Collaborative Research: A Combined Geophysics-Mineral Physics Approach To Investigating The Mid-Lithosphere Discontinuity Within A Stable Continent

在世界范围内稳定的大陆上，在大约100公里的深度（因地区而不同）和大约1000摄氏度的预期温度下，已经检测到速度的大幅下降。这种速度的下降大约为3-5%或更多，被称为中岩石圈不连续（MLD）。一个稳定大陆的岩石圈（坚硬的岩石）被认为是古老而寒冷的，因此对岩石圈内部地震速度的地质广泛不连续性的观察是令人困惑的。这导致了稳定岩石圈内波速下降的各种不同的、往往是相互矛盾的解释模型，例如，部分熔融、各向异性、亚固体流变转变和化学分层。该项目将利用EarthScope数据、实验室实验和计算机建模，根据新的地球物理和地质限制来评估这些拟议的因果模型。该项目将重点关注(1)整个MLD的弹性和非弹性特性以及电导率的变化，(2)MLD的全球存在，而不考虑地质历史，(3)水（水合作用）对可能导致速度增加的岩石特性的影响的实验室研究，以及(3)地幔包体的成分和质地，即随岩浆上升的固体地幔岩石样本。该项目将招收早期职业科学家、博士生和本科生。该项目还将促进EarthScope的教育和推广目标，通过在IRIS少数族裔招募演讲者系列和面向K-12教育工作者和学生的Nifty Fifty科学讲座上展示科学成果和研究机会。该项目将：(1)使用新的接收函数估计和贝叶斯方法扩展地震观测，这些方法可以在更广泛的地震台站范围内量化各向异性的大小和速度下降的尖锐程度；(2)获取表面波幅值和Pg混响尾值，判断MLD深度附近是否存在衰减峰值；(3)将大地电磁（MT）电导率估算与新的矿物物理和地震学约束结合起来，以确定整个MLD是否存在熔融或水化作用。研究人员将把研究重点放在稳定的前寒武纪北美克拉通上，该克拉通被地球范围可移动阵列低48层部署的后半部分覆盖。该项目还将涉及新的实验室实验，研究水如何影响地幔岩石的颗粒边界流动性。提高对MLD的了解对于将EarthScope的结果与大陆的演化联系起来至关重要。地震观测的扩展及其与大地电磁学和矿物物理学的结合是一种独特的方法，将为MLD的起源提供新的见解。这些处理地震数据的新策略以及将大地电磁学数据与地震学和矿物物理学相结合的新策略将对一般地球物理学界有用。通过这种跨学科的假设检验方法，研究人员建议更好地了解MLD的原因，将USArray数据的初步研究扩展到北美大陆的结构和演化，并通过类比，扩展到其他大陆。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

In stable continents worldwide, a substantial velocity decrease has been detected at about 100 km depth (varying depending on region) and at an expected temperature of about 1000 degrees C. This decrease in velocity of roughly 3-5% or more, is called the mid-lithosphere discontinuity (MLD). The lithosphere (hard rocks) of a stable continent is expected to be old, and cold, therefore observations of a geological wide-spread discontinuity in seismic velocity internal to the lithosphere is puzzling. This has led to a variety of different, often contradictory, explanatory models for a wavespeed drop within stable lithosphere, e.g., partial melt, anisotropy, sub-solidus rheology transitions, and chemical stratification. This project will evaluate these proposed causative models against new geophysical and geological constraints, using EarthScope data, laboratory experiments and computer modelling. The project will focus on (1) variation in elastic and anelastic properties and electrical conductivity across the MLD, (2) a global presence of the MLD, regardless of geological history, (3) laboratory studies of the influence of water (hydration) on properties of rock that could cause the velocity to increase, and (3) composition and textures of mantle xenoliths, samples of solid mantle rock that hitch a ride with rising magma. This project will engage early career scientists, Ph.D. students, and undergraduate students. The project will also promote EarthScope's education and outreach goals, by presenting the science results and research opportunities at the IRIS minority recruitment speaker series and the Nifty Fifty science lectures to K-12 educators and students.The project will: (1) extend the seismological observations using new receiver-function estimates and Bayesian methodology that can quantify the magnitude of anisotropy and the sharpness of the velocity drop over a more extensive footprint of seismic stations; (2) acquire measurements of surface wave amplitudes and Pg reverberation coda to identify whether there is a peak in attenuation around the MLD depth; and (3) jointly integrate magnetotelluric (MT) conductivity estimates with new mineral-physics and seismological constraints, to identify the presence of melt or hydration across the MLD. The investigators will focus the study on the stable Precambrian North American Craton, which was covered by the second half of the lower-48 deployment of the EarthScope Transportable Array. The project will also involve new lab experiments on how water influences grain-boundary mobility in mantle rocks. An improved understanding of the MLD is crucial for relating EarthScope results to the evolution of continents. The extension of the seismological observation and its integration with MT and mineral physics is a unique approach that will provide new insights into the origin of the MLD. These new strategies for processing seismic data and integrating MT data with seismology and mineral physics will be useful to the general geophysical community. With this interdisciplinary hypothesis-testing approach, the investigators propose to obtain a better understanding of the cause of the MLD that will extend the initial studies of USArray data to the structure and evolution of the North American continent, and by analogy, to other continents.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.