40Ar/39Ar age of the Jurassic North Mountain Basalt, southwestern Nova Scotia
Bibliographic record
Abstract
Two whole-rock samples of the Jurassic North Mountain Basalt have been dated using the 40Ar/39Ar method. A single sample of fresh, medium-grained, holocrystalline basalt from the lower flow unit of this thick (i.e., 400 m) sequence gave coincident plateau and isochron correlation ages of 201 ± 2.5 Ma, in agreement with a zircon U-Pb age of 202 ± 1 Ma for this same flow unit. This 40Ar/39Ar age contrasts with earlier conventional K-Ar whole-rock ages of ca. 192 Ma for the North Mountain Basalt, which are similar to other K-Ar and Ar/Ar ages for correlative basalts of eastern North America. The second dated sample is a zeolite-bearing basalt from the middle flow unit of the North Mountain Basalt. This sample gave a discordant age spectrum with excess argon, but the isochron correlation age of 206 Ma, albeit with a large error (i.e., 56 Ma), is similar to that for the fresh sample. The data indicate that reliable whole-rock 40Ar/39Ar ages for the basalts are attainable, but that fresh samples must be used. In light of this, it is suggested that the younger 192 Ma ages may reflect a widespread thermal event related to zeolite formation. RÉSUMÉ Deux échantillons de roche totale de basalte jurassique du mont North ont été datés au moyen de la méthode 40Ar/39Ar. Un échantillon unitaire de basalte holocristallin à grain moyen frais de l'unité d'écoulement inférieure de cette séquence épaisse (c.-à-d. 400 m) a fourni des âges concordants par corrélation tabulaire et isochrone de 201 ± 2,5 Ma, ce qui correspond à la datation au U-Pb à partir de zircon situant la même unité d'écoulement à 202 ± 1 Ma. Cette datation 40Ar/39Ar fait contraste avec les datations de la roche totale conventionnelles antérieures au K-Ar situant à environ 192 Ma le basalte du mont North, soit des âges semblables à d'autres datations au K-Ar et Ar/Ar de basaltes corrélatifs de l'Est de l'Amérique du Nord. Le deuxième échantillon daté est un basalte renfermant de la zéolite et provenant du milieu de l'unité d'écoulement du basalte du mont North. Cet échantillon a fourni un spectre d'âges discordant présentant un excès d'argon, mais la datation par corrélation isochrone le situant à 206 Ma, en dépit d'une erreur importante (c.-à-d. 56 Ma), fournit un âge semblable à celui de l'échantillon frais. Les données révèlent qu'il est possible d'obtenir des dates de roche totale fiables au moyen de la méthode 40Ar/39Ar, mais qu'il faut utiliser des échantillons frais. Compte tenu de ce fait, on suppose que les âges plus récents de 192 Ma pourraient correspondre à un phénomène thermique répandu apparenté à la formation de zéolite.
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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 | 0.001 | 0.001 |
| Science and technology studies | 0.001 | 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) | 0.001 | 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".