Short Communication: Nanoscale heterogeneity of U and Pb in baddeleyite – implications for nanogeochronology and <sup>238</sup> U series alpha recoil effects
Bibliographic record
Abstract
Abstract. Atom probe tomography (APT) of 238U and 206Pb has been applied to baddeleyite crystals from the Hart Dolerite (1791 ± 1 Ma) and the Great Dyke of Mauritania (2732 ± 2 Ma) in an effort to map U and Pb concentration at the nanometre scale. The purpose was to constrain the average nuclear recoil distance of 238U-series daughter nuclei in order to correct U-Pb ages determined on small baddeleyite crystals for alpha-recoil loss of Pb. Both crystals were thought to expose natural crystal surfaces providing a boundary where maximum effects of recoil loss could be observed. The Hart Dolerite sample showed no variations in Pb concentrations near the edge. The Great Dyke sample shows U zoning and the associated 206Pb zoning is affected by alpha recoil, apparently adjacent to a natural grain surface. The sample also shows 10 nm-scale apparently primary clusters of U atoms that contain about 40 % of the U. These are too small to constrain alpha recoil distance beyond a few nm but are apparently primary and their formation mechanism poses a dilemma. To constrain alpha recoil distance, a forward modelling approach is presented where 206Pb redistribution functions were determined for a range of possible distances and synthetic 206Pb/238U profiles were determined from the convolution of the observed U profile with the redistribution functions that were compared to the observed 206Pb/238U profile. A complication is the fact that the 40 by 400 nm size of the sample is lower than the range of possible alpha recoil redistribution effects so it was necessary to extrapolate the observed U zoning. An oscillatory pattern gives the best fit to the observed profile but any reasonable extrapolation constrains the average alpha recoil distance to be close to 80–90 nm, which is much larger than previous estimates using other methods. Either recoil distances can be highly anisotropic within small crystal samples or surface roughness was a factor that modified the recoiled Pb distribution. APT is a potentially useful approach to determining average alpha recoil distance but requires sampling of primary, smooth crystal faces.
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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 | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 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) | 0.002 | 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".