Genesis of strongly peraluminous granites facilitates nitrogen retention in the crust
Bibliographic record
Abstract
Sediments/sedimentary rocks are a major sink of atmospheric N 2 , which is mainly fixed into diazotrophic biomass and recycled into other organic matters or diagenetically transferred (in the form of NH 4 + ) into the crystal structures of phyllosilicate minerals in sediments. Driven by tectonic activities, sedimentary rocks may experience various degrees of metamorphism with some eventually being melted to form S-type granitoids. The destiny of sedimentary N during these processes, i.e., retained in the crust or returned to the atmosphere, determines the long-term N evolution in the crust and the atmosphere, which profoundly impacts not only volatile properties in the lithosphere but also the surface climatic and environmental conditions. Previous studies have reported various extents of N loss (from very subtle to up to 70 %) from regional meta -sedimentary rocks. However, detailed studies on the N-losing mechanism during crustal anatexis and S-type granitoid formation are rare. Here, we examined the Permian–Triassic strongly peraluminous granites (SPGs) in the Diancangshan-Ailaoshan area in SW China, which were formed by extensional decompression melting of Precambrian pelitic sediments in the upper–middle crust within a back-arc basin triggered by slab rollback of the subducted Paleo-Tethyan oceanic crust. The results show that the SPGs have a N-content range of 6.8 ppm to 58.2 ppm (mean = 37.9 ± 11.3 ppm; 1σ; n = 19). The δ 15 N values of the samples mostly cluster between 0.0 ‰ and + 3.1 ‰ with 3 samples showing higher values of + 4.2 ‰, +5.2 ‰ and + 7.5 ‰ (mean = +2.4 ± 1.7 ‰; 1σ; n = 19). For comparison, one altered sample has much higher N content of 128.7 ppm with a δ 15 N value of + 0.1 ‰. Compared with Precambrian sedimentary rocks, these SPGs contain statistically at least an order of magnitude less N. Data modeling suggests that decompression-induced magmatic N 2 degassing preponderates metamorphic N devolatilization for the major N loss from the Precambrian sedimentary rocks, which were also observed to control the N signatures of sediment-derived PGs in subduction-zone settings. Further compilation of the N contents of global granitoids show that SPGs contain 2–4 times higher N than non-SPGs, suggesting better retention of crustal N in SPGs. This can be attributed to the crystallization of hydrous peraluminous minerals (micas in particular, which have higher N-hosting capacities) in the H 2 O-rich strongly peraluminous melts. We further estimated the N inventory in global SPGs as 0.9 ± 0.5 × 10 17 kg N, which accounts for 4–9 % of the N budget in the upper continental crust.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.001 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 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 teacher head, 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".