a concise presentation. However I can see the value of an additional figure of BSE images that might reveal more clearly some of the textures encountered in these rocks. This is easily done if the editor feels this would improve the manuscript. I will also double-check that figures are better alluded to in the text.
  2. A statement about the calculation of model ages can be added to the methods sections. This is simply equivalent to a two-point isochron between the measured Rb-Sr ratios and an assumed initial 87Sr/86Sr. For all analyses an initial value of 0.73 is assumed. This initial of is probably inappropriate for biotite neoblasts in the inner aureole but it makes the discussion a bit easier to approach. The errors assigned to these model ages are proportion to the error on the measured 87Rb/86Sr. There were no data with sufficiently low 86Sr to assume that all 87Sr was radiogenic.
  3. I fully appreciate the issue of propagating error for this type of Rb-Sr analysis. I feel this is currently one of the outstanding issues that needs to be addressed in the community (and I appreciate KL's efforts to lead this charge). But I think it is more nuanced than simply adding an excess error based on the secondary standards. For example, the largish error on the isochron age for the Loch Borralan is primarily a result of the narrow spread of 87Rb/86Sr; this reaches a maximum of about 75. In contrast, the biotite in the MLP aureole ranges up to 87Rb/86Sr of >2000. So there is a significant mis-match between the material I used for correction of Rb/Sr mass bias (i.e. Loch Borralan) and the unknown targets. Propagating the error from the former unto unknowns could arguably add inappropriate excessive error; this could potentially obscure real (and subtle) variability. So I consider Loch Borralan phlogopite as an standard to ensure accuracy of the results (by correcting for bias). In reality, I think a standard used to assess propagated error should have the same spread of Rb/Sr as the unknowns. Unfortunately such a material doesn't currently exist. For example, if Loch Borralan contained grains with Rb/Sr between 500 and 1500 the error on the isochron age could easily be reduced to <1%. In that case, error on the standard could be more usefully propagated to the unknowns. So I am a bit reluctant to add significant excess error to the MLP isochrons using the error measured on Loch Borralan. It is easily to incorporate language to the effect described above. It is also easy to cite the long-term reproducibility of MicaMg-NP and/or NIST610 analyzed during each session. I am open to suggestions on this matter.
  
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  Citation: https://doi.org/10.5194/egusphere-2025-6320-AC1

Christopher R.M. McFarlane

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Short summary

The mineral biotite is a common rock-forming mineral and notable for its ability to incorporate Rb into its structure. Microanalytical tools such a laser ablation and tandem mass spectrometry, allows Rb-Sr biotite dating with context preserved. This study reveals Sr mobility that is controlled by location in a rock. Reconstructing the timing of geological events using biotite Rb-Sr geochronology is, therefore, contingent on grain-scale evaluation of biotite textures.


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