地学前缘 ›› 2010, Vol. 17 ›› Issue (3): 114-126.

• 论文 • 上一篇    下一篇

地幔转换带中的水及其地球动力学意义

杨翠平,金振民,吴耀   

  1. 1中国地质大学(武汉) 研究生院, 湖北 武汉 430074
    2中国地质大学(武汉) 地球科学学院, 湖北 武汉 430074
    3中国地质大学(武汉) 地质过程与矿产资源国家重点实验室, 湖北 武汉 430074
  • 收稿日期:2010-02-09 修回日期:2010-04-05 出版日期:2010-05-15 发布日期:2010-06-18
  • 作者简介:杨翠平(1986—),女,硕士研究生,构造地质学专业。Email: ycpw@msn.com
  • 基金资助:

    国家自然科学基金“华北克拉通破坏”重大研究计划重点资助项目(90714005)

Water in the mantle transition zone and its geodynamic implications.

 YANG  Cui-Beng, JIN  Zhen-Min, TUN  Yao   

  1. 1Graduate School, China University of Geosciences(Wuhan), Wuhan 430074, China
    2Faculty of Earth Sciences, China University of Geosciences(Wuhan), Wuhan 430074, China
    3State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences(Wuhan), Wuhan 430074, China
  • Received:2010-02-09 Revised:2010-04-05 Online:2010-05-15 Published:2010-06-18

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摘要：

综述了近20年国际上地幔转换带中水的研究进展。前人研究表明,地球深部的水主要以OH-(hydroxyl)形式存储在名义上无水矿物(NAMs)中。高温高压实验研究表明,地幔转换带中的主要矿物均具有较高的储水能力,且在转换带的温压条件下,其储水能力随着温度的升高而降低,其中瓦兹利石(βOl)和林伍德石(γOl)的储水能力为2%~3％,超硅石榴子石(Mj)的储水能力为01％左右,据此估算地幔转换带的储水能力约为12%~191％,是地表水总量的39~62倍;而转换带除外的上地幔和下地幔主要矿物的含水量或储水能力均小于01％,因此与上、下地幔相比,地幔转换带可能是地幔的主要储水库。尽管地幔转换带具有较强的储水能力,但对地幔转换带的实际含水量还存在干、湿两方面的地质和地球物理证据和争议。地幔转换带中的水会对转换带中一系列的过程产生重要影响,当水含量增加时,橄榄石(Ol)向βOl、γOl分解以及超硅石榴石的分解反应分别向低压、高压和低压方向迁移,从而由橄榄石向βOl和γOl分解两个相变反应界定的转换带宽度也会增加;水还会使地幔深部的部分熔融温度降低,熔体的密度降低;同时,水的加入可以很好地解释地幔岩“pyrolite”模型在410 km不连续面处产生的与地震波测量不相符突变,也可以解决全地幔对流模式所不能解释的地幔成分分层问题。因此,深入研究和探讨转换带中的水对地球深部动力学过程的影响,包括中国东部地区受太平洋板块深俯冲作用的影响,均具有重要的约束和研究意义。

关键词: 名义上无水矿物(NAMs), 结构水, 地幔转换带

Abstract:

This article presents an overview on progress in the studies of water in mantle transition zone during the past two decades. It has been shown that water in the Earths deep interior is stored in nominal anhydrous minerals (NAMs) in the form of OH-(hydroxyl). HP/HT experiments revealed high solubility of hydroxyl in the minerals of mantle transition zone, which decreases with increasing temperature. The solubility of hydroxyl is up to 2%3% in β and γolivine, and about 01％ in majorite. So we can estimate that the mantle transition zone can dissolve 12%191％ water, about 3962 times of the water in ocean. The mantle transition zone is potentially the largest water reservoir in the mantle because the water solubility of the upper mantle (excluding the transition zone) and the lower mantle is not more than 01％. However, high water solubility may not lead to high water contents in the transition zone. A hydrous and dry transition zone have been argued and supported by various pieces of geological and geophysical evidence. If mantle transition zone is hydrous, many processes in deep Earth can be influenced. Under hydrous conditions, the pressure will be lower for α to β and postgarnet transitions and higher for postspinel transition compared with anhydrous conditions, resulting in a thicker mantle transition zone. Water can also lower the temperature of partial melting and increase the degree of melting. A hydrous mantle transition zone can also well explain the sharp “410 km” seismic discontinuity, the chemical difference between ocean island basalts (OIB) and midocean ridge basalts (MORB). The studies of the water in transition zone are of great importance for a better understanding of many geodynamic processes in the deep interior of the Earth including the deep subduction in Eastern China.

Key words: Nominal Anhydrous Minerals (NAMs), water solubility, mantle transition zone

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