A fresh look at the Sm-Nd record of Earth’s oldest rocks: the Acasta Gneiss Complex (Northwest Canada)
Bibliographic record
Abstract
The earliest differentiation of the silicate Earth remains poorly constrained due to the scarcity and imperfect preservation of Earth's oldest rocks.These limitations can be circumvented, however, using the short-lived 146 Sm-142 Nd system, which provides selective chronological information on Hadean crustmantle differentiation processes [1] .The oldest rock record on Earth is represented by a chemically diverse suite from the 3.6-4.0Ga Acasta Gneiss Complex (AGC) in northwest Canada [2,3] .Previous 146 Sm-142 Nd studies of the AGC revealed the presence of negative 142 Nd anomalies, inherited from a so-far elusive Hadean crustal component [4,5,6] .However, the majority of AGC samples define a 147 Sm-143 Nd regression age of 3.37 ± 0.14 Ga [7] , suggesting late disturbance of their Sm-Nd systematics.This issue limits the potential of combined 146,147 143 Nd investigations to reveal the timing of Hadean crust-mantle differentiation.We will present coupled 146,147 Sm- 142,143 Nd systematics of layered gneisses from the AGC in combination with zircon U-Pb age constraints.The results for the short-lived 146 Sm-142 Nd system show µ 142 Nd values ranging from +2.7 to -8.5 ppm (±2.5 ppm, 2 S.D.) and document the existence of both mantle and Hadean crustal components within the sources of the AGC.The data for the long-lived 147 Sm-143 Nd system deviate from the ~3.4 Ga regional array, defining an apparent age of 4.01 Ga [8] , which may be geologically meaningless.Although the actual sample ages still need to be further constrained by forthcoming zircon data, our results suggest that the studied samples preserve a record of primary 142,143 Nd heterogeneities in the AGC and shed some light on the petrogenetic history of Earth's oldest rocks.
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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 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.002 | 0.003 |
| Science and technology studies | 0.002 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.000 | 0.001 |
| Insufficient payload (model declined to judge) | 0.004 | 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".