蛇纹石化作用对超镁铁岩地震波性质的影响及意义

Previous interpretation of seismic data in terms of lithology and composition in the upper mantle was essentially made by comparison of in-situ seismic velocities with laboratory-measured properties of ultramafic rock samples, ignored the influence of serpentinization. As the main hydrous mineral in the upper mantle, serpentine has distinct physical properties, such as very low P- and S-wave velocities, high seismic anisotropy and shear wave splitting. Serpentinization could significantly change the seismic properties of ultramafic rocks. In this item, a detailed study of seismic properties will be carried out on serpentinized peridotite collected from Songshugou area (Shangnan, Shaanxi) in the Eastern Qinling. Measurements of P- and S-wave velocities and their directional dependence (P-wave anisotropy and shear wave splitting) will be performed at hydrostatic pressure ranging 10-800 MPa using the pulse transmission technique. Electron back-scattering diffraction (EBSD) will be applied to the measurements of the lattice preferred orientations of olivine and serpentine. The research objective is to explore the quantitative relationships between serpentinization (the forms and content of serpentine, serpentine LPO, etc.) and seismic properties of ultramafic rocks (P- and S-wave velocites, seismic anisotropy, shear wave splitting, Poisson's ratio，elastic moduli, etc.), to provide necessary experiment evidence and theory constraints of seismic interpretation of the upper mantle, and to advance the quantitative research of mantle lithosphere.

过去对于上地幔地震波资料的解释大多建立在超镁铁岩高压物性实验数据的基础上，而忽略了蛇纹石化作用的影响。蛇纹石作为上地幔最主要的水化矿物，具有很低的地震波速、高的波速各向异性和剪切波分裂，能够显著地改变超镁铁岩的地震波性质。本项目拟开展陕西商南县东秦岭松树沟的蛇纹石化橄榄岩的地震波性质研究工作，以高压岩石物性实验和电子背散射衍射技术（EBSD）为主要实验手段，试图建立蛇纹石化作用（蛇纹石类型、含量，晶格优选定向等）与超镁铁岩的地震波性质（纵波和横波速度、各向异性、剪切波分裂、泊松比、弹性模量等）的定量关系，为上地幔地震资料的正确解释提供必不可少的、定量的实验支撑和理论约束，推动岩石圈地幔成分、结构、流变学和动力学研究的定量化发展。

地震波资料是了解上地幔物质和结构的最重要的途径，而对于地震波数据的解释是建立在超镁铁岩地震波性质的基础上的。蛇纹石作为上地幔最主要的水化矿物，具有很低的地震波速和高波速各向异性，能够显著地改变超镁铁岩的地震波性质。本项目以陕西省商南县松树沟超镁铁岩和西藏自治区曲松县罗布莎超镁铁岩为研究对象，主要研究蛇纹石化作用、矿物晶格优选定向和熔-岩反应对地幔岩地震波性质的影响。组构测试显示，松树沟超镁铁岩中存在橄榄石的A型、B型和C型晶格优选定向（CPO），罗布莎超镁铁岩的橄榄石则主要是A型CPO。地幔岩的P波波速各向异性主要由橄榄石的组构控制：由(010)[100]位错滑移形成的A型组构导致Vp(X)>Vp(Y)>Vp(Z)。由于A型CPO是正常上地幔最流行的组构，天然橄榄岩样品主要都是这样的波速各向异性。B型组构会形成Vp(Y)>Vp(X)>Vp(Z)，C型组构会形成Vp(Z)>Vp(X)>Vp(Y), E型组构会形成 Vp(X)>Vp(Z)>Vp(Y)，而D型组构会导致 Vp(X)>Vp(Y)≈Vp(Z)。在高温塑性变形导致的橄榄石的CPO被“冻结”后，温度降低到~800-970 °C，松树沟和罗布莎的超基性岩都与玻安质熔体发生反应，溶解斜方辉石，生成橄榄石新晶。由于熔-岩反应生成的橄榄石没有发生位错蠕变，其晶格优选定向是随机的，所以，熔-岩反应会削弱橄榄石的CPO并降低地震波各向异性。在温度低于300 °C的环境中，地幔岩的水化会生成低温蛇纹石：利蛇纹石和纤蛇纹石。超镁铁岩的蛇纹石化作用遵从Birch定律，即波速与密度呈线性相关。超镁铁岩波速随低温蛇纹石含量的增加线性降低，其的经验公式为：Vp=-3.09ϕ+8.17和 Vs=-2.23ϕ+4.56。由于低温蛇纹石化在橄榄岩中形成了三维网状结构，地震波速在X、Y 和Z方向降低的程度相当，所以，低温蛇纹石化对超镁铁岩波速各向异性影响不大。本研究可以为上地幔地震资料的正确解释提供必不可少的、定量的实验支撑和理论约束，其成果已经在《Journal of Geophysical Research: Solid Earth》发表一篇论文，另一篇论文正在投稿中。