Igneous Rock Associations 4. Oceanic Island Volcanism I Mineralogy and Petrology
Bibliographic record
Abstract
Oceanic islands tend to occur at the young ends of hotspot trails because they record the passage of oceanic plates over rising convection cells (plumes) in the mantle, or the propagation of cracks in the lithosphere. Basaltic volcanism on oceanic islands is generally unexplosive and, although potentially destructive, poses less threat to human life than volcanism in other tectonic environments. However, the possibility of giant tsunamis from the catastrophic gravitational collapse of islands is of real concern for major cities surrounding the ocean basins. Two series of magmas are recognized in oceanic islands. Tholeiites form at lower pressures than alkali basalts, from higher percentages of decompression melting. The former contain a low-Ca pyroxene and the latter can crystallize nepheline. Furthermore, minerals common to both series (chromite, olivine, augite, plagioclase, magnetite and ilmenite) are compositionally distinct reflecting fundamental chemical differences between the two magma series. Mineral compositions vary as magmas evolve in sub-volcanic, lithospheric magma chambers by assimilation and differentiation. Magmas assimilate wall rocks in these chambers. Time-scales for differentiation (mostly crystal fractionation) are generally less than a few thousand years. Early olivine, pyroxene, chromite and immiscible sulfide formation cause compatible elements(e.g., Ni, Co, Cr) to decline rapidly as differentiation proceeds. Plagioclase dramatically removes Sr at intermediate stages and alkali feldspars sequester Ba and Rb as late-stage trachytes and phonolites form in alkaline magmas. The high-field-strength elements are generally incompatible but locally decline reflecting apatite (P) and Fe-Ti oxide (Ti, Nb, Ta) removal. Studies of layering in individual lava flows suggest that rising volatiles may effect mass transfer of complexed ions during differentiation in magma chambers. SOMMAIRE Les iles oceaniques se trouvent generalement a l'extremite la plus jeune des trainees de points chauds parce qu'elles sont la marque du passage de plaques oceaniques au-dessus de cellules de convection ascendantes (panaches) dans le manteau ou de la propagation de fissures de la lithosphere. En general, le volcanisme basaltique des iles oceaniques n'est pas explosif, et bien qu'il puisse etre destructeur, il presente moins de danger pour les humains que le volcanisme d'autres environnements tectoniques. Cependant, la formation possible de tsunamis geants provoques par l'effondrement d'iles constitue un danger reel pour les grandes agglomerations situees au pourtour des basins oceaniques. On distingue deux series magmatiques dans les materiaux constitutifs des iles oceaniques. Les tholeiites qui se forment a des pressions plus faibles que les basaltes alcalins, et plus souvent sous des conditions de fusion par decompression, et les basaltes alcalins. Les premieres contiennent un pyroxene a faible teneur en calcium et l'autre permet la cristallisation de la nepheline. De plus, les mineraux communs a ces deux series (chromite, olivine, augite, plagioclase, magnetite et ilmenite) ont des compositions distinctes qui refletent les differences de composition chimique intrinseques de ces deux series magmatiques. Les compositions minerales varient puisque les magmas changent de composition dans les chambres magmatiques lithospheriques pre-volcaniques par assimilation et differentiation. Les magmas absorbent les roches des murs d'enceinte dans ces chambres. Les echelles de temps de ces differentiations (principalement de fractionnement cristallin) s'etalent generalement sur quelques milliers d'annees. D'abord, la formation d'olivine, de pyroxene, de chromite et de sulfures immiscibles entraine un appauvrissement rapide du magma en elements compatibles (Ni,Co, Cr par ex.). Puis, aux stades intermediaires, la formation de plagioclases reduit considerablement la teneur en Sr et, aux stades finaux, la formation de feldspaths alcalins en extrait le Ba et le Rb alors que se forment les trachytes et les phonolites a partir de magmas alcalins. Les elements les plus fortement polarises sont generalement incompatibles mais, localement, leur teneur decline, refletant ainsi la formation d'apatite (P) et d'oxydes de Fe-Ti (Ti, Nb, Ta). Les resultats d'etudes sur la formation en couche de coulees de laves indiquent que l'ascension gravitation-nelle des volatils pourrait avoir un effet sur le transfert de masse d'ions complexes durant la differenciation dans les chambres magmatiques.
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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 | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.004 | 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".