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The seismic signature of serpentinite in subduction zones: A rock physics approach

俯冲带蛇纹岩的地震特征：岩石物理方法

基本信息

批准号：
NE/M015629/1
负责人：
Lars Norman Hansen
金额：
$ 6.72万
依托单位：
University of Oxford
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2016
资助国家：
英国
起止时间：
2016 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=NE%2FM015629%2F1
关键词：
seismic signature serpentinite subduction zones

项目摘要

Serpentinites are rocks that contain a significant proportion of serpentines, which form by hydrothermal alteration of basic silicates (e.g., olivine). These rocks form primarily in the upper oceanic crust, due to hydrothermal circulation of oceanic water along the mid-oceanic ridges where new oceanic crust is generated. As a consequence, the oceanic crust that enters subduction zones is thought to be serpentinised extensively, at least in its upper part. The presence of serpentinite near the subduction interface is expected to have a key influence on subduction zone dynamics, because serpentine minerals have peculiar mechanical and physical properties: they are very weak compared to other crustal and mantle rocks, and they dehydrate (i.e., undergo chemical transformations and release free water) upon heating. The latter effect has dramatic consequences on the effective stress state in the subducting slab, and is thought to play a fundamental role in the generation of slow slip events, intermediate-depth earthquakes, arc volcanism, and water recycling in the mantle.The exact role of serpentinites in subdcution processes is however difficult to quantify precisely since the exact location and amount of serpentine minerals in subduction zones remains poorly known. In order to test whether serpentinites are indeed responsible for the aforementioned features of subduction zones, it is of primary importance to be able to demonstrate their presence or absence at depth. Seismic imaging is the most robust observational constraint available, but the precise identification of serpentinites using seismic methods is difficult. Significant progress has been achieved in the determination of the elastic properties and seismic speeds of serpentine (antigorite, lizardite) single crystals. However, the deformation and dehydration of serpentinites has been shown to systematically induce significant cracking. The microcracks generated by deformation and dehydration may well remain open at depth in subdcution zones, at least temporarily, due to the elevated fluid pressures arising from dehydration itself and buoyancy-driven fluid migration. Microcracking can potentially have strong, first order effects on seismic properties and anisotropy, but remains poorly quantified in serpentinites. In this project we propose to dramatically improve our ability to link seismic observables to the presence of serpentinite by (1) experimentally measure the seismic properties of serpentinites during deformation and dehydration, (2) quantify the microstructural evolution and the relationships between microcrack orientation and crystallographic preferred orientation, and (3) model the effects of microcracks on seismic wave speeds using effective medium approaches. Our study is expected to provide a robust chracterisation of the seismic signature of deformed and dehydrating serpentinites, and thus have a direct impact on the intrepretation of seismic images. In addition, our data will contribute to a better understanding of the deformation and dehydration mechanisms that are key aspects of subduction zone dynamics.

蛇纹岩是含有相当大比例的蛇纹岩的岩石，蛇纹岩是通过基性硅酸盐的热液蚀变形成的（例如，橄榄石）。这些岩石主要形成于上层洋壳，这是由于大洋水沿着洋中脊的热液循环，在那里产生了新的洋壳。因此，进入俯冲带的洋壳被认为是广泛的蛇纹化，至少在其上部。俯冲界面附近蛇纹岩的存在预计将对俯冲带动力学产生关键影响，因为蛇纹岩矿物具有特殊的机械和物理性质：与其他地壳和地幔岩石相比，它们非常脆弱，并且它们具有（即，在加热时发生化学转化并释放游离水）。后一种效应对俯冲板片的有效应力状态有显著影响，并被认为在慢滑动事件、中深地震、弧火山作用和地幔中水循环的产生中起着重要作用。然而，由于俯冲带中蛇纹石矿物的确切位置和数量仍然知之甚少，因此很难精确量化蛇纹岩在俯冲过程中的确切作用。为了检验蛇纹岩是否确实造成了俯冲带的上述特征，能够证明它们在深度上的存在或不存在是至关重要的。地震成像是目前最可靠的观测手段，但利用地震方法对蛇纹岩进行精确识别是困难的。蛇纹岩（叶蛇纹石、利蛇纹石）单晶体弹性性质和地震速度的测定取得了重大进展。然而，蛇纹岩的变形和脱水已被证明系统地诱导显着的开裂。由于脱水本身和浮力驱动的流体迁移引起的流体压力升高，变形和脱水产生的微裂纹可能在俯冲带的深度处至少暂时保持开放。微裂纹可能对地震属性和各向异性产生强烈的一阶效应，但在蛇纹岩中仍然难以量化。在这个项目中，我们建议通过（1）实验测量蛇纹岩在变形和脱水过程中的地震特性，（2）量化微结构演化以及微裂纹取向和晶体学择优取向之间的关系，（3）用有效介质方法模拟微裂纹对地震波速度的影响。我们的研究有望提供一个强大的变形和脱水蛇纹岩的地震特征，从而对地震图像的解释有直接的影响。此外，我们的数据将有助于更好地了解变形和脱水机制，这是俯冲带动力学的关键方面。