Water partitioning in the Earth's mantle

Abstract

We have conducted H2O partitioning experiments between wadsleyite and ringwoodite and between ringwoodite and perovskite at 1673K and 1873K, respectively. These experiments were performed in order to constrain the relative distribution of H2O in the upper mantle, the mantle transition zone, and the lower mantle. We successfully synthesized coexisting mineral assemblages of wadsleyite–ringwoodite and ringwoodite–perovskite that were large enough to measure the H2O contents by secondary ion mass spectrometry (SIMS). Combining our previous H2O partitioning data (Chen et al., 2002) with the present results, the determined water partitioning between olivine, wadsleyite, ringwoodite, and perovskite under H2O-rich fluid saturated conditions are 6:30:15:1, respectively. Because the maximum H2O storage capacity in wadsleyite is ∼3.3wt% (e.g. Inoue et al., 1995), the possible maximum H2O storage capacity in the olivine high-pressure polymorphs are as follows: ∼0.7wt% in olivine (upper mantle just above 410km depth), ∼3.3wt% in wadsleyite (410–520km depth), ∼1.7wt% in ringwoodite (520–660km depth), and ∼0.1wt% in perovskite (lower mantle). If we assume ∼0.2wt% of the H2O content in wadsleyite in the mantle transition zone estimated by recent electrical conductivity measurements (e.g. Dai and Karato, 2009), the estimated H2O contents throughout the mantle are as follows; ∼0.04wt% in olivine (upper mantle just above 410km depth), ∼0.2wt% in wadsleyite (410–520km depth), ∼0.1wt% in ringwoodite (520–660km depth) and ∼0.007wt% in perovskite (lower mantle). Thus, the mantle transition zone should contain a large water reservoir in the Earth's mantle compared to the upper mantle and the lower mantle.