The stability of anhydrous phase B, Mg14Si5O24, has been determined in the pressure range of 14-21 GPa and in the temperature range of 1100-1700 °C with both normal and reversal experiments at high pressures and high temperatures. Our results demonstrate that anhydrous phase B is stable at P-T conditions corresponding to the shallow depth region of the mantle's transition zone and it decomposes into periclase and wadsleyite at greater depth. The decomposition boundary of anhydrous phase B into wadsleyite and periclase has a positive phase transition slope and can be expressed by the following equation, P(GPa)= 7.5 + 6.6 × 10-3T(°C). This result is consistent with a recent result on the decomposition boundary of anhydrous phase B (Kojitani et al., 2017). However, our phase boundary deviates significantly from theirs at temperatures < 1400 °C.

Considering the spatial and temporal heterogeneity of mantle compositions, several mechanisms are proposed to produce MgO-rich conditions for the formation of anhydrous phase B in the deep mantle. Upon the cooling of a magma ocean likely formed during the early history of the Earth, a distinctive anhydrous phase B enriched layer might have formed through crystallization and accumulation in the upper part of the mantle transition zone. Subducting carbonate can be reduced in the metal-saturated mantle at depth > 250 km and abundant (Mg, Fe)O oxides should be introduced into the surrounding mantle. Hydrous melting of peridotite may also produce MgO-enriched components. In addition, dissociation of chromite in natural high-P chromitite is likely to produced lots of oxides. We propose that directly touching ferropericlase-olivine inclusions found in natural diamonds might be the retrogressive products of anhydrous phase B decomposing via the reaction Anh-B = Olivine + Periclase. This decomposition may occur during the transportation of the host diamonds from their formation depths of < 500 km in the upper part of the mantle transition zone to the surface.