Extent of the São Francisco Craton, South America: a mantle perspective
Bibliographic record
Abstract
Abstract Although cratons owe their longevity to thick, depleted and consequently buoyant sub-continental lithospheric mantle, their extents are commonly indicated by the known distribution of crustal rocks. However, the limits of a mapped cratonic area often do not match the seismically imaged thick sub-continental lithospheric mantle roots. New mantle xenolith petrology and geochemistry, combined with continental scale geophysical data, confirm the interpretation that the São Francisco Craton extends beyond the extent of surface exposures of Archean rocks. Mantle xenoliths from outside the typically recognised São Francisco Craton boundaries from the Limeira-1 and Redondão kimberlite diatremes have refractory olivine [Mg# ≤ 93, with Mg# = 100× molar Mg/(Mg + Fe); Limeira], high-Cr pyrope garnets (Cr 2 O 3 ≤ 7 wt% from Redondão), low bulk Al 2 O 3 < 1 wt%, and 187 Os/ 188 Os i = 0.11202–0.11916 (Limeira) and 0.10964–0.11576 (Redondão) that equate to Mesoproterozoic minimum T RD ages. Geothermobarometry indicates that the lithospheric thickness from which the Redondão xenolith suite was exhumed, extended to > 150 km at the time of eruption, similar to the lithosphere thickness estimated from garnet pyroxenites from the ‘on-craton’ Braúna field. The occurrence of thick, refractory and ancient lithosphere extending outside the known surface extent of the São Francisco Craton suggests that this cratonic nucleus is significantly larger than recognised at the surface. Based on the evidence reported in this study coupled with existing literature, we propose that the current limits of the São Francisco Craton nucleus should be revised to consider deep lithosphere to the north and southwest edges, despite these areas having younger rocks at the surface.
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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 | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.003 | 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".