Thallium isotope ratios record a transition to melting of hydrated oceanic crust at the Hadean-Archean boundary
Bibliographic record
Abstract
Earth's oldest continental crust is found in cratonic nuclei comprised of tonalite-trondhjemite-granodiorites (TTGs). However, the nature of the source materials that formed TTGs, as well as the timing and mechanisms of its formation remain controversial. The two endmember tectonic models for the formation of Archean TTGs are (i) vertical subduction and partial melting of the subducting hydrated oceanic crust, (ii) partial melting at the base of plume-generated oceanic plateaux [1, 2]. The thallium stable isotope composition of magmatic rocks can identify distinct components in modern subduction zone settings ( 205 Tl of primitive mantle = -2, altered oceanic crust = -2 to -16; [3]) and thus can be exploited to address early continental crust formation. Here we report Tl isotope compositions for rocks from the Acasta Gneiss Complex (AGC) in Canada, which contains the oldest known evolved crust, with igneous crystallization ages ranging from 4.03 to 2.95 Gyr [4]. Orthogneisses of the AGC have 205 Tl values in the range of -3.1 0.2 to -1.1 0.4, where lower values are consistent with the mixing of mantle melts and hydrothermally altered, hydrated oceanic crust.
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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) | 0.002 | 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".