High Pressure-Temperature Single-Crystal Elasticity of the Lower-Mantle Bridgmanite

地球的下地幔从670公里深一直延伸到2900公里深的核-地幔边界。在那里，压力和温度超过130万大气压和3500 K(5840华氏度)。了解它的性质对于约束行星动力学至关重要。下地幔将热量从外核传导出去。这有助于为地球磁场提供能量，使我们免受太阳风的影响。此外，地幔中的热对流驱动板块构造和相关的灾害，如地震和火山喷发。地震学是对地震(弹性)波的研究，它允许直接观察下地幔结构。但它们的解释需要了解构成矿物的弹性性质。桥辉橄榄石占下地幔体积的四分之三以上，是地球上储量最丰富的矿物。在研究下地幔性质时，它也是至关重要的。在这里，该团队通过实验量化了桥辉橄榄石在地球深处极端条件下的弹性性质。结合高压设备和最先进的分析技术，它们提供了能够解释下地幔结构的数据；特别是显示低地震剪切速度的神秘大省的数据。该项目具有强烈的地震学意义和广泛的地球动力学影响。它还为几名研究生和本科生提供矿物物理方面的支持和培训，并向当地中小学提供教育推广。在这项研究中，该团队在实验室合成了含(Al，Fe)的大单晶。在压力和温度的极端条件下，研究了金刚石顶压室中的晶体性质。这种装置在两个相对的钻石尖端产生高压。高温是通过外部加热或使用聚焦的激光光束获得的。使用实验室布里渊散射和脉冲受激光散射以及国家同步加速器设施的X射线衍射来现场测量晶体的弹性性质。这是可以实现的，因为该团队正在开发时间分辨脉冲激光光谱学的新技术。这些新技术将与社区分享，用于未来对极端条件下材料特性的研究。研究人员使用获得的数据来限制布里奇曼石的全弹性模数作为压力、温度及其在铁和铝中的含量的函数。将结果外推到与下地幔有关的条件，以及建模，可以解释地震学观测。该项目的成果进一步加深了对地球深处地震速度、温度剖面和化学成分的了解。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。

Earth's lower mantle extends from 670 km depth down to the core-mantle boundary, 2900 km deep. There, pressure and temperature exceed 1.3 million atm and 3500 K (5840 degree Fahrenheit). Understanding its properties is critical to constrain the planet dynamics. The lower mantle conducts heat away from the outer core. This contributes to power the Earth's magnetic field which shields us from the solar wind. Furthermore, thermal convection in the mantle drives plate tectonics and associated hazards, such as earthquakes and volcanic eruptions. Seismology, the study of seismic (elastic) waves, allows to observe directly the lower-mantle structures. But their interpretation requires knowledge of the elastic properties of the constitutive minerals. Bridgmanite accounts for more than 3/4th of the volume of the lower mantle, making it the most abundant mineral in the Earth. It is also of crucial interest when investigating lower-mantle properties. Here, the team quantifies experimentally the elastic properties of bridgmanite at the extreme conditions of the deep Earth. Coupling high pressure devices and state-of-the-art analytical techniques, they provide data allowing the interpretation of lower-mantle structures; notably that of large enigmatic provinces showing low seismic shear velocities. The project has strong implications in Seismology and broad impacts in Geodynamics. It also provides support and training in Mineral Physics for several graduate and undergraduate students, as well as educational outreach toward local elementary and middle schools. In this study, the team synthesize large single crystals of (Al,Fe)-bearing bridgmanite in the laboratory. Crystal properties are investigated at extreme conditions of pressure and temperature in the diamond-anvil cell. This apparatus generates high pressures at the tip of two opposing diamonds. The high temperatures are obtained by external heating or using focused laser beams. Crystal elastic properties are measured in situ using a combination of laboratory Brillouin and impulsive stimulated light scattering, as well as X-ray diffraction at national synchrotron facilities. This is achievable because the team is developing new technology in time-resolved impulsive laser spectroscopy. These new techniques will be shared with the community for future studies of material properties at extreme conditions. The researchers use the obtained data to constrain bridgmanite full elastic moduli as a function of pressure, temperature and of its contents in iron and aluminum. Extrapolation of the results to conditions relevant to the lower mantle, as well as modeling, allows interpreting seismological observations. The project outcomes further the understanding of seismic velocities, temperature profiles and chemical compositions in the deep Earth.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.