Lead isotope systematics of sulfide minerals in the Middle Valley hydrothermal system, northern Juan de Fuca Ridge
Bibliographic record
Abstract
The sources of metals for the sediment‐hosted massive sulfide deposits currently forming at the sediment‐filled Middle Valley segment of the northern Juan de Fuca Ridge have been investigated using Pb isotopes. Leg 139 of the Ocean Drilling Program drilled three sites in Middle Valley at which basaltic rocks, sediments, and sulfide mineralization were recovered. At site 856, massive sulfides composed of pyrite, pyrrhotite, minor chalcopyrite, and sphalerite have isotopic compositions intermediate between Juan de Fuca basalts and Middle Valley turbiditic sediments, with the exception of two samples that are relatively nonradiogenic. Primary pyrrhotite‐dominated and secondary pyrite‐dominated sulfides overlap in isotopic composition, indicating that both high‐ and low‐temperature hydrothermal fluids have interacted with the sediment pile. Drilling at sites 857 and 858 intersected hydrothermally altered basaltic sill‐sediment complexes containing disseminated aggregates and veins of secondary hydrothermal sulfides. The sulfide minerals have highly variable, continental crust‐like Pb ratios with Stacey‐Kramers model ages ranging from zero to 1.5 Ga ( 206 Pb/ 204 Pb ∼ 16.2). Some massive sulfide samples from site 856 also include an “old” sedimentary Pb component. The source of this old Pb is most likely Proterozoic detrital sulfides from turbidites within Middle Valley, even though the dominant source of detritus is thought to be the Mesozoic accreted terranes of Vancouver Island. The relative abundances of Cu and Zn in Middle Valley massive sulfides do not correlate with Pb isotopic composition, probably due to similar Cu/Zn in basalt and sediment components.
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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 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.001 | 0.000 |
| Bibliometrics | 0.000 | 0.001 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 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 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".