Isotopic and geochemical studies of kimberlite and included xenoliths
Bibliographic record
Abstract
InitialSr, Nd and Pb isotopic compositions of southern African Jurassic-Cretaceous kimberlites define two distinctive varieties that must be derived from dif¬ ferent types of upper mantle sources (Smith, 1983).Group I kimberlites, commonly re¬ ferred to as "basaltic", generally lack appreciable mica and with some exceptions have consistent Nd isotopic compositions slightly higher than the bulk earth value.87sr/86sr ratios are somewhat more variable ranging from about .7035 to .7050 in reas¬ onably fresh hypabyssal samples.Pb isotopic compositions are more variable, spanning nearly the full range of oceanic island basalts (OIB).Group II kimberlites have high groundmass mica contents and tend to lack ilmenite, zircon and perovskite, minerals characteristic of Group I kimberlites, as well as having somewhat older emplacement ages (Smith et al., in press).In addition, inclusions of the Cr-poor subcalcic megacryst assemblage and high P-T sheared Iherzolite xenoliths with isotopic character¬ istics suggestive of asthenospheric origins are apparently absent in Group II intrusives.The Group II kimberlites have radiogenic Sr (^^Sr/°^Sr = .7075to greater than .710)and non-radiogenic Nd (^^^Nd/^^'^Nd = .5120to .5123,relative to .51264 in standard BCR-1), plotting in the enriched mantle quadrant of the Sr-Nd correlation diagram.Compared to Group I, Group II source rocks have had high time-averaged Rb/Sr and LREE enrichment, with minimum model source ages from Sm-Nd systematics being of the order of 600 million years.Pb isotopic compositions are considerably less radio¬ genic and less variable than Group I, OIB or most mid-ocean ridge basalts (MORE).
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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 | 0.002 | 0.001 |
| Science and technology studies | 0.001 | 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) | 0.002 | 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".