Diamonds<subtitle>Crustal Distribution and Formation Processes in Time and Space and an Integrated Deposit Model</subtitle>
Bibliographic record
Abstract
Abstract Diamonds have been brought to the Earth’s surface from at least 2.82 Ga onward by igneous and tectonic processes, and they have been redistributed since then by sedimentary processes into secondary diamond deposits. None of the known tectonically emplaced diamond deposits are economically viable, and only two types of igneous rock, kimberlite and lamproite, sometimes carry diamonds and can occasionally be economic to mine. Where diamonds are present in kimberlite and lamproite, the concentrations are –3 ppm, acquired by random sampling of diamond source rocks in the subcontinental lithosphere. Diamond-forming processes in the lithosphere were episodic since ~3.57 Ga, and all primary diamond deposits show evidence of two or more diamond generations. The earliest diamond-forming episode at ~3.4 ± 0.2 Ga appears to have been a worldwide metasomatic event triggered by CO2-rich, probably subduction-derived fluids that produced diamonds associated with garnet harzburgite. Further diamond populations have formed in association with craton accretion, subduction, slab melting, magmatic modifications of the lithospheric mantle, obduction tectonics, and metasomatic infiltration. In the process, diamonds formed in association with metasomatized harzburgite were supplemented predominantly by metasomatic diamond growth in eclogite, with occasional significant contributions from grospyditic, lherzolitic, and websteritic sources as well as sublithospheric ultradeep sources, notably majorite. Diamond-bearing igneous bodies exploit preexisting zones of weakness in the crust. They probably traverse most of the distance from the mantle to the surface as thin dikes or dike swarms, with nearurface expressions dictated by multiple intrusions, their volatile content, the presence or absence of cap rocks, local structures, and the ambient stress field, interaction with ground water , and degrees of preservation from erosion. Average diamond values per carat for a given diamond occurrence vary by approximately three orders of magnitude (US$1-$1,000/carat). A multistage model for the formation of diamond deposits is presented for the Kaapvaal craton that takes into account the tectonic history of the craton as well as the complexities observed within diamond populations of its various primary diamond deposits. Although the details of this model are craton specific, the general features of the model are applicable to other cratons.
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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 | 0.001 | 0.002 |
| Meta-epidemiology (narrow) | 0.001 | 0.001 |
| Meta-epidemiology (broad) | 0.001 | 0.001 |
| Bibliometrics | 0.002 | 0.003 |
| Science and technology studies | 0.002 | 0.003 |
| Scholarly communication | 0.006 | 0.004 |
| Open science | 0.002 | 0.002 |
| Research integrity | 0.002 | 0.003 |
| Insufficient payload (model declined to judge) | 0.102 | 0.027 |
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 source (direct Gemma or distilled Codex), 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".