Textural Character of Gabbroic Rocks from Pito Deep: a Record of Magmatic Processes and the Genesis of the Upper Plutonic Crust at Fast-Spreading Mid-Ocean Ridges
Bibliographic record
Abstract
Abstract The tectonic window at Pito Deep, in the southern Pacific Ocean, permits study of the formative processes of uppermost East Pacific Rise (EPR) gabbroic ocean crust. Here we present a detailed microstructural and crystallographic study of 17 gabbroic samples from the uppermost ∼800 m of plutonic crust exposed in the Pito Deep Rift. We integrate two- and three-dimensional measurements of crystal size, shape, spatial distribution and orientation, with petrographic observations and geochemical data to constrain the formation of fast spread gabbroic ocean crust. The shallowest samples, collected < 55 metres below the sheeted dikes (mbsd), have evolved bulk-rock compositions, elongate plagioclase crystals, a clear plagioclase shape- and crystallographic-preferred orientation, and preserve only minor amounts of intracrystalline strain. The characteristics of these rocks and their proximity to the sheeted dike complex, suggests they formed by crystallization at the lateral tip of an axial melt lens that solidified as it moved off axis. Underlying samples from 96–724 mbsd, record more primitive bulk-rock compositions, less elongate plagioclase crystals and exhibit increasing strength of both plagioclase shape- and crystallographic-preferred orientation with depth below the sheeted dikes. These samples host plagioclase crystals that show increasing intracrystalline strain with depth, suggesting magmatic to hypersolidus submagmatic flow within the mush zone beneath the axial melt lens. These observations, together with inclined-to-steeply dipping mineral layering preserved below ∼180 mbsd, are interpreted to record the downward transport of crystal-rich magma originating at the bottom of the melt lens through the uppermost kilometre of the mush zone at the EPR. The location of initial crystallization along the floor of the axial melt lens determines the magmatic processes that affect the crystal-rich magma en route to solidification as lower ocean crust.
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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 | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.001 | 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) | 0.003 | 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".