Source and evolution of 4.0 – 3.9 Ga crust revealed by U-Pb, Lu-Hf and trace element analyses of xenocrystic zircon, Superior Province
Bibliographic record
Abstract
In the Superior Province, Eoarchean magmatism sourced from partially melted Hadean mafic crust has been inferred from published ca. 3.9 Ga detrital zircon Hf isotope compositions and whole-rock 142 Nd isotopes. We report simultaneously acquired U-Pb/Lu-Hf and U-Pb/trace element analyses of 4.0 – 3.9 Ga xenocrystic zircon grains from a polyphase gneiss that provide direct evidence for Eoarchean partial melting of Hadean crust in the Hudson Bay terrane, Superior Province. Zircon domains record age groups at ca. 3.99 – 3.94 Ga, 3.92 – 3.84 Ga, 3.59 Ga, 3.48 Ga, 3.32 Ga, 3.27 Ga, 3.1 – 2.95 Ga, 2.84 Ga and 2.73 Ga. The ca. 3.99 – 3.84 Ga core domains have subchondritic Hf isotope signatures yielding an estimated mantle extraction age between 4.23 Ga and 4.18 Ga. The ca. 3.92 – 3.84 Ga zircon core and mantle domains are faintly or homogeneously zoned (ghost zonation), they inherited the Hf isotope compositions of the older ca. 3.99 – 3.94 Ga protolithic grains, and have trace element signatures consistent with expulsion of LREE over HREE and Th over U, which altogether reflect solid-state recrystallization of the older protolith zircon population. The mechanism for solid-state recrystallization requires a high-grade metamorphic event. Therefore, the ca. 3.92 – 3.84 Ga zircon population appears to record the earliest known metamorphic event on Earth, which was previously identified and inferred from detrital zircon in the Jack Hills meta-conglomerate.
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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 | 0.000 |
| 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) | 0.001 | 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".