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Monazite-(Ce) and xenotime-(Y) occur as secondary minerals in iron-oxide-apatite (IOA) deposits, and their stability and composition are important indicators of timing and conditions of metasomatism. Both of these minerals occur as replacement of apatite and display slight but important variations in light (e.g. La, Ce, Pr, Nd, etc.) and heavy (e.g. Y, Er, Dy, Yb, etc.) REE concentrations [1,2]. The causes for these chemical variations can be quantified by combining thermodynamic modeling with field observations. Major challenges for determining the stability of these minerals in hydrothermal solutions are the underlying models for calculating the thermodynamic properties of REE-bearing mineral solid solutions and aqueous species as a function of temperature and pressure. The thermodynamic properties of monazite and xenotime have been determined using several calorimetric methods [3], but only a few hydrothermal solubility studies have been undertaken, which test the reliability and compatibility of both the calorimetric data and thermodynamic properties of associated REE aqueous species [4,5]. Here, we evaluate the conditions of REE metasomatism in the Pea Ridge IOA-REE deposit in Missouri, and combine newly available experimental solubility data to simulate the speciation of LREE vs. HREE, and the partitioning of REE as a function of varying fluid compositions and temperatures. Our new experimental data will be implemented in the MINES thermodynamic database (http:// tdb.mines.edu) for modeling the chemistry of crustal fluid-rock equilibria [6]. [1] Harlov et al. (2016), Econ. Geol. 111, 1963-1984;[2] Hofstra et al. (2016), Econ. Geol. 111, 1985-2016; [3] Navrotsky et al. (2015), J. Chem. Thermodyn. 88, 126-141; [4] Gysi et al. (2015), Chem. Geol. 83-95; [5] Gysi et al. (2018), Geochim. Cosmochim. Acta 242, 143-164; [6] Gysi (2017), Pure and Appl. Chem. 89, 581-596.
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Reaction calorimetry and structural crystal properties of non-ideal binary rhabdophane solid solutions (Ce1−xREExPO4·nH2O)
Rhabdophane is a hydrous phosphate that commonly replaces monazite as a weathering product in critical mineral deposits during the alteration of rare earth elements (REE) bearing carbonatites and alkaline igneous complexes. It is an important host to the light (L)REE (i.e., La to Gd) but the stability and structure of binary solid solutions between the Ce and the other LREE endmembers have not yet been determined experimentally. Here we present room temperature calorimetric experiments that were used to measure the enthalpy of precipitation of rhabdophane (Ce1−xREExPO4·nH2O; REE = La, Pr, Nd, Sm, Eu, and Gd). The solids were characterized using X-ray diffraction, scanning electron microscopy, Raman spectroscopy, and the role of water in the rhabdophane structure was further determined using thermogravimetric analysis coupled with differential scanning calorimetry. The calorimetric experiments indicate a non-ideal behavior for all of the binary solid solutions investigated with an excess enthalpy of mixing (ΔHex) described by a 2- to 3-term Guggenheim parameters equation. The solid solutions were categorized into three groups: (1) binary Ce-La and Ce-Pr which display positive ΔHex values with a slight asymmetry; (2) binary Ce-Nd and Ce-Sm which display negative ΔHex values with a nearly symmetric shape; (3) Ce-Eu and Ce-Gd which display both negative and positive ΔHex values with nearly symmetric shape. The excess Gibbs energy (ΔGex) of the solid solutions was further investigated using a thermodynamic analysis approach of aqueous-solid solution equilibria and the optimization programs GEMS and GEMSFITS. The resulting ΔGex values combined with the calorimetric ΔHex values indicate that there is likely an excess entropy contribution implying important short-range structural modifications in the solid solutions dependent on the deviation of the REE ionic radii from the size of Ce3+. These observations corroborate with the trends in the Raman v1 stretching bands of the PO4-site. The excess molar volumes determined from X-ray diffraction analysis further indicate an overall asymmetric behavior in all of the studied binary solid solutions, which becomes increasingly important from La to Gd. The pronounced short-range order–disorder occurring in groups 2 and 3 solid solutions mimics some of the behavior observed from previous studies in anhydrous monazite solid solutions. This study highlights the potential to use the chemistry and the structural modifications of rhabdophane as potential indicators of formation conditions in geologic systems and permits improving our modeling capabilities of REE partitioning in critical minerals systems.
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- PAR ID:
- 10506366
- Publisher / Repository:
- Mendeley Data
- Date Published:
- Journal Name:
- Geochimica et Cosmochimica Acta
- ISSN:
- 0016-7037
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Charlier, B (Ed.)Quantifying the oxygen fugacity (fo2) of high temperature lithospheric fluids, including hydrothermal systems, presents a challenge because these fluids are difficult to capture and measure in the same manner as quenched glasses of silicate melts. The chemical properties of fluids can however be inferred through mineral proxies that interacted with the fluids through precipitation or recrystallization. Here, we present hydrothermal experiments to quantify the partition coefficients of rare earth elements (REEs) – including redox-sensitive Ce and Eu – between zircon and fluid. Experiments were conducted in a piston cylinder device at temperatures that range from 1200 to 800 ◦C under fo2-buffered conditions in a SiO2-ZrO2-NaCl-REE-oxide system, and similar experiments were performed in the absence of NaCl (31 total experiments). The fo2 was buffered to values that range from approximately 3 log units below to 7 log units above the fayalite magnetite quartz equilibrium. Zircon REE concentrations were quantified using laser ablation inductively coupled plasma mass spectrometry whereas the quenched fluids were extracted and measured by solution-based inductively coupled plasma mass spectrometry. Zircon Ce anomalies, quantified relative to La and Pr, exhibit sensitivity to oxygen fugacity and temperature and our preferred calibration is: log [ Ce Ce* ) D 1 ] = (0.237 ± 0.040)× log(fo2) + 9437±640 T(K) 5.02 ± 0.38 where the Ce anomalies are calculated from the partition coefficients for La, Ce, and Pr. Zircon Eu anomalies are also a function of oxygen fugacity though they exhibit no systematic dependence on T. Our preferred calibration is described by: Eu Eu* ) D = 1 1+100.30±0.04 [0.27±0.03]×ΔFMQ We performed additional calculations, in which lattice strain parabolas were fit to all non-redox sensitive rare earth elements that were added to the starting composition (i.e., La, Pr, Sm, Gd, Dy, Ho, Tm, Lu) as an alternate means to calculate anomalies. This method yields broadly similar results, though we prefer the La-Pr calibrations due to the non-systematic REE patterns frequently encountered with hydrothermal zircons; e.g., LREE zircon enrichment relative to other REEs. These experiments are applied to quantify the fo2 of fluids during mineralization of critical element-bearing systems, and separately to calculate the oxygen fugacity values of fluids formed during plate boundary processes.more » « less
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1016/j.gca.2024.04.034
