OCCURRENCE, ALTERATION PATTERNS AND COMPOSITIONAL VARIATION OF PEROVSKITE IN KIMBERLITES
Bibliographic record
Abstract
The present work summarizes a detailed investigation of perovskite from a representative collection of kimberlites, including samples from over forty localities worldwide. The most common modes of occurrence of perovskite in archetypal kimberlites are discrete crystals set in a serpentine–calcite mesostasis, and reaction-induced rims on earlier-crystallized oxide minerals (typically ferroan geikielite or magnesian ilmenite). Perovskite precipitates later than macrocrystal spinel (aluminous magnesian chromite), and nearly simultaneously with “reaction” Fe-rich spinel ( sensu stricto ), and groundmass spinels belonging to the magnesian ulvospinel – magnetite series. In most cases, perovskite crystallization ceases prior to the resorption of groundmass spinels and formation of the atoll rim. During the final evolutionary stages, perovskite commonly becomes unstable and reacts with a CO 2 - rich fluid. Alteration of perovskite in kimberlites involves resorption, cation leaching and replacement by late-stage minerals, typically TiO 2 , ilmenite, titanite and calcite. Replacement reactions are believed to take place at temperatures below 350°C, P a (Mg 2+ ) values. Perovskite from kimberlites approaches the ideal formula CaTiO 3 , and normally contains less than 7 mol.% of other end-members, primarily lueshite (NaNbO 3 ), loparite (Na 0.5 Ce 0.5 TiO 3 ), and CeFeO 3 . Evolutionary trends exhibited by perovskite from most localities are relatively insignificant and typically involve a decrease in REE and Th contents toward the rim (normal pattern of zonation). A reversed pattern is much less common, and probably results from re-equilibration of perovskite with a kimberlitic magma modified by assimilation or contamination processes. Oscillatory zonation on a fine scale is comparatively uncommon, and involves subtle variations in LREE , Th, Nb and Fe. Relatively high levels of LREE , Th and Nb observed in perovskite from some occurrences (Lac de Gras and Kirkland Lake in Canada, Obnazhennaya in Yakutia) probably result from inherent enrichment of the host kimberlites in “incompatible” elements. In some cases (Benfontein in South Africa), differentiation processes may have contributed to the accumulation of “incompatible” elements in perovskite.
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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 | 0.004 | 0.002 |
| Science and technology studies | 0.000 | 0.000 |
| 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".