Vigorous Exsolution of Volatiles in the Magma Chamber beneath a Hydrothermal System on the Modern Sea Floor of the Eastern Manus Back-Arc Basin, Western Pacific: Evidence from Melt Inclusions
Bibliographic record
Abstract
Image analyses and ion probe analyses were carried out on melt inclusions in olivine, pyroxene, and plagioclase phenocrysts of vesicular volcanic rocks that host the actively forming massive sulfides at the PACMANUS hydrothermal field in the eastern Manus back-arc basin, western Pacific. The melt inclusions, ranging in size from 150 μm, contain one or more bubbles. Most of them have bubble/melt inclusion volume ratios (Vb/Vm) of >5.5 vol percent, higher than those for the melt inclusions with their bubbles generated from shrinkage (Vb/Vm, 3.6 vol %) or postentrapment exsolution of volatiles (Vb/Vm, 3.7 vol %). This, together with the presence of vapor bubbles with little melt (Vb/Vm, 80 to >95 vol %) in pyroxene and olivine phenocrysts, suggests that a vapor phase was formed from vigorous exsolution of volatiles or the boiling of a magma in the crystallizing magma chamber. The Vb/Vm ratios show distinct distribution patterns in the phenocryst minerals, indicating that there was more than one vesiculation event in the magma chamber. Melt inclusions with no visible bubbles have the highest contents of H2O (2.1–2.5 wt %). The H2O contents of the glass in the melt inclusions are inversely related to the size of the coeval bubbles. The significant variation of H2O contents (1.0–2.5%) in the melt inclusions suggests that the H2O was degassed from the crystallizing magma. The positive correlations of S, Cl, and possibly F with H2O indicate that all these volatiles were exsolved from the melt into the vapor phase. The negative correlation of H2O with alkali contents in the glass of melt inclusions suggests that the magma was crystallizing in the presence of a fluid phase. It is estimated that the magma might have lost up to 65 percent of its volatiles while crystallizing. Significant amounts of magmatic fluid can be degassed from a magma during its crystallization in a shallow magma chamber. An individual vesiculation event may last at least 5 to 48 years, as estimated from the growth of the phenocrysts. This estimate is a minimum because not all the phenocrysts crystallize at the same time. The estimated life of a black smoker chimney is 2 to 10 years, which is within the duration of an individual vesiculation event. This implies that the magmatic fluid could be a major source of ore metals during the growth of individual sulfide chimneys. Through multiple vesiculation events in a crystallizing magma chamber, the magma could provide large quantities of ore metals and volatiles to a hydrothermal system over a long period of time to form a large massive sulfide deposit on the sea floor.
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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 | 0.000 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 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".