铁对D″层铁方镁石矿中弹性波影响机理的模拟研究及对该矿组分的约束意义

Ferropericlase ((Mg, Fe) O) is an important mineral in the D" layers. But its chemical composition is not yet clear, the effective method we can use to study the mineral composition are: we can preliminarily constrain its component by experimental or theoretical; we first get its elastic waves by experiment or theory, and then we further constrain the composition by comparing with the observed seismic wave. Recent studies show that ferropericlase may provide an important contribution to the seismic shear-wave anisotropy and ultra-low velocity zones in the D" layer. High temperature and high-pressure laboratory are limited. Experimental temperature and pressure are difficult to measure accurately. And less of them can reach D" layer conditions. Few studies on the effects of iron on the elastic wave of ferropericlase. And the microscopic mechanism is not clear. The first-principles simulation can address these problems easily. We intend to use the first principles to calculate the entropies of ferropericlase in the D" layer. According to the minimum entropy principle, we can preliminarily constrain its chemical composition; we can study the impact mechanism of the content and the valence of the iron on the elastic waves. We also can calculate the elastic wave of ferropericlase in the D" layer. We can deduct the contribution of other components of seismic waves reasonably. We further constrain the chemical composition of ferropericlase by experiment and seismic observation of the Western Pacific. Our project not only can effectively reduce the uncertainty of the chemical composition of the ferropericlase, but also can provide a basis and reference to related researches.

铁方镁石矿((Mg,Fe)O)是地球深部D″层的重要矿物，但其化学组分尚不十分清楚。研究该矿组分的有效手段是：实验或理论对其组分初步约束；实验测定或理论计算其中的弹性波，结合地震波观测结果做进一步约束。最近研究表明：铁方镁石可能对D″层中地震剪切波各向异性和超低速区有重要贡献。高温高压实验室较少，实验中温度和压强不易测准，且较少达到D″层条件，铁对铁方镁石矿弹性波影响的研究较少，微观机理不大清楚，第一性原理模拟可以方便地解决这些问题。我们拟利用第一性原理计算D″层中的铁方镁石矿的熵，根据熵最小原理对该矿的化学组分进行初步约束；研究铁的含量及价态对该矿弹性波的影响机理，计算D″层铁方镁石矿中的弹性波，合理扣除其他成分对D″层地震波异常的贡献，结合实验，并以西太平洋地震波观测结果为例，对D″层铁方镁石矿中的化学组分做进一步约束。本项目可以有效减少该矿组分的不确定性，还可为相关研究提供一定参考。

本项目背景：方镁石、铁方镁石是地球内部的主要成分，在地球内部，特别是下地幔（D″层）中分布广泛，对地球的形成、演化具有重要意义，但铁在其中的分布形态难以确定。一般来说，主要采用地震波观测、来自下地幔金刚石包裹体中的铁方镁石的微区分析、光学性质等，反推铁方镁石在D″层的分布形式，通过对其地震波、光学性质的研究，反推铁在其中的分布，铁方镁石在地球形成，演化过程中所扮演的角色。. 本项目的主要研究内容是通过计算机模拟的方法，利用熵最小原理和Cluster expansion的方法，找出铁在铁方镁石中的分布，研究出铁对其弹性波，光学性质的影响等，结合地震波观测数据，高温高压实验结果，对铁方镁石在D″层的分布提供约束。通过本项目的研究，研究出铁在MgO中的分布，铁对MgO的弹性波、光学性质等影响，还带动了类似相关矿物材料如红锌矿，红柱石等研究。通过对D″层铁方镁石的约束，本项目对D″层的其他相关研究（如热性质和磁性等）提供一定的参考价值。