Bibliographic record
Abstract
As the asthenospheric mantle rises at oceanic spreading centres, it undergoes partial melting, producing oceanic crust and depleted mantle, both of which have lower intrinsic density than the asthenospheric mantle from which they were derived. With a warmer asthenosphere in the Archean, these effects are enhanced, leading to the possibility that subduction was no longer feasible. I investigate the density of the oceanic crust and underlying mantle for a mantle with temperatures 200 °C higher than today, using models of the chemistry of melting and the mineralogy of the ensuing rocks. For the melting model used, crustal thicknesses are 21 km and the depth to which the mantle is partially melted is 114 km, compared with 7 and 54 km for a comparable model of modern Earth. Two thermal-evolution models for Archean oceanic lithosphere are examined. One assumes twice the heat flow into the base of the plates, which severely restricts the depths to which the plates can cool with age. A second assumes the plates can cool to the depth to which the asthenosphere undergoes partial melting, resulting in heat flow into the base of the plates only 1.3 times as large as today. With the first model, oceanic plates do not become denser than an equivalent column of asthenosphere. With the second, they do after ∼50 Ma of cooling. In both cases, however, the cooling is sufficient to provide a significant driving force for the initiation of subduction because the sole requirement for a subduction-initiation drive is that the cooled lithosphere be denser than the column of differentiated asthenosphere that would replace it. This, combined with the low flexural rigidity of Archean plates, makes the initiation of subduction probably slightly easier than it is today. The relatively low density of oceanic plates results in lower slab pull, but this effect is counterbalanced both by the likelihood that some of the low-density crust may have been delaminated, and by the effect of passage of the thicker crust through the eclogite transition. Given our present knowledge of Archean thermal conditions, there does not appear to be a compelling theoretical argument against efficient subduction processes at that time.
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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 | 0.001 | 0.002 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.001 |
| Bibliometrics | 0.000 | 0.001 |
| Science and technology studies | 0.001 | 0.003 |
| Scholarly communication | 0.003 | 0.004 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.005 | 0.001 |
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".