Lead Isotope Age of Chondrules in the CR2 Chondrite NWA801 by a Progressive Stepwise Dissolution
Bibliographic record
Abstract
CR2 chondrites are a unique class of carbonaceous meteorites characterized by an abundance of metallic iron that forms multi-layered rims around silicate chondrules (Fig. 1). We have experimented with a progressive stepwise dissolution of chondrules from the CR2 chondrite NWA801 for the purpose of obtaining Pb/Pb ages from the corresponding radiogenic leachates and residues. Following an extensive pre-cleaning to remove extraneous lead, three multi-chondrule fractions (12.6, 31.5 and 35.7 mg) were leached with acids of increasing strength starting from weak HCl and ending with HF/HNO3. ID-TIMS analyses of the progressively stronger leachates (on a VG354 thermal ionization mass spectrometer in Daly and multi-dynamic-Faraday modes using a U-Pb-spike) showed that both HF and HF/HNO3 are efficient at extracting radiogenic Pb components from the chondrules (Pb/Pb = 324 to 1114 after fractionation, blank and spike corrections). A weighted regression through the final three leaches and residues of each of the three multi-chondrule fractions (nine points total) yields a Pb/Pb isochron age of 4564.6±1.0 Ma (95% confidence, MSWD=1.3) assuming that the lead is a mixture between blank and primordial Canyon Diablo Troilite Pb (CDT; Fig. 2). Our age for NWA801 chondrules is identical to the Pb/Pb age of chondrules in the CR2 chondrite Acfer 059 (4564.7±0.6 Ma; Amelin et al. 2002) whereas chondrules from CB chondrites are ~2 Myr younger, and those from Allende (CV3) are at most ~2 Myr older (Fig. 3). This suggests that all chondrules in CR2 chondrites formed as a result of the same nebular event and may be from the same parent body. However the complex multi-rimmed textures of the chondrules in NWA801 (i.e. Fig. 1) suggest that multiple and as yet unresolved processes were involved during their formation.
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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 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.001 |
| 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".