Nature of Sub-volcanic Magma Chambers, Deccan Province, India: Evidence from Quantitative Textural Analysis of Plagioclase Megacrysts in the Giant Plagioclase Basalts
Bibliographic record
Abstract
Sub-volcanic magma chambers might be a widespread component of flood basalt provinces, and their presence can be revealed in some cases by plagioclase megacrystic basalts. In at least four levels within the Deccan flood basalt sequence the generally low abundance of small plagioclase crystals increases to 5–25%, with some as large as 30 mm long. These Giant Plagioclase Basalt (GPB) flows were formed by mixing of megacryst-rich and megacryst-poor magmas. The crystal size distributions (CSD) of these megacrysts mostly plot as almost straight lines on a classic CSD diagram. For a plagioclase growth rate of 10−10 mm/s steady-state magma chamber models and simple continuous growth suggest residence times of 500–1500 years. However, the lack of crystals smaller than 2 mm suggests that coarsening may have been involved and crystal shape can help define the environment where this happened. Plagioclase megacrysts are very tabular and commonly form clusters of sub-parallel crystals, characteristics that are also found in the plagioclase of anorthosites formed by flotation at the top of shallow magma chambers and crystallization in a high Peclet number regime (e.g. Skaergaard; Sept Îles). A possible history is as follows. (1) Plagioclase megacrysts crystallize in a convecting magma chamber just below the lava pile. (2) Currents sweep the crystals to the top of the chamber, where they accumulate as a result of their buoyancy. The crystals coarsen in response to the continuous supply of hot magma. (3) New magma sweeps through the plagioclase mush, mingles and mixes, then erupts to form the GPBs. The residence time recorded by the megacrysts in the GPBs is that of the magma chamber where the megacrysts formed, not that of the magmas that make up the megacryst-poor part of the GPBs or the other megacryst-poor lavas. Lavas with megacrysts similar to the GPBs are uncommon but widespread (Galapagos, Surtsey, etc.), and suggest the presence of sub-volcanic magma chambers elsewhere.
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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 | 0.000 | 0.001 |
| Scholarly communication | 0.001 | 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".