CHEMICAL AND PHYSICAL CHARACTERISTICS OF DIAMOND CRYSTALS FROM GARNET LAKE, SARFARTOQ, WEST GREENLAND: AN ASSOCIATION WITH CARBONATITIC MAGMATISM
Bibliographic record
Abstract
A shallow-dipping kimberlite – ultramafic lamprophyre sheet at Garnet Lake, Sarfartoq, West Greenland, contains diamond crystals in abundance. The crystals are most commonly colorless, and irregular or octahedral in shape. Nitrogen concentrations are unusually high, averaging 1100 ppm, compared to diamond crystals from elsewhere; the nitrogen is not strongly aggregated, however, averaging 26% IaB. This observation points to a short residence-time in the mantle, low temperature of mantle residence, or likely a combination of both conditions. Evidence of two populations is apparent in the relative abundance of different states of aggregation of the nitrogen in the stones. The common octahedral morphologies constrain the minimum temperature of formation for the majority of the crystals to 1375°C, assuming a 41 mWm−2 geotherm. Nitrogen concentrations and aggregation states only allow for very short residence-times, however, up to 1 Ma at this temperature. Hence a model of diamond formation quickly followed by ponding within a shallower, cooler mantle at a temperature of approximately 1252°C, supported by the tight clustering of associated mantle xenoliths, is preferred. Under these cooler conditions, the unusually high concentrations of nitrogen, combined with relatively low aggregation, give a calculated spread of diamond residence up to a maximum of 42.3 Ma. An emplacement age of 568 ± 11 Ma (2σ) obtained for the Garnet Lake kimberlite is based on the weighted average 207Pb/206Pb age for two perovskite fractions. This age is coincident with revised data on emplacement of the neighboring 565 Ma Sarfartoq carbonatite complex. Combined with diamond residence-time, the maximum likely age for diamond formation is ca . 610 Ma. Geographic and petrological similarities between kimberlitic hosts and the Sarfartoq carbonatite are apparent; in view of the close temporal similarity, we propose that CO2-rich fertilization of mantle rocks giving rise to the formation of diamond has in this case assisted in the subsequent production of carbonatite magmatism and associated kimberlitic melts acting as transporting medium for the diamond.
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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 | 0.001 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 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
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".