Excellence in Research: Effect of Hydration on the Thermo-elastic Properties of Mantle Minerals and the Geophysical Implications.

地球的深层内部无法直接取样。当温度和压力随着深度的增加而增加时，人造仪器就无法使用了。最直接的观测来自于研究地震产生的振动，即地震波。地震波在地球内部传播，并在地表被地震仪收集。对地震信号进行分析，以了解地球内部的结构和组成，就像超声技术用于医学成像一样。地震波的速度取决于它们遇到的岩石类型。地震学研究与实验相结合，可以识别地幔中的岩石。研究表明，在410至660公里（255至410英里）的深度——在所谓的过渡带——存在两种致密矿物：瓦德斯莱岩和环伍德岩。这些矿物质可以以OH分子（羟基）的形式将大量的水结合到它们的结构中。过渡带可能含有和海洋一样多的水。这对推动板块构造的地幔热对流有影响。然而，目前还不清楚在过渡区储存了多少水。这部分是由于羟基如何影响地震波在矿物中的传播的不确定性。在这里，研究人员研究了水在瓦德斯莱岩和环伍德岩中的掺入如何影响地震波的速度。他们在实验室里合成各种成分和含水量的矿物。他们在地球上普遍存在的极端压力和温度下进行超声波测量。这些实验在国家同步加速器设施中进行，以确保样品质量，并在测量期间通过射线照相测量其尺寸。研究结果对于更好地理解过渡带的性质至关重要。这对理解地球上的热对流有意义。该项目促进地球科学、物理、化学和数学的多学科合作。它为特拉华州立大学（DSU）的博士后助理和本科生培训提供支持。DSU是一所历史悠久的黑人大学，以本科生为主。该项目为来自科学领域代表性不足的群体的学生提供了独特的机会。它促进了地球科学的多样性和包容性。它是由美国国家科学基金会地球科学理事会和传统黑人学院和大学-卓越研究（HBCU-EiR）计划共同资助的。实验和理论研究表明，沃德斯莱岩和环伍德岩可以在其结构中加入高达2- 3%重量百分比的羟基（OH-）。在一颗源自过渡带的钻石中捕获的环伍德石晶体中，测量到了高达1.5%重量的水。水的掺入强烈地影响矿物的物理和化学性质——例如导电性和导热性、熔化性和流动性——以及弹性波的传播。在这里，研究人员合成了含有受控结构水的瓦德斯莱石和环伍德石的多晶样品。他们使用的是石溪大学的2000吨单轴裂筒装置。通过x射线衍射、扫描透射电子显微镜、体积密度测量和台式声速测量来验证热压样品的质量。然后，在高压和高温下，在矿物稳定性场中，通过超声波测量来量化试样的弹性波速。这些测量是在先进光子源（阿贡国家实验室）的6-B-MB光束线上进行的。光束线配备了一个立方砧高压装置，结合了原位超声干涉测量、x射线衍射和成像。采用红外光谱法、二次离子质谱法和电子探针微量分析仪技术对高压实验前后试样的含水量进行测定。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。

Earth’s deep interior is not accessible to direct sampling. As temperature and pressure increase with depth, man-made instruments become unusable. The most direct observations arise from studying vibrations generated by earthquakes, called seismic waves. The waves travel within the Earth and are collected at the surface using seismographs. The seismic signal is analyzed to inform the structure and composition of Earth’s interior, as sonography is used in medical imaging. The velocity of seismic waves depends on the type of rocks they encounter. Seismological studies combined with experimentation allow identifying rocks in the Earth’s mantle. It was shown that at depths of 410 to 660 km (255 to 410 miles) - in the so-called transition zone - two dense minerals are present: wadsleyite and ringwoodite. These minerals can incorporate large amount of water in their structure under the form of OH molecules (hydroxyls). The transition zone may contain as much water as that contained in the oceans. This has implications for Earth’s mantle thermal convection, which drives plate tectonics. Yet, it is unclear how much water is stored in the transition zone. This is partly due to uncertainties on how hydroxyls affect seismic-wave propagation in minerals. Here, the researchers investigate how water incorporation in wadsleyite and ringwoodite affects the velocity of seismic waves. They synthetize in the laboratory minerals with various compositions and water contents. They carry out ultrasonic measurements at the extreme pressures and temperatures prevailing in the Earth. These experiments are performed at a national synchrotron facility, to ensure specimen quality and measure their size by radiography during the measurements. The study outcomes are critical to better understand the properties of the transition zone. It has implications for the understanding of thermal convection in the Earth. This project promotes multidisciplinary collaborations across Earth Sciences, Physics, Chemistry, and Mathematics. It provides support for a post-doctoral associate and training for undergraduate students at Delaware State University (DSU). DSU is a Historically Black University and a predominantly undergraduate institution. The project offers unique opportunities to students from groups underrepresented in Science. It fosters diversity and inclusion in Geosciences. It is co-funded by NSF Directorate for Geosciences and Historically Black Colleges and Universities - Excellence in Research (HBCU-EiR) Program. Experimental and theoretical studies indicate that wadsleyite and ringwoodite can incorporate up to 2-3 weight percent of hydroxyl (OH-) in their structures. Up to 1.5 weight percent of water was measured in a ringwoodite crystal trapped in a diamond which originated from the transition zone. Water incorporation strongly affects mineral physical and chemical properties – such as electrical and thermal conductivity, melting and flow – as well as elastic wave propagation. Here, the researchers synthetize polycrystalline samples of wadsleyite and ringwoodite containing controlled structural water. They use the 2000-ton uniaxial split-cylinder apparatus at Stony Brook University. The quality of the hot-pressed specimens is verified using X-ray diffraction, scanning transmission electron microscopy, bulk density measurements, and bench-top acoustic velocity measurements. Specimen elastic wave velocities is then quantified by ultrasonic measurements at high pressure and temperature, in the mineral stability fields. These measurements are carried out at the 6-B-MB beamline of the Advanced Photon Source (Argonne National Laboratory). The beamline is equipped with a cubic anvil high-pressure apparatus coupled with in situ ultrasonic interferometry, X-ray diffraction and imaging. Specimen water content is measured before and after the high-pressure experiments by infrared spectroscopy, secondary ion mass spectrometry, and using the Electron Probe Micro-Analyzer techniques.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.