Ultrasonic Measurement Of The Transition Zone's Seismic Velocities

过渡带地震速度的超声波测量

• 批准号: NE/T007737/1
• 负责人: Andrew Thomson
• 金额: $ 64.82万
• 依托单位: University College London
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2021
• 资助国家: 英国
• 起止时间: 2021 至 无数据
• 项目状态: 未结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FT007737%2F1
• 关键词: Ultrasonic; Measurement; Transition; Zone; Seismic

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.

地球的地幔过渡带从大约410公里到660公里的深度延伸，是地球上一个巨大的和不可接近的层。过渡带没有直接的样本，所以我们对这个地区的了解都是从地震声波穿过这个地区的速度推断出来的。通过使用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，结构和含水量。此外，地震数据可以类似于医学超声波的方式用于对横向变化进行成像，这揭示了过渡带的地震特性在地球仪周围发生微妙变化，这可能反映了温度、矿物学或成分的变化。过渡区对于理解行星地球的大规模动态和化学演化特别重要，因为（i）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，（ii）过渡区具有巨大的水容量，可能比表面海洋多10倍。如果我们能够利用地震观测直接对过渡带的温度和成分进行成像，我们将朝着理解地球深部内部的行为迈出巨大的一步。虽然地震学为我们提供了越来越详细的地球内部视图，但仍然存在的挑战是根据地幔和核心的矿物学，化学和温度来解释这些数据。然而，令人惊讶的是，很少有实验数据的地幔矿物在适当的压力和温度条件下的过渡带的地震性质。即使是橄榄石，最常见的上地幔矿物，几乎占地幔岩石的60%，深度为410公里，数据仅限于< 1300 K，而上地幔温度范围为1500至2000 K。其他地幔阶段的情况要糟糕得多。代替数据，使用热力学数据库已经是司空见惯的，假设这些可以安全地外推到过渡区条件。然而，我们发现这些数据库并不可靠，因为它们甚至不匹配现有的数据。在高压和高温条件下的实验数据的缺乏仅仅是由于实验的限制。在这个建议中，我们将采用一种新的方法，在真实的过渡带条件下，使用基于大学的设备，用高频超声波常规测量地震速度。我们将使用它来测量最重要的上地幔和过渡带矿物的压缩和剪切波速度，作为变化的压力，温度，成分和含水量的函数。我们相信这将是我们对地球深处的理解的变革。

项目成果

Incorporation of tetrahedral ferric iron into hydrous ringwoodite

四面体三价铁掺入水合尖晶石中

• DOI: 10.2138/am-2021-7539
• 发表时间: 2021
• 期刊: American Mineralogist
• 影响因子: 3.1
• 作者: Thomson A