Enriched Hadean reservoir recorded by Archean rocks from NE Brazil: Insights from <sup>142</sup>Nd isotopes
Bibliographic record
Abstract
A complex crustal history has been recorded by the northeastern Brazil Precambrian crust, from the northern end of the So Francisco Archean Craton to the northern domain of the Neoproterozoic Borborema Province. Archean rocks from this region are mostly spanning from 3.5 Ga to 2.5 Ga, but the occurrence of a 4.1 Ga detrital zircon suggests the incorporation of Hadean material in the So Francisco Craton. Despite the lack of Hadean rocks, the short-lived 146 Sm-142 Nd isotopic system can provide further evidence of potential Hadean silicate differentiation events because variations in 142 Nd isotopic compositions can only be produced by Sm-Nd fractionation within the first ~500 Ma of Earth's history, given the short halflife (103 Ma) of 146 Sm. Here we present 142 Nd data for Paleo-to Mesoarchean rocks to constrain the timing and extent of the potential involvement of Hadean material in the formation of the NE Brazilian Archean crust. Paleoarchean amphibolites from the So Jos do Campestre massif show lower 142 Nd/ 144 Nd ratios compared to the Nd standard, with an average 142 Nd value of -10.0 4.8 (n = 17, 2SD). The 142 Nd isotopic composition theses amphibolites, together with their geochemistry and unusually high concentrations in incompatible trace elements, provide evidence for a Hadean enriched mantle reservoir. Additional six mafic samples from small Paleoarchean basement inliers within the Serid Neoproterozoic belt, as well as samples from the So Tom differentiated mafic-ultramafic intrusion (-9.0 6.0, n= 13, 2SD) are also consistent with a similar early enriched source, with 142 Nd values as low as -14.6 4.2. Although four felsic samples from the So Francisco Craton exhibit 142 Nd values closer to zero, additional measurements in felsic and metasedimentary rocks have values as low as 142 Nd = -10.5 4.0 and are consistent with large-scale reworking of this mafic crust sourced from an enriched Hadean mantle. The wellresolved negative 142 Nd values measured in rocks of distinct nature and covering a regional extent Precambrian crust provide strong evidence for the widespread involvement of an early enriched source in the formation and evolution of the of the NE Brazilian Archean crust.
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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 | 0.001 |
| Science and technology studies | 0.001 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.001 |
| 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
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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".