Minor Minerals, Major Implications: Using Key Mineral Phases to Unravel the Formation and Evolution of Earth's Crust
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Key minerals, including zircon, apatite, titanite, rutile, monazite, xenotime, allanite and garnet, can retain critical information about petrogenetic and geodynamic processes. This Special Publication showcases snapshots of the latest developments using key minerals in igneous, metamorphic and detrital rocks through current-state reviews, contributions focused on case studies and newly developed techniques.
Experimental alteration of allanite at 200°C: the role of pH and aqueous ligands Available to Purchase
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Published:January 03, 2024
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CitationAxel Denys, Anne-Line Auzende, Emilie Janots, German Montes-Hernandez, Nathaniel Findling, Pierre Lanari, Valérie Magnin, 2024. "Experimental alteration of allanite at 200°C: the role of pH and aqueous ligands", Minor Minerals, Major Implications: Using Key Mineral Phases to Unravel the Formation and Evolution of Earth's Crust, V. van Schijndel, K. Cutts, I. Pereira, M. Guitreau, S. Volante, M. Tedeschi
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Abstract
Allanite is a major host of rare earth elements (REEs) in the continental crust. In this study, reaction mechanisms behind allanite alteration are investigated through batch experiment runs on natural allanite grains in carbonate-bearing hydrothermal fluids at 200°C, with initial acidic (pH = 4) or alkaline (pH = 8) conditions and with different aqueous ligands (120 mmol kg−1 of F, Cl, P or S). Time-series experiment runs in F-doped systems at different durations between 15 and 180 days reached a steady state at 120 days. The pH efficiently controls the allanite alteration process, with initial high pH, alkaline conditions being more reactive (75% alteration compared with 25% under acidic conditions). The ligand also significantly influences the alteration process under initial acidic conditions with the P-doped system (70%) almost non-reactive for the Cl- and S-doped systems (<5%). In the alteration rim, REEs are mainly redistributed in REE-bearing phases either as carbonates (F-doped) or phosphates (P-doped). The relatively flat REE-normalized patterns of the recovered experimental fluids suggest a fractionation of light rare earth elements (LREEs) over heavy rare earth elements (HREEs) during the course of the alteration reactions. It is proposed that secondary REE mineral precipitation at the reaction front creates a local disequilibrium in the solution and a steep chemical gradient promoting allanite dissolution and thus its alterability.
- allanite
- alteration
- aqueous solutions
- continental crust
- crust
- electron microscopy data
- epidote
- epidote group
- experimental studies
- ICP mass spectra
- ligands
- mass spectra
- metals
- mineral deposits, genesis
- ore-forming fluids
- orthosilicates
- pH
- rare earths
- SEM data
- silicates
- sorosilicates
- spectra
- statistical analysis
- time series analysis
- X-ray diffraction data