Hydrothermal alteration and microbial sulfate reduction in peridotite and gabbro exposed by detachment faulting at the Mid‐Atlantic Ridge, 15°20′N (ODP Leg 209): A sulfur and oxygen isotope study
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Bibliographic record
Abstract
Whole rock sulfur and oxygen isotope compositions of altered peridotites and gabbros from near the 15°20′N Fracture Zone on the Mid‐Atlantic Ridge were analyzed to investigate hydrothermal alteration processes and test for a subsurface biosphere in oceanic basement. Three processes are identified. (1) High‐temperature hydrothermal alteration (∼250–350°C) at Sites 1268 and 1271 is characterized by 18 O depletion (2.6–4.4‰), elevated sulfide‐S, and high δ 34 S (up to ∼2 wt% and 4.4–10.8‰). Fluids were derived from high‐temperature (>350°C) reaction of seawater with gabbro at depth. These cores contain gabbroic rocks, suggesting that associated heat may influence serpentinization. (2) Low‐temperature (<150°C) serpentinization at Sites 1272 and 1274 is characterized by elevated δ 18 O (up to 8.1‰), high sulfide‐S (up to ∼3000 ppm), and negative δ 34 S (to −32.1‰) that reflect microbial reduction of seawater sulfate. These holes penetrate faults at depth, suggesting links between faulting and temperatures of serpentinization. (3) Late low‐temperature oxidation of sulfide minerals caused loss of sulfur from rocks close to the seafloor. Sulfate at all sites contains a component of oxidized sulfide minerals. Low δ 34 S of sulfate may result from kinetic isotope fractionation during oxidation or may indicate readily oxidized low‐ δ 34 S sulfide derived from microbial sulfate reduction. Results show that peridotite alteration may be commonly affected by fluids ± heat derived from mafic intrusions and that microbial sulfate reduction is widespread in mantle exposed at the seafloor.
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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) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 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) | 0.000 | 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 it