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

蛇纹岩是含有大量蛇纹石的岩石，蛇纹石是由碱性硅酸盐(如橄榄石)的水热蚀变形成的。这些岩石主要形成于洋壳上部，这是由于海水沿着大洋中脊的热液循环而形成的，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岩，至少在其上部是这样。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，因为蛇纹石矿物具有特殊的力学和物理性质：与其他地壳和地幔岩石相比，它们非常弱，而且它们在受热时脱水(即经历化学转化并释放自由水)。后一种效应对俯冲板的有效应力状态有很大的影响，并被认为在地幔的慢滑事件、中深地震、弧火山作用和水循环中发挥了基础性作用。然而，蛇纹岩在俯冲过程中的确切作用很难准确量化，因为蛇纹石矿物在俯冲带中的确切位置和数量仍然鲜为人知。为了测试蛇纹岩是否真的对俯冲带的上述特征负责，最重要的是能够在深部证明它们的存在或不存在。地震成像是现有的最可靠的观测约束，但使用地震方法精确识别蛇纹岩是困难的。蛇纹石(蛇纹石、文石)单晶的弹性性质和地震速度的测定已取得重大进展。然而，蛇纹岩的变形和脱水已被证明系统地导致显著的破裂。变形和脱水产生的微裂缝很可能在俯冲带的深处保持开放，至少是暂时的，这是因为脱水本身引起的流体压力升高和浮力驱动的流体迁移。微破裂可能会对地震性质和各向异性产生强烈的一阶影响，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性质，(2)量化微结构演变和微裂纹取向与晶体择优取向之间的关系，以及(3)使用有效介质方法模拟微裂纹对地震波速的影响，来显著提高我们将地震观测数据与蛇纹岩的存在联系起来的能力。我们的研究有望对变形和脱水的蛇纹岩的地震特征提供可靠的表征，从而对地震图像的解释产生直接影响。此外，我们的数据将有助于更好地理解俯冲带动力学的关键方面--变形和脱水机制。

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 subduction 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 subduction 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 characterisation of the seismic signature of deformed and dehydrating serpentinites, and thus have a direct impact on the interpretation 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.