The stability of anhydrous phase B, Mg14Si5O24, at mantle transition zone conditions
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The stability of anhydrous phase B, Mg14Si5O24, has been determined in the pressure range of 14–21 GPa and the temperature range of 1100–1700 °C with both normal and reversal experiments using multi-anvil apparatus. Our results demonstrate that anhydrous phase B is stable at pressure–temperature conditions corresponding to the shallow depth region of the mantle’s transition zone and it decomposes into periclase and wadsleyite at greater depths. 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 × 10−3 T (°C). This result is consistent with a recent result on the decomposition boundary of anhydrous phase B (Kojitani et al., Am Miner 102:2032–2044, 2017). However, our phase boundary deviates significantly from this previous study at temperatures < 1400 °C. Subducting carbonates may be reduced at depths > 250 km, which could contribute ferropericlase (Mg, Fe)O or magnesiowustite (Fe, Mg)O into the deep mantle. Incongruent melting of hydrous peridotite may also produce MgO-rich compounds. Anh-B could form in these conditions due to reactions between Mg-rich oxides and silicates. Anh-B might provide a new interpretation for the origin of diamonds containing ferropericlase–olivine inclusions and chromitites which have been found to have ultrahigh-pressure characteristics. 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 (Mg,Fe)14Si5O24 (Anh-B) = (Mg,Fe)2SiO4 (olivine) + (Mg,Fe)O (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.
KeywordsAnhydrous phase B Phase transition Raman spectroscopy High-P chromite Diamond inclusions
E.O. acknowledges the financial support from the Japan Society for the Promotion of Science (No. 15H05748). A.S. acknowledges the financial support from the Ministry of Education, Science, Sports and Culture, Grant-in-Aid for Scientific Research on Innovative Areas (15H05826, 15H05828, and 15K21712). Z.J. thanks the financial support from the National Natural Science Foundation of China (41174076). L.Y. is supported by the International Joint Graduate Program in Earth and Environmental Science (GP-EES), Tohoku University. D.F. acknowledges DFG funding of the International Research and Training Group “Deep Volatile Cycles”.
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