Systematic study of hydrogen incorporation into Fe-free wadsleyite

The H2O content of wadsleyite were measured in a wide pressure (13-20 GPa) and temperature range (1,200-1,900 degrees C) using FTIR method. We confirmed significant decrease of the H2O content of wadsleyite with increasing temperature and reported first systematic data for temperature interval of 1,400-1,900 degrees C. Wadsleyite contains 0.37-0.55 wt% H2O at 1,600 degrees C, which may be close to its water storage capacity along average mantle geotherm in the transition zone. Accordingly, water storage capacity of the average mantle in the transition zone may be estimated as 0.2-0.3 wt% H2O. The H2O contents of wadsleyite at 1,800-1,900 degrees C are 0.22-0.39 wt%, indicating that it can store significant amount of water even under the hot mantle environments. Temperature dependence of the H2O content of wadsleyite can be described by exponential equation C-H2O = 637.07e(-0.0048T), where T is in degrees C. This equation is valid for temperature range 1,200-2, 100 degrees C with the coefficient of determination R-2 = 0.954. Temperature dependence of H2O partition coefficient between wadsleyite and forsterite (D-wd/fo) is complex. According to our data apparent D-wd/fo decreases with increasing temperature from D-wd/fo = 4-5 at 1,200 degrees C, reaches a minimum of D-wd/fo = 2.0 at 1,400-1,500 degrees C, and then again increases to D-wd/fo = 4-6 at 1,700-1,900 degrees C.