Major-element, trace-element and sulfur-isotope evidence for arc-like magmatism in the 4.0–2.9 Ga Acasta Gneiss Complex
Bibliographic record
Abstract
Abstract The Acasta Gneiss Complex (AGC) in northwestern Canada comprises Earth’s oldest known evolved crust, with zircon U–Pb ages up to 4.03 Ga. Several pulses of crustal generation and metamorphism are preserved in tonalitic and granitic gneisses spanning over one billion years, along with mafic and ultramafic rocks of unknown age. Major elements, trace elements and radiogenic isotope signatures have been invoked to suggest that these rocks preserve the local onset of horizontal tectonic processes. However, the behavior and influence of volatiles, which have a defining role in modern arc magmatism, remain unconstrained. Here we combine new whole-rock major- and trace-element data with multiple sulfur isotope analyses in 4.0–2.9 Ga Acasta gneisses and spatially associated mafic and ultramafic rocks to investigate the petrogenesis of the AGC. We use a recently-published major element-based melt hygrometer to estimate dissolved water contents for all published plagioclase-saturated Acasta meta-igneous rocks, and find modes at < 0.5 wt.% and 5 wt.% H2O, similar to modern arc magmas. Tholeiitic and calc-alkaline trends are both present, with the former being more prominent in the oldest (ca. 4.0 Ga) samples and in mafic rocks. Zircon trace element oxybarometry reveals a shift towards more oxidized magmatic conditions by 3.75 Ga. Sulfur isotopes record a limited range in δ34S values, suggesting a common igneous end-member at ~ + 1 ‰, and positively correlate with calculated H2O contents, with more positive values (up to + 5‰) appearing in the Paleoarchean (< 3.6 Ga). The Eoarchean (4.0–3.6 Ga) δ34S values are consistent with a precursor Hadean crust having an enriched sulfur isotope signature, possibly resulting from hydrous alteration or from isotopic fractionation during its formation. The temporal progression to more positive δ34S values is consistent with a shift towards more hydrous and oxidized magmatic differentiation. Most samples have near-zero Δ33S that fall along a mass-dependent fractionation (MDF) array, but one 3.5 Ga metasedimentary sample has a negative MIF Δ33S signature of -0.60 ± 0.01 ‰. Additionally, two granitic gneisses dated at 3.3 and 2.9 Ga preserve small positive MIF Δ33S values of + 0.08 ± 0.02 ‰, which could reflect recycling of sedimentary material via subduction by 3.3 Ga. Overall, our data indicate that the Acasta Gneiss Complex preserves several modes of crustal generation evolving over time, with an increasing importance of deep hydrous magmatism by 3.75 Ga and of sedimentary inputs by 3.3 Ga.
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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 | 0.002 | 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".