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

“合作研究：地震学和岩石学综合研究中地幔过渡带的成分和热变化”

• 批准号: 0551117
• 负责人: Yang Shen
• 金额: $ 26.42万
• 依托单位: University of Rhode Island
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2006
• 资助国家: 美国
• 起止时间: 2006-08-01 至 2012-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0551117&HistoricalAwards=false
• 关键词: COLLABORATIVE; RESEARCH; Compositional; thermal; variations

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).

地幔过渡带在地幔动力学中的重要作用早已为人们所认识。 近410和660公里的深度，标志着过渡带的顶部和底部的地震不连续性，通常与矿物相变从橄榄石wadsleyite和从ringwoodite钙钛矿加镁橄榄石，分别。 由于在660 km深度附近相变的负克拉珀龙斜率，与相变相关联的密度变化通过抵抗通过过渡带的地幔上涌和下涌而对地幔对流产生强烈影响。 过渡带的知识主要来自两个来源：高压实验，其将可能的地幔矿物的相关系和弹性系数约束为压力、温度和成分的函数;以及地震观测，其深度和地震不连续性的锐度以及过渡带的速度结构。 这两种方法的结合产生了地幔的基本结构和组成。在这个合作项目中，来自罗得岛大学、华盛顿卡内基研究所、国立台湾大学和爱媛大学（日本）的科学家将地震学和岩石学方法结合起来，研究南部非洲和南美洲中部三个明显不同构造背景（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：南部非洲大陆根的厚度是多少？ 在这些大陆根下面的过渡带中是否有过量的挥发物（水）？ 坦桑尼亚热点的起源深度是多少？过渡区是滤水器吗？ 亏损冷俯冲板片如何影响过渡带结构？本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带速度结构的图像。 新方法产生了一个更强大的速度结构比传统的方法基于射线理论。 改进的层析成像图像对于从地幔不连续面获得更多的相干地震相位以及更好地估计过渡带厚度和深度到不连续面是重要的。 地震观测及其初步解释为最佳设计的高压实验提供了基础，以了解不同构造特征的地幔条件。 这项研究的重点是测量的厚度的过渡带，这是一个参数，可以最精确地测量地震数据分析和高压实验，通过测量的压差的橄榄石wadsleyite和后尖晶石过渡边界。 结合热力学模拟，对简化的和真实的地幔成分进行高压实验的结果对地幔温度和成分的小尺度横向变化提供了限制。 通过地震数据分析和高压实验的研究生培训，该项目的地震学家和实验人员对彼此领域的问题和优势有了深刻的了解。 这种理解对于地球深层内部的合作研究（CSEDI）至关重要。