Deep Water: Hydrous Silicate Melts and the Transition Zone Water Filter

地球是一个潮湿的星球，它的可居住性本质上与表面的水有关。水在地球内部也起着关键作用，因为它具有大幅降低地幔岩石熔点的作用，事实上，地表的水最终通过岩浆作用来自内部。水返回到俯冲带的内部，在地质时期，地表和地幔水的存量受到深水循环的调节。我们在地球表面看到的几乎所有火山活动都是由浅地幔的熔化引起的，特别是在俯冲带上方，这构成了深水循环的重要组成部分。然而，水也被输送到地幔深处，它发生了什么，以及它对内部过程的控制，是一个谜。深地幔岩石通常温度太低而无法熔化，但在地幔中有一些区域，最明显的是在大约400和700公里处，地震信号被解释为部分熔化。在上地幔和下地幔中，水使地幔的固相线降低了数百度，但在地幔过渡带（410-660 km）中的降低程度要小得多，这是因为水可溶于构成该区域的主要矿物，即水硅铝石和水硅铝石，但不溶于构成过渡带上下区域的矿物。这意味着，如果在过渡带中有水，并且它可能存储几个海洋的价值，当地幔从过渡带转移到上地幔或下地幔时，预计会在水释放时融化。这个概念最初是由Bercovici和Karato在2003年的一篇里程碑式的论文中应用于过渡带上方的区域，并被称为“过渡带水过滤器”，因为它对地幔地球化学的重要预测影响。有趣的是，从那时起，地震证据一直在增加，表明过渡带上方和下方的熔融区域。如果这些信号确实代表了这些地区存在含水熔体，那么过渡带可能充当了一个双面的中地幔水过滤器，在其边界发生的熔融可能在整个历史上调制了地幔的化学和地球动力学。目前，我们不能充分测试这个模型或理解其含义，因为我们不知道准确的组成含水硅酸盐熔体的地幔在这些深度，我们也不知道他们的物理性质，如密度和粘度。正因为如此，我们目前无法准确地模拟在这些深度的含水硅酸盐熔融预期的地震响应，我们不能模拟这种熔体的动态行为，如果它们存在于这些位置。在此，我们建议收集这些数据，进行高P-T实验测量，以确定对应于上地幔深部、过渡带和下地幔上部的地幔中含水硅酸盐熔体的组成。我们还将使用新的实验，我们结合联合收割机金刚石砧室技术与同步加速器X射线散射方法，以确定熔体密度。同时，我们将使用第一性原理分子动力学方法来计算这些熔体的物理和地震性质，并辅以实验来测量其润湿性。有了这些数据，我们将能够首次开发动力学和地震模型，以明确测试过渡带水过滤模型，并预测其对地幔的化学和动力学影响，以及整个地质时期的深水循环。

Earth is a wet planet, and its habitability is intrinsically tied to water at the surface. Water also plays a key role inside the Earth because it has the affect of drastically lowering the melting point of mantle rocks, and indeed, the water at the surface ultimately comes from the interior through magmatism. Water is returned to the interior at subduction zones, and over geological time the surface and mantle water inventories are regulated by a deep water cycle. Nearly all of the volcanism we see at the Earth's surface is caused by melting in the shallow mantle, especially above subduction zones, and this constitutes an important part of the deep water cycle. However, water is also transported deeper down into the mantle, and what happens to it, and the controls it has on interior processes, is a mystery. Deep mantle rocks are generally too cool to melt, but there are regions in the mantle, most notably at about 400 and 700 km, where seismic signals are interpreted to represent partial melting. Water reduces the solidus of the mantle by hundreds of degrees in the upper and lower mantle, but much less so in the mantle transition zone (410-660 km), and this is because water is soluble in the main minerals that make up this region, wadsleyite and ringwoodite, but it is not soluble in the minerals that constitute the regions above and below the transition zone. This means that if there is water in the transition zone, and it potentially can store a couple of oceans worth, when mantle is transported from the transition zone into either the upper or lower mantle it is expected to melt when water is released. This concept was originally applied to the region above the transition zone by Bercovici and Karato in a landmark paper in 2003, and was called the 'transition zone water filter' because of the important predicted affects on mantle geochemistry. Interestingly, since then seismic evidence has been mounting indicating melted regions above and below the transition zone. If these signals do indeed represent the presence of hydrous melt in these regions, then the transition zone may act as a double-sided mid-mantle water filter, and the melting that occurs at its boundaries could have modulated the chemistry and geodynamics of the mantle throughout its history. Currently we cannot adequately test this model or understand its implications because we do not know accurately the composition of hydrous silicate melts of the mantle at these depths, nor do we know their physical properties, such as the density and viscosity. Because of this, we are currently unable to model accurately the seismic response expected for hydrous silicate melting at these depths, and we cannot model the dynamic behavior of such melts should they exist in these locations. Here we propose to collect this data.We propose to make high P-T experimental measurements to determine the compositions of hydrous silicate melts in the mantle at depths corresponding to the deep upper mantle, transition zone and upper part of the lower mantle. We will also use novel experiments where we combine diamond anvil cell techniques with synchrotron X-ray scattering methods to determine melt densities. Simultaneously we will use first principles molecular dynamics methods to calculate the physical and seismic properties of these melts, supplemented with experiments to measure their wetting properties. With these data, we will be able for the first time to develop dynamic and seismic models to explicitly test the transition zone water filter model, and make predictions about its chemical and dynamical affects on the mantle, and on the deep water cycle, throughout geological time.