Collaborative Research: Understanding the Origin of the mid-lithospheric discontinuity within a stable continent from a combined geophysics-mineral physics approach

在全世界稳定的大陆上，在大约100公里的深度（视区域而定）和大约1000摄氏度的预期温度下，已经检测到速度大幅下降。这种速度降低约3-5%或更多，称为岩石圈中部不连续（MLD）。 一个稳定大陆的岩石圈（坚硬的岩石）应该是古老而寒冷的，因此，对岩石圈内部地震速度的地质广泛不连续性的观测令人困惑。 这导致了各种不同的，往往是相互矛盾的，解释稳定岩石圈内波速下降的模型，例如，部分熔融、各向异性、亚固相线流变转变和化学分层。该项目将利用地球观测数据、实验室实验和计算机建模，对照新的地球物理和地质制约因素，评价这些拟议的成因模型。该项目将重点关注（1）整个MLD的弹性和滞弹性特性以及电导率的变化，（2）MLD的全球存在，无论地质历史如何，（3）水的影响的实验室研究（水化作用）对岩石性质的影响，可能导致速度增加;（3）地幔捕虏体的成分和结构，固体地幔岩的样本与上升的岩浆搭便车。该项目将吸引早期职业科学家，博士。学生和本科生。该项目还将通过在IRIS少数民族招聘演讲者系列和面向K-12教育工作者和学生的Nifty Fifty科学讲座中展示科学成果和研究机会，促进EarthScope的教育和外联目标。（1）使用新的接收器扩大地震观测范围-函数估计和贝叶斯方法，可以量化各向异性的大小和更广泛的地震台站足迹上速度下降的尖锐度;（2）获取表面波振幅和Pg混响尾波的测量值，以确定MLD深度周围是否存在衰减峰值;以及（3）将大地电磁（MT）电导率估计与新的矿物物理学和地震学约束联合整合，以确定MLD上是否存在熔融或水合作用。研究人员将把研究重点放在稳定的前寒武纪北美板块上，这一板块被EarthScope可移动阵列部署的下半年所覆盖。该项目还将涉及关于水如何影响地幔岩石中晶界流动性的新实验室实验。更好地理解MLD对于将EarthScope结果与大陆演化联系起来至关重要。地震观测的扩展及其与MT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。 这些处理地震数据和将MT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USAray数据的初步研究扩展到北美大陆的结构和演化，并以此类推，该奖项反映了NSF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进行评估，被认为值得支持审查标准。

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.