The Effect of Seawater <scp>Sr</scp> Concentration on the Hydrothermal Alteration of Oceanic Crust: <scp>Sr</scp> Isotopes in Dikes of the Early Paleozoic Bay of Islands Ophiolite
Bibliographic record
Abstract
Oceanic crust is composed of basaltic rocks that have been hydrothermally altered at mid-ocean ridges by circulating heated seawater. Alteration changes the 87 Sr/ 86 Sr of oceanic crust, especially in sheeted dike sections, because seawater has high 87 Sr/ 86 Sr and sufficient dissolved Sr to affect the rocks. The inference that paleoseawater Sr concentrations have been higher than in the modern ocean suggests that ancient examples of altered oceanic crust should record evidence of elevated seawater Sr concentrations, a possibility supported by comparison of Cretaceous ophiolites and Neogene oceanic crust. To further evaluate this relationship, we measured Sr isotopes in sheeted dikes and other oceanic crustal samples of the 488 Ma Bay of Islands (BOI) ophiolite, which formed when seawater may have had the highest Sr concentration of any time during the Phanerozoic, 7 times higher than modern oceans and ∼2.5 times higher than mid-Cretaceous oceans. Our results are consistent with the inferred high Ordovician seawater Sr concentrations. The Sr isotope effects in the BOI ophiolite are not much larger than those in the Cretaceous ophiolites, but the BOI rock Sr concentrations are unusually high, suggesting that a high seawater Sr concentration is needed to account for the Sr isotope effects.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.001 |
| Meta-epidemiology (narrow) | 0.001 | 0.000 |
| Meta-epidemiology (broad) | 0.002 | 0.001 |
| Bibliometrics | 0.001 | 0.002 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.000 | 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 teacher head, 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".