Metasomatism in Subduction Zones of Subducted Oceanic Slabs, Mantle Wedges, and the Slab-Mantle Interface
Metasomatism in Subduction Zones of Subducted Oceanic Slabs, Mantle Wedges, and the Slab-Mantle Interface
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DOI：
10.1007/978-3-642-28394-9_9
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发表时间：
2013
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影响因子：
--
通讯作者：
G. Bebout
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作者：
G. Bebout
化学性质不同的岩石的物理并置，以及高达数十公里尺度的“流体”的流动性，导致俯冲带中无数的交代作用，更大规模的表现包括与质量通量相关的交代作用，导致弧岩浆作用和汇聚边缘挥发物循环。俯冲带交代作用是在非常浅的层面开始的，因为进入海沟的海洋板块弯曲并可能被海水渗透，并且随着沉积部分开始进入弧前，导致广泛的物理压实、流体排出和成岩蚀变。弧前流体地球化学研究（例如，在增生杂岩中，以及在马里亚纳边缘，在蛇纹岩海山中）追踪这种浅层交代蚀变，而高压和超高压变质岩套提供了流体生成和流动的记录，以及接近火山锋下方深度的相关交代作用（在少数情况下，深度超出了弧下的深度）。关于沿着板片-地幔界面（即“俯冲通道”）的强烈机械混合带的地球化学影响仍然存在不确定性，这些影响以许多变质岩套中的混杂岩带为代表。实验研究预测俯冲带“流体”的物理化学性质和交代能力随着深度的变化而发生巨大变化。然而，变质岩组的研究尚未记录这种变化的结果，为了实现这一目标，需要对代表 100 公里或更大俯冲带深度的 UHP 组进行进一步的工作。为了检验普遍接受的假设，即俯冲带变质作用充当地球化学“过滤器”，改变深俯冲岩石的成分，从而导致弧岩浆作用和洋岛玄武岩采样的更深地幔的地球化学异质性，对更高 P 套件的研究也是必要的。
Physical juxtaposition of chemically disparate rocks, and the mobility of “fluids” at up to tens of kilometer scales, lead to myriad metasomatic effects in subduction zones, with larger scale manifestations including the metasomatism related to mass flux leading to arc magmatism and convergent margin volatiles cycling. Subduction-zone metasomatism is initiated at very shallow levels, as oceanic slabs entering trenches bend and are potentially infiltrated by seawater and as sedimentary sections begin their journey into forearcs resulting in extensive physical compaction, fluid expulsion, and diagenetic alteration. Studies of forearc fluid geochemistry (e.g., in accretionary complexes and, for the Marianas margin, in serpentinite seamounts) track this shallow-level metasomatic alteration, whereas high- and ultrahigh-pressure metamorphic suites provide records of fluid generation and flow, and related metasomatism to depths approaching those beneath volcanic fronts (and, in a smaller number of cases, depths beyond those beneath arcs). Uncertainty remains regarding the geochemical influence of strongly mechanically mixed zones along the slab-mantle interface (i.e., in the “subduction channel”), represented by mélange zones in many metamorphic suites. Experimental studies predict dramatic change in the physicochemical properties, and metasomatic capabilities, of subduction-zone “fluids,” as a function of depth. However, the studies of metamorphic suites have yet to document results of this change and, toward this goal, further work is warranted on UHP suites representing subduction-zone depths of 100 km or greater. Work on higher-P suites is also necessary in order to test the generally accepted hypothesis that subduction-zone metamorphism serves as a geochemical “filter” altering the compositions of deeply subducting rocks that then contribute to arc magmatism and the geochemical heterogeneity of the deeper mantle sampled by ocean-island basalts.