Hydrothermal calcite-fluid REE partitioning experiments at 200 °C and saturated water vapor pressure
Hydrothermal calcite-fluid REE partitioning experiments at 200 °C and saturated water vapor pressure
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DOI：
10.1016/j.gca.2020.07.018
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发表时间：
2020-07
影响因子：
5
通讯作者：
Emily P. Perry;A. Gysi
中科院分区：
文献类型：
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作者：
Emily P. Perry;A. Gysi
方解石是一种常见的与矿床和蚀变带地热系统有关的脉状矿物。方解石中稀土元素的含量可以作为热液成矿过程中物理化学条件变化的特征。以前的方解石流体分配实验已在环境温度下进行，但尚未开发出旨在预测稀土元素在高温下的行为的模型。本研究的目的是确定稀土元素掺入方解石矿化热液含水流体高于100° C的控制。在此，我们提出了一系列的热液批式稀土分配实验在200° C和饱和水蒸气压（15.5巴）。实验的目的是合成稀土掺杂方解石从流体混合不同的初始稀土浓度（250 ppb至1000 ppb），并允许在原位采样的含水流体。方解石-流体分配系数（KD）随稀土离子半径和初始稀土浓度的变化而变化：250 ppb稀土时，logKD为0.86-1.67; 500 ppb稀土时，logKD为1.05-1.70; 1000 ppb稀土时，logKD为0.47-1.07。这些数据符合抛物线根据晶格应变模型，从计算的杨氏模量（ES）的值范围在21.6和63.2 GPa之间。拟合结果表明，轻稀土的配分受应变引起的Ca 2+在方解石结构中的置换作用的控制，而重稀土则偏离这一趋势。基于双热力学（DualTh）方法建立的REE（OH）3-CaCO 3固溶体模型表明，在pH = 6时，REE分配受以下可能的耦合置换控制：Eu 3++ 3 OH-惠Ca 2++ CO 3 2-Eu 3++ O 2-+ OH-惠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混合行为。由于实验矿物-流体KD值随含水流体中REE浓度的变化，该模型在自然系统中的应用产生了一定的局限性。DualTh方法考虑了含水流体的水相形态和活度-浓度关系，因此，提供了一种有效的方法来评估矿物和含水流体之间的稀土分配。这项研究是在热液条件下建立方解石-流体分配数据实验数据库的第一步，旨在开发更准确的模型来预测天然矿物-流体系统中稀土元素的行为。
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 Ca 2+ 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.