HYPOGENE TITANIAN, VANADIAN MAGHEMITE IN REWORKED OXIDE CUMULATES IN THE BEJA LAYERED GABBRO COMPLEX, ODIVELAS, SOUTHEASTERN PORTUGAL
Bibliographic record
Abstract
At Odivelas (Beja, Portugal), massive accumulations of coarse-grained magnetic oxides occur within the lower group of a layered, synorogenic Variscan gabbroic sequence. These oxide bodies, irregular in shape, at approximately right angles to the regional layering and of considerable size (up to an estimated 50 t each), are mainly composed of vanadium-bearing titanian maghemite, as large equant grains forming a polygonal texture, poikilitic ilmenite and accessory amounts of maghemite. No Fe 2 + was detected by Mossbauer spectroscopy in either maghemite and titanian maghemite. Strongly weathered samples show hematite and goethite replacing the maghemite or forming late veinlets cutting all oxide phases. Apart from these supergene veinlets, no fracturing is discernible within the oxide bodies; also, no zonation was found in the oxide phases, despite an examination by several techniques (reflected light microscopy, EPMA and micro-PIXE). The irregular and discordant shape of these bodies must have resulted from localized settling of primary oxide grains in the magma chamber or from gravitational destabilization of previously formed layers of oxide minerals. A supergene origin for these accumulations of maghemite is discounted because of textural considerations and the total absence of hydrous alteration. The surrounding gabbroic rocks show no alteration and host an oxide assemblage composed of titanian magnetite and ilmenite, which closed chemically at around 650-675°C, thus above the experimental threshold for maghemite formation. The oxide bodies of Odivelas thus underwent an oxidation process that has not affected their host rocks and that must have been caused by the focusing to their margins of late degassing fluids owing to rheological contrasts. As soon as the oxidizing fluids reached the oxide accumulations, oxidation proceeded by intergranular diffusion until all primary spinels were converted to maghemite, but did not spread to the country rocks, owing to the sluggishness of diffusion through silicates at these temperatures. The development of part of the observed ilmenite and of the precursor of Ti-free maghemite seems to result from the early stages of the oxidation process, according to the equation normally describing oxidation-induced exsolution in titanian magnetite. The ilmenite and spinel structures remained intact under further strong oxidation, probably owing to the incorporation of minor cations inherited from the original spinel.
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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 | 0.000 |
| 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) | 0.000 | 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 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".