Nd, Pb and Sr Isotopic Compositions of East African Carbonatites: Evidence for Mantle Mixing and Plume Inhomogeneity
Bibliographic record
Abstract
New Pb isotopic data are presented for 10 young Mesozoic to Cenozoic (0–116 Ma) carbonatites from a 1400 km long segment of the East African Rift. Patterns observed in Pb vs Pb, Sr vs Pb and Nd vs Pb isotope diagrams define unusual, nearly linear, trends that are interpreted as mixing between two components that are broadly similar to the two mantle end-member components, HIMU and EM1, which were first recognized from ocean-island basalts. The two plutons with isotope signatures closest to HIMU and EM1 crop out within 140 km of each other. From these data, EM1 and HIMU are now known to occur in both continental and oceanic settings that are associated with plumes or rifts. Moreover, these isotopic signatures tend to occur in regions where seismic tomography indicates prominent low-velocity zones in the lower mantle. For these reasons, we favour a model for the origin of the East African Rift carbonatites that involves melting and mixing of HIMU and EM1 components contained within an isotopically heterogeneous mantle plume. We consider the HIMU and EM1 sources to be stored within the deep (lower 1000 km) mantle, possibly the core–mantle boundary. The role that continental lithosphere plays in carbonatite generation is probably one of concentrating volatiles at the upper levels of an ascending mantle plume.
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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 | 0.000 | 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) | 0.001 | 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".