摘要
橄榄石在不同热动力学条件下形成的晶格优选定向是认识上地幔塑性变形与地震波各向异性的基础。本文通过总结橄榄石的晶格优选定向、含水量与地震波各向异性的研究进展,探讨大陆俯冲带的变形环境。绝大部分天然变形橄榄岩中的橄榄石都发育了[100](010)组构:[100]轴近平行于线理,(010)面平行或近平行于面理,使橄榄岩的最快P波速度近平行于线理,最大S波分裂平行面理并垂直线理。但来自超高压变质带的石榴石橄榄岩可发育[001](100)组构,使橄榄岩的最快P波速度和最小S波分裂方向垂直面理。近年来的变形实验与理论计算表明:超高压和低温是橄榄石组构从[100](010)向[001](100)转变的关键因素,而水对橄榄石流变行为的影响还有待进一步研究。对170个天然橄榄石结构水含量的统计结果表明:橄榄石含水量变化很大(0～170×10^(-6)H_2O),玄武岩中橄榄岩包体的橄榄石比较贫水,而所有富水橄榄石均来自克拉通金伯利岩中的石榴石橄榄岩。因此,除了水在上地幔的不均匀分布,橄榄岩折返过程中氢的扩散会强烈影响橄榄石的含水量。苏鲁芝麻房橄榄岩提供了以超高压、低温、贫流体为特征的大陆俯冲带中[001](100)橄榄石组构的实例。在俯冲的大陆板片中,橄榄石组构在120～220 km从[100](010)向[001](100)的转变可导致地震波各向异性突然降低,而且最快P波速度垂直于俯冲方向。
The lattice-preferred orientation (LPO) of olivine under different thermal-mechanical conditions is critical to understand ductile deformation and seismic anisotropy of the upper mantle. This paper reviews the latest research progress in the LPO, water content and seismic anisotropy of olivine, which provides an opportunity to trace deformation environment of continental subduction zones. Most naturally deformed peridotites develop [100] (010) olivine fabric, which is characterized by the [100] axis of olivine subparallel to the lineation and the (010) plane parallel to the foliation. As a result, the fastest P-wave velocity of peridotites is parallel to the lineation and the maximum S-wave splitting parallel to the foliation but normal to the lineation. However, some garnet peridotites from the ultrahigh-pressure metamorphic terranes display [001] (100) olivine fabric and have the fastest P-wave velocity and the minimum S-wave splitting normal to the foliation. Recent deformation experiments and theoretical calculation suggest that ultrahigh pressure and/or low temperature can produce a fabric transition in olivine from [100] (010) to [001] (100), while the effects of water on olivine rheological behavior needs further study. Statistical analysis of water contents in 170 natural olivine samples reveals a wide variation of 0 similar to 170 x 10(-6) H(2)O. Olivine from peridotite xenoliths in basalts contains relatively low water content, while all water-rich olivine crystals are from garnet peridotite xenoliths in craton kimberlites, implying that besides the heterogeneous water distribution in the upper mantle, hydrogen diffusion during transport of peridotites can significantly affect the water concentration in olivine. The Zhimafang garnet peridotite in the Sulu terrane provides a natural example of [001] (100) olivine fabric under the ultrahigh-pressure, low temperature and low fluid activity of continental subduction zones. In a subducted continental slab, the change of olivine LPO from [100] (010) to [001] (100) between 120 similar to 220 km may lead to a decrease of seismic anisotropy and the fastest P-wave velocity normal to the subduction direction.
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2007年第12期3065-3077,共13页
Acta Petrologica Sinica
基金
国家自然科学基金委青年基金项目(编号40502022)
重大项目(编号40399140)
重点项目(编号40634021)资助
关键词
橄榄石
晶格优选定向
地震波各向异性
结构水
苏鲁
大陆俯冲带
olivine
lattice-preferred orientation
seismic anisotropy
structural water
Sulu
continental subduction zone
