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

蛇纹岩是一种含有大量蛇纹岩的岩石，由基性硅酸盐（如橄榄石）热液蚀变而形成。这些岩石主要形成于大洋上部地壳，这是由于大洋中脊的海水热液循环，在那里产生了新的大洋地壳。因此，进入俯冲带的海洋地壳被认为是广泛的蛇纹岩，至少在其上部是如此。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，它们非常弱，并且在加热时脱水（即发生化学转化并释放游离水）。后一种作用对俯冲板块的有效应力状态有显著影响，并被认为在慢滑事件、中深地震、弧火山活动和地幔水循环的产生中起着根本作用。然而，蛇纹岩在俯冲过程中的确切作用很难精确量化，因为在俯冲带中蛇纹岩矿物的确切位置和数量仍然知之甚少。为了检验蛇纹岩是否确实造成了上述俯冲带的特征，能够在深度上证明蛇纹岩的存在与否是最重要的。地震成像是最可靠的观测约束，但利用地震方法精确识别蛇纹岩是困难的。在测定蛇纹石（反长辉石、蜥蜴石）单晶的弹性特性和地震速度方面取得了重大进展。然而，蛇纹岩的变形和脱水已被证明会系统性地诱发显著的开裂。由于脱水本身和浮力驱动的流体运移引起的流体压力升高，变形和脱水产生的微裂缝很可能在俯冲带深处保持开放状态，至少是暂时的。微裂缝可能会对地震性质和各向异性产生强烈的一阶影响，但在蛇纹岩中仍然很难量化。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)量化微观结构演变以及微裂纹取向与晶体学首选取向之间的关系，以及(3)利用有效介质方法模拟微裂纹对地震波速度的影响，从而显著提高我们将地震观测结果与蛇纹岩存在联系起来的能力。我们的研究有望为变形和脱水蛇纹岩的地震特征提供一个强有力的特征，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变形和脱水机制，这是俯冲带动力学的关键方面。

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.