What is the Relationship Between Mineral Fabric, Seismic Anisotropy, and Mantle Flow in Partially Molten Regions? The Ethiopian Rift as a Testing Ground

地震各向异性是检验地幔变形和流动模型最广泛使用的工具之一。它的广泛使用是因为地幔流动的方向和地震快方向之间明显简单的对应，这是因为地幔橄榄岩中的橄榄石晶体在变形时产生的共格组构。然而，最近的实验室研究令人怀疑，当地幔岩石在少量水或熔体存在的情况下受到应变时，流动与快速波速方向之间的这种关系。在水和熔体存在下对橄榄石岩石的变形研究表明，地震快速方向可以从剪切方向旋转到90度，而不是通常假设的平行于剪切方向。这些观测可能会颠覆我们在熔体丰富地区的地幔流动模型，但到目前为止，这些观测只是来自实验室实验，还没有证明它们与地幔规模和/或条件有关。我们将测试部分熔融和/或水可能在多大程度上控制熔体丰富地区的地幔组构，特别是埃塞俄比亚的东非裂谷。这个位置非常适合进行这次实验，因为明显的熔体富集区被很好地界定了，而且存在着丰富的地震数据。然而，目前尚不清楚现有数据是否具有足够的各向异性空间分辨率，以区分相互竞争的模型。在这项为期一年的概念验证研究中，我们将进行地震面波分析，并测试几个简单的地幔组构和流动模型。我们的主要目的是确定裂缝下的各向异性是否表明在干燥、无熔融的样品中发现的“标准”面料，或者观察到的各向异性是否与在富熔体或潮湿条件下观察到的变形面料更一致。在这个过程中，我们将极大地提高我们对该地区产生伸展和火山作用的地幔动力学过程的理解。在更广泛的影响中，这项工作将为LDEO博士后研究员Ben Holtzman提供地震学培训，他是该项目的联合PI。此外，我们将与布里斯托尔大学的迈克尔·肯德尔教授和利兹大学的格雷厄姆·斯图尔特博士合作。

Seismic anisotropy is one of the most widely utilized tools for testing models of mantle deformation and flow. Its widespread use results from the apparently simple correspondence between the orientation of mantle flow and seismic fast-direction, due to the coherent fabric that is produced in olivine crystals in mantle peridotites when they deform. However, recent laboratory studies have cast doubt on this relationship between flow and fast-wavespeed directions when mantle rocks are strained in the presence of a small amount of water or melt. Deformation studies of olivine rocks in the presence of water and melt suggest that the seismic fast direction can rotate to 90 degrees from the shear direction, rather than parallel to the shear direction as is commonly assumed. These observations potentially turn on their heads our models of mantle flow in melt-rich regions, but to date the observations are only from laboratory experiments, and it has not been demonstrated that they are relevant to mantle scales and/or conditions. We will test the degree to which partial melt and/or water may control the mantle fabric in a melt-rich region, specifically the East African Rift in Ethiopia. This location is ideal for this experiment, because the apparently melt-rich region is very well defined, and abundant seismic data exists. However, it is unclear whether the existing data have sufficient spatial resolution of anisotropy to differentiate between competing models. In this one-year proof-of-concept study, we will perform the seismic surface-wave analysis and test several simple models of mantle fabric and flow. Our primary aim is to determine wither the anisotropy beneath the rift is indicative of the "standard" fabric found in dry, melt-free samples, or whether the observed anisotropy is more consistent with deformation fabrics observed under melt-rich or wet conditions. In the process, we will greatly improve our understanding of the mantle dynamic processes that are producing extension and volcanism in this region. Among the broader impacts are that the work will provide training in seismology for LDEO Post-Doctoral Fellow Ben Holtzman, who is serving as a co-PI on the project. In addition, we will be collaborating with Prof. Michael Kendall of the University of Bristol, and Dr. Graham Stuart of University of Leeds.