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Calcite is a common vein mineral associated to ore deposits and alteration zones geothermal systems. The concentration of rare earth elements (REE) in calcite can potentially be used to fingerprint the changing physicochemical conditions during mineralization from hydrothermal fluids. Previous calcite-fluid partitioning experiments have been carried out at ambient temperature but models aiming at predicting the behavior of REE at elevated temperature have not yet been developed. This study aims at determining the controls on REE incorporation into calcite mineralized from hydrothermal aqueous fluids above 100 °C. Here, we present a series of hydrothermal batch-type REE partitioning experiments at 200 °C and saturated water vapor pressure (15.5 bar). The experiments were designed to synthesize REE-doped calcite from fluid mixing with varying initial REE concentrations (250 ppb to 1000 ppb) and permit in situ sampling of the aqueous fluids. Calcite-fluid partition coefficients (K D) were observed to depend on the ionic radius of the REE and vary as a function of initial REE concentrations: log K D of 0.86–1.67 at 250 ppb REE; log K D of 1.05–1.70 at 500 ppb REE; log K D of 0.47–1.07 at 1000 ppb REE. These data fit a parabola according to the lattice strain model from which calculated Young's modulus ( E S ) values range between 21.6 and 63.2 GPa. The fits indicate that the partitioning of the light REE is controlled by the strain-induced Ca2+ substitution in the calcite structure, whereas the heavy REE deviate from these trends. The development of a binary REE(OH) 3 - CaCO 3 solid solution model based on the dual-thermodynamic (DualTh) approach suggests that the REE partitioning is controlled by the following possible coupled substitutions at pH of 6: Eu 3 + + 3 OH - ⇔ Ca 2 + + CO 3 2 - Eu 3 + + O 2 - + OH - ⇔ Ca 2 + + CO 3 2 - The lattice strain fits are useful to interpret deviations of the experimental data from the solid solution model and possible non-ideal mixing behavior. Application of this model to natural systems yields some limitations because of the variability of experimental mineral-fluid K D values with REE concentrations in the aqueous fluids. The DualTh approach considers the aqueous speciation and activity-concentration relationships of the aqueous fluids, and therefore, provides an efficient method to evaluate the partitioning of REE between mineral and aqueous fluids. This research is a first step in building an experimental database of calcite-fluid partitioning data at hydrothermal conditions and aims at developing more accurate models for predicting the behavior of the REE in natural mineral-fluid systems. [ABSTRACT FROM AUTHOR]

Monazite is a light rare earth element (REE) phosphate found in REE mineral deposits, such as those formed in (per)alkaline and carbonatite magmatic-hydrothermal systems, where it occurs in association to the development of alteration zones and hydrothermal veins. Although it has been recognized that monazite may undergo replacement by coupled dissolution-precipitation processes, currently there is no model describing the compositional REE variations in monazite resulting from direct interaction with or precipitation from hydrothermal fluids. To develop such a model requires quantification of the thermodynamic properties of the aqueous REE species and the properties of the monazite endmembers and their solid solutions. The thermodynamic properties of monazite endmembers have been determined previously using calorimetric methods and low temperature solubility studies, but only a few solubility studies have been conducted at >100 °C. In this study, the solubility products (log K s0) of LaPO 4 , PrPO 4 , NdPO 4 , and EuPO 4 monazite endmembers have been measured at temperatures between 100 and 250 °C and saturated water vapor pressure. The solubility products are reported with an uncertainty of ±0.2 (95% confidence) according to the reaction, REEPO 4 (s) = REE3+ + PO 4 3−. The REE phosphates display a retrograde solubility, with the measured K s0 values varying several orders of magnitude over the experimental temperature range. Discrepancies were observed between the experimental solubility products and the calculated values resulting from combining calorimetric data of monazite with the properties of the aqueous REE3+ and PO 4 3− species available in the literature. The differences between the calculated and measured standard Gibbs energy of reaction (Δ r G0) for PrPO 4 , NdPO 4 , and EuPO 4 increased with higher temperatures (up to 15 kJ mol−1 at 250 °C), whereas for LaPO 4 these differences increased at lower temperatures (up to 8 kJ mol−1 at 100 °C). To reconcile these discrepancies, the standard enthalpy of formation (Δ f H0) of monazite was optimized by fitting the experimental solubility data and extrapolating these fits to reference conditions of 25 °C and 1 bar. The optimized thermodynamic data provide the first internally consistent dataset for the solubility of all the monazite endmembers, and can be used to model REE partitioning between monazite and hydrothermal fluids at >100 °C. [ABSTRACT FROM AUTHOR]

Abstract Monazite (CePO 4) 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 CePO 4 , SmPO 4 , and GdPO 4 in aqueous perchloric acid solutions at temperatures from 100 to 250 °C at saturated water vapor pressure (swvp). The solubility products (K s0) were determined according to the reaction: REEPO 4 = REE3+ + PO 4 3−. 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 REEPO 4 (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. [ABSTRACT FROM AUTHOR]

Petrological and geochemical observations of pegmatites in the Strange Lake pluton, Canada, have been combined with numerical simulations to improve our understanding of fluid–rock interaction in peralkaline granitic systems. In particular, they have made it possible to evaluate reaction paths responsible for hydrothermal mobilization and mineralization of rare earth elements (REE) and Zr. The focus of the study was the B-Zone in the northwest of the pluton, which contains a pegmatite swarm and is the target of exploration for an economically exploitable REE deposit. Many of the pegmatites are mineralogically zoned into a border consisting of variably altered primary K-feldspar, arfvedsonite, quartz, and zirconosilicates, and a core rich in quartz, fluorite and exotic REE minerals. Textural relationships indicate that the primary silicate minerals in the pegmatites were leached and/or replaced during acidic alteration by K-, Fe- and Al-phyllosilicates, aegirine, hematite, fluorite and/or quartz, and that primary zirconosilicates (e.g., elpidite) were replaced by gittinsite and/or zircon. Reaction textures recording coupled dissolution of silicate minerals and crystallization of secondary REE-silicates indicate hydrothermal mobilization of the REE. The mobility of the light (L)REE was limited by the stability of REE-F-(CO2)-minerals (basnäsite-(Ce) and fluocerite-(Ce)), whereas zirconosilicates and secondary gadolinite-group minerals controlled the mobility of Zr and the heavy (H)REE. Hydrothermal fluorite and fluorite–fluocerite-(Ce) solid solutions are interpreted to indicate the former presence of F-bearing saline fluids in the pegmatites. Numerical simulations show that the mobilization of REE and Zr in saline HCl–HF-bearing fluids is controlled by pH, ligand activity and temperature. Mobilization of Zr is significant in both saline HF- and HCl–HF-bearing fluids at low temperature (250°C). In contrast, the REE are mobilized by saline HCl-bearing fluids, particularly at high temperature (400°C). The LREE are more mobile than the HREE in saline HCl-bearing fluids due to the greater stability of LREE-chloride complexes. The simulated mineralogy is consistent with the zonation observed in the pegmatites and with fluid–rock interaction at conditions that were rock-buffered in the pegmatite borders (low fluid/rock ratio; and pH>4) and fluid-buffered in the cores (high fluid/rock ratio; pH⩽2). We propose a model in which saline HCl–HF-bearing fluids created pathways during acidic alteration from the pegmatite cores outward. This led to the mobilization of REE and Zr due to progressive alteration of primary silicate minerals and increased acidity upon cooling. The key requirement for REE and Zr mobilization in peralkaline igneous intrusions is the formation of an acidic subsystem with high fluid/rock ratios that increases the overall permeability of the rocks. In these zones, the extent of late stage hydrothermal redistribution and concentration of REE and Zr depends on the buffering capacity of the rocks and the availability of fluids that may produce autometasomatic rock alteration, interact with external rock units and/or mix with fluids from other sources. [ABSTRACT FROM AUTHOR]

Abstract: The alteration of basaltic glass was studied experimentally at elevated CO2 pressures (~10–25bar) and hydrothermal conditions (75–250°C) to determine the effects of temperature and extent of reaction (ξ) on the secondary mineral formation and compositional evolution of alteration assemblages resulting from CO2–water–basalt interaction. At <100°C, the alteration products consisted of concentric layers of ankerite and dolomite–ankerite solid solutions (Ca–Mg–Fe carbonates) and amorphous silica. At ≥150°C, mixed Ca–Mg–Fe smectites and chlorite, calcite, amorphous silica and zeolites formed instead of Ca–Mg–Fe carbonates. Competing reactions between carbonates and clays for major divalent cations (Ca, Mg and Fe) were affected by the extent of reaction with smectites formed initially and progressively being replaced by calcite and chlorite. The basaltic glass dissolution rate and mechanism were also affected by temperature and reaction time. At lower temperatures (≤150°C), a hydrated leached layer formed on the glass surface and the mass fluxes in the system were largely controlled by the dissolution rate and mechanism of the glass, whereas at higher temperature (250°C) the dissolution rate of the basaltic glass was fast with the reactions primarily driven by secondary mineral replacement and growth. We conclude that the driving force for carbonate mineralization in basaltic glass at elevated CO2 conditions is linked to the geochemical behavior and mobility of Ca, Mg and Fe together with the availability of Si for Al–Si-mineral formation. At <100°C, the divalent cations are available for Ca–Mg–Fe carbonate formation, whereas at ≥150°C abundant clays limit the availability of Mg and Fe resulting in calcite being the only carbonate formed. This implies a quantitatively more effective fixation of CO2 at <100°C associated with the formation of Ca–Mg–Fe carbonates. [Copyright &y& Elsevier]

Abstract: CO2-water–basaltic glass batch experiments were performed in order to study the feasibility of low temperature CO2 sequestration into basalts including the key reactions and chemical mass transfer associated to progressive water–rock interaction. The experiments were carried out at 40°C for up to 260days with initial dissolved CO2 concentrations ranging from 24 to 305mmol/kg. Alteration minerals were identified on the basaltic glass grain surfaces and in the matrix, consisting of poorly crystalline Ca–Mg–Fe carbonates, Fe-hydroxides and/or oxy-hydroxides and Ca–Mg–Fe clays. Other cryptocrystalline phases were identified by variable amounts of Al, Si and Fe reflected by the secondary mineral compositions. The water chemistry was monitored during the experiments and was characterized by an increase in Si, Ca, Mg and Na with time, whereas Al, Fe and CO2 decreased. The dissolution of basaltic glass in CO2-rich waters was observed to be incongruent with the overall water composition and secondary mineralogy depending on reaction progress and pH. The pH was determined primarily by the initial CO2 concentration and its ionization constants, the amount of dissolved basaltic glass and by the mass and composition of secondary minerals formed. Initially, the pH increased rapidly from <4.5 to ∼4.5–6. Under these conditions, Mg and Ca were observed to be mobile and dissolved stoichiometrically relative to Na, whereas Si, Al and Fe were immobile. Upon quantitative CO2 mineralization, the pH increased to >6.5 and the mobility of most elements consequently decreased including Mg and Ca. The experimental results indicate that increased aqueous CO2 concentrations modify considerably the natural water–basalt reaction path. The mineralization of CO2 into carbonates was rapid and controlled by the initial CO2 concentration and rock to water ratio, with the composition of the carbonates depending on the availability of Ca, Mg and Fe and the oxidation state of Fe. At pH <5.5, Fe was in the ferrous oxidation state resulting in the formation of Fe-rich carbonates with the incorporation of Ca and Mg as well as the formation of Ca–(Mg)–Fe smectites. At pH >5.5, the mobility of Fe decreased due to oxidation of Fe(II) to Fe(III) and subsequent formation of ferrihydrite. At pH >6.5, the experimental solutions became progressively supersaturated with calcite, zeolites and Mg-rich clays limiting the mobility of Ca and Mg. The results indicate that reactions between clays (Ca–(Mg)–Fe smectites) and carbonates at pH <6.5, and zeolites, clays (Ca–Mg–Fe smectites) and carbonates at pH >6.5, control together the availability of Ca, Mg and Fe, playing a key role for low temperature CO2 mineralization and sequestration into mafic rocks. [Copyright &y& Elsevier]

Abstract: The interaction between CO2-rich waters and basaltic glass was studied using reaction path modeling in order to get insight into the water–rock reaction process including secondary mineral composition, water chemistry and mass transfer as a function of CO2 concentration and reaction progress (ξ). The calculations were carried out at 25–90°C and pCO2 to 30bars and the results were compared to recent experimental observations and natural systems. A thermodynamic dataset was compiled from 25 to 300°C in order to simulate mineral saturations relevant to basalt alteration in CO2-rich environment including revised key aqueous species for mineral dissolution reactions and apparent Gibbs energies for clay and carbonate solid solutions observed to form in nature. The dissolution of basaltic glass in CO2-rich waters was found to be incongruent with the overall water composition and secondary mineral formation depending on reaction progress and pH. Under mildly acid conditions in CO2 enriched waters (pH <6.5), SiO2 and simple Al–Si minerals, Ca–Mg–Fe smectites and Ca–Mg–Fe carbonates predominated. Iron, Al and Si were immobile whereas the Mg and Ca mobility depended on the mass of carbonate formed and water pH. Upon quantitative CO2 mineralization, the pH increased to >8 resulting in Ca–Mg–Fe smectite, zeolites and calcite formation, reducing the mobility of most dissolved elements. The dominant factor determining the reaction path of basalt alteration and the associated element mobility was the pH of the water. In turn, the pH value was determined by the concentration of CO2 and extent of reaction. The composition of the carbonates depended on the mobility of Ca, Mg and Fe. At pH <6.5, Fe was in the ferrous oxidation state resulting in the formation of Fe-rich carbonates with the incorporation of Ca and Mg. At pH >8, the mobility of Fe and Mg was limited due to the formation of clays whereas Ca was incorporated into calcite, zeolites and clays. Competing reactions between clays (Ca–Fe smectites) and carbonates at low pH, and zeolites and clays (Mg–Fe smectites) and carbonates at high pH, controlled the availability of Ca, Mg and Fe, playing a key role for low temperature CO2 mineralization and sequestration into basalts. Several problems of the present model point to the need of improvement in future work. The determinant factors linking time to low temperature reaction path modeling may not only be controlled by the primary dissolving phase, which presents challenges concerning non-stoichiometric dissolution, the leached layer model and reactive surface area, but may include secondary mineral precipitation kinetics as rate limiting step for specific reactions such as retrieved from the present reaction path study. [Copyright &y& Elsevier]