"COLLABORATIVE RESEARCH: Compositional and thermal variations in the mantle transition zone from integrated seismological and petrological investigations"

长期以来，人们认识到地幔过渡带在地幔动力学中起着重要作用。410 km深度和660 km深度附近的地震不连续标志着过渡带的顶部和底部，通常与橄榄石到沃兹利矿和环木矿到钙钛矿+镁武矿的矿物相转变有关。在660 km深度附近，由于相变的负Clapeyron斜率，与相变相关的密度变化对地幔内的对流有很强的影响，通过过渡带抵抗地幔的上升流和下升流。过渡带的知识主要来自两个来源：高压实验，它约束了可能的地幔矿物的相关系和弹性系数作为压力、温度和成分的函数；并对地震不连续面的深度和清晰度以及过渡带的速度结构进行了地震观测。这两种方法的结合得到了地幔的基本结构和组成。在这个合作项目中，来自罗德岛大学、华盛顿卡内基研究所、国立台湾大学和爱元大学（日本）的科学家们将地震学和岩石学方法结合起来，研究了非洲南部和南美洲中部三种截然不同的构造环境（大陆龙骨、热点和俯冲带）下的地幔过渡带。该项目旨在解决地球科学中的几个重要问题：南部非洲大陆根部的厚度是多少？在这些大陆根下的过渡区是否有多余的挥发物（水）？坦桑尼亚热点的起源深度是多少？过渡区是水过滤器吗？贫冷俯冲板块如何影响过渡带构造？该项目的研究人员使用最近开发的有限频率地震层析成像方法来锐化过渡区的速度结构图像。与基于射线理论的传统方法相比，新方法产生了更稳健的速度结构。改进后的层析成像对于从地幔不连续面获得更相干的地震相位以及更好地估计到不连续面的过渡带厚度和深度具有重要意义。地震观测及其初步解释为高压实验的优化设计提供了基础，以了解不同构造特征的地幔条件。研究的重点是过渡带厚度的测量，这是地震资料分析和高压实验中最精确测量的参数之一，通过测量橄榄石-瓦兹利岩和尖晶石后过渡边界的压差。结合热力学模拟，对简化和真实地幔成分的高压实验结果为地幔温度和成分的小规模横向变化提供了约束。通过对地震数据分析和高压实验的研究生训练，本项目的地震学家和实验学家对彼此领域的问题和优势有了深入的了解。这样的理解对于地球深层内部合作研究（CSEDI）是必不可少的。

It has long been recognized that the mantle transition zone plays an important role in mantle dynamics. The seismic discontinuities near 410 and 660 km depth, which mark the top and bottom of the transition zone, are commonly associated with the mineralogical phase transformations from olivine to wadsleyite and from ringwoodite to perovskite plus magnesiowustite, respectively. Because of the negative Clapeyron slope of the phase transformation near 660 km depth, the density change associated with the phase transformation has a strong influence on convection in the mantle, by resisting mantle upwelling and downwelling through the transition zone. Knowledge of the transition zone comes primarily from two sources: High-pressure experiments that constrain the phase relations and elastic coefficients of likely mantle minerals as a function of pressure, temperature, and composition; and seismic observations of the depth and sharpness of the seismic discontinuities and the velocity structure of the transition zone. The combination of these two approaches has yielded the basic structure and composition of the mantle. In this collaborative project, scientists from University of Rhode Island, Carnegie Institution of Washington, National Taiwan University and Ehime University (Japan) integrate seismological and petrological approaches to study the mantle transition zone in three distinctly different tectonic settings (continental keel, hotspot, and subduction zone) beneath southern Africa and central South America. The project aims to address several important questions in earth sciences: What is the thickness of the southern African continental roots? Are there excess volatiles (water) in the transition zone beneath these continental roots? What is the depth of origin of the Tanzania hotspot? Is the transition zone a water filter? How does a depleted and cold subducting slab affect the transition zone structure?Researchers in this project use a recently developed finite-frequency seismic tomography method to sharpen the images of the velocity structure in the transition zone. The new method yields a more robust velocity structure than conventional methods based on ray theory. The improved tomographic images are important for obtaining more coherent seismic phases from mantle discontinuities and better estimates of the transition zone thickness and depth to the discontinuities. The seismic observations and their initial interpretations provide the basis for high-pressure experiments optimally designed to understand the mantle conditions of the different tectonic features. The study focuses on measurements of the thickness of the transition zone, which is one of the parameters that can be measured most precisely by seismic data analyses and by high-pressure experiments through the measurements of the differential pressure of the olivine-wadsleyite and the postspinel transition boundaries. Together with thermodynamic modeling, results from high-pressure experiments on both simplified and real mantle compositions provide constraints on small-scale lateral variations in mantle temperature and composition. Through graduate student training in seismic data analyses and high-pressure experiments, the seismologists and experimentalists in this project gain deep understanding of the problems and strengths in each other's field. Such understanding is essential to the Collaborative Study of Earth's Deep Interior (CSEDI).