化学、矿物学及温度变化对下地幔密度和体波波速的影响

发布时间：2018-01-28 20:33

本文关键词： 下地幔底部密度体波波速化学不均一性温度　出处：《中国科学技术大学》2017年硕士论文　论文类型：学位论文

【摘要】：下地幔底部是地球内部最为复杂的区域之一。在过去的几十年里,地震学在下地幔底部研究观测到了许多波速异常,比如剪切波的各向异性的增强、核幔边界上方的超低速带异常、大剪切波低速异常体中密度(体波波速)和横波波速反相关等等。为了更好的理解下地幔波速异常的形成机制,在本研究中我们利用高温高压矿物学实验获得的下地幔主要构成矿物的热力学状态方程,包括:布里奇曼石、铁方镁石、硅酸钙钙钛矿以及后钙钛矿,建立了下地幔密度及体波波速的模型。需要指出的是,不同实验组确定的热力学状态方程来自不同的压标,致使无法在同一标准下获得成分变化对热力学参数的影响。因此,我们首先采用Fei et al(2007)所提出的统一自洽压标,对来自不同实验组的结果进行压标统一,并由此对实验结果进行重新分析,以便更准确的获得成分变化对下地幔矿物热力学状态参数的影响。利用新得到的下地幔矿物热力学参数,我们建立了在不同的化学、矿物组成及温度条件下下地幔底部的密度和体波波速模型。结果显示化学、矿物学及温度变化对下地幔模型有不同的地球物理学影响。其中,铁元素和铝元素的富集会增加下地幔密度,但是会极大的降低下地幔体波波速。我们将下地幔底部划分为:俯冲板片区,热柱区,以及平均地幔区。与平均地幔相比,俯冲板片由于其温度较低以及在较浅区域后钙钛矿的提前出现,密度会比平均地幔要高。而温度较高区域的密度则会比平均地幔小1-2%。当布里奇曼石是平均地幔的主要矿物时,俯冲板片以及较热E域的体波波速都会比平均地幔要小。但是一旦在平均地幔中布里奇曼石相变为后钙钛矿后,俯冲板片以及较热区域的体波波速则高于平均地幔。我们的模型还显示了,当核幔边界上方有富含铁、铝元素的布里奇曼石热堆的出现时,下地幔的密度和体波波速都会增加,而剪切波速会随之减少,这与地震学观测到的大剪切波低速异常体的异常相一致。这种富含铁、铝元素的布里奇曼石热堆有助于理解大剪切波低速异常体的出现。因此,本项研究结果能够帮助我们更好的理解下地幔的结构及化学性质的异常。
[Abstract]:The bottom of the lower mantle is one of the most complex regions in the earth's interior. Over the past few decades, seismology has observed many wave velocity anomalies at the bottom of the lower mantle, such as the enhancement of anisotropy of shear waves. In order to better understand the formation mechanism of mantle velocity anomaly, the ultra-low velocity zone anomaly over the core and mantle boundary, the density (bulk wave velocity) and the shear wave velocity inverse correlation between the large shear wave low velocity anomaly and so on. In this study, the thermodynamics equation of state of the lower mantle was obtained by using the high temperature and high pressure mineralogical experiments, including Bridgman, ferroperovskite, calcium silicate perovskite and post-perovskite. The model of density and body wave velocity of the lower mantle is established. It should be pointed out that the thermodynamic equation of state determined by different experimental groups comes from different pressure scales. As a result, it is impossible to obtain the influence of composition change on thermodynamic parameters under the same standard. Therefore, we first adopt the uniform self-consistent pressure scale proposed by Fei et al 2007). The results from different experimental groups were unified, and the experimental results were analyzed again. In order to obtain more accurately the influence of composition changes on the thermodynamic state parameters of the lower mantle minerals, we have established different chemical parameters using the new thermodynamic parameters of the lower mantle minerals. The density and bulk wave velocity model at the bottom of the lower mantle under mineral composition and temperature conditions. The results show that chemical mineralogical and temperature changes have different geophysical effects on the lower mantle model. The enrichment of Fe and Al can increase the density of the lower mantle, but decrease the wave velocity of the lower mantle greatly. We divide the bottom of the lower mantle into subduction plate region and hot plume area. Compared with the average mantle, the subduction plate plate appeared earlier because of its lower temperature and earlier occurrence of perovskite in the shallow region. The density will be higher than the average mantle, and the density of the higher temperature region will be 1-2 less than the average mantle. When Bridgmanite is the main mineral of the average mantle. Both the subduction plate and the hot E domain have smaller body wave velocities than the average mantle, but once the Bridgman phase in the average mantle becomes post-perovskite. The body wave velocities of subduction plates and hot regions are higher than the average mantle. Our model also shows that when there is an iron-rich and aluminum-rich Bridgman hot pile at the upper boundary of the core and mantle. The density and bulk wave velocity of the lower mantle will increase, but the shear wave velocity will decrease, which is consistent with the anomaly of large shear wave and low velocity anomaly observed by seismology, which is rich in iron. The Bridgman thermal reactor of aluminum element is helpful to understand the appearance of large shear wave low velocity anomaly. Therefore, this study can help us to better understand the anomalies of the structure and chemical properties of the lower mantle.
【学位授予单位】：中国科学技术大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：P542.5

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