Exploring the advantages and limitations of in situ U–Pb carbonate geochronology using speleothems
Bibliographic record
Abstract
Abstract. The recent development of methods for in situ U–Pb age determination in carbonates has found widespread application, but the benefits and limitations of the method over bulk analysis (isotope dilution – ID) approaches have yet to be fully explored. Here we use speleothems – cave carbonates such as stalagmites and flowstones – to investigate the utility of in situ dating methodologies for “challenging” matrices with typically low U and Pb contents and predominantly late Cenozoic ages. Using samples for which ID data have already been published, we show that accurate ages can be obtained for many speleothem types by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). Consideration of our own and literature data suggest that most carbonates with >1 ppm uranium and a few hundred parts per billion of Pb should be good targets for in situ methodologies, regardless of age. In situ analysis often provides a larger spread in U∕Pb ratios, which can be advantageous for isochron construction, but isochron ages rarely achieve the ultimate precision of ID analyses conducted on the same samples simply because signal sizes are dramatically reduced. LA analysis is faster than ID and thus will play a significant role in reconnaissance studies. The major advantage of the in situ methodology appears to be the potential for successful dating outcomes in sample types requiring high spatial-resolution analysis or those with a high common-Pb component where LA approaches may facilitate identification of the most radiogenic regions for analysis.
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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 | 0.002 |
| 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 | 0.001 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.001 | 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".