Brucite as an important phase of the shallow mantle wedge: Evidence from the Shiraga unit of the Sanbagawa subduction zone, SW Japan
Brucite as an important phase of the shallow mantle wedge: Evidence from the Shiraga unit of the Sanbagawa subduction zone, SW Japan
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水镁石作为浅地幔楔的重要相：来自日本西南部三场川俯冲带白发单元的证据
DOI：
10.1016/j.lithos.2016.02.022
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发表时间：
2016
期刊：
影响因子：
3.5
通讯作者：
Yoshihiro Asahara
中科院分区：
文献类型：
--
作者：
Hirokazu Kawahara;Shunsuke Endo;Simon R. Wallis;Takayoshi Nagaya;Hiroshi Mori;Yoshihiro Asahara
大部分的浅地幔楔被认为是水化的，由于流体从俯冲板片和上覆地幔岩石的蛇纹化释放。如果蛇纹石化作用在低SiO2活性下进行，水镁石可能是蛇纹石化地幔楔低温（< 450 °C）部分的主要物相，但只有很少的自然例子被记录。综合岩石学、地球化学和地质学研究表明，水镁石广泛分布于日本西南部三坝川变质带的楔状幔源白贺变麻粒岩体中。热力学模型结合大块岩石成分和点计数表明，原始完全水合的三坝川地幔楔的浅部含有约10-15体积%的水镁石在白贺体折返开始之前和峰变质条件之前。不含水镁石、基本上为单矿物叶蛇纹石蛇纹岩的明显区域仅限于矿体100米厚的边缘区域。这表明浅地幔楔中板片流体对硅的交代作用是有限的。水镁石的存在可能会强烈影响蛇纹岩的水收支和力学性质;在研究浅地幔楔的行为时，应考虑这些因素。
Large parts of the shallow mantle wedge are thought to be hydrated due to release of fluids from the subducting slab and serpentinization of the overlying mantle rocks. If serpentinization proceeds under low SiO2activity, brucite can be a major phase in the low-temperature (< 450 °C) part of the serpentinized mantle wedge, but only very few natural examples have been documented. A combined petrological, geochemical, and geological study shows that brucite is widely distributed in the wedge mantle-derived Shiraga metaserpentinite body in the Sanbagawa metamorphic belt of SW Japan. Thermodynamic modeling combined with bulk rock composition and point counting indicates that the original fully hydrated shallow parts of the Sanbagawa mantle wedge contained ~ 10–15 vol.% brucite before the onset of exhumation of the Shiraga body and before peak metamorphic conditions. A distinct zone of brucite-free essentially monomineralic antigorite serpentinite occurs limited to a 100-m-thick marginal zone of the body. This indicates a limited degree of Si-metasomatism by slab-derived fluids in the shallow mantle wedge. The presence of brucite may strongly affect the H2O budget and mechanical properties of serpentinite; these should be taken into consideration when examining the behavior of the shallow mantle wedge.