Discovery of a hexagonal hydrous phase in the deep lower mantle - Dr. Li Zhang

MARCH 7, 2018

The Earth’s lower mantle comprises >55% by volume of our planet, extending from 670 to 2900 kilometers in depth. The lower mantle is potentially the most massive water reservoir in our planet, which largely depends on availability of hydrous minerals which can store and transport water down to the deep lower mantle. Through high-pressure-temperature experiments in a laser-heated diamond anvil cell at 107–136 GPa and 2,400 K, a research team at HPSTAR discovered an ultradense hydrous phase in (Fe0.8Al0.2)OOH with a previously unknown hexagonal lattice. The team includes Li Zhang, Hongsheng Yuan, Ho-kwang Mao of HPSTAR, and Yue Meng of High Pressure Collaborative Access Team (HPCAT) of the Advanced Photon Source (APS). The discovery is reported in Proceeding of the National Academic of Sciences, USA.

Experimentally, it is a great challenge to identify an unknown hydrous phase in a multiphase assemblage under extreme high-pressure-temperature conditions corresponding to the deep Earth. By combining powder x-ray diffraction techniques with multigrain indexation, the research team was able to determine the hexagonal hydrous phase with a=10.5803(6) Å and c=2.5897(3) Å stable up to 2400 K at 110 GPa. The high-pressure x-ray multigrain technique was established in previous studies from this group (ZHANG et al., 2013; ZHANG et al., 2016). Tens of individual crystallites, each with its unique orientation matrix, confirm the existence of the hexagonal hydrous phase, referred to as the “HH-phase”.

After quench from laser heating to room temperature, the nearly pure HH-phase gradually transformed to the cubic pyrite-phase (Py-phase) with the same density with increasing time. The high brilliance monochromatic x-ray beam available at the synchrotron radiation sources allowed us to in situ monitor the phase changes, as shown in the Figure. The Py-phase in (Fe0.8Al0.2)OOH is only stable at low temperature whereas the pyrite-phase FeOOH was found stable at least up to 2,500 K, suggesting that the stability of the Py-phase expands to high temperature as its composition approaches the FeOOH endmember. A recent study led by Dr. Mao found that the Py-phase FeOOH can be produced by reaction between water and Fe at the core mantle boundary (MAO et al., 2017). The HH-phase can be formed when δ-AlOOH incorporates Py-phase FeOOH and may store a substantial quantity of water in the deep lower mantle.

Experiments were performed at the High Pressure Collaborative Access Team (HPCAT 16-IDB) of the APS, Argonne National Laboratory, and the BL15U1 beamline, Shanghai Synchrotron Radiation Facility in China. Link to the paper: https://doi.org/10.1073/pnas.1720510115

Caption: In situ x-ray diffraction observation of the gradual growth of the cubic pyrite phase at expense of the hexagonal hydrous phase in (Fe,Al)OOH within 10 minutes after quench to room temperature at 107 GPa. Py: the cubic pyrite phase; HH: the hexagonal hydrous phase; Ne: neon was used as pressure medium.

References:

Mao, H.-K., Hu, Q., Yang, L., Liu, J., Kim, D.Y., Meng, Y., Zhang, L., Prakapenka, V.B., Yang, W., Mao, W.L. (2017) When water meets iron at Earth's core–mantle boundary. National Science Review.

Zhang, L., Meng, Y., Dera, P., Yang, W., Mao, W.L., Mao, H.K. (2013) Single-crystal structure determination of (Mg,Fe)SiO3 postperovskite. Proceedings of the National Academy of Sciences 110, 6292-6295.

Zhang, L., Popov, D., Meng, Y., Wang, J., Ji, C., Li, B., Mao, H.-k. (2016) In-situ crystal structure determination of seifertite SiO2at 129 GPa: Studying a minor phase near Earth’s core–mantle boundary. American Mineralogist 101, 231-234.

张莉研究员课题组与合作团队利用同步辐射X射线衍射技术，在百万大气压和2400卡尔文所对应的下地幔高温高压条件，发现了(Fe0.8Al0.2)OOH具有致密的六方结构，该六方相在110 GPa的晶胞参数为a=10.5803(6) Å 和 c=2.5897(3) Å，该结构类型属首次发现。该研究应用高压同步辐射多晶X射线衍射法，确定了数十个符合该六方晶胞的亚微米晶粒，各自具有不同的晶面取向，从而确定该六方含水相的存在及其基本性质，简称为”HH-phase”。在107 GPa的压力下从激光加温卸温到室温后，六方相逐步转化为立方相(黄铁矿结构)。通过X射线衍射原位监控样品的变化，发现10分钟后，接近三分之二的六方相转化为立方相（如图）。近期由毛河光院士领导的研究工作已经证明了含水矿物FeOOH具有黄铁矿结构，且可由铁和其它储水载体在核幔边界释放出来的水反应生成。该六方相和立方相含水矿物随温度的相互转化是可逆并且可重复的，证明铝元素的加入改变了下地幔深部含水矿物的性质和相稳定性。