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A nearly water-saturated mantle transition zone inferred from mineral viscosity

Hongzhan Fei1,2, Daisuke Yamazaki1, Moe Sakurai1,3, Nobuyoshi Miyajima2, Hiroaki Ohfuji4, Tomoo Katsura2, and Takafumi Yamamoto5

1Institute for Study of the Earth’s Interior, Okayama University, Misasa, Tottori 682-0193, Japan.
2Bayerisches Geoinstitut, Universität Bayreuth, Bayreuth D95440, Germany.
3Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo 152-8551, Japan.
4Geodynamics Research Center, Ehime University, Matsuyama 790-8577, Japan.
5Department of Earth and Planetary Systems Science, Hiroshima University, Hiroshima 739-8526, Japan

 

Citation: Fei, H., et al. (2017), Science Advances 3, e1603024. doi: 10.1126/sciadv.1603024.

Summary:

An open question for solid-earth scientists is the amount of water in Earth’s interior. The uppermost mantle and lower mantle contain little water because their dominant minerals, olivine and bridgmanite, have limited water storage capacity. In contrast, the mantle transition zone (MTZ) at a depth of 410 to 660 km is considered to be a potential water reservoir because its dominant minerals, wadsleyite and ringwoodite, can contain large amounts of water [up to 3 weight % (wt %)]. However, the actual amount of water in the MTZ is unknown. Given that water incorporated into mantle minerals can lower their viscosity, we evaluate the water content of the MTZ by measuring dislocation mobility, a property that is inversely proportional to viscosity, as a function of temperature and water content in ringwoodite and bridgmanite. We find that dislocation mobility in bridgmanite is faster by two orders of magnitude than in anhydrous ringwoodite but 1.5 orders of magnitude slower than in water-saturated ringwoodite. To fit the observed mantle viscosity profiles, ringwoodite in the MTZ should contain 1 to 2 wt % water. It means the total amount of water in the mantle transition zone is nearly he same as all the world’s ocean water.

 

Fig. 1.The dislocation mobility (k) profile fits well with the mantle viscosity (η) when ringwoodite and wadsleyite in the mantle transition zone incorporated 1-2 wt.% water. Dislocation mobility of olivine from Wang et al. (EPSL 2016), viscosity profile from Mitrovica et al. (EPSL 2004).