铁方镁石中金属铁生成的合金反应实验及其地球物理涵义

Alloy reaction experiment of metal iron formation in ferropericlase and its geophysical implications

研究下地幔物质组成和性质是地球动力学的重要课题.铁方镁石(Mg_(0.8)Fe_(0.2)O)是下地幔中的主要组成矿物之一,在适当的温度、压强和氧逸度条件下铁方镁石中的二价铁可以通过化学还原反应得到电子形成0价的金属铁.如果有金属生成,则它对下地幔的环境和矿物的物理化学性质具有重要的影响,金属铁会更容易捕获水而降低下地幔中名义无水矿物熔点导致出现部分熔融,金属铁的出现还会影响C、N、H等元素的分布.而实际下地幔中具有等诸多金属元素,会与形成的金属铁形成合金.目前没有关于金属铁形成的合金反应的公开研究,因此本文进行高温高压实验研究铁方镁石中的金属铁生成以及伴随的合金反应,实验温度范围为1573~1773 K,压强范围0~23 GPa.并对高温高压实验后回收的样品进行扫描电镜分析和电子探针分析,通过分析发现采用的钛、铬和碳化钛控制氧逸度的材料会在高温高压实验过程中与铁方镁石形成的金属铁发生合金反应,实验中所有生成的金属铁均以合金的形式存在且合金中的铁含量分布不均匀,同时在样品区域和氧逸度控制材料中出现了反应区,反应区主要成分为氧化镁和控制氧逸度的金属材料氧化物反应得到的尖晶石相矿物.从本文的实验看来采用金属材料控制氧逸度不可避免与铁方镁石产生的金属铁发生反应,需要根据合金的成分制定新的氧逸度计算方法或者采用其他氧逸度控制材料.

It’s an important subject of geodynamics to study the composition and properties of lower mantle materials.Ferropericlase is one of the main constituent minerals in the lower mantle.Under appropriate temperature,pressure,and oxygen fugacity,the ferrous iron of Ferropericlase can be reduced into zero-valent metallic iron by a chemical reduction reaction.If metallic iron is formed,it has an important impact on the environment of the lower mantle and the physical and chemical properties of minerals.Metallic iron will be easier to capture water and reduce the melting point of anhydrous minerals in the lower mantle,resulting in partial melting.The appearance of metallic iron will also affect the distribution of C,N,H,and other elements.There are many metal elements in the lower mantle,which will form alloys with the formed metal iron.At present,there is no public research on the alloy reaction to metallic iron formation.Thus,this paper carries out high-temperature and high-pressure experiments to study the metal iron formation and accompanying alloy reaction in ferropericlase.The experimental temperature range is 1573~1773 K and the pressure range is 0~23GPa.The samples recovered after the high temperature and highpressure experiment were analyzed by scanning electron microscope and electron probe microanalysis.According to the analysis,the titanium,chromium,and titanium carbide would react with the metallic iron formed from ferropericlase during the high temperature and high-pressure experiment.All the generated metallic iron existed as alloy after the experiment and the iron content of the alloy was heterogeneous.At the same time,there is a reaction zone between the sample area and the oxygen fugacity buffer.The main component of the reaction zone is spinel minerals obtained by the reaction to magnesium oxide and metal material oxide controlling oxygen fugacity.During the experiment,the metal-oxygen fugacity buffer will react with the metallic iron produced from ferropericlase.It’s necessary to find a new calculation method of oxygen fugacity according to the composition of the alloy or use other oxygen fugacity buffers.