Ultrasonic Measurement Of The Transition Zone's Seismic Velocities

地球的地幔过渡带从大约410公里到660公里深，是地球上一个辽阔而难以到达的层。没有来自过渡带的直接样本，所以我们对该地区的一切了解都是从地震声波通过该地区的速度推断出来的。通过实验限制潜在矿物组合的声学特性，地球科学家可以推断其温度、化学、结构和水分含量。此外，地震数据可以用类似于医学超声的方式来成像横向变化，这表明过渡带的地震性质在全球范围内发生了微妙的变化，这可能反映了温度、矿物学或成分的变化。过渡带对于了解地球的大规模动力学和化学演化特别重要，因为(1)许多俯冲板块在继续下降到下地幔之前在那里停滞了数百万年，(2)过渡带具有巨大的水容量，可能容纳的水量是表层海洋的10倍。如果我们能利用地震观测直接成像过渡带的温度和成分，我们将朝着了解地球深处的行为迈出巨大的一步。虽然地震学为我们提供了对地球内部越来越详细的看法，但仍然存在的挑战是从地幔和地核的矿物学、化学和温度角度来解释这些数据。然而，在过渡带适宜的压力和温度条件下，关于地幔矿物地震性质的实验数据令人惊讶地很少。即使是橄榄石，这种最常见的上地幔矿物，在410公里以下的地幔岩石中几乎占60%，数据也仅限于1300K，而上地幔温度在1500至2000K之间。其他地幔阶段的情况要糟糕得多。用热力学数据库来代替数据是很常见的，并假定这些数据可以安全地外推到过渡区的条件。然而，我们发现这些数据库是不可靠的，因为甚至不匹配现有的数据。高压和高温条件下实验数据的缺乏只是由于实验的限制。在这项提案中，我们将采用一种新的方法，在实际过渡区条件下使用基于大学的设备来常规地使用高频超声波测量地震速度。我们将用它来测量最重要的上地幔和过渡带矿物的纵波和横波速度，作为压力、温度、成分和水含量变化的函数。我们相信这将对我们对地球深处的理解产生革命性的影响。

The Earth's mantle transition zone extends from approximately 410 to 660 km depth, is a vast and inaccessible layer of the Earth. There are no direct samples from the transition zone, so everything we know about this region is inferred from the speed that seismic sound waves transit this region. By constraining the acoustic properties of potential mineral assemblages using experiments, Earth Scientists can infer its temperature, chemistry, structure and water content. Additionally, seismic data can be used in an analogous way to medical ultrasound, to image lateral variations, which reveal that the seismic properties of the transition zone change subtly around the globe, which presumably reflect variations in temperature, mineralogy or composition. The transition zone is particularly important for understanding the large-scale dynamic and chemical evolution of planet Earth because (i) many subducting slabs stall there for millions of years before continuing descent into the lower mantle and (ii) the transition zone has a huge water capacity, potentially holding 10 times more water than the surface oceans. If we could use seismic observations to directly image the temperature and composition of the transition zone we would take a gigantic step towards understanding the behaviour of Earth's deep interior. Whilst seismology provides us with ever increasingly detailed views of the Earth's interior, the challenge that remains is to interpret this data in terms of the mineralogy, chemistry and temperature of the mantle and core. However, there is surprisingly little experimental data on the seismic properties of mantle minerals under the appropriate pressure and temperature conditions for the transition zone. Even for olivine, the most common upper mantle mineral making up almost 60% of mantle rocks down to depths of 410 km, data are restricted to < 1300 K, whereas upper mantle temperatures range from 1500 to 2000 K. The situation for other mantle phases is far worse. In lieu of data it has been commonplace to use thermodynamic databases, with the assumption that the these can be safely extrapolated to transition zone conditions. However, we have discovered that these databases are not reliable because do not even match the available data.The paucity of experimental data at high pressure and temperature conditions is simply due to experimental constraints. In this proposal we will employ a new method to routinely measure seismic velocities with high-frequency ultrasonic waves at real transition zone conditions using university-based equipment. We will use this to measure compressional and shear wave velocities of the most significant upper mantle and transition zone minerals as a function of changing pressure, temperature, composition and water content. We believe this will be transformational for our understanding of the deep Earth.