CSEDI Collaborative Research: Understanding the nature of water transport between the transition zone and the lower mantle through the interdisciplinary studies

地球是一颗水行星，它的大部分水化学物质都被束缚在岩石内部。该项目的目标是为水的循环方式提供新的见解，这种循环方式是由地球硅酸盐地幔缓慢的热对流携带的。地球物理学家现在有强有力的证据表明，地球内部有大量的水，可能比目前海洋的质量还要多。有强有力的证据表明，这些水大部分存在于地幔过渡带(MTZ)内，深度在地球410公里到660公里之间。MTZ跨越了矿物的稳定范围，这些矿物可以在其晶体结构中结合水，质量分数高达1%或更多。看起来，过渡带在地球内部扮演着矿物质结合水的可再灌装水库的角色。地球表层海洋中的水可以通过地幔对流与地幔过渡带的水进行交换，时间尺度为1,000-3,000万年。当地幔岩石上下流动时，过渡带如何保持较高的水分含量，在其边界，特别是在约660公里深的下边界，转化为憎水矿物？该项目的目标是在实验室和计算机模拟的高压下研究MTZ和下地幔矿物，以确定水在660公里深附近的行为，并利用散射地震波探测从下降的岩石中排出的水。该奖项将在一个广泛的跨学科研究项目中为几名学生提供研究生和本科生培训，包括暑期研究项目。了解地球内部大规模水分运输的性质是研究地球(和其他行星)演化的关键问题之一。由于扩散是低效的，控制水大规模传输的最重要的过程是熔融和随后的熔固分离。本研究的目的是提高我们对这两个问题的认识。当过渡带中的富水物质被带入矿物中水溶解度较低的上地幔或下地幔时，可能发生熔融。然而，当金属铁存在时，大多数水就会分配到它里面。因此，金属铁对熔化行为具有控制作用。这项研究的一个方面是为了更好地了解下地幔中的金属铁含量。当熔体形成时，大部分水(氢)进入熔体。熔体向上或向下迁移取决于其相对于未熔化残余岩石的密度。深部地幔条件下的熔体密度没有得到很好的限制，特别是考虑到可能的化学成分的范围。该项目将研究深部地幔条件下的熔体密度，具有不同的氧化还原值和铁镁比的合理变化。结合这些，研究人员将对地球深处的水循环有一个更好的看法。对于散射的地震波，主要使用接收函数技术，该团队将绘制出地幔过渡带下存在水增强部分融化的位置。这一奖项反映了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。

Earth is the water planet, with much of its H2O chemically bound within its rocky interior. The goal of this project is to provide new insight into the way in which water circulates, carried by the slow heat convection of Earth's silicate mantle. Geophysicists now have strong evidence that there is a substantial amount of water in Earth's interior, perhaps more than the current mass of the oceans. There is strong evidence that most of this water resides within the mantle transition zone (MTZ), between 410 km and 660 km depth in the Earth. The MTZ spans the stability range of minerals that can incorporate water within their crystal structures, up to a mass-fraction of a percent or more. It appears that the transition zone acts as a refillable reservoir of mineral-bound water within Earth's interior. The water in Earth's surface oceans can exchange with the water in the mantle transition zone via mantle convection, on time scales of 10-30 million years. How does the transition zone maintain its elevated water content as mantle rock flows upward and downward through it, converting to water-phobic minerals at its boundaries, particularly at its lower boundary at ~660-km depth? The goal of this project is to study the MTZ and lower-mantle minerals at high pressure in the lab and with computer simulations to determine how the water behaves near 660-km depth, and to detect water expulsion from descending rock masses using scattered seismic waves. The award will provide graduate and undergraduate training, including summer research projects, for several students in a broadly interdisciplinary research project.Understanding the nature of large-scale water transport in Earth's deep interior is one of the key issues in the study of evolution of Earth (and other planets). Since diffusion is inefficient, the most important process to control the large-scale transport of water is melting and subsequent melt-solid separation. The goal of this study is to improve our understanding on these two issues. Melting likely occurs when water-rich materials in the transition zone are brought into upper or the lower mantle where the water solubility in minerals is low. However, when metallic iron is present, then a majority of water partitions into it. Hence metallic iron has a controlling effect on the melting behavior. One aspect of this research is to obtain a better understanding of metallic iron content in the lower mantle. When melt is formed, most of water (hydrogen) goes to the melt. Melt migrates up or down depending on its density relative to the unmelted residual rock. Melt density under deep-mantle conditions has not been constrained well, particularly considering the range of likely chemical compositions. This project will investigate melt density under deep-mantle conditions with different redox values and plausible variations in the iron-magnesium ratio. Combining these, the investigators will have an improved view of deep-Earth water circulation. With scattered seismic waves, primarily using the receiver-function technique, the team will map out locations beneath the mantle transition zone where water-enhanced partial melt is present.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.