Geochemistry of the impact-generated melt sheet at Manicouagan: Evidence for fractional crystallization
Bibliographic record
Abstract
[1] Recent exploration drilling of the Manicouagan impact structure has revealed local developments of impact melt that are substantially thicker than the previously accepted average of ∼300 m and a more complex melt sheet–basement interface showing considerable “topography.” The thickest section (1045 m of clast-free to clast-poor impact melt overlying 425 m of clast-laden impact melt) is associated with a centrally located, fault-bounded graben that was in place prior to melt crystallization. Here we report the results of a geochemical investigation of 115 samples obtained from nine drill holes and a set of field samples. The results reveal that most of the drill core comprises an undifferentiated unit, showing minimal geochemical variation. This is typically ∼300 m thick and equates with previous studies performed on samples from exposed rock. In contrast, the deeper section exhibits differentiation and is divided into three layers based on chemical, mineralogical, and textural variations: a quartz monzonite to quartz monzodiorite upper zone (276 m), a quartz monzodioritic middle zone (244 m), and a mainly monzodioritic lower zone (525 m). The mineralogy is defined by plagioclase > orthoclase > clinopyroxene > orthopyroxene, with ubiquitous amphibole and biotite and olivine locally developed in the lower zone and below. Our results reveal that the thicker section of the impact melt sheet has undergone fractional crystallization. Differentiation of parts of the Manicouagan impact melt indicates that lunar impact melt samples possessing different chemistries, previously considered to be derived from distinct impact basins, may be related via the fractionation of common, much smaller diameter impact structures.
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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 | 0.001 |
| Scholarly communication | 0.001 | 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".