Constraints on the emplacement age of Yakutian province kimberlites from U-Pb perovskite dating
Bibliographic record
Abstract
Numerous radiometric ages for kimberlites from the Yakutian province in eastern Siberia have been reported (e.g.Brakhfogel ' and Kovol'skiy, 1979;Davis et al., 1980) but many of these ages are older than that permitted by the geological relationships of the pipes.Despite the inherent limitations in some of these determinations, at least five periods of kimberlite emplacement have been proposed ranging from Late Ordovician to Late Jurassic (Davis et ah, 1980;Sobolev, 1985).In order to better understand the tectonic significance of the Yakutian kimberlites, precise U-Pb perovskite dating is in progress on kimberlite samples from the Malo-Batuobinsk and Daldyn-Alakit fields in the south and from some of the diamond-poor kimberlite fields (Kuika, Lower Olenek) to the north.In many ways, perovskite is an ideal choice for determining the timing and origin of kimberlites because it is generally one of the few mineral constituents that contains appreciable amounts of U, Sr and Nd and can be shown to be a primary magmatic phase.The first phase of this project will focus on the temporal relationship between Yakutian kimberlite fields while phase two will involve a comparison of the nature of the subcontinental mantle source regions between diamond-rich and diamond-poor pipes from Sr-Nd isotopic studies of kimberlitic perovskite.Preliminary U-Pb perovskite results for small (<0.5 kg) samples from two pipes located within the Kuika kimberlite field indicate that the diamond-poor kimberlites in this northern field are Jurassic in age.Perovskite from the Velikan II pipe are small, rounded, dark orange anhedral grains that are rimmed with a thin white coating (leucoxene?).The results for one analysis of 190 grains indicate moderate U (114 ppm) and Th (659 ppm) abundances and a 206Pb/238U age of 159.1+2.4Ma (2a).This 159.1 Ma age is considered a good estimate for the emplacement age of the Velikan II kimberlite.Abundant tiny tan perovskite fragments were recovered from a sample of the Monticellitovaya kimberlite and one analysis consisting of 469 grains yielded a slightly older 206Pb/238U age of 169.6±3.0Ma(2a) and slightly lower U (73 ppm) and Th (252 ppm) abundances.These Jurassic emplacement ages for kimberlites from the Kuika field overlap with U-Pb mantle zircon ages (159-146 Ma) reported for several kimberlites from fields further south (Davis et al., 1980) indicating that this period of Jurassic kimberlite magmatism occurs in a number of the Yakutian fields.It remains to be established whether all Yakutian Jurassic kimberlites, like Velikan and Monticellitovaya, are diamond-poor and hence poor exploration targets.Diamond-poor Jurassic kimberlite magmatism is not unique to the Yukutian fields as temporally equivalent, diamond-poor kimberlitic magmatism (159-152 Ma) is preserved in the Kirkland Lake field, North America (Heaman, 1989;Heaman and Kjarsgaard, 1995).The existence of temporally distinct diamond-rich and diamond-poor kimberlites in the same geographic location may
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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 | 0.001 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.002 | 0.001 |
| Science and technology studies | 0.001 | 0.000 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.000 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 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".