Calcium Perovskite: the forgotten mantle phase

下地幔，从大约660公里延伸到2900公里深，是地球上一个巨大而难以接近的层。下地幔没有直接的样本，所以我们对这个区域的了解都是从地震声波通过这个区域的速度推断出来的。通过实验限制候选矿物组合的声学特性，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非均质性。在非洲和太平洋下面的地核顶部，有两个巨大的慢声速区域（直径约1000公里）。更小的异常慢速物质碎片遍布下地幔的其余部分。据信，这种异常物质中的大部分是被回收的海洋地壳，它们被俯冲并混合回地球的下地幔。这种非均质性的分布，如果它确实是再循环的地壳，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，地震非均质性是否是再循环的地壳，以及它们告诉我们的关于地球过程的信息，目前仍不确定。这是因为钙钛矿的声学和流变特性尚不清楚。钙钛矿在下地幔条件下几乎占海洋地壳的三分之一。事实上，即使是钙钛矿最基本的性质，它的晶体结构，也不为人所知，因为它不能在压力释放过程中不分解而恢复到室内条件。钙钛矿的这种性质使得它的研究极具挑战性，并且要求我们在样品保持在高压和高温条件下测量其性质。如果我能确定钙钛矿的结构、声学和流变特性，我将能够解开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实验，即多砧和金刚石砧细胞，来做到这一点。多砧是一种大型液压机，可以对一毫米大小的样品施加高达1000吨的力，相当于约170头非洲象的力。它可以模拟地球深处约700公里（3000万巴）的情况，这是下地幔的最顶端。使用这种设备，结合额外的“麦克风”传感器，可以测量声波穿过钙钛矿样品的速度，当它处于下地幔条件下。当钙钛矿样品处于较低的地幔压力时，通过在一个方向上缩短它，也可以使其变形。这样就可以测定样品的强度或流变性。钻石砧室由两个相对的宝石级钻石组成，它们的扁平尖端被压缩在一起，可以产生更高的压力，超过2亿巴。但样品很小，直径比人类头发还细（约100微米），厚度为5微米。然而，奇怪的是，如此微小的样品却有一些巨大的优势，因为它对可见光是透明的。这使得利用光谱学（称为布里渊）和x射线可以同时测量钙钛矿在下地幔条件下的结构和声速。通过对钙钛矿的声速和流变学的了解，可以确定下地幔的非均质性是否来自回收的海底，并预测它将如何被搅拌回地幔。目前尚不清楚板块是否因为坚硬而保持完整，还是因为柔软和延展性而迅速搅拌到地幔中。最终，这些行为控制着我们星球的宜居性。

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.