Volatiles in silicate melts: From geophysical detection to primordial reservoirs

地震学是研究地球内部深处的主要工具。通过分析地震波的速度和形式，科学家可以可视化和解释深层的结构和成分；一种与医学超声相媲美的技术。地幔中部分熔融的存在以异常低的地震速度为特征。在构造板块下，岩石圈-软流圈边界处也观察到低速。在一个板块俯冲到另一个板块下面的俯冲带也可以观察到它们。人们怀疑在地幔的其他几个区域也发生了熔融，特别是在410公里至670公里深度的过渡带上下。然而，在许多这些地区，估计的温度低于岩石的熔化温度；除非岩石中含有降低其熔化温度的挥发物（水或二氧化碳）。在这里，科学家们在地球深处的极端压力和温度下进行实验。他们的目标是确定观测到的地震异常是否可以归因于富含挥发物的硅酸盐熔体的存在。他们使用最先进的技术来压缩和加热玻璃样品，同时测量它们的密度和弹性性能。这些特性与地震速度直接相关。高压是在两个相对的钻石的尖端产生的，或者是在桑迪亚国家实验室（SNL，新墨西哥州）的Thor和Z机器中由巨大的电力和磁力脉冲产生的。研究结果解决了地球科学中的基本问题。在地幔中能探测到含水硅酸盐熔体吗？它们的物理性质和对地幔演化的可能影响是什么？在大型行星撞击中产生的硅酸盐熔体能否保留原始挥发物？这个项目支持一位早期职业生涯的女科学家。它为科罗拉多大学博尔德分校的研究生和本科生提供支持和培训，并向科学界代表性不足的群体提供服务。它还促进了学术界、SNL和工业界之间的跨学科合作。在硅酸盐矿物中，水主要以羟基（OH）的形式储存，并通过缺陷平衡电荷。在一个大气压下，随着含水量的增加，弹性波速降低，弹性模量变软。然而，在压缩时，富含挥发物的硅酸盐玻璃比无水玻璃更坚硬。这种意想不到的效果归因于挥发性物质填充间隙和/或以硅烷醇（Si-O-H）基团键合的能力。这种效应对压力、温度和成分的依赖性目前尚不清楚。然而，量化这些依赖关系对于约束地幔中非晶硅酸盐的状态方程模型至关重要。在这里，可行的假设是含水硅酸盐玻璃和熔体在地幔压力下比无水硅酸盐玻璃和熔体弹性更硬。为了验证这一假设，该团队使用了静态和动态压缩方法的组合。在激光加热的金刚石砧池中进行了原位（高压和高温）ghz -超声干涉测量。SNL Thor和Z机器用于斜坡和冲击斜坡压缩实验，压力可达100万atm （100+ GPa）。斜坡压缩是一种新的动态压缩方法，它遵循等熵路径，而不是通常在激波实验中探索的Hugoniot路径。在SNL机器的动态压缩过程中，机器产生的强大压力波允许通过激光干涉测量法（例如VISAR）在原位（在高压和高温下）探测压缩后的非晶硅酸盐的弹性特性。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。

Seismology is a major tool when investigating Earth’s deep interior. By analyzing the velocity and form of seismic waves, scientists can visualize and interpret structures and compositions at depth; a technique which is comparable to medical sonograms. The presence of partial melts in the Earth’s mantle is characterized by anomalously low seismic velocities. Low velocities have been observed under tectonic plates, at the lithosphere-asthenosphere boundary. They are also observed in subduction zones where one plate dives underneath an other. Melting is suspected to occur in several other regions of the mantle, notably above and below the transition zone which extends from 410 km to 670 km depth. However, in many of these regions the estimated temperature is below the melting temperature of rocks; unless rocks contain volatiles (water or CO2) which lower their melting temperatures. Here, the scientists are carrying out experiments at the extreme pressures and temperatures of the deep Earth. Their goal is to determine if the observed seismic anomalies can be attributed to the presence of volatile-rich silicate melts. They use state-of-the-art techniques to compress and heat up glass specimens while measuring their density and elastic properties. These properties are directly linked to seismic velocities. The high pressures are produced at the tip of two opposing diamonds or generated by huge electric and magnetic power pulses in the Thor and Z machines at Sandia National Laboratories (SNL, New Mexico). The results address fundamental questions in Earth Sciences. Can hydrous silicate melts be detected in the mantle? What are their physically properties and possible impact on mantle evolution? Could silicate melts generated during large planetary impacts retain primordial volatiles? This project supports an early-career female scientist. It provides support and training to graduate and undergraduate students at University of Colorado Boulder, and outreach to underrepresented groups in science. It also fosters an interdisciplinary collaboration between academia, SNL and industry.In silicate minerals water is mostly stored as hydroxyl (OH) and charge balanced by defects. At one atmosphere, this leads to lower elastic wave velocities and softer elastic moduli with increasing water content. However, on compression, volatile-rich silicate glasses are stiffer than their anhydrous counterparts. This unexpected effect has been attributed to the ability of volatile species to fill interstitial voids and/or bond as silanol (Si-O-H) groups. The dependences on pressure, temperature and composition of this effect is currently unknown. Yet, quantifying these dependences is critical to constrain equation-of-state models for amorphous silicates in the Earth‘s mantle. Here, the working hypothesis is that hydrous silicate glasses and melts are elastically stiffer at mantle pressures than their anhydrous counterparts. To test this hypothesis, the team uses a combination of static and dynamic compression methods. GHz-ultrasonic interferometry measurement are carried out in situ (at high pressure and temperature) in the laser-heated diamond-anvil cell. SNL Thor and Z machines are used for ramp and shock-ramp compression experiments up to pressures of one million atm (100+ GPa). Ramp-compression is a new method of dynamic compression that follows isentropic paths rather than the Hugoniot, which is usually probed in shock experiments. During dynamic compression in SNL machines, powerful pressure waves generated by the machines allow probing in situ (at high pressure and temperature) the elastic properties of the compressed amorphous silicates by laser interferometry (e.g. VISAR).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.