Speciation in carbonatitic brine-melts and fluids from molecular simulations

碳酸盐岩是最重要的岩石之一，含有经济上重要的矿石矿物。它们的形成通常被认为是来自地幔来源。尽管进行了深入的研究，但原始地幔熔体的组成和最终的碳酸盐岩成因仍然存在争议。赋存于不同碳酸盐矿物中的流体和熔体包裹体提供了有关圈闭过程中流体和熔体成分的独特信息，但由于圈闭后的修改和对通常复杂的多组分化学体系的热力学相关系缺乏了解，它们的解释很复杂。在本项目中，我们将使用分子模拟方法研究在相应的压力和温度条件下演化的碳酸盐岩卤水熔体和相关热液的分子结构和热力学性质。基于现有的反应力场，将对H2O-CO2-Na2CO3-NaCl模型体系的颗粒相互作用势进行参数化，该模型体系允许在从高盐度、高密度、碱性碳酸盐岩流体到低密度、富CO2水溶液的组成范围内进行模拟，这些流体在伴生的流体包裹体中观察到。物种形成将直接来自分子动力学模拟和/或先进的自由能采样方法，如热力学积分。结果将通过参考观测和从头计算模拟来确定基准。为了与Dome计划第一个资助期正在进行的项目的结果相联系，我们将在碳酸盐体系中添加稀土元素(REE)La和Y，以研究稀土元素与碳酸盐配位体的可能形态。通过这个项目，我们希望对晚期碳酸盐岩熔体的演化有新的了解，在此期间，不相容的和经济上重要的元素，如稀土，预计将显著浓缩。

Carbonatites are among the most important rocks that contain economically important ore minerals. Their formation is usually assumed to arise from mantle sources. Despite intensive research, primary mantle melt compositions and ultimate carbonatite genesis are still controversial. Fluid and melt inclusions hosted in different carbonatite minerals provide unique information about fluid and melt compositions during entrapment but their interpretation is complicated due to post-entrapment modifications and insufficient knowledge of the thermodynamic phase relations of the usually complex multicomponent chemical systems. In this project, we will use molecular simulation methods to study the molecular structure and thermodynamic properties of evolved carbonatitic brine-melts and related hydrothermal fluids at relevant pressure and temperature conditions. Based on existing reactive force fields, the particle interaction potential will be parameterized for the model system H2O-CO2-Na2CO3-NaCl, which allows simulation in a composition range from high-salinity, high-density, alkali carbonatite fluids to low density, aqueous CO2-rich fluids, which are observed in associated fluid inclusions. The speciation will be derived directly from molecular dynamics simulations and/or from advanced free energy sampling methods such as thermodynamic integration. Results will be benchmarked by reference to observations and ab initio simulations. To connect to results from the ongoing project of the first funding period of the DOME program, we will add the rare earth elements (REE) La and Y to the carbonatitic system to investigate possible REE speciation with carbonate ligands. With this project, we expect to shed new light on the evolution of late-stage carbonatitic melts, during which significant enrichment of incompatible and economically important elements such as the REEs is expected.