Nitrogen isotopes as a robust tracer of fluid activities and mineral reactions in regional metamorphism
Bibliographic record
Abstract
Regional metamorphism is characterized by complicated mineral-mineral reactions and fluid-mineral interactions, which may promote elemental migration and ore genesis. A robust tracer of metamorphic reactions and fluid activities is a key for identifying material sources and quantifying mass flow along metamorphic pathways. Here, through examinations of not only bulk-rock nitrogen (N) isotopic variation along six prograde metamorphic zones but more importantly inter-mineral comparison of isotopic equilibration/disequilibration in the classic Barrovian metamorphic sequence in the Mica Creek area in the Canadian Cordillera, we show that N isotopes are a sensitive tracer for probing metamorphic devolatilization, mineral reaction and external fluid infiltration at various spatial scales from prograde to retrograde stages. In detail, our results show that, despite a relatively large variation in bulk-rock N/Al molar ratio, low-grade rocks (slates and phyllites) display a narrow δ15N range, which suggests relatively homogeneous N isotope compositions in their protoliths. Biotite-zone schists show reduced N/Al ratios with elevated δ15N values, which is consistent with devolatilization of 15N-depleted NH3 from rocks during prograde metamorphism at T < 500 °C. In contrast, samples from high-grade rocks (kyanite zone and sillimanite–K-feldspar zone) do not follow the metamorphic devolatilization trend but decrease in both N/Al ratios and δ15N values, indicating overprinting by a relatively large-scale flow (over at least tens of kilometers) of a 15N-depleted external fluid during the high-grade stage. The fluid was likely derived from the granitic magmas that intruded into these rocks during or slightly post peak metamorphism. The δ15N values of minerals in kyanite and sillimanite–K-feldspar zones displays negative correlations between biotite and both muscovite and plagioclase, with high δ15Nbiotite values close to isotope equilibrium but low δ15Nbiotite values at isotope disequilibrium. These negative correlations can be best explained by a kinetic isotopic effect associated with metamorphic reactions that consume muscovite and plagioclase to produce biotite. This suggests that plagioclase may be more involved in the metamorphic reactions in pelitic rocks than previously assumed. This kinetic isotopic effect can result in a Δ15Nplagioclase-biotite > 5 ‰, much larger than the theoretically predicted equilibrium N isotope fractionation factor (Δ15Nplagioclase-biotite < 0.5 ‰ at T > 350 °C) but close to the large N isotope fractionation (Δ15Nplagioclase-biotite ≈ 8 ‰) recently observed in field samples. Our data also imply that plagioclase is susceptible to retrograde alteration. Overall, combined N signatures of bulk rocks and coexisting minerals can provide multiple lines of constraints on mineral reactions and fluid activities from prograde to retrograde metamorphism.
Fetched live from OpenAlex and de-inverted. Abstracts are not stored in this database: the inverted indexes are 8.6 GB of the frame’s 9.3 GB of text, and the host has 13 GB free.
How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.000 | 0.001 |
| Research integrity | 0.001 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".