Fluids and trace element transport in subduction zones
Fluids and trace element transport in subduction zones
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DOI：
10.2138/am-2017-5716
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发表时间：
2017
影响因子：
3.1
通讯作者：
H. Keppler
中科院分区：
文献类型：
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作者：
H. Keppler
来自墨西哥和堪察加的原始弧玄武岩的熔体包裹体数据表明，“流体活动元素”/H2O比值与Cl/H2O比值呈明显的正相关，表明与氯化物的络合作用显著提高了俯冲带流体中的微量元素含量。这种效应不仅存在于大离子亲石元素(LILE)中(如Rb和Sr)，而且还存在于轻稀土元素(REE，如La和Ce)以及U中。这些元素与Cl/H2O的相关性不能用向地幔源区添加沉积熔体或板片熔体来解释，因为Cl对这些元素在硅酸盐熔体体系中的溶解度或分配没有影响。另一方面，观察到的微量元素丰度与氯的关系与大量实验数据一致，这些数据表明，加入氯化物后，分配到水溶液中的能力大大增强。因此，一种稀薄的含氯水流体似乎是LILE、轻稀土和U从板条到弧形熔融源的主要载体。此外，Ce/H2O比值升高与流体盐度明显相关，因此不适合作为“平板地温计”。从实验数据和熔体包裹体数据的概要来看，沉积物或板片熔融在弧岩浆形成中的重要性可能被高估了，而从地幔楔体中清除微量元素的作用可能被低估了。此外，建立作为压力、温度和盐度函数的微量元素的流体/矿物分配系数的可靠数据集需要额外的努力，因为大多数常用的实验策略都有严重的缺点和潜在的陷阱。
Abstract Melt inclusion data from primitive arc basalts from Mexico and Kamchatka show clear positive correlations of “fluid mobile element”/H2O ratios with the Cl/H2O ratio, suggesting that the trace element content of subduction zone fluids is strongly enhanced by complexing with chloride. This effect is observed for large-ion lithophile (LILE) elements, (e.g., Rb and Sr), but also for the light rare earth elements (REE, e.g., La and Ce) as well as for U. The correlations of these elements with Cl/H2O cannot be explained by the addition of sediment melts or slab melts to the mantle source, since Cl has no effect on the solubility or partitioning of these elements in silicate melt systems. On the other hand, the observed relationship of trace element abundance with Cl is consistent with a large body of experimental data showing greatly enhanced partitioning into aqueous fluid upon addition of chloride. Accordingly, it appears that a dilute, Cl-bearing aqueous fluid is the main carrier of LILE, light REE, and U from the slab to the source of melting in arcs. Moreover, elevated Ce/H2O ratios clearly correlate with fluid salinity and therefore are not suitable as a “slab geothermometer.” From a synopsis of experimental and melt inclusion data, it is suggested that the importance of sediment or slab melting in the generation of arc magmas is likely overestimated, while the effects of trace element scavenging from the mantle wedge may be underestimated. Moreover, establishing reliable data sets for the fluid/mineral partition coefficients of trace elements as a function of pressure, temperature, and salinity requires additional efforts, since most of the commonly used experimental strategies have severe drawbacks and potential pitfalls.