Water in the Deep Earth

每个人都熟悉水在地球水圈中的重要性。水圈的存在可能是由于岩石中的水，因为水是通过地球内部岩浆的脱气作用输送到地球表面的。我们不知道地球上现在或过去有多少水。根据我们目前对构成地幔的矿物中水的最大容量的了解，地幔中可能有几个海洋的水，并且可能有足够的氢溶解在地核中。在地球最早的历史时期，当深层硅酸盐岩浆海洋首次结晶时，可能有更多的水。我们目前还不知道是否有一个或多个早期的，可能是富含水的大气层在吸积过程中由于撞击侵蚀而丢失。一个或多个海洋的水可能已经参与了地球的内部分化的含水流体中的溶解物质的转移。我们都知道水在地表的惊人特性。事实证明，当溶解在地幔中的矿物、熔体和流体中时，它具有不同但同样令人惊讶的特性。即使是一点点，也会极大地改变矿物的强度或岩石的熔点。在高温下，水作为地幔中的流体，就像一种腐蚀性溶剂，可以溶解岩石。水作为一种流体也很容易在地幔的岩石中移动，因为它的密度和粘度很低，所以它可以运输它溶解的物质。水被认为是上地幔物质转移的重要媒介，它改变了岩石的化学和物理性质。实验学家已经能够确定水在构成地球上地幔的岩石和矿物中的行为。例如，在高温和高压下，水和熔体不再是截然不同的，而是地幔中的热水溶解了如此多的硅酸盐，以至于它很像熔体。这种“超临界”含水流体可能在地幔中向上移动，并改变上面岩石的化学性质。硅酸盐地幔的底部在~ 2900 km处，目前我们对构成地球上最大硅酸盐储层的下地幔深度范围（~ 660 - 2900 km）内含水流体的行为知之甚少。在这里提出的研究中，我们想做的是弄清楚含水流体如何影响下地幔矿物镁和钙钛矿的稳定性。我们还想知道在高温高压下溶解下地幔矿物的含水流体的化学性质。我们将使用多砧压力装置（~ 24 GPa）以及激光加热金刚石砧室（24-100 GPa）进行高P-T实验。我们将在含有MgO、CaO、SiO2和H2O的系统中加热含水成分。在多砧实验中，我们可以利用现代微探针分析技术直接测量含水流体的常量和微量元素组成。在金刚石对顶砧实验中，我们将使用原位同步加速器X射线衍射技术来仔细跟踪相位关系，以分离流体的主要元素组成。通过收集到的数据，我们希望能够回答一些重要的问题，比如：含水流体从下地幔到上地幔的运动是否会显著改变其组成？微量元素如何在钙钛矿相和含水流体之间分配，它们能否作为地幔流体迁移的示踪剂？下地幔中的MgO（也许还有CaO）是否已经耗尽到现在几乎100%为钙钛矿的程度？俯冲板块是否能渗透到下地幔中，提供足够的流体，从而在整个地球历史中改变地幔的化学性质？我们提出的研究将提供第一个系统的实验数据集，使我们能够解决这些关于水在地球进化和分化中的作用的重要问题。

Everyone is familiar with the importance of water in Earth's hydrosphere. The hydrosphere likely owes its existence to water in rocks, because water is delivered to the Earth's surface by degassing of magmas derived from Earth's interior. We don't know how much water the Earth currently contains, or contained in the past. Based on our present knowledge of the maximum capacity of water in the minerals that make up Earth's mantle, there could be several oceans of water in the mantle, and possibly enough hydrogen dissolved in the core for several more. There may have been much more water during the earliest history of the Earth when deep silicate magma oceans first crystallized. We currently do not know if one or more early, possibly water-rich, atmospheres were lost due to impact erosion during accretion. One or many oceans of water may have been involved in the internal differentiation of Earth by transfer of dissolved materials in hydrous fluid. We all know of the amazing properties of water at the surface. It turns out to have different but equally amazing properties when dissolved in minerals, melts and fluids in the mantle. Even a little bit can change dramatically the strength of a mineral or the melting point of a rock. At high temperatures water as a fluid in the mantle acts like a corrosive solvent and can dissolve rock. Water as a fluid also moves around easily in the rocks of the mantle because of its low density and viscosity, so that it can transport the materials that it dissolves. Water is known to be an important agent of mass transfer in the upper mantle, changing the chemistry and physical properties of the rocks it moves through. Experimentalists have been able to determine much about how water behaves in the rocks and mineral that comprise the Earth's upper mantle. For example, at high temperatures and pressures water and melts are no longer distinct, but instead hot water in the mantle dissolves so much silicate that it much like a melt. This 'supercritical' hydrous fluid may move upward in the mantle and change the chemistry of the rocks above. The base of the silicate mantle is at ~ 2900 km, and currently we know very little about the behaviour of hydrous fluids in the depth range of the lower mantle (~ 660 - 2900 km), which constitutes the largest silicate reservoir in Earth. What we want to do in the research proposed here is to work out how hydrous fluids affect the stability of the lower mantle minerals magnesium and calcium perovskites. We also want to know the chemistry of hydrous fluids that dissolve lower mantle minerals at high P and T. We will make high P-T experiments using a multi-anvil pressure apparatus (~ 24 GPa), as well as a laser-heated diamond anvil cell (24-100 GPa). We will heat water-bearing compositions in systems containing MgO, CaO, SiO2 and H2O. In multi-anvil experiments we can measure the major and trace element composition of the hydrous fluid directly using modern micro-probe analytical techniques. In the diamond anvil cell experiments we will use in situ synchrotron X-ray diffraction techniques to carefully track phase relationships to isolate the major element composition of the fluids. With the data we collect we hope to answer important questions like: Can the movement of hydrous fluid from the lower to upper mantle significantly alter its composition? How do trace elements partition between perovskite phases and the hydrous fluid, and can they be used as tracers of fluid transfer in the mantle? Could the lower mantle have been depleted in MgO (and perhaps CaO) to the extent that it is now nearly 100% perovskite? Could subducted slabs that penetrate into the lower mantle provide enough fluid to have altered mantle chemistry through Earth history? The research we propose will provide the first systematic experimental data set that will allow us to address these important questions about the role of water in the evolution and differentiation of our planet.