Metasomatism at a metapelite?ultramafic rock contact at the subduction interface: Insights into mass transfer and fluid flow at the mantle wedge corner
Metasomatism at a metapelite?ultramafic rock contact at the subduction interface: Insights into mass transfer and fluid flow at the mantle wedge corner
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俯冲界面变泥岩-超镁铁质岩石接触处的交代作用：深入了解地幔楔角处的质量传递和流体流动
DOI：
10.1007/s00410-023-02011-1
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发表时间：
2023
影响因子：
3.5
通讯作者：
Okamoto Atsushi
中科院分区：
文献类型：
--
作者：
Oyanagi Ryosuke;Uno Masaoki;Okamoto Atsushi
俯冲板片和地幔楔的交代作用影响俯冲带界面岩石的流变学和化学性质。我们调查了一个蛇纹岩体，起源于地幔楔在富里地区的三坝川变质带，日本西南部。在蛇纹岩体与泥质片岩的交界处，发育有泥质片岩、钠长石片岩、白云母岩、滑石岩、透闪石片岩、滑石片岩、滑石+蛇纹岩和蛇纹岩等交代反应带。通过岩石学观察，我们发现界面上的交代作用发生在P-T峰值（0.8-1.0 GPa，530-570 °C）附近，这与地幔楔角处的交代作用相对应。质量平衡计算表明，泥质片岩的交代作用伴随着Si和Ca的去除和Mg的加入，而蛇纹岩的交代作用伴随着Ca和Al的加入，Ca由外部供给，Na、K和C则释放到外部体系中。流体介导的物质迁移导致了蛇纹岩之后的富透闪石岩石和变质沉积之后的富绿泥石岩石的形成，它们可能沿着俯冲界面和地幔楔角广泛分布。我们认为，交代作用在地幔楔角重新分配流体和岩石之间的元素，引入空间不均匀性的矿物组合在地幔楔和俯冲板，它可以产生流体超压。因此，这种交代作用将影响板幔分离和地震活动。
Metasomatism of the subducting slab and mantle wedge influences the rheological and chemical properties of rocks at the subduction zone interface. We investigated a serpentinite body that originated from the mantle wedge in the Tomisato area of the Sanbagawa metamorphic belt, SW Japan. At the boundary between the serpentinite body and a pelitic schist, metasomatic reaction zones have developed, consisting of pelitic schist, albite schist, muscovite rock, chlorite rock, tremolite schist, talc schist, talc + serpentine rock, and serpentinite. Using petrological observations, we showed that metasomatism at the interface occurred around the peakP–Tconditions (0.8–1.0 GPa and 530–570 °C), which correspond to those at the mantle wedge corner. Mass balance calculations revealed that the metasomatism of the pelitic schist was accompanied by the removal of Si and Ca and the addition of Mg, whereas the metasomatism of the serpentinite was accompanied by the addition of Ca and Al. The Ca was supplied externally, and Na, K, and C were released to an external system. Fluid-mediated mass transfer induces formation of tremolite-rich rocks after serpentinite and chlorite-rich rocks after metasediments, which could be widespread along the subduction interface and the mantle wedge corner. We propose that metasomatism at the mantle wedge corner redistributes elements between fluids and rocks, introduces spatial heterogeneities in the mineral assemblages in the mantle wedge and subducting slab, and that it could produce fluid overpressure. Consequently, such metasomatism would influence slab–mantle decoupling and seismicity.