Enrichment mechanism of heavy rare earth elements in magmatic-hydrothermal titanite: Insights from SXAS/XPS experiments and first-principles calculations and implications for regolith-hosted HREE deposits
Bibliographic record
Abstract
Abstract Heavy rare earth elements (HREE), as critical materials for the global transition to a low-carbon economy, are mostly supplied by regolith-hosted ion adsorption REE deposits formed from the weathering of granites. Magmatic-hydrothermal titanite of pronounced HREE enrichment in granites has been suggested to play an important role in forming HREE-dominated regolith-hosted deposits. However, the crystal-chemical mechanism and geological process responsible for HREE enrichment in titanite remain unknown. In this study, we investigated two texturally distinct types of HREE-enriched titanite (titanite I and II) in granites from the Gucheng regolith-hosted HREE deposit in South China. Secondary ion mass spectrometry (SIMS) U-Pb dating of rutile associated with titanite I and II yielded similar ages of 101.0 ± 1.7 and 97.9 ± 6.2 Ma, respectively, supporting HREE remobilization during auto-metasomatism. Microbeam synchrotron X-ray absorption spectroscopic analyses show that titanite I and II have distinct Y K-edge X-ray absorption near-edge structure spectra. The best-fit results of Y K-edge extended X-ray absorption fine structure data suggest that titanite I and II have a Y-O first shell with coordination numbers and distances of 7.8 and 7.2 Å and 2.31–2.51 Å and 2.30–2.47 Å, respectively. Measured Y 3d and Dy 3d X-ray photoelectron spectroscopic data also support different Y,HREE-O first shells between titanite I and II. These results suggest that Y3+ and REE3+ occupy the 7-coordinated Ca site via three substitutions (Y,REE)3+ + (Al,Fe)3+ ↔ Ca2+ + Ti4+, 2(Y,REE)3+ + □Ca ↔ 3Ca2+, and 2(Y,REE)3+ + O2− ↔ 2Ca2+. First-principles calculations and the lattice strain model predict preferential uptakes of Y3+ and HREE3+ as the (Y,HREE)O8 polyhedra arising from the latter substitution mechanism at the Ca site in titanite. Moreover, we link the 2Y3+ + O2− ↔ 2Ca2+ substitution in titanite to highly oxidized and HREE-enriched melts/fluids originating from the subducted slab. These findings further support our previous suggestion that the crystallization of HREE-enriched titanite in granites plays an important role in forming HREE-dominated regolith-hosted deposits.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| 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 | 0.000 | 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 teacher head, 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".