Water partitioning between coexisting olivine (ol) and wadsleyite (wads) and the effect of the OH-defect concentrations on the P-T-x coordinates of the ol-wads phase transformation

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The seismic discontinuity at 410 km depth within the Earth¿s mantle is believed to be caused by the transformation of olivine (ol) to wadsleyite (wads) (e.g. Dziewonski and Anderson, 1981). Recent seismic observations indicate that the discontinuity is not as sharp as observed in experiments (e.g. Zhang and Herzberg, 1994) but shows a wide range of thicknesses from about 4 km (e.g. Benz and Vidale, 1993).) to 35 km (van der Meijde et al. (2003). Based on theoretical studies (Wood, 1995; Helffrich and Wood, 1996) van der Meijde et al. (2003) interpret the observed broadening of the 410 km discontinuity as being caused by water incorporation in the coexisting mantle minerals ol and wads. If this is true, varying thicknesses of the discontinuity indicate variable and inhomogenous distribution of water in the upper mantle. However, up to now it has not been confirmed unquestioningly by experiments how water incorporation into the mantle minerals ol and wads influence the phase boundaries between the minerals (Smyth and Frost, 2002; Chen et al., 2002). To fully understand the water cylcle of the Earth and its effect on the Earth¿s structure more experimental data are needed. The goals of this project are to experimentally determine (i) the water saturation of coexisting olivine and wadsleyite under mantle conditions, and (ii) the effect of water on the P-T-x coordinates of the ol-wads phase transition in quenched multi-anvil experiments as well as in-situ in laser heated diamond anvil experiments. On the base of these studies a future project in the second application period will deal with (iii) the influence of water on the mechanisms and kinetics of the ol-wads phase transition.