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

在全球稳定的大陆上，在大约100公里的深度(根据地区的不同而不同)和大约1000摄氏度的预期温度下检测到了显著的速度下降，这种大约3-5%或更多的速度下降被称为中岩石圈不连续(MLD)。稳定大陆的岩石圈(坚硬岩石)预计是古老而寒冷的，因此观察到岩石圈内部地震速度的地质大范围不连续是令人费解的。这导致了对稳定岩石圈内波速液滴的各种不同的、往往相互矛盾的解释模型，例如部分熔融、各向异性、亚固相线流变性转变和化学分层。该项目将利用EarthScope数据、实验室实验和计算机模拟，针对新的地球物理和地质限制，评估这些提出的因果模型。该项目将侧重于(1)整个MLD的弹性和滞弹性性质以及电导率的变化，(2)MLD的全球存在，而不考虑地质历史，(3)水(水化)对岩石性质的影响的实验室研究，这可能导致速度增加，以及(3)地幔捕虏体的成分和结构，即与上升的岩浆搭便车的固体地幔岩石样品。这个项目将吸引早期职业科学家、博士生和本科生。该项目还将通过在IRIS少数民族招募演讲者系列和面向K-12教育工作者和学生的Nifty 50科学讲座上展示科学成果和研究机会，促进EarthScope的教育和推广目标。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，这些方法可以量化更广泛的地震台覆盖范围内各向异性的大小和速度下降的尖锐程度；(2)获得面波幅度和PG混响尾波的测量，以确定MLD深度附近是否存在衰减峰值；以及(3)将大地电磁(MT)电导率估计与新的矿物物理和地震约束相结合，以识别整个MLD是否存在熔融或水化作用。调查人员将把研究重点放在稳定的前寒武纪北美克拉通，它被地球范围可移动阵列部署的下半部分所覆盖。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中的晶界流动性。改善对MLD的理解对于将地球范围的结果与大陆的演化联系起来至关重要。地震观测的延伸及其与大地电磁和矿物物理的结合是一种独特的方法，将为MLD的起源提供新的见解。这些处理地震数据和将MT数据与地震学和矿物物理相结合的新策略将对一般地球物理界有用。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，这将把对USArray数据的初步研究扩展到北美大陆的结构和演化，以此类推，扩展到其他大陆。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。

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.