Abstract
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–SiO2–H2O 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.
Highlights
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 series of high-pressure experiments[13] has demonstrated, that the MgO–SiO2–H2O forms of these dense hydrous magnesium silicates (DHMS) phases are stable at slab temperatures but decompose at lower temperatures than the normal mantle-geotherm
Super-aluminous phase D with extremely high Al-content ( Mg0.2Fe0.15Al1.8SiO6H1.8)[10] was first synthesized at 1300 °C and 25 GPa from a bulk composition similar to that reported for bridgmanite formed from a mid-ocean ridge basalt (MORB) composition
Summary
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 series of high-pressure experiments[13] has demonstrated, that the MgO–SiO2–H2O forms of these DHMS phases are stable at slab temperatures but decompose at lower temperatures than the normal mantle-geotherm.
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