(U‐Th)/He dating of terrestrial impact structures: The Manicouagan example
Bibliographic record
Abstract
The accurate dating of meteorite impact structures on Earth has proven to be challenging. Melt sheets are amenable to high‐precision dating by the U‐Pb and 40 Ar/ 39 Ar methods, but many impact events do not produce them, or they are not preserved. In cases where high‐temperature shock metamorphism of the target materials has occurred without widespread melting, these isotopic chronometers may be partially reset and yield dates that are difficult to interpret unambiguously as the age of impact. However, the (U‐Th)/He chronometer is sensitive to thermal resetting and can provide a powerful new tool for dating impactites. We report (U‐Th)/He dates for accessory minerals from the Manicouagan impact structure in Quebec, Canada. Nine zircons from a melt sheet sample yield a weighted mean age of 213.2 ± 5.4 Ma (2SE), indistinguishable from the published 214 ± 1 Ma (2 σ ) U‐Pb zircon age for the impact. In contrast, five apatites from this sample yield dates between 205.9 ± 6.5 and 162.0 ± 5.3 Ma (2 σ ), indicating variable postimpact helium loss due to low‐temperature thermal disturbance. Preimpact titanite crystals from a shocked meta‐anorthosite sample yield two dates consistent with the impact age, at 212 ± 27 and 214 ± 13 Ma (2 σ ), and two younger dates of 189.6 ± 6.9 and 192.2 ± 9.8 Ma (2 σ ), suggestive of postimpact helium loss. These results indicate that (U‐Th)/He chronometry is a suitable method for dating impact events, although interpretation of the results requires recognition of possible 4 He loss related to reheating subsequent to impact.
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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 | 0.002 |
| Science and technology studies | 0.002 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 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".