U–Pb geochronology and clumped isotope thermometry study of Neoproterozoic dolomites from China
Bibliographic record
Abstract
Abstract The higher abundance of dolomite in the Neoproterozoic compared with that of the Phanerozoic has been linked to elevated marine Mg/Ca ratios, and a hypothesized ‘dolomite sea’ that promoted widespread syn‐sedimentary fibrous dolomite cement precipitation. This article provides new uranium–lead (U–Pb) chronology, clumped isotope thermometry (TΔ47), combined with δ13C and δ18O, and trace element geochemistry measurements for different types of dolomite, including bulk dolomite matrix (BD), fibrous dolomite cements (DC1) and late sparry dolomite cements (DC2 and DC3), in three Neoproterozoic dolostone profiles from China. The Ediacaran U–Pb ages coupled with nearly identical, low TΔ47 temperatures (average at ca 55‐60°C) for BD and DC1 dolomites, imply that these dolomites were either syn‐depositional or early diagenetic. Some DC1 dolomites yield slightly younger (ca 20 to 50 Ma) U–Pb ages than the BD dolomites, suggesting that they were diagenetic products formed after matrix dolomitization. The estimates of δ18Owater values for dolomitizing fluids suggest that both DC1 and BD dolomites were precipitated from highly evaporated seawater. In contrast, DC2 and DC3 dolomites filling in vugs and fractures have much younger U–Pb ages (mostly <525 Ma), higher TΔ47 temperatures (>95°C), and lower Sr and U contents, which is consistent with the paragenetic sequence that they were deeper burial diagenetic products. A synthesized diagenetic model with evaporative dolomitizing water evolved from anoxic, ferruginous to sub‐oxic and/or oxic, non‐ferruginous conditions is responsible for the decrease of Fe and Mn concentrations in global DC1 and BD dolomites throughout Neoproterozoic time. This study suggests that the widely distributed Neoproterozoic fibrous dolomite cements may be a later diagenetic feature, and thus calls into question their use in palaeoclimate reconstruction and as a principal line of evidence for the Neoproterozoic ‘dolomite sea’ hypothesis.
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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 | 0.003 | 0.002 |
| 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".