Sediment and ocean crust both melt at subduction zones
Sediment and ocean crust both melt at subduction zones
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沉积物和洋壳均在俯冲带融化
DOI：
10.1016/j.epsl.2022.117424
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发表时间：
2022
影响因子：
5.3
通讯作者：
Langmuir, Charles H.
中科院分区：
文献类型：
--
作者：
Turner, Stephen J.;Langmuir, Charles H.
当来自俯冲的大洋板块的含水物质被输送到火山弧下的地幔时，就会产生含水弧状岩浆，尽管物质输送的机制仍然存在争议。板片组分的地球化学特征对下沉板块的热结构和元素进入深部地幔的通量具有重要的意义。如果板坯温度较低，则板坯中的元素通量将在稀薄的流体中携带。如果温度较高，板材可能反而会融化。虽然长期以来，弧形火山作用的一个范例是沉积物融化，洋壳脱水，但越来越多的来自弧形地球化学和实验岩石学的证据表明，沉积物和洋壳都融化了。许多元素在水中的低溶解度阻碍了它们对电弧质量平衡的实质性贡献。受锶浓度和87锶/86锶比值的限制，需要来自洋壳的大量锶，而这只有在地壳融化时才有可能。弧形火山岩的H2O/Sr比值与板条流体也不一致。这些结论得到了热力学模型的支持，这些模型表明，对于大洋地壳和沉积物来说，板坯温度都超过了含水固相线。对实验数据的检验表明，氧逸度对板坯熔炼过程中的残留相可能有很大的影响。如果洋壳在FMQ和NNO+2之间的氧化条件下在所有弧下熔化，那么弧光数据就能得到最好的解释。对沉积物的实验限制也需要熔化，沉积物熔体的组成取决于主体成分和温度。如果这些实验类似于弧下熔化的沉积物，那么沉积物的主体成分是任何基于稀土元素的板材温度计的必要输入。我们根据现有的实验、物理模型和弧形数据，提出了洋壳部分熔体的组成和沉积物熔融的分配系数，可用于弧火山作用的地球化学模型。
Hydrous arc magmas are produced when water-bearing materials from subducted oceanic plates are transported to the mantle beneath volcanic arcs, though the mechanism of mass transport remains debated. The geochemical characteristics of the slab component have important implications for the thermal structures of down-going plates and the fluxes of elements into the deep mantle. If slab temperatures are low, then elemental fluxes from the slab will be carried in a dilute fluid. If temperatures are high, the slab may melt instead. While a long-standing paradigm for arc volcanism has been that sediments melt and ocean crust dehydrates, a growing body of evidence from arc geochemistry and experimental petrology suggests both sediment and ocean crust melt. The low solubility of many elements in aqueous fluids prevents them from making a substantial contribution to arc mass-balance. Constraints from Sr concentrations and87Sr/86Sr ratios require a large flux of Sr from the ocean crust, which is only possible if the crust melts. H2O/Sr ratios of arc volcanics are also inconsistent with slab fluids. These conclusions are supported by thermo-mechanical models indicating that slab temperatures exceed the hydrous solidus for both ocean crust and sediments. Examination of experimental data shows a likely strong effect of oxygen fugacity on residual phases during slab melting. Arc data are best explained if the ocean crust melts beneath all arcs under oxidizing conditions somewhere between FMQ and NNO+2. Experimental constraints on sediments also require melting and that sediment melt compositions depend on bulk composition as well as temperature. If these experiments serve as analogs to sediment melting beneath arcs, then sediment bulk compositions are a necessary input for any rare earth element-based slab thermometer. We present compositions for ocean crust partial melts and partition coefficients for sediment melting based on existing experiments, physical models, and arc data, that can be used in geochemical models of arc volcanism.
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发表时间：
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