Fei Hongzhan, Yamazaki Daisuke, Sakurai Moe, Miyajima Nobuyoshi, Ohfuji Hiroaki, Katsura Tomoo, Yamamoto Takafumi
Institute for Study of the Earth's Interior, Okayama University, Misasa, Tottori 682-0193, Japan.
Bayerisches Geoinstitut, Universität Bayreuth, Bayreuth D95440, Germany.
Sci Adv. 2017 Jun 7;3(6):e1603024. doi: 10.1126/sciadv.1603024. eCollection 2017 Jun.
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. The MTZ should thus be nearly water-saturated globally.
对于固体地球科学家来说，一个悬而未决的问题是地球内部的水量。上地幔和下地幔含水量很少，因为它们的主要矿物橄榄石和布里奇曼石的储水能力有限。相比之下，深度在410至660公里的地幔过渡带（MTZ）被认为是一个潜在的水库，因为其主要矿物林伍德石和沃兹利石可以容纳大量的水[高达3重量%（wt%）]。然而，MTZ中的实际水量尚不清楚。鉴于纳入地幔矿物中的水会降低其粘度，我们通过测量位错迁移率来评估MTZ的含水量，位错迁移率是一种与粘度成反比的性质，它是林伍德石和布里奇曼石中温度和含水量的函数。我们发现，布里奇曼石中的位错迁移率比无水林伍德石快两个数量级，但比水饱和林伍德石慢1.5个数量级。为了拟合观测到的地幔粘度剖面，MTZ中的林伍德石应含有1至2 wt%的水。因此，MTZ在全球范围内应该几乎是水饱和的。
