[SIT23-P04] Stability of anhydrous phase B, Mg14Si5O24, at the mantle transition zone conditions
キーワード：Anhydrous phase B, phase relations, Clapeyron slope, diamond inclusions, Raman spectra
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 reverse experiments at high pressures and high temperatures. Our results imply that anhydrous phase B is stable at the P-T conditions corresponding to the shallow depth of the mantle 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 x 10-3T(°C). Configuration disorder might account for an increase of entropy for anhydrous phase B at high-temperature conditions. It is suggested that MgO-rich conditions can be available in the deep mantle during hydrous melting of peridotite and the reduction of subduction carbonates by the metal-saturated mantle at depth > 250 km. Anhydrous phase B might become an important phase in the area where SiO2 activity is low. We propose that the paragenesis of directly touched ferropericlase-olivine inclusions in natural diamonds might be the retrogression products of anhydrous phase B via the decomposition reaction Anh-B = Olivine + Periclase during the transportation of host diamonds from the deep to the surface. Our experimental results put a constraint on the origin of such diamonds at a depth less than 500 km. On the cooling of the magma ocean during the early history of the Earth, a distinctive layer that was concentrated with hydrogen and other elements such as Fe, Ca, Mn substituting Mg might exist with the crystallization and accumulation of anhydrous phase B at depth equivalent to the upper part of the mantle transition zone.