Compositional evolution of tourmaline-supergroup minerals from granitic pegmatites in the Larsemann Hills, East Antarctica
Bibliographic record
Abstract
In the Larsemann Hills, Prydz Bay, East Antarctica, granulite-facies metasedimentary units containing tourmaline, prismatine and grandidierite are cut by three generations (D 2 , D 3 and D 4 ) of granitic pegmatites, several of which are also unusually enriched in B-rich phases, including tourmaline (schorl – dravite – foitite solid solutions), prismatine, grandidierite, werdingite, boralsilite and dumortierite. Tourmaline can be used to gain information about the crystallization history of the pegmatites. Six microstructural varieties of tourmaline have been recognized: 1) primary tourmaline in a graphic intergrowth with quartz, 2) tourmaline overgrowths on tourmaline in the graphic intergrowth, 3) prisms of tourmaline with oscillatory zoning, 4) tourmaline that has replaced boralsilite, 5) tourmaline that has replaced prismatine, and 6) chaotically zoned tourmaline. Tourmaline compositions evolved as crystallization proceeded in the D 2 and D 3 pegmatites (considered together) and in the D 4 pegmatites, resulting in an increase in X -site vacancy, a decrease in Ti and F contents, an increase in Al at the ( Y + Z ) sites, and a decrease in the ratio Mg/(Mg + Fe). The first three changes are consistent with decreasing temperature. However, oscillatory zoning is characteristic of open systems, where there had been a mixing from different chemical sources; we consider it more likely that X -site vacancy and F content varied as a function of fluid composition rather than temperature per se. On the basis of the compositional variation and microstructural relations and comparison with the retrograde path inferred for the host rocks, we suggest that the pegmatites evolved as temperatures decreased from 700–750°C, 3–4.5 kbar when the graphic tourmaline + quartz crystallized, through 600–700°C, 3–4 kbar, the stability range considered appropriate for boralsilite, to below 600°C at~3 kbar for secondary tourmaline and dumortierite, conditions consistent with the presence of secondary andalusite.
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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 | 0.002 | 0.001 |
| Science and technology studies | 0.001 | 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) | 0.001 | 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".