Proterozoic highly siderophile element enrichment of the Kaapvaal cratonic Archean mantle
Bibliographic record
Abstract
The c. 86 Ma Jagersfontein kimberlite in the southern portion of the Kalahari craton in South Africa erupted a suite of refractory peridotite xenoliths with regionally anomalous rhenium+osmium (Re Os) isotope systematics and highly siderophile element (HSE) abundances. A portion of this suite comprises peridotites with 2.7 to 3.0 Ga Re- depletion 187 Os/ 188 Os model ages, refractory major elements and HSE abundances that represent some of the most depleted and least metasomatised known components of Archean cratonic nuclei. Other peridotites in the Jagersfontein suite show enrichment in Os, Ir, Pd, Pt and, especially, Ru, as well as having more radiogenic 187 Os/ 188 Os. Ruthenium abundances reach up to ∼22 times “Primitive Upper Mantle” levels and correspond to samples containing microscopic laurite plus pentlandite. The more radiogenic 187 Os/ 188 Os in HSE-enriched samples, along with the modern mantle like Ru isotope composition (ε 100 0.00 ± 0.12, ε 102 –0.03 ± 0.16), indicate that the prominent HSE enrichment is not related to an excess of unequilibrated Late Veneer material in the mantle source. Instead, the HSE characteristics and Os isotope ratios of these peridotites are inferred to have been caused by metasomatic addition of small volumes of HSE-rich sulfide/alloy to the refractory Archean peridotites. Although there is some variation in the metasomatic characteristics, the calculated 187 Os/ 188 Os of the contaminant causing the strongly Ru-enriched peridotites is ∼0.116, which is equivalent to the convecting mantle composition in the early Mesoproterozoic. Unlike the erosion and Proterozoic replacement of older mantle interpreted to explain post-Archean 187 Os/ 188 Os mantle beneath the central Kaapvaal nucleus, the Archean mantle lithosphere beneath Jagersfontein therefore appears to have been modified by the passage of Proterozoic melts. The 2400 km 2 1.9 Ga layered mafic Trompsburg Complex, less than 10 km southeast of Jagersfontein, indicates that the mantle lithosphere in the area has experienced substantive transfer of large volumes of post-Archean melt, and we suggest that the passage of those melts may have caused the mantle HSE enrichment.
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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) | 0.006 | 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".