B-type olivine fabrics developed in the fore-arc side of the mantle wedge along a subducting slab
B-type olivine fabrics developed in the fore-arc side of the mantle wedge along a subducting slab
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DOI：
10.1016/j.epsl.2008.06.014
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发表时间：
2008-08
影响因子：
5.3
通讯作者：
M. Tasaka;K. Michibayashi;D. Mainprice
中科院分区：
文献类型：
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作者：
M. Tasaka;K. Michibayashi;D. Mainprice
b型橄榄石组构普遍存在于日本西南弧俯冲型三川变质带内的小型伊莫诺橄榄岩体的高度贫化砾岩中。邓恩岩含有各种微观结构，从斑岩碎屑到细粒的强烈剪切结构。这些白云石中橄榄石的Mg/(Mg+Fe)原子比（Fo数）一直在0.9左右，铬尖晶石的Cr/(Cr+Al)原子比（Cr数）也一直在0.9左右，表明它们是从高度枯竭的岩浆（宝钢岩）演化而来的。这些数据为高度枯竭的dunites的形成提供了强有力的热约束，因为它们的形成需要地幔楔中热的、含水的浅地幔（<30 km深度的>1250°C）。由于三巴川变质带在渐退过程中最终带走了这些橄榄岩，因此b型橄榄岩组构可能发育在俯冲带的弧前侧、俯冲板块上方或沿俯冲板块发育，可能与板块的脱水流体有关。先前记录的小震级s波分裂可以用b型橄榄岩的地震特性来解释，这些橄榄岩位于厚度约为几公里的各向异性层中，由平行于俯冲板块的流动定向，根据流动应力的不同，最高温度为880-1030°C。这些发现表明，这种b型层可能是地幔楔块区地震S波和p波各向异性的主要来源。
B-type olivine fabrics are pervasive within highly depleted dunites of the small-sized Imono peridotite body located within the subduction-type Sanbagawa metamorphic belt of the southwest Japan arc. The dunites contain various microstructures, ranging from porphyroclastic to fine-grained intensely sheared textures. The Mg/(Mg+Fe) atomic ratios (Fo number) of olivine within these dunites are consistently around 0.9, as are the Cr/(Cr+Al) atomic ratios (Cr number) of chromian spinel, suggesting their evolution from a highly depleted magma (boninite). These data provide strong thermal constraints on the formation of the highly depleted dunites, as their formation requires hot, hydrous, shallow mantle (>1250 °C at <30 km depth) in the mantle wedge. Because the Sanbagawa metamorphic belt finally entrained these peridotites during progressive retrogression, B-type olivine fabrics probably developed in the fore-arc side of the subduction zone, above or along the subducting slab, possibly in association with dehydration fluids derived from the slab. The previously documented small magnitude of S-wave splitting can be explained by the seismic properties of B-type peridotites within an anisotropic layer of approximately several kilometers in thickness, oriented by flow parallel to the subducting slab, under maximum temperatures of 880–1030 °C depending on the flow stress. These findings indicate that such a B-type layer could constitute a dominant source of seismic S- and P-wave anisotropy in mantle wedge regions.