Water in the Deep Earth

地球深处的水

• 批准号: NE/H006362/1
• 负责人: Michael Walter
• 金额: $ 26.79万
• 依托单位: University of Bristol
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2010
• 资助国家: 英国
• 起止时间: 2010 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FH006362%2F1
• 关键词: Water; Deep; Earth

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.

每个人都熟悉水在地球水圈中的重要性。水圈可能归功于岩石中的水，因为水是通过从地球内部衍生而来的岩浆脱落到地球表面的。我们不知道地球目前含有多少水或过去的水。基于我们目前对构成地球地幔的矿物质水的最大能力的了解，地幔中可能有几个海洋的水，并且可能足够多的氢溶解在核心中。在地球最早的历史中，深硅酸盐岩浆海洋首先结晶时可能会有更多的水。我们目前不知道由于积聚期间的影响侵蚀而丧失了一个或多个可能的水丰富的气氛。通过在含水液中转移溶解的材料，可能已经参与了一个或多个水海洋的内部分化。我们都知道水在地面上的惊人特性。事实证明，当矿物质，融化和地幔中的流体溶解时，它具有不同但同样令人惊奇的特性。即使一点一点也可以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腐蚀性溶剂一样，可以溶解岩石。水作为流体，由于其低密度和粘度，水也很容易在地幔岩石中移动，因此它可以运输其溶解的材料。众所周知，水是上地幔中传质的重要药物，改变了它通过的岩石的化学和物理特性。实验者已经能够确定构成地球上地幔的岩石和矿物的水的表现。例如，在高温和压力下，水和熔体不再截然不同，而是地幔中的热水溶解了太多的硅质，就像熔体一样。这种“超临界”的含水流体可能会向上移动，并改变上面的岩石的化学反应。硅酸盐地幔的底部为〜2900 km，目前我们对下层深度（〜660-2900 km）深度范围内的含水液的行为知之甚少，这构成了地球中最大的硅酸盐储层。我们想在此提出的研究中要做的是弄清含水液如何影响下地幔矿物质镁和钙壶岩的稳定性。我们还想了解溶解高P和T下较低地幔矿物的含水液的化学性质。我们将使用多隔离压力设备（〜24 GPA）进行高P-T实验，以及激光加热的钻石钻石砧木细胞（24-100 GPA）。我们将在包含MGO，CAO，SIO2和H2O的系统中加热水分组成。在多动物实验中，我们可以使用现代的微型探针分析技术直接测量液压流体的主要和微量元素组成。在钻石砧细胞实验中，我们将使用原位同步加速器X射线衍射技术仔细跟踪相位关系以隔离流体的主要元素组成。通过收集的数据，我们希望回答重要的问题，例如：含水从下层到上地幔的运动能否显着改变其成分？钙钛矿相和含水液之间的痕量元件分区如何用作地幔中流体转移的示踪剂？现在的MGO（也许是CAO）耗尽了下层地幔，现在它已经接近100％的钙钛矿了？穿透到下地幔中的俯冲平板可以提供足够的液体以改变地球化学在地球历史上的改变吗？我们提出的研究将提供第一个系统的实验数据集，该数据集将使我们能够解决有关水在地球演化和分化中的作用的重要问题。

项目成果

Phase Relations In The System MgO-SiO2-H2O At Lower Mantle Conditions

下地幔条件下 MgO-SiO2-H2O 体系中的相关系

• 发表时间: 2013
• 期刊: AGU Fall Meeting Abstracts
• 作者: Walter M. J.

The stability of hydrous silicates in Earth's lower mantle: Experimental constraints from the systems MgO-SiO2-H2O and MgO-Al2O3-SiO2-H2O

• DOI: 10.1016/j.chemgeo.2015.05.001
• 发表时间: 2015-12-15
• 期刊: CHEMICAL GEOLOGY
• 影响因子: 3.9
• 作者: Walter, M. J.;Thomson, A. R.;Kohn, S. C.
• 通讯作者: Kohn, S. C.