A COMBINED GEOCHEMICAL AND GEOCHRONOLOGICAL INVESTIGATION OF NIOCALITE FROM THE OKA CARBONATITE COMPLEX, CANADA
Bibliographic record
Abstract
This study is the first to report a detailed geochemical investigation and in situ U-Pb ages for niocalite, which occurs within carbonatite from the Bond Zone area of the Oka Carbonatite Complex (Canada). Niocalite is a Nb-disilicate member of the lavenite–cuspidine group. The major element composition of the niocalite studied here is relatively homogeneous with the average formula of: (Na 0.34 Fe 0.06 Mn 0.19 Mg 0.09 Ca 13.40 REE 0.15 Ti 0.02 ) ∑14.25 Nb 2.12 Ta 0.06 (Si 2 O 7 ) 4 O 7.65 F 2.44 . Niocalite is enriched in minor and trace elements [ i.e. , Ta, Ti, and rare earth elements (REEs) up to 4.35 wt.%], with double- and triple-valenced elements ( i.e. , Sr, Y, REEs) substituting at different Ca-occupied lattice sites. The chondrite-normalized REE patterns for niocalite are LREE-enriched (~10 4 times chondrite) and negatively sloped. In addition, niocalite has higher HREE contents compared to those for co-existing apatite from the identical carbonatite sample. This result is expected based on bond valence data because of differing cation sizes. In situ U-Pb ages for niocalite from three carbonatite samples obtained by LA-ICP-MS define a wide range of ages, between ~111 and ~133 Ma. Niocalite from one carbonatite sample yields a bimodal distribution with weighted mean 206 Pb/ 238 U ages of 110.1 ± 5.0 Ma and 133.2 ± 6.1 Ma, and overlap those of co-existing apatite for the same sample. Two other samples of carbonatite yield weighted mean 206 Pb/ 238 U ages of 110.6 ± 1.2 Ma and 115.0 ± 1.9 Ma. The geochronological data for niocalite reported here suggests a protracted petrogenetic history for Oka, which corroborates the relatively large range of ages (~20 Myr) previously determined for apatite from the Oka complex. In situ 87 Sr/ 86 Sr isotope ratios for niocalite obtained by LA-MC-ICP-MS are variable and range between 0.70314 (±7) and 0.70343 (±8). Based on the combined U-Pb ages, major and trace element abundances, and Sr isotope ratios, the history of niocalite formation cannot be attributed to either simple closed-system fractional crystallization or carbonate-silicate liquid immiscibility. Rather, these are consistent with periodic carbonatite melt derivation from a heterogeneous mantle source, and suggest that some of niocalite investigated here represent cognate crystals.
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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 | 0.003 | 0.002 |
| Science and technology studies | 0.003 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.001 | 0.001 |
| 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".