In-situ phosphate U-Pb ages of the L chondrites
Bibliographic record
Abstract
The thermal history of asteroids is recorded by the radioisotopic ages of meteorites that derive from them. Radioisotopic ages may date a number of events, such as the cooling of a parent body during waning radiogenic metamorphism, rapid cooling experienced upon parent body break-up, and/or subsequent collision-induced reheating of material. However, sampling statistics for meteorite radioisotope ages are currently relatively low and most are derived from analyses of bulk material, therefore lacking the in-situ microtextural context that aids in distinguishing collisional events. Here, we present new in-situ apatite U-Pb ages for nine L chondrite meteorites using secondary ionisation mass spectrometry. Our measurements greatly expand the L chondrite phosphate U-Pb age record and provide evidence for distinct stages in the thermal evolution of the L chondrite parent asteroid, including: early collisions driving parent body fragmentation- and/or exhumation-associated cooling at > 4530 Ma; onion-shell-style cooling with waning radiogenic metamorphism until 4500 Ma; late collisional reheating from 4480–4460 Ma; parent body break-up at 474± 22 Ma; and recent ejection events within several 10s of Myr of present day. We show that meteorite shock stage correlates with upper intercept age but is uncorrelated with lower intercept age. This outcome links the upper intercept ages alone to the preserved high-energy impact-related features in strongly shocked meteorites, which has important implications for our interpretation of the L chondrite U-Pb record. We see no evidence in our record for collisional episodes between 3000–4400 Ma, i.e., the Late Heavy Bombardment. Our upper intercept age record hints that collision rates changed as a result of some dynamical instability at 4460–4480 Ma, which may have strongly depleted the main asteroid belt, and/or that L asteroid physical structure changed such that the shock metamorphic response to collisions was muted after this time, e.g., by the formation of weak rubble pile bodies. L chondrite phosphate U-Pb ages provide evidence for a heterogeneous early and shared late (less than 500 Ma) thermal history for the majority of L chondrite meteorites falling to Earth today. From this observation, we infer that most L chondrites derive from a single parent asteroid (in existence from around 4500–4440 Ma to 474± 22 Ma), which has since been disturbed to create an asteroid family. Our record of meteorite U-Pb ages traces out the thermal and dynamical evolution of the L chondrite asteroid. These observations can be used in future to benchmark dynamical models of Solar System evolution.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.001 | 0.000 |
| Bibliometrics | 0.000 | 0.001 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.000 | 0.001 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 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 teacher head, 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".