Elemental and isotopic geochemistry of kimberlites from the Lac de Gras field, northwest territories, Canada
Bibliographic record
Abstract
Detailed major/trace element and high-precision Hf-Nd-Sr isotope data has been acquired for 86 samples of kimberlites from the recently discovered Lac de Gras kimberlite field in the Northwest Territories, Canada, plus 23 samples from other Canadian kimberlite occurrences. This constitutes the most comprehensive geochemical database available for kimberlites at the present time, and allows detailed comparisons to be made with the well-documented kimberlites of the Kaapvaal craton, southern Africa. Major and trace element data shows that some Lac de Gras kimberlites have interacted extensively with continental crust, whereas others are minimally contaminated with crust but have physically incorporated large quantities (˃30%) of lithospheric mantle peridotite. Fresh, minimally-contaminated kimberlites from Lac de Gras have both elemental and isotopic characteristics that are transitional relative to those typical of southern African Group I and II kimberlites. The Hf-Nd isotope variations of these samples also define a linear array that strongly suggests mixing of two or more components/reservoirs within the mantle. Isotopic mixing models and mass balance considerations constrain the most likely candidate components/reservoirs to be depleted sub-continental lithospheric mantle and a component with an isotopically-enriched, negative ΔɛHf signature that is derived from beneath the lithosphere. Such a component has previously been identified in southern African kimberlites, indicating that it is globally extensive within the mantle. Its resfriction to magmas generated at great depths, and its unusual Hf isotope signature also suggest that it may reside in isolation at some mantle boundary layer. Ancient oceanic crust, generated by melting in the presence of garnet and subsequently subducted and stored below the lithosphere, could evolve to negative ΔɛHf compositions. Melts of this material, variably recombined with the depleted, garnet- rich melting residua that constitute the lithosphere, can then account for the Hf-Nd kimberiite isotopic array. Trace element characteristics, such as K and Sr anomalies, are consistent with those of OIB-like magmas derived from within the convecting mantle.
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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 | 0.004 |
| Science and technology studies | 0.002 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.001 | 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".