Renewal: Petrology and Geochemistry of the Deep Lower Mantle

下地幔从地球表面以下670公里深处延伸到2,900公里深处的核幔边界，按体积计算，它占了地球的一半以上。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，但非常稀缺，特别是对于1800公里以下无法进入的深层下地幔（DLM）。DLM极高的压力和温度条件使得在实验室中模拟和研究非常困难，并且仍然存在许多无法解决的谜团。Ho-kwang Mao建议使用他新开发的高压多晶粒晶体学（MGC）技术在DLM条件下原位描绘地幔岩石的晶体结构，并使用聚焦离子束技术结合电子显微镜来探测淬火样品中各种矿物相的组成。 研究结果将揭示DLM的许多关键特征，包括地球化学储集层、核幔交换、地幔对流源、上升柱的根源和俯冲板块的墓地。 当时流行的理论是，下地幔的大部分是由一种铁镁硅酸盐，（Mg，Fe）SiO 3，称为硼镁石。人们认为这种矿物在下地幔中发现的巨大压力和温度范围内不会改变结构，除了在超过110吉帕斯卡的最底部D”层。最近，在使用激光加热的金刚石砧室模拟下地幔的条件时，压力在95至101千帕之间，温度在2，200至2，400开尔文之间，Mao的团队发现铁镁硼镁石是不稳定的。它分解成两个相：一个是不含铁的硅酸镁硼镁石，由化学式的无铁部分表示，另一个是富含铁的六方结构，称为H相。进一步的测试表明，这种富含铁的H相比铁被纳入铁镁布里奇曼石时更稳定，这让所有人都感到惊讶。在DLM中可能还有更多未识别的阶段，等待由当前项目识别。 这些阶段的发现和特征将导致一个全新的地球深部范例。

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.