Carbon–Strontium Isotope Decoupling in Carbonatites from Caotan (Qinling, China): Implications for the Origin of Calcite Carbonatite in Orogenic Settings
Bibliographic record
Abstract
Abstract Mantle-derived carbonatites emplaced in orogenic belts and some extensional settings are hypothesized to contain recycled crustal material. However, these carbonatites are typically composed of calcite showing a typical mantle range of C–O isotopic values devoid of recognizable sedimentary fingerprints. Here, we report the first known instance of C–Sr isotope decoupling between intimately associated dolomite carbonatites and magnetite–forsterite–calcite carbonatites from the northern Qinling orogen, central China. The calcite-dominant variety is developed at the contact between the dolomite carbonatite and metasomatized wall-rock gneiss. The two types of carbonatites have similar δ18OVSMOW (6·98‰ to 9·96‰), εNd(i) (-3·01 to -6·47) and Pb (206Pb/204Pb(i) = 17·369–17·584, 207Pb/204Pb(i) = 15·443–15·466) isotopic compositions, but significantly different C and Sr isotopic signatures (δ13CVPDB = -3·09 to -3·58‰ and -6·11 to -7·19‰; 87Sr/86Sr(i) = 0·70373 to 0·70565 vs 0·70565 to 0·70624 for the dolomite and calcite rocks, respectively). The relative enrichment of the early-crystallizing dolomite carbonatite in 13C and its depletion in 87Sr are primary isotopic characteristics inherited from its mantle source. The observed field relations, petrographic and geochemical characteristics of the Caotan dolomite and calcite carbonatites imply that the strong C–Sr isotopic decoupling between them could not result from mixing of different mantle reservoirs (e.g. HIMU and EM1), or from magma fractionation processes. We propose that the calcite carbonatites were a by-product of metasomatic reactions between primary dolomitic melts and felsic wall-rock. These reactions involved the loss of Mg and CO2 from the magma, leading to depletion of the evolved calcite-saturated liquid in 13C and its enrichment in radiogenic Sr. We conclude that calcite carbonatites in plate-collision zones may not represent primary melts even if their isotopic signature is recognizably ‘mantle-like’.
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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".