Seismic evidence for flow in the hydrated mantle wedge of the Ryukyu subduction zone.
Seismic evidence for flow in the hydrated mantle wedge of the Ryukyu subduction zone.
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DOI：
10.1038/srep29981
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发表时间：
2016-07-20
影响因子：
4.6
通讯作者：
Kendall JM
中科院分区：
文献类型：
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作者：
Nagaya T;Walker AM;Wookey J;Wallis SR;Ishii K;Kendall JM
人们普遍认为，富含水的蛇纹岩域普遍存在于浅俯冲板片上方的地幔中，在控制俯冲带的地球化学循环和物理性质方面发挥着关键作用。热学和岩石学模型显示，主要的蛇纹石矿物是叶蛇纹石。然而，对于这种矿物的数量、分布和排列还没有达成良好的共识。地震速度通常用于识别富含叶蛇纹石的区域，但叶蛇纹石具有高度各向异性，并且根据地震射线路径，其性质很难与非水合富含橄榄石的地幔区分开来。在这里，我们利用这种各向异性，并展示了结合琉球弧中测量的射线路径几何形状的地震各向异性分析如何限制叶蛇纹石的分布、方向和数量。我们发现超过 54% 的楔子必须由叶蛇纹组成，并且排列必须从垂直排列变为与板平行。这种方向的变化表明水合的弧前地幔中存在对流。其他俯冲带的剪切波分裂分析表明，大规模蛇纹石化和弧前地幔对流可能比普遍认为的更为广泛。冷俯冲带弧前地幔是干燥的观点需要重新评估。
It is widely accepted that water-rich serpentinite domains are commonly present in the mantle above shallow subducting slabs and play key roles in controlling the geochemical cycling and physical properties of subduction zones. Thermal and petrological models show the dominant serpentine mineral is antigorite. However, there is no good consensus on the amount, distribution and alignment of this mineral. Seismic velocities are commonly used to identify antigorite-rich domains, but antigorite is highly-anisotropic and depending on the seismic ray path, its properties can be very difficult to distinguish from non-hydrated olivine-rich mantle. Here, we utilize this anisotropy and show how an analysis of seismic anisotropy that incorporates measured ray path geometries in the Ryukyu arc can constrain the distribution, orientation and amount of antigorite. We find more than 54% of the wedge must consist of antigorite and the alignment must change from vertically aligned to parallel to the slab. This orientation change suggests convective flow in the hydrated forearc mantle. Shear wave splitting analysis in other subduction zones indicates large-scale serpentinization and forearc mantle convection are likely to be more widespread than generally recognized. The view that the forearc mantle of cold subduction zones is dry needs to be reassessed.