Bibliographic record
Abstract
Abstract— The radiogenic 207Pb/206Pb ratio is the only extant nuclide chronometer with sufficient time resolution for studies of the solar nebula accretion and early asteroidal differentiation and metamorphism. Pb isotopic dates can be used to link the dates obtained from extinct nuclide chronometers to the absolute time scale. The factors that control precision and accuracy of Pb isotopic dates of meteorites: instrumental mass fractionation in isotopic analysis, mass spectrometer sensitivity, removal of common Pb, multi‐stage evolution of U‐Pb systems, disturbances caused by diffusion, alteration, and shock metamorphism, and uncertainties in decay constants and the natural ratio of the U isotopes are reviewed. The precision of Pb isotopic dates of meteorites attained with currently available techniques and methodology is ±0.5–1.0 Myr in favorable cases. The accuracy of time interval measurements is approximately the same. The most serious limitation on precision and accuracy of Pb isotopic dates is placed by the presence of common Pb of uncertain and/or variable isotopic composition. Improvement in precision and accuracy of Pb isotopic dates would be possible through combined advancement of techniques of isotopic analysis (most importantly, better control over instrumental mass fractionation) and more effective techniques for the removal of common Pb, together with a better understanding of the effects of thermal metamorphism, shock metamorphism, and aqueous alteration on the U‐Pb system in meteorites.
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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 | 0.004 | 0.003 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.001 | 0.002 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.001 | 0.001 |
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".