Discovery of an ultradense hydrous phase in the deep lower mantle: Application of multigrain crystallography in megabar experiments

Water enters the Earth's interior through hydrated subducting slabs. The Al-rich hydrous phase is stable throughout the high pressure-temperature (P-T) conditions of the deep lower mantle (DLM), but with a density 10% less than the surrounding mantle (1); it cannot form as a sizeable water reservoir. Our experiments at 107-136 GPa and 2400 K showed that AlOOH could incorporate up to 75% FeOOH_x in a previously unknown hexagonal structure and the product is 17% denser than the mantle and 31% less dense than the core. Applying the multigrain crystallography, 27 individual crystallites—each with 50 to 120 reflections consistent with the HH-phase—were discovered, unequivocally confirming the 12-formula unit of the HH-phase. The hexagonal (Fe,Al)OOH_x (HH-phase) can transform to the cubic pyrite structure at low T with the same density, isostructural to the pyrite (Py) structured FeO₂ and FeOOH (2). The HH-phase can be formed when delta-AlOOH incorporates FeOOH_x produced by reaction between Fe and water at the DLM conditions, thus storing a substantial quantity of water at the CMB.

1. I. Ohira, et al., Earth and Planetary Science Letters 401, 12-17 (2014).
2. Q. Hu, et al., Nature 534, 241-244 (2016).