The Lu-Hf isotope system in the Acasta gneiss complex (NWT, Canada)
Bibliographic record
Abstract
The Acasta gneiss complex (AGC) is an outcrop of poly-phase Hadean/Eoarchean rocks that records a multistage history. We explore the complexities encountered in such an old crustal remnant at the mineral- to outcrop scale and discuss the preservation and disturbance of the Lu-Hf isotope system in these rocks. Twenty-one new Lu-Hf isotope whole rock measurements are used in combination with previous data to show that some samples have been disturbed by metamorphic garnet growth and/or migmatization/mineral segregation while others clearly preserve their Lu-Hf isotope systematics. Two main Archean magmatic events to be identified at around 3600 and 3960 Ma and an almost purely metamorphic event at about 3750 Ma. Least-contaminated gneisses indicate a Lu-Hf isochron age range of 3929 84 to 3945 91 Ma for the oldest rocks that range from mafic to felsic. This age discrepancy relative to 3960 Ma is likely due to crust assimilation. This process is significant in both magmatic groups in the AGC as also observed by previous authors. However, our study demonstrates that crustal contamination is widespread but not ubiquitous and, according to assimilation calculations, the least contaminated samples identified here indicate a near-CHUR Hf isotope composition for the mantle source of 3960 Ma group. Comparison between Hf isotopes in >3.9 Ga detrital zircons from North-American Archean meta-sedimentary basins and those in Acasta gneisses indicate a possible connection that would in turn imply that the AGC was volumetrically significant. Analogy with western Australian Hadean zircon bearing meta-sedimentary belts would point out to local effects in early crustal record in favor of progressive (but not continuous) crustal growth as independent crustal segments in the Hadean-Eoarchean.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 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 teacher head, 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".