Effect Of Minor Component On Water Contents Of Nominally Anhydrous Minerals In The Upper Mantle

The research on the water abundance of nominally anhydrous minerals （NAMs）was one of the most significant topics in geosciences, while it was little known up todate. NAMs was an important host of water （hydrogen） in the mantle, and there weremany factors that could affect the water abundance in the NAMs, such as temperature,pressure, chemical composition and oxygen fugacity. To date, there were not enoughhigh pressure and high temperature experimental researches related to the effect ofminor composition on the water abundance of the NAMs in the upper mantle, so theoccurrence state of NAMs in the upper mantle, the water recycling in the deep earthand some geodynamic processes were poorly understood. The experiment on theinfluence of minor composition on the water contents in the NAMs was performed,and the reaction mechanisms were discussed. The studies included the influence of Alon the water content of clinopyroxene in the alkali basalt system, the influence of Tion the water content of garnet in SiO₂-Al₂O₃-CaO-TiO₂+H₂O system, and theinfluence of Al on the water content of orthopyroxene in SiO₂-MgO-Al₂O₃+H₂Osystem.The experiments using alkali basalt system with different Al₂O₃contents （0,4,7,11wt.%） were conducted at3GPa and1473K to study the influence of Al on thewater content in clinopyroxene. The results indicated that clinopyroxenes were themain products and their water contents varied from781×10^-6to2181×10^-6. Watercontent was much lower in the clinopyroxene with rich Na₂O+K₂O and TiO₂, poorFeO^*and Al₂O₃than that with rich FeO^*and Al₂O₃, poor Na₂O+K₂O and TiO₂. TheAl-number （100Al/（Al+Na+K）） remarkably affected clinopyroxene’s water content.There were two ways for hydrogen incorporating into clinopyroxene:（1） occupyingthe metal vacancies in the M2site and （2） occupying the isolated hydroxyl defectscreated by a coupled substitution of different valence cations. The first mechanismwas prevailing in the Al-free or nominally Al-free alkali basalt system, while inAl-rich alkali basalt system, the second mechanism was dominant.The experiments using SiO₂-Al₂O₃-CaO-TiO₂+H₂O with different TiO₂contents（0,0.5,1wt.%） were conducted at3GPa and1673K to study the influence of Ti onthe water content in garnet. The results indicated that garnets were the main products and their water contents varied from398×10^-6to817×10^-6. Water content in garnetwas dependent on Ti content in the system, but the water content was highest whenthere was no TiO₂added to the system. The water content decreased at first, thenincreased with increasing TiO₂added into SiO₂-Al₂O₃-CaO-TiO₂+H₂O system. Thediversity of titanic coordination number may be responded to this phenomenon. Thehydrogen incorporated into clinopyroxene in the way of hydro-garnet component([O₄H₄]^4-=Si[O₄]^4-), which could be expressed as: Si4O V4OH. Tiion easily caused the infrared spectrum characteristic peaks split occurring.The experiments using SiO₂-MgO-Al₂O₃+H₂O with different Al₂O₃contents （0,4,12wt.%） were conducted at3GPa and1773K to study the influence of Al on thewater content in orthopyroxene. The results indicated that orthopyroxenes were themain products and their water contents varied from139×10^-6to452×10^-6. The Al₂O₃remarkably affected clinopyroxene’s water content. Water content was the lowest inthe clinopyroxene with Al₂O₃=0wt.%, then water content increased from139×10^-6to452×10^-6as Al₂O₃increased from0wt.%to12wt.%. This phenomenon wasprobably contributed to the similarity of ionic radius between Al₃₊and Si₄₊, and thediversity of aluminium coordination number. Hydrogen incorporated intoorthopyroxene through occupying the metal vacancies in the M2site in the Al-free ornominally Al-free system and the reaction could be expressed as Mg²⁺2H⁺.Hydrogen entered into orthopyroxene by occupying the isolated hydroxyl defectscreated by a coupled substitution in Al-rich SiO₂-MgO-Al₂O₃+H₂O system, and thereaction could be expressed as Si⁴⁺ Al³⁺+H⁺and2Mg²⁺ Al³⁺+H⁺. Inaddition, the most likely host way for Al and hydrogen incorporating intoorthopyroxene was solid solution with Mg-Tschermarks component as MgAl₂SiO₆.