地幔中主要硫化物矿物热力学性质的第一性原理研究

Sulfides participant actively in the deep sulfur cycle of the Earth, and they are closely related to geodynamic evolution. Studying on the thermodynamic properties of sulfide minerals under high temperature and high pressure is important for understanding the related geophysical and geochemical processes in the Earth's interior. Moreover, studying on sulfide minerals shows such information of the physical properties, the dynamic process, the formation and evolution of the mantle, which cannot be shown in the study of the silicate and oxide minerals. However, due to the limitations of experimental conditions, previous experimental studies on thermodynamic properties of sulfide minerals are mainly under atmospheric pressure conditions; the theoretical studies under high temperature and pressure are also still far from sufficient, and studies of sulfide system which suitable for deep earth are very limited from the perspective of sulfide species. In this study, we calculate the lattice vibration properties of the major sulfide minerals in the mantle based on density functional perturbation theory. The calculated frequencies are used to determine the vibrational free energy within the quasi-harmonic approximation. Then through combining thermal electronic and magnetic contribution to the free energy, we study the thermodynamic properties of sulfides under high temperature and high pressure. The results from this study will make up the gaps and shortcomings of previous studies on mantle sulfides, enrich the thermodynamic database of sulfides, and provide more accurate constraints on understanding the properties of the Earth's interior and the distribution and role of sulfides in the mantle.

硫化物在地球深部的硫循环过程中非常活跃，同地球动力学演化密切相关。研究硫化物矿物在高温高压下的热力学性质，对于理解有硫参与的相关地球物理和地球化学过程非常重要，它所揭示的地幔物理性质、形成演化和动力学活动等信息，是研究地球内部广泛分布的硅酸盐矿物和氧化物矿物所无法替代的。然而由于实验条件的限制，关于硫化物热力学性质的实验研究主要集中在常压下，高温高压下的理论研究也十分有限，而且从研究的硫化物种类来看，适用于地球深部的硫化物体系的研究极少。本课题基于密度泛函微扰理论计算地幔中主要硫化物矿物的晶格振动性质，然后利用计算出的声子频率根据准谐近似确定晶格振动自由能，再结合热电子及磁性对自由能的贡献计算硫化物在高温高压下的热力学性质。本课题的研究结果能够弥补前人关于地幔硫化物研究的空白与不足，丰富硫化物热力学数据库，为了解硫化物在地幔中的分布与作用、地球内部性质提供更准确的约束。

地幔硫化物在岩浆演化、地幔部分熔融、地幔交代及地幔流体成矿等动力学过程中发挥着重要作用。研究地幔硫化物在高温高压下的热力学性质，对于理解有硫参与的地球深部动力学过程非常重要。然而由于实验条件的限制，关于硫化物热力学性质的实验研究主要集中在常压下，高温高压下的理论研究也十分有限，而且从研究的硫化物种类来看，适用于地球深部的硫化物体系的研究很少。鉴于此，本项目基于密度泛函微扰理论的第一性原理方法，采用准谐近似计算了地幔主要硫化物矿物如针硫镍矿、六方硫镍矿、辉镍矿、紫硫镍矿和黄铁矿等在高温高压下的热力学性质。我们计算出了不同压力下这些地幔主要硫化物矿物的晶体结构和振动性质，确定了密度、热膨胀系数、等容热容、等压热容、等温体积模量、绝热体积模量、熵等热力学参数与温度、压力变化之间的关系。这些数据的获得为人们开展地球深部的地球动力学模拟和建立地球物理模型提供了有用的信息，能够为了解硫化物在地幔中的分布与作用、地球内部性质提供更准确的约束。