Ferric iron and water incorporation in wadsleyite at 410-km depth

The 410 km-discontinuity is often invoked to bare a great deal of information about the chemical and thermal state of the Earth’s mantle. Indeed the olivine to wadsleyite transformation is sensitive to many parameters such as temperature, iron content, water content and oxygen fugacity. However, it is yet difficult to state on what is the chemistry of wadsleyite in the mantle near a subducting slab where oxygen and water fugacities are higher than in the mantle away from subducting slabs. In this study we have studied the incorporation of both water and ferric iron in wadsleyite under conditions of the 410 km discontinuity. The samples were synthesized in the multi-anvil press at 13.5 GPa and 1400 degrees C. The ferric iron contents were analysed by means of micro-XANES analysis (X-ray Absorption Near Edge Structure) at ID24 beamline of ESRF performed on samples of wadsleyites coexisting with pyroxene +/- olivine. The results show that ferric iron incorporation increases with increasing iron content up to 40% Fe3+/Fetotal under Re-ReO2 buffering conditions of oxygen fugacity. The XANES data show that ferric iron partitions preferentially into wadsleyite compared to pyroxene or olivine, pointing out that under oxidizing conditions wadsleyite will be stabilised over olivine. Moreover, the Fourier transform infrared spectra display a new mode of incorporation of water into oxidized wadsleyite demonstrating a coupled substitution between water and Fe3+ in wadsleyite. These new data indicate that in the mantle transition zone close to subduction zones, wadsleyite should have a chemical state, oxidized and hydrous, that was never evidenced before. The implications are important for the olivine to wadsleyite transformation as well as for the density constrast of the 410-km discontinuity. In addition, the electrical conductivity signal of such hydrous oxidised wadsleyite should be very high.