Calcium Perovskite: the forgotten mantle phase

下地幔，从大约660公里到2900公里深，是地球上一个巨大的和难以接近的层。由于没有来自下地幔的直接样本，因此我们对该地区的了解都是从地震声波穿过该地区的速度推断出来的。通过使用实验来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性质。此外，地震数据可以以类似于医学超声的方式用于成像横向变化，这表明下地幔充满了不均匀性。两个巨大的区域（直径> 1000公里）的声速慢坐在非洲和太平洋下面的核心顶部。在下地幔的其余部分，到处都可以观察到小得多的慢物质碎片。据信，这些异常物质中的大部分是再循环的海洋地壳，它们已经俯冲并混合回到地球的下地幔中。这种不均匀性的分布，如果它确实是再生地壳，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，无论是地震不均匀性是否是再循环的地壳，以及它们告诉我们的地球过程目前仍然不确定。这是因为钙钛矿的声学和流变学特性尚不清楚，钙钛矿在下地幔条件下占海洋地壳的近三分之一。事实上，即使是钙钛矿的最基本性质，即其晶体结构，也是未知的，因为它不能在压力释放期间不分解而恢复到室温条件。钙钛矿的这种性质使其研究极具挑战性，并要求我们在样品保持在高压和高温条件下的同时测量其性质。如果我能确定钙钛矿的结构，声学和流变学特性，我将能够解开许多关于地幔深部工作方式的秘密。我的研究旨在通过使用在两种不同设备中进行的实验来做到这一点，多砧和金刚石砧室。多砧是一种大型液压机，可以对毫米大小的样品施加高达1000吨的力，相当于170头非洲大象。它可以模拟地球深处约700公里（3000万巴）的条件，这是下地幔的顶部。使用这种设备，结合额外的“麦克风状”传感器，可以测量声波在下地幔条件下穿过钙钛矿样品的速度。钙钛矿样品也可以变形，一旦它处于下地幔压力下，就可以在一个方向上缩短它。这可以确定样品的强度或流变性。金刚石压砧单元由两个相对的宝石级金刚石组成，其扁平尖端压缩在一起，可以产生更高的压力，超过2亿巴。但这些样本很小，直径比人的头发丝还细（约100微米），厚度为5微米。然而，奇怪的是，这样一个微小的样品有一些很大的优势，因为它对可见光是透明的。这允许使用光谱学（称为布里渊散射）和X射线，在下地幔条件下同时测量钙钛矿的结构和声速。总之，钙钛矿的声速和流变学的知识将允许识别下地幔的异质性是否是由再循环的海底制成的，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，或者它们是否因为柔软和可延展而迅速被搅动到地幔中。最终，这些行为控制着我们星球的可居住性。

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.