水在下地幔深部的赋存形式

The knowledge of the water cycling and storage is crucial for understanding the evolution and dynamics of the Earth. Important experimental results have been obtained on the structure, stability and thermal equation of states of the hydrous phases under the deep lower mantle conditions using diamond anvil cells coupled with laser-heating techniques, but investigation on the hydrous multiple-component systems remains limited. Because of the tiny sample in a diamond anvil cell, identification of phase assemblages and mineral chemistry in different potential lower-mantle compositions has been a long-term challenge in mineral physics. In this project, we will combine synchrotron X-ray diffraction with multigrain analysis to identify the phase assemblages and determine the pressure dependent unit-cell volume, and further transmission electron microscopy analysis will be conducted on the recovered samples. We will carry out high pressure-temperature experiments on different hydrous lower-mantle compositions. This project will focus on (i) exploration of a hydrous phase in Fe-bearing systems; (ii) identification of phase assemblages and mineral chemistry in hydrous peridotitic and basaltic systems; (iii) elemental partitioning in the hydrous systems at different pressure-temperature conditions corresponding to different depths in Earth’s deep interior. The outcome of this project may provide new implications for understanding the storage of water in the deep lower mantle.

地球内部水的循环与储存对深入理解地球演变及动力学至关重要。迄今，利用金刚石对顶砧与激光加热技术，科研工作者们对下地幔深部单相含水矿物的结构、稳定性和热状态方程的研究取得了丰硕的成果，但对于含水多元体系的研究却进展缓慢。由于金刚石对顶砧合成的样品微小，准确鉴别复杂多元体系中的矿物组成与化学成分是矿物物理学的一项长期挑战。为克服这一困难，本申请项目将结合同步辐射X射线衍射实验与多晶粒晶体结构解析方法，原位鉴定多相矿物结构及压强体积关系；进一步利用微区元素分析手段对矿物化学成分进行表征。本申请项目将对不同含水岩石成分展开系统的实验研究。研究内容具体涵括三个方面：（1）含铁体系中潜在含水相的探索及稳定性研究；（2）含水橄榄岩质和含水玄武岩质成分在下地幔深部条件的矿物相组成；（3）不同深度下含水体系中矿物间的元素分配。本研究项目的结果有望使人们对下地幔深部水的存储产生新的认识。

数十亿年的板块运动可将一定量的水通过俯冲板片的储水相输运至地球内部。俯冲岩石圈板块由海洋沉积物、玄武质洋壳和橄榄岩地幔组成。水在俯冲板片中的分布是不均匀的，这取决于相应储水相在压力-温度和局部矿物学条件下的稳定性。我们按计划针对不同岩石成分模型中潜在储水相为研究对象，对其高温高压条件下的稳定性和晶体化学成分做了探索性研究。得到了以下创新结果：(1)首次在下地幔中底部发现了冷俯冲板片的玄武岩成分中高含铝的NiAs型二氧化硅储水相。结果发表在《地球物理研究通讯》。(2)观察到俯冲板片的洋中脊玄武岩成分中含水“后斯石英”型二氧化硅沿着地幔地热线82 GPa附近发生了铝和氢含量陡增的有趣现象。（3）实验论证了了高压黄铁矿相过氧化铁与镁的不相容特性。结果发表在《美国矿物学家》。我们的研究为理解地球深部水循环提供了重要的启示.