Radiogenic isotope characteristics of the Mesoproterozoic intrusive rocks of the Nipigon Embayment, northwestern Ontario
Bibliographic record
Abstract
The ca. 1110–1120 Ma diabase sills of the Nipigon Embayment represent the oldest phase of the Midcontinent Rift recognized to date. The sills can be subdivided into five geochemically distinct suites: Nipigon, McIntyre, Inspiration, Shillabeer, and Jackfish. Sm–Nd, Rb–Sr, and Pb–Pb isotope data for the sills can be used to evaluate the source regions and emplacement histories of these sills. The εNdT values for all the sill suites are consistently negative (–0.5 to –6.6) but show coherent variations both within and between suites. The negative εNdT values can be interpreted as the result of contamination of a plume-related mantle source by older crustal material. The initial strontium (Sri) values (0.7032–0.7068) for the sills indicate that at least two distinct contaminants are required: (1) a source with strongly negative εNdT and lower Sri, likely Archean metasediment or granite, and (2) one with elevated Sri, likely sedimentary rocks of the Proterozoic Sibley Group. The Pb data for the Nipigon and Inspiration sills form distinct linear arrays consistent with variable contamination of a source comparable to that of other igneous suites of the Midcontinent Rift. The radiogenic isotopes preserve a complex contamination history best interpreted as the result of <5% contamination by a variety of Archean and Proterozoic sources. The ultramafic Jackfish sill shows the least evidence of contamination and is interpreted to have been emplaced along a crustal-scale fault with little interaction with crustal material. In contrast, the other sill suites have undergone complex contamination histories requiring variable crustal residence times and assimilation of material both at depth in large magma chambers and during emplacement.
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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 | 0.000 | 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".