Melt Network Geometry in Stressed, Partially Molten Mantle Rocks: Implications for Seismic Anisotropy

熔融和变形在本质上是紧密耦合的，它们协同工作以适应板块构造运动。这些运动与地震和火山爆发有关，而地震和火山爆发是人类住区的主要自然灾害之一。因此，重要的是我们要了解融化和变形在地球深处是如何相互作用的。由于我们无法直接观察到这种相互作用，我们依赖于各种遥感调查的解释。研究地球内部的一个强有力的方法是研究地震后在地球上传播的声波。波的传播速度以及它们的衰减取决于熔融和结晶矿物如何排列的细节。因此，这些信息可以用来探测部分熔融区域的空间范围以及它们在地球内部深处的流动方向。这位研究人员的研究旨在通过一系列实验室实验，限制控制熔体和矿物排列的基本物理原理，使部分熔融的岩石变形。由这项建议资助的研究将成为国际和平研究所实验室研究生的博士论文的基础，并将向学生介绍科学方法和思维。此外，还将进行模拟实验，以说明部分熔融对声波传播的影响。这项实验将用于PI教授的本科生和研究生课程，以及在剑桥科学节等活动期间向普通公众推广。熔融发生在地球上活跃的变形区域，如俯冲带和大洋中脊，并影响到地幔、地壳和大气之间的大部分化学交换。熔体的存在强烈影响部分熔融岩石的机械和运输性质，并可产生遥感可探测到的特征。板块边界的遥感为我们提供了关于驱动板块构造的基本力的丰富信息，并形成了检验地球动力学假说的基础。本研究的主要目的是阐明(I)变形对颗粒尺度熔体网络拓扑的影响以及(Ii)随熔体存在的变形对岩石圈地幔中最丰富的矿物橄榄石结晶择优取向(CPO)演化的影响。应力驱动熔体择优取向(MPO)和CPO在不同的长度和时间尺度上演化，对自然界中部分熔融岩石变形的综合地震特征做出了贡献。该团队将合成细粒橄榄石，加入约3vol%的大洋中脊玄武岩作为熔融相，并在上地幔典型的压力和温度条件下(T=900-1200摄氏度，P=0.3-2 GPA)在一般剪切条件下使这些岩石变形。在至少一个数量级的载荷水平下进行恒定载荷实验，以研究应力对颗粒尺度熔体网络拓扑的影响。在恒应力水平下，对不同的有限应变进行多次实验，以阐明应变对MPO和CPO的影响。进行这样一系列控制良好的综合实验将限制应力和应变对部分熔融地幔岩石中MPO和CPO发展的影响。所得到的定量微构造数据将被用于基于几种理论模型计算岩石的地震特征。在实验中建立测量的微观结构量和外力之间的相关性将为固-液复合材料变形的理论模型提供严格的约束，并将有助于将实验室数据外推到自然条件。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Melting and deformation are intimately coupled in nature and work in concert to accommodate plate tectonic movements. These movements are associated with earthquakes and volcanic eruptions which are one of the primary natural hazards to human settlements. It is therefore important that we understand how melt and deformation interact deep in our planet. Since we are unable to directly observe this interaction, we rely on the interpretation of various remote sensing surveys. One powerful means of interrogating our planet's interior is to study the sound waves propagating through the Earth after an earthquake. The speed at which waves propagate as well as their damping depends on the details of how melt and crystallized minerals are aligned. This information can therefore be used to detect the spatial extent of partially molten domains as well as their flow direction deep in Earth's interior. The investigator's research aims at constraining the basic physical principles that govern melt and mineral alignment in deforming partially molten rocks through a series of laboratory experiments. The research funded by this proposal will form the basis of a doctoral dissertation of a graduate student in the PI's laboratory and will introduce the student to the scientific method and thinking. Additionally, an analogue experiment will be developed to illustrate the influence of partial melting on sound wave propagation. This experiment will be used in undergraduate and graduate classes taught by the PI as well as for outreach to general public during events like the Cambridge Science Festival. Melting occurs in actively deforming regions of the Earth such as subduction zones and mid-oceanic ridges and affects much of the chemical exchange between mantle, crust and atmosphere. The presence of melt strongly influences the mechanical and transport properties of partially molten rocks and can produce a signature detectable by remote sensing. Remote sensing of plate boundaries gives us a wealth of information about the fundamental forces that drive plate tectonics and forms the basis for testing geodynamic hypotheses. The primary goal of this research is to elucidate the (i) influence of deformation on grain-scale melt network topology and (ii) the influence of deformation with melt present on the evolution of crystallographic preferred orientation (CPO) in olivine, the most abundant mineral in the lithospheric mantle. Both stress-driven melt preferred orientation (MPO) as well as CPO evolve on different length- and time-scales and contribute to the integrated seismic signature of deforming partially molten rocks in nature. The team will synthesize fine-grained olivine rocks with approximately 3 vol% of mid ocean ridge basalt added as a melt phase and deform these rocks in general shear under pressure and temperature conditions typical for the upper mantle (T = 900 - 1200 degrees C, P = 0.3 - 2 GPa). Constant load experiments at load levels covering at least one order of magnitude will be performed to study the effect of stress on the grain-scale melt network topology. Several experiments at a constant stress level will be taken to different finite strains to elucidate the influence of strain on MPO and CPO. Conducting such a comprehensive series of well-controlled experiments will constrain the influence of stress and strain on both MPO as well as CPO development in partially molten mantle rocks. The derived quantitative microstructural data will be used to calculate the seismic signature of the rocks based on several theoretical models. Establishing correlations between measured microstructural quantities and external forces in experiments will provide tight constraints for theoretical models of deforming solid-liquid composites and will facilitate extrapolation of laboratory data to natural conditions.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.