Applications of LA-ICP-MS analysis to zircon : assessing downhole fractionation and pre-treatment effects for U-Pb geochronology and trace element variations in accessory minerals from the Bushveld Complex
Bibliographic record
Abstract
Zircon and other U-Th-Pb-bearing minerals are now recognized as key geochemical and geochronological tracers of the evolution of late-stage fractionated interstitial melt in mafic layered intrusions. Two separate, yet complementary, applications of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) on zircon from layered intrusions are presented with the goals of advancing the analytical capabilities of LA-ICP-MS and showcasing the powerful geochemical fingerprinting of zircon. The effects of downhole fractionation, the time-dependent evolution of Pb-U ratios during laser ablation, represents a significant limitation on the accuracy of U-Th-Pb zircon geochronology by LA-ICP-MS. Exponential downhole correction models developed from the analyses of three common zircon reference materials (Plešovice, Temora-2, 91500) and applied to low-U zircon from Precambrian mafic intrusions (Laramie, Bushveld, Stillwater) indicate that successful correction requires careful matrix-matching the reference zircon to the unknowns. Pre-treatment protocols, including annealing and leaching, applied to all analyzed zircon produces strong effects on downhole fractionation with correlative impact on the relative accuracy of the calculated ages as a function of the downhole behaviour in the reference material used. In the Paleoproterozoic Bushveld Complex, the world’s largest layered intrusion, the trace element systematics of zircon provide temperature-composition constraints on the near-solidus crystallization of mafic-ultramafic cumulates and overlying granitic rocks. Zircon occurs with other late-stage interstitial minerals (e.g., quartz, biotite, Na-plagioclase) and crystallized at temperatures ranging from 950°C down to 690°C based on Ti-in-zircon thermometry. Forward modeling using rhyolite-MELTS of proposed Bushveld parental magmas yields similar zircon saturation temperatures from melts of intermediate-silicic composition, representing less than ~15% remaining melt, and reproduces the observed mineral assemblages. Anomalously high and variable Th/U in zircon from the Critical Zone (e.g., UG2 chromitite, Merensky Reef) reflects U loss to late, oxidized Cl-rich fluids that exsolved from the fractionated interstitial melt, a process that may be a characteristic feature of large open-system layered intrusions (e.g., Neoarchean Stillwater Complex). The presence of late-stage interstitial zircon and other accessory minerals in layered intrusions provides new in situ geochemical and geochronological tools for evaluating the origin and evolution of mafic-ultramafic magmatism in the Earth’s crust throughout geological time.
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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 | 0.001 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 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) | 0.000 | 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".