Timing of lunar Mg-suite magmatism constrained by SIMS U-Pb dating of Apollo norite 78238
Bibliographic record
Abstract
The lunar Mg-suite magmatic rocks are commonly thought to represent mafic intrusions into the anorthositic flotation crust of the lunar magma ocean (LMO). Their geochronology is, therefore, important for constraining evolution models of the LMO. Petrogenetic models of the Mg-suite hold that their parent magmas were derived from primary LMO sources (Mg-cumulates, An-rich plagioclase, and melts enriched in KREEP—potassium, rare earth elements, and phosphorus). Previous radiogenic isotopic age interpretations of Mg-suite and putatively older, related ferroan anorthosites (FANs) overlap over a 200-million-year interval. The Apollo 78238 norite is an exemplary Mg-suite rock, with a relict coarse igneous texture modified by shock metamorphism. In-situ secondary ion mass spectrometry U-Pb analyses of zircon and baddeleyite in 78238 yield discordant arrays, attributed to recent impact metamorphism, with upper intercepts that constrain its crystallization age. The four oldest baddeleyite analyses give a weighted mean 207Pb/206Pb age of 4332 ± 18 Ma (2σ, MSWD = 0.06, P = 0.98), which is interpreted as the crystallization age of the norite. The overlap of the baddeleyite age with previously reported Sm-Nd and Pb-Pb mineral isochron ages for 78238 (Edmunson et al., 2009) supports a moderately fast cooling of the norite. Moreover, it is distinguishably younger than the most precisely dated sample of FAN (Apollo 60025), measured at 4360 ± 3 Ma by Sm-Nd and Pb-Pb mineral isochrons (Borg et al., 2011). Together with the baddeleyite 207Pb/206Pb age of Apollo Mg-suite troctolite 76535 at 4328 ± 8 Ma (White et al., 2020), the chronological record of the 78238 norite indicates a significant Mg-suite magmatic event at 4.33 Ga and a lower age limit on LMO differentiation.
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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 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| 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".