Nakatsuka Akihiko, Yoshiasa Akira, Ohkawa Makio, Ito Eiji
Graduate School of Sciences and Technology for Innovation, Yamaguchi University, Ube, 755-8611, Japan.
Faculty of Advanced Science and Technology, Kumamoto University, Kumamoto, 860-8555, Japan.
Sci Rep. 2022 Mar 4;12(1):3594. doi: 10.1038/s41598-022-07007-8.
The potential for storage of a large quantity of water/hydrogen in the lower mantle has important implications for the dynamics and evolution of the Earth. A dense hydrous magnesium silicate called phase D is a potential candidate for such a hydrogen reservoir. Its MgO-SiO-HO form has been believed to be stable at lower-mantle pressures but only in low-temperature regimes such as subducting slabs because of decomposition below mantle geotherm. Meanwhile, the presence of Al was reported to be a key to enhancing the thermal stability of phase D; however, the detailed Al-incorporation effect on its stability remains unclear. Here we report on Al-bearing phase D (Al-phase D) synthesized from a bridgmanite composition, with Al content expected in bridgmanite formed from a representative mantle composition, under over-saturation of water. We find that the incorporation of Al, despite smaller amounts, into phase D increases its hydrogen content and moreover extends its stability field not only to higher temperatures but also presumably to higher pressures. This leads to that Al-phase D can be one of the most potential reservoirs for a large quantity of hydrogen in the lower mantle. Further, Al-phase D formed by reaction between bridgmanite and water could play an important role in material transport in the lower mantle.
在下地幔中储存大量水/氢的可能性对地球的动力学和演化具有重要意义。一种名为D相的致密含水镁硅酸盐是这种氢储存库的潜在候选物。其MgO-SiO-HO形式被认为在下地幔压力下是稳定的，但由于在地幔地热线下会分解，所以只在俯冲板块等低温区域稳定。同时，据报道铝的存在是提高D相热稳定性的关键；然而，铝的掺入对其稳定性的详细影响仍不清楚。在此，我们报告了在水过饱和条件下，由布里奇曼石成分合成的含铝D相（Al-D相），其铝含量与由代表性地幔成分形成的布里奇曼石中的预期含量相同。我们发现，尽管铝的掺入量较少，但进入D相后会增加其氢含量，而且不仅将其稳定场扩展到更高温度，还可能扩展到更高压力。这使得Al-D相可能成为下地幔中大量氢的最具潜力的储存库之一。此外，由布里奇曼石与水反应形成的Al-D相可能在下地幔的物质传输中发挥重要作用。
