CSEDI Collaborative Research: Carbon-Oxygen-Hydrogen Volatile Metasomatism in the Cratonic Mantle - Implications for Mid-Lithospheric Discontinuities

地球是太阳系中唯一一颗表面分为海洋和大陆的行星。因此，了解大陆的起源以及它们的部分如何在数十亿年内保持稳定，对于理解地球的演化是至关重要的。古老而稳定的大陆核心(克拉通)被厚厚的地幔(位于地壳下方的地球硅酸盐层)所覆盖，这些地幔根部被认为有助于大陆的稳定。然而，关于大陆地幔是如何形成和演化的，仍存在争议。特别是，在大陆形成期间或之后，岩浆或流体与地幔的相互作用所起的作用尚不清楚，尽管地震成像在克拉通地幔中观察到的广泛分层可能是由岩浆和流体相关的过程造成的。这项研究将通过将熔体/流体和地幔反应的情景与岩石圈中部深处的地球物理观测相一致来限制克拉通的形成和演化。这些研究成果通过同行评议出版物、会议报告和座谈会以及课堂讲座和活动传播，将影响广泛的地球科学分学科，包括岩石学-地球化学学、矿物物理学、地球动力学和地震学。该提案将支持高P-T实验岩石学、矿物物理和地震学专业的女性博士生，并支持几名本科生的研究活动，包括来自代表性不足群体的学生。除了他们自己的研究，所有的学生将参与讨论，并与所有三个PI一起撰写研究文章，学习如何结合固体地球科学三个不同但相关的子学科的概念和方法。因此，这项提议将直接促进几名学生在其职业生涯的不同阶段的教育经验和专业发展。这项拟议的研究还将支持矿物物理领域的早期职业PI，并将帮助研究人员让K-12学生参与科学研究。大陆内部20-30亿年的稳定性是地球动力学和热演化的关键特征。因此，了解克拉通地幔的地球物理结构、岩石学和材料特性以及产生它们的构造历史是地球科学面临的重大挑战之一。次大陆地幔的一个奇怪的特征是在60-150公里深处存在明显的地震剪切速度减慢，通常被称为岩石圈中部不连续(MLD)。鉴于克拉通中普遍存在多重LD，它们的起源可能与克拉通形成有关；然而，多重LD和克拉通形成之间的因果联系尚未确定，而多重LD的起源仍然存在很大争议。一种假设是，MLD代表富含CO2-H2O挥发分的区域，这导致了含水和/或碳酸盐矿物或部分熔体的稳定性。这个项目将综合实验岩石学、矿物物理学和地震学的结果来检验这一假设。通过实验室实验，该项目将限制在MLD深处由混合CO2-H2O挥发分熔融的克拉通地幔橄榄岩的相平衡，考虑到挥发分引入的因素可能随着克拉通形成模型的不同而变化。其目标将是确定部分熔体上含水和碳酸盐矿物相的稳定性，并获得每种情况下矿物相的组成和比例随深度、温度和熔体/流体：橄榄岩比的函数。在相平衡实验和汇编的捕虏体数据的指导下，将利用共振超声光谱(RUS)和第一性原理模拟来确定角闪石和云母的适当端元和中间固溶体的热弹性性质。利用实验中的矿物组合和新矿物物理中最新的弹性数据，可以预测地幔速度模型。这些将与南部非洲克拉通(通过联合反演面波、Sp和Ps数据获得)和北美的地幔模型进行比较，并用于计算将与全球MLD观测进行比较的合成Sp和Ps接收器函数栈。拟议的研究将测试一个或几个流体/熔体渗入克拉通的情景是否满足岩石圈中部大陆地幔的地球物理和岩石学特征。我们的研究还将产生特定的产品，如枯竭的地幔CO2 H2O的固相线参数化；交代地幔矿物组合作为P-T的函数；渗透的流体/熔体组成和流体/熔体：岩石比；使用更新的热弹性数据计算与MLD相关的组合的地震速度的代码；以及观测到的地幔不连续参数和地幔速度模型。该奖项反映了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。

Earth is the only planet in the Solar System whose surface is divided into oceans and continents. Hence understanding the origin of continents and how portions of them remain stable for billions of years is central to understanding the evolution of the Earth. The old, stable cores of continents (cratons) are underlain by thick layers of mantle (the silicate layer of the Earth that lies below the crust), and these mantle roots are thought to contribute to continental stability. Yet, it remains debated how the continental mantle formed and evolved. In particular, the roles of magma or fluid interaction with the mantle, either during or after the continent formation, are unclear, although widespread layering observed in the cratonic mantle by seismic imaging may have been created by magma and fluid related processes. This research will constrain the formation and evolution of cratons by bracketing scenarios of melt/fluid and mantle reactions that are consistent with geophysical observations at mid-lithospheric depths. The research findings - disseminated through peer-reviewed publications, conference presentations and colloquia, and class room lectures and activities - will impact a wide range of Earth science sub-disciplines including petrology-geochemistry, mineral physics, geodynamics, and seismology. The proposal will support female PhD students in high P-T experimental petrology, mineral physics, and seismology, and support the research activities of several undergraduate students, including students from underrepresented groups. In addition to their own research, all the students will participate in discussions and write research articles together with all 3 PIs, learning how to combine concepts and methods from three different yet related sub-disciplines of solid Earth science. The proposal will thus directly further the educational experience and professional development of several students at various stages of their careers. The proposed research will also support an early career PI in the field of mineral physics and will aid the investigators in engaging K-12 students in science.The stability of the continental interiors for 2-3 billion years is a key feature of the Earth's dynamical and thermal evolution. Understanding the geophysical structure, petrology, and material properties of cratonic mantles and the tectonic history that produced them is therefore one of the grand challenges of Earth science. A curious feature of subcontinental mantles is the presence of distinct reductions in seismic shear velocity at depths of 60-150 km, often called mid-lithospheric discontinuities (MLDs). Given the widespread occurrence of MLDs within cratons, their origins may be linked to craton formation; however, the causal link between MLD and craton formation is yet to be established, and MLD origins remain highly debated. One hypothesis is that MLDs represent zones enriched in CO2-H2O volatiles, which led to stability of hydrous and/or carbonate minerals or partial melts. This project will test this hypothesis by integrating results from experimental petrology, mineral physics, and seismology. Through laboratory experiments, this project will constrain the phase equilibria of cratonic mantle peridotites fluxed by mixed CO2-H2O volatiles at MLD depths, taking into consideration how the agent of volatile introduction may vary with craton formation models. The goal will be to determine the stability of hydrous and carbonate mineral phases over partial melts and obtain the compositions and proportions of mineral phases as a function of depth, temperature, and melt/fluid:peridotite ratio for each of the cases. Guided by the phase equilibria experiments and compiled xenolith data, thermoelastic properties of the appropriate end members and intermediate solid solutions of amphibole and mica will be determined using Resonant Ultrasound Spectroscopy (RUS) and first principles simulation. Using the mineral assemblages from experiments and updated elasticity data from new mineral physics, mantle velocity models will be predicted. These will be compared to mantle models for southern African craton (to be obtained by joint inversion of surface wave, Sp and Ps data) and North America, and used to calculate synthetic Sp and Ps receiver function stacks that will be compared to MLD observations globally. The proposed research will test whether one or several scenarios of fluid/melt infiltration into cratons, if any, satisfy the geophysical and petrologic characters of continental mantles at mid-lithospheric depths. Our research will also lead to specific products such as solidus parameterizations of depleted mantle+CO2+H2O; metasomatized mantle mineral assemblages as a function of P-T, infiltrated fluid/melt composition and fluid/melt:rock ratio; codes for computing seismic velocities for MLD-related assemblages with updated thermoelastic data; and observed mantle discontinuity parameters and mantle velocity models.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.