Transportation of H2O beneath the Japan arcs and its implications for global water circulation
Transportation of H2O beneath the Japan arcs and its implications for global water circulation
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DOI：
10.1016/j.chemgeo.2006.08.011
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发表时间：
2007-04
期刊：
影响因子：
3.9
通讯作者：
H. Iwamori
中科院分区：
文献类型：
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作者：
H. Iwamori
为了了解俯冲带中H2O的运输和循环，对地质和地震观测以及相应的定量建模进行了回顾和巩固，特别强调了日本弧。日本弧是理想的这样一个研究，（1）他们是非常活跃的岩浆和地震与丰富的高质量的观测，（2）俯冲参数变化显着沿着日本弧允许一个调查的变化“输入”参数如何影响所产生的结构和过程。首先，介绍了水的传输和熔化的数值模型。根据橄榄岩和玄武质系统的实际相关系，考虑了含水流体的生成和迁移，与对流固体的相互作用以及熔融作用。然后讨论了模型在日本岛弧上的应用以及模型结果与沿着日本岛弧的观测结果（火山和地震构造的分布）之间的比较。来自日本东北弧、日本中部弧和日本西南弧的三个案例阐明了以下几点。首先，从俯冲洋壳释放的含水流体在俯冲板片上方的地幔楔中形成蛇纹岩层，在含水流体迁移过程中保持接近平衡的水化-脱水反应。第二，大部分的水被俯冲到蛇纹岩层中的蛇纹岩和碳酸盐岩分解的深度。这个深度取决于板块的热结构，对于日本弧下的较老板块来说更大。因此，含水流体和熔体，一般来说，不直接向上供应到火山前。这一结果与传统观点大相径庭。最后，根据对日本弧的认识，讨论了全球H_2O的收支和环流。即使在俯冲物质中的主要含水矿物相脱水完成后，特别是在俯冲板片上方的地幔楔底部，上地幔组合的名义上无水相（即，橄榄石，辉石，石榴石）可以携带大量的H2O（1.1× 1011至7.8× 1011千克/年）进入地幔深处。最小估计值与大洋中脊和热点地区地幔中H2O的流出量相当，而最大估计值大大超过了它。估计范围大主要是由于上地幔名义无水矿物中最大H2O含量的不确定性，这控制了H2O通过俯冲带净流入深部地幔。水;俯冲;岩浆活动;全球环流
In order to understand the transportation and circulation of H2O in subduction zones, geological and seismological observations and the corresponding quantitative modeling are reviewed and consolidated, with a special emphasis on the Japan arcs. The Japan arcs are ideal for such a study in that (1) they are very active magmatically and seismically with abundant high-quality observations, and (2) subduction parameters change significantly along the Japan arcs allowing one to investigate how the variations of ‘input’ parameters affect the resultant structures and processes. First, numerical models for the transportation of H2O and melting are introduced. The generation and migration of aqueous fluid, its interaction with the convecting solid, and melting are considered, based on the realistic phase relationships of the peridotitic and basaltic systems. Application of the models to the Japan arcs and comparison between the model results and the observations (distribution of volcanoes and seismic structures) along the Japan arcs are then discussed. The three cases from the NE, central and SW Japan arcs clarify the following points. First, an aqueous fluid released from the subducting oceanic crust forms a serpentinite layer in the mantle wedge just above the subducting slab, maintaining a condition close to equilibrium in terms of hydration–dehydration reactions during aqueous fluid migration. Second, most of the H2O is subducted to a depth where serpentine and chlorite in the serpentinite layer break down. This depth depends on the thermal structure of the slab, and is greater for the older plate beneath the Japan arcs. As a consequence, aqueous fluid and melt are, in general, not supplied straight upward to the volcanic front. This result is rather different from the conventional view. Finally, based on the understanding obtained from the Japan arcs, the global budget and circulation of H2O are discussed. Even after the completion of dehydration of major hydrous mineral phases in the subducting materials, especially at the base of the mantle wedge just above the subducting slab, nominally anhydrous phases of the upper mantle assemblage (i.e., olivine, pyroxenes, garnet) can carry a significant amount of H2O (1.1×1011to 7.8×1011kg yr−1) into the deep mantle. The minimum estimate is comparable to the outflow of H2O from the mantle at mid-ocean ridges and hotspots, while the maximum estimate greatly exceeds it. The large estimated range is mainly due to the uncertainty of the maximum H2O contents in the nominally anhydrous minerals of the upper mantle, which controls the net influx of H2O into the deep mantle through subduction zones.Water; Subduction; Magmatism; Global circulation