Coda of the snowball: combined U-Pb LA-ICPMS dating of calcite-after-aragonite crystal fans and clumped isotope thermometry of Ediacaran cap carbonates
Bibliographic record
Abstract
Direct dating of sedimentary successions is a main challenge in geochronology, key for the establishment of chronostratigraphic frameworks for both regional and global events. U-Pb in-situ LA-ICPMS direct dating of carbonate samples is emerging as a promising tool, but complications such as mobility and low U contents hinder most of the attempts on common carbonate rocks. We present new U-Pb in-situ LA-ICPMS data for Ediacaran cap carbonate and related successions from Brazil, China and Canada, along with stable carbon, oxygen, and clumped isotope data for the same samples. The novel dataset reveals that in some instances, especially within calcite-after-aragonite crystal fans and microbialite facies, U is retained from early diagenesis through intermediate to deep burial, resulting in tightly constrained and well-spread linear fits in the Concordia space. Calcite-after-aragonite crystal fan samples from the Guia Fm. (Brazil) and Hayhook Fm. (Canada) caps, sitting immediately above glacial diamictite, yielded 632 ± 14 Ma and 631 ± 6 Ma, respectively, supporting quick deposition and diagenesis following Marinoan deglaciation. Clumped isotope apparent equilibrium temperatures ( T Δ47 ) of 79 (+12/−11) °C and 181 (+14/−13) °C (95 % confidence level), respectively, indicate that the U-Pb system remained unreset within the crystal fans even through the deep burial realm. In the Sete Lagoas Formation of the Bambuí Group (Brazil), crystal fans are not restricted to the immediate cap carbonate sitting above glacial deposits, but instead occur throughout ca. 400 m of carbonate-dominated facies, in distinct stratigraphic intervals corresponding to the Pedro Leopoldo and Lagoa Santa members. Samples from the basal Pedro Leopoldo member yielded U-Pb ages between 625 Ma and 605 Ma. A crystal-fan bearing sample of the Acauã Formation in the Sergipano Belt (Brazil) yielded similar results, suggesting protracted deposition/diagenesis of the negative δ 13 C-bearing limestone above the basal cap dolostone. Crystal fans in the topmost Lagoa Santa member, just below the contact with the mudstone-rich Serra de Santa Helena Formation and 330 m above the contact with the glacials, yielded late Ediacaran ages at ca. 570‒550 Ma. All of these yielded T Δ47 of around 110‒149 °C. These ages are identical within uncertainty to U-Pb ages obtained in stromatolites at the same stratigraphic level, and from the phosphorite-bearing stromatolites of the Salitre Formation, Una Group, further north in the São Francisco craton, which yielded a lower T Δ47 of 91 ± 7 °C. Finally, both the cap dolostone matrix and isopachous cement filling sheet-cavities from a sample of the basal Doushantuo Formation of South China align in a regression with a lower intercept at ca. 619 Ma. The new U-Pb carbonate data are highly coherent with available U-Pb zircon and Re-Os whole-rock data worldwide, and also with previous U-Pb LA-ICPMS carbonate dating, suggesting that Ediacaran mixed carbonate-siliciclastic units preserve a common global depositional trend, characterized by condensed (few tens of meters-thick) lower Ediacaran (635‒600 Ma) cap carbonate successions followed by more well-developed and continuous late Ediacaran (575‒540 Ma) fossil-bearing mixed carbonate-siliciclastic successions. This study tests the feasibility of a relatively new method benefiting from samples of globally distributed and correlated units with depositional ages independently established through other methods, demonstrating the potential of direct U-Pb carbonate dates.
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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) | 0.001 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.001 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.001 | 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".