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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. 

Monazite (CePO4) is a light rare earth element (REE) phosphate occurring as accessory mineral in metamorphic, igneous and sedimentary rocks, and is also a common mineral in REE mineral deposits. Metasomatism of monazite yields important clues about fluid-rock interaction in the crust, in particular, because its compositional variations may enable us to determine conditions of mineralization. The thermodynamic properties of monazite have been determined using several calorimetric methods, but up to the present time only a few solubility studies have been undertaken, which test the reliability of both, the thermodynamic properties of the REE phosphates and associated REE aqueous species. In this study, we have measured the solubility of the monoclinic REE phosphate end-members CePO4, SmPO4, and GdPO4 in aqueous perchloric acid solutions at temperatures from 100 to 250 °C at saturated water vapor pressure (swvp). The solubility products (Ks0) were determined according to the reaction: REEPO4 = REE3+ + PO43−. Combining available calorimetric data for the REE phosphates with the REE aqueous species from the Supcrt92 (slop98.dat) dataset, yields several orders of magnitude differences when compared with our solubility measurements. We have investigated ways to reconcile these discrepancies and propose a consistent set of provisional thermodynamic properties for REE aqueous species and REE phosphates that reproduce our measured solubility values. To reconcile these discrepancies, we have used the GEMS code package and GEMSFITS for parameter optimization by adjusting the standard Gibbs energy of REE3+ and REEOH2+ at 25 °C and 1 bar. An alternative optimization could involve adjustment of the standard Gibbs energy of REEPO4(s) and REEOH2+. Independently of the optimization method used, this study points to a need to revise the thermodynamic properties of REEOH2+ and possibly other REE hydroxyl species in future potentiometric studies. These revisions will have an impact on calculated solubilities of REE phosphates and our understanding of the mobility of REE in natural hydrothermal fluids. 

CePO4 and YPO4 are major components in monazite and xenotime, respectively, which are common hydrothermal phases in REE mineral deposits. Both minerals also occur as secondary minerals in iron-oxide-apatite deposits [1,2], and as accessory phases in high-grade metamorphic rocks where they display varying degrees of metasomatism. Studying the cause of their compositional variations using thermodynamic modeling may provide geochemical signals for interpreting P- T-x of crustal fluid-rock interaction. The thermodynamic properties of monazite and xenotime have been determined using several calorimetric methods [3], but only a few solubility studies have been undertaken, which test the reliability of both the calorimetric data and thermodynamic properties of associated REE aqueous species [4]. Combining available calorimetric data with the REE aqueous species from Haas et al. [5], implemented in the Supcrt92 database [6], yields several orders of magnitude differences when compared with our solubility measurements. To reconcile these discrepancies, we have used the GEMS code package [7,8] and GEMSFITS [9] for parameter optimization, and re- evaluated the standard Gibbs energies for aqueous REE species, while maintaining consistency with available calorimetric measurement of the REE phosphates. This study points to a need to revise the thermodynamic properties of the REE hydroxyl species, which will have an impact on the calculated solubilities of the REE phosphates and our understanding of the mobility of REE in hydrothermal fluids. Our new experimental data will be implemented in the MINES thermodynamic database (http:// tdb.mines.edu) [10] for modeling the chemistry of crustal fluid-rock equilibria.