Calcium Perovskite: the forgotten mantle phase

钙钛矿：被遗忘的地幔相

• 批准号: NE/P017657/1
• 负责人: Andrew Thomson
• 金额: $ 93.9万
• 依托单位: University College London
• 依托单位国家: 英国
• 项目类别: Fellowship
• 财政年份: 2017
• 资助国家: 英国
• 起止时间: 2017 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FP017657%2F1
• 关键词: Calcium; Perovskite; forgotten; mantle; phase

The lower mantle, extending from approximately 660 to 2900 km depth, is a vast and inaccessible layer of the Earth. There are no direct samples from the lower mantle, 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 candidate mineral assemblages using experiments, Earth Scientists can infer the chemistry of the lower mantle. Additionally, seismic data can be used in an analogous way to medical ultrasound, to image lateral variations, which reveal that the lower mantle is full of heterogeneity. Two massive regions (> 1000 km in diameter) of slow acoustic velocity sit on top of the core beneath Africa and the Pacific Ocean. Much smaller fragments of anomalously slow material are observed pervasively throughout the remainder of the lower mantle. It is believed that much of this anomalous material is recycled oceanic crust, which has been subducted and mixed back into the Earth's lower mantle. The distribution of this heterogeneity, if it is indeed recycled crust, combined with knowledge of mechanical properties will tell us about the vigour and style of mantle convection. However, both whether or not the seismic heterogeneities are recycled crust and what they tell us about Earth processes currently remains uncertain. This is because the acoustic and rheological properties of calcium perovskite, which makes up almost a third of oceanic crust at lower mantle conditions, are not known. Indeed, even the most basic property of calcium perovskite, its crystallographic structure, is not known because it cannot be recovered to room conditions without decomposing during pressure release. This property of calcium perovskite makes it extremely challenging to study, and requires that we measure its properties whilst the sample remains at high pressure and temperature conditions. If I can determine the structure, acoustic and rheological properties of calcium perovskite, I will be able to unlock many secrets about the way the deep mantle works. My research aims to do exactly this by using experiments performed in two different apparatuses, the multi anvil and the diamond anvil cell. A multi-anvil is a large hydraulic press that can apply a force of up to 1000 tonnes, the equivalent of ~ 170 African elephants, to a millimetre sized sample. It allows simulation of conditions up to ~ 700 km depth in the Earth (30 million bar), which is the very top of the lower mantle. Using this equipment, in combination with additional "microphone-like" sensors, it is possible to measure the speed that sounds waves traverse through a calcium perovskite sample whilst it is at lower mantle conditions. It is also possible to deform a sample of calcium perovskite, by shortening it in one direction once it is at lower mantle pressure. This allows determination of the strength, or rheology of the sample. The diamond anvil cell, consisting of two opposing gem-quality diamonds with flat tips compressed together, can generate much higher pressures, more than 200 million bar. But the samples are tiny, with a diameter thinner than a human hair (approximately 100 microns) and a thickness of 5 microns. However, strangely, such a tiny sample has some big advantages because it is transparent to optical light. This allows, using spectroscopy (called Brillouin) and x-rays, for the structure and acoustic velocities of calcium perovskite to be measured simultaneously at lower mantle conditions. Together, knowledge of the acoustic velocity and rheology of calcium perovskite will allow identification of whether heterogeneity in the lower mantle is made from recycled ocean floor, and to predict how it would be stirred back into the mantle. It is currently unknown whether slabs remain intact because they are rigid, or whether they get rapidly stirred into the mantle because are soft and malleable. Ultimately these behaviours control the habitability of our planet.

下地幔大约从660公里到2900公里深，是地球上一个巨大而难以接近的层。没有来自下地幔的直接样本，所以我们所知道的关于这个地区的一切都是从地震声波通过这个地区的速度推断出来的。通过实验限制候选矿物组合的声学性质，地球科学家可以推断下地幔的化学成分。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，这表明下地幔充满了不均质性。位于非洲和太平洋地核顶部的两个巨大的低声速区域(直径1000千米)。异常缓慢的物质的小得多的碎片在下地幔的其余部分被普遍观察到。据信，这种异常物质大部分是回收的洋壳，它们被俯冲并混合回到地球下地幔中。这种不均质性的分布，如果它真的是再循环地壳，结合力学性质的知识，将告诉我们地幔对流的活力和风格。然而，地震不均质性是否是再循环地壳，以及它们告诉我们的关于地球过程的信息，目前都还不确定。这是因为钙钙钛矿的声学和流变性尚不清楚，在下地幔条件下，钙钙钛矿几乎占洋壳的三分之一。事实上，即使钙钛矿最基本的性质，它的晶体结构，也是未知的，因为它不可能在压力释放过程中不分解就恢复到室温条件。钙钛矿型钙钛矿的这一特性使其研究极具挑战性，并要求我们在样品保持高压和高温条件下测量其特性。如果我能确定钙钙钛矿的结构、声学和流变性，我就能解开关于深部地幔工作方式的许多秘密。我的研究旨在通过在两种不同的设备上进行的实验来准确地做到这一点，这两种设备分别是多砧座和钻石砧座单元。多锤是一种大型液压压力机，可以对一毫米大小的样品施加高达1000吨的力，相当于约170头非洲大象。它允许模拟地球最深达700公里(3000万巴)的条件，这是下地幔的最高端。使用这种设备，再加上额外的“麦克风状”传感器，就有可能在钙钙钛矿样品处于下地幔条件下时测量声波通过它的速度。钙钙钛矿样品一旦处于较低的地幔压力下，也可以通过向一个方向缩短来使其变形。这使得可以确定样品的强度或流变性。钻石顶盒由两颗相对的宝石级钻石组成，顶端扁平地压在一起，可以产生更高的压力，超过2亿巴。但这些样品很小，直径比人的头发还细(约100微米)，厚度为5微米。然而，奇怪的是，这么小的样本有一些很大的优势，因为它是光学透明的。这使得可以利用光谱学(称为布里渊)和X射线，在较低的地幔条件下同时测量钙钙钛矿的结构和声速。总而言之，钙钙钛矿的声速和流变学知识将有助于确定下地幔中的不均质性是否由再循环的洋底形成，并预测它将如何被搅回地幔。目前尚不清楚是因为它们坚硬而保持完好，还是因为它们柔软和可延展性而迅速搅拌到地幔中。归根结底，这些行为控制着我们这个星球的宜居性。

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

The speciation, distribution, transport, and impact of volatile elements in the Earth's interior

地球内部挥发性元素的形态、分布、运输和影响

• DOI: 10.1016/j.chemgeo.2018.01.007
• 发表时间: 2018
• 期刊: Chemical Geology
• 影响因子: 3.9
• 作者: Ni H

Diamonds from the lower mantle?

来自下地幔的钻石？

• DOI: 10.2138/am-2017-6061
• 发表时间: 2017
• 期刊: American Mineralogist
• 影响因子: 3.1
• 作者: Thomson A

Peritectic Melting of Mica in Fault-Related Pseudotachylite Melts and Potassium Mass Balance as an Indicator of Fluid-Absent Source Conditions.

与断层相关的假太石熔体中云母的包晶熔化和作为无流体源条件指标的钾质量平衡。

• DOI: 10.1029/2020gc009217
• 发表时间: 2021
• 期刊: Geochemistry, Geophysics, Geosystems
• 作者: Dobson D

Deep Earth carbon reactions through time and space

穿越时空的地球深部碳反应

• DOI: 10.2138/am-2020-6888ccby
• 发表时间: 2020
• 期刊: American Mineralogist
• 影响因子: 3.1
• 作者: McCammon, Catherine;Bureau, Hélène;Cleaves, James H.;Cottrell, Elizabeth;Dorfman, Susannah M.;Kellogg, Louise H.;Li, Jie;Mikhail, Sami;Moussallam, Yves;Sanloup, Chrystele
• 通讯作者: Sanloup, Chrystele