The Kimberlite Olivine Phenocryst/Macrocryst/Xenocryst Problem, Re-Visited
Bibliographic record
Abstract
Detailed petrography, SEM and electron microprobe studies of olivine in the hypabyssal Jos kimberlite dyke, Somerset Island, Canada, were undertaken to re-examine the olivine phenocryst/macrocryst/xenocryst “problem”. A parallel integrated trace element and isotopic study was also undertaken (Malarkey et al., submitted). Euhedral to subhedral groundmass olivine phenocrysts (less than ~1 mm in size and typically .4 to <1 mm) anhedral to subhedral olivine grains of similar compositions. However, we suggest that a sub-set of the olivine macrocrysts (Mg# 9092) likely represent high pressure phenocrysts, but this distinction could not be made with the analytical techniques utilized in this study. This difference is however evident in Sr isotope/trace element systematics. That two distinct olivine populations in the Jos dyke can be discerned on the basis of petrographic/SEM studies and microprobe data is certainly not new. Previously, similar observations were reported from the De Beers pipe „peripheral‟ kimberlite (Boyd and Clement, 1977); and a variety of hypabyssal kimberlites on Somerset Island (Mitchell, 1986) and in the Lac de Gras field (Fedortchouk and Canil, 2005; this study). These studies, along with a variety of other lines of evidence (e.g., experimental petrology and Sr isotopes), are consistent with the idea that the kimberlite liquidus is defined by crystallization of olivine, or that olivine is a very early crystallizing phase (after Cr-rich spinel) and continues to co-crystallize with spinel over a protracted temperature interval. In contrast, recent studies by Brett et al. (2009) on Diavik kimberlites have concluded that primary magmatic olivine crystallization is limited to „thin‟ rims on olivine xenocrysts. This conclusion is then universally applied to all kimberlites worldwide, with the suggestion that virtually all kimberlite olivine is of xenocrystic origin. This interpretation, however, ignores a significant body of work (see above) on a number of kimberlites, and is not supported by our detailed isotopic study (Malarkey et al., submitted). Perhaps the most outstanding problem with the idea that olivine in a kimberlite is a „late‟ magmatic crystallizing phase is the implication that the source region of kimberlite cannot be olivine-rich.
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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 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.002 | 0.002 |
| Scholarly communication | 0.002 | 0.001 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.001 | 0.001 |
| 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 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".