Abstract Calcite is a common vein mineral associated to ore deposits and alteration zones in active geothermal systems. The concentration of rare earth elements (REE) in calcite can potentially be used to fingerprint 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 (KD) 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 - CaCO3 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 seems to yield 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 in 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 REE partitioning data at hydrothermal conditions that will enable developing more accurate models for predicting the behavior of the REE in natural mineral-fluid systems.

中文翻译：

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200 °C和饱和水蒸气压下的热液方解石-流体REE分配实验

摘要 方解石是一种常见的矿脉矿物，与活跃地热系统中的矿床和蚀变带有关。方解石中稀土元素 (REE) 的浓度可潜在地用于在热液流体成矿过程中对物理化学条件的变化进行指纹识别。以前的方解石-流体分配实验是在环境温度下进行的，但尚未开发旨在预测 REE 在高温下的行为的模型。本研究旨在确定对 REE 掺入从 100 °C 以上的热液水性流体矿化的方解石中的控制。在这里，我们在 200 °C 和饱和水蒸气压 (15.5 bar) 下进行了一系列水热间歇式 REE 分配实验。这些实验旨在从流体混合中合成 REE 掺杂的方解石，并具有不同的初始 REE 浓度（250 ppb 至 1000 ppb），并允许对含水流体进行原位采样。观察到方解石-流体分配系数 (KD) 取决于 REE 的离子半径，并作为初始 REE 浓度的函数而变化：250 ppb REE 时的 log KD 为 0.86 – 1.67；500 ppb REE 的 log KD 为 1.05 – 1.70；1000 ppb REE 时的 log KD 为 0.47 – 1.07。这些数据根据晶格应变模型拟合抛物线，从中计算出的杨氏模量 (ES) 值范围在 21.6 和 63.2 GPa 之间。拟合表明轻稀土的分配受方解石结构中应变诱导的 Ca2+ 取代控制，而重稀土偏离这些趋势。基于双热力学 (DualTh) 方法的二元 REE(OH)3 - CaCO3 固溶体模型的开发表明，在 pH 值为 6 时，REE 分配受以下可能的耦合取代控制：Eu 3 + + 3 OH - = Ca 2 + + CO 3 2 - Eu 3 + + O 2 - + OH - + = Ca 2 + + CO 3 2 - 晶格应变拟合有助于解释实验数据与固溶体模型的偏差和可能的非- 理想的混合行为。由于实验矿物流体 KD 值随水性流体中 REE 浓度的变化，将该模型应用于自然系统似乎会产生一些限制。DualTh 方法考虑了水性流体中的水性物种形成和活性-浓度关系，因此，提供了一种有效的方法来评估 REE 在矿物和水性流体之间的分配。这项研究是在热液条件下建立方解石流体 REE 分区数据实验数据库的第一步，这将有助于开发更准确的模型来预测天然矿物流体系统中 REE 的行为。