Large Oxygen and hafnium isotopic variations in zircon from the Saglek Block (North Atlantic Craton) document reworking of mature supracrustal rocks as early as 3.5 Ga
Bibliographic record
Abstract
The most ancient rocks in the geological record provide insights into the processes that shaped the evolution and composition of the first continental masses. Due to both the scarcity and the polymetamorphic history of exposed Eoarchean (>3.5 Ga) crust, the study of early geodynamic processes is very challenging and most of our knowledge has been learned from only a few localities on Earth. The present study focuses on felsic meta-igneous rock from the Saglek Block (North Atlantic Craton), a locality where recent zircon U-Pb dating studies indicate earliest crust formation in the Eoarchean (Komiya et al., 2017; Sałacińska et al., 2018; Vezinet et al., 2018). We performed in situ oxygen isotopes measurement (SIMS analyses) in zircon grains that have been carefully selected from CL-imaging for the good preservation of their internal structure and for their pristine composition in rare Earth element. We then performed U-Pb/Hf isotopes by laser ablation split stream (LASS)-ICP-MS. The results indicate 3 distinct crystallization events: (1) an Eoarchean event at ca 3.86 Ga; (2) an early Paleoarchean metamorphic event at ca. 3.5 Ga, and (3) a Neoarchean event (ca. 2.7-2.8 Ga) with zircon domains showing complex zoned overgrowths. While the 3.86 Ga magmatic domains display mantle-like δ18O(+4.9±0.2‰ to +6.8.0±0.2‰, n=30), large O-isotope fractionation (δ18Ovalues up to +9‰) characterise the Paleoarchean metamorphic event. Such elevated δ18O signatures provide unequivocal evidence for hydrosphere–crust interactions and reworking processes resulting in metamorphic zircon growth at ca. 3.5 Ga, namely 1 Ga before the Archean-Proterozoic transition (Vezinet et al., 2019). Interestingly, the two oldest age groups have chondritic to sub-chondritic εHfi values: +1.0 ± 2.2 to –5.5 ± 1.8 whereas large variations in Hf isotope composition (εHfi value from –11.2 ± 2.5 to –20.3 ± 1.5) are found in the 2.8–2.7 Ga zircon domains. Such intra-sample heterogeneities implies a significant perturbation of Hf-isotope composition during metamorphic events related to mixing of fluid with inherited (older) Hf isotope source. In the light of these results, we will discuss the potential consequences of isotope perturbation on whole-rock isochrones interpretation. Komiya, T., et al. "A prolonged granitoid formation in Saglek Block, Labrador: Zonal growth and crustal reworking of continental crust in the Eoarchean." Geoscience Frontiers 8.2 (2017): 355-385. Sałacińska, A., et al. "Complexity of the early Archean Uivak Gneiss: Insights from Tigigakyuk Inlet, Saglek Block, Labrador, Canada and possible correlations with south West Greenland." Precambrian Res. 315 (2018): 103-119. Vezinet, A, et al. "Hydrothermally-altered mafic crust as source for early Earth TTG: Pb/Hf/O isotope and trace element evidence in zircon from TTG of the Eoarchean Saglek Block, N. Labrador. EPSL 503 (2018): 95-107. Vezinet, A., et al. "Extreme δ18O signatures in zircon from the Saglek Block (North Atlantic Craton) document reworking of mature supracrustal rocks as early as 3.5 Ga." Geology 47.7 (2019): 605-608.
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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".