Liquidus Phase Relations in the System CaO–MgO–Al2O3–SiO2 at 2·0 GPa: Applications to Basalt Fractionation, Eclogites, and Igneous Sapphirine
Bibliographic record
Abstract
To model magmatic crystallization processes for mafic to intermediate compositions at high pressure, liquidus phase relations in the forsterite–anorthite–diopside–silica (FADS) tetrahedron within the CaO–MgO–Al2O3–SiO2 system have been determined at 2·0 GPa. Compositions of five liquidus invariant points have been determined and the approximate compositions of five others have been inferred. These involve primary phase volumes for forsterite (fo), enstatite (en), diopside (di), high quartz (qz), spinel (sp), sapphirine (sa), garnet (gt), anorthite (an), and corundum (cor). The determined (with wt % coefficients) and inferred reactions (without coefficients) that define each isobaric invariant point are as follows: 23 en + 68 di + 9 sp = 84 liq + 16 fo 37 di + 63 sa = 47 liq + 40 sp + 13 en 100 gt = 21 liq + 27 sa + 55 en + 18 di 1 di + 59 en + 41 an = 43 liq + 57 gt 18 di + 21 qz + 15 en + 47 an = 100 liq di + an + gt = liq + sa an + gt = liq + sa + en sa + an + di = liq + sp sa + an = liq + cor + sp di + cor = liq + an + sp. These phase relations provide a diverse range of constraints on igneous processes at pressures near 2 GPa. They show that fractional crystallization of a model basalt gives a residual liquid strongly enriched in SiO2, strongly depleted in MgO, and mildly enriched in Al2O3. Such a trend is consistent with the calc-alkaline fractionation trend observed at subduction zones, but is in disagreement with suggestions that fractionation of tholeiitic basalt in this pressure range yields an alkalic basalt. Both trends may occur for natural basalts depending on the Na2O content of the parental magma. Also, the data show that the minimum pressure for the formation of cumulate eclogites and garnet pyroxenites is about 1·8–1·9 GPa. The lower limit of pressure at which sapphirine can crystallize from a liquid in the FADS tetrahedron is estimated to be 1·1–1·5 GPa and the upper limit is >3 GPa. Sapphirine crystallizes from magmas intermediate in composition between basalt and andesite. Probable igneous sapphirine in mafic associations is rare, but it occurs as part of a pyroxenite xenolith from Delegate, Australia, that we suggest is a cumulate assemblage and in a sapphirine norite at Wilson Lake, Labrador, Canada.
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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 | 0.000 | 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) | 0.001 | 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".