Iron isotope evidence of an impact origin for main-group pallasites
Bibliographic record
Abstract
Despite decades of work, the origin of pallasite meteorites has remained enigmatic.Long thought to be samples of the core-mantle boundary of differentiated asteroids, more recent studies have suggested a range of mechanisms for pallasite formation.These include olivine-metal mixing during a planetesimal collision and the intrusion of over-pressured core liquids into a planetesimal mantle.Establishing if the olivine and metal that comprise pallasites were once equilibrated at high temperature remains key to discriminating between these hypotheses.To this end, we determined the iron isotope compositions of olivine and metal in eleven main-group pallasites and found, in all cases, that olivine is isotopically lighter than metal.To interpret these data, we constrained the olivine-metal equilibrium Fe isotope fractionation with ab initio calculations and high temperature experiments.These independent approaches show that olivine preferentially incorporates the heavy isotopes of iron relative to metal.Our results demonstrate that pallasitic olivine and metal never achieved isotopic equilibrium with respect to iron.This precludes extended cooling at high temperature and is best reconciled with an impact origin for the main-group pallasites.
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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 | 0.000 | 0.000 |
| Open science | 0.001 | 0.001 |
| 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".