A Geochemical Investigation of Oxygen Fugacity in the Marianas Subduction Factory
Bibliographic record
Abstract
Oxygen fugacity is a fundamental thermodynamic property that describes reduction-oxidation (redox) equilibria in the solid Earth. It controls material transfer from the interior to the exterior of the planet by dictating the speciation of multi-valent elements (e.g., Fe, V, S). Oceanic crust ages and oxidizes as it moves from spreading centers to subduction zones, where it returns to the mantle in modified form. Subducting slabs release H2O-rich fluids and SiO2-rich melts to the mantle wedge in subduction zones, contributing significantly to the isotopic and major, trace, and volatile element composition of arc and back-arc magmas, however the effect that the oxidized nature of subducting slabs on arc basalts remains unclear. Arc basalts have a higher proportion of oxidized (Fe3+) relative to reduced (Fe2+) iron, expressed as the Fe3+/ΣFe ratio, than do mid-ocean ridge basalts (MORB) but there is disagreement as to whether this arises due to shallow level differentiation processes (e.g., crystal fractionation, crustal assimilation, degassing) in the arc crust or to differences in the f O2 of the mantle source. This thesis addresses this problem by examining the oxidation state of Fe and other transition element proxies for f O2 in (1) altered oceanic crust prior to subduction, (2) modern eruptive products from the active Mariana arc and back-arc, and (3) eruptive products representative of subduction initiation and margin evolution in the Marianas. Melt inclusions and submarine glasses record variable magmatic compositions that have the potential to record changes in magma chemistry during crystal fractionation and volcanic degassing. Recent innovations in synchrotron technologies have made studies of Fe redox possible in situ, on small scales (>10 microns), allowing direct observation of changes in Fe redox during shallow level differentiation processes in arc and back-arc magmas. This study reports observations of Fe redox variation from several Mariana arc volcanic centers as well as from the Mariana trough, demonstrating that shallow level differentiation processes are not responsible for generating the oxidized nature of arc basalts. Constraints for mantle source f O2 show that the mantle wedge is more oxidized than MORB source mantle and link this oxidation to influence from recycling slab fluids. Additionally, we explore other transition row element proxies for fO2 and show that these proxies need not preclude oxidized mantle wedge conditions in the Marianas. We examine changes in Fe redox in samples that record subduction initiation and margin evolution, demonstrating that in zones of melt generation within
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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 | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.001 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.001 |
| 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 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".