Reaction Rims on Ilmenite and Chromite: Implications for Volatile Behavior and Crystallization Conditions of Kimberlite Magma
Bibliographic record
Abstract
Abstract Significant uncertainty surrounds the crystallization conditions and the composition of kimberlite melts, including the role of volatiles (H2O and CO2) due to their hybrid nature, intense alteration, and volatile loss during emplacement. In this study, we address these uncertainties by investigating the interaction between oxides (ilmenite and chromite) and kimberlite magma. During kimberlite ascent, mantle minerals react with the magma and develop dissolution textures, compositional zoning, and rims of secondary mineral phases in response to crystallization conditions and the composition of kimberlite magma. We examined oxides from several lithologies within the BK1 and AK15 kimberlites of the Orapa cluster in Botswana, where diamonds demonstrate distinct dissolution styles in each lithological unit owing to differences in magma saturation with volatiles. Here we discovered a strong correlation of the reaction products on ilmenite and chromite with the dissolution style of diamonds in the same kimberlite unit. Diamonds with glossy, low-relief surface features indicative of fluid-rich magma occur in the kimberlite units where ilmenite and chromite develop reaction rims of Ti-bearing phases. Diamonds with corrosion sculptures implying a volatile-undersaturated magma occur in kimberlite units with heavily resorbed chromite and ilmenite completely replaced by a MUM (magnesio-ulvöspinel-magnetite)–perovskite symplectite. Furthermore, the composition of ilmenite reaction rims depends on kimberlite lithology, where MUM co-exists with perovskite or its break-down product anatase in the two coherent kimberlite units, or with perovskite and titanite in the massive volcaniclastic unit. We examine how decompression, cooling, degassing, or assimilation of crustal rocks by kimberlite magma could have shifted conditions from perovskite to titanite stability in the volcaniclastic kimberlite unit. We propose perovskite replacement by anatase-calcite pseudomorphs in the top coherent unit, from which diamonds exhibit an overprint of fluid resorption with a melt resorption. Composition of ilmenite reaction rims provides estimates of kimberlite crystallization temperatures of 730–1275 °C and oxygen fugacities of +0.5 to −3.5 relative to the nickel-nickel oxide buffer, which are validated through controlled experiments. Our study shows that preservation of ilmenite, the type of Ti-phase in its reaction rim, the relative rate of chromite dissolution, and compositional re-equilibration with kimberlite can help model the eruption process as well as the style and rate of diamond dissolution.
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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) | 0.000 | 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".