Renewal: Petrology and Geochemistry of the Deep Lower Mantle

从地球表面下670公里的深度延伸到2900公里深处的核心-地幔边界，下地幔的体积占地球的一半以上。关于其化学成分和岩石学成分的信息对于了解固体地球至关重要，但非常缺乏，特别是对于1800公里以下难以进入的深下地幔（DLM）。DLM极高的压力和温度条件使其很难在实验室中进行模拟和研究，并且仍然存在许多无法解决的谜团。Mao Ho-kwang提出利用其新开发的高压多粒晶体技术（MGC）在DLM条件下原位圈定地幔岩石的晶体结构，并利用聚焦离子束技术结合电子显微镜探测淬火样品中各种矿物相的组成。研究结果将揭示该地区的地球化学储层、核幔交换、地幔对流源、上升羽流的根源和俯冲板块的墓地等重要特征。流行的理论是，下地幔的大部分是由镁铁硅酸盐（Mg,Fe）SiO3组成的，称为桥菱铁矿。人们认为，这种矿物在巨大的压力和温度范围内不会改变下地幔的结构，除了在超过110千兆帕斯卡的最底部D层。最近，在模拟下地幔的条件时，毛的团队使用激光加热的金刚石砧细胞，使其压力在95到101千兆帕斯卡之间，温度在2200到2400开尔文之间，发现镁铁菱镁矿是不稳定的。它分解成两相：一种是不含铁的硅酸镁桥菱石，化学式中不含铁的部分表示；另一种是富铁的六角形结构，称为h相。进一步的测试表明，这种富铁的h相比铁掺入镁铁桥菱铁矿时更稳定，这让所有人都感到惊讶。在DLM中也可能有更多未确定的阶段，等待由当前项目确定。这些阶段的发现和特征将导致地球深处的一个根本的新范式。

Stretching from 670 kilometers depth below the Earth's surface to the core-mantle boundary at 2,900 kilometers depth, the lower mantle comprises more than half of our planet by volume. Information on its chemical composition and petrological constituents is crucial for understanding the solid Earth, but is very scarce, especially for the inaccessible deep lower mantle (DLM) underneath 1800 kilometers. The extremely high pressure and temperature conditions of the DLM make it very difficult to simulate and investigate in the laboratory, and numerous unresolvable mysteries remain. Ho-kwang Mao proposes to use his newly developed high-pressure multigrain crystallographic (MGC) technology to delineate the crystal structures of the mantle rock in-situ under the DLM conditions, and use the focused ion beam technique combined with electron microscope to probe the compositions of various mineral phases in the quenched samples. The results will shed light on many key features of the DLM, including the geochemical reservoir, the core-mantle exchange, the source of mantle convection, the root of rising plumes and the graveyard of subducted plates. The prevailing theory was that the majority of the lower mantle is made up of a ferromagnesian silicate, (Mg,Fe)SiO3, called bridgmanite. It was thought that this mineral did not change structure over the enormous range of pressures and temperatures found in the lower mantle except at the very bottom D" layer beyond 110 gigapascals. Very recently, in simulating the conditions of the lower mantle using laser-heated diamond anvil cells brought to pressures between 95 and 101 gigapascals and temperatures between 2,200 and 2,400 Kelvin, Mao's team found that ferromagnesian bridgmanite is unstable. It disassociates into two phases: one magnesium silicate bridgmanite without the iron, which is represented by the Fe-free portion of the chemical formula, and one iron-rich and hexagonal in structure, called the H-phase. Further tests showed that this iron-rich H-phase is more stable than when the iron is incorporated in the ferromagnesian bridgmanite, much to everyone's surprise. There could be many more unidentified phases down there in the DLM as well, waiting to be identified by the present project. Discoveries and characterizations of these phases will lead to a fundamentally new paradigm of the deep Earth.