Microstructures in Hole 1274A peridotites, ODP Leg 209, Mid‐Atlantic Ridge: Tracking the fate of melts percolating in peridotite as the lithosphere is intercepted
Bibliographic record
Abstract
We report the microstructures of harzburgites and dunites from ODP Leg 209, Hole 1274A, 15°39′N on the Mid‐Atlantic Ridge. A set of features in these peridotites is so unaffected by plastic flow that it must have formed very late by magmatic processes. We believe that the microstructures record the interaction between a peridotite and a percolating melt as the thermal boundary layer was progressively intercepted. The following chronology for the microstructures is derived: Group 1, resorption of orthopyroxene (opx) associated with olivine precipitation, enhanced by a migrating melt; Group 2, conversion of opx to clinopyroxene (cpx) by a percolating melt; Group 3, precipitation of cpx and spinel as intricate intergrowth (symplectite), associated with olivine dissolution, from an interstitial melt. For these reactions the ratio of melt generated to melt consumed is progressively decreasing, as expected during progressive freezing of asthenosphere. Pristine reaction microstructures analogous to Site 1274A are also found in the Lanzo Massif (Italy) and the Little Port Complex (Canada). For all three settings we infer (1) slow spreading conditions and (2) an ancient depletion event. We thus infer that preservation of the described microstructures is favored by slow spreading and a previous depletion. Slow spreading may be required since it avoids the tectonic overprint during ductile corner flow generally inferred for fast spreading ridges. Slow spreading does not tend, however, to generate highly depleted residues because a thick lithospheric lid limits the extent of melting. Residues will thus tend to be lherzolitic, not harzburgitic. However, lherzolites have a lower permeability for mafic melts than harzburgites. In order to generate the percolation‐dominated microstructures of Site 1274A, a harzburgitic host rock appears favorable. This may now explain the need for an ancient depletion event since only then can harzburgitic residues occur at the top of the mantle column during slow spreading conditions.
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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 | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 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 | 0.000 | 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 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".