The mineralogy and crystal chemistry of alkaline pegmatites in the Larvik Plutonic Complex, Oslo rift valley, Norway. Part 1. Magmatic and secondary zircon: implications for petrogenesis from trace-element geochemistry
Bibliographic record
Abstract
Abstract A detailed electron microprobe (EMP) and laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) study of zircon from six types of miaskitic and agpaitic alkaline pegmatite from the Larvik Plutonic Complex, Oslo rift valley, Norway, was undertaken to shed light on the pegmatite petrogenesis. Detailed rare earth element (REE) analyses indicate important differences between the zircon from each type of pegmatite. Primary zircon from miaskitic Stavern-, Tvedalen- and Stålaker-type pegmatites has a mean ΣREE = 704 ppm, is depleted in LREE and has a significant positive Ce anomaly (Ce/Ce* = 44–67) and negative Eu anomaly (Eu/Eu* = 0.15–0.18). Secondary Tvedalen-type zircon is REE-enriched (ΣREE = 5035 ppm), with a flatter REE pattern, Ce/Ce* = 0.97 and a Eu anomaly similar to primary Tvedalen-type zircon (Eu/Eu* = 0.21). Secondary zircon from agpaitic Langesundsfjord-type pegmatites display a distinctive flat REE pattern characterized by overall REE enrichment (ΣREE = 967), Ce/Ce* = 1.92, and a minor negative Eu anomaly (Eu/Eu* = 0.37). Zircon from agpaitic Bratthagen-type pegmatites occurs as both altered primary and secondary phases and is strongly enriched in REE relative to other zircon (ΣREE = 4178 and 8388, respectively). Primary Bratthagen-type zircon has a similar REE pattern to miaskitic zircon, with a steeper HREE profile and smaller Ce and Eu anomalies (Eu/Eu* = 0.73; Ce/Ce* = 6.22). Secondary Bratthagen-type zircon is strongly enriched in LREE compared to primary zircon, does not display a positive Ce anomaly and has Eu/Eu* = 0.56. The altered primary and secondary Bratthagen-type zircons have elevated Th/UN ratios, suggesting a different melt source for Bratthagen-type agpaitic pegmatites. Zircon from external pegmatites has trace-element signatures similar to Stavern-, Tvedalen- and Staålaker-type primary zircon with Ce/Ce* = 214 and Nb/Ta and Th/U ratios that are similar to those of secondary Langesundsfjord- and Bratthagen-type zircon. It is suggested that the parental melt of the external pegmatites is the same as the miaskitic pegmatites, but that it has undergone alteration by hydrothermal fluids derived from the host basalt, or by post-magmatic F-rich fluids which mobilize Nb and Th. On the basis of texture, morphology and geochemistry, two populations of zircon can be recognized: (1) primary zircon from miaskitic pegmatites; and (2) secondary zircon from post-magmatic, hydrothermal assemblages. The U–Th–Pb isotope analyses indicate that the secondary and altered zircon are depleted in 238U, and enriched in LREE. Interaction of a post-magmatic hydrothermal fluid with an externally derived meteoric fluid is suggested to have influenced the REE signatures, and in particular the Eu and Ce anomalies of the late-stage zircons.
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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 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| 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".