Magmatic N2 degassing dominates nitrogen loss during sediment subduction and granitoid genesis
Bibliographic record
Abstract
• Sediment-derived granitoids are surprisingly N-poor. • N loss during granitoid genesis is mainly through magmatic N 2 degassing. • N isotopes of sediment-derived granitoids can serve for source tracing. • Water content may control N retention in granitoids. Subducting sediments contain much more abundant nitrogen (N) than the other slab components. The fate of sedimentary N along subduction directly determines the quantity of surficial N that can be released back to the atmosphere/crust or further delivered to the deep mantle, which has a strong impact on the long-term evolution of the N budgets and isotopic signatures of Earth's major reservoirs. Previous studies on the fate of subducted sedimentary N have been intensively focused on assessing the extent of metamorphic N devolatilization from metasedimentary rocks varying from ultrahigh-pressure eclogites facies (subducted to as deep as 90 km) in cold subduction zones to medium-pressure amphibolite facies in extremely hot subduction zones. However, the fate of sedimentary N after these metamorphic stages, particularly during sediment melting and subsequent magmatic crystallization, has not been examined so far. Here, we present the N data of the Cretaceous peraluminous granitoids in the Oman-United Arab Emirates area, which were formed by melting of subducted pelagic sediments followed by intrusion and crystallization in the lithospheric mantle before the entire lithospheric section was obducted to form the Oman-United Arab Emirates ophiolites. The 27 studied samples show surprisingly low N contents (4.0 - 13.7 ppm; mean: 6.9 ± 2.3 ppm; 1σ) and a narrow δ 15 N range (mostly in the range of –1.4‰ to +2.2‰; mean: +0.4 ± 1.5‰; 1σ). Data modeling indicates that the N in these granitoids contains little mantle contribution and was predominantly inherited from their source rocks. Compared with potential source rocks, these granitoids display orders of magnitude lower N contents but comparable δ 15 N values, indicating that the significant N loss during the genesis of these granitoids was associated with little N isotope fractionation. This suggests that magmatic N 2 degassing (with minor N isotope fractionation) rather than metamorphic N devolatilization (with much larger N isotope fractionation) is the dominant mechanism driving the N loss from subducted sediments.
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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 | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.001 | 0.000 |
| Scholarly communication | 0.000 | 0.001 |
| Open science | 0.000 | 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".