高压下温度对铁的声速的影响

The composition and motion state of the core are the keys to geophysical and geochemical modeling, because they are strongly coupled with many major events that happened in deep Earth, such as the mantle plume, the evolution of the geodynamo. According to the high pressure experiments, geophysical, geochemical researches, the main component of earth's core is iron alloyed with other light elements. However, the identity and amounts of the light elements are still unclear. To solve above controversy, the ultimate method is to measure the densities and sound velocities of core material under high pressure and temperature, and compare them with the values of the core. The major premise is to clear the sound velocity of iron under high temperature and pressure conditions. However, there are few reports on the sound velocities of iron under high pressure and temperature. And the temperature effect on the sound velocities of iron is still unclear. Using dynamic compression technique, we plan to adopt new diagnostic techniques, such as the reverse impact technique and improved optical analyzer technique, to measure the longitudinal and shear sound velocity of iron under high pressure and high temperature simultaneously. Comparing them with existing data at room temperature, we can theoretically model the effect of temperature on sound velocity of iron under high pressure. Then the estimated sound velocities of iron under inner core condition will provide the major data constraining the identity and amounts of light elements in the core.

地核物质组成、运动状态与地幔热柱、地磁场运行的机制等发生在地球深部的重大地球物理事件密切相关，是地球物理和地球化学建模的关键。根据高压实验、地球物理、地球化学等多方面的研究，地核的主要成分是铁，同时含有少量的轻元素。然而有关轻元素的种类和含量至今仍存在大量争议。将地核候选物质在高温高压下的密度、声速与地震波探测结果相比较，是约束轻元素的最终方法。因此厘清铁在高温高压下的声速是限定地核中轻元素种类和含量的前提。然而高温高压下铁的声速报道非常少，并且温度对铁的纵波、剪切波声速的影响存在争议。本项目拟利用动高压实验装置，采用反向碰撞技术和改进的光分析技术等新的诊断技术，研究高温高压环境下铁的纵波、剪切波声速，并与室温下的实验数据对比，建立理论模型分析高压下温度对声速的影响。将上述理论模型应用于内地核高温高压条件，估算铁的纵波声速与剪切波声速，为限定轻元素的种类和含量提供重要的支持。

地核主要是由铁和原子量小于铁的轻元素组成。虽然轻元素的含量很少，但是却对外地核释放的热流、固体内地核的生长速率、地球磁流体发电机的运行机制等有重要影响。但是有关地核中轻元素的种类和含量一直存在争议。解决上述争议的有效方法之一是将地球物理和地球化学所给出的地核组分模型与地震波数据比较。但是有关高温高压下铁的声速数据存在争议。本项目利用动高压实验装置——二级气体炮，研究了高温高压下铁的纵波声速、横波声速和泊松比，并和室温高压下的实验数据比较，探索温度对铁的声速和泊松比的影响。发现在相同密度下,高温下铁的纵波声速与密度依然满足线性关系，并且和室温下测量结果一致，说明在高温下铁的纵波声速依然满足Birch定律。与室温下测量的横波声速相比，高温下横波声速随温度升高而减小；在等温条件下横波声速温度系数与密度呈指数函数。与室温下测量结果相比，在相同密度下泊松比随温度升高而升高，在等温条件下其温度系数与密度亦呈指数关系。在内地核温度和压强环境下，铁的密度、声速和泊松比与地震波数据有明显差异，而温度影响和Si元素的掺入并不能完全解决这些问题。结合静高压Fe-C体系声速的实验结果，根据混合物法则计算发现Fe-5wt%C-1wt%Si的密度、纵波声速、横波声速和泊松比与内地核的相关数据相匹配。即内地核有可能是Fe-C-Si体系，但是还需要高温高压实验进一步验证。