In situ X‐ray observation of decomposition of superhydrous phase B at high pressure and temperature

High pressure and high temperature in situ X‐ray diffraction experiments on the stability of superhydrous phase B have been conducted to clarify the host of water in the slabs descending into the lower mantle. We observed the decomposition reaction of superhydrous phase B into phase G (= phase D and F), periclase, and MgSiO 3 perovskite at around 700 ∼1100°C and above 30 GPa, and the boundary of the decomposition reaction can be expressed as P(GPa) = −0.0037(±0.0017)T(°C) + 32.2(±0.5). This reaction can occur in slabs descending into the uppermost part of the lower mantle.