Thermal expansion of wadsleyite, ringwoodite, hydrous wadsleyite and hydrous ringwoodite

Abstract

The thermal expansions of wadsleyite, ringwoodite, hydrous wadsleyite, and hydrous ringwoodite have been determined by high temperature powder X-ray diffraction at ambient pressure. Wadsleyite and ringwoodite persist up to 700 °C, and their thermal expansion coefficients are measured as 34.0(5)×10 −6 and 30.7(6)×10 −6 K −1, respectively, values that are slightly larger than those measured by [J. Phys. Earth 27 (1979) 53; J. Phys. Earth 28 (1980) 273]. Hydrous wadsleyite and hydrous ringwoodite were dehydrated at 450 and 400 °C, respectively. Below their dehydration temperatures, their thermal expansion coefficients are 30.1(14)×10 −6 and 27.3(9)×10 −6 K −1, respectively, which are slightly lower than those of their anhydrous forms. Using the present results combined with the previous thermoelastic data, the seismic velocity contrast at the 410 km discontinuity has been calculated for both dry and wet mantle. If a pyrolite composition is assumed, a hydrous mantle transition zone better explains seismic constraints.