Significant variability in the δ44/40Ca of global carbonatites: implications for carbonate recycling, magma differentiation and source-mantle mineralogy
Bibliographic record
Abstract
Stable Ca isotopic composition (δ44/40Ca) of crustal carbonates are typically lighter than that of the bulk silicate Earth value (~1.05 ‰). Hence, δ44/40Ca of mantle-derived rocks can potentially trace recycled crustal carbonates into the mantle. We report the Ca isotopic compositions of globally distributed carbonatites (n = 46), which are unique igneous rocks with more than 50% modal carbonate minerals, with eruption ages ranging from Precambrian until recent. The δ44/40Ca (w.r.t. SRM915a) of these carbonatites show a large range (0.35 ‰ to 1.26 ‰), which is significantly higher than the analytical uncertainty (0.08‰) of the measurements performed using TIMS at CEaS, IISc. These samples are well-characterized in terms of their major and trace element geochemistry as well as Nd, Sr, B, C, and O isotopic compositions for selected samples. No systematic trend is observed between δ44/40Ca of the carbonatites and their eruption ages. Significant variability is observed in δ44/40Ca values in samples from individual provinces including those from the Oka complex in Canada (0.44 ‰ – 1.26 ‰, n= 8), Ambadongar (0.53 ‰ – 1.1 ‰, n= 8) and the Newania complexes (0.44 ‰ – 0.83 ‰, n= 4) in north-west India, the South Indian carbonatites (0.65 ‰ – 0.91 ‰, n= 3) and carbonatites from the Palabora complex in South Africa (0.35 ‰ – 0.84‰, n= 3). The δ44/40Ca of carbonatites from Oka, Newania and the Ambadongar show strong correlations with Ca/Mg, Ca/Fe as well as CaO and MgO contents. The δ44/40Ca of the Oka and Ambadongar carbonatites show correlated variations with their Mg# and K/Rb ratios, respectively. The large variability in δ44/40Ca of global carbonatites is explained in terms of: (1) presence of isotopically lighter ancient subducted carbonates in the mantle-source regions and carbonate metasomatism of the mantle, (2) partial melting and differentiation of the carbonatite magma and (3) heterogeneity in the source-mantle mineralogy of carbonatites.
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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 | 0.001 | 0.001 |
| Science and technology studies | 0.001 | 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".