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

长期以来，人们一直认为地幔过渡带在地幔动力学中起着重要的作用。位于过渡带顶部和底部的410和660 km深度附近的地震不连续面，通常分别与橄榄石-水滑石和环状辉石-钙钛矿+镁闪石的矿物学相变有关。由于660 km深度附近相变的负Clapeyron斜率，与相变相关的密度变化通过抵抗地幔上涌和穿过过渡带的下沉，对地幔对流有很大的影响。对过渡带的了解主要来自两个来源：高压实验，它限制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；以及地震观测，即地震不连续的深度和锐度以及过渡带的速度结构。这两种方法的结合已经得出了地幔的基本结构和组成。在这项合作项目中，来自罗德岛大学、华盛顿卡内基研究所、台湾国立大学和日本爱慕大学的科学家结合地震学和岩石学的方法，研究了南部非洲和南美洲中部三个截然不同的构造背景(大陆龙骨、热点和俯冲带)中的地幔过渡带。该项目旨在解决地球科学中的几个重要问题：南部非洲大陆根的厚度是多少？在这些大陆根下面的过渡带中是否有多余的挥发物(水)？坦桑尼亚热点的起源有多深？过渡区是滤水器吗？枯竭的冷俯冲板块对过渡带结构有何影响？该项目的研究人员使用最近发展起来的有限频率地震层析成像方法来锐化过渡带的速度结构图像。与基于射线理论的传统方法相比，新方法得到了更稳健的速度结构。改进的层析成像对于从地幔不连续面获得更多的相干地震相，以及更好地估计到不连续面的过渡带的厚度和深度是重要的。地震观测及其初步解释为高压实验提供了基础，这些高压实验旨在了解不同构造特征的地幔条件。这项研究的重点是测量过渡带的厚度，这是通过测量橄榄石-瓦兹利石和尖晶石后过渡边界的压差，通过地震数据分析和高压实验可以最精确地测量的参数之一。与热力学模拟一起，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范围横向变化提供了限制。通过地震数据分析和高压实验方面的研究生培训，该项目的地震学家和实验者对彼此领域的问题和优势有了深刻的了解。这样的理解对于地球深部合作研究(CSEDI)至关重要。

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).