Bibliographic record
Abstract
Large, single crystals (> 1cm) are a familiar component of mantle xenolith suites of many kimberlites. Confusion between different suites exists in the literature, however, which affects petrogenetic models, and some clarification is warranted. Megacrysts of the Cr-poor suite[1] are most common. Cr-poor silicates (garnet, clinopyroxene, orthopyroxene, olivine) are characterized by lower Mg/(Mg+Fe) and Cr2O3 and higher TiO2 values than typical mantle peridotite minerals. Strong geochemical trends in most occurrences of Cr-poor megacryst suites (e.g., concomitant decrease in Mg/(Mg+Fe) and Cr2O3) are interpreted by most authors as the result of fractional crystallization of a kimberlite, or kimberlite-like, magma. The Cr-rich megacryst suite, comprising garnet, clinopyroxene, orthopyroxene and olivine, but not ilmenite, was described from the Sloan/Nix kimberlites in northern Colorado[1]. Constituent minerals, all four of which are essential to the definition of the suite, are characterized, in part, by high and restricted values of Mg/(Mg+Fe) and wt% Cr2O3 (e.g., 0.791 to 0.837 and 6.1 to 13.0, respectively, in garnet [2]). Elsewhere, large crystals with Mg/(Mg+Fe) and Cr2O3 values higher than Cr-poor suite minerals do occur, but none correspond to the Sloan-Nix Cr-rich suite in paragenesis, size and/or composition[2]. For example, almost no garnet megacrysts described as “Cr-rich” or “high-Cr” from other localities (e.g., refs 3-6) contain >6 wt% Cr2O3 and even garnets with <2 wt% Cr2O3 are termed “Cr-rich” or “high-Cr”. Most, or all, of these so-called “Cr-rich garnet megacrysts” are simply xenocrysts from coarse-grained peridotite. The “Granny Smith” suite, first described from Kimberley and Jagersfontein [7], is dominated by Cr-clinopyroxene associated with phlogopite (and ilmenite at Kimberley), with uncommon olivine or rutile. Garnet and orthopyroxene do not occur in this suite, which is neither equivalent to nor a subset of the Cr-rich megacryst suite. Other suites dominated by Cr-clinopyroxene, also not shown to coexist with garnet and orthopyroxene, have been described from Orapa and Bobbejaan [6] and Grib [8], though analogies have been drawn with the Cr-rich megacryst suite despite compositional and paragenetic differences. A similar megacrystalline assemblage (Cr-cpx, ilmenite, phlogopite, olivine) has been described from Attawapiskat [9] and at Balmoral megacrysts of Cr-cpx occur with ilmenite, Nb-Cr rutile and zircon [10].All of these suites of Cr-cpx +/- ilmenite, rutile, phlogopite, olivine, zircon (lacking garnet/opx), though varied, have more in common with each other than with the Cr-rich megacryst suite. All might be best termed “Granny Smith”, and may have common origins. The only feature they share with the Sloan-Nix Cr-rich megacryst suite is the presence of large chromian clinopyroxene. Use of such populations as equivalents of the Sloan-Nix Cr-rich megacryst suite in mantle petrogenetic schemes can lead to faulty conclusions. References: 1) Eggler et al. (1979) The Mantle Sample, 2) Schulze (2022) Goldschmidt Conf. Abstr., 3) Hunter and Taylor (1984) Am. Min., 4) Kopylova et al. (2009) Lithos, 5) Bussweiller et al. (2018) Min. Pet., 6) Nkere et al. (2021) Lithos, 7) Boyd et al. (1984) GCA, 8) Kargin et al. (2017) Lithos, 9) Hetman (1996) MSc., 10) Schulze, unpub. data.
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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 | 0.002 | 0.001 |
| Science and technology studies | 0.001 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.004 | 0.001 |
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".