An Early Triassic 40Ar/39Ar age for a camptonite dyke in Cambridge, Massachusetts
Bibliographic record
Abstract
A 2.4 m-thick megacryst- and xenolith-rich camptonite dyke briefly exposed during excavation of a subway station at Porter Square in Cambridge, Massachusetts, was sampled for petrological study and radiometric dating to constrain its age. The sample yielded a well-defined 40Ar/39Ar age of 246 ± 4 Ma using separates prepared from a ferro-kaersutite mega-phenocryst. This age is the oldest yet reported for a Coastal New England province lamprophyric dyke. The dyke likely reflects regional extension in Avalonia during the initial rifting stage of Pangea to form the North Atlantic Ocean basin. This new age further confirms that mantle upwelling began early in the Late Permian to Triassic as a precursor to the breakup of Pangea some 40 million years later.
 
 RÉSUMÉ
 
 Au cours de travaux d’excavation d’une station de métro à Porter Square, à Cambridge, au Massachusetts, un dyke de campronite d’une épaisseur de 2,4 m composé de mégacristaux et riche en xénolite a été exposé brièvement. On y a prélevé un échantillon aux fins d’une étude pétrologique et d’une datation par radiométrie. Par la méthode 40Ar/39Ar, il a été possible de déterminer avec relativement de précision un âge de 246 ± 4 Ma, grâce à des fractions obtenues par préparation d’un mégaphénocristal de ferro-kaersutite. Il s’agit de la plus ancienne datation établie pour un dyke lamprophyrique de la province géologique du littoral de la Nouvelle-Angleterre. Ce dyke rendrait vraisemblablement compte de l’extension régionale de l’Avalonien au cours du stade initial de distension de la masse continentale de Pangée, qui allait entraîner la création du bassin océanique de l’Atlantique Nord. Cette nouvelle datation vient confirmer une fois de plus que la remontée du manteau terrestre a débuté très tôt entre la fin du Permien et le Trias, cet événement géomorphologique annonçant le fractionnement ultérieur du continent de Pangée quelque 40 millions d’années plus tard.[Traduit par la redaction]
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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 | 0.000 | 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 | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.003 | 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 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".