Research highlights

Presence of hydrous phases explains the sharp D'' discontinuity in the deepest lower mantle

In a lower mantle system, phase changes are associated with element partitioning  in a mineral assemblage under high pressure-temperature (P-T) conditions. On the other hand, the lower mantle is the most massive potential water reservoir in Earth. Some amounts of water may be stored in nominally anhydrous lower mantle phases, and the presence of water components may also stabilize hydrous phases. Due to the technical challenges, how water can alter phase relations in the lower mantle system remains unclear.

In this study, we performed high P-T experiments on a model composition of subducted slabs in the system MgO-Al₂O₃-Fe₂O₃-SiO₂-H₂O containing ~7 wt.% water in laser-heated diamond anvil cells over a P-T range of 104‒126 GPa and 1900‒2500 K. The phase assemblages were determined by the combination of in-situ synchrotron-based X-ray diffraction (XRD) and ex situ transmission electron microscope (TEM) analysis.

In this hydrous system, the Fe-bearing δ-phase and the pyrite-type FeOOH_x phase were found coexisting with bridgmanite (Bdg), post-perovskite (pPv), or both. we observed that Al depletion in both the Bdg and pPv phases can significantly reduce the width of the Bdg to pPv transition in contrast to a wide two-phase coexistence region in a dry Al-rich system. Meanwhile, the Fe enrichment in the pPv phase relative to the coexisting Bdg phase lowers the transition pressure to the depth of the D'' discontinuity. Accordingly, the depth and thickness of the Bdg to pPv transition in subducted basaltic crustal materials can explain the seismically detected D'' discontinuity.

 This study was published in Earth and Planetary Science Letters. EPSL(2019)115714.pdf

Caption: The (Fe,Al)-bearing Bdg to pPv phase transition thickness in our hydrous system is determined to be less than 150 km (~8 GPa). Applying the paritioning coefficients obtained in this study, the Al₂O₃ content in both Bdg and pPv phases can be further reduced from the currect value of ~4 mol% to ~1‒2 mol% in hydrous subducted slabs, and accordingly, the transition thickness can be further reduced.

Discovery of a hexagonal hydrous phase in the deep lower mantle

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, we discovered an ultradense hydrous phase in (Fe_0.8Al_0.2)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).

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. Tens of individual crystallites, each with its unique orientation matrix, confirm the existence of the hexagonal hydrous phase, referred to as the “HH-phase”. 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.

The discovery is reported in the Proceeding of the National Academic of Sciences, USA. 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 quenching to room temperature at 107 GPa. Py: the cubic pyrite phase; HH: the hexagonal hydrous phase; Ne: neon was used as pressure medium.