Combined<sup>147,146</sup>Sm‐<sup>143,142</sup>Nd constraints on the longevity and residence time of early terrestrial crust
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Bibliographic record
Abstract
Abstract Primordial silicate differentiation controlled the composition of Earth's oldest crust. Inherited 142 Nd anomalies in Archean rocks are vestiges of the mantle‐crust differentiation before ca. 4300 Ma. Here we report new whole‐rock 147,146 Sm‐ 143,142 Nd data for the Acasta Gneiss Complex (AGC; Northwest Territories, Canada). Our 147 Sm‐ 143 Nd data combined with literature data define an age of 3371 ± 141 Ma (2 SD) and yield an initial ε 143 Nd of −5.6 ± 2.1. These results are at odds with the Acasta zircon U‐Pb record, which comprises emplacement ages of 3920–3960 Ma. Ten of our thirteen samples show 142 Nd deficits of −9.6 ± 4.8 ppm (2 SD) relative to the modern Earth. The discrepancy between 142 Nd anomalies and a mid‐Archean 147 Sm‐ 143 Nd age can be reconciled with Nd isotope reequilibration of the AGC during metamorphic perturbations at ca. 3400 Ma. A model age of ca. 4310 Ma is derived for the early enrichment of the Acasta source. Two compositional end‐members can be identified: a felsic component with 142 Nd/ 144 Nd identical to the modern Earth and a mafic component with 142 Nd/ 144 Nd as low as −14.1 ppm. The ca. 4310 Ma AGC source is ∼200 Myr younger than those estimated for Nuvvuagittuq (northern Québec) and Isua (Itsaq Gneiss Complex, West Greenland). The AGC does not have the same decoupled Nd‐Hf isotope systematics as these other two terranes, which have been attributed to the crystallization of an early magma ocean. The Acasta signature rather is ascribed to the formation of Hadean crust that was preserved for several hundred Myr. Its longevity can be linked to 142 Nd evolution in the mantle and does not require slow mantle stirring times nor modification of its convective mode.
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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 | 0.001 | 0.001 |
| Meta-epidemiology (narrow) | 0.001 | 0.000 |
| Meta-epidemiology (broad) | 0.001 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.001 |
| Insufficient payload (model declined to judge) | 0.004 | 0.001 |
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 it