Role of Subducted Basalt in the Genesis Island Arc Magmas: Evidence from Western Aleutian Seafloor Lavas
Bibliographic record
Abstract
Western Aleutian seafloor lavas define a highly calc-alkaline series, with Mg numbers (Mg#, Mg/Mg+Fe) greater than 0.65 in dacitic lavas with 2-4% MgO at 63-70% SiO2. These lavas have uniformly radiogenic Hf and Nd and variable, but relatively unradiogenic, Sr and Pb, at the MORB-like end of the spectrum of island-arc lavas. Andesites and dacites have high Sr >1000 ppm, fractionated trace element patterns (Sr/Y=50-350, La/Yb=8-35, Dy/Yb=2-3.5), and low relative abundances of Nb and Ta (La/Ta=100-300), consistent with an enhanced role for residual or cumulate garnet + rutile. MORB-like isotope compositions and high MgO and Mg# relative to silica, rule out an origin for the andesites and dacites by fractional crystallization from basalt, except perhaps, by a process of melt-rock reaction with peridotite. The most fractionated trace element patterns are in western seafloor rhyodacites (69-70% SiO2), which were dredged from volcanic cones built on Bering Sea oceanic lithosphere, where the crust is probably no more than 10 km thick, and so unlikely to produce garnet during crustal melting. We interpret the western seafloor andesites and dacites to have been produced by melting of subducted MORB-like basalt in the eclogite facies, followed by interaction of the resulting high-silica melt with mantle peridotite. This interpretation is consistent with the tectonic setting in the western Aleutians, which is dominated by oblique convergence, capable of producing a relatively hot subducting plate. Western seafloor lavas define an end-member composition with MORB-like isotope compositions and fractionated trace element ratios, which falls at the end of the continuum of compositions for all Aleutian lavas. The end-member character of western seafloor lavas is clearest in plots highlighting their radiogenic Hf, Nd and strong relative depletions in Ta and Yb. The western seafloor lavas also define an end-member composition for Pb isotopes Some western seafloor samples have high Nd/Hf, as required by Hf-Nd mixing scenarios, which indicate that a source component with radiogenic Hf and Nd and Nd/Hf greater than ~8, is present in lavas throughout the Aleutian arc (Brown et al., 2005 Fall AGU). Abundances of radiogenic Nd and Hf in the eclogite-melt component are relatively high, and so offset the unradiogenic Hf and Nd from subducted sediment. The result is a source mixture with much higher contributions of both subducted basalt and subducted sediment relative to the mantle end-member, than is produced when these elements are modeled as binary mixtures of mantle and sediment.
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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 | 0.001 |
| 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".