Water transport into the deep mantle and formation of a hydrous transition zone
Water transport into the deep mantle and formation of a hydrous transition zone
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DOI：
10.1016/j.pepi.2003.09.015
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发表时间：
2004-06-15
影响因子：
2.3
通讯作者：
Kondo, T
中科院分区：
文献类型：
--
作者：
Ohtani, E;Litasov, K;Kondo, T
下地幔顶部橄榄岩-水体系的物相关系表明，各种含水矿物可在过渡带中输送和储存一定量的水，从而形成含水过渡带。本文给出了水对橄榄石-瓦德斯莱石相变动力学影响的初步实验结果，结果表明水对橄榄石-瓦德斯莱石相变动力学有显著的促进作用。深部地幔存在潜在的脱水部位；根据温度剖面，沿下降板块划分出4个位置，即俯冲带的地幔楔、地幔过渡带、下地幔顶部含水环伍德岩和超含水B相分解的位置，以及下地幔深部约1200 ~ 1500 km处D(G)相分解发生的位置。我们还期望在上地幔底部有一个脱水熔融点，在那里过渡带的热地幔柱可以脱水。过渡带可以作为地幔尺度水循环系统的吸收体。最近的一些地震工作，包括p波层析成像显示，在过渡带中存在低速异常，可能是一个湿过渡带。根据我们对湿条件下橄榄石向瓦兹利石转变的动力学研究，410km不连续面的高程和冷板中亚稳橄榄石楔块的缺乏明确证据可能与冷板和湿板中橄榄石向瓦兹利石的平衡相变相一致。(C) 2004 Elsevier B.V.版权所有
The phase relations in the peridotite-water system to the top of the lower mantle reveal that a certain amount of water can be transported and stored in the transition zone by various hydrous minerals resulting in formation of a hydrous transition zone. Results of preliminary experiments to clarify the effect of water on kinetics of the olivine-wadsleyite transformation are given, and show that water enhances the transformation kinetics significantly.There are potential dehydration sites in the deep mantle; four sites along the descending slabs depending on their temperature profiles, i.e., the mantle wedge of the subduction zone, mantle transition zone, top of the lower mantle where hydrous ringwoodite and superhydrous phase B decompose, and deep in the lower mantle around 1200-1500 km depth where decomposition of phase D(G) occurs. We also expect one site of dehydration melting at the base of the upper mantle where the hot mantle plume hydrated in the transition zone can be dehydrated. The transition zone can serve as a water absorber for the mantle scale water circulation system.Some of the recent seismic works including the P-wave tomography images reveal the existence of low velocity anomalies in the transition zone suggesting possibly a wet transition zone. Elevation of the 4 10 km discontinuity and lack of clear evidence for a metastable olivine wedge in cold slabs may be consistent with the equilibrium phase transformation from olivine to wadsleyite in cold and wet slabs on the basis of our kinetic study of the transformation under wet conditions. (C) 2004 Elsevier B.V. All rights reserved.