Using oxygen (δ18O, δ17O) and hydrogen (δ2H) isotopes in the Northern Limb of the Bushveld Igneous Complex to assess the extent of footwall contamination
Bibliographic record
Abstract
The Platreef in the Northern Limb is in the same stratigraphic position as the Critical Zone (i.e. Merensky Reef) in the Eastern and Western limbs of the Bushveld Igneous Complex (BIC). However, in some areas, the Platreef is in direct contact with Archean granitic gneisses and Transvaal Supergroup metasedimentary rocks. Although there is significant evidence of assimilation of Transvaal Supergroup metasedimentary rocks in the southern parts of the Platreef, there is no consensus on the extent of contamination and if contamination is proximal or distal to the Pt, Pd, Au mineralized zone/Merensky Reef equivalent. For this study, we report δ’ 18 O, Δ’ 17 O, and δ 2 H values of samples from the downdip portion of the Flatreef (part of the Platreef) on Turfspruit. The results show that δ’ 18 O values range from ∼4 to 7 ‰ in intervals with little to no evidence of assimilation and can reach up to 17 ‰ in intervals with evidence of pervasive assimilation. The δ’ 18 O values in areas with no evidence of assimilation are similar to previously reported values in the Eastern and Western limbs of the BIC. The Δ’ 17 O values range from −0.200 to 0.050 ‰ throughout the stratigraphy. Most of the positive Δ’ 17 O values are associated with intervals showing evidence of assimilation. The δ 2 H values (which range from −92 to −30 ‰) show no correlation between intervals with pervasive assimilation and those with little to no contamination. Samples from the Turfspruit area of the Northern Limb have δ’ 18 O values of -5.53 to 17.14 ‰ which shows the extent of footwall assimilation of the Transvaal metasedimentary rocks. The triple oxygen isotope values show that the Northern Limb of the BIC records two separate episodes of crustal contamination: (1) a proximal contamination by local footwall lithologies; and (2) a deeper, more pervasive (with respect to the entire BIC) contamination, prior to injection of the BIC magmas into the middle/upper crust.
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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) | 0.000 | 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".